Synthesis and properties of novel low band-gap polymers bearing squaraine units

Article abstract of DOI:10.1016/j.cclet.2009.09.003

Novel main-chain-conjugated poly(carbazol-alt-squaraine) and poly(dipyridyl-alt-squaraine) were successfully synthesized through direct polycondensation of 9-(2-ethylhexyl)carbazole-bridged or dipyridyl-bridged bispyrrole and squaric acid. The structures

Full text of DOI:10.1016/j.cclet.2009.09.003

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Chinese Chemical Letters 21 (2010) 245–248
www.elsevier.com/locate/cclet
Synthesis and properties of novel low band-gap
polymers bearing squaraine units
*
Wei Zhang, Zheng Wang, Yu Sheng Tang, Zhi Gang Xu, Ying Li , Qing Jiang
College of Chemistry, Sichuan University, Chengdu 610064, China
Received 23 June 2009
Abstract
Novel main-chain-conjugated poly(carbazol-alt-squaraine) and poly(dipyridyl-alt-squaraine) were successfully synthesized
through direct polycondensation of 9-(2-ethylhexyl)carbazole-bridged or dipyridyl-bridged bispyrrole and squaric acid. The
structures and properties of the polymers were characterized using 1H NMR, FT-IR, UV–vis and cyclic voltammetry. Both polymers
exhibit excellent solubility in common organic solvents and good thermal stability. Their UV–vis absorption spectra indicated the
polymers have broad and strong spectral responses from 200 nm to 900 nm, which reveals a low optical band gap around 1.38 eV,
suggesting that they may be promising candidates for organic solar cells.
# 2009 Ying Li. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Squaraine; Polymer; Synthesis; Low band gap; Organic solar cell material
For the past few years, solar cells based on conjugated polymers have been of great interest because of their low
cost, ease of processing and flexibility [1–3]. However, polymer-based solar cells are, in general, of low efficiency
compared to their inorganic counterparts due to their relatively low mobility of charge carriers, narrow absorption
bands and large band gaps [4].
Squaraines, 1,3-disubstituted derivatives of squaric acid, which were extensively used for dye-sensitized solar cells
[5], exhibit intense absorption bands in the visible and near IR regions. Ajayghost [6] have reported some low band gap
polymers bearing squaraine units. Herein, we reported an alternative strategy to synthesize novel low band-gap polymers
with a donor-acceptor structure where carbazole units or dipyridyl units were employed as donor and squaric acid was
used as acceptor. We expected that squaraines unit introduced to the main chain of the polymers could harvest a larger
fraction of the solar photo flux, carbazole or dipyridyl units could improve the transporting of charge carriers. Moreover,
bulky side chains were attached onto the conjugated backbone to achieve good solubilities and high molecular weights.
1. Experimental
Scheme 1 showed the synthesis route of the monomers and polymers. The monomer M2 [7] was synthesized
according to the literature. M1 [8] was prepared by the reaction of N-hexadecylpyrrole-2-carbaldehyde and
* Corresponding author.
E-mail address: profliying@sina.com (Y. Li).
1001-8417/$ – see front matter # 2009 Ying Li. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
doi:10.1016/j.cclet.2009.09.003

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W. Zhang et al. / Chinese Chemical Letters 21 (2010) 245–248
Scheme 1. The synthetic route of the monomers and the polymers.
N-(2-ethylhexyl)-3,6-bis(methylphosphonate)carbazole 2 [9]. P1 [10] was synthesized through direct polycondensa-
tion of M1 and squaric acid using toluene and n-butanol as solvents. The synthesis of P2 [10] has the similar procedure
with P1. The comonomer feed molar ratio of M2 to N-(2-ethylhexyl)pyrrole was 1:1.
M1: Light yellow oil. Yield 80%. 1H NMR (400 MHz, CDCl3): d 8.17 (s, 2H), 7.63 (d, 2H, J = 8.4 Hz), 7.35 (d, 2H,
J = 8.4 Hz), 7.13 (d, 2H, J = 8.0 Hz), 7.00 (d, 2H, J = 8.0 Hz), 6.71(s, 2H), 6.53 (d, 2H, J = 3.2 Hz), 6.21 (t, 2H,
J = 6.0 Hz), 4.16(m, 2H), 4.03 (t, 4H, J = 14.4 Hz), 2.08 (m, 1H), 1.83 (m, 4H), 1.26–1.46 (m, 60H), 0.90 (m, 12H).
HRMS (ESI) calcd. for C64H99N3 909.7839, found: 910.7923 [M + H]+.
P1: Deep blue solid. Yield 75%. 1H NMR (400 MHz, CDCl3): d 0.85 (s, 12H), 0.95–2.11 (m, 65H), 4.0–4.2 (s, 4H),
4.92 (s, 2H), 6.20–8.20 (m, 14H); IR (KBr, pellet, cmÀ1): 3439, 2922, 2851, 1622, 1488, 1382, 1262, 1098, 801, 620.
P2: Deep blue solid. Yield 72%. 1H NMR (400 MHz, CDCl3): d 0.88 (s, 18H), 1.10–1.60 (m, 27H), 4.08 (m, 4H),
4.86 (s, 2H), 6.9 (s, 1H), 7.0 (s, 1H), 7.38 (s, 3H), 7.53 (s, 3H), 7.72 (s, 2H), 7.96 (s, 1H), 8.10 (s, 1H), 8.39 (s, 2H), 8.67
(s, 2H); IR (KBr, pellet, cmÀ1): 3426, 2927, 2864, 1623, 1463, 1385, 1262, 1103, 819, 620.
2. Results and discussion
As shown in Scheme 1, polymers were obtained through direct condensation of monomer with squaric acid. They
have good solubility in common organic solvents such as CHCl3, THF and toluene due to the long alkyl side chain was
introduced to pyrrole unit. GPC results, summarized in Table 1, showed that the polymers had high weight-average
molecular weight (Mw) which is higher than 20000 with polydispersity indices around 2.0 and 2.5. The thermal
stability of the polymers was studied by TGA under a nitrogen atmosphere. Fig. 1 shows that the onset temperatures of

W. Zhang et al. / Chinese Chemical Letters 21 (2010) 245–248
247
Table 1
The properties of P1 and P2.
elec
Polymer
P1
Mw
Mn
8268
10019
PDI
2.53
l
maxsol (nm)
HOMO (eV)
LUMO (eV)
Eg
(eV)
Td (8C)
20866
685
727
636
718
À5.28
À3.83
1.45
1.53
388
P2
20196
2.02
À5.33
À3.80
317
Fig. 1. Thermal gravimetric analysis curves of the polymers.
the weight loss of P1 and P2 are 388 8C and 317 8C, indicating that the target polymers have good thermal stability, this
probably because of the donor-acceptor-structured backbone.
Cyclic voltammetry which is shown in Fig. 2 was applied to estimate the values of the highest occupied molecular
orbital (HOMO), the lowest unoccupied molecular orbital (LUMO) and band gap. According to empirical formulas
(EHOMO = À(Eonset,ox + 4.4) eV and ELUMO = À(Eonset,red + 4.4) eV), where Eonset,ox and Eonset,red are the onset
potentials for oxidation and reduction vs SCE, 4.4 is a scale factor relating SCE to vacuum [11], HOMO and LUMO
elec
levels calculated were shown in Table 1. Eg
was 1.45 eV for P1 and 1.53 eV for P2.
The UV–vis absorption spectra of polymers P1 and P2 in the solution of THF are shown in Fig. 3 where two distinct
absorption bands can be observed, one locates at 200–520 nm and the other at 520–900 nm. The shorter wavelength
absorption band could be ascribed to the p–p* transition of the conjugated polymer backbone, while the longer
Fig. 2. Cyclic voltammograms of the polymers.

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W. Zhang et al. / Chinese Chemical Letters 21 (2010) 245–248
Fig. 3. Uv–vis absorption spectra for the polymers in THF solution (1 Â 10À5 mol/L).
wavelength band should be attributed to a charge transfer state in the D-A-D segment. From the onset of absorption
(ꢀ900 nm), the optical band gaps of P1 and P2 can be estimated to be as low as 1.38 eV [12]. They are basically in
accordance with the electrochemical energy gaps. Also it confirms that the introduction of squaric acid as a strong
acceptor in the D-A-D-segment could give the polymers low band-gaps.
In conclusion, we have synthesized a series of low optical Eg polymers with intense absorption in the visible and
near IR regions. The incorporation of carbazole and dipyridyl as bridging unit with squaraine moieties not only
increased the solubilities and stabilities of the polymers, but also improved their electronic and optical properties. The
photophysical and electrochemical properties show that they may be potential candidates for solar cells. Future studies
on the application as photovoltaic materials are now in progress.
Acknowledgments
This work was supported by the key foundation of education ministry of China (No. 20070610053) and Sichuan
Province Foundation for Youths (No. 2008JY0050). The authors also acknowledge the Analytical & Testing Center of
Sichuan University for the NMR measurements.
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