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Radicals, with their unpaired electrons, exhibit unique chemical and physical properties that have long intrigued chemists. Despite early skepticism about their stability, the discovery of persistent radicals has opened new possibilities for molecular interactions. This review examines the mechanisms and applications of radically driven self-assembly, focusing on key motifs such as naphthalene diimides, tetrathiafulvalenes, and viologens, which serve as models for radical assembly. The potential of radical interactions in the development of artificial molecular machines (AMMs) are also discussed. These AMMs, powered by radical-radical interactions, represent significant advancements in non-equilibrium chemistry, mimicking the functionalities of biological systems. From molecular switches to ratchets and pumps, the versatility and unique properties of radically powered AMMs are highlighted. Additionally, the applications of radical assembly in materials science are explored, particularly in creating smart materials with redox-responsive properties. The review concludes by comparing AMMs to biological molecular machines, offering insights into future directions. This overview underscores the impact of radical chemistry on molecular assembly and its promising applications in both synthetic and biological systems., (© 2024 The Author(s). Advanced Materials published by Wiley‐VCH GmbH.)

Imine synthesis has enjoyed a long history as the dynamic covalent reaction of choice for the construction of purely covalent molecular architectures. In organic solvents, the formation of imine bonds is reversible but leads to thermodynamically stable products. In the presence of water, however, imine bonds are labile, a fact which limits their utility as mediators of self-assembly in aqueous and biological media. In this Review, we discuss water-compatible dynamic covalent bonds based on N-substituted imine derivatives, namely hydrazones and oximes, for the self-assembly of metal-free organic architectures with well-defined structures. The reasons why hydrazones and oximes are more robust in water than their parent imines are explained. Recent progress in the self-assembly, characterization, and design principles of a variety of complex molecules including macrocycles, cages, catenanes, and knots in aqueous media is highlighted. Emerging applications for these molecules, including guest recognition and separations, are also discussed., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The promiscuous encapsulation of π-electron-rich guests by the π-electron-deficient host, cyclobis(paraquat-p-phenylene) (CBPQT(4+)), involves the formation of 1:1 inclusion complexes. One of the most intensely investigated charge-transfer (CT) bands, assumed to result from inclusion of a guest molecule inside the cavity of CBPQT(4+), is an emerald-green band associated with the complexation of tetrathiafulvalene (TTF) and its derivatives. This interpretation was called into question recently in this journal based on theoretical gas-phase calculations that reinterpreted this CT band in terms of an intermolecular side-on interaction of TTF with one of the bipyridinium (BIPY(2+)) units of CBPQT(4+), rather than the encapsulation of TTF inside the cavity of CBPQT(4+). We carried out DFT calculations, including solvation, that reveal conclusively that the CT band emerging upon mixing TTF with CBPQT(4+) arises from the formation of a 1:1 inclusion complex. In support of this conclusion, we have performed additional experiments on a [2]rotaxane in which a TTF unit, located in the middle of its short dumbbell, is prevented sterically from interacting with either one of the two BIPY(2+) units of a CBPQT(4+) ring residing on a separate [2]rotaxane in a side-on fashion. This [2]rotaxane has similar UV/Vis and (1)H NMR spectroscopic properties with those of 1:1 inclusion complexes of TTF and its derivatives with CBPQT(4+). The [2]rotaxane exists as an equimolar mixture of cis- and trans-isomers associated with the disubstituted TTF unit in its dumbbell component. Solid-state structures were obtained for both isomers, validating the conclusion that the TTF unit, which gives rise to the CT band, resides inside CBPQT(4+)., (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Suppression of the dimerization of the viologen radical cation by cucurbit[7]uril (CB7) in water is a well-known phenomenon. Herein, two counter-examples are presented. Two viologen-containing thread molecules were designed, synthesized, and thoroughly characterized by (1)H DOSY NMR spectrometry, UV/Vis absorption spectrophotometry, square-wave voltammetry, and chronocoulometry: BV(4+), which contains two viologen subunits, and HV(12+), which contains six. In both threads, the viologen subunits are covalently bonded to a hexavalent phosphazene core. The corresponding [3]- and [7]pseudorotaxanes that form on complexation with CB7, that is, BV(4+)⊂(CB7)2 and HV(12+)⊂(CB7)6, were also analyzed. The properties of two monomeric control threads, namely, methyl viologen (MV(2+)) and benzyl methyl viologen (BMV(2+)), as well as their [2]pseudorotaxane complexes with CB7 (MV(2+)⊂CB7 and BMV(2+)⊂CB7) were also investigated. As expected, the control pseudorotaxanes remained intact after one-electron reduction of their viologen-recognition stations. In contrast, analogous reduction of BV(4+)⊂(CB7)2 and HV(12+)⊂(CB7)6 led to host-guest decomplexation and release of the free threads BV(2(·+)) and HV(6(·+)), respectively. (1)H DOSY NMR spectrometric and chronocoulometric measurements showed that BV(2(·+)) and HV(6(·+)) have larger diffusion coefficients than the corresponding [3]- and [7]pseudorotaxanes, and UV/Vis absorption studies provided evidence for intramolecular radical-cation dimerization. These results demonstrate that radical-cation dimerization, a relatively weak interaction, can be used as a driving force in novel molecular switches., (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The pursuit to achieve miniaturization has tantalized researchers across the fields of chemistry, physics, biology, materials science and engineering for over half a century because of its many alluring potential applications. As alternatives to traditional "top-down" manufacturing, "bottom-up" approaches, originating from the (supra)molecular level, have enabled researchers to develop switches which can be manipulated on surfaces at nanoscale dimensions with deft precision using simple external triggers. Once on surfaces, these organic switches have been shown to modulate both the physical and chemical surface properties. In this Progress Report, we shed light on recent advances made in our laboratories towards integrated systems using all-organic switches on a variety of substrates. Design concepts are revealed, as well as the overall impact of all-organic switches on the properties of their substrates, while emphasizing the considerable promise and formidable challenges these advanced composite materials pose when it comes to conferring function on them., (Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Supramolecular chemistry and mechanostereochemistry have been major beneficiaries of the concepts and reactions pioneered under the "click chemistry" philosophy. The success of the copper(I) 1,3-dipolar cycloaddition between azides and alkynes, resulting in the triazole ring has inspired the application of other emerging click reactions, for example, Diels-Alder cycloadditions, thiol-ene/yne chemistry, and nitrile N-oxide cycloadditions, towards the creation of advanced functional supramolecular and mechanostereochemical systems. In this Focus Review, recent advances in the use of click chemistry in these fields are highlighted., (Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The ability to control the kinetic barriers governing the relative motions of the components in mechanically interlocked molecules is important for future applications of these compounds in molecular electronic devices. In this Full Paper, we demonstrate that bipyridinium (BIPY(2+)) dications fulfill the role as effective electrostatic barriers for controlling the shuttling and threading behavior for rotaxanes and pseudorotaxanes in aqueous environments. A degenerate [2]rotaxane, composed of two 1,5-dioxynaphthalene (DNP) units flanking a central BIPY(2+) unit in the dumbbell component and encircled by the cyclobis(paraquat-p-phenylene) (CBPQT(4+)) tetracationic cyclophane, has been synthesized employing a threading-followed-by-stoppering approach. Variable-temperature (1)H NMR spectroscopy reveals that the barrier to shuttling of the CBPQT(4+) ring over the central BIPY(2+) unit is in excess of 17 kcal mol(-1) at 343 K. Further information about the nature of the BIPY(2+) unit as an electrostatic barrier was gleaned from related supramolecular systems, utilizing two threads composed of either two DNP units flanking a central BIPY(2+) moiety or a central DNP unit flanked by a BIPY(2+) moiety. The threading and dethreading processes of the CBPQT(4+) ring with these compounds, which were investigated by spectrophotometric techniques, reveal that the BIPY(2+) unit is responsible for affecting both the thermodynamics and kinetics of pseudorotaxane formation by means of an intramolecular self-folding (through donor-acceptor interactions with the DNP unit), in addition to Coulombic repulsion. In particular, the free energy barrier to threading (ΔG(f)(++)) of the CBPQT(4+) for the case of the thread composed of a DNP flanked by two BIPY(2+) units was found to be as high as 21.7 kcal mol(-1) at room temperature. These results demonstrate that we can effectively employ the BIPY(2+) unit to serve as electrostatic barriers in water in order to gain control over the motions of the CBPQT(4+) ring in both mechanically interlocked and supramolecular systems., (Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Five donor-acceptor oligorotaxanes made up of dumbbells composed of tetraethylene glycol chains, interspersed with three and five 1,5-dioxynaphthalene units, and terminated by 2,6-diisopropylphenoxy stoppers, have been prepared by the threading of discrete numbers of cyclobis(paraquat-p-phenylene) rings, followed by a kinetically controlled stoppering protocol that relies on click chemistry. The well-known copper(I)-catalyzed alkyne-azide cycloaddition between azide functions placed at the ends of the polyether chains and alkyne-bearing stopper precursors was employed during the final kinetically controlled template-directed synthesis of the five oligorotaxanes, which were characterized subsequently by (1)H NMR spectroscopy at low temperature (233 K) in deuterated acetonitrile. The secondary structures, as well as the conformations, of the five oligorotaxanes were unraveled by spectroscopic comparison with the dumbbell and ring components. By focusing attention on the changes in chemical shifts of some key probe protons, obtained from a wide range of low-temperature spectra, a picture emerges of a high degree of folding within the thread protons of the dumbbells of four of the five oligorotaxanes-the fifth oligorotaxane represents a control compound in effect-brought about by a combination of C-H···O and π-π stacking interactions between the π-electron-deficient bipyridinium units in the rings and the π-electron-rich 1,5-dioxynaphthalene units and polyether chains in the dumbbells. The secondary structures of a foldamer-like nature have received further support from a solid-state superstructure of a related [3]pseudorotaxane and density functional calculations performed thereon., (Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)