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CoVT_Manuscript_revised.pdf (601.15 kB)
Photoinduced Valence Tautomerism of a Cobalt-Dioxolene Complex Revealed with Femtosecond M-Edge XANES
Preprints are manuscripts made publicly available before they have been submitted for formal peer review and publication. They might contain new research findings or data. Preprints can be a draft or final version of an author's research but must not have been accepted for publication at the time of submission.
revised on 14.08.2019 and posted on 15.08.2019by Ryan Ash, Kaili Zhang, Josh Vura-Weis
Cobalt complexes that undergo charge-transfer induced spin-transitions (CTIST) or valence tautomerism (VT) from low spin (LS) Co(III) to high spin (HS) Co(II) are potential candidates for magneto-optical switches. We use M-edge XANES spectroscopy with 40 fs time resolution to measure the excited-state dynamics of Co(III)(Cat-N-SQ)(Cat-N-BQ), where Cat-N-BQ and Cat-N-SQ are the singly and doubly reduced forms of the 2-(2-hydroxy-3,5-di-tert-butylphenyl-imino)-4,6-di-tert-butylcyclohexa-3,5-dienone ligand. The extreme ultraviolet probe pulses, produced using a tabletop high-harmonic generation light source, measure 3p3d transitions and are sensitive to the spin and oxidation state of the Co center. Photoexcitation at 525 nm produces a low-spin Co(II) ligand-to-metal charge transfer state which undergoes intersystem crossing to high-spin Co(II) in 67 fs. Vibrational cooling from this hot HS Co(II) state competes on the hundreds-of-fs timescale with back-intersystem crossing to the ground state, with 60% of the population trapped in a cold HS Co(II) state for 24 ps. Ligand field multiplet simulations accurately reproduce the ground-state spectra and support the excited-state assignments. This work demonstrates the ability of M-edge XANES to measure ultrafast photophysics of molecular Co complexes.