The demonstration of light-powered unidirectional molecular motion suggests that a new breed of artificial nanoscale motors and pumps could be a reality in the near future. Giulio Ragazzon and co-workers at the Photochemical Nanosciences Laboratory at the University of Bologna in Italy report how a molecular 'axle' can be made to move through a macrocycle 'ring' following illumination with ultraviolet light (Nature Nanotech. http://dx.doi.org/10.1038/nnano.2014.260; 2014). The axle is composed of three parts: a photoswitchable E-azobenzene unit at one end (green/red structure in image), a central ammonium recognition site (purple) and a passive methylcylopentyl pseudo-stopper at the other end (white sphere). The macrocycle ring (pink) is composed of 2,3-dinaphtho[24]crown-8 ether 124. With the appropriate illumination, a photoisomerization reaction is triggered and the photoswitchable end of the axle changes from an E-2+ configuration (green) to a bent Z-2+ state (red) and the axle moves relative to the ring. For irradiation with 365 nm light, the E to Z state conversion is estimated at 96% and experimental tests indicate that almost complete conversion occurs within an illumination period of five minutes. As the E-2+ and Z-2+ states of the axle have different absorption and luminescence spectra, the motion can be observed by monitoring the time-dependent changes in emission. The authors report that the effect is reversible as the same light can also trigger a state-change back to the initial E-2+ state because the E- and Z-azobenzene have overlapping absorption spectra. Simulations suggest that around 430 photons of 365 nm light are needed to complete a motion cycle. The cycle is expected to be highly resistant to fatigue and by using a series of periodic flashes the authors say that it should be possible to produce directional and repetitive movements. In the future it is postulated that the light-driven assembly could be used to create miniature molecular pumps capable of generating concentration gradients across membranes. The team is now investigating the design of more complex three-dimensional assemblies.

Credit: NPG