Linear and nonlinear optical behavior in hemicyanine Langmuir—Blodgett monolayers

Abstract

Hemicyanine dyes, N-(3-sulfopropyl-4-( p-dioctylaminostyryl)) pyridinium ( I) and 4-(4-dihexadecylaminostyryl)- N-methylpyridinium iodide ( II), fabricated in Langmuir—Blodgett monolayer films have been studied by means of absorption and fluorescence spectroscopy, as well as second harmonic generation (SHG). The two hemicyanine dyes exhibit substantially different aggregation behavior during monolayer formation. While monolayers of I contain predominantly the aggregate species, the composition of pure monolayers of II can be managed to contain either pure aggregate, pure monomer, or any intermediate composition. SHG studies indicate that films with significant monomer population give a nonzero second-order nonlinear response, while films containing only aggregates give zero nonlinear response, χ (2) = 0. Monolayers of I sandwiched between two layers of stearic acid were obtained, and the spectroscopic behavior is compared with a single monolayer on a quartz substrate. Fluorescence experiments indicate the presence of a species with twisted tertiary amine in the sandwiched structure. This might be the result of stresses experienced by the N-bound long alkyl chains originating from tail—tail interactions with the neighbouring layer of stearic acid.