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 Fe II 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 ν 3 and ν 4 act as “bottleneck” vibrational states, while ν CH and ν 7 couple more effectively to the environment.