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Operational Nanovalves

In everyday life, macroscopic valves control the flow of fluids and gases by opening and closing passageways. These valves are essential regulatory devices that maintain the balance of fluids in living beings as well as in macroscopic machines. Construction of such a device on the nanoscale level requires (i) controllable switching elements, (ii) a method for operating them on demand, and (iii) appropriately sized passageways. These three conditions can be fulfilled by self-assembling addressable mechanically interlocked, linear motor-molecules on top of an inorganic chassis and then controlling them chemical, electrochemical, or photochemical stimuli. The Stoddart and Zink groups have demonstrated (Proc. Nat. Acad. Sci. 2005, 102, 10029–10034) that a molecular nanovalve can be turned on and off reversibly by redox chemistry. It traps and releases molecules from a maze of nanoscopic passageways in the mesoporous silica by controlling the operation of redox-activated bistable [2]rotaxane molecules (a in the Box) tethered to the openings of nanopores leading out of the nanoscale reservoirs.

In the research described in this report, the movable element that has been used to control the flow of molecules in a nanovalve is the bistable, redox-controllable [2]rotaxane R4+. In R4+, the movable part of the molecule is the tetracationic cyclophane, cyclobis(paraquat-p-phenylene) (CBPQT4+), component that can be induced to move between two different recognition sites on a dumbbell component. In its ground state, the CBPQT4+ ring prefers to encircle the tetrathiafulvalene (TTF) unit, rather than the dioxynaphthalene (DNP) one on the dumbbell component. The DNP unit is separated from the TTF unit by an oligoethyleneglycol chain incorporating a rigid terphenylene spacer. Because the stabilization energy between CBPQT4+ and TTF is ~2 kcal/mol more than that between CBPQT4+ and DNP, it follows that, in ~95% of the molecules, the CBPQT4+ ring encircles the TTF unit. Twoelectron oxidation of the TTF unit with Fe(ClO4)3 to give the TTF2+ dication destabilizes its interaction (Coulombic repulsion) with the CBPQT4+ ring, which moves to the DNP unit in its electromechanically excited state. The preference for the CBPQT4+ ring to encircle the DNP unit instead of the TTF2+ dication is even greater on account of the large difference (~8 kcal mol, at least) in their stabilization energies. Reduction of the TTF2+ dication back to the neutral TTF unit by ascorbic acid heralds the return of the CBPQT4+ ring to the TTF unit in a thermally activated process. This sequence of reactions is employed (b in the Box) to regulate the opening and closing the nanopores—a gate-keeping process that, in turn, controls the loading and releasing of guest molecules into the silica pores.

 

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