Control of formation and molecular orientation of J-aggregates in Langmuir–Blodgett films of mixed merocyanine dyes

We have fabricated Langmuir–Blodgett (LB) films of mixed-merocyanine dyes, DS and its methyl-substituted analog (6-Me-DS), diluted with arachidic acid (AA), expressed as [6-Me-DS]1−X[DS]X[AA]2, in order to control the J-aggregation of dye molecules. For most of XX-values, the red-shifted J-band peak appeared in the optical absorption spectra with its peak position depending on the mixing ratio of the dyes. In particular, J-band anomalously disappears around X=0.9X=0.9, suggesting the dominance of monomer state. We have also characterized the in-plane preferential orientation of the dye molecules along the dipping direction in the mixed-dye films by the orientation-dependent ESR signals and polarized optical absorption spectra. The optical dichroic ratios were semiquantitatively reproduced by the angular distribution function of the molecular orientation determined by the ESR spectrum simulation. The in-plane orientation of the dye molecules was clearly observed for DS, 6-Me-DS and X=0.2X=0.2 films, whereas it was nearly random for X=0.5X=0.5 film, suggesting the possibility to control the molecular orientational order of dye molecules by employing mixed-dye system.