Ultrafast Dynamics of 2E State Formation in Cr(acac)3.

Femtosecond time-resolved absorption spectroscopy has been used to elucidate the excited- state dynamics associated with formation of the ²E excited state in a CrIII transition metal complex. Cr- (acac)3 (where acac is the deprotonated monoanion of acetylacetone) exhibits monophasic decay kinetics with τ = 1.1 ± 0.1 Ps following excitation into the lowest-energy ligand-field absorption band; the time constant is found to be independent of both excitation and probe wavelength across the entire 4A2 → 4T2absorption envelope. The lack of a significant shift in the excited-state absorption spectrum combined with the observed spectral narrowing is consistent with an assignment of this process as vibrational cooling (kvib) in the ²E state. The data on Cr(acac)3 indicate that intersystem crossing associated with the 4T2→ ²E conversion occurs at a rate kISC > 1013 S-1 and furthermore competes effectively with vibrational relaxation in the initially formed 4T2 state. Excitation into the higher energy 4LMCT state (&lemda;ex = 336 nm) gives rise to biphasic kinetics with τ1 = 50 ± 20 fs and τ2 2 = 1.2 ± 0.2 Ps. The slower component is again assigned to vibrational cooling in the ²E state, whereas the subpicosecond process is attributed to conversion from the charge-transfer to the ligand-field manifold. In addition to detailing a process central to the photophysics of CrIII, these results reinforce the notion that the conventional picture of excited-state dynamics in which kvib > kIC > kISC does not generally apply when describing excited-state formation in transition metal complexes. [ABSTRACT FROM AUTHOR]