Femtosecond spectroscopic study of MLCT excited-state dynamics of Cr(CO)4(bpy):: Excitation-energy-dependent branching between CO dissociation and relaxation

The excited-state dynamics of Cr(CO)(4)(bpy) were studied with pump-probe time-resolved absorption spectroscopy in pyridine and CH2Cl2 solutions. Samples were excited into the strong Cr --> bpy metal-to-ligand charge-transfer (MLCT) absorption band by tunable laser pulses. It was found that the Cr(S)(CO)(3)(bpy) photoproduct (S = solvent) is formed completely within the time resolution of the experiment, that is ca. 600 fs, parallel with the population of two unreactive excited states which act as excitation energy traps. The photoproduct does not undergo any changes during the time range investigated (0-2 ns), while the two "trapping states" decay, in pyridine, with lifetimes of 8 and 87 ps, respectively. Branching of the evolution of the optically prepared Franck-Condon MLCT state between the reactive and relaxation channels is highly dependent on the excitation energy and limits the photochemical quantum yield of CO dissociation. The course and outcome of the photochemistry of Cr(CO)(4)(bpy), and of many analogous MLCT-active organometallic compounds, is obviously determined at the earliest times after optical excitation. In addition to the excited-state dynamics, the reorientation time of ground-state Cr(CO)(4)(bpy) molecules was measured to be 32 ps inpyridine at room temperature.