TY - JOUR
T1 - Tracking the Metal-Centered Triplet in Photoinduced Spin Crossover of Fe(phen)3 2+ with Tabletop Femtosecond M-Edge X-ray Absorption Near-Edge Structure Spectroscopy
AU - Zhang, Kaili
AU - Ash, Ryan
AU - Girolami, Gregory S.
AU - Vura-Weis, Josh
N1 - This material is based upon work supported by the National Science Foundation under Grant no. 1555245. The transient XUV instrument, including the nonlinear optical parametric amplifier, was built with funding from the Air Force Office of Scientific Research under AFOSR Award no. FA9550-14-1-0314. G.S.G. acknowledges the support of the William and Janet Lycan Fund at the University of Illinois.
PY - 2019/10/30
Y1 - 2019/10/30
N2 - 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 long-lived quintet 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)3 2+ and conclusively identify a 3T intermediate that forms in 170 fs and decays to a vibrationally hot 5T2g state in 39 fs. A coherent vibrational wavepacket with a period of 249 fs and damping time of 0.63 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.
AB - 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 long-lived quintet 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)3 2+ and conclusively identify a 3T intermediate that forms in 170 fs and decays to a vibrationally hot 5T2g state in 39 fs. A coherent vibrational wavepacket with a period of 249 fs and damping time of 0.63 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.
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U2 - 10.1021/jacs.9b07332
DO - 10.1021/jacs.9b07332
M3 - Article
C2 - 31587557
AN - SCOPUS:85073737631
SN - 0002-7863
VL - 141
SP - 17180
EP - 17188
JO - Journal of the American Chemical Society
JF - Journal of the American Chemical Society
IS - 43
ER -