Molecular and electronic structure in the metal-to-ligand charge transfer excited states of fac-[Re(4,4'-X2bpy)(CO)3(4-Etpy)]+* (X = CH3, H, Co2Et). Application of density functional theory and time-resolved infrared spectroscopy

Density functional theory (DFT) calculations have been conducted on the ground and metal-to-ligand charge transfer (MLCT) excited states of the series fac-[Re(4,4'-X 2 bpy)(CO) 3 (4-Etpy)](PF 6 ) (X = CH 3 , H, and CO 2 Et; 4-Etpy is 4-ethylpyridine). The energy gap varies across this series, influencing excited-state geometries and electronic structures. The DFT calculations assist in assigning v(CO) bands in the infrared and give insight into variations in the experimental values. The predicted bond length and angle changes in the excited state point to the importance of Re-CO a bond polarization in the excited states as well as π*(4,4'-X 2 -bpy)-π*(CO) mixing suggested previously.