Spectroscopic study of J aggregates of amphiphilic merocyanine dyes formed in their pure Langmuir films

It is known that an amphiphilic merocyanine dye, 3-carboxymethyl-5-2-(3-octadecyl-2(3H)- benzothiazolylidene)ethylidene]-2-thioxo-4-thiazolidinone (DS) and its derivatives DSe and 6MeDS form J aggregates under the presence of metallic cations, although the dyes' static dipole moments must be favorable for H aggregates. A metal-free J aggregate of 6MeDS has been generated in its pure Langmuir films and transferred onto CaF2 substrates to investigate the molecular environment around the carboxylic group born by the dye. Combining visible and infrared spectroscopy, geometrical consideration based on ab initio calculations, and simulation of the excitation energy, a structural model of this J aggregate has been proposed. With this model, formation of intermolecular carboxyl-keto hydrogen bonds, which can compensate the electrostatic disadvantage of the J aggregate under the presence of water, has been suggested. As for another derivative of DS, DO, similar discussion has been made for its Mg2+ -containing J aggregate, which is found in this work. In addition, the proposed structural model can tell the cause of the difference in the tendency to J aggregate among 6MeDS, DSe, DS, and DO.