Molecular recognition and switching via radical dimerization

Abstract

This article highlights the emerging use of the interactions of radical π-dimers to drive both molecular recognition and switching processes within supramolecular systems and mechanically interlocked molecular architectures. The enhanced stability experienced by dimers of radical cation species when encapsulated, as compared to when they are free in solution, is driving their useful incorporation into functional systems. The redox stimulation used in the production of radical cation species provides the ideal trigger for molecular switching events. Moreover, the nature and strength of the radical dimerization events introduces a completely novel recognition motif within supramolecular and mechanically interlocked molecular systems, complementing well-established techniques and enabling new research opportunities to blossom.atom-centered radical dimerization and, thereafter, I will highlight the recent advances that have led to the use of this unusual class of bonding as a practical and fully orthogonal molecular recognition motif. EARLY EXAMPLES OF RADICAL DIMERSThe π-dimerization of planar radical species was first observed in the middle of the last century [1][2][3][4][5][6] in both the solid state and in concentrated solutions at low temperatures. Under these conditions, equilibria are established between two paramagnetic free radical species and a diamagnetic dimer, most often with a particular equilibrium favoring the dissociated, non-dimer state. These types of dimeric associations have been observed for a whole range of different types of planar radical species, varying in both the size of the planar systems and the charges of the dimer components. Both radical anions and cations, as well as neutral free radicals, have been observed to associate with one another in this manner. Some of the most common radical π-dimeric species are illustrated in Fig. 1, including the associated radical anions of singly reduced tetracyanoethylene (TCNE) [7], and the neutral radical pairs of phenalenyl [8], as well as the radical cation π-dimers of singly oxidized tetrathiafulvalene (TTF) [9,10] and singly reduced bipyridium (BIPY 2+ ) units [11,12].The π-association of phenalenyl has received much attention [8,13] as a model for the radical dimerization of large planar π-systems. The persistent (i.e., stable) neutral radical is readily generated during the oxidation of phenylene. Its stability can be attributed to the delocalization of the radical throughout the fused polycyclic structure, its character residing predominately on 6 of the 12 peripheral carbons [14,15]. When substituted with bulky side groups to prevent σ-binding (the radical-radical coupling to form a traditional covalent sigma (σ) bond), π-dimer formation is observed in concentrated deoxygenated solutions. Moreover, this π-dimer is comprised of two perfectly overlapping delocalized singly occupied molecular orbitals (SOMOs) on each phenalenyl unit in such a way that the two formal free radical electrons pair to form what has been described as a two-electron...

Symbiotic Control in Mechanical Bond Formation

“…1 HNMR (500 MHz, CD 3 8, 147.4, 138.1, 132.3, 129.3, 129.2, 128.8, 128.6, 66.7, 64.2, 64.61, 63.2, 51.0, 28.2, 27.6, 26.9 3·6 PF 6 :A ne xcess of Cu dust was added to 4·2 PF 6 (14 mg, 0.025 mmol) and CBPQT·4 PF 6 (27 mg,0 .025 mmol) dissolved in degassed MeCN (20 mL) in an Ar-filled glovebox. Cu Dust in excess was also added to another flask containing 4·2 PF 6 (14 mg, 0.025 mmol) and CBPQT·4 PF 6 (27 mg,0 .025 mmol) which were dissolved in degassed MeCN (10 mL) in an Ar-filled glovebox.…”

“…For several decades,m echanically interlocked molecules [1] (MIMs), such as bistable catenanes and rotaxanes,h ave attracted [2] much attention from the broader community of scientists and engineers.A pplications in directing the construction of molecular switches [3] and machines, [4] as well as in drug delivery [5] and molecular electronic devices, [6] are already well established. With our rapidly growing insight into molecular recognition and its use in aiding self-assembly processes,M IMs can now be synthesized in high yields by employing template-directed approaches [7] that rely on noncovalent bonding interactions.A mong the redox-based approaches to templation, radical-pairing interactions [8] have been shown to provide au nique form of covalent assistance in the formation of mechanical bonds.Subsequent eradication of the radical-pairing interactions is more often than not followed by the formation of repulsive electrostatic interactions.I np articular, recognition [9] between BIPYC + units (the radical state of 4,4'-bipyridinium BIPY 2+ )h as captured our attention because of our interest in the syntheses of MIMs where the component parts repel each other in the final analysis.…”

Template-directed nonenzymatic oligonucleotide synthesis: lessons from synthetic chemistry

“…Kinetic template-directed protocols have employed a large range of noncovalent interactions to hold together the template and reactive substrates. These templation strategies make use of a wide range of noncovalent bonding interactions, including hydrogen bonding [27,28], π-π donor-acceptor forces between π-electron-rich and π-electron-poor species [29], metal-ligand coordination [30,31] and radical pairing [32,33].…”

Mechanostereochemistry and the mechanical bond

“…The development of functional devices, for example optical chloride sensors (Gassensmith et al 2010) based on anion-templated MIMs, underscores their potential for applications in the future. One of the most recently uncovered modes of templation is the one that relies on radical-radical interactions (Spruell 2010). The radical organocations of TTF and methyl viologen (MV 2+ )-generated by oxidation and reduction, respectively-have been long known to interact with one another.…”