A spectroscopic study of nonamphiphilic pyrene assembled in Langmuir-Blodgett films: formation of aggregates

Abstract

Nonamphiphilic pyrene molecules mixed with a fatty acid have been incorporated in Langmuir-Blodgett (LB) films. The surface pressure versus area per molecule isotherms show that the average area per molecule at first increases and then decreases with increasing mole fraction of pyrene in mixtures of pyrene and stearic acid (SA), which indicates that the pyrene molecules are very likely squeezed out of the air-water interface in between the fatty acid chains and are on the surface of the monolayer. A positive deviation from the additivity rule has been observed in the plot of the average area per molecule versus the mole fraction of pyrene, which suggests a repulsive type of interaction between the pyrene and the fatty acid molecules. Spectroscopic studies reveal the formation of I-aggregates. The emission spectra of mixed films of pyrene and stearic acid show a single broad band at 420 nm which has been assigned to a partial excimeric emission band originating from an incomplete or partial overlap of the pyrene molecules. Such an incomplete or partial overlap along with its small area per molecule at the air-water interface add credence to the thesis of formation of I-aggregates of pyrene molecules in mixed LB films. The possible aggregation has further been verified by spectroscopic studies of pyrene in ethanol-water mixtures. The partial excimer to monomer intensity ratio is seen to increase sharply with increasing volume fraction of water in the binary mixture of ethanol and water above a critical composition of the mixture corresponding to a volume fraction of 0.75 for water, suggesting the onset of formation of aggregates. The discussion on the excitation spectra of pyrene in LB films and in ethanol-water mixtures also supports the propositions of the formation of aggregates. Finally, scanning electron microscopy has been employed to reveal the existence of microcrystalline aggregates in the LB films.