Linear and nonlinear optical responses of a dye anchored to gold nanoparticles dispersed in liquid and polymeric matrixes

The compound [4-CH 3 C 6 H 4 -SS-CH 2 CH 2 -N(CH 3 )C 6 H 4 C(C 6 H 4 N(CH 3 ) 2 )-4) 2 ]Cl (1a) consists of the triarylmethyl chromophore of crystal violet appended to an organic disulfide, to entail direct attachment of this dye to gold surfaces. Addition of 1a to colloidal dispersions of the metal produced a stable dispersion in methanol–water mixtures. Transmission electron microscopy of Freeze-Fracture sections shadowed with carbon and platinum confirmed that the dispersion remains unaggregated. Local-field effects and coupling to the surface plasmon of gold magnified the interaction of the chromophore with optical fields. A large oscillator strength in the visible spectrum (f ca. 35) and a first hyperpolarizability (β ca. 10 –26 esu, hyper-Rayleigh scattering) were observed as the colloid adsorbed dye molecules. A Hill-type analysis indicated that the adsorption process is cooperative. This combination of organic chromophore and nanoparticle was adapted for incorporation into PMMA-based composite films whose electrooptic coefficient (r 113 = 2.20 pm V –1 ) is larger than the sum of the individual contributions of dye and metallic particles.Key words: chromophores, colloids, gold, local-field effects, nonlinear optical materials.