AD-A236 360REPORT DOCUMENTAl ION P'AGE

0- - lb RE S RIL I'VE NMARKINGS

'a SECURITY CLASSIFICATION A~lJHORITy 3 DIST P.1IUTION /AVAILABILIT Y Of REPORT

20 DECLASSiFICATION /DOVNCRADING SCHfEDULE

4 PERFORMING ORGAN'ZAtION REPORT NUMBER(S) 5 MONI!OR'NG O 4 A ZAflON RPrORT NUMBER(S)

'a NA VE OF FIRFOR,%"NG ORGANIZATIO N EbOFFICE SyMP-OL 7a NA%'[ OF %MONJTOT1%G ORGANIZATION

Thie University o Alabama 16 If.r'b)

Q' ADDDESS (Cit-y. State, and 11ZIP e1' 'b At.[Wf SS (City State. and lip Cod.)

Box 870104 ONR Re.-iide~nt Rpeu

Tuscaloosa, AL 35467-O1O4 (vor 4i Institute of luclino boy

_________________________ ____________ 06 O'Keefe Building,, Atlanta', (A 303)2- :jo.Y

a, PJAPME OF FU'NDING /SPONSC'lING I b 0tFI( SiMP-J 9 rkiOC,,I.(MENT INSIRUMENI lot NTIIICATION NUMBER

ORGANIZATION 'o pplaab'o)

Office of Naval Research- ColIit B:DW C N00014-86-(;-0215

9C ADDRESS (City. State, and ZIPI Co1 0 SOURCE Of FUNDING NUMB8ETIS

800 North QuLinc% Street PPOGRFAM PRJC AKI~)KUI

Arlington, VA _22-217-5000 ELEMENT NO No NO ACSINN

11 TTLE(IncudeSecvrity Clasp,catlon)IIIUltra Thin Film Characterization of the", Or'ai Retfer ro t

12 PE RSONAL AUTHOR(S)Robert M. Metz.oer

!" TPE Oc RPPPT lilt) TII'E COVrE.L 14 ')ATE flt PcPORT (Year. Month.D.Day 1 '5 PA.r rOON4T'-i na L F-;OA0/01/ 86 oO 7/ 31/S~ 910-0

'SUPPLEMENTARY NOTATION II1 910'0

17 CO-SAT CODE-, 18 SUBJECT TEPNMS (Continue on re.'e'se if necejsary and identi ty by block number)

)FlLD GROUP SU8GP~JP

'I '.P'.'PACT (Continue on reverse if r -esiary and identify by block nunt-~er)

A Fourier Transform Infrared Spectrometer (Bruker IFS-88), extended wave number range

400 - 40,000 I/cm, with FTIR Microscope), an automated Elupsometer (Rudolph Auto-EL-II),

anda Lbortor Sperminicomputer (Digital Equipment Corp. Microvax - 1I') were purchased.

Seven publications can be I'dentified as deriving in part from this major equipment purchase.

20 DSTPVUTIO*4 /AVAILABILITY OF A9SIRACT 21 ABSTRACT SECURITY CLASSIFIC.ATION

M UNCLA5SSFIEDI"UNLIPAITIF0 0 SAME AS RPT 0 OTIC USERS

22a NAM-E OF RESPONSIBLE INDIVIDUAL 22b IELEPHONE (include Are# Code) 22C OW(IE SYMBOL

Robert M. Metzg~er (205) 348-5952 1

DD FORM 1473.84PMAR ~ 83 APR edt' mnay be used unt.1exhausled SECURITY CLASSIFICATION Or THIS PAGEAll other editions are obolete91 550 2

Organic and InorganicLow-DimensionalCrystalline MaterialsEdited by

Pierre DelhaesCNRS, Paul Pascal CenterUniversity of Bordeaux ITalence. France

and

Marc DrillonEHICSStrasbourg. France

Plenum PressNew York and LondonPublished in cooperation with NATO Scientific Affairs Division

Proceedings of a NATO Advanced Research Workshop onOrganic and Inorganic Low-Dimensional Crystalline Materials.held May 3-8, 1987.in Minorca, Baleares. Spain

Library of Congress Cataloging in Publication Data

NATO Advanced Research Workshop on Organic and Inorganic Low-DimensionalCrystalline Materials (1987: Minorca. Spain)

Organic and inorganic low-dimensional crystalline materials / edited by PierreOeihaes and Marc Drillon.

Q. cm.-(NATO ASI series. Series B. Physics; v. 168)"Proceedings of a NATO Advanced Research Workshop on Organic and Inorganic

Low-Dimensional Crystalline Materials, held May 3-8, 1987, in Minorca. Baleares.Spain"-T p. verso.

"Published in cooperation with NATO Scientific Affairs Division."Bibliography: p.Includes index.ISBN 0-30642783-4

1. Solid state physics-Congresses. 2. Solid state chemistry-Cc,.igresses. 3. One-dimensional conductors-Congresses. 4. Superconductors -Congresses. 5.Magnetic materials -Congresses. I. Delhaes. Pierre. I1. Drillon, Marc. Ill. North Atlan-tic Treaty Organization. Scientific Affairs Division. IV. Title. V. Series.QC176.A1N28 1987530.41 -dc19 87-28561

CIP

1987 Plenum Press, New YorkA Division of Plenum Publishing Corporation233 Spnng Street, New York, N.Y 10013

All rights reserved

No part of this book may be reproduced, stored in a retrieval system. or transmittedin any form or by any means, electronic, mechanical. photocopying. microfilming.recording, or otherwise, without written permission from the Publisher

Printed in the United States of America

FINAL REPORTon

UNITED STATES OFFICE OF NAVAL RESEARCHGRANT

N00014-86-G-0215"Ultrathin Film Characterization of the Organic Rectifier Project"

Award: $ 96,800.00Grant Duration: 01 August 1986 - 31 July 1988

Date of Report: 5 May 1991

Partial support (50%) was received to purchase major instruments to assist inscientific measurements for the Organic Rectifier Project. The grant was awarded on15 April 1986 to Prof. Robert M. Metzger when he was still at the University ofMississippi, but was transferred, with the consent of the Scientific Officer, Dr. KennethJ. Wynne, Code 1113, USONR Arlington, to the University of Alabama, where Dr.Metzger moved in August 1986. The original grant was for the acquisition of threeinstruments: (1) Fourier Transform Infrared Spectrometer, (2) Automatic Ellipsometer,3) Solid State Probe for planned acquisition of a Nuclear Magnetic Resonance (NMR)pectrometer. Since at the University of Alabama (Tuscaloosa) there already was a

Nicolet 200 MHz NMR with solid-state capabilities, item (3) was replaced, withpermission of Dr. Wynne, by item (3') A MicroVAX-II laboratory superminicomputer.The grant was supplemented by $ 96,800 as matching costs from the University ofAlabama, for a total budget of $ 193,600.

The following instruments were purchased:(1) Bruker IFS-88 Fourier Transform Infra-Red Spectrometer, with additional

wavelength capabilities ranging from the ultraviolet (UV), through the visible, to thenear and mid-Infrared range (frequency range 40,000 cm-1 to 400 cm-1 , or wavelengthrange 250 nm to 25 Im, resolution 0.25 cm-1), and with a mid-IR microscope (4000 -400 cm-1, or 2.5 - 25 gim). Approximate cost $ 135,000 (with discount)

(2) Rudolph Auto-EL-Ill Automatic Ellipsometer, 632.8 nm source, 700 fixedincidence and refraction. Approximate cost $ 25,400.

(3) Digital Equipment Corporation (DEC) MicroVAX-Il, with 9 MB memory, Emulex380 MB disk storage, 800-1600 bpi Kennedy 9000 magnetic tape recorder, LN03laser printer, TK-50 cartridge tape system, 16-terminal multiplexor, modem.Approximate cost $ 33,000.

The equipment has been used since 1987, and the following publications havebenefitted directly: FTIR [1-4], Ellipsometer [3], MicroVAX [5].

The use of the FTIR for detection of domains within LB films has not yet reachedfull maturity: our use sofar has been more conventional, in the spectralcharacterization of LB films [1-4]. (The recent purchase of a scanning tunnelingmicroscopy, to probe electrical conductivity through the film without worrying aboutdomain realignment, represents a shift in our research approach). Nevertheless, theFTIR has represented a growing research tool in our laboratory. and we are nowturning to studies of conducting LB films and of second-harmonic generating crystals(MAP-MNA).

The ellipsometer has been used to characterize film thicknesses [3], and will beused to obtain the index of refraction of a new second-harmonic generation crystal(MAP-MNA).

The MicroVAX-II has been used to compute fits between observed and calculatedX-ray powder diffraction data (program TAUPIN) [5], and for band structurecalculations [6], which were ancillary to investigations about the metal-to-insulatorboundary in transition metal oxidas [7]. These band structure calculations will bepublished soon.

REFERENCES.

[1] R. M. Metzger and C. A. Panetta, "Langmuir-Blodgett Films ofDonor-Sigma-Acceptor Molecules and Prospects for Organic Rectifiers", NATOASI Series, B168, 271-285 (1987).

91 5 15 012

[2] R. M. Metzger and C. A. Panetta, "Langmuir-Blodgett Films of PotentialDonor-Sigma-Acceptor Organic Rectifiers" J. Mol. Electronics 5, 1-17 (1989).

[3] R. M. Metzger, D. C. Wiser, R. K. Laidlaw, M. A. Takassi, D. L. Mattern, and C. A.Panetta, "Monolayers and Z-Type Multilayers of Donor-a-Acceptor Molecules withOne, Two, and Three Dodecoxy Tails", Langmuir 6, 350-357 (1990).

[4] R. M. Metzger and C. A. Panetta, "The Quest for Unimolecular Devices", New J.Chem. 15, 209-221 (1991).

[5] D. A. Robinson, C. Asavaroengchai, A. Morrobel-Sosa, C. Alexander, Jr., R. M.Metzger, J. S. Thrasher, M. Abbot Maginnis, and D. A. Stanley, "Zero Resistance at100 K in the Superconductor GdBa 2Cu 3O7 .x", J. Phys. C: Solid State Phys. 21,4091-4096 (1988).

[6] C. Asavaroengchai, Ph. D. dissertation, Univ. of Alabama, 1990.

[7] J. B. Torrance, P. Lacorre, C. Asavaroengchai, and R. M. Metzger, J. Soid StateChem. 90, 168-172 (1990).

W.

/U/

1isL A. pc:r tlc con lhr. K. VNi'nnc, , . ..

_/ /(

101 R%* kI'f s~til I T'.t A Mr 4I NT I 41 t !N- 1- 1441

LETTERS TO THE EDITOR

Simple and Perovskite Oxides of Transition -Metalis: Why Some AreMetallic, While Most Are Insulating

JERRY B. TORRANCE VND PHILIPPE LAC(.RRO*

IBMt Re search DaLisunt Aitnaden Re searc h Cenrer. 050 1larr Roiad.San Jo e. Calibrnia 95/20-6099

%.%D CHINNARONG ASAVAROENGCHAI %-,D ROBERT MI. METZGER

Departnent oj Chemzisirx. L nit-ersav of A lahanr. Tiscaloova.Alahaina 3_5487

Communicated b I M. Honig. October 1. 1990

Some '6 simple and perovskite transition-metal oxides are classified as "metals." "insulators. *andthose exhibiting metal-insulator transit(ions Using the framework of Zaanen. Sawatzky. and Allen anda simple ionic model to estimate the two relev'ant energies fa and L'.we can find boundlanes whichseparate the insulating oxides from two types of metals. low-%, metals and low U0, metals In addition.compounds w~ith me~al-insulator transitions are found to he on -or neari these houndanes It isco~ncluded that the large Jitferen es in conductivity behavior of oxides are largely due to ditTercrncesin the ionization potentials of the transition metal cations e 4 swJemnl P*c, In,

The discovery (1-3) of superconductivity tronic energies that determine why some ox-at high temperatures has generated consid- ides are metallic. whereas the majority haveerable effort at understanding the physical low conductivity. It is these fundamentalproperties of bismuth and copper oxides. It energies for the undoped oxide s,,,stems thatis important to recognize that these oxides provide the basis for understanding the hole-are members of the broad class of perovskite doped systems and lie behind the assump-and simple transition metal oxides that usu- tions of the various theories for high-tem-ally do not exhibit metallic conductivity and perature superconductivity,even less often exhibit superconductivity. The conductivity of a largrime ofnimAs part of an overall understanding of high pie and perovskite oxide compounds hastemperature superconductivity, it is im- been compiled in several review articlesportant to understand the fundamental elec- (4-9). In Table I we list (10) 76 such systems

involving transition metals (including rare

Permnanent address. Laboratore des Fluorures. earths) in formally divalent (11), trivalentFacultt des Sciences. L'niversiti! du Maine. F-72017 (111). and tetravalent (IV) oxidation states.Le Mans. France. Here the conductivity behavior of these ox-002-459Ni91 53.00 1IM 0022-4596191 S3.00(oVvnihi C~ 1991 hv%.dr Pvew. in Copynithi C 1991 by Academic Pir,,. ifc.Ai l~ls no o reprimduc.ion ,n inv N1,m reserve~d All niilim of reprodnciton mi any form rrereed

LI-FIERS r() THE EDl1OR I

T -B t F I I -d ! E I-( " u ni ued

%i4 ~ 1\fj 1 I) % 1) I'FRfI)%SI It 0R rion nut- %PE R' Sk ITI- 1 1 , I I () 0 DI N .4 Vt I) I R IOR . . .....

AROI. I NL oIBIR. THI OI I)RM %I % At%( I tAut 1 14 I - I.

CONDL(rf 4IT M M[T T l I( I l%SL I A f%i, Rlf) l IJ "

()XYGE%-M ET L A Iro m N.6 t I) t t N,, sirt Poi ti ki rk ,L I uqh MI I

IF. DP.I 1 IIHR( Rf %( F AV. 16 1 S M % I) P % M II-I

N% ) I. ') Rh.( I . 'b.4Rh(O I , 4" : i."

O r'vund o nduAIv1Is .1. L . I A 1,1 Rho . 'o6 is

Tt1i48 fl I ki I '%n

I ~ Ti M I ~ R i ~ 'Ifl i I'. 2L. F ri Mt I <R 2 122 I' 1 %r'ni i I 2" - 1 I1i

To(). 1 1 1 2 ~ Sr.Snhj, M ~ 4 2(I 1 2SrTtO, I 69 4 2 I I ISrJTli), I0_4 '2: 0 LaO 4 1 42 14

L ( 4411 2' I 6sV'y M 44 6 4 14 IV.0 M I 1 116 (IS C M s '4 1LAvO I 'I t 44 () I 4 '

" 1R X

LSr'(, I 94 V - 4 (ci). I i19 24 IVO,. M I l1 '" I ' sit) 3 632' 21' r20

Sr.V(), I ') 414 1 1M 400 " 6 I, I

Cr0 I I to 4 Pr.o, I 500, 1 ; , 14 4

LCrO, 1 19) 4 4 141 PrO. 625 14' 96L.aCrO, I g 14 4 14.3 SrP,'O, I 63,4' I I I0 0LaSo'ro, 1 60 '-" 14 4 14.1 SPo 34 0

Cr. M '2 I 1 NdO M 401 '3 10 1SrCrO M 71,01 164 67 N1.O I 066 145 146

MnO I 454 134 15 64-Mn.O, 1 602 129 11 6 EuO I 39 9 9' 146

L4MnO, 1 599 11 8 112 Eu.O, I 51,0 1 3 121(La.SriMnO, I 59 3' I 8 10 5

-MnO. M 'I A 16 2 6,8 Ybo I 413 8' 8 'SrMnO, I 68I ' i

120 Yh.O, I 'R 146 I'

SrMnO. I 'I , 17 4 I La'b") I 54 3 I 4 14 9

FeO I 46 9 ' Ih I Averaged between ,= ,lalhographicall, inlui~jleni slesa.FeO I 59I 19" 119 Ref 12)LaFeO, I 58 4- 20 5 12 9" In-plane f and 0 onlyLaSrFcO. I 59 2 20 4 1 14 Estimated ifrom knon Calculations, or for approkirnaicSrFeO. M 70 1 If 5 0 1 crstal structure

(o) I 47 '2 I I " I ILa.CoO, I s0 4 12 7 182La'oO, I fvJ 9 14 I 12 ides has been crudely and arbitrarily dividedLajSrCo0, I 60 5 14 0 11 -S, ,, M 70 2 24' -; into "metals" and "insulators" on the basis

N10 I " 12 1 isI of the magnitude of their conductivity,La.N#O. I 496 131 16 2 ro(300 K), at room temperature: "metals"I.rnl MI 6,1 W it 16 9 are defined as having o(300 K) > I Sicm.LaSr~l(), M 64)1 16I (! 96h

while "insulators" have cr(300 K) < I SiuO I 441 118 1 6 cm. A few oxides have metal-insulator tran-

La-C'u(), I 492 128 I'

I (,, M , 146 P h sitions and are so labeled in Table I.LSrt ui, M4 146 S A simple, and yet powerful. framework

146 ( 41 1 9 1_ 1 which includes correlation effects has beenNWo. I 1' 8 14 - introduced and developed by Zaanen. Sa-

t-MoO. m 69 2' lo i - 9 watzky, and Allen (ZSA) (I1). AccordingSr-Mo, M 682 -112 1 to this picture, oxides (as well as halides,SrMoo). M 6c2 I I I i_______________________________ sulfides. etc.) can be described in terms of

170 LETERS To) THE EDIIOR

A metal results when either of these twogaps approaches zero. Hence. there existtwo tpe, of correlated electron metals.

1) Low-.1 metak .w hen A < 4 (far leftof Fig. Ii. in Ahich the lowet-Iking metal

-, , . onduction hand o.erlap,, the occupied ow,..gen 2p-,,alence states. and

12) "Lo,-U' metals". when L' < W'(farright of Fig. 1). in % hich case the two metalorbital, merge to form a partiallh filled band.

". ... ........... -he goal of this paper is to obtain valuesof U' and ._ for each of the 76 compoundsin Table l and examine these w ithin the ZSAframework. While experimental values for

FIG. I A schematic diagram of the energ !e,,els of a few of these compounds are available, athe ZSA frame..ork. -,pec&i ing the definitions of .1. comparison among all oxide systems is moreU,). and W. appropriately carried out with a self-consis-

the relative energies of three electronic en- tent set of approximate values, obtained inergy levels near the Fermi level, shown in the same manner for each. Such a set mayFig. 1. The fully occupied oxygen 2p states be obtained, if we approximate these oxidesare shown as the shaded band on the left as ionic solids containing transition metalside of the vertical energy axis. On the right cations M " and oxygen anions 02-. with aside of the vertical energy axis and shown negligible wave function overlap. The val-unshaded is the lowest unoccupied metal ues of U' and A obtained in this approxima-orbital (corresponding to the metal conduc- tion are called U; and .10, where the zerotion band) which lies at an energy A above subscript serves as a reminder that they arethe former. A is seen to increase in going theoretically calculated values for the sim-toward the right in Fig. I. This unoccupied pie ionic model. The energy U; correspondsmetal orbital lies at an energy U' above the to the excitation of an electron from onehighest occupied (shaded) 3d- or 4d-metal transition metal cation to its neighbor (at astates, as shown in Fig. I. (The prime on U' distance d,-,,). and is specified in terms ofsignifies that this energy (defined in Fig. I) the ionization potential 1,., of M' " and itsis not always the Hubbard U.) For simplic- electron affinity A 1,:ity. we assume that the width. W, of thesethree bands is the same. U,' = 1, -1 M) - e-w (I)

For the case of W < .A. U'. the compounds Correspondingly, the energy . to excite anare insulating, and ZSA (II ) distinguish two electron from 0-' to a neighboring transi-different types of insulators: tion metal (at a distance dw-o) involves the

(I) Charge transfer insulators when W < ionization potential 1(02 - ) of 02- (the nega-- < U' (left of Fig. I). In this case, the gap tive of the electron affinity IA(O-)) and the-(A - W) is dominated by the value of A; electron affinity A = -1, of M". In addi-and tion, there exists a term AVf. the difference

(2) Mott-Hubbard insulators, when W < in electrostatic Madelung site potentials,U' < a (right of Fig. 1). Here the gap -(U' that the electron experiences when it- W) is dominated by U'. changes sites:

LETTERS TO THE EIITOR

horizontal one. such that the metals haseeither A, --- A = 10 eV or U, E U,* = I1eV. Thus, there are eight metals in Fig. 2

54 shich %e can classify as low-U' metals and16 metals which are classified as lo%,-A met-als, Within this framewsork. oxides with

- metal-insulator transitions should fall on tornear) the boundaries in Fig. 2. Such is thecase for each of the fi%,e examples in Table

NP , _I (shown in Fig. 2 as smbols with a dot inthe center).

We should recall the approximationsmade in the ionic model calculations of ..

1 and U,. We have neglected the effects of theelectronic overlap between ions (covalency.crystal field splittings. screening. electronicpolarizability) and the motion of the ions

- ' (lattice relaxation, ionic polanzability). Our• : :.., basic assumption is not that these effects

0 5 0 5 Ze are small, rather, that they are similar for allthe oxides in Table I, so that the differencesin % and U' are caused by the large differ-

Fic. 2. A plot of the calculated values of U; and 10 ences in Iv and AV, via Eqs. (I) and (2).for the 76 oxides in Table t. The lines attempt to sepa- The main consequence of these neglectedrate the "insulators" (open symbols) from the "met-als" I solid symbols) according to the ZSA framework. effects is to reduce (or screen) the actual

values of . and U' well below the ionicvalues of Ao and U; in Table 1. For example.o= , - /I.(M) - (O . o xape

12) for an electron or a hole in La,CuO4,. thedecrease in energy due to screening is calcu-

In this very simple ionic model, the values lated (161 to be -5-6 eV. For an excitationfor U; and A0 depend only on the electro- of a neighboring electron and hole (as forstatic interactions between ions and the gas- U and A,). the decrease should be consider-phase ionization potentials. In Table I, we ably less than twice this value. This screen-show values of AV 4 , calculated earlier (12) ing will decrease the metal-insulator bound-or here (13), together with the values of U; aries at A - 10 eV and b'k - II eV toand A, calculated from Eqs. (1) and (2), us- values comparable with estimates (/7) of theing the experimental (14) gas-phase values electronic bandwidth W - 6 eV. Thus. thefor /, , while a value (15) of 7.70 eV was used magnitudes of AB and U.' in Fig. 2 are notfor A(-). In Fig. 2, we plot U0 versus A, unreasonable.foreach of the simple and perovskite oxides. There are a few insulating oxides that lieIn this letter. we present the major features deep in the metallic regions of Fig. 2. suchof these data and discuss the details else- as SrMnO,, Sr.,MnO,. and NbO, (which haswhere (/0). It is clearly seen in Fig. 2 that the a strong pairing of metal cations). There are"metallic" oxides tend to have low values of several more minor exceptions nearer theeither A or U;. In fact, one can separate boundary. many of which can (10) be relatedmost of the "insulators" from the "metals" to the assumption that the bandwidths Wby drawing a vertical boundary line and a and hence the boundaries U and AB are the

172 TIrR~S To rHF EDITOR

same for all three types of orbitals in Fig. I I'R I ~vBHi ~.IIK~jand for all oxides. Among the oxide-, in Ta- F k Ri PP \tor WI 312

ble I there are significant difference, in VVW 1 I'4XI

betv~een the rare earth and the first ,eries 4 ri r1&In B I4.~ c

irafltmon meild osides. fore simple. as ~elI Lig. Berlin P,() Si %.% I ki k I li

.1" difference, due to di-stort ions and to dif- lirnocen % Aele II l I 2. m

terent dimensionalities. Taking these fac- lag Berlin itly-i J H %."If 1i 14

tors into consideration I1/0). there still re- ,li sit ( hem 5, 1-19119-11

mains an uncertaintY of -1-2 eV in the 1) %D R, Radi ff 4. 1",

location of the boundanes in Fig. 2. "hich .di A . 59 * 19-01. (C NH R 9.oAn , tis an estimate of the relative errors in the Phv. Chemn 40. 291 119MY1agreement between the data and our 'xiic M Pot. ( H RDetl 'tai, , R t im'p P-, 1%.

model of the ZSA framework. Considering 011989) Id~~~.i.nh.

the~~~~~~ ~ ~ ~ ~ sipiiyo h oi oe n h SAtso.%o% l- e1)ieHnhwlthe implcit of he onicmodl an th I Plenum. Ne% 't ,rk Ii '

number of factors neglected, the combina- A~ W Si EiGHT. -Prceedirigs of the Wekh Contion of an ionic model and the ZSA frame- ference on Vaiencsj- p i23 1989)work forms a remarkably good starting point lii. For a more comnplete account of this wNork, 'cc

for an understanding of the electronic struc- JB ToRRLANCE. P LA(ORRE. C As A1.RIN,

ture of both simple and perovskite oxides (HAI. and R. M. %IFIGFR. submeitc for putqi-

of transition metals. In this way, a simple ,/, ZASEN . G, A. SAWATZKIV. ADJ 'A %I I F%

picture is able to account for the dilfferences Phv5. Rev. Lett 55, 418 u1985).in conductivity for a wide range of oxides 12. J. Q. BROUGHTON AND P. S. BAGL'S. J. Ve( ron

and to attribute these differences pfimarily Spectrosc. Relai. Phenotn 20, 261 (19M)

to differences in the ionization potentials of 13, R. M. METZGER. J. Chemn. Phvs. 57. 1870 11972).J. Chiem. P/evs. 64, 2069 11976). J. B TORRANCE

the transition-metal cations. It is hoped that and R. M. METZGER. Phvs, Rev. Lett 63. 1515such a broad perspective of oxides will com- ( 1989).plement the common approach of examining 14. -CRC Handbook of Physics and Chemisry." CRCeach particular compound in detail. PreCss. Boca Raton, FL (19811: W, C. MARTIN. L.

HAGANJ. READER. andJ. SUGAR.J. Phvs. Chem.Rey' Data 3. 771 (1974).

15, M1. KEEFFE. J Solid State Chem. 85. 108

Refernces 1990).Refeence 16.M.. S. IsLA-A. M1. Ltsi,%. S M TomiiNSN. AND

IA. W. SLEIGHT. J. L. GILLSON. AND P. E. BIER- C. R. A. CArLOW. J. Phvs. C Solid State P.'vs 21.STEDT. Solid State Commun. 17, 27 11975). L 109 (1988).

2. J. G. BEDNORZ AND K. A. MOLLER. Z. Phys. B 17. See.tfor example, W.E.PNcKE-r. Rev.MWod. Phvs.64, 189 i1986). 61. 433 (1989).

J Ph%, C Solid sirce Ph%, 21 1 5s 4iIl-i PrintedI in the L K

Zero resistance at 100 K in the superconductorGdBa,Cu1,O-.

Dw, id A- Roihins, ii hi nnarome, -\ d ~r ci ch 'itAnn%~ Norro bcI-Soxj (hester Alcsindcr Jr Robert NJ \ietjcer7.

Joseph S Thri~hcr \1 \bbot MLiiinnis indD)onald A 1)inlc,% 1

L '

'.h~trai. '~ *.. .. i" ' " -

nt .. tv -t '.rt-tcn \r i :[' i, t. v ~ t ndit!- 'h,: :t: ajrnpic

1. Introduction

Since the reports bh Bedniirz ind Muller ( 195sh) on superconducti\ it% n the quaternarNmixed- alcnce s~stem La. Ba- *CuO, _. 'ktth ons'et temperatures in excess~ cit 10 K. andh% Wku etal 19,1s on superconducti\ tx in singlt:-phasc 'Ba-('u 0- _. v ith in onset at-Q3 K and T ('zero* resistance) = NO) K. man\ laboratories arounld the wkorld. includinvour o%%n. embarked oin a desperate search tor higher critical temperatures throughoutthe entire lanthanide series Wkithin a tew. months there \scre Lecral reports cit such

compoundso )xith values cit T In the 9( 100~l K raneI Ca~a ei at 19S-. Stacy er a! 1987.Takavi etra! 19s-). i.e. \sell abose the boiling ternp-rature of liquid nitrogen.

As a result of these efforts. superconducttv it\ has been obser%ed in materials wviththegieneral formula R.A Cu(). .in %k hich acertamn amount .(ifothe trv alent rare-earth

ion (R1. or its analog'ue. :oeists sk ith a di' alent ilkaline-earth ion (A. usually Ba or Sr)

A possible mixed-valence ('u( l1I-Cul 111) state, in ss hich the copper (a Jahn-Teller ton)can he either squaire-piainar-,-oordinated or square-p\ ramidallx coordinated in a one- ort~ko-dimensional arra%. results from the acaic\ kot a certain traction oft the oxide

sites 16.7 w ) .Y) The almost planar (,dimpled*) arrays of Cu and 0 atoms, of

stoichtometrv CuO.. surmournid by, apical 0 atoms, yield a square-pyramidal Wie.

distorted octahedral) coordination about Cu. These ---r planes' ma\, be responsible forT iround 401 K. There is also a , D[ ribbon' motif of square-planar Cu and 0 atoms, of

WO22-371998/224091 -06 S(I2 ;0 Z, 1988 lop Publishing Ltd 4091

4092 D A Robinson et al

stoichiometrv' CuO,. which ma, be responsible tar F atound ) K in the Ya.(u()-structure.

To aid in our studies on the lanthanide effect on superconducti it.\ we consideredthe published onset and zero-resistance alues ot o ' Terus the ionic radii of the )an-thanide ions. ClearN. e% aluationsot this nature should consider the ,ariousexperimentaldifferences amongst the man% reports. but it is ne% ertneless usetul tor obtainin , aali-tative information from the resultirg trenls Other than tor )i there exists an optimis-ation window between Ho. Gd and Eu tor consistentl achiexing the highest %alues ,1T in compounds with the RBaCuI() composition Our obsersations also indicae thistrend. e\en for two-phase materials %kith s crall compostion R Ba icu*O, ( Robinson Ctal 1987). in agreement with others ! Brok n etl L),- . Fisk et a/ 117<S

We have recenthl reported our result, on superconductiixi, in single-phaseYBa:CuO- ( Morrobel-Sosa era! Q,,-) Herc kke present our studies on the sx nthesis,.powder x-ray diffraction and conducti xit% o GdBa.Cu O,. tor w hich w e rind - =-- 141Kionset at 108 K). We also present ., comparison ot these Ondinas with our pre% ouswork on YBa-CuO- :. prepared under similar conditions and studied usinv the sameexperimental configuration and equipment A prehminar\ and partial acount ot thiseffort is given else\ here (Asa, aroen!chai er al 1 ,,

2. Experimental methods

The GdBa.CuO, and YBa.Cu O, samples used tor this work were prepared b% con-ventional ceramic powder methods Appropriate molar mixtures ot fine powders otGd.O, (Aesar). Y'O, Mol.corp). BaCO. I Mallinckrodt . and CuP Aldrich). All withpurity >95%7. were mixed and sround in ceramic media Samples ot the Y-Ba-Cu-Omixture %%ere pre-fired in air at 9)5i C for 24 h and cooled in a mutfle furnace io roomtemperature. Cylindrical pellets (t\pical dimensions. 1 25 cm in diameter and (1.1 to0.2 cm thick) of the materialere pressed at 1 2 kbar and placed in a ceramic (alumina)boat, inside a Lindberg tube furnace, for sinter,na at 95() :C for 8 h. The samples \erethen annealed at 500 'C for 6 h and slow-cooled. The Gd-Ba-Cu-O samples werepressed at 1.2 kbar and sintered at 950 -C for 12 h. slow,-cooled, reground and pressed.followed by further sintering for 16 h. These pellets were then annealed at 5W0 oC for 5 hand oven-cooled to room temperature. The entire heating annealing schedules wereperformed under flowing oxygen. If preliminary electrical resistance (DC) measurementsdid not indicate the presence of a superconducting transition, the samples were submittedto further heating after being reground and pressed. Some of the Y-Ba-Cu-O samplesrequired further heating at 9W5 'C for 12 h and at 5(5) 'C for 5 h. while some Gd-Ba-Cu-0 samples required an additional 18 h at 980 'C. in flowing oxygen. The ,esulting pelletswere black throughout. The experiments we report here were performed on thosesamples.

Four-probe measurements of electrical resistance versus temperature were per-formed on pellet samples that Aere attached, from the bottom, to the cold finger of aclosed-cycle refrigerator unit (CTI-Cryogenics model 22C) with electrical insulatingvarnish (General Electric 71)31 I. The temperature was measured with a calibratedchromel versus Au-0.07% Fe thermocouple attached to the top surtace of the samplewith the same varnish. Four parallel leads were painted onto the top surface of the pelletwith conductive silver paint (DuPont 7'"13). The small amount of solvent present in the

Superconducrivir. im Gd8a0 0- 41( 3

paint Aa e'.aporated b%. heating the pellet', to 40~ ( in a tiowk ot o.~ecn li";r approvi-mateI 11) min Copper wire contacts vwere soldered directl'. onto the leads

The [) measurement', Aere made b% reahl1 nL the '.oliaLe dro p across, the inner leadsJot the sample usinL a niano'. ltage ampliner IKeithlc% Model 14111 A constant samplecurrent ot less than I mA ma,~ maintained throughout the e\periment ito diminish thepos;sible suppression ot the superconductimne transition Phe \.1 measurements werepertormed using the reteren-ce output 1.'lt1. ,'_ \ ij at trequenc% ot _' kzi Iot a

lock-in i~mplihier (Princeton Applied Re~sc.irch NI. ie I 'm as the constant currentsource. throuvih a I kQ resistor in ceries, w ith the sample Ihe Nample x olave Asi pre-amplified and detected bh\ the sigrial ,inncl,,l the 1I ~k-inl Data acquisition And anal'.si,for both measurement,, %ere pertormed with a personal computer (Hewxlett-Packaird

Mode YS t~anda dta-cqusiton .ontrol unit H lewlett-Paickard Model 34'Ci* Thetemperature dta wxere dieitised with scintihe rinstruments, temperature indi,.It. rwhich asalso intertaccd to the si:quisition :ontrol unit All measurement,, were per-tornied in hot h decreasin e aind mrsinetempiera'tures, ror holh sample' No incas-ureable h'. steresis was Jete~tce ! r either o! the twomaterials small specimens oreach

sampe. btaied rum .irshedpeiets erestuded ' - ra'. dittraction (Phillips Modlel.Sl(X1Lcenerator) with (iu Kai raidiaton- where /A Cu Ka, i §ti9 A. /Ii u K~

1 §.41 2A The Idat.i rinee_ '.'.is _' )' to -11 1Mlii w -ith a step site ot 11 (Cil and ,)untti mes )I I I 0

3. Results and discussion

The \-trl'. powk der Jilttrii. rn tr 10cms tIr N Ba *( u. ) - aind tfor ( id Ba -( u 1)are -,i'.enin tables Iand 2. respecii'.ci'. )ni'. \,:r\. wealk line-, out ot Scould not be indesed tor

Table 1. 1 h, !,'.s.c.l~~~m r HJ Hul

I Aif- I 14 iti I A 4

IN"3 91I AIM 1"~ .1 I IMN

a 1p NO I I 4i INN)

.4).' - P 4N) 'l's II I

*~~~~ I. iss~e

%trinhuted i, t hc fl ,:!ect nn t u 0 10 4 _- A3 I-( .1

*Attributed tthe 0 t rec tnt C (t) S3- 5 A

4094 D A Robinson et al

Table 2. The x-ra, powder diftractoeram for GdBa ,Cu O-

hkt Ji. d, ,, - d (A) II hkl d.",IA) d,_ - d., (A) I I

t li l1 " [) 1112 IIlit) I .02 It fW I t I ( W 2

1 3544 .- 1 - 14

h I I 3t, )Hhl '11 I0 1 6t4 o INN2 21 210 1) r1 0010 3

IIl 0 009

11t3 2 , t t 0009 3

LUnassiened

Attributed to the 22' reflec'tion of (Od- - ia = 10 S1O A. d_,, = 3 121 A). or to the IIIreflection o BO (a = 5 -k, d _,, -- I 5A -

Attributedto theH 1) i reflec:tion ,t ('uO la -1 hS4 A. h = 1425 -k. = 5 120 A. t0

Q9 I - - 214 11 S I; t\ I

Attibuted to the I1I> I r.ict 4on ,)C- = " 522.A)

< Attributed to the I I ! refl]ection oI CuO id-,. " ' 22-Attributed to the 22 reletion ol uOi , = I 81 A .

the known structure of Y-Ba-Cu-O. with some of the intensities matching closelythouth not exactlh-. The calculated lattice parameters yield: a = 3.824(5) AA b =

3.891(1) A and c = 11.6()3)A. in very good agreement with the values a-=3.8218(7) A,, b = 3.8913(7) A and c = 11.t77(2) A (Cava et al 1987). The x-ray dif-fraction pattern for GdBa.Cu,O, consists of_,2" peaks. of which 16 belong to a unit cellof dimensions ver similar to those of YBaCi0 - Four of the remaining reflectionscan be attributed to Cu. and one to either GdO, or to BaO. while the rest are weak.The calculated lattice parameters obtained from this pattern are: a = 3.855(5) A. b =3.872(4) A and c = 11.348(l) A. in eyood agreement with the reported values a =3.89 7A. b = 3.89 37 and c = 11(.73 A attributed to GdBaaCuO, by Horetal(1987).

The normalised Plots Of Dc resistance versus temperature for YBaCuO, andGdBa,Cu- are shown in figure 1. The data were normalised with respect to R(T =265 K) = 20.99 g for YBatCuou, and R(T = 265 K ) = 6.38 whie for GdBa,Cu3 w :,to compare the qualitative featuresof these mat is Tter r-temperature data show

the metallic behaviour of both samples. The onset temperature; for superconductivity inthe YBaCu30, pellet was 102 K and the zero-resistance temperature was 96 K. Theresult is aonsistent. within a few degrees kelvine with values of T reported by othergroups. In comparison the onset temperature for GdBaCuO- -, was 108 K. with zeroresistance reached at 100 K. The measurement Of Ac resistance vrsus temperature forGdBaICuO -: is shown in figure 2. For this caste onset temperature is also 108 K.but zer o re sistance was achieved at 99 K.

These values are a few degrees higher than those reported by others (Hor eral 1987.Mei et al 1987) which, when coupled with the presence of some CuO reflections in thex-ray pattern, may be indicative of slight deviations from stoichiometry, for example.

SuperconductictY in GdBaCuO- 4095

A

• U B

ernere re K

Figure 1. The normaled I)t j restance Ri T i R(i'h, K, as a tunction oI temperature tor

pelietI , ,)t i YB -(u (). and (Bi GdBa,('u ()

the oxygen composition for the "YBaCuO, material was determined to be 6.8 to6.9 from thermogravimetric analysis .Morrobel-Sosa et al 1987). Nevertheless. themeasurements are reproducible for both the Ac and DC methods and sho, no detectablehysteresis effects upon the cycling of temperature. and the x-ra% data do indicate thepresence of a mostly pure phase in these samples.

ot

|U

4,...in

I

I2 I

. 80 120 160 200 2.0 280

Temperoture (K)

Figure 2. The resistance (A( of a pellet of GdBaCuO,, as a function of temperature

4096 DA. Robinson et a!

4. Conclusions

We have examined the temperature dependence of the electncal resistance, by both AC

and DC methods. for mostly single-pahise GdBa-Cu,;O, prepared (sintered and annealed)in the presence of a flox~ing oxygen atmosphere. by comparisoi, with similar measure-ments performed under identical conditions for NYBaCu,O,. Both electrical resistancedetection methods yielded the same T. (onset) of 10)8 K. T ' (zero resistance) was achievedat 99 K (.-C) and at 10( K (Do. These results support the hypothesis of a T,-optimisationwindow for the trivalent lanthanide ions, betmeen Ho. Gd and Eu.

Acknowledgments

We are grateful to Professor G N Coleman. of the Department of Chemistry, forproviding many of the starting materials and to Professor J H Fang. of the Departmentof Geology, for providing the x-ray data. One of us (DAR) would also like to acknowl-edge the award of a Research Fellowship under the auspices of the US Bureau of IMines.

References

Asavaroengchat C. Mletzger R M . Robinson D A. Niorrobel-Sosa A. Thrasher I S. Alexander C Jr. StanleyD A and %Macinn~s MI A t9W? Supercondtictii'irx S . nhesis. Properies and Processing ed. W Hatfield(Net% York Mlarcel-Dekker) submitted

Bednorz G and.Muller KA 1980 Z Phs B 64 189Brown S E. Thompson J D. Wills J 0. Aikin R M. Zirngiebl I E. Smith J1 L. Fisk ZLand Schwartz R B 1987

PhVrs Re, B 36229SCa~a RI3. Badlogg B. %an Dover R B. Murpb' D W. Sunshine S. Siegrist T. Remeika J P. Rietman E A.

Lahurak S and Espinosa G P 14JN Phn, Re, Lett 58 1(,'tFisk Z. Thompson I D. Zirneiebl E. Smith J L and Cheong S-W 19W? .Solid State (Commun. submittedHor P H. NMeng R L. Wang Y Q, Gao L. Huang LIJ. Bechtold J. Forster K and Chu C \k 1987 Phv5 Rev

Lett 58 1891%lei Y. Green S M. Rev nolds G G. Wicz,.nski T. Luo H L and Politi. C 198' Z Ph%5 B 673103Niorrobel-Sosa A. Robinson D A. Asaroengchai C. Mletzger R M. Thrasher J S. Alexander C Jr. Standle .

D A and %laginnis M4 A 19878uipercortductuiit Svntlieso.. Properties andi Proce55ine ed Me'Hatfield (NewYork Marcel -Dekker

Robinson D A. Morrobel-Sosa A. Alexander C Jr. Thrasher J S. Asa\ aroenitchat C and I etz~er R !V! 1987Proc N.-ITO W.orkshop lInorganic and Origanic LoK -Dimensiional( Critalline Materials (Minorca) 198'

ed P Delhacs New York Plenum)Stacy A M. Baddiune J V. Geselbracht MI J. Ham W K. Holland G F. Hoskins R L. Keller S W. Millikan C

F and zur Love H-C 19h- ] Am ( hem Soc 108 _52STakagi H. U'chida S-I. Kishio K. KOiZZIza'a K. Fueki K and Tanaka S 198-7 Japan. J Appi PhyT 26 L320Wu MI K. Ashburn J R. Tomne C J Hor P H. Mieng R L. Gao L. Huang Z J. Wang Y 0 and Chu C W 1987

Phvs Ret, Lett 58'X)8

NewJ Chem. 1991, 15, 209-221

THE QUEST FOR UNIMOLECULAR DEVICES lo

Robert M. Metzger * and Charles A. Panetta

* Department of Chemistry. University of Alabama, Tuscaloosa, AL 35487-0336 USA

Department of Chemistry, University of Mississippi. University, MS 38677 U S A

Received April 24, accepted November 8, 1990

ABSTRACT. - We review here the progress made towards the goal of truly unimolecular devices, that is,electronic or other devices, whose active principle is based on the manipulation of the molecular geometry, orthe molecular conformation, of a single organic molecule, or small cluster. This quest is at the forefront of anascent field of research, called "molecular electronics", which aims to fill a future gap in conventional"inorganic" m:croelectronics: as devices based on silicon or gallium arsenide or germanium get smaller andsmaller, because of the need for faster electronic circuits, the fabrication difficulties will rise rapidly, and, atthe nanometer level, organic molecules, with the tunability of their molecular orbitals, should offer significantadvantages, along with new technological challenges. The issue of the potential speed of organic rectifiersand transistors can be resolved by the use of strong oxidizing and reducing molecules, through which electronictransport can be very rapid. At present one cannot routinely address a group of several isolated singlemolecules: as yet there are no "molecular wires". However, one can use the tip of a scanning tunnelingmicroscope to address a single molecule, and thus several candidates for molecular electronic devices canfinally be studied. The review focusses mainly on the efforts to realize the Aviram-Ratner unimolecular rectifier,and chronicles the progress of the Organic Rectifier Project.

I. Introduction working thickness promised by such a D-rT-A device is ofthe order of one or two molecular lengths, i. e. about 5 nm:

There are two current definitions of "molecular dec- such a small size is predicted to he unattainable, even by thetronics' (ME) 1: rosiest forecasts for silicon or gallium arsenide technology.

ii) the "wider" definition, which includes the interesting The progress of the ORP' -2S has been reviewed often

electrical behaviour (conductivity. superconductivit. etc.) of bef'ore d I. I - 21) 22. 2.. 2. 2, but new results areall known loer-dimensional systems, LB films, and clusters, incorporated in this review.and As discussed in Section 9. the rectification by a single

(ii) the "stricter and narrower" definition, which focusses organic molecule, or by an organized Langmuir-Blodgett

on the properties of the single molecule. These two definitions (LB) monolayer of such molecules. has not yet beenare discussed in Section 2. demonstrated by our group. The revolutionary scanning tun-xarepdiscus in Se or d s aneling microscope " has an obvious potential for confirming

Examples of "bulk'" molecular devices are presented in thAirm nst:(T :teealaddsponigthe Aviram Ansatz. (STMp: the earl,, and disappointingSection 3. Efforts to mimic intramolecular electron transfer efforts to detect rectification by an ad hoc modification ofin biosynthetic molecules are reviewed in Section 4. "Trul, the STM 2-' 22. 24 will be reviewed: our own continuingunimolecular" electronic device proposals are re\,iewed in efforts will be mentioned briefly. We mention also the resultsSection 5' device assembl, ideas are presented in Section 6. of Sambles' group at Exeter and Ashwell's group at Cranfield,The ideas for "'connecting'" to such molecular devices are who may have achieved rectification by an LB film 1' .-discussed in Section 7.

Sections 8-9 chronicle the progress made by our own group["the Organic Rectifier Project" (ORP)] towards the reali-/ation of the organic rectifier, a proposal advanced in 1973 2. Broad and narrow definitions of molecular electronicsby Ar Aviram, Mark A. Ratner and coworkers 2 " that a%inl' or'anc al,h'cui' of the type D-7-A could he a rmct/ier The term ME has been popularized by the late Forrest L.of te,'trjcal turrent. This molecule D-rr-A would do so,. Carter, who organized international workshops on MEbecause the ) end is a good organi, one-electron donor. CT devices in Washington in 1981 31, 1983 ", and 1986 4": thisis a covalent saturated ("sigma") bridge, and A is a good was followed by conferences in Varna, Bulgaria in 1986 " '.organic one-electron acceptor. The driving force for the ORP. and in Hawaii in 1989. which outlined the present chal-which may be one of the key experiments in ME,. is that the lenges '2.

NiW i1 R% iF O ( HIfMISIRY \,) 15. '2-1-1991, 019x-9836 91 2.3 20) 13A S 130 c (NRS - (jauthier-Vitlars

21W R %I %if f/itR \iNt( x MP-i r I

In the period 1983-19S4. Nonie unsciCntific propoak about I c. because of the hand ,tructure of the solids. or because,elf-assembling biological 'conputers' "biochips" reced.ed of phase change properties,. or other hulk clffc.tiunfortunate. undesers ed and uncritical , orld-\,.ide mediaattention: the critical reaction to such exau¢crations almostdrowned, in the ocean of righteous disbchlicf. the infant Iicld l It t R1,, k1i 11111K,

of IF- A s ers sobering note outlined lhat technical accotli- Afer the discosers of the unction diode and he transistorplishments .ere still needed . Since then. inore consersaichemists, phsicists., and material, scientists hasc broadenedthe definition of MF. re-labelled omeC it' their rescarch arealecls could function H bulk pn rectifiers diodes or asnpn transistors This w+ould occur if+ a film or cr5,,tal of anlas M,1. and thus hase gis en NI its present nascent respecta-

ilt\organic electron donor wcre brought in iontact A th that tain organic electron acceptor This w as Indeed ,eriticd in theAt prcent. two interpretation', of ,iF esist In its first. 9

6t,,"broader, definition. st 1n,1 10 NIF inc Iudese I ,'(itateoof all pnalccu'-ieba.sd 'ct troni protc,,w oi l,,i 'r-dimoi',mional

i 'tef.t, to %it, in approximate historical order 3 2. so 11 )t I . B R(, \%I( R1 I II RsiAl ) Intercalated graphites W(,K, etc.)Ia Kuhn ci al. show ed that one can obtain a pn ior I).XiiA21 Intercalated 2-D chalcouenides TaS. etc.) 4' rectifier in a LB multilaver sandwich

A3t Charge-transfer salts ( organic conductors and super-conductors) " -UAl('-D,, ('Ci, (A-A), Al

A+41 Conducting low-dimensional polsmeor,, " +where denotes an interface boundar\. Al denotes a bulk Al

(A5t Inorganic linear-chain compounds " contact, ((_A-D ,, denotes the elctron donor sstcmconact LaigA-Dtodet denotes theiwyr elcto doodssA) Langmuir-Blodgett ( LB muttila,,ers 'D[ = q LB monola.ers of cadmium arachidate (CA) randoml,

These research areas ha\,e been studied world-widc for doped in the ratio :I w ith suitable organic 7z electron donorsalmost two decades, and relabelling them as ME may be a DI (CAt, denotes a spacer la.er of r undoped monolasers ofconsenient short-hand, an attention-getting method for fund- CA. and ICA-At, denotes the electron acceptor system Aing research proposals, or just a current fad. = LB monolaers of CA randoml. doped wi th suitable

In the second, more narrow definition. s&nsu strrbo) ME organic it electron acceptors Al. This work Aas repeateddesices ("zero-dimensional device") utilize the eleetron' and confirmed by Sugi et al. "'. who obsersed rectificationproperlieN ot sinile molecules or elnster.s. Examples of propo- properties. but onlt if q _-3. r > I and s 2 3. In these LBsals are: films, the nearest-neighbor distance between D and A in

successive layers cannot be controlled.(B It Airam and Ratner's rectifier. s hich is the main sub-ject of this review\ -" 4

(B2) A,,iram and Ratner's hydroLen atom switch P t. 3. \P HtSi- R .Ni %I( sNi 1 ( Ii

B3) Mitani's proton transfer system 4" In 1979 a fast switch ywas discosered b\ Potember inI1B4) Carter's soliton switch " CuTCNQ ". This vas due to the thermodnamic meta-

1B5)* Fujihira's LB monolaer photodiode stability tin crstals or amorpho,, powderst of the iolet.Of these. onl, the last tindicated b\ an asterisk) is an exper- low-conductivity ionic state IS) Cu'TCNQ-tct. relative toimentall, prosen desice. the yellow. low%-conductivit%. neutral state (NS) CuOtt

As "passive" molecular connectors which may be useful to TCNQ(0t(c). with. presumably. an intermediate, mixed-interrogate unimolecular ME dez'i(es, the following proposals salent, higher-conductvt state (CS): one could switchhase been brought forth: between the two states IS-CS either with an applied ,oltage

(CI) molecular wi tres and antennas. e g. carotenes and over a certain threshold ,alue. or by a moderate laser beam.other polsenes - while heat will restore the IS. This is found also in AgTCNQ.

and in a f'e\. other related systems ". The switching rate isW2) Miller's lkecl spaers" connec" quite fast. and can be esen used for optical data storage ,(C3) Miller's "molecular spacers" " but has not been incorporated in commercial devices.t('4) molecular inclusion compounds, c. g. cxclodextrins "

and calixarencs ". 3.4 \ Si - ti i .i.\i D i R.V"Isif A I SI*,t* ( o'i t riN(, P<i N Mt Rs

These ideas and potential devices, which represent the bestpresent long-range hope for ME, should exhibit the inherent Wrighton et al. developed a "molecule-based transistor"speed of intramolecular electron transfer (much less than which uses conducting polymers: either chemically dopedI ns. It is very likely that ME can compete with present- polvaniline layers deposited on Au interdigitated electrodes "

'

d it semiconductor electronics onl, if' ME can couple the or a 50-100 nm "'gate" polyaniline polymer between two Auad, antages of rno'oular voz' and la.st tntraoieular eletron electrodes shadowed with SiO.: this device still has a rathertranther (faster than I nst: there is little use in making a small slow switching rate (10 kHz). due to the slowness of ionicbut slow deice. conduction, and a gain of almost .1)O ". Recently Stubb

et al. showed that a single Langmuir-Blodgett monolayer canbe used in a "molecule-based" transistor "s

3. M olecular devices that depend on bulk properties 3 . . Pt S iN(, )PiI( Ai. %I-MORIiP

First. "we will discuss ,ome "bulk" molecular devices, esen There are three established technologies for mass informa-though they are molecular desices only in the larger sense tion storage and retrieval "" "H

"I A ilt R%-\i OF (Iit MiSIR+

%il 15, -,, 2-3-191

fll 0)1 IS[1 I i .W 1 I I \R ii 1)I51 I

(if ferrite, iron oside and chronmin oside-based nmagnetic t Ion I 1*1I R IIt is hoped. Ili orne qul~arltr. thatit 1on1ememories. disks and tape, t\1\1I these arie erasable, or s% rite orearnl/cs sUCh [)-- - m cculCs in I B multi lascrN or oitherMani. read mans, iW\IRM) media . IINCUmllics. thle B( IR n..is be Suppressed

ift Si and (ia.As- based semicond uctor computer memnoriesISMi: these can be considered as %ers. tast skrite mans%, read HI *' ii %1''I Ik , Ii I~Is l ii ,1 D-7- A

mnain IIA R:\1R media,

lt Al pits on pol'carbonatc-hascd laser-readable c0Ip~ \ature produ-ICs a Cons1(ilan rd abundant LiippI\ Of :cir-

disk I(l)i techrtolo% thfisl, is a permanent. or %% rite onkx. ICTICsA a-i. l srticii I .ritI- idAnts I in is12 t, oreansmtt. these

read man\ t\)R.\I) Medium .:arolcncs lhase dliCUSsCd either -. 'molecular antennas '-or as no111leUlar s ties ( arotenes fiase been used is tile

[he 'present limitations in Sioraec deiis. access time, and pitbridc behiss ecn 1) and A, t0 teld D---A sItruct nrc-.cost of the abose three tCchIqIlC uc' as spurred ani esten-.se i)I ori\rt.- artn.- tihrqioc l-Search for alternatise Storage stralcgics I or instanlce. mien- boh(1R ndi(-Rocurd t npmbertseto-optical recordinig s till pros ide \I R\1 capabilities in s iliii A ind B( I h lase:rr siait o ipithert

CD[. but %iti no itncrease in storage denSits -\lierriaiiscntet hods Of stori ng W()R NI data ot kil )L'-i-\pe 'uILIac- depenidon phase-change SNtents (opaque ito iratspiret. ec.. s Mii

can use Inorg'anic sthstrates (e. 1- [ci or orc-anic uih-.tlaic-. . [rul.s unimokecular de~icesor N,,stemN ss hicli depend on local melting and msincII asidean organic film i c g'. phthaloc-%anincs FilTe tnced ito find Iis ticL discussed ciunai etrt, to ininii. Nither NatureWl R NI opticall% addressable mnedia us acuitels felt. and niaii\ Ili p liti \tfei-. se tciurn Ito iIdea toAi ach IeCe Liln'0Imeiu1.11

oreanaric ss stents arcne cL onsidered tese mas hlo%% A cos)t elect iii. des ices, r, /cr-dimets)ni l des ices

10sil' iR\I~ I Bfist'.Fihe Vnsai/ of As rant and Raier 2 starts troni thle

The groups of Barraud arid Kahn haw dcmion-traied dis~c en 1 t ighf% cotiductine fo%%cr-dim ensional ori-anlichs stereti bistabilit, in an LB film of' 200f nonolaser. of cfta,iruu-iransf .er N.s tems based onl good one-electroni donor,

Let ntsa~k latd- .1 i-phnanhroine I CS (D) such as tetalitlflsAeti t1i11, 1) and vood orgLanicOmie-clctiron aCCept 'rs I At such as - .X.-leiracr 'anoquJinodil-

met han I R S. 2) (Good donor molecules io molecules4. Biosnthetic electron transfer molecules Nkit refatisels lo%% Las-pltase tir-.t ioni/atton potentials 1,,i

are. at thle Samte tline. poor accepltors (thle\ hase lo~x electron4 I si si 55,SiR551 0)L5RI t i I isSI I aflinit. AN), 200d acceptors ii C mole1cules suith a relatisels

higzh F-irst electron atfinits A\).tare, at the Sante time, ratherFaubse pioneered the tunderstandintg tnt ramll"0Cenlar elect rot poor donors (has e high Ili) ft us' thle gas-phase etterg. required

transfer in Solutiotns ol ig bminucear tranition metal corn- J'r reaction 3 ( both cottponents at inf -inite Separation f isplexes. both 1)-cT- A and [)-r--A Ni ore recenils . efforts about 4 e\. s ittl reaction 4 ssould needL os er 9) eV,base been made ito control tfte electron transfer in solution %Q,het\scn 1)-T=C reuti-[auihe Ion [Ruibp. ),]' bhp. is2.2'-hip. ridinef and A mcntth. s ol ogen KMiller. (loss ef 4 '

a! shossed that the intramolecular electron transfer rate Ni. NN~C Nthrougzh a molecule 1)-ct-A at first Increases steadil\ with an /IIIncea sing (\-&.~I shere Ili is the ion/ation potential of [the C >< )1 S3 MCAi

donor moitl D).AN is the electron atfinits of' the acceptor N C C N NC aA

ntlotet, At. Then, if t- IS Isncreased furthter. the eleetron -2 S6.-

transfer rate diecreases, bcause of' an increase in the Franck-Condon reorvati/ation hbecause noss the geonties of' 1) ' I hus. if one makes, I i-c- A molecule liL,. 5., and assembhle'and .A are quite different from the geometirs of' 1) and A it soniehoss bictssceit t1%o ntetal electrodes, M, and Ni.. asroespectis.els I, Marcus "inoerted revton - I in 6 (discussed a IN N1)-t \11 I.i Section X beloAs . then

thle dirctiont of eass electron flokk is from MNI( to1 bIecause

4 2 %r~ \1RSi i i'iw)SN555itiII i Ml I i D-o-l- It uitili/es the /sstttertotttc state 1) -ct-A tshtfe the electronfloss f rom %1i to M ,ssotuld be nef-ficett. hecause the barrier

Seseral largec research groups hase tried t it ake artificial to f'orm the tss itterion 1) -o-,-A 'ssould be seserul eV higher)photos. nthetic molecules, of the I. pe 1)-ct-A. "shere 1) is an I sing terns popularied hi, Hloffnman -. the As iam-Rainerclectron donor, Such is a porph. rt. phthaloc. aiine. or des ice can \xork if' the tunneling of' electron from -A to 1) usrelated molecule. ct is, a cos alent sNigma" bridgue. i e at through tile bond ssstem. and %%ill fa ill if' the tUrnehtni

strtdlinkage, Such as one or ses.eral [2 2 2[-bicsclmoctatte het%%ceit the metal electrodes M, and N I is, predomntinntlIrings, and A is the electront acceptor t Usuall, p-bett/o- throughl space Molecule 5 %%as neser ss nthesiwed. and theq uin Inc I one should nmentionu the groups oft NI au/eral ILI ide languinshed until tiftc OR P started in earnest, as, is diSeul-Rockefeller)i '. Weedon and Bolton (Western Ontario) tSef in Sections 8 and 9) below.

Ders an and Hopield t (aliech) M Nalaga O~saka) I. (s t(A ri/ona Sta tel ")- Staab t Heidelber) f I, attd Verftoes en 5 .IR1) H11 \I)1\S 'HIt 'S tR I R sAT HI sIAmsterdam) I hfe results sotar base Shotss i rapid charge-tra nsfer reactiton It I R t, but alIso a scr\ ra pid recombintatiott Asrn has also proposed an intramolecular hsdroven(if Ite c.harc-sepauraiid -tat ie . ,u back c~hargve trantsfer rete- atom transfer sssttch. based on Ht bonding in orh/o-lumnone-

If\ J1,1 WIN S I 'I f i sII STR %,,I S . 1991

212 R M MLF/Gt;R AND( -5 PAN[I--

catechol systems ' It has been confirmed that there is to-photon illumination at 540 nm and 635 nn- into K ol(intramolecular H atom transler in such a system '4 Ml or bathordohopsin; at lower laser powers, the same two-

photon illumination scheme can interrogate which moleculeshave converted to K 610. and which remain in the bR 56S(HaC),C" OH ------- c (C H 3 )3 state. The K 610 state maV consist of a ct.o-retinal segment

HQ0oC -1S o -----OH The harnessing of such ,'i.%-tran. isoneri/ations (also insolsedCH 3 in the physiolog, of the Visual pigments) maV some da sicldCCH3 CH3 interesting molecular devices.

Miltan is working tow ard a proton transfer switch "'

6. Device assembly ideas5. 3 PRt)POSI i) ';ILIhI)N N A1l Ht

Carter "' proposed that soliton motion in polacct\lcncs 6. I MWt Kos(oPiL (oNNit tijONs-LB HiLMs 5N". Pv)l't)l~ i 1i1 -could be harnessed in Various switches. gates, and logic cir- Before the proposed unimolecular devices can be used a,cuits. However, apart from the difficulties of s\nthesizmg or such. one must confront the serious issues of controlledaddressing such molecules, soliton s\witches are expected to issemblv of device interrogation, and of desice resetting athe relat,.el slow, dc'lces.asmlo eieitroain n fdlerstigathe molecular level. In this section we deal with bulk molecu-

lar de ,ices. If one cannot address a single molecule, one cani 4. . JIHIR, s LB pp',ntt)ioiiI perhaps address a monolaser of identical molecules transfer-

red to a suitable (metal) substrate by the Langmuir-BlodgettFujihira and co-workers hase demonstrated that a single (LB) film method - then man\ identical molecules can

IB monolayer can function as a photodiodc ': tht. I proh- be addressed electrically, provided that electrical short circuitsably the firt truld uninolecular d,-cie. Thes s nthesized a through defects do not occur. However, LB monolavers arcD-cfl-A-o2-S molecule, where D)=electron donor=ferro- only weakly physisorbed to surfaces, and thus can desorbcene. a I ((H.), chain. A = final electron acceptor Viol- with time. This difficulty can be circumvented, ifa photopol\-ogen. (72=(CHO, chain. S=sensitizer =p rene. This mole- merizable diacetylene " group is included in the monola~er-cule was transferred as an LB monolayer onto a semitranspa- forming molecule, and if the molecular geometry is so engi-rent Au electrode (with the viologen. or A. part of molecule neered. as to yield topotactic polymerization: this can resultclosest to Au): this electrode was used as a window of an in a very robust, yet electroactive. polymer.electrochemical cell which also contained a 0.1 M KCI sol-ution and a Pt counter electrode. Under bias, an electron istransferred from solution to the ferrocene end of the LB film. 6.2 M.\Rao(OPi( (ONN-(i tONS HE SILS L BRiD[-and then to the ground state of the pyrene molecule. Light An alternate connection strategy. devised by Murra? "' toat 330 nm excited the pyrene radical cation from the ground study the electrochemistry at the electrode surface morestate to an excited state. from which the electron is transferred closely, consists of derivatizing an oxide-coated metal elec-to the viologen. thus completing the circuit. A photocurrent trode surface with trichlorosilyl groups. then reacting thisof 2 nA at 0.0 V vs SUE was obsersed only when the light surface with the desired molecule which has a terminal alco-was turned on 5

hol group: this, however, does not usually give full monolayercoverage on the metal. The reverse strategy, advanced by

5. 5. PRISpsi I) Kt-I(,-\",oI s'Nhir 1CH Sagiv. of'sitanizing the molecule and attaching it to an oxideand hydroxyl-group-covered metal surface, claims to achieve

Sixi focussed interest on the photochromism of N-salicyli- full monolayer coverage q.dencaniline. where the enol configuration can be convertedto the tran.-keto conformation by using light of frequency 6.3. PROPO5l-[) M5(ROSCOPI( ()NNii(iiONS ('iji.ODE\IRINS ANt)v,. and conserted back by either heat or by light of frequency (ALXARENESv,. In this case keto-enol tautomerism is accompanied by an:if!:zL7. re:i'. n " The existence of cavities of precisely controlled size within

cyclodextrins and calixarenes allows the inclusion of smallelectroactive molecules within these cavities, and yet provide

5.6. 1 ((,-,t i 1 1tpt' n,,.- IN, LB Ti.Ms ii t [t GHt-INO (i well-formed macroscopic crystals with the desired inclusions()NI-,.IRM. tN. I-.\N'(, precisely oriented within them ''

Kawabata ct al. hase showed recently that a light-inducedi.%-trans conformattonal change ;n azobenzene could affect

the electrical conductisity of a conductive LB film ' 7. How to connect to a unimolecular electronic device

5 . 7 POI IN III ( (ON(ORM, IIWN.AI ( FIANGE SYSrUi.s 7. 1. PROPOSED MOLCi tAR WIR-S

The idea of using organic molecules which undergo con- The carotenes and other conjugated linear polyenes haveformational changes in optical storage devices is widespread. been touted as "molecular wires" or as "molecular anten-Birge " has studied the primary step in the photocycle of nas" 52. and certainly will provide fast electronic access tobacteriorhodopsin (the light-harvesting protein of Halobac- single molecules. These polyenes are, unfortunately, veryterium halobiuml, in which the bactertorhodopsin containing susceptible to air oxidation (as is the simplest conductingall-trans retinal (bR 568) can be converted (at 77 K) with polymer, doped polyacetylene).

NEW J0 RNAL O1 ( H.MISTRY. VOL YS. N' 2-3-19Q1

fill Q1 Itll 1()k NI t \ 1([I (I I \R lit \k I 213

PR P ),I'R I si l iROLi INKi tI I If1(01 \ Ni NIlI R(I R(I Ii I NNIj I, Is I-able 1. -- Fsperimenial ioni/aion potentialsIt l or selected V(olddonors 1). experimental electron affinities .A, tor selected goo d acccp-

As iram has proposed a spiro-linked not ' thiophene tor,,. A. and Aork lunctions Q lor selected rnials 0_\,tern as a possible interconnection betwkeen conductIneL rn]ol-ecular Aires" based kin ni or p-doped polsthiophenc. 1, eV A0 1 uCV

3. '-t 1l1 I I \ R silAtI IRs F r i I I 3 rCN\Q 2) 2 u 4

Psrene i -4) (hlo. .nil 110) ' PMiller et ail hase show n how seseral SUCeesNiseC liels-Aldler

condensation reactions can teld In sulaiting. ,et stuird\ LBfilin-fOrnuini oltoonierN of controlled lenI HC N OH,

'4 m0 \NNiNi, it N-,i I [N(, Sllit R~s( W

The recent adsecnt of alfordable scatnring 1.tnelitiv, icro- H30 N. CH I0 CN C0z

scop\ i STM1 " should let us address elect rotiica Ill a siri~c 7 8 9 1 0molecule. If' this become,, easily controllatble aind reproduc- Gh!I y DOIible. then a real res olutton in ME I des ices, should be ls

arudthe corner"tin In a molecule in which the "sigma- hridge has alread 0

been bult., it is estremels ditficult ito consecrt. in sitLI h

~. e~i~* f te oganc rctiierproedchemical ss nihesis. a weak donor into a strongz donor, or aweak acceptor Into a -strong acceptor Therefore, instead, onemust sOnthest,'e a series of' mono-substituted strono donors.

Thissecio eies the p Ln hroitrsstiond b\ the O~RamP and mono-substituted strong~ acceptors. which can be joinedtowads he ntest an chractrt~tto of he s tam- h-, some coupling reaction which can avoid the usual. and

ractnirfcatifi ae rciAe in Seto L)r- ) teefrsodtct undesirable, formation of ionic charge-transf'er complexes.recifiatin ae rsisse inSecion'~such a coupling reaction is the urethane. or carbamnate cou-

pling reaction, pioneered for a l-rT-TTF-n--TC\Q- 1 , copols.-XI . s IRs Ai I I i( 1() ssI MBi s D-(-.A,- in% I mer bs Hertler '~and adapted to monof'unctional dens arises

b,, Baizhdadcht iAS discussed above, to address a sinle molecule electri- i%-Temlces utpakfiinl noslfse-

cali.) The moleule mus packcla efirien" int selt-assem-dccall, oe nedsa 'mleclarwir (C g. po'.aet~ene bliniz monolavers. If' the destened molecule does not form

strand) or a ''molecular antenna'' (e. vt. the con~ugated por-tion of Ve-arotenel, neither of swhich can be casils connected PcesIagurIP " el-smbI mooarstto a extrnalpotntia souceat pesen.ite air-v. ater interlace, then either long aliphatic *'reasr

.1d.cn ofth ST, oe cul no cone titilsi te roecue tails' must be added to form a hydrophobic tail. or an ionic.advet o th ST. on cold ot onnct asinle oleule hsdrophiltc -*head"* should be added. The nmolecules should

to an external circuit. Therefore. Ahen the ()RP ).sas Initiated, befrlfatsosin about 1981, to realize the Asiram-Ranter rectifier, one ofr opc im.rtfeil nuh

so as transfer well (hr the vertical dipping method) as LBhad to content oneself with assemblies of molecules;. The films.three techniques that shiowed promise were (I) the LBtechnique '.and the technique of covalentlv bonding Ivl) The srnthesis of the acceptor should be relatively. eass,.molecules to electrode surfaces, either hK fit) silantiniz a aind should oceur in high sield. The acceptor used in the early

rdroxrl-coated electrode, then atcigmluesoa- work of the ORP was BHTCNQ, 1 ).-whose synthesis '

lent]% or (it) by sila ntiing the molecule. aind attaching' it contained a s.ery inefficient, low-yield step. A new. better

directly by spin-coating by the oleophobic method '2 ito a monofunctionalized TCNQ acceptor. prepared in high y ield.

hsdrox\sl-coated electrode ". As discussed above, good was H-ET('NQ. 12 ~monolaver coverage is claimed for the latter method "H butnot for the former method 'lNC ON NC ON

The ORP wxas committed to the L-B techniqueC. Then, of Bcolurse. one then needs LB films that are defect-free in theH0,%BHO^lateral dimension, at least to within the area probed electri-calls. Of course. LB monolarers do have manr microscopicNC Oand' macroscopic defects NG. NG 12

BHTCNO HTCNQ

X .2. iiii( iR(oNf NNmii NfIIIH R IH I iA

rhce ae s- cal ntrlokin crtera tatniuf h saisfcd (vi) The electron transfer through the D-(7-A molecule.Ther ar seera inerlckig citeia hatnmum h saisted ind through its hydrophobic or hydrophilic tails, must be

for the rational sO, nthesis of suitable D-rr-A ss stems: at ssi bvaroiclrdzc httvsalhtso

tI) It, for the donor -nd D must be as small, and ais close Is ?lot predicted to be uselul. That electron transfer is fastas possible to the work function (p of' the metal laser Mi. through properly designed molecules. e. g. the photosyntheticTypical values are listed in Table 1. reaction center, is well known. The work of Miller et al. -3 -

(it) A, for the acceptor end (Table 1) mist be as large as. reviewed above, shows that a balance must be struck betweenpossible, and as close as possible to the work function (p ol' the requirements implied by the ionization potentials andthe metal laver M.. It is clear from Table I that reqtiirement,, electron affinities of Table 1. and the resulting 10-A, values.(I) and (ii) can be met only approximately, aind those of the Franck-Condon (actor.

N.I 'A 0i RN-Si )F ( Isltt 51W) 15. N 2-1-1'99t

214 R \1 M T/GIR \NI) a. t p\,T f.I-\

Table II. - Pressure-Are isotherm data tor Pocke-Langmuir filmt' II and A are ihe prcsure and nmolculai a repecio e, at the

collapse point S indicat s that the film make, Z-tipe LB ultlla ers (,ubNraie at 22(. ai t it ¢(

T I1. A,Miolecule No T pe K MN m RetITF-('-BItT('NQ 13 .trong ) 22 12

,irong .1%D )P (- 3B1I1 CNQ 14 A ak 1) 29'2 2) 2 ,) I

,arong -\Bt)D-P-t.-BII r(tN) 15 medium DI 2t 4D 5

,trong N

P\ ( -fIl TC'NQ 16 medium 1) 2s3 2s2 53,trong .

BDDAP-(-tIFT('NQ 17 mdCd I U11 1) 2')13 400 44 1 1,iroiC .A

Pt,-( -HEI('NQ 18 Tedium 1) 291 46,trolng -

B1)D.-P-(-IIMT(.\Q 19 TIMedInt I) 293 223 5 "%k.ik

8Hf.\P-('-IMT('.SQ 21) medium 1) ' 5 s 42 I%keaik \

DDOP-('-[NP , 21 %k .k I) 2-s 2 istl%k eak .A

FDD)P-C-ENP $ 22 A.,eak I) 2'S 14 o 6%, eak -N

I'DDOP-('-HEi(NQ 523 sAeak D 2,3 47 5 ,4strong -

\ITDAP-ENP S 24 weak I) 2s 16.5 3 ±1 4

,Aeak A

NC CN (vii) The desice 6 must have a finite tolerance for hwh

H Br voltages or heating.SN- Ir N, N1N NC CN S. 3. 1 NGM IR-BOi)iErt HI MS

13 H We present in Table II an updated catalog of molecule-sraerc.. N . . .O - (13-24) prepared by the ORP. which form PL monola)ers

oj'

0 NC CN at the air-water interface, and which transfer well onto Al14 NC CN or glass or other slides as LB monolayers. TTF-C-BHTCNQ.

H D0.,G Hr 13. was difficult to purify the "neutral" form seemed toN -c-O_,, O_ I deposit "pancake-style" onto the water, and synthetic diffi-

N . 0 CN culties forced its abandonment. The strongest films (highest

Nc N 1 collapse pressure. most vertical pressure-area isotherm) wereC Br BWAPaTCNQ obtained with BDDAP-C-BHTCNQ, 15 (Fig. 1). The accep-

H i*l "Htor HMTCAQ used in 19 and 20 was easy to prepare 0

NCON but is well known to be a weak two-electron acceptor. with0 NC O N 2H a highly non-planar geometry3.

N u_,,_'_ '__"_ _Molecules 21-24 are model systems for a related project.on' 0 CNC N which aims to incorporate D-a-A systems into LB-film-form-

I,

NC ON AP'CHETCAD ing diacetylenes. which may be polymerized in situ on the

. r.t film balance, for the purpose of preparing new systems withN, C 'O__ promise as non-linear optical devices. Interestingly. 21-24

0 NCCN -He form Z-type multilayers on a glass substrate (the film sub-NC CN phase is held at 5°C but the slide is at room temperature).

H An attempt was made to see whether any second harmonicN CIOsignals could be detected from Z multilayers of 23. but the

n 0 NC CN result was negative, maybe because the Z-type multilayersmay have reorganized with the time elapsed between deposi-

BOCAPC -,,fC N CC N tion and measurementai

H

- - N 6'

NC C N

20 One donor (DMAP-C-Me, 25). several acceptors (2. 11.P C-HWCA. 12. and HMTCAQ, 26) and several D-G-A molecules (14,

NFW 1t(RNAL OF CHEMISTRY. VOL 15, N, 2-3-1991

rHti QI FST I-OR I NIMl() I (t I \R I) \ IS 2 15

HNC CN H

_ _ -I- NO?

TDOO c ENP40 NC_ CN

30 -o N C N C

-0 3

TlOO C TCNQ H

22

TrO"PAP C NP

40

0 20 40 (50 so 100 120Area m (21at oecule)

17. 18. 21, 22 and 23) were characterized by cyclic voltamme- Figure 1. -Pressure-area isotherm of' BDDAP-(" BHT('NQ, 15. attr , (CV). The results. -summarized in Table Il confirm that 219- K [151

the carbamnate linkage does preservec the oxidation (reduction 1potentials of the D (A) ends of the D-<'r-A molecules. cular effects. NaturallN, amphiphilic molecules that form LB

films will not usuallh, crystallize, because of the usual aliphatic

X. 5. (RNSt.\L SIRI ( 11 RIS O)F MOD.tO I)DO"OR. .((FPOR \',.1 "Ltaids" added to them. Hayever. we have sol,.ed the struc-

D-rT-A %tot l i ii ,, tares of' two D-(-T-A molecules which do not forrm PL orLB films: Ph-C-BHTCNQ. 28- (Fig. 6), and DMAP-C-

A fe%% crystal structures ha~e been solvedc: for the donor HMTCAQ, 29 23 (Fiq. 7). Both structures show an extendedDMAP-C-Me, 25 2((.3) for the acceptor BHT(-NQ, carbamate linkage: in Ph-C-BHTCNQ, the dihedral angle1! " jFiq. 4), and for the methyl ester (AETCNQ, 27) of between the phenyl ring and the six-memered central ringthe acceptor HETCNQ, 12 (Fiq. 5). The small dlllr,,: of BHTCNQ is only 8* ". This gives hope that in LB filmsin conformation between AETCNQ and BHTCNQ can be of related D-cy-A molecules the carbarnate linkage "ill alsoattributed to crystal packing forces. rather than to intramole- be extended.

Table Ill. - Solution c~chic ,.oltarnmetric potentials : " All data were obtained at a Pt electrode, and are gp~en in Vlils %.s SCE

Oxid.(l1) D -D- Oxid. (2) D" D" Re& (1)A -A Red. (2)A A-

tf,,le~ule NVo E, El E, E, I :r [-1. E, EF, Rel.

Dor

DMAP-(-Me 25' 0,58 0.';5514t( ept(,r-(NQ 2 h 0.19 -0 i

1(-\Q 2e 0).11 0.13 -0,46 -0.43 :BHITCNQ ill' 0.305 -0 170 ,HETCNQ 1W+ O.107 -0()398 ,HMHTCAQ 26' -. 172 -0331

D-rT-.4DDOP-('-BHTCNQ 14 d 1.21 0 25 -0.07BDDAP-C-HIETCNQ 17' 0 661 1. I0 0.02 - 0.491

P,,-C-HETCNO 18" 1.04 1.01 1.18 1.15 0.11 04Mg -0.32 -0.35BHAP-(-H MT('AQ 20' 061 0,60 -0(.19 -0. 36 ''TDDOP-C-HETCNQ 23' 1.021 0.10 007 -0.50 --0.47 2DDOP-C'-ENP 21 ' 1 42 I ;q 1.16 1 13

TDDOP-(C-ENP 22' 1,171 I 12 1 15 z2MTDAP-C-ENP 204' 0.57 0 4 1.09 106

Solvent: CHCN. Reference electrode: SCE. A peak at 0 3 7 V ( return scan) grows with successive cycles. and is indicative of dimer or polymerformation " ' Solvent. CH CN. Reference electrode: SCE. ' Solvent: C'H2,CHC'I. Reference electrode: Ag IAgCl, An offset correction of0 15 V has been applied to convert the values to V vs. SCE. ' Solvent: CHjCN. Reference electrode: Ag IAgNO , An offset correction of0.320 V has been applied to convert the values to V vs. SCE. F Solventi CH ICCI. Reference electrode: Agof AgC. An offset correction of() 19 V has been applied to convert the values ot V vs. SCE

NLEW l: RNh B OF (THFMIS2RY VOL 15, .- 2-3-1991

NC O N atomnic radii fI_' A f'or H-. 17. A f'Or Cj. I5 A for 0. 1 35 AH I f'or F. and 1 6S A f'Or SI are show n in F-igures S- 1- i the %an

N " 0CH HOder Wak shape is viewed from the A end of' the molecule).H 3 ,, Q Alsko given are the HOMO) (-I,) and LiMO (-A,) energies.

INC ON and also the differences l,,-A, and 1,-A,) discussed in [qua-OH3 226tions 3 and 4 ahove.

DMAPCMe HMTCAO Overall. the MNI)( structure,, are extended. aS expcted.hut there are some small surprises (I) while the data ot'

F ~cTable 11 suggeSt that in a P'L film PxN-C'-BHTCNQ is arelatisex hadt molecleI. Fiegure I I introduces, a iw ist in thecarbamnate litkaue: (i) ss hile the dlata of' Table 11 saggest

a ell-packed Bl-DAP-('-BHTCN\Q molecule. F-igure 11)0,2,v shows a conf'ormer in which the bis-hexs, " tails" are not well

Awined %kith each other: (I II) while the cr5 stal structure oh

BMAP---H\II(AQ shows that hoth diexanomethsleneEf-) VOTS 'subtituecnts on the anthracene ring deviate from the plane in

tile same direction. kI NDO c'ase them a "corkscrew', tMINiin Figure 12. The \I N ~ioni/ation energies lu)are prohabl\1 to 2 elk high, as expected fromt a Koopman's theoremcalculation: the su/e of' thle difference (l1,-A , -ilAIj 11 is a

IpA ~large ais expected. but some of' the 1, value,, are not as low%as hoped.

X. 7. -1 otRu i 1R N\si (R \i 1iI R NRi I) NPIi(F- I R I ) -'- 50

H. lclo --C( raing-angle Fourier transh'Orm inf'rared IFTIRI spec-

___________ ira ofI monolaxers of' BDDA,-P-('-HMTCA-Q. BDDAP-C-0 NC CIN BHTCNQ. and TDDOP-C-HF TCNQ. have been measured.

and reported pres iouslv. 2"-''. The C-H stretch bands areFigure 2. - (selc soiat.Innograrn of B[)t)AP-( -l.TI( \Q. 17 well resolved. even for a single monolawer, and a broad

structure at about 3 5001 cm is seen f'or ''fresh" samples.

NC ON NC CIN but disappears tor samples older than about 60. daxs. thisBrTa be water trapped between the L-B film las.er and the

CoH aluminium laver, bitt the identification is not certain theHC.O0_- 0_ ,C,-( films were not studied in vacuum).

o N CN NC ON 0 NCON27 28

AETGNO H PSI-C-BHTCNQ

HC, , rkllco IC9. Rectification attemptsH 3~N~ N CON

6H, 29 We discuss here seven attempts. by the ORP and by others.BkWP-CHMTCAO to detect rectification.

In the first experiment ~.a 2 mm diameter droplet of' Hgca C11was used to probe the conduetisity across a single monolas.er

1 ~LB film deposited on either Pt or conducting tin oxide glass:64: the sandwiches:

j N7 010 i') Pt IDDOP-C-BHTCNQ'IHFg,Cq (~in) Pt IP\;-C-BHTCNQ I Hg, and

(iii) conducting SnO. glass!I DDOP-C-BHTCNQ Hgwere thus tested: in all cases the backgzround conductivit\

Figre . OTEP11 lo ot (h crstalin srucure(it DMP- of the solid support was measured. presumably. because ohigue 2. Spc gRT rop oPhe Irs i#il strutu. oa - 1,2. , microscopic pinholes in the LB film. which max' have been

1.4 854 8 R~ 5. R, 11, r '86 refXlcionl lbred. in part, by the damage that the Hg drop can do onthe film surf'ace '.

In the .second experiment 21,in the hope that maybe def'ectsmay be avoided. statistically'. in a domain of the order (if

\Mu oi I (ill 1 \1S (ltt [ II (SI \11(l about 0.5 mm x 0.5 mm. if one searched through enoughsamples. the left-hand hall' of fi'teen glass microscope slides

As repolrted presiousls2 semi-empirical mnoleculair orbital was coated fusing a mask) with five parallel fingers of Al at(MO) calculations, using the MNDO algorithm, with I'ill least 501) nm thick, 3.5 mm long, and 1.6 mm wide. Then thegeometry optimitation. in program MOPAC. base been per- fifteen slides were coated w'ith a single LB monolayer of'formed on D-rr-A molecules. to predict their eometry, and BDDAP-C--BHTCNQ. 15. at room temperature. Then thealso their HOMO and [IAMO energies. The structure as slides were coated again with five lingers of AL but this timedrawni by program ORTFP-Il using typical %an der Waals on the right hand side of' the slide. so that the vertical overlap

Ni1 \kJo RNAI M ( HI Mist R5 \,0I I S. N' 2-3-1991

rm: QtI sr H)R I \1'iW [ F( tI A\R iFAR I~ S 17

N16 N5 C23N I

C2 C4

020 C19 C5C 2

cis 02202Ca 1 C13 C6 0W7 I

N5i

C4o caCI

C31C IO C

C2N14 C315N16

Figure 4. - ORTEP-11 plot of BHI('.\Q. 11 " Space grou p P-2,. n Figure 5. ORTi P-Il plot ot- XfT('\Q. 27 Spaice group Ii . 4-1. j 4 25tS. h = I 1 018, 10947A.[i=-9214 ./ 4. R? 011 ha r # 2 j. a 7 165. h )-9uS. - I;'4A

for 1 1 reflections - 68 2.' R -- 1 4'. 1"or 1 141 reflection,

CZ6I

r:.

logure~~~ 21. - RF-1po ftecvtltesctueO h-25Q 8 Sac ru ~n~ 4.a~ 1.697.c 23396~~~~~~~~ 5' / .R491Fr229relcin

Figure~2 7.(RF-1poto h2r~altsrcue4 l)A-.MCQ 2 pc rupP a 1) a7X 09913541~C3 A.: C19 C9

67' fC69l5.' 9 6' .R 17 r 3 e ut

NEW ~ ~ ~ ~ ~ 12 C2I CI[S HIMSR Vl1.'2319

-1S IN Mt M I RN \\ I '\)It

F-igure~ 8. Structure itop . Ni NDO conformation middil. i, igu~!re 9. Structure i top., \lN [)) conformation miiddic) and

lN I)() Nan let Wais Ahape. %ie~ed tromn the acceptor e:nd (hot- NIN)) Nan der Waak Ahape. \eNed from the accephir end ihot-

tntm hor M -( CL IfTCNQ. a %.riant of- FTI- -13111 R M). 13. tom). hor I'h-C I iT(N\Q. a variant of Ph-( -B11 R NC). 28.f", N -Ie . AN I[)2A( -222. IN-%,- W e'41,\ )-41~ c\ 294; c\. 1,, hf'ie 21 1) . ) A,' IN2 11e

N\

NS<'

Figure 10. Structure (top). %vlND() contormation iniddlet. an Figure 11. Structure (top). MAND0( conformation imiddle), andMNI)() van der Waals shape. %iewed from the acceptor end hoorn M1ND() Nan der Waals shape, viewed from the acceptor end (bot-

tor BIIAP-('-FIIT(-sQ. a * variant of BFDDAP-C>BIT(\Q, 15 torn), for Py-C-FHTN.avrin fP--BT1Q'1

1, 6 V , 296e.1)A ii 834 [,A,)- 0 eV 1,) 8192 eV, A, = 2957 eV. 1,-A, 5 215eV, ],AD 8 147 ev "

N14 \k Jut RNA[ Oif (tFMtSt RS VOL5 ,- 2~-3- 1091

THE QUI ST FOR L \I%1(I H I I AR f)FVI( PS 219

Later, Sambles' and Ashwell's group found. in a sixthexperiment, that an LB film of' Z-r3- (I -hexadecyl- 4-quino-linium )-ax-cvano-4- trv rvldicyanometanic- (C,,,H'33-Q3CNQ.

~ J~"''~- ~77). a D-it-A molecule. similariv sandwiched between Pt and~ ~\~4Mg electrodes (the latter shad'owed with Ag) also showed

-0-~ macroscopic rectification behavior '. However it has not yet1~ '0,, been excluded that the observed I-V curves are not due'to

~ ~ dipolar association (e. g. between Mg and TCNQ) or to otherSchottks barriers 3'

~O'~ -CThe ORP has recentiv acquired a STM (Digital Instrumentc., ~Nanoscope I of its ow'n, and. in a setventh experiment, has

studied monolayer LB films of BDDAP-C-HETCNQ andPs-C-H ETCNQ. transferred onto highly oriented pyrolv ticgraphite (Union Carbide grade ZYA) by a horizontal liftingtechnique. Pt Ir "nanotips'* Digital Instruments) were usedAt low set-point currents, some asymmetric,. in the Current-%oltage plots can be seen. In preliminary work with BDDAP-

. . C-HETCNQ and Py-C-HETCNQ, the image of' graphite is~ replaced by what seems to be an image of- the film

7N

\. Conclusion

We have outlined here several topics in MF. and hope to~ ~ .have convinced the reader that many exciting prospects exist

in [his infant field.

Figure 12. - MNDO conformatton itopi. structure fmiddle., and\IND() san der Waals shape. %tewed from the acceptor end ihot- REF1 Ri No EStomn) of BMAP-C-HMTCAQ, 29. 1,, =9 02 eVA, 2.617 e%'.1,-A, -6425. 1A-AD = 9 220 eV ZIMetzger R. M.., in I"ov'er-Dimensional Sistems and Molecular

Electronic". Metzger R M Day P. Papavassiliou Gi C Eds_NATO ASI Series. Plenum, New York, 1991. p 691Asiram A.. F-reiser M. J.. Seiden P. E . Young W~ R . U SPatent LS-3.953. 974, 27 April 1976

Ad 1 BD AP-CBHTNQ! l wuld e i an reaof' nlvAsiram A.. Ratner MI. A.. Chem. Phis. Iett . 1974. 29. 2'7Al DDA-C-HTCQ~A wold e i anare ofonl ~ Asiram A.. Seiden P F.. Ratner MA ,in" Molecular Elec-

about 0.5 mm x 0.5 mm. Of the 75 junctions thus prepared. tronic Devices". Carter F L. Ed.. Dekker, New York. 1982. p. 5.many were open circuits the rest were short circuits. Thus. / Metzger R. M.. Panetta C A.. J Phlv, Les ls Fr., Colloque.a defect-free domain of BDDAP-C-BHTCNQ had not been 1983. 44. C3-1605.found. Metzger R. M.. Panetta C. A., in "Molecular Electronic Devices",

Vol. It. Carter F L. Ed.. Dekker. New York. 1987. p. IIn the third experiment. Aviram. Joachim, and Pomerantz Panetia C. A.. Baghdadchi J., Metzger R. M.ifol. (rtst Lig.

reported in 1988 " (and later retracted ") that rectification &irtt. 1984, 107. 103.had been observed in a modified STM. for the molecule Metzger R. M , Panetta C. A.. Heimer N E . Bhatti A M..

shown in Section 5.2 above, which had been originally desi- Torres E.. Blackburn G. F . Tripathy S. K.. Samuelson L. A.gnedas ntenalhydrgenato trnsfr swtch(an no asJ. MoI. Electronics, 1986, 2. 119gnedas ntenalhydrgenato trnsfr swtch(an no asMetzger R. M., Panetta C. A.. Miura Y., Torres E . Svnth.

a rectifier). The claim for molecular rectification was later Metals. 1987. 18. 797.retracted . Torres E.. Panetta C. A.. Meizger R. M., J. Org (Chem., 1987,

In te furthexprimnt. r. omerntzstuied ithhis52, 2944.In te lurt exprimntDr.Pomeant stdiedwit hi 11Metzger R. M.. Panetta C. A.. in ~Proceedings of the Eighth

modified STM. an LB mololayer of BDDAP-('- BHTCNQ. Winter Conference on Low- Temperature Physics. Cuernavaca.15. deposited on a AulAglmica substrate, using an ato- Mlexico", 1987. 81.mically sharp W tip, as the couple W BDDAP-C- 2, Laidlaw R. K.. Miura Y.. Panetta C. A., Metzger R. M.. ActaBHTCNQ IAu. Large "rectification" currents were observ- Crvst., 1988. C44. 2009.ed 2.- 2', but later control experiments by Dr. Pomerantz It' aidlaw R. K.. Miura Y., Grant J. L., Cooray L.. Clark M..showed that this "rectification effect', with very large tcur- Kisperi L. D., Metzger R. M., J. Chem. Ph 'vs., 1987. 87, 4967.rents, could occur in the absence of anyv moleeule. Disclaimers 14 Laidlaw R. K.. Baghdadchi J.. Panetta C. A.. Muira Y..

for the preliminary results were issued 21. 2 2 Torres E., Metzger R. M., 4cta Crv-st., 1988. B44, 645.Miura Y.. Laidlaw R. K.. Panetta C. A.. Metzger R. M.. .4cta

Recently, in a filth experiment, Sambles et al. at the iUni- Cri-st.. 1988, C44. 2007.Metzger R. M.. Schumaker R. R., Cava M. P.. Laidlaw R. K.

versity of Exeter found that a monolayer of BDDOP-C- Panetta C. A., Torres E., Langmuir, 1988, 4. 298.BHTCNQ, sandwiched between Pt and Mg electrodes, Metzger R. M.. Panetta C. A.. in "Organic and Inorganic Lower-behaved as a rectifying LB film ". Dimenstional Materials', NATO ASI Series. Vol. B168,

NEW JOI.RNAL OF (HEMISTRY VOL 15, N' 2-3-1991

220 R %I %ltT/(FR ANI)C A PXNIFTTA

Delhaes P,. Drillon M4. Eds ,Plenum. News York, 19SX. p. "'I F-ujihira M . Nishivaama K . Yamada H ,Thin Solid Films. 19X5.

" Miura Y., Tocrres E_. Panetta U A . Metzger R M I J Org' 132. 77.

Chem.. 19S8. 53, 439' Lehn i.-M. ,.4ngesc. (hem. Intl. E-d. Engl.. 1988, 27, 89

"' Miura Y.. Panetta C.Metzger R. M , J Liquid (hroni , Aviram A.. J. Am. Chem. Soc.. 1988, 110. 5687988. 11, 24i Kennv P. W . Miller L. L., Rick S F.. Jozefiak T. R_

2'Metzger R MI Panetta C A , J1 Mot hicc rcncc s. 1999. 5. 1. Christopfel W C.. Kim J. H , Uphaus R. A.. J, -4m Chem

Metzger R. MI Pacnetta C.A. Ch tin. Phics,, 198 8, 85. 1125. Soc., 1988. 110. 4445.M~etzger R. M_. Panetta C A. cpu/c .1lft. 19S9. 28. C807 Szejtli J., "Cic/odextrin Tec/inclogi". Kluwser. Dordrecht, 1988.

2'Metiver R MI.. Laidlaw R. K . Torres F , Panetta C. A ~ cVicens J., Bohmer V. Eds., -Ca/xarenes a Versatile (/as.ss o/

J Crvis Np.; Re, . 1989. 19. 47S Mfacroccc/ctic Compounds. K!usser. Dosrdrecht. Holland. 1990

M4Netzger R. M . Panetia C A . in tb~i/, 0/ct F/cc ironci iecnhard J. E. AppI. Phi s Lett.. 1964. 35. 3059Sitere and lF.chno/cgi ,Asirai -A., Bross A Fds . .Nessk York '~Polymeropoulos F. E , Mohius D.. Kuhn H . Thcn So/cd Fclms.

Engineering Foundation. 1990 p. 293. 1990. 68. 17V2'Metzger R. Ni.. Wiser D. C_, Laidlass R. K.. lakassi MI A.. " Sugi M.. Sakai K., Saito M.. Kaviahata Y . lizima S , Thin Socd

Nlattern D L.. Panetta C. Langrmuir. 1190, 6. 1 515. Ft/ms. 1985. 132. 69.Mi \etzger R Mi.. Panetta C. .in "Lcicer-Dinienvconal Si~tcfms 'Potember R. S . Poehier T 0,. Cowsan D. 0). .4ppl Pht.s. Lett

and Afi'/cculur Electronicsv" Metzger R. M.. Day P.. Papicsassi- 19'9, 34. 405.hou ci. CI. Eds S AA1 .4St Series, Plenum. Ne" York. 1 11~. '" goiember R . Hoffman R. C.. Hu H. S.. Cocchiaro J. E£

p. 64 1. Viands C. A.. Murphy R. A.. Poehler T. 0 . Po/imer. 1987. 28.M Netzger R. Mi.. Panetta C. in '-Adzancei/ Orecjni S'olid/ State 574.

l'iaerials. Chiang L Y.. Cowan D. 0., Chaikin P. Eds.. ~ 2Potember R. S., Hoffman R. C.. Benson R. C.. Poehler T 0.Materia/ls Re~search .Simieti .SiPnipiisiurm Prcc'edinzc Seric's. 1990. J. Phis., Les C/cs. 1983. 44. C3-1597.

173. 5311' White H. S.. Kittleson G. P . Wrightion M.SJ, -m. (hem.

See e.g. Gamnes Jr 6 L . lfnsocubli Vtonolaers tit L.iqidii -- Soc. 1984. 106. 5375

(jas Interfaic"-. Intersecence. New York. 1966 '~Turner Jones E. T.. Chvan 0. M.. Wrighton M. S. J Am

NBlodgett K. B.. J Am. (Chem. Sc'c... 1935. 57. 1007 Chem. Soc., 1987. 109. 5526.Blodgett K. B.. Langmuir L . Phis, Rev.. 1937, -51. 964. Paloheimo J., Kuivalainen P.. Stubb H , Vuonmica E . Yli-

Kuhn H .Mi hius D , Bucher H.. in "Tee ntques of C'hc'm.stri. Laht, P.,.-lpp/. Phi's. Leti.. 1990, 56. 1157,

Vol. I -- Phisica/ Mfethodv cci Chemisti - Part V - Determina- "'R~u H ., Akagane K.. Kori H.. Chem. Economni and Eng Rev.

ticcn oft Thermod inamic and Surface Properties., Weissherger A.. 1()86, 18(12), 26.Rossiter B. W. Eds.. WileN-Interscience, Ness York. 1972. Slonim J., Mole D., Bauer M.. Librari' Hi-Tech. 1985, 3(4). 27.

p. 577. i8Kalanaraman P. S., Kuder J. E.. Jones R. S.. in "Functionali2 Kuh [.PueAp (h .,17,5,31 oimr"Brgbreiter D E.. Martin C. R. Eds . Plenum. New

iKuhn H.. Pure AppI. (hem.. 1981, 53. 2105 York. 1989. p. 173.

SSee e. g. Thin Sol/id Films. Vols. 68. 1981), 99. 19S3, 132-134. '~Nikles D. E.. Forbes C. E., Goldberg H. A.. Johnson R. E..

1985, 159-160, 1987. Kohn R. S., Onorato F. J , Pro(. Soc. Photo-optical Instrurn.

'~Binnig G9.. Rohrer H., Gerber Ch.. Weibel E.. ['his. Rez,. Lett. Engineers. 1989. 1078, 43.

1982. 49, 57. ~~Coronel P., Barraud A., Claude R., Kahn 0.. Ruicudel-

0. Geddes N. J., Sambles J. R., Jarvis D. J . Parker W G., Teisxier A.. Zarembowitch J., J. Chem, Soc ('hem Commun..

Sandman D. J., AppI. Phvys. Lett., 1990. 56. 1916. 193. 1989.

- Ashwell G9. J., Sambles J. R., Martin A. S., Parker W. 6.. -iTaube H.. Pure .4pp/. Chem.. 1975. 44.

Siablewski MI.. J (hem.So(. Chem. (omiun.. 1990, 1374. -Meyer T. J.. in -Photchemic al Enerv Conv ersion"

3s5 Carter F. L. Ed.. "Molecular Electronic Devices., Dekker. Nesw Norris Jr. J. R.. Meisel D. Eds.. Elsevier, New York, 1989.York,1982.p. 75.

31c Carter F. L. Ed.. "Molecular Electronic Devices II". Dekker. actr LT.ClsGLMierJR,.Am hmS ,

New ork.19871983, 105. 670.

'~Carter F. L.. Siatkoski R. E. , Wohltjen H. Eds.. Mu/cc u/lar 'Miller J. R., Caleaterra L. T., (loss G. L.. J. Amn. Chem. Soc..

Electronic Devices. Proceedings of the 3rd International Sympo- Marcu4, A06 34.Do Fad Sc.19,2.2.

scuim'. North-Holland. Amsterdam. 1988. Mru .ADs aaa o. 90 9 1

Bonssov M. Ed.. Mci/lecu/ar Electronics - lVth International ~ "Lindsay J., Mauzerall D., Linschitz H.. J. Am. Chem. Soc., 1993,

School on Condensed Matter', World Scientific. Singapore., - 105. 6528.1987.McIntosh A. R., Bolton J. R., Conolly J. S., Marsh K. L..1987 Cook D. R.. Ho T -F.. Weedon A. C.. J,. Phis. Chem.. 1986. 90,

42Aviram A. Ed.. MAo/ecu/ar E/ectrocnics -Scepne and Tcechnic- 5640.

loigv. Ness York Engineering Foundation. 1990. Joran A. D.. Leland B. A.. Geller G9. G.. Hopfield J J.SHaddon R. C.. Lamola A C., Price .. at/. lead. Sec. I.. 1985. Dervan P B.. J. Am. (hem. Soc., 1984. 106. 6090.

82. 1874. '~Nishitani S.. Kurata N.. Sakata Y.. Misum S., Karen A.Davidov D., Selig H Eds.. "G;raphite Interccc/atiiin Comnpiiunds.C. Okada T., Mataga N... J. Am Chem. Soc., 1983. 105, 7771

N inth Mfetals. 1988. 23. Moore T. A., Gust D., Mathis P.. Mialocq J -C.. Chachaty C.."Acrivos J. V .l VTO AS! Ser. 1984. (330. 479, Bensasson R. V.. Land E. J., Doizi D. Lidell P. A.."For the proceedings ccl a receni conference, Nee A.4ivantec/ Lehman W. R.. Nemweth G. A., Moore A. L., Nature. 1984.

Orgunit Sic/id State Mlaterials'. Chiang L Y . ('osAdn D. 0.. 307. 630.Chaikin P Eds . Materials Research S'iceti Sc mpim Priicce- Krieger C.. Weiser J.. Stacab H A.. Tetrahedron Leit. 1985. 26.dings Series. 1990. 173. 531 6050.

SSkotheim T. A. Ed. Hand/cook if 'conc/uctinc' Piiltmeri". Vol. I [lHush N S.. Paddon-Row M. N.. Cotsaris E.. Oevering H..

arid 11, Dekker. New Yosrk, 1986 Verhoeven J. W , Heppener M , Chem. Phsvs Lett., 1985. 117.~'Relevant ai tidles in Miller J S. Ed.. ''Es ended /.iear-Chioin 8

(Cimnundi. Vols 1, 1H. and 111. Plenum, Ness Yoirk, 1982. ~'Wasieleski M. R., Johnson D. G . Ssec W. A.. Kersey K. M.

li983 Cragg D E_. Minsek D. W.. in 'Photochemica/ Energi, Corner-

Inabe F . Iloihino N.. Mitani T., Maruaina Y . Bu/l (hem stun". Norris Jr. J R., Meisel D Eds , Elsevier, New York.Sicc Ipn . 1989. 62. 2245 1989. p 135.Carter F L.. in Carter F L Ed . 'b/ecu/ar 1:/itrcnite- evice'". ~ 'Aviram A . Joachim C . Pomerantz M.. Chem Phi's. Lem . 1988,D~ekker. New York. 19)(2. p 11. 146 490

%F'e JOt RNAi. OF c'HUIiSTRY. VOL 15. -4-2-3-l9"l

THE QIEST FOR UINMOIECULAR DEVICES 221

85 Aviram A.. Joachim M., Pomerantz M., Chem. Phvs. Lett. ' Peterson 1. R. J Chin. Phis. 1988, 85, 997.

1989. 162, 416. 9' Hertler W. R , J. Or', (hem.. 1976. 41, 1412o SixI H., Higelin D.. in Carter F L. Ed.. "'tdoleculur Ele'tronic " Baghdadch J., Ph. D. dissertation, Univ. of Mississippi. Dec

Devices ir. Dekker. New York. 1987, p. 27. 1982.'" Tachibana H.. Nakamura T., Matsumoto M., Komizu H.. " Wu X. -L.. Parakka J. L.. Metzger R. M.. Unpublished results

Manda E., Niino H.. Yabe A., Kawabata Y.. J A.m. Chem. '" Cephalas A. C., private communication

Soc.. 1989. III, 3080.98 Birge R. R.. Cooper T. M.. Biophvs. J., 181. 42. 61. Anderson J. R., Jorgensen 0.. J (hem. S, Pterk Trni I.

89 Wilson E G . ,1ol Crt-'t. Liq. Cryvt. 1985. 121, 271. 1979. 3095.

90 Murray R. W c, c. Chem. Res., 1980, 13, 135. Takassi M. A.- Ph. D. Disertation. University of Missis ippi.

Maoz R., Netzer L.. Gun J., Sagiv J., J ('him. Phv.s 1988, 85, Aug. 1989.1059. 'u" Wu X. -L., Metzgcr R M., Unpublished results.

, Bigelow W C., Pickett D L.. Zisman W A . J (,dlotd Sci.. Supported by the N\tional Science Foundation. Grant DMR-

1946, I. 513 88-01924.

NF-W 1()1.RNAI. OF ( HEMIStR- VfOL 15. N" 2-3-1991

Reprinted from LANGMUIR, 1990, 6, 350.Copyright D 1990 by the American Chemical Society and reprinted by permission of the copyright owner.

Monolayers and Z-Type Multilayers of Donor-a-AcceptorMolecules with One, Two, and Three Dodecoxy Tails'

Robert M. Metzger,* Dawn C. Wiser,' Richard K. Laidlaw,5 and

Mohammad A. Takassi

Department of Chemistry, University of Alabama, Tuscaloosa, Alabama 35487-0336

Daniell L. Mattern" and Charles A. Panetta'

Department of Chemistry, University of Mississippi, University, Mississippi 38677

Received March 10, 1989. In Final Form: June 7, 1989

Pockels-Langmuir and Langmuir-Blodgett monolayer films have been found for the 2-(4-nitrophe-nyl)ethyl esters of N-(4-dodecoxyphenyl)carbamic acid (DDOP-C-ENP, 7) and N-t3,4,5-tridodecoxyphenyl)carbamic acid (TDDOP-C-ENP, 9) and also for the (2-hydroxyethoxy)-7,7,8,8-tetra-cyanoquinodimethane ester of N-(3,4,5-tridodecoxyphenvl)carbamic acid (TDDOP-C-HETCNQ, 10). How-ever, monolayers were not formed for the 2-(4-nitrophenyl)ethyl esters of N-(3,5-didodecoxyphenyl)carbamic acid (BDDOP-C-ENP, 8). The pressure-area isotherms vary greatly dependingon the number of dodecoxy groups attached to these donor-a-acceptor molecules. The film strengthimproves dramatically by lowering the subphase temperature. The molecular area appears to be definedby the acceptor end for DDOP-C-ENP, 7, but by the dodecyoxy donor end (for both TDDOP-C-ENP,9, and TDDOP-C-HETCNQ, 10). Noncentrosymmetric Z-type LB multilayers are obtained for DDOP-C-ENP, TDDOP-C-ENP, and TDDOP-C-HETCNQ. This suggests some interesting nonlinear opticalproperties for these multilayers.

I. Introduction D-a-A, where D is an organic one-electron donor, a is a

A recent report from these laboratories discusses the covalent a bridge, and A is an organic one-electronacceptor.' These molecules are of interest in the con-

formation of monolayer films of molecules of the type struction of unimolecular organic rectifiers,- 13 which are

Authors to whom inquiries should be addressed.Supported in part by the National Science Foundation, Grants (1) Metzger, R. M.; Schumaker, R. R.; Cava, M. P.; Laidlaw, R. K.;

DMR-84-17563 and 88-01924. and by a grant from Nippon Tele- Panetta, C. A.; Torres, E. Langmuir 1988, 4, 298-304.phone and Telegraph Co. (2) Metzger, R. 'A.; Panetta, C. A. J. Phys., Colloq. 1983, 44, C3-

1605-C3-1611.£1987 Summer Undergraduate Participant, NSF Research Employ- (3) Metzger, R. M.; Panetta, C. A. In Molecular Electronic Devices;

ment for Undergraduates (REU) Program from Muskingum Col- Carter. F. L., Ed.; Marcel Dekker: New York, 1987; pp 5-25.lege, New Concord, OH 43762. Current address: Department of (4) Panetta, C. A.; Baghdadchi, J.; Metzger, R. M. Mol. Cryst. Liq.Chemistry, Colorado State University, Fort Collins, CO 80523. Cryst. 1984, 107, 103-113.

1 Permanent address: Science Department, Laramie County Con- 15) Metzger. R. M.; Panetta, C. A.; Heimer. N. E.; Bhatti, A. M.;munity College, Cheyenne, WY 82007-3299. Torres, E.; Blackburn, G. F.; Tripathy, S. K.; Samuelson. L. A. J. Mol.Electron, 1986, 2, 119-124.

Current address: U.S. Army Chemical Research and Develop- (6) Metzger, R. M.; Panetta, C. A.; Milra, Y.; Torres, E. Synth.ment Center, Aberdeen Proving Ground, MD 21010-5423. Met. 1987, 18, 797-802.

0743-7463/90/2406-0350$02.50/0 e, 1990 American Chemical Society

Mono- and Mfultila-vers (if Donor- a- Acceptor Molecules Lan gmuir. Vol. 6, No. 2. 1 99Y0 51

based on the idea"' that electrical current path will beunsymmetrical within molecules D-aT-A, because the zwit-terionic state D*-aT-A- is much lower in energy than the - ~ ~ ,state D--a-A . If these D-a-A compounds can be orga- -

nized as a monolayer between two conventional metal L -

films M, and M, as ordered structures M,ID-cT-AIM.,, 1,then the asymmetry of the molecular orbitals of the D-aT-A molecules would ensure that electron transfer is pre-ferred in one direction over the other.

. D-a-A M2.-y-.. f1D-o-A M2

The present efforts' 3 to synthesize and test the organic Jrectifier concept assume that the best technique for it, Iassembly is the Langmuir-Blodgett film technique.' 2

For this, the D-aT-A molecule must be an amphiphilic rmol-ecule; i.e.. either the D end or the A end must be madehydrophilic and the opposite end hydrophobic. On a Lang- .,

muir trough, or Pockels-Langmuir-Adam-Wilson-McBain balance. or film balance, the amphiphilic mole-cules self-assemble as a monolayer (hereinafter called' a-Pockels-Langmuir" (PL) monolayer in honor of Agnes Figure 1. Structures of D-a-A molecules that do form PL mono-Pockels (1862-1935)"6 and Irving Langmuir (1881- lavers, from ref 1 and 11.1957)). The monolayer must transfer well (i.e.. with aT),transfer ratio close to 1.00) to a solid substrate, thus form- .---.-- { -

ing a Langmuir-Blodgett (LB) film (named after Lang-muir and Kathleen Blodgett (1898-1979))" As reportedelsewhere.,1. 0 - 12 we have prepared several such mole-cules (1-6), shown in Figure 1: all of them form both PLand LB films.

Recently, indications'. 2,17

.18 that two such molecules

icluding BDDAP-C-BHTCNQ, 4) did exhibit rectifica- a K:- ..

tion, when deposited on a AulAgImica support, and probedby scanning tunneling microscopy. were later found tobe due to some unexplained artifact of the W'A'tipjAujAgjmica couple.2

(7) Torres, E.; Panetta, C. A.; Metzger. R. M. J. (Jrg. (Chem. 1987,.52, 2944-2945.

8) Metzger, R. M.; Panetta, C. A. Proc. of the Eighth Winter Meet-ing on Low-Temp. Physics. Cuernavaca Mexico. 1987. pp 81-100.------------------~i .

9 Metzger, R. . Panetta, C. A. in Organic and Inorganic Lou- -. -T-. ..- -'

er-Dimensional Materials; Delhaes. P.. Drillon. M., Eds.; Plenum: New -

York. 1988; pp 271-286. - --.

01 Metzger. R. M.: Panetta, CA. J. Mol Electron. 1989. 5. 1-17. 1_g-(11) Metzger, R. M.: Panetta, C. A. .1. Chim Ph vs. 1988. 85.1t125- Fiue2SrutesoD-Amlclererednthstdy

1134. Fgr .Srcue fD(- oeue eotdi hssuy1 12) Metzger. R. M.; Panetta. C. A. S.nth. Met. 1989. 28. C807-erweeptonP adLBflfrminofou

C814. Hr erpr nP n Bfl omto ffu13) Miura. Y.; Torres, E.: Panetta, C. A: Metzger. R. M. .1. Org. novel D-T-A compounds (Figure 2): 0i) the 2-(4-

Chem. 1988.,53, 439-440. nitrophenNl (ethyl ester of tN-(4-dodecoxvphenyl )-14) Aviram. A.:' Freiser. M.J4.: Seiden. P. E.: Young, W. R. US Patent crai cd(DPCEP ) i)te2(-irpe

.3,953,874, April 27. 19716. abmcai DO C-N,7,G)te2(ntrp -15) Aviram, A., Ratner. M. A. Chem. Ph vs. L.ett 1974, 29. 27 ,7- nyl)ethyl ester of N-4-):3.5-didodecoxyphenyl)carbami c acid

283. (DO--N,8,(i)te2(-irpey~t se'16) Aviram, ., Seiden. P. E.; Hatner, M. A. fn Molecular Elec- (DO--N,8,(i)te2(-irpey~ty se

tronic Deu ces; Carter. F. L.. Ed.. Marcel Dekker: New York. 1982: p 5. of N-(3.4.5-tridodecoxyphenvl)carbamic acid (TDDOP-(17) Aviram, A.; -Joachimn, C.. Pomerantz. M. ('hem. PhNs. Lett, C-ENP. 9). and (iv) the (2- hyd roxyethoxy) -7,7,8,8- tetra-

1988. 146. 490-495. ynqioiehneseofN(,,-rd eoxp -18) Aviram. :-Joachim, C., Pomerantz. M. unpuh Ilished. n c rmanoq dthe esterC-ETNQ of) T-345tioeohe-no

(19) See. c g.: Gaines, CG. U.. -Jr. Insoluble Monolayers at Liquid- nycab-iccd(T O--HT Q,1)ThdorGas Interfaces; inmersciEnce Publishers: New York. !')4. ends of 7-10 are in fact rather weak electron donors. sim-

20) Blodgett, K. B. 41. Am. Chem. Soc. 1935,5 ;s:07 -1022. ilar to the donor end of DDOP-C-BHTCNQ, 3; the accep-(21) Blodgett, K. B.; Langmuir. 1. Ph vs. Reu. 19:17. .51, 964-982. tor end of 7-9 is again a weak electron acceptor, useful(22) Kuhn. H.: Mhhius, D.; Buicher, H. in Techniques of ('hemis-

tr',. Vol I-Physical Methods of Chemistn'-.Part V-Determrnation of Ther- only as a readily accessible model acceptor group. Themnod~namir anid Surface Properties: Weissherger, A., Rossiter. B. W.. acceptor end of 10 is the strong one-electron acceptorEdls.: Wiley-Interscience: New York, 1972; pp .577-70)2. CQ Prlmnydaafr1hvebnpe

(23) Kuhn. H. Pure Appl. Chem. 1979. .51, 34.-352.TNQ 2 - 9 Peinaydtfo10hv benp-(24) Kuhn. H. Pure AppI. Chem. 1981. 53. 2105- 2122. sented elsewhere.''(2.5) See, e.g.: Thin Solid Films 1980. 68; 1983, 99: 1985 1.32-134.(26) See biographical note in: Derrick. M. E. J. (hem. Ed. 1982.59. 28) Acker. D). S.: Harder. R. .. Hertler. W. R.; Mahler, W.; Melhv.

1030-1031. L.. R.: Benson. H. E.; Mochel. W. E. J. Am. Chemn. Soc. 1960, 82.,6408-(271 Pomerantz, M.; Aviram. A., private communication. 64)19.

Metzger et al

,.... ()C., with TMS as the internal standard) with a Perkin-•- _ '-, .. Elmer R24B spectrometer, except for 7, which was determined

-. . on a Varian XL-300 instrument. Mass spectra MS) were deter-mined with a Hewlett-Packard 5895 GC/ MS instrument (heat-ed probe): parent (M+ } and base (100%) peaks are given. IR

'L , spectra were determined with a Beckman Acculab-1 spectrom-eter: weak peaks are not given. Microanalyses were performed

- . - ..... by Desert Analytics, Tucson, AZ. Starting materials were obtainedfrom Aldrich Chemical Co.. except for 12a, which was prepared

- by Fischer esterification of p-hydroxybenzoic acid.- General Procedure for Alkylation of Phenols 12. The

phenol t10 mmolb and l-bromododecane 15% excess) were sus-pended in 60 mL of toluene in a round-bottom flask fitted witha Dean-Stark trap. After the mixture was heated to the refluxtemperature for 1.5 h to remove water, crushed anhydrous potas-

_-sium carbonate 11017 excess) was added. ThenTDA (about 5'7 ), - ! _ was added. and the mixture was heated to the reflux tempera-

.ure. If the reaction seemed incomplete by TLC analysis afterI or 2 days. an additional charge of K2C0 1 was added. Reac-tions were typically stopped after 3-4 days. The mixture was

Figure 3. Synthetic scheme. cooled and washed with water 3 times, and the aqueous layerswere back-extracted with hexanes. The combined organic lay-

Our rationale in choosing these D-T-A molecules was ers were dried over MgSO 4 and filtered, and the solvents wereto rate ifstronger filmsi ouldhesuhe n tA m o esws removed by rotary evaporation to give crude product.to see if stronger films would result when the cross-sec- Methyl 4-Dodecoxybenzoate (13a). The light-orange prod-

tional area of the alkyI tails better matched that of other uct was crystallized from 10 mL of MeOH to give a shiny whiteportions of the molecule. Thus BDDAP-C-BHTCNQ, solid in 60% yield: mp 54-56.5 'C: TLC 0.56: 'H NMR 5 7.94, with a TCNQ acceptor and two alkyl tails, forms more (d. 2 H. Ar-H), 6.85 (d. 2 H. Ar-H), 4.2-3.8 (5 H, 0-CH, 2.1-rigid PL monolayers than DDOP-C-BHTCNQ, 3, which 0.6 (23 H. alkyl; MS rle:320 M,+ , 11c%), 57 (100%); IR )KBr)also has a wide TCNQ acceptor but only one alkyl tail.'" 2960, 292), 2860, 1720. 1605, 1505, 1455. 1430, 1315, 1280, 125,On this basis, we might expect DDOP-C-ENP, 7 (one 1190. 1170. 1105. 1020, 840. 760 cm-.tail), to make the weakest film, with BDDOP-C-ENP, 8 Anal. Calcd for C2.,H.,20,: C, 74.96; H. 10.06. Found: C, 74.88;(two tails), TDDOP-C-ENP, 9, or TDDOP-C-HETCNQ, H, 10.33.

Methyl 3,5-Didodecoxybenzoate (13b). The crude prod-10 (three tails), forming better ones. We have found that uct was crystallized from 12 mL of EtOAc to give a 77% yieldmonolayer formation does indeed depend strongly on the of a waxy, off-whtc :A id: mp 62-65.2 'C; TLC 0.67; 'H NMRnumber of tails, but the relationship is complex. 5 7.1 (d, 2 H, o-Ar-H, J = 3 Hz). 6.55 (br s, 1 H, p-Ar-H), 4.2-

In addition, we report that LB multilayers formed from :3.7 47 H. O-,,H), 2.0--0.6 (46 H, alkyl); MS. m/e 505 (M, 1% ,these compounds are of the -Z type-; i.e.. they are non- 336 (M-C,H.,5, 1%). 69 110%): IR (KBr) 2920, 2850, 1720,centros.vmmetnc. with a head-to-tail packing as in I I below 1600, 1465, 1440. 1:390, 1325, 12:35, 1160, 1050 cm'.(where the bars indicate the solid substrate, e.g., glass). Anal. Calcd for C:,2H,,O,: C, 76.14; H, 11.18. Found: C, 75.93;These multilavers may be expected to have unusual non- H, 11.49.linear optical )NLO) properties, such as frequency dou- Methyl 3,4,5-Tridodecoxybenzoate (13c). The crude oilbling. was triturated twice with CH:,OH to remove incompletely alky-

lated starting material; the methanol was back-extracted with

ID-a-A D-q-A D-a-A D-a-A hexanes and the extract added to the crude product (78%). ThisID-a-A D-aT-A D-a-A D-a-A was crystallized twice from 1:3 i-PrOH:EtOH, with cooling on

ice, to give a :38% yield of cream-colored powder: mp 39-42.5ID-,a-A D-,-A D-a-A D-,a-A 'C; TLC 0.66; 'H NMR ,5 7.15 (s. 2 H, Ar-H), 4.2-3.8 (9 H, 0-11 CH), 2.1-0.7 (69 H, alkyl); MS, re 689 (M', 1%), 57 (100%);

IR (KBr) 2920, 2850. 1715, 1585. 1460. 1435, 1330, 1215, 11202. Experimental Section cm I.

Anal. Calcd for C4HoO,: C, 76.69; H, 11.70. Found: C, 77.06;Synthesis. Summary. As shown in Figure 3, the synthe- H, 11.97.

ses started from the phenols 12, which were activated by the General Procedure for Saponification of Esters 13. Thetrident trisf.3.6-dioxaheptyl)amine (TDA. tris[2-)2-methoxyethox- ester was dissolved in EtOH; for 14b and 14. about 20% tet-v)ethvljamine) as a phase-transfer catalyst, as described by rahvdrofuran was added as a cosolvent. A 2.5-fold excess ofSoula.M The resulting alkoxy esters 13 were saponified to give NaGH was added as a 2.5 M aqueous solution. The solutionthe alkoxy acids 14, which were converted to the acyl chlorides was heated for several hours on steam and then cooled and par-15 with thionvl chloride. Treatment with sodium azide in titioned between CHCIX2 and 5% HCI (aqueous). The aqueoustetrahydrofuran /acetonitrile gave the acyl azides 16. and Cur- layer was extracted twice with CHCl 2, and the combined organictius rearrangement in benzene at the reflux temperature gave layers were dried over MgSO, and filtered. The solvents werethe isocvanates 17. These were coupled with 4-nitrophenethyl removed by rotary evaporation.alcohol. 18. in benzene in the presence of the catalyst dibutyl- 4-Dodecoxybenzoic Acid (14a). The crude product (90%)tin dilaurate9 to give the carbamate products 19 (i.e., 7-10). was crystallized from 50 parts hexanes to give a white solid inThe acceptor HETCNQ used to obtain 10 has been described two crops: 85% mp 93-140 °C (liquid crystal, lit.32 mp 95-137elsewhere. 0C); TLC 0.05; 'H NMR 6 8.9 (br s, 1 H, COOH). 8.0 (d, 2 H,Synthetic Details. Melting temperatures were determined Ar-H), 6.85 (d, 2 H. Ar-H), 4.0 )t, 2 H, 0-CH). 2.0-0.7 (23 H,oin a Mel-Temp apparatus and are uncorrected. Thin-laver chro- alkyl); MS m ,306 (M+ , 3%). 57 (100%); IR (KBr) 2920. 2850,matographv ,TLC) was performed on Whatman silica gel plates: 2640 )br), 1680, 1600, 1570, 1510. 1465, 1430, 1330, 1300, 1260,R, values are given for development with CHCi, and inspec- 1170. 1125, 1060. 990, 965, 935, 845, 770, 710, 690, 640 cm - .tion under ultraviolet light. NMR spectra were determined Iin 3,5-Didodecoxybenzoic Acid (14b). The crude prodL

Iquantitative) was crystallized from 8 parts EtOH to give a white'29) Acker. D. S.: Hertier. W. H J1. Am. ('hem. Soc. 1962,84,3:70- solid (857): mp 65-66 °C; TLC 0.44; iH NMR 6 9.3 (br s, 1 H,

3374.'30) Soula. , Org. (hem. 1985. .50. 3717-3721.:31) Francis. T.; Thorne. M. P. Can J. (hem. 1976, i4,24-30, 32) Bennett. G. M.: Jones, B. .1. Chem. Soc. 1939, 420-424.

Mono- and Multilayers of Donor-a-.Acceptor Molecules Langmur, Vol. 6. No, 2, 1990 353

COOH, 7.2 (d, 2 H, o-Ar-H, J = 3 Hz), 6.65 (br s, 1 H, p-Ar- Anal. Cacd for C:-i 62ON,: C, 71.52; H, 9.54; N, 4.28. Found:H), 3.95 (t, 4 H, O-CH), 2.1-0.7 146 H, alkvl); MS mie 490.5 C, 71.45: H, 9.80; N. 4.19.(M . 6%), 57 (100%); IR iKBr) 2920, 2850, 2640 (brl, 1690, 2-(4-Nitrophenyl)ethyl N-(3,4,5-Tridodecoxyphenyl)-1590. 1465, 1440, 1420. 1380, 1350, 1300, 1270, 1240, 1170, 1050. carbamate (TDDOP-C-ENP, 9). The crude yellow oil solid-940, 880, 835, 770, 740, 730, 720, 670 cm -'. ified on standing. It was crystallized from 1:10 i-PrOH:EtOH

Anal. Calcd for C3,HO 4 : C, 75.87; H, 11.09. Found: C, 76.32: and then twice from acetic acid to give a cream-colored solid:H, 11.41. 39% yield, mp 60.5-64.5 'C; TLC 0.44; 'H NMR 6 8.1 (d, 2 H,

3,4,5-Tridodecoxybenzoic Acid (14c). The crude product Ar-NO 2), 7.3 (d. 2 H. Ar-NO2 ), 6.5 (br s, 3H. o-Ar-OR + NH),(quantitative) was crystallized from 2.5 parts EtOH to give a 4.35 it, 2 H, COO-CH2 ), 3.9 (t, 6 H, ArO-CH2(, 3.05 (t, 2 H.white solid (76% 1: mp 57.5-59 'C; TLC 0.10: 'H NMR 6 8.1 (br Ar-CH29 2.0-0.7 (69 H, alkyl); IR (KBr) 3380 (NH), 2920, 2850,s. i H. COOH), 7.2 is. 2 H, Ar-H). 4.0 it, 6 H. O-CH), 2.0-0.7 1690, 1595, 1520, 1465, 1430, 1380, 1:340, 1295, 1235, 1120, 1085,69 H. alky); MS m e 675 (M. 1% .57 P.01); IR (KBr) 2920. 150 cm H

2850. 1675. 1575. 146)). 1425. 1:375. 1325, 1270, 1220, 1115cm '. Anal. Calcd for C.IH.O1 N2 : C, 72.99; H, 10.33; N, 3.34.Found: C. 73.41; H, 10.77; N, 3.24.

Anal. Calcd for C 43H:Oy C. 76.50: H, 11.65. Found: C, 76.58; N-(3,4,5-Tridodecoxyphenyl)carbamateof2-(2'-Hydrox-H.nerl 2e fyet hoxy)-7,7,8,8-tetracyanoquinodimethane) (TDDOP-C-General Procedure for Curtius Rearrangement and HETCNQ, 10). TLC showed incomplete conversion to prod-

Coupling. The substituted benzoic acid 14 11 mmob was dis- HTN,1) L hwdicmlt ovrint rdColding. T2 e sub benzene. ozoic clide 1I 1m was d, uct. so the mixture was allowed to sit 2 weeks before workup.solved in 20 mL of benzene. Thionyl chloride (1 mL) was added. The crude oil was triturated with 80% methanol to remove unre-and the solution was heated to the reflux temperature over- acted HETCNQ. The remainder was taken into 125 parts ofnight. A drop of the solution was removed for iR analysis, which hot ethanol and cooled to give a dark gum (40% yield). Thisshowed loss of the carbonyl peak for 14 at 1675-1690 cm'1 and was chromatographed on TLC-mesh silica gel; elution with 1%the appearance of the acyl chloride carbonyl peak(s) for 15 at THF in CH.,C12 gave a 15% yield of material pure to TLC (0.10).1735-1765 cm-. The benzene was removed by rotary evapo- Crystallization from CH3CN gave a 7% yield of a puce solid:ration, and the acyl chloride 15 was dissolved in 6 mL of dry mp 67.5-70.5 °C; IR (KBr) 3360 (NH), 2920, 2850, 2200,1725,tetrahydrofuran and 2 mL of acetonitrile. Sodium azide (1.5 1595, 1530, 1500, 1455, 1420, 1375, 1275, 1230, 1210, 1170, 1110,equiv( was added, and the mixture was stirred overnight at room 990, 800 cmtemperature. The mixture was then diluted with CH2CI., and Anal. Calcd for C57HsN 506 C, 73.12; H. 9.15; N, 7.47; Found:filtered, and the solvents were removed by rotary evaporation. C, 73.79; H, 9.62; N, 6.66.

IR analysis showed that acvl chloride remained, so the NaN, The compound blackened on standing in air; wafting bro-treatment was repeated. The resulting product showed loss of mine vapors over a solution, which typically restores the orangethe acyl chloride peak for 15 and the appearance of carbonyl color of darkened TCNQs, in this case apparently formed a new(1690 cm ) and azide t2130-2150 cm ') peaks for acyl azide compound (TLC 0.15), perhaps by bromination of the anilide16. The acyl azide 16 was dissolved in 20 mL of benzene in adry round-bottom flask protected with a CaSO, drying tube Characterization. Cyclic Voltammetry. Cyclic voltam-and heated to the reflux temperature overnight. A drop of the mograms were obtained by using a Bioanalytical Systems CV-solution was removed for IR analysis, which showed no car- 27 cyclic voltammograph with a Houston Instruments Omni-bonvl or acyl azide peaks but an isocyanate peak for 17 at 2260- graphic X-Y recorder; all measurements were performed in2280 cm-' (TLC: 17a, 0.70; 17b, 0.76; 17c, 0.85). 4-Nitrophen- grapic sovet at wasureMen were by inethCHCICH 2 C1 solvent that was 0.1 M in (NH 4)4PF by using aetycohol mml was dissolved in dryTHFnandzad ed he sor- three-electrode cell, with a Pt disk working electrode, a Pt wiretionQ faiscyanate 17,sollwed byH aut 20dd o diteln auxiliary electrode, and a AgjAgCI electrode. The emf data weretion o f isocyanate 17, followed by about '20 4L of dibutyltin converted to volts vs the saturated calomel electrode (SCE) bydilaurate. After 1-2 days, TLC showed a substantial conver- adding 0.15 V to the data vs AgjAgCI.sion to product. The solvent was removed by rotary evapora- Pockels-Langmuir Monolayers. As indicated elsewhere'tion to give the crude carbamate product. a Lauda FW- I film balance (Langmuir trough) was used to obtain

2-(4-Nitrophenyl)ethyl N-(4-Dodecoxyphenyl)carba- pressure-area (iI-A) isotherms; LB films were prepared by usingmate (DDOP-C-ENP, 7). The cream-colored crude product a .Joyce-Loebl vertical film dipping mechanism. The film bal-was crystallized twice from EtOH to give a 30% yield of white ance was housed in a special room equipped with a HEPA fil-powder: mp 126-127 °C; TLC 0.20; 'H NMR 6 8.2 (d, 2 H. Ar- ter to provide dust- and oil-free air under positive pressure. ANO2), 7.4 id. 2 H. Ar-NO2i. 7.25 (br s. 2 H, Ar-OR), 6.8 id, 2 Millipore Milli-Q Z040 system, supplied with house deionizedH. Ar-OR), 6.45 (br s, 1 H, NH). 4.4 it, 2 H. COO-CH 2i, 3.9 it, water, provided pyrogen-free water for the aqueous subphase.2 H, ArO-CH 2). 3.1 it, 2 H. Ar-CH,), 1.8 im. 2 H, ArOCH 2 - with a resistivity of 18 MfU cm. The temperature of the sub-CH ),1.5-1.2 (br. 18 H. alkyl. 0.9 it, 3H, CH ): MS m/e 470.5 phase (5-35 oC) was controlled to 0.1 oC by a Lauda RM6 vari-Mi. 12% . 135(100% ): IR- KBr) 3 340 !NH), 2920. 2850, 1680. able-temperature bath. A Brinkmann BR 1101 XY recorder

1590, 1520, 1460, 1410, 1340, 1285, 1225, 1160, 1075, 815 cm . provided a graph of the H-A curves. The compounds to beAnal. Calcd for CH, sON 2: C, 68.91; H. 8.14; N. 5.95. Found: studied were weighed on a Mettler M5SA microbalance and

C, 68.75; H. 8.30; N. 6.08. dissolved in reagent-grade deuterochloroform. The solutions2-(4-Nitrophenyl)ethyl N-(3,5-Didodecoxyphenyl)car- were spread on the subphase (-cast-) with a Hamilton 100-ML

bamate (BDDOP-C-ENP. 8). The crude syrup was purified syringe. With the syringe held above the subphase, small dropsby column chromatography on Florosil, with CHC12 in hex- were dispersed at the air-water interface, as the syringe wasanes as eluant. A gum was obtained in 727 yield. This was passed over the subphase. Typical quantities of solution werecrystallized by dissolving the gum in 5 mL of CH2CIbd adding 20 Il, containing approximately 1 0 i7 molecules. To ensure quan-50 mL of EtOH. and allowing the mixture to concentrate by titative transfer, the syringe was rinsed twice with solvent, andevaporation over many days. A first crop of brown gummy mate- the wash solvent was spread over the subphase. The micro-rial was discarded; the second crop represented a 56% yield of scope slides used for LB studies were soaked in soap and Milli-white solid: mp 47.5-51.5 °C; TLC 0.50: 'H NMR 6 -.0 (d. 2 H. Q water for 1 day and rinsed with Milli-Q water before use.Ar-NO,). 7.3 td, 2 H, Ar-NO2 ). 6.45 (br s. 3 H. o-Ar-OR + Molecular Areas of PL Monolayers. As discussedNH), 6.1 (br s, 1 H. p-Ar-OR). 4.4 it. 2 H. COO-CH,, 3.9 )t, 4 previously,' one can define molecular areas from a pressure-H. ArO-CH,), 3.1 it. 2 H. Ar-CH,), 2.0-4).7 (46 H. alkyl): IR area isotherm in three ways: A0 is the area per molecule at extrap-(KBr 3310 (NH). 2920, 2850. 1695. 1605. 1515. 1460. 14:30. 1:380, olated zero differential surface tension; A, is the minimum area1340. 1310, 1255, 1230, 1155. 1090, 1050, 820 cm 1. per molecule at the collapse point, i.e., at the point in the fl-A

isotherm where the pressure is the maximum reversible pres-331 Riddle. M.B. Rickert. S H , Lando, .1 B Thin Sd Fils sure (or "collapse pressure" H,); and A. is the area at the mid-

1985, 1.34, 121-134. point pressure 11. = 0.5H c.(34) Daniel. M. F.; Lettington, 0. C, Small, S M Thin Solid Films LB Monolayers and Multilayers on Glass. Glass micro-

1983. 99. 61--69. scope slides were precleaned with soapy water and rinsed with

354 Langmuir, Vol. 6. No. 2, 1990 Metzger et al

70

60

50

2. 40

35"I3

Figure 4. Cyclic voltammogram for DDOP-C-ENP (7): sol- 2vent, CH.,CICH 2CI; supporting electrolyte. ( 4 H,)4NPF: ref- berence electrode, AglAgCI; sensitivity. 2 u'A. division; scan speed, ,00.1 V .s \

0 125 25 375 AREA A. molecule,

Figure 6. Pressure-area isotherm for DDOP-C-ENP (7: (a)at 27S.2. ib) at 293.2. and Ic) at 306.3 K.

' 2'

mN m,

40

1 5., X 30

20

Figure 5. Cyclic voltammogram for TDDOP-C-HETCNQ110): solvent, CH2ClCHCI; supporting electrolyte. (C4Hi -

NPF,; reference electrode, AgJAgCI: sensitivity. 2 uA division:scan speed. 0. 1 V s. 0 25 50 75 AREA (A. molecule)

Figure 7. Pressure-area isotherm for BDDOP-C-ENP (8): (a)Table I. Half-Wave Solution Potentials E,,.at 279.1. (h) at 293.2. and (c) at 306.3 K.

E, E1 'l

DDOP-C-ENP (7) 1.39 rev -1.)13 Calculated Molecular Geometry. The optimal molecularBDDOP.C-ENP 181 1.67 irr I. 19 ,,emetry of 9 was computed by using the MNDO algorithm ofTDDOP-C-ENP 19) 1.14 irr 112 Ihe computer program MOPAC (Version 4.02) on the AlabamaTDDOP-C-HETCNQ 1101 0)99 irr ( 07, 047 Supercomputer Authority Cray X/'MP-24 computer. The esti-TCNQ 00I2, A.55 mate of molecular packing area and thickness of 9 was obtained

' In V versus SCE. quoted as oxidation peak )-o3O V ,or as b, first plotting the computed geometry by using the programreduction peak + 0.030 V and measured in CH..CICH,('I ;olution oRTEP-II, using the computed coordinates, and accosting sev-with 01 M iC4H-9 4NPF6 supporting electrolyte E, - refers to eral molecules with the SYBYL software system on an Evansthe irreversible oxidation of the donor lanisole) muoiet\: h ,, and Sutherland PS-300 terminal driven by a MicroVAX-IT.refers to the reversible reduction of the acceptor ip-nitrophenol Infrared Studies of LB Multilayers. The transmittanceor TCNQ) moiety ithe TCNQ moiety has two reduction waves). spectra were measured in the range 4000-2000 cm - 1 (the latter

is a silica absorption edge) by using a BioRad FTS-40 spectro-photometer at ambient temperature under purge N2 gas. Also,

Milli-Q water: they were then dipped at an intermediate speed the grazing angle reflectance of a glass slide covered by Al and(0.2 mm/st while maintaining a constant film pressure of approx- bv three monolayers of 9 was measured with a Bruker IFS-88imately 0.7511,. To monitor the film transfer ratios more closely. FT IR spectrometer, equipped with a Specac grazing-angle acces-a digital voltmeter, connected to the output of the X-Y recorder. s,,ry: this preliminary spectrum is given as Figure 6 of ref 11was used to follow the barrier movement (when this barrier was and is not reproduced here.held electronically at constant film pressure). The transfer wasconducted with the film held at the lowest temperature (278.2 3. ResultsK for 7 and 9; 277.7 K for 10) out with the substrate kept at Cyclic Voltammetry. Figures 4 and 5 show the cyclicroom temperature. voltammograms for 7 and 10, respectively. The data for

LB Film Thickness Monitored by X-ray Di'fraction. The 7 and in respev. The datorthicknesses of several LB multilavr films. on lass microscope 7, 8, 9. and 10 are reported in Table 1. The oxidationslides, were monitored by X-ray diffraction (Cu K, radiation) wave for the donor parts is irreversible for 8-10 but revers-using a Phillips Model 31 0) X-ray powder diffractometer. The ible for 7. The reduction waves for 7-9 are (as they shouldestimates of crvstallitesize, L. were obtained by using the Sc-her- be) the same to within the experimental error of +0.02rer line-width equation: :'" u = 0.94\ (L cos 81 (where 0 is 'he V. As expected for the TCNQ-bearing molecule 10, twoBragg angle and u is the line width at half height). reduction waves appear, shifted slightly (relatively to pure

LB Film Thickness Monitored by Ellipsometry. A TCNQ) to more negative voltages No significant impu-Rudolph Research Auto-EL-Ill automatic ellipsometer, with a ritv was detected in any of the compounds.imall HeNe lasersource. microspot optics, and a XN'stage adjust, PL Monolayers. Figures 6-9 show the Hl-A iso-able to ±0.001 in. was used to monitor the film thickness at therms for 7-10, respectively. The monolayer data areabout 20 spots over a silicon substrate and to obtain a histo-gram of the computed values, measured for the bare substrate given in Table II. The formation of monolayers was deter-and for substrate with LB films, measured by a micrometer at mined on the basis of the measured area per molecule.the same spots. Since it is fairly obvious from Figures S-10 that the dif-

ferential surface tension regime between H = 0 and I =35) Cf: Warren. B. E. X-Rax Diffraction. Addison.WeeV\ Read- I11 corresponds to that of a "two-dimensional fluid- (rath-

ing, PA. 1969; p 253 er than a two-dimensional incompressible solid), we present

Mono- and Afuila.ers of Donor-a-Acceptor Mlecuhl'.s Langmuir Vol. 6, No 2, 1990 355

,mN n, phenyl group is the hydrophilic end of the molecule, onecan safely assume that the dodecoxy group is hydropho-bic. The molecular area of 38 A2 is quite reasonable, ifone assumes that the van der Waals width (W) of the

60 benzene ring (or either end of DDOP-C-ENP) is 6 A anda that the molecule has a length L of about 6 A, across the50 benzene rings, with all substituents folded into an approx-

40- b imately extended geometry.A different situation exists for BDDOP-C-ENP, 8 (Fig-

,10, ure 7. At all temperatures an initial rise in H is observed,until a plateau is reached. The area A, does not corre-

20 late well with the area of 8, and the flatness of H1, as the0 '-area is reduced, indicates that no monolayer is formed

at all, but rather a peculiar clustering into aggregates,AREA" which are not monolayers. This has been observed for

0 0 0 150 AREA A- oecle films of l-cvano-4"-n-pentyl-p-terphenyl (where, how-

Figure S. Pressure-area isotherm for TI))(OP-C-ENP (9): (a) ever. monolayers do form and the constant H indepen-at 278.2. (b at 293.2. and 1c) at 306.2 K. Small dots indicate dent of A is shown only at 293.2 K, but a more usual H-the points (11, A,) A curve appears at 281.2 K). 3 4 One possibility is that

,N m,, the molecules of 8 cluster on a slant (thus explaining the-large" A,) and then "ride on each other" with no exten-sive property such as a characteristic film pressure.

The situation changes again for TDDOP-C-ENP, 9 (Fig-Sure 8). This time the area per molecule A, is very tem-

ba perature-dependent, and the collapse pressure increases50 to a reasonable value only at 278.2 K. In fact, the situ-

ation at 306.2 K is reminiscent of DDOP-C-ENP, 7, whileat 293.2 and 278.2 K true monolayer behavior seems tohave been reasserted. The area per molecule at the low-30 . "est temperature, 76 A 2 , is 38 A2 larger than that of 7:

20- . - since the area per molecule A, of cadmium arachidate isb about 20 A2, and there are two more alkyl chains for 9

10. than for 7, we observe some deviation from additivity ofmolecular areas (38 + 20 + 20 = 78 A > 76 A).

0 50 100 ,50 200 AREA A- olec le The strong electron acceptor end of 10 makes the pres-Figure 9. Pressure-area isotherm for TDDOP-C-HETCNQ sure-area isotherm change again (Figure 9). The film1 I0 : ia3 at 277.7. t b at 282.8. ci at 288.3. . d) at 293.4. (e) at strength improves dramatically with decreasing temper-297.7. and if) at 30:3.1 K ithese curves supersede those quoted ature (as was the case for 9). At the highest tempera-in ref 11. Small dots indicate the points 1[1. A,). ture (297.7 K), there is only one ordering transition at

in Table II also a value of the isothermal compressibil- fairly low I11, but, as the temperature is lowered, two order-ity. defined as & = -(1/Ad,,),A/,H)T, and measured as ing transitions set in; at the lowest temperature (277.7the slope of the 11-A isotherms at I1 = 0.5171. K only one transition remains, at a fairly high H. One

The shape of the Il-A isotherms for DDOP-C-ENP may want to speculate whether the intermediate order-(7. Figure 6) is rather peculiar at both 306.3 and 293.2 ing transitions are due to uncoiling of the three dode-K: the monolayer forms and then. under further corn- coxy groups. The minimum area at collapse, A¢, of 10 ispression. the differential surface tension drops to zero. very similar to that of 9: this indicates that the limitingIt may be that some other collective ordering mode sets area is due not to the TCNQ end but to the three bulkyin. so that the molecules aggregate as isolated -islands" chains on the donor end. Indeed, the previously mea-with no lateral cohesion. One is led to think of liquid sured A, for TCNQ-bearing D-a-A molecules was betweencrvstals, Scheibe-Jelly aggregates, micelles, etc., but from 50 and 58 A2.i '-O-11

the present data it is not safe to speculate too far. By Transfer Ratios and Z-Type Deposition. The LBlowering the temperature to 278.2 K. one sees that the film transfers were carried out at a film pressure heldpressure for areas below 4c no longer drops to zero but constant at about three-quarters of the measured He and,sinks to about half of I11. This behavior has been seen as stated above, at the lowest possible subphase temper-before, for brassidic acid, :" where this region was termed ature (but, since a glass slide at room temperature is movedthe "post-collapse region", and the amount of decline of through the PL monolayer, there is an unavoidable tem-the differential surface tension was considered as a mea- perature gradient). For 7. the barrier moves in slowlysure of the relative toughness of the PL film. It is quite over a 2-h period even when no films are being trans-clear that for 7 the toughness, as well as the critical sur- ferred (i.e.. the PL film is marginally stable); neverthe-face pressure H, is increased by reducing the aqueous less. by keeping the pressure at 0.75H, and the area abovesubphase temperature. The area per molecule A, is fairly A, LB transfer becomes possible. For 8 and 9, the bar-temperature-independent. so one can safely assume that rier does not creep appreciably by itself, and transfer isthis is indeed a monolayer, but its strength and tough- performed at about 0.75H.ness are extremely sensitive to temperature. Since the For the molecules 7, 9, and 10, which did transfer asslope of the 1l-A curve is not steep, we have a two-di- LP films, transfer ratios close to 1.0 were observed whenmensional fluid rather than a two-dimensional solid: there the slide was withdrawn from the subphase, with the polaris a slight temperature dependence to the film compress- end of the first LB monolayer adhering to the glass sub-ibility. Although it is not quite certain that the p-nitro- strate. whereas the transfer ratio was close to zero when

356 Langmuir, Vol. 6, No. 2, 1990) Metzger et al

Table 11. Monolayer Data from Pressure-Area (fl-A) Isotherms for 7-10 at Water Subphase Temperature 7'molecule T. K [I,, mN m A,. A .,, A .4,,, A2 m,, 'mN PL LB Z

DDOP-C-ENP 171 278.2 23.7 38 444 42 0.019 Y Y Y293.2 20.0 :88 41) 41 0.017 Y Y306.3 10.4 39 39 41 0.024 Y Y

BDDOP-C-ENP 184 279.1 14.3 1751 (85) (95) 0.047 N N N293.2 13.8 47:0 182) 1914 0.048 N N N306.3 12.6 (79? 1-7 1 1964 !.045 N N N

TDDOP-C-ENP 19) 278.2 34.0 76 S2 S9 0.066 Y Y Y29:3.2 11.5 99 117 132 0.037 Y Y806 2 15.2 107 1144 151 0.033 Y Y

TDD0P-('-HET('NQ (10 277.7 55.5 75 1)7 227 Y Y Y282.8 52.5 8:1 125 188 Y Y288.6 47.5 1;5 115 27., Y Y29:3.4 4:1.0 73 121 254 Y Y297.7 20.)0 96 125 2)7 Y Y34)8 .1 19.:3 1:2 164 235 Y Y

'The estimated precision in 11 is ±0.1 mN m: the estimated precision in the 3rea per molecule (A,. A, A,) is ±5%. except whereindicated. As indicated in the text, II, indicates the differential surface pressure at the collapse point; A,... and A0 are the molecularareas at the collapse point, at 11 = 4).511 . and at the extrapolated zer, pressure, respectively: finally. ,, is an estimate of the isothermalc(,mpressibility at 4..511,. A Y under PL indicates that a Pockels l.angmuir monolayer at the air-water interface does form IN meansno,. A Y under LB indicates that a Langmuir Blodgett film wa, transferred to a glass substrate (microscope slide). A Y under Zindicates that the multilavers are of the Z-type P1. films transfer t, microscope glass only during upstroke. i.e., withdrawal of slide fromthe subphasei. The data for TDDOP-C-HET('NQ 410) supersede those reported in ref 10.

the slide was dipped downward into the suhphase )Z- -

type deposition).FTIR Spectra. The FTIR spectra of a 15-multilaver

film of 7 failed to show characteristic bands. This indi-cates that the LB monolayers are extremely disorderedand are more typical of a fluid with a low degree (of ori-entation within the film, rather than of a two-dimen-sional solid (as one sees, e.g., for FTIR spectra of multi-layers of cadmium arachidate). However. a 15-multi-layer LB film of 9 )TDDOP-C-ENP). transferred to amicroscope slide at 278 K. but studied at room temper-ature, showed two weak and broad bands peaked at 2925+ 2 and at 2855 ± 2 cm '. which are characteristic C-Hstretch frequencies (we have observed much sharper sig-nals. at the same frequencies. for 15 monolavers of themolecule 4, BDDAP-C-BHTCNQ'' which forms PL mono-layers with a much steeper II-.4 isotherm than does 9, Figure 10. O'RTEP-1 plot of the MNDO-computed geometry ofTDDOP-C-ENP). Thus. we can claim that an interme- 7 obtained by using van der Waals atomic radii (1.2 A for H,diate degree (of order exists within LB films of TDDOP- 1.7 . for C. 1.5 A for N. 1.4 A for 0).('-ENP.

LB Film Thickness Monitored by X-ray Diffrac- puted dipole moment was 5.992 D. The highest occu-tion. For a multilaver of cadmium arachidate trans- pied molecular orbital has energy -9.172 eV; the lowestferred onto a glass slide, nine X-rav diffraction Cu Koo unoccupied molecular orbital has energy -1.117 eV. Thispeaks were observed. ,f which the lowest angle is at 20 correlates well with similar calculations reported= 1.62'. giving a repeat distance itwo monolaver thick- elsewhere:'' the HOMO should be localized mainly onnessesi of 54.5 A. The Scherrer line-width equation gave the donor end of the molecule, and the LUMO shoulda correlation length ,of 2:35 A. be localized on the acceptor. As expected, the LUMO of

A multilaver of 7, transferred to a glass substrate, gave 7 implies a lower electron affinity for ENP than. e.g..nly one X-ray diffraction peak iCti Kw at 20 = 2.160. TCNQ Ifor which LUMO energies in related D-a-A mol-giving a repeat distance of 40.9 A. The Scherrer line- ecules are about -2.95 eV)." The MNDO-optimized geom-width equation gave a correlation length of 438 A. etry did yield an extended configuration (as MNDO tends

A L.B multilaver of 9 on glass yielded a single diffrac- to do). but there was a significant twist in the ethyl car-tion peak at 2,9 = 1.94' 'repeat distance oft 4.'1.5 A). The bon atoms linked to the nitrophenyl. namely around theScherrer correlation length was 47:3 A. atom indicated in boldface in the group

LB Film Thickness Monitored by Ellipsometry. OCHCH.,C5HNO,. An ORTEP-II plot, obtained by usingBy use of the literature value for the refractive index of van der Waals atomic radii, is given in Figure 10. Thecadmium arachidate in = 1.542, the thickness for two molecular length of the bent molecule is 30 A; when1.B layers of cadmium arachidate on i was obtained as .'unfolded- (not shown), the moleci'!ar length becomes5: ± 8 A fincertaintv = width of distrihution of thick- :39 A. The molecular cross section o( the ENP end (Fig-ness values at half the peak ot the (istribution). whence ore 1)) is only about 23 A 2 (nitro group eclipsed withthe thickness per monolayer becomes 27 ± -4 A. For 4) phenyl ring) or :33 A2 Initro group bent as shown).LB layers of 7 on Si. the refractive index wi-s n = 1.58 5.and the measured film thickness was l:ifl ± 1-5)A, whence 4. Discussionthe monolayer thickness is :33 ± 4 A. While the acceptor ENP of molecules 7, 8, and 9 is

Calculated Molecular Geometry. A MND() geom- fairly weak, compared to the TCNQ acceptor of 10, itetry-optimized structure of 7 was obtained The coin- was sufficiently polar to let 7 and 9 (but not 8) form mono-

35,

layers. The monolayers can be transferred to a glass sub- These molecules seem polar enough to orient and packstrate. if the PL monolayer is held at low temperature, as PL monolayers at the air-water interface, but not polarbut the films probably are not well ordered. enough to produce the typical centrosymmetric Y-type

The large intralayer Scherrer correlation length is sur- layers formed, e.g., by LB multilayer films of cadmiumprising, but the difficulties in observing good FTIR spec- arachidate.tra indicate a low degree of interlayer registry. Clearlysome further FTIR studies are needed. Acknowledgment. We are grateful to Nippon Tele-

The experimental molecular area of 7 {38 A2 ) corre- phone and Telegraph for their initial support. We thanklates approximately with the -thickness" of the ENP ring, the Geology Department of the University of Alabamaviewed as in Figure 10. The X-ray monolayer thickness for the use of their X-ray powder diffractometer and theof 7 agrees within two standard deviations with the ellip- Alabama Superconmputer Authority for computer timesometrV result. The X-ray monolayer thickness of 7 and on the Alabama Cray X-MP-2/4. Shankar Krishnamoor-9 is consistent with a Z-type deposition. The X-ray thick- thy performed the ellipsometry measurements. Chinn-ness of 7 correlates also very well with the -molecular arong Asavaroengchai helped with the theoretical calcu-length" of 7: one must presume that under the condi- lations. Dora Fracchiolla (NSF REU student in 1988)tions of the film balance all -kinks- are indeed taken out, helped with some of the LB film work.and that the molecule assumes the most extended geom-etry it can. Registry No. 7, 123126-34-1; 8, 123126-35-2; 9, 123126-36-

the finding that Z-type monolayers are formed is sur- 3; 10, 123126-37-4; 12a, 99-76-3: 12b, 2150-44-9; 12c, 99-24-1;prising, and indicates that some of these molecules (par- 13a, 40654-49-7: 13b, 123126-38-5; 13c, 123126-39-6; 14a, 2312-ticularly 10) should be very intersting as noncentrosym- 15-4; 14b, 123126-40-9; 14c, 117241-31-3; 15a. 50909-50-7; 15b,metric systems with aromatic rings and potentially very 123126-41-0; 15c, 117241-33-5; 16a. 123126-42-1; 16b. 123126-large transition dipoles, which should make them prom- 4:3-2; 16c, 12:3126-44-3; 17a, 123126-45-4; 17b, 123126-46-5; 17c,ising for nonlinear optical devices (frequency doublers). 123126-47-6.

JOURNAL OF MOLECULAR ELECTRONICS VOL 5 1 17 1989)

Langmuir- Blodgett Films of PotentialDonor-Sigma-Acceptor Organic Rectifiers *

Robert M. MetzgerS.- .. A~ f.-~..P~v-..Alabama 35,487 9671 USA

Charles A. Panetta.. ~ .- , . ".1>.. ~-~;~; ~ 1ss,sppi 38677, USA

The Organic Rectifier Project (ORP) at the Universities of Mississippi and Alabama aims at therealization of a one-molecule-thick rectifier of electrical current, which could be part of a 5 nm-thick electronic device. Such a device, suggested in 1973 by Aviram. relies on the asymmetryof molecules D3-o -A. where D is a good one-electron donor (but poor acceptor), A is a good one-electron acceptor (but poor donor) and a is a covalent bridge that insulates the molecular orbitalsof D from those of A. Six molecules have been found to self-assemble as monolayers; fourcontain the TCNQ moiety; three contain 'greasy' dodecyl groups on the donor end (which helpsin monolayer formation), but one contains only hexyl groups. All of them can be transferred toa glass, conducting tin oxide or Al substrate as Langmuir- Blodgett films. In preliminary trials, norectification was observed. Recent FTIR data for LB films as a function of aging are presented.

INTRODUCTION 1tle mo11Clecle 5 nes ~er ,rirthesiZed) '4101.11d he 0111\2 im thick. If ,ery thin cofl\entional fImri, \I. N

I Ilki paper ouIth ile proc rcvN, of t rhe Organic Rc- (1.5 nrn each) canl be Used, then a 5 timl thic:kie r Proiecci ( )RP1) t0\\ ,trd' the s it \1ieCil of des icc bCcome"IS Possible, mu1Lch t hinrier thlan the

ritoleculc' of tiel [I) 1e ) -0 - .V~ l0icli. illccc \s oikiiie direct ion of" comntim onal Si or (iaA.tree orcai ed as~cmhhiihl In~ ay be red fii- her 0 de\ ices (Ii -3 11n).dCci111d hie urei ire I) IN a stronLe one-elect ron OH3 CH NC ONoruailicL donor. SLuC1 a, f I F (let rat hiaful\ alene. 1) 3" Yor IN! PD ( .. N. -tet ramethyl-p-phelnen-d aminte. 2). o i,, a cos alent sigmna bridvee anid A 5 <,,I .tionL, one-electron acceptor. such a,, TC'NQ I..(7- . - 8.8-itraic\v noquinlodii met hane. 3) or chlor-

andj (2,.3.5 .6-ici rachloro)-/p-beitoquiinonie. 4). Ini I C N N1973, \\iram~ and co-sorkers' proposed that C 3 0 3 NC O1)-i-A miolecuiles. suich as, lie (iedlankennmolelsul 5, 2sarids ichecd as an oriented mionlolayer hetse en or- 0ditiar-. metallic thin filnis \1 and N. a,, inl 6. s oldI C1 C1rectif'y alteriat incL elect rica) currenC~t . The obk ousa~dsantWC of' SUch a des ice i,, its smnall t hickness: 1. C1

"'rr l d il rr h% \',I -MIRt 0 ii

Viho (io M T1i orrc~poridcnThc hoiulJ hx,,- dJi!", 4

1)45 -L9qi 's'/(IX)I I- SOS 51)

198 hS I lhr \\ ilc% & ~Soil. l id ReceIied t4 tanuar\. 19,1s'

2 ROBERT M METZGER AND CHARLES A PANETTA

NC CN chemixorpt Ixe barrier fronm 1) to M, and from N to

A JEqn fI a) I thle /x\ it terionic molecular state1) -Y- A form,,: t his i,, folloxxed' bx t hrougli-7 ond ( 1-11 inelastic tunnelinu [called Ill', EqnI ~ ~f~b), or torxxard inelastic tunieliei fromn A toD' thiix \ieldx charge Neparation. Inrder rve e

5 elect rical b iax., the format ion of /xx itenionNC N D) -,I -- A [ Lqn (2)1 is % er\ Unlikel\ because

good orguanic onle-elect ron donors are \er\ poor-~ acceptors. and good acceptOrs are poor donors.

IndeedLI In tile ,,a, phase. thle encrg\% rqi-CIred toI form thfe ions [Ii- Ig fu. F('NQ (g) at I ntitite

6 In t al se paratitoni i,,. e\pLrimL titdll, 4.1) e\:in cottras(t tilie energ\ required to formi thle ion

The incremental rexultx of the ()rganie: Rectifier pair VIE- ) IgI VCNQ (g)I ix esti-ated atProtect hax e been reported e[ex ee.' Ihe 9. 6 e Vpresent rcex ex olloxxN cloxeix anot her rex cxx yF j:( f FCQN)-TI L -'Q )precpared for- a di fferent audleenc NI\ mnont l ago NQ I cII

I3 MI -6.s3 - 2. 8eV 4.0) -A' (4

TTI-(gf - TCN\Q~uI -- TTF Ig) -TCQ

THEORETICAL, SYNTHETIC AND ASSEMBLY -1 9.6 e\ (ext.) f'

CRIERIHFoxsex er. undet- large iexerse electrical biax.

lIn this Nectiott are x mr~dthle dexiVen criteria another react ion channtel open,, 1Eqti I 3a : athathax cxolx d i t le C R / xit tenion D) -(-A . forms first, Iolloxx ed bythatha~cewl~d i) tie Ok).4charge separat ion fromn \1 to Ns: t hiN iN re, erxc. or-

Iuphill' tunneling fIR, and is nmuch lexx likel.x than

Simplified mechanism IF.2 Of' course. itunnehiig here i,, betx'eii statesof' differet etieretex. xo one Nees t hexe ax elast ic

It' dcx ice 6 is xuLcee1stfUll\ aNxxembled. lhen the1 processes betw eet \irtaal states of' eq!ial erierp.

mechnismfor ectiicai onIN L~cn Fqs follow\ed by itnelastic 'throug-h-bond' procexxex.

Frx ard bias: Langmuir- Blodgett films

GO: N 1)--A N- ~ he ORP lias concentrated otil mo1CLel 1)---A

NI 1) - - a-A N - MN D-a-A N - xx hich self-a,etnble at the air-xxater itnterface axIl'Pockels- [angmlui r (P L) inotolaverx It h ix

IIname"- h ~lonorN i rx Ig atinluir (1881 -1I95' IRcx.crxc biax. and Eirautlein A LttneN Pockelx S SuIch PL.

NC) GO0: tiiouolayerx e at, uxuall,, be transferred to ulass or-metal or ot her solid subxtrate b thie

NI D - a- \ N -'-*\I t) -- AL N - Langtnuir-Blodgett ILB13 tchnliqueC pioneered by(2) Larigmuir atid bx Katharine B Blodgett (1898-

Rex erse bias: 1979). '_ I-The typical molecule that forms excel-lent PL and LB1 films is cadmium arachidate. or

06: 1i [1-1-\I cadmiuIm eicosatoate. Cd~ti-CiyH,,COOI:1 7).At the air-v ater itnterface a PL nionolayer of'

NI I) -A N -- N NI' -1)- -A N' cationx 7 Ixhaped like tadpoles) poitts x% itfi tile(9(3)

Under x anixhitie or nioderatc: forxx ard electrical 3 2~-biax,~~~~~~~~ th lcrncrnfrIE cnocrbhias th elctrn tansfr (T) an ccu- '7

elastic, thlrough-space (TS)"2 (Utnl ing throughi a

LB M"',1' PC)t- T %.0 ORCA% IC RECT IIFRS 3

- ------ Synthetic and device assembly criteria

- 'I hie criteria tor the Nrtirc'is and assernhbir oi an)NID jA \ dle\icc 6 hak e been discu''cd

heloic ' anid' can1 be enIunCIarCi Ua' toliokk41) fle donlor 1) MNI t ia%. e a relati\~l ci', N

101l/,itioli p t~l d 4<, C% e ): it should he a- -- --- aIr I flat,1 Ileen.,le. To ro~ ide L'Ood lateral

(2) 1ie acceptor A\ trln't hrae a relati~ eIL hum

carhN ~ ~ I laeed i',k ire N',aCr LI h' IN~ II elctiroir atttiitk, A (>2I e\, ). anld NhOUld allo he8: ire cadwirir comitril, are inl the ~iiiae a taijil\ flat 1ioee.L1, t0 1l11r0%% tor corlrpaitcootditt11ated Itail\ clo'cl% %\,ithl tte carhoN,% late P'.ickiitg'read' erotip. L Ntiall\ , scli \'ork iN caried 01.n 10- 43) Fihe o htidl've nr~t be at Iea~rt partialir

,1,\ h\ L1i~ii1 111 aLitt0rtttate I a11rot111nir troug1_h k) 'atItLraled IP rio ciItjnate. It nmust also be hntim akitce.erronih (more than three carbonl-like atoin !kt

:K'I ehItrikque :oii,!,t, or Iro\\i\ n'ert lIt a rc% CTr C~renlsrkC 2rOUIrnd-NIale Wi\'an of thTie

ltit1 ],, 0 N,:0 iti:11eta. gla, . si Iict or ~ther JC donor II mlecuLla r orhitals \ t ni thle acceptor~itht rte troieh lie troil jritto t lie Niih- molecniar orbital', \et Nhurt enouob 4 lie,, thanl

piia'e. cihrott iertiotr Into tire N', ater orol ott% itine! carboni-like aromis or rigid eniough ito pre~enrtN', IidraN' ,il !rout ilk 'A atetl. thle P I rioiola\ cr, thIe culurrcn ot thie I) end o~ er the A\ endt of the:

tPii'tcr -IIIlnt1IaII\Ctl% ttie'hrae,,it)IC %irl little itioleCnIeC. I[Be or si'. carbonl a10onN CeerrN ito be rthe01r Ik dr'toriorr: tin'u arIe formled tire I 13 oprirral lenlth. Ilie a bridine nutai'o he tlar

11t01n01ai kr1. Iilkth 'ithttaile IN .ipp~ed NO11 Cra 11roLILrh to pro~ ide good Li eral L B fint packing.art e' tltroitlitre mttnttiarcr. 1,13 rrnlrilaet' cart1 44 ie Ii) arid A1 N %aiueC1 of' ID and A- rru't apl-he fi' eredto tie uih'ltle r ~ roach a, clo'ei v aN po'. bie thle vk ork fnLncrtiort' oft

tire mnetak \1 anrd N(N-Iee Table I ). L ri-tortitriatel' , on can1oC hope (or a perfect matrc

Cell er or tire he't dlorors anld acceptor,, kno , ri ToOther techniques for monolayer coverage darte. -iof metals 5 ) YIe org-aiC cul reaction (6). leadimne

o, tire littked D -o--A, must be more probai -Ie, or\iroricri itetliod of trttrite'eif-a''emlbiTirtIc ta~ter, tiran tire corrpetirrg charge trarter (C Iiotioia1% el~ tIo 'nth't raIteN %\ Ithonit Ut'Ie1 a 'alltfottratiorr ThI atietriltir1 tronoit1 iN tire Bijuglo\%' 0eoiroi01c: (130 )nilttl-caN[IILrtgechiriCILI. Ire1 1o'0C bliltdrrre ai D -- A -- -D-oj --- (6

1 It or 130 ) ttrottoa~eI to rite ~Nbstrate arc UNdtIi11'A c:,I k Itl %,I Nor pt I % e or c irerrriNor pt I~ \ ftorce'. I lette D -- 1) - Y -A .- X~ - D *Y- A -

aIdieiorli V acirie~ ed bk direct cox aicrir honldirre itoa litrI 2 1 ae -iLorii t to NiiaIiCi~e Mtiral 'Urface', AN dexerihed belo',', tire e~Ner arid carbarnateia civ act-i cxc' tiorolar er co% erage. .-\Il~o, COpl inrg react orb, hiake been found to be accept -

at chIti tr 'ilt,ed1C riroiccIc'l to o\jIide-hari rr! able. \fail% othier couplintg react iorns are riot I eas-tictal -Nurtaccs prodce'CC c01111pait "ttno ver , IN thi)hCaire of' the high reacti', Litof TCNQ.doe, attacitir~reirtlkecuLe heairreI dis~tide 11011d,to g Old ~Nr faces Ariot icr trier itod of prox durte Table 1.- Ionization potentials, /D, for the donors

rtoite, ti i' IN to pol rtlerl/C I B t int'i dit ni itl (D), electron affinities, AA, for the acceptors (A) andd iacer\ IC1 eric i C M W11't Li1i1t12tilt rakxolet work funcations, 0, for the metals (MA)N

r adiat lolrl-Ice'ptte tire evi'terice oft tie~e 'titer potential 'VA.% *ev e V,

ItkcIhIIrtItI Ite~ for a Ner:rltIi ' art o r c'arnttc TMPD 6 26 DDO 3t13 Al1 3 74"

1 1) -A- N rectifier. %%.e concenltrate belo\% o lF 63-- TO 280' A 4 8

I) - tolecn le.x thrat \kill citf-a~scriihle a, Ill Fl fener 7 11 I~oa 2 7 6' Pi 5 29-"arid LBI itorrolarcr', -________________________________________

4 ROBERT M MEiTZGER AND CIKARLES A PANETTA

(6) Vhec I- trnk C ondon reoreant.ation of the r ix i of ittak incv react ion (1I) much more like]\molecular comect r of 1) to D' and of1 A to A t hall reaction., (2) and (3); InI inelastic electron

4 t tuIIt he 1ttal ke ItaI No hfat thle ox erall elec- t unneline2 NPect roxcop flTs)'' through1 rNIM1outlion trattxtev 0 1) It \itl hin ) (1 tat oxxins- to tllhx of' inxu4latorx placed betxxeeni xuperconduc-thei laree, dit'leretice I 1) does not lo\x doxx n (inc, Ph And normal Al electrodex at 4.2 K, thebcauxe otihe I ! ankI - (ondon aco. "' r atio 1II I3jI1I - [S) is olxl 0i.01 I-or thle oreanicL nder thle riehlt contdit iotts 1: F thrOULh thle rc:tifier, t his ratio I bor compact tIonola\erx) muxtINmolec:ule :Itca he axter than I itx. 'It 5, impor- be mu1Lch cloxer to 1 .0. ()lne should remnemlber that1,111tt ot cotitNc. toac0 e e a a,1t 1: Il 0rocxx xi ce e0 enl itt Com1pact t1onol0axerx ofI cadtn ill)ia smlall bilt NlOxx molecuNlar elec:tricl1 Il c(ax arachidate. a 'backer outt1d' Conductixitx, itde pet-to adx arnae oxk~ erhe lareer bilt x! e f all co0tiCen- dent Of ite ilumber of tnionolaxerx. \\a,, alxx axtional (Si or ('1a \S) elec:tronlic dc c.prcertt this %\ ax prexullltual\ due to IS tittiteline!

C File D)-0 A IIoIlec trmn't pack itt P[ processes.itottolax erN. 1-or this, onc cild mIIIN be 1I0) Heating- tile miotolaxer abox e itsx chemricalh\ydrophobic and tilie otI,: hetlt\drophilic. Il thek Ill dc:opoxit ionl tetuperatUre (cojnxerxatix el\tuottt0la\ er, otte m1u11t prexci :t (i e packi tt2 of extinmated ax, 100(-) s hould be ax oided, bothII) T- -\ ox er \ -0 -1 ) (xx hlil xx ould canlC di pole xxlile laxi11 r 10\e do t ile secontd metal laver N atoplotnlentN attd dextrox thie directioltalit x of [the \l D)- I - \ as a xpffttered or ex aporated flm.l atiddex ice 6 ). fThe lateral ~- - attlacuxC e teatou alodriu thle 113 tururte6LIu process Itt thle dex iceatld pONib111 cxett a tnt Nd -xalencL: erountd x ate 6. xxitich x\%ill be e\ot herntic,, atnd xx ill terefore

Cxxhilte 11t inl thle Sux-ted euira alt require Notme ntecltanisrn for (teat dixxupation.I--I\0) u11a1 heilp reduc thle :Oxt ot 10ttt/- Sitnilarlx, h tultx l ae that rna\ cause dIelct crue

attort of 1)-c A to 1) A . trottt 4 e\ to breakdoxxT 1u111uxt be ax\0idd.possibh. 1 -2 eV

(8) File tnotbola\ erN niuxi be cloxe-packed attddetect -ree oxer tlite regtort of ox erlap of m tetal RESULTSlaxur NI atop 1)-i- A atop mectal laxer N (toprexettt electrtical ltortitu2 Of' Mtetal laver \1 x\%hett Coupling reactionsit Ix, depoxited atop mectal lax 'er N). \\hett LBI filttmof tcadfili(utn ar-achidate are transferred ott to Itt 1976 Hert ler - prepared earbatnate. ortxdropltobic. oxvde-free met als (Pt, \ut AL,) tie\~ tire) Iate.ptlur of* F171 xkith Ii CNQ if' ottehaxe relatixel% (arize defects probabl\ at dornainr reacts t(lie 2.2'- -bixixocxattate ot' FTF x it h atbouttdariex(.' attd tia\ (taxe a disordered, f an- 2. -dili\dro\vethlo\';-_ V \NQ: t he restultirug inxolu-like iiLI ruce.' 0%x er hdroptiflic, o~ide-cox ered ble blaek polytier xxax, xenncondueting.1 rtottutet all (AL Sru) or 2lax, or qttart/ tilie defects iti ntetalli. H-oxx ex r. tlie lt ilit\ tf' Hert ler'x eott-te Cadrttiutn1 araehidate L1B ftlmtx seetm srtaller, t ribut ion xx\ax to xhoxx that earbanatex of' st Rifl

alloxxn it or at percolat ix eCeurret acroxx, thle filtit donors xx itl I t rotig acceptors eould be prepared,anrd t ItrxtuuLt di xclitiotix tite x ol t age artd that reactiott (6) xxitax preferred ox er react iondepentdettce of' j Ix lot, j at artd tiot lo- (7) for earbatnates. Ballhdadchi prepared the

o-/"1 ) Otte xxax of achiex ie- freedoni frontt rnonoisocx anate of TTF. 9. attd coupled itdeftects on the xcale ot a t'exx microns xxould be to x% it It Hert ler's 2-bronio-h\ dro\\et ho\ v-TC NQuxe a thin ittterdi~itated layer MI. x%\tere tile film (BHTCNQ), 10." - to yield to earbanate 11 ofperfection is satmpled on xex eral small disks of' -rFF xwith TCNQ. Hoxxexer, txwo f'orns xxereradius 3 pmi (these disks xx ould be cotnttected to found, of' w~hich one seemed txx itterionic and themacroscopic electrodes). Some crude, preliminaryand tieatixe results (Ott t ite 11ntn scale) are des- NC CNcrihed below. 3 S NCO

(9) At an NI-N distance of'. tv picallx. 2 nm., TB H, . rtuinneling must be much tmore likelx thati TS HO~ Itunneling [the latter is undirected, and depends 9 O

oruly on barrier height and distance, i.e. it is Nequally likely from NM to N as it ts from N to M:NC NTB tunneling shows the destued 'cherittcal' selec- 10

LB FILMS OF POTENTIAL ORGANIC RECTIFIERS 5

NC CNt\ o-electron acceptor, rather than a stronilL one-NC ON electron accceptor. In a miajor breakthrougth, the

HBr s~rithesis of' 13 has been accomplished in hichp %\ield. ' and the cr\stal structure of' its meth\ II-s S_ N0 ester. 14 has been determined. 16

NC CC

Donors

other nleutral (as es idenced b\ thle inf'rared (N After thle ,Iarl%, s ork onl TTE isocs anate (a stronLstretching, frequencies). Neither product could be donor), the isocS anate of' pvrene, 15 (aisolated in acceptable purin .Medium donor), a orepared: later, %arious1

Esters of TTE v it h BH T(NQ. 10, "e re also dens atis es of- pher. ocs anate. 16 (a %% eakprepared. using the 2-acs I chloriLdc 0f11 1T. but, donor), ss crc prepared; adding! a dodecvlo\%again. t 550 product,, s crc ob i tted . [lie neutral group ease thle 5\ eak donor 17: addinLe theform of the VIE carbamnate s it i RH TC\Q did dimietlhvlamino grou p gas e the medium donlorform L-B nionola\ crs. but \% it h1 thle xs rong D.\ I A 1 N CO, 18. ihe crs stal struct ure ot itsgeom etr\ (probably x it h thle mole1cule ill nl ethyl carbaniate. D.\1APC\Ie, 19, \\ asalmos10t flat onl the ss arer sUhphaNC, rat her t han nor- determined:' its calculated (Mi )ontiationmnal to 0. potential (7.17 eV) arid cyclic- oltarnnietr '

Sinice that time, miost ol* OUr ctforis hasc heen~t shoss ed that 19 is a donor weaker than T\IPD orcoticent rated On the carbamiate coupillng.L' although1 TIE but stronger thant pyrene. '2 There is eterrstseone mlaN conceis e of' ot her possible couling11 hy~drogen bonding in tile crystal of D\I.APCNle.react ioins, in solution '2 and also in the solution of its electro-

chemicallv izenerated radical anion. 2The

bisdodecyl and the bishexyl derisatises of 18 haseAcceptors also been synrthesized.

M ost of' tilie s\ork has been carried out s it hHertler's BHTCNQ. 10., thle cr\ tal StrUct nrc ot NCO

Mi ich ss as rccetitl\ determined. - Its electron N. )NCO

affinity is estimate'd b\ cscicsoltamrinetry as.9t0.2 eV. I-Ho\\ es er, its % ers inefficient sn

thesis spurred a search for a better, or more 15 16accessible acceptor. The ati~hraqUinotie analog 12 NOhas been prepared in excellent ield, "' but theIImolecule is sesercl\ bent, and 12 is a ss eak N

NOC ON NC ON 17H

I HO NO OH0 H 3C. H3CN 0

NO ON NC C N H C HGC

12 13 18 19NO OCN

Crystal structures of dlono r- sigma -acceptorH 3 0 ~molecules

4T To test thle proposition that thle a bridge alloss s forN C \art emtended structure of the D-a--A adducts,

14 crystal structures liase been determined for

6 ROBERT M. METZGER AND CHARLES A PANETTA

c2l N5C4 NI

C26

99 C528 62 25 C3 12

C0 C22 C19 C7 C6

0 a foC20 C8 11 a n t- ' -018

69 CI:O

C12

N14

Figure 1. ORTEP Plot of Dhenvi C BHTCNO, 20 (monochnic, space group P2./n., 8 310 A. b 9 278 A,

025 383A. 95 15 Z 4 R 7 9-

C37

Kd C-33s C34 1.122

C38 e , -3 4 ,) =

( 32, (72

; ~~~ ~- C12 C1, 862Z2 1

phenyI-C-BHTCNQ, 20, (Fi2. I),- and for but tile cr , tructure re,,eal anl emtended con-

DMAP-C-HNITCAQ, 21 (Fig. 2). " ' cO Or,,C. tornianion [lie ct.,.,tal, belong to tlh en~om

in both caws the molecule,, do not form [.B flm,,, metric ,pace -,roup /11. the mole:ule', pajck in the,:r-% al ,o I lat 1) 7 \ i~ .A, divlost O\er .A-)--D.

20 21 . .

LB FILMS OF POTENTIAL ORGANIC RECTIFIERS 7

Pockels-Langmuir and Langmuir-Blodgett %ant pressure-area isotherms are sho,,n in Figsfilms 3-7,

The collapse pressure 11, is defined as the differ-It ,.sa, found that only certain D-o-A molecules, ential surface tension, which, if exceeded, leads to22-26, form monola.ers at the air-water inter- the collapse of the film, and to the irreversibleface; the data are given in Table 2. and the rele- 'riding of ice floes over each other.' The area per

NC CN

22 NC CN

I Br

N-< 3-H (l,

%C ONi 23 ri' IC N

Br

I1 NC ON

24 ti c N

-. ~ ~ ~ N - H 0 I

25 NC ON

'iC CN

tC ONJ26

Table 2. Molecular areas and collapse pressures for PL monolayers

T 11 A A,. A-MoI~c.jie %. oK) mNm \

2 "

2) Ref

TTF C BHTCNQ 11 292 12.7 134 50 6

DDOP C BHTCNQ 22 292 20 2 50 55 60 6.14BDDAP C BHTCNQ 23 293 47 3 57 69 82 14

303 459 54 70 82Py C BHTCNQ 24 283 28.2 53 60 66 14BDDAP C HMTCAQ 25 293 22 3 58 71 83 14BHAP C HMTCAO 26 293 35 8 42 47 53 13

8 ROBERT M METZGER AND CHARLES A PANETTA

NC ¢

40 ____Ny_____Cr o

C"CDOOP-C-SHTCNQ

30

nMImNl.m

20

10

10 20 30 40 50 60 70 s0

Area I (A2 j Molecule)

Figure 3 Pressure area isotherm for DDOP C BHTCNQ. 22.'

60

14 0's0 NC CN80AP-C.WfTCpNQ

40n

mN Fm

30

NC CN20 - 8,

4 0 - 0 _ o

10 -- y-c a TCNO

OL ~3020 40 60 00 100 120Area I (A2

1 molecule) __

mN F mFigure 4. Pressure area isotherm for BDDAP C BHTCNQ.23. at (11 293K and (2) 303K 20

10

10 20 30 40 so 60 70 80Area I (A

2 1 molecule)

Figure 5. Presure- area isotherm for Py -C -BHTCNQ,24

LE3 FILMS OF POTENTIAL ORGANIC RECTIFIERS 9

moeuecanl be defined ilc rl '.~l %a'.s: thle area Thle ,lope of tile 1- A isotherm in the t her-at the :ollallse point iN < I.And is tie xmallest: thie inod'.namiicall'. re'.ersible monola'. er reline

araper molecule at /cro 1r~ r,-, s [lhe p-oijil IF I 11 and A -> -I.). help, to determine theinl hie I1 -1 Ixot hemf obtaine1d '.s en thie xeepex t tUiiditv or rigcdity \\i.thin thle filmi. For cadmiuml11,1t Of the cur'.C 1, ix e'.tra lted lilean'. to ,'em ataehidate. (he slope is almost ertical. because

1rxne >1,i Jeflned ax hle in id-poit bet ecue the tfilmis almost crystalline in its t '.%o-dimenlsional

1. antd l''. 0t all these poits \\flex'..at p1acking'. For thle Molecules, 22-26 W~ig, 3--) theprobal'.% is thle best ext iinate of ttile lat eral di men - slope indicate,, a more fluid-li ke en'wIronment inlstuns of thle mole1cule inl a \' el pacd film1. %\s hich the molecule,, probably do not pack as

iiLidl\ a,, they '.%ould in a crystal. Of these, ol\NC CN P%-CB HT( , Q, 24, lacks anl alk'. Icreas.

60 chainlC DDOP-C-BHTC.\Q, 22. BDDA\P-C-N N11 o, BHIFCNQ. 23. and BDDAP-C-HN1TCAQ. 25

so ~ '~f'y all ha'.e dlodecyl groups that help pro'.ide a80A HMTCAO hydrophobic region inl the molecule (it the dodec'.I

,-!roups are replaced hr meth\l grou1-ps, 11040 nlonola'.ers are formed). Thle 'most ricid' of these

mn filmrs are the film of BDD,,P-C-BHT-(NQ, 23MNIM(F-i. 4).

30Since, ho'.'e'er, the presence of' long alk'.lchain,, may retard the TB ET. it may be an ad'. ant-

20 -age to have the shortest possible alk% I chain \0jhch,,till canl pro'.ide monolayer formation. Thuis \. Care pleased with the recent result that the bishe\'.l

10 -chain in BHAP-C-H.\ITCA-Q, 26. w\as, 'suffi-cient' to pro'.ide 'good PL monola.ers. Work onl'similar molecules is continuing. .\ preliminar\0 2 0 4 0 6 0 a90 100 search for an inter\.alence banld in the optical spec-

Area I W J mol~cule) tra of 22-26 has failed,;-' it ma'. be that itsFigure 6 P'T;uu d Su'i s1'rm for BDDAP C HMTCAO oscillator strength is so '.%eak that a more careful25 search for it should be made: it is also possible

60 -NC CN

so N NH 0 -

SH$AP C HUTCAO

40

mN~m

30

20

20 30 40 so Go 70 soAreaI(Azi molecule)

Figure 7 Pressure area isotherm for BHAP-C-HMTCAQ. 26. 13

10 ROBERT M METZGER AND CHARLES A PANETTA

that a ,ix-atom carbamate link is 'too long' andthat an ester linkage (one atom shorter) isdesirable.

Preliminary tests of rectification, ,iorro Ser /

Efforts %%ere made to find rectification hehaior in g,and%% iches of molecules 11, 4 22' and 24. F-orI I, sandsiches Pt I I Hi and SnO Ii H \ere Figure 8. Schematic diagram of Ai 23 Al sandvich user;electrical shorts. 4 This s as not ,urpriing, ,ince in a preliminary test of rectification A monolaser of

the large molecular .4, for I I indicated that it \kas BDDAP C BHTCNO. 23. was transferred as an LB fim'sitting' \kith the lo taxis most parallel to the atop a glass microscope slide provided wlon five bottom

longest strips of Al (left, shaded for simplicty only one -s showm,water subphase, and therefore almost parallel to Then five top strips of Al fright, snaded only onp ,s

the Pt of SnO substrate; if this is true, then the shovvno .,: depul.:. ,. r n ioiaver. but wth Small

film thickness, probably onk 6 A, 'a, so ,mall over!ap o.,th the bottom strip

that TS tunneling, or tunneling throughi defects.could not be aoided. For 22 and 24, electrical bulk AI, i.e. \%ere electrical shorts. B'en this,horts ~%ere observed in similar sand\kiches.' 'refined crude experiment' %, as not a suitable test.Hosever, in the same laboratory, sandxiches or else there \%ere too man, defect,, in theA4 7 Hg using monolayers of cadmium monolayer. Naturally. more refined experiment,arachidate were not insulating, een though they are planned.had been prepared preiousl, as high-resistancesandwAiches in other laboratories." Therefore, itcan be stated that a proper test of rectification of Fourier transform infrared (FTIR) spectra of 2522 or 24 had not been made.

More recently an effort sas made to detect Important structural information *or I.B films isrectification by LB monolayers of BDDAP-C- - proided by Fourier transform infrared If-TIR)BHTCNQ, 23, in a .41 23 .4/ sands, iches con- spectroscopy. '' ,, h2 In a comparatise test ofstructed as sho%.n in Fig. 8. and explained below%. se%.eral FTIR spectrometers, spectra otFifteen glass microscope slides (.5 e 2.5 cm)r sere BDDAP-C-HNITCAQ(25) monolayers on AI-cleaned and coated, using a slitted brass mask, coated glass slides \\ere obtained (rhg 9-- in\%ith fie fingers each of shiny Al 3.5 mm long, the grazing angle ,pecular reflectance mode, at1.6 mm ,aide, and probably 100-500 rm thick different time, after the receipt of identical(bottom strips). A single LB film %%as then samples from the same batch, and their subse-transferred to each coated slide, oerlapping the quent storage in air. Figures 9-11. 12- 15 and 16bottom strips. By using the same mask turned and 17 \%ere obtained, in a lov, in boil-off N-180". each slide receiked a second coating of fi'.e atmosphere, on Mattson, Nicolet and Brukermore fingers of Al of the same dimensions (top FTIR spectrometers,. respectisel. Table 3 lists thestrips), but so that the top strips overlapped details of the instruments used, the sample age.(through the inter,,ening monolayer) o, ith the bot- the resolution. scan time, number of scans and thetom strips only within a disc region about angle ofincidence used (Nicolet stated 'Bre\,ster',1.5 x 1.5 mm (see Fig. 3). Of course, the align- angle' but did not gie its ska\elength).ment of the D-a-A molecules around the edges of It is clear that LB rnonolayers ha\e a finitethe disc is poor, ov, ing to the irregular contour of lifetime on Al substrates in fact, whereas theglass and Al that they must follo. Electrodes Mattson instrument sak, the single monolasersere attached to the long aluminium strips and easily (Fig. 9). the Nicolet instrument barely couldthrough-film conductiity sas monitored using a (Figs. 13-15) and the Bruker instrument, notBAS CV-22 potentiostat for the 15 slides, i.e. for seeing it at all, used the monolaser slide as a75 Al 23 Al sand, iches. Many of these had in- 'blank' to determine the 'background' absorbancefinite resistance, i.e. good contact %,as somehow. of Al for Figs 16 and 17. In Figs 12-14 a singlenot achieved. The rest of these sandvAiches monolayer is present, and one can resole theexhibited the linear, ohmic I- V characteristics of aliphatic CH stretch bands at 2951, 2910 and

LB FILMS OF POTENTIAL ORGANIC RECTIFIERS 11

30 40 5 S 20 15

--- -[ -fi __ _ I I L

0 014 CN

A 0012NH

s 0010 BDAP C HMTCAQ

e 00O

0.000e 0 0000 ~ 0010

Wavenumber

Figure 9. FTiR Mattsor, of on, LB ronoiaer of BDDAP C HMTCAQ 25 on Al

microm

3 0 4 0 6 8 10 15

020 N N H .

b .1 .OAPC HITCAO CN

O 0010

0 014I

0 0102

c 0 oco-e,

C o ooI

e

Figure 10. FTP Mattson' of four LB monolayers of BDDAP-C HMTCAO 25, on Al

12 ROBERT M METZGER AND CHARLES A. PANETTA

30 4 0 to 1 15

WC CN

A 0

AODOAP C H&4TCAQ

0 0200

b 0O j I P1

n

e

0 00

Wavenumber

Figure 11 F TIR %1itT of s -~LB ')orlolfrs of BDDAP C HMTCAQ. 25. on Al

NC CN

Figure ~ ~ ~ ~ ~ F2 12. 1T 0~clt of one) LBm2lcr)fBOPCMTC 5 nA

LB FILMS OF POTENTIAL ORGANIC RE3-TIF.RS 1

, c ch,

800"P C UTCAO

Figure 13. FTIR NicoIl t o~ LB monna~er ,,f BDDAP C HMTCAQ 25 on A] detali showint C H stretcni ha,)O5

8O~ M~ANC c

Figure 14 FTIR Nicolet of one LB monolaver of BDDAP C HMTCAQ 25. on Al, detail showing C N band

14 ROBERT M METZGER AND CHARLES A PANETTA

W- CN

SVDAP C HU7CAC)-

Figure 15. FTIR Nicolmt of six won LB monoiavers of BDDAP C HMTCAQ, 25 on Al.

NC CN

OVENJMBER5 (iMI

Figure 16 FTfR Brukr, of four LB mooolavers manus spectrum of One monolayer on Al of BODAP C HMTCAQ, 25, Al

LB FILMS OF POTENTIAL ORGANIC RECTIFIERS 15

BOA- ONC CNT

9OOAP4W7C0 N +-Jfr

Figure 17 FTIR Bruk or .f s nLB mvonolav-rs ni,us ,t),-ctrurn if one monolaver on Ah of BODAP C HMTCAO

2 5 nn AI

Table 3. Sampe age and instrumental conditions for FTIR measurements

SaImple iqe R(25olution1 No. Scan time Angle of incideceO

,itrirn u ns mt hs cm of scains mnin fromy normah

VNtrli C Jnus 100 0 1 1 13 8 6 82

.'O60SXR 12 13 2 13 42048 9 N aB'uk-er IFS 88 16 1 - 1 83 9 1000 2 70

,8-49 :m an1d tihe muc h x\ caker CN st rctc:h at nionit or sample cx aporation, annealing and aging.2221 cm11 and obtain data which shfould be uISefuLl in

Suuilarl\, [I s 11. 15 anid I- should all he the exIalatiti thle Lualit\ of' the LB film,,.eai\ cent I 6-ntonolaver pect rutti. Itt prac:t ic, alltltrce 1i1tr11ttt1cttt do slto\. thle (-H stecdoublet (at 292- and 2944 cml ' that is also) "Cell WORK OF OTHER GROUPSin irtotolakers ot cadmium arichidate: hosx exer.i heir absorbance IN not thle same. Also, the Miatt- Conceptuallv, our synithetic program is related tosori tist rumnetit does01 10t rels0l thle dOulet110. as efforts, b manl\ groups io model the photosv n-\%0Lild he e\pected. thetic reaction center: linked D-oj-A systemls

Ilt all 'attt pie' the \ li oti istirlitent obserx Cs ( D = porphv riti. A quiniones) are uinder intensix eat broad ahibrpi ion peakcd at 34'5 cm Ml ic:h st ud\ in sex eral laboratories. i6

uta0 be d10t ~ 01 Ot Ler. cit her ill tiltc P'LITL It shoul1d be mentioned that Polymeropouloscotttpar iticilt~eS or al c6 t s: e prepared rectifiers,

tte:t her the liruiker nor t hc Ni:olet instrumecnt see based onl sexen or more monola\ ers of cadmiumthis.- ~resUmnabl\ because thle samiples has e agLed arachidlate. three of them randomly doped (5 : I)and dried for a lonver time. Inl conclIon. onie can xx ith an electron donor dye, one a pure cadmium

resume111 that FTI R pectiroscop% Canl be used to arachidate laxer anid tile 'other three doped (5 : I)

16 ROBERT M METZGER AND CHARLES A PANETTA

'.'.it h an electron acc:eptor d\ e. H-o\%0QI C'.e 11t i' depo'.ited atop ALI n11ierdlINated elect rode,, or a".de\. ice doe,, nlot '.oi ok ft" there ar c te.%.er than '.\ci a 50- 100 nin at pok aniline pols mer het.%% cenlmionola\ cry, t'.'. V1\ elect rode,, '.hado'.'. d '. ibh Sj(), -t Thle

Recent l%, F-ujiiira e'i al. annonnIced anl 13 de' cc ha'. been impro. ed ito thle point \\'.here itphotodiode u'.inct, a '.inulc mloiiola\ er of molecule '. clnerate k' 10 k H., and It,. vain i'. ahno't27. '.'.hichl con'.i'.t' of a donor 1) ( ferrocene). a '.en- 1000.,itiier S (p renle) and anl acceptor A (\i ologen 1.

depo'ited a,, an LB filmn onl a '.emlit ran'.parentl Loldelectrode i'.. it the \i ologen onl the Anl 'Lurtace). CONCLUSIONSand placed in 0.1 1 pota'.'iium chloride '.oli oil.'.'.ith a platilnm counter elect rode. Thflct ronII01 \\ C ha'.e dem~on'.t rated that I1.111iiInt of [ ) Aran'.fer ik from '.olu-t ion to ID, then to the groun,11d mo11Clec' can he \%ci h'ied '' are onl our \\'.a\

'.tate of S; onl irradiation Of the filmn at 33)1 mim, the ito fatbricatitto mletal orl-anic metal ,and\%i.chte,elect rotn i' pu'.hed to t he e".cited stiate of S. )'' Itch:1 \1.1ud Li\CLl It u'.areli',Iiic te'.t tor the ore'-anicMl. ich is, hichcr in etter'.\ than thle [l"()of A, rcct ficr .Ml. ich1 finahl\ aClquire, the elect ron arid traitfer'. it

to the ALI laer. A. current of 2 nA (at (t0o V \. e r, 1 cko legmetSCE) '.'.as mcasured \\'hen licLht \.'a'. ott. and no Ako lde etcurrent \.'as regt'.ered \%. ith the light off. Ho'.'.e'. r.i10 suLb'.equenil full pu~blication of the'.e re'.uilt' ha'.i I, I plvea'iiie wn ihik all ot owr ollaho'rxor,. na'i and pre-Comle to our11 attention. crll: DI 1,111ii1 ttiehjajjri mou 1 .i ttBu iial"o. ), DI \t\

-~~ "~~'\ 0/0~**'' \ I4 Io \Iiini o)'.ika ( i t s L Ili \ r'ii .%. P'rof, Norma~n I Ici tttim- ~ ~ .,) aMill tDaniell t \tattern it'i'ipi.Prio t .P( aua. I+. I .

// \ , '\ / 2 ~ \il111d~ t1). Ki'.tittL ldaam. Dr,. R.'chiniIakut. I

loie \\.i'p n. 'c ,cre uijrteliil it) \t tDiid IIre'.'.e i7 ~ ~ -KJ \t11i011111,in tii'.riitr', \t1 Ken kenctieii and tDr Rohect Ron'.

hi7 I'm I \i1ileNttili Ili'riieit' anid \It .tuteii (a'I~l0

Finall\k \ right0on and cn-.'. oker'. hla'. real i/ed tiuk ci Vi~~ c e e-r,:Jiik !or ine~in iic i I FItR teh'L ie %\01k Antl!.

4 not I'ieheenl p''- hic %kiciolti J1ie

thle 'noeue-a.d tran'.i'.or, '.. ich relies. otl __-t~ f.. h i a ee I t iicUlion alid ;heeitther chemlicall'. doped polyaniline la\~ er'. o ~h Ii \,i Reecai cli

REFERENCES

I -N *\urtanwi. \t 1. t rci'ei .P I 1,ciden arid N'. K, )4 mt . 10, R \1 tiue iaid Pane t't a . Ili I'i,',/upieu, ofit ,tLN /lilt 19 11-- ' 19-6). uI:'ht/jIf itnit,', ( ,cI'r' tc, In m l - te'ciic'atrtl , 'i 'I,

V N. N' ci aInid \t \ Rawner, ( hem, l'.i cit. 29. Y' ( uw,tot Ocd. lltuu i, hiiur 08. 1)t. XI 14I 19-4). 1t 1> t, e. m c . tPantta anid R, Nt. Ntger, ./+ Of"'.V VN N' tnt. tP I. "eidcti and \t -N. Ramner. i, Ilif. o ar hem ;2i 529-14 (1S.1/,'itrm, 1) i, ' edited 4''. t I al.iter, p. 5. taicl 12. R.- K. t .iI i la \%,) \tiia, .1 t . (icUa . tI (oorjx NI.,tDekker. Ness N ork 1 9',). ( lark. t , ) Ki~pcii anid R \t Ntet/rer, .1. Chemn. llIii'.

.4 R Mt \teilcr md IsN P'anteit,. I Pits. tcs ti' ,. h 1 87. -49- it 9s-),oiuiii .44. 3- 160) 19S53, 13. R \1. Ntei/gr and nttaiii Ii l'r~clioe'ms 01 iOiW

tiara'-tdadjii1. I-ht) tDi'-elI'l tii . t niuei'ii'. it \Ii1-11. N-o) itn ' iirk,1hnp on? Iiiorj',ijic jtIil(ruoi-'ippi (~: 1981 cr Xlcc, i iaNItriols . edited h\ P. t Deh ac'

6 R. Nt1 Nscele and P.NFaiieiia. In %.Iileu/or 1h'cirolii P1011.i1i1 Pre-.'. Nc%\ Nork. pp. 2-1 Z'S6.IDci ii''-. \i It1. edited I- I (arler. lip. ; 25 \Iarcl 14. R. Nt. Ntei/ier. R. R. Se:hiiikei. Mt. 11. (a a,. R. K.D~ekker, c\ Ne' rk I198-). 1 aidtas\. P.Ntaiietta anld I. torre. I.,imocuir. 4, 2981( N Ptw.n. 1. ttaghdad-hi iand R Mt. \Ieiver. .%loti. 191,8).('Iii /uq ( iii/. 107, 1(13 it 0)4 1I ' \Iiim. . I oie'.. ( V. Pamta anid R. Nt. \lei,,rer.

JVR I tuce .Faiei.N tiic NN. .. (In.' ( hit. 53. 439qI 198),)tt3maiii. L Virre'.. G I. ttllaAliut. S. K. Lnipaihs and 16 't . \timia, R. K. t aidl,ms V. N. t antd R. \t

I N SAnUCole'.on I. cliii I-j'ctnnm 2. I119 19S16), \slt ii'c , I( tow ri ituiihmr St,' I. 4.Ili pre'.9 R. Nt. Vt/ier. ( N F'ucietta. 1 , NIt,ic and LI. tor e. I -, R. K t xcdlaus J . ttacihd1ih . N. P'anetita. )i . Niuira.

Si t ith NWc'i.Ill,99 It198') 1.. Torte. and R. Nt. Mcteizc Attu ('rviiufoii'r. . Sect. Ft

E iLIS OF POTE,.'TIAL oRkAVC RF(-Tff !FqS 17

1144. ,- [)- :1,.1.-,, 1 I\ x1j \1M. P"- 1-!r

P 1 1) k P- Ii- 1) \1 I-i- k \1 P K

M- 8" 111, 1J I I,' jp,: I Ri:s-,1!. 1),!, 134,

R I 1 1, A', 4 1 1!,)- !,

I k I

Hc

p,I k I LI j I J I. I

VK H W-:-:: t!ld I I Io,'i

N IL. -:1, Pi-elH I"

19,6)

t I cr 132. -4 (Ilik' I \1 ' N IA AIILI 1) N A'1'1i. I/'/',

".1 H-0) 39. 1 1')I I Pc IPP 53. 'Ii) 6! 1 1 ktho!i. I Bu!n,. 1,,HwIe, :TIJ 1 1)

08 1 1 -o 99 1 132 134 / ( hon Pi; I . 7M, 9-it) 1,1,, 1)W 1 1) ,,,tiei, arld I I Raloli. in 1,,m \ .......

I,!, k";! wd I\ %ol 4.Pic". )"Ik (191o)

k Is ...... i xiJ I om, 91). 1 1 ind"o . 1) \Iaulcraii and [I [ 11""All." Im. ( il"'),0,;. 191,

R I. R Bolton. I K, IVarh, D R ook. F. -1 , Ho and kcedor-

J 1 1 Ll 1 90 . 5 64( ) 119"'1",

1111 2(11 1,),4) t I o: I ri I t c I'l [I J, I I o r t I C: J

R I- iww N, 13. 1 1 9NO) P B. 1 Im ( hem S,,( 100. 60140 1 N ', 4)R \L1,1z ind I /h/,: 1 132. 1 P), w, -hiwr;. kiiii[a. ) ',akiij. Miums. V Kcn,R I cl, 1) Vlx,, IP: I ()ktda tlij V %It;j--,a. I Iw /?,,111 105. --- 1

I. .... .. il ") I P)w)) h- I Moo!c. 1) (,11,[. 1) Vathl'. IA \11,11o"j.41 \1 Iix ;, i! j I I I If,, 3. ', - III)(- ) ( !A1111ti%, R 111:11 ',1 1 1. 1 Jild, 1) D 01/1. P4' R 1-Hoe": IIIJ I% I jJv1I, I\ R I chnixi. (I V Ncm, ,!h and \ I MooiQ.

103 1 I'll il Nl) \xwI il wdwo 307. 610 1 1 9S-4)I :rJ I\ I \% J:, ?I PPL, 03. t, \\clei xi, I "[w.0" [coabcdom I

K I P"Mile;opoillo'. 1) "Ild H K'111r. li:.,-P 69 2'-, o8. 1 19"M,

.1 J, p" 1, xio. ) Kx,,.0,.cI r d 11.111-1 4 1 Io:, 132. 69 1 19

R \h: 1). 1 29 '1 \1 I i ::,!. K, 'ind 11 N amajj. 11?1,: ,,1"1J I R 1 132, 19"5)

1115, (,-,1 I\ 'H on. I Imid \1

w I 100, 1 9S-; I1110 , I j-4 1, 1 \1 ( 11 1,111 M ill \1

109.

LANGMUTR-BLODGETT FILMS OF DONOR-SIGMA-ACCEPTOR

MOLECULES AND PROSPECTS FOR ORGANIC RECTIFIERS §

Robert M. Metzger* and Charles A.Panetta

t Department of Chemistry - University of AlabamaTuscaloosa AL 35487-9671 USADepartment of Chemistry - University of MississippiUniversity MS 38677 USA

INTRODUCTION

Our goal is to assemble and te-' i unimolecular rectifier of electrical current.which could be part of a very thin , nm thick) electronic device. This idea.originated by Aviram in 1973. depends on the asymmetry of molecules D-a-A.where D is a good one-electron donor (but poor acceptor), A is a good one-electronacceptor ibut poor donor), and a is a covalent bridge that keeps the molecularorbitals of D separate from those of A. We have found five molecules whichself-assemble as monolayers. three contain the TCNQ moiety: three contain"greasy' dodecyl groups on the donor end (which helps in monolayer formation),but one contains only hexyl groups. All of them can be transferred to a glass orAl substrate as Langmuir-Blodgett films. Recent FTIR data for a single monolayerare presented.

THE ORGANIC RECTIFIER - THEORETICAL. SYNTHETIC, AND ASSEMBLY CRITERIA

This is a progress report on the Organic Rectifier Project. i.e. on our efforts to,ynthesize molecules of the type D-a-A. which may be potential rectifiers of-lectrical current. Here D = strong one-electron organic donor, such as TTF

(tetrathiafulvalene. I ). or TMPD (N.NN'.N'-tetramethyl-para-phenvlenediamine.2). a = covalent sigma bridge. A = strong one-electron acceptor, such as TCNQ17 ,7,8,8-tetracyanoquinodimethan. 3) or chloranil (2.3.5,6-tetrachloro-para-ben-zoquinone. 4). Aviram and co workers predicted in 1973 [1-3] that D-o-Amolecules, such as the proposed molecule 5. sandwiched as an oriented monolayerbetween ordinary metallic thin films M and N, as in 6. would act as a rectifier ofelectrical current. The obvious advantage of such a device is its small thickness:the molecule 5 should be only 2 nm thick. If very thin conventional films M, N(1.5 nm each) can be used, then a 5 nm thick device becomes possible. muchthinner than the working direction of conventional Si or GaAs devices (I to 3 .m).

The incremental results of the Organic Rectifier Project have been reportedelsewhere [4-15]. Here we summarize the design criteria that have evolved, anddiscuss the implications of our most recent results.

§Supported in part by NSF-DMR Grant 84-17563

271

0H 3 % N C 3 N C C N C 0 C

N I0CH3 ' CH 3 NC C.

NC CN

D-a-A

s S D-a-A~NC I CN

Simplified Mechanism

If device 6 can be assembled, then the mechanism for rectification is givenby [2-5.10.16]:

FORWARD BIAS: GO: M I D-a-A I N ---- > M-i D+-a-A'I N --... > M-i D-a-A I N+ 77a 7b (ITF)

REVERSE BIAS: NO GO: M I D-o-A I N ---- xxx--> M+I D'-a-A+I N- 8

REVERSE BIAS: GO: M I D-c-A I N ---- > M I D+-a-A-i N ---- > M+I D-a-A I N 9

9a (ITR) 9b

Under zero or moderate forward bias the electron transfer (ET) can occur byelastic, through-space (TS) [17] tunneling though a chemisorptive barrier from Dto N. and from N to A (7a), thus is formed the zwitterionic molecular state D+-o-A':this is followed by through-bond (TB)[171 inelastic tunneling (call it ITF, 7b, orforward inelastic tunneling) from A' to D+: this produces charge separation.Under reverse bias, the formation of zwitterion D-a-A + (8) is very unlikely,because good organic one-electron donors are miserable acceptors, and goodacceptors are poor donors. Indeed. in the gas phase, the energy required to formthe ions TTF+(g). TCNQ'(g) at infinite mutual separation is known to be 4.0eV[18.19]. whereas the energy required to form the ion pair TTF-(g). TCNQ+(g) isestimated at over 9 eV [20,21]:

lF(g) + TCNQ(g) = TTF+g) + TCNQ-(g) AH = 683 - 2.8 eV = 4.0 eV 10

TTF(g) + TCNQ(g) = TTF(g) + TCNQ+(g) 6H = 9.6 eV (est) 11

However, under reverse bias another mechanism is possible (9a ): a zwitterionD+-a-A forms first, then charge separation from %I to N occurs: this is reverse-uphill" tunneling ITR. and is much less likely than ITF Of course, tunneling ishere between states of different energies, so one sees these inelastic processes aselastic processes between virtual states of equal energy, followed by inelastic.through- bond" processes.

272

Langmuir-Blodgett Films

So far, our efforts have been centered upon molecules D-a-A whichself-assemble at the air-water interface as "Pockels-Langmuir" monolayers [ourterm for such monolayers. honoring Irving Langmuir (1881-1957) and AgnesPockelsl.[221. Such monolayers can. usually, be transferred to glass or metal orother substrates by the Langmuir-Blodgett (LB) technique pioneered byLangmuir and by Katharine B. Blodgett (1898-1979) [23-261. The typical moleculethat forms excellent PL and LB films is cadmium arachidate. Cd(n-C 19 H 3 9 COO) 2 , 12.

(H 3 C COO) 2 Cd

air

13

water---------------

At the air-water interface a PL monolayer of cations 12 (shaped liketadpoles) points with the carboxylate ends toward the water subphase, as in 13:the cadmium counterions are in the subphase. coordinated rather closely with thecarboxylate head group. Usually, such work is done today by using an automatedLangmuir trough or film balance.

The LB technique consists of slowly inserting a macroscopic metal, glass,silicon, or othe:r substrate through the monolayer into the subphase; either uponinsertion into the water, or upon slow withdrawal from the water, the PLmonolayers transfer quantitatively to the substrate with little or no distortion:thus are formed the LB monolayers. If the substrate is dipped several timesthrough the monolayer, LB multilayers can be transferred to the substrate'23-281.

Other techniques for monolayer coverage of metals

Another method of transferring self-assembling monolayers to substrateswithout using a Langmuir trough is the Bigelow oleophobic (BO) film-castingtechnique [29-321. The forces binding an LB or BO monolayer to the substrate areusually weak physisorptive or chemisorptive forces. A better adhesion is bydirect covalent bonding to a surface. Bonding to silanized metal surfaces rarelyachieves monolayer coverage [331. Also, attaching silanized molecules tooxide-bearing metal surfaces produces compact monolayers [341, as does attachingmolecules bearing disulfide bonds to gold surfaces [35]. Another method ofproviding "strong" films is to polymerize LB films containing diacetylenelinkages in situ using ultraviolet radiation [361.

273

Despite the existence of these other potential techniques for assembling anorganic M I D-o-A I N rectifier, we concentrate below on D-o-A molecules that willself-assemble as PL and LB monolayers.

Synthetic and device assembly criteria

The criteria for successful synthesis of candidate D-o-A molecules and fortheir assembly into the organic rectifier 6 have been stated before [4,5,8.10,161and can be summarized as follows:

(1) The donor D must have a low ionization potential ID (<7.5 eV) and befairly flat.

(2) The acceptor A must have a high electron affinity AA (>2 eV) and must befairly flat.

(3) The o bridge must be saturated (not conjugated). It must be long enoughto prevent extensive ground-state mixing of the donor molecular orbitals with theacceptor molecular orbitals (> 3 carbon-like atoms), yet short enough (< 9carbon-like atoms) or rigid enough to prevent the curling of the D end over the Aend of the molecule. Molecular modeling shows that 5 or 6 carbon atoms seems tobe the optimal length. The a bridge must be flat enough to provide good lateral LBfilm packing.

(4) The D-a-A molecule must pack well in monolayers. For this. one end mustbe hydrophobic, the other hydrophilic. There should be no overlap of D-a-A overA-a-D (which would cancel dipole moments and destroy the directionality of thedevice 6). The lateral n-n attractive interactions, and maybe a mixed-valenceground state, as exists in the q uasi-one-dimensional salt TTF TCNQ, may reduce thecost of ionization of D-a-A to D"'-a-A" from 4 eV to maybe I or 2 eV.

(5) The ID and AA values must match as closely as possible the workfunctions of the metal layers M and N (see Table 1). As can be seen, the match isfar from perfect., even for the best donors and acceptors known to date.

Table 1. Ionization potentials ID for the donors D,electron affinities AA for the acceptors A,and work functions for the metals M. N

ID (eV) AA(eV) * (eV)

TMPD 6.25 [37] DDQ 3.13 1391 Al 3.74 [41]TTF 6.83 [181 TCNQ 2.8 [19] Au 4.58 [411pyrene 7.41 [381 Chloranil 2.76 [40] Pt 5.29 [41]

(6) The bridge-building organic reaction 14 leading to D-a-A must be morelikely, or more efficient, than the competing reaction of charge transfer (CT) saltformation 15:

D-X + Y-A -------------- > D-a-A 14

D-X + Y-A -------------- > X-D + Y-A" 15

(7) The Franck-Condon reorganization of the molecular geometry of D to Dand of A to A- [4Z] must be small, or fast, so that the overall ET within D-a-A doesnot become slow [42-44]. Under the right conditions. ET through the moleculecould be faster than I ns [43-46].

274

Tgi 'he monola.ers must be close-packed and defect-free for at least a few .im

n the lateral direction ito prevent electrical shorting of metal layer %1 to metal

la.cr N. It is known that over hydrophobic, oxide-free metals (Pt. Au. Ag

cadmium arachildate films have lareer defects (probably at domain boundaries i

and have inaybe a disordered. fan-like structure 4 [. Over hydrophilic.

0,,tde-covered metals (Al. Sn) or glass or quartz, the defects seem smaller. allo',.ing

for a percolative curent I across the film and through disclnations [49j (the

,oltage dependence of j is Iog l = a% 50j and not log j = b V1" [511). One %a, of

achieving freedom from detects on the scale of a few microns would be to use a

thin interdicitated layer M. v here the film perfection is sampled on several small

disks of radius 3 I m radius ithese disks would be connected to macroscopic

e Ie c t rode s i.

(9) At the %I to N distance of, typically. 2 nm, the directed TB tunneling must

be much more likely than the undirected TS tunneling. Sofar, in inelastic

electron tunneling spectroscopy (IETS) {521 for random films of insulators placed

betveen superconducting Pb and normal AI electrodes at 4.2 K. the ratio

TB/iTB-TS i is only 001. For the organic rectifier to succeed. this ratio (for

compact monolayersi must be much closer to 1.0.

(lO) The process of laying down the second metal layer N atop .I I D-0-A as a

sputtered or evaporated film. and also the exothermicity of the TB tunneling in

the device 6. must both avoid heating the monolayer above 100°C. Similarly. high

% ,oltages that may cause dielectric breakdown must be avoided.

RESULTS

Couplin! reactions

In 1976 Hertler prepared carbamate. or urethane, polymers of TTF with TCNQ

it one reacts the 2.2'-bisisocyanate of TTF with a 2.5-dihydroxyethoxyTCNQ the

resulting insoluble black polymer was semiconducting, not metallic [531.HowAever, the utility of Herilers contribution was to show that carbamates of

strong donors with strong acceptors could be prepared, and that reaction 14 was

preferred over reaction 1 5 for carbamates, Baghdadchi prepared the

monoisocyanate of TTF. 16. and coupled it with 2-bromo-hydroxyetho.yTCNQ. 17

-- 6. 541 to yield the carbamate 18 of TTF with TCNQ. However. two forms were

found, of which one seemed zwitierionic. the other neutral (as evidenced by the

infrared CN stretching frequencies). Neither product could be isolated in

acceptable purity

NC CN NC CN

cSS<NC4 Br HB

aNNC CN

Esters of TTF with TCNQ 17 were also prepared, using the 2-acyl chloride of

TTF. but, again, two products were obtained [6]. The neutral form of the TTF

carbamate with TCNQ did form LB monolayers, but with the wrong geometry(probably with the molecule lying almost flat on the water subphase. rather thannormal to it).

275

Since that time, most of our efforts have been concentrated on the carbamatecoupling, although one may conceive of other possible coupling reactions.

Acceptors

Most of the work has been done with Hertler's TCNQ alcohol 17 (BHTCNQ)."hose crystal structure was recently determined (Fig. 1) [13]. From cyclicvoltammetric studies we estimate that the electron affinity of 17 is 2.9 ± 0.2 eV[13 The inefficient synthesis of 17 has caused us to seek better, or moreaccessible acceptors. The anthraquinone analog 19 has been prepared inexcellent yield [91, but the molecule is severely bent. and 19 is a weaktwo-electron acceptor. rather than a strong one-electron acceptor. The synthesisof 20 has been accomplished [551. and the crystal structure of its methyl ester. 21,has been determined [14] (Fig. 2).

NC CN NC CN NC CNIHO HO,^ 0 H3C , 0 %-0, 0

NC ON NC ON NC CN19 _3

_ _ ;~N.is -Co3

Fig. 1. ORTEP plot of BHTCNQ. 17 Fig. 2. ORTEP plot of AETCNQ. 21monoclinic. space group P2 1/n, (triclinic, space roup Plbar.

a= 9.258 A/. b= 13.618 A., c- 10.947 A/. a= 7.165 A, b= 9.058 A. c-- 13.244A.,

-3 92.14 °, Z = 4. R = 39% [1301 = 70.06", 3= 87.140. y= 68.22 ° , Z =2R = 3.4% 114>,.

Donors

After the earl,, work on TTF isocyanate (a strong donor) [4-6. our attention

turned to the isoc.,anate of pyrene. 22 ta medium donor), and to substituents ofphenyl isocy'anate (a weak donor). adding a dodecyloxy group made weak donor

24; adding he dimethylamino group made the medium donor DMAP-NCO, 2. Thecrsal structure of its methyl carbamate. DMAPCMe. 26. was determined (Fig. 3)

[111. its calculated (AMl ) ionization potential (7 17 eV) and its cyclic voltammetr'yshowed that 26 is a donor weaker than TMPD or TTF but stronger than pyrene [12.There is extensive hydrogen bonding in the crs'stal of DMAPCMe [111. in solution

276

I-,and also in solution of its electrochemically generated radical anion 1-2 We~e also prepared the bis-dodec,,l and the bis-hexvl deri~atives of 25.

IkNONCO NCO

H

NCO Q N ) 0CH

H3C.NC O CN'~H3C Z ~ H30 0j

Fi. .ORE o. otellC.2

N ONP412 ~ ~ N ONI 4T

OH2 ,

Fig4. RTE plt o Pienl-CBHTNQ.27Fig. 3. ORTEP plot of DMIAP-C-HMCAQ 281(monahnic spae grup P 1/n.(thorhoic, sbp.ac gr874ou.pa=8.310A~~~b=9278 a~=5.8A 1O99= 13.41 A.b= 9 999 .~3=6.1 0 ,=4.=79%[1c ~ = 149.IS , = 862RZ= 5 911.

Cr~sal sructres f Door-~zm AccolorW 151)t

To tst he ropsiton hatthea bidg alowsforan xteded strct 277o

the D-c-A adducts, crystal structures have been determined for pheayl-C-BHTCNQ,27 (Fig. 4) [11] and for DMAP-C-HMTCAQ. 28 (Fig. 5) [151. In both cases themolecules do not form LB films, but the crystal structure reveals an extendedconformation. the crystals belong to the centrosymmetric space group PI bar: themolecules pack in the crystal so that D-o-A packs nearly over A-r-D.

Pockels-Langmuir and Langmuir-Blodgett Films

It \,as found that only certain D-o-A molecules 29-33 form monolayers at theair-%.ater interface. the data are given in Table 2. and the relevant pressure-areaisotherms (and molecular structures) are shown in Figs. 6-10.

The collapse pressure flc is defined to be the differential surface tension.vhich. if exceeded, leads to the collapse of the film, and to the irreversible "ridingof ie floes over each other". The area per molecule can be' defined in severalva , the area at the collapse point is Ac . and is the smallest; the area per moleculeat /ero pressure, A. , is the point in the 11 -A isotherm obtained when the steepestpart of the curve is extrapolated linearly to zero pressure: Am is defined as themid-point between A. and A0 . Of all these points, A c gives what probably is thebest estimate of the lateral dimensions of the molecule in a well-packed film.

The shape of the 17,-A isotherm is indicative of the fluidity or rigidity withinthe film. For cadmium arachidate, the slope is almost vertical, because the film isalmost crystalline in its two-dimensional packing. For the molecules described inFi s 6-10, the slope indicates a more fluid-like environment, in which themolecules probably do not pack as rigidly as they would in a crystal. Of themolecules given above, only Py-C-BHTCNQ lacks an alkyl "greasy chain":DDOP-C-BHTCNQ. BDDAP-C-BHTCNQ and BDDAP-C-HMTCAQ all have dodecyl groupsthat help provide a hydrophobic region in the molecule (if the dodecyl groups arereplaced by methyl groups, no monolayers are formed). Since. however, thepresence of long alkyl chains may retard the TB ET. it was important to find ashorter alkyl chain which would help in providing monolayer formation. Thus\,e are pleased with the recent result that the bis-hexyl chain in 33 was'sufficient' to provide good PL monolayers. Work on similar molecules iscontinuing.

Lanermuir-Blodeett films and tests on rectification

Efforts were made to find rectification behavior in sandwiches of molecules18 [4.5]. 29 [7], and 31[7]. For 18. sandwiches Ptll8lHg and SnOII8lHg wereelectrical shorts [4.5]. This was not surprising, since the large molecular A. for18 indicated that it was "sitting" with the longest axis almost parallel to the watersubphase. and therefore almost parallel to the Pt of SnO substrate: if this is true.then the fir thickness, probably only 6 A, was so small that TS tunneling, ortunneling through defects could not be avoided. For 29 and 31, electrical shortswere observed in similar sandwiches [71. However, in the same laboratory.sandwiches Al12111g using monolayers of cadmium arachidate were notinsulating[7], even though they had been prepared previously as high-resistance sandwiches in other laboratories [511. Therefore it can be stated that aproper test of rectification of 29 or 31 had not been made.

Important structural information is provided by Fourier Transform Infrared(FTIR) spectroscopy [32.56-60). Fi s. 11-13 were obtained on a Nicolet 60SXRspectrometer, operated at 4 cm- resolution, using grazing angle specularreflectance and a Brewster angle polarizer. In all cases the samples weretransferred as LB films onto aluminium-coated glass slides. The molecule studiedwas BDDAP-C-HMTCAQ, 32; in Figs. 11.12 a single monolaver is present, yet onecan resolve the aliphatic CH stretch bands at .951, 2910. and 2849 cm" and the CNstretch at 2221 cm 1 . In Fig. 13 the LB films consists of 16 monolayers.

278

Fable 2. Molecular areas and collapse pressures for PL monolayers

NMolecule No. T fl A A A Ref.

T-TF-C-BHTC-,Q is 292 12.7 134±50 -- -- 51

DDOP-C-BHTCNQ 29 292 20 2 50 55 60 [5,161

BDDA-P-C-BHTCNQ 30 293 473 57 69 82 [161303 -5.9 54 7,0 82

R -C-BHTCNQ 31 283 28.2 53 60 66 [161

BIDDAP-C-HMNTCAQ 32 293 22.3 58 71 83 [16]

BHAP-C-HMTCAQ 33 293 35 8 42 47 53

30 NCii 'C -

20

10 20 30 40 so 60 70 goArta I (A21 molecule)

Fig. 6. Pressure-area isotherm for DDOP-C-BHTCNQ, 29 [5,161.

279

60 N ~ H~

"C C050,

40

mMlm

30

20

2120 40 60 80 100 120

Arta I (A 21 M0l9CUl.)

Fig. 7. Pressure-area isotherm for BDDA.P-C-BHTCNQ, 30. a! 293 K (curve 1) and at 303 K(curve 2) [16].

40

300

mNIm

20V

10

o L

10 20 30 40 s0 60 70 90Area I (A21 molecule)

Fig. 8. Pressure-area isotherm for Py-C-BHTCNQ, 31 [16].

280

60

400

nmNimn

30

20

10

0 20 40 60 so 100

Arta /(AZIMOICUI*)

Fig. 9. Pressure-area isotherm for BDDAP-C-HMTCAQ, 32 [161.

60-

500

I

20 30 40 so 60 70 soArta I (A2 rooc uI.)

Fig. 10. Pressure-area isotherm for BHAP-C-HMTCAQ. 33 (this study).

281

Fig, 11. FTIR of single LB monolayer of BDDAP-C-HMvTCAQ. 32. on aluminum,sho~k ng aliphatic CH- stretch bands.

Fig.12 1-TIR of single LB monolayer of BDDAP-C-HMlTCAQ, 32. on aluminum,shoAing CN stretch bands.

282

\ORK OF OTHER GROUPS

Conceptuall,. our -synthetic program is relat d1 to effor-ti b- man" groups tomdlthe photosynthetic reaction center: linked D-a-A systems 1D = porphyrinl A

qutnones are under intzrnse ,iud\. in ,ev.eral laboratories 61 -661

It slhould be mentioned that Kuhn et al. 61 and later Supt et al. 1681 haveprepared rc~tifiers biased on cven or more monola ,ers of' cadmium arachidlate.,hree of them random[,, doped 5:1I) %4ith an electron donor d-,e. one a pure.:admium arachiliate layer. the other three doped 1; ) whith an electron acceptor,.e However, the de ice does not work if' there are fewer than seven monolayers

Fg1,FTIR of 16 LB monolavers of BDDAP-C-HMTCAQ .32. on aluminum.

Recently Fujihira prepared a LB photodiode (68] using a single monolayer ofmolecule 34. which consists of a donor D (fer~ocenc). a sensitizer S (pyrene). andan acceptor A (viologen). deposited as an l.B fiim onto a semitransparent goldelectrode (with the viologen on the Au surface), anid placed in a 0.1 molarpotassium chloride solution, with a platinum counter ele,'rode. The electrontransfer is from solution to D. then to the ground state of S: upon irradiation of thefilm at 330 nm, the electron is pushed to the excited state of S. S*, which is abovethe ILUMO of A. which finally gets the electron and transfers it to the Au layer. 2nA of current at 0.0 V versus SCE was measured when light was on [69].

283

Br- Br-

~((CH4 )e N+ --(CH 2 ) 1 1

CONCLUSIONS

We have demonstrated that LB films of D-a-A molecules can be synthesized;we are on our way to fabricating metal IOrganic I metal sandwiches which shouldgive us a realistic test for the organic rectifier.

ACKNOWLEDGMENTS

It is a pleasure to thank all of our collaborators past and present: Dr. JamilBaghdadchi (now at BASF Buffalo), Dr. A. M. Bhatti (now at Univ. of Southern.Mississippi). Prof. Sukant Tripathy (GTEL, now Univ. of Lowell), Prof. Yozo Miura(Osaka City University). Profs. Norman E. Heimer and Daniell L. Mattern(Mississippi), Profs. M. P. Cava, 1. L. Atwood, and L. D. IKisper (Alabama), and Drs.R. Schumaker, J. L. Grant and R. K. Laidlaw (Alabama), and Mvr Epifanio Torres(Mississippi). We are grateful to Mr. Ken Kempfert and Dr. Robert Rosenthal ofNicolct Analytical Instruments for measuring the FTIR spectra. The work wouldnot have been possible without the generous support of the National ScienceFoundation and the Office of Naval Research.

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