Radically Enhanced Molecular Switches

The mechanism governing the redox-stimulated switching
behavior
of a tristable [2]­rotaxane consisting of a cyclobis­(paraquat-p-phenylene) (CBPQT4+) ring encircling a dumbbell,
containing tetrathiafulvalene (TTF) and 1,5-dioxynaphthalene (DNP)
recognition units which are separated from each other along a polyether
chain carrying 2,6-diisopropylphenyl stoppers by a 4,4′-bipyridinium
(BIPY2+) unit, is described. The BIPY2+ unit
acts to increase the lifetime of the metastable state coconformation
(MSCC) significantly by restricting the shuttling motion of the CBPQT4+ ring to such an extent that the MSCC can be isolated in
the solid state and is stable for weeks on end. As controls, the redox-induced
mechanism of switching of two bistable [2]­rotaxanes and one bistable
[2]­catenane composed of CBPQT4+ rings encircling dumbbells
or macrocyclic polyethers, respectively, that contain a BIPY2+ unit with either a TTF or DNP unit, is investigated. Variable scan-rate
cyclic voltammetry and digital simulations of the tristable and bistable
[2]­rotaxanes and [2]­catenane reveal a mechanism which involves a bisradical
state coconformation (BRCC) in which only one of the BIPY•+ units in the CBPQT2(•+) ring is oxidized to the
BIPY2+ dication. This observation of the BRCC was further
confirmed by theoretical calculations as well as by X-ray crystallography
of the [2]­catenane in its bisradical tetracationic redox state. It
is evident that the incorporation of a kinetic barrier between the
donor recognition units in the tristable [2]­rotaxane can prolong the
lifetime and stability of the MSCC, an observation which augurs well
for the development of nonvolatile molecular flash memory devices.