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

Vibrational coherence was observed following excitation into the lowest-energy spin-allowed (4)A(2) -> T-4(2) 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 T-4(2) -> E-2 intersystem crossing that immediately follows (4)A(2) -> T-4(2) 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 T-4(2) -> E-2 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.