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 acceptors 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.
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