Influence of Constitution and Charge on Radical Pairing Interactions in Tris-radical Tricationic Complexes

Harnessing Radicals: Advances in Self-Assembly and Molecular Machinery

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.

Supramolecular design as a route to high-performing organic electrodes

Organic electrodes may someday replace transition metals oxides, the current standard in electrochemical energy storage, including those with severe issues of availability, cost, and recyclability. To realize this more sustainable future, a thorough understanding of structure-property relationships and design rules for organic electrodes must be developed. Further, it is imperative that supramolecular interactions between organic species, which are often overlooked, be included in organic electrode design. In this review, we showcase how molecular and polymeric electrodes that host non-covalent interactions outperform materials without these features. Using select examples from the literature, we emphasize how dispersion forces, hydrogen-bonding, and radical pairing can be leveraged to improve the stability, capacity, and energy density of organic electrodes. Throughout this review, we identify potential next-generation designs and opportunities for continued investigation. We hope that this review will serve as a catalyst for collaboration between synthetic chemists and the energy storage community, which we view as a prerequisite to achieving high-performing supramolecular electrode materials.

Radical Pairing Interactions and Donor-Acceptor Interactions in Cyclobis(paraquat-p-phenylene) Inclusion Complexes

Understanding molecular interactions in mechanically interlocked molecules (MIMs) is challenging because they can be either donor-acceptor interactions or radical pairing interactions, depending on the charge states and multiplicities in the different components of the MIMs. In this work, for the first time, the interactions between cyclobis(paraquat-p-phenylene) (abbreviated as CBPQTn+ (n = 0-4)) and a series of recognition units (RUs) were investigated using the energy decomposition analysis approach (EDA). These RUs include bipyridinium radical cation (BIPY•+), naphthalene-1,8:4,5-bis(dicarboximide) radical anion (NDI•-), their oxidized states (BIPY2+ and NDI), neutral electron-rich tetrathiafulvalene (TTF) and neutral bis-dithiazolyl radical (BTA•). The results of generalized Kohn-Sham energy decomposition analysis (GKS-EDA) reveal that for the CBPQTn+···RU interactions, correlation/dispersion terms always have large contributions, while electrostatic and desolvation terms are sensitive to the variation in charge states in CBPQTn+ and RU. For all the CBPQTn+···RU interactions, desolvation terms always tend to overcome the repulsive electrostatic interactions between the CBPQT cation and RU cation. Electrostatic interaction is important when RU has the negative charge. Moreover, the different physical origins of donor-acceptor interactions and radical pairing interactions are compared and discussed. Compared to donor-acceptor interactions, in radical pairing interactions, the polarization term is always small, while the correlation/dispersion term is important. With regard to donor-acceptor interactions, in some cases, polarization terms could be quite large due to the electron transfer between the CBPQT ring and RU, which responds to the large geometrical relaxation of the whole systems.

The Story of the Little Blue Box: A Tribute to Siegfried Hünig

The tetracationic cyclophane, cyclobis(paraquat-p-phenylene), also known as the little blue box, constitutes a modular receptor that has facilitated the discovery of many host-guest complexes and mechanically interlocked molecules during the past 35 years. Its versatility in binding small π-donors in its tetracationic state, as well as forming trisradical tricationic complexes with viologen radical cations in its doubly reduced bisradical dicationic state, renders it valuable for the construction of various stimuli-responsive materials. Since the first reports in 1988, the little blue box has been featured in over 500 publications in the literature. All this research activity would not have been possible without the seminal contributions carried out by Siegfried Hünig, who not only pioneered the syntheses of viologen-containing cyclophanes, but also revealed their rich redox chemistry in addition to their ability to undergo intramolecular π-dimerization. This Review describes how his pioneering research led to the design and synthesis of the little blue box, and how this redox-active host evolved into the key component of molecular shuttles, switches, and machines.

Quantifying the barrier for the movement of cyclobis(paraquat-p-phenylene) over the dication of monopyrrolotetrathiafulvalene

A bistable [2]pseudorotaxane 1⊂CBPQT·4PF₆ and a bistable [2]rotaxane 2·4PF₆ have been synthesised to measure the height of an electrostatic barrier produced by double molecular oxidation (0 to +2). Both systems have monopyrrolotetrathiafulvalene (MPTTF) and oxyphenylene (OP) as stations for cyclobis(paraquat-p-phenylene) (CBPQT⁴⁺). They have a large stopper at one end while the second stopper in 2⁴⁺ is composed of a thioethyl (SEt) group and a thiodiethyleneglycol (TDEG) substituent, whereas in 1⊂CBPQT⁴⁺, the SEt group has been replaced with a less bulky thiomethyl (SMe) group. This seemingly small difference in the substituents on the MPTTF unit leads to profound changes when comparing the physical properties of the two systems allowing for the first measurement of the deslipping of the CBPQT⁴⁺ ring over an MPTTF²⁺ unit in the [2]pseudorotaxane. Cyclic voltammetry and ¹H NMR spectroscopy were used to investigate the switching mechanism for 1⊂CBPQT·MPTTF⁴⁺ and 2·MPTTF⁴⁺, and it was found that CBPQT⁴⁺ moves first to the OP station producing 1⊂CBPQT·OP⁶⁺ and 2·OP⁶⁺, respectively, upon oxidation of the MPTTF unit. The kinetics of the complexation/decomplexation process occurring in 1⊂CBPQT·MPTTF⁴⁺ and in 1⊂CBPQT·OP⁶⁺ were studied, allowing the free energy of the transition state when CBPQT⁴⁺ moves across a neutral MPTTF unit (17.0 kcal mol^-1) or a di-oxidised MPTTF²⁺ unit (24.0 kcal mol^-1) to be determined. These results demonstrate that oxidation of the MPTTF unit to MPTTF²⁺ increases the energy barrier that the CBPQT⁴⁺ ring must overcome for decomplexation to occur by 7.0 kcal mol^-1.

Radically Enhanced Dual Recognition

Complexation between a viologen radical cation (V^.+ ) and cyclobis(paraquat-p-phenylene) diradical dication (CBPQT^2(.+) ) has been investigated and utilized extensively in the construction of mechanically interlocked molecules (MIMs) and artificial molecular machines (AMMs). The selective recognition of a pair of V^.+ using radical-pairing interactions, however, remains a formidable challenge. Herein, we report the efficient encapsulation of two methyl viologen radical cations (MV^.+ ) in a size-matched bisradical dicationic host - namely, cyclobis(paraquat-2,6-naphthalene)^2(.+) , i.e., CBPQN^2(.+) . Central to this dual recognition process was the choice of 2,6-bismethylenenaphthalene linkers for incorporation into the bisradical dicationic host. They provide the space between the two bipyridinium radical cations in CBPQN^2(.+) suitable for binding two MV^.+ with relatively short (3.05-3.25 Å) radical-pairing distances. The size-matched bisradical dicationic host was found to exhibit highly selective and cooperative association with the two MV^.+ in MeCN at room temperature. The formation of the tetrakisradical tetracationic inclusion complex - namely, [(MV)₂ ⊂CBPQN]^{4( .+)} - in MeCN was confirmed by VT ¹ H NMR, as well as by EPR spectroscopy. The solid-state superstructure of [(MV)₂ ⊂CBPQN]^{4( .+)} reveals an uneven distribution of the binding distances (3.05, 3.24, 3.05 Å) between the three different V^.+ , suggesting that localization of the radical-pairing interactions has a strong influence on the packing of the two MV^.+ inside the bisradical dicationic host. Our findings constitute a rare example of binding two radical guests with high affinity and cooperativity using host-guest radical-pairing interactions. Moreover, they open up possibilities of harnessing the tetrakisradical tetracationic inclusion complex as a new, orthogonal and redox-switchable recognition motif for the construction of MIMs and AMMs.

Radical-pairing-induced molecular assembly and motion

Radical-pairing interactions between conjugated organic π-radicals are relative newcomers to the inventory of molecular recognition motifs explored in supramolecular chemistry. The unique electronic, magnetic, optical and redox-responsive properties of the conjugated π-radicals render molecules designed with radical-pairing interactions useful for applications in various areas of chemistry and materials science. In particular, the ability to control formation of radical cationic or anionic species, by redox stimulation, provides a flexible trigger for directed assembly and controlled molecular motions, as well as a convenient means of inputting energy to fuel non-equilibrium processes. In this Review, we provide an overview of different examples of radical-pairing-based recognition processes and of their emerging use in (1) supramolecular assembly, (2) templation of mechanically interlocked molecules, (3) stimuli-controlled molecular switches and, by incorporation of kinetic asymmetry in the design, (4) the creation of unidirectional molecular transporters based on pumping cassettes powered by fuelled switching of radical-pairing interactions. We conclude the discussion with an outlook on future directions for the field.

A Self-Assembled Homochiral Radical Cage with Paramagnetic Behaviors

Condensation of an inherently C₃ -symmetric polychlorotriphenylmethyl (PTM) radical trisaldehyde with tris(2-aminoethyl)amine (TREN) yields a [4+4] tetrahedral radical cage as a racemic pair of homochiral enantiomers in 75 % isolated yield. The structure was characterized by X-ray crystallography, confirming the homochirality of each cage framework. The homochirality results from intramolecular [CH⋅⋅⋅π] and hydrogen-bonding interactions within the cage framework. The four PTM radicals in a cage undergo weak through-space coupling. Magnetic measurements demonstrated that each cage bears 3.58 spins.

Cyclophane-Sustained Ultrastable Porphyrins

We report the encapsulation of free-base and zinc porphyrins by a tricyclic cyclophane receptor with subnanomolar binding affinities in water. The high affinities are sustained by the hydrophobic effect and multiple [CH···π] interactions covering large [π···π] stacking surfaces between the substrate porphyrins and the receptor. We discovered two co-conformational isomers of the 1:1 complex, where the porphyrin is orientated differently inside the binding cavity of the receptor on account of its tricyclic nature. The photophysical properties and chemical reactivities of the encapsulated porphyrins are modulated to a considerable extent by the receptor. Improved fluorescence quantum yields, red-shifted absorptions and emissions, and nearly quantitative energy transfer processes highlight the emergent photophysical enhancements. The encapsulated porphyrins enjoy unprecedented chemical stabilities, where their D/H exchange, protonation, and solvolysis under extremely acidic conditions are completely blocked. We anticipate that the ultrahigh stabilities and improved optical properties of these encapsulated porphyrins will find applications in single-molecule materials, artificial photodevices, and biomedical appliances.

A Zr-cluster based thermostable, self-healing and adaptive metallogel with chromogenic properties responds to multiple stimuli with reversible radical interaction

A Zr-cluster based metallogel is synthesized via an unusual one-pot solvothermal method. The resulting metallogel is robust, adaptive, self-healing, highly thermostable and conductive. Moreover, the metallogel exhibits reversible stimuli-responsive properties. The gel could respond to at least four kinds of stimuli such as light, aliphatic amines, electricity and metals with color and fluorescence tunability. Importantly, the metallogel with electrochromic properties could be used as soft electrochromic devices for smart windows and electro display boards, and metalchromism provides a practical way for coating corrosion monitoring of metal materials.

Anion-Coordination-Assisted Assembly of Supramolecular Charge-Transfer Complexes Based on Tris(urea) Ligands

Charge-transfer (CT) complexes, formed by noncovalent bonding between electron-rich (donor, D) and electron-deficient (acceptor, A) molecules (or moieties) have attracted considerable attention due to their fascinating structures and potential applications. Herein, we demonstrate that anion coordination is a promising strategy to promote CT complex formation between anion-binding, electron-rich tris(urea) donor ligands (D) and electron-deficient viologen cation acceptors (A), which form co-crystals featuring infinite ⋅⋅⋅DADA⋅⋅⋅ or discrete (circular DADA or three-decker DAD) π-stacking interactions. These CT complexes were studied by X-ray diffraction, UV/Vis spectroscopy, electric conductivity measurements, charge displacement curve (CDC) calculations, and DFT computations.

Tuning radical interactions in trisradical tricationic complexes by varying host-cavity sizes

Although host–guest pairing interactions between bisradical dicationic cyclobis(paraquat-p-phenylene) (BB²⁽˙⁺⁾) and the bipyridinium radical cation (BIPY˙⁺) have been studied extensively, host molecules other than BB²⁽˙⁺⁾ are few and far between.

Although host–guest pairing interactions between bisradical dicationic cyclobis(paraquat-p-phenylene) (BB²⁽˙⁺⁾) and the bipyridinium radical cation (BIPY˙⁺) have been studied extensively, host molecules other than BB²⁽˙⁺⁾ are few and far between. Herein, four bisradical dicationic cyclophanes with tunable cavity sizes are investigated as new bisradical dicationic hosts for accommodating the methyl viologen radical cation (MV˙⁺) to form trisradical tricationic complexes. The structure–property relationships between cavity sizes and binding affinities have been established by comprehensive solution and solid-state characterizations as well as DFT calculations. The association constants of the four new trisradical tricationic complexes are found to range between 7400 and 170 000 M^–1, with the strongest one being 4.3 times higher than that for [MV⊂BB]³⁽˙⁺⁾. The facile accessibility and tunable stability of these new trisradical tricationic complexes make them attractive redox-controlled recognition motifs for further use in supramolecular chemistry and mechanostereochemistry.

Molecular Russian dolls

The host-guest recognition between two macrocycles to form hierarchical non-intertwined ring-in-ring assemblies remains an interesting and challenging target in noncovalent synthesis. Herein, we report the design and characterization of a box-in-box assembly on the basis of host-guest radical-pairing interactions between two rigid diradical dicationic cyclophanes. One striking feature of the box-in-box complex is its ability to host various 1,4-disubstituted benzene derivatives inside as a third component in the cavity of the smaller of the two diradical dicationic cyclophanes to produce hierarchical Russian doll like assemblies. These results highlight the utility of matching the dimensions of two different cyclophanes as an efficient approach for developing new hybrid supramolecular assemblies with radical-paired ring-in-ring complexes and smaller neutral guest molecules.

Guest Back-Folding: A Molecular Design Strategy That Produces a Deep-Red Fluorescent Host/Guest Pair with Picomolar Affinity in Water

One of the major goals of modern supramolecular chemistry, with important practical relevance in many technical fields, is the development of synthetic host/guest partners with ultrahigh affinity and selectivity in water. Currently, most association pairs exhibit micromolar affinity or weaker, and there are very few host/guest systems with K_a > 10⁹ M^-1, apparently due to a barrier imposed by enthalpy/entropy compensation. This present study investigated the threading of a water-soluble tetralactam cyclophane by a deep-red fluorescent squaraine guest with flanking polyethylene glycol chains, an association process that is dominated by a highly favorable enthalpic driving force. A squaraine structure was rationally designed to permit guest back-folding as a strategy to greatly expand the hydrophobic surface area that could be buried upon complexation. Guided by computational modeling, an increasing number of N-benzyl groups were appended to the squaraine core, so that, after threading, the aromatic rings could fold back and stack against the cyclophane periphery. The final design iteration exhibited an impressive combination of fluorescence and supramolecular properties, including ratiometric change in deep-red emission, picomolar affinity ( K_a = 5.1 × 10¹⁰ M^-1), and very rapid threading ( k_on = 7.9 × 10⁷ M^-1 s^-1) in water at 25 °C. Similar excellent behavior was observed in serum solution. A tangible outcome of this study is a new cyclophane/squaraine association pair that will be a versatile platform for many different types of fluorescence-based imaging and diagnostics applications. From a broader perspective, guest back-folding of aromatic groups is a promising new supramolecular stabilization strategy to overcome enthalpy/entropy compensation and produce ultrahigh affinity [2]pseudorotaxane complexes in water and biological media.

Supramolecular polymer hydrogels induced by host-guest interactions with di-[cyclobis(paraquat-p-phenylene)] cross-linkers: from molecular complexation to viscoelastic properties

Supramolecular polymer networks have been designed on the basis of a π-electron donor/acceptor complex: naphthalene (N)/cyclobis(paraquat-p-phenylene) (CBPQT⁴⁺ = B). For this purpose, a copolymer of N,N-dimethylacrylamide P(DMA-N1), lightly decorated with 1 mol% of naphthalene pendant groups, has been studied in semi-dilute un-entangled solution in the presence of di-CBPQT⁴⁺ (BB) crosslinker type molecules. While calorimetric experiments demonstrate the quantitative binding between N and B groups up to 60 °C, the introduction of BB crosslinkers into the polymer solution gives rise to gel formation above the overlap concentration. From a comprehensive investigation of viscoelastic properties, performed at different concentrations, host/guest stoichiometric ratios and temperatures, the supramolecular hydrogels are shown to follow a Maxwellian behavior with a strong correlation of the plateau modulus and the relaxation time with the effective amount of interchain cross-linkers and their dissociation dynamics, respectively. The calculation of the dissociation rate constant of the supramolecular complex, by extrapolation of the relaxation time of the network back to the beginning of the gel regime, is discussed in the framework of theoretical and experimental works on associating polymers.

Stepwise, Protecting Group Free Synthesis of [4]Rotaxanes

Despite significant advances in the last three decades towards high yielding syntheses of rotaxanes, the preparation of systems constructed from more than two components remains a challenge. Herein we build upon our previous report of an active template copper-catalyzed azide-alkyne cycloaddition (CuAAC) rotaxane synthesis with a diyne in which, following the formation of the first mechanical bond, the steric bulk of the macrocycle tempers the reactivity of the second alkyne unit. We have now extended this approach to the use of 1,3,5-triethynylbenzene in order to successively prepare [2]-, [3]- and [4]rotaxanes without the need for protecting group chemistry. Whilst the first two iterations proceeded in good yield, the steric shielding that affords this selectivity also significantly reduces the efficacy of the active template (AT)-CuAAC reaction of the third alkyne towards the preparation of [4]rotaxanes, resulting in severely diminished yields.

Threading of various ‘U’ shaped bidentate axles into a heteroditopic macrocyclic wheel via NiII/CuII templation

The Ni^II/Cu^II templated threading of various terminal group embedded ‘U’ shaped axles into an amido–amine macrocyclic wheel towards the development of a new generation of [2]pseudorotaxanes via [3 + 2] coordination assisted by other non-covalent interactions.