Bis(azacrown ether) and bis(benzocrown ether) dyes: butterflies, tweezers and rods in cation binding

This paper focuses on various chromophores that have been covalently linked to two azacrown or benzocrown ether units. These compounds can interact with near-UV or visible light. By comparison with monocrown dyes, they display particular behaviour in the presence of cations. The crowns may have the possibility to act in concert, thus forming sandwich complexes around the guest. This often leads to increased selectivity towards large cations. This is the case for photochromic crown ethers, which allow the photochemical control of cation binding, as well as for some optical sensors, designed for cation recognition. Biscrown dyes can also be rigid structures in which the two spatially separated crowns cannot co-operate intramolecularly. However, intermolecular sandwich complexes can be detected. They range from discrete face-to-face arrangements to extended supramolecular architectures, which can generate nanosized aggregates and materials. The variety of possible combinations with cations bestows a particular interest on biscrown dyes.