LINEAR AND NONLINEAR-OPTICAL PROPERTIES OF Z-TYPE LANGMUIR-BLODGETT-FILMS OF A BENZOTHIAZOLIUM DYE

Langmuir-Blodgett films of the two-legged dye, E-N-hexadecyl-2-[2-(4-d ocosylaminophenyl ethenyl] benzothiazolium octadecylsulphate, are non- centrosymmetric (Z-type). The second-harmonic generation (SHG) increas es with film thickness, albeit subquadratically, the intensity from a 200 layer film being 600 times that from the monolayer. The tilt angle of the chromophore relative to the normal to the substrate increases in the first few layers. This is exemplified by a decrease in the SHG polarization dependence, I-2 omega(p --> p)/I-2 omega(s --> p), from 1 7 +/- 2 for the monolayer to 4 +/- 1 for the multilayer at 45 degrees, and is accompanied by a bathochromic shift of the charge transfer ban d from 506 +/- 2 nm to 565 +/- 5 nm. The second-order susceptibility, tilt angle, thickness and dielectric constants of the monolayer are ch i(ZZZ)((2)) = 70 pm V-1, phi = 32 degrees, l = 3.51 nm, epsilon(r) = 2 .62 and epsilon(i) = 0.45 at 532 nm (lambda(2 omega)).