Monolayer films of U-shaped molecules: suppression of the aggregation-induced second-harmonic generation of squaraine dyes by guest-host interactions

Langmuir-Blodgett (LB) monolayers of 2,4-bis[4-(N-methyl-N-docosylamino)phe nyl]squaraine dagger (I), which has a centric donor-acceptor-donor chromoph ore, exhibit strong second-harmonic generation (SHG) with an effective susc eptibility of 26 pm V-1 at 1.064 mum. The squaraine molecule adopts an acen tric U-conformation, with the chromophore parallel to the substrate and the docosyl groups contributing to the layer thickness of 2.41 nm. However, th e SHG is attributed to non-centrosymmetric aggregation rather than the mole cule itself and is suppressed when the dye is co-deposited with 2-{4-[4,5-b is(octadecylsulfanyl)[1,3]dithiol-2-ylidene]cyclohexa-2,5-dienylidene}malon onitrile (II). The latter adopts a piggyback arrangement and is partially e nclosed by the two docosyl tails of the squaraine: the areas in contact wit h the substrate are 0.63 +/- 0.05 nm(2) molecule(-1) for I and 0.67 +/- 0.0 5 nm(2) unit(-1) for (I)(II)(n), the combined molecular area being independ ent of composition for 0 < n < 0.5. Thus, the suppression of the second-ord er properties may be explained by an altered film structure with self-aggre gation of the squaraines being inhibited by competitive heteromolecular int eractions 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'-bipyri dinium dibromide (IV), affect the second-order properties in a similar mann er.