Extended Dipolar Nonlinear Optical Chromophores Based on <i>trans</i>-Bis[1,2-phenylenebis(dimethylarsine)]chlororuthenium(II) Centers

Four new complex salts trans-[RuIICl(pdma)2LA][PF6]n [pdma = 1,2-phenylenebis(dimethylarsine); LA = 1,4-bis[E-2-(4-pyridyl)ethenyl]benzene (bpvb), n = 1, 1; LA = N-methyl-1,4-bis(E-2-(4-pyridyl)ethenyl)benzene (Mebpvb+), n = 2, 2; LA = N-phenyl-1,4-bis(E-2-(4-pyridyl)ethenyl)benzene (Phbpvb+), n = 2, 3; LA = N-(2-pyrimidyl)-1,4-bis(E-2-(4-pyridyl)ethenyl)benzene (Pymbpvb+), n = 2, 4] have been prepared. The electronic absorption spectra of 1−4 display intense, visible metal-to-ligand charge-transfer (MLCT) bands, with λmax values in the range 432−474 nm in acetonitrile. Intense intraligand charge-transfer (ILCT) bands due to LA are also observed, with λmax values in the range 350−416 nm. Cyclic voltammetric studies in acetonitrile reveal reversible RuIII/II waves with E1/2 values of ca. 1.05 V vs Ag/AgCl, together with LA-based reduction processes that are irreversible with the exception of 1. Salts 1−4 have been investigated by using Stark (electroabsorption) spectroscopy in butyronitrile glasses at 77 K. These studies have afforded dipole moment changes, Δμ12, for the MLCT and ILCT transitions which have been used to calculate molecular static first hyperpolarizabilities, β0, according to the two-state equation β0 = 3Δμ12(μ12)2/(Emax)2 (μ12 = transition dipole moment, Emax = MLCT/ILCT energy). In contrast with related RuII ammine complexes, replacement of a central E-ethylene bond with a 1,4-phenylene unit does not appear to be an especially effective strategy for combating the NLO transparency-efficiency tradeoff in these pdma complexes. Single-crystal X-ray studies with the complex salts 2 and 3 and also with the pro-ligand salt [Phbpvb+]PF6·0.5HPF6 show that these materials all adopt centrosymmetric packing structures.