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

The cationic dye, E-N-hexadecyl-4-[2-(4-octadecyloxynaphthyl)ethenyl]quinol inium bromide, has hydrophobic n-alkyl groups at opposite ends of a wide-bo died donor-(pi -bridge)-acceptor chromophore. When deposited by the Langmui r-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 s tretched rather than a U-shaped conformation. In contrast, the mean thickne ss within multilayer films is 2.5 nm layer(-1). This discrepancy arises fro m an interdigitating layer arrangement not applicable to the monolayer, as well as a slightly different molecular tilt in the bulk film. The second-ha rmonic intensity increases quadratically with the number of deposited layer s, i.e. as I-(N)(2 omega) = I2((1))(omega)N(2). Thus, it is the first examp le of a Lego-type structure with non-centrosymmetric alignment of the optic ally nonlinear chromophores.