Theoretical studies of ground and excited electronic states in a series of halide rhenium(I) bipyridine complexes

Density functional theory (DFT) is applied to analyze ground- and excited-state properties of the Re(l) halide bipyridine complex ReCl(CO)(3)(bpy) (1) and the related complexes ReCl(CO)3(5,5'-dibromo-bpy) (2), ReCl(CO)(3)(4,4'-dimethyl-bpy) (3), and ReCl(CO)(3)(4,4'-dimethylformyl-bpy) (4) (where bpy = 2,2'bipyridine). The electronic properties of the neutral molecules, in addition to the positive and negative ions, are studied using the B3LYP functional. Excited singlet and triplet states are examined using time-dependent DFT (TDDFT). The low-lying excited-state geometries are optimized at the ab initio configuration interaction singlets (CIS) level. As shown, the occupied orbitals involved in the transitions have a significant mixture of the metal Re and the group Cl, by the amount of metal 5d character which varies from 30 to 65%. The lowest unoccupied molecular orbital (LUMO) is a pi* orbital of the ligand bpy for the series of molecules. The TDDFT result indicates that the absorption maxima are at relatively high energy and are mainly assigned to bpy-based pipi* transitions with somewhat metal-to-ligand charge transfer (MLCT) [d(Re) --> pi*(bpy)] and ligand-to-ligand charge transfer (LLCT) [p(Cl) --> pi*(bpy)] except for complex 3, in which this band is mainly assigned to mixed MLCT/LLCT, and overlaps bpy pipi* character. All the low-lying transitions are categorized as mixed MLCT/LLCT. The absorption bands are blue shifted when substituted by an electron-releasing group (-CH3) and they are red shifted when substituted by an electron-withdrawing group (-Br or -COOCH3). The luminescence of all complexes is assigned as a triplet metal/chlorine to bpy charge transfer (MLCT/LLCT).