Cyclopenta[c]thiophene-Based D–A Conjugated Copolymers: Effect of

Article pubs.acs.org/Macromolecules

Cyclopenta[c]thiophene-Based D−A Conjugated Copolymers: Effect of Heteroatoms (S, Se, and N) of Benzazole Acceptors on the Properties of Polymers Soumyajit Das, Palas Baran Pati, and Sanjio S. Zade* Department of Chemical Sciences, Indian Institute of Science Education and Research, Kolkata, PO: BCKV campus main office, Mohanpur 741252, Nadia, West Bengal, India S Supporting Information *

ABSTRACT: Three new donor−acceptor (D−A) type copolymers P1, P2, and P3 have been synthesized by Stille condensation between the distannyl derivative of thiophenecapped cyclopenta[c]thiophene (CPT) with 4,7-dibromo[2,1,3]benzothiadiazole, 4,7-dibromo[2,1,3]benzoselenadiazole, and 4,7-dibromo[2,1,3]benzotriazole, respectively. These new CPT-based D−A copolymers showed an interesting trend of visible color (red, green, and blue) in solution as the acceptor was varied keeping the donor constant. The optical band gaps of the polymers, which were estimated by measuring the absorption onset in the UV−vis spectra of the film, were found to be 1.57, 1.44, and 1.86 eV for P1, P2, and P3, respectively. DFT calculations correlated the strength of the acceptors with the interesting trend in the colors of these (D)nonvariant−(A)variant copolymers. Compared with the solution, the film state absorption of P2 and P3 was significantly red-shifted compared to that of P1, indicating the presence of strong interchain interactions due to efficient self-π-stacking in the solid state.

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INTRODUCTION The potential use of π-conjugated polymers in electrochromic devices, field-effect transistors, light-emitting diodes, and photovoltaic cells has encouraged researchers globally to develop new optoelectronic materials with unique properties.1−3 Structural variation is the key in order to tune the band gap in conjugated polymers, and thus the optical and electronic properties which are possible by replacing heteroatom in a homopolymers4−7 or by constructing a π-conjugated chain consisting alternating electron-rich donor (D) and electrondeficient acceptor (A) units into one system that would broaden its valence and conduction bands to afford a low-bandgap polymer.8−11 In such polymer systems, the HOMO (highest occupied molecular orbital) is mainly located at the donor unit and the LUMO (lowest unoccupied molecular orbital) is located at the acceptor unit, and thus both HOMO and LUMO energy levels can be narrowed or widened on the basis of the selection of donor and acceptor units.8,12−15 It is theoretically proved by Salzner et al. that a proper selection of donor and acceptor in D−A copolymer can tune the band gap relying on the moderate to excellent intramolecular charge transfer (ICT) interaction between donor and acceptor units.16 This D−A approach has afforded many low-band-gap polymers which may produce high power conversion efficiency in organic/polymer solar cells.17−22 Electron-withdrawing imine nitrogens (CN) are known to act as acceptor-type building blocks, and among the three most popular benzazoles[2,1,3]benzothiadiazole (BDT), [2,1,3]© 2012 American Chemical Society

benzoselenadiazole (BDS), and [2,1,3]benzotriazole (BTAz) BDS was reported to be the best electron-accepting unit which can be attributed to the large polarizability and the electrochemical amphotericity of selenium atoms, followed by BDT, whereas BTAz is the poorest among them. The D−A strategy was successfully studied on BDT,23 BDS,24 and BTAz25 based systems. Polythiophenes are red in their neutral forms, but based on this D−A strategy, several green,23,26−30 blue,31,32 yellow,33 and black34 neutral-state polymers were synthesized electrochemically as well as chemically, and some of them are also reported to be solution processable.26,28,30 Among the many π-conjugated thiophene derivatives,3 cyclopenta[c]thiophene (CPT) based oligothiophenes35 or quinoidal oligomers36,37 have been successfully explored in sensor and optoelectronic applications. Surprisingly, although the first polycyclopenta[c]thiophene (PCPT) was reported in 1987,38,39 not much study on PCPT derivatives was found in the literature until our recent investigations revealed that cyclopentane substitution on thiophene or selenophene unit can increase their respective polymer’s oxidative stability than one of the most sought-after classes of thiophene and selenophene derivatives 3,4-ethylenedioxythiophene (EDOT) and 3,4-ethylenedioxyselenophene (EDOS) which possess very low oxidation potential that restrict their application as a lightReceived: April 2, 2012 Revised: June 12, 2012 Published: June 26, 2012 5410

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MHz, CDCl3): δ 7.17 (dd, J = 5.5 Hz, 1.3 Hz, 2H), 7.12 (dd, J = 3.6 Hz, 0.9 Hz, 2H), 6.94 (dd, J = 5.0 Hz, 3.6 Hz, 2H), 3.45−3.40 (m, 8H), 2.62 (s, 4H), 1.55 (m, 4H), 1.29−1.24 (m, 36H), 0.87 (t, J = 6.4 Hz, 6H). 13C NMR (100 MHz, CDCl3): δ 131.1, 126.8, 126.0, 124.2, 91.3, 75.9, 71.9, 71.7, 43.1, 32.0, 29.8, 29.7, 29.6, 29.5, 26.3, 23.6, 22.8, 14.2. HRMS calculated for C41H64O2S2 [M + H]+, 653.4426; found 653.4416. Synthesis of 3. To Cp2ZrCl2 (807 mg, 2.76 mmol) in 40 mL of dry THF was added n-BuLi (3.5 mL, 5.52 mmol, 1.6 M in hexane) at −78 °C under nitrogen. After the solution was stirred for about 10 min at the same temperature, thiophene-capped diyne 2 (1.5 g, 2.3 mmol) in 5 mL of dry THF was added dropwise. The resulting solution was allowed to warm to room temperature and stirred for an additional 2 h. The color of the solution changed from pale yellow to dark brown-red. S2Cl2 (200 μL, 2.53 mmol) was added slowly via syringe at 0 °C, and the solution was stirred overnight. The reaction mixture was then exposed to sunlight for ∼5 h following rotary evaporation of the solvent, and the resulting mixture was then loaded on a 230−400 mesh silica gel to perform column chromatography using hexanes and 1−2% ethyl acetate in hexanes. After column, a yellow-red solid was isolated which was redissolved in dichloromethane and exposed to sunlight for ∼2 h. Then the reaction mixture was again column chromatographed with 1−2% ethyl acetate in hexanes to obtain 3 as pure yellow solid (yield = 40%). 1H NMR (400 MHz, CDCl3): δ 7.21 (d, J = 5.0 Hz, 2H), 7.10 (d, J = 3.6 Hz, 2H), 7.02 (dd, J = 5.0 Hz, 3.6 Hz, 2H), 3.43− 3.40 (m, 8H), 2.75 (s, 4H), 1.53 (m, 4H), 1.24 (m, 36H), 0.87 (t, J = 6.4 Hz, 6H). 13C NMR (100 MHz, CDCl3): δ 143.7, 137.2, 127.6, 125.0, 124.0, 123.1, 74.0, 71.5, 55.3, 34.7, 31.9, 29.7−29.3, 26.2, 22.6, 14.1. HRMS calculated for C41H64O2S3 [M + H]+, 685.4147; found 685.4129. Synthesis of 4. NBS (363 mg, 2.04 mmol) was added in 30 mL of DMF solution of 3 (700 mg, 1.02 mmol) at 0 °C. The reaction mixture was warmed to room temperature and left for 12 h. Saturated sodium bicarbonate solution was added and worked up using chloroform and water. The organic layer was dried over sodium sulfate, and solvent was evaporated to afford the dibromo derivative 4 as yellow solid (yield = 80%). 1H NMR (400 MHz, CDCl3): δ 6.96 (d, J = 3.6 Hz, 2H), 6.83 (d, J = 4.1 Hz, 2H), 3.43−3.40 (m, 8H), 2.68 (s, 4H), 1.54 (m, 4H) 1.32−1.21 (m, 36H), 0.87 (t, J = 6.8 Hz, 6H). 13C NMR (125 MHz, CDCl3): δ 144.3, 138.5, 130.4, 124.4, 123.2, 110.9, 73.8, 71.5, 55.4, 34.3, 31.9, 29.7−29.3, 26.2, 22.6, 14.1. HRMS calculated for C41H62Br2O2S3 [M + Na]+, 863.2176; found 863.2161. Synthesis of 5. n-BuLi (0.18 mL, 0.28 mmol) was added to a THF solution of 4 (110 mg, 0.13 mmol) at −78 °C under nitrogen and stirred for 2 h at the same temperature. Then SnMe3Cl (286 μL, 0.286 mmol, 1 M solution in hexane) was added at −78 °C and stirred at the same temperature for an hour and left overnight at room temperature. The reaction mixture was washed with copious amounts of water and extracted with diethyl ether. The organic layer was dried over sodium sulfate and evaporated to dryness to afford desired distannyl derivative 5 in nearly quantitative yield. This was used further without any purification. 1H NMR (400 MHz, CDCl3): δ 7.20 (d, J = 3.6 Hz, 2H), 7.08 (d, J = 3.6 Hz, 2H), 3.44−3.41 (m, 8H), 2.76 (s, 4H), 1.55 (m, 4H) 1.28−1.25 (m, 36H), 0.87 (t, J = 6.8 Hz, 6H), 0.38 (s, 9H). Synthesis of P1. To an oven-dried 50 mL two neck round-bottom flask equipped with a stir bar under nitrogen was added dry toluene and charged with 5 (120 mg, 0.118 mmol) and 6 (35 mg, 0.118 mmol). Solution was purged with nitrogen for 15−20 min. Pd2(dba)3 (10 mol %) and P(o-tolyl)3 (50 mol %) were added into the solution mixture. Color changes within a few minutes as the reaction progresses under refluxing condition. The reaction mixture was stirred at 110 °C and formation of polymer was monitored by precipitating some reaction mixture taken in a capillary into methanol. Finally, after 24 h the reaction vessel was cooled to room temperature, and the resulting polymer was precipitated with methanol. The precipitate was collected by filtration, and collected polymer was subjected to Soxhlet extraction with methanol and acetone for several hours. The polymer was extracted with chloroform and collected after precipitating into methanol. The dark-blue polymer P1 (86 mg, yield = 89%) was then isolated after drying in vacuo. 1H NMR (400 MHz, CDCl3): δ

absorbing donor in polymer solar cells. Recently, we synthesized a didodecyloxymethyl-functionalized CPT-based π-conjugated quasi-planar polythiophene with reasonable oxidative stability that showed good hole mobility in PFET devices.40 We also reported the application of CPT-based D−A copolymers in field-effect transistors.41 Herein, we report the synthesis of three new D−A copolymers which comprise of a common donor, didodecyloxymethyl-functionalized thiophenecapped CPT, but different acceptors; as a consequence, the most important three colors (red, green, blue) are produced in neutral state as solution under visible light. Additionally, DFT calculations were performed to understand their spectral properties.

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EXPERIMENTAL DETAILS

Materials. Diethyl malonate, diisopropylamine, and sodium sulfate were purchased from Merck. n-Butyllithium (n-BuLi, 1.6 M in hexane) was purchased from Neo-Synth. 2-Bromothiophene, tetrakis(triphenylphosphine)palladium Pd(PPh3)4, cuprous iodide (CuI), Nbromosuccinimide (NBS), N,N-dimethylformamide (DMF), tetrabutylammonium perchlorate (TBAPC), tetrabutylammonium hexafluorophosphate (TBAPF6), 1-bromododecane, bis(cyclopentadienyl)zirconium(IV) dichloride (Cp2ZrCl2), tris(dibenzylideneacetone)dipalladium (Pd 2 (dba) 3 ), (o-tolyl) 3 phosphine (P(o-tolyl) 3 ), trimethyltin chloride (Me3SnCl), and ferrocene were obtained from Aldrich and were used as received. Disulfur dichloride (S2Cl2) was purchased from Merck (Germany). Toluene and tetrahydrofuran (THF) were freshly distilled from sodium/benzophenone prior to use. Compound 1 was synthesized according to our previous report.40 The dibromo derivative of BDT (6),42 BDS (7),42 and BTAz (8)25 were prepared following literature methods. Characterization. 1H NMR and 13C NMR spectra were collected on a Jeol JNM-ECS400 spectrometer, with tetramethylsilane (TMS) as the internal reference; chemical shifts (δ) were recorded in parts per million (ppm). Molecular weight and polydispersity of the polymer were determined by gel permeation chromatography (GPC, Waters 2414) analysis with polystyrene as the standard using THF (HPLC grade) as eluent at a flow rate of 0.3 mL/min at room temperature. Thermogravimetric analysis (TGA) was conducted on a Mettler Toledo TGA/SDTA 851 thermogravimetric analyzer with a heating rate of 10 °C min−1 under nitrogen. Differential scanning calorimetry (DSC) was recorded with a Mettler Toledo DSC 1 under nitrogen at a heating rate of 10 °C min−1. The UV−vis absorption spectra were recorded on a Hitachi U-4100 spectrophotometer. For solid state measurements, polymer solution (5 mg/mL in chloroform) was dropcasted on glass plates and dried. Cyclic voltammetry was performed with a computer-controlled Princeton Applied Research 263A electrochemical workstation using polymer drop-cast film on platinum (Pt) disk as a working electrode, Pt wire as the counter electrode, and nonaqueous Ag/AgCl as the pseudoreference electrode. Nonaqueous Ag/AgCl wire was prepared by dipping silver wire in a solution of = FeCl3 and HCl. Ferrocene was used as an external standard, Eferrocene 1/2 0.35 V vs Ag/AgCl. 0.1 M TBAPC or TBAPF6 dissolved in acetonitrile (ACN) was used as supporting electrolyte. Spray coated films of polymers were prepared from chloroform solution by using Aztek A470 airbrush. Synthesis of 2. 2-Bromothiophene (1.1 g, 0.65 mL, 6.75 mmol), 4,4-bis(dodecyloxymethyl)hepta-1,6-diyne 1 (1.5 g, 3.06 mmol), Pd(PPh3)4 (347 mg, 10 mol %), and CuI (57 mg, 10 mol %) were stirred in 30 mL of dry THF under nitrogen. To this solution was added 20 mL of diisopropylamine. The resulting solution was warmed at 55 °C for 12 h using oil bath. The mixture was filtered through Celite pad. Filtrate was diluted with 50 mL of diethyl ether and washed with 10% NH4OH, water, 2 N HCl, and water. The organic layer was separated and dried over sodium sulfate. After the volatile material was removed by rotary evaporation, the residue was subjected to column chromatography (230−400 mesh silica gel, 1% ethyl acetate in hexanes) to isolate 2 as sticky liquid (yield = 70%). 1H NMR (400 5411

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Scheme 1. Synthetic Route to D−A Copolymers (P1, P2, P3) and Their Precursors

8.09 (br, 2H), 7.82 (br, 2H), 7.33 (br, 2H), 3.51−3.47 (br, 8H), 2.86 (br, 4H), 1.58−0.85 (br, 46H). GPC analysis: Mw = 51 000 Da, Mn = 23 000 Da, and PDI = 2.2. Synthesis of P2. Polymer P2 was synthesized following analogous method and using same catalyst as for P1. Starting with 5 (180 mg, 0.178 mmol) and 7 (61 mg, 0.178 mmol), P2 was isolated as a dark blue-green solid (97 mg, yield = 62%). 1H NMR (400 MHz, CDCl3): δ 7.98 (br, 2H), 7.76 (br, 2H), 7.61 (br, 2H), 3.49−3.46 (br, 8H), 2.86 (br, 4H), 1.57−0.85 (br, 46H). GPC analysis: Mw = 55 000 Da, Mn = 20 000 Da, and PDI = 2.7. Synthesis of P3. Polymer P3 was synthesized following analogous method and using same catalyst as for P1 and P2. Starting with 5 (130 mg, 0.128 mmol) and 8 (58 mg, 0.128 mmol), P3 was isolated as a brown solid (80 g, yield = 64%). 1H NMR (400 MHz, CDCl3): δ 8.07 (br, 2H), 7.56 (br, 2H), 7.44 (br, 2H), 4.81 (br, 2H) 3.49−3.46 (br, 8H), 2.85 (br, 4H), 2.23 (br, 4H), 1.6−0.85 (br, 65H). GPC analysis: Mw = 51 000 Da, Mn = 31 000 Da, and PDI = 1.6.

functionalized dialkyne 1 was prepared following our previously reported procedure.40 The terminal dialkyne 1 was converted into internal acetylene by capping thiophene units using Sonogashira coupling to afford 2 in 70% yield which was converted into 3 via in situ prepared “Cp2Zr” species (prepared by dropwise addition of n-BuLi into the solution of zirconocene dichloride) followed by quenching with disulfur dichloride what accompanied a color change from yellow to brown-red to finally deep red due to the presence of 1,2-dithiin compound.35 After stirring overnight under nitrogen, the reaction mixture was exposed to sunlight (to extrude one sulfur from 1,2-dithiin) for ∼5 h and then directly loaded on silica gel column. The flash column was performed using hexanes first (to take out sulfur and other impurities) followed by 1−2% ethyl acetate in hexanes to obtain the pure didodecyloxymethyl-functionalized thiophene-capped CPT 3 as yellow solid in 40% yield. We found it necessary to convert compound 3 to its corresponding dibromo derivative 4 as the dilithiation of 4 proceed smoothly compared to 3. Compound 3 was treated with NBS to afford the dibromo derivative 4 in 80% yield. Lithiation of 4 in dry THF followed by quenching with SnMe3Cl afforded the distannyl derivative 5 in nearly quantitative yield. The Pd2(dba)3 mediated Stille condensation of 5 with the dibromo derivatives 6, 7, and 8 afforded the D−A type polymers P1, P2, and P3, respectively, which interestingly resulted in blue (P1), green (P2), and red (P3) color in neutral state in chloroform solution satisfying the three complementary colors (RGB) suggesting their use as soluble conducting polymers for polymeric electrochromic devices. This observation could be attributed to different extent of ICT arising due to varying

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COMPUTATIONAL DETAILS Density functional theory (DFT) calculations were performed using the Gaussian 0943 program at the B3LYP/6-31G(d) level. The D−A co-oligomers 3TCPTTBDT, 3TCPTTBDS, and 3TCPTTBTAz were first optimized at B3LYP/6-31G(d) level by replacing dodecyl side chains by methyl groups to save computational time. Then the time-dependent density function theory (TD-DFT) calculations were performed on the optimized structures using same level and basis sets. No frequency calculations were performed.

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RESULTS AND DISCUSSION The synthetic routes to the D−A copolymers and their precursors are shown in Scheme 1. The didodecyloxymethyl5412

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Figure 1. (a) TGA thermogram of the polymers at a heating rate of 10 °C/min under nitrogen. (b) CV of P1 and P2 on a Pt working electrode in ACN/TBAPC solvent/electrolyte couple and P3 on a Pt working electrode in ACN solution of TBAPF6 at scan rate of 50 mV/s.

Figure 2. (a) UV−vis absorption spectra of P1−P3 in chloroform solution (1.2 × 10−5 M). (b) Normalized UV−vis spectra of P1−P3 in solution and film state.

and P3, respectively (Figure 1a). The DSC thermogram of P1, P2, and P3 show significant glass transition temperatures (Tg) at 56, 59, and 67 °C, respectively (see Supporting Information, Figure S1). The cyclic voltammogram (CV) of P1, P2, and P3 polymer films on a Pt electrode revealed quasi-reversible pdoping/dedoping processes over a positive potential range and irreversible n-doping/dedoping processes over a negative potential range in ACN/0.1 M TBAPC or TBAPF6 solvent/ electrolyte couple at 50 mV/s scan rate. The oxidation potentials of P1, P2, and P3 are +1.13, +1.07, and +1.08 V with their onset oxidation potentials observed at +0.76, +0.72, and +0.67 V, respectively, against Ag/AgCl (Figure 1b). The respective HOMO energy levels for P1, P2, and P3 are evaluated44 as −5.21, −5.17, and −5.12 eV which reveals their sufficient air stability. The LUMO level was estimated relative to the HOMO from the optical band gap. Compared to P1, the first oxidation potential of both P2 and P3 is shifted to a lower value which should be attributed to a more extended πconjugation in P2 and poor electron-acceptance nature of BTAz making the P3 more susceptible to oxidation. Alternatively, the ionization energy decreases as one moves from S to Se (larger atomic radius) what may also be a reason for lower oxidation potential of P2. The electrochemical bandgaps for P1, P2, and P3 are determined from the oxidation and reduction onset difference and are as 1.71, 1.64, and 1.73 eV, respectively. The UV−vis absorption spectra of the polymers as chloroform solution and in film state are shown in Figure 2

acceptance strength of the acceptors. The polymers were purified by continuous extractions with methanol and acetone for consecutively 4 days using Soxhlet apparatus. The chemical structure of the polymers was confirmed by 1H NMR (see Supporting Information) where the characteristic −CH2 protons of cyclopentane ring were shifted more downfield (2.86 ppm) in polymers than their precursor distannylterthiophene derivative 5 (appeared at 2.76 ppm). The signals for polymers at 8.09−7.3 ppm range may be assigned to the resonance of protons on thiophene and phenyl rings. The −N− CH2 protons appear at 4.81 ppm, and the signals for at 3.51− 3.46 ppm correspond to the protons of the −CH2−O−CH2 on cyclopentane ring. The signals at 1.6−0.85 ppm should be assigned to the protons of long dodecyl chain except the −O− CH2 protons. The more upfield shift of aromatic protons in P3 at 7.56 ppm compared to its S or Se analogue (appeared at 7.82−7.76 ppm) indicates the poor acceptance nature of BTAz. The number-average molecular weights (Mn) of the copolymers P1, P2, and P3 were determined as 23, 20, and 31 kDa with polydispersity index (PDI) value of 2.2, 2.7, and 1.6, respectively, as determined by GPC against a polystyrene standard in THF at room temperature. Nonetheless, these polymers are still highly soluble in common organic solvents (THF, chloroform, toluene, dichlorobenzene) due to the two dodecyl side chains. The thermal properties of the polymers P1, P2, and P3 were evaluated by TGA and DSC. The polymers exhibited good thermal stability under nitrogen, showing 5% weight loss on heating at ∼365, 280, and 300 °C for P1, P2, 5413

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Table 1. Characterization Data of P1−P3 λmax (nm) polymer

Mw (kDa)

Mn (kDa)

Tg (°C)

5% wt loss (°C)

HOMO (eV)

LUMO (eV)

solution

film

band gapexp (eV)

P1 P2 P3

51 55 51

23 20 31

56 59 67

365 280 300

−5.21 −5.17 −5.12

−3.64 −3.73 −3.26

418, 624 340, 428, 622 522

422, 639 432, 645 544

1.57 1.44 1.86

Figure 3. Spectroelectrochemistry of (a) P1, (b) P2, and (c) P3 polymer film drop-casted on ITO as a function of different applied potential.

in chloroform solution. Another electron-acceptor BDT strongly influences on the UV−vis spectra, absorbing strongly in the red region at λmax = 624 nm with a relatively weak absorbance at λmax = 418 nm reflecting blue (B) color of P1 in chloroform solution. The loss of intensity of the low-energy band in P2 could be due to the loss of electronegativity and the acceptor unit’s ability to separate charge. This phenomenon in even more pronounced in BTAz-based P3 polymer in which poor electron-acceptance property of BTAz moiety hardly be able to separate charge and results in red (R) color of the P3 in chloroform solution. Hence, the BTAz-based polymer P3 lacks the ICT band and is consistent with the literature.46,47 This reflects that a strong ICT interaction from thiophene-capped CPT to BDT compared to its Se counterpart although the band gap is much reduced in P2 due to high absorption onset which could be attributed to more dominating second resonance structure (−D+A−−) over neutral structure (−D−A−) that increases the double-bond character of the bonds between donor and acceptor units in the P2 polymer. Another reason for band gap reduction was suggested45 and should be the degree of bond length alternation (S−N or Se−N) in the benzene ring that increases in the order S to Se trending toward a decrease in aromaticity that affects the conjugation among donor and acceptor units resulting in destabilization of the HOMO, stabilization of the LUMO, and an overall decrease in the band gap of the P2 polymer. To establish a charge transfer

and summarized in Table 1. Both P1 and P2 exhibited wide absorption with two strong bands in blue and red region covering the entire visible region (300−800 nm), whereas P3 shows only one intense absorption band at λmax = 522 nm (Figure 2a). Similar to the BDS-based D−A copolymer reported by Seferos et al.,45 our BDS-based copolymer P2 shows two distinct absorptions at λmax = 340 and 428 nm in the high-energy region and also shows an intense absorbance at λmax = 622 nm in the low-energy region. The film state absorption of P2 and P3 is significantly red-shifted compared to that of P1, indicating the presence of strong interchain interactions due to efficient self-π-stacking in the solid state (Figure 2b). The red-shifted π−π* transition corresponding to triazole-based polymer P3 is, unlike P1 or P2, associated with a shoulder at 606 nm as a result of vibronic splitting. Although P1, P2, and P3 contain the same donor moiety, the polymers show different UV−vis absorption spectral pattern relying on the extent of ICT interaction in their polymer backbone because of covalently linked different acceptor moieties. A strong ICT is expected between the electronsufficient thiophene−CPT−thiophene and electron-deficient Sand Se-based BDT and BDS unit compared to its nitrogen counterpart. The electron-accepting property of BDS strongly influences the UV−vis spectra, resulting in an almost evenly balanced absorbance in both red and blue region that results in broad absorbance reflecting green (G) color of the polymer P2 5414

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state, we have done solvatochromic experiments.45 The absorption and emission spectra were recorded for polymers P1, P2, and P3 in solvents of increasing dielectric constants (benzene (2.3), chloroform (4.8), and THF (7.5)) (Figure S4). The absorption and emission spectra of P3 did not show any solvatochromic changes. The absorption spectra of polymer P1 remained same in all three solvents with λmax of 620 nm. However, emission spectra of P1 showed red shift of ∼15−20 nm by changing solvent benzene to chloroform or THF. The absorption spectra P2 showed blue shift of 15 nm in THF compared to that in chloroform and benzene. It can be attributed to the stabilization of ground state of P2 through the strong interactions between O (THF) and Se (BDT) in THF. Similar to the P1, the emission spectra of P2 showed red shift of ∼15−20 nm by changing solvent benzene to chloroform or THF. Another reason for band gap reduction was suggested45 and should be the degree of bond length alternation (S−N or Se−N) in the benzene ring that increases in the order S to Se trending toward a decrease in aromaticity that affects the conjugation among donor and acceptor units resulting in destabilization of the HOMO, stabilization of the LUMO, and an overall decrease in the band gap of the P2 polymer. The optical band gaps of the polymers, which were estimated by measuring the absorption onset in the UV−vis spectra of the film, were found to be 1.57, 1.44, and 1.86 eV for P1, P2, and P3, respectively. The optical band gap of P3 is higher than the electronic band gap due to the incomplete reduction of P3 film possibly caused by the more electron-rich BTAz. These results are again consistent with the CV results that the P2 has a longer conjugation length than P1. The optical band gaps are well in range of required narrow band gap (1.5 eV < Eg < 1.9 eV) polymers. It is worth noting that, similar to previous observation,45 the whole dual-band absorption spectrum shifts to lower energy as S in acceptor is substituted with Se and also the absorption coefficient of both the low-energy or highenergy band changes in intensity as heavier chalcogens are substituted into the acceptor. Our observations agree with the literature report45 that the low-energy band in the dual-band spectrum for D−A copolymer is due to an improved ICT.48 The electrochromic behavior was investigated with a UV−vis spectrometer at various applied potentials (Figure 3). Polymers were dissolved in chloroform (5 mg/mL) and drop-casted onto cleaned ITO-coated glass slide. Spectroelectrochemistry was carried out using TBAPF6 as supporting electrolyte in ACN solution. Polymers P1 and P2 showed two intense absorption bands in the visible region in the neutral state and P3 showed one strong absorption band in the visible region. As the polymers become oxidized, a depletion of the absorption peaks was observed and new peak intensified at the higher wavelength region. This process was associated with the formation of new charge carriers. As the applied potential increases, radical cations (polarons) and dications (bipolarons) were formed. These charge carriers were expected to have a planar quinoidal form of the polymer backbone which strengthens the conjugation. The spectroelectrochemical experiment of all these polymers was associated with color change from blue to transparent-gray for P1, green to transparent-gray for P2, and red to transparent-black for P3, respectively. The pictures of polymers P1−P3 in solution and spray-coated films on ITOcoated glass slides are shown in Figure 4, and the pictures of spray-cotaed films on ITO-coated glass slides in neutral and oxidized states are shown in Figure 5.

Figure 4. Pictures of polymers P3, P2, and P1 in soluation and spraycoated thin films.

Figure 5. Color change as observed for P3, P2, and P1 in neutral and oxidized states during spectroelectrochemistry.

To get more insight about band gap, frontier orbital’s participation in electronic transitions, and absorption spectra, we have performed DFT calculations on our CPT-based D−A copolymer series using the oligomer (3 repeating units which comprise conjugation of 24 double bonds) approach with alternating, more stable and cisoid−transoid conformations (see Figure S14).49,50 The decreasing order of dipole moment for the optimized ground-state geometry is 3TCPTTBDT (2.1 D) > 3TCPTTBDS (1.5 D) > 3TCPTTBTAz (0.76 D). All the optimized structures were found to be quasi-coplanar with approximately 7°−13° distortion at the linear conjugated backbone. The HOMO−LUMO gaps are 1.79, 1.69, and 2.12 eV for 3TCPTTBDT, 3TCPTTBDS, and 3TCPTTBTAz, respectively. Thus, the oligomer method accurately pointed out the increasing band gap trends in the order of P2 < P1 < P3 (theoretical as well as both electrochemical and optical), which is in agreement with the optical band gaps as all are within 0.26 eV.49 Our experimental data also agree well with our theoretical observation and previous reports that replacement of BDT with BDS in D−A type copolymers with constant donor groups provides reduced band gap polymers.51,52 The trends in the calculated energies of HOMO and LUMO are also consistent 5415

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with the experimental data. The increasing energy of HOMO in the order of P1 < P2 < P3 may be rationalized by the poor electron acceptance nature of the BTAz destabilizing HOMO whereas the incorporation of Se into acceptor of a D−A copolymer backbone stabilizes the LUMO in P2 compared to that of P1 or P3, which is again consistent with the literature reports.

Table 2. TD-DFT Calculated Transition Data for Model Compounds up to 12-mer (n = 3) oligomer

state

excitation

λcal [eV]

f

3TCPTTBDT

S1 S10 S1 S10 S1

H→L (83.9%) H→L+3 (69.9%) H→L (83.3%) H→L+3 (77.4%) H→L (85.4%)

1.51 2.59 1.41 2.50 1.83

2.84 1.61 2.50 1.83 4.05

3TCPTTBDS 3TCPTTBTAz

with a less intense absorption compared to P1 at the longer wavelength with maximum at 877 nm (f = 2.4965) and a more intense absorption compared to P1 at high-energy region with wavelength maximum at 495 nm (f = 1.8276). The oscillator strength of this low-energy transition also decreases as one moves from S to Se. The strong absorption maximum of P3 at 677 nm (f = 4.0497) is evident while its absorption coefficient increases in intensity as the S or Se is replaced with nitrogen into the acceptor of a similar oligomer structure. The high oscillator strengths are consistent with the high absorption intensity, and interestingly, the summation of low- and highenergy oscillator strengths of P1 or P2 almost equals to the oscillator strength for the intense absorption band of P3 (see Figure S3). Hence, calculation nicely predicts the shape and trend of the absorption spectra for P1 and P3 that agree well with our experimental results; however, the spectral pattern for P2 slightly differs from the experimental observations.

To get more insight into the absorption spectra P1−P3, TDDFT calculations (at B3LYP/6-31G(d)) were carried out. The calculated frontier molecular orbital (FMO) profiles of 3TCPTTBDT, 3TCPTTBDS, and 3TCPTTBTAz are shown in Figure 6. Unlike 3TCPTTBTAz, electron density distribution in HOMO and LUMO is nearly same for structurally identical both 3TCPTTBDT and 3TCPTTBDS, where the LUMO is mainly localized at the BDT or BDS unit and HOMO resides at the donor resulting in an ICT interaction. Calculations suggest that the intense absorption bands at the longer wavelength are mainly due to the electronic transitions from HOMO to LUMO. The calculations showed an intense absorption maximum at 818 nm (oscillator strength f = 2.8418) as well as moderate intensity absorption peak at the lower wavelength with maximum at 479 nm (f = 1.6132) for P1 (Table 2). P2 also reveals a similar spectral pattern, however,

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CONCLUSION A new series of low-band-gap D−A copolymers based on electroactive cyclopenta[c]thiophene have been synthesized and characterized. The dual-band absorption of BDT- and BDS-based polymers appeared as a result of improved ICT interaction compared to BTAz-based polymer; as a consequence, blue, green, and red colors were observed for neutral P1, P2, and P3 in solution under visible light. DFT calculation was successfully implemented to elucidate the unique

Figure 6. Frontier molecular orbital diagrams for D−A cooligomers up to 12-unit (up to 3 repeating units of BDT-, BDS-, and BTAz-based thiophene-capped CPT). 5416

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absorption spectral pattern which is consistent with the experimental investigations. The thermal as well as oxidative stability of the polymers is good enough for most of the devicebased applications.

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ASSOCIATED CONTENT

S Supporting Information *

1 H NMR and 13C NMR of all new compounds; DSC graph; concentration dependent absorption spectra of P1 and P2; absorption and emission spectra of P1-P3 in benzene, chloroform and THF; optimized geometries of D−A trimers, their coordinates and calculated excitation spectra. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax +91-33-25873020; e-mail [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support from DRDO, India, is gratefully acknowledged. S.D. acknowledges research fellowship from IISER-Kolkata. P.B.P. thanks UGC-India for research fellowship.

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REFERENCES

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