Organometallic complexes for nonlinear optics. X. ¹ Molecular quadratic and cubic hyperpolarizabilities of systematically varied (cyclopentadienyl) bis(phosphine) ruthenium σ-arylacetylides

The complexes Ru(4,4′-C ≡ CC₆H₄C₆H₄NO₂)(PPh₃)₂(η-C₅H₅) and Ru(4,4′-C ≡ CC₆H₄C ≡ CC₆H₄NO₂)(PPh₃)₂(η-C₅H₅) have been prepared and the latter structurally characterized; they belong to a series of organometallic donor-bridge-acceptor compounds containing (cyclopentadienyl)bis(phosphine)ruthenium(II) centres as donors, conjugated arylacetylide bridges, and nitro acceptor groups. Electrochemical data for the series of complexes Ru(C ≡ CR)(PR′₃)₂(η-C₅H₅) (R=Ph, 4-C₆H₄NO₂, R′=Ph, Me; R=4,4′-C₆H₄C₆H₄NO₂, (E)-4,4′-C₆H₄CH=CHC₆H₄NO₂, 4,4′-C₆H₄C ≡ CC₆H₄NO₂, 4,4′-C₆H₄N=CHC₆H₄NO₂, R′=Ph) are consistent with an Ru^II/III couple whose oxidation potentials vary strongly with chain-lengthening from one-ring to two-ring acetylide ligand, but show little variation with changes at the bridging unit of the two-ring acetylide ligand. The molecular quadratic and cubic optical nonlinearities of the series of complexes have been determined by hyper-Rayleigh scattering (HRS) and Z-scan techniques, respectively. Molecular first hyperpolarizabilities by HRS at 1064 nm are dispersively enhanced; experimental and two-level corrected data suggest an increase in nonlinearity on chain-lengthening of the bridge, in proceeding from C₆H₄ to C₆H₄C₆H₄ and then C₆H₄C ≡ CC₆H₄ and C₆H₄CH=CHC₆H₄, a general trend that is reproduced by semiempirical ZINDO computations. Cubic hyperpolarizabilities by Z-scan at 800 nm are negative for complexes with nitro acceptor groups, probably a result of two-photon dispersion, with absolute values (up to 850 × 10^-36 esu) large for small organometallic complexes; as with quadratic nonlinearities, cubic nonlinearities increase substantially on bridge lengthening, with little variation on phosphine substitution.