Orientation Order of Nonelectrically Poled FTC-Type Chromophores in PMMA on SiO2 Surfaces

We present the orientation order of push-pull chromophores consisting of phenyl vinylene thiophene vinylene (FTC) bridges between the N,-N-bis-(2-hydroxyethyl)-amine electron donor and phenyl-trifluoromethyl-tricyanofuran (TCF-Ph-CF3) acceptor in poly (methyl methacrylate) (PMMA). The chromophores have attracted great attention as a guest material for nonlinear optical polymers for near-IR applications. Our experimental results demonstrated that the chromophores with relatively large molecular weights were oriented in a polar order with the aid of adsorption onto the surfaces of the SiO2 substrates, even without the conventional electrical poling procedure. The polar order of the chromophores extended approximately 50 nm vertically from the substrate surfaces. Polarized linear absorption and second harmonic generation spectroscopies were performed to determine the orientation distribution function of the chromophores in the PMMA. The average molecular tilt angle Θ, indicating the degree of the orientation order, was wider than the magic angle, and the chromophores were preferably oriented horizontally to the substrate surfaces. A relatively low orientation order was explained in terms of the strong dipole-dipole repulsions among the neighboring guest chromophores.