Organometallic complexes for nonlinear optics. X. Molecular quadratic and cubic hyperpolarizabilities of systematically varied (cyclopentadienyl)bis(phosphine) ruthenium σ-arylacetylides: X-ray crystal structure of Ru(( E)-4,4′-C CC 6H 4CHC 6H 4NO 2)(PPh 3) 2(η-C 5H 5)

Abstract

The complexes Ru(4,4′-C  CC 6H 4C 6H 4NO 2)(PPh 3) 2(η-C 5H 5) and Ru(4,4′-C  CC 6H 4C  CC 6H 4NO 2)(PPh 3) 2(η-C 5H 5) 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-C 5H 5) (R  Ph, 4-C 6H 4NO 2, R′  Ph, Me; R = 4,4′-C 6H 4C 6H 4NO 2, ( E)-4,4′-C 6H 4CH  CHC 6H 4NO 2, 4,4′-C 6H 4C  CC 6H 4NO 2, 4,4′-C 6H 4N  CHC 6H 4NO 2, 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 C 6H 4 to C 6H 4C 6H 4 and then C 6H 4C  CC 6H 4 and C 6H 4CH  CHC 6H 4, 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.