Large birefringence of single J aggregate nanosheets of thiacyanine dye in solution

Concentration-dependent string-to-sheet morphological transition of supramolecular thiacyanine J aggregates was demonstrated in aqueous solutions. Fluorescence microscopy and atomic force microscopy revealed that the string-like J aggregate was a bundle of thinner strings, whereas the sheet-like aggregate had a rectangular structure with a two-dimensional monomolecular layer of molecules (nanosheet). The well-defined morphology of the single nanosheet allows us to evaluate its optical anisotropy with polarized-light microscopy. The rectangular nanosheet of J aggregates showed large negative birefringence of about −1.2 ± 0.2 (at 550 nm), indicating that the dye molecules are likely to orient along the short axis of the sheet.