Self-assembly Solid-State Enhanced Fluorescence Emission of GFP Chromophore Analogues: Formation of Microsheets and Microtubes Oriented by Molecular Skeleton.

The p-, m- and o-N,N-dimethylamino analogs of the green fluorescent protein (GFP) chromophore (denoted as pDBHI, m-DBHI and o-DBHI) were synthesized by 2,3-cycloaddition. These three compounds were structurally characterized by NMR, HRMS and single crystal X-ray diffraction and were shown to be in the Z-form in both the solid phase and solution. Their fluorescence properties and self-assembly behaviors were investigated by UV-Vis, photoluminescence spectroscopy, fluorescence microscopy and scanning electron microscopy. They exhibited low fluorescence quantum yields in both protic and aprotic solvents, which was consistent with the reported results, and strong emissions in the solid state, thus exhibiting aggregation-induced emission (AIE) behaviors. By a solvent exchange method, the p-DBHI and o-DBHI were assembled into microsheets, while the m-DBHI was assembled into microtubule-like structures. The photoluminescence properties of the assemblies were compared with those of the pristine microcrystalline powders obtained by evaporation from organic solvents. The fluorescence quantum yields of the microcrystals obtained by self-assembly were recorded to 9.86 %, 3.37 % and 31.65 %, respectively, which were much higher than those of the corresponding pristine powders (4.71 %, 2.51 % and 17.03 %). This indicated that the fluorescence properties in the solid state depended on the morphologies of the particles.