Spectroscopic properties of a self-assembled zinc porphyrin tetramer II. Time-resolved fluorescence spectroscopy

Excited-state kinetics of complexes of a functionalized zinc tetraphenylporphyrin (ZnTPP) derivative, zinc mono(4-pyridyl)triphenylporphyrin (ZnPyP) in toluene and polystyrene/toluene mixtures have been investigated by time-resolved fluorescence spectroscopy. In addition to the ~2.0 ns monomer fluorescence lifetime, a 1.5 ns component was found by applying global analysis to the time-resolved fluorescence decay. The 1.5 ns component is assigned to a cyclic porphyrin tetramer [Part I], with a 1 ns rotational correlation time at 10 C. The initial fluorescence anisotropy of the monomer is found to be 0.1. In the tetramer an additional depolarization process occurs with a correlation time of ~31 ps, resulting in a further decrease of the anisotropy from 0.1 to 0.025. This additional depolarization is ascribed to singlet energy transfer between the porphyrin units that constitute the tetramer. The intramolecular energy transfer processes have been simulated using the Monte Carlo method, yielding rate constants of (26 ± 4 ps)-1 and (180 ps)-1 for energy transfer between nearest neighbor and next nearest neighbor porphyrins in the tetramer