Spectroscopic studies of nonamphiphilic coronene assembled in langmuir-blodgett films: aggregation-induced reabsorption effects

This paper reports the incorporation of large flat molecules of coronene in Langmuir-Blodgett (LB) films mixed with stearic acid(SA). The spectroscopic characteristics of these films have been investigated here. An outstanding feature of this study is the appearance of the S-1-S-0 absorption band at 420 nm in the LB films. Interestingly, in solution this band is never observed owing to this transition being a electric-dipole forbidden arising from the D-6h symmetry of the molecule. Spectroscopic studies of these mixed LB films of coronene and SA indicate the formation of aggregates. The spectroscopic characteristics of the aggregates formed in the LB films were found to be identical to those formed in a binary solvent mixture of ethanol and water that confirm the formation of microcrystallites in the mixed LB films and binary solvent mixtures. A comparative study of the fluorescence excitation spectra of the aggregates formed in the binary solvent mixtures and in the LB films reveals the hidden S-0-S-1 transition in LB films in contrast to that in solution where the S-0-S-1 transition is not manifested. Furthermore, the excitation spectra of the aggregates formed in different systems were found to be different suggesting different packing configuration of the molecules in the aggregates formed in different systems.