Laser-power-dependent coordination and photo-oxidation of zinc tetraphenylporphin in alkyl chlorides probed by resonance Raman spectroscopy

Abstract

Changes in the axial ligation and π-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 π-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.