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-(π-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 2ω (N) = I 2ω (1) N 2 , Thus, it is the first example of a Lego-type structure with non-centrosymmetric alignment of the optically nonlinear chromophores.