Vibrational Energy Relaxation of Metalloporphyrins in a Condensed Phase Probed by Time-Resolved Resonance Raman Spectroscopy

Abstract

Abstract Recent experimental work on vibrational energy relaxation of metalloporphyrins in a condensed phase carried out in this laboratory is summarized. 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. Results related to intramolecular relaxation of octaethylporphyrinato nickel (NiOEP) are described. Stokes Raman bands due to a photoproduct of NiOEP instantaneously appeared upon the photoexcitation. Their intensities decayed with a time constant of ∼300 ps, which indicates an electronic relaxation from the (d, d) excited state (B1g) to the ground state (A1g), being consistent with the results of transient absorption measurements. Anti-Stokes ν4 and ν7 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 ν4 intensity was instrument-limited, the rise of anti-Stokes ν7 intensity was delayed by 2.0 ± 0.4 ps, which indicates that intramolecular vibrational energy redistribution has not been completed in the subpicosecond time regime. To study the 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 myoglobin, temporal changes of the anti-Stokes Raman intensity of the ν4 and ν7 bands demonstrated immediate generation of a vibrationally excited heme upon photodissociation and subsequent decays of the excited populations, whose time constants were 1.1 ± 0.6 and 1.9 ± 0.6 ps, respectively. This direct monitoring of the cooling dynamics of the heme cofactor within the protein matrix allows the characterization of the vibrational energy flow through the protein moiety and to the water bath. For solute–solvent energy transfer process, low-frequency modes of proteins seem to be less important.