Photoactivated Artificial Molecular Machines that Can Perform Tasks

Abstract

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.201906064.Research on artificial photoactivated molecular machines has moved in recent years from a basic scientific endeavor toward a more applicative effort. Nowadays, the prospect of reproducing the operation of natural nanomachines with artificial counterparts is no longer a dream but a concrete possibility. The progress toward the construction of molecular-machinebased devices and materials in which light irradiation results in the execution of a task as a result of nanoscale movements is illustrated here. After a brief description of a few basic types of photoactivated molecular machines, significant examples of their exploitation to perform predetermined functions are presented. These include switchable catalysts, nanoactuators that interact with cellular membranes, transporters of small molecular cargos, and active joints capable of mechanically coupling molecular-scale movements. Investigations aimed at harnessing the collective operation of a multitude of molecular machines organized in arrays to perform tasks at the microscale and macroscale in hard and soft materials are also reviewed. Surfaces, gels, liquid crystals, polymers, and self-assembled nanostructures are described wherein the nanoscale movement of embedded molecular machines is amplified, allowing the realization of muscle-like actuators, microfluidic devices, and polymeric materials for light energy transduction and storage.

Bottom-Up: Can Supramolecular Tools Deliver Responsiveness from Molecular Motors to Macroscopic Materials?

“…The macroscopic amplification of a collection of stimuli-responsive molecular switches/machines is a common challenge that has attracted widespread interest in the past decades. [56][57][58][59][60] Several strategies have been proved to be reliable with some photoswitches and molecular machines by covalently incorporating stimuliresponsive molecular units into polymeric networks, such as liquid crystal (LC) elastomers and multi-branch-crosslinked gels. [61][62][63] Giuseppone and co-workers, in a pioneering study, demonstrated that covalently crosslinked motorized gels can perform light-driven macroscopic volume shrinkage due to the intertwining of polymer chains as a result of the light-driven unidirectional rotation of motors.…”

“…This model can be considered to be a ''macroscopic supramolecular machine.'' 56,58 This concept of signal delivery from molecular motors to macroscopic materials can create a new generation of smart materials whose responsiveness originates at the molecular level.…”

Recent Advances of Photoresponsive Supramolecular Switches

“…Generally, switches can be divided into three groups: [2] molecular switch (e. g., pH indicators), supramolecular switches (e. g., pseudorotaxanes) and mechanically interlocked molecular (MIM) switches (e. g., rotaxanes) [3] . The term “molecular switch” originated in the early 1980s and was first used in biological field [4] .…”

A Light‐Operated Molecular Cable Car for Gated Ion Transport

“…It is well known that rotaxane-type systems can control the shuttle process by photoinduced conformational transformation of the photoswitch moieties. [16][17][18][19] If this lightcontrolled shuttling can be incorporated into the rotaxane transporter operated by our reported shuttle mechanism, [11a] it will be possible to realize light-gated ion transport across lipid membranes. As illustrated in Scheme 1, an azobenzene-based molecular shuttle (PR2, Scheme 1 a) was designed, which includes an azobenzene-modified amphiphilic thread (PT2) with two secondary ammonium ions as the terminal stations for the shuttle and a wheel (RCE) consisting of a K + selective receptor, i.e., a benzo [18]crown-6 (B18C6) ring.…”