Morphology and thermochromic phase transition of merocyanine J-aggregate monolayers at the air-water and solid-water interfaces

Thermal changes of the merocyanine dye (MD) monolayer at the air-water inte rface were investigated under various subphase conditions in order to eluci date the formation mechanism and to control morphological and chromatic pro perties of two-dimensional MD J-aggregate crystallites (JC) formed in the m onolayer. The dissociation temperature (T-d) of the JC to the monomer MD wa s measured for different counterions of MD molecules in the subphase. The J C size was found to be dependent on the subphase temperature; it becomes la rger when the JC is formed at a temperature closer to T-d. This phenomenon is qualitatively reproduced by the numerical simulation of the Cahn-Hilliar d equation. In the case of the MD monolayer on the subphase which contains two kinds of counterions, it exhibits a reversible thermochromic transition between two different JC states. The chromatic change is discrete, and is attributed to the structural phase transition of the JC induced by the mutu al recombination of two kinds of counterions to MD molecules. The structura l difference between the high and low temperature JC states is examined by the point dipole model. The transition temperature and thermal hysteresis w idth can be varied by the fraction of 2 counterions. In situ observations u sing a multipurpose nonlinear optical microscope revealed that the transiti on is of first order and the nucleation and growth process of the low tempe rature phase in the high temperature matrix was observed. The JC size of th e low temperature phase became much larger through the recrystallization pr ocess. For future application of this phenomenon, an airtight cell consisti ng of two monolayers at the solid-water interface and the subphase was deve loped. In the cell, the same reversible transition occurs, but with a slow relaxation. (C) 2001 American Institute of Physics.