Electrochemistry and photoelectrochemistry of two donor–acceptor polythiophene polymers with acceptor moieties in the main chain

The electrochemical, spectroelectrochemical and photoelectrochemical behavior was studied of two polymers of the polythiophene series containing electron–acceptor groups 1,10-phenanthroline and dimethylazine in the main chain. The polymers were obtained by electropolymerization of corresponding monomers, 3,8-bis(2,2′-bithien-5-yl)-1,10-phenanthroline (BT-phen) and 1,4-dimethyl-1,4-bis(2,2′-bithien-5-yl)-azine (BT-azine). It was shown that both polymers can undergo reversible anodic and cathodic doping in appropriate conditions. This was confirmed by using electrochemical impedance and in situ UV–vis absorption measurements. The optical bandgap values for both poly(BT-phen) and poly(BT-azine) were very close to that of the parent polymer, polybithiophene (PBT), while the electrochemical bandgap for poly(BT-azine) was 0.3–0.4 eV smaller than the electrochemical bandgap values for PBT and poly(BT-phen). A similar trend was derived from DFT calculations of the optimized geometry and electronic structure of corresponding oligomers. This was explained by the flexibility of the poly(BT-azine) backbone as compared to both PBT and poly(BT-phen), which facilitates the structural changes associated with the polymer doping. Both poly(BT-phen) and poly(BT-azine) exhibited cathodic photocurrents when illuminated in solution at negative potentials. The photoefficiency of these two polymers was found to be ca. 25% higher than the efficiency of non-modified polybithiophene, similarly to other donor–acceptor polythiophene polymers studied by us earlier [O.A. Semenikhin, M.M.D. Hossain, M.S. Workentin, J. Phys. Chem. B 110 (2006) 20189–20196].