A stimuli-responsive dimeric capsule built from an acridine-based metallacycle for ratiometric fluorescence sensing of TNP†

Stimulus-responsive luminescent metal–organic architectures have received a lot of attentions in supramolecular chemistry. Herein, we report the synthesis of an acridine-based metal–organic macrocycle that undergoes reversible interconversion between the monomer and the dimer states in response to variations in the concentration and solvent, resulting in a switch between blue and green fluorescence. X-ray structure analysis reveals that hydrogen bonds between benzimidazole C–H and NO₃⁻ anions, along with π–π interactions between acridines, are the primary driving forces behind this behavior of the assembly. The stimuli-responsive supramolecular fluorescence switching originates from the monomer and excimer states. The addition of 2,4,6-trinitrophenol (TNP) leads to a fluorescence “turn-off” at 430 nm for the monomer and a “turn-on” at 520 nm for the dimer, thus facilitating the ratiometric detection of TNP with the detection limit being as low as 13 ppb. Our work provides valuable insights into the construction of stimuli-responsive materials for fluorescence sensing.