Ac. Benniston et al., PHOTOACTIVE [2]ROTAXANES - STRUCTURE AND PHOTOPHYSICAL PROPERTIES OF ANTHRACENE-STOPPERED AND FERROCENE-STOPPERED [2]ROTAXANES, Journal of the American Chemical Society, 117(19), 1995, pp. 5275-5291
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.