화학공학소재연구정보센터
Journal of the American Chemical Society, Vol.127, No.18, 6857-6865, 2005
Ultrafast dynamics of E-2 state formation in Cr(acac)(3)
Fermtosecond time-resolved absorption spectroscopy has been used to elucidate the excitedtate 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 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 (4)A(2) → T-4(2) absorption envelope. The lack of a significant shift in the excited-state absorption spectrum combined with tie observed spectral narrowing is consistent with an assignment of this process as vibrational cooling (k(vib)) in the E-2 state. The data on Cr(acac)(3) indicate that intersystem crossing associated with the T-4(2) → E-2 conversion occurs at a rate k(ISC) > 10(13)s(-1) and furthermore competes effectively with vibrational relaxation in the initially formed T-4(2) state. Excitation into the higher energy (LMCT)-L-4 state (ℷ(ex) = 336 nm) gives rise to biphasic kinetics with τ (1) = 50 ± 20 fs and τ (2) = 1.2 ± 0.2 ps. The slower component is again assigned to vibrational cooling in the E-2 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.