Photoactivated Artificial Molecular Motors

Abstract

Accurate control of long-range motion at the molecular scale holds great potential for the development of ground-breaking applications in energy storage and bionanotechnology. The past decade has seen tremendous development in this area, with a focus on the directional operation away from thermal equilibrium, giving rise to tailored man-made molecular motors. As light is a highly tunable, controllable, clean, and renewable source of energy, photochemical processes are appealing to activate molecular motors. Nonetheless, the successful operation of molecular motors fueled by light is a highly challenging task, which requires a judicious coupling of thermal and photoinduced reactions. In this paper, we focus on the key aspects of light-driven artificial molecular motors with the aid of recent examples. A critical assessment of the criteria for the design, operation, and technological potential of such systems is provided, along with a perspective view on future advances in this exciting research area.

Figure 1

General four-step reaction cycle used to describe the basic operation of most light-driven molecular motors (a). Horizontal processes, marked with “Δ”, are thermally activated, while vertical processes, marked with “hν”, are photochemical steps. Simplified energy diagrams of photochemical (b) and thermal (c) processes.

Figure 2

(a) Schematic representation of the four-step operation cycle of alkene-based motors, comprising photochemical reactions (horizontal processes) and thermal helix inversion (THI, vertical processes). The black arrows indicate the relative movement of the rotor (blue) with respect to the stator (red). The direction of cycling is indicated by the gray dashed arrow. (b) First, second, and third generation Feringa-type motors (refs (−51)). The rotor (blue) and stator (red) portions of the motor molecule are highlighted where relevant.

Figure 3

(a) Examples of first, second, and third generation hemithioindigo motors (refs (53), (59), and (61)). The rotor and stator portions are colored blue and red, respectively. (b) Schematic representation of the reaction network traveled by a second generation hemithioindigo motor, comprising single bond rotation (horizontal processes), double bond isomerization (vertical processes), and hula twist (central crossing processes).

Figure 4

(a) Structure of the motor displaying “figure-of-eight” shaped motion of the methyl group marked in red (ref (59)). (b) Relative position of the methyl group with respect to the oxygen atom of the sulfoxide moiety. (c) Operational four-step cycle comprising photochemical double bond isomerizations (DBI, horizontal processes) and thermal hula twist (HT, vertical processes) highlighting the motion of the Me group relative to the stator (in plane). (d) Overall sequential motion of the Me group. Adapted with permission from ref (59). Copyright 2019 Springer Nature.

Figure 5

(a) Four-step operation cycle of imine-based motors, comprising photochemical reactions (horizontal processes) and thermal helix inversion (THI, vertical processes) (ref (65)). (b) Two-step operation cycle of an imine motor with a more rigid stator, which involves a photochemical out-of-plane rotation (upper pathway) and an in-plane nitrogen inversion (lower pathway). The transition states are represented in gray, while the black dashed arrows indicate the relative movement of the rotor with respect to the stator in the transition states (ref (67)). The gray dashed arrow indicates the preferred traveled direction of the network.

Figure 6

Molecular gearing with overcrowded alkene-based motors. Molecular “planetary gears” based on a Feringa-type motor (a), “chain drive” (b), and “bevel photogear” (c) developed by Dube and co-workers. The axis exhibiting synchronized (geared) motion to the motor/switch is colored red. The dashed arrows indicate the relative direction of motion of the two axes.

Figure 7

(a) “Figure-of-eight” overcrowded alkene-based motor containing inert (G = I) and dynamic (G = II) tethers. (b) Simplified energy diagram correlating the molecular topology (tension/torque) with the photoinduced unidirectional rotation and the potential energy of the molecule, similarly to a spring. Adapted from ref (75). Copyright 2022 American Chemical Society.

Figure 8

Maximum photoluminescence (PL) emission intensity at different irradiation times for the compound shown in Figure 5a (G = I). The initial and final (photostationary state) compositions of the topological isomer mixture are displayed on top. Adapted from ref (75). Copyright 2022 American Chemical Society.

Figure 9

Photoactive molecular motor-based materials. Left: First- (a) and second-generation (b) photoactive hydrogel based on molecular chain entanglement to drive the shrinkage of a macroscopic sample of hydrogel (c) (refs (74) and (81)). Right: structure of the motor amphiphile(d) and hierarchical self-assembly to form fibers (e). Reversible light-driven actuation of a centimeter-sized fiber (f) (ref (83)). Scale bar in (f) is 0.5 cm. Adapted with permission from refs (74), (81), and (83). Copyright 2015, 2017, and 2018 Springer Nature.

Figure 10

Transport across cell membranes using light-operated molecular rotary motors. (a) Potassium cation transporter developed by Qu and co-workers and (b) cartoon representation of the proposed working principle (ref (89)). (c) Molecular motors equipped with receptor-targeting oligopeptides and (d) proposed mechanism of membrane rupture induced after adsorption and UV light irradiation (ref (90)). Adapted with permission from refs (89) and (90). Copyright 2022 John Wiley and Sons and 2017 Springer Nature.

Figure 11

(a) Components of the redox-driven supramolecular pump described by Stoddart and co-workers: the cyclobis(paraquat-p-phenylene) (CBPQT) ring (left) and the nonsymmetric axle (right). (b) Scheme of the light-induced operation of the supramolecular pump based on photoinduced electron transfer. Blue arrows indicate the electron-transfer processes. Reprinted from ref (95). Copyright 2013 American Chemical Society.

Figure 12

(a) Four-state reaction network describing the operation of Credi’s supramolecular pump. The network includes two photochemical (vertical) and two self-assembly (horizontal) reactions. The gray dashed curved arrow shows the preferred traveled direction of the cycle. (b) Components of the first- and second-generation supramolecular pumps: nonsymmetric axles (left) and diaryl-24-crown-8 ether based rings (right) (refs (97), (99), (101), and (102)). (c) Cycling rate (black dots) and quantum yield (red dots) values at different light intensities. d) Energy dissipated by the self-assembly steps for E (green bars) and Z (red bars) configurations to keep the concentrations of species away from equilibrium at different light intensities. Adapted with permission from ref (103). Copyright 2022 Springer Nature.