Structural phase transition of J-aggregate crystallites on the binary counter-ion subphase

The recently found chromic phase transition of merocyanine J-aggregates monolayer on the aqueous subphase containing Cd²⁺ and Mg²⁺ was investigated by the grazing-incidence X-ray diffraction method. The evidence of the structural difference between the phases of J-band at 620 nm and 590 nm was obtained by the investigations on the monolayers prepared at the different molar ratios of Cd²⁺ and Mg²⁺. The change in a Bragg's peak was observed at the ratio of the ions, where the J-band wavelength also exhibited the discrete shift. The relationship between the obtained results and the phase diagram determined by the shift of the J-band wavelength was also discussed from the viewpoint of the formation mechanism of J-aggregate crystallites.