Metal Coordination and Isomerization of a Hydrazone Switch

Molecular photoswitches in aqueous environments

Molecular photoswitches enable dynamic control of processes with high spatiotemporal precision, using light as external stimulus, and hence are ideal tools for different research areas spanning from chemical biology to smart materials. Photoswitches are typically organic molecules that feature extended aromatic systems to make them responsive to (visible) light. However, this renders them inherently lipophilic, while water-solubility is of crucial importance to apply photoswitchable organic molecules in biological systems, like in the rapidly emerging field of photopharmacology. Several strategies for solubilizing organic molecules in water are known, but there are not yet clear rules for applying them to photoswitchable molecules. Importantly, rendering photoswitches water-soluble has a serious impact on both their photophysical and biological properties, which must be taken into consideration when designing new systems. Altogether, these aspects pose considerable challenges for successfully applying molecular photoswitches in aqueous systems, and in particular in biologically relevant media. In this review, we focus on fully water-soluble photoswitches, such as those used in biological environments, in both in vitro and in vivo studies. We discuss the design principles and prospects for water-soluble photoswitches to inspire and enable their future applications.

Multistimuli-Responsive Enaminitrile Molecular Switches Displaying H+-Induced Aggregate Emission, Metal Ion-Induced Turn-On Fluorescence, and Organogelation Properties

Multistimuli-responsive enaminitrile-based configurational switches displaying aggregation-induced emission (AIE), fluorescence turn-on effects, and supergelation properties are presented. The E-isomers dominated (>97%) in neutral/basic solution, and the structures underwent precisely controlled switching around the enamine C═C bond upon addition of acid/base. Specific fluorescence output was observed in response to different external input in the solution and solid states. In response to H⁺, configurational switching resulted in complete formation of the nonemissive Z-H⁺-isomers in solution, however displaying deep-blue to blue fluorescence (Φ_F up to 0.41) in the solid state. In response to Cu^II in the solution state, the E-isomers exhibited intense, turn-on, blue-green fluorescence, which could be turned off by addition of competitive coordination. The acid/base-activated switching, together with the induced AIE-effects, further enabled the accomplishment of a responsive superorganogelator. In nonpolar solvents, a blue-fluorescent supramolecular gel was formed upon addition of acid to the E-isomer suspension. The gelation could be reversed by addition of base, and the overall, reversible process could be repeated at least five cycles.

Hydrazone switches and things in between

This feature article surveys the various ways by which a structurally simple hydrazone can be used in accessing different functional materials, mainly photo/chemically activated switches, fluorophores and sensors.

Multiresponsive Dynamic Covalent Assemblies for the Selective Sensing of Both Cu(2+) and CN(-) in Water

The use of dynamic assembly for molecular sensing is an intensive area of research in supramolecular chemistry. However, the development of self-assembly architectures for the detection of multiple signals remains challenging. Here, we present dynamic covalent assemblies with multiresponsive properties that also show unique selectivity profiles in water. The receptors were generated in a single step with modularly designed building blocks through acylhydrazone linkages, and their orthogonal assembly with a series of external stimuli was investigated. Notably, the system exhibits responses toward cations, anions, solvents, pH, and amphiphilicity. The discrimination of Cu(2+) from other divalent metal ions was achieved by simply changing the solvent and was evidenced by a Cu(2+)-induced pKa shift. The selective recognition of CN(-) in pure aqueous media was also accomplished through a cooperative effect in conjunction with Zn(2+). Furthermore, the assembly and its responsiveness are functional both in solution and the solid state. The aggregation ability of these dynamic covalent systems supports their binding and sensing properties.