A photochemically driven molecular-level abacus

A molecular-level abacus-like system driven by light inputs has been design ed in the form of a [2]rotaxane, comprising the pi-electron-donating macroc yclic polyether bis-p-phenylene-34-crown-10 (BPP34C10) and a dumbbell-shape d component that contains 1) a Ru-II polypyridine complex as one of its sto ppers 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'-bipy ridinium unit as pi-electron-accepting stations, and 4) a tetraarylmethane group as the second stopper. The synthesis of the [2]rotaxane was accomplis hed in four successive stages. First of all, the dumbbell-shaped component of the [2]rotaxane was constructed by using conventional synthetic methodol ogy to make 1) the so-called "west-side" comprised of the Ru-II polypyridin e complex linked by a bismethylene spacer to the p-terphenyl-type ring syst em terminated by a benzylic bromomethyl function and 2) the so-called "east -side" comprised of the tetraarylmethane group, attached by a polyether lin kage to the bipyridinium unit, itself joined in turn by a trismethylene spa cer to an incipient 3,3'-dimethyl-4,4'-bipyridinium unit. Next, 3) the "wes t-side" and "east-side" were fused together by means of an alkylation to gi ve the dumbbell-shaped compound, which was 4) finally subjected to a thermo dynamically driven slippage reaction, with BPP34C10 as the ring, to afford the [2]rotaxane. The structure of this interlocked molecular compound was c haracterized by mass spectrometry and NMR spectroscopy, which also establis hed, along with cyclic voltammetry, the co-conformational behavior of the m olecular shuttle. The stable translational isomer is the one in which the B PP34C10 component encircles the 4,4'-bipyridinium unit, in keeping with the fact that this station is a better pi-electron acceptor than the other sta tion. This observation raises the question-can the BPP34C10 macrocycle be m ade to shuttle between the two stations by a sequence of photoinduced elect ron transfer processes? In order to find an answer to this question, electr o-chemical, photophysical, and photo-chemical (under continuous and pulsed excitation) properties of the [2]rotaxane, its dumbbell-shaped component, a nd some model compounds containing electro- and photoactive units have been investigated. In an attempt to obtain the photoinduced abacus-like movemen t of the BPP34C10 macrocycle between the two stations, two strategics have been employed-one was based fully on processes that involved only the rotax ane components (intramolecular mechanism), 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 isome r, macrocyclic ring displacement, electronic reset, and nuclear reset) that have to compete with energy-wasteful steps. The results have demonstrated that photochemically driven switching can be performed successfully by the sacrificial mechanism, whereas, in the case of the intramolecular mechanism , it would appear that the electronic reset of the system is faster than th e ring displacement.