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

Langmuir-Blodgett (LB) monolayers of oligomeric dyes (I), obtained from the reaction of N-octadecylpyrrole and squaric acid, exhibit second-harmonic g eneration (SHG). This arises from the inherent noncentrosymmetry but, impor tantly, the intensity is 4 orders of magnitude weaker than the optimum sign al 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 a cetate) are SHG-active. The intensity increases quadratically with the conc entration of dye but is extinguished by the incorporation of electron accep ters into the film. The second-order properties are attributed to a noncent rosymmetric aggregate structure rather than the molecule itself or the inte rface between the film and the underlying glass substrate. The latter is ru led out by the direct observation of SHG from a free-standing film of the s olid solution.