Vibrational coherence in the excited state dynamics of Cr(acac)3: probing the reaction coordinate for ultrafast intersystem crossing
- 25 June 2010
- journal article
- research article
- Published by Royal Society of Chemistry (RSC) in Chemical Science
- Vol. 1 (3), 405-410
- https://doi.org/10.1039/c0sc00262c
Abstract
Vibrational coherence was observed following excitation into the lowest-energy spin-allowed 4A2 → 4T2 ligand-field absorption of Cr(acac)3 . The transient kinetics were fit to a rapidly damped 164 cm−1 oscillatory component, the frequency of which is not associated with the ground state of the molecule. The signal is assigned as an excited-state vibrational coherence; the timescale of the event suggests that this vibrational coherence is retained during the 4T2 → 2E intersystem crossing that immediately follows 4A2 → 4T2 excitation. DFT calculations indicate that the 164 cm−1 oscillation likely corresponds to a combination of Cr–O bond stretching in the ligand-field excited state as well as large amplitude motion of the ligand backbone. This hypothesis is supported by ultrafast time-resolved absorption measurements on Cr(t-Bu-acac)3 (where t-Bu-acac is the monoanionic form of 2,2,6,6-tetramethyl-3,5-heptanedione) – an electronically similar but more sterically encumbered molecule – which exhibits a 4T2 → 2E conversion that is more than an order of magnitude slower than that observed for Cr(acac)3 . These results provide important insights into the nature of the reaction coordinate that underlies ultrafast excited-state evolution in this prototypical coordination complex.Keywords
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