LASER-POWER-DEPENDENT COORDINATION AND PHOTOOXIDATION OF ZINC TETRAPHENYLPORPHIN IN ALKYL CHLORIDES PROBED BY RESONANCE RAMAN-SPECTROSCOPY

Changes in the axial ligation and pi-cation radical formation of zinc tetraphenylporphin (ZnTPP) in alkyl chlorides as a function of laser power have been probed by the resonance Raman technique. ZnTPP forms weakly bound pentacoordinate species in the ground state at low laser power (< 8 mW at the sample) which dissociate mainly by a thermal process into tetracoordinate, neutral ZnTPP or its pi-radical cation due to electron transfer from excited ZnTPP to CCl4 at higher laser powers. From the dependence of photo-oxidation on the concentration of electron acceptors, excitation wavelength and solvents, it is inferred that a weak triplet exciplex is formed between the excited ZnTPP and electron acceptor which serves as transient species and that light-induced intermolecular charge transfer from ZnTPP to CCl4 is the primary proces involved in photo-oxidation.