Radically Enhanced Molecular Switches

Mechanistic studies of isomeric [2]rotaxanes consisting of two different tetrathiafulvalene units reveal that the movement of cyclobis(paraquat-p-phenylene) can be controlled

Controlling the movement in artificial molecular machines is a key challenge that needs to be solved before their full potential can be harnessed. In this study, two isomeric tri-stable [2]rotaxanes 1·4PF6 and 2·4PF6 incorporating both a tetrathiafulvalene (TTF) and a monopyrrolotetrathiafulvalene (MPTTF) unit in the dumbbell component have been synthesised to measure the energy barriers when the tetracationic cyclobis(paraquat-p-phenylene) (CBPQT4+) ring moves across either a TTF2+ or an MPTTF2+ dication. By strategically exchanging one of the thiomethyl barriers on either the TTF unit or the MPTTF unit with the bulkier thioethyl group, the movement of the CBPQT4+ ring in 14+ and 24+ can be controlled to take place in only one direction upon tetra-oxidation. Cyclic voltammetry and 1H NMR spectroscopy were used to investigate the switching mechanism and it was found that upon tetra-oxidation of 14+ and 24+, the CBPQT4+ ring moves first to a position where it is located between the TTF2+ and MPTTF2+ dications producing high-energy co-conformations which slowly interconvert into thermodynamically more stable co-conformations. The kinetics of the movement occurring in the tetra-oxidised [2]rotaxanes 18+ and 28+ were studied at different temperatures allowing the free energy of the transition state, when CBPQT4+ moves across TTF2+ (21.5 kcal mol-1) and MPTTF2+ (20.3 kcal mol-1) at 298 K, to be determined. These results demonstrate for the first time that the combination of a TTF and an MPTTF unit can be used to induce directional movement of the CBPQT4+ ring in molecular machines with a 90% efficiency.

Evaluating the energy landscape of an out-of-equilibrium bistable [2]rotaxane containing monopyrrolotetrathiafulvalene

The unique redox properties of monopyrrolotetrathiafulvalene can be used to induce directional movement in interlocked molecules. In this study, the kinetics for the directional movement of cyclobis(paraquat-p-phenylene) across the dioxidised monopyrrolotetrathiafulvalene in a [2]rotaxane is quantified by time-resolved ¹H NMR spectroscopy.

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.

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.

Probing the Electrostatic Barrier of Tetrathiafulvalene Dications using a Tetra-stable Donor-Acceptor [2]Rotaxane

A tetra-stable donor-acceptor [2]rotaxane 1⋅4PF₆ has been synthesized. The dumbbell component is comprised of an oxyphenylene (OP), a tetrathiafulvalene (TTF), a monopyrrolo-TTF (MPTTF), and a hydroquinone (HQ) unit, which can act as recognition sites (stations) for the tetra-cationic cyclophane cyclobis(paraquat-p-phenylene) (CBPQT⁴⁺ ). The TTF and the MPTTF stations are located in the middle of the dumbbell component and are connected by a triethylene glycol (TEG) chain in such a way that the pyrrole moiety of the MPTTF station points toward the TTF station, while the TTF and MPTTF stations are flanked by the OP and HQ stations on their left hand side and right hand side, respectively. The [2]rotaxane was characterized in solution by ¹ H NMR spectroscopy and cyclic voltammetry. The spectroscopic data revealed that the majority (77 %) of the tetra-stable [2]rotaxane 1⁴⁺ exist as the translational isomer 1⋅MPTTF⁴⁺ in which the CBPQT⁴⁺ ring encircles the MPTTF station. The electrochemical studies showed that CBPQT⁴⁺ in 1⋅MPTTF⁴⁺ undergoes ring translation as result of electrostatic repulsion from the oxidized MPTTF unit. Following tetra-oxidation of 1⋅MPTTF⁴⁺ , a high-energy state of 1⁸⁺ was obtained (i.e., 1⋅TEG⁸⁺ ) in which the CBPQT⁴⁺ ring was located on the TEG linker connecting the di-oxidized TTF²⁺ and MPTTF²⁺ units. ¹ H NMR spectroscopy carried out in CD₃ CN at 298 K on a chemically oxidized sample of 1⋅MPTTF⁴⁺ revealed that the metastable state 1⋅TEG⁸⁺ is only short-lived with a lifetime of a few minutes and it was found that 70 % of the positively charged CBPQT⁴⁺ ring moved from 1⋅TEG⁸⁺ to the HQ station, while 30 % moved to the much weaker OP station. These results clearly demonstrate that the CBPQT⁴⁺ ring can cross both an MPTTF²⁺ and a TTF²⁺ electrostatic barrier and that the free energy of activation required to cross MPTTF²⁺ is ca. 0.5 kcal mol^-1 smaller as compared to TTF²⁺ .

A polycyclic aromatic hydrocarbon diradical with pH-responsive magnetic properties

By integrating azulene with a quinoid moiety, a novel non-alternant polycyclic aromatic hydrocarbon molecule BCHF1 exhibiting manifold zwitterionic, quinoidal and diradical behaviors is designed and synthesized. Its zwitterionic feature is evidenced by the changes shown by the ¹H-NMR and absorption spectra when the molecule undergoes reversible protonation and deprotonation reactions at varied pH. The diradical facet, manifesting a small singlet-triplet energy gap (ΔE _S-T), is characterized with a paramagnetic resonance signal detected by the EPR spectroscopy at room temperature. As the diradical properties are not observed in the protonated form, BCHF1+H⁺ , a pH-controlled reversible magnetic switching behavior is illustrated by monitoring the on and off cycles of EPR signals upon successively adding bases and acids to a solution or exposing a thin film of BCHF1+H⁺ to base vapor followed by acid vapor.

Mono- and Di-Quaternized 4,4'-Bipyridine Derivatives as Key Building Blocks for Medium- and Environment-Responsive Compounds and Materials

Mono- and di-quaternized 4,4'-bipyridine derivatives constitute a family of heterocyclic compounds, which in recent years have been employed in numerous applications. These applications correspond to various disciplines of research and technology. In their majority, two key features of these 4,4'-bipyridine-based derivatives are exploited: their redox activity and their electrochromic aptitude. Contemporary materials and compounds encompassing these skeletons as building blocks are often characterized as multifunctional, as their presence often gives rise to interesting phenomena, e.g., various types of chromism. This research trend is acknowledged, and, in this review article, recent examples of multifunctional chromic materials/compounds of this class are presented. Emphasis is placed on solvent-/medium- and environment-responsive 4,4'-bipyridine derivatives. Two important classes of 4,4'-bipyridine-based products with solvatochromic and/or environment-responsive character are reviewed: viologens (i.e., N,N'-disubstituted derivatives) and monoquats (i.e., monosubstituted 4,4'-bipyridine derivatives). The multifunctional nature of these derivatives is analyzed and structure-property relations are discussed in connection to the role of these derivatives in various novel applications.

Exploring and Exploiting the Symmetry-Breaking Effect of Cyclodextrins in Mechanomolecules

Cyclodextrins (CDs) are cone-shaped molecular rings that have been widely employed in supramolecular/host–guest chemistry because of their low cost, high biocompatibility, stability, wide availability in multiple sizes, and their promiscuity for binding a range of molecular guests in water. Consequently, CD-based host–guest complexes are often employed as templates for the synthesis of mechanically bonded molecules (mechanomolecules) such as catenanes, rotaxanes, and polyrotaxanes in particular. The conical shape and cyclodirectionality of the CD “bead” gives rise to a symmetry-breaking effect when it is threaded onto a molecular “string”; even symmetrical guests are rendered asymmetric by the presence of an encircling CD host. This review focuses on the stereochemical implications of this symmetry-breaking effect in mechanomolecules, including orientational isomerism, mechanically planar chirality, and topological chirality, as well as how they support applications in regioselective and stereoselective chemical synthesis, the design of molecular machine prototypes, and the development of advanced materials.

Facile Construction of a Supramolecular Organic Framework Using Naphthyl Viologen Guests and CB[8] Host via Charge-Transfer Complexation

Herein, we report the synthesis of guest-host systems comprising naphthyl-viologen-naphthyl (Np-Vio-Np) and viologen-naphthyl-viologen (Vio-Np-Vio) guest molecules and their subsequent supramolecular polymerization in the presence of a CB[8] host in water. In addition, the guest complexation of ethyl-terminated trimeric viologen (ETV) with Np-Vio-Np and CB[8] was investigated. As a result of supramolecular interactions, 2D supramolecular organic frameworks with high internal periodicity were constructed. 1H NMR studies clearly demonstrated the formation of a host-stabilized charge-transfer complex via folding back (Np-Vio-Np and Vio-Np-Vio) in the presence of CB[8]. In the case of ETV + Np-Vio-Np + CB[8], a large polymeric network was formed as indicated by the NMR titrations. UV-vis and fluorescence studies clearly confirm the formation of an inter/intra molecular CT complex upon complexation with cucurbit[8]uril. The size obtained using the dynamic light scattering (DLS) method pinpoints the formation of larger supramolecular aggregates in the order of μm through host-guest assembly, which is further complemented by FESEM and TEM. SAXS measurements indicate the formation of a 2D supramolecular polymer/polymer aggregate with long-range order.

An aryl-fused redox-active tetrathiafulvalene with enhanced mixed-valence and radical-cation dimer stabilities

Molecular recognition of stable organic radicals is a relatively novel, but important structural binding motif in supramolecular chemistry. Here, we report on a redox-switchable veratrole-fused tetrathiafulvalene derivative VTTF which is ideally suited for this purpose and for the incorporation into stimuli-responsive systems. As revealed by electrochemistry, UV/Vis measurements, X-ray analysis, and electrocrystallisation, VTTF can be reversibly oxidised to the corresponding radical-cation or dication which shows optoelectronic and structural propterties similar to tetrathiafulvalene and tetrakis(methylthio)tetrathiafulvalene. However, theoretical calculations, variable temperature EPR, and NIR spectroscopy indicate that the dispersion-driven binding in the mixed-valence dimer (VTTF2)˙+ (KMV = 69 M-1 in CH2Cl2) and the radical-cation dimer (VTTF˙+)2 (KRC = 38 M-1 in CH3CN) is significantly enhanced by the additional veratrole π-surface in comparison to pristine tetrathiafulvalene.

Redox-Triggered Folding of Self-Assembled Coordination Polymers incorporating Viologen Units

We report the study of stimuli-responsive Zn^II and Fe^II coordination polymers (MC3⁴⁺ or MC2⁴⁺ with M=Fe²⁺ or Zn²⁺ ). These soluble metallopolymers were formed spontaneously by reaction of an organic ligand (C3⁴⁺ or C2⁴⁺ ) with one molar equivalent of metal ions. The C3⁴⁺ and C2⁴⁺ ligands incorporate two chelating terpyridine groups bridged by a redox responsive hinge featuring two viologen units (viologen=N,N'-dialkyl-4,4'-bipyridinium) linked either with propyl (C3⁴⁺ ) or ethyl (C2⁴⁺ ) chains. The viologen units in the polymer chains were reduced (1 e^- per viologen group) either by bulk electrolysis or by visible-light irradiation carried out in the presence of a photosensitizer. The 1 e^- reduction of the viologen units in the MC2⁴⁺ polymers induced a slight decrease in the viscosity of the solutions due to a modification of the overall charge carried by the metallopolymers. In strong contrast, reduction of coordination polymers involving propyl linkers (MC3⁴⁺ ) led to a remarkable increase (≈+400 %) in observed viscosity. This reversible effect was attributed to a folding of the polymer chains triggered by π-dimerization of the photo-generated viologen cation radicals.

Shuttling Rates, Electronic States, and Hysteresis in a Ring-in-Ring Rotaxane

The trisradical recognition motif between a 4,4'-bipyridinium radical cation and a cyclo-bis-4,4'-bipyridinium diradical dication has been employed previously in rotaxanes to control their nanomechanical and electronic properties. Herein, we describe the synthesis and characterization of a redox-active ring-in-ring [2]rotaxane BBR·8PF6 that employs a tetraradical variant of this recognition motif. A square-shaped bis-4,4'-bipyridinium cyclophane is mechanically interlocked around the dumbbell component of this rotaxane, and the dumbbell itself incorporates a smaller bis-4,4'-bipyridinium cyclophane into its covalently bonded structure. This small cyclophane serves as a significant impediment to the shuttling of the larger ring across the dumbbell component of BBR8+ , whereas reduction to the tetraradical tetracationic state BBR4(+•) results in strong association of the two cyclophanes driven by two radical-pairing interactions. In these respects, BBR·8PF6 exhibits qualitatively similar behavior to its predecessors that interconvert between hexacationic and trisradical tricationic states. The rigid preorganization of two bipyridinium groups within the dumbbell of BBR·8PF6 confers, however, two distinct properties upon this rotaxane: (1) the rate of shuttling is reduced significantly relative to those of its predecessors, resulting in marked electrochemical hysteresis observed by cyclic voltammetry for switching between the BBR8+/BBR4(+•) states, and (2) the formally tetraradical form of the rotaxane, BBR4(+•) , exhibits a diamagnetic ground state, which, as a result of the slow shuttling motions within BBR4(+•) , has a long enough lifetime to be characterized by 1H NMR spectroscopy.

Stable Radical Cations and Their π-Dimers Prepared from Ethylene- and Propylene-3,4-dioxythiophene Co-oligomers: Combined Experimental and Theoretical Investigations

Co-oligomers composed of two 3,4-ethylenedioxythiophene (EDOT) units and two or three 3,4-propylenedioxythiophene (ProDOT) units, i.e., 2E2P_Et and 2E3P_Et, were newly synthesized together with the ProDOT trimer 3P_Me. On the basis of cyclic voltammetry, the gaps between the first and second oxidation potentials (ΔE^1-2) of 2E2P_Et and 2E3P_Et were found to be larger than that of the previously synthesized ProDOT tetramer 4P_Hex. These co-oligomers gave the fairly stable radical cations 2E2P_Et^•+ and 2E3P_Et^•+ by chemical oxidation with AgSbF₆. The disproportionation of 2E2P_Et^•+ and 2E3P_Et^•+ into neutral and dicationic species, which was observed for 4P_Hex^•+, was inhibited in accord with the larger ΔE^1-2. Additionally, the formation of the π-dimers (3P_Me)₂²⁺, (2E2P_Et)₂²⁺, and (2E3P_E)₂²⁺ was clearly observed in dichloromethane solution at low temperatures with UV-vis-NIR spectroscopy. Furthermore, the π-dimerization enthalpies of 2E2P_Et^•+ and 2E3P_Et^•+ were greater than that of 3P_Me^•+, suggesting the formation of fully π-contacted structures. The structures of the π-dimers were optimized at the B97D3 method, and the calculated absorption spectra of the π-dimers obtained using TD-DFT methods were in reasonable agreement with the observed ones, supporting the reliability of the calculated structures.

Optical Distinction between "Slow" and "Fast" Translational Motion in Degenerate Molecular Shuttles

A series of six [2]rotaxane molecular shuttles was designed which contain an axle with a benzo-bis(imidazole) core (in either a neutral or dicationic form) and a single 24-membered, crown ether wheel (24C6, B24C6, or DMB24C6), and the shuttling rates of the ring along the axle were determined. The charged versions showed much slower shuttling rates as a result of the increase in noncovalent interactions between the axle and wheel. The [2]rotaxane with a B24C6 wheel shows a difference in fluorescence between the charged and neutral species, while the [2]rotaxane with a DMB24C6 wheel exhibits a difference in color between the charged and neutral compounds. These changes in optical properties can be attributed to the structural differences in the co-conformations of the [2]rotaxane as they adapt to the changes in acid/base chemistry. This allowed the relative rate of the translational motion of a molecular shuttle to be determined by observation of a simple optical probe.

Paramagnetic Molecular Grippers: The Elements of Six-State Redox Switches

The development of semiquinone-based resorcin[4]arene cavitands expands the toolbox of switchable molecular grippers by introducing the first paramagnetic representatives. The semiquinone (SQ) states were generated electrochemically, chemically, and photochemically. We analyzed their electronic, conformational, and binding properties by cyclic voltammetry, ultraviolet/visible (UV/vis) spectroelectrochemistry, electron paramagnetic resonance (EPR) and transient absorption spectroscopy, in conjunction with density functional theory (DFT) calculations. The utility of UV/vis spectroelectrochemistry and EPR spectroscopy in evaluating the conformational features of resorcin[4]arene cavitands is demonstrated. Guest binding properties were found to be enhanced in the SQ state as compared to the quinone (Q) or the hydroquinone (HQ) states of the cavitands. Thus, these paramagnetic SQ intermediates open the way to six-state redox switches provided by two conformations (open and closed) in three redox states (Q, SQ, and HQ) possessing distinct binding ability. The switchable magnetic properties of these molecular grippers and their responsiveness to electrical stimuli has the potential for development of efficient molecular devices.

Synthesis and Photoisomerization of Substituted Dibenzofulvene Molecular Rotors

The synthesis, spectral and structural characterization, and photoisomerization of a family of 2-substituted dibenzofulvene molecular actuators based on (2,2,2-triphenylethylidene)fluorene (TEF) are reported. The 2-substituted species investigated are nitro (NTEF), cyano (CTEF), and iodo (ITEF). X-ray structures of these three compounds and three intermediates were determined to assign alkene configuration and investigate the effects of the 2-substituents on steric gearing. The addition-elimination reaction of Z-9 with trityl anion to form Z-10 proceeded with complete retention of configuration. Rates of photoisomerization were measured at irradiation wavelengths between 266-355 nm in acetonitrile/dioxane solutions at room temperature. Photoisomerization quantum yields (φ) were calculated by means of a mathematical model that accounts for a certain degree of photodecomposition in the cases of CTEF and ITEF. Quantum yields vary significantly with substituent, having maximum values of φ=0.26 for NTEF, 0.39 for CTEF, and 0.50 for ITEF. NTEF is photochemically robust and has a large quantum yield for photoisomerization in the near-UV, making it a particularly promising drive rotor moiety for light-powered molecular devices.

Oligorotaxane Radicals under Orders

A strategy for creating foldameric oligorotaxanes composed of only positively charged components is reported. Threadlike components-namely oligoviologens-in which different numbers of 4,4'-bipyridinium (BIPY(2+)) subunits are linked by p-xylylene bridges, are shown to be capable of being threaded by cyclobis(paraquat-p-phenylene) (CBPQT(4+)) rings following the introduction of radical-pairing interactions under reducing conditions. UV/vis/NIR spectroscopic and electrochemical investigations suggest that the reduced oligopseudorotaxanes fold into highly ordered secondary structures as a result of the formation of BIPY(•+) radical cation pairs. Furthermore, by installing bulky stoppers at each end of the oligopseudorotaxanes by means of Cu-free alkyne-azide cycloadditions, their analogous oligorotaxanes, which retain the same stoichiometries as their progenitors, can be prepared. Solution-state studies of the oligorotaxanes indicate that their mechanically interlocked structures lead to the enforced interactions between the dumbbell and ring components, allowing them to fold (contract) in their reduced states and unfold (expand) in their fully oxidized states as a result of Coulombic repulsions. This electrochemically controlled reversible folding and unfolding process, during which the oligorotaxanes experience length contractions and expansions, is reminiscent of the mechanisms of actuation associated with muscle fibers.

Supramolecular radical polymers self-assembled from the stacking of radical cations of rod-like viologen di- and trimers

A series of π-conjugated oligomeric viologens have been synthesized, from which supramolecular radical polymers were constructed through the stacking of their radical cations.

A tristable [2]rotaxane that is doubly gated by foldamer and azobenzene kinetic barriers

Hydrogen bonded foldamer and azobenzene units have been incorporated into a donor–acceptor-type [2]rotaxane to assemble a doubly gated switching system.

Metal-induced dynamic conformational and fluorescence switch of quinone-appended Zn-porphyrin

In this report, we have designed and synthesized a novel switching molecule whose fluorescence can be switched via dynamic conformational change between expanded and shrunk states induced by metal complexation and decomplexation. The switching molecule is composed of three kinds of components, namely, a Zn 2+-porphyrin fluorophore, two quinone quenchers, and their linkers containing a 4,4′-bipyridine moiety. UV-vis and fluorescence titration studies revealed that metal complexation of the bipyridine units with Zn 2+ ions induced the dynamic structural change of the molecular shape and simultaneous enhancement of fluorescence of the Zn 2+-porphyrin fluorophore.

Aromatic sulfonate anion-induced pseudorotaxanes: environmentally benign synthesis, selectivity, and structural characterization

Aromatic sulfonates allow the effective construction of anion-containing pseudorotaxanes from a tetracationic macrocycle known as the “Texas box” in organic media and under organic-free aqueous conditions.

Relative contractile motion of the rings in a switchable palindromic [3]rotaxane in aqueous solution driven by radical-pairing interactions

Artificial muscles are an essential component for the development of next-generation prosthetic devices, minimally invasive surgical tools, and robotics. This communication describes the design, synthesis, and characterisation of a mechanically interlocked molecule (MIM), capable of switchable and reversible linear molecular motion in aqueous solution that mimics muscular contraction and extension. Compatibility with aqueous solution was achieved in the doubly bistable palindromic [3]rotaxane design by using radical-based molecular recognition as the driving force to induce switching.