Substituent-dependent self-assembly: Two-dimensional aggregate formation in cyanine dye-adsorbed Langmuir-Blodgett films

Interface-adsorbed complex Langmuir-Blodgett (LB) films of arachidic acid and water-soluble, mesosubstituted cyanine dyes were fabricated using the diffusion-adsorption method to examine the substituent-dependent self-assembly at the water-monolayer interface. Either a J aggregate or an H aggregate was found to be formed in the LB films depending on the substituent introduced, the ethyl or methyl group. The H aggregate is accompanied by Davydov splitting, which is characterized as the split of the band into two bands, whose transition moments are orthogonal to each other. Measurements of photoluminescence and microscopic spectroscopy were performed to identify the type of the aggregates. The molecular arrangements in the aggregates were estimated based on Davydov's theory and the extended dipole model. Dependence of photoelectric properties of the LB films on the type of aggregates has been studied employing Schottky-type cells.