Controlling the structure of transparent Langmuir–Blodgett films for nonlinear optical applications

Abstract

Langmuir–Blodgett (LB) multilayers of (E)-N-alkyl-4-[2-(4-docosyloxyphenyl)ethenyl]pyridinium bromide, C_nH_{2n+ 1}—Py⁺—CHCH—C₆H₄—O—C₂₂H₄₅Br^–(dye I), are centrosymmetric (Y-type) for n⩽ 6 and non-centrosymmetric (Z-type) for 8 ⩽n⩽ 20. The areas obtained from grazing incidence synchrotron X-ray diffraction indicate that the molecules adopt a ‘stretched’ rather than a ‘U-shaped’ configuration and, unlike conventional amphiphilic materials, the long chain homologues do not invert during deposition. The second-harmonic intensity from films of the higher homologues (n 10) increases quadratically with the number of LB layers (I_2ω(N)=I_2ω(1)N²) whereas for the lower homologues (n⩽ 6) the intensity is negligible for even numbers of layers. The nonlinear optical properties of the sulfur (dye II), selenium (dye III) and quinolinium (dye IV) analogues are also reported and, in each case, quadratic enhancement of the second-harmonic intensity has been observed for suitable combinations of alkyl chain lengths. The second-order susceptibility, χ⁽²⁾_zzz, of a 100 layer LB film of (E)-N-dodecyl-4-[2-(4-docosyloxyphenyl)ethenyl]quinolinium bromide (dye IV) is 20 pm V^–1.