Design and Optimization of Molecular Nanovalves Based on Redox-Switchable Bistable Rotaxanes Thoi D. Nguyen, Yi Liu, Sourav Saha, Ken C.-F. Leung, J. Fraser Stoddart,* and Jeffrey I. Zink* Contribution from the California NanoSystems Institute and Department of Chemistry and Biochemistry, UniVersity of California, Los Angeles, 405 Hilgard AVenue, Los Angeles, California 90095-1569 Received July 29, 2006; E-mail: zink@chem.ucla.edu Abstract: Redox-controllable molecular nanovalves based on mesoporous silica nanoparticles have been fabricated, using two bistable [2]rotaxanes with different spacer lengths between their recognition sites as the gatekeepers. Three different linkers with varying chain lengths have been employed to attach the bistable [2]rotaxane molecules covalently to the silica substrate. These nanovalves can be classified as having IN or OUT locations, based on the positions of the tethered bistable [2]rotaxanes with respect to the entrances to the nanopores. The nanovalves are more efficient when the bistable [2]rotaxane-based gatekeepers are anchored deep within (IN) the pores than when they are attached closer to (OUT) the pores’ orifices. The silica nanopores can be closed and opened by moving the mechanically interlocked ring component of the bistable [2]rotaxane closer to and away from the pores’ orifices, respectively, a process which allows luminescent probe molecules, such as coumarins, tris(2-phenylpyridine)iridium, and rhodamine B, to be loaded into or released from the mesoporous silica substrate on demand. The lengths of the linkers between the surface and the rotaxane molecules also play a critical role in determining the effectiveness of the nanovalves. The shorter the linkers, the less leaky are the nanovalves. However, the distance between the recognition units on the rod section of the rotaxane molecules does not have any significant influence on the nanovalves’ leakiness. The controlled release of the probe molecules was investigated by measuring their luminescence intensities in response to ascorbic acid, which induces the ring’s movement away from the pores’ orifices, and consequently opens the nanovalves. Introduction A valve is a machine constructed by combining a movable element that regulates the flow of gases or liquids with a reservoir which can also serve as a supporting platform for the movable element. The effectiveness of the valve in controlling the flow is highly dependent on the fitting and matching of these components; if too loose, the valve leaks, and if too tight, it will not open. Construction of such a device on the nanoscale 1-3 requires the integration of stable and inert nanocontainers with appropriate moving parts that can act as gatekeepers to regulate molecular transport in to and out of the containers. Strategies for the assembly of nanovalves in which nanoscale movable elements are attached to mesoporous silicate reservoirs have been demonstrated. These movable elements have been shown, for example, to function as a result of the cis/trans isomerization of azobenzene, 4 intermolecular dimerization of coumarins, 5 tethering and untethering of cadmium sulfide nanoparticles, 6 and the expanding and collapsing of heat-responsive polymers. 7 Other systems involve the electrochemical corrosion of a gold membrane in micro-electromechanical systems 8 and the ap- pending of an addressable photosensitive compound to naturally occurring channel proteins. 9 Generally, the mechanical properties of these nanovalves have not been optimized for the release of a variety of probe molecules. In our own work, we have demonstrated that the integration of mesoporous silica with interpenetrating supermolecules (pseudorotaxanes) or interlocked molecules (bistable rotaxanes) with movable and switchable properties 10-12 produces operating nanovalves. Redox-switchable [2]pseudorotaxanes and bistable [2]rotaxanes, having a cyclobis(paraquat-p-phenylene) (CB- PQT 4+ ) tetracationic ring, can be tethered to porous silica thin films 13 and to MCM-41 14 to act together as supramolecular and molecular nanovalves, respectively. In addition, we have reported 15 a supramolecular nanovalve system (switchable pseudorotaxane) based on dibenzo[24]crown-8/dialkylammo- nium ion complexation that responds to a range of bases. Underscoring of the importance of structure/property relation- ships, the dimensions of the bases play a vital role in the outcome of the operation of these nanovalves, resulting in a supramolecular system that can release luminescent probe molecules at different rates. In this paper, we (i) report a comparative study of the molecular nanovalves, synthesized from bistable [2]rotaxanes and attached at different positions on the nanostructrued silica, and (ii) expand the structure-property relationships of this class of molecular nanovalves. The effects of constructing nanovalves using silane linkers with different lengths positioned in different regions relative to the silicate pores’ orifices on the effective Published on Web 01/03/2007 626 9 J. AM. CHEM. SOC. 2007, 129, 626-634 10.1021/ja065485r CCC: $37.00 © 2007 American Chemical Society