para-Phenylenediamine Dimer as a Redox-Active Guest for Supramolecular Systems.

Redox-active components are highly valuable in the construction of molecular devices. We combined two p-phenylenediamines (p-PDA) with a biphenyl (BiPhe) unit to prepare a supramolecular guest 4 consisting of three binding sites for cucurbit[7/8]uril (CBn) and/or cyclodextrins (CD). Supramolecular properties of 4 were investigated using NMR, UV-vis, mass spectrometry and isothermal titration calorimetry. Our analysis revealed that 4 forms higher-order host-guest complexes, wherein a CD unit occupies the central BiPhe site, secured by two CBn units at the terminal p-PDA sites. Additionally, 1 : 1 complexes with α-CD and ß-CD, a 1 : 2 complex with γ-CD and 2 : 1 complexes with CB7 and CB8 were identified. Through UV-vis and cyclic voltammetry, redox processes leading to the formation of a stable, deep blue dication diradical of 4 are elucidated. Furthermore, it is demonstrated that CB7 selectively protects oxidised 4 from reduction in the presence of a reducing agent. The supramolecular and redox properties of the structural motif represented by 4 render it an interesting candidate for the construction of supramolecular devices.

Encapsulation in Charged Droplets Generates Distorted Host-Guest Complexes.

The ability of various hydrogen-bonded resorcinarene-based capsules to bind α,ω-alkylbisDABCOnium (DnD) guests of different lengths was investigated in solution and in the gas-phase. While no host-guest interactions were detected in solution, encapsulation could be achieved in the charged droplets formed during electrospray ionisation (ESI). This included guests, which are far too long in their most stable conformation to fit inside the cavity of the capsules. A combination of three mass spectrometric techniques, namely, collision-induced dissociation, hydrogen/deuterium exchange, and ion-mobility mass spectrometry, together with computational modelling allow us to determine the binding mode of the DnD guests inside the cavity of the capsules. Significant distortions of the guest into horseshoe-like arrangements are required to optimise cation-π interactions with the host, which also adopt distorted geometries with partially open hydrogen-bonding seams when binding longer guests. Such quasi "spring-loaded" capsules can form in the charged droplets during the ESI process as there is no competition between guest encapsulation and ion pair formation with the counterions that preclude encapsulation in solution. The encapsulation complexes are sufficiently stable in the gas-phase - even when strained - because non-covalent interactions significantly strengthen in the absence of solvent.

The Interplay of Weakly Coordinating Anions and the Mechanical Bond: A Systematic Study of the Explicit Influence of Counterions on the Properties of (Pseudo)rotaxanes.

Weakly coordinating anions (WCAs) have attracted much attention in recent years due to their ability to stabilise highly reactive cations. It may well be argued, however, that a profound understanding of what truly defines a WCA is still lacking, and systematic studies to unravel counterion effects are scarce. In this work, we investigate a supramolecular pseudorotaxane formation reaction, subject to a selection of anions, ranging from strongly to weakly coordinating, which not only aids in fostering our knowledge about anion coordination properties, but also provides valuable theoretical insight into the nature of the mechanical bond. We employ state-of-the-art DFT-based methods and tools, combined with isothermal calorimetry and 1H NMR experiments, to compute anion-dependent Gibbs free association energies ΔGa, as well as to evaluate intermolecular interactions. We find correlations between ΔGa and the anions' solvation energies, which are exploited to calculate physico-chemical reaction parameters in the context of coordinating anions. Furthermore, we show that the binding situation within the (pseudo)rotaxanes can be mostly understood by straight-forward electrostatic considerations. However, quantum-chemical effects such as dispersion and charge-transfer interactions become more and more relevant when WCAs are employed.

Interfaces with Fluorinated Amphiphiles: Superstructures and Microfluidics.

This Minireview discusses recent developments in research on the interfacial phenomena of fluorinated amphiphiles, with a focus on applications that exploit the unique and manifold interfacial properties associated with these amphiphiles. Most notably, fluorinated amphiphiles form stable aggregates with often distinctly different morphologies compared to their nonfluorinated counterparts. Consequently, fluorinated surfactants have found wide use in high-performance applications such as microfluidic-assisted screening. Additionally, their fluorine-specific behaviour at solid/liquid interfaces, such as the formation of superhydrophobic coatings after deposition on surfaces, will be discussed. As fluorinated surfactants and perfluorinated materials in general pose potential environmental threats, recent developments in their remediation based on their adsorption onto fluorinated surfaces will be evaluated.

Gas-Phase Structural Analysis of Supramolecular Assemblies.

Ion mobility spectrometry and gas-phase IR action spectroscopy are two structure-sensitive mass-spectrometric methods becoming more popular recently. While ion mobility spectrometry provides collision cross sections as a size and shape dependent parameter of an ion of interest, gas-phase spectroscopy identifies functional groups and is capable of distinguishing different isomers. Both methods have recently found application for the investigation of supramolecular assemblies. We here highlight several aspects.Starting with the characterization of switching states in azobenzene photoswitches as well as redox-switchable lasso-type pseudorotaxanes, structures of isomers can be distinguished and mechanistic details analyzed. Ion mobility mass spectrometry in combination with gas-phase H/D-exchange reactions unravels subtle structural details as described for the chiral recognition of crown ether amino acid complexes. Gas-phase IR spectroscopy allows identification of details of the binding patterns in dimeric amino acid clusters as well as the serine octamer. This research can be extended into the analysis of peptide assemblies that are of medical relevance, for example, in Alzheimer's disease, and into a general hydrophobicity scale for natural as well as synthetic amino acids. The development of ultracold gas-phase spectroscopy that for example makes use of ions trapped in liquid helium droplets provides access to very well resolved spectra. The combination of ion mobility separation of ions with subsequent spectroscopic analysis even permits separation of different isomers and studying them separately with respect to their structure. This represents a great advantage of these gas-phase methods over solution experiments, in which the supramolecular complexes under study typically equilibrate and thus prevent a separate investigation of different isomers. At the end of this overview, we will discuss larger and more complex supramolecules, among them giant halogen-bonded cages and complex intertwined topologies such as molecular knots and Solomon links.

Effect of Perfluorinated Side-Chain Length on the Morphology, Hydrophobicity, and Stability of Xerogel Coatings.

Superhydrophobic surfaces can be quickly formed with supramolecular materials. Incorporating low-molecular-weight gelators (LMWGs) with perfluorinated chains generates xerogel coatings with low surface energies and high roughness. Here, we examine and compare the properties of the xerogel coatings formed with eight different LMWGs. These LMWGs all have a trans-1,2-diamidocyclohexane core and two perfluorinated ponytails, whose lengths vary from three to ten carbon atoms (CF3 to CF10). Investigation of the xerogels aims to provide in-depth information on the chain length effect. LMWGs with a higher degree of fluorination (CF7 to CF10) form superhydrophobic xerogel coatings with very low surface energies. Scanning electron microscopy images of the coatings show that the aggregates of CF5 and CF7 are fibrous, while the others are crystal-like. Aggregates of CF10 are particularly small and further assemble into a porous structure on the micrometer scale. To test their stabilities, the xerogel coatings were flushed multiple times with a standardized water flush test. The removal of material from the surface in these flushes was monitored by a combination of the water contact angle, contact angle hysteresis, and coating thickness measurements. A new method based on image processing techniques was developed to reliably determine the change of the coating thickness. The CF7, CF9, and CF10 surfaces show consistent hydrophobicity and coating durability after repetitive flushing tests. The length of the perfluorinated side chains thus has a significant effect on the morphology of the deposited xerogel coatings, their roughness, and, in consequence, their hydrophobicity and mechanical durability.

Redox-Responsive Host-Guest Chemistry of a Flexible Cage with Naphthalene Walls.

"Naphthocage", a naphthalene-based organic cage, reveals very strong binding (up to 1010 M-1) to aromatic (di)cationic guests, i.e., the tetrathiafulvalene mono- and dication and methyl viologen. Intercalation of the guests between two naphthalene walls is mediated by C-H···O, C-H···π, and cation···π interactions. The guests can be switched into and out of the cage by redox processes with high binding selectivity. Oxidation of the flexible cage itself in the absence of a guest leads to a stable radical cation with the oxidized naphthalene intercalated between and stabilized by the other two. Encapsulated guest cations are released from the cavity upon cage oxidation, paving the way to future applications in redox-controlled guest release or novel stimuli-responsive materials.

From a Cerium-Doped Polynuclear Bismuth Oxido Cluster to ß-Bi2O3:Ce.

The simultaneous hydrolysis of Bi(NO3)3·5H2O and Ce(NO3)3·6H2O results in the formation of novel heterometallic bismuth oxido clusters with the general formula [Bi38O45(NO3)24(DMSO)28+δ]:Ce (DMSO = dimethyl sulfoxide; cerium content <1.50%), which is demonstrated by single-crystal X-ray diffraction analysis. The incorporation of cerium into the cluster core is a result of the interplay of hydrolysis and condensation of the metal nitrates in the presence of oxygen. Diffuse-reflectance UV-vis and X-ray photoelectron spectroscopy reveal the presence of CeIV in the final bismuth oxido clusters as a result of oxidation of the cerium source. The cerium atoms are statistically distributed mainly on the bismuth atom positions of the central [Bi6O9] motif of the [Bi38O45] cluster core. Hydrolysis and subsequent annealing of the bismuth oxido clusters in the temperature range of 300-400 °C provides ß-Bi2O3:Ce samples with slightly lowered band gaps of approximately 2.3 eV compared to the undoped ß-Bi2O3 (approximately 2.4 eV). The sintering behavior of ß-Bi2O3 is significantly affected by the cerium dopant. Finally, differences in the efficiency of the as-prepared ß-Bi2O3:Ce and undoped ß-Bi2O3 samples in the photocatalytic decomposition of the biocide triclosan in an aqueous solution under visible-light irradiation are demonstrated.

Study on the Influence of Chirality in the Threading of Calix[6]arene Hosts with Dialkylammonium Axles.

: The influence of chirality in calixarene threading has been studied by exploiting the "superweak anion approach". In particular, the formation of chiral pseudo[2]rotaxanes bearing a classical stereogenic center in their axle and/or wheel components has been considered. Two kind of pseudo[2]rotaxane stereoadducts, the "endo-chiral" and "exo-chiral" ones, having the stereogenic center of a cationic axle inside or outside, respectively, the calix-cavity of a chiral calixarene were preferentially formed with specifically designed chiral axles by a fine exploitation of the so-called "endo-alkyl rule" and a newly defined "endo-α-methyl-benzyl rule" (threading of a hexaalkoxycalix[6]arene with a directional (α-methyl-benzyl)benzylammonium axle occurs with an endo-α-methyl-benzyl preference). The obtained pseudorotaxanes were studied in solution by 1D and 2D NMR, and in the gas-phase by means of the enantiomer-labeled (EL) mass spectrometry method, by combining enantiopure hosts with pseudoracemates of one deuterated and one unlabeled chiral axle enantiomer. In both instances, there was not a clear enantiodiscrimination in the threading process with the studied host/guest systems. Possible rationales are given to explain the scarce reciprocal influence between the guest and host chiral centers.

Thermodynamic and electrochemical study of tailor-made crown ethers for redox-switchable (pseudo)rotaxanes.

Crown ethers are common building blocks in supramolecular chemistry and are frequently applied as cation sensors or as subunits in synthetic molecular machines. Developing switchable and specifically designed crown ethers enables the implementation of function into molecular assemblies. Seven tailor-made redox-active crown ethers incorporating tetrathiafulvalene (TTF) or naphthalene diimide (NDI) as redox-switchable building blocks are described with regard to their potential to form redox-switchable rotaxanes. A combination of isothermal titration calorimetry and voltammetric techniques reveals correlations between the binding energies and redox-switching properties of the corresponding pseudorotaxanes with secondary ammonium ions. For two different weakly coordinating anions, a surprising relation between the enthalpic and entropic binding contributions of the pseudorotaxanes was discovered. These findings were applied to the synthesis of an NDI-[2]rotaxane, which retains similar spectroelectrochemical properties compared to the corresponding free macrocycle. The detailed understanding of the thermodynamic and electrochemical properties of the tailor-made crown ethers lays the foundation for the construction of new types of molecular redox switches with emergent properties.

Naphthocage: A Flexible yet Extremely Strong Binder for Singly Charged Organic Cations.

We report a quite flexible naphthol-based cage (so-called "naphthocage") which adopts a self-inclusion conformation in its free state and is able to bind singly charged organic cations extremely strongly ( Ka > 107 M-1). Ion-selective electrodes prepared with this naphthocage show a super-Nernstian response to acetylcholine. In addition, the highly stable complex (1010 M-1) between ferrocenium and the naphthocage can be switched electrochemically, which lays a basis for its application in stimuli-responsive materials.

Strong Emission Enhancement in pH-Responsive 2:2 Cucurbit[8]uril Complexes.

Organic fluorophores, particularly stimuli-responsive molecules, are very interesting for biological and material sciences applications, but frequently limited by aggregation- and rotation-caused photoluminescence quenching. A series of easily accessible bipyridinium fluorophores, whose emission is quenched by a twisted intramolecular charge-transfer (TICT) mechanism, is reported. Encapsulation in a cucurbit[7]uril host gave a 1:1 complex exhibiting a moderate emission increase due to destabilization of the TICT state inside the apolar cucurbituril cavity. A much stronger fluorescence enhancement is observed in 2:2 complexes with the larger cucurbit[8]uril, which is caused by additional conformational restriction of rotations around the aryl/aryl bonds. Because the cucurbituril complexes are pH switchable, this system represents an efficient supramolecular ON/OFF fluorescence switch.

Accordion-Like Motion in Electrochemically Switchable Crown Ether/Ammonium Oligorotaxanes.

Reversible oxidation reactions in electrochemically switchable oligorotaxanes with tetrathiafulvalene (TTF) decorated 24-crown-8 ether wheels generate intramolecular mixed-valence and radical-cation interactions between the wheels. This induces shuttling of the wheels and a contraction of inter-wheel distances. Further oxidation generates repulsive forces between the TTFs and maximizes the inter-wheel distances instead. These interactions and co-conformational changes were not observed for structurally similar controls in which acetyl groups along the axle prevent translational motion of the wheels. This operation mode of oligorotaxanes, which is reminiscent of an accordion-like motion, is promising for functional materials and nanodevices such as piston-type rotaxane motors.

Ion-Mobility Mass Spectrometry for the Rapid Determination of the Topology of Interlocked and Knotted Molecules.

A rapid screening method based on traveling-wave ion-mobility spectrometry (TWIMS) combined with tandem mass spectrometry provides insight into the topology of interlocked and knotted molecules, even when they exist in complex mixtures, such as interconverting dynamic combinatorial libraries. A TWIMS characterization of structure-indicative fragments generated by collision-induced dissociation (CID) together with a floppiness parameter defined based on parent- and fragment-ion arrival times provide a straightforward topology identification. To demonstrate its broad applicability, this approach is applied here to six Hopf and two Solomon links, a trefoil knot, and a [3]catenate.

Anion-driven encapsulation of cationic guests inside pyridine[4]arene dimers.

Pyridine[4]arenes have previously been considered as anion binding hosts due to the electron-poor nature of the pyridine ring. Herein, we demonstrate the encapsulation of Me4N+ cations inside a dimeric hydrogen-bonded pyridine[4]arene capsule, which contradicts with earlier assumptions. The complexation of a cationic guest inside the pyridine[4]arene dimer has been detected and studied by multiple gas-phase techniques, ESI-QTOF-MS, IRMPD, and DT-IMMS experiments, as well as DFT calculations. The comparison of classical resorcinarenes with pyridinearenes by MS and NMR experiments reveals clear differences in their host-guest chemistry and implies that cation encapsulation in pyridine[4]arene is an anion-driven process.

New Water-Soluble Cluster Compound {Zn(en)3 }3 [V15 Sb6 O42 (H2 O)]⋠(Ethylenediamine)3 ⋠10 H2 O as a Synthon for the Generation of Two New Antimonato Polyoxovanadates.

A new antimonato polyoxovanadate {Zn(en)3 }3 [V15 Sb6 O42 (H2 O)]⋅3 en⋅10 H2 O (en=ethylenediamine) synthesized under hydrothermal conditions exhibits remarkable solubility in water. Electrospray ionization mass spectrometry (ESI-MS) investigations on an aqueous solution demonstrate that the cluster core remains fully intact for 72 h. At longer times, slow transformation into a {V14 Sb8 O42 } cluster is observed. The conversion reaches 50 % after 14 days and is complete after approximately 20 days. The rate of this {V15 Sb6 }→{V14 Sb8 } cluster transformation is significantly increased in the presence of ammonium acetate. Applying the new compound as a synthon in the presence of 1,10-phenanthroline (phen) led to crystallization of {Zn(phen)3 }2 [Zn(en)2 V15 Sb6 O42 (H2 O)]⋅23 H2 O after a short reaction time, whereas addition of Sb2 O3 led to fast crystallization of {(Zn(en)2 (H2 O)2 )(Zn(en)2 )}[Zn(en)2 V15 Sb6 O42 (H2 O)] ⋅8.5 H2 O. In the crystal structure of {Zn(en)3 }3 [V15 Sb6 O42 (H2 O)]⋅3 en⋅10 H2 O, the en molecules are seen to be attached to the cluster anion through Sb-N bonds. In the structures of the two new compounds obtained, the [V15 Sb6 O42 (H2 O)]6- anions are expanded by Zn2+ -centered complexes through Zn-O-V bond formation.

To Anion-π or not to Anion-π: The Case of Anion-Binding to Divalent Fluorinated Pyridines in the Gas Phase.

A series of mono- and divalent fluorinated pyridine derivatives is investigated by electrospray ionization (tandem) mass spectrometry and quantum chemical calculations with respect to their capability to bind anions in the gas phase. The pyridine derivatives differ not only in valency, but also with regard to the degree of fluorination of the pyridine rings, the positions of the fluorine atoms, the rigidity of the spacers connecting the two pyridines in the divalent compounds, and the relative configuration. While the monovalent compounds did not form anion complexes, the divalent analogues exhibit anion binding even to weakly coordinating anions such as tetrafluoroborate. Three different tandem mass spectrometric experiments were applied to rank the gas-phase binding energies: (i) collision-induced dissociation (CID) experiments in a Fourier transform ion-cyclotron-resonance (FTICR) mass spectrometer on two different, simultaneously mass-selected complexes with different receptors, (ii) determination of the collision energy required to fragment 50 % of the mass-selected complexes in an ESI-QToF mass spectrometer, and (iii) CID of heterodimers formed from two different, competing pyridine receptors and indigo carmine, a dianion with two identical binding sites. All three experiments result in consistent binding energy ranking. This ranking reveals surprising features, which are not in agreement with binding through anion-π interactions. Density functional theory (DFT) calculations comparing different potential binding modes provide evidence that the ranking can instead nicely be explained, when C-H⋅⋅⋅anion interactions with the spacers are invoked. These results are supported by gas-phase IR spectroscopy and ion mobility-mass spectrometry (IM-MS) on a selected set of chloride pyridine complexes.

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.

Assessing cooperativity in supramolecular systems.

This tutorial review summarises different aspects of cooperativity in supramolecular complexes. We propose a systematic categorisation of cooperativity into cooperative aggregation, intermolecular (allosteric) cooperativity, intramolecular (chelate) cooperativity and interannular cooperativity and discuss approaches to quantify them thermodynamically using cooperativity factors. A brief summary of methods to determine the necessary thermodynamic data is given with emphasis on isothermal titration calorimetry (ITC), a method still underrepresented in supramolecular chemistry, which however offers some advantages over others. Finally, a discussion of very few selected examples, which highlight different aspects to illustrate why such an analysis is useful, rounds up this review.

Multivalent Crown Ether Receptors Enable Allosteric Regulation of Anion Exchange in an Fe4 L6 Tetrahedron.

We report a strategy for regulating the rate of internally bound anion exchange within an Fe4 L6 metal-organic tetrahedron through external coordination of tripodal tris(alkylammonium) cations. The cage features three flexible 18-crown-6 receptors at each of its FeII vertices, facilitating strong tritopic interactions with tris(ammonium) cations to "cap" the vertices of the tetrahedron. This capping mechanism restricts the flexibility of the cage framework, thereby reducing the rate of anion exchange within its central cavity by 20-fold. Thus, we demonstrate the first use of an externally bound multivalent effector to allosterically control internal guest binding in a molecular cage.