Thermally activated isomeric all-hydrocarbon molecular receptors for fullerene separation

Synergistic regulation of metal-organic cage architectures via temperature- and solvent-driven atropisomerism

Regulating multistimulus responses in artificial systems remains a challenge in smart material development. We present a versatile chemical switching system that precisely controls the self-assembly of metal-organic cages via temperature and solvent changes. The key component, cyclo[2](1,3-(4,6-dimethyl)benzene) (4-pyridine)[6](1,3-(4,6-dimethyl)benzene) (CP2), was generated as three atropisomers (1, 2, and 3) with Cs, C1, and C2v symmetries. Thermally, metastable isomers (1 and 2) convert into the stable isomer (3), which reacts with Pd2+ to form specific molecular cages. Depending on the solvent, either rectangular M2L2 cages (5' and 5) form in 1,4-dioxane or hexagonal M3L3 cages (6) in 1,1',2,2'-tetrachloroethane. The solvent dictates the cage type and enables reversible transformation between cages 5 and 6. Additionally, cage 5', formed from metastable isomer 1, can switch to other cage types (i.e., 5 or 6) depending on temperature and solvent conditions. This multipathway system offers a precise strategy for controlling self-assembly in smart 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, namely 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.

Atroposelective Formal [2 + 5] Macrocyclization Synthesis for a Novel All-Hydrocarbon Cyclo[7] Meta-Benzene Macrocycle

A novel axially chiral all-hydrocarbon cyclo[7] (1,3-(4,6-dimethyl)benzene (CDMB-7) was designed and synthesized using atroposelective[2 + 5] cyclization through Suzuki-Miyaura coupling. CDMB-7 adopts an irregular bowl-like shape with C2 symmetry and exhibits two diastereoisomers in its crystallographic structure. The conformational isomers of CDMB-7 racemates remain stable at high temperatures (393 K). High-performance liquid chromatography (HPLC) confirmed that a single chiral isomer will spontaneously undergo racemization within 30 min at room temperature. This finding opens up possibilities for achieving adaptive chirality in all-hydrocarbon cyclo[7] m-benzene macrocycles.

Recent advances in supramolecular fullerene chemistry

Fullerene chemistry has come a long way since 1990, when the first bulk production of C60 was reported. In the past decade, progress in supramolecular chemistry has opened some remarkable and previously unexpected opportunities regarding the selective (multiple) functionalization of fullerenes and their (self)assembly into larger structures and frameworks. The purpose of this review article is to provide a comprehensive overview of these recent developments. We describe how macrocycles and cages that bind strongly to C60 can be used to block undesired addition patterns and thus allow the selective preparation of single-isomer addition products. We also discuss how the emergence of highly shape-persistent macrocycles has opened opportunities for the study of photoactive fullerene dyads and triads as well as the preparation of mechanically interlocked compounds. The preparation of two- or three-dimensional fullerene materials is another research area that has seen remarkable progress over the past few years. Due to the rapidly decreasing price of C60 and C70, we believe that these achievements will translate into all fields where fullerenes have traditionally (third-generation solar cells) and more recently been applied (catalysis, spintronics).

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) (D_4d-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.

Supramolecular Ensembles Formed via Calix[5]arene-Fullerene Host-Guest Interactions

This minireview introduces the research directions for the synthesis of supramolecular fullerene polymers. First, the discovery of host-guest complexes of pristine fullerenes is briefed. We focus on progress in supramolecular fullerene polymers directed by the use of calix[5]arene-fullerene interactions, which comprise linear, networked, helical arrays of fullerenes in supramolecular ensembles. The unique self-sorting behavior of right-handed and left-handed helical supramolecular fullerene arrays is discussed. Thereafter, an extensive investigation of the calix[5]arene-fullerene interaction for control over the chain structures of covalent polymers is introduced.

Lanthanide-organic pincer hosts with allosteric-controlled metal ion binding specificity

A series of lanthanide-organic pincer hosts were synthesized, which showed allosteric-controlled metal ion binding selectivities due to the lanthanide-induced subtle changes of the central vacant binding site.

Multifunctional Anthracene-Based Ni-MOF with Encapsulated Fullerenes: Polarized Fluorescence Emission and Selective Separation of C70 from C60

We report an anthracene-based Ni-MOF [Ni(II) metal-organic framework, {[Ni(μ₂-L)₂Cl₂]·x(C₆H₆)·y(MeOH)}_n (1), L = anthracene-9,10-diylbis(methylene)diisonicotinate] whose crystal structure reveals the presence of hexagonal channels with a pore size of 1.4 nm that can accommodate guests such as C₆₀ and C₇₀. Both confocal fluorescence and Raman microscopy results are in agreement with a homogeneous distribution of fullerenes throughout the single crystals of 1. Efficient energy transfer from 1 to the fullerenes was observed, which emitted partially polarized fluorescence emission. Stronger binding between 1 and C₇₀ versus C₆₀ was confirmed from HPLC analysis of the dissolved material and provides a basis for the selective retention of C₇₀ in liquid chromatography columns packed with 1.

Macrocyclic host molecules with aromatic building blocks: the state of the art and progress

Macrocyclic host molecules play the central role in host-guest chemistry and supramolecular chemistry. The highly structural symmetry of macrocyclic host molecules can meet people's pursuit of aesthetics in molecular design, and generally means a balance of design, synthesis, properties and applications. For macrocyclic host molecules with highly symmetrical structures, building blocks, which could be described as repeat units as well, are the most fundamental elements for molecular design. The structural features and recognition ability of macrocyclic host molecules are determined by the building blocks and their connection patterns. Using different building blocks, different macrocyclic host molecules could be designed and synthesized. With decades of developments of host-guest chemistry and supramolecular chemistry, diverse macrocyclic host molecules with different building blocks have been designed and synthesized. Aromatic building blocks are a big family among the various building blocks used in constructing macrocyclic host molecules. In this feature article, the recent developments of macrocyclic host molecules with aromatic building blocks were summarized and discussed.

Graphitic supramolecular architectures based on corannulene, fullerene, and beyond

In this Feature Article, we survey the advances made in the field of fulleretic materials over the last five years. Merging the intriguing characteristics of fulleretic molecules with hierarchical materials can lead to enhanced properties of the latter for applications in optoelectronic, biomaterial, and heterogeneous catalysis sectors. As there has been significant growth in the development of fullerene- and corannulene-containing materials, this article will focus on studies performed during the last five years exclusively, and highlight the recent trends in designing fulleretic compounds and understanding their properties, that has enriched the repertoire of carbon-rich functional materials.

On the Many-Body Expansion of an Interaction Energy of Some Supramolecular Halogen-Containing Capsules

A tetramer model was investigated of a remarkably stable iodine-containing supramolecular capsule that was most recently characterized by other authors, who described emergent features of the capsule's formation. In an attempt to address the surprising fact that no strong pair-wise interactions between any of the respective components were experimentally detected in condensed phases, the DFT (density-functional theory) computational model was used to decompose the total stabilization energy as a sum of two-, three- and four-body contributions. This model considers complexes formed between either iodine or bromine and the crucial D4d-symmetric form of octaaryl macrocyclic compound cyclo[8](1,3-(4,6-dimethyl)benzene that is surrounded by arenes of a suitable size, namely, either corannulene or coronene. A significant enthalpic gain associated with the formation of investigated tetramers was revealed. Furthermore, it is shown that the total stabilization of these complexes is dominated by binary interactions. Based on these findings, comments are made regarding the experimentally observed behavior of related multicomponent mixtures.

One-Pot Synthesis of a Cyclic Pyrene Octamer from Two Bifunctionalized Pyrene Monomers

A 2,2′-tert-butyl-5,9-6′,8′-cyclooctameric pyrenylene ([8]CP) was synthesized by a one-pot Suzuki–Miyaura cross-coupling reaction from two kinds of bifunctionalized monomers, as a rare example of a cyclic octamer. The octameric molecular structure of [8]CP was revealed by single-crystal X-ray diffraction analysis.

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 (D_4d-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, ((D_4d-CDMB-8)₃⊃(Cora)₂)⊃I₂, is obtained. This all-hydrocarbon capsule consists of three D_4d-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 D_4d-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.

Time-dependent solid-state molecular motion and colour tuning of host-guest systems by organic solvents

Host-guest complex solid state molecular motion is a critical but underexplored phenomenon. In principle, it can be used to control molecular machines that function in the solid state. Here we describe a solid state system that operates on the basis of complexation between an all-hydrocarbon macrocycle, D_4d-CDMB-8, and perylene. Molecular motion in this solid state machine is induced by exposure to organic solvents or grinding and gives rise to different co-crystalline, mixed crystalline, or amorphous forms. Distinct time-dependent emissive responses are seen for different organic solvents as their respective vapours or when the solid forms are subject to grinding. This temporal feature allows the present D_4d-CDMB-8⊃perylene-based system to be used as a time-dependent, colour-based 4th dimension response element in pattern-based information codes. This work highlights how dynamic control over solid-state host-guest molecular motion may be used to induce a tuneable temporal response and provide materials with information storage capability.

Host-guest solid state molecular motion is a critical but underexplored phenomenon which can be used to control molecular machines that function in the solid state. Here, the authors describe a solid state machine that shows solvent vapour- and mechanically-induced molecular motion that allows access to different crystalline and amorphous forms.

Analytical and preparative separation and isolation of functionalized fullerenes by conventional HPLC stationary phases: method development and column screening

Isolation and purification of functionalized fullerenes from often complex reaction mixtures is challenging due to the hydrophobic nature and low solubility in regular organic solvents. We have developed an HPLC method that efficiently, employing regular reversed phase stationary phases, separates not only C₆₀ from C₇₀ in a model mixture, but also C₆₀ monoadducts from polyadducts and unreacted C₆₀ from fulleropyrrolidine and hydroarylation example reaction mixtures. Six HPLC columns with regular reversed phase stationary phases were evaluated using varying proportions of acetonitrile in toluene as eluent; with C18 and C12 stationary phases with high surface area (450–400 m² g⁻¹) being the most efficient regarding separation efficiency and analysis time for all mixtures. The analytical method is effectively transferrable to a preparative scale to isolate the monoaddition products from complex fullerene reaction mixtures.

Isolation and purification of functionalized fullerenes from often complex reaction mixtures is challenging. Here, a simple and efficient HPLC method is presented.