Stoddart Mechanostereochemistry Group

 

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Stoddart Mechanostereochemistry Group
 

molecular machines

 

With the advent of nanotechnology, the fascination of achieving the miniaturization of macroscale machines, with the prospect of matching- if not eventually emulating- the performance of nanoscale biological machines, has acted as an incentive to chemists and engineers to develop artificial nanoscale machines at the molecular and supramolecular levels. A machine can be defined as a device with stationary and movable parts that modifies mechanical energy to do work when fueled by an external force. Mechanically interlocked molecules and supermolecules- bistable rotaxanes and catenanes as well as pseudorotaxanes- are some of the most intriguing systems which can be considered as nanoscale machines because of the stimuli-induced, controlled mechanical movements of one part of the molecule- in this case, one interlocked ring component- with respect to the other stationary part.



We have designed and synthesized a number of molecular machines based on chemically, electrochemically, and photochemically switchable bistable interlocked molecules for a variety of applications, for example, pH-driven molecular elevator,3 molecular muscles that can bend microcantilever beams,4 as well as molecular nanovalves5 for control release of molecules from nanopores. We have also developed a light-harvesting molecular triad,6 which produces photocurrents, an energy which we have utilized to power supramolecular machines in the form of a pseudorotaxanes.

The molecular elevator3 (Box 1) comprises a tripodal rig-like component bearing two competitive recognition sites on each of its three legs, positioned at two different levels. The bistable tripod-shaped component is triply interlocked with a platform component, which contains three macrocyclic rings fused with a trigonal core, forming the molecular elevator. The platform could be switched mechanically between two recognition units on the triply bistable rig component by switching the pH of the medium. In response to the chemical stimuli, the platform component travels a distance of 0.7 nm from the upper to the lower level of the rig, generating a force up to 200 pN, a number which is one order of magnitude higher than that exerted by the natural motors, like myosin and kinesin. Furthermore, the base-induced "power stroke" of the platform from the upper to the lower level opens up a cavity (1.5 nm by 0.8 nm), which can potentially act as a host for a variety of guest molecules.



The concept of a bistable\]otaxane has been expanded4 into a doubly bistable palindromic\]otaxane (Box 2) to develop a molecular muscle. These molecular muscles, when self-assembled on microcantilever beams (500x100x1 μm), are capable of bending and stretching the beams when appropriate redox-reagents are injected into the device environment in a microfluidic cell. The palindromic bistable\]otaxane is composed of two cyclobisparaquat(p-phenylene) (CBPQT4+) rings- each carrying a disulfide tether for gold-surface attachment- interlocked onto a symmetrical dumbbell component which bears two π-electron rich tetrathiafulvalene (TTF) units close to its ends and two 1,5-dioxynaphthalene (DNP) units bridged by a rigid di-alkyne spacer at its center, and two 2,6-diisopropyl-phenol stoppers at both termini. Chemical oxidation of both TTF units to TTF2+ dications induces an electrostatic charge repulsion between the TTF2+ dications and CBPQT4+ rings, which drives them towards the DNP sites near the center, a process which bends the underlying microcantilever beams onto which the\]otaxane molecules are attached covalently upward by ca. 35 nm to their apparent saturation point. Reduction of the TTF units back to their neutral states moves both CBPQT4+ rings back to their original location, making the cantilevers straight.

We reported the construction of redox-switchable and reversible molecular nanovalves5 (Box 3), employing bistable\]otaxanes as the redox-controllable gatekeepers and mesoporous silica nanoparticles as nanoreservoirs. To estimate the overall performance of the nanopores they were filled with different luminescent probe molecules. The nanovalves can be closed by adding two equivalents of oxidant to oxidize the TTF unit on the rotaxane backbone, a process which forces the CBPQT4+ ring to shuttle mechanically from the oxidized TTF unit to the DNP unit on account of the charge repulsion between the CBPQT4+ ring and the oxidized TTF2+ dicationic unit. The controlled release of probe molecules can be demonstrated by adding reducing agents to open the nanovalves. This process reduces the oxidized TTF units back to their neutral state, so that the CBPQT4+ rings move away from the openings of the nanopores and once again complex with the neutral TTF unit, causing a subsequent release of the probe molecules.

A molecular triad6 (Box 4)- composed of three unique electroactive components, namely, (i) an electron-donating TTF unit, (ii) a chromophoric porphyrin (P) unit, and (iii) an electron-accepting C60 unit- has been developed to harness light and convert it into electrical energy. A disulfide-based anchoring group was tagged to the TTF end of the triad in order to promote its self-assembly onto gold surfaces. When irradiated near the absorption maximum (Soret band) of the chromophore P at 413 nm, the triad undergoes a photo-induced electron transfer (PET) from the singlet excited state of porphyrin (*P) to the electron-accepting C60 unit, followed by a charge shift to the better electron-donating TTF unit to generate the final charge-separated state, TTF.+-P-C60.-. This charge-separated state generates photocurrents in a closed circuit in the form of a unidirectional electron flow from the working cathode through (i) the photoactive triad, (ii) the electrolyte solution, to (iii) the counter electrode, and through (iv) the outer circuit where the current is measured (ΔI ~1 μA cm-2, Φphotocurrent ~ 1%). The triad has been utilized as a nanoscale power supply to drive the dethreading of the BHEEN C CBPQT4+ pseudorotaxane in the presence of 413 nm light at an applied potential (Vap = 0 V) that is much lower than is required for direct electrochemical reduction (E1/2 = -300 mV) of the CBPQT4+ ring. In accordance with the PET mechanism at Vap = 0 V, the charge-separated state of the triad affords a C60 unit on the triad-functionalized Au-working electrode, resulting in an effective terminal potential of -550 mV which is the reduction potential of the C600/-. unit. This potential is high enough to reduce the CBPQT4+ ring and induce its dissociation from the BHEEN stalk- a process which has been monitored by detecting the BHEEN-based fluorescence intensity. Based on the increase in the fluorescence intensity (320-370 nm), 6.7% of the 0.37 mM\]seudorotaxane in MeCN underwent dissociation in the presence of triad-excitation over 2900 s, an estimation which is commensurate with the triad's ability to photoreduce 7% of the CBPQT4+ ring by generating 1.1 μA cm-2 photocurrent during the period of irradiation.


References:


  1. Balzani, V.; Credi, A.; Venturi, M. Molecular Devices and Machines- A Journey into the Nano World, Wiley-VCH: Weinheim, 2003.

  2. Kay, E. R.; Leigh, D. A. Top. Curr. Chem. 2005, 262, 133-177.

  3. Badjic, J. D.; Balzani, V.; Credi, A.; Silvi, S.; Stoddart, J. F. Science 2004, 303, 1845-1849.

  4. Huang, T. J.; Brough, B.; Ho, C.-M.; Liu, Y.; Flood, A. H.; Bonvallet, P. A.; Tseng, H.-R.; Stoddart, J. F.; Baller, M.; Magonov, S. Appl. Phys. Lett. 2004, 85, 5391-5393.

  5. Nguyen, T. D.; Tseng, H.-R.; Celestre, P. C.; Flood, A. H.; Liu, Y.; Stoddart, J. F.; Zink, J. I. Proc. Natl. Acad. Sci. USA 2005, 102, 10029-10034.

  6. Saha, S.; Johansson, E.; Flood, A. H.; Tseng, H.-R.; Zink, J. I.; Stoddart, J. F. Chem. Eur. J. 2005, 11, 6846-6858.

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