Femtosecond X-Ray Scattering Study of Ultrafast Photoinduced Structural Dynamics in Solvated [Co(terpy)2]2+

TL;DR

This study uses ultrafast x-ray scattering to observe the real-time structural changes in a cobalt complex after photoexcitation, revealing bond elongation, vibrational oscillations, and the formation of a high spin state within a few picoseconds.

Contribution

First direct observation of ultrafast structural dynamics in [Co(terpy)2]2+ using femtosecond x-ray scattering combined with DFT calculations.

Findings

Co-N bonds elongate upon photoexcitation

Co-N vibrational mode oscillates with a 0.33 ps period

High spin state forms within 1 ps and lasts about 7 ps

Abstract

We study the structural dynamics of photoexcited [Co(terpy)2]2+ in an aqueous solution with ultrafast x-ray diffuse scattering experiments conducted at the Linac Coherent Light Source. Through direct comparisons with density functional theory calculations, our analysis shows that the photoexcitation event leads to elongation of the Co-N bonds, followed by coherent Co-N bond length oscillations arising from the impulsive excitation of a vibrational mode dominated by the symmetrical stretch of all six Co-N bonds. This mode has a period of 0.33 ps and decays on a subpicosecond time scale. We find that the equilibrium bond-elongated structure of the high spin state is established on a single-picosecond time scale and that this state has a lifetime of ~ 7 ps.