A novel approach toward low optical band gap polysquaraines

Article abstract of DOI:10.1021/ol016284p

(Equation presented) Polycondensation of 2,5-dialkoxydivinylbenzene-bridged bispyrroles and squaric acid resulted in extensively conjugated polymers with strong near-infrared (NIR) absorption around 800-1000 nm, which is a signature of their low band gap

Full text of DOI:10.1021/ol016284p

ORGANIC
LETTERS
2001
Vol. 3, No. 16
2595-2598
A Novel Approach Toward Low Optical
Band Gap Polysquaraines
A. Ajayaghosh* and J. Eldo
Photochemistry Research Unit, Regional Research Laboratory, CSIR,
TriVandrum 695 019, India
aajayaghosh@rediffmail.com
Received June 15, 2001
ABSTRACT
Polycondensation of 2,5-dialkoxydivinylbenzene-bridged bispyrroles and squaric acid resulted in extensively conjugated polymers with strong
near-infrared (NIR) absorption around 800−1000 nm, which is a signature of their low band gap (E ). Conjugated polymers with such a strong
g
NIR absorption are rare. One of the synthesized polysquaraines showed a significantly low E of 0.79 eV with an intrinsic conductivity of 5.3
g
× 10-4 S/cm.
Since optical absorption in π-conjugated systems plays a key
role in determining the HOMO-LUMO energy gap and the
intrinsic electronic properties, control of these parameters
by molecular engineering is of great significance to the
design of low band gap (Eg) polymers.1 To a large extent,
this has been achieved with the help of strategies such as
rigidification of the polymer backbone and enhancement of
a polymer’s nonclassical quinoid character.2,3 Among several
recent approaches, the twist inhibition between consecutive
repeating units of conjugated polymers using ladder-type
bonds,4 reversible noncovalent linkages,5 and strong donor-
acceptor interactions6 are shown to be effective in achieving
reduced Eg. Even though several oxidative polymerization
methods are available for the synthesis of conjugated
polymers with Eg below 1 eV, a reasonably viable nonoxi-
dative polymerization pathway towards intrinsically semi-
conducting polymers with adequate solubility in organic
solvents remains elusive.7
A novel class of polymers that has generated considerable
interest in recent years is polysquaraines in anticipation of
their low HOMO-LUMO separation due to a strong donor-
acceptor interaction.8,9 Unfortunately, none of these reports
(5) (a) Marsella, M. J.; Swager, T. M. J. Am. Chem. Soc. 1993, 115,
12214. (b) McCullough, R. D.; Williams, S. P. J. Am. Chem. Soc. 1993,
115, 11608. (c) Brockmann, T. W.; Tour, J. M. J. Am. Chem. Soc. 1995,
117, 4437. (d) McCullough, R. D.; Williams, S. P. Chem. Mater. 1995, 7,
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(f) Tarkka, R. M.; Zhang, X.; Jenekhe, S. A. J. Am. Chem. Soc. 1996, 118,
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Chem. Commun. 1996, 2163. (h) Delnoye, D. A. P.; Sijbesma, R. P.;
Vekemans, J. A. J. M.; Meijer, E. W. J. Am. Chem. Soc. 1996, 118, 8717.
(6) Donor-acceptor strategies reported earlier are not very successful
in producing extremely low Eg polymers; see: (a) Yamamoto, T.; Zhou,
Z.-h.; Kanbara, M.; Kizu, K.; Maruyama, T.; Nakamura, Y.; Fukuda, T.;
Lee, B.-L.; Ooba, N.; Tomaru, S.; Kurihara, T.; Kaino, T.; Kubota, K.;
Sasaki, S. J. Am. Chem. Soc. 1996, 118, 10389. (b) Van Mullekom,
Vekemans, J. A. J. M.; Meijer, E. W. Chem. Eur. J. 1998, 4, 1235. (c)
Zhang, T. Q.; Tour, J. M. J. Am. Chem. Soc. 1997, 119, 5065. (d) Zhang,
T. Q.; Tour, J. M. J. Am. Chem. Soc. 1997, 120, 5355.
* Fax: +91 471 490 186.
(1) Roncali, J. Chem. ReV. 1997, 97, 173.
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Chem. Commun. 1994, 991. (b) Roncali, J.; Thobie-Gautier, C.; Elanda-
loussi, E. H.; Frere, F. J. Chem. Soc., Chem. Commun. 1994, 2249. (c)
Roncali, J.; Thobie-Gautier, C. AdV. Mater. 1994, 6, 846.
(3) (a) Bredas, J. L. J. Chem. Phys. 1985, 82, 3808. (b) Wudl, F.;
Kobayashi, M.; Heeger, A. J. J. Org. Chem. 1984, 49, 3382. (c) Jenekhe,
S. A. Nature 1986, 322, 345. (d) Aota, H.; Reikan, T.; Matsumoto, A.;
Kamachi, M. Chem. Lett. 1997, 527. (e) Aota, H.; Reikan, T.; Matsumoto,
A.; Kamachi, M. Chem. Lett. 1998, 335.
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(b) Godt, A.; Schluter, A.-D. AdV. Mater. 1991, 3, 497. (c) Scherf, U.;
Mullen, K. Synthesis 1992, 23. (d) Lamba, J. J. S.; Tour, J. M. J. Am. Chem.
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S.; Roncali, J. Chem. Commun. 1998, 2081.
10.1021/ol016284p CCC: $20.00 © 2001 American Chemical Society
Published on Web 07/06/2001

were successful in realizing a soluble low Eg polymer despite
the recent theoretical prediction that squaraine dyes can be
used to construct intrinsically semiconducting polymers with
extremely small band gaps.10 Here, we describe a rational
approach toward the synthesis of low Eg polysquaraines that
are reasonably soluble and intrinsically semiconducting with
intense NIR sensitivity.11 The idea involves a non-oxidative
polycondensation of logically selected molecular components
via the in situ generation of an organic dye A at well-defined
positions of a conjugated backbone, which is capable of
electronically communicating with each repeating segment
with the aid of a strongly electron-donating moiety B. This
has been illustrated by the polycondensation of electron-rich
bispyrroles with squaric acid, leading to the formation of
low Eg polymers 3a-c (Scheme 1).
maxima due to π-π* transition around 400-420 nm with
strong fluorescence emission around 470-480 nm. Poly-
condensation of 1a-c with squaric acid in stoichiometric
quantities resulted in the formation of intensely colored
products (3a-c), which were isolated in 69-75% yields after
repeated precipitation and washing with hexane and diethyl
ether, respectively, in the form of powders or ribbons with
metallic luster. The reaction conditions could be optimized
by changing the dilution of the reactants and composition
of the solvents. The progress of the reaction was monitored
by following the changes in the absorption spectra of the
reaction mixtures at different time intervals (Figure 1).
Scheme 1
Figure 1. The evolution of the absorption spectra of polysquaraines
3c with time.
Polysquaraines with long alkyl chains on the benzene and
the pyrrole moieties were soluble in organic solvents such
as dichloromethane, chloroform, tetrahydrofuran, and tolu-
ene.
We speculated that the positive charge on the zwitterionic
dye moieties could be delocalized along the conjugated
backbone, thus generating quinoid resonance structures 3′a-
c. For this reason, bispyrroles 1a-c were prepared by the
Wittig-Horner-Emmons reaction of the corresponding
N-alkylpyrrole-2-carboxaldehyde with appropriate bisphos-
phonate esters.12 Compounds 1a-c showed absorption
All new compounds were characterized by spectral data,
which were in agreement with their structures (see Support-
ing Information). 1H NMR spectrum of the soluble polymer
3a showed considerable broadening of the resonance bands
corresponding to the aromatic protons, indicating strong π-π
interaction of the rigid backbone. For a better insight into
the structure of the polymers, the model compound 4 was
prepared under controlled conditions (Scheme 2). Spectral
characteristics of this compound were identical with that of
the intermediate compound corresponding to the 762 nm
band in Figure 1. Structures of these compounds were
established by NMR and HRMS analyses. These compounds
were subsequently used for the synthesis of 3c (route B),
which showed identical spectral properties and thereby
established their structural identity. The average molecular
weight (Mn) of the most soluble polymer 3a was around
17000-18000 g/mol (Mw/Mn ≈ 10.2) as determined by the
size exclusion chromatography (SEC) using polystyrene
(8) (a) Treibs, A.; Jacob, K. Angew. Chem., Int. Ed. Engl. 1965, 4, 694.
(b) Hall, H. K., Jr.; Chen. Y.-Y. Polym. Bull. 1986, 16, 419-425. (c)
Havinga, E. E.; ten Hoeve, W.; Wynberg, H. Polym. Bull. 1992, 29, 119.
(d) Havinga, E. E.; ten Hoeve, W.; Wynberg, H. Synth. Met. 1993, 55-57,
299. (e) Havinga, E. E.; Pomp, A.; ten Hoeve, W.; Wynberg, W. Synth.
Met. 1995, 69, 581. (f) Lynch, D. E.; Geissler, U.; Kwiatkowski, J.;
Whittaker, A. K. Polym. Bull. 1997, 38, 493. (g) Lynch, D. E.; Geissler,
U.; Peterson, I. R.; Floersheimer, M.; Terbrack, R.; Chi, L. F.; Fuchs, H.;
Calos, N. J.; Wood, B.; Kennard, C. H. L.; Langley, G. J. J. Chem. Soc.,
Perkin Trans. 2, 1997, 827.
(9) (a) Ajayaghosh, A.; Chenthamarakshan, C. R.; Das, S.; George, M.
V. Chem. Mater. 1997, 9, 644. (b) Chenthamarakshan, C. R.; Ajayaghosh,
A. Chem. Mater. 1998, 10, 1657. (c) Chenthamarakshan, C. R.; Eldo, J.;
Ajayaghosh, A. Macromolecules 1999, 32, 251.
(10) (a) Brocks, G. J. Chem. Phys. 1995, 102, 2522. (b) Brocks, G.;
Tol, A. J. Phys. Chem. 1996, 100, 1838.
(11) Polymers with intense near-IR sensitivity are important as dyes for
optical data storage; see: Zollinger, H. Color Chemistry; VCH: New York,
1991.
(12) Eldo, J.; Arunkumar, E.; Ajayaghosh, A. Tetrahedron Lett. 2000,
41, 6241.
2596
Org. Lett., Vol. 3, No. 16, 2001

charge transfer states of the donor-acceptor-donor type
interaction, we anticipated that the introduction of a strong
conjugated electron donor bridge and elongation of the
conjugation length will substantially improve the absorption
properties of the resulting polysquaraines. The observed
spectral properties strongly support this argument. The optical
and electronic properties of 3a-c are summarized in Table
1 and reveal that the alkoxy substituents have considerable
Scheme 2
Table 1. Solution and Solid-State Electronic Properties of
3a-c
solutiona
solid-stateb
intrinsic
conductivity
(S/cm)d
c
c
λmax λonset Eg
polymer (nm) (nm) (eV) (nm) (nm) (eV)
λmax λonset Eg
3a
3b
3c
998 1160 1.07 905 1208 1.02
870 1170 1.06 870 1390 0.89
871 1300 0.95 880 1570 0.79
6.2 × 10-6
2.2 × 10-5
5.3 × 10-4
a In toluene. b Film cast from chloroform solution. c Calculated from the
onset of absorption. d Measured on rectangular pellets by two-point probe
method using a Kiethley Electrometer.
influence on their absorption properties, Eg, and electrical
conductivities. The solution band gap of 3c is one of the
lowest values reported for any conjugated polymer.14
The intrinsic conductivity of 3a is 6.2 × 10-6 S/cm,
whereas 3c showed an enhanced conductivity of 5.3 × 10-4
S/cm, which reveals that the length of the alkyl chains plays
a key role in the conducting properties of these materials.
X-ray data reveal that steady increase in the alkyl chain
length could produce disruption of the stacking by increasing
the d-spacing within the layered assemblies as observed in
the case of regioregular polythiophenes.15 For example, 3c
showed an interplanar distance of 11 Å with an interlayer
π-stacking distance of 3.4 Å, indicating compact molecular
packing with ordered microstructural domains. The ordered
domains present in such self-organized microstructures of
π-conjugated polymers have a crucial role in their charge
carrier mobilities and photopysical properties.16 On increasing
the length of the hydrocarbon side chains from 3b to 3a,
both interplanar and interlayer spacing increased from 17 to
20.5 and 3.8 to 4.3 Å, respectively, indicating considerable
disruption of the π-stacking. Consequently, the conductivities
showed a drastic decrease while going from 3c to 3a.
standards.13 Further purification of the polymer did not show
any change in the broad dispersity, indicating that this could
be due to the possible aggregation of the polysquaraines.
Thermogravimetric analysis (TGA) under nitrogen atmo-
sphere revealed a decomposition temperature around 300 °C
for 3a-c.
The UV-vis-NIR absorption spectra of 3a-c revealed
several interesting properties, which are unique to the new
polymers. For example, the intense and broad NIR absorption
of 3a-c in toluene with vibronic features around 750-1050
nm in comparison with the sharp absorption of 4 is an
indication of a high degree of conjugation and planarity of
the former (Figure 2). Since the ground-state S0 and the first
excited singlet state S1 of squaraines represent intramolecular
(13) As a result of the zwitterionic and rigid structure of 3a its molecular
weight calibrated against polystyrene standards may not reflect the true
value. (a) Grell, M.; Bradley, D. D. C.; Long, X.; Chamberlain, T.;
Inbasekaran, M.; Woo, E. P.; Soliman, M. Acta Polym. 1998, 49, 439. (b)
Huang, S. L.; Tour, J. M. J. Am. Chem. Soc. 1999, 121, 4908.
(14) Eg determined from the onset of absorption of a polymer film is
strongly influenced by the thickness and the mode of preparation. On the
other hand, Eg determined from the onset of solution spectrum is more
reliable.
(15) (a) McCullough, R. D.; Tristram-Nagle, S.; Williams, S. P.; Lowe,
R. D.; Jayaraman, M. J. Am. Chem. Soc. 1993, 115, 4910. (b) Chen, T.-A.;
Wu, X.; Rieke, R. D. J. Am. Chem. Soc. 1995, 117, 233.
(16) (a) Sirringhaus, H.; Brown, P. J.; Friend, R. H.; Nielsen, M. M.;
Bechgaard, K.; Langeveld-Voss, B. M. W.; Spiering, A. J. H.; Janssen, R.
A. J.; Meijer, E. W.; Herwing, P.; de Leeuw, D. M. Nature 1999, 401,
685. (b) McQuade, D. T.; Kim, J.; Swager, T. M. J. Am. Chem. Soc. 2000,
122, 5885.
Figure 2. Absorption spectra of polysquaraines 3a-c and 4 in
toluene.
Org. Lett., Vol. 3, No. 16, 2001
2597

In conclusion, we have shown that the optical and
electronic properties of polysquaraines can be significantly
altered to the low band gap region when coupled with a
strongly electron-donating conjugated moiety. The broad and
structured NIR absorption spectra of the new polysquaraines
suggest a high degree of conjugation and planarization of
the polymer backbone. This is one of the simplest examples
to demonstrate how optical and electronic properties of
donor-acceptor-type conjugated polymers can be tuned by
the logical selection of their molecular components. We
foresee a wider use of the present approach for the designing
of new π-conjugated materials with novel optoelectronic
properties.
Acknowledgment. This is manuscript RRLT-PRU- 130.
This work was supported by the Department of Science and
Technology (DST Grant-in-aid SP/S1/G-11/97) and the
Council of Scientific and Industrial Research, Government
of India (SRF to J.E.).
Supporting Information Available: Experimental pro-
cedures and characterization data of 1a-c, 3a-c, and 4. This
material is available free of charge via the Internet at
http://pubs.acs.org.
OL016284P
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