AGGREGATION-INDUCED REABSORPTION OF P-TERPHENYL IN LANGMUIR-BLODGETT-FILMS - A SPECTRAL STUDY

Mono- and multilayers of nonamphiphilic p-terphenyl (TP) mixed with a fatty acid have been incorporated in Langmuir-Blodgett (LB) films. The surface pressure-area isotherm suggests that the molecules stand almost vertically at the air-water interface. In LB films and in ethanol-water binary mixtures, both (1)L(a) and (1)L(b) bands appear in absorption spectra, the latter being absent in pure ethanol. The appearance of the (1)L(b) band has been attributed to the organization of TP molecules. The higher energy bands of the emission spectra in solution are not manifested in the LB film. This phenomenon has been attributed to reabsorption induced by aggregation. This has been further verified by the emission spectra in ethanol-water mixtures. The role of the (1)L(b) band as the reabsorbing state in LB films has been supported by excitation spectra. Comparison of the emission and excitation spectra of TP in solution, in LB films, and in ethanol-water mixtures suggests formation of low-dimensional aggregates of TP molecules in LB films. Scanning electron microscopy also supports the existence of such aggregates in LB films.