PHOTOACTIVE [2]ROTAXANES - STRUCTURE AND PHOTOPHYSICAL PROPERTIES OF ANTHRACENE-STOPPERED AND FERROCENE-STOPPERED [2]ROTAXANES

The structures of several one- and two-station [2]rotaxanes have been probed in both solid and solution phases. It is shown that the cycloph ane, comprising two 4,4'-bipyridinium dications Linked via 1,4-xylyl g roups, resides preferentially over a dialkoxybenzene subunit incorpora ted into the ''string''. The terminal stoppers, anthracene or ferrocen e subunits, associate with the exterior of the cyclophane, via pi-stac king, so as to form closed conformations. Excitation into the anthrace ne stoppers is followed by rapid, but reversible, electron transfer to the cyclophane. Direct excitation into the charge-transfer absorption band results in formation of an intimate radical ion pair in which an electron has been transferred from the central dialkoxybenzene donor to the cyclophane acceptor. Rapid charge recombination occurs but can be partially intercepted by hole transfer to an appended ferrocene sto pper. In this latter case, the ground state is restored by relatively slow electron transfer from the reduced cyclophane to the terminal fer rocenium cation. It is proposed that the closed conformation unwinds s ubsequent to this second electron-transfer step in order to minimize e lectrostatic repulsion. To some extent, therefore, the rates of electr on transfer are controlled by conformational exchange.