Optical sensing of anions by macrocyclic and interlocked hosts

The ubiquity of anions in biological and environmental systems has motivated the development of many novel anion receptors and sensors over the past two decades. Optical anion sensors, which undergo a spectral change in response to anion binding, are particularly desirable due to the technical simplicity and fast response time of such systems. A myriad of macrocyclic host molecules have been shown to be effective anion receptors and present a promising platform for elaboration into optical anion sensors by incorporation of an appropriate fluorogenic or chromogenic group. The enhanced anion binding properties of the three-dimensional binding cavities in mechanically interlocked host molecules have also been exploited to design anion sensors that exhibit remarkable selectivity and sensitivity. This review summarises recent progress in the development of optical anion sensors based on macrocyclic and interlocked hosts. The major classes of macrocyclic receptors possessing neutral, cationic and metal-based binding motifs are examined, followed by a survey of optically-responsive interlocked anion hosts.