Spectroscopic study of nonamphiphilic carbazole assembled in Langmuir-Blodgett films: Aggregation induced reabsorption effects

Mono- and multilayers of carbazole mixed with stearic acid (SA) have been prepared by the Langmuit-Blodgett (LB) technique. Surface pressure studies at the air-water interface show that pure carbazole does not form a stable film and collapses at a low surface pressure of about 12 mN/m only. However, carbazole mixed with SA forms excellent films that may be easily transferred onto solid supports. The surface pressure versus area per molecule isotherm studies show that the average area per molecule initially increases and then decreases with increasing mole fraction of carbazole. These results indicate that the carbazole moieties are accommodated in the SA matrix at low concentrations but with increasing concentration are very likely squeezed out of the air-water interface to remain sandwiched in between the fatty acid chains and on the surface of the monolayer. The positive deviation of the observed area per molecule from the ideal curve corresponding to the additivity rule suggests a repulsive interaction between the components in the mixed films. Spectroscopic studies of the LB films reveal a broadening and red shift of the absorption spectra compared to the solution absorption spectrum that confirms the formation of organized aggregates of carbazole in the mixed LB films. Fluorescence studies of the mixed LB films reveal new bands in the low-energy region of the fluorescence emission spectrum. Increasing the concentration of carbazole in the mixed films results in the enhancement of the intensity of these low-energy emission bands and quenching of the high energy bands that maybe attributed to aggregation-induced reabsorption effects indicating the formation of crystallites in the LB films. A comparative study of the spectroscopic characteristics of these aggregates formed in the LB films and in the binary solvent mixtures of ethanol and water reveals identical spectra that confirm formation of crystallites in both the systems.