A light controlled cavitand wall regulates guest binding

Glucose-derived receptors for photo-controlled binding of amino acid esters in water

Selective receptors of amino acids in aqueous media are highly sought after as they may enable the creation of novel diagnostic and sensing tools. Photoswitchable receptors are particularly attractive for such purposes as their response and selectivity towards bioanalytes can be modulated using light. Herein we report glucose-based photoswitchable receptors of amino-acid methyl esters and biogenic amines in water. The tetra-ortho-fluoroazobenzene unit in the receptors structure allows to control the distance between their binding sites using light. The Z-isomers of both receptors, having these sites in closer proximity, bind lysine, ornithine and arginine esters significantly stronger compared to E-isomers, where the binding sites are further apart.

Supramolecular Handshakes: Characterization of Urea-Carboxylate Interactions Within Calixarene Frameworks

The research of molecular capsules offers high application potential and numerous benefits in various fields. With the aim of forming supramolecular capsules which can be reversibly assembled and dissociated by simple external stimuli, we studied interactions between calixarenes containing urea and carboxylate moieties. To this end two ureido-derivatives of p-tert-butylcalix[4]arene comprising phenylureido-moieties and diacetate-calix[4]arenes were prepared. The binding of acetate by ureido-derivatives of calixarene in acetonitrile was characterized, revealing high affinity of ureido-calixarenes for carboxylates. This suggested high potential for uniting the complementary calix[4]arenes via H-bonds between carboxylic groups and urea moieties. The assembly of calixarenes was examined in detail by means of UV, 1H NMR, ITC, DOSY, MS, and conductometry providing insight in the structure-stability relationship. The tetraureido-calixarene derivative formed the most stable heterodimers with diacetate-calix[4]arenes featuring practically quantitative association upon mixing the two calixarene counterparts. The possibility of controlling the formation of the heterodimer by protonating the carboxylates, thereby hindering the interactions critical for capsule assembly, was investigated. Indeed, the reversibility of breaking and re-forming the heterodimer by addition of an acid and base to the solution containing urea- and carboxylate-derivative calix[4]arene was demonstrated using NMR spectroscopy.

Photoswitchable molecular tweezers: isomerization to control substrate binding, and what about vice versa?

The linkage of two identical binding motifs by a molecular photoswitch has proven to be a straightforward and versatile strategy to control substrate binding affinity by light. Stimulus control of binding properties in artificial receptors is partly inspired by the dynamic behavior of proteins and is highly attractive as it could, for example, improve extraction processes and allow (de)activation of membrane transport on demand. This feature article summarizes the development and design principles of molecular tweezers containing a molecular photoswitch as the core unit. Besides the control of binding affinity by isomerization, the effect of substrate binding on the isomerization behavior is discussed where data is available. While the latter often receives less attention, it could be of benefit in the future creation of multi-stimuli-controlled molecular switching and machine-like systems.

Stimuli-Responsive Resorcin[4]arene Cavitands: Toward Visible-Light-Activated Molecular Grippers

Resorcin[4]arene cavitands, equipped with diverse quinone (Q) and [Ru(bpy)₂ dppz]²⁺ (bpy=2,2'-bipyridine, dppz=dipyrido[3,2-a:2',3'-c]phenazine) photosensitizing walls in different configurations, were synthesized. Upon visible-light irradiation at 420 nm, electron transfer from the [Ru(bpy)₂ dppz]²⁺ to the Q generates the semiquinone (SQ) radical anion, triggering a large conformational switching from a flat kite to a vase with a cavity for the encapsulation of small guests, such as cyclohexane and heteroalicyclic derivatives, in CD₃ CN. Depending on the molecular design, the SQ radical anion can live for several minutes (≈10 min) and the vase can be generated in a secondary process without need for addition of a sacrificial electron donor to accumulate the SQ state. Switching can also be triggered by other stimuli, such as changes in solvent, host-guest complexation, and chemical and electrochemical processes. This comprehensive investigation benefits the development of stimuli-responsive nanodevices, such as light-activated molecular grippers.

Controlled binding of organic guests by stimuli-responsive macrocycles

Synthetic supramolecular chemistry pursues not only the construction of new matter, but also control over its inherently dynamic behaviour.

Polyaromatic nanocapsules as photoresponsive hosts in water

Molecular containers that provide both stimuli-responsive assembly/disassembly properties and wide-ranging host capabilities in aqueous medium still remain a current synthetic challenge. Herein we report polyaromatic nanocapsules assembled from V-shaped amphiphilic molecules bearing a photoresponsive ortho-dianthrylbenzene unit in water. Unlike previously reported supramolecular capsules and cages, the nanocapsules quickly and quantitatively disassemble into monomeric species by a non-invasive light stimulus through structural conversion from the open to the closed form of the amphiphiles. Regeneration of the nanocapsules is demonstrated by light irradiation or heating of the closed amphiphiles. With the aid of the wide-ranging host capability, the photo-induced release of various encapsulated guest molecules (e.g., Nile red, Cu(II)-phthalocyanine, and fullerene C₆₀) can be achieved by using the present nanocapsule in water. This feature can furthermore be utilized to switch the fluorescence of encapsulated coumarin guests through their controlled release.

Photoresponsive molecular capsules that can be used in water are rare. Here, the authors construct polyaromatic nanocapsules via self-assembly from photoswitch-bearing amphiphilic molecules in water. Light induces a structural change in the amphiphiles, triggering the capsule to disassemble into monomers and release encapsulated guests.

Azobenzene-Bridged Expanded "Texas-sized" Box: A Dual-Responsive Receptor for Aryl Dianion Encapsulation

We report an expanded "Texas-sized" molecular box (AzoTxSB) that incorporates photoresponsive azobenzene bridging subunits and anion recognition motifs. The shape of this box can be switched through light induced E ↔ Z photoisomerization of the constituent azobenzenes. This allows various anionic substrates to be bound and released by using different forms of the box. Control can also be achieved using other environmental stimuli, such as pH and anion competition.

Structure-switching M3L2 Ir(iii) coordination cages with photo-isomerising azo-aromatic linkers

Cyclotriguaiacylene has been functionalised with 3- or 4-pyridyl-azo-phenyl groups to form a series of molecular hosts with three azobenzene-type groups that exhibit reversible photo-isomerisation. Reaction of the host molecules with [Ir(C^N)2(NCMe)2]+ where C^N is the cyclometallating 2-phenylpyridinato, 2-(4-methylphenyl)pyridinato or 2-(4,5,6-trifluorophenyl)pyridinato results in the self-assembly of a family of five different [{Ir(C^N)2}3(L)2]3+ coordination cages. Photo-irradiation of each of the cages with a high energy laser results in E → Z photo-isomerisation of the pyridyl-azo-phenyl groups with up to 40% of groups isomerising. Isomerisation can be reversed by exposure to blue light. Thus, the cages show reversible structure-switching while maintaining their compositional integrity. This represents the largest photo-induced structural change yet reported for a structurally-integral component of a coordination cage. Energy minimised molecular models indicate a switched cage has a smaller internal space than the initial all-E isomer. The [Ir(C^N)2(NCMe)2]+ cages are weakly emissive, each with a deep blue luminescence at ca. 450 nm.

Hydrogen-bonded six-component assembly for capsule formation based on tetra(4-pyridyl)cavitand and isophthalic acid linker and its application to photoresponsive capsule

Two molecules of tetra(4-pyridyl)cavitand 1 and four molecules of isophthalic acid linker 2a with a triethylene glycol monomethyl ether (TEG) group self-assembled into a six-component capsule 1₂·2a₄ through eight pyNHO₂C hydrogen bonds, which encapsulates one molecule of guest G such as bis(4-acetoxyphenyl)acetylene and hexakis(4-iodophenyl)benzene to form G@(1₂·2a₄). Guest-encapsulation ability and selectivity of 1₂·2a₄ were revealed. trans-5-(p-Substituted-phenylazo)isophthalic acid with two dichotomous branching TEG groups trans-2b serves as a photoresponsive linker to form 1₂·(trans-2b)₄, which moderately reduced guest-encapsulation ability upon photoisomerization (at the photostationary state, 10% guest release upon subunit-trans-2b/subunit-cis-2b = 18 : 82).

Azo group(s) in selected macrocyclic compounds

Azobenzene derivatives due to their photo- and electroactive properties are an important group of compounds finding applications in diverse fields. Due to the possibility of controlling the trans-cis isomerization, azo-bearing structures are ideal building blocks for development of e.g. nanomaterials, smart polymers, molecular containers, photoswitches, and sensors. Important role play also macrocyclic compounds well known for their interesting binding properties. In this article selected macrocyclic compounds bearing azo group(s) are comprehensively described. Here, the relationship between compounds' structure and their properties (as e.g. ability to guest complexation, supramolecular structure formation, switching and motion) is reviewed.

Photocontrol of Anion Binding Affinity to a Bis-urea Receptor Derived from Stiff-Stilbene

Toward the development of photoresponsive anion receptors, a stiff-stilbene photoswitch has been equipped with two urea anion-binding motifs. Photoinduced E/Z isomerization has been studied in detail by UV–vis and NMR spectroscopy. Titration experiments (¹H NMR) reveal strong binding of acetate and phosphate to the (Z)-isomer, in which the urea groups are closely together. Isomerization to the (E)-form separates the urea motifs, resulting in much weaker binding. Additionally, geometry optimizations by density functional theory (DFT) illustrate that oxo-anion binding to the (Z)-form involves four hydrogen bonds.

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.

Development of redox-switchable resorcin[4]arene cavitands

CONSPECTUS: Within the framework of miniaturization of electromechanical devices, the development of a redox-switchable molecular gripper as a tool for nanorobotics is appealing both from an academic and from a practical perspective. Such a tool should be able to controllably grab a molecular cargo, translocate it over considerable distances and time scales, and release it. Resorcin[4]arene cavitands seem to be an ideal platform for the development of molecular grippers due to their ability to adopt two spatially well-defined conformations: an expanded kite and a contracted vase. Furthermore, they possess "legs" for functionalization and attachment to metal surfaces. While changes in temperature, pH, and metal-ion concentration were known to induce conformational switching, redox-switchable cavitands remained a challenge. In this Account, we describe our efforts toward the development of a new class of resorcin[4]arene cavitands that are redox-switchable. First, we introduced the naphthoquinone moiety as a redox-active wall component and showed that cavitands containing four quinone walls strongly prefer the open kite conformation in both the quinone and hydroquinone redox states, while cavitands that contain two quinone and two quinoxaline walls can adopt both the vase and the kite conformations depending on solvent but not on redox state. Next, in order to introduce a driving force for the conformational switching process in diquinone cavitands, we designed cavitands with hydrogen bond acceptor groups on the quinoxaline walls. These acceptors were sought to establish hydrogen bonds with the hydroquinone groups in the reduced redox state, thereby stabilizing the vase form. Oxidation to the quinone state would remove these interactions, switching the cavitand back to the kite form. Cavitands equipped with different hydrogen bond acceptor groups were synthesized and evaluated. We found that carboxamide moieties are best suited to assist redox-induced switching of conformational and binding properties. With the goal of further increasing association constants and reducing guest-exchange rates via steric congestion, we exchanged the naphthoquinone with the triptycene-quinone moiety. The congesting influence of the triptycene-quinone moiety on the binding properties was quantified both in the presence and in the absence of additional hydrogen bond interactions that stabilize the vase form. X-ray crystallographic studies provided insights into the solid-state structures of the cavitands in different solvents and redox states. A significant enhancement of association constants and reduction in guest release rates was observed in the reduced redox state compared with the top-open system, yielding redox-switchable cavitand baskets. These studies represent a step towards the development of redox-switchable molecular grippers on metal surfaces. Future challenges will consist in the development of cavitands that will no longer rely on an external proton source for the switching process, allowing redox-switching to be performed in purely aprotic media. Finally, suitable leg functionalization would enable the grippers to be interfaced with metal surfaces.

Experimental and computational study of BODIPY dye-labeled cavitand dynamics

Understanding the distance distribution and dynamics between moieties attached to the walls of a resorcin[4]arene cavitand, which is switchable between an expanded kite and a contracted vase form, might enable the use of this molecular system for the study of fundamental distance-dependent interactions. Toward this goal, a combined experimental and molecular dynamics (MD) simulation study on donor/acceptor borondipyrromethene (BODIPY) dye-labeled cavitands present in the vase and kite forms was performed. Direct comparison between anisotropy decays calculated from MD simulations with experimental fluorescence anisotropy data showed excellent agreement, indicating that the simulations provide an accurate representation of the dynamics of the system. Distance distributions between the BODIPY dyes were established by comparing time-resolved Förster resonance energy transfer experiments and MD simulations. Fluorescence intensity decay curves emulated on the basis of the MD trajectories showed good agreement with the experimental data, suggesting that the simulations present an accurate picture of the distance distributions and dynamics in this molecular system and provide an important tool for understanding the behavior of extended molecular systems and designing future applications.

Reversible switching between self-assembled homomeric and hybrid capsules

We present here the reversible, guest-controlled disproportionation of homomeric and hybrid capsules using photochemistry. The supramolecular containers are self-assembled from shallow and deep cavitand modules.

Photophysics Applied to Cavitands and Capsules

The use of light as a stimulus to control functional materials or nano-devices is appealing as it provides convenient control of triggering events where and when they are desired without introducing extra components to the system. Many photophysical and photochemical processes are extremely fast, giving rise to nearly instantaneous onset of events. However, these fast processes can be challenging to engineer into chemical systems. Supramolecular chemistry offers a convenient way to study and control photoprocesses. Given the reversible and self-programmed nature of modern host-guest systems, a modular approach can be considered in which different photoprocesses are coupled to obtain complex functions that emerge and are controlled solely by light inputs. In this review, we highlight recent examples of photoswitching and photophysics applied in the context of supramolecular host-guest systems, with a particular emphasis on resorcinarene based cavitands and hydrogen bonded capsules.