Electronic States of Tris(bipyridine) Ruthenium(II) Complexes in Neat Solid Films Investigated by Electroabsorption Spectroscopy

We present the electric field-induced absorption (electroabsorption, EA) spectra of the solid neat films of tris(bipyridine) Ru(II) complexes, which were recently functionalized in our group as photosensitizers in dye-sensitized solar cells, and we compare them with the results obtained for an archetypal [Ru(bpy)3]2+ ion (RBY). We argue that it is difficult to establish a unique set of molecular parameter values by discrete parametrization of the EA spectra under the Liptay formalism for non-degenerate excited states. Therefore, the experimental EA spectra are compared with the spectra computed by the TDDFT (time-dependent density-functional theory) method, which for the first time explains the mechanism of electroabsorption in tris(bipyridine) Ru complexes without any additional assumptions about the spectral lineshape of the EA signal. We have shown that the main EA feature, in a form close to the absorption second derivative observed in the spectral range of the first MLCT (metal-to-ligand charge transfer) absorption band in Ru(bpy)3(PF6)2, can be attributed to a delocalized and orbitally degenerate excited state. This result may have key implications for the EA mechanism in RBY-based systems that exhibit similar EA spectra due to the robust nature of MLCT electronic states in such systems.