Ultrafast excitation transfer and trapping in a thin polymer film

Abstract

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 $\ensuremath{\sim}40\mathrm{fs}$ 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 $({\ensuremath{\tau}}_{h}=1\ifmmode\pm\else\textpm\fi{}0.1\mathrm{}\mathrm{ps}),$ the fraction of the interchain aggregates $(\ensuremath{\sim}10%)$ and the structural disorder of the polymer. The initial $\ensuremath{\sim}30\mathrm{fs}$ anisotropy decay does not originate from incoherent hopping energy transfer but from some other relaxation among electronic excited states within a spectroscopic unit.