Benzocarborano[2,1-b:3,4-b′]dithiophene Containing Conjugated

Dec 20, 2013 - We report the stannylation of a benzocarborano[2,1-b:3,4-b′]dithiophene monomer and its polymerization by Stille polycondensation wit...
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Benzocarborano[2,1‑b:3,4‑b′]dithiophene Containing Conjugated Polymers: Synthesis, Characterization, and Optoelectronic Properties Jonathan Marshall,*,† Bob C. Schroeder,† Hugo Bronstein,‡ Iain Meager,† Stephan Rossbauer,§ Nir Yaacobi-Gross,§ Ester Buchaca-Domingo,∥ Thomas D. Anthopoulos,§ Natalie Stingelin,∥ Peter Beavis,⊥ and Martin Heeney*,† †

Department of Chemistry and Centre for Plastic Electronics, Imperial College London, London SW7 2AZ, U.K. Department of Chemistry, University College London, London WC1H 0AJ, U.K. § Department of Physics and Centre for Plastic Electronics, Imperial College London, London SW7 2AZ, U.K. ∥ Department of Materials and Centre for Plastic Electronics, Imperial College London, London SW7 2AZ, U.K. ⊥ AWE, Aldermaston, Reading RG7 4PR, U.K. ‡

S Supporting Information *

ABSTRACT: We report the stannylation of a benzocarborano[2,1-b:3,4-b′]dithiophene monomer and its polymerization by Stille polycondensation with solubilized cyclopentadithiophene and diketopyrrolopyrrole derivatives. The physical, material, and optoelectronic properties of the resultant conjugated copolymers are reported, demonstrating that benzocarboranodithiophene acts as a mildly electron-withdrawing monomer.

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neutron exposure.13 Carborane has been used as both the boron and carbon source to grow 250 nm thin films of boron carbide, which were able to detect single neutrons in real time in diode device configurations.14 Carboranes have also found applications in heat-resistant and flame-retardant materials. A range of m-carborane containing polyamides and polyesters have been developed that show high heat and chemical resistance, and carborane containing polysiloxanes that offer excellent thermal stability have already been commercialized.15 Surprisingly, for a molecule with such unique electronic properties and a rigid, symmetrical structure, research into carborane containing materials for application in optoelectronic devices has been relatively limited. To date, most of the effort has focused upon polymers containing o-carborane. These exhibit enhanced delocalization between adjoined aromatic systems compared to the meta and para analogues.3,6,7,16 Aggregation-induced emission (AIE) has also been observed in some o-carborane systems, where luminescence efficiency was enhanced under conditions of high concentration or in the solid state.17 Such o-carborane-substituted systems have also

INTRODUCTION Carboranes are icosahedral clusters of boron, carbon and hydrogen atoms, existing in discrete isomers; ortho, meta, and para, dependent on the relative positions of the carbon atoms within the cage structure (Figure 1).1,2 Known for their

Figure 1. Chemical structures of o-carborane (left), m-carborane (center), and p-carborane (right). Unlabeled vertices represent BH units.

extremely high thermal and oxidative stability that arises as a result of three-center−two-electron bonding throughout the cage structure, they have found use in multiple areas of chemistry.1,3−9 In particular, the high boron density of carboranes in combination with the high neutron capture radius of 10B has led to the investigation of carboranes in boron neutron capture therapy (BNCT) treatments for cancer,10−12 and for similar reasons boron-rich semiconductors have also been investigated as solid-state neutron detectors. For example, boron carbide has been successfully used as a detector based upon resistivity changes resulting from lithium doping upon © XXXX American Chemical Society

Received: November 5, 2013 Revised: December 11, 2013

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thiophene units leading to improved π-orbital overlap between neighboring rings and therefore increased conjugation.24,25 Furthermore, compared to BZ, the terminal thiophenes in BZTT are expected to minimize steric interactions between αhydrogen atoms on adjacent monomers which can cause unfavorable out-of-plane twisting of the backbone.21 Such twisting would not only impede conjugation length but also potentially suppress intermolecular π-stacking and therefore reduce device performance. We were interested to incorporate BZTT into semiconducting conjugated polymers: first as a possible route toward the direct electronic detection of neutrons in large area thin film transistor devices and second to explore its potential as a new weakly, accepting comonomer. In particular, we identified the development of ambipolar semiconducting polymers containing carborane as a particularly interesting target because the sensitivity of ambipolar transistors has been shown to be improved over unipolar devices in the related detection of light (phototransistors).26 As such, we chose two comonomers with which to copolymerize BZZT: the relatively electron-poor bis(2-thien-5-yl)2,5-pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (DPP)27−32 which has been shown to be an effective comonomer for a range of donor− acceptor polymers exhibiting ambipolar behavior and the more electron-rich cyclopenta[2,1-b;3,4-b′]dithiophene (CDT) in which we could investigate whether BZTT was able to act as an electron accepting comonomer.33 Here we report the synthesis of conjugated polymers containing BZTT, their optoelectronic characteristics, and preliminary transistor results.

been noted to show interesting optical effects upon exposure to a variety of organic vapors.18 However, one potentially detrimental aspect of direct incorporation of o-carborane into a conjugated backbone is the formation of a significant kink in the polymer backbone as well as the nonplanarity of the groups directly bonded to the cage, which can lead to the formation of amorphous polymers. We were interested in the preparation of carborane containing conjugated polymers that possessed more linear backbones, in the belief that this would improve solid state aggregation and benefit charge transport. The most obvious strategy to improve backbone linearity is to include pcarborane as the linking unit instead of o-carborane. However, it has been shown that the direct incorporation of p-carborane inhibits aromatic conjugation between units bonded to the antipodal carbon atoms.7 Therefore, as an alternative, we were interested to append the carborane unit directly to the aromatic backbone, where it could still influence the electronic properties of the conjugated polymer. One system of interest is 1,2-(buta-1′,3′-diene-1′,4′-diyl)-1,2dicarbadodecaborane (benzocarborane, BZ, Figure 2), in which

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Figure 2. Chemical structures of benzocarborane (left) and benzocarborano[2,1-b:3,4-b′]bithiophene (right). Unlabeled vertices represent BH units.

EXPERIMENTAL SECTION

General. Decaborane was purchased from Katchem and sublimed before use. All other reagents were purchased from Sigma-Aldrich, VWR, Alfa Aesar, or Apollo Scientific and were used without further purification. Dry solvents for anhydrous reactions were purchased from Sigma-Aldrich. All reactions were carried out under an inert argon atmosphere unless otherwise stated. 1H NMR and 13C NMR spectra were recorded on a Bruker 400 spectrometer in CDCl3 solution at 298 K unless otherwise stated. Proton NMRs were recorded after boron decoupling. Number-average (Mn) and weightaverage (Mw) molecular weights were determined with an Agilent Technologies 1200 series GPC in chlorobenzene at 80 °C using two PL mixed B columns in series and calibrated against narrow polydispersity polystyrene standards. UV−vis absorption spectra were recorded on a UV-1601 Shimadzu UV−vis spectrometer. Column chromatography was carried out on silica gel (VWR) or using a Biotage Isolera system. Microwave reactions were performed in a Biotage initiator v.2.3. Photo Electron Spectroscopy in Air (PESA) measurements were recorded using a Riken Keiki AC-2 PESA spectrometer with a power setting of 5 nW and a power number of 0.5. Differential scanning calorimetry (DSC) measurements were made using a TA Instruments DSC TZero Q20 instrument and analyzed using TA universal analysis software. Thermal gravimetric analysis measurements were recorded using a PerkinElmer Pyris 1 TGA. X-ray diffraction (XRD) measurements were carried out using a Panalytical X’pert-pro MRD diffractometer fitted with a nickel-filtered Cu Kα1 beam and an X’celerator detector using a current of 40 mA and an accelerating voltage of 40 kV. 2,6-Dibromo-4,4-bis(dodecyl)4H-cyclopenta[2,1-b;3,4-b′]dithiophene (CDT)34 and 3,6-bis(2-bromothien-5-yl)2,5-bis(2-octyldodecyl)pyrolo[3,4-c]pyrole-1,4(2H,5H)dione (DPP)35 were synthesized according to the previously published procedures. The synthesis of benzocarborano[2,1-b:3,4-b′]dithiophene (BZTT)22 is reported in the Supporting Information. S yn t h e t i c P r o c e d u r e s . 5 , 5 ′ - B i s ( t r i m e t h y l s t a nn y l ) benzocarborano[2,1-b:3,4-b′]dithiophene (1). Benzocarborano[2,1b:3,4-b′]dithiophene (65 mg, 0.21 mmol) was dissolved in anhydrous THF (10 mL) and cooled to −78 °C before n-BuLi (2.5 M in hexanes,

a benzenoid ring is fused to the carborane cage. This has been identified as a means of incorporating o-carborane into the backbone of the polymer without impeding electronic delocalization, and several benzocarborane containing aromatic systems and polymers have been reported.8,19−21 However, in most of these examples, the benzocarborane unit was linked to the aromatic backbone by meta-substitution in the benzenoid ring, which impedes through conjugation and also results in the introduction of an unfavorable kink in the polymer backbone, a feature which is likely to impede intermolecular interactions and lead to amorphous polymers. Morisaki and co-workers recently reported the synthesis of benzocarborano[2,1-b:3,4-b′]bithiophene (BZTT) (Figure 2). Fusing the carborane to the 3,3′-positions of the bithiophene ensured coplanarity of the thiophene rings.22 Upon extending the conjugated system by coupling thiophene oligomers to the BZTT unit, they observed that the BZTT containing oligomers absorbed at longer wavelengths than the corresponding thiophene analogues and were easier to reduce and harder to oxidize. These observations were ascribed to the inductively electron-withdrawing effect of the carborane, in addition to the enforced planarity of the system. Further computational investigations by Li and co-workers reported high hyperpolarizability in asymmetric BZTT-based systems, concluding that materials based on this unit may find use in nonlinear optical applications.23 Compared to previously reported benzocarborane containing aromatic species, the structure of BZTT lends itself for use in semiconducting polymers. The benefits of fused aromatic systems are well-known; with increased coplanarity between B

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Table 1. Properties of Polymers pBZTT-CDT and pBZTT-DPP λmax (nm) polymer

Mna (kDa)

Mwa (kDa)

PDIa

DP

filmb

solutionc

HOMOd (eV)

calc HOMOe (eV)

band gap (eV)

pBZTT-CDT pBZTT-DPP

41 63

78 99

1.9 1.6

45 54

543 793

550 791

−5.0 −5.4

−4.91 −5.55

1.91 1.29

a

Determined by GPC and reported as their polystyrene equivalents. bSpin-coated from 5 mg/mL chlorobenzene solution. cMeasured in dilute chlorobenzene solution. dThe HOMO energy was measured as a thin film by PESA (error ±0.05 eV). eDetermined by DFT on the energyminimized trimers (B3LYP/6-31G*); alkyl chains were substituted for methyl groups. Mw = 99 kDa, PDI = 1.6. 1H NMR (400 MHz, (CDCl3): δ 8.8 (br, 2H), 7.7 (br, 2H), 7.1 (br 2H), 2.7−0.5 (m, br, 78H). Transistor Fabrication Details. Top Gate Devices. All film preparation steps were carried out under an inert atmosphere. The 2 × 2 cm glass slides were cleaned in a DECON90 deionized (DI) water solution in an ultrasonic bath twice for 10 min and then rinsed with DI water. To help with the adhesion of the gold on the glass substrate, 5 nm of aluminum was evaporated prior to the evaporation of 25 nm of gold. Polymeric chlorobenzene solution and substrates were heated to 80 °C followed by spin coating for 10 s at 500 rpm followed by 30−60 s at 2000 rpm. The films were then dried at 100 °C for 5 min. A perfluorinated polymer (commercial name CYTOP from Ashani Glass) was used as gate dielectric and applied via spin coating for 60 s at 2000 rpm and cured at 100 °C for 90 min. 50 nm aluminum was evaporated on top of the dielectric as a gate electrode. Bottom Gate Devices. Bottom-gate, bottom-contact (BG, BC) organic field-effect transistors (OFETs) were fabricated on a highly doped silicon substrate, which acted as a common gate electrode. A thermally grown 200 nm layer of silicon dioxide was then used as the gate dielectric. Gold source and drain electrodes were patterned using standard photolithography. Substrates were cleaned in an ultrasonic bath (acetone, 10 min; isopropanol, 10 min). The SiO2 layer was treated with the primer hexamethyldisilazane (HMDS) to passivate the surface. The devices were spun from same solution concentrations and processing parameters as described for top gate devices.

0.35 mL, 0.84 mmol) was added dropwise. The solution was stirred at −78 °C for 2 h before trimethylstannyl chloride (1.0 M in hexanes, 1.25 mL, 1.25 mmol) was added. The solution was allowed to warm to room temperature overnight before being poured into water and extracted with diethyl ether. The organic layers were combined, washed with water and brine, and dried over anhydrous sodium sulfate. Purification by recycling preparative gel permeation chromatography yielded the desired product as a white solid (101 mg, 16 mmol, 76%). 1 H NMR (400 MHz, (CD3)2CO): δ 7.47 (s, 2H), 3.2−1.65 (m, 10H), 0.47 (s, 18H). 13C NMR (100 MHz, (CD3)2CO): δ 136.3, 134.5, 90.1, 88.2, 88.0, 33.6. 11B {1H} NMR (400 MHz, (CD3)2CO): δ 28.86, −5.46, −9.20, −12.77. MS (EI): m/z 636.124 (M+). pBZTT-CDT. 5,5′-Bis(trimethylstannyl)benzocarborano[2,1-b:3,4b′]dithiophene 1 (78.5 mg, 0.12 mmol), 2,6-dibromo-4,4′-bis(2dodecyl)-4H-cyclopenta[1,2-b:5,4-b]dithiophene (82.5 mg, 0.12 mmol), tris (dibenzylideneacetone)dipalladium(0) (2.3 mg, 0.002 mmol), and tri-o-tolylphosphine (2.4 mg, 0.008 mmol) were added to a dry microwave vial which was flushed thoroughly with argon. Anhydrous, degassed chlorobenzene (2 mL) was added, and the resulting mixture was degassed for 45 min. The vial was sealed and the reaction mixture was heated in a microwave in successive intervals of 5 min at 100 °C, 5 min at 140 °C, 5 min at 170 °C, and finally 30 min at 200 °C. After cooling to room temperature the reaction mixture was poured into vigorously stirring acidified methanol, and the resulting precipitate was filtered. The precipitate was purified by successive Soxhlet extractions with methanol (24 h), acetone (24 h), n-hexane (24 h), and finally chloroform (24 h). The chloroform extract was stirred at 50 °C in the presence of aqueous sodium diethyldithiocarbamate trihydrate for 2 h. After cooling to room temperature, the organic layer was separated and washed with water, dried, and concentrated under reduced pressure. Final purification was by preparative size exclusion chromatography. Following concentration under reduced pressure, the desired polymer was afforded as a dark purple solid (67 mg, 61%). GPC (chlorobenzene): Mn = 41 kDa, Mw = 78 kDa, PDI = 1.9. 1H NMR (400 MHz, (CDCl3): δ 7.1 (br, 1H), 7.0 (br, 1H), 2.8−2.1 (m, 10H), 1.2−0.6 (m, 54H). pBZTT-DPP. 5,5′-Bis(trimethylstannyl)benzocarborano[2,1-b:3,4b′]dithiophene (1) (85.3 mg, 0.134 mmol), 3,6-bis(2-bromothien-5yl)2,5-bis(2-octyldodecyl)pyrolo[3,4-c]pyrole-1,4(2H,5H)-dione (146.0 mg, 0.134 mmol), and tris (dibenzylideneacetone)dipalladium(0) (2.5 mg, 0.003 mmol) and tri-o-tolylphosphine (3.6 mg, 0.012 mmol) were added to a dry microwave vial which was flushed thoroughly with argon. Anhydrous, degassed chlorobenzene (2 mL) was added, and the resulting mixture was degassed for 45 min. The vial was sealed, and the reaction mixture was heated in a microwave in successive intervals of 5 min at 100 °C, 5 min at 140 °C, 5 min at 170 °C, and finally 30 min at 200 °C. After cooling to room temperature the reaction mixture was poured into vigorously stirring methanol, and the resulting precipitate was filtered. The precipitate was purified by successive Soxhlet extractions with methanol (24 h), acetone (24 h), n-hexane (24 h), and finally chloroform (24 h). The chloroform extract was stirred at 50 °C in the presence of an aqueous sodium diethyldithiocarbamate trihydrate for 2 h. After cooling to room temperature, the organic layer was separated and washed with water, dried, and concentrated under pressure. Final purification was by preparative size exclusion chromatography. Following concentration under reduced pressure, the desired polymer was afforded as a dark green solid (128 mg, 82%). GPC (Chlorobenzene): Mn = 63 kDa,

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RESULTS AND DISCUSSION Benzocarborano[2,1-b:3,4-b′]dithiophene was synthesized according to previously reported methods, with only slight modifications to the precursor synthesis (see Supporting Information).22 In particular, the use of a higher reaction temperatures during the electrophilic bromination of 3iodothiophene improved the yield of 2-bromo-3-idothiophene. 36 We found that benzocarborano[2,1-b:3,4-b′]dithiophene could be readily stannylated to afford monomer 1 by dilithiation with 2 equiv of n-BuLi followed by treatment with anhydrous trimethyltin chloride solution. The crude tin monomer was not sufficiently stable on silica to allow for purification by column chromatography, and therefore 1 was purified by recycling preparative GPC prior to copolymerization. Stille polymerizations with 2,6-dibromo-4,4-bis(dodecyl)4H-cyclopenta[2,1-b;3,4-b′]dithiophene (CDT) and 3,6-bis(2bromothien-5-yl)2,5-bis(2-octyldodecyl)pyrolo[3,4-c]pyrole1,4(2H,5H)-dione (DPP) were performed under microwaveassisted conditions by heating in chlorobenzene at 200 °C for 30 min.37 Branched side chains were necessary on the DPP derivative to ensure good solubility of the resultant polymer. However, for the CDT polymer we found that linear alkyl chains were sufficient to provide good solubility. Following polymerization, the materials were purified by washing (Soxhlet) with methanol, acetone, and n-hexane to remove catalyst residues and low weight oligomers, before the polymer was extracted into hot chloroform. After precipitation, pBZTTCDT was afforded as a dark purple solid in 61% yield and pBZTT-DPP as a dark green solid in 68% yield. Potential C

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Both polymers were soluble in chloroform upon heating and in chlorobenzene at room temperature. The optical properties of the polymers were determined by UV−vis absorption spectroscopy, with pBZTT-CDT exhibiting a broad, featureless absorption with a λmax at 550 nm in dilute chlorobenzene solution (Figure 3). Upon film formation the spectra broadened slightly, and a small hypsochromic shift of λmax to 543 nm was observed. The absence of any red-shift in absorption suggests that there is not a significant change in order as the polymer solidifies, in agreement with some previously reported CDT copolymers.42−44 Annealing the film did not change the absorption spectra. From the onset of absorption in the solid state around 650 nm, the optical band gap was calculated to be 1.91 eV. This is similar to that reported for CDT copolymers with either a bithiophene bridged with film benzene (λfilm 1.91) or a pyrazine ring (λfilm max 555 nm, Eg max 554 45 film nm, Eg 1.94) and significantly wider than for CDT copolymers with strong donor−acceptor character like the benzothiadiazole copolymer in which intramolecular charge transfer occurs.46 This suggests that BZTT is not acting as a strong electron acceptor. The λmax of pBZTT-DPP was significantly red-shifted compared to pBZTT-CDT, occurring at 784 nm in chlorobenzene solution (Figure 3), with a significant shoulder at 729 nm. This red-shift reflects the more electron-accepting character of DPP compared to CDT. The pronounced double peak observed is seen in many DPP polymers and is often associated with aggregation in solution. In our case heating the solution results in a decrease in the relative intensity of the long wavelength peak around 784 nm to that of the peak 729 nm as well as a slight hypsochromic shift, supporting the fact that the longer wavelength peak is associated with aggregation. Upon film formation, the spectrum is remarkably similar to that of the room temperature solution, although there is a small broadening of the absorption and a slight red-shift in the maximum absorption to 793 nm. These results suggest that the inclusion of three-dimensional carborane cage does not significantly inhibit polymer aggregation, possibly due to the strong tendency for DPP to aggregate as previously noted.28 From the absorption onset in the solid state around 960 nm, the

catalytic impurities were removed by heating a solution of the polymer in the presence of sodium diethyldithiocarbamate, followed by preparative size-exclusion chromatography in chlorobenzene.38,39 Molecular weights and polydispersities were determined by GPC in hot (80 °C) chlorobenzene (Table 1), demonstrating reasonable degrees of polymerization. These molecular weights are significantly higher than previously reported instances of carborane containing polymers.17,40,41 Both pBZTT-CDT and pBZTT-DPP showed excellent thermal stability, with 5% mass loss occurring at temperatures above 375 and 400 °C, respectively (see Supporting Information). Scheme 1. Synthesis of Poly(benzocarborane[2,1-b:3,4b′]dithiophene-4,4-bis(dodecyl)-4H-cyclopenta[2,1-b;3,4b′]dithiophene (pBZTT-CDT) and Poly(benzocarborane[2,1-b:3,4-b′]dithiophene-bis(2bromothien-5-yl)2,5-bis(2-octyldodecyl)pyrolo[3,4c]pyrole-1,4(2H,5H)-dione (pBZTT-DPP)a

a

Reagents and conditions. (i) pBZTT-CDT: 2,6-dibromo-4,4-bis(dodecyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene, Pd2(dba)3, P(otol)3, chlorobenzene, μW, 61%. pBZTT-DPP: 3,6-bis(2-bromothien5-yl)2,5-bis(2-octyldodecyl)pyrolo[3,4-c]pyrole-1,4(2H,5H)-dione, Pd2(dba)3, P(o-tol)3, chlorobenzene, μW, 68%.

Figure 3. UV−vis spectra of pBZTT-CDT (left) and pBZTT-DPP (right) in chlorobenzene solutions and thin films. D

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Figure 4. Minimum-energy conformation of pBZTT-CDT (A) and pBZTT-DPP (B) Trimers: face-on view (top) and side-on view (second). Visualized frontier molecular orbitals: LUMO (second from bottom) and HOMO (bottom) calculated in Gaussian 09 with the B3LYP/6-31G* basis set.

optical band gap is significantly smaller than the CDT copolymer at 1.3 eV, in agreement with the enhanced donor−acceptor character. Comparing the absorption and band gap to the analogous DPP bithiophene polymer (λfilm max 790 1.2 eV),47 there is no significant difference upon nm, Efilm g inclusion of the BZTT, particularly considering the difficulty in getting an accurate measurement of the absorption edge. The ionization potential of the two polymers films was measured by PESA to be 5.0 eV for pBZTT-CDT and 5.4 eV for pBZTT-DPP with an error of ±0.05 eV in both cases. For the DPP copolymer this is slightly larger than the ionization potential of the DPP bithiophene copolymer measured by the same technique (5.25 eV),48 suggesting that that carborane cage is exhibiting a slight inductive electron-withdrawing effect, in agreement with the results of Morisaki. To calculate the optimized molecular geometry and electronic distribution of the frontier orbitals of the polymers, density functional theory (DFT) calculations were carried out using Gaussian0949 at the B3LYP level of theory with the 631G* basis set.50 Trimers of polymers with methyl group instead of longer side chains were employed as models in order to simplify the calculations. As shown in Figure 4, pBZTT-DPP showed a planar backbone structure, with no out-of-plane twisting between the monomer units in the polymer backbone. As envisaged, the presence of the terminal thienyl groups on the carborane containing monomer does indeed prevent any torsional strain with α-hydrogen atoms on adjacent monomers, thereby facilitating backbone planarization. However, for pBZTT-CDT we observed small deviations from planarity, with a torsion angle of 10° between the BZTT and CDT units for the outmost rings and 6° for the inner BZTT/CDT monomers. The reduction in torsion angle is a reflection of the enhanced LUMO delocalization over the central portion of the trimer compared to the outermost rings. It is notable that the most energetically favorable backbone conformation for pBZTT-CDT has the sulfurs in adjacent monomer units arranged in an “anti” configuration, resulting in a polymer conformation that has significant backbone curvature. In this

conformation all of the carborane rings exist on the same side of the polymer backbone. Similarly pBZTT-DPP also has an “anti” configuration for adjacent thiophene rings; however, in this case the backbone is able to adopt a more linear conformation with less backbone curvature, in which the carborane units pack on alternative sides of the polymer main chain. However, this conformation does still exhibit notable “crankshaft”-like geometry along the chain. The differences in backbone curvature are principally due to the different symmetries of CDT and T-DPP-T monomer, which are axisymmetric and centrosymmetric, respectively. Visualization of the frontier molecular orbitals (Figure 4) shows that although for both trimers the HOMO and LUMO are extensively delocalized over the π-conjugated polymer backbone including the BZTT, in neither case is there any orbital density on the carborane cage. In addition, there is no orbital density on the carborane cage for the HOMO−1, HOMO−2 (the first two orbitals lower in energy than the HOMO) or the LUMO+1, LUMO+2 orbitals (see Supporting Information). These findings are in agreement with earlier reports on BZTT and other carborane containing polymers which found that the frontier molecular orbitals were not delocalized over the carborane.21,22 They also support the conclusions made by Matteson and Davis regarding the nonaromatic character of the fused diene in related benzocarborane systems.6,51 The calculations are in agreement with the optical absorption data in which we did not see evidence of an internal charge transfer (ICT) absorption band. The predicted ionization potentials and band gaps are 4.91 and 1.76 eV for the CDT trimer and 5.55 and 1.41 eV for the DPP trimer, which are in reasonable agreement with that observed experimentally. To investigate the film morphology, DSC and X-ray diffraction measurements were performed. For pBZTT-CDT no obvious thermal transitions were observed by DSC between 0 and 250 °C with a heating rate of 10 °C/min. This absence of crystallinity was further confirmed by X-ray diffraction analysis of drop-cast films, showing no reflections that could be E

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Figure 5. XRD spectra of pBZTT-CDT (left) and pBZTT-DPP (right) films; as-cast and postannealing (1 h at 150 °C).

attributed to lamellar or π−π stacking. DSC analysis of pBZTTDPP also showed no obvious thermal transitions up to 250 °C (see Supporting Information). However, in this case, X-ray diffraction measurements of drop-cast pBZTT-DPP films revealed a diffraction peak at 2θ = 4.0°, corresponding to a d-spacing of 22.1 nm (Figure 5). Annealing at 150 °C for 1 h resulted in a sharpening of the peak and an increase in intensity, suggesting an improvement in order. The d-spacing reduced slightly after annealing to 20.1 Å This improved crystallinity compared to pBZTT-CDT is likely to be due to the strong aggregating properties of DPP, a trait that has led to its widespread use in organic electronics.28 The diffraction results clearly support the UV/vis evidence that the polymer is able to order and aggregate in the solid state, despite the presence of the three-dimensional carborane cages and long, branched alkyl side-chains. A broad peak was also observed at 24.4° (2θ) in annealed films, which corresponds to a spacing of 3.6 Å and may relate to π−π spacing of the polymer backbone. The spacings observed are similar to the 20.4 Å lamellar spacings and 3.71 Å π−π distances reported by Li and co-workers for a copolymer of DPP and thieno[3,2-b]thiophene,35 suggesting that the presence of the carborane unit does not notably impede intermolecular packing. The charge-transporting behavior of both polymers was investigated in either top gate, bottom contact or bottom gate, bottom contact devices with HMDS-treated SiO2 dielectric and Au source drain electrodes. P-type behavior was observed for pBZTT-CDT, with bottom gate devices exhibiting well-defined output characteristics. However, the charge carrier mobility was very low in both device configurations, on the order of 5 × 10−7 cm2 V−1 s−1, with an on/off ratio of 103. Annealing did not improve performance. We ascribe the low mobility to the amorphous nature of the films, similar to that observed for other CDT copolymers, in combination with high degree of backbone curvature which may limit overlap of the conjugated backbones in the solid state.52,53 More encouraging performance was observed for pBZTTDPP, in agreement with the evidence of enhanced ordering for this polymer. Thus, top gate devices exhibited ambipolar behavior, with hole and electron mobilities of 1 × 10−3 and 1 × 10−4 cm2 V−1 s−1, respectively, albeit at high voltages (Figure S4). The observation of ambipolar charge transport is in agreement with the low band gap of this polymer, which facilitates the injection of both holes and electrons from the

common source/drain electrode, and has been observed for several other DPP polymers. Although higher mobility DPP copolymers have been reported, this is the first time, to the best of our knowledge, that ambipolar charge transport has been observed in a carborane containing polymer. We further note that the transistor mobilities are similar to that reported for DPP copolymer with other bridged bithiophene comonomers containing bulky solubilizing groups like dialkylated or dialkoxybenzo[2,1-b;3,4-b′]dithiophene.54,55 In common with BZTT, these comonomers are all axisymmetric, which contributes to a crankshaft-type morphology of the polymer backbone, potentially limiting overlap of the conjugated backbones and thereby limiting performance compared to the state-of-the-art DPP copolymer which utilize centrosymmetric comonomers like 2,2-bithiophene. The ambipolar transport displayed by pBZTT-DPP may be of benefit for the development of transistor devices which can directly detect neutrons, and experiments to explore this behavior are planned.

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CONCLUSIONS

In conclusion, we report the stannylation of a benzocarborano[2,1-b:3,4-b′]bithiophene monomer and describe its polymerization by Stille polycondensation with electron-rich cyclopenta[2,1-b;3,4-b′]dithiophene (CDT) and electron-poor bis(2-thien-5-yl)2,5-pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (DPP) comonomers. The resulting polymers exhibit fully delocalized conjugated backbones and absorb in the visible region. Density functional theory calculations and optical absorption data suggest the BZTT acts as a mild, inductively electron-withdrawing comonomer. The pBZTT-CDT polymer exhibits no evidence of solid state aggregation, and only low charge carrier mobilities were observed in field effect transistor devices. On the other hand, pBZZT-DPP exhibited clear evidence of crystallinity and demonstrated ambipolar transistor performance in top gated structures, the first time such behavior has been observed in a carborane containing conjugated polymer. Such behavior may be useful for the development of electronic sensors, and future studies investigating the direct detection of neutrons are in development. F

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ASSOCIATED CONTENT

S Supporting Information *

Synthetic procedure for the synthesis of the BZTT monomer, OFET transfer and output curves, polymer TGA, and polymer DSC. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (J.M.). *E-mail [email protected] (M.H.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank AWE for financial support of an EPSRC CASE award (EP/I501444/1) and for their input into this project. We also thank Scott E. Watkins (CSIRO Materials Science and Engineering, Victoria, Australia) for his help in the UV-PESA measurements.

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