PHOTOOXIDATION AND ELECTRON-TRANSFER PROCESSES IN FREE-BASE TETRAPHENYLPORPHIN PROBED BY RESONANCE RAMAN-SPECTROSCOPY

We report here the resonance Raman studies of photooxidation of free base tetraphenylporphin (H2TPP) in the presence of external electron acceptors such as CCl4 and chloranil under selective laser irradiation. From the dependence of photooxidation on the concentration of electron acceptors, polarity of solvents, excitation lines and temperatures, we have inferred that a weak triplet exciplex formed between the excited H2TPP and electron acceptor in non-polar solvents serves as transient species and the light-induced intermolecular charge transfer from HTPP to the electron acceptor is the primary process involved in photooxidation. Observation of partial photooxidation in the rigid matrix at low temperatures has been interpreted to be due to long-range quantum mechanical electron tunneling process. Almost complete photooxidation is observed in a soft matrix as the donor and acceptor molecules can attain favorable relative orientation and separation for electron transfer during the excited state lifetime of the exciplex.