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. 2021 Dec 16;17(2):159–165. doi: 10.1038/s41565-021-01021-z

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© The Author(s), under exclusive licence to Springer Nature Limited 2021

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Fig. 1 — a, A photoresponsive molecular motor is constrained by two imine macrocycles, allowing AMM ±n to adopt distinct topologies by forming twists in the strands of the bicyclic molecule. Crossings can be established by either thermal nucleophile exchange of the imines (brown), leading to temporary ring-opening of one macrocycle, or by light-driven unidirectional rotation (blue) via photochemical-induced E–Z isomerization and subsequent thermal helix inversion (blue), which increases the number of chiral crossing per half-turn by +1 in the case of (S,R,R) configured ±n. b, Thermal equilibration (brown) takes place by temporary ring-opening of the imines and will populate all accessible, distinct topological states so that the Gibbs free energy of the system is minimized. The expected population is indicated by the brown bar diagram. Light-driven winding (blue) twists the bicyclic molecule in a unidirectional and sequential manner, increasing the number of crossings in the system above its equilibrium level until mechanical resistance in the system hinders further winding. The expected population is indicated by the blue bar diagram. The maximum number of crossings that was experimentally observed is ±3. Lighter colour refers to lower energy, whereas darker colour refers to higher energy. Thermal nucleophile exchange (Fig. 1a, brown) leads to restoration of the initial equilibrium.