Length-Dependent Convergence and Saturation Behavior of Electrochemical, Linear Optical, Quadratic Nonlinear Optical, and Cubic Nonlinear Optical Properties of Dipolar Alkynylruthenium Complexes with Oligo(phenyleneethynylene) Bridges

The syntheses of trans-[Ru{4,4'-C CC6H2[2,5-(OEt)(2)]C CC6H4NO2}Cl(dppm)(2)] (19), trans-[Ru{4,4',4 ''-C CC6H4C CC6H2[2,5-(OEt)(2)]C CC6H4NO2}Cl(dPPm)(2)] (20), trans-[Ru{f4,4',4 '',4 '''-C CC6H4-C CC6H2[2,5-(OEt)(2)]C CC6H2[2,5-(OEt)(2)]C CC6H4NO2}Cl(dppe)(2)] (21), trans-[Ru{4,4',4 '',4 '''-C CC6H4C CC6H2[2,5-(OEt)(2)]C CC6H2[2,5-(OEt)(2)]C CC6H4NO2)C1(dppm)(2)] (22), trans-[Ru{(4,4',4 '',4 '''-C CC6H4-C CC6H4C CC6H2[2,5-(OEt)(2)]C CC6H4NO2]Cl(dppm)(2)] (23), and trans-[Ru(4,4',4 '',4 ''',4 ''''-C CC6H4-C CC6H4C=CC6H2[2,5-(OEt)(2)]C CC6H2[2,5-(OEt)(2)]C CC6H4NO2)Cl(dppm)(2)] (24) are reported, together with those of precursor alkynes, complexes with the donor-7-pi-bridge-acceptor formulation that affords efficient quadratic and cubic NLO compounds; the identity of 19 was confirmed by a structural study. The electrochemical properties of 19-24 and related complexes with shorter pi-bridge ligands were assessed by cyclic voltarnmetry, and the linear optical, quadratic nonlinear optical, and cubic nonlinear optical properties were assayed by UV-vis-NIR spectroscopy, hyper-Rayleigh scattering studies at 1064 and 1300 nm, and broad spectral range femtosecond Z-scan studies, respectively. The Ru-II/III oxidation potentials and wavelengths of the optical absorption maxima decrease on;rr-bridge lengthening, until the tri(phenylene-ethynylene) complex is reached, further chain lengthening leaving these parameters invariant; theoretical studies employing time-dependent density functional theory have shed light on this behavior. The quadratic nonlinearity beta(1064) and two-photon absorption cross-section reach maximal values at this same pi-bridge length, a similar saturation behavior that may reflect a common importance of ruthenium-to-alkynyl ligand charge transfer in electronic and optical behavior in these molecules.