Long lived photoinduced charges in donor-acceptor anthraquinone substituted thiophene copolymers

The photoinduced charge-transfer properties of a series of polyalkylthiophene copolymers, carryinganthraquinone substituents covalently linked to the conjugated backbone, have been studied in the solid stateby photoinduced absorption (PA) and light-induced electron spin resonance (LESR) spectroscopy. Themeasurements indicate the formation of metastable charges arising from the photoinduced electron transferfrom the polythiophene backbone to the anthraquinone moieties. At low temperatures (below 200 K), longlivedpersistent charges are formed, exhibiting lifetimes that extend for several minutes; their recombinationkinetics has been studied by following the formation and decay of the PA and LESR signals. The results arerationalized using a model originally proposed to describe the low-temperature recombination kinetics oflong-lived photoexcited carriers in amorphous inorganic semiconductors. It is clearly evidenced that, in thesepolymers, the number of acceptor substituents in the chain, easily tuned by chemical tailoring, plays a keyrole in the photoexcitation scenario.