Anthracene-thieno[3,4- c ]pyrrole-4,6-dione based donor – acceptor conjugated copolymers for applications in optoelectronic devices

Three novel alternating copolymers of thieno[3,4- c ]pyrrole-4,6-dione (TPD) and triisopropylsilylacetylene- functionalized anthracene were prepared via Suzuki polymerization. Various solubilizing substituents were attached to the TPD moiety in order to ascertain the impact they have upon the optical, electrochemical, and thermal properties of the resulting polymers. All copolymers showed good solubility and thermal stability with decomposition temperatures in excess of 300°C. Optical properties revealed that PTATPD(O), PTATPD(DMO), and PTATPD(BP) displayed optical energy gaps in excess of 2.0eV. It is speculated that steric repulsion between solubilizing groups on repeat units along polymer chains reduces their planarity and decreases their electronic conjugation. The amorphous nature of the polymers was con ﬁ rmed with differential scanning calorimetry and powder X-ray diffraction. The highest occupied molecular orbital levels of the three polymers are unaffected by the different solubilizing chains. However, they exert some in ﬂ uence over the lowest unoccupied molecular orbital (LUMO) levels with PTATPD(BP) and PTATPD(O) displaying the lowest LUMO levels ( (cid:1) 3.4eV). In contrast, PTATPD(DMO) displayed the highest LUMO level ( (cid:1) 3.3eV). © 2015 The Authors. Polymers for Advanced Technologies Published by John Wiley & Sons Ltd. Supporting information may be found in the online version of this paper.


INTRODUCTION
Semiconducting polymers offer distinct advantages over their inorganic counterparts including the following: light weight, improved mechanical flexibility, large active layers, and can be manufactured using low-cost solution processing. [1,2] It is hypothesized that conjugated polymers will fulfill a larger number of functions providing large-scale industrial manufacture of organic electronics becomes feasible. [1] In actuality, the commercial success of organic light-emitting diodes (OLEDs) should serve as a platform to promote the efficacy of organic optoelectronic devices. [3][4][5][6] Aside from OLEDs, the electrical, magnetic, and optical properties of conjugated polymers makes them promising candidates for organic photovoltaic devices, organic field-effect transistors, electrochromic devices, and stable electronic memories on flexible circuit boards. [7][8][9][10][11][12][13][14][15] Furthermore, they have proven to be suitable candidates for gas and humidity sensing and bio-sensing, which can be attributed to their sensitivity to biological, chemical, and physical perturbations. [16,17] The attractiveness of conjugated polymers lies in the facile tunability of their optical, electronic, and morphological properties. [18] Previous literature has proven that copolymerising an electron-rich donor monomer with an electron-deficient acceptor unit in a so-called donor-π-acceptor (D-π-A) arrangement is an effective method of tuning these properties. [19][20][21] The alternation of donor and acceptor units results in hybridization of the lowest unoccupied molecular orbital (LUMO) of the acceptor and highest occupied molecular orbital (HOMO) of the donor, resulting in a decreased band gap. [12] Moreover, it is believed that the D-π-A arrangement promotes intramolecular charge transfer along the polymer backbone, which can enhance charge carrier mobility. [12] This D-π-A methodology has been used to produce a range of conjugated chromophores that absorb light over the whole solar spectrum.
Literature reports have speculated that the highly planar and symmetrical structure of thieno [3,4-c]pyrrole-4,6-dione (TPD) repeat units along polymer chains could improve electron delocalization thereby enhancing interchain interactions and increasing their hole mobility. [22,23] Furthermore, the nitrogen of the imide functionality can be substituted with solubilizing groups, facilitating solution processing of the final polymer. Finally, the electron-withdrawing nature of the TPD moiety allows it, once polymerized with alternate electron donor units, to form the highly desirable D-π-A arrangement. There is currently a large body of work covering the use of TPD in optoelectronic devices, mainly in photovoltaic devices and organic field-effect transistors. [22][23][24][25][26] TPD-based copolymers have displayed efficiencies up to 8% when fabricated into a bulk heterojunction solar cell using PC 70 BM as the acceptor. [27] Additionally, TPD-containing copolymers have displayed high hole mobilities (1.29 cm 2 /Vs) when fabricated into field-effect transistors. [28] 2,6-Linked anthracene units are finding use in D-π-A conjugated polymers. [29][30][31][32][33] Iraqi and co-workers presented the preparation of D-π-A polymers with alternating 2,6-linked anthracene units with aryloxy substituents at their 9,10-positions and various benzothiadiazole alternate repeat units. [33] Bulk heterojunction solar cells fabricated from these polymers displayed efficiencies ranging from 1.93% to 4.17% when blended with PC 70 BM. [33] Copolymers comprising alternating benzo[1,2-b:4,5-b']dithiophene units and electron-donating units have been reported in literature. [34][35][36][37] However, to the best of our knowledge, nobody has reported any alternating TPD-anthracene copolymers. In the present contribution, we report the synthesis of three alternating copolymers comprising TPD with varying substituents as electronaccepting moieties and triisopropylsilylacetylene-functionalized anthracene as electron-donor moieties. The preparation of polymers PTATPD(O), PTATPD(DMO), and PTATPD(BP) (Fig. 1) is presented along with a study of their optical, electrochemical, thermal, and structural properties.

Polymer synthesis
The synthetic route for the preparation of the three TPD monomers is outlined in Scheme 1. It is perhaps worth mentioning that M2 and M3 were obtained in slightly lower yields, relative to M1. This was attributed to the additional steric hindrance associated with 3,7-dimethyloctan-1-amine and 4-butylaniline.  2 and tri(o-tolyl)phosphine were used as the catalyst and sodium hydrogen carbonate as the base. All polymerizations were left for 24 hr with large quantities of bright red precipitate forming. The polymers were fractionated via Soxhlet extraction using methanol, acetone, hexane, and toluene. The toluene fractions were collected, reduced in vacuo, and precipitated in methanol. Subsequent studies were conducted on the toluene fractions only. The chemical structures of PTATPD(O), PTATPD(DMO), and PTATPD(BP) were confirmed via 1 H nuclear magnetic resonance (NMR) (supplementary information) and elemental analysis. The number-average molecular weight (M n ) and weight-average molecular weight (M w ) were estimated using gel permeation chromatography (GPC) using 1,2,4-trichlorobenzene as the eluent at 140°C (Table 1). Substituting n-octyl chains in PTATPD(O) for dimethyloctyl chains in PTATPD(DMO) on the TPD moiety results in a significantly higher M w . It is speculated that the branched alkyl chain disrupts intermolecular interactions in solution, increasing the solubility of the resulting polymer. Thus, the final polymer product is able to achieve a higher  wileyonlinelibrary.com/journal/pat

Optical properties
The ultraviolet-visible absorption properties of the polymers were investigated in solution and film state (Fig. 2). The results are summarized in Table 1. All polymers display an intense transition band at 383 nm in solution, corresponding to a π-π* transition. This band is red-shifted to 388, 390, and 389 nm for PTATPD(O), PTATPD(DMO), and PTATPD(BP), respectively, in films states. PTATPD(O), PTATPD(DMO), and PTATPD(BP) display a second, less intense transition at 529, 533, and 532 nm in solution, respectively. This is red-shifted to 548, 553, and 550 nm for PTATPD(O), PTATPD(DMO), and PTATPD(BP) in film states, respectively. This transition corresponds to intramolecular charge transfer between the TPD-acceptor unit and the anthracene-donor moiety. The small bathochromic shift (~20 nm) that is observed from solution to films indicates that the polymers adopt a similar conformation in both states. The optical band gaps, as estimated from the onset of absorption wavelengths, are 2.16, 2.14, and 2.12 eV for PTATPD(O), PTATPD(DMO), and PTATPD(BP), respectively.
The results indicate that the optical properties of the polymers are not significantly affected by the molecular weight or the nature of the substituent attached to the TPD moiety. It is speculated that there is a large torsion angle between the anthracene moiety and the TPD unit, arising from the intramolecular repulsion between the bulky TIPS group and functionalized-imide on the TPD. Consequently, the planarity of the polymer is disrupted. Thus, orbital overlap between non-coplanar aromatics is poor, leading to localization of the π-electron wave functions and a decreased electronic conjugation. Therefore, the true effects the different substituents have on the optical properties are not revealed. It is hypothesized that the lack of a regular, planar structure will result in an amorphous polymer.
Najari and co-workers synthesized a series of thieno [3,4-c]pyrrole-4,6-dione-alt-2,7-carbazole polymers. The lowest optical band gap reported by the group was 1.97 eV. They speculated that the alkyl chains twist the polymer backbone decreasing the effective conjugation length resulting in a wider optical band gap. [38] The homopolymer, regioregular poly(3-hexylthiophene-2,5diyl) (P3HT), displays a reduced optical band gap of 1.9 eV and a larger bathochromic shift from solution to film. [39,40] It is hypothesized that the localization of the π-electron wavefunction, decreased electronic conjugation, and amorphous nature of the polymers synthesized in this report are responsible for the wider optical band gaps, relative to P3HT. Furthermore, it is known that the solar harvesting of P3HT is restricted by its mismatch with the solar spectrum. [39,40] The optical band gaps of PTATPD(O), PTATPD(DMO), and PTATPD(BP) are wider than that of P3HT. Thus, these polymers are not optimized with respect to the maximum photon flux of the solar spectrum.
The photoluminescence (PL) emission spectra of the polymers in solution and solid films are illustrated in Fig. 2. The PL spectra of all polymers in chloroform solution and thin films were excited at incident wavelengths of their absorption λ max . In solution, Table 1. A summary of the GPC, UV-vis absorption, and photoluminescence data for PTATPD(O), PTATPD(DMO), and PTATPD(BP)

UV-vis absorption Photoluminescence
Polymer  The electrochemical band gap of the polymers is significantly larger than their corresponding optical band gap. It is believed that the additional energy is the result of the strong Coulomb attraction of excitons, which needs to be overcome in order to generate free charge carriers.
Previous literature has estimated the HOMO levels of P3HT and PCPDTBT to be À5.0 and À5.3 eV, respectively. [39,40] All polymers synthesized within this report display deeper HOMO levels than P3HT and PCPDTBT. Furthermore, all polymers synthesized in this contribution demonstrate significantly deeper HOMO levels than the polymers synthesized by Najari and co-workers. [38] Thermal properties Thermogravimetric analysis of PTATPD(BP), PTATPD(O), and PTATPD(DMO) revealed that all polymers possess good thermal stability with degradation temperatures (5% weight loss) occurring at 354°C, 360°C, and 309°C, respectively (Fig. 4a, Table 2). Differential scanning calorimetry (DSC) revealed that PTATPD(O) and PTATPD(BP) exhibit a broad, weak glass transition temperature (T g ) at 56.4°C and 48.4°C, respectively (Fig. 4b, Table 2). No polymers exhibited any clear melting endotherms up to 220°C on the DSC thermograms. This result supports the hypothesis that the polymers adopt an amorphous structure in the solid state.
The results obtained suggest that the thermal properties of the conjugated polymers are influenced by the size of the pendant group attached to the imide functionality, the larger the group the lower the T d and T g . PTATPD(DMO) and PTATPD(BP) display lower T d and T g temperatures relative to PTATPD(O). We tentatively hypothesize that the larger groups create "free volume" within the polymer by increasing separation between polymer chains, allowing the polymer to reorganize more easily within solid state resulting in lower T d and T g temperatures.

Powder X-ray diffraction
Powder x-ray diffraction patterns of polymers PTATPD(O), PTATPD(DMO) and PTATPD(BP) were obtained to investigate the molecular organization of polymers in solid state (Fig. 5). The X-ray diffraction (XRD) patterns of all polymers display a single broad diffuse feature, which is consistent with the random scatter of an amorphous solid. Furthermore, the lack of a peak  in the low angle region suggests that the polymer does not possess any long-range translational order; providing further evidence that the polymer has an amorphous structure in solid state. The results obtained from the XRD patterns agree with the DSC data.

CONCLUSIONS
In summary, the preparation of three alternating copolymers comprising TPD functionalized units and 9,10-bis(triisopro pylsilylacetylene) anthracene units using Suzuki cross-coupling reactions was reported and yielded the polymers PTATPD(O), PTATPD(DMO), and PTATPD(BP). Their optical, thermal, electrochemical, and structural properties in solid state were characterized. The amorphous nature of the polymers was confirmed by DSC and powder XRD studies. It has been demonstrated that the M w , electronic, and thermal properties are dependent upon the pendant-group attached to the TPD-moiety. The larger the pendant group, the higher the M w and lower the T d . Surprisingly, the optical properties, largely the absorption profiles, were not influenced by the pendant-group attached to the TPD-moiety.
It is speculated that the intramolecular repulsion between solubilizing groups within the polymer reduces the planarity and decreases the electronic conjugation. Thus, changing the pendant-group has little impact on the optical properties of the resulting polymer.

1
H and 13 C NMR spectra were recorded on a Bruker AV 400 (400 MHz) using chloroform-d (CDCl 3 ) or acetone-d as the solvent. 1 H NMR spectra of the polymers were recorded on a Bruker Avance III HD 500 (Bruker, Coventry, United Kingdom) (500 MHz) spectrometer at 100°C using 1,2-dideutrotetrachloroethance (C 2 D 2 Cl 4 ) as the solvent. Coupling constants are given in Hertz (Hz). Carbon, hydrogen, nitrogen, and sulfur elemental analysis was performed on a Perkin Elmer 2400 series 11 CHNS/O analyzer (Perkin Elmer, Buckingham shire, United Kingdom). Analysis of halides was undertaken using the Schöniger flask combustion method. GPC analysis was conducted on polymer solutions using 1,2,4-trichlorobenzene at 140°C as the eluent. Polymer samples were spiked with toluene as a reference. GPC curves were obtained using a Viscotek GPCmax VE2001 GPC solvent/sample module and a Waters 410 Differential Refractometer (Waters, Hertfordshire, United Kingdom), which was calibrated using a series of narrow polystyrene standards (Polymer Laboratories). Thermogravimetric analyses (TGAs) were obtained using a Perkin Elmer TGA-1 Thermogravimetric Analyzer (Perkin Elmer, Buckinghamshire, United Kingdom) at a scan rate of 10°C min À1 under an inert nitrogen atmosphere. DSCs were obtained using a Perkin-Elmer Pyris 1DSC (Perkin Elmer, Buckinghamshire, United Kingdom) in the temperature range À50-220°C. Powder X-ray diffraction samples were recorded on a Bruker D8 advance diffractometer (Bruker, Coventry, United Kingdom) with a CuKα radiation source (1.5418 Å, rated as 1.6 kW). The scanning angle was conducted over the range 2-40°. Ultraviolet-visible absorption spectra were recorded using a Hitachi U-2010 Double Bean UV/Visible Spectrophotometer (Hitachi, Berckshire, United Kingdom). Polymer solutions were made using chloroform and measured using quartz cuvettes (path length = 1 × 10 À2 m). Thin films, used for absorption spectra, were prepared by drop-casting solutions onto quartz plates using 1 mg cm À3 polymer solutions that were prepared with chloroform. Photoluminescence spectra were recorded on a Horiba FluoroMax 4 spectrometer (Horiba, Middlesex, United Kingdom). Polymer solutions were made using chloroform and measured using quartz cuvettes (path length = 1 × 10 À2 m). Thin films were prepared by drop-casting solutions onto quartz plates using 5 mg cm À3 polymer solutions that were prepared with chlorobenzene. Cyclic voltammograms were recorded using a Princeton Applied  Research Model 263A Potentiostat/Galvanostat (Princeton Applied Research, Cambridge, United Kingdom). A three electrode system was employed comprising a Pt disk, platinum wire, and Ag/AgCl as the working electrode, counter electrode, and reference electrode, respectively. Measurements were conducted in a tetrabutylammonium perchlorate acetonitrile solution (0.1 mol dm À3 ) on polymer films that were prepared by drop casting polymer solution. Ferrocene was employed as the reference redox system, in accordance with International Union of Pure and Applied Chemistry's recommandations. [44] 1,3-Dibromo-5-(4-butylphenyl)thieno [3,4-c]pyrrole-4,6-dione (M3) THF (12 cm 3 ) was added to a round bottom flask containing 4,6-dibromothieno[3,4-c]furan-1,3-dione (1.50 g, 4.81 mmol) and 4-butylaniline (789 mg, 5.29 mmol). The mixture was heated to 50°C for 3 hr. Upon completion, the reaction mixture was cooled to room temperature. Thionyl chloride (5 cm 3 ) was added, and the reaction was stirred at 55°C for a further 4 hr. The reaction was precipitated into methanol (150 cm 3 ). The product was filtered off and washed with methanol to give the title compound as white needles (1.73 g, 3.9 mmol, 81%). 1