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

Abstract

Seven new complex salts trans-[RuII(NCS)(NH3)4(LA)][PF6]2 [LA = 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 (Emax) of which decrease in the order 2 > 1 > 3 > 4 > 6 > 5 > 7, as the electron-accepting ability of LA increases. The Emax 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-{RuII(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/(Emax)2 (μ12 = transition dipole moment). Both the β0[H] and β0[S] data confirm that the increased electron donor ability of a trans-{RuII(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 LA.