STRUCTURE AND LINEAR DICHROISM OF LANGMUIR-BLODGETT-FILMS OF URACILIC AMPHIPHILES

Surface-pressure studies of mono- and di-substituted uracilic amphiphiles and their transfers in Langmuir-Blodgett (LB) films are presented. These compounds give very stable films at the air/water interface and are, in turn, easily transferable to quartz plates giving stable and reproducible Y-type LB multilayers. The shapes and maxima of the UV absorption spectra of these films, in contrast to the case in solution, depend greatly on the substitution of the chromophore, which influences its orientation. In addition, the LB films show strong dichroic behaviour which also depends on the substitution and on the absorption band. The variations of the polarisation plane of the incident beam and of the incidence angle lead to four dependences (two for each absorption band) for each film. A simple orientation model of the transition dipoles in the multilayer, which enables four angular dependences per film to be fitted and the different orientations of the planes of the chromophores to be determined, is presented. These orientations (with the help of molecular exciton theory) facilitate interpretation of the differences in the shapes of the absorption bands and their dependence on substitution, allow confirmation of the orientation obtained by surface-pressure measurements, and also suggest that no [2 + 2]-photodimerization can occur in these films, contrary to the results obtained for these compounds in the solid state and in solution.