Radically Enhanced Molecular Switches

다국어 초록 (Multilingual Abstract)

<P>The mechanism governing the redox-stimulated switching behavior of a tristable [2]rotaxane consisting of a cyclobis(paraquat-<I>p</I>-phenylene) (CBPQT<SUP>4+</SUP>) 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 (BIPY<SUP>2+</SUP>) unit, is described. The BIPY<SUP>2+</SUP> unit acts to increase the lifetime of the metastable state coconformation (MSCC) significantly by restricting the shuttling motion of the CBPQT<SUP>4+</SUP> 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 CBPQT<SUP>4+</SUP> rings encircling dumbbells or macrocyclic polyethers, respectively, that contain a BIPY<SUP>2+</SUP> 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<SUP>•+</SUP> units in the CBPQT<SUP>2(•+)</SUP> ring is oxidized to the BIPY<SUP>2+</SUP> 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.</P><P><B>Graphic Abstract</B>
<IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jacsat/2012/jacsat.2012.134.issue-39/ja306044r/production/images/medium/ja-2012-06044r_0014.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/ja306044r'>ACS Electronic Supporting Info</A></P>