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Use of pre-programmed anion binding sites provide structure-directing elements in subcomponent self-assembly that operate in an orthogonal way to the traditional role of cations.

Preorganization is a key concept in supramolecular chemistry. Preorganized receptors enhance binding by minimizing the organization costs associated with adopting the conformation needed to orient the binding sites toward the guest. Conversely, poorly organized receptors show affinities below what is possible based on the potential of their specific binding interactions. Despite the fact that the organization energy is paid each time like a tax, its value has never been measured directly, though many compounds have been developed to measure its effects. We present a method to quantify the hidden costs of receptor organization by independently measuring the contribution it makes to chloride complexation by a flexible foldameric receptor. This method uses folding energy to approximate organization energy and relies on measurement of the coil-helix equilibrium as a function of solvent. We also rely on the finding, established with rigid receptors, that affinity is inversely related to the solvent dielectric and expect the same for the foldamer's helically organized state. Increasing solvent polarity across nine dichloromethane-acetonitrile mixtures we see an unusual V-shape in affinity (decrease then increase). Quantitatively, this shape arises from weakened hydrogen-bonding interactions with solvent polarity followed by solvent-driven folding into an organized helix. We confirm that dielectric screening impacts the stability of host-guest complexes of flexible foldamers just like rigid receptors. These results experimentally verify the canonical model of binding (affinity depends on the sum of organization and noncovalent interactions). The picture of how solvent impacts complex stability and conformational organization thereby helps lay the groundwork for de novo receptor design.

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

The triggered self-assembly of surfactants into organized layers at aqueous interfaces is important for creating adaptive nanosystems and understanding selective ion extraction. While these transformations require molecular recognition, the underlying driving forces are modified by the local environment in ways that are not well understood. Herein, we investigate the role of ion binding and ion hydration using cyanosurf, which is composed of the cyanostar macrocycle, and its binding to anions that are either size-matched or mis-matched and either weakly or highly hydrated. We utilize the supra-amphiphile concept where anion binding converts cyanosurf into a charged and amphiphilic complex triggering its self-organization into monolayers at the air-water interface. Initially, cyanosurf forms aggregates at the surface of a pure water solution. When the weakly hydrated and size-matched hexafluorophosphate (PF

Brightly fluorescent solid-state materials are highly desirable tools in bioimaging, optoelectronic applications and energy harvesting. However, close contact between π-systems leads to strong electronic coupling and often subse-quent quenching of the emitters in dense solid-state materials. Recently, we developed a method to prohibit strong coupling based on small molecule ionic isolation lattices (SMILES) that efficiently isolate fluorophores while in paral-lel ensuring very high densities of the dyes. Nevertheless, efficient FRET energy migration in such dense systems is inevitable. While attractive for energy harvesting applications, FRET also significantly compromises quantum yields of fluorescent solids by funneling excitation energy to dark trap states like crystal defects. Here we investigate the underlying property of FRET and exploit it to our favor by intentionally introducing fluorescent dopants into SMILES materials, acting as FRET acceptors with favorable photophysical properties. This doping is shown to outcompete en-ergy migration to dark trap states, while also ruling out reabsorption effects in dense SMILES materials, resulting in universal fluorescent solid state materials (thin films, powders, crystals) with superior properties. These include emission quantum yields reaching as high as 50-65 %, programmable fluorescence lifetimes with mono-exponential decay, and independent selection of absorption and emission maxima. The volume normalized brightness of these FRET-based SMILES now reach values up to 32200 M-1 cm-1 nm-3 and can deliver freely tunable spectroscopic properties for the fabrication of super bright advanced optical materials.

CH-hydrogen bonding provides access to new building blocks for making macrocyclic ionophores with high degrees of preorganization and selective anion recognition. In this study, an anion-binding ionophore in the shape of a clamshell (ClS) was employed that is composed of two cyanostar (CNstar) macrocycles with preorganized cavities linked with a 12-carbon chain. This ionophore allows for anion complexation by CH-hydrogen bonding. The potentiometric performance of membrane-based ion-selective electrodes incorporating this ionophore was evaluated. Different membrane compositions were prepared to determine the optimum concentrations of the ionophore and lipophilic additive in the membrane. The optimized electrode had a slope of -58.2 mV/decade and demonstrated an anti-Hofmeister selectivity pattern toward iodide with a nanomolar detection limit. Electrospray ionization mass spectrometry was employed to study the relative association strengths of ClS with various anions. The observed mass peaks of the ion-ionophore complexes were found to be consistent with the potentiometric selectivity pattern of the corresponding electrodes. Overall, the selectivity of the electrode could be altered by using an ionophore in which the two CNstar macrocycles are linked together with a flexible 12-carbon chain to control the molecularity of the binding event.

Ultra-bright fluorescent nanoparticles (NPs) hold great promise for demanding bioimaging applications. Recently, extremely bright molecular crystals of cationic fluorophores were obtained by hierarchical co-assembly with cyanostar anion-receptor complexes of associated counterions. These small-molecule ionic isolation lattices (SMILES) ensure spatial and electronic isolation to prohibit dye aggregation quenching. We report a simple, one-step supramolecular approach to formulate SMILES materials into NPs. Rhodamine-based SMILES NPs stabilized by glycol amphiphiles show high fluorescence quantum yield (30%) and brightness per volume (5000 M -1 cm -1 / nm 3 ) with 400 dyes packed into 16-nm particles, corresponding to a particle absorption coefficient of 4 × 10 7 M -1 cm -1 . UV excitation of the cyanostar component leads to highest brightness (>6000 M -1 cm -1 / nm 3 ) by energy transfer to rhodamine emitters. Coated NPs stain cells and are thus promising for bioimaging.

The recognition of boron compounds is well developed as boronic acids but untapped as organotrifluoroborate anions (R-BF

The selectivities and driving forces governing phosphate recognition by charged receptors at prevalent aqueous interfaces is unexplored relative to the many studies in homogeneous solutions. Here w...

Bimetallic magnesium(II) complexes are gaining significant interest in catalysis, yet their fundamental formation and behavior in organic media remain surprisingly unexplored relative to other divalent cations. To understand key principles of their formation, we investigate symmetric ditopic ligands bearing a phenolic backbone and characterize their ability to form dinuclear magnesium(II) cascade complexes with two bridging anions. High-fidelity production of bimetallic magnesium complexes relative to the monometallic complexes is indicative of positive cooperativity. Binding and recognition of analytes or substrates is a key characteristic of metal cascade complexes and relies on anion exchange, but this is also rarely studied with bimetallic magnesium complexes. Investigations with acetate, phosphate, and pyrophosphate reveal exchange of bridging nitrates using the bis-dipicolylamine complex. Rare seven-coordinate magnesium centers are found for the ester complex. The findings in this study provide formative steps to establish design principles for future generations of bimetallic magnesium(II) complexes.

Self-organizations of supramolecular assemblies at surfaces typically achieve highly-ordered two-dimensional packing structures due to the negotiation of various intermolecular interactions into a ...

Sequence-controlled supramolecular polymers offer new design paradigms for generating stimuli-responsive macromolecules with enhanced functionalities. The dynamic character of supramolecular links present challenges to sequence definition in extended supramolecular macromolecules, and design principles remain nascent. Here, we demonstrate the first example of using stoichiometry-control to specify the monomer sequence in a linear supramolecular polymer by synthesizing both a homopolymer and an alternating copolymer from the same glycol-substituted cyanostar macrocycle and phenylene-linked diphosphate monomers. A 2:1 stoichiometry between macrocycle and diphosphate produces a supramolecular homopolymer of general formula (A)n comprised of repeating units of cyanostar-stabilized phosphate-phosphate dimers. Using a 1:1 stoichiometry, an alternating (AB)n structure is produced with half the phosphate dimers now stabilized by the additional counter cations that emerge hierarchically after forming the stronger cyanostar-stabilized phosphate dimers. These new polymer materials and binding motifs are sufficient to bear normal and shear stress to promote significant and tunable adhesive properties. The homopolymer (A)n, consisting of cyanostar-stabilized anti-electrostatic linkages, shows adhesion strength comparable to commercial superglue formulations based on polycyanoacrylate but is thermally reversible. Unexpectedly, and despite including traditional ionic linkages, the alternating copolymer (AB)n shows weaker adhesion strength more similar to commercial white glue based on poly(vinyl acetate). Thus, the adhesion properties can be tuned over a wide range by simply controlling the stoichiometric ratio of monomers. This study offers new insight into supramolecular polymers composed of custom-designed anion and receptor monomers and demonstrates the utility of emerging functional materials based on anion-anion linkages.

Based on Coulomb's Law alone, electrostatic repulsion between two anions is expected to prevent their dimerization. Contrary to that idea, this Tutorial Review will present evidence showing that anion-anion dimers of protic hydroxyanions can form readily, and describe conditions that facilitate their formation. From X-ray crystal structures, we learn that hydroxyanions dimerize and oligomerize by overcoming long-range electrostatic opposition. Common examples are hydroxyanions of phosphate, sulfate, and carbonate, often in partnership with charged and neutral receptors. Short-range hydrogen bonds between anionic donors and acceptors are defined as anti-electrostatic hydrogen bonds (AEHBs) with insight from theoretical studies. While anion dimers are difficult to identify unequivocally in solution, these solution dimers have recently been definitively identified. The development of the supramolecular chemistry of anion-anion dimers has led to applications in hierarchical assemblies, such as supramolecular polymers and hydrogen bonded organic frameworks.

Persistent anion binding in a wide range of solution environments is a key challenge that continues to motivate and demand new strategies in synthetic receptor design. Though strong binding in low-polarity solvents has become routine, our ability to maintain high affinities in high-polarity solvents has not yet reached the standard set by nature. Anions are bound and transported regularly in aqueous environments by proteins that use secondary and tertiary structure to isolate anion binding sites from water. Inspired by this principle of solvent exclusion, we created a sequence-defined foldameric capsule whose global minimum conformation displays a helical folded state and is preorganized for 1:1 anion complexation. The high stability of the folded geometry and its ability to exclude solvent were supported by solid-state and solution phase studies. This capsule then withstood a 4-fold increase in solvent dielectric constant (er) from dichloromethane (9) to acetonitrile (36) while maintaining a high and solvent-independent affinity of 105 M-1; ΔG ∼ 28 kJ mol-1. This behavior is unusual. More typical of solvent-dependent behavior, Cl- affinities were seen to plummet in control compounds, such as aryl-triazole macrocycles and pentads, with their solvent-exposed binding cavities susceptible to dielectric screening. Finally, dimethyl sulfoxide denatures the foldamer by putative solvent binding, which then lowers the foldamer's Cl- affinity to normal levels. The design of this capsule demonstrates a new prototype for the development of potent receptors that can operate in polar solvents and has the potential to help manage hydrophilic anions present in the hydrosphere and biosphere.

International audience; The recognition of substituted phosphates underpins many processes including DNA binding, enantioselective catalysis, and recently template-directed rotaxane synthesis. Beyond ATP and a few commercial substrates, however, little is known about how substituents effect organophosphate recognition. Here, we examined alcohol substituents and their impact on recognition by cyanostar macrocycles. The organophosphates were disubstituted by alcohols of various chain lengths, dipropanol, dihexanol, and didecanol phosphate, each accessed using modular solid-phases syntheses. Based on the known size-selective binding of phosphates by π-stacked dimers of cyanostars, threaded [3]pseudorotaxanes were anticipated. While seen with butyl substituents, pseudorotaxane formation was disrupted by competitive OH···O– hydrogen bonding between both terminal hydroxyls and the anionic phosphate unit. Crystallography also showed formation of a backfolded propanol conformation resulting in an 8-membered ring and a perched cyanostar assembly. Motivated by established entropic penalties accompanying ring formation, we reinstated [3]pseudorotaxanes by extending the size of the substituent to hexanol and decanol. Chain entropy overcomes the enthalpically favored OH···O– contacts to favor random-coil conformations required for seamless, high-fidelity threading of dihexanol and didecanol phosphates inside cyanostars. These studies highlight how chain length and functional groups on phosphate’s substituents can be powerful design tools to regulate binding and control assembly formation during phosphate recognition.

There is a critical need for receptors that are designed to enhance anion binding selectivity at aqueous interfaces in light of the growing importance of separation technologies for environmental sustainability. Here, we conducted the first study of anion binding selectivity across a series of prevalent inorganic oxoanions and halides that bind to a positively charged guanidinium receptor anchored to an aqueous interface. Vibrational sum frequency generation spectroscopy and infrared reflection absorption spectroscopy studies at the water-air interface reveal that the guanidinium receptor binds to an oxoanion series in the order SO42- > H2PO4- > NO3- > NO2- while harboring very weak interactions with the halides in the order I- > Cl- ≈ Br-. In spite of large dehydration penalties for sulfate and phosphate, the more weakly hydrated guanidinium receptor was selective for these oxoanions in contradiction to predictions made from ion partitioning alone, like the Hofmeister series and Collins's rules. Instead, sulfate binding is likely favored by the suppression of dielectric screening at the interface that consequently boosts Coulombic attractions, and thus helps offset the costs of anion dehydration. Geometric factors also favor the oxoanions. Furthermore, the unique placement of iodide in our halide series ahead of the stronger hydrogen-bond acceptors (Cl-, Br-) suggests that the binding interaction also depends upon single-ion surface partitioning from bulk water to the interface. Knowledge of the anion binding preferences displayed by a guanidinium receptor sheds light on the receptor architectures needed within designer interfaces to control selectivity.

Summary Fluorescence is critical to applications in optical materials including OLEDs and photonics. While fluorescent dyes are potential key components of these materials, electronic coupling between them in the solid state quenches their emission, preventing their reliable translation to applications. We report a universal solution to this long-standing problem with the discovery of a class of materials called small-molecule ionic isolation lattices (SMILES). SMILES perfectly transfer the optical properties of dyes to solids, are simple to make by mixing cationic dyes with anion-binding cyanostar macrocycles, and work with major classes of commercial dyes, including xanthenes, oxazines, styryls, cyanines, and trianguleniums. Dyes are decoupled spatially and electronically in the lattice by using cyanostar with its wide band gap. Toward applications, SMILES crystals have the highest known brightness per volume and solve concentration quenching to impart fluorescence to commercial polymers. SMILES materials enable predictable fluorophore crystallization to fulfill the promise of optical materials by design.

Nanoporous thin films formed on electrodes are considered functional elements of electrochemical sensing systems, thus motivating methods for their development. We report a preparative strategy detailing the effects of surface modification of gold substrates with thiolate self-assembled monolayers (SAMs) on the properties of nanoporous thin films derived from polystyrene-block-poly(ethylene oxide) having a photocleavable o-nitrobenzyl ester junction (PS-hν-PEO). Two PS-hν-PEO having similar PEO volume fractions (≈0.2) but different molecular weights (10 and 23 kg/mol) were used to prepare films (30–100 nm thick) spin-cast on gold substrates unmodified and modified with cysteamine, thioctic acid, and 6-hydroxy-1-hexanethiol SAMs. Solvent vapor annealing followed by PEO removal led to the formation of nanopores with average diameters of 12 and 19 nm from the smaller and larger PS-hν-PEO, respectively. Cyclic voltammograms of 1,1′-ferrocenedimethanol showed that nanoporous films on cysteamine SAMs afforded nanopores reaching the underlying substrates at higher density than those on the other substrates. This result was attributed to balanced affinity of the cysteamine SAM surface with PS and PEO, which enhanced the vertical orientation of PEO microdomains. The generation of carboxyl groups associated with the photocleavage reaction was revealed by pH-dependent changes in the voltammogram of Fe(CN)₆³– that reflected electrostatic effects regulated by the protonation state of the carboxyl groups. The SAMs underneath the nanoporous films could be replaced by treatment with a thiol solution, as verified by voltammograms of l-ascorbic acid. These results suggest that thiolate SAM modification provides a simple means to control the interfacial orientation of PEO microdomains in thin PS-hν-PEO films.

Fluorophores are powerful tools for the study of chemistry, biology, and physics. However, fluorescence is severely impaired when concentrations climb above 5 μM as a result of effects like self-absorption and chromatic shifts in the emitted light. Herein, we report the creation of a charge-transfer (CT) fluorophore and the discovery that its emission color seen at low concentrations is unchanged even at 5 mM, some 3 orders of magnitude beyond typical limits. The fluorophore is composed of a triphenylamine-substituted cyanostar macrocycle, and it exhibits a remarkable Stokes shift of 15 000 cm

The selectivities and driving forces governing phosphate recognition by charged receptors at prevalent aqueous interfaces is unexplored relative to the many studies in homogeneous solutions. Here we report on electrostatic binding versus hydrogen-bond-assisted electrostatic binding of phosphate (H

Amines are ubiquitous in the chemical industry and are present in a wide range of biological processes, motivating the development of amine-sensitive sensors. There are many turn-on amine sensors, however there are no examples of turn-on sensors that utilize the amine's ability to react by single electron transfer (SET). We investigated a new turn-on amine probe with a 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) fluorophore. BODIPY fluorescence is first preprogrammed into an off state by internal photoinduced electron transfer (PET) to an electron-deficient quinolinium ring, resulting in fluorescence quenching. At low concentrations of aliphatic amine (0 to 10 mM), this PET pathway is shut down by external SET from the amine to the photoexcited charge-transfer state of the probe and the fluorescence is turned on. At high concentrations of amine (50 mM to 1 M), we observed collisional quenching of the BODIPY fluorescence. The probe is selective for aliphatic amines over aromatic amines, and aliphatic thiols or alcohols. The three molecular processes modulate the BODIPY fluorescence in a multi-mechanistic way with two of them producing a direct response to amine concentrations. The totality of the three molecular processes produced the first example of a multi-state and dose-responsive amine sensor.

Allosteric regulation of protein structure and function is a hallmark of biology. The structures of protein-like abiological foldamers have been subject to allosteric control, however, regulation of their function is rare. We report this behavior using a photoactive foldamer following the discovery that small and large anions select between single and double helical structures, respectively. Correspondingly, these anions activate different functions in the photofoldamer; small anions turn on photoregulation of anion concentrations while large anions turn on chiroptical switching of quaternary structure. For this demonstration, we used an aryl-triazole based photofoldamer in which the light-driven trans–cis isomerization of azobenzenes alters intrastrand π–π contacts while the triazoles define the allosteric anion-binding site. Binding to 11 anions of increasing size was quantified (Cl–, Br–, NO2–, I–, NO3–, SCN–, BF4–, ClO4–, ReO4–, PF6–, SbF6–). Contrary to expectations that single helices will expand to...

The primary sequence in biopolymers carries the information to direct folded secondary structures, to modulate their stabilities, and to control the resultant functions. Our ability to encode such information into nonbiological oligomers and polymers, however, is still limited. Here, we describe a C

Due to the growing world population, there is an ever-increasing need to develop better receptors to recover and recycle phosphate for use in agricultural processes. This need is driven by agricultural demand and environmental concerns because phosphate eutrophication has a damaging effect on fresh water supplies by fueling algal blooms. The air/water interface provides a unique region with a dielectric constant (e) that diminishes from high in bulk water (e = 80) to significantly lower (e.g., e < 40) near the monolayer surface to potentially enhance affinities during molecular recognition. The work presented here uses a model system of phosphate binding to an amino acid, arginine, and utilizes the interfacial properties of the phospholipid monolayer, 1,2-dipalmitoyl-sn-glycero-3-phosphatidic acid, as the phosphate source to quantify binding. Employing arginine as a probe molecule allows for the evaluation of its guanidinium moiety for phosphate chelation. Surface pressure–area isotherms from Langmuir mon...

The importance of phosphate in biology and chemistry has long motivated investigation of its recognition. Despite this interest, phosphate's facile oligomerization is only now being examined following the discovery of complexes of anion–anion dimers of hydroxyanions. Here we address how oligomerization dictates phosphate's recognition properties when engaged with planar cyanostar macrocycles that can also oligomerize by stacking. The crystal structure of cyanostar with phosphate shows an unprecedented tetrameric stack of cyanostar macrocycles threaded by a phosphate trimer, [H2PO4⋯H2PO4⋯H2PO4]3−. The solution behaviour, studied as a function of solvent quality, highlights how dimers and trimers of phosphate drive formation of higher order stacks of cyanostar into dimer, trimer and tetramer co-assemblies. Solution behaviors differ significantly from simpler complexes of bisulfate hydroxyanion dimers. Phosphate oligomerization is: (1) preferred over ion pairing with tetrabutylammonium cations, (2) inhibits disassembly of the complexes upon dilution, and (3) resists interference from competitive anion solvation. The phosphate oligomers also appear critical for stability; complexation of just one phosphate with cyanostars is unfavored. The cyanostar's ability to self-assemble is found to create a tubular, highly electropositive cavity that complements the size and shape of the phosphate oligomers as well as their higher charge. When given the opportunity, phosphate will cooperate with the receptor to form co-assembled architectures.

Amphiphilic alkoxybenzonitriles (ABNs) of varying chain length are studied at the solution/graphite interface to analyze dynamics of assembly. Competitive self-assembly between ABNs and alkanoic acid solvent is shown by scanning tunneling microscopy (STM) to be controlled by concentration and molecular size. Molecular dynamics (MD) simulations reveal key roles of the sub-nanosecond fundamental steps of desorption, adsorption, and on-surface motion. We discovered asymmetry in desorption-adsorption steps. Desorption starting from alkyl chain detachment from the surface is favored due to dynamic occlusion by neighbouring chains. Even though the nitrile head has a strong solvent affinity, it more frequently re-adsorbs following a detachment event.

The acid-base switching of complexes formed from anti-electrostatic anion-anion homodimers of organophosphates and cyanostar macrocycles was investigated for the first time. High-fidelity 2:2 complexes were selected by using suitably sized organo substituents. Reversible and direct switching occurs with triflic acid and hydroxide base. An unexpected acid⋅⋅⋅anion heterodimer was discovered with weaker picric acid, which helped reveal some of the elementary steps. Switching can also proceed in a cooperative (strong anion then weak acid) or stepwise manner (weak acid then strong anion).

Summary A fundamental understanding of anion binding by receptors is essential for managing salts during energy, water, and food production. However, the limited understanding of solvent effects in ion recognition leads to a persistent blind spot that prevents effective receptor design. We experimentally discovered an underlying 1/ ɛ r dependence of anion affinity on solvent dielectric constant ( ɛ r ). We found this relationship by measuring how chloride binds to macrocyclic triazolophane receptors across a wide range of solvents: ɛ r = 4.7–56.2. Solvent weakens affinity by screening electrostatics; electrostatics dominates when ɛ r ɛ r – affinity in solvents used in liquid-liquid extractions in the nuclear fuel cycle. This model offers a general foundation for anion recognition and electrostatically driven complexation.

The interaction and mobility of ions in complex systems are fundamental to processes throughout chemistry, biology, and physics. However, nanoscale characterization of ion stability and migration remains poorly understood. Here, we examine ion movements to and from physisorbed molecular receptors at solution–graphite interfaces by developing a theoretical model alongside experimental scanning tunneling microscopy (STM) results. The model includes van der Waals forces and electrostatic interactions originating from the surface, tip, and physisorbed receptors, as well as a tip–surface electric field arising from the STM bias voltage (Vb). Our model reveals how both the electric field and tip–surface distance, dtip, can influence anion stability at the receptor binding sites on the surface or at the STM tip, as well as the size of the barrier for anion transitions between those locations. These predictions agree well with prior and new STM results from the interactions of anions with aryl-triazole receptors that order into functional monolayers on graphite. Scanning produces clear resolution at large magnitude negative surface biases (−0.8 V) while resolution degrades at small negative surface biases (−0.4 V). The loss in resolution arises from frequent tip retractions assigned to anion migration within the tip–surface tunneling region. This experimental evidence in combination with support from the model demonstrates a local voltage gating of anions with the STM tip inside physisorbed receptors. This generalized model and experimental evidence may help to provide a basis to understand the nanoscale details of related chemical transformations and their underlying thermodynamic and kinetic preferences.

A cage molecule incorporating three electron donating monopyrrolotetrathiafulvalene units was synthesised to host electron accepting [60]fullerenes. Formation of a strong 1 : 1 donor-acceptor (D-A) complex C60⊂1 was confirmed by solid state X-ray analysis as well as (1)H NMR and absorption spectroscopic analyses of the arising charge-transfer (CT) band (λ = 735 nm, ε ≈ 840 M(-1) cm(-1)). Inserting Li(+) inside the [60]fullerene increased the binding 28-fold (Ka = 3.7 × 10(6) M(-1)) and a large bathochromic shift of the CT band to the near infrared (NIR) region (λ = 1104 nm, ε ≈ 4800 M(-1) cm(-1)) was observed.

Fluorescence is critical to many advanced materials including OLEDs and bioimaging. While molecular fluorophores that show bright emission in solution are potential sources of these materials, their emission is frequently lost in the solid state preventing their direct translation to optical applications. Unpredictable packing and coupling of dyes leads to uncontrolled spectral shifts and quenched emission. No universal solution has been found since Perkin made the first synthetic dye 150 years ago. We report the serendipitous discovery of a new type of material that we call small-molecule ionic isolation lattices(SMILES) tackling this long-standing problem. SMILES are easily prepared by adding two equivalents of the anion receptor cyanostar to cationic dyes binding the counter anions and inducing alternating packing of dyes and cyanostar-anion complexes. SMILES materials reinstate solution-like spectral properties and bright fluorescence to thin films and crystals. These positive outcomes derive from the cyanostar. Its wide 3.45-eV band gap allows the HOMO and LUMO levels of the dye to nest inside those of the complex as verified by electrochemistry. This feature simultaneously enables spatial and electronic isolation to decouple the fluorophores from each other and from the cyanostar-anion lattice. Representative dyes from major families of fluorophores, viz, xanthenes, oxazines, styryls, cyanines, and trianguleniums, all crystalize in the characteristic structure and regain their attractive fluorescence. SMILES crystals of rhodamine and cyanine display unsurpassed brightness per volume pointing to uses in demanding applications such as bioimaging. SMILES materials enable predictable fluorophore crystallization to fulfil the promise of optical materials by design.

Anfinsen’s dogma that sequence dictates structure is fundamental to understanding the activity and assembly of proteins. This idea has been applied to all manner of oligomers but not to the behavior of cyclic oligomers, aka macrocycles. We do this here by providing the first proofs that sequence controls the hierarchical assembly of nonbiological macrocycles, in this case, at graphite surfaces. To design macrocycles with one (AAA), two (AAB), or three (ABC) different carbazole units, we needed to subvert the synthetic preferences for one-pot macrocyclizations. We developed a new stepwise synthesis with sequence-defined targets made in 11, 17, and 22 steps with 25, 10, and 5% yields, respectively. The linear build up of primary sequence (1°) also enabled a thermal Huisgen cycloaddition to proceed regioselectively for the first time using geometric control. The resulting macrocycles are planar (2° structure) and form H-bonded dimers (3°) at surfaces. Primary sequences encoded into the suite of tricarb macrocycles were shown by scanning-tunneling microscopy (STM) to impact the next levels of supramolecular ordering (4°) and 2D crystalline polymorphs (5°) at solution-graphite interfaces. STM imaging of an AAB macrocycle revealed the formation of a new gap phase that was inaccessible using only C(3)-symmetric macrocycles. STM imaging of two additional sequence-controlled macrocycles (AAD, ABE) allowed us to identify the factors driving the formation of this new polymorph. This demonstration of how sequence controls the hierarchical patterning of macrocycles raises the importance of stepwise syntheses relative to one-pot macrocyclizations to offer new approaches for greater understanding and control of hierarchical assembly.

Phosphate remediation is important for preventing eutrophication in fresh waters and maintaining water quality. One approach for phosphate removal involves the utilization of molecular receptors. However, our understanding of anion recognition in aqueous solution and at aqueous interfaces is underdeveloped, and the rational design of surface-immobilized receptors is still largely unexplored. Herein, we evaluated the driving forces controlling phosphate binding to elementary amphiphilic receptors anchored at air-water interfaces. We designed biologically inspired receptors with neutral thiourea, positively charged guanidinium, and thiouronium units that all formed Langmuir monolayers. Phosphate binding was quantitatively examined using surface pressure-area isotherms and infrared reflection-absorption spectroscopy (IRRAS). The receptors within this homologous series differ in functional group, charge, and number of alkyl chains to help distinguish the fundamental components influencing anion recognition at aqueous interfaces. The two charged receptors bearing two alkyl chains each displayed strong phosphate affinities and 103- and 101-fold anti-Hofmeister selectivity over chloride, respectively. Neutral thiourea and the single-chain guanidinium receptor did not bind phosphate, revealing the importance of electrostatic interactions and supramolecular organization. Consistently, charge screening at high ionic strength weakens binding. Spectroscopic results confirmed phosphate binding to the double alkyl chain guanidinium receptor, whereas surface pressure isotherm results alone showed a minimal change, thus emphasizing the importance of interfacial spectroscopy. We found that the binding site identity, charged interface created by the electrical double layer, and supramolecular superstructure all affect interfacial binding. These detailed insights into phosphate recognition at aqueous interfaces provide a foundation to develop efficient receptors for phosphate capture.

Tight binding and high selectivity are hallmarks of biomolecular recognition. Achieving these behaviors with synthetic receptors has usually been associated with OH and NH hydrogen bonding. Contrary to this conventional wisdom, we designed a chloride-selective receptor in the form of a cryptand-like cage using only CH hydrogen bonding. Crystallography showed chloride stabilized by six short 2.7-angstrom hydrogen bonds originating from the cage's six 1,2,3-triazoles. Attomolar affinity (10