Ultramicroscopy 97 (2003) 271–278

Highly ordered thin films prepared with octabutoxy copperphthalocyanine complexes

Kelly Stevensona, Naoko Miyashitaa, Joanne Smiejab, Ursula Mazura,*aDepartment of Chemistry and Materials Science Program, Washington State University, Pullman, WA 99164-4630, USA

bDepartment of Chemistry, Gonzaga University, Spokane, WA 99258, USA

Received 11 September 2002; received in revised form 31 October 2002

Abstract

Langmuir–Blodgett (LB) films of copper (II) 1,4,8,11,15,18,22,25-octabutoxyphthalocyanine, nCuPc(OBu)8, (non-

peripheral substitution) and copper (II) 2,3,9,10,16,17,23,24-octabutoxyphthalocyanine, pCuPc(OBu)8, (peripheral

substitution), were fabricated and characterized by optical spectroscopy and scanning probe microscopy. The LB films

were transferred onto hydrophilic substrates by vertical dipping. Although they posses relatively short polar

substituents both compounds form smooth, uniform, dense, and highly stable LB monolayers composed of linear

arrays of cofacial oligomers. The long range discotic assemblies of LB and spun cast films of pCuPc(OBu)8 and

nCuPc(OBu)8 posses physical and chemical properties favorable for molecular electronic device application.

r 2003 Elsevier Science B.V. All rights reserved.

1. Introduction

Of past, present, and future interest as compo-nents in molecular electronic devices are thephthalocyanine-like molecules. Because of theirthermal, chemical, and physical stability (in excessof 500�C), and useful photophysical, redox, andcatalytic properties, they are used in manyadvanced technological applications [1]. They areemployed in laser printers [2], in optical datastorage systems [3], and in electronic devices suchas WORM drives [4]. They are also of continuinginterest as active elements in photovoltaic cells [5],fuel cells [6], and chemical sensors [7].

The exploitation of the desirable optical andelectrical or electrochemical properties of phtha-locyanines (Pc), relies on the precise control overthe molecular packing and ordering in the solidphase. Unsubstituted Pc self-organize into avariety of crystalline polymorphs and have beenused to prepare ordered thin films by vapordeposition [1,8–10]. In addition, many Pc deriva-tives with flexible side-chains make excellentcandidates for Langmuir–Blodgett (LB) film fab-rication and also films generated by spin coating,and self-assembly [1,11–15].Our interest in Pc materials is in employing

them as model systems for studying electrontransport processes by orbital-mediated tunnelingspectroscopy (OMTS) both in the scanning tunnel-ing microscopy (STM) and the metal–insulator–metal diode (M–I–M) environments. We have

*Corresponding author. Fax: +1-509-335-8867.

E-mail address: umazur@wsu.edu (U. Mazur).

0304-3991/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0304-3991(03)00052-4

investigated vapor deposited monolayer, and sub-monolayer films of several unsubstituted metalphthalocyanines (MPc) on Au(1 1 1) substratesand incorporated into tunnel diodes [16–21]. Ourstudies produced tunneling spectra that show adirect correlation between orbital mediated elec-tron transfer processes and the expected HOMOand LUMO energies in single molecules (STM)and in collections of 109 molecules (M–I–M)[16–21]. We are now extending our efforts to thestudy of single LB layers of substituted Pc in whichthe macrocyclic rings are oriented in linearcofacially stacked domains. Electrochemical prop-erties of linear cofacial arrays of Pc polymers areknown to be strongly affected by the ring–ringspacing, twist angle, and position relative to theneighboring rings [1,13,22–25].Functionalized and cofacially arranged Pc

suitable for study either in the STM or M–I–M’environment, need to have relatively short R

groups in order to facilitate the tunneling pro-cesses. Many of the excellent candidates reportedin the literature, however, proved unsuitable forour purposes because of their long and bulky R

groups [1,11–15,26,27]. Two molecules thatshow great promise for our OMTS studies arethe non-peripherally substituted copper (II) 1,4,8,11,15,18,22,25-octabutoxyphthalocyanine, nCuP-c(OBu)8 and the peripherally substituted copper(II) 2,3,9,10,16,17,23,24-octabutoxyphthalocyanine,pCuPc(OBu)8 [27]. In this article we report on thefabrication and structure of LB and spin-castmonolayer films of these complexes.

2. Experimental

2.1. Materials

CuPc and the non-peripheral CuPc(OBu)8 wereobtained from Aldrich Chemical Co.1 The parentcomplex was twice sublimed and the nCuPc(OBu)8was recrystallized from tetrahydrofuran and etha-nol prior to usage. The peripheral CuPc(OBu)8analogue was prepared using a previously reported

general synthetic method [28]. All solvents usedwere either reagent or spectroscopic grade andwere acquired either from Aldrich Chemical Co.(see footnote 1) or J.T. Baker.2 Corning3 micro-scope slides were cleaned in a HNO3/H2O2 bath,and stored in high purity water until usage. Micastrips 1� 4 cm (catalog 54) from purchased fromTed Pella.4

2.2. Film preparation

Langmuir-Blodgett monolayers were preparedusing the KSV 50005 LB double trough systemequipped with two Whilhelmy balances. Eachbalance was mounted at the midpoint betweentwo moving barriers. Films were spread on aMilliPore Milli Q-purified water subphase typicallykept at approximately 20�C. Aliquots (200–700 ml)of 2� 10�4M solutions of the complexes inchloroform were employed. The Pc layer wasallowed to stabilize for several minutes and wasthen compressed at a barrier speed of 20mm/minuntil the desired surface pressure was reached.Mono- and multi-LB layers were transferred viaZ-type deposition onto freshly cleaved micasubstrates or pre-cleaned microscope slides. Thedipping speed was 20mm/min and the transferratios ranged between 0.95 and 1.05.Spin-cast films on mica and glass slides were

made with the same solutions used for the LB filmpreparation. The substrates were exposed to theindividual solutions for 15–30 s and then spun dryfor 60 s to remove the solvent.

2.3. UV-vis

LB films of the CuPc(OBu)8 complexes weredeposited on glass slides employing vertical dip-ping. CuPc was vapor deposited on glass slides in aHV deposition chamber at o6� 10�7 Torr. Film

1Aldrich Chemical Co. Inc., 1001 West Saint Paul Avenue

Milwaukee, WI 53233, USA. Tel.: 1-800-558-9260.

2KSV Instruments USA, P.O. Box 192 Monroe, CT 06468,

USA. Tel.: +1-800-280 6216.3 J.T. Baker, 222 Red School Lane, Phillipsburg NJ 08865,

USA. Tel.: +1-800-582-2537.4Ted Pella, Inc., P.O. Box 492477, Redding, CA 96049-2477,

USA. Tel.: +1-800-237-3526.5GBC Scientific Equipment, 3930 Ventura Drive, Arlington

Heights, IL 60004, USA. Tel.: +1-800-445-1902.

K. Stevenson et al. / Ultramicroscopy 97 (2003) 271–278272

thickness was monitored with a quartz crystalmicrobalance. Film deposition rate was 0.01 nm/sand a CuPc density of 1.6 g/cm3 was assumed. Aclean glass slide was used as a reference.Electronic spectra of Pc compounds were

recorded with a Cintra 406 UV-visible spectro-meter from GBC Scientific. Data was collected at150 nm/s and a slit opening of 2 nm.

2.4. AFM

Monolayers of CuPc(OBu)8 LB films wereprepared on mica substrates. The specimen wasthen mounted on a metal disk with a double sticktape and placed on the top of the scanner tube.The samples were always set in such a way that thefast scan direction and the dipping direction werethe same. Spin-doped MPc samples were alsoprepared on mica. A precut mica substrate wasexposed to 2� 10�4M solutions of CuPc(OB)8complexes in chloroform solutions for 10–15 s andspun dry for 30 s.The AFM used for topographic imaging of thin

films was Nanoscope IIIA multimode microscopefrom Digital Instruments, DI.7 All micrographs

were acquired in tapping mode with etched silicontips having a diameter of 5�10 nm. The specifiedcantilever length was 127 mm and a force constantof the order of 50N/m. The setpoint variedbetween 1.85 and 2.75V and the resonancefrequency of the cantilever was 260–340 kHz.The standard DI software was used for dataacquisition.

3. Results and discussion

Langmuir films of pCuPc(OBu)8 on waterexhibit two distinct phase transitions (Fig. 1).Armstrong et al. also observed two sharply definedphase transitions in the LB isotherms of aperipherally substituted Pc derivative, 2,3,9,10,16,17,23,24-octakis(2-benzyloethoxy) phthalocya-nato copper (II), CuPc(OC2OBz)8 [13,14]. Theyassociated these two transitions with formation ofa monolayer at the lower surface pressure (firstphase transition) and a bilayer of the complex atthe second phase transition. The very stable andrigid CuPc(OC2OBz)8 monolayer was determinedto consist of coherent rigid Pc columns. Thedriving forces for this strong structural stabilityare both the Pc–Pc macrocycle interactions andthe associations between the terminal benzylgroups. The bilayer structures on the subphase

Fig. 1. Surface pressure-area isotherms (ca. 20�C, pH 7) for pCuPc(OBu)8, thick line and for nCuPc(OBu)8, thin line.

6VWR, P.O. Box 5229, Buffalo Grove, IL 60089-5229, USA.

Tel.: +1-800-727-4368.7Digital Instruments. 112 Robin Hill Road, Santa Barbara,

CA 93117, USA. Tel.: +1-800-873-9750.

K. Stevenson et al. / Ultramicroscopy 97 (2003) 271–278 273

were found to consist of tightly folded bilayersheets.In Fig. 1, we assign an area of 96 (A2/molecule

at the first phase transition for pCuPc(OBu)8(20mN/m), where the Pc macrocycles are cofa-cially stacked into columnar aggregates at a smallangle to the normal of the subphase surface. Theobserved molecular area for the second phasetransition is 52 (A2 and is associated with theformation of a MPc bilayer of ordered aggregates.The compression of nCuPc(OBu)8 films pro-

duced well-defined isotherms with an apparentsingle-phase transition (Fig. 1). Thus, for the non-peripheral derivative the molecular area at anestimated first phase transition at 20mN/m is82 (A2/molecule. Cook and coworkers reported avalue 97 (A2/molecule for the same molecule mea-sured at a higher surface pressure of 30mN/m [27].LB monolayers of pCuPc(OBu)8 and nCuPc

(OBu)8 were deposited onto freshly cleaved micaand imaged by tapping mode AFM. A surfacepressure between 12 and 18mN/m with barrierspeed and the substrate withdrawal rate of 20mm/min were determined to be the optimum para-meters for pCuPc(OBu)8 film deposition, yieldinga reproducible transfer ratio of 1.0702. Filmcompression–relaxation studies showed no hyster-esis in the surface pressure vs. area expansioncurves if the film was manipulated in the pressureregion of the first phase transition. The film qualitywas not compromised by a series of sequentialcompression–relaxation cycles. However, if the

compression of the film advances to the secondphase change, considerable hysteresis is observedin the isotherm curve and the bilayer film whichwas then formed, did not relax to its precom-pressed state. This was also found to be true in thecase of CuPc(OC2OBz)8 [13,14].A surface pressure of 15–20mN/m and barrier

compression and substrate dipping speeds of20mm/min furnished the best quality nCuPc(O-Bu)8 monolayers. The transfer ratios varied from0.89 to 1.05. Unlike the monolayer of pCuPc(O-Bu)8, the nCuPc(OBu)8 Langmuir film did notsurvive multiple compression–relaxation cycles.The film of the non-peripheral complex did showsmall hysteresis in the surface pressure vs. areaexpansion curve for a single compression–relaxa-tion cycle. Cook also observed significant hyster-esis in the compression and relaxation isothermsplots of non-peripherally substituted octaalkoxyPc monolayers and reported transfer ratios of0.50–0.75 for those complexes with Z and Y-typefilm deposition employed [27].Fig. 2 displays AFM images of monolayers of

pCuPc(OBu)8 after a vertical transfer onto mica.The films were air-dried but not annealed. Thevertical axis (slow scan direction) of the imagescorrespond to the direction of the film withdrawalfrom the LB through. The films were smooth,dense, and essentially pinhole free. Film thicknesswas uniform and averaged 1.2 nm over areas ofmany m2. The monolayer height was determinedby making cross-sectional measurements through

(A)(A) (B)(B)

Fig. 2. Tapping mode AFM images of a pCuPc(OBu)8 LB monolayer on hydrophilic mica substrate obtained in an ambient

environment. Image A represents a 3.5� 3.5mm area while image B that of 500� 500nm section. In B, strands of Pc columns can be

observed with inter-column distances of about 0.2 nm. The film height in both micrographs is 1.2 nm.

K. Stevenson et al. / Ultramicroscopy 97 (2003) 271–278274

the rare perforations in the film. The measuredfilm height is consistent with the Pc moleculesorienting at an angle relative to the substratenormal [12–15]. Close-up AFM images of the film(Fig. 2B) exhibit the expected long range columnarpacking extending for distances of tens of nan-ometers. The observed molecular ordering wasgenerally disrupted by defects in the substrate.AFM images of LB monolayer and bilayer films ofperipherally substituted CuPc(OC2OBz)8 depos-ited via vertical and horizontal methods onto silicashowed similar characteristics [15]. The inter-column spacing in the pCuPc(OBu)8 film wasabout 2 nm which suggests that the butoxy groupson the Pc ring are not fully extended (molecularmodeling predicts they will stretch over 2.7 nm inthe fully extended configuration) but are eithercontracted or interdigitated with adjacent aggre-gates. It is surprising that pCuPc(OBu)8, a com-plex with relatively short ring substituents andnone of the ancillary terminal group interactionspresent in CuPc(OC2OBz)8 gives rise to such long-range ordered and stable columnar films.The non-peripheral complex also formed stable

nearly pinhole free films on mica substrates butnot with the same reproducibility as pCuPc(OBu)8,Fig. 3A. The AFM images of the nCuPc(OBu)8monolayer (1.5 nm thick) appear grainy and showless organization than the pCuPc(OBu)8. Thisresult is to be expected for films made fromnon-peripherally derivitized Pc. Cook notes that

non-peripherally substituted Pc produce discoticmesophase LB layers in which there is disorderwithin the column aggregates [12,15]. The bilayerfilms of nCuPc(OBu)8 appear to be highlystructured as shown in Fig. 3B. Many of the Pcfibers we imaged were more than 5 mm long andmeasured 20 nm across, on average. The height ofthe bilayer was about 3.0 nm. The fibers, therefore,consist of bundles of individual cofacial strandsone-molecule in thickness.The morphological images of spin-cast films

(from chloroform solutions) made from theoctabutoxy complexes are depicted in Fig. 4. BothpCuPc(OBu)8 and nCuPc(OBu)8 produced simi-larly uniform net-like films on mica. Varying theconcentration of the material in solution, theevaporation rate of the solvent, the exposure timeof the surface to the solution, and the rate ofsubstrate spin can control the thickness anddensity of this network. Using Pc concentrationsof 10�6M, 10 s contact with mica, and spinningdry for 60 s at 2000 rpm produced films onemolecular layer thick. The higher magnificationinset, in Fig. 4B, shows formation of columnar-like assemblies with structural characteristicssimilar to those prepared by LB deposition.Thicker, 3 nm spin-cast films of pCuPc(OBu)8with larger surface area were obtained from moreconcentrated solutions, Fig. 5. This very largesurface to volume ratio may make these cast filmsespecially useful for sensor applications. There is

(A)(A) (B)(B)

Fig. 3. Tapping mode AFM images of nCuPc(OBu)8 monolayers on hydrophilic mica substrate acquired in air. Image A represents a

3.5� 3.5 mm area. Image B displays a part of a bilayer of the complex on a 1500� 1500 nm scale. The fibers visible in the image appear

as twisted strands each about 20 nm wide and microns in length. The film thickness of each layer is about 1.5 nm.

K. Stevenson et al. / Ultramicroscopy 97 (2003) 271–278 275

very little fundamental work on the structuralorder within spin-coated films for comparison.Spin coating of asymmetric Pc derivatives contain-ing both polar and non-polar side chains producedfilms with multilayer structure analogous to thoseobtained from the LB technique [1,12]. These filmswere largely amorphous and contained crystallinedomains. Substantial ordering was introduced into

the multilayers by post-deposition annealing asdetermined by X-ray diffraction.The UV-visible spectrum of 10 LB layers of

pCuPc(OBu)8 shown in Fig. 6 is almost identicalto the spectrum of 3 nm thick vapor deposited filmof the unsubstituted parent compound, CuPc, interms of the position and the characteristicsplitting of the Q-band, 624 and 690 nm. The

Fig. 5. A 2� 2mm image of spin coated pCuPc(OBu)8 film on

mica prepared with 10�4M chloroform solution. The substrate

was exposed to the dissolved complex for 30 s and spun dry for

60 s. This high surface area rough bilayer film has an average

thickness of 2.5–3.0 nm.

Fig. 6. UV-visible spectra obtained from both parent CuPc

sublimed onto a glass substrate and pCuPc(OBu)8 and

nCuPc(OBu)8 LB films deposited on microscope slides. The

LB multilayers were prepared employing Z-type deposition

with a transfer ratio of 0.9270.05 for pCuPc(OBu)8, and a

transfer ratio of 0.8770.07 for nCuPc(OBu)8. The surface

pressures used in fabrication the samples were consisted with

the formation of monolayer films for each complex.

(A)(A) (B)(B)

Fig. 4. AFM tapping mode images of pCuPc(OBu)8 and nCuPc(OBu)8 films spin-coated on mica, A and B, respectively. The mica

substrates were exposed to 10�6M chloroform solutions of the compounds for 15 s and spun dry for 1min. Both images are

3.5� 3.5 mm in size. Films in graphs A and B are 1.3 and 1.57 nm thick, respectively. The inset in graph A is a 1.5� 1.5mm scan of the

pCuPc(OBu)8 and the fibers are 1.3 nm in thickness. The fiber bundles are 10 nm in diameter.

K. Stevenson et al. / Ultramicroscopy 97 (2003) 271–278276

CuPc polycrystalline film evaporated under va-cuum conditions is the a-type [29]. In this form thePc rings are stacked like a deck of cards andspaced 3.76 (A apart [9]. This molecular stacking isassisted by the Cu metal in one macrocycleinteracting with aza nitrogens on the adjacentPc ring [9]. Because of the close similarity of theabsorbance spectra of the CuPc film andpCuPc(OBu)8 LB monolayer the molecular orien-tation in these films is expected to be the essentiallysame. The peripheral butoxy substituents appearto have very little effect on the optical spectra ofthe CuPc complex. The absorption spectrum of thenCuPc(OBu)8 LB layers, on the other hand,displays a significant shift of the Q bands to thered. These results are consistent with reports thatfunctionalities in the peripheral positions bringabout only minor perturbation in the Q-bandenergy, whereas spectra of Pc derivatives withalkoxy groups located closest to the porphyrazinering, exhibit significant bathochromic shifts of thep-p* transition [1,11,26,30].

4. Conclusions

Both the peripherally and non-peripherallysymmetrically substituted CuPc(OBu)8 complexesproduce high quality well ordered LB films. Thecolumnar aggregation of these films is consistentwith the position of the polar side-chains on the Pcmacrocycle. The quality of the monolayers andtheir long-range ordering is unexpected for Pc withsuch relatively short ring substituents and noassisting terminal group interactions. Spin-coatedfilms of the butoxy Pc derivatives produced filmswith aggregate structure analogous to thoseobtained from the LB technique. Absorptionspectra of LB films of pCuPc(OBu)8 and nCuP-c(OBu)8 are consistent with literature UV-visibledata on octaalkoxy substituted Pc complexes. Weare presently working on obtaining STM of ourLB films both air-dried and annealed to obtainbetter-resolved structural information. We havegood evidence that the octabutoxy Pc LB andspin-doped films are suitable candidates forincorporation into M–I–M junctions [31].

Acknowledgements

This work was supported by the PetroleumResearch Fund (ACS-PRF#32317-AC6,5) and wegratefully acknowledge their assistance. We alsothank Tammy Oshiro for help with obtainingAFM images of the spin-cast films.

References

[1] N.B. McKeown, Phthalocyanine Materials: Synthesis,

Structure, and Function, Cambridge University Press,

Cambridge UK, 1998.

[2] P. Gregory, J. Porh. Phthal. 4 (2000) 432.

[3] R. Ao, L. Kilmmert, D. Haarer, Adv. Mater. 7 (1995) 495.

[4] Q. Chen, D. Gu, F. Gan, L. Xu, M. Li, Appl. Surf. Sci. 93

(1996) 151.

[5] D. W .ohrle, D. Meissner, Adv. Mater. 3 (1991) 129.

[6] A.B.P. Lever, M.R. Hempstead, C.C. Leznoff, W. Lui, M.

Melnik, W.A. Nevin, P. Seymour, Pure Appl. Chem. 58

(1986) 1467.

[7] C. Hamann, M. Hietschold, A. Marwa, M. Mueller, M.

Starke, R. Kilper, Top. Mol. Organ. Eng. 7 (1991) 129.

[8] M.K. Engel, Report Kawamura Institute of Chemical

Research, 1997, pp. 11–54.

[9] J. Simons, J.A. Andre, Molecular Semiconductors, Spring-

er, New York, 1985.

[10] N.B. McKeown, Chem. Ind. (1999) 92.

[11] M.J. Cooke, A.J. Dunn, M.F. Daniel, R.C.O. Hart, R.M.

Richardson, S.J. Foster, J. Chem. Soc. Perkin Trans. I

(1988) 2453.

[12] M.J. Cook, Pure Appl. Chem. 71 (1999) 2145.

[13] P.E. Smolenyak, E.J. Osburn, S.Y. Chen, L.K. Chau, D.F.

O’Brian, N.R. Armstrong, Langmuir 13 (1997) 6568.

[14] P.E. Smolenyak, R. Peterson, K. Nebesny, M. T +orker,

D.F. O’Brian, N.R. Armstrong, J. Amer. Chem. Soc. 121

(1999) 8628.

[15] M. V!elez, S. Viera, I. Chambrier, M.J. Cook, Langmuir 14

(1998) 4227.

[16] U. Mazur, K.W. Hipps, J. Phys. Chem. 98 (1994) 8169.

[17] X. Lu, K.W. Hipps, X.D. Wang, U. Mazur, J. Amer.

Chem. Soc. 118 (1996) 7197.

[18] X. Lu, K.W. Hipps, J. Phys. Chem. B 101 (1997) 5391.

[19] D. Barlow, K.W. Hipps, J. Phys. Chem. B 104 (2000) 5993.

[20] L. Scudiero, D.E. Barlow, K.W. Hipps, J. Phys. Chem. B

104 (2000) 11899.

[21] L. Scudiero, D.E. Barlow, U. Mazur, K.W. Hipps,

J. Amer. Chem. Soc. 123 (2001) 4073.

[22] N.B. McKeown, J. Mater. Chem. 10 (2000) 1979.

[23] B. Simic-Glavinski, A.A. Tanaka, M.E. Kenny, Y. Yeager,

J. Electroanal. Chem. 229 (1987) 285.

[24] H. Rengel, P. Gattinger, R. !Silerov!a (Beck), D. Naher,

J. Phys. Chem. B 103 (1999) 6822.

K. Stevenson et al. / Ultramicroscopy 97 (2003) 271–278 277

[25] A.B. Anderson, T.L. Gordon, M.E. Kenney, J. Amer.

Chem. Soc. 107 (1995) 192.

[26] A.N. Cammidge, I. Chambrier, M.J. Cook, A.D. Garland,

M.J. Heeney, K. Welford, J. Porh. Phthal. 1 (1997) 77.

[27] M.J. Cook, A.J. Dunn, M.F. Daniel, R.C.O. Hart,

R.M. Richardson, S.J. Foster, Thin Solid Films 159

(1988) 395.

[28] S. Foley, J. Chem. Soc. Perkin Trans. 2 (1997) 1725.

[29] J.J. Hoagland, J. Dowdy, K.WW. Hipps, J. Phys. Chem.

95 (1991) 2246.

[30] D. W .ohrle, V. Schmidt, J. Chem. Soc. Dalton Trans.

(1988) 549.

[31] K. Stevenson, N. Miyashita, J. Smieja, U. Mazur, to be

published.

K. Stevenson et al. / Ultramicroscopy 97 (2003) 271–278278