TRIPLET-STATE ELECTRON-TRANSFER PATHWAY FOR TETRABENZOPORPHYRIN DIPHENYLIODONIUM SALT PHOTOINITIATOR SYSTEM

A triplet state electron transfer mechanism for the photoreaction between meso-diphenyltetrabenzoporphyrin (P2TBP) metallo complexes, eg. magnesium (1a, MgP2TBP), zinc (1b, ZnP2TBP) and cadmium (1c, CdP2TBP), and diphenyliodonium (DPI) salt was confirmed by the detection of the cation radicals of P2TBP using nanosecond laser flash photolysis equipment, as expected from the estimation of the free energy changes (DELTA-G(T)). Rate constants ((T)k(q)) of the triplet state quenching of metallo P2TBP complexes by DPI, determined from the slopes of Stern-Volmer plots, approximately obeyed a diffusion-controlled limit in accordance with fully exothermic DELTA-G(T) values. Since the fluorescence of the magnesium and zinc complexes was quenched by DPI at high concentrations (above 10(-3) M), as previously reported, both excited states were deactivated by the onium salt, depending on its concentration. However, it was presumed that a triplet state electron transfer pathway was preferred for the cadmium complex because of its short fluorescence lifetime.