Structural model of merocyanine dye Langmuir–Blodgett films with blue-shifted band

We have studied the red-shifted and the blue-shifted bands in the mixed Langmuir–Blodgett (LB) films of the merocyanine dye (MS)–arachidic acid (C20)–n-octadecane (AL18) ternary systems with the molar mixing ratio [MS]:[C20]:[AL18]=1:2:x (0.5≤x≤1.5). The overlapping spectra of the red-shifted and the blue-shifted bands are typically seen in the ternary systems and they are deconvoluted into the J- and H-bands. We compare the aggregation number N and the other structural parameters of the J- and H-aggregates estimated using the point dipole model and the extended dipole model referring to the deconvoluted spectra. For the J-aggregate of MS, the minimum aggregation number is estimated to be Nmin=40 in both models. The slip angle between the long axis of the aggregate and the MS transition dipole moment is estimated as α=40° and α=30° for the point dipole model and the extended dipole model, respectively. The latter α-value is in good agreement with those in the earlier works, while the former α-value is appreciably larger than the earlier estimates, reconfirming the validity of the extended dipole model. The α-value is estimated to be 50° for the H-aggregate, assuming the same Nmin-value based on the results of the extended dipole model.