Unification of molecular NIR fluorescence and aggregation-induced blue emission via novel dendritic zinc phthalocyanines

Abstract

In this work, novel dendritic zinc phthalocyanines showing both molecular NIR fluorescence and aggregation-induced emission characteristics have been successfully synthesized via a two-step reaction on the basis of bisphthalonitrile precursors with different electron donor–conjugation–acceptor (D–π–A) structures. The luminescent properties of the resultant dendritic zinc phthalocyanines were fully investigated in N, N-dimethylformamide (DMF) solution, in DMF/H2O mixed solvent, in the solid state as well as polymethylmethacrylate composite film. The dendritic zinc phthalocyanines substituted with hydroquinone, naphthalenediol, or dihydroxybiphenyl moieties exhibited an unusual luminescence changes from highly near infrared (NIR) emitting in molecular state to strongly blue fluorescence in aggregated state as well as solid state, where the maximum fluorescent emission wavelength and highest quantum yield were detected from dendritic zinc phthalocyanines containing naphthalenediol and dihydroxybiphenyl groups, respectively. On the contrary, dendritic zinc phthalocyanines bearing diphenylmethane or bisphenol A units only exhibit NIR fluorescent emission in their molecular state. The distinct fluorescent emissions for these dendritic zinc phthalocyanines are rationalized in the framework of molecular configuration and electron density distribution as clarified by density functional theory (DFT) calculation. Moreover, the well-organized three-dimensional microspheres composed of abundant rod-like building blocks were discovered via the self-assembly of dendritic zinc phthalocyanine molecules in DMF/H2O (50/50 vol%) mixture.