Photoreactivity of Cr(CO)₄(2,2‘-Bipyridine): Quantum Chemistry and Photodissociation Dynamics

Metal-to-ligand charge-transfer (MLCT) excited-state dynamics of Cr(CO)4(bpy) (bpy = 2,2‘-bipyridine) have been investigated through wave packet propagations on CASSCF/MR-CCI potentials and compared with the results of recent femtosecond time-resolved spectroscopic study. The calculated spectrum is characterized by two peaks, one centered at 21 000 cm-1 and assigned to low-lying metal-to-bipyridine charge transfer states, the second one centered at 29 000 cm-1 and assigned to a second series of metal-to-bipyridine charge transfer states and to the lowest metal-centered (MC) states. Visible irradiation into the first band brings the molecule in the b1A1 MLCT excited state calculated at 21 710 cm-1, with an oscillator strength f ≈ 0.24 and corresponding to the 3dxz → excitation. The evolution of the wave packet along the Cr−COax bond elongation shows an ultrafast direct dissociation (in a few hundred femtoseconds) of the axial carbonyl ligand, in agreement with the observed photodissociation dynamics, which points to a carbonyl departure in less than 400 fs. The calculated probability of dissociation of the carbonyl ligand after excitation into the second band is lower than 10% in 1ps.