Tracking the Metal-Centered Triplet in Photoinduced Spin Crossover of Fe(phen)32+ with Tabletop Femtosecond M-edge XANES
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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 5T2g
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 5T2g 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 5T2g 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 M2,3-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.