Theoretical investigation of the first hyperpolarizability redox-switching in a ruthenium complex

The first hyperpolarizability (β) of [Ru-(NH )(4-AcPhQ)] and of its oxidized form [Ru-(NH)(4-AcPhQ)], previously synthesized and characterized by Coe and co-workers by means of hyper-Rayleigh scattering measurements, has been calculated using time-dependent density functional theory and analyzed resorting to the summation-over-states scheme. Besides confirming a β contrast of at least one order of magnitude, calculations point out the key role of the metal-to-ligand charge transfer state, which is responsible for the larger β response of the reduced Ru(II) form. Results also evidence the small contribution to β of the higher-energy intra-ligand charge transfer excited states, as well as the small changes of the depolarization ratio upon oxidation.