J-aggregate formation of 6-methyl-merocyanine dye in mixed Langmuir-Blodgett films with arachidic acid in a wide range of mixing ratio

By changing the dye concentration with respect to the fatty acid matrix, J-aggregate formation of the 6-methyl-merocyanine dye (6Me-DS) molecules in the mixed Langmuir-Blodgett film is studied by optical absorption and atomic force microscopy (AFM). The J-band is observed in the film with a wide range of molar ratios of the dye to the matrix down to 1:120. The absorption peak shift of approximately 80 nm remains almost unchanged with the decrease of the molar ratio. The dye domains are identified by AFM as the region with low height. This is confirmed by comparing the total area of low-height domains with that calculated with the molecular occupation area ratio on the water surface. Typical dye domains with more than 103 molecules are observed, even in the films of 1:120 molar ratio. The size is consistent with the expected size of J-aggregates calculated with the extended dipole model using the observed wavelength shift.