Molecular orientation of nonlinear optical polymer nanosheets on silica nanoparticle monolayer studied by optical waveguide spectroscopy

The paper describes molecular orientation of nonlinear optical (NLO) polymer monolayer transferred onto a sphere-shaped silica nanoparticle monolayer using optical waveguide spectroscopy. Structurally well-defined hybrid polymer nanoassemblies were constructed through bottom-up approaches: Langmuir-Blodgett technique and immersion method. Silica nanoparticles (SiO₂ NPs, 40-50 nm diameter) were immobilized on a quartz waveguide using cationic polymer Langmuir-Blodgett films (nanosheets) as a template. The SiO₂ NPs took a uniformly distributed monolayer formation without any aggregates, which minimizes light scattering. This allows us to gain reproducible absorption spectra of dye molecules embedded in polymer nanosheet monolayer on the nanoscale rough surface using optical waveguide spectroscopy. The NLO polymer nanosheets containing disperse red 1 (DR) were transferred onto the SiO₂ NP monolayer. The polarized absorption spectra were obtained; the s-light absorption was larger than the p-light absorption, indicating that polymer nanosheets are wrapped around SiO₂ NPs so that DR moieties undergo molecular disorientation not to form H-aggregates. This method provides us with useful information on structure-property relationship between nanoshaped inorganic nanoparticle and organic functional molecules in hybrid nanoassemblies.