Dispersive relaxation dynamics of photoexcitations in a polyfluorene film involving energy transfer: Experiment and Monte Carlo simulations

Time-resolved fluorescence spectroscopy is used to investigate relaxation o f electronic excitations in films of pi -conjugated polymer 1 in the ps tim e domain. The position of the fluorescence band and its width are measured as a function of time and excitation energy. Both low (15 K) and room-tempe rature behavior are investigated. For high energy excitation, the fluoresce nce band shows a continuous red shift with time. The energy associated with the maximum of the fluorescence band E is proportional to log(t), with t b eing the time after excitation. For excitation in the tail of the lowest ab sorption band, the fluorescence remains stationary and selective excitation of a subset of chromophoric chain segments is possible. At intermediate ex citation energy the time required for the excitations to make their first j ump depends on the excitation energy and is longer at lower energy. At low temperature and high energy excitation the fluorescence bands are found to narrow with time, while for low energy excitation a broadening with time is observed. The experimental data are consistent with dispersive relaxation dynamics for the photoexcitations by incoherent hopping between localized s tates. Monte Carlo simulations are performed to obtain the average energy a nd the width of the energy distribution for an ensemble of photoexcitations in an energetically disordered molecular solid assuming Forster type energ y transfer. A Forster radius R-0 similar to 30 Angstrom is found to give go od agreement between experiment and simulations. In addition, the measureme nts indicate that for excitation energies >2.94 eV additional relaxation pr ocesses, ascribed to ultrafast intrachain vibrational relaxation, are opera tive.