Scheme 6.5.4. A topologically chiral rotaxane. Arrows indicate the directionality imposed by the sulfonamide group in the wheel.

6.5.5). Photochemical E ! Z isomerization produces a sterically strained state in which the methyl substituents are positioned in close proximity. The strain is released when the methyl groups and the naphthyl moieties switch their positions. This second step is followed by back-isomerization from the Z to the E form of the double bond and again steric hindrance provides an energetically unfavorable state which can reach an energetically more favorable state by a second flip of the sub-

Scheme 6.5.5. Light-driven unidirectional rotation around an overcrowded double bond.

stituents. This system is driven by light inducing the isomerization reactions of the double bond. Each photoinduced step leads upwards in energy and is followed by a second comforting flip of the substituents.

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