A novel family of AIE-active meso-2-ketopyrrolyl BODIPYs: bright solid-state red fluorescence, morphological properties and application as viscosimeters in live cells
Fluorescent probes towards viscosity determination can be used to report on variations in local molecular viscosity and have become valuable tools for the study of intracellular microenvironments. Herein, we develop a family of remarkable meso-2-ketopyrrolyl-derived BODIPY rotors and report a new strategy for building them via simple condensation reactions between oxalyl chloride and substituted pyrroles. They are well characterized by NMR, HRMS, crystal structure, spectroscopic and morphological studies. These uncommon meso-2-ketopyrrolyl-derived BODIPYs are weakly fluorescent in their organic solutions but exhibit bright solid-state red fluorescence from 620 to 661 nm with a maximum fluorescence quantum yield of 25%. All of them also exhibit notable aggregation-induced emission (AIE)-active features. Interestingly, the formed nanoaggregates were observed to exhibit morphological distinctions owing to different substituents. The fluorescence enhancement towards the increased viscosity of the environment might be mainly attributed to the restriction of their intramolecular rotations of the meso-ketopyrrolyl groups. More interestingly, the increase in the fluorescence lifetime of these representative rotors appears to perfectly correlate with the increase in the viscosity of the media, and they have been used as viscosimeters for real-time quantitative determination of the variation of the intracellular viscosity in live cells. The easy synthetic strategy and real-time quantitative determination of the variation pave a new way for creating fluorescent molecular rotor materials.