Green light powered molecular state motor enabling eight-shaped unidirectional rotation

Abstract

Molecular motors convert external energy into directional motions at the nano-scales. To date unidirectional circular rotations and linear motions have been realized but more complex directional trajectories remain unexplored on the molecular level. In this work we present a molecular motor powered by green light allowing to produce an eight-shaped geometry change during its unidirectional rotation around the central molecular axis. Motor motion proceeds in four different steps, which alternate between light powered double bond isomerizations and thermal hula-twist isomerizations. The result is a fixed sequence of populating four different isomers in a fully unidirectional trajectory possessing one crossing point. This motor system opens up unexplored avenues for the construction and mechanisms of molecular machines and will therefore not only significantly expand the toolbox of responsive molecular devices but also enable very different applications in the field of miniaturized technology than currently possible.

Fig. 1

Structures of isomers A to D of motor 1 and ground-state properties. a Schematic representation of the molecular structures of A to D. b Structures of isomers A to D in the crystalline state and thermal interconversion in solution. c Ground-state energy profile of 1 experimentally determined in 12DCB-d₄ (red) and in acetonitrile-d₃/D₂O (8/2, green). d Ground-state energy profile of 1 calculated at the ω-B97XD/6-311 G(d,p) PCM(DMSO) level of theory (black) and experimentally determined in DMSO-d₆ (blue)

Fig. 2

Photoreactions of HTI 1 during 520 nm irradiation. a Molar absorption coefficients in cyclohexane solution. b Experimental quantum yields determined for the photoreactions of isomers A and B in cyclohexane-d₁₂ solution at 27 °C under 520 nm irradiation. c Beginning of the photoreaction of isomer A in cyclohexane-d₁₂ at 27 °C. d Beginning of the photoreaction of isomer B in cyclohexane-d₁₂ at 27 °C

Fig. 3

Molecular motor properties of 1. a Schematic representation of the geometry of A. b Four-step process of the motor motion and associated fluorescence changes. c Individual steps of motor operation followed by ¹H NMR spectroscopy, (1) B in cyclohexane-d₁₂, (2) after 520 nm irradiation, (3) D in acetonitrile-d₃/D₂O (8/2), (4) after heating to 27 °C, (5) A in cyclohexane-d₁₂, (6) after 520 nm irradiation, (7) C in acetonitrile-d₃/D₂O (8/2), (8) after heating to 60 °C. d One full cycle of motor operation followed by ¹H NMR spectroscopy, (1) B in cyclohexane-d₁₂, (2) after 520 nm irradiation, (3) after solvent change to acetonitrile-d₃/D₂O (8/2), (4) after heating to 60 °C, (5) after solvent change to cyclohexane-d₁₂, (6) after irradiation with 520 nm, (7) after solvent change to acetonitrile-d₃/D₂O (8/2), (8) after heating to 60 °C, (9) after solvent change to cyclohexane-d₁₂. e Individual steps of autonomous motor operation followed by ¹H NMR spectroscopy in 12DCB-d₄, (1) pure B, (2) after 520 nm irradiation, (3) pure D, (4) after heating to 130 °C, (5) pure A, (6) after 520 nm irradiation, (7) pure C, (8) after heating to 130 °C. f Positional changes of the methyl group with respect to the static thioindigo fragment during one full cycle of directional motion