Kinetics of Hydrothermally Induced Reorganization of J-Aggregate

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We have investigated the kinetics of the J-band reorganization process in merocyanine (MS)–Cd arachidate mixed Langmuir–Blodgett (LB) films induced by heat treatments at 100% humidity (HTTs). The as-deposited J-band is reorganized by HTT to form a new phase associated with a narrowing and a slight redshift of the peak. The change in the UV–visible absorption spectrum $A$ is found to involve two different stages: an increase in the absorbance $A$ at wavelengths shorter than ${\sim}540$ nm at the expense of the as-deposited J-component and a decrease in $A$ for the shorter wavelengths compensated by the growth of the new phase of the J-band. By introducing a three-component model, each spectrum is deconvoluted into three bands, Bands I, II, and III, which are presumably assigned as the blueshifted dimer, monomer and redshifted J-bands, respectively. The Band II fraction remains almost unchanged throughout the HTT process, indicating that the first and second stages of the spectral change can be interpreted as the dissociation of the as-deposited J-band into the dimers and as the reorganization of the J-band out of the dimers, respectively. The second-stage kinetics is found to obey a simple first-order law. The HTT-induced superstructures have been quantitatively examined referring to the average size $\langle d\rangle$ of the domains observed in the microscopic images. It has been suggested that the domain growth is a phenomenon occurring subsequent to the J-band reorganization process rather than being an aspect of the process, since a superlinear increase in $\langle d\rangle$ with heating time is observed with a long delay after the saturation of J-band growth.
2008-02-25

Kinetics of Hydrothermally Induced Reorganization of J-Aggregate

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