Controlling rotary motion of molecular motors based on oxindole

Abstract

Molecular motors are essential components of artificial molecular machines, which can be used to manipulate and amplify mechanical motion at the nanoscale to create machine-like function. Since the discovery of light-driven rotary molecular motors, the field has been widely developed, including the introduction of molecular motors based on oxindole by our group in 2019. The rotational properties of molecular motors, e.g. absorption wavelength, quantum yield and rotation speed, often critically depend on substituent effects. Up to now, the substituent effects of oxindole-based molecular motors have not yet been investigated. Herein, we present a family of oxindole-based molecular motors functionalised at three different positions on the motor core, with either CN or OMe groups. The motors prepared in this work retain the favourable features of oxindole-based motors, i.e. simple synthesis and visible light addressability. We find that functionalisation has substantial effects on the absorption wavelength of the motors, meanwhile the rotation speed is unaffected. Furthermore, we found that functionalisation of the oxindole molecular motors increases their quantum efficiency considerably in comparison to previous motors of their class.

Scheme 1. A) Oxindole-based molecular motors 1–9 investigated in this study; (B) four-step rotation cycle of the motors.

Fig. 1. Optimised structures and Frontier orbitals of E_S-1 and Z_M-1. Calculated with DFT at the PBE0/def2-TZVP level with the SMD DMSO solvent model.

Scheme 2. General procedure for the Knoevenagel reaction in the synthesis of oxindole-based motors.

Fig. 2. ORTEP images (ellipsoid probability at 50%) of X-ray crystal structures of motors E_S-3, E_S-6 and E_S-9.

Fig. 3. (A) UV-Vis irradiation cycle of 4 in DMSO (c = ∼2 × 10⁻⁵ M). (i) irradiation to PSS with λ = 365 nm; (ii) heating to 60 °C, THI; (iii) irradiation to PSS with λ = 365 nm; (iv) heating to 100 °C, THI. (B) ¹H NMR irradiation studies of 4 in DMSO-d₆ (c = 2.4 × 10⁻³ M). (i) E_S-4 before irradiation; (ii) PSS₃₉₅, 52 : 48 (E_S-4 : Z_M-4); (iii) THI, 70 °C, 60 min; (iv) PSS₃₉₅, 4 : 96 (Z_S-4 : E_M-4); (v) THI, 100 °C, 180 min.