Photoswitchable rotaxanes using the photolysis of alkoxyacridanes
作者:Werner Abraham、Andre Wlosnewski、Karin Buck、Sabine Jacob
DOI:10.1039/b815848g
日期:——
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.
Novel rotaxanes containing two 9-aryl-9-methoxy-10-methyl-9,10-dihydroacridine moieties (acridanes) at both ends of the molecular axle as recognition stations for the tetracationic ring CBQT4+ were synthesized together with their acridinium counterparts. A new concept of controlling the ring movement within rotaxanes has been realized with these rotaxanes. Owing to Brownian molecular movement, the ring shuttles from one end of the axle to the other one in acridane rotaxanes. The shuttle process is stopped by converting two or one of the acridane stations into the corresponding acridinium unit. If both acridanes are transformed by addition of an acid, the ring resides on evasive stations present in the center of the axle. Photons convert only the unoccupied acridane station, thus the ring remains on the unchanged acridane station. The shuttle process can be switched on by addition of a base and by the thermal reaction of the methoxide with the formed acridinium ion, respectively.