Moving from single molecules to dynamic molecular systems and responsive materials requires control of molecules that can induce motion and enable machine-like functions. Light-driven rotary molecular motors are compounds that can undergo unidirectional rotational motion, using light as a power source. Their 360° rotation cycle is driven by the successive interconversion of four unique isomers of the motor, undergoing two energetically uphill photochemical E-Z isomerisation (PEZ) steps and two downhill thermal helix inversion (THI) steps. Thanks to this unidirectional rotational behaviour, molecular motors can be used to produce useful physical work on the nanoscale – finding a multitude of applications in fields of catalysis, smart materials, and nanotechnology.
The physical characteristics governing the rotational motion of molecular motors include their rotational speed, the wavelength of light which the motors can consume to power their motion, and their photochemical efficiency (or quantum yield). Investigating these characteristics will allow us to understand the dynamic behaviour of molecular motors – and ultimately how to tune this behaviour – will allow molecular motors to be used as tuneable actuators and molecular machines.
Figure 1: A) Oxindole-based molecular motors functionalised on the upper half or lower half of the motor scaffold, with either cyano or methoxy groups; B) Unidirectional rotation cycle of the molecular motors, consisting of two photochemical E-Z isomersations (PEZ) and two thermal helix inversions (THI).
Molecular motors containing the heterocyclic oxindole moiety were discovered in 2019 and it was found that they have desirable rotational properties – they can be driven with benign visible light, and they can be synthesised easily and quickly, relative to traditional hydrocarbon-based molecular motors. Due to these advantages, more research into the functionalisation of oxindole-based motors could help to further improve these promising motors.
Recently, researchers in the group of Ben Feringa have carried out a systematic benchmark study on oxindole-based molecular motors; through functionalisation with cyano or methoxy groups at three different positions along the motor backbone, the rotational properties of oxindole-based molecular motors can be fine-tuned (Figure 1).
By functionalising the motors at these positions (Figure 1, R1, R2 and R3), the wavelength of light used to power the molecular motors can be further red-shifted into the visible region of the electromagnetic spectrum, which is a lower energy source of light. In some cases, the use of longer wavelengths even improved the photochemical efficiency of the motors. In addition, the photochemical quantum yields of the motors could be tuned, with electron withdrawing cyano groups improving the quantum yields of the photochemical isomerisation steps of the motor rotation cycle.
The favourable properties fulfilled by the oxindole-based molecular motors investigated in this work show the great potential of these molecules to be used as viable visible light-driven actuators, which can be fine-tuned to accurately control nanoscale motion in light responsive systems.
Corresponding author:
Professor Ben Feringa
Ben Feringa obtained his PhD degree in 1978 at the University of Groningen in the Netherlands under the guidance of Prof. Hans Wynberg. After working as a research scientist at Shell he was appointed full professor at the University of Groningen in 1988 and named the distinguished Jacobus H. van’t Hoff Professor of Molecular Sciences in 2004. He was elected foreign member of the American Academy of Arts and Sciences, and the Royal Society and is member of the Royal Netherlands Academy of Sciences. His research interests include stereochemistry, organic synthesis, asymmetric catalysis, molecular switches and motors, photopharmacology, self-assembly and nanosystems.
