Molecular Meccano, Part 61 [ = ] A Photochemically Driven Molecular-Level Abacus Peter R. Ashton, [a] Roberto Ballardini, [d] Vincenzo Balzani,* [b] Alberto Credi, [b] Klaus Ruprecht Dress, [a] ElØna Ishow, [b] Cornelis J. Kleverlaan, [c] Oldrich Kocian, [a] Jon A. Preece, [a] Neil Spencer, [a] J. Fraser Stoddart,* [a, e] Margherita Venturi, [b] and Sabine Wenger [a, e] [a] Prof. J.F. Stoddart, P.R. Ashton, Dr. K. R. Dress, Dr. O. Kocian, Dr. J.A. Preece, Dr. N. Spencer, Dr. S. Wenger School of Chemistry, University of Birmingham Edgbaston, Birmingham, B15 2TT (UK) [b] Prof. V. Balzani, Dr. A. Credi, Dr. E. Ishow, Prof. M. Venturi Dipartimento di Chimica ªG. Ciamicianº, Universita Á di Bologna Via Selmi 2, 40126 Bologna (Italy) Fax: ( 39)051-209-9456 E-mail: vbalzani@ciam.unibo.it [c] Dr. C. J. Kleverlaan Dipartimento di Chimica, Universita Á di Ferrara via L. Borsari 46, 44100 Ferrara (Italy) [d] Dr. R. Ballardini Istituto FRAE-CNR, via Gobetti 101, 40129 (Italy) [e] Prof. J.F. Stoddart, Dr. S. Wenger Current address: Department of Chemistry and Biochemistry University of California, Los Angeles 405 Hilgard Avenue, Los Angeles, CA 90095-1569 (USA) Fax: ( 1)310-206-1843 E-mail: stoddart@chem.ucla.edu [ = ] For Part 60 see: S. J. Cantrill, D. A. Fulton, A. M. Heiss, A. R. Pease, J. F. Stoddart, A. J. P. White, D. J. Williams, Chem. Eur. J . 2000, 6, 2274. FULL PAPER  WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2000 0947-6539/00/0619-3558 $ 17.50+.50/0 Chem. Eur. J. 2000, 6, No. 19 3558 Abstract: A molecular-level abacus- like system driven by light inputs has been designed in the form of a [2]rotax- ane, comprising the p-electron-donating macrocyclic polyether bis-p-phenylene- 34-crown-10 (BPP34C10) and a dumb- bell-shaped component that contains 1) aRu II polypyridine complex as one of its stoppers in the form of a photoactive unit,2)a p-terphenyl-type ring system as a rigid spacer, 3) a 4,4'-bipyridinium unit and a 3,3'-dimethyl-4,4'-bipyridinium unit as p-electron-accepting stations, and 4) a tetraarylmethane group as the second stopper. The synthesis of the [2]rotaxane was accomplished in four successive stages. First of all, the dumb- bell-shaped component of the [2]rotax- ane was constructed by using conven- tional synthetic methodology to make 1) the so-called ªwest-sideº comprised of the Ru II polypyridine complex linked by a bismethylene spacer to the p-terphen- yl-type ring system terminated by a benzylic bromomethyl function and 2) the so-called ªeast-sideº comprised of the tetraarylmethane group, attached by a polyether linkage to the bipyridinium unit, itself joined in turn by a trismeth- ylene spacer to an incipient 3,3'-dimeth- yl-4,4'-bipyridinium unit. Next, 3) the ªwest-sideº and ªeast-sideº were fused together by means of an alkylation to give the dumbbell-shaped compound, which was 4) finally subjected to a thermodynamically driven slippage re- action, with BPP34C10 as the ring, to afford the [2]rotaxane. The structure of this interlocked molecular compound was characterized by mass spectrometry and NMR spectroscopy, which also es- tablished,alongwithcyclicvoltammetry, the co-conformational behavior of the molecular shuttle. The stable transla- tional isomer is the one in which the BPP34C10 component encircles the 4,4'- bipyridinium unit, in keeping with the fact that this station is a better p- electron acceptor than the other station. This observation raises the questionÐ can the BPP34C10 macrocycle be made to shuttle between the two stations by a sequence of photoinduced electron transfer processes? In order to find an answer to this question, the electro- chemical, photophysical, and photo- chemical (under continuous and pulsed excitation) properties of the [2]rotaxane, its dumbbell-shaped component, and somemodelcompoundscontainingelec- tro- and photoactive units have been investigated. In an attempt to obtain the photoinduced abacus-like movement of the BPP34C10 macrocycle between the two stations, two strategies have been employedÐone was based fully on pro- cesses that involved only the rotaxane components (intramolecular mecha- nism), while the other one required the help of external reactants (sacrificial mechanism). Both mechanisms imply a sequence of four steps (destabilization of the stable translational isomer, mac- rocyclic ring displacement, electronic reset, and nuclear reset) that have to compete with energy-wasteful steps. The results have demonstrated that photo- chemically driven switching can be per- formed successfully by the sacrificial mechanism, whereas, in the case of the intramolecular mechanism, it would ap- pear that the electronic reset of the system is faster than the ring displace- ment. Keywords: electron transfer ´ mo- lecular devices ´ molecular switch ´ photochemistry ´ redox chemistry ´ rotaxanes ´ self-assembly