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

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-4,4ʹ-bipyridinium (PhQ+)4, N-phenyl-4-[trans-2-(4pyridyl)ethenyl]pyridinium (Phbpe+)5, N-(4-acetylphenyl)-4,4ʹ-bipyridinium (4-AcPhQ+)6, or N-(2-pyrimidyl)-4,4ʹ-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 (beta) have been measured by using the hyper-Rayleigh scattering technique with acetonitrile solutions and a 1064 nm laser, and static first hyperpolarizabilities (beta)0[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 (delta) (mu)12 for the MLCT transitions which have been used to calculate (beta)0[S] values according to the two-state equation (beta)0= 3 (delta) (mu)12((mu)12)2/(Emax)2((mu)12= transition dipole moment). Both the (beta)0[H] and (beta)0[S] data confirm that the increased electron donor ability of a trans-{RuII(NCS)(NH3)4}+ centre affords enhanced (beta)0 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 (beta)0 increase with respect to the analogous mim complexes varies over a range of ca. 25-120%, depending upon LA.