Diminished second-harmonic generation from oligomeric squaraine derivatives of N-octadecylpyrrole relative to the intensity from films of the centric anilinosquaraines

Langmuir-Blodgett (LB) monolayers of oligomeric dyes (I), obtained from the reaction of N-octadecylpyrrole and squaric acid, exhibit second-harmonic generation (SHG). This arises from the inherent noncentrosymmetry but, importantly, the intensity is 4 orders of magnitude weaker than the optimum signal from LB films of 2,4-bis[4-(N-methyl-N-hexylamino)phenyl]squaraine (II). The anilinosquaraine is centric, but solid solutions of II in poly(vinyl acetate) are SHG-active. The intensity increases quadratically with the concentration of dye but is extinguished by the incorporation of electron accepters into the film. The second-order properties are attributed to a noncentrosymmetric aggregate structure rather than the molecule itself or the interface between the film and the underlying glass substrate. The latter is ruled out by the direct observation of SHG from a free-standing film of the solid solution.