Cruciform π-systems: Hybrid phenylene-ethynylene/phenylene-vinylene oligomers

Article abstract of DOI:10.1039/b312156a

The cruciform pentamers 3a-g were synthesized by a combined Horner-Sonogashira approach; their band gaps vary significantly with emission varying from blue to red depending upon their substituent pattern.

Full text of DOI:10.1039/b312156a

Cruciform p-systems: hybrid phenylene-ethynylene/phenylene-vinylene
oligomers†
James N. Wilson, Mira Josowicz, Yiqing Wang and Uwe H. F. Bunz*
Department of Chemistry and Biochemistry, Georgia Institute of Technology, 770 State St., Atlanta, GA,
30332, USA. E-mail: uwe.bunz@chemistry.gatech.edu; Fax: 01 404 385 1795; Tel: 01 404 385 1795
Received (in Columbia, MO, USA) 30th September 2003, Accepted 22nd October 2003
First published as an Advance Article on the web 11th November 2003
The cruciform pentamers 3a–g were synthesized by a
combined Horner–Sonogashira approach; their band gaps
vary significantly with emission varying from blue to red
depending upon their substituent pattern.
case of the synthesis of 3g triethylamine was utilized to avoid
nucleophilic addition of piperidine to the aromatic nucleus. In
the cases of 3c,d the products were quite insoluble and therefore
the yield was lower than average (53% and 63%); 3a–g were
purified by double crystallization.
Conjugated materials are important as active layers in device
applications. Poly(paraphenylenevinylene)s (PPV) have
proved to be tremendously successful in device fabrication, due
to their balanced hole and electron injection capabilities.1
Recently, polymers that combine the structural features of
poly(paraphenyleneethynylene)s (PPE), and those of PPVs
have been reported.2 Formal hybrids such as A behave
considerably more like PPEs3 than like PPVs. However, if
styryl groups are laterally attached to the benzene rings the
electronic properties of the resulting polymer B are different
from both PPV and PPEs.4 Is the change in properties
indigenous to the polymer backbone, or are single, isolated,
cruciforms “cut out” of B responsible for the observed
electronic effects? We find that the optical, electronic and redox
properties of B reside mostly in their pentameric cruciform5
modules 3.
The absorption data of 3a–g are shown in Figs. 1 and 2 and
in Table 1. The lmax values correspond qualitatively with the
expected ordering predicted from the substituent patterns.
Cruciform 3d has the largest and 3g the lowest optical band gap.
The three oligomers 3e–g show a nice correlation of decreasing
band gap with increasing CF3 substitution. The donor–acceptor
interaction decreases the band gap. To obtain more information
we investigated the electrochemistry of 3a–g. The cruciforms
3a–g show oxidation potentials that are in qualitative agreement
with their calculated (RHF 6-31G**, Fig. 1) HOMO values. The
electrochemical reduction data is difficult to interpret for 3e–g
due to electron-transfer induced reactions that lead to dull,
Starting from the bisphosphonate 1 a Horner6 reaction
produced the distyrylbenzenes 2a–e in good to excellent yields
(Scheme 1). In the second step 2a–e were coupled to terminal
alkynes utilizing (Ph3P)2PdCl2 and CuI in piperidine.7 In the
Fig. 1 Top: electrochemical bandgap with reduction in green and oxidation
in red (3a–g). Faded regions correspond to an onset of oxidation/reduction
that does not reach a peak value. The range of each bar corresponds to the
onset and peak values. Middle: calculated bandgaps (reduction: green,
oxidation: red). Bottom: comparison of optical (green, eV), electrochemical
(red, V), and calculated (blue, scale at right, eV) bandgaps.
Scheme 1 Two-step reaction scheme, substituent key and yields of
compounds 2a–e and 3a–g.
† Electronic supplementary information (ESI) available: details of the
synthesis of 3. See http://www.rsc.org/suppdata/cc/b3/b312156a/
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This journal is © The Royal Society of Chemistry 2003

Table 1 Summary of the absorbance and emission data of cruciforms 3 in chloroform and hexane; 3d is insoluble in hexane. 3a–d show similar spectra in
both hexane and chloroform, while 3e–g show dramatic solvatochromicity in emission
3a
3b
3c
3d
3e
3f
3g
Ab. CHCl3
Em. CHCl3
F CHCl3
Ab. hex
Em. hex
F hexa
331, 365 sh
420, 442
0.83
326, 352 sh
414, 432
0.78
330
446, 526 sh
0.28
324, 348 sh
424, 444 sh
0.45
339,374 sh
432, 454
0.88
334, 376 sh
420, 442
0.78
330, 363 sh
419, 434 sh
0.92
—
—
339, 439
514
0.16
332, 422
472, 498
0.94
342, 444
543
0.20
344, 416
502, 526 sh
0.70
345, 458
563
0.14
346, 420
524
0.53
a
—
a Vary by ±5%.
yields were lower for 3b and 3e–g. The cruciforms 3a–d are
blue emitters but 3e–g show dramatic differences in their
emission that are also solvent dependent. In chloroform (Fig. 2
top) the emission of 3e–g changes from green to orange, while
in hexane a similar trend is observed, however, the color
changes from blue–green to yellow (Fig. 2 bottom, Table 1). In
methanol the emission of 3g is weak and red. Similar effects are
observed for 3e and 3f.
The frontier orbitals of 3g were inspected (RHF 6-31G**,
Spartan). The HOMO is localized on the distyrylbenzene
branch of the cruciform while the LUMO is localized on the
phenyleneethynylene part (Fig. 3). HO and LU orbitals overlap
only in the central benzene ring. The insensitivity of the
oxidation potential of 3e–g upon introduction of CF3 groups
into the molecule is a consequence of the spatial separation of
the HOMO and the LUMO. For 3a this type of de-mixing of the
HOMO and LUMO is not observed and both orbitals are almost
evenly distributed over the whole molecule. The excited states
of 3e–g must show charge separation, which explains the
sensitivity of their emission wavelength towards the polarity of
the solvent. An increasingly polar solvent stabilizes the excited
state and leads to a bathochromically shifted emission.
In summary we have made cruciforms 3 and examined their
electronic properties. They are model compounds for polymers
of the type B. Conjugation along the backbone does not seem to
have a significant effect on B. The cruciforms 3 are versatile and
tuneable chromophores where the position of the HOMO and
LUMO can be changed almost at will by the introduction of
electron donating and electron accepting substituents. The
localization of the HOMO on the PV branch and that of the
LUMO at the PE branch makes 3 cross-conjugated in a non-
classical sense and extends this attractive concept.8,9
Fig. 2 The series of dibutylamino compounds (top) in chloroform: 3e (grn),
3f (yel), 3g (org). Absorbance (triangles) shows varying peak height, but the
same position, while emission (squares) clearly shows a 20+ nm shift on
inclusion of CF3 substituents. These compounds also show high sol-
vatochromicity: the emissions of the compounds in chloroform are
substantially red-shifted from their emissions in hexane (bottom), 3e (blu),
3f (grn), 3g (yel).
colored deposits on the electrodes. Thus, only onset values are
given for 3e–g. The reduction of 3c is easier than expected in
comparison to the calculated value, while the reduction
potential of 3d is higher than expected. Not surprisingly, 3d is
the most difficult, and 3e is most easily oxidized while the
electron-rich cruciforms 3e–g show a second irreversible
oxidation wave.
The authors thank the National Science Foundation (CHE
0138-659, PI Bunz) for funding. UHFB is a Camille Dreyfus
Teacher-Scholar (2000-2004).
Notes and references
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4 J. N. Wilson, P. M. Windscheif, U. Evans, M. L. Myrick and U. H. F.
Bunz, Macromolecules, 2002, 35, 8681.
In solution all of the cruciforms were highly fluorescent (0.45
< F < 0.94) in hexane. In chloroform the emission quantum
5 J. E. Klare, G. S. Tulevski, K. Sugo, A. de Picciotto, K. A. White and C.
Nuckolls, J. Am. Chem. Soc., 2003, 125, 6030.
6 L. Horner, H. Hoffmann and H. G. Wippel, Chem. Ber., 1958, 91, 61; L.
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Fig. 3 RHF 6-31G** calculated structure of 3g (butyl groups omitted). Left:
HOMO of 3g. Right: LUMO of 3g. The HOMO is almost completely
localized on the distyrylbenzene branch of the cruciform, while the LUMO
is almost fully localized on the aryleneethynylene substructure.
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