Femtosecond fluorescence dynamics of porphyrin in solution and solid films: The effects of aggregation and interfacial electron transfer between porphyrin and TiO₂

The excited-state relaxation dynamics of a synthetic porphyrin, ZnCAPEBPP, in solution, coated on a glass substrate as solid films, mixed with PMMA and coated on a glass substrate as solid films, and sensitized on nanocrystalline TiO₂ films were investigated by using femtosecond fluorescence up-conversion spectroscopy with excitation in the Soret band, S₂. We found that the S₂→S₁ electronic relaxation of ZnCAPEBPP in solution and on PMMA films occurs- in 910 and 690 fs, respectively, but it becomes extremely rapid, <100 fs, in solid films and TiO₂ films due to formation of porphyrin aggregates. When probed in the Si state of porphyrin, the fluorescence transients of the solid films show a biphasic kinetic feature with the rapid and slow components decaying in 1.9-2.4 and 19-26 ps, respectively. The transients in ZnCAPEBPP/TiO₂ films also feature two relaxation processes but they occur on different time scales, 100-300 fs and 0.8-4.1 ps, and contain a small offset. According to the variation of relaxation period as a function of molecular density on a TiO ₂ surface, we assigned the femtosecond component of the TiO ₂ films as due to indirect interfacial electron transfer through a phenylethynyl bridge attached to one of four meso positions of the porphyrin ring, and the picosecond component arising from intermolecular energy transfer among porphyrins. The observed variation of aggregate-induced relaxation periods between solid and TiO₂ films is due mainly to aggregation of two types: J-type aggregation is dominant in the former case whereas H-type aggregation prevails in the latter case.