Rigidity Matters: Hydrogen Bonding and Inner Filter Effect Govern Selective Sensing of 4-Nitrophenol by Coumarin[4]arenes

Abstract

The sensitive and selective detection of nitrophenolic pollutants is essential for environmental monitoring but remains challenging due to their structural similarity and strong electron-deficient nature. Here, we present coumarin[4]arenes as a new class of macrocyclic fluorescent probes, featuring upper-rim carbonyl and hydroxyl groups that enable nitroaromatic guest recognition within a confined cavity. Rigid and flexible analogues were synthesized and compared to assess the role of macrocyclic conformation in sensing. Steady-state and time-resolved studies reveal a dual quenching mechanism-hydrogen-bond-assisted ground-state complexation coupled with an inner filter effect-with rigidity markedly enhancing discrimination. The rigid coumarin[4]arene exhibits nearly two-fold stronger binding selectivity toward 4nitrophenol relative to flexible one, along with superior emission intensity, improved sensitivity, and a detection limit of 6.9 μM. Beyond solution-phase studies, paper-strip and TLC-based assays enabled rapid, low-cost, and naked-eye detection of 4NP in the 10⁻⁸-10⁻⁶ M range. These results highlight coumarin[4]arenes as versatile supramolecular scaffolds that merge photophysical tunability with practical sensing formats, offering a promising functional macrocycle-based platform for selective, fielddepolyable detection of toxic nitrophenolic contaminants.