LINEAR AND NONLINEAR-OPTICAL BEHAVIOR IN HEMICYANINE LANGMUIR-BLODGETT MONOLAYERS

Hemicyanine dyes, N-(3-sulfopropyl-4-(p-dioctylaminostyryl)) pyridiniu m (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 h armonic generation (SHG). The two hemicyanine dyes exhibit substantial ly different aggregation behavior during monolayer formation. While mo nolayers of I contain predominantly the aggregate species, the composi tion of pure monolayers of II can be managed to contain either pure ag gregate, pure monomer, or any intermediate composition. SHG studies in dicate that films with significant monomer population give a nonzero s econd-order nonlinear response, while films containing only aggregates give zero nonlinear response, %((2))= 0. Monolayers of I sandwiched b etween two layers of stearic acid were obtained, and the spectroscopic behavior is compared with a single monolayer on a quartz substrate. F luorescence experiments indicate the presence of a species with twiste d 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 neighboring layer of stearic acid .