Tracking the Metal-Centered Triplet in Photoinduced Spin Crossover of Fe(phen)32+ with Tabletop Femtosecond M-edge XANES

Fe(II) coordination complexes are promising alternatives to Ru(II) and Ir(III) chromophores for photoredox chemistry and solar energy conversion, but rapid deactivation of the initial metal-to-ligand charge transfer (MLCT) state to low-lying (d,d) states limits their performance. Relaxation to a ⁵T_2g state is postulated to occur via a metal-centered triplet state, but this mechanism remains controversial. We use femtosecond extreme ultraviolet (XUV) transient absorption spectroscopy to measure the excited-state relaxation of Fe(phen)₃²⁺ and conclusively identify a ³T intermediate that forms in 170 fs and decays to a vibrationally hot ⁵T_2g state in 40 fs. A coherent vibrational wavepacket with a period of 250 fs and damping time of 0.66 ps is observed on the ⁵T_2g surface, and the spectrum of this oscillation serves as a fingerprint for the Fe-N symmetric stretch. The results show that the shape of the M_2,3-edge X-ray absorption near edge structure (XANES) spectrum is sensitive to the electronic structure of the metal center, and the high spin sensitivity, fast time resolution, and tabletop convenience of XUV transient absorption make it a powerful tool for studying the complex photophysics of transition metal complexes.