FEMTOSECOND AND CW PHOTOEXCITATIONS OF THIOPHENE BASED CONJUGATED POLYMERS

We present an extensive photoinduced absorption study of alkyl substituted poly(2,5-thienylenevinylene)s in both the solid state and in solution. The optical and conformational properties of the samples are monitored with UV-Vis absorption and emission, IR absorption and Raman scattering. Three photoinduced states are detected under steady state condition which display lifetimes of the order of 100 ms in the solid state. The two low-energy bands are assigned to bipolarons, while a third band peaked near the band edge has a different origin. In solution very long-lived photoexcitated states are observed and they are assumed to recombine via a solvent-assisted photo-doping mechanism. Femtosecond transient bleaching decay indicate that fast dynamics is a sensitive probe of the backbone rigidity, the conjugation length and the supermolecular organization of the chain. The experimental data are interpreted in terms of the formation and decay of bound pairs (polarons-excitons) produced by the absorption of above gap photons