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′-CtCC6H2[2,5-(OEt)2]CtCC6H4NO2}Cl(dppm)2] (19), trans-[Ru{4,4′,4′′-CtCC6H4CtCC6H2[2,5-(OEt)2]CtCC6H4NO2}Cl(dppm)2] (20), trans-[Ru{4,4′,4′′,4′′′-CtCC6H4-CtCC6H2[2,5-(OEt)2]CtCC6H2[2,5-(OEt)2]CtCC6H4NO2}Cl(dppe)2] (21), trans-[Ru{4,4′,4′′,4′′′- CtCC6H4-CtCC6H2[2,5-(OEt)2]CtCC6H2[2,5-(OEt)2]CtCC6H4NO2}Cl(dppm)2] (22), trans-[Ru{4,4′,4′′,4′′′-CtCC6H4CtCC6H4CtCC6H2[2,5-(OEt)2]CtCC6H4NO2}Cl(dppm)2] (23), and trans-[Ru{4,4′,4′′,4′′′,4′′′′-CtCC6H4-CtCC6H4CtCC6H2[2,5-(OEt)2]CtCC6H2[2,5-(OEt)2]CtCC6H4NO2}Cl(dppm)2] (24) are reported, together with those of precursor alkynes, complexes with the donor-π-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 π-bridge ligands were assessed by cyclic voltammetry, 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 RuII/III oxidation potentials and wavelengths of the optical absorption maxima decrease on π-bridge lengthening, until the tri(phenyleneethynylene) 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 1064 and two-photon absorption cross-section reach maximal values at this same π-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.