Efficient, Reversible Redox-Switching of Molecular First Hyperpolarizabilities in Ruthenium(II) Complexes Possessing Large Quadratic Optical Nonlinearities

Low-energy metal-to-ligand charge-transfer (MLCT) excitations are associated with the very large molecular nonlinear optical (NLO) coefficients, β, of the complexes RuII(NH3)5(N-R-4,4′-bipyridinium)3+ (R=methyl, phenyl, or 4-acetylphenyl). Chemical oxidation to the RuIII forms causes bleaching of the MLCT absorptions and marked attenuation of the NLO responses. Both effects are completely reversed upon re-reduction, and the extent of the β switching is about 10- to 20-fold.