Abstract

Langmuir–Blodgett (LB) monolayers of 2,4-bis[4-(N-methyl-N-docosylamino)phenyl]squaraine (I), which has a centric donor–acceptor–donor chromophore, exhibit strong second-harmonic generation (SHG) with an effective susceptibility of 26 pm V⁻¹ at 1.064 µm. The squaraine molecule adopts an acentric U-conformation, with the chromophore parallel to the substrate and the docosyl groups contributing to the layer thickness of 2.41 nm. However, the SHG is attributed to non-centrosymmetric aggregation rather than the molecule itself and is suppressed when the dye is co-deposited with 2-{4-[4,5-bis(octadecylsulfanyl)[1,3]dithiol-2-ylidene]cyclohexa-2,5-dienylidene}malononitrile (II). The latter adopts a piggyback arrangement and is partially enclosed by the two docosyl tails of the squaraine: the areas in contact with the substrate are 0.63 ± 0.05 nm² molecule⁻¹ for I and 0.67 ± 0.05 nm² unit⁻¹ for (I)(II)_n, the combined molecular area being independent of composition for 0 < n < 0.5. Thus, the suppression of the second-order properties may be explained by an altered film structure with self-aggregation of the squaraines being inhibited by competitive heteromolecular interactions with the adjacent acceptor group of II. The electron acceptors, 2-methyl-3-phytyl-1,4-naphthoquinone (III) and N,N′-bis(docosyl)-4,4′-bipyridinium dibromide (IV), affect the second-order properties in a similar manner.