Competition Between Intersystem Crossing and Vibrational Cooling in Photoinduced Valence Tautomerism of a Cobalt-Dioxolene Complex Revealed with Femtosecond M-Edge XANES

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 3p3d 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.