A Photoswitchable Rotaxane with an Unfolded Molecular Thread

Abstract

AbstractNovel [2]rotaxanes containing the tetracationic cyclophane cyclobis(paraquat‐4,4‐biphenylene) and a dumbbell‐shaped molecular thread incorporating a photoactive diarylcycloheptatriene station as well as a photo‐inactive aryl station have been synthesized with yields of nearly 30% by the coupling of two halves of the molecular thread. Each half incorporates one of the two stations which are recognized by the tetracationic ring. The switch principle is based on photoheterolysis of a diarylcycloheptatriene bearing a suitable leaving group such as a methoxy substituent. The rotaxanes were transformed into the related rotaxanes incorporating a diaryl tropylium unit by electrochemical oxidation. The co‐conformation of all rotaxanes was determined by 1H NMR and UV/Vis spectroscopy. It has been shown that the occupation of the two different stations by the tetracationic ring depends both on the electron donor capacity of these stations and on the chain units between the stations. The desired preferred occupation of the diarylcycloheptatriene station was achieved by fine tuning both the stopper group at the end of the molecular thread and the number of oxygen atoms within the chain. By tuning the chain length it has also been achieved that both of the rotaxanes incorporating the diarylcycloheptatriene unit and the tropylium‐containing rotaxanes adopt an unfolded structure in solution. The tetracationic ring resides exclusively on the second aryl station within the tropylium rotaxanes. Switching between co‐conformations of the cycloheptatrienyl‐containing rotaxanes and the tropylium‐containing rotaxanes can be achieved by photoheterolysis of the methoxy‐substituted diarylcycloheptatriene, yielding the tropylium rotaxane, which reacts thermally back to the cycloheptatriene rotaxane, and thus closes the switching cycle. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)