Molecular Machines: Generating Work from a Nanoscopic Level

The concepts of machines and work are familiar to practically everyone. Molecular machines function similarly at the nanoscopic level. Although their practical applications are still being investigated, they have the potential to generate useful work through motion generation. These molecular systems contain an operator that imparts energy and directionality. The operator is integrated into its environment so that work can be performed on a coupled load.¹

Molecular machines can be understood using the bicycle analogy proposed by Dr. Aprahamian in his comprehensive 2020 review. He distinguishes between a switch and a machine based on the work performed. Pedaling a bicycle in exactly repetitive motions back and forth does not move the bicycle beyond where it started, meaning that no work was produced. In this case, the bicycle is a switch, and the bicyclist is the operator. In contrast, a bicycle would qualify as a motor vehicle when the bicyclist achieves directionality by pedaling across a distance.¹

Several obstacles in the synthesis and utilization of molecular machines have hindered their widespread application. Researchers have been working on the best ways to magnify and direct nanoscopic motion to efficiently generate work at a macroscopic scale. They have developed several types of molecular machine systems consisting of various materials, stimuli, and mechanisms.¹

To begin with, crystalline machines are orderly and allow correlated motion across their structure. Two types of crystal machines are most prominent: diarylethenes and metal-organic frameworks (MOFs). Diarylethenes (DA) undergo rapid cyclization/cycloreversion reactions at their surface through isomerization when exposed to ultraviolet (UV) and visible light. These reactions lead to an uneven mix of structurally different molecules that cause stress and reversible back-and-forth bending. The motion can then be converted into work by channeling it to rotate a gearwheel with a millimeter-sized diameter. Diarylethenes are advantageous because of their small and controllable structural changes that can occur in a solid, confined state. However, their rigid structure and limited light penetration depth restrict the amount of work they can produce. In contrast, metal-organic frameworks (MOFs) contain greater inherent free volume, allowing larger amplitudes of motion and thus more work to be done. Mechanically interlocked struts, such as rotaxanes (mechanically interlocked structures of a dumbbell-shaped molecule threaded through a macrocycle ring of twelve carbon members or greater), help maintain their framework. However, they also have drawbacks. Their larger voids make cooperative motion and environmental integration more difficult to achieve. Scientists have suggested filling the free volume with active materials to enhance correlated motion and using surface-attached MOFs (SURMOFs) to assist the MOFs in interacting with their environments.¹

Surface-mounted machines, where the switches and motors are anchored on surfaces, have several advantages. One such machine contains surface-bound rotaxanes assembled on Au cantilevers (rigid structures that extend horizontally while supported only on one end) using positively charged CBPQT4+ rings (C₃₆H₃₂N₄). First, the rings favor the outer tetrathiafulvalene (TTF) stations. After TTF is chemically oxidized to its dicationic form, the rings are pushed inward through coulombic repulsion. Consequently, the distance between the rings decreases, which puts stress on the cantilevers to which they are attached. The cantilevers thus bend and deflect by approximately 35 nm. This process can be repeated through multiple oxidation–reduction cycles to generate movement and work. Another machine system utilizes azobenzene compounds that are attached to small peptides. In its trans form, azobenzene inhibits the kinesin activity. When EM radiation activates fast cis/trans isomerization, the corresponding movement controls the kinesin motors bound to a glass surface. However, these kinesin systems have a limited number of isomerization cycles. They are unable to regenerate as they would in nature.¹

Some of the most developed and promising machines are liquid crystal (LC) polymers, which include photo switches and hydrazones. By allowing order within soft materials, LC polymers have been shown to amplify molecular motion into microscopic changes. One such machine is a light-driven motor containing a bi-pulley that can rotate continuously in a clockwise direction through the contraction and expansion of a connected LC film. The application of UV and visible light at different pulley positions enabled rotation. The varied radiation enables this system to circumvent the photostationary state (PSS), which otherwise prevents continuous movement. Other methods take advantage of transitions, such as the trans/cis isomerization of azobenzene, which builds stress that, in turn, deforms a polymer. The concept of azobenzene deformations is applied to capillary tubes that control microfluidic devices known as fluid slugs. The tubes were created from linear LC polymers that were deformed asymmetrically by visible light in a process in which the tube y-axis dimensions were elongated and the x-axis thickness was decreased. The resulting asymmetric cone generates a capillary force that can propel the liquid and move the solvents. Dr. Aprahamian believes that LC polymers will result in the first successful real-life applications of this technology. He discusses how they may be coupled with 3D printing technology to expand beyond one-dimensional motion.¹

Other categories of machines have also been investigated. Helical LC machines take advantage of chiral changes to invert a helical LC structure by unwinding it and rewinding it with the opposite handedness. Chirality refers to the inability to superimpose a mirror image and describes a pairing similar to that of the left and right hands. The helical movements then cause macroscopic rotations of the object on top of the LC surface. This system would become more useful if the rotational movements were converted into translational motion. In addition, amorphous polymers include daisy chains and polymer chain braids. Daisy chains are interlocked cyclic dimers with units that can slide past each other to create efficient molecular contractions and expansions. Braided polymer chains can incorporate a modulator to form a tension-release mechanism. Furthermore, researchers have experimented with chemical stimuli in addition to light stimuli for the mechanisms mentioned above. Chemical stimuli require a method to control kinetic barriers that allow specific molecular pathways to occur, which has proven to be a challenge. They may rely on differences in the kinetics and speed of reactions to generate motion. Although they are not limited by a photostationary state or physical barriers that block light, chemical stimuli pathways have difficulties in organizing motion into work.¹

Despite solid theoretical understanding of molecular machines, practical difficulties exist in their utilization in real-life applications. The variety of molecular machines indicates that they may be implemented for a wide range of applications. The progress made in the field of molecular motion generates great intrigue and significant potential for the future.