Rotaxane Formation With Intramolecular Charge Transfer Properties for Phosphate Sensing

Mechanically interlocked molecules (MIMs), specifically rotaxanes, have been demonstrated to have immense utility as sensing materials with a wide array of analytes, owing to their unique topologies and properties afforded by the presence of a mechanical bond. Among other more conventional sensor compounds, aromatic amide functionalities have been employed to induce intramolecular charge transfer (ICT) processes, where a crown ether donor oxygen atom donates electron density to an aromatic amide acceptor, resulting in reduced or no emission; this methodology has received less attention for its application in rotaxanes, particularly to detect anionic species. Hence, in this work, we were inspired to construct a novel kinetically stable [1]rotaxane comprising dibenzo[24]crown-8 donor and N-benzylbenzamide acceptor moieties, namely 1-H(Rot)·PF₆, using a template-directed “slippage” approach. Before [1]rotaxane formation, the linear molecule showed ICT response. Fluorescence response was restored after the [1]rotaxane formed as the dialkylammonium group threaded through the crown ether, inhibiting ICT from the oxygen atoms to the aromatic amide. The [1]rotaxane showed marginal fluorescence quenching when titrated with different metal cations by a photoinduced electron transfer (PeT) mechanism; however, when titrated against various inorganic anions, 1-H(Rot)·PF₆ exhibited substantial emission quenching (up to ca. 50%) in the presence of phosphate (PO₄^3‒).