Organometallic Complexes for Nonlinear Optics. 8.1 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)