Abstract: | 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. |