Dipolar character of l&and-centered transitions in transition metal tris-bipyridyl complexes’

The Stark spectra of bipyridine (bpy) and the metaltris(2,2’-bipyridine) complexes of Zn, Fe, Ru and OS have been measured in the ligand-centered region in the near UV. The results show that excitation into these ligand-centered absorption bands of the metal complexes leads to excited states with dipole moments that are substantially larger than would be expected were the transitions purely ligand-centered. The difference dipoles between ground and excited states are 4.6 f 0.3 D/j for Ru, 3.5 f 0.3 D/j for OS, 2.2 f 0.2 Dlf for Fe, 1.4 i 0.2 Dlffor Zn and 0.5 f 0.5 Dlffor free bpy (fis the local field correction: 1.1-1.3). The values of the excited state dipole moments roughly correlate with the separation between the W ligand-centered bands and the visible MLCT bands. Possible mixing of ligand-centered excited states with Mm states is discussed. Keyworrfs: Tris-bipyridyl complexes; Stark spectra; Zn complexes; UV spectroscopy