Direct observation of competitive ultrafast CO dissociation and relaxation of an MLCT excited state: picosecond time-resolved infrared spectroscopic study of [Cr(CO)(4)(2,2'-bipyridine)].
Early excited-state dynamics of [Cr(CO)(4)(bpy)] were studied in a CH(2)Cl(2) solution by picosecond time-resolved IR spectroscopy, which made it possible to characterize structurally the individual species involved and to follow separately the temporal evolution of the IR bands due to the bleached ground-state absorption, the fac-[Cr(CO)(3)(Sol)(bpy)] photoproduct, and two (3)MLCT states. It was found that the fac-[Cr(CO)(3)(Sol)(bpy)] photoproduct is formed alongside population of two (3)MLCT states during the first picosecond after excitation at 400 or 500 nm by a branched evolution of the optically populated excited state. Vibrationally relaxed (3)MLCT excited states are unreactive, decaying directly to the ground state on a picosecond time scale. The photoproduct is long-lived, persistent into the nanosecond time domain. Changing the excitation wavelength from 400 to 500 nm strongly increases the extent of the bleach recovery and decreases the yield of the photoproduct formation relative to the initial yield of the population of the unreactive (3)MLCT states. The photochemical quantum yield of CO dissociation also decreases with increasing excitation wavelength (Víchová, J.; Hartl, F.; Vlcek, A., Jr. J. Am. Chem. Soc. 1992, 114, 10903). These observations demonstrate the relationship between the early dynamics of optically populated excited states and the overall outcome of a photochemical reaction and identify the limiting role of the branching of the initial excited-state evolution between reactive and relaxation pathways as a more general principle of organometallic photochemistry.