Ultrafast energy relaxation and excitation delocalization in excited states of zinc porphyrin dimers and trimer

Ultrafast excited-state relaxation process has been studied with zinc porph yrin dimers and circular trimer. Following 80 fs excitation at Sorer band ( 420 nm) or Q band (580 nm) of zinc porphyrin, the fluorescence decay curves exhibit ultrafast decays with lifetimes of 80 fs in o-dimer, 450 fs in tri mer and 540 fs in m-dimer. The time-resolved fluorescence spectra show that the fast decay process correspond to disappearance of monomer-like emissio n followed by red-shifted and broaden spectra. These ultrafast processes ar e assigned as due to excitation transfer among monomers and delocalization of excitation yielding excitonic states.