Linear and nonlinear optical behavior in hemicyanine Langmuir—Blodgett monolayers

Enhanced optical and nonlinear optical responses in a polyelectrolyte templated Langmuir-Blodgett film

Optical and nonlinear optical properties like fluorescence and second harmonic generation (SHG) of molecular materials can be strongly influenced by the mode of assembly of the molecules. The Langmuir-Blodgett (LB) technique is an elegant route to the controlled assembly of molecules in ultrathin films, and complexation of ionic amphiphiles in the Langmuir film by polyions introduced in the aqueous subphase provides a simple and efficient access to further control, stabilization, and optimization. The monolayer LB film of the hemicyanine-based amphiphile, N-n-octadecyl-4-[2-(4-(N,N-ethyloctadecylamino)phenyl)ethenyl]pyridinium possessing a "tail-head-tail" structure, shows fluorescence as well as SHG response. The concomitant enhancement of both of these linear and nonlinear optical attributes is achieved through templating with the polyanion of carboxymethylcellulose. Brewster angle and atomic force microscopy reveal the influence of polyelectrolyte templating on the morphology of the Langmuir and LB films. Polarized absorption and fluorescence spectroscopy provide insight into the impact of complexation with the polyelectrolyte on the orientation and deaggregation of the hemicyanine headgroup leading to fluorescence and SHG enhancement in the LB film.

Fluorescence enhancement in Langmuir-Blodgett films: role of amphiphile structure, orientation, and assembly

Suppression of self-quenching of fluorescence emission in supramolecular systems is of fundamental interest in the design and development of novel optical materials. While a number of molecular design and assembly strategies have been formulated for the fabrication of molecular crystals with enhanced light emission, parallel explorations based on systematic approaches in the domain of molecular ultrathin films are rare. We have investigated Langmuir-Blodgett films of amphiphilic molecules bearing the same hemicyanine choromophore headgroup but with different possibilities of attachment of the octadecyl hydrocarbon chain, under different deposition conditions. Fluorescence spectroscopy studies indicate enhanced light emission in the derivative with a "tail-head-tail" structure. The observed trends are attributed to the amphiphile structure, nature of deposition, and the chromophore orientations revealed through polarized absorption spectra of the films. The study suggests a simple design strategy toward realization of molecular ultrathin films with enhanced light emission.