Five Types of J-Aggregates in Single-Chain Merocyanine Langmuir Monolayers on an Aqueous Subphase

The formation of J-aggregates in Langmuir monolayers of a neat merocyanine dye (3-carboxymethyl-5-[2-(3-octadecyl-2-benzothiazolinyldene)ethylidene]-2-thioxo-4-thiazolidinone) has been examined by measuring the normal incidence absorption spectra. Five types of J-aggregates were found to be formed, depending on the temperature and the concentration and type of cations present in the subphase. The shift in the absorption peak wavelength on the J-aggregates formation is accounted for in terms of the number and the geometrical arrangement of the chromophores in each J-aggregate. The spectral shifts were analyzed numerically using both a point dipole model and a transition density model. These calculations indicate that the five different J-aggregates are linear ones with aggregation numbers ranging from 2 to 6.