J-aggregates in Langmuir-Blodgett films of a merocyanine dye without metallic cations: Using FT-IR to observe changes in the electronic structure of the molecules upon aggregation

Langmuir-Blodgett films of mixtures of fatty acids (FA) and a merocyanine dye (DS), bearing one long alkyl chain and one carboxylic group, were prepared at a nitrogen-pure water interface. The films exhibit a distinct and highly anisotropic J-band near 605 nm, even though they do not contain any metallic or ammonium cations. Although an isotropic band was seen in the region of 500-550 nm, the contribution of the J-aggregates was extracted from the observed visible and IR absorption spectra by using the difference in the polarization of the bands. This analysis indicates a change in the electronic structure of DS in the J-aggregates, probably corresponding to intramolecular charge transfer. Furthermore, this analysis also suggests that in this metal-free case, the formation of the J-aggregates is strongly affected by the hydrogen bonding between the carboxylic groups in two DS and that between the carboxylic groups in DS and FA.