The molecular structures and electrochemical response of "twisted" tetra(aryl)benzidenes

Article abstract of DOI:10.1039/b404731a

The compounds N,N,N′,N′-tetra(4-methylphenyl)-(1,1′- biphenyl)-4,4′-diamine (4), N,N,N′,N′-tetra(4-methylphenyl)- (2,2′-dimethyl)-(1,1′-biphenyl)-4,4′-diamine (5a) and N,N,N′,N′-tetra(4-methylphenyl)-(2,2′,6,6′-tetramethyl) -(1,1′-biphenyl)-4,4′-diamine (

Full text of DOI:10.1039/b404731a

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A R T I C L E
The molecular structures and electrochemical response of
‘
‘twisted’’ tetra(aryl)benzidenes
a
Paul J. Low,* Michael A. J. Paterson, Andr e´ s E. Goeta, Dmitry S. Yufit,
a
a
a
a
b
b
Judith A. K. Howard, Julian C. Cherryman, Daniel R. Tackley and Bev Brown
b
a
Department of Chemistry, University of Durham, South Road, Durham, UK DH1 3LE.
E-mail: p.j.low@durham.ac.uk
b
Avecia Ltd., PO Box 42, Hexagon House, Blackley, Manchester, UK M9 8ZS
Received 30th March 2004, Accepted 28th May 2004
First published as an Advance Article on the web 28th June 2004
The compounds N,N,N’,N’-tetra(4-methylphenyl)-(1,1’-biphenyl)-4,4’-diamine (4), N,N,N’,N’-tetra(4-
methylphenyl)-(2,2’-dimethyl)-(1,1’-biphenyl)-4,4’-diamine (5a) and N,N,N’,N’-tetra(4-methylphenyl)-
(
2,2’,6,6’-tetramethyl)-(1,1’-biphenyl)-4,4’-diamine (6a) undergo two reversible one electron oxidations.
The first oxidation potential increases in the order 4 v 5a v 6a, while the separation between the first
and second oxidation decreases in the reverse order 4 (0.30 V) w 5a (0.16 V) w 6a (0.00 V), reflecting
1
1
1
the decreasing thermodynamic stability of the radical cations [4 ] w [5 ] w [6a ]. Electronic spectroscopy
and spectroelectrochemistry (UV-Vis-NIR) confirm expectations, and the introduction of methyl groups
at the 2,2’ and 6,6’ positions of the 1,1’-biphenyl moiety electronically decouple the arylamine moieties.
In contrast, N,N,N’,N’-tetra(phenyl)-(2,2’-dimethyl)-(1,1’-biphenyl)-4,4’-diamine (5b) and N,N,N’,N’-
tetra(phenyl)-(2,2’,6,6’-tetramethyl)-(1,1’-biphenyl)-4,4’-diamine (6b) give much less kinetically stable
radical cations upon oxidation, which oligomerise/polymerise through the 4 positions of the
N-phenyl groups via a step-growth process. The molecular and crystal structures of 4 and 6b are
also reported.
Introduction
Tetra(aryl)benzidenes, such as TPD (1) represent a basic
structural motif often found in hole transport materials which
are used in a wide variety of applications, ranging from
1
photocopiers to electroluminescent display devices. These
small molecular materials are often incorporated into multi-
layer devices as dispersions in an inert polymer matrix in order
to reduce problems associated with crystallisation during
device fabrication or device operation. Alternatively, linear
or star shaped oligomers, dendrimers and polymeric polyaryl-
amine derivatives may be used, as these relatively high
molecular weight materials are more resistant to crystallisation
than smaller molecular analogues, and have better electronic
properties which may be attributed to the modified interchain
2
interactions, yet typically remain sufficiently soluble for ready
processing. Hence, there is considerable contemporary interest
not only in the synthesis of these polyarylamine based materials
but also in the various molecular structure–property relation-
ships which govern the oxidation (or ionisation) potentials of
the resulting materials and the intermolecular interactions
which occur in the solid state.
Polymeric polyarylamines are usually prepared from Ull-
3
mann-style coupling reactions, or nickel catalysed amine–aryl
4
halide cross coupling reactions. In either case, any metal
catalyst residue must be removed from the product polymer
before it can be employed in electronics based applications. The
oxidative dimerisation of triphenylamine (2) has long been
5
known, and Dunsch et al. have reported the polymerisation of
triphenylamine by anodic oxidation at high current densities in
6
non-polar solvents. In addition, oxidation of a small number
of diphenylamines has been shown to afford oligomeric mate-
7
rials in low yield, More recently, Lambert and N o¨ ll, as well as
Leung’s team, have demonstrated that bis-triarylamines spanned
by a variety of conjugated bridges can be polymerised electro-
chemically to give conjugated poly{tetra(aryl)benzidene}
2
5 1 6
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 2 5 1 6 – 2 5 2 3
T h i s j o u r n a l i s ß T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

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˚
Table 1 Selected bond lengths (A), bond angles and torsion angles (u)
for compounds 4 and 6b
4
6b
N(1)–C(4)
N(1)–C(7)
N(1)–C(14)
C(1)–C(2)
C(2)–C(3)
C(3)–C(4)
C(4)–C(5)
C(5)–C(6)
1.410(1)
1.426(1)
1.429(1)
1.398(2)
1.391(2)
1.397(2)
1.401(2)
1.383(2)
1.405(2)
1.480(2)
119.52(9)
120.03(9)
116.51(9)
236.0(1)
268.2(1)
149.9(2)
1.428(1)
1.411(2)
1.424(2)
1.404(2)
1.397(2)
1.390(2)
1.390(2)
1.395(2)
1.404(2)
1.500(2)
120.1(1)
117.9(1)
120.8(1)
35.6(2)
C(6)–C(1)
C(1)–C(1A)
C(4)–N(1)–C(7)
C(4)–N(1)–C(14)
C(7)–N(1)–C(14)
C(4)–N(1)–C(7)–C(12)
C(4)–N(1)–C(14)–C(15)
C(2)–C(1)–C(1A)–C(2A)
39.7(2)
298.3(2)
more thoroughly the influence of molecular conformation on
the properties of these systems.
Fig. 1 A graphical representation of the DFT calculated Semi-
Occupied Molecular Orbital (SOMO) of the triphenylamine radical
Polyarylamines and derivatives are most often prepared via
Ullmann condensation protocols, which, in the most general
form, involves copper catalysed condensation of an unacti-
vated aryl halide with a primary or secondary arylamine in the
1
cation, 2 .
8
,9
polymers. These results have prompted us to disclose our
own work on the structural, electrochemical and spectroscopic
properties of tetra(aryl)benzidenes in which the conjugation
between the two triarylamine moieties is disrupted through
the introduction of methyl groups into the common biphenyl
moiety.
3
presence of some additional base. The reference compound 4
was readily prepared in good yield from the Ullmann-style
condensation of 4,4’-diiodobenzene and di(p-tolyl)amine in the
presence of K
dimethylbiphenyl was diazotised, and converted to the corres-
ponding diiodide by treatment with aqueous KI , followed by
2 3
CO . Commercially available 4,4’-diamino-2,2’-
3
copper catalysed condensation with ditolylamine and diphenyl-
amine to give 5a and 5b in good yield. A similar sequence
beginning with 4,4’-diamino-2,2’,6,6’-tetramethylbiphenyl gave
6a and 6b.
Results and discussion
The distribution of the SOMO of triphenylamine radical cation
10,11
1
?
1?
([2] ), calculated using the BPW91 method
[
conjunction with the 6–31G(d,p) basis set in Gaussian 98,
NPh
3
]
in
2,13
Despite the great interest in tetra(aryl)benzidenes and related
systems as hole transporting materials, there is remarkably
little solid state data relating to the molecular structures of
1
1
4
and visualised using the Molekel program, suggests that an
appreciable amount of the unpaired electron character resides
on the aromatic ring carbons para to the nitrogen centre
1
6,17
these compounds.
omethane solvate 1 : 1) and 6b suitable for X-ray diffraction
were obtained by slow evaporation of CH Cl (4) or THF (6b)
Single crystals of 4 (in form of dichlor-
(
Fig. 1), and is clearly the underlying electronic basis for the
2
2
5
observed chemical reactivity of this species. In contrast, the
SOMO of radical cations derived from tetra(aryl)benzidenes
are delocalised over the benzidene-like central portion of the
molecule, which lends significantly greater chemical stability to
solutions. The molecular structure of 4 is illustrated in Fig. 2,
and selected bond lengths and angles are summarised in
Table 1. A C -axis, which passes through the midpoint of the
2
central biphenyl C–C single bond, relates the two halves of
the molecule. There are no unusual bond lengths within the
molecular structure, although it is worth noting that the
shortest N–C bond length is associated with C(4) and reflects
the predominant conjugation pathway in the molecule. The
biphenyl moiety is characterised by a C(1)–C(1A) bond length
1
5,16
these species.
This orbital description demands a relatively
planar structure to the benzidene core in these radical cations,
and indeed this is observed in the solid state structure of
1
3]SbCl6. Given the wide range of molecular conformations
6
[
likely to be found in an amorphous film of device-grade TPD-
style hole transporting material, we were interested in probing
˚
of 1.480(2) A. Each nitrogen centre is approximately planar,
Fig. 2 An ORTEP diagram (50%) illustrating the molecular structure of 4, and the atom labelling scheme. Hydrogen atoms have been omitted
for clarity.
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 2 5 1 6 – 2 5 2 3
2 5 1 7

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Fig. 3 An ORTEP plot (50%) showing the molecular structure of 6b and the atom labelling scheme, with hydrogen atoms omitted for clarity.
and the aryl groups are arranged in the usual propeller like
fashion (Table 1). The biphenyl geometry is typical, with an
inter-ring torsion angle C(2)–C(1)–C(1A)–C(6A) of 232.8u.
The comparable biphenyl inter-ring angles in TPD (1), 3,
and N,N’-di(p-tolyl)-N,N’-diphenyl-4,4’-diaminobiphenyl are
1
6,17
2
34.7u, 237.1u and 244.5u,
respectively. The molecular
backbone is distorted by intermolecular contacts, with H(16)
˚
making close contacts with both C(1) [2.878 A] and C(2)
˚
2.851 A] on an adjacent molecule.
The molecular geometry of 6b (Fig. 3, Table 1) reveals
[
clearly the influence of the methyl groups C(13) and C(20), with
the torsion angle C(6)–C(1)–C(1A)–C(6A) observed as 85.0u.
The amine centres are approximately planar, with the aryl ring
systems defining the usual propeller geometry associated with
triarylamines. The N–Caryl bond lengths span a range identical
˚
to that found in 4 [1.411(2)–1.428(2) A], although the shortest
N–C bond in 6b is associated with C(7). The C(1)–C(1A) bond
˚
length in 6b [1.500(2) A] is longer than the equivalent separa-
tion in 4.
The packing of these molecules within the crystal lattice is
also of interest, given the scarcity of reports dealing with the
intermolecular relationships of TPD-type molecules in the solid
state. Molecules 4 form a loose framework, with the solvent of
crystallisation located in the cavities between molecules of the
arylamine (Fig. 4). The framework is supported by a number of
Fig. 4 Packing of the molecules of 4 within the crystal lattice. Viewed
along b-axis, H-atoms omitted for clarity.
…
weak intermolecular C–H p interactions, the shortest being
…
…
˚
C(16)–H(16) C(2’)(0.5 2 x, 0.5 2 y, 2z) (H C 2.85(1) A).
…
Such weak C–H p interactions are commonly observed in the
solid state structures of aromatic compounds, but it should be
…
linked together by p p interactions (the shortest C
distance between parallel Ph-rings is C16 C16’ 3.282 A),
… …
while CH p interactions (the shortest being C15–H15 C8,
…
˚
C
…
…
noted that there is no evidence for p p stacking type
…
˚
interactions in 4. Not surprisingly, the solvent molecule of
…
C8 H15 2.828 A) connect the molecules of the different
layers.
2 2
CH Cl also takes part in a number of weak CH C and
…
CH Cl interactions. In contrast to 4, the molecules of 6b
are found as layers perpendicular to the crystallographic
a-direction (Fig. 5). The individual molecules in each layer are
While crystallographic studies reveal detail about solid state
structure in a crystalline environment, other methods can
provide indirect evidence about the physical and electronic
Fig. 5 Crystal structure of 6b, viewed in the 011 direction, H-atoms omitted for clarity.
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 2 5 1 6 – 2 5 2 3
2
5 1 8

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Fig. 6 The UV-Vis absorption spectra (l/e) of (a) 2 (302/23400), (b) 4 (307/29700; 355/36600), (c) 5a (309/38700) and (d) 6a (309/40300). The spectra
of 5b and 6b are similar to those of their tolyl-substituted analogues 5a and 6a.
structure of these materials in solution. The electronic spectra
of simple triarylamines, such as 2, typically feature a single UV
p–p* absorption band (Fig. 6a). In contrast, tetra(aryl)benzi-
denes such as 1 exhibit two absorption bands arising from p–p*
transitions which may be more precisely defined as transitions
between the delocalised HOMO to unoccupied orbitals more or
required during the oxidation of these more sterically restricted
systems. The electrochemical response of 6b was complicated
by the chemical instability of the redox product. Thus while the
initial oxidative sweep displayed an oxidation wave at 0.90 V,
as the potential was cycled between 2.0 and 21.5 V this wave
became broader, and a new wave at 0.58 V was detected. A thin
film of material was clearly evident when the surface of the
working electrode was examined at the end of the experiment.
The spectroelectrochemical response of a small number of
tetra(aryl)benzidenes have been reported recently, and not
less localised on the biphenyl (l
max
1
ca. 350 nm) and peripheral
aryl (lmax ca. 300 nm) groups. This characteristic profile is
also observed for the conformationally unrestricted system 4
6
(
Fig. 6b). However, in the case of 5 (Fig. 6c) and 6 (Fig. 6d)
n1
surprisingly the spectral profiles obtained for 4 (n ~ 0–2)
n1 n1 16
only a single absorption band is observed near 300 nm indicat-
ing that the triarylamine moieties in these compounds are
electronically decoupled.
The electrochemical response of 4, 5a, 5b, 6a and 6b were
examined by cyclic and differential pulse voltammetry
were very similar to those of 1 , 3
methoxyphenyl)-1,1’-diaminobiphenyl. Oxidation of 4 to 4
was characterised by collapse of the parent spectrum, and the
, and N,N,N’,N’-tetra(p-
18 1
growth of a lower energy p–p* transition at 490 nm and the
appearance of an intense, asymmetric NIR band (lmax ~
(
Table 2). The unrestricted system 4 displayed two reversible,
1
transition (Fig. 7a). Further oxidation to 4 results in the
470 nm), which arises from the (HOMO-1) A SOMO
one electron oxidation waves, which were separated by 0.30 V.
Introduction of methyl groups at the 2 and 6 positions in 5a, 5b
and 6a resulted in an increase in the first oxidation potential,
and a decrease in the separation of the redox events, becoming
a single, apparently two electron, process in 6a. Therefore,
despite the presence of the inductively electron donating methyl
groups in the 2 and 6 positions in 5a, 5b and 6a, the oxidation
of these compounds is thermodynamically less favourable
than 4 and probably reflects the greater reorganisation energy
21
1
collapse of the spectral transitions characteristic of 4 and the
shift of the p–p* band to 780 nm (Fig. 7b), which is entirely in
21
keeping with the spectral profiles observed for 1 and the
related systems mentioned above.
In general terms, the spectroelectrochemical response of 5a
was similar to that of 4, but in keeping with the lower com-
1
proportionation constant associated with 5a this species could
21
not be generated free of 5a and 5a . As the potential applied
to the working electrode increased, the p–p* bands observed in
the neutral species at 308 nm collapsed, with a concomitant
increase in the bands associated with the radical cation at 439
and 1863 nm (Fig. 8a), while the p–p* bands from the dication
dominate at higher potentials still (Fig. 8b). The chemical
reversibility of the system was verified by the recovery of the
spectrum of 5a after exhaustive reduction. Oxidation of 6a
proceeded smoothly to the dication, with the observation of
a single isosbestic point at 430 nm (Fig. 9). The NIR region
Table 2 The electrochemical response of 1, 4, 5a, 5b, 6a and 6b
E ’ vs. Fc/Fc1
0
DE0’
CH
Comproportion
a
Compound
CH
2
Cl
2
/V
2
Cl
2
/V
constant K
4.320
133,140
c
1
4
E
E
E
E
E
E
E
E
E
E
1
2
1
2
1
2
1
2
1
1
~ 0.292
~ 0.507
~ 0.251
~ 0.554
~ 0.390
~ 0.550
~ 0.442
~ 0.588
~ 0.463
~ 0.90 (irrev)
b
0.215
0.303
0.158
0.146
5
5
a
b
550
295
1
remained transparent during this process, and 6a was not
detected.
1
While 5b was shown to be sufficiently stable kinetically to
give a chemically reversible response under conditions of the
6
6
a
a
b
—
—
—
—
2
1
cyclic voltammetry experiment (100–200 mV s ), the chemical
reactivity of this species was apparent on the longer timescale
of the OTTLE-based spectroelectrochemical experiments.
0
K
c 2 2 4 4
~ exp(DE RT/nF) Data recorded in CH Cl (0.1 M NBu BF )
1
on a Pt working electrode. Potentials are referenced against Fc/Fc .
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 2 5 1 6 – 2 5 2 3
2 5 1 9

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Fig. 9 The spectroelectrochemical response (l/e) of 6a, illustrating the
smooth conversion to [6a] (347/25300; 691/16900).
21
band. During this oxidation process, the peak maximum of the
NIR band shifted from 1600 nm to approximately 1400 nm
(
Fig. 10a). Continued oxidation at higher potentials caused the
collapse of the bands at 485 and the NIR envelope and the
continued growth of the band at 740 nm (Fig. 10b). When
the potential was reversed, there was a smooth transition to a
species characterised by intense bands centred at 1400 nm and
4
80 nm, reminiscent of a tetra(aryl)benzidene monocation,
with an isosbestic point near 820 nm (Fig. 10c). Continued
reduction saw these bands collapse and the growth of two UV
transitions at lmax 320 and 350 nm, with an isosbestic point
near 400 nm (Fig. 10d).
Fig. 7 The spectroelectrochemical response (l/e) of 4 (307/29700, 355/
3
(
1
6600) upon oxidation to the (a) mono (4 : 484/30100; 1467/32620) and
Compound 6b behaved in a manner similar to that illustrated
for 5b. Thus, oxidation of 6b gave a new spectral trace which,
despite the absence of a conjugated p-system linking the amine
centres in the starting material, exhibited an absorption
21
b) di (4 : 784/64000) cations.
1
spectrum similar to that of a 4 (lmax ~ 480, 1350 nm) in
addition to a band near 700 nm. Further oxidation resulted in
the collapse of the bands at 480 and 1400 nm and the continued
growth of the band at 700 nm. Reduction of the exhaustively
oxidised solution resulted in the collapse of the band at 700 nm
and re-appearance of the bands at 480 and 1350 nm with an
associated isosbestic point at 920 nm. The band energy and
shape of the NIR transition was almost identical to that
1
1
observed for 1 and 4 electrochemically generated in an
identical solvent and electrolyte solution. Further reduction
gave a clean spectral profile with new band maxima at l ~ 320
and 353 nm (Fig. 11).
The clearest evidence for the reactivity of the oxidised forms
of 5b and 6b is arguably the spectroscopic profiles of the
authentic material before oxidation, and after the complete
cycle of oxidation and reduction, which are illustrated in
Fig. 11 for 6b. The spectra derived from 5b are similar. The
similarity of the spectral profile obtained from samples of 6b
(and 5b) after the cycle of electrochemical oxidation and
reduction to that of 4 is obvious.
Taken together, the spectroelectrochemical and electro-
chemical results across the series 4, 5, 6 indicate that the
dominant factor in determining the kinetic and thermodynamic
stability of the redox products lies in the relative conforma-
tional flexibility of the biphenyl moiety. The introduction of
steric constraints which limit rotation of the aryl rings around
the C(1)–C(1A) bond serves to decrease the thermodynamic
stability of the oxidised forms, as evidenced by the increase in
oxidation potentials. It is also clear that the kinetic stability of
the oxidised forms of the more restricted systems 5 and 6 is
dependent upon the presence of substituents at the C(10) and
C(17) positions. This observation leads to the suggestion that
inclusion of simple blocking groups at the equivalent positions
at the termini of hole transport materials may lead to increased
stability and more consistent device performance over time.
Fig. 8 The spectroelectrochemical (l/e) response of 5a illustrating
(
5
1
a) the maximum equilibrium concentration of 5a (430/48000; 1870/
21
7600) and (b) the conversion to 5a (685/25900).
1
Thus, attempts to oxidise 5b to 5b in the OTTLE cell resulted
in a collapse of the original spectral profile which was replaced
by new bands at 485 and 740 nm together with a broad NIR
2
5 2 0
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 2 5 1 6 – 2 5 2 3

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Fig. 10 (a) The spectroelectrochemical response of 5b upon oxidation at lower potentials during the early stages. Note the shift in the low energy
band maximum. (b) The spectroelectrochemical response during the latter stages of the complete oxidation of 5b. (c) The spectral profiles generated
during early stages of the back-reduction. (d) The spectral profile after complete back-reduction.
The spectroscopic profiles of the products derived from electro-
chemical oxidation of 5b and 6b, which contain two UV
absorption bands, are indicative of an unrestricted tetraaryl-
benzidene moiety. It seems likely that the increased barrier to
rotation caused by the methyl groups at the 2,2’ (and 6,6’)
positions destabilises the planar arrangement of the biphenyl
moiety, effectively decoupling the triarylamine moieties and
restoring triphenylamine-like chemical reactivity to the oxi-
dised forms of these compounds. The chemical stability of the
tolyl-based analogues 5a and 6a clearly indicates that the site of
reaction lies at the carbon centres on the peripheral rings trans
to the nitrogen centre [i.e. C(10) and C(17)].
the spectroelectrochemically generated profiles. The electro-
spray mass spectrum of the crude reaction mixture clearly
indicated the presence of dimeric (m/z 1086) and trimeric (m/z
1628) products, although higher oligomers were not observed,
possibly due to the lower solubility of the longer chain mate-
rials. The reaction mixture was allowed to stir for 1 h at
ambient temperature, before being quenched by addition of
SnCl . Filtration of the reaction mixture through a silica pad
2
to remove the tin and antimony by-products gave a colourless
solution from which a white solid (7) was obtained after
concentration and precipitation into ethanol.
1
In the H NMR spectrum of the resulting white solid, singlet
resonances at d 2.20 and d 6.91 ppm characteristic of the
Guided by the electrochemical and spectroelectrochemical
results, preliminary preparative scale chemical oxidation of
2
,2’,6,6’-tetramethylbenzidene fragment were observed. In the
aromatic region, an AB resonance centred at d 7.13 ppm indi-
cated the presence of a 4,4’-disubstituted 1,1’-biphenyl moiety
in the product, 7 as well. This suggestion is further supported
by the UV/Vis spectrum of 7 prepared in this manner, which
was characterised by two absorption bands at 315 and 351 nm,
similar to the spectral profile of 4, and other simple TPD-type
molecules. Estimates of the weight average of 7 obtained from
several reactions by GPC against polystyrene standards fell in
the range 1740 (3-mer) to 30500 (58-mer) depending upon the
rate of addition of the oxidising agent and the concentration of
the reagents. While the conditions were not optimised, it is clear
that oxidation of 6b, either chemically or electrochemically,
results in the production of oligo and polymeric derivatives.
6
b was undertaken using an equimolar quantity of SbCl
CH Cl . The addition of the oxidising agent was marked by an
5
in
2
2
instant deep blue colouration of the solution, which rapidly
faded to orange. The progress of the reaction could be followed
by UV-Vis spectroscopy, with two absorption bands at 480 and
1
400 nm observed from these orange solutions, consistent with
Conclusion
The introduction of steric bulk at the 2 and 6 positions in
4
,4’-bis(diphenylamino)biphenyls can influence both the oxida-
tion potential of the material and the thermodynamic stability
of the oxidation products. The use of steric constraints to
break the extended p-conjugated system in the benzidene
moiety results in materials which are susceptible to oxidative
polymerisation, either electrochemically or by chemical oxida-
tion. The production of a polymeric polyarylamine in
Fig. 11 The UV-Vis spectra (l/e) of 6b before (solid line, 310/39400)
and after (dashed line, 320/28700; 346/31900) the oxidation/reduction
cycle.
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 2 5 1 6 – 2 5 2 3
2 5 2 1

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electrochemical fashion, in which the only ‘‘reagent’’ is the solid
electrode, and the ‘‘by-products’’ are protons is an intriguing
observation. Studies aimed at optimisation of the benign
electrochemical synthesis, and investigation of the transport
properties of the resulting materials, are underway.
Crystal data for 6b. C H N , M ~ 544.71, monoclinic,
4
0
36
2
space group C 2/c, a ~ 9.7412(8), b ~ 16.670(1), c ~ 18.784(2)
3
˚
˚
A, b ~ 97.068(4)u, U ~ 3027.1(4) A , F(000) ~ 1160, Z ~ 4,
c
2
~ 1.195 mg m , m ~ 0.069 mm . 20949 reflections
3
21
D
(2.18 ¡ h ¡ 30.51u) were collected yielding 4619 unique data
2
2
(Rmerg ~ 0.046). Final wR (F ) ~ 0.1364 for all data (262
refined parameters), conventional R (F) ~ 0.0505 for 3278
1
reflections with I w 2s, GOF ~ 1.021. The largest peak on the
residual map is 0.32 e A . CCDC reference number CCDC
Experimental
2
3
˚
All reactions were carried out under a nitrogen atmosphere in
flame-dried glassware. Solvents were dried from an appropriate
agent and distilled. Di(4-methylphenyl)amine, 4,4’-diamino-
235058. See http://www.rsc.org/suppdata/jm/b4/b404731a/ for
crystallographic data in .cif or other electronic format.
2
,2’-dimethylbiphenyl dihydrochloride and ortho-dichloroben-
zene were purchased from Aldrich; diphenylamine, 18-crown-6
and K CO were purchased from Avocado. All commercial
reagents were used as received. The diiodides were prepared as
Synthesis and characterisation
N,N,N’,N’-tetra(4-methylphenyl)-(1,1’-biphenyl)-4,4’-diamine
4). A solution of diiodobiphenyl (2.0 g, 4.93 mmol), di(4-
2
3
(
methylphenyl)amine (2.14 g, 10.9 mmol), K CO (5.44 g,
1
9–21
previously described.
out within the Gaussian 98 package. Molecular geometries
were optimised for both the neutral and radical cation states of
The computational work was carried
3
2
3
1
39.4 mmol), copper powder (0.67 g, 10.6 mmol) and 18-crown-
(0.29 g, 0.99 mmol) in o-dichlorobenzene (10 ml) was heated
at reflux for 15 h. The reaction mixture was filtered and the
6
2
at a DFT level using the BPW91 functional (Becke’s 1998
solvent volume reduced in vacuo. The crude mixture was
purified by column chromatography (silica, hexanes) (1.25 g,
exchange functional, with Perdew and Wang’s 1991 gradient-
corrected correlation functional) in conjunction with the
6
the molecular orbitals generated by the calculations was carried
1
47%). H NMR: d 7.31 (d, J
1
0–12
HH
~ 8 Hz, 4H, H2/H3), 6.97 (m,
–31G(d,p) basis set.
Post-processing for visualisation of
1
0H), 2.2 (s, 12H, CH3). C NMR: d 147.22, 145.54, 134.20,
3
2
132.61, 130.08, 127.28, 124.74, 123.16, 21.07. EI MS m/z 544
1
4
out using the Molekel programme.
1
1
[
M] , 349 [M 2 NAr ] . m.p. 203–205u. Calculated for
2
40 36 2
C H N : C 88.19%; H 7.66%; N 5.14%; found: C 88.18%; H
Instrumentation
7.74%; N 4.49%.
Mass spectra were recorded on a Micromass Autospec instru-
ment operating in EI mode. NMR spectra were recorded from
N,N,N’,N’-tetra(4-methylphenyl)-(2,2’-dimethyl)-(1,1’-biphenyl)-
4,4’-diamine (5a). The compounds 2,2’-dimethyl-4,4’-diiodobi-
phenyl (2.17 g, 5.0 mmol), di(4-methylphenyl)amine (2.36 g,
12.0 mmol), K CO (5.52 g, 40.0 mmol), copper powder (0.68 g,
1
CDCl solutions on Varian XL-200 ( H), Varian Unity-300 ( H
1
3
1
3
1
1
13
1
and C{ H}) or Varian VXR-400 ( H, C, H– H COSY
1
1
13
2
3
and H– C HETCOR). 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
1
0.75 mmol) and 18-crown-6 (0.27 g, 1.0 mmol) were dissolved
in o-dichlorobenzene (10 ml) and heated at reflux for 45 h. The
reaction mixture was filtered and the solvent volume reduced
in vacuo. The crude mixture was purified by column chromato-
PGSTAT30 from CH
2
Cl
2
solutions containing 0.1 M
NBu BF as supporting electrolyte and potentials cited are
1
graphy (silica, hexanes) (2.35 g, 82%). H NMR d 7.05 (m,
4
4
1
2H), 2.32 (s, 12H, H13), 1.99 (s, 6H, H20). C NMR d 145.79,
3
2
1
1
referenced against ferrocene such that the ferrocene couple falls
at 0 V. Spectroelectrochemical experiments were also carried
44.54, 135.90, 134.08, 129.27, 128.76, 122.79, 118.96, 19.78,
1
1
21
9.05. EI MS m/z 572 [M] , 377 [M 2 NAr2] , 286 [M] . Mp
4 4 2 2
out in 0.1 M NBu BF /CH Cl solutions using an OTTLE
2
2
248–250u.
cell, fitted with a Pt gauze mesh semi-transparent working
electrode, a Pt wire counter and pseudo-reference electrodes,
and a home-built potentiostat.
N,N,N’,N’-tetraphenyl-(2,2’-dimethyl)-(1,1’-biphenyl)-4,4’-
diamine (5b). A stirred solution of 2,2’-dimethyl-4,4’-diiodobi-
phenyl (1.0 g, 2.3 mmol), diphenylamine (0.78 g, 4.6 mmol),
K CO (2.54 g, 18.4 mmol), copper powder (0.58 g, 9.2 mmol)
Crystallographic details
2
3
and 18-crown-6 (0.12 g, 0.46 mmol) in o-dichlorobenzene
10 ml) was heated at reflux for 20 h. The reaction mixture was
Single-crystal X-ray data for both compounds 4 and 6b were
collected at 120(1) K on a Bruker SMART-CCD 6000
diffractometer (v-scan, 0.3u/frame) using Mo-Ka- radiation
(
filtered and the solvent volume reduced in vacuo. The crude
mixture was purified by column chromatography (silica,
hexanes/CH Cl ) to afford the title compound as a yellow
˚
l ~ 0.71073 A). No absorption correction was applied. The
(
2
2
structure was solved by direct methods and refined by full-
2
matrix least squares on F for all data using SHELXTL soft-
ware. All non-hydrogen atoms were refined with anisotropic
displacement parameters, H-atoms were located on the diffe-
rence map and refined isotropically.
1
powder. (0.38 g, 35%). H NMR d 7.22 (s, 2H, H5), 7.19 (m,
H, H8), 7.06 (m, 8H, H9), 6.93 (m, 2H, H2/H3), 6.92 (m, 4H,
8
1
3
3
H10), 6.85 (m, 2H, H2/H3), 1.94 (s, 6H, CH ). C NMR d
1
1
47.95, 146.48, 137.10, 135.93, 130.40, 129.17, 125.10, 124.17,
1
1
2
22.64, 121.23, 20.06. EI MS m/z 516 [M] , 349 [M 2 NPh ] .
Mp 206–209u.
Crystal data for 4. C H N . CH Cl , M ~ 629.63, mono-
2
4
0
36
2
2
clinic, space group C 2/c, a ~ 18.5659(3), b ~ 13.6399(2), c ~
N,N,N’,N’-tetra(4-methylphenyl)-(2,2’,6,6’-tetramethyl)-(1,1’-
biphenyl)-4,4’-diamine (6a). A stirred solution of 2,2’,6,6’-
tetramethyl-4,4’-diiodobiphenyl (2.0 g, 4.33 mmol),
di(4-methyl)phenylamine (1.87 g, 9.52 mmol), K CO (4.78 g,
34.63 mmol), copper powder (0.59 g, 9.3 mmol) and 18-crown-
6 (0.27 g, 1.0 mmol) in o-dichlorobenzene (10 ml) was heated to
reflux for 18 h. The reaction mixture was filtered and the
solvent volume reduced in vacuo. The crude mixture was
3
˚
˚
1
1
4.6862(2) A, b ~ 116.78(1)u, U ~ 3320.2(2) A , F(000) ~
23 21
c
328, Z ~ 4, D ~ 1.260 mg m , m ~ 0.228 mm . 16149
reflections (2.11 ¡ h ¡ 29.49u) were collected yielding 4596
2
3
2
(F ) ~ 0.1190 for all
unique data (Rmerg ~ 0.041). Final wR
data (280 refined parameters), conventional R (F) ~ 0.0426 for
2
1
3
703 reflections with I w 2s, GOF ~ 1.069. The largest peak
23
˚
on the residual map is 0.54 e A . CCDC reference number
CCDC 235057. See http://www.rsc.org/suppdata/jm/b4/
b404731a/ for crystallographic data in .cif or other electronic
format.
2 2
purified by column chromatography (silica, hexanes, CH Cl ),
1
(0.29 g, 11%). H NMR d 6.99 (d, J ~ 8 Hz, 8H, H8), 6.93
(d, JAB ~ 8 Hz, 8H, H9), 6.73 (s, 4H, H3), 2.25 (s, 12H,
AB
2
5 2 2
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 2 5 1 6 – 2 5 2 3

View Article Online
1
ArCH ), 1.76 (s, 12H, biphenyl CH ). C NMR d 146.88,
3
3624; H. Weingarten, J. Org. Chem., 1964, 29, 977; (i) J. Lindley,
Tetrahedron, 1984, 40, 1433.
(a) S. Tanaka, T. Iso and Y. Doke, Chem. Commun., 1997, 2063;
3
3
1
2
46.10, 132.08, 130.89, 130.07, 128.06, 124.55, 122.56, 21.16,
1 1
0.34. EI MS m/z 600 [M] , 405 [M 2 NAr ] .
4
2
(b) M. Ishikawa, M. Kawai and Y. Ohsawa, Synth. Met., 1991, 40,
31.
2
N,N,N’,N’-tetra(phenyl)-(2,2’,6,6’-tetramethyl)-(1,1’-biphenyl)-
,4’-diamine (6b). A stirred solution of 2,2’,6,6’-tetramethyl-
,4’-diiodobiphenyl (2.0 g, 4.33 mmol), diphenylamine (1.61 g,
.5 mmol), K CO (5.52 g, 40.0 mmol), copper powder (0.6 g,
2 3
.3 mmol) and 18-crown-6 (0.23 g, 0.86 mmol) in ortho-
5
6
7
E. T. Seo, R. F. Nelson, J. M. Fritsch, L. S. Marcoux, D. W. Leedy
and R. N. Adams, J. Am. Chem. Soc., 1966, 88, 3498.
A. Petr, C. Kvarnstr o¨ m, L. Dunsch and A. Ivaska, Synth. Met.,
4
4
9
9
2
000, 108, 245.
H. Sato, A. Kanegae, R. Yamaguchi, K. Ogino and J. Kurjata,
Chem. Lett., 1999, 79.
dichlorobenzene (10 ml) was heated at reflux for 19 h. The
reaction mixture was filtered and the solvent volume reduced
in vacuo. The crude mixture was treated MeOH and placed in
8
9
C. Lambert and G. N o¨ ll, 2003, personal communication.
M. Leung, M.-Y. Chou, Y. O. Su, C. L. Chiang, H.-L. Chen,
C. F. Yang, C.-C. Yang, C.-C. Lin and H.-T. Chen, Org. Lett.,
2
003, 5, 839.
10 A. D. Becke, Phys. Rev. A, 1988, 38, 3098.
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6533; Erratum: Phys. Rev. B, 1998, 57, 14999.
the freezer, producing a pale precipitate, which was collected
1
and recrystallised from benzene/hexanes (1.64 g, 70%).
NMR d 7.26 (pseudo t, JHH ~ 16 Hz, 8H, H9), 7.12 (d, JHH
H
~
1
1
1
7
Hz, 8H, H8), 7.00 (t, J
~ 14 Hz, 4H, H10), 6.86 (s, 4H,
HH
12 (a) R. Ditchfield, W. J. Hehre and J. A. Pople, J. Chem. Phys.,
1971, 54, 724; (b) W. J. Hehre, R. Ditchfield and J. A. Pople,
J. Chem. Phys., 1972, 56, 2257; (c) P. C. Hariharan and J. A. Pople,
Mol. Phys., 1974, 27, 209; (d) M. S. Gordon, Chem. Phys. Lett.,
1
3
3
H3), 1.86 (s, 12H, CH ). C NMR d 148.34, 146.24, 137.14,
1
[
6
35.07, 129.37, 124.19, 123.63, 122.48, 20.24. EI MS m/z 544
1
M] , 377 [M 2 NAr
1
] . Calculated for C40
.66%; N 5.14%; found: C 87.80%; H 6.95%; N 5.05%.
2
36 2
H N : C 88.19; H
1980, 76, 163; (e) P. C. Hariharan and J. A. Pople, Theor. Chim.
Acta., 1973, 28, 213.
1
3
M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, V. G. Zakrzewski,
J. A. Montgomery, Jr., R. E. Stratmann, J. C. Burant,
S. Dapprich, J. M. Millam, A. D. Daniels, K. N. Kudin,
M. C. Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi,
R. Cammi, B. Mennucci, C. Pomelli, C. Adamo, S. Clifford,
J. Ochterski, G. A. Petersson, P. Y. Ayala, Q. Cui, K. Morokuma,
P. Salvador, J. J. Dannenberg, D. K. Malick, A. D. Rabuck,
K. Raghavachari, J. B. Foresman, J. Cioslowski, J. V. Ortiz,
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I. Komaromi, R. Gomperts, R. L. Martin, D. J. Fox, T. Keith,
M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, M. Challacombe,
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GAUSSIAN 98 (Revision A.11), Gaussian, Inc., Pittsburgh, PA,
2001.
Acknowledgements
This work was supported by the University of Durham, the
EPSRC, ONE-North East and Avecia Ltd. JAKH holds an
EPSRC Senior Research Fellowship. MAJP held an EPSRC/
Avecia CASE award. We are pleased to acknowledge helpful
discussions with Professor Dr C. Lambert (University of
W u¨ rzburg) and communication of results prior to publication.
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J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 2 5 1 6 – 2 5 2 3
2 5 2 3