Reversible Acid-Base Responsive Fluorescence Changes of Solutions and Crystals Based on Anthracenyl Pyridyl Derivatives

Organic compounds that can respond to external stimuli and exhibit fluorescence changes have drawn increasing attention recently because of their potential applications in intelligent displays, optical data storage, anticounterfeiting, bioimaging, and sensors. Herein, we have synthesized two new organic compounds based on frameworks of anthracene and pyridine groups: 4-(anthracen-9-yl) pyridine (AN9P) and 4-(anthracen-2-yl) pyridine (AN2P). Both compounds, in solution and solid state, including polycrystals and single crystals, display reversible fluorescence transformations under alternate acid and base treatments. AN9P and AN2P solutions could be regulated to emit white-light luminescence. The photoluminescence of the AN9P and AN2P polycrystals showed fast fluorescence changes with wide ranges (>300 nm) upon alternate acid and base stimuli and still exhibited remarkable fluorescence emission with almost no attenuation after 15 cycles of the reversible process. Both experimental and computational results suggested that the heteroatom nitrogen in the AN9P and AN2P molecules significantly influenced the intra- and intermolecular electronic interactions during the reversible protonation and deprotonation processes, resulting in changes in their frontier molecular orbitals and fluorescence emission characteristics. Our results provide a new facile approach to design molecular structures that realize highly dynamic photoluminescence changes in both liquid solution and solid crystal.