Modulation of the Electronic Properties in Polydithienotiiophene Materials
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ABSTRACT Conjugated polymers with narrow band gap are promising candidates for symmetric electrochemical devices, such as electrochromics and redox supercapacitors, which involve the pand n-doped state of the polymer. Polydithienothiophene materials which are prepared from isomer monomers show this characteristic. The copolymerisation of two dithienothiophene isomers leads to materials whose electronic properties depend on the monomer ratio. Optical and electrochemical characterizations of different copolymers are reported and discussed. INTRODUCTION Conjugated polymeric materials with narrow band gap have been receiving considerable attention due to their capacity to be both n- and p- doped, along with a suitable transparency in the doped state [1]. This opens the possibility of application in symmetric electrochemical devices such as electrochromics and redox supercapacitors. We have recently described a series of narrow band gap isomeric polydithienotiophenes, which can undergo p- and n-doping processes [2-4]. A powerful strategy for chemical modification of the conjugated materials and for electronic property control is the copolymerisation of different monomers. This strategy produced thiophene-based materials with tunable optical properties [5], narrow band gap [6], and which are stable in both the neutral and doped states [7]. The copolymerisation of two dithienothiophene isomers in well defined ratios leads to materials with properties depending on the chemical composition of the chain. Synthesis of the copolymers and their preliminary optical and electrochemical characterization are presented. EXPERIMENT Dithienothiophene polymers and copolymers were electrochemically grown on transparent ITO or TO glass and on Pt electrodes in galvanostatic conditions at lmA cm2. The electrosyntheses were performed at room temperature in acetonitrile - 0.1 M tetrabutylammonium tetrafluoroborate (Et 4NBF 4) - 0.015 M monomer (or monomers). The electrochemical measurements were performed with EG&G PAR M270A potentiostat/galvanostat and with a Solartron 1255 frequency response analyzer in propylene carbonate (PC) - I M Et 4NBF 4 in a MBraun Labmaster 130 dry-box (02 and 1120 contents < lppm). All chemicals were reagent-grade products, dried and purified before use. The capacitance values were evaluated by impedance spectroscopy after charging the electrode for 150 s at the peak potential of the voltammetric waves in the p- and n-doping potential domains, and by maintaining this potential during the impedance measurement. The reference electrode was an Ag wire (+100 mV vs saturated calomel electrode, SCE). The UV-Vis spectra were recorded using a Varian Cary 2400 spectrophotometer. 317
Mat. Res. Soc. Symp. Proc. Vol. 488 ©1998 Materials Research Society
RESULTS Dithienothiophene (DTT) molecules (Scheme 1) consist of three heteroaromatic fused rings i) and have a high it-delocalisation. The four DTT isomers have different condensation types: in dithieno[3,2-b:2',3'-d]thiophene (DTF0) the lateral thiophene moieties are fused
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