Vibronic effects in photolysis of metalloporphyrins and hemoproteins

A discussion of the origin and mechanism of photolysis of 3dⁿ metalloporphyrin and hemoprotein complexes with diatomics (CN, CO, NO and O₂) based on the pseudo Jahn-Teller effect is given. It is shown that the vibronic mixing of the photoexcited state with a higher one, formed by an electron transition of the e type MO to the a₁ (d₂2)MO, is most essential in the phenomenon under consideration. This mixing induced by E type nuclear displacements causes the photodecay of the complexes under consideration along the normal E coordinate, which is a superposition of two symmetrized distortions corresponding to the ligand shift as a whole in parallel to the porphyrin ring and its rotation about the axis perpendicular to both the molecule and its shift. A number of experimental data including the fact of high photolysis quantum yield (φ{symbol}L ≈ 1) in the case of ligand linear-end-on coordination and low quantum yield (φ{symbol}L ≈ 10^-4-10^-2) in the case of bent coordination are qualitatively understood in this scheme. Some experimental possibilities are proposed in order to get further verification of the suggested photolysis mechanism and to clarify its origin in more detail.