# Rotaxane Formation With Intramolecular Charge Transfer Properties for Phosphate Sensing

## TL;DR

A new rotaxane molecule was designed to detect phosphate through changes in fluorescence, using intramolecular charge transfer.

## Contribution

A novel [1]rotaxane with intramolecular charge transfer properties was developed for selective phosphate sensing.

## Key findings

- The linear precursor molecule showed intramolecular charge transfer (ICT) response.
- The [1]rotaxane exhibited significant fluorescence quenching (up to 50%) in the presence of phosphate ions.
- The rotaxane structure inhibits ICT after formation, restoring fluorescence.

## Abstract

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‒).

“ON‐OFF” fluorescence array in [1]rotaxane formation for selective phosphate chemosensing.