PHOTOCHEMISTRY OF METALLOPORPHYRINS IN POLYMER MATRICES - LASER PHOTOLYSIS STUDIES OF CHLOROCHROMIUM(III) TETRAPHENYLPORPHYRIN IN POLYSTYRENE FILMS IN THE TEMPERATURE-RANGE 77-300 K

Nanosecond laser photolysis studies of chloropyridinato- and chloroaqu achromium(III) tetraphenylporphyrin, (ClCrTPP)-T-III(Py) and (ClCrTPP) -T-III(H2O), dissolved in polystyrene films have been carried out in t he temperature range 77-300 K. The transient observed for (ClCrTPP)-T- III(Py) after laser pulsing is solely the excited states (the tripquar tet, T-4(1), and tripsextet, T-6(1), states). The emission from the tr ipquartet state, T-4(1), is detected upon laser excitation of (ClCrTPP )-T-III(Py). No photodissociation of axial pyridine is detected in the temperature range studied. The laser photolysis of (ClCrTPP)-T-III(H2 O) in polystyrene films gives two transients; the excited states (T-4( 1) and T-6(1)) Of (ClCrTPP)-T-III(H2O) and the five coordinate (ClCrTP P)-T-III yielded by the photodissociation of axial H2O from (ClCrTPP)- T-III(H2O). The yields of the excited states (T-4(1) and T-6(1)) for b oth (ClCrTPP)-T-III(Py) and (ClCrTPP)-T-III(H2O) were found to increas e with a decrease in temperature. The lifetimes and the relative yield s of the excited states and the relative yields of the photodissociati on of the axial ligands were measured in the temperature range 77-300 K, These results lead to the conclusions that (1) the energy gaps betw een the T-4(1) and T-6(1) states are 1.5 and 1.1 kcal mol(-1) for (ClC rTPP)-T-III(Py) and (ClCrTPP)-T-III(H2O), respectively, (2) the full p hotodissociation of axial H2O from (ClCrTPP)-T-III(H2O) occurs from th e singquartet state, S-4(1), and (3) the photodissociation of the axia l ligand Is the major pathway for excitation energy dissipation at the S-4(1) state of six-coordinate chromium(lII) porphyrin.