Molecular quadratic non-linear optical properties of dipolar trans-tetraammineruthenium(ii) complexes with pyridinium and thiocyanate ligands

Abstract

Seven new complex salts trans-[Ru^II(NCS)(NH₃)₄(L^A)][PF₆]₂ [L^A = N-methyl-4,4′-bipyridinium (MeQ⁺) 1, N-methyl-2,7-diazapyrenium (Medap⁺) 2, N-methyl-4-[trans-2-(4-pyridyl)ethenyl]pyridinium (Mebpe⁺) 3, N-phenyl-4,4′-bipyridinium (PhQ⁺) 4, N-phenyl-4-[trans-2-(4-pyridyl)ethenyl]pyridinium (Phbpe⁺) 5, N-(4-acetylphenyl)-4,4′-bipyridinium (4-AcPhQ⁺) 6, or N-(2-pyrimidyl)-4,4′-bipyridinium (2-PymQ⁺) 7] have been prepared. The electronic absorption spectra of 1–7 display intense, visible metal-to-ligand charge-transfer (MLCT) bands, the energies (E_max) of which decrease in the order 2 > 1 > 3 > 4 > 6 > 5 > 7, as the electron-accepting ability of L^A increases. The E_max values generally correlate with cyclic voltammetric and ¹H NMR data. Comparisons of the MLCT absorption and electrochemical data for 1–7 show that a trans-{Ru^II(NCS)(NH₃)₄}⁺ centre is a stronger electron donor than analogous groups containing only neutral ligands. Molecular first hyperpolarizabilities β have been measured by using the hyper-Rayleigh scattering technique with acetonitrile solutions and a 1064 nm laser, and static first hyperpolarizabilities β₀[H] were obtained by application of the two-state model. Stark (electroabsorption) spectroscopic studies in butyronitrile glasses at 77 K have afforded dipole moment changes Δμ₁₂ for the MLCT transitions which have been used to calculate β₀[S] values according to the two-state equation β₀ = 3Δμ₁₂(μ₁₂)²/(E_max)² (μ₁₂ = transition dipole moment). Both the β₀[H] and β₀[S] data confirm that the increased electron donor ability of a trans-{Ru^II(NCS)(NH₃)₄}⁺ centre affords enhanced β₀ values, when compared with related complexes containing the neutral trans ligands NH₃, 4-(dimethylamino)pyridine, N-methylimidazole (mim) or pyridine. However, the magnitude of the observed β₀ increase with respect to the analogous mim complexes varies over a range of ca. 25–120%, depending upon L^A.