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

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.