Molecular Lego: non-centrosymmetric Langmuir-Blodgett films for nonlinear optical applications

The cationic dye, E-N-hexadecyl-4-[2-(4-octadecyloxynaphthyl)ethenyl]quinolinium bromide, has hydrophobic n-alkyl groups at opposite ends of a wide-bodied donor-(pi -bridge)-acceptor chromophore. When deposited by the Langmuir-Blodgett technique at 40 mN m(-1), the LB monolayer has a thickness of 4.9 nm, this being compatible with the molecular length if the dye adopts a stretched rather than a U-shaped conformation. In contrast, the mean thickness within multilayer films is 2.5 nm layer(-1). This discrepancy arises from an interdigitating layer arrangement not applicable to the monolayer, as well as a slightly different molecular tilt in the bulk film. The second-harmonic intensity increases quadratically with the number of deposited layers, i.e. as I-(N)(2 omega) = I2((1))(omega)N(2). Thus, it is the first example of a Lego-type structure with non-centrosymmetric alignment of the optically nonlinear chromophores.