Redox-triggered Switching in Three-Dimensional Covalent Organic Frameworks

We reported the first switchable 3D COF, which can undergo a reversible transformation in the skeleton through a hydroquinone/quinone redox reaction. In addition, the desinged 3D COF can show interesting stimuli-responsive properties.

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The incorporation of molecular switches into solid-state material toward promising applications has attracted great interests in recent years. Unfortunately, their solution-state chemistry does not always translate into solid state, presumably owing to the spatial confinement effect. In this regard, porous materials have provided a promising platform to preserve their switchable functions in solid state, as the pore structure can offer the essential space for transformation. For example, metal-organic frameworks (MOFs) have shown the ability in maintaining the property of molecular switches in solid state, by immobilizing them into the framework as organic components. However, the moderate stability of MOFs will be a fatal obstacle to the development of this field, especially after considering their practical use as stimuli-responsive porous materials.

In principle, covalent organic frameworks (COFs) can also be used as the platform for efficient tuning of molecular switches in solid state. More importantly, they can show enhanced stability in contrast to most MOFs, due to their robust covalent linkages in the framework. There are several attempts to construct such systems and very few stimuli-responsive two-dimensional (2D) COFs have been announced. Unfortunately, although these systems have shown interesting stimuli-responsive properties, there still has some problems that need to be further improved, such as inefficient conversion. These drawbacks may probably be explained by their inherent 2D structures, as the strong π-π interaction in the adjacent layers can strongly restrict the transformation of molecular switches.

Our group in Wuhan University (Website: http://chengwang.whu.edu.cn) is specialized in design and construction of three-dimensional (3D) COFs. Unlike the layered structures of 2D COFs, the molecular building blocks in 3D COFs are three-dimensionally organized to form the extended network. Therefore, 3D COFs can characteristically possess numerous isolated sites and abundant open channels, which will theoretically enable 3D COFs more suitable than 2D COFs for switching molecules in solid state. However, due to the synthetic difficulty and complicated structural determination, the construction of 3D COFs has been considered as a big challenge.

In this paper, we designed and synthesized of the first switchable 3D COF (3D-TPB-COF-HQ), which can undergo a reversible transformation in the skeleton through a hydroquinone/quinone redox reaction. In collaboration with Prof. Junliang Sun from Peking University, 3D-TPB-COF-HQ and 3D-TPB-COF-Q were determined to adopt a five-fold interpenetrated pts topology with a high resolution database of ~1.0 Å by using continuous rotation electron diffraction (cRED) technique. Interestingly, this redox-triggered transformation in the framework can modify the pore environment, which will further lead to drastic changes in gas separation property. From this study, we strongly believe 3D COFs can provide an ideal platform for efficient tuning of molecular switches in solid state, and the obtained materials can show interesting stimuli-responsive properties.

To read more about our work at Nature Communications: https://www.nature.com/articles/s41467-020-18588-1

Cheng Wang

Professor, Wuhan University

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