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 MS18aggregation state in the pure MS18system are closely and mildly linked with the MS18intramolecular charge transfer and the behavior of the packing, orientation, conformation, and thermal mobility of MS18hydrocarbon chain, respectively. The J-aggregate in the pure MS18system 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 MS18aggregation, appear at around 520 nm in the pure MS18system 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 MS18with 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 MS18system arises from the partial melting of hydrocarbon chain in MS18with Cd2+. [ABSTRACT FROM AUTHOR]