A tetraphenylmethane based starburst triarylamine cluster: Spectroscopy, electrochemistry and morphological studies

Article abstract of DOI:10.1016/S0040-4039(02)00589-0

While tetraphenylmethane and triphenylamine are crystalline molecules, a starburst hybrid of the two leads to a low band-gap molecular glass which retains the optical and electrochemical properties of the individual components.

Full text of DOI:10.1016/S0040-4039(02)00589-0

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 3521–3524
A tetraphenylmethane based starburst triarylamine cluster:
spectroscopy, electrochemistry and morphological studies
Saumitra Sengupta,* Subir Kumar Sadhukhan and Sanjukta Muhuri
Department of Chemistry, Jadavpur University, Kolkata 700 032, India
Received 22 February 2002; accepted 21 March 2002
Abstract—While tetraphenylmethane and triphenylamine are crystalline molecules, a starburst hybrid of the two leads to a low
band-gap molecular glass which retains the optical and electrochemical properties of the individual components. © 2002 Elsevier
Science Ltd. All rights reserved.
There is considerable current interest in low band-gap
organic molecules for applications in various electrolu-
minescent (EL) devices.1–3 Along with their optical and
electrical properties, the morphological stability of such
compounds plays an important role in determining
their potential applications in EL devices. Crystalline
compounds or materials having large crystalline
domains tend to degrade EL devices.4 Moreover, they
form aggregates or excimer states resulting in dimin-
ished emission efficiencies. To circumvent these prob-
lems, many studies have focussed on the use of low
band-gap amorphous materials in EL devices and
indeed, it has been shown recently that amorphous
materials when used as the active layer in EL devices
can lead to significant improvements in the efficiency of
multilayer devices.5,6 In view of such promise, in recent
years, several studies have been directed towards
design, synthesis and physical studies of new amor-
phous molecules.7
the fact that starburst derivatives of 1 and 2 readily
form amorphous glasses must be due to their large
molecular volume, non-planar structure and the pres-
ence of a large number of conformers which prevent
easy crystallization. Based on these arguments, we rea-
soned that a starburst hybrid comprised of 1 and 2 may
also show a stable amorphous state. Moreover, in such
a hybrid, the triarylamine moieties would provide
means for efficient hole transporting abilities. We there-
fore embarked on the synthesis and physical studies of
a tetraphenylmethane based starburst triphenylamine
cluster viz. 6, the results of which are reported in this
letter.
Shirota and co-workers have systematically developed
several starburst triarylamine derivatives that form sta-
ble amorphous glasses with glass transition tempera-
tures (Tg) much above room temperature.7 Some of
these have been used as efficient hole-transporting com-
ponents in multilayer EL devices. In an interesting
development, Bazan et al. have recently reported a few
starburst tetraphenylmethane derivatives which show
similar amorphous properties.8 Notably, both triphenyl-
amine (1) and tetraphenylmethane (2) and their simple
derivatives are highly crystalline materials. Therefore,
In designing our starburst hybrid 6, a stilbenoid unit
was chosen as the linker between 1 and 2. This, we
thought, would impart structural rigidity as well as a
strong fluorescence property to the cluster. The synthe-
sis of 6 is shown in Scheme 1. Thus, monoformylation
of triphenylamine with POCl3/DMF gave the aldehyde
3 (90%) which upon Wittig olefination led to p-
diphenylamino styrene (4) in 70% yield. A fourfold
Heck reaction of 4 with tetra-(p-iodo)phenylmethane
(5)9,10 under Jeffery’s phase transfer conditions
(Pd(OAc)2, Bu4NBr, KOAc, DMF, 100°C)8a,11 then
produced the starburst cluster 6 (35%) after purification
by preparative thin layer chromatography over silica
gel. The product was characterized by its spectral data
(1H and 13C NMR) and elemental analysis.12
Keywords: triarylamine; tetraphenylmethane; molecular glass.
* Corresponding author. Fax: 91 033 4734266; e-mail: jusaumitra@
yahoo.co.uk
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)00589-0

3522
S. Sengupta et al. / Tetrahedron Letters 43 (2002) 3521–3524
Scheme 1.
The solution optical properties of 6 recorded in CH2Cl2
are shown in Fig. 1. The absorption maximum was
found at 378 nm with a high molar absorptivity (mmax
1.52×105 M−1 cm−1) whereas the emission maximum
was found at 423 nm with some red tailing up to 600
nm. Compared to tetrastilbenylmethane,8a both these
maxima are considerably red shifted (Dumax(abs)=56 nm;
Dumax(ems)=44 nm) due to increased charge delocaliza-
tion in the styryl triarylamine chromophore of 6. In
fact, the absorption and emission maxima of 6 match
very
well
with
those
reported
for
tetra-
(styrylstilbenyl)methane8a indicating similar delocaliza-
tion efficiencies in diphenylamino stilbenes and distyryl
benzene chromophores. Interestingly, the absorption
maximum of 6 is also red shifted from that of the
model chromophore arm 713 (umax 366 nm) suggesting a
small degree of homoconjugation through the central
tetrahedral carbon of 6. Moreover, the molar absorp-
tivity of 6 is much greater than the fourfold value of 7
(mmax 21 000 M−1 cm−1), indicating constructive excitonic
coupling between the four stilbenoid arms in 6. The
band-gap energy (Eg) of 6, estimated from the onset of
the absorption curve, was calculated to be 2.91 eV,
which is comparable to those reported by Shirota et al.
for other triarylamine clusters.7
Figure 1. Absorption and fluorescence spectra of 6 in CH2Cl2.

S. Sengupta et al. / Tetrahedron Letters 43 (2002) 3521–3524
3523
Cyclic voltammetric (CV) studies on 6 (Pt anode,
CH2Cl2, Bu4NClO4 as supporting electrolyte) produced
a reversible oxidation wave (E1/2=1.00 V versus SCE)
due to the peripheral triarylamine moieties. The oxida-
tion takes place at a higher potential than those
reported for common triarylamine derivatives, pre-
sumably due to charge delocalization from the triaryl-
amine groups to the conjugated styryl double bonds.
This, in turn, lowers the HOMO level of 6, which is
also reflected in its optical properties mentioned above.
Interestingly, 6 showed only one redox wave in the
vicinity of 1.00 V, suggesting that all the four triaryl-
amine units in 6 are electrochemically equivalent. It
thus appears that the central sp3-carbon of 6 causes a
total disruption in conjugation in the molecule and does
not allow any electronic communication whatsoever
between the peripheral triarylamine units.14 It may also
be noted that the four triarylamine moieties in 6 are
rigidly held by the stilbenoid linkages and hence are
placed at a maximum distance apart, despite which they
show only one redox wave. Therefore, during CV scan-
ning, the molecule must undergo fast rotational
changes so that all four triarylamine ends are able to
reach the electrode within the electrochemical time
scale. A second oxidation peak (irreversible) was also
found for 6 at 1.23 V (versus SCE) due to oxidation of
the stilbene units.
Acknowledgements
Professor Samaresh Bhattacharya is thanked for the
electrochemical measurements. One of us (S.M.) thanks
CSIR, New Delhi for a research fellowship.
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viz. tetraphenylmethane and triphenylamine when held
together by a covalent linkage provide an amorphous
character to the hybrid while retaining their individual
optical and electrochemical properties. Based on these
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small molecular weight molecular glasses for optoelec-
tronic applications.

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S. Sengupta et al. / Tetrahedron Letters 43 (2002) 3521–3524
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