Evidence for Cation-Controlled Excited-State Localization in a Ruthenium Polypyridyl Compound

The visible absorption and photoluminescence (PL) properties of the four neutral ruthenium diimine compounds [Ru­(bpy)₂(dcb)] (B2B), [Ru­(dtb)₂(dcb)] (D2B), [Ru­(bpy)₂(dcbq)] (B2Q), and [Ru­(dtb)₂(dcbq)] (D2Q), where bpy is 2,2′-bipyridine, dcb is 4,4′-(CO₂^–)₂-bpy, dtb is 4,4′-(tert-butyl)₂-bpy, and dcbq is 4,4′-(CO₂^–)₂-2,2′-biquinoline, are reported in the presence of Lewis acidic cations present in fluid solutions at room temperature. In methanol solutions, the measured spectra were insensitive to the presence of these cations, while in acetonitrile a significant red shift in the PL spectra (≤1400 cm^–1) was observed consistent with stabilization of the metal-to-ligand charge transfer (MLCT) excited state through Lewis acid–base adduct formation. No significant spectral changes were observed in control experiments with the tetrabutylammonium cation. Titration data with Li⁺, Na⁺, Mg²⁺, Ca²⁺, Zn²⁺, Al³⁺, Y³⁺, and La³⁺ showed that the extent of stabilization saturated at high cation concentration with magnitudes that scaled roughly with the cation charge-to-size ratio. The visible absorption spectra of D2Q was particularly informative due to the presence of two well-resolved MLCT absorption bands: (1) Ru → bpy, λ_max ≈ 450 nm; and (2) Ru → dcbq, λ_max ≈ 540 nm. The higher-energy band blue-shifted and the lower-energy band red-shifted upon cation addition. The PL intensity and lifetime of the excited state of B2B first increased with cation addition without significant shifts in the measured spectra, behavior attributed to a cation-induced change in the localization of the emissive excited state from bpy to dcb. The importance of excited-state localization and stabilization for solar energy conversion is discussed.