Guest-Induced Vibration Locking and Fluorescence Switching in a Pyrene-Functionalized Covalent Organic Framework (COF) for Perfluorooctanoic Acid (PFOA) Recognition.

Light-up luminescent covalent organic framework (COF) sensors hold promise for rapid and selective detection of molecular pollutants, yet their performance is often limited by insufficient guest-induced rigidity and incomplete mechanistic understanding. Here, we introduce a pyrene-functionalized flexible covalent organic framework (PY-TTA COF) that undergoes a guest-induced molecular-locking process upon exposure to perfluorooctanoic acid (PFOA), producing a pronounced fluorescence turn-on response in aqueous media. The pendent pyrene chromophores, which project into the pore interior, create an adaptable microenvironment in which PFOA binding, involving hydrogen-bonding interactions, possible proton-transfer processes at imine/pyridine nitrogen sites, and C-F···π interactions, restricts local molecular motions and enhances radiative emission. Solid-state NMR spectroscopy, supported by semiempirical calculations, provides molecular-level insight into the hydrogen-bonding and fluorine-aromatic proximities associated with this locking effect, while PXRD and TEM reveal guest-induced structural modulation while confirming preservation of framework crystallinity. This guest-locked luminescence enhancement enables fluorescence-based and recyclable detection of PFOA with a limit of detection of 2.7 μg L-1. The demonstrated mechanism establishes a design strategy for developing stimuli-responsive COFs with tunable photophysical properties and environmentally relevant sensing capabilities.