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 unitstetrathiafulvalene (TTF)
and a dioxynaphthalene (DNP). Bistable catenanestwo mechanically interlocked
ringsand bistable rotaxanesa ring mechanically trapped on a dumbbell shaped
componentconstructed 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 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 unitspyromellitic
diimide (PmI) and naphtho-diimide (NpI).
Several 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 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.