Stoddart Mechanostereochemistry Group

 

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The Next Generation of Switchable Rotaxanes for Nanoelectronics

In recent years, developments in the area of mechanically interlocked molecules designed to act as artificial molecular switches and machines have proceeded apace. The potential applications of these molecules varies widely and includes nanoelectronics and nanoelectromechanical devices. Many different control stimuli have also been demonstrated, including light, electricity and chemicals.

One of the most prolific mechanically-interlocked systems to be incorporated into nano-devices is based on the donor-acceptor interaction between an electron-poor tetracationic cyclophane (CBPQT4+) and two electron-rich aromatic unitstetrathiafulvalene (TTF) and a dioxynaphthalene (DNP). Bistable [2]catenanestwo mechanically interlocked ringsand bistable [2]rotaxanesa ring mechanically trapped on a dumbbell shaped componentconstructed using this recognition motif have been incorporated into molecular switch tunnel junctions (MSTJs) and shown to switch reversibly between high and low conductivity states by application of a bias across the junction. A working 64-bit memory was assembled (ChemPhysChem, 2002, 3, 519-525) using this technology and at least 56 bits shown to operate.

One of the consequences of the four positive charges carried by the electron-poor cyclophane component of this recognition motif is the presence of four anions to counterbalance the charge. In condensed phases, such as in a MSTJ device, these counterions may induce drag on the cyclophane component and limit the accessible switching speeds. In addition, these counterions can exchange with other anions present in the processing steps for device assembly, leading to a degree of uncertainty regarding the final composition of the molecular monolayer. Thus, it was desired to design and synthesize neutral equivalents.

The Stoddart group in collaboration with the Sanders group at the University of Cambridge in the UK, and the Balzani group in Bologna have investigated the feasibility of incorporating uncharged recognition motif into bistable [2]rotaxanes. The Cambridge group has worked for many years on a neutral recognition motif, where a electron-rich crown ether ring (1/5DNP38C10) is paired with two electron-poor aromatic unitspyromellitic diimide (PmI) and naphtho-diimide (NpI).

Several [2]rotaxanes (Box) have since been synthesized (J. Am. Chem. Soc. 2004, 126, 9884-9885), including model compounds and bistable ones, and their properties investigated. Kinetic and thermodynamic parameters for the relevant mechanical motions were determined in order to facilitate optimization of the molecular design. In addition, both the chemical and electrochemical control mechanisms have been shown (Chem. Eur. J. 2004, 10, 6375-6392) to operate effectively in the bistable neutral [2]rotaxanes.

The speed with which this new system has been developed simply could not have been achieved without the knowledge of all those involved. The importance of this type of collaboration is expected to become even more important as the various components used to assemble nano-devices become ever more complex.

 

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