Organometallic Complexes for Nonlinear Optics. 8. Syntheses and Molecular Quadratic Hyperpolarizabilities of Systematically Varied (Triphenylphosphine)gold σ-Arylacetylides:  X-ray Crystal Structures of Au(C⋮CR)(PPh₃) (R = 4-C₆H₄NO₂, 4,4‘-C₆H₄C₆H₄NO₂)

The series of complexes Au(C⋮CR)(PPh₃) (R = Ph (2), 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 synthesized by reaction of AuCl(PPh₃) with the corresponding acetylene and methoxide, and complexes 3−8 have been structurally characterized. The molecular first hyperpolarizabilities for the complexes have been determined by hyper-Rayleigh scattering at 1064 nm. Introduction of the nitro substituent (in proceeding from 2 to 3) leads to a significant increase in nonlinearity. Experimental β values increase as 3 < 4 < 6 ≈ 7 < 8 < 5 consistent with nonlinearity increasing with (i) chain lengthening, (ii) replacing biphenyl (4) or yne linkage (7) by ene linkage (5), (iii) replacing (Z)-ene stereochemistry (6) with (E)-ene stereochemistry (5), and (iv) ene linkage (5) being more efficient than imino linkage (8). The same trend is observed with two-level-corrected data. A linear correlation of both experimentally-determined and two-level-corrected nonlinearities of the acetylides with precursor acetylenes is observed.