New Twists and Turns in Radical Organic Chemistry

Date & time

4–5pm 11 October 2013

Location

RSC Lecture Theatre

Speakers

Sir Professor Fraser Stoddart

Contacts

 Gavin Perri
 61252391

The observation of π–π radical-cation dimers—sometimes referred to as pimers—on the redox stimulation of electron rich and poor heterocycles is long precedented. Investigations of such systems have been hindered, however, by the inherent instability of even the simplest radical-cation dimers, such that they have only been observed at low temperatures or in the solid state, an attribute that has caused them to be considered as mere novelties, rather than the fundamental cornerstone of functional devices. Specifically, methyl viologen (MV2+), the predominant member of a class of organic salts, valued in previous times for their herbicidal activities, was observed long ago to form diamagnetic radical-cation dimers following the one-electron reduction of the dication. These dimers, however, are only stable at low temperatures and high concentrations in air-free environments. Similar π–π radical-cation and mixed valence dimers of tetrathiafulvalene (TTF) are known to form upon the one-electron oxidation of the neutral compound to its stable radical-cation. As the simple parent compound, however, the dimeric complex is once again only soluble at low temperatures and in the solid state. In recent years, a renaissance in the radical-cation dimer chemistry of both MV2+ and TTF has been spurred on by the discovery that inclusion complexes of each species exhibit exceptionally high stabilities in solution at room temperature. Recently, we have harnessed the tendency of MV•+ and TTF•+ radical-cations to undergo dimerization in [2]rotaxanes, [2]- and [3]catenanes and Solomon links in order to generate a new portfolio of radically enhanced molecular switches. It transpires that, within the confines of these mechanically interlocked molecules, both highly stable MV•+ and TTF•+ mixed-valence, as well as radical-cation dimers, are formed. The topologies of the catenanes and links, in addition to the relative dispositions of the dimeric units, can affect the stabilities of the mixed-valence and radical-cation dimers. The question arises—to what use can we put radically enhanced molecular switches? The answer is to introduce them, in an appropriately modified manner, into the design and construction of artificial molecular motors employing energy ratchet mechanisms. In my lecture, I will describe a wholly synthetic, smallmolecule system which, under the influence of chemical reagents, electrical potential, visible light, undergoes relative translational motion. Altering the redox state of a cyclobis(paraquat-p-phenylene) ring simultaneously (i) inverts the relative heights of kinetic barriers presented by two termini—one a neutral 2-isopropylphenyl group and the other a positively charged 3,5-dimethylpyridinium unit—of a constitutionally asymmetric dumbbell, which can impair the threading / dethreading of a [2]pseudorotaxane, and (ii) controls the ring’s affinity for a 1,5-dioxynaphthalene binding site located at the dumbbell’s central core. I will provide both experimental and computational evidence for the formation and subsequent dissociation of the [2]pseudorotaxane by passage of the ring over the neutral and positively charged termini of the dumbbell component in one, and only one, direction. I will show that, in the presence of a photosensitizer, visible light energy is the only fuel source that is needed to drive the unidirectional molecular translation, making it feasible to repeat the operation numerous times without the buildup of byproducts.
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