Modification of aggregate formation in arachidic-acid–cyanine-dye complex Langmuir–Blodgett films by substituent groups

Water-soluble cyanine dye derivatives can be adsorbed from aqueous solution to an arachidic acid monolayer at the air-water interface. Interface-adsorbed complex Langmuir–Blodgett films of arachidic acid and four different derivatives of thiacarbocyanine dyes were fabricated using this method, and their optical properties were examined employing a linearly polarized incident. Clear differences in both spectral shape and in-plane anisotropy were found among the films of different derivatives, each associated with an aggregate form characteristic of the substituent group. They can be classified as a dimer with Davydov splitting, a monomer, and a J aggregate. Estimation of the configuration of the molecules in the dimers and measurement of the photoluminescence spectrum for the J aggregate were performed to clarify the nature of aggregates. The results suggest the possibility of systematic control of aggregate formation in the system by introducing substituent groups for dyes.