Synthesis and characterization of polythiophenes with alkenyl substituents

Abstract

Synthesis of polythiophenes with alkenyl side chains is reported for the first time. The alkenyl side chains are versatile functional groups, allowing facile chemical modification to generate novel materials with tunable opto-electronic properties. Poly(3-pentenylthiophene), poly(3-undecenylthiophene), poly(3-hexylthiophene-ran-3-pentenylthiophene) and poly(3-hexylthiophene-ran-3-undecenylthiophene) were synthesized by a nickel mediated cross-coupling polymerization. Poly(3-alkenylthiophene) homopolymers and random copolymers were prepared from 2-bromo-5-chlorozinc-3-alkenylthiophene monomers using Ni(dppp)Cl₂ as catalyst. The field-effect mobilities and photovoltaic response in bulk heterojunction solar cells were measured for poly(3-hexylthiophene-ran-3-pentenylthiophene) and poly(3-hexylthiophene-ran-3-undecenylthiophene).

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Introduction

Semiconducting polymers have generated extensive interest in the scientific community over the last few decades.^1–3 Among semiconducting polymers, polythiophenes have been extensively investigated due to a relatively small band gap, environmental stability and good processability.^3,4 The initial syntheses of polythiophenes by oxidative polymerization yielded insoluble polymers.^5–8 In 1986, Elsenbaumer reported the synthesis of poly(3-alkylthiophenes) which are soluble and processable.⁹ The asymmetric structure of 3-alkylthiophene monomer gives rise to four spectroscopically distinct regioisomers. The regioregular, or head-to-tail coupled, isomer exhibits high co-planarity and improved optoelectronic properties due to the supramolecular assembly of the polymer chains in highly-ordered solid-state structures.^10–23 A few polymerization techniques are employed for the synthesis of regioregular poly(3-alkylthiophenes).³ All these reported techniques use various transition metal mediated cross-coupling reactions.^4,24–27 The McCullough method was the first reported procedure for the synthesis of regioregular poly(3-hexylthiophene).^28–30 This method employs a Kumada cross-coupling protocol. In the original McCullough method, 2-bromo-3-hexylthiophene was lithiated to generate 2-bromo-3-alkyl-5-lithiothiophene, which was subsequently transmetallated with MgBr₂ to generate the corresponding organomagnesium thiophene derivative. Addition of Ni(dppp)Cl₂ catalyst generates regioregular poly(3-hexylthiophene). An alternate and more efficient modification of the original McCullough method involves the Negishi cross-coupling of the corresponding organozinc thiophene monomer.³¹ The polymers generated are regioregular with well-defined molecular weights and a narrow polydispersity index. The Rieke method, which was reported at the same time as the McCullough method, also generates regioregular poly(3-hexylthiophene).^32,33 In this method, the activated Rieke zinc is reacted with 2,5-dibromo-3-hexylthiophene and Ni(dppp)Cl₂ catalyst to generate regioregular poly(3-hexylthiophene). Both McCullough and Rieke methods require low temperatures during the metal halogen exchange step. Stille and Suzuki cross-coupling protocols have also been employed for the synthesis of regioregular polythiophenes.^34–36 Grignard metathesis polymerization (GRIM) developed in McCullough's group in 1999 represented a breakthrough, as it allows the synthesis of regioregular poly(3-alkylthiophenes) in large scale without the need of cryogenic temperatures.^37,38 Development of living nickel mediated cross-coupling polymerizations (McCullough and GRIM methods) has provided a versatile tool for controlling the molecular parameters of regioregular poly(3-alkylthiophenes).^31,39–41 Regioregular poly(3-hexylthiophene) with various molecular weights and narrow molecular weight distributions have been synthesized and the field-effect mobilities were correlated with the polymer length.²⁰ The AFM indicated that the polymers form nanofibrils with contour length corresponding to the polymer length. Field-effect mobility increased exponentially with nanofibril width.²⁰ The width of nanofibrils, (determined by Fourier analysis of AFM images) initially increased linearly with the molecular weight and then leveled off.²⁰ Furthermore, a versatile and simple method for in situ end-group functionalization of regioregular poly(3-alkylthiophenes) using the Grignard metathesis (GRIM) method has been reported.^42,43 The facile end group functionalization method developed in McCullough group triggered the development of various techniques to synthesize block copolymers containing regioregular poly(3-alkylthiophenes).^44–56 Post-polymerization functionalization has also been employed to functionalize polythiophenes.^57,58

While the polymerization and applications of regioregular poly(3-alkythiophene)s are well developed and quite successful, it will be desirable to add the ability to modify the polymer side chains via post-polymerization chemical processes. For example, such modifications could allow one to enhance the interaction between polythiophene and acceptors in photovoltaic polymer blends. A terminal alkene group is a versatile chemical handle which allows subsequent modification to a variety of functional groups, including hydroxyl and thiol groups. With this goal, we are reporting for the first time a thiophene monomer with either a 3-pentenyl or 3-undecenyl chain which can be polymerized. Previously an alkenyl substituted polythiophene was synthesized through a post-polymerization elimination route, which caused cross-linking due to the elevated temperatures.⁵⁹ In this study the alkene substituted polythiophenes are amenable to subsequent modification. The synthesized polymers have been tested in field effect transistors and bulk heterojunction solar cells. Tapping mode Atomic Force Microscopy (TMAFM) and X-ray diffraction were employed to investigate the morphology of the synthesized polymers.

Results and discussion

Introduction of alkenyl substituents in the 3-position of polythiophenes generates functional semiconducting polymers. The alkenyl substituent is a versatile functional group that allows chemical modification via chemical reactions specific to double bonds. Among many possible routes for chemical modification, hydroboration/oxidation has been employed to convert the double bonds to hydroxyl functional groups.⁴⁴

3-Pentenylthiophene and 3-undecenylthiophene were synthesized by Kumada cross-coupling of 3-bromothiophene with the corresponding alkenyl Grignard reagents. Bromination of 3-alkenyl substituted thiophenes with NBS failed to generated the 2,5-dibromo compounds. However, lithiation of 3-alkenylthiophene followed by the reaction with carbon tetrabromide generated the 2-bromo-3-alkenylthiophenes (Scheme 1). The McCullough method was used for homopolymerization of both 3-pentenylthiophene and 3-undecenylthiophene.^28,29 2-Bromo-3-alkenylthiophene monomers were reacted with LDA, followed by transmetallation with ZnCl₂ to generate 2-bromo-3-alkenyl-5-bromozinc thiophene. Addition of Ni(dppp)Cl₂ catalyst initiated the polymerization to generate poly(3-alkenylthiophene) polymers. An important feature of the homopolymerization reactions was the inability to generate high molecular weight polymers. The synthesized poly(3-pentenylthiophene) had a M_n = 1130 g/mol, while the poly(3-undecenylthiophene) had a M_n = 2600 g/mol. We speculate that this behavior is due to the competing interaction of the alkene substituent with the Ni(0) which is generated during the catalytic cycle.³⁹ This competing interaction could thus break the Ni(0) from its proposed η²-thiophene complex resulting in low molecular weight polymers.³⁹

Scheme 1 Synthesis and polymerization of 3-alkenylthiophene monomers; P1 = poly(3-pentenylthiophene) P2 = poly(3-undecenylthiophene); P3 = poly(3-hexylthiophene-ran-3-pentenylthiophene); P4 = poly(3-hexylthiophene-ran-3-undecenylthiophene)

Due to this behavior of 3-alkenylthiophene monomers we decided to synthesize copolymers of 3-hexylthiophene with 3-pentenylthiophene and 3-undecenylthiophene to generate polymers with higher molecular weights. Poly(3-hexylthiophene-ran-3-pentenylthiophene) (P3) and poly(3-hexylthiophene-ran-3-undecenylthiophene) (P4) were synthesized by the McCullough method. The kinetic behavior of these polymerizations was investigated and indicated that 3-alkenylthiophene monomers incorporate into the copolymers at a slower rate than 3-hexylthiophene (Supporting Information). This observed behavior is consistent with the proposed Ni-alkene interaction discussed above.

At reaction time lower than 120 min the preferential incorporation of the 3-hexylthiophene monomer has been observed, while only 18% of the 3-undecenylthiophene monomer has been consumed at that time. However, the final copolymer obtained after 12 h contained 21 mol% of 3-undecenylthiophene. Molecular weight vs. conversion plot (Supporting information) shows the increase of molecular weight with conversion.

The UV-vis solution spectra of homopolymers P1 and P2 display maximum absorption bands at 400 nm and 410 nm, respectively. This can be explained by taking into consideration the low regioregularity and low molecular weight of polymers P1 and P2. It has been demonstrated that regioregular poly(3-hexylthiophene) show a progressive shift of λ_max as the proportion of head-to-tail couplings increases as compared to their regioirregular analogues.^60,61 In soluble α- thiophene oligomers it has been shown that the UV-vis spectra progresively changes from 396 nm for quarterthiophene to 418 nm for pentathiophene to 430 nm for hexathiophene oligomer.⁶⁰ The solution UV-vis spectra of P3 and P4 copolymers show broad, featureless long wavelength absorption band with a steep absorption edge, consistent with the presence of a nearly coplanar backbone structure. The maximum absorption of P3 was found at 420 nm and P4 at 440 nm (Table 1). The blue shifted absortion maxima of polymers P3 and P4 as compared to regioregular poly(3-hexylthiophene) is due to lower molecular weights which result in shorter conjugation length.^60,61 Absorbance maxima for both copolymers in thin films were red shifted to 501 nm for P3 and 541 nm for P4, indicating a better electronic delocalization relative to the solution spectra which may result from a greater effective π-conjugated length due to locked molecular conformation and the coplanar arrangements of the thiophene rings. Coplanarity of the thiophene rings gives rise to lower HOMO–LUMO gaps and a red shifted π–π* transition due to the vibronic coupling in which a ring vibration is excited simultaneously with the electronic π–π* transition.

Table 1 Molecular weight, optical and electronic energy levels of poly(3-pentenylthiophene) (P1), poly(3-undecenylthiophene) (P2), poly(3-pentenylthiophene-ran-3-hexylthiophene)(P3) and poly(3-undecenylthiophene-ran-3-hexylthiophene)(P4)

Polymer	Mn^a	PDI^a	λmaxL^b/nm	λmaxS^c/nm	Eox /V	Ered /V	HOMO^d/eV	LUMO^e/eV
a Mn (g/mol) and PDI estimated from SEC using polystyrene calibration. b in chloroform. c in thin films. d estimated from onset of oxidation peak. e estimated from onset of reduction peak. P3 contains 16 mol% 3-pentenylthiophene estimated from ^1HNMR. P4 contains 21 mol% 3-undecenylthiophene estimated from ^1HNMR.
P1	1130	1.6	400	N/A	N/A	N/A	N/A	N/A
P2	2600	1.4	410	N/A	N/A	N/A	N/A	N/A
P3	7090	2.2	420	501	0.74	-0.92	−5.45	−3.79
P4	6460	2.1	440	541	0.77	−1.00	−5.48	−3.71

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Thin film X-ray diffraction data were collected for poly(3-hexylthiophene-ran-3-pentenylthiophene) and for poly(3-hexylthiophene-ran-3-undecenylthiophene) as shown in Fig. 1. The sharp low-angle crystalline peak corresponds to the first order reflection (100) near 2θ = 5°, and it is due to the lamellar planes formed by the side-by-side stacking of molecules. The higher order peaks, second order (200) and third order (300) are also present. This corresponds to an interlayer d-spacing of 16 Å of the lamellar structure. The peak near 2θ = 24° (d = 3.8 Å) is a composite peak of the 020 reflection and is strongly dependent on alkyl chain length. This peak confirms the π–π stacking.

Fig. 1 X-Ray Diffraction pattern of: (a) poly(3-hexylthiophene-ran-3-pentenylthiophene) and (b) poly(3-hexylthiophene-ran-3-undecenylthiophene).

The surface morphology of the copolymers was investigated by atomic force microscopy (AFM) (Fig. 2 and Supporting information). Thin films of the copolymers were drop-cast onto mica substrate and the solvent was slowly evaporated in a chloroform chamber. Nanofibrillar morphology was observed for thin films of both copolymers P3 and P4. The morphology of the films revealed that the polymers form highly ordered nanostructures. The polymer films contained long nanofibrils which are due to the favorable π–π stacking interactions of the polymer chains. As expected, the morphology of these copolymers is similar to the morphology of regioregular poly(3-hexylthiophene).

Fig. 2 Tapping mode AFM image poly(3-hexylthiophene-ran-3-undecenylthiophene); (a) phase image, (b) height image, scan size: 5 × 5 μm.

Cyclic voltammograms of thin films were obtained for P3 and P4. For P4 the first oxidation peak was observed ca. 0.94 V and the second oxidation occurred ca. 1.59V. The return scan shows a broad return wave for both P3 and P4 copolymers. From the value of oxidation potential and reduction potential of the copolymers, the HOMO and LUMO energy levels were determined as shown in Table 1.

Field-effect mobilities of copolymers P3 and P4 were measured on bottom contact thin-film transistor devices fabricated on highly doped n-type silicon wafer. The plots of source–drain current (I_DS) vs. source–drain voltage (V_DS) at different negative gate voltages are shown in Fig. 3. The charge carrier mobilities were obtained from a plot of I^1/2_DSvs V_GS (shown in the Supporting information) using the following equation: , where W is the channel width, L is the channel length, C_i is the capacitance per unit area of the dielectric, and V_T is the threshold voltage. For poly(3-hexylthiophene-ran-3-pentenylthiophene) the measured mobility was 1.36 × 10⁻⁴ cm²/Vs, while for poly(3-hexylthiophene-ran-3-undecenylthiophene) the mobility value was 1.29 × 10⁻³ cm²/Vs.

Fig. 3 Current–voltage characteristics of poly(3-hexylthiophene-ran-3-pentenylthiophene): (a) output curves at different gate voltages; (b) transfer curve at V_DS = −75 V (right), (μ = 1.36 × 10⁻⁴ cm²V⁻¹s⁻¹, V_T = − 5 V, on/off ratio = 10², W = 475 μm, L = 10 μm). Current–voltage characteristics of poly(3-hexylthiophene-ran-3-undecenylthiophene): (c) output curves at different gate voltages; (d) transfer curve at V_DS = −55 V, (μ = 1.29 × 10⁻³ cm²V⁻¹s⁻¹, V_T = − 5 V, on/off ratio = 10², W = 475 μm, L = 10 μm).

To further investigate the electronic properties, copolymers P3 and P4 were used as donors in bulk heterojunction solar cells with PCBM and CdSe acceptors. The general structure of the fabricated devices was ITO/PEDOT:PSS (20 nm)/blend (∼100 nm)/Ca.Table 2 and supporting information show a comparison of I–V data obtained from the solar cell devices, where power conversion efficiencies (PCE) as high as 0.53% and 0.39%, were measured for copolymers P3 and P4 respectively, with PCBM acceptor. Similar power conversion efficiencies were measured for bulk heterojunction solar cells with CdSe acceptor for both P3 and P4 copolymers. Fig. 4 shows the current–voltage response of P3 and P4 blends with PCBM acceptor under AM 1.5 illumination at 100 mW/cm². The performance of the reported polythiophenes with alkenyl substituents is lower when compared with regioregular poly(3-hexythiophene). However, if one compares the lower molecular weight of copolymers P3 and P4 with the regioregular poly(3-hexylthiophene) that is usually used in solar cells, synthesizing alkenyl substituted polythiophenes with higher molecular weight should improve the electronic properties of these materials.

Table 2 Photovoltaic properties of poly(3-pentenylthiophene-ran-3-hexylthiophene) (P3) and poly(3-undecenylthiophene-ran-3-hexylthiophene)(P4)

Polymer	Acceptor	Voc/V	Isc/mA	FF(%)	η (%)
a spin coated films of polymer/PCBM (1 : 1 wt:wt). b spin coated films of CdSe/polymer (5 : 1 wt:wt).
P3	PCBM^a	0.71	−1.44	51.77	0.53
P3	CdSe^b	0.79	−0.48	79.72	0.30
P4	PCBM^a	0.71	−1.21	45.62	0.39
P4	CdSe^b	0.73	−1.11	51.90	0.42

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Fig. 4 I–V plot of the polymer solar cell with: (a) poly(3-hexylthiophene-ran-3-pentenylthiophene (P3) and (b) poly poly(3-hexylthiophene-ran-3-undecenylthiophene) (P4) and PCBM acceptor, ([P3]: [PCBM] = 1: 1 (w/w), [P4]: [PCBM] = 1: 1 (w/w).

To demonstrate that the alkenyl side chains can be easily functionalized we performed a hydroboration/oxidation of poly(3-hexylthiophene-ran-3-undecenylthiophene) with almost quantitative yield. The ¹H NMR analysis confirmed the complete conversion of the alkenyl substituents to hydroxyalkyl functional groups (¹H NMR spectra before and after chemical modification are shown in the Supporting information).

Current effort in our group are directed towards the synthesis of alkenyl substituted polythiophenes with higher molecular weights and larger content of 3-alkenylthiophene. These materials will be subjected to chemical modification to generate thiol side chains, which will make these polymers compatible with CdSe semiconductor nanocrystals.

Experimental

Materials

All commercial chemicals were purchased from Aldrich Chemical Co., Inc. and were used without further purification unless otherwise noted. All reactions were conducted under nitrogen. The polymerization glassware and syringes were dried at 120 °C for at least 24 h before use and cooled under a nitrogen atmosphere. Tetrahydrofuran was dried over sodium/benzophenone ketyl and freshly distilled prior to use.

Structural analysis

¹H NMR spectra of the synthesized monomers were recorded on a JEOL-Delta 270 MHz spectrometer at 25 °C. ¹H NMR spectra of the polymers were recorded on a VARIAN-INOVA-500 MHz spectrometer at 30 °C. ¹H NMR data are reported in parts per million as chemical shift relative to tetramethylsilane (TMS) as the internal standard. Spectra were recorded in CDCl₃.

GC/MS was performed on an Agilent 6890-5973 GC-MS workstation. The GC column was a Hewlett-Packard fused silica capillary column cross-linked with 5% phenylmethyl siloxane. Helium was the carrier gas (1 mL/min). The following conditions were used for all GC/MS analyses: injector and detector temperature, 250 °C; initial temperature, 70 °C; temperature ramp, 10 °C/min; final temperature, 280 °C.

The UV-Vis spectra of polymer solutions in chloroform solvent were carried out in 1 cm cuvettes using an Agilent 8453 UV-VIS spectrometer. Thin-films of polymer were obtained by evaporation of chloroform solvent on glass microscope slides.

Molecular weights of the synthesized polymers were measured by Size Exclusion Chromatography (SEC) analysis on a Viscotek VE 3580 system equipped with ViscoGEL™ columns (GMHHR-M), connected to a refractive index (RI) detectors. GPC solvent/sample module (GPCmax) was used with HPLC grade THF as the eluent and calibration was based on polystyrene standards. Running conditions for SEC analysis were: flow rate = 1.0 mL/min, injector volume = 100 μL, detector temperature = 30 °C, column temperature = 35 °C. All the polymers samples were dissolved in THF and the solutions were filtered through PTFE filters (0.45 μm) prior to injection.

Synthesis of 3-pentenylthiophene

A dry 100 mL three neck round bottom flask equipped with a reflux condenser was flushed with nitrogen and charged with magnesium (3.72 g, 155 mmol), one crystal of I₂, and 60 mL of anhydrous diethyl ether. To this solution, 5-bromo-1-pentene (17.7 mL, 150 mmol) was added dropwise under stirring. The reaction mixture was gently refluxed for 1 h under a nitrogen atmosphere. In a separate flask, 3-bromothiophene (14.2 mL, 150 mmol) and Ni(dppp)Cl₂ (50 mg, 0.09 mmol) were dissolved in 30 mL of anhydrous diethyl ether. The solution of 5-magnesiobromo-1-pentene was cannulated to the flask containing the 3-bromothiophene and the Ni(dppp)Cl₂ catalyst. The reaction was gently refluxed overnight under a nitrogen atmosphere. The reaction mixture was quenched by pouring into a beaker containing ice and HCl. The organic phase was washed with brine and water three times and was extracted with diethyl ether. The ether phase was dried over MgSO₄, filtered and the solvent removed. 3-Pentenylthiophene was purified by vacuum fractional distillation (55 °C, 3 Torr).

¹H NMR (CDCl₃, 270 MHz): δ 1.55 (m, 2H), 2.09 (m, 2H), 2.64 (t, 2H), 4.98 (m, 2H), 5.88 (m, 1H), 6.93 (s, 1H), 6.96 (d, 1H) 7.22 (m, 1H). EI MS m/z: [M+] 152.1.

3-Undecenylthiophene was synthesized with a similar procedure as described above (90 °C, 3 Torr).

¹H NMR (CDCl3, 270 MHz): δ 0.96 (m, 2H), 1.35 (m, 12H), 1.62 (m, 4H), 2.10 (m, 2H), 2.62 (t, 2H), 4.95 (m, 2H), 5.85 (m, 1H), 6.93 (d, 1H), 7.03 (m, 1H), 7.23 (m, 1H). EI MS m/z: [M+] 236.1.

Synthesis of 2-bromo-3-pentenylthiophene

A 250 mL three neck round bottom flask was placed in a dry ice/acetone bath (−78 °C) and 50 mL of dry THF was added via a gas tight syringe under a nitrogen atmosphere. t-Butyllithium (1.7 M in hexane) (25 mL, 42.5 mmol) was added to the reaction vessel followed by drop-wise addition of 3-pentenylthiophene (6.32 g, 40 mmol). The mixture was allowed to react for one hour at −78 °C. The reaction was then allowed to warm up to −40 °C and maintained at this temperature for 1 h. After 1 h the reaction mixture was cooled to −78 °C and carbon tetrabromide (27.8 g, 84 mmol) was added as a solution in 20 mL dry THF. The dry ice/acetone bath was then removed and the reaction mixture was stirred one hour followed by quenching with ice. The organic phase was extracted with diethyl ether, dried over MgSO₄, filtered and concentrated. The crude product was purified using column chromatography with hexane as the mobile phase.

¹H NMR (CDCl₃, 270 MHz): δ 1.55 (m, 2H), 2.09 (m, 2H), 2.63 (t, 2H), 4.98 (m, 2H), 5.88 (m, 1H), 6.81 (d, 1H), 6.87 (d, 1H). EI MS m/z: [M+] 232.0.

2-bromo-3-undecenylthiophene was synthesized using the same procedure described above.

¹H NMR (CDCl₃, 270 MHz): δ 0.95 (m, 2H), 1.27 (m, 8H), 1.55 (m, 4H), 2.10 (m, 2H), 2.65 (m, 2H), 4.94 (m, 2H), 5.82 (m, 1H), 6.80 (d, 1H), 6.90 (d, 1H). EI MS m/z: [M+] 316.00.

Homopolymerization of 2-bromo-3-pentenylthiophene

10 mL of dry THF and diisopropylamine (0.47 mL, 3.33 mmol) were added to a round bottom flask under a nitrogen atmosphere. The flask was then placed in a dry ice/acetone bath (−78 °C) and n-butyllithium (2.5M in hexanes) (0.6 mL, 1.5 mmol) was added drop-wise. The reaction was stirred for 30 min at −78 °C after which the reaction mixture was allowed to warm up to −40 °C and maintained at this temperature for one hour. In a separate flask, a solution of 2-bromo-3-pentenylthiophene (0.35 g, 1.5 mmol) and 10 mL THF was degassed by bubbling with N₂. This solution was added to the previously prepared LDA mixture and stirred for one hour at −78 °C. ZnCl₂ (0.27 g, 2.0 mmol) dissolved in 5 mL of dry THF was degassed with nitrogen and added to the lithiated monomer solution. The reaction mixture was stirred at −78 °C for 4 h. The reaction was then allowed to warm to room temperature and Ni(dppp)Cl₂ (40 mg, 0.07 mmol) was added to the reaction mixture to initiate the polymerization. The polymerization was allowed to proceed at room temperature for 12 h. The reaction mixture was quenched in 50 mL of methanol and the polymer was separated by filtration. The product was purified by Soxhlet extraction with methanol and chloroform. The polymer was separated from the chloroform fraction by evaporation of the solvent. Molecular weight of the copolymer was determined by SEC and the polymer structure was confirmed by ¹H NMR.

¹H NMR (CDCl₃, 500 MHz): δ 1.7 (br, 2H), 2.12 (br, 2H), 2.45 (br, 2H), 2.75 (br, 2H), 4.93 (m, 2H), 5.80 (m, 1H), 6.91 (br, 1H). SEC: M_n = 1130 g/mol, PDI = 1.59.

Homopolymerization of of 2-bromo-3-undecenylthiophene

2-Bromo-3-undecenylthiophene was polymerized using a similar procedure as discussed above. ¹H NMR (CDCl₃, 500 MHz): δ 1.28 (br, 12H), 1.57 (br, 4H), 2.03 (m, 2H), 2.54 (t, 2H), 4.97 (m, 2H), 5.04 (m, 1H), 6.88 (br, 1H); SEC: M_n = 2880 g/mol, PDI = 1.20.

Synthesis of poly(3-hexylthiophene-ran-3-pentenylthiophene)

The monomers 2-bromo-5-chlorozinc-3-pentenylthiophene and 2-bromo-5-chlorozinc-3-hexylthiophene were prepared in two different flasks under a nitrogen atmosphere.

Solution A was prepared by reacting diisopropylamine (0.45 mL, 3.2 mmol) and n-butyllithium (2.5 M in hexane) (1.2 mL, 3 mmol) in 10 mL of dry THF at −78 °C under a N₂ atmosphere for one hour to generate LDA. 2-Bromo-3-pentenylthiophene (0.7 g, 3.0 mmol) was dissolved in 5 mL of dry THF and the mixture was added drop-wise to the LDA solution. The solution was stirred for one hour at −78 °C, followed by the addition of ZnCl₂ (0.44 g, 3.2 mmol) dissolved in 5 mL of dry THF. The reaction mixture was stirred at −78 °C for 3 h. The reaction was then allowed to warm to room temperature. Solution B was prepared by reacting diisopropylamine (0.7 mL, 5.2 mmol), n-butylithium (2.5 M in hexane) (2.0 mL, 5 mmol) in 10 mL of dry THF at −78 °C under a N₂ atmosphere for one hour to generate LDA. 2-Bromo-3-hexylthiophene (1.24 g, 5.0 mmol) was dissolved in 5 mL dry THF and the mixture was added drop-wise to the LDA solution. The lithiated monomer solution was stirred for 1 h at −78 °C, followed by the addition of ZnCl₂ (0.7 g, 5.2 mmol) dissolved in 5 mL of dry THF. The reaction mixture was stirred at −78 °C for 3 h. The reaction was then allowed to warm to room temperature. Solution B was cannulated to solution A under nitrogen atmosphere. Ni(dppp)Cl₂ catalyst (80 mg, 0.15 mmol) was added to the reaction mixture to initiate the polymerization. The reaction mixture was stirred for 12 h at room temperature and then quenched in methanol and filtered. The product was purified by Soxhlet extraction with methanol and chloroform. Molecular weight of the copolymer was determined by SEC and copolymer structure was confirmed by ¹H NMR.

¹H NMR (CDCl₃, 500 MHz): δ 0.90(t, 3H), 1.25 (m, 4H), 1.43 (m, 2H), 1.55(m, 2H), 1.73 (m, 2H), 2.18 (m, 2H), 2.57 (m, 2H), 2.79 (m, 2H), 5.02 (m, 2H), 5.87 (m, 1H), 6.98 (s, 2H); 16 mol% 3-pentenylthiophene; SEC: M_n = 7090 g/mol, PDI = 2.2.

The composition of the copolymer was determined by integrating the methyl protons of the 3-hexylthiophene vs. the methine protons of the 3-pentenylthiophene.

Poly(3-hexylthiophene-ran-3-undecenylthiophene) was synthesized by the same procedure as described above. Molecular weight of the copolymer was determined by SEC and copolymer structure was confirmed by ¹H NMR.

¹H NMR (CDCl₃, 500 MHz): δ 0.90 (t, 3H), 1.25 (m, 4H),, 1.43 (m, 20H), 1.70 (m, 2H), 2.18 (m, 2H), 2.57 (m, 2H), 2.80 (m, 2H), 5.02 (m, 2H), 5.87 (m, 1H), 6.98 (s, 2H); 21 mol% 3-undecenylthiophene; SEC: M_n = 6460 g/mol, PDI = 2.1.

Hydroboration/oxidation of poly(3-hexylthiophene-ran-3-undecenylthiophene)

Poly(3-hexylthiophene-ran-3-undecenylthiophene) (0.3 g, M_n = 6460 g/mol) was dissolved in anhydrous THF (20 mL) under nitrogen. To this reaction mixture, a 0.5 M solution of 9-BBN (4 mL, 2 mmol) in anhydrous THF was added via a syringe. The reaction mixture was stirred for 12 h at 40 °C, at which point a 6M solution of NaOH (2 mL) was added to the reaction flask. The reaction mixture was stirred for additional 15 min at 40 °C. The reaction mixture was allowed to cool to room temperature followed by slow addition of a 33% aqueous solution of hydrogen peroxide (2 mL), and the reaction was heated at 40 °C for 12 h. The copolymer was isolated by precipitation in methanol. The copolymer was filtered and purified by a Soxhlet extraction with methanol. The copolymer was characterized by 1H NMR.

¹H NMR (CDCl3, 500 MHz): δ 0.90 (t, 3H), 1.45 (m, 24H), 1.6 (m, 4H), 2.57 (m, 2H), 2.80 (m, 2H), 3.6 (m, 2H).

Opto-electronic properties

Cyclic voltammetry. Electrochemical grade tetrabutylammonium perchlorate (TBAP) was used as the electrolyte without further purification. Acetonitrile (low water 99.9% grade) was distilled over calcium hydride (CaH₂). Electrochemical experiments were performed using a BAS CV-50W Voltammetric Analyzer (Bioanalytical Systems, INC.). The electrochemical cell was comprised of a platinum electrode, a platinum wire auxiliary electrode and an Ag/AgCl reference electrode. Acetonitrile solutions containing 0.1 M of TBAP were placed in a cell and purged with N₂. A drop of the polymer solution in chloroform was placed on tip of the platinum electrode. The solvent was evaporated in air. The film was immersed in the electrochemical cell containing the electrolyte prior to measurements. All electrochemical shifts were standardized to the ferrocene redox couple at 0.471 V.

X-Ray diffraction. X-Ray diffraction patterns were obtained on a RIGAKU Ultima III diffractometer. Samples were subjected to Cu-K_α radiation (λ ∼1.54 Å) and scanned from 3 to 25 degrees (2θ) at 0.01 degree intervals at a rate of 1 degree/min. Glass cover slips (22 × 22 mm) were used as the sample substrates.

Field effect transistor fabrication and mobility determination. Organic thin film transistors with bottom contact configuration were fabricated on a 4 inch silicon wafer with a 200 nm SiO₂ layer, which was thermally grown. A 50 Å layer of Cr metal was deposited on top of the SiO₂ layer followed by a deposition of 1000 Å of Au. Source and drain electrodes were patterned using standard photolithography. The channel width of the field effect transistors was 475 μm, with channel lengths from 2 μm to 80 μm. Surface treatment of the devices was performed with a 5 mmol solution of octyltrichlorosilane (OTS) in toluene at 65 °C for 1 h, followed by a rinse with toluene. Then, the devices were kept in a chamber containing NH₄OH in a vial overnight and rinsed with DI water, toluene, and then sonicated in toluene for 10 min. Lastly, devices were vacuum dried at 90 °C for 30 min. The active layer was deposited by drop casting filtered (0.2μm PTFE syringe filters) chloroform solutions of the polymers (0.75 mg/mL). The solvent was slowly evaporated in a covered Petri dish filled with chloroform. The devices were annealed at 120 °C for 30 min prior to measurement. Mobilities were measured using a Keithley 4200 parameter analyzer under ambient conditions.

TMAFM (tappping mode atomic force microscopy). AFM studies were carried out on a VEECO-dimension 5000 Scanning Probe Microscope with a hybrid xyz head equipped with NanoScope Software. AFM images were obtained using silicon cantilevers with nominal spring constant of 42 N/m and nominal resonance frequency of 300 KHz (OTESPa). Image analysis software Nanoscope 7.30 was used for surface imaging and image analysis. All AFM measurements were conducted under ambient conditions. All cantilever oscillation amplitude was equal to ca 375 mV and all images were acquired at 1H_z scan frequency. Sample scan area varied from 1 μm to 3 μm. Samples were prepared from chloroform solutions (0.75 mg/mL) via drop casting method onto a mica substrate. The solvent was slowly evaporated in a cover Petri dish filled with chloroform. Samples were annealed at 120 °C for 30 min.

Solar cell fabrication and measurement of I–V characteristics. Patterned ITO/glass coupons (Luminescence Technology Corp.) were cleaned successively with deionized water, acetone and isopropanol by sonication for 5 min each, and then dried and ashed for 10 min in oxygen plasma. Immediately after ashing, poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) (Clevios PVP 4083) was spin coated (3000 min⁻¹, 60 s) and the coupons were annealed under nitrogen for 30 min at 180 °C. The CdSe/polymer blend was prepared in dichlorobenzene/pyridine solvent mixture (95/5, v/v) with the weight ratio of 5 parts CdSe to 1 part polymer. The PCBM/polymer blends were prepared by making separate 30 mg/mL solutions of polymer and PCBM in dichlorobenzene, then combining equal volumes of the polymer and PCBM solutions and blending for one hour. The CdSe/polymer blend was spin-casted (500 min⁻¹, 40 s), placed in a covered petri dish for 90 s, and then annealed at 150 °C for 30 min. The polymer/PCBM blend was spin-casted (1000 min⁻¹, 40 s). Cathodes were deposited by thermally evaporating calcium (25 nm) followed by silver (75 nm) through shadow masks at a rate of 0.2–0.6 Å/s. The coupons were encapusulated with epoxy resin (Addison) and cured under UV-light for 15 min before IV testing. I–V testing was carried out using a Keithley 2400 interfaced with LabView software. The solar simulator used was a ORIEL (model # 67005) equipped with xenon lamp; the intensity of the light was calibrated to 100 mWcm-2 with a Si-photodiode purchased from PV measurements Inc (PV 242) and calibrated by NERL in Colorado.

Conclusions

In conclusion, we have synthesized new semiconducting polymers by introducing alkenyl substituents into polythiophenes. The synthesized copolymers show nanofibrillar morphology with long and densely packed nanofibrils. The copolymers were used as an active layer in field effect transistors and in bulk-heterojunction solar cells confirming their good electronic properties. The ability to chemically modify the alkene group was demonstrated with the synthesis of a terminal hydroxy group. Furthermore, alkenyl substituents will be subjected to further chemical modification to generate thiol side chains.

Acknowledgements

Mihaela C. Stefan gratefully acknowledge the financial support from University of Texas at Dallas (start-up funds) and NSF (Career, DMR -0956116).

Notes and references

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Footnote

† Electronic Supplementary Information (ESI) available: NMR, UV-Vis, AFM, cyclic voltammetry, I–V plots for solar cells. See DOI: 10.1039/c0py00176g/

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