To find out how best to optimize shuttling of the macrocycle in a particular class of photochemically driven molecular abacus, which has the molecular structure of BR-I6+ in its Mark I prototype (Ashton et al., Chem. Eur. J. 2000, 6, 3558), we have synthesized and characterized a Mark II version of this kind of two-station rotaxane comprised of six molecular modules, namely (a) a bisparaphenylenecrown-10 electron donor macrocycle M and its dumbbell-shaped component which contains (b) a Ru(ii)-polypyridine photoactive unit P2+ as one of its stoppers, (c) a p-terphenyl-type ring system as a rigid spacer S, (d) 4,4?-bipyridinium (A12+) and (e) 3,3?-dimethyl-4,4?-bipyridinium (A22+) electron acceptor units that can play the role of stations for the macrocycle M, and (f) a tetraarylmethane group T as the second stopper. This Mark II version is identical with BR-I6+ in the Mark I series that works as a sunlight-powered nanomotor (Balzani et al., Proc. Natl. Acad. Sci. USA 2006, 103, 1178), except for the swapping of the two stations A1 2+ and A22+ along the dumbbell-shaped component, i.e. the Mark I and II bistable rotaxanes are constitutionally isomeric. We have found the closer the juxtaposition of the electron transfer photosensitizer P2+ to the better (A12+) of the two electron acceptors, namely the situation in BR-II6+ compared with that in BR-I6+ results in an increase in the rate ? and hence the efficiency ? of the photoinduced electron-transfer step. The rate of the back electron transfer, however, also increases. As a consequence, BR-II 6+ performs better than BR-I6+ in the fuel-assisted system, but much worse when it is powered by visible light (e.g. sunlight) alone. By contrast, when shuttling is electrochemically driven, the only difference between the two bistable rotaxanes in the Mark I and Mark II series is that the macrocycle M moves in opposite directions.
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