Dipolar character of ligand-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 +/- 0.3 D/f for Ru, 3.5 +/- 0.3 D/f for Os, 2.2 +/- 0.2 D/f for Fe, 1.4 +/- 0.2 D/f for Zn and 0.5 +/- 0.5 D/f for free bpy (f is the local field correction: 1.1-1.3). The values of the excited state dipole moments roughly correlate with the separation between the UV ligand-centered bands and the visible MLCT bands. possible mixing of ligand-centered excited states with MLCT states is discussed.