Vibrational coherence in the excited state dynamics of Cr(acac)3: probing the reaction coordinate for ultrafast intersystem crossing

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.