Effect of Side-Chain Architecture on the Optical and Crystalline

Jul 29, 2013 - Control of vertical distribution of thiophene-based copolymers containing 4 ... D. Hendsbee , Seth M. McAfee , Devproshad K. Paul , Kun...
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Effect of Side-Chain Architecture on the Optical and Crystalline Properties of Two-Dimensional Polythiophenes Cheng-Yu Kuo,†,‡ Yu-Chen Huang,† Chuen-Yo Hsiow,† Yu-Wen Yang,† Ching-I Huang,† Syang-Peng Rwei,§ Hsing-Lin Wang,*,‡ and Leeyih Wang*,†,∥ †

Institute of Polymer Science and Engineering, National Taiwan University, Taipei 106 Taiwan CPC-S, Los Alamos National Laboratory, Los Alamos, New Mexico 87544, United States § Institute of Organic and Polymeric Materials, National Taipei University of Technology, Taipei 106, Taiwan ∥ Center for Condense Matter Science, National Taiwan University, Taipei 106, Taiwan ‡

S Supporting Information *

ABSTRACT: The present study reported here synthesis of three novel two-dimensional (2D) polythiophene derivatives with conjugated terthiophene−vinylene side chainpoly{3(5″-hexyl-2,2′:5′,2″-terthiophenyl-5-vinyl)thiophene-alt-thiophene} (P1), poly{3-(5,5″-dihexyl-2,2′:5′,2″-terthiophenyl-3′vinyl)thiophene-alt-thiophene} (P2), and poly{3-(4,4″-dihexyl-2,2:5′,2″-terthiophene-3′-vinyl)thiophene-alt-thiophene} (P3)that were synthesized via stille coupling reaction. The terthiophene side chain with different conformations conjugated to the polythiophene main chain via vinyl linkage provided the ability to control the molecular organization, hence affecting the optoelectronic and electrochemical properties of 2D polymers. TD-DFT calculation with the B3LYP/631+g(d) function on electronic structures of the monomers was consistent with the experimental results. It suggested that the energetic states of HOMO and LUMO were highly dependent on the side-chain architectures. These polythiophene thin films fabricated by spin-casting show a broader absorption ranges from 300 to 700 nm which was significantly wider than the absorption of pure poly(3-hexylthiophene). When comparing the solid-state absorption spectra of these polymers before and after thermal annealing, P3 displayed the most red-shift in the wavelength range between 450 and 700 nm. It was presumably due to an extended conjugation length resulting from the linear conformation and preferred chain packing, as manifested in the X-ray diffraction. Molecular dynamics (MD) simulation on polymers with different side chains in isolated and packed states suggests planar conformation of the main chain was adopted and regulated by the side chains which were placed in parallel with the mainchain direction. Interestingly, P1 solution revealed an excitation-dependent emission property, suggesting a structural inhomogeneity in solution. Contrary to P1, the PL spectra of P2 and P3 showed only one emission peak at 460 nm, regardless of the excitation energy. Orientation and regiochemistry of the terthiophene side chain had a major impact on the overall optical and electronic properties of the polymer. Moreover, the HOMO and LUMO of these three polymers had been determined through cyclic voltammetry. HOMO of the three polymers were in the following order: P1 > P2 > P3. It implied that the energy level was regiochemistry dependent and directly associated with the linked position between backbone and conjugated side chain. Most importantly, through mesogen-jacketed-like design strategy employed in the present study, the improved packing of these two-dimensional polymers offered insights into structure design to enhance properties that have strong ties to the electronic devices.

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INTRODUCTION

which gives excellent hole mobility and adequate energy levels

Over the past decades, π-conjugated polymers have received lots of attention due to their applications in fabricating organic optical and electronic devices.1−5 Many carefully designed polymer systems that tailored to fulfill the requirements of the electronic devices have been developed and studied.6 Among those semiconducting polymers, the regioregular poly(3hexylthiophene) (rr-P3HT), a symbolic and well-known material, has been widely used in these organic-based devices.7,8 It is mainly due to its highly oriented semicrystalline structure, © XXXX American Chemical Society

to ensure effectively charge transfer. Consequently, several structural modifications to the geometry of polythiophene have been proposed for optimizing the physical and electronic properties of specific optoelectronics.9 Received: April 18, 2013 Revised: July 5, 2013

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Scheme 1. Synthetic Routes of Two-Dimensional Polythiophenes by Polymerizing M1, M2, and M3 with 2,5Bis(trimethylstannyl)thiophene

backbone via a vinyl linkage with different spatial arrangements. These terthiophenes are either perpendicular or parallel positioned with respect to the polythiophene backbone, as shown in Scheme 1. The geometrical effect of the side chain on the optical, physical, and electrochemical properties of the 2D polymer were systematically examined. Experimental results suggested strong correlations between the side-chain geometry and the physical/electronic properties of the conjugated polymers. Such understanding of structure−property relationship in 2D polymers offers insights into designing materials with enhanced optical and solid state properties for future usage.

Of particular interest is to study the side-chain architecture− property relationship through incorporating conjugated chromophores onto the polythiophene backbone. This unique twodimensional conjugated system shows promises in solar application as reported by Li et al. 10 The attached chromophoric side chains improve the light-harvesting properties of the parent polymer by broadening its absorption range to maximize the overlap with solar emission and modulate optoelectronic properties of the conjugated polymers, especially in D−A (donor−acceptor) type low band gap polymers.11−13 Moreover, 2D polymers possess larger π-conjugated plane than their linear counterparts that allow the improvement of the isotropic charge transport to achieve higher hole mobility.14 However, a challenge remains as the oversized conjugated branches disrupts the coplanarity of the backbone, thereby decreasing the conjugation length and chain packing of the polymer system.15 In an attempt to achieve the desirable optoelectronic properties without sacrificing the structural order, an interesting strategy arises by adopting diverse mesogen arrangement to resolve this packing issue. Based on the paper reported by Zhou et al.,16 while the mesogenic groups are laterally attached to main chain of the polymer at the mass center without long spacer, the crowded rigid side groups will force the backbone to be extended like a “jacket”. This concept offers a great opportunity to elongate the conjugation via chain alignment through incorporating rigid side chains into the 2D πconjugated polymer, which may not only improve the overlap with solar emission but also retain the structural order for the 2D conjugated polymer system. The present study designed three different kinds of 2D polythiophene derivatives that were given names as poly{3-(5″hexyl-2,2′:5′,2″-terthiophenyl-5-vinyl)thiophene-alt-thiophene} (P1), poly{3-(5,5″-dihexyl-2,2′:5′,2″-terthiophenyl-3′-vinyl)thiophene-alt-thiophene} (P2), and poly{3-(4,4″-dihexyl2,2′:5′,2″-terthiophene-3′-vinyl)thiophene-alt-thiophene} (P3). They were synthesized by attaching terthiophene groups to the 3-position of the thiophene rings in the polymer

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EXPERIMENTAL SECTION

Materals. The reactants used herein were purchased from Arcos Organics and used as received without further purification. All organic solvents were AR grade and purchased either from TEDIA Co. (Fairfield, OH) or Mallinckrodt Inc. (Hazelwood, MO). Toluene was dried over calcium hydride under nitrogen gas (N2) before usage. Tetrahydrofuran (THF) and diethyl ether were distilled over sodium/ benzophenone prior to use. N,N-Dimethylformamide (DMF) was purchased from Acros, dried over magnesium sulfate (MgSO4) followed by barium oxide (BaO), and then distilled and stored in a Schlenk flask in an inert atmosphere. Measurement. Ultraviolet−visible (UV−vis) absorption spectra were recorded on a JASCO MD-2010 spectrometer. Gel permeation chromatography (GPC) was conducted at 40 °C using two Jordi DVB mixed-bed columns (250 × 10 mm; suitable for separating polymers with molecular weights from 1 × 102 to 1 × 107 g mol−1). THF was used as the eluent at a flow rate of 1.0 mL/min on a JASCO instrument that was equipped with UV−vis and refractive index (RI) detectors connected in series. Thirteen linear polystyrene samples with molecular weights from 7 × 102 to 2 × 106 g mol−1 that were purchased from Aldrich were used as standard. 1H (300 and 400 MHz) and 13C (100 MHz) nuclear magnetic resonance (NMR) spectra were recorded using a Bruker SPECTROSPIN spectrometer at room temperature and using CDCl3 as the solvent. The solvent signal was adopted as an internal standard. Photoluminescence was conducted using Jobin Yvon Fluorolog-Tau-3. The X-ray diffraction patterns were measured by a Bruker D8 ADVANCED diffractometer operated at a 40 kV voltage and 200 mA current with Cu Kα radiation B

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(λ = 1.542 Å, scan rate: 0.6 s/step). The electrochemical cyclic voltammetry was recorded on CH Instruments CHI 614 A with carbon, platinum (Pt), and Ag/Ag+ as working, counter, and reference electrode, respectively. The solvent is acetonitrile, ferrocene is used as internal standard, and tetrabutylammonium hexafluorophosphate is the electrolyte (scan rate 100 mV/s). Synthesis of Monomers and Polymers. Synthesis of 2Hexylthiophene (1).17 One equivalent (52 mL) of n-butyllithium (1.6 M in hexane) was added dropwisely into a solution of thiophene (7 g, 0.08 mol) in 50 mL of dry THF at −78 °C under a N2 atmosphere and kept stirring for 1 h. Then 1-bromohexane (15.1 g, 0.09 mol) was added in one portion, and the solution was warm to room temperature. After 12 h, the solution was purged into cold water and extracted with 50 mL of ether. The combined organic phase was washed with water, dried over anhydrous MgSO4, filtered, and concentrated. Distillation under vacuum gave 10.8 g of compound 1 (0.064 mol, yield 80%) as a colorless oil. 1H NMR (δ, CDCl3): 7.10 (m, 1H), 6.90 (m, 1H), 6.77 (m, 1H), 2.82 (t, 2H), 1.67 (m, 2H), 1.34 (m, 6H), 0.89 (t, 3H). Synthesis of 2-Hexyl-5-tributylstannylthiophene (2).18 Compound 1 (5 g, 0.03 mol) was dissolved in 30 mL of dry THF in a roundedbottom flask under a N2 atmosphere. 18.5 mL of n-butyllithium (1.6 M in hexane) was added dropwisely at −78 °C. After stirring for 1 h, tributyltin chloride (9.66 g, 0.03 mol) was added in one portion. Then the reaction was warm to room temperature and stirred overnight. Finally, the solution was purged into cold water. The organic phase was separated, and the aqueous layer was twice extracted with 50 mL of ether. The organic layers were collected and dried over anhydrous MgSO4. After removing the solvent, the crude product was purified by passing Al2O3 column and hexane was used as eluent. A colorless oil (12.62 g) 5-tributylstannyl-2 hexylthiophene was obtained (0.027 mol, yield 92%). 1H NMR (δ, CDCl3): 7.98 (d, 1H), 6.90 (d, 1H), 2.85 (t, 2H), 1.72 (m, 2H), 1.34 (m, 12H), 1.28 (m, 6H), 1.08 (m, 6H), 0.92 (m, 12H). Synthesis of 3-Hexylthiophene (3). To a solution of Mg (2.98 g, 0.12 mol) in 50 mL of dry ether, 1-bromohexane (10.12 g, 0.06 mol) was slowly added at 0 °C under N2 gas, and then the solution was heated to reflux for 2 h. Prepared Grignard reagent was purged into an one-necked flask containing 3-bromothiophene (5 g, 0.03 mol) and Ni(dppp)Cl2 (0.083 g, 0.2 mmol) in 50 mL of dry ether, and the reaction was stirred overnight at room temperature. After completing the reaction cycle, the solution was purged into cold water and twice extracted with 50 mL of ether. The combined organic phase was washed with water, dried over anhydrous MgSO4, filtered, and concentrated. The purification step was carried out via silica column chromatography, using hexane as eluent. After concentration, the colorless oil (4.39 g) 3-hexylthiophene was obtained (0.026 mol, yield 87%). 1H NMR (δ, CDCl3): 7.21 (m, 1H), 6.90 (m, 2H), 2.61 (t, 2H), 1.61 (m, 2H), 1.29 (m, 6H), 0.87 (t, 3H). 13C NMR (δ, CDCl3): 143.2; 128.2; 125.0; 119.7; 31.7; 30.6; 30.3; 29.0; 22.6; 14.1. Synthesis of 5-Tributylstannyl-3-hexylthiophene (4).19 Compound 3 (3 g, 0.017 mol) was dissolved in 25 mL of dry THF in a rounded-bottom flask under a N2 atmosphere. 11.1 mL of nbutyllithium (1.6 M in hexane) was added dropwisely at −78 °C. After the solution was stirred for 1 h, tributyltin chloride (5.8 g, 0.017 mol) was added in one portion. Then the reaction was warm to room temperature and stirred overnight. Finally, the solution was purged into cold water. The organic phase was separated, and the aqueous layer was extracted with 50 mL of ether twice. The organic layers were collected and dried over anhydrous MgSO4. After the solvent was removed, the crude product was purified by passing through Al2O3 column, using hexane as eluent. A colorless oil (7.54 g) of 5tributylstannyl-3-hexylthiophene was obtained (0.016 mol, yield 97%). 1 H NMR (δ, CDCl3): 7.20 (s, 1H), 6.90 (s, 1H), 2.97 (t, 2H), 1.61 (m, 8H), 1.36 (m, 12H), 1.03 (m, 6H), 0.91 (m, 12H). Synthesis of 2,2′-Bithiophene (5). To a solution of Mg (0.89 g, 0.036 mol) in 25 mL of dry ether, 2-bromothiophene (3 g, 0.018 mol) was slowly added at 0 °C under N2 gas protection, and then the solution was heated to reflux for 2 h. Prepared Grignard reagent was purged into an one necked-flask contained 2-bromothiophene (3 g,

0.018 mol) and Ni(dppp)Cl2 (0.048 g, 0.09 mmol) in 25 mL of dry ether, and the reaction was stirred overnight at room temperature. After completing the reaction, the solution was then purged into cold water and extracted twice with 50 mL of ether. The combined organic phase was washed with water, dried over anhydrous MgSO4, filtered, and concentrated. Purification was carried out via silica column chromatography, using hexane as eluent. After concentration, the light blue solid (2.6 g) 2,2′-bithiophene was obtained (0.015 mol, yield 85%). 1H NMR (δ, CDCl3): 7.23 (dd, 1H), 7.19 (dd, 1H), 7.03 (m, 1H). Synthesis of 5,5′-Dibromo-2,2′-bithiophene (6). N-Bromosuccinimide (NBS) (4.49 g, 0.025 mol) was slowly added to a solution of 2,2′-bithiophene (2 g, 0.012 mol) in 30 mL of acetic acid at room temperature. After the precipitate was formed, the solution was kept stirring for 3 h. Then the reaction was purged into cold water, and a large amount of solid was formed. The crude product was collected by filtration and washed with water until neutral. Finally, the residue was purified via silica column chromatography, using hexane as eluent to give a white solid (3.5 g) title compound (0.011 mol, yield 90%). 1H NMR (δ, CDCl3): 6.97 (d, 1H), 6.86 (d, 1H). Synthesis of 5′-Bromo-2,2′-bithiophene-5-carbaldehyde (7). To a solution of compound 6 (2 g, 6 mmol) in 20 mL of dry THF, 3.8 mL of n-butyllithium (1.6 M in hexane) was slowly added at −78 °C under a N2 atmosphere. One hour later, DMF (0.45 g, 6 mmol) was added in one portion and warmed to room temperature. After 2 h, the solution was purged into cold water and kept stirring for 1 h. The organic phase was separated, and the aqueous layer was extracted with 50 mL of ether twice. The organic layers were collected and dried over anhydrous MgSO4. After the solvent was removed, the crude product was purified by passing through SiO2 column, using CH2Cl2:hexane (2:1) as eluent. A light yellow solid (1.13 g) 5′-bromo-2,2′bithiophene-5-carbaldehyde was obtained20 (4 mmol, yield 67%). 1H NMR (δ, CDCl3): 9.82 (s, 1H), 7.63 (d, 1H), 7.22 (d, 1H), 7.18 (d, 1H), 7.01 (d, 1H). Synthesis of 5″-Hexyl-2,2′:5′,2″-terthiophene-5-carbaldehyde (8).21 To a solution of compound 2 (2 g, 4.3 mmol) and compound 7 (1 g, 3.6 mmol) in 25 mL of dry THF, Pd(PPh3)2Cl2 (0.12 g, 0.18 mmol) was added as catalyst under a N2 atmosphere, and the solution was heated to reflux overnight. After removal of the solvent, the mixture was directly purified by column chromatography (SiO2), using CH2Cl2:hexane (2:1) as a mobile phase. 0.94 g of title compound was obtained as a yellow solid (2.6 mmol, yield 71%). 1H NMR (δ, CDCl3): 9.84 (s, 1H), 7.63 (d, 1H), 7.15 (d, 1H), 7.07 (d, 1H), 7.01 (d, 1H), 6.68 (d, 1H), 2.77 (t, 2H), 1.65 (m, 2H), 1.33 (m, 6H), 0.89 (t, 3H). 13C NMR (δ, CDCl3): 182.40, 147.07, 146.72, 141.32, 133.73, 133.71, 128.78, 126.78, 126.89, 125.04, 124.023, 123.82, 123.79, 31.52, 30.19, 29.7, 28.7, 22.5, 14.06. Synthesis of 2,5-Dibromothiophene-3-carbaldehyde (9). Thiophene-3-carbaldehyde (3 g, 0.027 mol) was dissolved in 30 mL of DMF in an one-necked flask, and the NBS (9.99 g, 0.056 mol) was then slowly added to the solution. After stirring overnight, the solution was purged into cold water and extracted twice with 50 mL of ether. The combined organic phase was then dried over anhydrous MgSO4, filtered, and concentrated. The crude product was purified via silica column chromatography, using CH2Cl2:hexane (1:1) as mobile phase to get 3.68 g of title compound22 (0.014 mol, yield 51%). 1H NMR (δ, CDCl3): 9.78 (s, 1H), 7.33 (s, 1H). 13C NMR (δ, CDCl3): 189.14, 139.35, 128.69, 124.20, 113.41. Synthesis of 5,5″-Dihexyl-2,2′:5′,2″-terthiophene-3-carbaldehyde (10). To a solution of compound 9 (1 g, 3.7 mmol) and compound 2 (3.72 g, 8.1 mmol) in 25 mL of dry THF, Pd(PPh3)2Cl2 (0.29 g, 0.4 mmol) was added as catalyst under a N2 atmosphere, and the solution was heated to reflux overnight. After removal of the solvent, the mixture was directly purified by column chromatography (SiO2), using CH2Cl2:hexane (1:1) as mobile phase. 0.86 g of title compound was obtained as a yellow liquid (1.9 mmol, yield 52%). 1H NMR (δ, CDCl3): 10.06 (s, 1H), 7.43 (s, 1H), 7.09 (d, 1H), 6.99 (d, 1H), 6.80 (d, 1H), 6.68 (d, 1H), 2.81 (m, 4H), 1.67 (m, 4H), 1.33 (m, 12H), 0.89 (m, 6H). 13C NMR (δ, CDCl3): 185.20, 150.00, 146.82, 146.29, 137.03, 136.62, 132.96, 129.57, 128.88, 125.34, 124.88, 124.46, 121.35, C

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Scheme 2. Synthetic Routes of the Monomersa

Conditions and reagents: (i) n-BuLi, THF, −78 °C, 1 h; then 1-bromohexane, rt, overnight; (ii) n-BuLi, THF, −78 °C, 1 h; then tributyltin chloride, rt, overnight; (iii) 1-bromohexane, Mg, ether, reflux, 2 h; then Ni(dppp)Cl2, rt, overnight; (iv) n-BuLi, THF, −78 °C, 1 h; then tributyltin chloride, rt, overnight; (v) Mg, ether, reflux, 2 h; then 2-bromothiophene, Ni(dppp)Cl2, rt, overnight; (vi) NBS, acetic acid, 3 h; (vii) n-BuLi, THF, −78 °C, 1 h; then DMF, rt, 2 h; (viii) compound 2, Pd(PPh3)2Cl2, THF, overnight; (ix) NBS, DMF, rt, overnight; (x) compound 2, Pd(PPh3)2Cl2, THF, overnight; (xi) compound 4, Pd(PPh3)2Cl2, THF, overnight; (xii) NBS, acetic acid, rt, 3 h; (xiii) NBS, BPO, CCl4, reflux, 3 h; (xiv) P(OC2H5)3, 160 °C, 2 h; then compound 8, NaOCH3, DMF, rt, overnight; (xv) P(OC2H5)3, 160 °C, 2 h; then compound 10, NaOCH3, DMF, rt, overnight; (xvi) P(OC2H5)3, 160 °C, 2 h; then compound 11, NaOCH3, DMF, rt, overnight. a

Synthesis of 2,5-Dibromo-3-(bromomethyl)thiophene (13).23 Compound 13 was prepared by reacting compound 12 (5 g, 0.02 mol) with NBS (3.48 g, 0.02 mol) in 30 mL of carbon tetrachloride, and benzoyl peroxide (BPO) was added as the catalyst. After the solution was heated and refluxed for 3 h, the precipitate was filtered, and then the filtrate was concentrated under vacuum. Purification was carried out via silica column chromatography, using hexane as eluent. After concentration, 3.61 g of 2,5-dibromo-3-(bromomethyl)thiophene was obtained as a colorless oil (0.011 mol, yield 54%). 1 H NMR (δ, CDCl3): 6.42 (s,1H), 4.52 (s, 2H). 13C NMR (δ, CDCl3): 142.71, 134.53, 115.12, 113.12, 21.44. Synthesis of 2,5-Dibromo-3-(5″-hexyl-2,2′:5′,2″-terthiophenyl-5vinyl)thiophene, M1 (14). Compound 13 (1.11 g, 3.3 mmol) and phosphorous acid triethyl ester (0.66 g, 3.9 mmol) were put into a 25 mL two-necked flask and heated to 160 °C for 2 h. The crude product was obtained and used without further purification. To this product, 10 mL of DMF was added and purged into a 50 mL round-bottomed flask containing NaOCH3 (0.37 g, 6.6 mmol) under ice−water bath. After 10 min, compound 8 (0.8 g, 2.2 mmol) dissolved in 20 mL of DMF was added dropwisely to the mixture and stirred overnight at room temperature. Finally, the solution was purged into the cold water and extracted twice with 50 mL ether. The combined organic layers was washed with water, dried over anhydrous MgSO4, filtered, and then concentrated. Purification was carried out via SiO2 column chromatography, using CH2Cl2:hexane (1:2) as eluent. 1.14 g of yellow solid was obtained after evaporation (1.9 mmol, yield 86%). 1H

31.46, 30.16, 28.72, 28.70, 22.52, 14.03. EI MASS (m/z): calcd, 444.72; found, 444. Synthesis of 4,4″-Dihexyl-2,2′:5′,2″-terthiophene-3-carbaldehyde (11). To a solution of compound 9 (1 g, 3.7 mmol) and compound 4 (3.72 g, 8.1 mmol) in 25 mL of dry THF, Pd(PPh3)2Cl2 (0.29 g, 0.4 mmol) was added as catalyst under a N2 atmosphere, and the solution was heated to reflux overnight. After removal of the solvent, the mixture was directly purified by column chromatography (SiO2), using CH2Cl2:hexane (1:1) as mobile phase. 0.66 g of title compound was obtained as a yellow liquid (1.4 mmol, yield 40%). 1H NMR (δ, CDCl3): 10.05 (s, 1H), 7.47 (1, 1H), 7.08 (s, 1H), 7.00 (d, 2H), 6.81 (s, 1H), 2.56 (m, 4H), 1.60 (m, 4H), 1.30 (m, 12H), 0.88 (m, 6H). 13 C NMR (δ, CDCl3): 184.75, 145.97, 144.45, 144.07, 137.15, 136.60, 135.00, 131.69, 130.17, 125.91, 123.20, 121.73, 120.16, 31.49, 30.22, 30.18, 30.14, 28.83, 22.47, 13.95. EI MASS (m/z): calcd, 444.72; found, 444. Synthesis of 2,5-Dibromo-3-methylthiophene (12).23 N-Bromosuccinimide (NBS) (11.42 g, 0.064 mol) was slowly added to a solution of 3-methylthiophene (3 g, 0.03 mol) in 50 mL of acetic acid at room temperature. After 3 h, the solution was purged into cold water and neutralized with 2 M NaOH(aq) and then twice extracted with 50 mL of ether. The combined organic phase was then dried over anhydrous MgSO4, filtered, and concentrated. Finally, the crude product was purified via silica column chromatography, using hexane as eluent to give a colorless (6.72 g) compound (0.026 mol, yield 86%). 1H NMR (δ, CDCl3): 6.78 (s, 1H), 2.51 (s, 3H). D

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Figure 1. 1H NMR of the corresponding monomers in the aromatic region: (a) M1 monomer, (b) M2 monomer, and (c) M3 monomer. The residue solvent is indicated by an asterisk. NMR (δ, CDCl3): 7.12 (s, 1H), 7.05 (d, 1H), 7.01 (d, 1H), 6.90−6.97 (m, 3H), 6.69−6.73 (d, 1H), 6.66 (d, 1H), 2.77 (t, 2H), 1.64 (m, 2H), 1.35 (m, 6H), 0.88 (t, 3H). 13C NMR (δ, CDCl3): 145.87, 140.68, 138.75, 137.29, 136.97, 135.24, 134.33, 128.06, 127.16, 124.87, 124.52, 124.12, 123.87, 123.63, 123.52, 119.39, 111.98, 109.90, 31.55, 30.20, 28.75, 22.56, 14.06 EI MASS (m/z): calcd, 598.50; found, 598. Synthesis of 2,5-Dibromo-3-(5,5″-dihexyl-2,2′:5′,2″-terthiophenyl-3′-vinyl)thiophene, M2 (15). Compound 13 (0.79 g, 2.4 mmol) and phosphorous acid triethyl ester (0.48 g, 2.8 mmol) were put into a 25 mL two-necked flask and heated to 160 °C for 2 h. The crude product was obtained and used without further purification. To the product 10 mL of DMF was added and purged into a 50 mL roundbottomed flask containing NaOCH3 (0.26 g, 4.8 mmol) under ice− water bath. After 10 min, compound 10 (0.7 g, 1.6 mmol) dissolved in 20 mL of DMF was added dropwisely to the mixture and stirred overnight at room temperature. Finally, the solution was purged into the cold water and extracted twice with 50 mL of ether. The combined organic layers was washed with water, dried over anhydrous MgSO4, filtered, and then concentrated. Purification was carried out via SiO2 column chromatography, using CH2Cl2:hexane (1:2) as eluent. 0.9 g of a yellow sticky oil was obtained after evaporation (1.3 mmol, yield 84%). 1H NMR (δ, CDCl3): 7.27 (s, 1H), 7.09−7.14 (d, 1H), 7.12 (s, 1H), 6.99 (d, 1H), 6.94 (d, 1H), 6.80−6.86 (d, 1H), 6.76 (d, 1H), 6.68 (d, 1H), 2.81 (m, 4H), 1.67 (m, 4H), 1.34 (m, 12H), 0.88 (m, 6H). 13C NMR (δ, CDCl3): 147.66, 146.02, 139.38, 136.36, 134.97, 134.02, 132.82, 132.37, 127.43, 126.79, 124.92, 124.88, 124.55, 123.88, 121.12, 121.02, 111.86, 109.81, 31.61, 30.26, 28.88, 28.82, 22.64, 14.16. EI MASS (m/z): calcd, 682.66; found, 682. Synthesis of 2,5-Dibromo-3-(4,4″-dihexyl-2,2′:5′,2″-terthiophene3′-vinyl)thiophene, M3 (16). Compound 13 (0.68 g, 2.0 mmol) and

phosphorous acid triethyl ester (0.4 g, 2.4 mmol) were put into a 25 mL two-necked flask and heated to 160 °C for 2 h. The crude product was obtained and used without further purification. To the product 10 mL of DMF was added and purged into a 50 mL round-bottomed flask containing NaOCH3 (0.22 g, 4 mmol) under an ice−water bath. After 10 min, compound 11 (0.6 g, 1.3 mmol) dissolved in 20 mL of DMF was added dropwisely to the mixture and stirred overnight at room temperature. Finally, the solution was purged into the cold water and extracted twice with 50 mL of ether. The combined organic layers was washed with water, dried over anhydrous MgSO4, filtered, and then concentrated. Purification was carried out via SiO2 column chromatography, using CH2Cl2:hexane (1:2) as eluent. 0.76 g of yellowish oil was obtained after evaporation (1.1 mmol, yield 82%). 1H NMR (δ, CDCl3): 7.32 (s, 1H), 7.14 (d, 1H), 7.10 (s, 1H), 7.03 (d, 1H), 6.97 (d, 2H), 6.85 (d, 1H), 6.82 (d, 2H), 2.60 (m, 4H), 1.63 (m, 6H), 1.34 (m, 12H), 0.89 (t, 6H). 13C NMR (δ, CDCl3): 144.02, 143.97, 139.20, 136.22, 136.10, 135.14, 134.51, 132.71, 128.18, 127.24, 125.37, 124.27, 121.36, 121.01, 119.43, 111.81, 109.80, 31.66, 30.45, 30.38, 30.26, 29.00, 22.61, 14.12. EI MASS (m/z): calcd, 682.66; found, 682. Preparation of P1, P2, and P3. The synthesis of P1, P2, and P3 was carried out following the same procedure using the Stille coupling reaction.24 The standard procedure was described as follows. A solution of 1 equiv of monomer (compound 14, 15, or 16), 1 equiv of 2,5-bis(trimethylstannyl)thiophene, and 5 mol % Pd(PPh3)4 in 20 mL of dry toluene was purged with N2 to remove oxygen. The mixture was heated to reflux for 2 days. Thus, formed solution was poured into methanol, and the precipitate was collected with a membrane filter. The polymer was then washed through Soxhlet extraction with E

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methanol, hexane, and CHCl3. The chloroform fraction was then concentrated under vacuum to yield the polymer. P1: 1H NMR (δ, CDCl3): 6.30−7.24 (m br, 11H), 2.4−2.9 (br, 2H), 1.4−1.6 (m br, 2H), 1.05−1.40 (m br, 6H), 0.5−1 (m br, 3H). P2: 1H NMR (δ, CDCl3): 6.4−7.3 (m br, 10H), 2.3−2.8 (br, 4H), 1.45−1.8 (m br, 4H), 1−1.45 (m br, 12H), 0.8−0.9 (m br, 6H). P3: 1H NMR (δ, CDCl3): 6.50−7.40 (m br, 10H), 2.3−2.6 (br, 4H), 1.4−1.7 (m br, 4H), 0.9−1.4 (m br, 12H), 0.7−0.9 (m br, 6H).

Table 1. Molecular Weight as Well as Electrochemical and Thermal Properties of the Polymers

P1 P2 P3

■

Mwa (g/mol)

PDIb

Eox/HOMOc (eV)

Egd (eV)

LUMOe (eV)

Tgf (°C)

6 500 16 000 21 000

1.9 1.86 2.17

−4.98 −5.25 −5.31

1.77 1.92 1.85

−3.21 −3.33 −3.46

102 105

a

Determined by GPC with polystyrene as standard and THF as eluent. Polydispersity (PDI) was calculated by Mw/Mn. cCyclic voltammetry was recorded in 0.1 M TBAPF6 solution (acetonitrile) vs Fc/Fc+. dThe optical band gap was decided by λonset of the polymer film absorption. e Determined by HOMO plus optical band gap. fAnalysis by DSC (second round). b

RESULTS AND DISCUSSION Synthesis and Characterization of the Polymers. The synthetic routes of the monomers 2,5-dibromo-3-(5″-hexyl2,2′:5′,2″-terthiophenyl-5-vinyl)thiophene (M1), 2,5-dibromo3-(5,5″-dihexyl-2,2′:5′,2″-terthiophenyl-3′-vinyl)thiophene (M2), and 2,5-dibromo-3-(4,4″-dihexyl-2,2′:5′,2″-terthiophene3′-vinyl)thiophene (M3) are shown in Scheme 2. The bithiophene prepared from the Grignard reaction of 2bromothiophene with a yield greater than 88% was brominated with 2 equiv of NBS in DMF, followed by treating with 1 equiv of n-BuLi to form thienyl anion. It was then reacting with the carbonyl group of DMF to form a stabilized lithium salt and then working up with acid to eliminate dimethylamine from lithium salt to produce compound 7 with a moderate yield of 67%. Afterward, compound 8, a yellow solid, was generated through coupling compound 7 with (5-hexylthiophen-2-yl)trimethylstannane. Similarly, two other formylated terthiophenes, compounds 10 and 11, were prepared from the Stille coupling of 2,5-dibromothiophene-3-carbaldehyde with the corresponding stannic derivatives with a yield of 52% and 40%, respectively. The lower yields for compounds 10 and 11 are mainly attributed to the steric hindrance between the carbonyl-substituted side in compound 9 and stannylated thiophene. It might be worse if 5-tributylstannyl-4-hexylthiophene was used instead of 5-tributylstannyl-3-hexylthiophene. Finally, the monomers M1, M2, and M3 were obtained by the Wittig−Horner reaction of the corresponding terthiophene− carbaldehyde and phosphorus ylides under base condition at 0 °C. Figure 1 displays the 1NMR spectra of three monomers. The coupling constant of peaks i and ii corresponding to the two protons of the vinylic linkage was determined to be 16 Hz for M1, M2, and M3, which fell in between the reported range (12−18 Hz) for the trans conformation.25 This observation confirmed that all M1, M2, and M3 possessed a trans conformation without the contamination of the cis form. However, this stereoselectivity showed a temperature-dependent property. It was noted that if reactions were performed under room temperature, the cis-form product would appear and its amount increased with increasing reaction temperature. The polymerizations of these 2D polymers were carried out via the Stille coupling reaction using Pd(PPh3)4 as a catalyst, which has been widely used in preparing the alternating conjugated copolymer for many years.24 As listed in Table 1, the molecular weights of P2 and P3 were higher than P1, presumably due to fewer alkyl chains on the terthiophene units that restrict the solubility of P1. This solubilizing side chain directly impacts the polymer molecular weight and PDItwo dominant factors in determining the chain packing and mechanical properties of the polymer thin films. Absorption Properties of the Polymer Solution. Figure 2a−c shows the normalized UV−vis spectra of the polymers together with their corresponding monomers recorded under diluted chloroform solution (1 × 10−5 M). In such a diluted solution, the intermolecular π−π interaction between aromatic

rings would be minimized because of isolation between polymer chains by solvent molecules. In other words, the optical and electronic properties measured may be regarded as the properties of single chain species which can rotate in solvent with minimum interchain interaction. The experimental results had demonstrated that there were two distinct absorption bands in the low- and high-energy regions for these three polymers accompanied by broad spectral absorption. First, the high-energy band in the UV region was in perfect agreement with the monomer which suggested the high-energy absorption was resulting from the terthienyl-vinyl side chain, and the other absorption peak in the visible region is mainly assigned to the π−π* transition of their conjugated main chain that are the typical features of the 2D thiophenebased polymers.10 Besides, a detailed comparison between the absorption spectra of as-synthesized conjugated polymers was shown by using poly(3-hexylthiophene) as a standard. As can be seen in Figure 2d, these 2D polythiophenes showed a wider absorption ranges from 300 to 700 nm; P1, P2, and P3 all had a red-shifted absorption edge (smaller band gap) as opposed to poly(3-hexylthiophene). The above results had indicated that the absorption band in the visible region is not only contributed by the polymer main chain but also benefited from the extended conjugation which was provided by the conjugated side chain, hence leading to a smaller band gap.15 To better understand the variation in band gap energy between P1, P2, and P3, theoretical calculation was conducted on the electronic structures of the monomers to observe how they impact the band gap energy. For the sake of simplicity, the hexyl side chains on the monomer were replaced by the methyl groups since such simplification had minimal influence on the electronic structure. Figure 3a showed the UV−vis spectra of M1 and M2 derived from the experiment and time-dependent density functional theory (TD-DFT) with B3LYP/6-31+g(d) functional modeling method, which was very useful to understand, qualitatively, the contribution of the molecular orbitals to their absorption spectrum.26,27 As shown in Figure 3a, the results from simulation were overestimated in comparison with experiment, which could be due to the limitation of the current approximate exchange-correlation functional in studying extended system.26 Even so, the absorption pattern and the trend of band gap difference between M1 and M2 were still consistent with experimental observation; the band gap of M1 monomer remained smaller than M2 (M3 was not shown here because it had the similar molecular structure and absorption spectrum as M2). The TD-DFT calculation results showed that the absorption edge corresponded almost exclusively to the promotion of an electron from HOMO to LUMO, which had the strongest F

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Figure 2. Absorption spectra of the 2D conjugated polymer in chloroform solution: (a) P1, (b) P2, (c) P3, and (d) P3HT with as-synthesized polymers.

Figure 3. Effective conjugation length difference can be clearly derived from the UV spectrum and molecular orbital level. (a) UV absorption spectrum of M1 and M2 observed from experiment and modeling. (b) Energy diagram and frontier orbital of HOMO and LUMO on M1 and M2.

electron transitions with the largest oscillator strength in the low-energy region (please refer to Tables S1 and S2 in the Supporting Information). Thus, in Figure 3b, we plotted the frontier orbital of the HOMO and LUMO of M1 and M2 to elucidate the optical property change under different side-chain arrangement. It had been observed that HOMO of M1 and M2 were almost localized on the same fragment of the monomer with similar antibonding character between the bridge atoms of each conjugated segment. As for the LUMO, the electron cloud in both monomers was widely spread over the molecule, but the inter-ring connection character also turned into bonding, implying that the LUMO could be easily disrupted by increasing the inter-ring torsion angle.26,27 In other words, planar conformation could stabilize and lead to lower LUMO level.28 Next, a significant difference between LUMOs of M1 and M2 was observed (Figure 3b). It was obvious that M1 possessed a more planar conformation with reduced dihedral

angle in the terthiophene units as compared to M2. This kind of structural feature prompted π electrons to easily delocalize over the whole conjugated system, which was beneficial to stabilize the LUMO. Therefore, the LUMO level of M1 (−2.38 eV) obtained from calculation was lower than M2 (−2.00 eV). On the other hand, owing to the antibonding inter-ring conjunction, the influence of structural planarity on HOMO was not as predominant as on LUMO. Consequently, the modeling results clearly showed that the HOMOcal level difference (ΔHOMOM1cal − HOMOM2cal ∼ 0.12 eV) originated from geometrical transformation between M1 and M2 was much smaller than the difference in LUMOcal (ΔLUMOM1cal−M2 cal ∼ −0.38 eV), as manifested in the experimental results recorded in Table 2; the HOMOexp level of M1 was 0.14 eV higher than M2, and the LUMOexp is 0.20 eV lower. The theoretical modeling offered a reasonable explanation to the greater drop of LUMO level in M1, which apparently G

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Table 2. Calculated and Experimental Frontier Orbital Level of M1 and M2

M1 M2

HOMOcala (eV)

HOMOexpb (eV)

Egopt c (eV)

LUMOcala (eV)

LUMOexpd (eV)

−5.22 −5.34

−5.30 −5.44

2.58 2.92

−2.38 −2.00

−2.72 −2.52

a

Obtained from TDDFT. bThe data were obtained from electrochemical measurement. cThe optical band gap was decided by λonset of the monomer solution absorption. dDetermined by HOMO plus optical band gap.

leaded to a larger decrease in band gap energy when compare to M2. Importantly, the observation was consistent with the experimental absorption spectra which showed the band gap of M1 was smaller than M2 and M3. Hence, benefiting from the monomers, the band gap of P1 is smaller than P2 and P3. Moreover, when comparing P2 to P3, the steric hindrance from alkyl chains between neighboring M2 caused twisting of the polythiphene backbone and resulted in slight decrease in conjugation length. Therefore, the magnitude of band gap was in the following order: P1 < P3 < P2 < P3HT. Besides, it was observed that the energy absorbance of P2 (∼490 nm) and P3 (∼500 nm) in solution (Figure 2d) is significantly higher than that of P1 (∼600 nm), presumably due to the fact that the terthiophene units in P2 and P3, which were placed in parallel to the backbone, formed a jacket-like structure and forced the main chain to extend, resulting in stronger main chain absorption.16 A similar phenomenon was also found in the paper reported by Zhou et al.29 A plausible explanation given to that was the oversized side chains could force the main chain to bend out of plane and lead to weak absorbance.29,30 To examine our jacket-like hypothesis, these polymer conformation were theoretically calculated by the molecular dynamics (MD) method, which offers insights into the chain-folding model and conformation in the polymeric system.31 When simulating the molecular conformation in the disordered state, one repeat unit was set within the simulation box (P1 = 12 repeated units/per chain; P2 = 26 repeated units/per chain; P2 = 34 repeated units/per chain) under vacuum to mimic the chain behavior in solution. In addition, periodic boundary conditions were removed and the simulations were carried out in the canonical ensemble (NVT) with the time step equal to 1 fs, and the system temperature was set to 300 K. As can be seen from Figures S23−S25 in the Supporting Information, P1, P2, and P3 all adopted a coil-like chain conformation without the presence of π−π interchain interaction. However, after statistically analyzing the torsional angles between thiophene units in the main chain, it is interesting to find strong correlation between torsional angles and the orientation of the side chains, evolve from perpendicular to parallel as shown in Figure 4. The values of torsional angles (τ) are summarized in Table S3, and τ =180° represented that the two adjacent thiophenes were in a coplanar trans conformation. The distribution plot clearly showed that the deviation of P1 was larger than P2 and P3, in which τP1 = 180 ± 150°, τP2 = 180 ± 83°, and τP3 = 180 ± 90°, indicating that changing the orientation of the side chain from parallel to perpendicular could intensify the degree of torsional angle of the main chain and force the thiophene rings to bend out of coplanarity. The above results suggested that the red-shift of polymer absorption due to restriction of chain conformation as in P2 and P3 was not nearly as significant as having a side chain which

Figure 4. Torsional angle (τ) distribution of the isolated polymer main chain.

could be effectively integrated with the main chain to have an extended conjugation, as in P1. Moreover, the seemly anomalous absorbance property had created an excitationdependent emission (PL) property, which is discussed later in the article. Solid-State Properties of the Polymer Film. In order to validate the hypothesis of our design principle in achieving broad absorption and good crystallinity, the solid state UV−vis spectra and X-ray spectroscopy of our 2D polymers were measured to assess the extent of polymer conjugation and packing structure. The UV−vis spectra for these three polymers in chloroform solvent, as-cast films, and in annealed films are shown in Figure 5, and the λmax for the absorption spectra are also listed in Table 3. The X-ray spectroscopy was used to determine the relative crystallinity (chain packing) of our polymers as shown in Figure 6. And the d-spacing was interpreted from the XRD spectra by Bragg’s equation, nλ = 2dhkl sin θ, in which λ is the wavelength of radiation, dhkl is equal to specific lattice spacing, and θ is the diffraction angle; the detailed results before and after heat treatment are listed in Table 4. Comparing the absorption spectra of the polymers in chloroform solution and in thin film, P2 and P3 revealed an obvious red shift and a rise of low energy absorbance (as shown in Figure 5b,c). The above results are similar to the highly crystallized solid film formed by regioregular P3HT, which exhibited red-shifted thin film spectra, originated from planarization of the main chain and enhanced interchain stacking interaction.32 However, it could be seen from Figure 5a, the absorption spectra of P1 films, before or after annealing, showed no red-shift phenomenon when compared to its solution, as manifested in Figure 6a. The XRD spectrum of P1 was featureless, implying that while the oversized side chains were perpendicularly attached to the main chain, larger interchain distance was created, which hinder the chain packing into order lamella structure and impeded the π−π interaction between the aromatic rings. The disrupted chain packing resulted in amorphous structure which was consistent with the fact that absorption spectrum of P1 in cast thin film revealed minimum changes as compared to that of solution. Interestingly, in the case of P2 and P3, with the terthiophene moieties that were placed in parallel to the main chain, the lowenergy absorbance increased in intensity and red-shifted were H

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Figure 5. Absorption spectrum of the polymer film for the 2D polythiophenes: (a) P1, (b) P2, (c) P3, and (d) P1, P2, and P3 film after heat treatment.

above each polymer’s glass transition temperature as determined by the DSC scan (Figure S20). The extended red-shift of P3 was in agreement with the results we got from Figure 6c. Thermal annealing of P3 leaded to a smaller π−π stacking distance (4.0−3.7 Å) as derived from the diffraction peaks before and after annealing at 22.36° and 24.31°, respectively. Significant increase in the packing density after thermal annealing was only observed in P3, suggesting a structural preference allowing relaxation of chain packing upon thermal annealing. The heat treatment provided P3 with the energy to relax into a denser and ordered structure, which could facilitate the electron transfer through hopping and further influence energy level contributed to the absorption; thus, P3 showed a much more intense absorption band and red-shifted λmax (∼19 nm) absorbance in the long wavelength region. In contrary to P3, XRD spectra of P2 revealed no change in the distance of interchain π−π stacking, but an increase in the dspacing of the lamellae (1,0,0) direction after thermal treatment, suggested increased disorder in polymer film. Thermal annealing for P2 thin film caused disorder and absorbance decrease in the low energy range, which was exactly opposite to that of P3. This could be rationalized as polymer P2 had alkyl chains extended to the side of the terthiophene moiety, which created a steric hindrance between neighboring M2 monomers and force the side chain and polythiophene main chain to bend out of the plane. Such steric hindrance not only shortened the conjugation length as compared to P3 but also leaded to a larger π stacking distance, as validated by the XRD results. To better understand the experimental results, we perform simulations on all three polymers. We adopted the DREIDING force-field method of the Forcite module which was an all-atom

Table 3. UV−Vis Absorption Data for the 2D Conjugated Polymers λmax (nm) P1 P2 P3

solution

film (as-cast)

film (annealed)

417/557 332/475 333/492

431/557 346/519 344/541

434/557 348/524 348/560

presumably due to the elongation of π-conjugation aided by the terthiophene jacket structure that restricted the polymer to a linear and planar conformation,16 and further enhanced the interchain packing. As expected, it was clearly observed that the as-cast film of P2 and P3 exhibited two distinct diffraction peaks in the 2θ range between 3.5° and 4.5° and another one at ∼20° in Figure 6b,c, confirming that the P2 and P3 have better packing properties than P1. The peak at low angle region were attributed to the lamellar (100) d-spacing between polythiophene main chain,33 giving the distance of 20 and 25 Å for P2 and P3, respectively. The peaks centered at 22.02° (∼4.1 Å) for P2 and 22.36° (∼4.0 Å) for P3 correspond to the interchain π−π stacking distance, which indicated the relative packing density in solid state. The d-spacing values clearly pointed out that P3 had a denser π stacking and a larger interchain lamellae distance as compared to that of P2, mainly resulting from the spatial arrangement of the alkyl chain on the terthiophene as depicted in Figure S26. It was important to note that the diffraction peak of P3 was more pronounced than P2 (Figure 5b,c), suggesting an ordered structure in P3 was facilitated through the alignment of the alkyl chain. Moreover, Figure 5b,c shows the red-shift of low-energy absorbance in the solid state that derived from π−π * transition of the main chain became more pronounced in P3 than in P2 after annealing at ∼120 °C, I

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Figure 6. XRD pattern of the 2D polythiophenes: (a) P1, (b) P2, and (c) P3.

approximation of the experimental method. The Qeq method required several parameters such as electron affinity, atomic radius, atomic free energy, and the relative position of atoms for calculation which enabled to reflect changes in the molecular surroundings via received the charge distribution. All the details of the potential energy function terms can be found in refs 34 and 35. In simulating the ordered packing structures, a large simulation box was first built up, which contains 4 molecules (P1 = 12 repeated units/per chain; P2 and P3 = 26 and 34 repeated units/per chain, respectively). The steepest descent minimization method was then adopted to relax and equilibrate

Table 4. XRD Data for the 2D Conjugated Polymers 2θ (deg)

P1 P2 P3

lamellae (as-cast/annealed) 4.36/4.45 3.50/3.33

spacing (Å) π−π

lamellae (as-cast/annealed)

π−π

22.02/21.82 22.36/24.31

20/20 25/27

4.0/4.1 4.0/3.7

model to calculate the atomic interaction parameters.34 Charge needed to be given from other method when DREIDING force field occurred. The charge equilibration (QEq) method was an

Figure 7. Torsional angle (τ) distribution of the polymer (a) main chain and (b) side chain in the condensed state. J

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Figure 8. PL spectra of monomers (M1, M2, and M3) and polymers (P1, P2, and P3) in chloroform solution: (a) P1, (b) P2, and (c) P3.

the initial structures. The minimized system was then relaxed and followed by a series of annealing processes. The annealing temperature was first raised from 300 to 393 K at a rate of 4.65 K/ps and then quenched to 300 K at the same rate. This annealing cycle was repeated 30 times to ensure that the system had been equilibrated. Finally, it was followed by a long MD relaxation period of 1.0 ns (1.0 × 106 time steps) at the setup temperature of each system and checked if the variation of the system temperature is smaller than 1%. After these processes were performed so that the system energy has reached the equilibrium value, then the trajectory is collected to analyze the distribution profile of the torsional angle between the adjacent thiophene rings. All the simulations were carried out in the isothermal−isobaric ensemble (NPT) with the time step equal to 1 fs, and the system pressure was set at 10−4 GPa (1 atm). The equilibrated structures of the polymers in the condensed state are shown in Figures S27−29. The distributions of the torsional angle of the conjugated side chains and main chains are also plotted in Figures S30 and S31, and the values are summarized in Table S3. As could be seen from Figures 4 and 7a, with the presence of π−π interchain interaction, the torsional angle distribution of the main chains were decreased from 180 ± 150° to 180 ± 100°, 180 ± 83° to 180 ± 80°, and 180 ± 90° to 180 ± 60° in P1, P2, and P3, respectively. The largest distribution of torsional angle in P1 (±100°) revealed that P1 had the most irregular conformation which could disrupt the chain packing. In comparison, the smallest deviation from the coplanarity (the value of τ = 180° and 0° indicated that the two neighboring thiophenes were in the same plane with trans and cis conformation, respectively, as depicted in

Figure 7a) in P3 indicated that the main chain was more planar than P1 and P2 in the condensed state as manifested in Figure 5. In addition, Figure 7b showed the distribution of inter-ring torsional angle of the conjugated side chain in P2 and P3. The distribution peak centered at the 180° was contributed by the τside1 and τside2, and the other peak in the lower angle region mainly resulted from the τside1. It could be clearly observed that the fraction of the peak centered around 180° in P3 was much higher than P2, implying that most of the inter-ring connections of the side chain adopted trans or gauche conformation that slightly deviated from trans (±60°) to maintain the structural planarity. In contrast, due to the steric hindrance between the alkyl chains on terthiophene, τside1 of P2 experienced a bond rotation from trans/gauche to cis/gauche conformation that could act as a trigger to force the terthiophenes to bend out of plane and induce the deformation of main chain, which also reflected in the absorption spectrum and XRD results. The phenomena observed from solid state absorption spectrum and XRD were in agreement with our hypothesis and molecular model. More importantly, our design strategy improved the structural order in traditional 2D polythiophenes without compromising their broad absorption and lower band gap energy. Photoluminescence Spectra. The normalized PL spectra of P1−P3 and their corresponding monomers in diluted chloroform solution (10−5 M) were measured to better understand their electronic properties. All the polymers were excited by incident laser with two different wavelengths, which coincided with the λmax of their monomers and backbone K

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design based on two-dimensional “mesogen-jacketed”-like concept. Our preliminary studies revealed a strong correlation between the structure and orientation of the conjugated terthiophene side chain with the final optical, electronic, and electrochemical properties. The side-chain orientation also impacted interchain distance and crystallinity as UV−vis and XRD spectra suggested. The parallel side chains manifested a better self-assembling property in comparison with the vertical ones. Furthermore, the alkyl chains on the terthiophene moiety also played an important role in affecting the molecular packing. Herein, the P3 with the alkyl chain on the 4-position of the terthiophene moiety allowed better organized chain folding and kept the coplanarity of the aromatic rings on the side chain and main chain which were in agreement with MD modeling results. Of particular interest was polymer P1 which exhibited excitation-dependent fluorescent properties which suggested incomplete energy transfer and/or some level of structure inhomogeniety. In addition, our novel design in molecular structure which was different than the traditional 2D model had not only led to the wide range absorption and low HOMO level but also rendered self-organization of the larger side group.

absorption as shown in Figure 2. The comparison between monomer and polymer revealed how monomers got integrated into the polymer and contributed to the overall absorbance and emission properties. As could be seen in Figure 7, the PL of M1, M2, and M3 showed a λmax at 500, 477, and 467 nm, respectively. Those phenomena were not observed in the PL spectra of their corresponding polymers. Moreover, the PL spectra of P2 and P3 were independent of the excitation energy. It was believed that the energy transfer occurred as the emission generated by the terthiophene moiety was then absorbed by their backbone and leaded to the emission. It was dominated by the polymer backbone as previously discussed.36 Interestingly, the pattern of P1 deviated from P2 and P3. The PL spectra of P1 revealed excitation wavelength dependent optical properties: two emission peaks were found at 573 and 595 nm when irradiated by 425 and 525 nm light, respectively. This phenomenon could be rationalized as the presence of structure inhomogeneity, resulting from the distribution of various aggregated and amorphous states which responded predominantly to the excitation energy resonate to the respective electronic structure. Another reason might be due to incomplete energy transfer between conjugated side chain and main chain due to the weak absorption in the low-energy region.30 Electrical Properties. Cyclic voltammetry was used to determine the oxidation and reduction potentials of the conjugated polymers thin films made by dip-coating the working electrode with corresponding polymer in chlorobenzene solution, and the results are shown in Table 1. By using ferrocene as an internal standard, the HOMO and LUMO levels were determined using the equation HOMO = −(Eox + 4.8);

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ASSOCIATED CONTENT

S Supporting Information *

Figures give characterization results of the DSC, CV, and 1H NMR of the polymers; structural information on M1, M2, and M3 monomer and its aldehyde derivatives; simulation results of the monomers and the chain conformation of polymers. This material is available free of charge via the Internet at http:// pubs.acs.org.

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LUMO = HOMO + Eg opt

opt

AUTHOR INFORMATION

Corresponding Author

where Eg (optical band gap) was estimated from 1240/λonset (nm), and the unit of Eox is eV vs Ag/Ag+. The values obtained are listed in Table 1, and P1 turned out to have the higher HOMO level (−4.98 eV) than P2 (−5.25 eV) and P3 (−5.31 eV). It was important to note that P1 had the highest HOMO level and smallest band gap with the same number of repeated unit on the side chain. The reasons that leaded to this disparity might be complex and most likely due to the fact that P1 has a more extended conjugated double bond (see Figure 3), contributed to the overall system. Hence, it leaded to a narrowed band gap energy which was typically accompanied by the rising of HOMO and/or lowering of LUMO energetics. On the contrary, for P2 and P3, only part of the side chain was conjugated to the polythiopene backbone as manifested in Figure 3b. Therefore, the optical band gap was larger and HOMO was lower. It was also being noticed that P2 and P3 both had lower HOMO level than P3HT (∼−5.0 eV). This energetic variation might be attributed to the density of electron-donating alkyl substituents and backbone twisting which resulted from bulky side chain. Hence, it diminished the interchain interaction and lowered the HOMO level. Based on our results, the broader absorption and lower HOMO level suggested that the P3 might be a promising polymer photovoltaic material.

*E-mail: [email protected] (L.W.); [email protected] (H.L.W.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the National Science Council of the Republic of China for financially supporting this research. H.L.W. thanks the financial support from LDRD program under the auspices of Department of Energy. Part of this work is supported by Basic Energy Science, Biomaterials Program.

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REFERENCES

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CONCLUSIONS The present study had demonstrated synthesis of a series of 2D polythiophene derivatives with terthiophene as the side chain moiety attached to the main chain via vinyl linkagea structure L

dx.doi.org/10.1021/ma4007945 | Macromolecules XXXX, XXX, XXX−XXX

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dx.doi.org/10.1021/ma4007945 | Macromolecules XXXX, XXX, XXX−XXX