Contrasting linear and quadratic nonlinear optical behavior of dipolar pyridinium chromophores with 4-(dimethylamino)phenyl of ruthenium(II) ammine electron donor groups

In this article, we contrast the optical properties of dipolar chromophores having 4-(dimethylamino)-phenyl electron donor (D) and pyridinium acceptor (A) groups with those of closely related cations having pyridyl-coordinated Ru II donors. A range of physical data, including that from Stark (electroabsorption) spectroscopy, permits unprecedented quantitative comparisons, most notably regarding the effects of extension of bridging polyene chains. The purely organic compounds display normal optical properties in that their intense, visible π → π* intramolecular charge-transfer (ICT) bands red-shift as the number of E-ethylene units (n) increases from 1 to 3 and the associated static first hyperpolarizabilities β0 increase steadily with n. The related RuII complexes show intense, visible d → π* metal-to-ligand charge-transfer (MLCT) bands, which are found to lower energy when compared with the ICT transitions of the corresponding organics. Abnormally, these MLCT bands blue-shift as n increases, and β0 maximizes at n = 2. Time-dependent density-functional theory and finite field calculations verify these empirical trends for both types of compound, which can be rationalized as arising from the differing orbital structures of the chromophores and the associated degrees of D-A electronic coupling.