Crystallographic coherent lengths of J-aggregates and their absorption spectra

The Langmuir (L) and Langmuir–Blodgett (LB) films of the J-aggregates formed by amphiphilic merocyanine dye (MD) differ their absorption bands (J-bands), and the J-band in the LB film is located in shorter wavelength than that in the L film. From the grazing incidence X-ray diffraction patterns, the lattice structures and the crystallographic coherent lengths of the MD J-aggregates in the L and LB films were evaluated. Besides a 7% shrinkage of the 2-dimensional lattice in the LB film, more than two-order of magnitude shorter coherent lengths were observed in the LB film, suggesting that the difference of the J-band wavelength was attributed to the size effect of the J-aggregate crystal.