Non-centrosymmetric alignment of optically nonlinear dyes

Langmuir-Blodgett (LB) film structures of the two-legged dye, (E)-4-[2-alkanoyloxyphenyl) ethenyl]-N-docosylpyridinium iodide, CHC(O)OCHCHCHCHNCH I, are dependent upon the alkanoyl group, there being a cross-over from a centrosymmetric Y-type arrangement to a non-centrosymmetric Z-type arrangement as n increases. The second-harmonic intensity from films comprising an even number of layers is negligible for 1 ≤ n ≤ 10, increases with the number of LB layers when n ≥ 12 and is greatly reduced when n = 21. The two-legged molecules adopt a 'stretched' rather than a 'U-shaped' configuration and there is a tendency towards an antiparallel alignment within layers as the length of the alkanoyl group approaches that of the second alkyl chain. The stearoyloxy homologue (n = 17) has a layer thickness, from ellipsometry, of 52 Å and an in-plane area, from grazing incidence synchrotron diffraction, of 19.72 ± 0.04 Å molecule