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 (2)E excited state in a Cr(III) transition metal complex. Cr(acac)(3) (where acac is the deprotonated monoanion of acetylacetone) exhibits monophasic decay kinetics with tau = 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 (4)A(2) --> (4)T(2) absorption 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 (k(vib)) in the (2)E state. The data on Cr(acac)(3) indicate that intersystem crossing associated with the (4)T(2) --> (2)E conversion occurs at a rate k(ISC) > 10(13) s(-)(1) and furthermore competes effectively with vibrational relaxation in the initially formed (4)T(2) state. Excitation into the higher energy (4)LMCT state (lambda(ex) = 336 nm) gives rise to biphasic kinetics with tau( 1) = 50 +/- 20 fs and tau( 2) = 1.2 +/- 0.2 ps. The slower component is again assigned to vibrational cooling in the (2)E 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 Cr(III), these results reinforce the notion that the conventional picture of excited-state dynamics in which k(vib) > k(IC) > k(ISC) does not generally apply when describing excited-state formation in transition metal complexes.