Modification of the local structures of dye aggregates in Langmuir-Blodgett films as probed by electron spin resonance

The effect of heat treatment on mixed Langmuir-Blodgett (LB) films of a merocyanine dye and arachidic acid has been studied by electron spin resonance (ESR) measurements at the X and K bands. The ESR spectra of the stable radicals, associated with g and hyperfine anisotropies, directly provide information about the orientation of molecular axes. In as-deposited films, the ppi orbital axis and the long axis of the chromophore have been shown to be well confined in the film plane, reflecting a high orientational order along the plane normal. On the contrary, the heat treatment, which brings about the dissociation of J aggregates, has been found to cause a clear out-of-plane deviation of both axes of the radical molecules. In-plane anisotropy has been found to be retained in the initial stage of heating. The observed orientation is discussed in relation to the structural modification of dye aggregates in LB films.