Towards an understanding of structure–property relationships in hole-transport materials: The influence of molecular conformation on oxidationpotential in poly(aryl)amines{

Paul J. Low,*a Michael A. J. Paterson,a Dmitry S. Yufit,a Judith A. K. Howard,a Julian C. Cherryman,b

Daniel R. Tackley,b Robert Brookc and Bev Brownc

Received 29th November 2004, Accepted 1st March 2005

First published as an Advance Article on the web 4th April 2005

DOI: 10.1039/b417962e

The influence of molecular conformation on the oxidation (ionisation) potential and electronic

structure associated with several TPD-style hole transport materials has been assessed through a

combination of single crystal X-ray diffraction, electrochemical and spectroelectrochemical

methods and DFT calculations. The introduction of methyl groups can be used to tune the

ionisation potential of these molecular species through a combination of electronic (inductive)

and thermodynamic effects, while the conformation of the biphenyl portion of the molecular

framework is found to play the greatest role in determining the Marcus-type reorganisation

energy associated with the charge transport process on the molecular level.

Introduction

Poly(aryl)amines are now widely used as hole-transport

materials in applications ranging from the Xerox process to

multi-layer organic light emitting diode based devices. In

order to maximise charge transport through a bulk sample

of these materials, it is necessary to minimise grain boundaries

caused by crystallisation in the layers of organic material.

In much work reported to date, there has therefore been a

heavy emphasis placed on the preparation and use of

amorphous polymeric compounds, and also on composite

materials derived from low molecular weight poly(aryl)amines

with an amorphous polymer support. Thus, while poly-

(aryl)amines such as TPD (1) have been the subject of

numerous experimental and computational studies, relatively

little is known about the molecular and solid-state structure

of even simple examples. Consequently, despite the intense

interest that is generated by these readily oxidised

organic materials, definitive structure–property relationships

remain elusive.

Several authors have used computational methods to

address the structure–property issues in TPD-derived mole-

cular materials, with particular emphasis on the geometric and

electronic changes which accompany oxidation/hole transport

phenomena.1–7 In the case of N,N,N9,N9-tetra(aryl)-1,19-

biphenyl-4,49-diamines, which form one of the most widely

employed classes of hole transport material the key conforma-

tional parameters (Fig. 1) can be considered to be: the torsion

angle between the ring systems of the biphenyl moiety, i.e. the

relative orientation of rings A and B (Fig. 1a); the relative

orientation of the NAr2 system with respect to the biphenyl

moiety i.e. the orientation of the Cx–N–Cy plane with respect

to ring A (Fig. 1b); the relative orientation of the peripheral

aromatic groups C with respect to the planar nitrogen centre

(Fig. 1c). Bond length variation should also be expected upon

oxidation, which will reflect the different localisation and

occupation of the HOMO in the neutral and hole-containing

radical cation (i.e. oxidised) form of the material.

We have recently embarked upon a study of poly(aryl)amine

based compounds with a view to obtaining experimental

evidence for how the electronic structure of TPD-related

compounds and important physical properties associated with

them, such as oxidation (or ionisation) potential and the

re-organisation energy associated with the charge transfer

process, l, are influenced by conformational changes in the

molecular structure. We considered that a firmer grasp of the

role molecular conformation plays on the molecular and bulk

properties of TPD-type systems would prove valuable in

understanding the properties of as-cast organic films of these

materials. With this goal in mind we have prepared a series of

compounds based upon the prototypical TPD framework in

which methyl groups are used to impose a degree of control

over the relative orientation of the various six-membered ring

systems. We report here the synthesis of several TPD-

derivatives and correlate the electrochemical response, the

electronic spectra of the compounds in each electrochemically

{ Electronic supplementary information (ESI) available: The opti-mised geometries of [1-H]n+–[9-H]n+ (n 5 0, 1) in chime format,together with plots of the HOMO (neutral molecules) and SOMO(oxidised species). See http://www.rsc.org/suppdata/jm/b4/b417962e/*p.j.low@durham.ac.uk

PAPER www.rsc.org/materials | Journal of Materials Chemistry

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accessible oxidation state and electronic structures with the

molecular structures of these compounds.

Results and discussion

Syntheses

The preparation of biaryl systems through reaction of two

molecules of an aryl halide in the presence of a copper source

was first developed by Ullmann and this C–C bond forming

protocol is now known as the Ullmann coupling. Despite the

uncertainty over the precise nature of the catalytically active

species the Ullmann coupling has been widely applied to the

synthesis of many symmetrically and unsymmetrically sub-

stituted biaryl systems.8 The extension of the Ullmann

coupling to the formation of C–N and C–O bonds via the

copper-catalysed reaction of an aryl amine or phenol with an

aryl halide has been termed the Ullmann condensation. While

the reaction is notoriously capricious, with side reactions such

as halide reduction and homocoupling often encountered, the

Ullmann condensation has been utilised for over 90 years with

considerable success. Recent modifications to the reaction

conditions include the addition of hydrolytically stable co-

ligands such as phenanthrolines and 2,29-bipyridyl when

copper(I) salts are utilised as the copper source, or the use of

18-crown-6/K2CO3 with copper powder.9 Of course, while

Ullmann protocols have been widely adopted, metal catalysed

aromatic C–N coupling reactions are not limited solely to

copper, and in the last ten years or so there have been many

interesting developments in this area, particularly with regard

to palladium-catalysed processes.10–20

The compounds 2 and 3 were prepared by Ullmann

condensation of 4,49-diiodo-1,19-biphenyl with diphenyl amine

and di(p-tolyl)amine, respectively, in refluxing ortho-dichloro-

benzene in the presence of potassium carbonate, copper

powder and 18-crown-6 (Scheme 1).

Compound 4, in which methyl groups at the 2-positions of

two peripheral rings are used to increase the pitch of these

rings relative to the biphenyl rings, was prepared in a similar

manner, as described previously.24

In the case of 5a and 5b the key reagent 4,49-diiodo-

3,39-dimethyl-1,19-biphenyl was prepared by diazotisation of

the corresponding diamine, which was subsequently coupled

with the appropriate diarylamine (Scheme 2).

The acidic methylene protons of fluorene were found to

complicate the Ullmann condensation reaction with diaryl-

amines, and consequently 4,49-diiododiethylfluorene was

employed in the preparation of the fluorene derivatives

6a and 6b,21 while carbazole was found to couple readily

with 4,49-diiodo-1,19-biphenyl to give 7 under comparable

conditions.

Compounds 8 and 9 were prepared from di(p-tolyl)amine

and 2,29-dimethyl-4,49-diiodo-1,19-biphenyl or 2,29,6,69-tetra-

methyl-4,49-diiodo-1,19-biphenyl, using Ullmann chemistry

entirely analogous to that indicated in Schemes 1 and 2.22

The products 2–9 were readily purified either by column

chromatography or simple recrystallisation of the reaction

mixture, and were fully characterised by the usual spectro-

scopic methods (see Experimental).

Fig. 1 The key torsional modes available to tetra(aryl)benzidenes.

Scheme 1 The synthesis of compounds 2 and 3 via the Ullmann

condensation.

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Molecular structures

Given the dearth of solid state structural data relating to the

tetra(aryl)benzidene motif, we were pleased to obtain crystals

of 5a, 6a and 7 suitable for X-ray diffraction analysis. In

keeping with the observations previously made on the solid

state structures of 1, 3, 4, 4[SbCl6] and 9a, the molecules 5a, 6a

and 7 were found to be packed into the crystal lattice without

any strong close intermolecular contacts or p-stacking motifs

in evidence.22–24 The crystals of 5a and 6a contained

disordered molecules of benzene and CH2Cl2, respectively,

filling channels formed between adjacent molecules.

Plots illustrating single molecules of 5a, 6a and 7 are given in

Fig. 2, with a comparative overlay of the molecular structures

provided in Fig. 3. Experimentally determined bond lengths

and important dihedral angles associated with 1, 3, 4, 5a, 6a, 7

and 9a are summarised in graphical format in Chart 1. The

introduction of methyl groups on the peripheral rings in

positions meta- or para- to the amine nitrogen centre have little

influence on the molecular geometry, and the geometries of 1

and 3 are similar. In each case, the twist angle between the

adjacent rings of the biphenyl moiety is approximately 35u.The DFT optimised geometries of 1 and 3 fail to reproduce the

distinct differences in orientation of the peripheral ring systems

relative to the plane defined by the N–C bonds (see below).

Since rotation of the aryl groups around the N–Caryl bonds are

relatively low energy processes, the precise geometry adopted

by the aromatic groups is probably a function of the packing

efficiency of the molecules in the solid state. However, the

geometry of 4 is remarkably well modelled by the calculation

for the gas-phase structure, suggesting that the orientation

of the 2,4-dimethylphenyl group is likely a property of the

molecule, rather than of the crystal lattice.

For 5a, the introduction of methyl groups on the biphenyl

moiety ortho- to the amine nitrogen centre (Fig. 2) results in

rotation of the diphenylamino group around the correspond-

ing N(1,2)–C(4,23) bond (Fig. 3). Obviously, this is the most

effective way to minimise unfavourable close contacts between

Scheme 2 The preparation of 5a and 5b.

Fig. 2 The molecular structure and labelling scheme of 5a (top), 6a

(middle) and 7 (bottom). Atomic displacement ellipsoids are drawn at

the 50% probability level and disordered solvent molecules of benzene

and CH2Cl2 in 5a and 6a, respectively, are omitted.

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the NPh2 moieties and the C(19) and C(38) methyl groups

whilst maintaining the propeller-like geometry around N(1)

and N(2). Curiously, the biphenyl group in molecule 5a is

almost planar, and the dihedral angle between the planes of the

adjacent aromatic rings is just 210.2u.The planar conformation of the central biphenyl group in

molecule 6a is enforced by the C(2)–C(19) bonds which form

the fluorene system (Fig. 2), and the molecule maintains the

usual propeller-like orientation of the aromatic rings about

the nitrogen centres. The same C(2)–C(19) bonds also restrict

the C(2)–C(1)–C(1A) bond angle in comparison with the other

molecules of this type (Chart 1, Fig. 3). The molecule 6a is

located in a special position in the crystal with a mirror plane,

which passes through the middle of the central bond, C(19)

and the ethyl groups, relating the two halves of the molecule.

Molecule 7 also occupies a special position in the crystal at

the centre of symmetry. The biphenyl fragment is remarkably

planar, the torsion angle around the central bond being 0.2u.The rigid carbazole groups formed by the C(12)–C(13) bond

are rotated around the N(1)–C(4) bond and give a torsion

angle of 46u with the plane of the bi-phenyl group, which is

similar to those found in other crystallographically charac-

terised tetraarylbenzidenes (Chart 1, Fig. 3).

Electrochemical response

The electrochemical response of 1–9 illustrate the marked

effect the substitution patterns play on the oxidation (ionisa-

tion) potential of these materials (Table 1). The parent

material 2 undergoes two sequential, one-electron oxidation

processes at E1/2 5 0.39 and 0.64 V (vs Fc/Fc+ 5 0.00 V). The

separation of the oxidation processes allows estimation of the

comproportionation constant (Kc) to be 18 000 and indicates

Fig. 3 Overlapped views of the molecules 5a, 6a and 7. Top:

view perpendicular to the biphenyl amine C–N bonds, atoms

of the symmetry related parts of the molecules and H

atoms omitted for clarity. Bottom: view perpendicular to the

mean plane of biphenyl fragment, H atoms also omitted.

Open lines correspond to molecule 5a, full lines to 6a and dashed

lines to 7.

Chart 1

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Table 1 Oxidation potentials and associated parameters with 1–9 (in CH2Cl2, 0.1M NBu4BF4, 100 mV s21, all platinum electrodes, vsFc/Fc+ 5 0.0 V

E1/2(1)/V E1/2(2)/V DE/V KC

0.29 0.51 0.22 4300

0.39 0.64 0.25 18 000

0.25 0.55 0.30 133 000

0.25 0.51 0.26 20 500

0.34 0.56 0.22 5900

0.35 0.47 0.12 100

0.28 0.65 0.37 1 955 000

0.12 0.47 0.35 970 000

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the significant thermodynamic stability of the radical cation 2+

with respect to disproportionation.25 These parameters pro-

vide a convenient data set against which to measure the

influence of the various structural modifications made in the

other compounds.

In the case of the established hole transport material 1, and

the related materials 3 and 4, the inductively donating methyl

groups around the peripheral rings result in the expected

decrease in the first [E1/2(1)] and second [E1/2(2)] oxidation

potentials (Table 1). Compounds 3 and 4, which contain

inductively electron donating methyl groups at the 4-positions

of the peripheral ring systems, have the lowest first oxidation

potentials in this subset. Thus, while the ortho- methyl groups

in 4 can influence the conformation of the molecular system in

the solid state, the influence of this substituent on the

electrochemical behaviour appears to be inconsequential.

When compared with the observed and calculated geome-

tries of TPD and closely related materials, the dominant

structural differences in the mono-oxidised states of these

molecules include a more planar biphenyl fragment, which is

co-planar with the N–C bonds, and an increase in the torsion

angles about the N–Caryl bonds such that the peripheral ring

systems adopt orientations which enhance the propeller

geometry about the nitrogen centres.6,24 The electrochemical

response of compounds featuring the generic tetra(aryl)benzi-

dene structure are therefore likely to be sensitive to the

introduction of groups at the 2 and 3 positions of the central

biphenyl moiety (compounds 5, 6, 8, 9).

The first oxidation potentials of 5a and 5b are similar to

each other, and more positive than E1/2(1) of 1, 3 or 4. It seems

that in 5a and 5b the C(19) methyl group, which hinders

rotation about the N(1)–C(4) bond and restricts conjugation of

the nitrogen lone pairs through the biphenyl moiety, necessi-

tates a greater thermodynamic driving force for oxidation than

is the case in the unrestricted systems 1, 3 and 4. The trend in

E1/2(2) values is more keeping with the expected inductive

effects (e.g. E1/2(2) 5b , 3), further supporting the hypothesis

that upon oxidation the peripheral aromatic systems adopt an

orientation more orthogonal to the central biphenyl group and

are therefore less affected by the steric influence of the C(19)

methyl group.

Similar trends are observed when methyl groups are used to

restrict the adoption of a planar conformation in the biphenyl

moiety (8, 9).22 Thus, despite the additional inductively

electron donating methyl groups at the 2 and 29 positions of

the 1,19-biphenyl fragment in 8 the first oxidation potential is

significantly higher than that of 3. The second oxidation

potential is broadly in keeping with the trends established for

the other compounds in Table 1.

The introduction of a fluorene moiety in 6a and 6b, which

confines the biphenyl moiety to be planar, results in a

significant decrease in the first oxidation potential relative to

the model materials 1 and 3. The inductive effect of the methyl

groups on the peripheral ring systems has the expected

influence on the first and second oxidation potentials of 6b

relative to those of 6a.

The electrochemical response of the carbazole derivative 7

was complicated by the relative insolubility of the radical

cation in the presence of common anions such as BF42 and

PF62. The most reproducible electrochemical results were

obtained from THF solutions containing 0.1 M NBu4BF4 as

supporting electrolyte. The initial voltammogram revealed two

1.03 (thf) 1.50 (thf) 0.470 —

0.44 0.59 0.15 300

0.46 — — —

Table 1 Oxidation potentials and associated parameters with 1–9 (in CH2Cl2, 0.1M NBu4BF4, 100 mV s21, all platinum electrodes, vsFc/Fc+ 5 0.0 V (Continued )

E1/2(1)/V E1/2(2)/V DE/V KC

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chemically irreversible oxidation processes at 1.03 and 1.50 V

(vs an internal Fc/Fc+ standard) and inspection of the working

electrode revealed an obvious film on its surface. These

chemical complications associated with 7 have been commen-

ted upon previously and we have been unable to find a

satisfactory combination of solvent and supporting electrolyte

to resolve these issues.26

Spectroelectrochemistry

Electronic spectroscopy (UV–vis–NIR) coupled with spectro-

electrochemical methods has proven to be a useful tool in the

analysis of the electronic structure of tetra(aryl)benzidenes and

closely related structures in various oxidation states27–29 and

the electronic spectra of 2n+, 3n+, 4n+ and 8n+ (n 5 0, 1, 2) have

been described in detail on previous occasions.22–24,30 The

monocations (n 5 1) derived from compounds 1–8 can be

regarded as organic ‘‘mixed-valence’’ compounds. Detailed

analyses of the NIR and/or Raman spectra of 2+, 3+, 4+ and

closely related systems which preserve the 4,49-diamino-

1,19-biphenyl substructure have concluded that these radical

cations lie at the border between Class II (valence trapped) and

Class III (delocalised) materials.24,27,30,31

The changes in the UV–vis–NIR spectral regions in the

series of compounds 1–8 that accompany oxidation to the

mono- and di-cations were followed using an OTTLE cell in

CH2Cl2 containing 0.1 M NBu4BF4 as supporting electro-

lyte.32 In general terms, oxidation of each compound to the

corresponding monocation was characterised by the shift of

the p–p* bands from 300–350 nm to 400–500 nm (Table 2) and

the appearance of a new spectral feature in the NIR region, the

energy and shape of which was sensitive to molecular structure

(Table 3). Further oxidation resulted in the collapse of the

monocation spectrum and the appearance of a new band

between 670–830 nm.

The spectroelectrochemical response of 5a was complicated

by the reactivity of the mono-oxidised form. While not

Table 2 UV–vis–NIR spectra [lmax/cm21 (e/M21dm23cm21)] of1–9n+ (in CH2Cl2, 0.1 M NBu4BF4)

Oxidation state/n

0 1 2

1 32 260 (24 280) 20 660 (28 000) 13 700 (74 800)28 300 (40 150) 71 40 (37 800) —

2 33 000 (33 100) 20 750 (26 100) 14 120 (49 300)28 500 (44 800) 7310 (27 300) —

3 33 100 (23 400) 20 660 (29 700) 12 750 (64 000)28 170 (36 600) 6820 (32 600) —

4 32 790 (24 280) 20 960 (28 000) 13 330 (56 590)28 570 (35 300) 7450 (30 680) —

5a 33 200 (40 200) — —29 950 (27 800) — —

5b 32 900 (34 450) 21 370 (11 000) 12 050 (56 600)29 400 (23 900) 14 620 (7230) —— 5260 (14 200) —

6a 31 850 (29 400) 20 200 (24 600) 14 900 (42 800)26 300 (42 200) 7520 (20 000) —

6b 31 650 (28 400) 20 700 (32 100) 12 820 (78 700)26 300 (40 200) 7520 (36 400) —

7 34 130 (38 300 — —31 550 (30 000) — —29 400 (19 600) — —

Ta

ble

3B

an

dsh

ap

ea

nd

der

ived

da

tafr

om

the

NIR

ba

nd

in1

+ –8

+(i

nC

H2C

l 2,

0.1

MN

Bu

4B

F4)

v max/c

m2

1e/

M2

1cm

21

v 1/2

(ob

s)/c

m2

1v 1

/2(H

TL

)/cm

21

v 1/2

(hig

h)

v 1/2

(lo

w)

v 1/2

(hig

h)/

v 1/2

(lo

w)

VA

B~

� nn ma

x

2/c

m2

1V

AB~

meg er

� nn ma

xV

AB~

0:0

20

5

r

�� e m

ax� nn 1=

2� nn m

ax

�� 1=2

/cm

21

17

14

03

78

00

30

80

40

50

37

20

24

40

1.5

23

57

01

90

01

89

02

73

10

27

30

03

00

54

10

03

54

02

47

01

.43

36

55

16

40

16

00

36

82

03

26

00

30

40

39

60

35

20

25

60

1.3

73

41

01

86

01

70

04

74

50

30

68

03

00

54

13

03

73

02

28

01

.63

37

25

—1

72

05

b5

26

04

72

00

32

00

34

70

37

20

26

80

1.3

92

63

01

30

01

85

06

a7

52

03

22

00

30

80

41

50

36

20

25

40

1.4

23

76

01

80

01

79

06

b7

52

03

64

00

30

85

41

60

35

30

26

40

1.3

43

76

01

78

01

90

08

53

50

57

60

03

22

03

50

03

64

02

80

01

.30

26

60

20

70

20

60

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apparent on the much shorter timescale of the cyclic

voltammetry experiment, [5a]+ proved to be reactive on the

longer timescales necessary for bulk electrolysis in the OTTLE

cell. Oxidative oligomerisation/polymerisation of triphenyl

amine based materials with unprotected para- positions has

been demonstrated on previous occasions.22,33–38 It is likely in

the present case that the chemical complications observed in

the spectroelectrochemical experiment are related to similar

coupling reactions.

According to the Hush model for weakly coupled (Class II)

systems, the electronic coupling parameter VAB can be deduced

from the band energy and shape of the HOMO-1ASOMO

transition. This absorption, which usually falls in the NIR

region, is often termed the ‘‘intervalence charge transfer’’, or

IVCT, band, and in the case of weakly coupled (Class II)

systems the IVCT transition corresponds to the photo-induced

intramolecular electron transfer process. In strongly coupled

(Class III) systems the optical absorption involves transitions

between delocalised molecular orbitals, and the IVCT nomen-

clature can be misleading, and the term ‘‘charge resonance

band’’ has been suggested as an alternative descriptor.39,40 In

practice, extraction of VAB from spectroscopic data using

traditional approaches requires accurate assignment of the

mixed-valence species to the correct Robin–Day class and a

reasonable estimate of the electron transfer distance, which can

be difficult when transitions which lead to the NIR absorption

band are delocalised over a significant portion of the molecular

framework.41 In this context it should be noted that Lambert

and colleagues have recently described an adaptation of the

semi-classical Marcus–Hush theory which allows calculation

of VAB in strongly coupled systems by numerical fitting of the

experimental spectrum without any assumption of the

electron-transfer distance.24,40

If we assume the radical cations 1+–8+ to lie on the Class II

side of the II–III borderline, the term VAB may be estimated

from eqn. (1), which makes no implicit assumption about the

shape of the NIR band. In eqn. (1), meg is the transition

moment (in C m) between the ground and excited states and

can be calculated (in Debye, 1 D 5 3.336 6 10230 C m) from

the integrated area under the curve from eqn. (2), vmax is the

transition energy, r is the distance between the diabatic redox

centres and e is the elementary charge.

VAB~meg

er�nnmax (1)

meg~0:09584

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiÐe(�nn)d�nn

�nnmax

s(2)

The N…N distance in the crystallographically determined

structure of the SbCl62 salt of 4+ was used to estimate r (9.89 A),

although the delocalised nature of the orbitals involved in the

transition together with very recent work by Nelson suggest that

such an approximation leads to an overestimation of the

adiabatic ET distance.41, 42 The calculated VAB terms calculated

using this approach are in good agreement with the values

calculated from the Hush relationship summarised by eqn. (3).

VAB~0:0205

r

� � ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiemax�nn1=2�nnmax

p(3)

Alternatively, if the radical cations are strongly coupled

with a vanishingly small thermal barrier to electron transfer

(i.e. the system is delocalised, Class III) then VAB is

determined from the band energy by the simple relationship

described in eqn. (4). There is an increasing body of experi-

mental support for the description of radical cations derived

from simple TPD type systems in terms of a delocalised

structure.24,30,40

VAB~�nnmax

2(4)

The shapes of the NIR bands from spectroelectrochemically

generated 1+–8+ (in CH2Cl2/0.1 M NBu4BF4) and the VAB

terms which may be calculated from eqns. (1), (3) and (4) are

summarised together with the appropriate parameters in

Table 3.

The value of VAB calculated from eqn. (4) for 1+ and 4+ is

only ca. 10% larger than that calculated using Lambert’s semi-

empirical method (1+ 3200 cm21; 4+ 3300 cm21), and therefore

eqn. (4) provides a reasonable estimate for this parameter. The

data collected in Table 3 reveal some interesting trends in

the value of VAB as a function of molecular structure. For the

mono-oxidised forms of compounds 1, 2, 3, 4 and 6a, 6b the

value of VAB calculated using eqn. (4) is essentially the same

(3650 cm21 ¡ 10%), although much lower values of the

coupling term are associated with 5b+ and 8+ (ca. 2650 cm21).

These experimental observations are consistent with con-

clusions drawn from computational work performed earlier by

others, and indicate that the electronic properties of these

redox-active tetraphenyl benzidenes are more heavily depen-

dent upon the planarity of the biphenyl core and the

conjugation of the nitrogen lone pair with the biphenyl

p-system than the orientation of the peripheral ring systems.

However, based on the much higher first oxidation potential of

the carbazole derivative 7 we are reluctant to totally neglect the

contribution of the peripheral rings to the overall stability of

the mono-oxidised forms of these materials.

Molecular modelling

Geometry optimisation (see the electronic supplementary

information (ESI) for 3D structures{) and electronic structure

calculations were undertaken for each compound 1–9 using

DFT methods (see Experimental). We designate the computa-

tional model systems as ‘‘-H’’ in order to distinguish them

from the physical samples. The calculated geometries are in

good agreement with the experimentally determined para-

meters, and the differences computed to occur following

oxidation are in broad general agreement with previous

computational4–6 and experimental studies.24

The role of the methyl substituents on the biphenyl ring

systems on the crystallographically determined structure 5a are

clearly reproduced in the calculated geometries of 5a-H and

5b-H. In the case of 8, for which no experimental structural

data are available, the calculated structure (8-H) indicates

that the 2,29-dimethyl substitution pattern imposes an almost

orthogonal orientation of the biphenyl rings, which is in full

agreement with the experimentally determined geometries of

other 2,29-dimethyl biphenyl compounds.43–45 The steric

influence of the 2,29-dimethyl groups is still evident in the

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calculated geometry of [8-H]+ with the torsion angle between

the rings of the biphenyl moiety being as large as 52u.The calculated first ionisation potentials (IPs) associated

with 1-H–9-H are given in Table 4. In the case of 1-H, the IP

value of 5.38 eV obtained from our 6-31G-BPW91-DFT

results may be compared with the value of 5.73 eV from

Bredas’ earlier 6-31G**-B3LYP based calculations,6 and the

experimental value of 5.38 ¡ 0.15 eV for 1.46 Although the

computational work neglects solvation factors there is an

essentially linear correlation between the calculated IP values

and the first oxidation potential of these materials, which

lends weight to the qualitative arguments developed in the

analysis of the electrochemical response based upon structural

variation.

One of the critical parameters involved in the description of

the hole-transport process based on a Marcus-type electron

transfer reaction is the reorganisation energy, l, associated

with the change in molecular geometry which accompanies the

transfer of charge. The reorganisation energy can be estimated

from the sum of the stabilisation energy achieved by geometry

relaxation of the radical cation and neutral form of the

material (i.e. the sum of the difference in energies associated

with the material in neutral and radical cation electronic

configuration in cation and neutral geometry).5,6 Where direct

comparisons are possible, the calculated reorganisation energy

from the BPW1-DFT calculations is in good agreement with

the data reported from AM1(UHF) based calculations (2:

l 5 0.14 eV (this work) vs. 0.132 eV (AM1(UHF)7), but

considerably lower than the B3LYP estimated value (1: l 5

0.13 eV (this work) vs. 0.29 eV (B3LYP)6), reflecting the

sensitivity of the absolute value of the calculated parameters to

the computational method employed.

Within the data series for our compounds, the lowest values

of l calculated from the BPW1 determined geometries are,

unsurprisingly, associated with the rigid molecular framework

of 7, and the triphenylamine-like compound 9. The introduc-

tion of the 3,39 methyl groups on the biphenyl moiety does

not have a significant influence on the reorganisation energy

associated with the oxidation of [5a/b-H], which can be

attributed to the lowest energy conformation of [5a/b-H]+

being one which avoids steric interaction between the peri-

pheral ring system and these groups.

The reorganisation energy of 6a is curiously high compared

with that of 6b, and other compounds in the series. Given the

significantly greater experimental oxidation potentials of 6a

relative to 6b this influence seems to be of real physical

significance and not an artefact of the calculations, although

the origin is not clear at present. The largest reorganisation

parameter in the series investigated here is associated with 8,

in which the 2,29-methyl groups restrict the attainment of a

planar conformation in the biphenyl portion of the molecule.

Therefore, the present work, which has investigated a

significant number of TPD derivatives, supports the conclu-

sions reached in Malagoli and Bredas’ study of TPD, and

indicates that the conformation of the biphenyl moiety is

the dominating molecular property in determining the relative

gas-phase reorganisation energy in these tetra(aryl)benzidene-

based hole-transport materials.

Conclusion

A combination of single crystal X-ray diffraction, electro-

chemical and spectroelectrochemical measurements and DFT

based calculations have been used to interrogate the impact of

molecular conformation on oxidation (ionisation) potential,

electronic structure and reorganisation barriers to the charge

transfer process in TPD-type hole transport materials. The

oxidation (ionisation) potentials of TPD-style materials may

be tuned not only through inductive effects, but also through

the influence of Marcus-type factors, with the conformation

of the biphenyl moiety playing a significant role in determining

the reorganisation energy associated with the hole-transfer

process on the molecular level.

Experimental

Instrumentation

Mass spectra were recorded on a Micromass Autospec

instrument operating in EI mode. NMR spectra were recorded

from CDCl3 solutions on Varian XL-200, Varian Unity-300

or Varian VXR-400 spectrometers. All chemical shifts are

reported in d (ppm), referenced against solvent resonances.

UV–vis–NIR spectra were recorded on a Varian Cary-5

spectrophotometer, electrochemical measurements were made

with an AutoLab PGSTAT30 from CH2Cl2 solutions contain-

ing 0.1 M NBu4BF4 as supporting electrolyte and potentials

cited are referenced against ferrocene such that the ferrocene

couple falls at 0 V. Spectroelectrochemical experiments were

Table 4 Calculated energies (in eV), ionisation potentials and reorganisation energies for compounds 1-H–9-H

Compound E(neu)/eV E(cat-scf)/eV E(cat)/eV E(neu-scf)/eV E1/2/V IP/eV l/eV

1-H 0 5.44 5.38 0.066 0.29 5.44 0.132-H 0 5.51 5.45 0.071 0.39 5.51 0.143-H 0 5.29 5.23 0.062 0.25 5.29 0.124-H 0 5.42 5.34 0.075 0.25 5.42 0.155a-H 0 5.58 5.52 0.076 0.34 5.58 0.145b-H 0 5.36 5.30 0.063 0.35 5.36 0.126a-H 0 5.41 5.33 0.093 0.28 5.41 0.186b-H 0 5.19 5.13 0.066 0.12 5.19 0.147-H 0 6.04 6.01 0.037 1.03 6.04 0.078-H 0 5.39 5.26 0.075 0.44 5.39 0.209-H 0 5.35 5.31 0.045 0.46 5.35 0.09a E(neu) energy of the neutral molecule in the neutral geometry, chosen to be the reference point; E(cat-scf) energy of the cation in the neutralgeometry; E(cat) energy of the cation in the cation geometry; E(neu-scf) energy of the neutral molecule in the cation geometry

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also carried out in 0.1 M NBu4BF4/CH2Cl2 solutions using an

OTTLE cell,32 fitted with a Pt gauze mesh semi-transparent

working electrode, and Pt wire counter and pseudo-reference

electrodes, and a home-built potentiostat.

All synthetic reactions were carried out under a nitrogen

atmosphere in flame-dried glassware. Solvents were dried from

an appropriate agent and distilled. Compounds 2,29-dimethyl-

4,49-diiodobiphenyl, 2,29,6,69-tetramethyl-4,49-diiodobiphenyl,

3,39dimethyl-4,49-diiodobiphenyl,47,48 9,99-diethyl-2,7-diiodo-

fluorene,49 3, 4, 8 and 922,24 were prepared by literature

methods. TPD (1) was obtained from Avecia Ltd. All other

reagents were purchased from commercial sources and used

as received.

General procedure. In our hands, the Ullmann condensation

reactions proceeded most readily and consistently when

employing 18-crown-6 as a solubilising agent for the active

copper species and ortho-dichlorobenzene as solvent.9 In a

typical procedure, a rigorously dried two-necked 100 ml

round-bottomed flask was fitted with a magnetic stir bar,

and a condenser topped with a nitrogen gas inlet. The

apparatus was purged with dry nitrogen, and charged with

ortho-dichlorobezene (10 ml), the appropriate di(aryl)amine

(5.5 mmol) and diiodobiaryl (2.5 mmol), K2CO3 (20 mmol),

copper powder (5.0 mmol) and 18-crown-6 (0.5 mmol). The

reaction mixture was rapidly heated at the reflux point of the

solvent (ca. 180 uC) and maintained at that temperature for

12–24 h. After this time, the reaction mixture was cooled

and filtered to remove the inorganic residues. The filtrate was

reduced in volume under vacuum, and the crude product

purified by column chromatography or precipitation and

recrystallisation. Satisfactory elemental analyses were obtained

for all compounds.

N,N,N9,N9-Tetraphenyl-1,19-phenyl-4,49-diamine (2).

Diphenylamine (0.916 g, 5.43 mmol), diiodobiphenyl (1.0 g,

2.46 mmol), K2CO3 (2.72 g, 19.68 mmol), copper powder

(0.34 g, 5.3 mmol) and 18-crown-6 (0.13 g, 0.49 mmol) were

dissolved in ortho-dichlorobenzene (10 ml) and heated at reflux

for 18.5 h. The crude reaction mixture was diluted with MeOH

to precipitate the crude product which was collected, dried and

recrystallised from CH2Cl2 to afford the pure title compound

(0.68 g, 56%). 1H NMR d 7.48 (d, JHH 5 9 Hz, 4H), 7.16

(pseudo-t, JHH 5 15 Hz, 8H), 7.03 (d, JHH 5 9 Hz, 8H), 7.03

(d, JHH 5 9 Hz, 4H), 6.93 (t, JHH 5 14 Hz, 4H). 13C NMR d

147.71, 146.73, 134.74, 129.25, 127.29, 124.30, 124.08, 122.81.

EI MS m/z 488 [M]+. mp 220–222u

N,N,N9,N9-Tetraphenyl-(3,39-dimethyl)-(1,19-biphenyl)-

4,49-diamine (5a). The compounds 3,39-dimethyl-4,49-diiodo-

biphenyl (1.0 g, 2.3 mmol), diphenylamine (0.92 g, 5.75 mmol),

K2CO3 (2.54 g, 18.43 mmol), copper powder (0.31 g,

4.94 mmol), and 18-crown-6 (0.12 g, 0.46 mmol) were

dissolved in ortho-dichlorobenzene (10 ml) and heated to

reflux for 18 h. The solution was filtered and the solvent

removed in vacuo, the residue was dissolved in MeOH and

placed in the freezer, producing a pale yellow precipitate which

was purified by column chromatography (silica, hexanes–

CH2Cl2) (0.42 g, 36%). 1H NMR d 7.52 (d, JHH 5 2 Hz, 2H),

7.47 (dd, JHH 5 8 Hz, 2H), 7.26 (pseudo-t, apparent JHH 5

8 Hz, 8H), 7.21 (d, JHH 5 8 Hz, 2H), 7.05 (d, JHH 5 8 Hz, 8H),

6.97 (t, JHH 5 14 Hz, 4H), 2.12 (s, 6H, CH3). 13C NMR d

147.48, 144.65, 138.03, 136.53, 130.21, 129.75, 129.07, 125.83,

121.69, 121.51, 18.83. EI MS m/z 516 [M]+, 349 [M-NPh2]+

mp 174–176u

N,N,N9,N9-Tetra(4-methylphenyl)-(3,39-dimethyl)-1,19-

biphenyl-4,49-diamine (5b). A solution of 3,39-dimethyl-4,49-

diiodobiphenyl (1.0 g, 2.3 mmol), di(4-methylphenyldiamine

(1.0 g, 5.07 mmol), K2CO3 (2.54 g, 18.43 mmol), copper

powder (0.31 g, 4.94 mmol), and 18-crown-6 (0.12 g,

0.46 mmol) in ortho-dichlorobenzene was heated at reflux for

27 h. The solution was filtered and the solvent removed

in vacuo, the residue was recrystallised from MeOH in the

freezer. The yellow precipitate was purified by flash chroma-

tography (SiO2, CH2Cl2–hexane 1 : 9), (0.43 g, 33%). 1H NMR

d 7.38 (m, 2H), 7.19 (m, 4H), 6.88 (m, 16H), 2.22 (s, 12H,

CH3), 2.00 (s, 6H, CH3). 13C NMR d 145.35, 144.97, 137.61,

136.19, 130.72, 130.04, 129.59, 129.35, 125.62, 121.69, 20.68,

18.91. EI MS m/z 572 [M]+, 480 [M-Ar]+, 377 [M-NAr2]+.

mp 170–172u.

9,9-Diethyl-2,7-diiodofluorene. To a stirred solution of 9,9-

diethylfluorene (5.5 g, 24.7 mmol) and finely divided I2 (6.92 g,

27.3 mmol) in acetic acid (100 ml), a mixture of red fuming

nitric acid (2 ml) and concentrated sulfuric acid (10 ml) was

added dropwise. The mixture was stirred for 30 min then

poured into H2O (700 ml), neutralised with a sodium hydrogen

carbonate solution and the organics extracted with Et2O. The

solution was washed with a sodium thiosulfate solution, dried

(MgSO4) and the solvents removed in vacuo, producing an off

white solid. (3.56 g, 30%). 1H NMR d 7.49 (m, 2H, Ar), 7.26

(m, 2H, Ar), 7.11 (s, 2H, Ar), 1.82 (q, JHH 5 20 Hz, 4H, CH2),

0.16 (t, JHH 5 16Hz, 6H, CH3). 13C NMR d 150.79, 140.35,

136.12, 132.45, 121.69, 93.04, 56.30, 32.72, 8.27. EI MS m/z

474 [M]+, 347 [M-I]+, 222 [M-2I]+

N,N,N9,N9-Tetraphenyl-9,9-diethyl-2,7-diaminofluorene

(6a). A solution of 9,9-diethyl-2,7-diiodofluorene (1.5 g,

3.15 mmol), diphenylamine (1.17 g, 6.93 mmol), K2CO3

(3.48 g, 25.21 mmol), copper powder (0.43 g, 6.77 mmol),

and 18-crown-6 (0.17 g, 0.63 mmol) in ortho-dichlorobenzene

was heated at reflux for 18 h. The solution was filtered and the

solvent removed in vacuo. Purified by chromatography (silica,

hexanes–CH2Cl2) (0.531 g, 30%). 1H NMR d 7.0 (m, 26H),

1.72 (q, JHH 5 21 Hz, 4H), 0.29 (t, JHH 5 14 Hz, 6H). 13C

NMR d 151.28, 146.48, 144.32, 130.55, 129.86, 129.11, 127.72,

123.84, 122.33, 119.54, 56.25, 32.55, 8.56. EI MS m/z 556 [M]+,

404 [M-2Ph]+, 389[M-N Ph2]+. mp 191–193u

N,N,N9,N9-Tetra(4-methylphenyl)-9,9-diethyl-2,7-diamino-

fluorene (6b). A solution of 9,9-diethyl-2,7-diiodofluorene

(1.5 g, 3.15 mmol), di(4-methylphenyl)amine (1.37 g, 6.93 mmol),

K2CO3 (3.48 g, 25.21 mmol), copper powder (0.43 g,

6.77 mmol), and 18-crown-6 (0.17 g, 0.63 mmol) in ortho-

dichlorobenzene was heated at reflux for 18 h. The solution

was filtered and the solvent removed in vacuo, the residue was

recrystallised from MeOH at 220 uC (0.87 g, 45%). 1H NMR d

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7.0 (m, 22H), 2.2(s, 12H), 1.71 (q, JHH 5 20 Hz, 4H), 0.29 (t,

JHH 5 20 Hz, 6H). 13C NMR d 151.28, 145.72, 142.58, 132.80,

130.56, 130.02, 127.97, 124.56, 121.69, 56.43, 32.81, 21.04,

18.31, 8.83. EI MS m/z 612 [M]+, 417 [M-NAr2]+. mp 187–189u

4,49-Dicarbazole-1,19-biphenyl (7). Carbazole (0.90 g,

5.42 mmol), diiodobiphenyl (1.0 g, 2.46 mmol), K2CO3

(2.72 g, 19.7 mmol), copper powder (0.43 g, 6.77 mmol), and

18-crown-6 (0.13 g, 0.49 mmol) were dissolved in ortho-

dichlorobenzene and heated to reflux for 19.5 h. The solution

was filtered and the solvent removed in vacuo, the residue

was recrystallised from MeOH at 220 uC (0.94 g, 79%). 1H

NMR d 8.18 (d, JHH 5 8 Hz, 4H), 7.93 (d, JHH 5 8 Hz, 4H),

7.73 (d, JHH 5 8 Hz, 4H), 7.53 (d, JHH 5 8 Hz, 4H), 7.47

(pseudo-t, apparent JHH 5 15 Hz, 4H, Hc), 7.33 (t, JHH 5

15 Hz, 4H). 13C NMR d 140.82, 139.30, 137.25, 128.52, 127.49,

126.02, 123.50, 120.38, 120.08, 109.83. EI MS m/z 484 [M]+.

mp 270–272u

Crystallographic details

The data were collected at 120(1) K on a Bruker SMART

CCD 6000 (Bruker SMART CCD 1K for 5a) diffractometer

(v-scan, 0.3u per frame) using Mo Ka radiation (l 5 0.71073 A).

No absorption correction was applied. The structures were

solved by direct method and refined by full-matrix least

squares on F2 for all data using SHELXTL software. All non-

hydrogen atoms (except disordered ones) were refined with

anisotropic displacement parameters, H atoms in structure 7

were located on the difference map and refined isotropically, in

the structures 5a and 6a H atoms were placed into ideal

calculated positions and refined in ‘‘riding-mode’’.

Crystal data for 5a. C38H32N2?C6H6, M 5 594.76, triclinic,

space group P1, a 5 10.1614(3), b 5 10.6110(4), c 5

16.4871(5) A, a 5 83.65(1), b 5 78.61(1), c 5 71.11(1)u,U 5 1646.7(1) A3, F(000) 5 632, Z 5 2, Dc 5 1.200 mg m23,

m 5 0.069 mm21. 19 694 reflections ( 2.15 ¡ h ¡ 28.5u) were

collected yielding 8276 unique data (Rmerg 5 0.048). Final

wR2(F2) 5 0.1704 for all data (412 refined parameters),

conventional R(F) 5 0.0522 for 6180 reflections with I . 2s,

GOF 5 1.064. The largest peak on the residual map (0.45 e A23)

is in the disordered benzene molecule area.{

Crystal data for 6a. C41H36N2?0.5CH3Cl, M 5 580.45,

orthorhombic, space group Pnma, a 5 8.4577(2), b 5

19.9091(5), c 5 19.2392(5) A, U 5 3239.6(1) A3, F(000) 5

1230, Z 5 4, Dc 5 1.190 mg m23, m 5 0.108 mm21. 20 716

reflections ( 2.05 ¡ h ¡ 26.0u) were collected yielding 3282

unique data (Rmerg 5 0.067). The H atom of the disordered

solvent CH3Cl molecule could not be located reliably and was

not included in the refinement. Final wR2(F2) 5 0.1600 for all

data (234 refined parameters), conventional R(F) 5 0.0522 for

2295 reflections with I . 2s, GOF 5 1.099. The largest peak

on the residual map (0.52 e A23) is located in the vicinity of the

disordered chlorine atoms.

Crystal data for 7. C36H24N2, M 5 484.57, monoclinic,

space group P21/c, a 5 8.0120(4), b 5 16.0080(7), c 5

10.2428(5) A, b 5 110.19(1)u, U 5 1232.9(1) A3, F(000) 5 508,

Z 5 2, Dc 5 1.305 mg m23, m 5 0.076 mm21. 7634 reflections

(2.47 ¡ h ¡ 29.0u) were collected yielding 3249 unique data

(Rmerg 5 0.062). Final wR2(F2) 5 0.0874 for all data

(220 refined parameters), conventional R(F) 5 0.0458 for

1739 reflections with I ¢ 2s, GOF 5 0.933. The largest peak

on the residual map is 0.20 e A23.

Computational work

All calculations were run using the Gaussian 98 or Gaussian 03

programs. For each molecule, the geometry was optimised for

both the neutral and radical cation states at a DFT level using

the BPW91 functional (Becke’s 1988 exchange functional,50

with Perdew and Wang’s 1991 gradient-corrected correlation

functional51) in conjunction with the 6-31G(d,p) basis set.52–56

Where possible, molecular symmetry constraints were used to

simplify the calculations and to aid the interpretation of the

results. Post-processing for visualisation of the molecular

orbitals generated by the DFT calculations was performed

using the Molekel programme.57

Acknowledgements

This work was supported by the University of Durham, the

EPSRC, ONE-North East and Avecia Ltd. MAJP held an

EPSRC/Avecia Ltd CASE award through the CASE for

New Academics programme (PJL).

Paul J. Low,*a Michael A. J. Paterson,a Dmitry S. Yufit,a

Judith A. K. Howard,a Julian C. Cherryman,b Daniel R. Tackley,b

Robert Brookc and Bev Brownc

aDepartment of Chemistry, University of Durham, South Road, Durham,UK DH1 3LE. E-mail: p.j.low@durham.ac.uk;Fax: +(44) (0)191 384 4737; Tel: +(44) (0)191 334 2114bIntertek ASG, PO Box 42, Hexagon House, Blackley,Manchester M9 8ZS, UKcAvecia Ltd., Hexagon House, Blackley, Manchester M9 8ZS, UK

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