Watching Excited State Dynamics with Optical and X-ray Probes: The Excited State Dynamics of Aquocobalamin and Hydroxocobalamin

Femtosecond time-resolved X-ray absorption(XANES) at the Co K-edge,X-ray emission (XES) in the Co K & beta; and valence-to-core regions,and broadband UV-vis transient absorption are combined to probethe femtosecond to picosecond sequential atomic and electronic dynamicsfollowing photoexcitation of two vitamin B-12 compounds,hydroxocobalamin and aquocobalamin. Polarized XANES difference spectraallow identification of sequential structural evolution involvingfirst the equatorial and then the axial ligands, with the latter showingrapid coherent bond elongation to the outer turning point of the excitedstate potential followed by recoil to a relaxed excited state structure.Time-resolved XES, especially in the valence-to-core region, alongwith polarized optical transient absorption suggests that the recoilresults in the formation of a metal-centered excited state with alifetime of 2-5 ps. This combination of methods provides auniquely powerful tool to probe the electronic and structural dynamicsof photoactive transition-metal complexes and will be applicable toa wide variety of systems.

Automated summary

Femtosecond time-resolved X-ray absorption, X-ray emission, and broadband UV-vis transient absorption were used to investigate the atomic and electronic dynamics of two vitamin B-12 compounds after photoexcitation. Sequential structural evolution and coherent bond elongation were observed, followed by recoil to a relaxed excited state structure. The formation of a metal-centered excited state with a lifetime of 2-5 ps was suggested by time-resolved X-ray emission and optical transient absorption.