STRUCTURE AND 2ND-HARMONIC GENERATION OF LANGMUIR-BLODGETT-FILMS OF 2 CHIRAL AMPHIPHILIC AZO DYES

The properties and structure of Langmuir-Blodgett films of two new amphiphilic nonlinear optical dyes containing stereogenic (chiral) centers were studied. The dye molecule 4-[[4'-[(3R)-palmitoylpyrrolidin-1-yl]phenyl]azo]-3-nitrobenzoic acid (KZ16) forms homogeneous structures in the plane of the substrate when the monolayers are transferred into multilayers. Multilayers of 4-[[4'-[(3R,4R)-dipalmitoylpyrrolidin-1-yl]phenyl]azo]-3-nitrobenzoic acid (KDEZ16) which contains two alkyl chains on the pyrrolidine ring showed flow-induced orientation in the plane of the film. The tilt angles of the molecules in the multilayer were determined by polarized FT-IR measurements combined with X-ray diffraction. Tilt angles of approximately 55-degrees to the surface normal for the chromophore unit and approximately 35-degrees for the alkyl chain of KZ16 were revealed. KDEZ16 molecules showed a tilt angle of approximately 35-degrees for both the chromophore unit and the alkyl chain with the surface normal in the multilayer. The second harmonic generation (SHG) properties of mono- and multilayers of KZ16 with different architectures on glass were studied. When a nonpolar alignment (Y-type bilayer) was measured, a complete cancellation of the second-order nonlinearity was observed. The stereogenic center did not contribute to a noncentrosymmetric structure at a measurable scale. Alternating layers of the KZ16 dye and arachidic acid behave almost ideal, giving a SHG depending quadratically on the number of bilayers.