Photoswitchable rotaxanes on gold nanoparticles.

We studied rotaxanes that consisted of a molecular axle, with a photoactive 9-Aryl-9-methoxy-acridane moiety at one end, and a tetracationic ring of cyclobis(paraquat-p-phenylene) (CBQT(4+)). The aim of the study was to deposit the axle ends onto gold nanoparticles (AuNPs). First, we introduced thioctic acid into the axle molecules. Then, rotaxanes were deposited on AuNPs by two methods: 1) Pseudorotaxanes were deposited on the gold surface by forming rotaxanes with the AuNP as a terminator to prevent unthreading of the ring structure; and 2) a chain containing the thioctic ester was introduced into a complete rotaxane, and then it was deposited on the AuNP with the aid of an exchange process. The photoheterolysis of the acridane unit resulted in formation of the corresponding acridinium methoxide; this, in turn, could thermally react to return to the acridane moiety. Due to the creation of a positive charge, the ring moved from the acridane station to a second, evasive station within the axle. This switching cycle could also take place when deposited on the gold surface. However, on the gold surface, the ring movement associated with the switching process was unidirectional.

Photoswitchable rotaxanes using the photolysis of alkoxyacridanes.

9-Aryl-9-alkoxy-acridanes and their counterparts, 9-aryl-acridinium ions, have been incorporated into the axles both of one- and two-station [2]rotaxanes. The ring component of the rotaxanes consists of the tetracationic ring cyclobis(paraquat-4,4'-bisphenylene). The electron-rich acridanes represent suitable recognition sites for the electron-poor ring because charge transfer interaction plays an important role. The 9-aryl groups at the acridane unit bearing substituents such as the alkoxy and the amino groups influence the strength of the recognition site. Photoexcited acridanes bearing a suitable leaving group such as the methoxy substituent in the 9-position undergo heterolysis, resulting in the formation of the acridinium methoxides. The acridanes are regenerated by the nucleophilic attack of the methoxide ion at the acridinium ion formed. The lifetime of the ionic state is strongly dependent on the solvent composition. Because the positively charged acridinium ions repel the positively charged ring component of the interlocked molecules, a movement of the ring is initiated provided the molecular axle contains an evasive recognition site. Two-station rotaxanes presented here possess as the second station an anisol unit. Both the photoreaction and the thermal back-reaction render the thermodynamic driving force of the interaction of the ring with one of the two recognition stations. Accordingly, movement of the ring forward and back, driven by Brownian motion, occurs. The switching cycle can also be triggered by acid-base titration. The photoexcitation of the acridane unit present in one-station rotaxanes leads to a very unfavourable acridinium recognition station. However, because of the absence of a second station, the ring remains at the unfavourable acridinium station having interaction with 9-aryl group.