Synergistic regulation of nonbinary molecular switches by protonation and light.

We report a molecular switching ensemble whose states may be regulated in synergistic fashion by both protonation and photoirradiation. This allows hierarchical control in both a kinetic and thermodynamic sense. These pseudorotaxane-based molecular devices exploit the so-called Texas-sized molecular box (cyclo[2]-(2,6-di(1H-imidazol-1-yl)pyridine)[2](1,4-dimethylenebenzene); 14+, studied as its tetrakis-PF6- salt) as the wheel component. Anions of azobenzene-4,4'-dicarboxylic acid (2H+•2) or 4,4'-stilbenedicarboxylic acid (2H+•3) serve as the threading rod elements. The various forms of 2 and 3 (neutral, monoprotonated, and diprotonated) interact differently with 14+, as do the photoinduced cis or trans forms of these classic photoactive guests. The net result is a multimodal molecular switch that can be regulated in synergistic fashion through protonation/deprotonation and photoirradiation. The degree of guest protonation is the dominating control factor, with light acting as a secondary regulatory stimulus. The present dual input strategy provides a complement to more traditional orthogonal stimulus-based approaches to molecular switching and allows for the creation of nonbinary stimulus-responsive functional materials.

1,3,5-2,4,6-Functionalized Benzene Molecular Cage: An Environmentally Responsive Scaffold that Supports Hierarchical Superstructures.

New stimulus responsive scaffolds are of interest as constituents of hierarchical supramolecular ensembles. 1,3,5-2,4,6-Functionalized, facially segregated benzene moieties have a time-honored role as building blocks for host molecules. However, their user as switchable motifs in the construction of multi-component supramolecular structures remains poorly explored. Here, we report a molecular cage 1, which consists of a bent anthracene dimer 3 paired with 1,3,5-tris(aminomethyl)-2,4,6-triethylbenzene 2. As the result of the pH-induced ababab ↔ bababa isomerization of the constituent functionalized benzene units derived from 2, this cage can reversibly convert between an open state and a closed form, both in solution and in the solid state. Cage 1 was used to create stimuli-responsive hierarchical superstructures, viz. Russian doll-like complexes with [K⊂18-crown-6⊂1]+ and [K⊂cryptand-222⊂1]+. The reversible assembly and disassembly of these superstructures could be induced by switching cage 1 from its open to closed form. The present study thus provides an unusual example where pH-triggered conformation motion within a cage-like scaffold is used to control the formation and disassociation of hierarchical ensembles.

Time-Dependent Solvent-Driven Solid-State Fluorescence-based Numeric Coding.

Controllable solid-state transformations can provide a basis for novel functional materials. Herein, we report a series of solid-state systems that can be readily transformed between amorphous, co-crystalline, and mixed crystalline states via grinding or exposure to solvent vapors. The present solid materials were constructed using an all-hydrocarbon macrocycle, cyclo[8](1,3-(4,6-dimethyl)benzene) (D4d-CDMB-8) (host), and neutral aggregation-caused quenching dyes (guests), including 9,10-dibromoanthracene (1), 1,8-naphtholactam (2), diisobutyl perylene-3,9-dicarboxylate (3), 4,4-difluoro-1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-s-indacene (4), 4,7-di(2-thienyl)-benzo[2,1,3]thiadiazole (5), and 4-imino-3-(pyridin-2-yl)-4H-quinolizine-1-carbonitrile (6). Seven co-crystals and six amorphous materials were obtained via host-guest complexation. Most of these materials displayed turn-on fluorescence emission (up to 20-fold enhancement relative to the corresponding solid-state guests). The interconversion between amorphous, co-crystalline states, and crystalline mixtures could be induced by exposure to solvent vapors or by subjecting to grinding. The transformations could be monitored readily by means of single-crystal and powder X-ray diffraction analyses, as well as solid-state fluorescent emission spectroscopy. The externally induced structural interconversions resulted in time-dependent fluorescence changes. This allowed sets of privileged number array codes to be generated.

Bismacrocycle: Structures and Applications.

In the past half-century, macrocycles with different structures and functions, have played a critical role in supramolecular chemistry. Two macrocyclic moieties can be linked to form bismacrocycle molecules. Compared with monomacrocycle, the unique structures of bismacrocycles led to their specific recognition and assembly properties, also a wide range of applications, including molecular recognition, supramolecular self-assembly, advanced optical material construction, etc. In this review, we focus on the structure of bismacrocycle and their applications. Our goal is to summarize and outline the possible future development directions of bismacrocycle research.

A π-Extended Pentadecabenzo[9]Helicene.

A novel chiral nanographene (i.e. EP9H) with a pentadecabenzo[9]helicene core fragment has been synthesized and fully characterized. Single-crystal X-ray diffraction unambiguously confirms the helical structure. The fluorescence emission of EP9H is located in the near infrared region (λem =684 nm) with a medium quantum yield (0.10) for helicene derivatives. Cyclic voltammetry reveals its seven quasi-reversible redox states from -2 to +5. Furthermore, enantiopure EP9H displays distinct CD signals in a broad spectral range from 300 to 700 nm. Notably, compared to the reported small organic molecules, EP9H displays an outstanding |glum | value of 4.50×10-2 and BCPL as 304 M-1 cm-1 .

Emergent Self-Assembly of a Multicomponent Capsule via Iodine Capture.

Described here is a three-component self-assembly system that displays emergent behavior that differs from that of its constituents. The system comprises an all-hydrocarbon octaaryl macrocycle cyclo[8](1,3-(4,6-dimethyl)benzene (D4d-CDMB-8), corannulene (Cora), and I2. No appreciable interaction is seen between any pair of these three-components, either in cyclohexane or under various crystallization conditions. On the other hand, when all three-components are mixed in cyclohexane and allowed to undergo crystallization, a supramolecular iodine-containing capsule, ((D4d-CDMB-8)3⊃(Cora)2)⊃I2, is obtained. This all-hydrocarbon capsule consists of three D4d-CDMB-8 and two Cora subunits and contains a centrally bound I2 molecule as inferred from single-crystal and powder X-ray diffraction studies as well as solid-state 13C NMR and Raman spectroscopy. These analyses were complemented by solution-phase 1H NMR and UV-vis spectroscopic studies. No evidence of I2 escape from the capsule is seen, even at high temperatures (e.g., up to 418 K). The bound I2 is likewise protected from reaction with alkali or standard reductants in aqueous solution (e.g., saturated NaOH(aq) or aqueous Na2S2O3). It was also found that a mixed powder containing D4d-CDMB-8 and Cora in a 3:2 molar ratio could capture saturated I2 vapor or iodine from aqueous sources (e.g., 1.0 mM I2 in NaCl (35 wt %) or I2 + NaI(aq) (1.0 mM each)). The present system displays structural and functional features that go beyond what would be expected on the basis of a simple sum-of-the-components analysis. As such, it illustrates a new approach to creating self-assembled ensembles with emergent features.

One-Pot Synthesis of 3-Substituted 4H-Quinolizin-4-ones via Alkyne Substrate Control Strategy.

Three-substituted 4H-quinolizin-4-ones were obtained via a facile method with good selectivity and high efficiency. On the basis of alkyne substrate control, the mild and cost-efficient reaction has a broad substrate scope (20 examples, up to 93% yield) and is also easy to scale up. Active sites on the products allow for further modifications. The alkyne substrate control strategy could be further extended to achieve more complex three-substituted 4H-quinolizin-4-one skeletons.

Pb2+-Containing Metal-Organic Rotaxane Frameworks (MORFs).

The metal-organic rotaxane framework (MORF) structures with the advantage of mechanically interlocking molecules (MIMs) have attracted intense interest from the chemical community. In this study, a set of MORFs (i.e., MORF-Pb-1 and MORF-Pb-2) are constructed using Pb2+, a tetraimidazolium macrocycle (Texas-sized molecular box; 14+), and aromatic dicarboxylate (p-phthalate dianions (PTADAs; 2) or 2,6-naphthalene dicarboxylate dianions (3)) via a one-pot three-layer diffusion protocol. In particular, an unusual Pb…Pb weak interaction was shown in MORF-Pb-1 (charactered with distance of 3.656 Å).

"Texas-Sized" Molecular Boxes: From Chemistry to Applications.

The design and synthesis of novel macrocyclic host molecules continues to attract attention because such species play important roles in supramolecular chemistry. However, the discovery of new classes of macrocycles presents a considerable challenge due to the need to embody by design effective molecular recognition features, as well as ideally the development of synthetic routes that permit further functionalization. In 2010, we reported a new class of macrocyclic hosts: a set of tetracationic imidazolium macrocycles, which we termed "Texas-sized" molecular boxes (TxSBs) in homage to Stoddart's classic "blue box" (CBPQT4+). Compared with the rigid blue box, the first generation TxSB displayed considerably greater conformational flexibility and a relatively large central cavity, making it a good host for a variety of electron-rich guests. In this review, we provide a comprehensive summary of TxSB chemistry, detailing our recent progress in the area of anion-responsive supramolecular self-assembly and applications of the underlying chemistry to water purification, information storage, and controlled drug release. Our objective is to provide not only a review of the fundamental findings, but also to outline future research directions where TxSBs and their constructs may have a role to play.

Regulating the Structures of Self-Assembled Mechanically Interlocked Moleculecular Constructs via Dianion Precursor Substituent Effects.

Substituent effects play critical roles in both modulating reaction chemistry and supramolecular self-assembly processes. Using substituted terephthalate dianions (p-phthalic acid dianions; PTADAs), the effect of varying the type, number, and position of the substituents was explored in terms of their ability to regulate the inherent anion complexation features of a tetracationic macrocycle, cyclo[2](2,6-di(1H-imidazol-1-yl)pyridine)[2](1,4-dimethylenebenzene) (referred to as the Texas-sized molecular box; 14+), in the form of its tetrakis-PF6- salt in DMSO. Several of the tested substituents, including 2-OH, 2,5-di(OH), 2,5-di(NH2), 2,5-di(Me), 2,5-di(Cl), 2,5-di(Br), and 2,5-di(I), were found to promote pseudorotaxane formation in contrast to what was seen for the parent PTADA system. Other derivatives of PTADA, including those with 2,3-di(OH), 2,6-di(OH), 2,5-di(OMe), 2,3,5,6-tetra(Cl), and 2,3,5,6-tetra(F) substituents, led only to so-called outside binding, where the anion interacts with 14+ on the outside of the macrocyclic cavity. The differing binding modes produced by the choice of PTADA derivative were found to regulate further supramolecular self-assembly when the reaction components included additional metal cations (M). Depending on the specific choice of PTADA derivatives and metal cations (M = Co2+, Ni2+, Zn2+, Cd2+, Gd3+, Nd3+, Eu3+, Sm3+, Tb3+), constructs involving one-dimensional polyrotaxanes, outside-type rotaxanated supramolecular organic frameworks (RSOFs), or two-dimensional metal-organic rotaxane frameworks (MORFs) could be stabilized. The presence and nature of the substituent were found to dictate which specific higher order self-assembled structure was obtained using a given cation. In the specific case of the 2,5-di(OH), 2,5-di(Cl), and 2,5-di(Br) PTADA derivatives and Eu3+, so-called MORFs with distinct fluorescence emission properties could be produced. The present work serves to illustrate how small changes in guest substitution patterns may be used to control structure well beyond the first interaction sphere.

Highly Selective Binding and Inhibition of Pyr-His-Pro-NH2 (TRH) Function using a Polypyridinyl Macrocyclic Receptor with an Amphiphilic Cavity.

Macrocycle, cyclo[4] [(1,3-(4,6)-dimethylbezene)[4](2,6-(3,5)-dimethylpyridine (B4P4), shows highly selective binding affinity with protirelin (Pyr-His-Pro-NH2 ; TRH) among the tested 26 drug or drug adductive substrates. The stable complexation in a 1:1 manner was fully characterized in solution, gas phase, and solid state study. Furthermore, B4P4 acts as an efficient TRH inhibitor even at [macrocycle]:[drug] <1:300, both in membrane transport and cellar incubation. The current work provides an unprecedented strategy for macrocycles to be efficiently used in drug target therapy.

Excimer Disaggregation Enhanced Emission: A Fluorescence "Turn-On" Approach to Oxoanion Recognition.

A new approach to anion sensing that involves excimer disaggregation induced emission (EDIE) is reported. It involves the anion-mediated disaggregation of the excimer formed from a cationic macrocycle. This leads to an increase in the observed fluorescence intensity. The macrocycle in question, cyclo[1] N2, N6-dimethyl- N2, N6-bis(6-(1 H-imidazolium-1-yl)pyridin-2-yl)pyridine-2,6-diamine[1]1,4-dimethylbenzene (12+; prepared as its PF6- salt), is obtained in ca. 70% yield via a simple cyclization. X-ray diffraction analyses of single crystals revealed that, as prepared, this macrocycle exists in a supramolecular polymeric form in the solid state. Macrocycle 12+ is weakly fluorescent in acetonitrile. The emission intensity is concentration dependent, with the maximum intensity being observed at [12+] ≈ 0.020 mM. This finding is ascribed to formation of an excimer, followed possibly by higher order aggregates as the concentration of 12+ is increased. Addition of tetrabutylammonium pyrophosphate (HP2O73-) to 12+ (0.020 mM in acetonitrile) produces a ca. 200-fold enhancement in the emission intensity (λex = 334 nm; λem = 390-650 nm). These findings are rationalized in terms of the HP2O73- serving to break up essentially non-fluorescent excited-state dimers of 12+ through formation of a highly fluorescent anion-bound monomeric complex, 12+·HP2O73-. A turn-on in the fluorescence intensity is also seen for H2PO4- and, to a lesser extent, HCO3-. Little (HSO4-, NO3-) or essentially no (N3-, SCN-, F-, Cl-, Br- and I-) response is seen for other anions. Solid-state structural analysis of single crystals obtained after treating 12+ with HP2O73- in the presence of water revealed a salt form wherein a H2P2O72- anion sits above the cone-like macrocycle.

From CO2 to 4 H-Quinolizin-4-ones: A One-Pot Multicomponent Approach via Ag2O/Cs2CO3 Orthogonal Tandem Catalysis.

Using carbon dioxide as a C1 precursor, here we report relatively simple and cost-effective orthogonal tandem catalysis, namely Ag2O in conjunction with Cs2CO3 serves to promote a multicomponent tandem reaction forming two new C-C and one new C-N bonds. 4 H-Quinolizin-4-ones, key skeletal components in a variety of biologically active molecules, were obtained with yields up to 99%. The present approach features a broad substrate scope and mild reaction conditions and benefits from using cost-effective reaction and catalysts.

Multicomponent Self-Assembled Metal-Organic [3]Rotaxanes.

A set of environmentally responsive metal-organic [3]rotaxanes is described. These mechanically interlocked macromolecules may be prepared in quantitative yield via a one-pot procedure involving treatment of a flexible tetracationic macrocycle, known as the Texas-sized molecular box, with tri-1,3,5-benzenetricarboxylate anion and silver cations (Ag(+)). The use of this three-component mixture gives rise to a metal-organic [3]rotaxane via a self-assembly process that occurs under ambient conditions in DMSO-d6 solution. The complex is stable in the presence of excess TFA. However, disassembly of the [3]rotaxane to produce anion-box associated entities may be triggered by adding a competitive counteranionic species (e.g., I(-)). Adding excess Ag(+) serves to reverse this decomplexation process. The nature of the [3]rotaxane complex could be fine-tuned via application of an external stimulus. Increasing the temperature or adding small molecules (e.g., D2O, methanol-d4, acetonitrile-d3, DMF-d7, acetone-d6, or THF-d8) to the initial DMSO-d6 solution induces conformational flipping of the macrocycle within the overall complex (e.g., from limiting chair to chairlike forms). Support for the molecular stimuli responsive nature of the various structures came from solution-phase one- and two-dimensional ((1)H, 1D and 2D NOESY) NMR spectroscopic studies carried out in DMSO-d6. The core metal-linked rotaxane unit was characterized via single-crystal X-ray diffraction analysis. Initial evidence that the present self-assembly process is not limited to the use of the Ag(+) cation came from studies involving Cd(2+); this replacement results in formation of 2D metal-organic rotaxane-containing frameworks (MORFs).

Chemically Mediated Artificial Electron Transport Chain.

Electron transport chains (ETCs) are ubiquitous in nearly all living systems. Replicating the complexity and control inherent in these multicomponent systems using ensembles of small molecules opens up promising avenues for molecular therapeutics, catalyst design, and the development of innovative energy conversion and storage systems. Here, we present a noncovalent, multistep artificial electron transport chains comprising cyclo[8]pyrrole (1), a meso-aryl hexaphyrin(1.0.1.0.1.0) (naphthorosarin 2), and the small molecules I2 and trifluoroacetic acid (TFA). Specifically, we show that 1) electron transfer occurs from 1 to give I3 - upon the addition of I2, 2) proton-coupled electron transfer (PCET) from 1 to give H 3 2 •2+ and H 3 2 + upon the addition of TFA to a dichloromethane mixture of 1 and 2, and 3) that further, stepwise treatment of 1 and 2 with I2 and TFA promotes electron transport from 1 to give first I3 - and then H 3 2 •2+ and H 3 2 + . The present findings are substantiated through UV-vis-NIR, 1H NMR, electron paramagnetic resonance (EPR) spectroscopic analyses, cyclic voltammetry studies, and DFT calculations. Single-crystal structure analyses were used to characterize compounds in varying redox states.

"Texas-sized" molecular boxes: building blocks for the construction of anion-induced supramolecular species via self-assembly.

It was previously established that the flexible tetraimidazolium macrocycle cyclo[2](2,6-bis(1H-imidazol-1-yl)pyridine)[2](1,4-dimethylenebenzene) (1(4+)) is capable of stabilizing higher-order supramolecular structures via both anion and cation recognition. Described herein is a set of structurally related imidazolium macrocycles (2(4+)-4(4+)) that contain modified central cores. The flexible nature of these new constructs is highlighted by the isolation of several independent crystalline forms for the same basic structure. Each of the individual receptors was found to bind the 2,6-naphthalenedicarboxylate dianion and to stabilize the formation of self-associated structures. The observed binding modes and resulting supramolecular organizational forms were found to differ dramatically depending on the nature of the bridging group present in the imidazolium macrocycle. This finding was established by solution studies involving, inter alia, one- and two-dimensional ((1)H, (1)H-(1)H COSY, DOSY, and NOESY) NMR spectroscopy as well as electrospray ionization mass spectrometry. The new systems in this report serve to expand the available "tool box" for the construction of complex self-assembled materials while providing insights into the determinants that regulate the formation of specific supramolecular structures from flexible receptors capable of adopting multiple stable conformations.

The "Texas-sized" molecular box: a versatile building block for the construction of anion-directed mechanically interlocked structures.

Over the last two decades, researchers have focused on the synthesis and development of mechanically interlocked molecules (MIMs). The intramolecular motion of mechanical bonds and the ability to induce this effect with the choice of the proper external stimuli has prompted the development of macromolecular systems that possess the ability to "perform work" at the molecular level. Currently, researchers are working to incorporate interlocked species into complex structural systems, such as molecular frameworks and nanoparticles, and to create ever more elegant noncovalent architectures. This effort provides an incentive to generate new building blocks for the construction of MIMs. In this Account, we describe progress in the development of a new cationic building block inspired by the "blue box" of Stoddart and collaborators. The blue box (cylcobis(paraquat-p-phenylene) or CBPQT(4+)) is a tetracationic, electron-deficient macrocycle widely recognized for its role in the construction of MIMs. This venerable receptor displays a high affinity for a variety of π-donor guests, and researchers have used them to construct a wide range of molecular and supramolecular structures, including rotaxanes, catenanes, pseudorotaxanes, polypseudorotaxanes, pseudo[n]polyrotaxanes, and electrochemically switchable molecules. To date, several synthetic analogues of the basic CBPQT(4+) structure have been reported, including systems containing biphenylene linkers and chiral tetracationic cyclophanes. However, researchers have not yet fully generalized the promise of the blue box. In this Account, we chronicle the development of a larger, more flexible tetracationic macrocycle, referred to as the "Texas-sized" molecular box. To highlight its relatively increased size and to distinguish it from CBPQT(4+), we have chosen to color this new receptor burnt orange. The Texas-sized box (cyclo[2](2,6-di(1H-imidazol-1-yl)pyridine)[2](1,4-dimethylenebenzene), 1(4+)·4PF(6)(-)) acts as a dynamic molecular receptor that displays an ability to adjust its shape and conformation to accommodate anionic guests of different size and charge within its central core. The use of different guests can favor different binding modes and promote the formation of different macromolecular aggregates. Furthermore, the proper selection of the guest allows for the "turning on" or "turning off" of molecular threading and can be used to produce new kinds of threaded species. This dynamic behavior is a special feature of the Texas-sized molecular box, as is its ability to stabilize a range of polypseudorotaxanes, rotaxane-containing metal-organic frameworks (MORFs), and rotaxane-based supramolecular organic frameworks (RSOFs).

Thermally-induced atropisomerism promotes metal-organic cage construction.

Molecular folding regulation with environmental stimuli is critical in living and artificial molecular machine systems. Herein, we described a macrocycle, cyclo[4] (1,3-(4,6-dimethyl)benzene)[4](1,3-(4,6-dimethyl)benzene)(4-pyridine). Under 298 K, it has three stable stiff atropisomers with names as 1 (Cs symmetry), 2 (Cs symmetry), and 3 (C4v symmetry). At 393 K, 1 can reversibly transform into 2, but at 473 K, it can irrevocably transform into 3. At 338 K, 3 and (PhCN)2PdCl2 complex to produce the metal-organic cage 4. Only at 338 K does the combination of 1 or 2 and (PhCN)2PdCl2 create a gel-like structure. Heating both gels to 473 K transforms them into 4. In addition to offering a thermally accelerated method for modifying self-assembled systems using macrocyclic building blocks, this study also has the potential to develop the nanoscale transformation material with a thermal response.

Cyclo[m]pyridine[n]pyrroles: hybrid macrocycles that display expanded π-conjugation upon protonation.

Novel hybrid cyclo[m]pyridine[n]pyrroles have been synthesized using Suzuki coupling. Their NMR and optical spectroscopic features and solid state structural parameters provide support for the proposal that these species are best described as locally aromatic compounds devoid of long-range intersubunit conjugation. However, an extension of the π-conjugation in the macrocycles can be realized through protonation, as inferred from optical spectroscopic and X-ray diffraction-based structural studies.

Neutral and anionic guests and their effect on the formation of pseudorotaxanes by using a flexible tetracationic imidazolium macrocycle.

The ability to control and direct molecular assembly has important implications in the design of environmentally responsive materials. Reported here is the use of competitive neutral- and anionic-guest recognition to control the formation, disruption, replacement-based construction and higher-order assembly properties of pseudorotaxane structures involving a large, cationic tetraimidazolium receptor. In particular, we showed that the chloride anion (as the tetrabutylammonium (TBA(+)) salt) serves to replace directly the 2,6-naphthalene dicarboxylate dianion from the preformed complex, involving this dianion. In contrast, the addition of the nitrate anion (as the TBA(+) salt) serves to effect displacement of a pre-bound 2,6-naphthalene dicarboxylate dianion in a stepwise manner allowing for stabilization of a so-called "outside"-binding mode under appropriate conditions. We have also found that by using biphenyl-3,4,3',4'-tetraamine as the guest, a 1D-donor-acceptor-donor coordination polymer can be stabilized, whereas the addition of 6-amino-naphthalene-2-sulfonate anion to the pre-formed complex between the tetraimidazolium receptor and the 2,6-naphthalene dicarboxylate dianion produces a new pseudorotaxane complex. This guest-based competition and subsequent molecular translocation is supported by solution-state NMR spectroscopic studies as well as solid-state single-crystal X-ray structural analyses. The results described herein provide initial evidence that guest competition can be used to control molecular "switching" and substrate binding within an appropriately designed anion receptor.