The lowest metal-to-bipyridine charge-transfer excited state of [Cr(2,2′-bipyridine)(CO)4]

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Abstract
The DV-Xα molecular-orbital calculations have been carried out on [Crbpy(CO)4](bpy,2,2′-bipyridine) and its electron attachment and detachment products. The one-electron attachment of [Crbpy(CO)4] yields a complex of bipyridine anion radical, while the one-electron detachment results in ionization of the central metal atom. The lowest excited state of [Crbpy(CO)4] is the metal-to-ligand charge-transfer (MLCT) excited state which consists of the bipyridine anion radical and the central metal ionized. The transition-state calculation predicts the lowest MLCT excited states at around 23∼30×103 cm−1 and the lowest bipyridine (π,π*) excitations at 36×103 cm−1 and 42∼45×103 cm−1. The calculation also concludes that the MLCT excitation induces a counter migration of the other electrons not directly involved in the charge-transfer excitation. The configuration-interaction calculations predict the lowest MLCT excited singlet states at around 21∼35×103 cm−1 and the lowest bipyridine (π, π*) excited singlet states at 42∼62×103 cm−1, while the emissive lowest MLCT triplet state is at 17×103 cm−1. The transition moments evaluated with the transition-state wavefunctions can reproduce qualitatively the observed absorption spectral profile. The intensity of the MLCT transitions is obtained from the allowed (π, π*) transitions of coordinated bipyridine but is not due to the intrinsic transition moments of MLCT excitations.