Facile morphological control of fluorescent nano/microstructures via self-assembly and phase separation of trigonal azobenzenes showing aggregation-induced emission enhancement in polymer matrices

Abstract

We report a facile and mild strategy for constructing diverse fluorescent nano/microstructures via self-assembly and phase separation of trigonal azobenzene chromophores (3N1s) showing aggregation-induced emission enhancement (AIEE) in polymer matrices [poly(methyl methacrylate) (PMMA) and/or poly(4-chlorostyrene) (PSCl)]. Thermal treatment above the glass transition temperature enhances the large-scale molecular motions of the polymer chains, which causes AIEE-active 3N1 molecules to assemble into fluorescent nanorods and long nanosticks in the confined homopolymer (PMMA and PSCl, respectively) matrices. Strikingly, as-prepared 3N1–PMMA–PSCl ternary mixtures exhibit a splendid raspberry-like morphology. In other words, the uneven island-like surfaces with micrometer-scale round protuberances are decorated with red fluorescent nanospheres. This result can be interpreted as surface-directed phase separation of immiscible PMMA and PSCl during quick solvent evaporation, which would help the 3N1 components instantaneously assemble into nanospheres on uneven surfaces. By annealing above the glass transition temperature, a distinct morphological transformation from a raspberry-like to a bead-like structure could readily be visualized via (i) the inherent assembly of 3N1 molecules into red fluorescent spherical or 1D aggregates and (ii) the selective fluorescence marking due to the difference in compatibility between 3N1 and PMMA or PSCl.