Transient Raman observations of heme vibrational dynamics in five-coordinate iron porphyrins

Abstract

Transient resonance Raman spectroscopy has been used to study vibrational dynamics in five-coordinate, high-spin FeII octaethyl porphyrin with a 2-methyl imidazole axial ligand. Vibrational populations of the porphyrin ground electronic state were probed by examining Stokes and anti-Stokes Raman scattering as a function of incident laser flux using ∼10 nanosecond pulses in resonance with the Soret electronic transition. Within a single pulse, each molecule goes through several excitation-decay cycles, building up a non-equilibrium, excited vibrational energy distribution that is exquisitely sensitive to the vibrational mode lifetimes and to the incident laser flux. A kinetic model illustrates these ideas and provides strong support for the interpretation of the results. The flux dependence of the Raman intensities, positions and linewidths suggests that ν3 and ν4 act as “bottleneck” vibrational states, while νCH and ν7 couple more effectively to the environment.