Light-Gated Ion Transport in Molecularly Engineered Covalent Organic Framework Membranes

With the growing popularity of smart materials, research on their design, properties and applications has also increased tremendously. Distinct from other materials, the main advantage of smart materials is that one or more of their physical and chemical properties are responsive and can be intelligently tuned in a controlled fashion under external stimuli. Moreover, the change is temporary and reversible. This class of materials forms the basis of numerous contemporary applications, including sensors, actuators and electroactive polymers.

Among the existing smart materials, light-responsive materials are promising candidates for constructing smart devices toward task-specific applications owing to their superior facile controllability, safety and environmental friendliness. These light-responsive materials have shown exceptional structural and functional tunability when irradiated at excitation wavelength. Additionally, smart materials that can respond to external light have drawn attention for application in tunable transport.

Recently, light-responsive materials have been researched as a prospective building block for designing smart membranes with tunable selectivity and permeability. Moreover, integrating stimulus-responsive materials with ordered channels can help achieve precise control over mass transport. A uniform configuration will guarantee not only the homogeneity of pore size change but also accurate grafting of functional groups. Unfortunately, existing light-responsive materials exhibit limited response efficiency and mass transfer tunability due to their lack of limited crystallinity.

Covalent organic frameworks (COFs) have emerged as a new class of crystalline materials with superior controllable transport, diverse and intelligent applications. Owing to their regular configuration, their pore environments can be customized to achieve desired functions and on-demand applications. However, accurate manipulation of mass transport in COF nanochannels is still a big challenge attributed to the difficulty in building viable ion transport channels.

To overcome the mentioned challenges, Dr. Congcong Yin, Dr. Xiansong Shi, and Professor Yong Wang from Nanjing Tech University engineered a light-responsive COF based on light-gated ion channels featuring dangling azobenzene intrapore groups to achieve a highly efficient and adjustable ion transportation. The authors commenced their research by exploring the fabrication and pore engineering of COFs. The addition of reactive functional groups was useful for decorating stimulus-responsive azobenzene groups into COFs. The work is currently published in the journal, Chemistry of Materials.

The research team showed that the azobenzene-tagged channels could afford a customizable configuration that allowed reversible trans-to-cis isomerization by irradiating alternatively with ultraviolet and visible light. It also permitted remote-controlled aperture sizes and could be switched at angstrom level without degradation in its crystallinity. Light-responsive COF membranes further exhibit high mechanical robustness and excellent selectivity and discrimination capacity between monovalent and multivalent ions. As a proof of concept, the synthesized membrane was successfully used to discriminate antibiotic molecules and ions. The selective removal of ionic catalysts was beneficial for mitigating environmental issues, yielding high-purity products, which is of extreme importance in pharmaceutical industry.

In summary, the authors reported the design and synthesis of a stimulus-responsive COF membrane with excellent selectivity, controllability and photophysical properties. The resulting TbDa-Azo membrane achieved remote-controlled and reversible ion transport for efficient antibiotic production. Overall, the findings provided a new approach to developing smart materials using the new COFs. In a statement to Advances in Engineering, the corresponding author Prof. Yong Wang highlighted that light-switchable materials are a promising platform for developing high-performance membranes to meet a variety of demands in various fields like pharmaceutical industry.