A reversible molecular valve

Abstract

In everyday life, a macroscopic valve is a device with a movable control element that regulates the flow of gases or liquids by blocking and opening passageways. Construction of such a device on the nanoscale level requires (i) suitably proportioned movable control elements, (ii) a method for operating them on demand, and (iii) appropriately sized passageways. These three conditions can be fulfilled by attaching organic, mechanically interlocked, linear motor molecules that can be operated under chemical, electrical, or optical stimuli to stable inorganic porous frameworks (i.e., by self-assembling organic machinery on top of an inorganic chassis). In this article, we demonstrate a reversibly operating nanovalve that can be turned on and off by redox chemistry. It traps and releases molecules from a maze of nanoscopic passageways in silica by controlling the operation of redox-activated bistable [2]rotaxane molecules tethered to the openings of nanopores leading out of a nanoscale reservoir.

Fig. 1.

Graphical representations of the surface attachment of bistable rotaxanes to silica particles along with a cycle for loading and release of guest molecules. (a) The structural formula of the bistable [2]rotaxane R⁴⁺ and the procedure used for tethering R⁴⁺ to the surface of mesoporous silica particles. (b) The proposed mechanism for the operation of the nanovalve. The moving part of the molecular valve is a CBPQT⁴⁺ ring (blue), which shuttles between a TTF station (green) and a DNP station (red) under redox control. The openings of the cylindrical pores on the silica are blocked by the CBPQT⁴⁺ ring when the valve is closed. Guest molecules (turquoise spheres) are loaded in Step 1 by diffusion into the open pores when the CBPQT⁴⁺ ring is located on the TTF station. The valve is closed in Step 2 by oxidation of the TTF unit to its dication, causing the CBPQT⁴⁺ ring to move to the DNP station, which is much closer to the openings of the pores. The valve can be opened (Step 3) by adding ascorbic acid to reduce the TTF dication back to its neutral state, whereupon the CBPQT⁴⁺ ring moves back from the DNP station to be relocated around the much more π electron-rich TTF station, releasing the guest molecules in Step 4. The valve is ready for recharging (i.e., returning to Step 1). Thus, the valve can be closed and opened reversibly. The silica particles are not drawn to scale, and only a few of the ordered pores are shown.

Scheme 1.

Reaction scheme illustrating the synthesis of the dumbbell component 9.

Scheme 2.

Reaction scheme illustrating the synthesis of the bistable [2]rotaxane R·4PF₆.

Fig. 2.

Fluorescence data demonstrating the controlled release of the Ir(ppy)₃ guest molecules from the nonovalve. (a) Release of the Ir(ppy)₃ molecules from the pores can be monitored by luminescence spectroscopy. When the valve is opened by reduction of the TTF²⁺ dication with ascorbic acid, the luminescence of the solution increases rapidly. (b) The visible spectrum of the released Ir(ppy)₃ molecules is shown.

Fig. 3.

Naphthalene luminescence is used to monitor the operation of the valve. In its closed state when the CBPQT⁴⁺ ring encircles the DNP station, naphthalene luminescence is quenched by the tetracatronic cyclophane. When the valve is in its open state when the CBPQT⁴⁺ ring encircles the TTF station, the luminescence intensity of the naphthalene ring system in the DNP station increases 4-fold.

Fig. 4.

Release of the rhodamine B guest molecules is monitored by following the luminescence intensity of the solution. (a) After the valve has been opened by reduction of the TTF²⁺ dication with ascorbic acid, the luminescence in solution increases rapidly. (b) More guest molecules can be loaded into the pores and the valve closed by oxidation of the TTF station and later released by another reduction with ascorbic acid.