(Invited) What Did the Metals Know, and When Did They Know It? Femtosecond M-Edge XANES Reveals Short-Lived States in Transition Metal Catalysts

Abstract

X-ray absorption near edge spectroscopy (XANES or NEXAFS) is a powerful technique for electronic structure determination. However, widespread use of femtosecond XANES is limited by the need for scare free-electron laser beamtime. We have developed M2,3-edge XANES, corresponding to 3p→3d transitions, as a reliable technique for measuring the electronic structure of first-row transition metal coordination complexes. The tabletop high-harmonic source provides femtosecond time resolution and 24/7 acessiblility. This talk present the photophysics of three transition metal complexes relevant to photocatalysis: Iron(III) tetraphenyl porphyrin chloride, which relaxes through a ligand-to-metal charge transfer state; Nickel (II) octaethylporphyrin, which relaxes through a 3(d,d) state; and a Co4O4 cubane that catalyzes water oxidation through one of two proposed pathways. We demonstrate M-edge XANES as a powerful new tool for identification of catalytic intermediates.