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 m 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 finger print 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.