Morphology transformation of mesoscopic supramolecular J aggregates in solution

Mesoscopic structures of 5,5'-dichloro-3,3'-disulfopropyl thiacyanine (TC) J aggregates in aqueous electrolyte solution were characterized by using fl uorescence microscopy, microspectroscopy and atomic force microscopy. The a ggregates formed large strings and rods, several tens of micrometres in len gth and sub-micrometres in width. The aggregates showed a spontaneous morph ology transformation from a string to a rod structure when they were kept ( or "aged") in the solution, and the transformation was promoted by the addi tion of an electrolyte (NaCl). This is the first observation of the dynamic transformation of the morphologies of J aggregates in solution. Fluorescen ce microscopy under polarized light excitation indicated that the string-li ke J aggregates had a molecular alignment in which the long axis of the con stituent TC molecules was approximately parallel to that of the string, whe reas the rod-like aggregates had randomly arranged structural units (or dom ains) that had well-ordered molecular structure with the spectroscopic char acteristics of J aggregates. Atomic force microscopy revealed that each rod -like J aggregate was an ensemble of striated domains. A packing geometry o f TC arising from the flexibility of the charged sulfopropyl substituents c ould cause the morphology transformation of the mesoscopic aggregates. In s itu fragmentation and transformation of a J aggregate structure induced by photoirradiation was also demonstrated.