Fused Ring Cyclopentadithienothiophenes as Novel Building Blocks

Article pubs.acs.org/Macromolecules

Fused Ring Cyclopentadithienothiophenes as Novel Building Blocks for High Field Effect Mobility Conjugated Polymers Hongliang Zhong,† Yang Han,*,† Jessica Shaw,† Thomas D. Anthopoulos,‡ and Martin Heeney*,† †

Departments of Chemistry and Centre for Plastic Electronics, Imperial College London, London, SW7 2AZ, U.K. Departments of Physics and Centre for Plastic Electronics, Imperial College London, London, SW7 2AZ, U.K.

‡

Downloaded by SUNY UPSTATE MEDICAL UNIV on August 25, 2015 | http://pubs.acs.org Publication Date (Web): August 6, 2015 | doi: 10.1021/acs.macromol.5b01278

S Supporting Information *

ABSTRACT: A novel electron-rich cyclopentadithienothiophene (9H-thieno[3,2-b]thieno[2″,3″:4′,5′]thieno[2′,3′:3,4]cyclopenta[1,2-d]thiophene, CDTT) is reported by an optimized one-pot procedure involving two sequential lithiation/acylation steps. Three novel copolymers containing the varied electron-deficient acceptors 2,1,3-benzothiadiazole (BT), 5,6-difluoro2,1,3-benzothiadiazole (DFBT) and naphtho[1,2-c:5,6-c]bis[1,2,5]-thiadiazole (NT) were prepared by Stille polymerization. These three polymers show promising charge transport properties in transistor devices, with PCDTT-BT exhibiting unipolar hole mobility up to 0.67 cm2 V−1 s−1 in top gate devices utilizing gold source drain electrodes. Changing to a bilayer electrode of Al/Au resulted in ambipolar transistor behavior, with PCDTT-DFBT exhibiting balanced hole and electron mobilities of 0.38 and 0.17 cm2 V−1 s−1 respectively. These results clearly demonstrate that CDTT is a promising new building block for conjugated polymers.

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INTRODUCTION Organic field-effect transistors (OFETs) have attracted great attention over the past decades due to their potential application as low-cost, lightweight, and flexible integrated circuits in large-area displays, sensors and detectors, and radio frequency identification (RFID) tags.1,2 In terms of organic semiconductors for OFETs, conjugated polymers are interesting candidates due to their attractive combination of tunable structure, solution processability and thermal stability.3 There has been impressive recent progress in the development of high charge carrier mobility polymers.4−7 However, in some cases the devices show pronounced deviations from ideal transistor behavior or require special deposition techniques to align the polymer chains to achieve this promising performance.8−10 Therefore, the development of new superior polymers which show robust performances under simple processing techniques is still an important challenge. For the development of organic circuits, the combination of p- and n-type transistors in a complementary metal−oxide− semiconductor (CMOS)-like approach is an attractive strategy. Several approaches to organic CMOS have been demonstrated, including the fabrication of discrete p- and n-type semiconductors and the use of blends.2,11 An alternative is to utilize low band gap ambipolar materials which can be either p- or ntype depending on the gate bias.11 This has the potential advantage that only a single deposition step is required for both p- and n-channel transistors. To obtain ambipolar properties, suitable HOMO/LUMO levels are essential to ensure efficient hole and electron injection. Previous studies have demonstrated that these energy levels can be readily tuned in donor−acceptor © 2015 American Chemical Society

(D−A) copolymers by the selection of appropriate donor and acceptor units. Numerous efforts have been devoted to synthesize new donors and acceptors in an effort to tune the performance of ambipolar polymers.12,13 Recently reports in D−A polymers with diketopyrrolopyrrole (DPP),14 naphthalene diimide (NDI),15 perylenediimide (PDI),16 and isoindigo17 as acceptors exhibited promising ambipolar properties with high hole and electron mobilities. Among the class of donors for D−A polymers, bridged bithiophenes units have attracted considerable interest. Bridging the bithiophene forces the two thiophene rings into coplanarity ensuring good delocalization and reducing reorganization energy. The bridging atom is also known to have an important influence on the polymer properties, influencing optoelectronic as well as physical properties like crystallinity and melting point.18−20 Arguably, the most studied monomer is based upon cyclopenta[2,1-b;3,4-b]dithiophene (CDT), in which the bithiophene is bridged by a carbon atom.21 Often this also serves as the point of attachment for solubilizing side chains. Polymers based upon CDT have demonstrated excellent performance in many applications.12 For example copolymers of CDT and [1,2,5]thiadiazolo[3,4-c]pyridine or 2,1,3-benzothiadiazole (BT) have shown field effect mobilities greater than 1 cm2 V−1 s−1, as well as promising performance in organic solar cells.9,22,23 This promising performance is believed to be due to a combination of factors, including the minimal Received: June 12, 2015 Revised: July 21, 2015 Published: August 6, 2015 5605

DOI: 10.1021/acs.macromol.5b01278 Macromolecules 2015, 48, 5605−5613

Article


Macromolecules

were obtained with a Picoscan PicoSPM LE scanning probe in tapping mode. Synthesis. 2,7-Bis(trimethylsilyl)-9H-thieno[3,2-b]thieno[2″,3″:4′,5′]thieno[2′,3′:3,4]cyclopenta[1,2-d]thiophen-9-one (2). To a solution of (3,3′-dibromo-2,2′-bithieno[3,2-b]thiene-5,5′-diyl)bis(trimethylsilane) (5.8 g, 10 mmol) in anhydrous diethyl ether (400 mL) at −90 °C under argon atmosphere was added n-BuLi (6.25 mL of 1.6 M solution in hexane, 10 mmol) dropwise. The mixture was stirred for 30 min, and then dimethylcarbamyl chloride (1.1 g, 10 mmol) was added dropwise. The reactant was allowed to warm to −50 °C slowly and kept stirring for 30 min, followed by the dropwise addition of t-BuLi (5.9 mL of 1.7 M solution in pentane, 10 mmol) at −90 °C. The resulting solution was warmed to 0 °C slowly and stirred for another 30 min, and then quenched by moist hexane. The mixture was extracted by ethyl acetate, and the organic layer was washed by water and brine, and then dried over anhydrous Na2SO4. After the solvent was removed under reduce pressure, the crude product was purified by flash chromatography on silica gel (eluent: hexane: dichloromethane 1:1, v/v) to afford compound 2 (3.1 g, 6.9 mmol, 69%) as a black solid. 1H NMR (400 MHz, CDCl3), δ (ppm): 7.23 (s, 2H), 0.35 (s, 18H). 13C NMR (100 MHz, CDCl3), δ (ppm): 180.33, 152.41, 144.52, 137.88, 131.46, 125.26, −0.14. HRMS (ESI, m/z): [M + H]+ calcd for C19H21OS4Si2, 449.0014; found, 449.0027. 9,9-Dihexadecyl-9H-thieno[3,2-b]thieno[2″,3″:4′,5′]thieno[2′,3′:3,4]cyclopenta[1,2-d]thiophene (4). Compound 2 (4.49 g, 10 mmol) and KOH (2.8 g, 50 mmol) were dissolved in diethylene glycol (100 mL), followed by the addition hydrazine monohydrate (64−65%, 2 mL). The solution was stirred at 140 °C for 72 h under argon protection, and then cooled to room temperature and poured into cold aqueous HCl solution (500 mL of 0.1 M). After filtration, the residue was washed by water and methanol to afford 1.2 g of 9H-thieno[3,2b]thieno[2″,3″:4′,5′]thieno[2′,3′:3,4]cyclopenta[1,2-d]thiophene (3) as a pale yellow solid. Compound 3 was used without further purification. NaH (60%, 12 mmol) in anhydrous THF (20 mL) was added slowly into a solution of compound 3 (1.2 g) in 50 mL anhydrous THF at 0 °C. The resulting mixture was allowed to warm to room temperature and kept stirring for 30 min, followed by the addition of a solution of 1bromohexadecane (3.7 g, 12 mmol) in anhydrous THF (10 mL). The solution was heated to reflux and stirred overnight. After cooling to 0 °C, the reaction was quenched by methanol and extracted with diethyl ether. The organic phase was washed by water and brine, dried over anhydrous Na2SO4 and concentrated. The crude product was purified by chromatography on silica gel with hexane as eluent to afford compound 4 (1.7 g, 2.3 mmol, 23%) as a yellow solid with a total yield 23% of two steps. 1H NMR (400 MHz, CDCl3), δ (ppm): 7.31 (d, J = 5.2 Hz, 2H), 7.30 (d, J = 5.2 Hz, 2H), 2.03 (m, 4H), 1.19 (m, 56H), 0.88 (t, J = 6.9 Hz, 6H). 13C NMR (100 MHz, CDCl3), δ (ppm): 147.27, 140.37, 139.19, 133.72, 125.05, 120.55, 54.18, 36.89, 32.08, 29.90−29.33, 24.45, 22.84, 14.27. HRMS (EI, m/z): [M + H]+ calcd for C45H71S4, 739.4439; found, 739.4454. 2,7-Dibromo-9,9-dihexadecyl-9H-thieno[3,2-b]thieno[2″,3″:4′,5′]thieno[2′,3′:3,4]cyclopenta[1,2-d]thiophene (5). N-Bromosuccinimide (NBS) (427 mg, 2.4 mmol) in dichloromethane (10 mL) was added slowly to a solution of compound 4 (887 mg, 1.2 mmol) in dichloromethane (40 mL). The resulting solution was stirred for 2 h, and then saturated Na2SO3(aq) was added until the solution turned colorless. The mixture was separated and the water layer was extracted with dichloromethane. The organics were combined and washed by water and brine, dried over anhydrous Na2SO4, and evaporated. The crude product was purified by chromatography on silica gel (hexane) to afford a yellow oil (0.92 g, 1.0 mmol) with a yield of 83%. 1 H NMR (400 MHz, CDCl3), δ (ppm) 7.29 (s, 2H), 1.94 (m, 4H), 1.23 (m, 56H), 0.88 (t, J = 6.9 Hz, 6H). 13C NMR (100 MHz, CDCl3), δ (ppm): 147.31, 138.73, 138.28, 133.94, 123.24, 111.16, 54.18, 36.84, 32.07, 29.82−29.29, 24.43, 22.83, 14.26. HRMS (EI, m/ z): [M + H]+ calcd for C45H69Br2S4, 895.2649; found, 895.2628. 2,7-Bis(trimethylstannyl)-9,9-dihexadecyl-9H-thieno[3,2-b]thieno[2″,3″:4′,5′]thieno[2′,3′:3,4]cyclopenta[1,2-d]thiophene (6).

steric twist between terminal thienyl groups (as in CDT) and the comonomer BT which facilitates backbone planarization, as well as the enforced coplanarity of the CDT comonomer. In the case of CDT-BT, the molecular weight of the polymer was also shown to be a critical factor in obtaining high charge carrier mobilities.22 These promising results suggested that analogues of CDT could be excellent building blocks for conjugated polymers. We recently reported that the replacement of thiophene with thieno[3,2-b]thiophene in germanium bridged dithiophene monomers (dithienogermole) was beneficial for enhancement of conjugation length and intermolecular packing,24 leading to hole mobility up to 0.26 cm2 V−1 s−1 in a transistor device for a dithienogemolodithiophene containing polymer.25 As far as we are aware, there is only one reported example of a ring extended CDT system, in which benzothiophene units are fused to the core to afford a seven membered heterocycle with terminal phenyl rings.26 Therefore, the development of a new donor monomer in which the thiophene rings of the CDT system were replaced with thieno[3,2-b]thiophene presented a novel and interesting target. Herein we report the first synthesis of new donor 9Hthieno[3,2-b]thieno[2″,3″:4′,5′]thieno[2′,3′:3,4]cyclopenta[1,2-d]thiophene (CDTT) in which the bis(thieno[3,2-b]thiophene) is fused by a bridging carbon atom. This novel building block was a challenging synthetic target which required the development of two sequential one-pot lithiation/acylation steps. We demonstrate the potential of this novel building block by copolymerizing with three acceptor monomers of varying strength, namely 2,1,3-benzothiadiazole, 5,6-difluoro-2,1,3benzothiadiazole (DFBT) and naphtho[1,2-c:5,6-c]bis[1,2,5]thiadiazole (NT). All the polymers showed promising charge transport properties, in particular for PCDTT-DFBT which exhibited good ambipolar performance with balanced hole mobilities of 0.38 cm2 V−1 s−1 and electron mobilities of 0.17 cm2 V−1 s−1, whereas PCDTT-BT exhibited unipolar hole mobilities up to 0.67 cm2 V−1 s−1.

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EXPERIMENTAL SECTION

Materials. Commercially available reagents were purchased and used without further purification unless otherwise stated. All solvents were anhydrous and handled in argon atmosphere. (3,3′-Dibromo2,2′-bithieno[3,2-b]thiene-5,5′-diyl)bis(trimethylsilane) (1) was synthesized following a similar method in previous reports.24 4,9Dibromonaphtho[1,2-c:5,6-c]bis[1,2,5]thiadiazole was purchased from Lumtec. Flash chromatography (FC) was carried out on silica gel (Merck Kieselgel 60 grade 40−63 μm F254) using flash techniques. Analytical thin layer chromatography (TLC) was performed on Merck Kieselgel 60 F254 aluminum sheets and visualization was effected with UV fluorescence (254 and 365 nm). 1 H NMR and 13C NMR spectra were recorded on Bruker DRX-400 or AV-500 spectrometers using TMS as internal standard with CDCl3 as solvent for monomers at room temperature and deuterated 1,1,2,2tetrachloroethane as solvent for polymers at 130 °C respectively. Electron Ionization mass spectra were taken with a Micromass AutoSpec Premier. UV−vis spectra were recorded with a PerkinElmer Lambda20 UV−vis spectrophotometer or a UV-1601 Shimadzu UV− vis spectrometer. Number-average (Mn) and weight-average molecular weight (Mw) were determined by Agilent Technologies 1200 series GPC running in chlorobenzene at 80 °C, using two PL mixed B columns in series, and calibrated against narrow polydispersity polystyrene standards. Recycling GPC was run with a Shimadzu recycling GPC system running in hexane at 40 °C. Microwave experiments were performed in a Biotage initiator V 2.3. AFM images 5606


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Macromolecules


Scheme 1. Synthetic Route for Monomers and Polymers

(br, 2H), 7.98 (br, 2H), 2.32 (br, 4H), 1.44 (br, 8H), 1.26 (m, 48H), 0.96 (m, 6H). Synthesis of PCDTT-DFBT. The PDTTC-DFBT was synthesized with a yield of 73% by a similar procedure as PDTTC-BT. GPC (chlorobenzene): Mn, 23 000; Mw, 50 000 g/mol. 1H NMR (500 MHz, 1,1,2,2-tetrachloroethane-d2, 130 °C), δ (ppm) 8.78 (br, 2H), 2.34 (br, 4H), 1.44 (br, 8H), 1.26 (m, 48H), 0.95 (m, 6H). Synthesis of PCDTT-NT. The PCDTT-NT was synthesized with a yield of 64% by a similar procedure as PDTTC-BT. GPC (chlorobenzene): Mn, 10 000; Mw, 16 000 g/mol. 1H NMR (500 MHz, 1,1,2,2-tetrachloroethane-d2, 130 °C), δ (ppm): 9.29 (br, 2H), 8.90 (br, 2H), 2.42 (br, 4H), 1.29 (m, 56H), 0.97 (m, 6H). Organic Field Effect Transistor (OFET) Devices Fabrication and Characterization. All film preparation and characterization steps were carried out under inert atmosphere. Top-gate/bottom-contact devices were fabricated on glass using device structures with either Au (30 nm) or sequentially evaporated bilayer source/drain (S/D) electrodes comprising 20/30 nm thick layers of Al/Au, where the asymmetric bilayer electrodes enables ambipolar injection in a single device, as previously described.27 The electrodes were treated with a self-assembled monolayer (SAM) of pentafluorobenzenethiol (PFBT) to modify the work function. Polymer solutions in o-dichlorobenzene with a concentration of 5 mg mL−1 were spin-casted on the substrates at 2000 rpm for 60 s, before annealed at 200 °C for 30 min. Perfluorinated polymer CYTOP (CTL-809 M from Asahi Glass) was used as the gate dielectric (900 nm) and was spun directly onto the semiconductor polymer film. Aluminum gate electrodes (40 nm) were then thermally evaporated under high vacuum. Bottom-gate/top-contact devices were fabricated on heavily doped n+-Si (100) wafers with 400 nm-thick thermally grown SiO2. The Si/ SiO2 substrates were treated with trichloro(octadecyl)silane (OTS) to form a SAM. The polymers were dissolved in o-dichlorobenzene (5 mg mL−1) and spin-casted at 2000 rpm for 60 s, before being annealed at

To a solution of compound 5 (0.92 g, 1.0 mmol) in anhydrous THF (50 mL) was added n-BuLi (1.25 mL of 1.6 M solution in hexane, 2 mmol) dropwise at −78 °C under argon atmosphere. The resulting solution was stirred for 30 min, followed by the dropwise addition of trimethyltin chloride solution (2.1 mL of 1 M solution in hexane, 2.1 mmmol). The solution was stirred and warmed to room temperature slowly, and then poured into water (100 mL). The mixture was extracted with hexane and the organic phase was washed by water and brine, dried over anhydrous Na2SO4 and evaporated. The crude product was purified by recycling GPC (hexane) to afford compound 6 (1.0 g, 0.94 mmol, 94%) as yellow oil. 1H NMR (400 MHz, CDCl3), δ (ppm): 7.31 (s, 2H), 2.03 (m, 4H), 1.19 (m, 56H), 0.88 (t, J = 8.8 Hz, 6H), 0.45 (s, 18H). 13C NMR (100 MHz, CDCl3), δ (ppm): 146.88, 142.53, 139.33, 138.76, 127.56, 54.10, 36.91, 32.08, 29.98− 29.36, 24.45, 22.84, 14.27, −7.98. MS (ESI, m/z): M+, 1066.4. Synthesis of PCDTT-BT. Compound 6 (245 mg, 0.23 mmol), 4,7dibromo-2,1,3-benzothiadiazole (68 mg, 0.23 mmol), tris(dibenzylideneacetone)dipalladium (4.5 mg, 0.005 mmol), and tri(otolyl)phosphine (6.1 mg, 0.02 mmol) were added to a 2.0 mL high pressure microwave reactor tube. After the tube was flushed with argon, degassed chlorobenzene (1.5 mL) was added with argon protection. Then the tube was submitted to the following temperature scheme in the microwave reactor: 5 min at 100 °C, 5 min at 120 °C, and 20 min at 150 °C. After cooling to room temperature, the crude polymer was precipitated into methanol and filtered. The polymer was subjected to sequential Soxhlet extraction with methanol, acetone, hexane, and THF. The polymer residue in thimble was then dissolved in hot chlorobenzene, and the hot solution was filtered through a plug of glass wool. The resulting solution was concentrated and precipitated in methanol, to afford the polymer (180 mg, 0.21 mmol, 90%) as a dark solid. GPC (chlorobenzene): Mn, 41 000; Mw, 86 000 g/mol. 1H NMR (500 MHz, 1,1,2,2-tetrachloroethane-d2, 130 °C), δ (ppm): 8.65 5607


Article

Macromolecules Table 1. Molecular Weights and Optoelectronic Properties of Polymers DFT polymers

Mna (kDa)

Đ

λabs max(sol)b (nm)

λabs max(film) (nm)

HOMO (eV)

LUMO (eV)

Eg (eV)

Egc (eV)

IPd (eV)

LUMOe (eV)

PCDTT-BT PCDTT-DFBT PCDTT-NT

41 23 10

2.1 2.2 1.6

683 675 678

735 725 729

−4.55 −4.71 −4.68

−3.14 −3.26 −3.37

1.41 1.45 1.31

1.56 1.59 1.55

5.29 5.38 5.38

−3.73 −3.79 −3.83

Molecular weights measured using gel permeation chromatography (against polystyrene standards) in chlorobenzene at 80 °C. bDetermined from solution UV−vis absorption spectroscopy in chlorobenzene. cDetermined from the absorption onset of the polymers in thin film. dDetermined by photoelectron spectroscopy in air (PESA) on thin films, error ±0.05 eV. eEstimated by adding Eg (opt) to the ionization potential.


a

200 °C for 30 min. Asymmetric bilayer Au/Al (30/30 nm) source and drain electrodes were deposited onto the polymer films under vacuum through shadow masks. The channel width and length of the final transistors were 1 mm and 30 μm, respectively, for both device structures. Transistor characterization was carried out under nitrogen using a Keithley 4200 parameter analyzer. Mobility was extracted from the slope of ID1/2 vs VG, from the upper 20% of the VG range (i.e., if VG is from +10 to −60 V, we use −60 to −48 V).

carbonate. However, despite numerous attempts with different solvents (THF and diethyl ether), organolithium reagents (nBuLi and tert-BuLi), and temperatures (−90 to −50 °C), only trace amounts of the target ketone were obtained. In all cases the starting material was consumed, and analysis of the mixtures formed suggested most of the byproducts were the result of ring-opening of the thienothiophene. This can be rationalized by the instability of dianion of compound 1 which has a tendency to decompose by ring-opening, even at low temperature due to its electron-rich nature. However, the required reaction with dimethylcarbamyl chloride or dimethylcarbonate was very slow at the low temperatures required to stabilize the dianion of 1 (85%). More importantly, further study revealed compound 2a could be prepared and treated in situ for the next step without isolation. In order to form the ketone a second lithiation step was required on the aryl bromide, followed by an intramolecular reaction on the amide to form the ring closed structure. We found that utilizing n-BuLi was inefficient for this lithiation step, possibly due to competing nucleophilic attack of the butyl group at the amide. Changing to the more sterically hindered tert-BuLi suppressed this undesired reaction, and allowed clean halogen exchange at −90 °C. Subsequent warming resulted in the intramolecular ring closing. Running the whole reaction in one pot gave compound 2 in an overall yield of 69%. Compound 2 was reduced by a modified Wolff−Kishner reaction with hydrazine and potassium hydroxide to afford compound 3 with a low yield. The low isolated yield was attributed to the rather poor oxidative stability of this electron rich compound, since oxidation back to the ketone was found to occur during chromatographic purification on silica gel column. To circumvent this instability, the crude compound was alkylated directly, by treatment with sodium hydride followed by the addition of 1-bromohexadecane to afford compound 4 in 23% yield from 2 after column chromatography. Conversion of 4 to the desired bis-stannylated monomer was achieved by bromination with NBS, followed by low temperature lithiation and reaction with trimethyltin chloride. The final stannylated monomer 6 was purified by preparative recycling GPC. As found with many tin monomers, attempted purification by chromatography on silica led to partial destannylation. We also investigated the synthesis of 6 by

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RESULTS AND DISCUSSION Design, Synthesis, and Characterization. The 2,1,3benzothiadiazole unit is one of most widely used acceptors for D−A conjugated polymers due to its strong electron affinity, which typically results in a low lying polymer LUMO and a small optical band gap via intramolecular charge transfer. In addition the planar and rigid structure combined with the small steric hindrance of BT can promote close intramolecular packing in some cases. Herein PCDTT-BT was synthesized by Stille copolymerization of the dibromo-BT moiety and the bis(trimethylstannyl)CDTT unit. Among BT derivatives, naphtho[1,2-c:5,6-c]bis[1,2,5]thiadiazole (NT) possesses extended aromatic structure, which potentially enhances the facial packing in solid state resulting in an increase of carrier mobility.28 Moreover, the electron deficiency of NT is slightly stronger compared with BT, which could be helpful to improve the electron mobility of the resultant polymer. Recently BT derivatives with difluoro substituents have drawn much attention and displayed interesting optoelectronic properties, in which the strong electron affinity of the fluorine atoms could lower the frontier energy levels, potentially enhancing electron mobility and air stability for OFETs. In addition the existence of fluorine atoms has been suggested to promote non-covalent interactions with adjacent heterocycles, resulting in higher order of semiconductors.29,30 Therefore, 5,6-difluoro-2,1,3benzothiadiazole (DFBT) was also chosen as an acceptor to copolymerize with CDTT moiety to afford polymer PCDTTDFBT. Because there is no solubilizing side chain in any of these acceptors, the choice of side chain on the CDTT is important. Although long branched side chains are typically used to ensure good solubility, here we chose linear hexadecyl groups, since linear chains have been shown to be beneficial for the packing of polymer chains in related CDT copolymers, and therefore can enhance charge carrier mobility.31 The preparation of CDTT was challenging and our final synthetic route is shown in Scheme 1. Starting from compound 1, we focused our efforts on the bridged ketone compound 2, the key precursor of CDTT. Our approach here was to follow similar methodology to that reported for the synthesis of dialkyl-CDT, in which the CDT ketone is the key precursor.32 Hence we initially investigated the dilithiation of 1 with two equivalents of n-butyllithium followed by reaction of the resulting dianion with dimethylcarbamyl chloride or dimethyl5608


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Macromolecules


Figure 1. (a) Normalized optical absorption spectra of polymers in chlorobenzene solution and as thin films. (b) Energy minimized conformations (B3LYP/6-31G*) of polymers showing HOMO and LUMO distribution.

direct lithiation of compound 4 with n-BuLi followed by reaction with the trimethyltin chloride, but found that mixtures of mono and distannylated products were formed which were difficult to separate. Polymerizations were performed with CDTT and the corresponding acceptors under microwave-assisted Stille coupling conditions.33 The crude polymers were purified by precipitation and Soxhlet extraction to afford the polymers as dark solids. All three polymers could dissolve in hot chlorobenzene and dichlorobenzene. The molecular weights were measured by GPC in chlorobenzene at 80 °C against polystyrene standards (Table 1). As can be seen, introducing DFBT or NT resulted in a significant reduction in the observed molecular weight from PCDTT-BT (41 kDa/2.1) to PCDTTDFBT (23 kDa/2.2), and PCDTT-NT (10 kDa/1.6). The differences can be explained by a reduction in solubility of the polymers in the order of PCDTT-BT > PCDTT-DFBT > PCDTT-NT, most readily observed as an increase in the dissolution temperature required to solubilize the polymer. The reduced solubility likely caused the polymer to precipitate early in the reaction preventing further chain growth. The chemical structures of monomers were proven by NMR and high resolution mass spectrum. Because of the poor solubility and strong intermolecular packing, high temperature NMR at 130 °C in 1,1,2,2-tetrachloroethane-d2 was utilized to afford 1H NMR for polymers. The thermal properties of polymers were measured by differential scanning calorimetry (DSC), showing no obvious thermal transition between 0 and 350 °C (see Figure S1). Optoelectronic Properties. The optical properties of polymers PCDTT-BT, PCDTT-DFBT, and PCDTT-NT were investigated by UV−vis absorption spectroscopy. Figure 1a and Table 1 display the optical data of polymers in chlorobenzene solution and as spin-coated films. The solution spectrum of PCDTT-BT shows a broad maximum at 683 nm, with an illdefined shoulder around 720 nm and two peaks at higher energy around 422 and 327 nm. The maximum absorption is attributed to intramolecular charge transfer in common with other donor−acceptor polymers, with the shoulder suggesting that aggregation is present, even in dilute solution. The optical spectra and band gap is very similar to the analogous germanium bridged copolymer.34 The fluorinated analogue PCDTT-DFBT exhibits a very similar spectrum to the nonfluorinated polymer in agreement with previous reports that demonstrated that fluorination of the BT did not significantly influence the optical band gap.35 There is a slight blue shift of

the main absorption peak to 675 nm, and a better defined shoulder at 725 nm. Previous reports have suggested that the replacement of BT with NT results in a substantial reduction in the optical band gap,28,36 but we do not observe this, with PCDTT-NT displaying a similar absorption as PCDTT-BT in solution with a slight 5 nm blue shift of maximum. The relatively low molecular weight and solubility of PCDTT-NT is the likely explanation, suggesting that the conjugation limit has not been reached at this molecular weight. Attempts to increase the molecular weight were not successful, however, probably due to the relatively poor purity of 4,9-dibromonaphtho[1,2c:5,6-c]bis[1,2,5]thiadiazole, which contained significant amounts of the non/mono-brominated impurity. The poor solubility of the monomer prevented the removal of this impurity by chromatographic means, and attempts at recrystallization did not improve the purity. In the thin film, all three polymers display new maximum absorption at longer wavelength, 735 nm for PCDTT-BT, 725 nm for PCDTT-DFBT, and 729 nm for PCDTT-NT, whereby the solution maxima decrease in intensity to now become shoulder peaks. The red-shift of absorption is a common feature of D−A polymers upon film formation and is attributable to enhanced aggregation in the solid state. Comparing the intensities of the longer wavelength aggregation absorption with those of shoulder peaks, the ratio of PCDTTDFBT is the largest, suggesting the intermolecular packing of PCDTT-DFBT is the strongest. From the onset of absorption for the thin films, the optical band gaps of PCDTT-BT, PCDTT-DFBT, and PCDTT-NT were calculated as 1.56, 1.59, and 1.55 eV, respectively. To investigate the frontier orbital energy levels of the polymers, the ionization potentials (IP) of thin films of polymers were measured by photoelectron spectroscopy in air (PESA) and the HOMO energy was approximated as the negative of the ionization potential. As shown in Table 1, PCDTT-DFBT and PCDTT-NT show the same HOMO value of −5.38 eV, lower than that of PCDTT-BT (−5.29 eV), resulting from the stronger electron-withdrawing capability of the DFBT and NT units. From the difference of the IP and the optical band gap, the LUMO levels of PCDTT-BT, PCDTTDFBT, and PCDTT-NT were estimated as −3.73, −3.79, and −3.83 eV, respectively. Although such an estimate does not take into account the exciton binding energy it is nevertheless still useful to compare between the different polymers. To understand the molecular geometry and energy levels of polymers, density functional theory (DFT) calculations of 5609


Article

Macromolecules Table 2. Summary of OFETs Data Measured from Different Transistor Geometries

saturated mobility (cm2 V−1 s−1) polymers PCDTT-BT

PCDTT-DFBT


PCDTT-NT

device configuration

source/drain electrodes

hole

electron

BG/TC TG/BC TG/BC BG/TC TG/BC TG/BC BG/TC TG/BC

Au/Al, Si/SiO2-OTS Au/Al/Cytop Au/Cytop Au/Al, Si/SiO2-OTS Au/Al/Cytop Au/Cytop Au/Al, Si/SiO2-OTS Au/Al/Cytop

0.048 0.19 0.67 0.054 0.38 0.47 0.0031 0.015

N/A N/A N/A N/A 0.17 N/A N/A N/A

Figure 2. Transfer characteristics of (a) PCDTT-BT and (b) PCDTT-DFBT with Au source/drain electrodes. Ambipolar transfer characteristics of PCDTT-DFBT with bilayer Al/Au source/drain electrodes under (c) negative drain voltage and (d) positive drain voltage. All devices had channel length = 30 μm and channel width = 1 mm with a Cytop dielectric.

trimers of polymers were carried out using Gaussian at the B3LYP/6-31G* level. Long side chains were replaced by methyl groups in order to simplify the calculations, and trimers were utilized to represent the polymer backbone. In each case, the structures were allowed to relax to an equilibrium geometry from either an all-cis or an all-trans geometry, with respect to the bond between the CDTT and adjacent BT/NT monomer. As seen in Figure 1b the minimum-energy conformations reveal all three trimers have planar conformations with very small dihedral angles (

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