Turning solid-state molecular machines into photoactuators

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As a miniaturization of macroscopic machines, molecular machines play an important role in the microscopic world. These molecular machines are composed of molecular-scale structural units, which can perform a specific function when stimulated and driven by external energy input. For example, in the living system, protein-based molecular machines participate in many key physiological processes and maintain the normal operation of life processes. Compared with biomolecular machines with complex structures, molecular machines constructed by chemical methods are easier to realize. Once the precise construction of molecular machines can be achieved through chemical methods, it may lead to a new round of changes in materials and information technology. The 2016 Nobel Prize in Chemistry was awarded to three supramolecular chemists for "inventing molecular machines that move in a controlled manner and perform tasks when given energy".

    In recent years, based on the common mechanical interlocking structures with dynamic behaviors such as rotaxanes and locked hydrocarbons, great progress has been made in the research on molecular machines, and scientists' manipulation of molecular motion has reached an unprecedented new height. However, there is still a gap between the research on molecular machines and practical applications. The key to whether molecular machines can become practical is to realize their integration, deviceization, and scaling. One of the most challenging problems is how to manipulate the microscopic changes of molecular units to achieve macro-mechanical responses.

Macrocycle-confined photodimerization reaction in photo-responsive uranyl-organic rotaxane framework turning solid-state molecular machines into photoactuators

    To this end, our team proposed a cucurbit[8]uril-based host-guest system with photodimerization activity into metal-organic rotaxane framework for the first time. A new photoactive actinide polyrotaxane framework material (named U-CB[8]-MPyVB) was successfully constructed via coordination bonding by uranyl nodes. As expected, this supramolecular framework system composed of basic units of molecular machines can exhibit good macroscopic mechanical responses under ultraviolet light irradiation. The microscopic mechanism study shows that this framework has two intriguing attributes: a) the confinement effect of the macrocycle and the steric hindrance of the metal coordination make the two vinyl units in the macrocycle close to each other and possess the ability of photodimerization; b) due to the ordered arrangement and collective motion of the various structural units in the U-CB[8]-MPyVB lattice, the internal stress generated by the local photochemical reaction can be effectively accumulated in the lattice, which finally induces the photo-triggered bending of these rod-like crystals. Compared with other photobendable crystals, the confinement effect and pillaring effect of macrocycles slows down the bending rate of the crystal by nearly two orders of magnitude, which is crucial to the capture of intermediate compounds during the photodimerization process of crystalline molecular machines and makes the precise control of macroscopic bending possible.

    We believe that our research has achieved a leap from the microscopic structural changes of molecular machines to macroscopic mechanical responses, and will be able to advance the practical application of molecular machines. By establishing the structure-activity relationship between microstructure changes and macroscopic controllable motion, it also provides new ideas and important references for the construction of other novel molecular machines with macroscopic response and controllable motion behavior.

Lei Mei

Associate research fellow, Institute of High Energy Physics, Chinese Academy of Sciences