Morphology transformation of mesoscopic supramolecular J aggregates in solution

Abstract

Mesoscopic structures of 5,5′-dichloro-3,3′-disulfopropyl thiacyanine (TC) J aggregates in aqueous electrolyte solution were characterized by using fluorescence microscopy, microspectroscopy and atomic force microscopy. The aggregates formed large strings and rods, several tens of micrometres in length and sub-micrometres in width. The aggregates showed a spontaneous morphology transformation from a string to a rod structure when they were kept (or “aged”) in the solution, and the transformation was promoted by the addition of an electrolyte (NaCl). This is the first observation of the dynamic transformation of the morphologies of J aggregates in solution. Fluorescence microscopy under polarized light excitation indicated that the string-like J aggregates had a molecular alignment in which the long axis of the constituent TC molecules was approximately parallel to that of the string, whereas the rod-like aggregates had randomly arranged structural units (or domains) that had well-ordered molecular structure with the spectroscopic characteristics of J aggregates. Atomic force microscopy revealed that each rod-like J aggregate was an ensemble of striated domains. A packing geometry of TC arising from the flexibility of the charged sulfopropyl substituents could cause the morphology transformation of the mesoscopic aggregates. In situ fragmentation and transformation of a J aggregate structure induced by photoirradiation was also demonstrated.