Organometallic complexes for nonlinear optics .11. Molecular quadratic and cubic hyperpolarizabilities of systematically varied (cyclopentadienyl)(triphenylphosphine)nickel sigma-arylacetylides

The complexes Ni(C=R)(PPh3)(eta-C5H5) (R=4-C6H4NO2 (3), 4,4'-C6H4C6H4NO2 (4), (E)4,4'-C6H4CH=CHC6H4NO2 (5), (Z)-4,4'-C6H4CH=CHC6H4NO2 (6), 4,4'-C6H4C=CC6H4NO2 (7), 4,4'-C6H4N=CHC6H4NO2 (8)) have been prepared. Electrochemical data for the series of complexes NiCl(PPh3)(eta-C5H5) (1), Ni(C=CPh)(PPh3)(eta-C6H5) (2), and 3-8 have been determined. Introduction of nitro substituent (in progressing from 2 to 3) results in a substantial increase in the Ni-II/III oxidation potential, all of which is lost on progressing from a one-ring (3) to two-ring (4-8) chromophore. 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. HRS measurements at 1064 nm are consistent with an increase in beta upon chromophore chain-lengthening (in progressing from 3 to 7 to 5), replacing Z by E stereochemistry (in progressing from 6 to 5), and in replacing N by CH (in progressing from 8 to 5), general trends that are maintained with the two-level-corrected data. Nonlinearities for the 18-electron nickel complexes are larger than those for related 14-electron (triphenylphosphine)gold acetylides, but smaller than those for the more easily oxidizable 18-electron (cyclopentadienyl)bis(phosphine)ruthenium acetylide analogues. Z-scan data at 800 nm reveal a negative gamma for the nitro-containing complexes, consistent with the dispersion effect of two-photon states contributing to the nonlinearities. An increase in gamma upon chromophore chain-lengthening (in progressing from 3 to 5, 4 and 7) and replacing Z by E stereochemistry (in progressing from 6 to 5) is observed. Nonlinearities for the extended-chromophore acetylide complexes 5 and 7 are significantly less than those for the (triphenylphosphine)gold acetylide analogues.