Ultrafast excitation transfer and trapping in a thin polymer film

Transient absorption anisotropy of a polythiophene polymer in a thin film was studied on a femtosecond time scale. The anisotropy has a non-exponential decay on the sub-picosecond time scale, with a fastest component characterized by an ~40fs time constant. To simulate the anisotropy decay an incoherent energy migration model has been used. Comparison between the simulated and experimental kinetics enabled us to estimate the nearest-neighbor pair wise hopping time (τₕ=1±0.1ps), the fraction of the interchain aggregates (~10%) and the structural disorder of the polymer. The initial ~30fs anisotropy decay does not originate from incoherent hopping energy transfer but from some other relaxation among electronic excited states within a spectroscopic unit.