First observation in the gas phase of the ultrafast electronic relaxation pathways of the S2 states of heme and hemin

Abstract

The time evolution of electronically excited heme (iron II protoporphyrin IX, [Fe^II PP]) and its associated salt hemin (iron III protoporphyrin IX chloride, [Fe^III PP–Cl]), has been investigated for the first time in the gas phase by femtosecond pump–probe spectroscopy. The porphyrins were excited at 400 nm in the S₂ state (Soret band) and their relaxation dynamics was probed by multiphoton ionization at 800 nm. This time evolution was compared with that of the excited state of zinc protoporphyrin IX [Zn PP] whose S₂ excited state likely decays to the long lived S₁ state through a conical intersection, in less than 100 fs. Instead, for [Fe^II PP] and [Fe^III PP–Cl], the key relaxation step from S₂ is interpreted as an ultrafast charge transfer from the porphyrin excited orbital π* to a vacant d orbital on the iron atom (ligand to metal charge transfer, LMCT). This intermediate LMCT state then relaxes to the ground state within 250 fs. Through this work a new, serendipitous, preparation step was found for Fe^II porphyrins, in the gas phase.