Conversion of the Aggregation State of Merocyanine Dye, Modification of the Subcell Packing of Arachidic Acid, and Removal of the Majority of <i>n</i>-Octadecane by Hydrothermal Treatment in the Liquid Phase in a Mixed Langmuir−Blodgett Film of the Ternary System

We have investigated the influence of heat treatment in an air atmosphere (HT) and hydrothermal treatment in the liquid phase (HTTL) on the H-aggregate in a mixed Langmuir−Blodgett (LB) film of merocyanine dye with an octadecyl group (MS18)−arachidic acid (C20)−n-octadecane (AL18) ternary system by means of polarized visible and IR absorption spectroscopy. HT causes the variation from the H-aggregate to the monomer, the increment in the number of gauche conformers in the MS18 hydrocarbon chain, the slight orientation change in the C20 hydrocarbon chain, and the complete evaporation of AL18. The dissociation of MS18 is probably ascribed to the complete evaporation of AL18 from the mixed LB film and the increase in thermal mobility of the long axis of the MS18 hydrocarbon chain during HT. However, HTTL can easily and rapidly induce the conversion of the MS18 aggregation state from H- to J-aggregates, the modification of the C20 subcell packing from hexagonal to orthorhombic, and the removal of most of the AL18 molecules. The conversion of the MS18 aggregation state can be interpreted to consist of two processes from the H-aggregate to the monomer and from the monomer to the J-aggregate. In the initial stage of HTTL, the MS18 aggregation state changes from the H-aggregate to the monomer, which is caused by the removal of almost all of the AL18 molecules from the mixed LB film to warm water via the thermal energy of warm water. Then, the large relative permittivity of warm water is expected to relate strongly to the subsequent variation from the monomer to the J-aggregate. This transformation results in the decrease in the total value of the electrostatic energy based on the MS18 permanent dipole interaction. Moreover, the modification of the C20 subcell packing is possibly due to the hydrophobic effect, where the C20 hydrocarbon chains cohere again in the warm water during HTTL. Consequently, it has been found that HTTL is quite effective to reorganize the chromophore alignment of MS18, to modify the subcell packing of C20 and to erase the majority of AL18 molecules in the mixed LB film of the MS18−C20−AL18 ternary system in a short time

Thermal Behavior of J-Aggregates in a Langmuir−Blodgett Film of Pure Merocyanine Dye Investigated by UV−visible and IR Absorption Spectroscopy

We have characterized the structure of J-aggregate in a Langmuir−Blodgett film of pure merocyanine dye (MS18) fabricated under an aqueous subphase containing a cadmium ion (Cd2+) and have investigated its thermal behavior by UV−visible and IR absorption spectroscopy in the range from 25 to 250 °C with a continuous scan. The results of both UV−visible and IR absorption spectra indicate that temperature-dependent changes in the MS18 aggregation state in the pure MS18 system are closely and mildly linked with the MS18 intramolecular charge transfer and the behavior of the packing, orientation, conformation, and thermal mobility of MS18 hydrocarbon chain, respectively. The J-aggregate in the pure MS18 system dissociates from 25 to 150 °C, and the dissociation temperature at 150 °C is higher by 50 °C than that in the previous MS18-arachidic acid (C20) binary system. The lower dissociation temperature in the binary system originates from the fact that temperature-dependent structural disorder of cadmium arachidate (CdC20), being phase-separated from MS18, has an influence on the dissociation of J-aggregate. From 160 to 180 °C, thermally induced blue-shifted bands, caused by the oligomeric MS18 aggregation, appear at around 520 nm in the pure MS18 system by contraries, regardless of the lack of driving force by the melting phenomenon of CdC20. The temperature at which the 520 nm bands occur is in good agreement with the melting point (160 °C) of hydrocarbon chain in MS18 with Cd2+, whereas its chromophore part is clearly observed to melt near 205 °C by UV−visible spectra. Therefore, it is suggested that the driving force that induces the 520 nm band in the pure MS18 system arises from the partial melting of hydrocarbon chain in MS18 with Cd2+

Structural Characterization of a Mixed Langmuir−Blodgett Film of a Merocyanine Dye Derivative−Deuterated Arachidic Acid Binary System and the Influence of Successive Hydrothermal Treatment in the Liquid Phase on the Film as Investigated by Polarized UV−Visible and IR Absorption Spectroscopy

We have investigated the structure of the mixed Langmuir−Blodgett (LB) film of a merocyanine dye derivative (MO18)−deuterated arachidic acid (C20-d) binary system and the influence of successive hydrothermal treatment in the liquid phase (HTTL) on the mixed LB film by means of polarized UV−visible and IR absorption spectroscopy. The visible absorption band with in-plane anisotropy at 503 nm before HTTL transforms into an absorption band with in-plane isotropy at 557 nm after HTTL for 16−18 min through a peak maximum near 520 nm after HTTL for 2−12 min. The degree of total MO18 intramolecular charge transfer for the 503 nm band is the largest among those for all of the bands. Therefore, the 503 nm band is ascribed to the MO18 H-like aggregation, based on its shape, peak height, and in-plane anisotropy, the subsequent change to two kinds of visible peaks by successive HTTL, and the most degree of MO18 intramolecular charge transfer among all of the aggregation states. While the MO18 hydrocarbon chain takes the all-trans conformation before HTTL, its conformation and orientation are most disarranged after HTTL for 2 min. Subsequently, the original conformation and orientation are recovered by degrees with successive HTTL, except after final HTTL for 18 min, when the orientation is again changed. On the other hand, the C20-d hydrocarbon chain maintains the all-trans conformation before and after HTTL. The orientation of the C20-d hydrocarbon chain after HTTL for 2 min is more ordered than that before HTTL, with the nature of the C20-d subcell packing changing from hexagonal to orthorhombic. During successive HTTL from 2 to 18 min, the C20-d orientation is gradually disorganized but with the orthorhombic nature remaining constant. Thus, the variations in the conformation and orientation of the MS18 hydrocarbon chain and in the orientation of the C20-d hydrocarbon chain tend to change from ordered and disordered structures and turn to more disordered and ordered ones, respectively, where the former is mainly caused by the priority action of thermal energy and the latter by hydrophobic effect due to the presence of warm water. Consequently, it is suggested that there is a correlation between the degree of structural order for both hydrocarbon chains and the preferential action that takes place during HTTL