J-aggregate to J-aggregate relaxations in langmuir films of amphiphilic merocyanine dye derivatives studied by optimum difference spectrum method

Langmuir (L) films of amphiphilic merocyanine dye derivatives (DS and DSe) mixed with arachidic acid have been prepared on a Mg2+-containing aqueous subphase. Time evolution of the molar absorption coefficient (ɛ) spectra observed for them has shown that J-aggregates with relatively large red shifts (ca. 2800 cm−1) are the majorities at the initial stage, but J-aggregates with smaller shifts (ca. 2400 cm−1) become major within few minutes of compression. Isosbestic points clearly seen in the time evolution of the spectra imply those films include no other appreciable components. A similar phenomenon has been observed for DS films on Ca2+-containing subphase. The pure spectra of the initial and final J-aggregates have been extracted from the experimentally obtained ɛ spectra by employing the optimum difference spectrum (ODS) method. Semi-quantitative discussion based on the extracted pure spectra has suggested that the observed phenomena are structural relaxations, i.e., changes in the molecular arrangements in the J-aggregates, and that the interactions between the static dipoles born by the dye molecules are not the main driving force of the J-aggregation of DS and DSe.