Influence of heat treatment in air, and subsequent hydrothermal treatment in the liquid phase or water treatment in the liquid phase on a mixed Langmuir–Blodgett film of merocyanine dye-arachidic acid-n-octadecane ternary system

We have investigated the influence of heat treatment in air (HT), and subsequent hydrothermal treatment in the liquid phase (HTTL) or water treatment in the liquid phase (WTL) on the H-aggregate of mixed Langmuir–Blodgett (LB) films of merocyanine dye (MS18)-arachidic acid (C20)-n-octadecane (AL18) ternary system by means of polarized visible and IR absorption spectroscopy. The MS18 monomer is obtained from the first application of HT to the H-aggregate, and the monomer rapidly changes into the J-aggregate upon subsequent HTTL. This demonstrates variation via the monomer for reorganization of the MS18 chromophore from H- to J-aggregates induced by directly performing HTTL to the H-aggregate in our previous study. While the number of gauche conformers in the MS18 hydrocarbon chain increases by initial HT, the hydrocarbon chain adopts an all-trans conformation after subsequent HTTL. In addition, the degree of orientation of the MS18 hydrocarbon chain after HT also approximates to that before HT. The C20 hexagonal packing after HT turns to orthorhombic one with subsequent HTTL, and the orientation disorder of C20 hydrocarbon chain caused by HT is renovated as well. The structural changes in the MS18 and C20 hydrocarbon chains resulting from latter HTTL arise from the hydrophobic effect in the presence of warm water. Moreover, it has been verified that the AL18 evaporation strongly relates to the dissociation of H-aggregate, but is not responsible for the variation from the monomer to J-aggregate. Comparing the results obtained upon application of HT/HTTL and HT/WTL, it has been concluded that both large relative permittivity and thermal energy inherent in warm water are quite essential in inducing the rapid reconstitution of MS18 aggregation state from the monomer to J-aggregate. These also promote the restoration of conformation and orientation changes in the MS18 hydrocarbon chain, and the modification of subcell packing and orientation disorder in the C20 hydrocarbon chain.