A role of solvent in vibrational energy relaxation of metalloporphyrins

Abstract

Abstract The formation of a vibrationally excited photoproduct of metalloporphyrins upon (π, π*) excitation and its subsequent vibrational energy relaxation were monitored by picosecond time-resolved resonance Raman spectroscopy. Stokes Raman bands due to a photoproduct of nickel octaethylporphyrin (NiOEP) instantaneously appeared upon the photoexcitation. Their intensities decayed with a time constant of ∼300 ps, which indicates electronic relaxation from the ( d, d ) excited state (B 1g ) to the ground state (A 1g ), being consistent with the results of transient absorption measurements by Holten and coworkers. Anti-Stokes ν 4 and ν 7 bands for vibrationally excited ( d, d ) state of NiOEP decayed with time constants of ∼10 and ∼300 ps. The former is ascribed to vibrational relaxation, while the latter corresponds to the electronic relaxation from the ( d, d ) excited state to the electronic ground state. While the rise of anti-Stokes ν 4 intensity was instrument-limited, the rise of anti-Stokes ν 7 intensity was delayed by 2.6±0.5 ps, which indicates that intramolecular vibrational energy redistribution has not been completed in subpicosecond time regime. To study a mechanism of intermolecular energy transfer, solvent dependence of the time constants of anti-Stokes kinetics was investigated using various solvents. No significant solvent dependence of the rise and decay constants was observed for NiOEP. For an iron porphyrin, we observed two phases in intermolecular energy transfer. The fast phase was insensitive to solvent and the slow phase depended on solvents. A model of classical thermal diffusion qualitatively reproduced this behavior. For solute-solvent energy transfer process, low-frequency modes of proteins seem to be less important.