Designing light-driven rotary molecular motors

Abstract

The ability to induce and amplify motion at the molecular scale has seen tremendous progress ranging from simple molecular rotors to responsive materials. In the two decades since the discovery of light-driven rotary molecular motors, the development of these molecules has been extensive; moving from the realm of molecular chemistry to integration into dynamic molecular systems. They have been identified as actuators holding great potential to precisely control the dynamics of nanoscale devices, but integrating molecular motors effectively into evermore complex artificial molecular machinery is not trivial. Maximising efficiency without compromising function requires conscious and judicious selection of the structures used. In this perspective, we focus on the key aspects of motor design and discuss how to manipulate these properties without impeding motor integrity. Herein, we describe these principles in the context of molecular rotary motors featuring a central double bond axle and emphasise the strengths and weaknesses of each design, providing a comprehensive evaluation of all artificial light-driven rotary motor scaffolds currently present in the literature. Based on this discussion, we will explore the trajectory of research into the field of molecular motors in the coming years, including challenges to be addressed, potential applications, and future prospects.

Fig. 1. Key examples of the various families of artificial light-driven molecular motors discussed in this perspective. (A) Overcrowded alkene-based motors; (B) hemithioindigo-based motors; and (C) imine motors.

Fig. 2. (A) Rotation cycle of overcrowded alkene-based first-generation motor 1; (B) rotation cycle of overcrowded alkene-based second-generation motor 9; (C) potential energy surface of the rotation cycle of overcrowded alkene-based motors, with photochemical E–Z (PEZ) and thermal helix inversion (THI) steps highlighted.

Fig. 3. All-photochemical rotation cycle of phosphine-based motors 10–12, including thermal phosphorus inversion “shortcut” cycle of motor 10.

Fig. 4. (A) Rotation cycle of a second-generation hemithioindigo-based motor. The single bond rotation (SBR), double bond isomerization (DBI) and hula twist motion (HT) of 7 are highlighted; (B) second-generation hemithioindigo motor 13 with a figure-of-eight rotation cycle.

Fig. 5. Schematic representation of the rotation cycle for (A) four-stroke imine-based motor 8 and (B) two-stroke imine-based motor 14.

Fig. 6. Various light-driven molecular motors with their irradiation wavelength quoted in brackets.

Fig. 7. Simplified potential energy surface (PES) along the isomerisation coordinate, showing the excited state processes occurring during the photochemical E–Z isomerisation. The region of the perpendicular minimum is marked as “dark”.

Fig. 8. (A) Modes associated to the excited state rotational movement of overcrowded alkenes and Rho-like compounds: the pyramidalization angle θ is highlighted. (B) Paradigmatic rotational motions: on the left the precessional (or hippopede-like) type, typical of switches and motors with zwitterionic character around the S1 global minimum/S1 → S0 CInt; on the right the axial type, typical of photoactuators with diradical character at the S1 global minimum/S1 → S0 CInt.

Fig. 9. Novel motor scaffolds investigated via computational methods.

Fig. 10. Half-lives of various molecular motors with thermal steps in the ground state at 293.15 K.

Fig. 11. Modulation of rotation speed of second-generation overcrowded alkene-based motors by allosteric binding. (A) Binding of transition metal ions to the lower half increases the rotation speed; (B) non-covalent binding of molecular tethers to the upper half decreases the rotation speed.

Fig. 12. General synthetic methods for synthesising artificial light-driven molecular motors. (A) McMurry coupling for first-generation overcrowded alkene-based motors; (B) Barton–Kellogg coupling for second- and third-generation overcrowded alkene-based motors; (C) aldol-type condensation chemistry for oxindole-based motors, biomimetic motors, first-generation HTI-based motors; (D) imine condensation for imine-based motors.

Fig. 13. Various methods used to access enantiomerically pure molecular motors. (A) Preparative chiral chromatography; (B) chiral resolution processes; (C) asymmetric catalysis; (D) use of a chiral auxiliary.

Anouk S. Lubbe, Daisy R. S. Pooler, Stefano Crespi and Ben L. Feringa (from left to right).