Excited-State Dipole Moments of Homoleptic [Ru(bpy′)3]2+ Complexes Measured by Stark Spectroscopy

The visible absorption and Stark spectra of five [Ru(4,4'-(R)(2)-2,2'-bipyridine)(3)](PF6)(2) and [Ru(bipyrazine)(3)](PF6)(2) complexes, where R = CH3O-, tert-butyl-, CH3-, H-, or CF3-, were obtained in butyronitrile glasses at 77 K as a function of an applied electric field in the 0.2-0.8 MV/cm range. Analysis of the metal-to-ligand charge -transfer (MLCT) absorption and Stark spectra with the Liptay treatment revealed dramatic light-induced dipole moment changes, Delta(mu) over bar = 5-11 D. The application of a two-state model to the Delta(mu) over bar values provided metal-ligand electronic coupling, H-DA = 3100-4500 cm(-1). The ground state of these complexes has no net dipole moment and with the Ru-II center as the point of reference, the dipole moment changes were reasonably assigned to the dipole present in the initially formed MLCT excited state. Further, the excited-state dipole moment was sensitive to the presence of electron-donating (MeO-, tert-butyl-, CH3-) or -withdrawing (CF3-) substituents and was correlated with the substituent Hammett parameters. Hence, the data show for the first time that substituents on the bipyridine ligands, which are often introduced to tune formal reduction potentials, can also induce significant changes in the excited-state dipole, behavior that should be taken into consideration for artificial photosynthesis applications.