Long-range singlet energy transfer in perylene bis(phenethylimide) films

The distance over which singlet energy is transferred in polycrystalline films of perylene bis(phenethylimide), PPEI, was measured by a surface quenching technique in films ranging in thickness from 0.04 to 2.3 mu m. Radiative energy transfer was not observed. Accurate values of the exciton transfer length could be obtained only with quenchers exhibiting rapid surface quenching velocities (> 10(5) cm/s), such as poly(3-methylthiophene). The measured singlet exciton transfer length of 2.5 +/- 0.5 mu m is apparently the longest yet reported. Its approximate value can be inferred directly from the experimental data and is therefore essentially independent of the assumed theoretical model. Our measurements contain no direct information about the mechanism of the exciton motion; however, if it is assumed to be diffusional, the calculated intermolecular exciton hopping time, tau(h) much less than 100 fs, is unusually fast. This suggests that excitons, in fact,may be delocalized over a number of molecules and that coherent energy transfer plays some role in the exciton motion. Energy is apparently transferred further and faster in PPEI films than in natural photosynthetic light-harvesting systems.