Intramolecular vibrational energy redistribution and intermolecular energy transfer in the (d, d) excited state of nickel octaethylporphyrin

The formation of a vibrationally excited photoproduct of nickel octaethylporphyrin (NiOEP) upon (pi, pi*) excitation and its subsequent vibrational energy relaxation were monitored by picosecond time-resolved resonance Raman spectroscopy. Stokes Raman bands due to the photoproduct instantaneously appeared upon the photoexcitation. Their intensities decayed with a time constant of similar to 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 co-workers [D. Kim, C. Kirmaier, and D. Holten, Chem. Phys. 75, 305 (1983); J. Rodriguez and D. Holten, J. Chem. Phys. 91, 3525 (1989)]. The Raman frequencies of NiOEP in the (d, d) excited state are shifted to lower frequencies compared to those of the ground state species, and it is reasonably interpreted by the core size expansion of the macrocycle by 0.05 Angstrom upon the electron promotion from the d(z2) to the d(x2-y2) orbital. Anti-Stokes nu(4) intensity in the vibrationally excited (d, d) state of NiOEP appeared promptly and decayed with time constants of 11 +/- 2 and 330 +/- 40 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. In contrast, the rise of anti-Stokes nu(7) intensity was not instantaneous, but delayed by 2.6 +/- 0.5 ps, which indicates that intramolecular vibrational energy redistribution has not been completed in subpicosecond time regime. The peak position of the nu(4) band shifted by nearly 5 cm(-1) between 0 and 50 ps. The time constant for the shift of the nu(4) band was 9.2 +/- 1.3 ps, which was close to that for the fast component of intensity decay of anti-Stokes bands. The nu(4) band became narrower and symmetric as the delay time increases. These can be ascribed to intramolecular anharmonic coupling of the nu(4) mode with the low frequency modes. The intra- and intermolecular vibrational energy relaxation in the metal excited state will be discussed. (C) 1999 American Institute of Physics. [S0021-9606(99)00343-8].