Mesodomain separation in amalgamated J aggregate formation of cyanine dyes at a mica/solution interface

Aqueous mixed solutions of pseudoisocyanine (PIC) and its structural analogous asymmetric cyanine (AC) dye formed self-assembled amalgamated J aggregates (mixed J aggregates with a single excitonic band) at a mica/solution interface, and the formation mechanism of the aggregates was clarified by determining the optical, structural, and morphological properties of such aggregates. In this study, mesodomain separation in amalgamated J aggregate formation of the cyanine dyes at a mica/solution interface was revealed for the first time. Fluorescence microscopy showed that the emissive amalgamated J aggregates possessed mesoscopic island domain morphology. The aggregates covered the interface completely when the AC mole fraction was small (below 0.25). With an increase in the AC mole fraction, the emissive J aggregate islands decreased in size and number and were discretely distributed, although atomic force microscopy (AFM) showed that the interface was still completely covered by adsorbed molecules. Our results show that dye adsorption from the PIC/AC mixture solutions generated two types of domain formation at the mica/solution interface (mesodomain separation): emissive amalgamated J aggregate island (PIC-rich domain) and non-emissive island in the absence of J aggregates (AC-rich domain). On the basis of the anisotropic growth behavior of the islands, we proposed the internal structures of the emissive and non-emissive domains at the interface. Fluorescence microspectroscopy also revealed that each J aggregate mesodomain exhibited a different emission maximum.