Organometallic Complexes for Nonlinear Optics. 11.1 Molecular Quadratic and Cubic Hyperpolarizabilities of Systematically Varied (Cyclopentadienyl)(triphenylphosphine)nickel σ-Arylacetylides

The complexes Ni(C⋮CR)(PPh 3 )(η-C 5 H 5 ) (R = 4-C 6 H 4 NO 2 ( 3 ), 4,4‘-C 6 H 4 C 6 H 4 NO 2 ( 4 ), ( E )-4,4‘-C 6 H 4 CH CHC 6 H 4 NO 2 ( 5 ), ( Z )-4,4‘-C 6 H 4 CH CHC 6 H 4 NO 2 ( 6 ), 4,4‘-C 6 H 4 C⋮CC 6 H 4 NO 2 ( 7 ), 4,4‘-C 6 H 4 N CHC 6 H 4 NO 2 ( 8 )) have been prepared. Electrochemical data for the series of complexes NiCl(PPh 3 )(η-C 5 H 5 ) ( 1 ), Ni(C⋮CPh)(PPh 3 )(η-C 5 H 5 ) ( 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 β 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 γ for the nitro-containing complexes, consistent with the dispersion effect of two-photon states contributing to the nonlinearities. An increase in γ 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.