Mode Dependence of Vibrational Energy Redistribution in Nickel Tetraphenylporphyrin Probed by Picosecond Time-Resolved Resonance Raman Spectroscopy: Slow IVR to Phenyl Peripherals

Abstract

Abstract The formation of the (d, d) excited state of (meso-tetraphenylporphyrinato)nickel(II) ([Ni(tpp)]) upon (π, π*) excitation, and its vibrational energy relaxation were monitored by picosecond time-resolved resonance Raman spectroscopy. Stokes resonance Raman bands due the (d, d) excited state instantaneously appeared upon the photoexcitation into the (π, π*) excited state. Their intensities decayed with a time constant of about 250 ps, which corresponds to electronic relaxation from the (d, d) excited state to the electronic ground state. This is consistent with the results of ultrafast absorption measurements reported by Eom et al. [H. S. Eom, S. C. Jeoung, D. Kim, J. H. Ha, and Y. R. Kim, J. Phys. Chem. A, 101, 3661 (1997)]. Anti-Stokes ν4 (macrocycle in-plane mode) intensities of [Ni(tpp)] in the (d, d) excited state appeared promptly and decayed with a time constant of 3.6 ± 0.6 ps. The rise and decay of anti-Stokes intensity are interpreted as vibrational excitation due to the excess energy and intermolecular vibrational energy transfer to the surrounding solvent molecules, respectively. The φ4 mode, which is mainly ν(CC) of the peripheral phenyl groups, gave no detectable anti-Stokes intensity although the mode gave appreciable Stokes intensity. This means that the φ4 mode is left vibrationally less excited than the ν4 mode in the process of vibrational energy relaxation and that intramolecular vibrational energy redistribution is not completed in a subpicosecond time regime. These results for [Ni(tpp)] demonstrate that the vibrational modes of peripheral groups are vibrationally less excited shortly after the formation of the (d, d) excited state and that energy redistribution in the peripheral groups takes place in picoseconds, such a short time is competitive with vibrational energy transfer to the surrounding solvent molecules.