Excitation delocalization and relaxation in linear and circular arrays of porphyrins

Photophysical processes such as excitation energy transfer and electron transfer are expected to occur very fast in organized molecular systems in which reactant molecules are arranged with close proximity and optimal orientation, and they can be coupled to an adjacent molecule with relatively strong molecular interaction. In an extreme of such strongly interconnected molecular systems, the photoinduced reaction can compete with or exceed the vibrational relaxation from vibrational levels initially photoexcited. We have investigated ultrafast excitation migration and relaxation in circular and linear arrangements of zinc porphyrin (ZnP) with a femtosecond fluorescence up-conversion method and a ps time-correlated photon-counting method. The time-resolved fluorescence spectra revealed non-equilibrium processes of excitation delocalization and transport in femtosecond and picosecond time scales.