Molecular Crankshaft Effect Converting Piston-like Molecular Motion to Continuous Rotation of Macro Objects

A molecular system inducing an accumulative unidirectional rotation motion of glass flakes with about 100 μm in size is introduced. The molecular system is a chiral nematic liquid crystal containing a chiral azobenzene derivative which shows a reversible E- Z photoisomerization accompanying a large helical twisting power change. A film of the molecular system shows different texture change paths upon UV and visible light irradiations inducing " E to Z" and " Z to E" photoisomerizations, respectively, of the chiral azobenzene dopant. Namely, a polygonal fingerprint texture inducing the rotation of glass flakes on the film surface was maintained during UV irradiation, while a focal conic texture inducing no rotation of glass flakes emerged during visible light irradiation. As a result, cycles of the alternative irradiation of UV and visible lights afforded many rotations toward a single direction of the glass flakes which can be considered as a continuous conversion of light energy to mechanical work. We may compare the effect of this molecular system converting "back and forth" structural change between E and Z isomers of the chiral azobenzene to a continuous rotational motion of glass flakes with the crankshaft effect converting a piston-like motion to a rotational motion seen in engines in the real world.