The Incorporation of Radical Pairing Interactions into Mechanically Interlocked Molecules

One of the more promising entries into molecular nanotechnology has followed in the wake of the advent of the mechanical bond in chemistry. Upon their integration into nanoelectro-mechanical systems (NEMs), mechanically interlocked molecules (MIMs), often in the form of bistable [2]catenanes or [2]rotaxanes, have attracted interest because of their potential applications in the burgeoning field of nanotechnology, ranging from molecular electronic devices (MEDs) to targeted delivery of therapeutic materials. One of the major challenges in this area for synthetic chemists includes the development of driving forces—usually in the form of noncovalent bonding interactions—to gather building blocks together, which lead to the construction of mechanically interlocked structures. A historical perspective of different approaches leading to mechanical bond formation is presented in the first Chapter.;In the second Chapter, a novel type of binding motif—namely, radical templation—for [2]rotaxane syntheses is described. An unlikely [2]rotaxane composed only of an electron-deficient cyclobis(paraquat-p-phenylene) (CBPQT4+) ring and a dumbbell component, bearing a BIPY 2+ unit with no complementary electron-rich components, has been prepared and fully characterized. Radical dimer interactions were utilized as the recognition motif to generate this rotaxane following reduction to its trisradical tricationic form. In its fully oxidized state, a combination of Coulombic repulsion and mechanical bonding forces the oligomethylene chain to play the role of a "pseudo-station" for the ring component.;The third Chapter records the syntheses of a homologous series of [2]rotaxanes, in which CBPQT4+ serves as the ring component, while the dumbbell components all contain single 4,4'-bipyridinium (BIPY2+) units, centrally located in the midst of oligomethylene chains of varying lengths. The syntheses of the rotaxanes take advantage of radical templation and copper-free azide-alkyne 1,3-dipolar cycloadditions in the formation of their stoppers. Cyclic voltammetry, UV/Vis spectroscopy and mass spectrometry reveal that the BIPY•+ radical cations in this series of [2]rotaxanes are stabilized towards oxidation, both electrochemically and by atmospheric oxygen. The enforced proximity between the BIPY2+ units in the ring and dumbbell components gives rise to enhanced Coulombic repulsion, destabilizing the ground-state co-conformations of the fully oxidized forms of these [2]rotaxanes. The smallest [2]rotaxane with only three methylene groups on each side of its dumbbell component is found to exist under ambient conditions in a monoradical state, a situation which does not persist in acetonitrile solution at least in the case of its longer analogues. 1H NMR Spectroscopy reveals that the activation energy barriers to the shuttling of the CBPQT4+ rings over the BIPY2+ units in the dumbbells rise linearly with increasing oligomethylene chain lengths across the series of [2]rotaxanes. These findings provide a new way of producing highly stabilized BIPY•+ radical cations and open up more opportunities to use stable organic radicals as building blocks for the construction of paramagnetic materials and conductive molecular electronic devices.;The fourth Chapter demonstrates a redox-controllable bistable [2]rotaxane—in which the ring component is CBPQT4+ and the dumbbell component comprises (i) ruthenium(II)-tris(2,2&feet;-bipyridine) as one of its two stoppers and also serves as a photocatalytic unit, (ii) a π-electron rich 1,5-dioxynaphthalene recognition site for the π-electron poor tetracationic cyclophane, and (iii) a 4,4'-bipyridinium unit—has been designed and synthesized using templation and click chemistry. With assistance from a sacrificial electron donor, light-triggered switching through numerous cycles can be initiated by radical-pairing interactions between the reduced forms of the cyclophane and the bipyridinium unit in aqueous solution in the absence of air and then reset by donor-acceptor charge-transfer interactions in the presence of air (O2). In a manner reminiscent of a macroscopic switch, the ring can be made to dart back and forth along the axle of the dumbbell from one recognition site to the other in the bistable [2]rotaxane.;In the final Chapter, I summarize the investigations presented in Chapters 2–4 based on BIPY•+ radical cationic derivatives, and speculate about the direction of future research in this area.