Electronic structure and circular dichroism of tris(bipyridyl) metal complexes within density functional theory

Time-dependent density functional theory (TD-DFT) has been employed to calculate the electronic circular dichroism (CD) spectra of tris-bidentate iron group complexes [M(L-L)₃]²⁺ (M = Fe, Ru, Os; L-L = 2, 20-bipyridine). The simulated CD spectra are compared with the experiment, and reasonably good agreement is obtained. In this study, much effort has been made to interpret the exciton CD arising from the long-axis-polarized φ → φ excitations in the ligands of the complexes. Metal-ligand orbital interactions as well as the origin of the optical activity of the exciton transitions have been elucidated in connection with the detailed analysis of the TD-DFT results within a general model that is applicable to similar chiral compounds.