Dye aggregates formed in Langmuir–Blodgett films of amphiphilic merocyanine dyes

The electronic absorption spectra of pure Langmuir (L) films of 6MeDS prepared upon MgCl2, CaCl2, CdCl2, and CuCl2 aqueous solutions have shown that the Mg2+, Ca2+, and Cd2+ cations promote the J-aggregation of the dye, but the Cu2+ cation inhibits it. Instead, the Cu2+ cation promotes the formation of another type of dye aggregate (D-aggregate), which exhibits a split electronic absorption band. These L films have been transferred onto solid substrates by the Langmuir–Blodgett (LB) technique. The vibronic absorption spectra observed for the obtained LB films have shown the similarity between the metal–cation-containing J-aggregate and the metal-free J-aggregate of the dye, the latter of which was characterized in the previous study [K. Ikegami, J. Chem. Phys. 121 (2004) 2337]. These spectra have also indicated that the bimolecular metal chelation plays an important role in the J-aggregation, like the intermolecular hydrogen bonding in the metal-free case.