Polymer-assisted crystallization and optical properties of uniform microrods of organic dye Sudan II.

The self-assembly of uniform single-crystalline microrods of organic dye Sudan II (1-(2,4- dimethylphenyl-azo)-2-naphthol) in solution was achieved by reprecipitation with the assistance of amphiphilic block copolymer Pluronic F127. It was found that the formation of F127 micelles was favorable for the production of uniform Sudan II microrods whereas individual F127 molecules favored the formation of Sudan II fibers. The optical absorption and fluorescence emission properties of the uniform microrods were characterized, and the time-dependent spectra of the precipitating solution for microrod formation were measured to monitor the self-assembly process of Sudan II molecules. The Sudan II microrods exhibited intense fluorescence emission, indicating an unusual aggregation-induced emission (AIE) phenomenon for Sudan II. The fluorescent microscopy investigation suggested that the obtained Sudan II microrods exhibited remarkable optical waveguide behavior, which is expected to serve as single rod active optical waveguides that can be used as building blocks for miniaturized photonic devices. Furthermore, novel Sudan II microplates, which exhibited interesting 2D optical waveguide behavior, were readily obtained by reprecipitation of Sudan II from cationic surfactant cetyltrimethylammonium bromide (CTAB) solution.