Combining Very Large Quadratic and Cubic Nonlinear Optical Responses in Extended, Tris-Chelate Metallochromophores with Six π-Conjugated Pyridinium Substituents

We describe a series of nine new complex salts in which electron-rich Ru^II or Fe^II centers are connected via π-conjugated bridges to six electron-accepting N-methyl-/N-arylpyridinium groups. This work builds upon our previous preliminary studies (Coe, B. J. J. Am. Chem. Soc. 2005, 127, 13399−13410; J. Phys. Chem. A 2007, 111, 472−478), with the aims of achieving greatly enhanced NLO properties and also combining large quadratic and cubic effects in potentially redox-switchable molecules. Characterization has involved various techniques, including electronic absorption spectroscopy and cyclic voltammetry. The complexes display intense, visible d → π* metal-to-ligand charge-transfer (MLCT) bands, and their π → π* intraligand charge-transfer (ILCT) absorptions in the near-UV region show molar extinction coefficients as high as ca. 3.5 × 10⁵ M⁻¹ cm⁻¹. Molecular quadratic nonlinear optical (NLO) responses β have been determined by using hyper-Rayleigh scattering at 800 and 1064 nm and also via Stark (electroabsorption) spectroscopic studies. The directly and indirectly derived β values are very large, with the Stark-based static first hyperpolarizabilities β₀ reaching as high as ca. 10⁻²⁷ esu, and generally increase on extending the π-conjugation and enhancing the electron-accepting strength of the ligands. Cubic NLO properties have also been measured by using the Z-scan technique, revealing relatively high two-photon absorption cross sections of up to 2500 GM at 750 nm.