Photomediated chemical systems have gained increasing attention due to their high spatiotemporal resolution and easy access to high-energy states. Photoswitches represent a class of bistable switches that respond to light and offer many opportunities for active regulation of assemblies, motors and catalysis. By coupling photoswitches with proteins, light-triggered allosteric control of ligand binding and transmembrane receptors has been realized. However, photoinduced dynamic multistep chemical cascades are still rare. 

In a study published in Journal of the American Chemical Society, a research group led by Prof. YOU Lei from the Fujian Institute of Research on the Structure of Matter of the Chinese Academy of Sciences demonstrated a versatile platform of photoswitchable covalent cascades for remote and bidirectional control of reversible covalent bonds and subsequent assemblies. The relay of a photochromic switch, keto-enol equilibrium and ring-chain equilibrium allows light-mediated reversible allosteric structural changes. The results resemble allosteric control in biological signaling networks and should set the stage for many endeavors. 

The researchers prepared a novel photoswitching molecule by the combination of 2-formylphenyl group and dithienylethene (DTE)-attached cyclic 1,3-diketone. Upon irradiation in different wavelength, they found that the nearly complete reversal of open aldehyde and cyclic hemiacetal tautomers was accomplished with the cascade of pericyclic 6π-cyclization/cycloreversion, keto-enol tautomerism, and ring-chain tautomerism.  

With a photoswitchable cascade for allosteric and reversible structural regulation in place, the researchers moved on the manipulation of dynamic covalent reactions towards bidirectional control, and discovered the unique reactivity between the open aldehyde and cyclic hemiacetal tautomers, which allows the realization of photoswitchable dynamic covalent capture/ release of alcohols/thiols in a bidirectional manner. 

Taking advantage of light-mediated covalent connection and disconnection, the researchers investigated the modulation of dynamic assemblies towards increased complexity, and constructed three types of dynamic polymers. The related photoswitchable creation/degradation and structural reconfiguration were characterized by the collection of gel permeation chromatography, Fourier transform infrared and differential scanning calorimetry analyses.  

The findings of this study may find applications in many aspects such as dynamic assemblies, molecular machines, adaptable polymers, and smart materials.