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Demonstration of macroscopic shape-change of a material through mechanical motion at the molecular level

The rotaxane synthesized by the Edinburgh group used in these experiments was grafted onto a carboxylic acid-terminated self-assembled monolayer. Its macrocycle can be translocated from one position (“station”) on the thread to a second site through exposure to UV light and this movement conceal fluoroalkane residues and thereby modify surface tension. When a drop was deposited on this surface, one could change the wettability of the surface in front of the drop by light-induced movement of the macrocycles and hence induce drop movement. Evidence for this is shown in the lateral photographs of light-driven directional transport of a 1.25µl diiodomethane drop along the surface of a E-1.11-MUA/Au(111) substrate on mica.

The direction of transportation was controlled by irradiation with a perpendicular beam of 240-400 nm light focused on one side of the drop and the adjacent surface.
- Before irradiation (pristine 111), contact angle 35 ± 2?. Immediately after this image was taken, the UV light beam positioned as indicated was switched on.
- After 900 s irradiation, contact angle 13 +/- 2° (illuminated side), 15 +/- 2° (non-illuminated side). The diiodomethane drop has spread from the high E/Z-1 ratio area in the direction of the low E/Z-1 ratio region, increasing the total wetted area, and is about to be transported.
- After 1010 s irradiation (just after transport), contact angle 13 +/- 2.
- After 1110 s irradiation (at the photostationary state), contact angle 12 +/- 2.

Reported by

University of Edinburgh
Forbes Chair of Organic Chemistry West Main's Road
EH9 3JJ Edinburgh
United Kingdom
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