Disparate Downstream Reactions Mediated by an Ionically Controlled Supramolecular Tristate Switch

Accessible Tetrathiafulvalene Moieties in a 3D Covalent Organic Framework for Enhanced Near-Infrared Photo-Thermal Conversion and Photo-Electrical Response

Redox-active tetrathiafulvalene (TTF)-based covalent organic frameworks (COFs) exhibit distinctive electrochemical and photoelectrical properties, but their prevalent two-dimensional (2D) structure with densely packed TTF moieties limits the accessibility of redox center and constrains their potential applications. To overcome this challenge, an 8-connected TTF linker (TTF-8CHO) is designed as a new building block for the construction of three-dimensional (3D) COFs. This approach led to the successful synthesis of a 3D COF with the bcu topology, designated as TTF-8CHO-COF. In comparison to its 2D counterpart employing a 4-connected TTF linker, the 3D COF design enhances access to redox sites, facilitating controlled oxidation by I2 or Au3+ to tune physical properties. When irradiated with a 0.7 W cm-2 808 nm laser, the oxidized 3D COF samples (I X - ${\mathrm{I}}_{\mathrm{X}}^{-}$ @TTF-8CHO-COF and Au NPs@TTF-8CHO-COF) demonstrated rapid temperature increases of 239.3 and 146.1 °C, respectively, which surpassed those of pristine 3D COF (65.6 °C) and the 2D COF counterpart (6.4 °C increment after I2 treatment). Furthermore, the oxidation of the 3D COF heightened its photoelectrical responsiveness under 808 nm laser irradiation. This augmentation in photothermal and photoelectrical response can be attributed to the higher concentration of TTF·+ radicals generated through the oxidation of well-exposed TTF moieties.

Switchable molecular tweezers: design and applications

Switchable molecular tweezers are a unique class of molecular switches that, like their macroscopic analogs, exhibit mechanical motion between an open and closed conformation in response to stimuli. Such systems constitute an essential component of artificial molecular machines. This review will present selected examples of switchable molecular tweezers and their potential applications. The first part will be devoted to chemically responsive tweezers, including stimuli such as pH, metal coordination, and anion binding. Then, redox-active and photochemical tweezers will be presented.

A Photoswitchable Macrocycle Controls Anion-Templated Pseudorotaxane Formation and Axle Relocalization

Important biological processes, such as signaling and transport, are regulated by dynamic binding events. The development of artificial supramolecular systems in which binding between different components is controlled could help emulate such processes. Herein, we describe stiff-stilbene-containing macrocycles that can be switched between (Z)- and (E)-isomers by light, as demonstrated by UV/Vis and 1 H NMR spectroscopy. The (Z)-isomers can be effectively threaded by pyridinium halide axles to give pseudorotaxane complexes, as confirmed by 1 H NMR titration studies and single-crystal X-ray crystallography. The overall stability of these complexes can be tuned by varying the templating counteranion. However, upon light-induced isomerization to the (E)-isomer, the threading capability is drastically reduced. The axle component, in addition, can form a heterodimeric complex with a secondary isophthalamide host. Therefore, when all components are combined, light irradiation triggers axle exchange between the macrocycle and this secondary host, which has been monitored by 1 H NMR spectroscopy and simulated computationally.

Minimalist Design for Solar Energy Conversion: Revamping the π-Grid of an Organic Framework into Open-Shell Superabsorbers

We apply a versatile reaction to a versatile solid: the former involves the electron-deficient alkene tetracyanoethylene (TCNE) as the guest reactant; the latter consists of stacked 2D honeycomb covalent networks based on the electron-rich β-ketoenamine hinges that also activate the conjugated, connecting alkyne units. The TCNE/alkyne reaction is a [2 + 2] cycloaddition-retroelectrocyclization (CA-RE) that forms strong push-pull units directly into the backbone of the framework-i.e., using only the minimalist "bare-bones" scaffold, without the need for additional side groups of alkynes or other functions. The ability of the stacked alkyne units (i.e., as part of the honeycomb mass) to undergo such extensive rearrangement highlights the structural flexibility of these covalent organic framework (COF) hosts. The COF solids remain porous, crystalline, and air-/water-stable after the CA-RE modification, while the resulting push-pull units feature distinct open-shell/free-radical character, are strongly light-absorbing, and shift the absorption ends from 590 nm to around 1900 nm (band gaps from 2.17-2.23 to 0.87-0.95 eV), so as to better capture sunlight (especially the infrared region which takes up 52% of the solar energy). As a result, the modified COF materials achieve the highest photothermal conversion performances, holding promise in thermoelectric power generation and solar steam generation (e.g., with solar-vapor conversion efficiencies >96%).

Emulsion-oriented assembly for Janus double-spherical mesoporous nanoparticles as biological logic gates

The ability of Janus nanoparticles to establish biological logic systems has been widely exploited, yet conventional non/uni-porous Janus nanoparticles are unable to fully mimic biological communications. Here we demonstrate an emulsion-oriented assembly approach for the fabrication of highly uniform Janus double-spherical MSN&mPDA (MSN, mesoporous silica nanoparticle; mPDA, mesoporous polydopamine) nanoparticles. The delicate Janus nanoparticle possesses a spherical MSN with a diameter of ~150 nm and an mPDA hemisphere with a diameter of ~120 nm. In addition, the mesopore size in the MSN compartment is tunable from ~3 to ~25 nm, while those in the mPDA compartments range from ~5 to ~50 nm. Due to the different chemical properties and mesopore sizes in the two compartments, we achieve selective loading of guests in different compartments, and successfully establish single-particle-level biological logic gates. The dual-mesoporous structure enables consecutive valve-opening and matter-releasing reactions within one single nanoparticle, facilitating the design of single-particle-level logic systems.

Phototransduction in a supramolecular cascade: a mimic for essential features of the vision process

The tailored design of a light-triggered supramolecular cascade results in an artificial machinery that assimilates the transduction of photons into chemical communication and the final release of a neurotransmitter. This is reminiscent of key steps in the natural vision process.

Organic binary charge-transfer compounds of 2,2' : 6',2'' : 6'',6-trioxotriphenylamine and a pyrene-annulated azaacene as donors

Three binary charge-transfer (CT) compounds resulting from the donor 2,2' : 6',2'' : 6'',6-trioxotriphenylamine (TOTA) and the acceptors F4TCNQ and F4BQ and of a pyrene-annulated azaacene (PAA) with the acceptor F4TCNQ are reported. The identity of these CT compounds are confirmed by single-crystal X-ray diffraction as well as by IR, UV-vis-NIR and EPR spectroscopy. X-ray diffraction analysis reveals a 1 : 1 stoichiometry for TOTA·F4TCNQ, a 2 : 1 donor : acceptor ratio in (TOTA)2·F4BQ, and a rare 4 : 1 stoichiometry in (PAA)4·F4TCNQ, respectively. Metrical parameters of the donor (D) and acceptor (A) constituents as well as IR spectra indicate full CT in TOTA·F4TCNQ, partial CT in (TOTA)2·F4BQ and only a very modest one in (PAA)4·F4TCNQ. Intricate packing motifs are present in the crystal lattice with encaged, π-stacked (F4TCNQ-)2 dimers in TOTA·F4TCNQ or mixed D/A stacks in the other two compounds. Their solid-state UV-vis-NIR spectra feature CT transitions. The CT compounds with F4TCNQ are electrical insulators, while (TOTA)2·F4BQ is weakly conducting.

Supramolecular Recognition within a Nanosized "Buckytrap" That Exhibits Substantial Photoconductivity

We report here a nanosized "buckytrap", 1, constructed from two bis-zinc(II) expanded-TTF (exTTF) porphyrin subunits. Two forms, 1a and 1b, differing in the axial ligands, H₂O vs tetrahydrofuran (THF), were isolated and characterized. Discrete host-guest inclusion complexes are formed upon treatment with fullerenes as inferred from a single-crystal X-ray structural analyses of 1a with C₇₀. The fullerene is found to be encapsulated within the inner pseudohexagonal cavity of 1a. In contrast, the corresponding free-base derivative (2) was found to form infinite ball-and-socket type supramolecular organic frameworks (3D-SOFs) with fullerenes, (2•C₆₀)_n or (2•C₇₀)_n. This difference is ascribed to the fact that in 1a and 1b the axial positions are blocked by a H₂O or THF ligand. Emission spectroscopic studies supported a 1:1 host-guest binding stoichiometry, allowing association constants of (2.0 ± 0.5) × 10⁴ M^-1 and (4.3 ± 0.9) × 10⁴ M^-1 to be calculated for C₆₀ and C₇₀, respectively. Flash-photolysis time-resolved microwave conductivity (FP-TRMC) studies of solid films of the Zn-complex 1a revealed that the intrinsic charge carrier transport, i.e., pseudo-photoconductivity (ϕ∑μ), increases upon fullerene inclusion (e.g., ϕ∑μ = 1.53 × 10^-4 cm² V^-1 s^-1 for C₆₀⊂(1a)₂ and ϕ∑μ = 1.45 × 10^-4 cm² V^-1 s^-1 for C₇₀⊂(1a)₂ vs ϕ∑μ = 2.49 × 10^-5 cm² V^-1 s^-1 for 1a) at 298 K. These findings provide support for the notion that controlling the nature of self-assembly supramolecular constructs formed from exTTF-porphyrin dimers through metalation or choice of fullerene can be used to regulate key functional features, including photoconductivity.

Semiconductive 2D arrays of pancake-bonded oligomers of partially charged TCNQ radicals

Multicentre two-electron (mc/2e or 'pancake bonding') bonding between 7,7,8,8-tetra-cyano-quinodi-methane (TCNQ) radical anions was studied on its 14 novel salts with planar organic cations. The formal charges of the TCNQ^δ- moieties are -1/2 and -2/3, and they form mc/2e bonded dimers, trimers and tetramers which are further stacked into extended arrays. Multicentre bonding within these oligomers is characterized by short interplanar separations of 2.9-3.2 Å; distances between the oligomers are larger, typically >3.3 Å. The stacks are laterally connected by C-H⋯N hydrogen bonding, forming 2D arrays. The nature of mc/2e bonding is characterized by structural, magnetic and electrical data. The compounds are found to be semiconductors, and high conductivity [10^-2 (Ω cm)^-1] correlates with short interplanar distances between pancake-bonded oligomers.

Single-crystal-to-single-crystal transformation of tetrathiafulvalene-based hydrogen-bonded organic frameworks

A TTF-based HOF undergoes SCSC transformation into another two isomers in different solvents.

Ratiometric Turn-On Fluorophore Displacement Ensembles for Nitroaromatic Explosives Detection

There is a recognized need in the area of explosives detection for fluorescence-based sensing systems that are capable of not only producing a turn-on response but also generating a distinctive spectral signature for a given analyte. Here, we report several supramolecular ensembles displaying efficient fluorophore displacement that give rise to an increase in fluorescence intensity upon exposure to various nitroaromatic compounds. The synthetic supramolecular constructs in question consist of a tetrathiafulvalene (TTF)-based pyrrolic macrocycle, benzo-TTF-calix[4]pyrrole (Bz-TTF-C4P), and fluorescent dyes, monomeric or dimeric naphthalenediimide (NDI) and perylenediimide (PDI) derivatives, as well as chloride or hexafluorophosphate (PF₆^-) salts of rhodamine 6G (Rh-6G). In chloroform solution, these assemblies exist in the form of discrete supramolecular complexes or oligomeric aggregates depending on the specific dye combinations in question. Each ensemble was tested as a potential explosive-responsive fluorescence indicator displacement assay (FIDA) by challenging it with a series of di- and trinitroaromatic compounds and examining the change in fluorescence spectral characteristics. Upon addition of nitroaromatic compounds (NACs), either a "turn-on" or a "turn-off" fluorescent response was observed depending on the nature of the constituent fluorophore and, where applicable, the counteranion. The FIDAs based on the PDI derivatives were found to display not only a ratiometric fluorescence enhancement but also analyte-dependent spectral changes when treated with NACs. The NAC-induced fluorescence spectral response of each ensemble was rationalized on the basis of various solution-phase spectroscopic studies, as well as single-crystal X-ray diffraction analyses.

Control of Photoisomerization of an Azoazacryptand by Anion Binding and Cucurbit[8]uril Encapsulation in an Aqueous Solution

Control of isomerization of a receptor bearing multiple light-switchable subunits in a confined space is critical for the design of synthetic molecular machines. Toward this goal, a new azacryptand containing three azobenzene subunits has been developed, and its photoisomerization in an aqueous solution has been studied depending on anion coordination and recognition by a larger host-cucurbit[8]uril (CB[8]). The cryptand in its hexaprotonated form shows considerable affinity for fluoride and perchlorate, which in turn affects the isomer distribution of the receptor under UV-light irradiation, stabilizing the isomers of the cryptand with Z-configurations. CB[8] was found to be able to encapsulate the isomers of the cryptand by forming a Matryoshka-type complex. The irradiation of a 10:1 CB[8]-cryptand mixture has led to a selective conversion of the cryptand to the E,E,Z isomer inside CB[8]. It has been demonstrated that the addition of fluoride to the resulted complex induces the release of the cryptand as a major E,E,E isomer, while other studied anions were ineffective in this reaction. To our knowledge, this work presents a first example of a host-controlled photoisomerization of an anion receptor bearing multiple switching azobenzenes that model the function of naturally occurring chaperones.

Chemical signal cascading in a supramolecular network

A chemically-triggered signalling cascade between cucurbituril host-guest complexes by means of multi-step competitive displacement is demonstrated. The inter-complex communication of chemical information yields the release of bio-relevant cargo, reminiscent of cellular signalling pathways.

Zirconium metal-organic frameworks incorporating tetrathiafulvalene linkers: robust and redox-active matrices for in situ confinement of metal nanoparticles

Redox-active metal-organic frameworks (MOFs), with highly ordered porous structures and redox tunability, have attracted research interest in the fields of catalysis, energy storage, and electrochemical sensing. However, the chemical lability has limited the application scope of many redox-active MOFs. Herein, we selected stable Zr₆ inorganic nodes and redox-active tetrathiafulvalene (TTF)-based linkers to construct two robust, redox-active MOFs, namely compounds 1 ([Zr₆(TTFTB)₂O₈(OH₂)₈]) and 2 ([Zr₆(Me-TTFTB)_1.5O₄(OH)₄(C₆H₅COO)₆]) (TTFTB = tetrathiafulvalene tetrabenzoate; Me-TTFTB = tetrathiafulvalene tetramethylbenzoate). The structure and topology of the MOFs were controlled by tuning the linker conformation through steric effects, resulting in a variety of pore structures from microporous channels (compound 1) to hierarchically micro/mesoporous cages (compound 2). Compound 2 shows high porosity with a BET surface area of 1932 m² g^-1 and strong chemical stability in aqueous solutions with pH ranging from 1 to 12. Furthermore, the reductive TTF moieties allow for in situ generation and stabilization of ultra-small noble metal (Ag, Pd, and Au) nanoparticles by incubating MOFs in the respective metal salt solution. Single crystal structures, TEM images, and pore size distribution data from N₂ adsorption measurements indicated that the metal nanoparticles were mostly placed in the small cubic cavities of hierarchically porous compound 2, leaving the large cages open for substrate diffusion. As a proof of concept, Pd NPs@compound 2 was utilized as a heterogeneous catalyst for aerobic oxidation of alcohols, showing noteworthy activity and recyclability.

Remote Control of the Synthesis of a [2]Rotaxane and its Shuttling via Metal-Ion Translocation

Remote control in an eight-component network commanded both the synthesis and shuttling of a [2]rotaxane via metal-ion translocation, the latter being easily monitored by distinct colorimetric and fluorimetric signals. Addition of zinc(II) ions to the red colored copper-ion relay station rapidly liberated copper(I) ions and afforded the corresponding zinc complex that was visualized by a bright sky blue fluorescence at 460 nm. In a mixture of all eight components of the network, the liberated copper(I) ions were translocated to a macrocycle that catalyzed formation of a rotaxane by a double-click reaction of acetylenic and diazide compounds. The shuttling frequency in the copper-loaded [2]rotaxane was determined to k 298=30 kHz (ΔH ≠=62.3±0.6 kJ mol-1, ΔS ≠=50.1±5.1 J mol-1 K-1, ΔG ≠ 298=47.4 kJ mol-1). Removal of zinc(II) ions from the mixture reversed the system back generating the metal-free rotaxane. Further alternate addition and removal of Zn2+ reversibly controlled the shuttling mode of the rotaxane in this eight-component network where the ion translocation status was monitored by the naked eye.

Tetrathiafulvalene-calix[4]pyrrole: a versatile synthetic receptor for electron-deficient planar and spherical guests

The chemistry of tetrathiafulvalene-calix[4]pyrrole is reviewed with focus on conformational behavior, receptor properties and ionically controlled electron transfer processes.

New dimensions in calix[4]pyrrole: the land of opportunity in supramolecular chemistry

The quest for receptors endowed with the selective complexation and detection of negatively charged species continues to receive substantial consideration within the scientific community worldwide. This study is encouraged by the utilization of anions in nature in a plethora of biological systems such as chloride channels and proteins and as polyanions for genetic information. The molecular recognition of anionic species is greatly interesting in terms of their favourable interactions. In this comprehensive review, in addition to giving accounts of some selected syntheses, we illustrated diverse applications ranging from molecular containers to ion transporters and drug carriers of a supramolecular receptor named calix[4]pyrrole. We believe that the present review may act as a catalyst in enhancing the novel applications of calix[4]pyrrole and its congeners in the other dimensions of science and technology.

The quest for receptors endowed with the selective complexation and detection of negatively charged species continues to receive substantial consideration within the scientific community worldwide.

Orthogonal Stimuli Trigger Self-Assembly and Phase Transfer of FeII4L4 Cages and Cargoes

Two differently protected aldehydes, A and B, were demonstrated to deprotect selectively through the application of light and heat, respectively. In the presence of iron(II) and a triamine, two distinct Fe^II₄L₄ cages, 1 and 2, were thus observed to form from the deprotected A and B, respectively. The alkyl tails of B and 2 render them preferentially soluble in cyclopentane, whereas A and 1 remain in acetonitrile. The stimulus applied (either light or heat) thus determines the outcome of self-assembly and dictates whether the cage and its ferrocene cargo remain in acetonitrile, or transport into cyclopentane. Cage self-assembly and cargo transport between phases can in this fashion be programmed using orthogonal stimuli.

Tetrathiafulvalene (TTF)-Annulated Calix[4]pyrroles: Chemically Switchable Systems with Encodable Allosteric Recognition and Logic Gate Functions

Molecular and supramolecular systems capable of switching between two or more states as the result of an applied chemical stimulus are attracting ever-increasing attention. They have seen wide application in the development of functional materials including, but not limited to, molecular and supramolecular switches, chemosensors, electronics, optoelectronics, and logic gates. A wide range of chemical stimuli have been used to control the switching within bi- and multiple state systems made up from either singular molecular entities or supramolecular ensembles. In general, chemically triggered switching systems contain at least two major functional components that provide for molecular recognition and signal transduction, respectively. These components can be connected to one another via either covalent or noncovalent linkages. Of particular interest are switchable systems displaying cooperative or allosteric features. Such advanced control over function is ubiquitous in nature and, in the case of synthetic systems, may allow the capture and release of a targeted chemical entity or permit the transduction of binding information from one recognition site to another. Allosterically controlled complexation and decomplexation could also permit the amplification or deamplification of analyte-specific binding affinity, lead to nonlinear binding characteristics, or permit a magnification of output signals. Our own efforts to develop chemically driven supramolecular switches, advanced logic gates, and multifunction cascade systems have focused on the use of tetrathiafulvalene (TTF) annulated calix[4]pyrroles (C4Ps). These systems, TTF-C4Ps, combine several orthogonal binding motifs within what are conformationally switchable receptor frameworks. Their basic structure and host-guest recognition functions can be controlled via application of an appropriate chemical stimulus. Homotropic or heterotropic allosteric molecular recognition behavior is often seen. This has allowed us to (1) produce self-assembled structures, (2) control switching between bi- and multistate constructs, (3) generate chemical logic gates performing chemical-based Boolean logic operations, (4) create ionically controlled three-state logic systems that release different chemical messengers and activate disparate downstream reactions, and (5) encode a variety advanced functional operations into what are relatively simple molecular-scale devices. Looking to the future, we believe that exploiting allosteric control will expand opportunities for supramolecular chemists and allow some of the complexity seen in biology to be reproduced in simple constructs. Of particular appeal would be a capacity to release chemical messengers at will, perhaps after a prior capture and chemical modification step, that then encode for further downstream functions as seen in the case of the small molecules, such as neurotransmitters and pheromones, used by nature for the purpose of intraentity communication. Molecular scale logic devices with allosteric functions are thus the potential vanguard of a new area of study involving interactions between multiple discrete components with an emphasis on functional outcomes.

Light-induced cargo release from a cucurbit[8]uril host by means of a sequential logic operation

A combination of a chalcone/flavylium photoswitch with a supramolecular host–guest complex that can be used to phototrigger the logically-controlled and selective release of cargo was devised.