Extended conjugation in poly(triarylamine)s: synthesis, structure and impact on field-effect mobility

Organic Materials Innovation Centre, S Manchester, Oxford Road, Manchester, M manchester.ac.uk; Fax: +44 0161 275 4273 Department of Physics, Lancaster Universit Instituto de Ciencia y Tecnoloǵıa de Poĺım Madrid, 28006, Spain Department of Chemistry, University of Su navarro@sussex.ac.uk; Fax: +44 01273 678 † Electronic supplementary information details of DFT calculations, FET prepara MS spectra, UV-Vis and PL spectra, FET further annealing experiments. See DOI: 1 Cite this: J. Mater. Chem. C, 2014, 2, 6520


Introduction
Semiconducting polymers are important components of the active layers of organic light-emitting diodes (OLEDs), 1 photovoltaics (OPVs) 2 and organic eld-effect transistors (OFETs). 3,4 These materials can be deposited from solution using conventional patterning processes such as screen, gravure or ink-jet printing and are compatible with fabrication on exible, plastic substrates and the development of mechanically robust, conformable devices. 5 Examples of p-type polymers include regioregular poly(3-hexylthiophene), 6 poly(2,5-bis(3-alkylthiophen-2-yl)thieno [3,2-b]thiophenes) 7 and recently very high performance donor-acceptor main chain polymers derived from diketopyrrolopyrrole units. 8,9 Semicrystalline polymers with short intermolecular p-p stacking distances generally show the highest performance in OFETs, with charge mobilities of up 10 cm 2 V À1 s À1 . 8 To achieve such high performance from semicrystalline polymer lms requires optimized surface treatments and prolonged high temperature annealing to improve the molecular orientation and the domain morphology of the lms. By contrast in OFET devices amorphous semiconducting polymers require little post-processing of the lms beyond simple solvent removal but in general exhibit more moderate charge-carrier mobilities (<10 À2 cm 2 V À1 s À1 ). A recent report of an indenouorene-phenanthrene copolymer (PIFPA, Fig. 1) showed a signicant improvement in the measured mobility for an amorphous polymer, up to 0.3 cm 2 V À1 s À1 in a bottom contact, top gate architecture. 10 Polytriarylamines (PTAAs) 11,12 are an important class of amorphous semiconducting polymers, as they show excellent ambient performance over extended periods of time, 13 even in bottom gate device architectures where the channel is directly exposed to the atmosphere. This stable ambient performance and amorphous solid-state structure is desirable for use in chemical sensors and allows for simple and reproducible processing of PTAA thin-lms when compared to the processes required for semicrystalline polymers. 13 PTAAs have been employed as hole conductor materials for the active layer of photocopiers, laser printers, 14 OLEDs, 15 and vapour sensors. 16,17 Furthermore, they have been used in electroluminescent devices, 18 cathode components 19 and as electroactive separator materials in rechargeable lithium batteries. 20 One of the major benets of conjugated polymers as organic semiconductors is the ability to precisely tune the optoelectronic properties by modifying the backbone of the polymer. This precise control can be achieved by the incorporation of different co-monomers into the polymer main chain or by increasing the coplanarity of fused structures in the monomer units. 2,5,7 For example, polymers derived from indenouorene units (IF) show enhanced p-conjugation and more extensive pp interactions leading to improved charge-carrier mobilities in OFETs when compared to those of the corresponding uorene polymers. 21 Similarly, incorporation of IF units into (bisthienyl) benzothiadiazole copolymers gave power conversion efficiencies in organic photovoltaic devices of up to 2.44%, whereas the corresponding diindenouorene (DIF) copolymers gave improved efficiencies of up to 4.50%. 22 In addition structurally related analogues of indenouorenes, the indolocarbazole units, have been copolymerized with uorenes, 23 bithiophenes, 24 and others units to tune the optoelectronic properties. The incorporation of repeating units with extended conjugation into the backbone of PTAAs, e.g. uorene and indenouorene (PIFTAA, Fig. 1), resulted in a signicant improvement of the charge transport. 11 Herein we present the use of the Buchwald-Hartwig polyamination to the synthesis and characterization of PTAAs incorporating bridged phenyl units IF, 25 DIF 26 and ICz. 27 By using this approach it is possible to obtain very rigid systems and the PTAAs derived from IF show the highest mobility in an OFET reported to date for this class of amorphous semiconducting polymer. The relationship between structure and performance in these materials was rationalized by DFT calculations of the electronic structure of oligomeric models.

General remarks
All reagents were obtained from commercial sources and used as received, except for 2,4-dimethylaniline and 4-methoxy-2methylaniline, these were distilled over KOH prior to usage under reduced pressure. Solvents were dried according to standard procedures or using an Innovative Technology Pure-Solv MD 7 system using aluminium oxide columns. Reactions were carried out under an argon atmosphere using standard Schlenk techniques. 1 H NMR spectra were recorded at 400 MHz on a Bruker Avance AV 400 MHz Ultrashield spectrometer in the solvent stated at 25 C. Elemental analysis was performed using a Carlo Erba Instruments EA1108 elemental analyser. Palladium content was determined using a Thermo Scientic iCAP 6000 Series ICP Spectrometer. Bromine analysis was performed using a Metrohm 888 Titrando Autotitration system with a combined silver ring electrode aer decomposition of the sample into solution. MALDI-TOF mass spectrometry was carried out using a Shimadzu Biotech AXIMA Condence MALDI mass spectrometer in linear (positive) mode, referencing against PPG or PEG standards (4 k-12 kDa). Gel permeation chromatography was carried out in tetrahydrofuran (THF) at 35 C using a Viscotek GPCmax VE2001 solvent/sample module with 2 Â PL gel 10 mm Mixed-B and PL gel 500A columns, a Viscotek VE3580 RI detector and a VE 3240 UV-Vis multichannel detector. The ow rate was 1 mL min À1 and the system was calibrated with low polydispersity polystyrene standards in the range of 200 to 180 Â 10 4 g mol À1 from Polymer Laboratories. The analysed samples contained n-dodecane as a ow marker. UV-Vis absorption spectra were recorded on a Varian Cary 5000 UV-Vis-NIR spectrophotometer in dichloromethane at room temperature. Fluorescence spectra were recorded on a Varian Cary Eclipse uorimeter in dichloromethane solution at room temperature. Cyclic voltammetry was performed in dichloromethane solution scanning at 100 mV s À1 on a BASI Epsilon electrochemical workstation with a three-electrode cell, Ag/ AgNO 3 (in dichloromethane-acetonitrile 1 : 1) as reference electrode, platinum wire as counter electrode and working electrode, in nitrogen-purged, 0.1 M solution of tetrabutylammonium hexauorophosphate [(C 4 H 9 ) 4 N]PF 6 as a supporting electrolyte at room temperature. Wafers with a 300 nm SiO 2 layer on n ++ Si were obtained from Si-Mat GmbH (Germany). Differential scanning calorimetry measurements were recorded using a Perkin Elmer Jade DSC instrument under nitrogen atmosphere, 5-10 mg of the sample was sealed in an aluminium pan with a crimping tool. The sample was heated from 30 C to 220 C or 350 C at a heating rate of 10 C min À1 , held for 2 minutes at 220 C or 350 C and then cooled to 30 C at a rate of 10 C min À1 . This cycle was repeated once. WAXS spectra were recorded on a Bruker AXS D8 Discover using the precipitated polymers or spin-coated on a WSi 2 -C coated silicon wafer. Materials 6,6 0 ,12,12 0 -Tetra-n-octyl-6,12-dihydroindeno [1,2b] uorene, 25,28 6,6,12,12,15,15-hexa-n-butyl-12,15-dihydro-6H-cyclopenta[1,2b:5,4-b 0 ]diuorene, 26 2,7-dibromocarbazole, 29 3,9-dibromo-5,11di-n-octylindolo[3,2-b]carbazole 30 and (IPr)Pd(allyl)Cl 31 were synthesized using literature procedures.

Polymerization procedure A
A Schlenk tube was charged with KO t Bu and ame dried with a heat gun under vacuum. Aer cooling to room temperature the tube was lled with argon and the dibromoarene and NHC-Pd complex were added. The tube was then degassed for 10 min under reduced pressure and purged with argon. Toluene (anhydrous and degassed), end-capper (p-bromoanisole, 50 mmol L À1 toluene stock solution) and the aniline were added and the reaction mixture was heated to 105 C for the time specied. Aer cooling to room temperature the crude product was precipitated into methanol, ltered off and dried. The crude polymer was ltered over an n-hexane wet silica pad using dichloromethane as eluent. The ltrate was extracted with an aqueous sodium N,N-diethyldithiocarbamate solution (0.1 M), then water and dried over MgSO 4 and ltered. Aer evaporation of the solvent, the polymer was precipitated from a concentrated dichloromethane solution into methanol and ltered off. An extraction with a Soxhlet apparatus was performed. The polymer was then precipitated from a concentrated solution into methanol, ltered off and dried.

Polymerization procedure B
A Schlenk tube was charged with KO t Bu and ame dried with a heat gun under vacuum. Aer cooling to room temperature the tube was lled with argon and the dibromoarene was added. The tube was then degassed for 10 min under reduced pressure and purged with argon. Toluene (anhydrous and degassed) and end-capper (p-bromoanisole, 50 mmol L À1 toluene stock solution) were added and the reaction mixture was heated to 105 C for 10 minutes. The aniline and (IPr)Pd(allyl)Cl (dissolved in toluene) were added and the reaction mixture was kept for the time specied at 105 C. Aer cooling to room temperature the crude product was precipitated into methanol, ltered off and dried. The crude polymer was ltered over a silica pad (wetted with n-hexane) using dichloromethane as eluent. The ltrate was extracted with an aqueous sodium N,N-diethyldithiocarbamate solution (0.1 M), then water and dried over MgSO 4 and ltered. Aer evaporation of the solvent, the polymer was precipitated from a concentrated dichloromethane solution into methanol and ltered off. An extraction with a Soxhlet apparatus was performed. The polymer was then precipitated from a concentrated solution into methanol, ltered off and dried.

Results and discussion
Synthesis A series of six new PTAAs (1-4, 7 and 8) was synthesized by the polyamination of commercially available 2,4-dimethylaniline and 2-methyl-4-methoxyaniline with the dibromoarene monomers shown in Fig. 2, using a palladium/N-heterocyclic carbene catalytic system based on either (IPr)Pd(allyl)Cl or [(IPr) Pd(naphthoquinone)] 2 (IPr ¼ bis(2,6-diisopropylphenyl)imidazol-2-ylidene) catalysts. 32,33 In addition derivatives of the 3,7linked carbazole based polymers 5 and 6 (ref. 32) were prepared for comparison with n-octyl chains instead of the branched 2ethyl hexyl chains reported previously. High molecular weight polymers of the less soluble diindenouorene and indolocarbazole monomers were obtained by adding the catalyst aer homogenization of the reaction mixture at 105 C. For the carbazole and indolocarbazole monomers higher catalyst loadings of 4 mol% resulted in higher molecular weight polymers than those obtained with a lower loading of 2 mol%. 32 The molecular weight of all polymers was controlled for optimum performance in OFETs, as previous work has shown that it is critical to avoid bimodal molecular weight distributions in PTAAs to be used in organic electronics. 34,35 Control was achieved by the use of shorter reaction times for polymer 1-4 and an in situ end-capping by the monofunctional 4-bromoanisole to limit the molecular weight of the step-growth polymerization. 34,35 All polymers were puried by ltration over silica and extraction with sodium diethyldithiocarbamate solution to remove palladium catalyst residue 10 followed by Soxhlet extraction to remove small molecules and oligomers giving number average molecular weights (M n ) in the range of 5100-21 500 g mol À1 (Table 1). Lower molecular weights were obtained for the carbazole derived polymers (5-8) and similar effects have been observed previously. 32 Due to the extensive work-up the isolated yields of the recovered polymers were generally <60%. In the cases of polymers 4, 5 and 7 a larger proportion of the material was lower molecular weight and this was removed during the Soxhlet extraction procedure.
However, the inuence of the polymer molecular weight is not expected to be signicant for these polymers, as previous work has shown that in polymers with a M n above 5000 g mol À1 the inuence is only marginal. 36 The new polymers are soluble in common organic solvents (CHCl 3 , CH 2 Cl 2 , THF and toluene) and were characterized by 1 H NMR spectroscopy, MALDI-TOF mass spectrometry, elemental analysis and their physical properties examined by UV-Vis spectroscopy, photoluminescence and cyclic voltammetry.

MALDI-TOF MS analysis
Polymers 1-8 were analyzed by MALDI-TOF mass spectrometry. This proved to be much simpler for polymers containing indenouorene (1, 2), diindenouorene (3,4) and indolocarbazole moieties (7,8) in the backbone than for the biphenyl series reported earlier. 32 The absorption maxima of these polymers are much further from the wavelength of the laser used for matrix desorption, resulting in less photoinduced fragmentation of the polymer main-chains than previously observed.
The MALDI-TOF mass spectrum of polymer 1 (Fig. 3) shows two main series of peaks associated with the expected repeating unit terminated by either anilines (H-[XY] n -X) or anisole and anilines (H-[XY] n -XA). Similar results were obtained for the methoxy derivatives 2, 3 and 4 (see ESI †). For polymers 7 and 8 where the end-capping was performed at the end of the reaction, predominantly aniline terminated main-chains with no anisole end-capped polymers are observed. End-capping of the N-H functionalities at the end of the polymerization was ineffective, as we also have found previously in polymerizations of biphenyls and uorenes. 33 Elemental analysis of the polymers revealed only trace levels of bromine or palladium were present in the polymers.

Photophysical properties
The optical properties of the polymer 1 to 8 were studied by UV-Vis and photoluminescence measurements in dichloromethane solution ( Fig. 4 and ESI †). The absorption maxima varied between 425 nm and 452 nm. The optical band gap was determined from the onset of the absorption spectra (see Table 2). 37 The absorption maximum of 1 is observed at l max ¼ 443 nm and this represents a bathochromic shi of 23 nm compared to the previously reported uorene derivative (l max ¼ 420 nm). 32 The absorption maximum of 3 is bathochromically shied by 9 nm over that of 1 (l max ¼ 452 nm), due to the extension of the pconjugated system on replacing the indenouorene unit with the diindenouorene unit in the polymer backbone. A similar effect was found for 7, which is bathochromically shied by 23 nm when compared to 5 due to the extended conjugation of the indolocarbazole over that of the carbazole derivative. For the methoxy series (polymers 2, 4, 6, and 8) the absorption maxima are further red-shied over those of the methyl series (polymers 1, 3, 5 and 7) as the electron-donating methoxy group raises the HOMO energy level. Small Stokes shis are observed for all polymers and the vibrational ne structure of the PL spectra indicate that rotational freedom of the arene units in the polymer backbone is limited (Fig. 5).
The electronic structures of oligomers based on polymer 1-8 (n ¼ 2, 3 and 4) were computed and the predicted HOMO/LUMO energy levels and the band gaps are collected in Table 3.
Comparing these values to the experimental values shows that the general trends are consistent between theory and experiment. For the tetramers the HOMO is delocalized (see Fig. 6 and ESI †) over the arylamine segments but it does not extend on to the p-system of the terminal arenes possibly due to the high   torsion angle between the main chain and these subunits or the fact that the molecule is no longer symmetric.

Field-effect transistor results
Polymers 1-8 were used to fabricate OFETs, initially in the bottom-gate, top-contact architecture. Highly doped silicon substrates were used, which acted as the gate electrode with a 300 nm silicon oxide layer as the gate dielectric. In order to enhance charge transport 32,38,39 the SiO 2 layer was treated with noctadecyltrichlorosilane (OTS) by spin-casting from 1,1,2trichloroethylene (TCE) solution, using a modied method reported by Bao and co-workers. 40 The polymers (1)(2)(3)(4)(5)(6)(7)(8) were deposited on the OTS-treated SiO 2 /Si substrates from TCE solution and the mobility in the saturation region was calculated from the transfer characteristics of the OFETs. All polymers gave statistically signicant numbers of working devices for parameter extraction and the results are summarized in Table 4.
The highest hole mobility was observed for polymer 1 (5.0 Â 10 À2 cm 2 V À1 s À1 ), and this is the highest mobility reported to date for polytriarylamine transistors. 11 Top-gate bottom-contact devices were also fabricated for this polymer using poly (1,1,2,4,4,5,5,6,7,7-decauoro-3-oxa-1,6-heptadiene) (CYTOP) as the gate dielectric, however, the conguration seems to have no inuence on the observed mobility (5.1 Â 10 À2 cm 2 V À1 s À1 ) as expected for amorphous semiconducting polymers. A typical transfer and output characteristic for polymer 1 is shown in Fig. 7. All other polymers showed more moderate hole mobilities on OTS-treated SiO 2 /Si substrates that ranged from 1.1 Â 10 À3 to 3.7 Â 10 À3 cm 2 V À1 s À1 , the hysteresis was low for all devices (<4 V), and the threshold voltages and on/off ratios are in the range previously reported for PTAAs. The holemobility of 2, 3 and 4 were signicantly lower than those of 1. The carbazole containing polymers 5 and 6 gave very similar mobilities of ca. 1 Â 10 À3 cm 2 V À1 s À1 , which increased for the indolocarbazole derivatives 7 and 8, with 7 giving a higher performance at 3.7 Â 10 À3 cm 2 V À1 s À1 .
Charge-transport in disordered amorphous materials is believed to occur via an intermolecular hopping mechanism and can therefore be described by the Marcus theory. 41 The two main contributions for charge-transport in Marcus theory are the internal reorganization energy and the charge transfer integral. The inuence of the charge transfer integral was found to be limited for analogous amorphous compounds 42, 43 and it has been demonstrated for tetraphenyldiamines that internal reorganization energies can be related to the observed hole mobilities. 44 The relationship between structure and performance in OFET devices of polymers 1-8 was investigated by calculation of the optimized geometries and reorganization energies for oligomers based on the repeating units (n ¼ 2, 3 and 4) of the polymers. Optimized geometries of oligomers show that the phenylene rings in the indenouorene (1, 2) and the   indolocarbazole (7,8) units are coplanar. However, in the diindenouorene unit in polymers 3 and 4 the phenylene rings are twisted with respect to each other. This distortion reduces the conjugation of the polymer backbone and increases the conformation rotational disorder of the backbone, both effects potentially having a negative impact on the expected hole mobility. Even though the trend in the absolute numbers does not correspond precisely with the measured mobility, the overall trend is that the polymers with the lowest calculated reorganization energies (1 and 7) showed the highest mobilities in OFET devices ( Table 4). The distorted diindenouorene derivative 3 has a higher reorganization energy than 1 and a lower measured mobility in an OFET. For the carbazole and indolocarbazole series a correlation between the internal reorganization energies and the observed hole-mobility can be found, even though for the reorganization energy for 6 seems out of proportion with l + ¼ 0.31 eV. However, precisely correlating the reorganization energies of the tetramers with the measured hole mobilities of the polymers is not possible as the polymers are polydisperse, end-groups are present and for ease of calculation the solubilising aliphatic side-chains were omitted. 45 Differential scanning calorimetry (DSC) was used to analyse the thermal behavior of the polymers. Polymers 3 and 4 showed no glass-transition (T g ) on heating or cooling, whilst all other polymers exhibit a second order transition (see Table 2). Powders of polymer 1 isolated by precipitation from solution show a small melting isotherm at 138 C in the rst scan of the DSC (see ESI †). However, the polymer cooled into a glassy state showed no endothermic transitions in the second scan. Wide angle X-ray scattering (WAXS) of 1 isolated by precipitation from solution shows reections associated with a low degree of crystallinity. WAXS of thin lms of 1 deposited by spin-coating on a silicon substrate showed no reections and was consistent with an amorphous lm. It appears that polymer 1 may partially crystallize during precipitation from dichloromethane solution into a non-solvent such as methanol but this crystallization is not possible when the polymer is deposited as a thin lm directly from a good solvent during spin-coating. Spin-coated thin lms of 1 remain amorphous even by extensive thermal or solvent annealing and the partial crystalline character of the powders cannot be restored (see ESI †).

Conclusions
PTAAs with extended fused backbones were synthesized by the (NHC)-Pd catalysed C-N coupling of anilines and dibromoarenes. The optical and electrochemical properties of these polymers showed an increase in the HOMO energies and the onset of absorption on extending the conjugation from indeno-uorene (1, 2) to diindenouorene (3,4) and from carbazole (5, 6) to diindolocarbazole (7,8). The highest OFET performance was found for a polymer (1) derived from a dibromoindeno-uorene and 2,4-dimethylaniline with hole mobilities of 5 Â 10 À2 cm 2 V À1 s À1 in top-and bottom-gated devices. This is the highest reported performance for a polytriarylamine. The lower performance of more extended diindenouorene polymers (3,4) and polymer 2 was rationalized by DFT calculations of model oligomers that show higher reorganization energies for these polymer structures. Polymers derived from indolocarbazole units (7) have the lowest predicted reorganization energies of those in this study, suggesting that optimization of OFET fabrication for these materials could lead to further improvement in polytriarylamine device performance.