Optical second harmonic generation in Langmuir-Blodgett films of novel donor-acceptor substituted pyridine and benzene derivatives

Abstract

A series of sixteen novel amphiphiles has been prepared containing substituted nitropyridine and nitroaniline units as polar headgroups. The film forming properties of the amphiphiles at the air-water interface are described. Langmuir-Blodgett-films are characterized by spectroscopic methods and by xray diffraction. A comprehensive study of measurements of optical second harmonic generation in the films is reported. Among the compounds investigated 2-alkylamino-5-nitropyridines with C₁₈ to C₂₆ alkyl chains are best suited for second harmonic generation. Our studies show for the first time that LB-films of a single material not forming an asymmetric head-to-tail (X- or Z-type) structure can produce significant second harmonic signals. Structure models are presented, which explain the observation of nonlinear optical properties in terms of formation of either a non-centrosymmetric herringbonetype structure or an intercalated structure of the molecules with preferential orientation of the polar headgroups in a unique direction.