Effects of interchain interactions, polarization anisotropy, and photo-oxidation on the ultrafast photoluminescence decay from a polyfluorene

The ultrafast dynamics of excitons and excimers in an aligned film of poly(dioctylfluorene) (F8) have been studied by femtosecond time-resolved photoluminescence spectroscopy. The results indicate that the excimer formed in F8 involves only a small change in intermolecular separation upon excitation and has a limited, but nonzero, mobility. From changes in the photoluminescence decay upon photooxidation of F8 we deduce that the exciton diffusion constant is larger than the excimer diffusion constant by a factor of $21\pm 2,$ thereby explaining why the excitonic luminescence is more strongly affected by the presence of quenching sites than the excimer luminescence. The decay of excitonic photoluminescence exhibits a significant polarization anisotropy, consistent with migration of excitons between regions with different orientation of the polymer chains. In contrast, excimer migration between such domains is inhibited since the excimer experiences the domain borders as potential barriers.