PhD in Organic Chemistry, Department of Chemistry, Northwestern University, Evanston, Illinois, USA
题目：The Incorporation of Radical Pairing Interactions into the Syntheses and Translational Isomerization of Mechanically Interlocked Molecules
In the domain of donor-acceptor mechanically interlocked molecules (MIMs), 4,4′-dialkylbipyridinium (BIPY2+)moieties and cyclobis(paraquat-p-phenylene) (CBPQT4+) have been investigated extensively as examples of π-electron-deficient guests and hosts, respectively. We have demonstrated1 that viologen radical cations can form strong inclusion complexes with CBPQT4+ in its diradical dicationic redox state (Figure 1a), as a consequence of radical-pairing stabilizing interactions. The synthesis2 of a rotaxane 1·6PF6, composed only of electron-deficient CBPQT4+ and BIPY2+ units with no complementary electron-rich components (Figure 1b), takes advantage of the marriage between templation and radical-pairing stabilizing interactions, promises to help chemists overcome the reliance on traditional template-directed protocols, and can be utilized as a general synthetic procedure for preparing MIMs composed of components with little or no binding affinities characterizing their ground states. Using this type of binding motif, a rotaxane R·8Cl (Figure 1c) has been designed, synthesized and characterized.3 The rotaxane R·8Cl contains a CBPQT4+ ring and a dumbbell bearing a π-electron rich 1,5-dioxynaphthalene (DNP) recognition site, a BIPY2+ unit, and a Ru(bpy)32+ complex as one of its two stoppers. Upon irradiation with visible light in the presence of the sacrificial electron donor, triethanolamine (TEOA), all three of the BIPY2+ units, two in the CBPQT4+ ring and one in the dumbbell are reduced to BIPY(?+) units by the Ru(bpy)32+ in its excited state. The resulting diradical dicationic CBPQT2(?+) ring undergoes translational motion in order to encircle the BIPY(?+) unit in the dumbbell, on account of stabilizing radical-pairing interactions. As soon as the BIPY(?+) units are oxidized by oxygen, the donor-acceptor interactionsare reinstated while the Coulombic repulsion between the CBPQT4+ ring and the BIPY2+ unit in the dumbbell induces the ring to shuttle back and encircle the DNP unit. The fact that the rotaxane R8+ can have its components driven back and forth relative to each other through many cycles without decomposition in aqueous solution represents a step forward in the design and operation of an artificial molecular machine that can operate in water. This demonstration opens up opportunities for developing integrated nanobiomechanical systems in the direction of applications such as molecular prosthetics.