Matthew E. Belowich

Radically enhanced molecular recognition

The tendency for viologen radical cations to dimerize has been harnessed to establish a recognition motif based on their ability to form extremely strong inclusion complexes with cyclobis(paraquat-p-phenylene) in its diradical dicationic redox state. This previously unreported complex involving three bipyridinium cation radicals increases the versatility of host-guest chemistry, extending its practice beyond the traditional reliance on neutral and charged guests and hosts. In particular, transporting the concept of radical dimerization into the field of mechanically interlocked molecules introduces a higher level of control within molecular switches and machines. Herein, we report that bistable and tristable [2]rotaxanes can be switched by altering electrochemical potentials. In a tristable [2]rotaxane composed of a cyclobis(paraquat-p-phenylene) ring and a dumbbell with tetrathiafulvalene, dioxynaphthalene and bipyridinium recognition sites, the position of the ring can be switched. On oxidation, it moves from the tetrathiafulvalene to the dioxynaphthalene, and on reduction, to the bipyridinium radical cation, provided the ring is also reduced simultaneously to the diradical dication.

Metal–organic frameworks with designed chiral recognition sites

Linking struts containing Cram-like bisbinaphthyl[22]crown-6 with Zn(4)O(CO(2))(6) joints affords metal-organic frameworks with chiral recognition sites that are highly designed, ordered and placed in a precise manner throughout the entire crystal.

pH-Responsive mechanised nanoparticles gated by semirotaxanes

A [2]pseudorotaxane-based mechanised nanoparticle system, which operates within an aqueous acidic environment, has been prepared and characterised; this integrated system affords both water-soluble stalk and ring components in an effort to improve the biocompatibility of these promising new drug delivery vehicles.

Redox-driven switching in pseudorotaxanes

Two donor–acceptor thread-like compounds incorporating viologen (V2+) units and 1,5-dihydroxynaphthalene (DNP) stations have been prepared. Their ability to form self-assembled charge-transfer (CT) complexes with cucurbit[8]uril (CB[8]) is evidenced by UV-Vis and NMR spectroscopies, as well as by mass spectrometry. Binding studies show the formation of 1 : 1 and 2 : 1 complexes between CB[8] and a thread-like compound containing two viologen units, while only a 1 : 1 inclusion complex was observed between CB[8] and a thread-like compound containing only a single viologen unit. The switching behavior of the threads within their pseudorotaxane frameworks was investigated by using cyclic voltammetry (CV) and UV-Vis spectroscopy.

Gated Electron Sharing Within Dynamic Naphthalene Diimide-Based Oligorotaxanes†

The controlled self-assembly of well-defined and spatially ordered π-systems has attracted considerable interest because of their potential applications in organic electronics. An important contemporary pursuit relates to the investigation of charge transport across noncovalently coupled components in a stepwise fashion. Dynamic oligorotaxanes, prepared by template-directed methods, provide a scaffold for directing the construction of monodisperse one-dimensional assemblies in which the functional units communicate electronically through-space by way of π-orbital interactions. Reported herein is a series of oligorotaxanes containing one, two, three and four naphthalene diimide (NDI) redox-active units, which have been shown by cyclic voltammetry, and by EPR and ENDOR spectroscopies, to share electrons across the NDI stacks. Thermally driven motions between the neighboring NDI units in the oligorotaxanes influence the passage of electrons through the NDI stacks in a manner reminiscent of the conformationally gated charge transfer observed in DNA.