Fabian Schwer

Designer hoops: By incorporating aromatic units with specific electronic (i.e. donor/acceptor) or structural (i.e. chirality) properties, it is possible to modify cycloparaphenylenes and conjugated hoops according to design. This Review provides an overview of such structurally designed hoops, their structural, optoelectronic, and host properties, and potential (future) applications.

Abstract

In the last 13 years several synthetic strategies were developed that provide access to [n]cycloparaphenylenes ([n]CPPs) and related conjugated nanohoops. A number of potential applications emerged, including optoelectronic devices, and their use as templates for carbon nanomaterials and in supramolecular chemistry. To tune the structural or optoelectronic properties of carbon nanohoops beyond the size-dependent effect known for [n]CPPs, a variety of aromatic rings other than benzene were introduced. In this Review, we provide an overview of the syntheses, properties, and applications of conjugated nanohoops beyond [n]CPPs with intrinsic donor/acceptor structure or such that contain acceptor, donor, heteroaromatic or polycyclic aromatic units within the hoop as well as conjugated nanobelts.

All‐silicon versions of crown ethers were isolated and characterized. These silicon‐based macrocycles are obtained from ²D₂/s‐block metaliodide/gallium(III)‐iodide (²D₂=(Me₄Si₂O)₂) Lewis‐acid systems which enable s‐block metal templated Si−O bond activation and subsequent oligomerization. The coordination ability of these macrocycles is evaluated and a mechanism of formation is suggested by means of DFT calculations.

Abstract

We herein report the synthesis, structures, coordination ability, and mechanism of formation of silicon analogs of crown ethers. An oligomerization of ²D₂ (I) (²D_n,=(Me₄Si₂O)_n) was achieved by the reaction with GaI₃ and MI_x (M=Li, Na, Mg, Ca, Sr). In these reactions the metal cations serve as template and the anions (I⁻/[GaI₄]⁻) are required as nucleophiles. In case of MI_x=LiI, [Li(²D₃)GaI₄] (1) is formed. In case of MI_x=NaI, MgI₂, CaI₂, and SrI₂ the compounds [M(²D₄)(GaI₄)_x] (M=Mg²⁺ (3), Ca²⁺ (4), Sr²⁺ (5) are obtained. Furthermore the proton complex [H(²D₃)][Ga₂I₇] (6) was isolated and structurally characterized. All complexes were characterized by means of multinuclear NMR spectroscopy, DOSY experiments and, except for compound 3, also by single crystal X‐ray diffraction. Quantum chemical calculations were carried out to compare the affinity of M⁺ to ²D_n and other ligands and to shed light on the formation of larger rings from smaller ones.

Starting from fluoren[3]arenes, a new kind of easily available macrocyclic arenes, a powerful approach to the highly efficient synthesis of C(sp³)-bridged [6]cycloparaphenylenes (C[6]CPPs) with aesthetic conjugated belt structures was developed. Moreover, the bridged methylene of C[6]CPPs could be selectively methylated to produce their fully outer-methyl-substituted derivatives in ≥96 % yields.

Abstract

An approach to the highly efficient synthesis of C(sp3)-bridged [6]cycloparaphenylenes (C[6]CPPs) from fluoren[3]arenes (F[3]As) was developed. Consequently, F[3]As as a new kind of macrocyclic arenes were synthesized. Followed by the demethylation, triflation and intramolecular aryl–aryl coupling reactions, C[6]CPPs were then conveniently obtained. Interestingly, C[6]CPPs could be selectively methylated to produce their fully outer-methyl-substituted derivatives. The crystal structures showed the hydroxyl-substituted F[3]As had bowl-shaped conformations, and the C[6]CPPs exhibited rigid belt-shaped structures with deep cavities. Moreover, C[6]CPPs exhibited high HOMO energies and narrow energy gaps. An unclosed belt was further obtained, and it not only showed a similar narrow energy gap to those of the aromatic belts, but also displayed strong fluorescence property, which can play a vital role in the design and synthesis of new aromatic belts.

Nature Chemistry, Published online: 01 March 2021; doi:10.1038/s41557-021-00642-0

Three cycloparaphenylene (CPP) dimers with rigid aromatic linker were synthesized from an efficient cyclocondensation reaction. The structures of those giant molecules are in rapid interconversion between cis and trans conformations. In solution, they emit brightly beyond 600 nm with quantum yield up to 80 %. Having two CPP rings within one molecule, the first 1:2 complex with C₆₀ is constructed.

Abstract

Dimeric cycloparaphenylene (CPP) architectures with well‐defined flipping motion are constructed taking advantage of an efficient cyclocondensation reaction. Variable‐temperature nuclear magnetic resonance (VT‐NMR) analyses and theoretical calculations indicate rapid interconversion of cis and trans conformers at room temperature, while energetically favorable trans conformer exists at low temperature with the metastable cis conformer hidden. The trihexylsilylethynyl‐substituted dimer exhibits bright emission in solution at 616 nm with quantum yield up to 80 %, representing the brightest CPP‐based emitter beyond 600 nm. A 1:2 host–guest complex of the dimer and C₆₀ is established with negative cooperativity, demonstrating the first example of 1:2 complex from CPP derivatives.

Nuclear hyperpolarization is a powerful method that can dramatically enhance the sensitivity of NMR spectroscopy. This technology has been applied to the biomedical molecular imaging of a range of small molecules with stable isotopes and has provided valuable metabolic and physiological information. This review focuses on strategies for the design of molecular probes for hyperpolarized bioimaging.

Abstract

Nuclear hyperpolarization has emerged as a method to dramatically enhance the sensitivity of NMR spectroscopy. By application of this powerful tool, small molecules with stable isotopes have been used for highly sensitive biomedical molecular imaging. The recent development of molecular probes for hyperpolarized in vivo analysis has demonstrated the ability of this technique to provide unique metabolic and physiological information. This review presents a brief introduction of hyperpolarization technology, approaches to the rational design of molecular probes for hyperpolarized analysis, and examples of molecules that have met with success in vitro or in vivo.

Addition of a bipyridyl embedded cycloparaphenylene (CPP) nanohoop to [Fe{H₂B(pyz)₂}] (pyz=pyrazolyl) produces the distorted octahedral complex [Fe(bipy[9]CPP){H₂B(pyz)₂}₂]. Its spin‐crossover (SCO) properties are strongly influenced by a distortion associated with the nanohoop, implying that variations in nanohoop size may provide a way of tuning the SCO properties of iron compounds.

Abstract

Addition of the bipyridyl‐embedded cycloparaphenylene nanohoop bipy[9]CPP to [Fe{H₂B(pyz)₂}] (pyz=pyrazolyl) produces the distorted octahedral complex [Fe(bipy[9]CPP){H₂B(pyz)₂}₂] (1). The molecular structure of 1 shows that the nanohoop ligand contains a non‐planar bipy unit. Magnetic susceptibility measurements indicate spin‐crossover (SCO) behaviour with a T _1/2 of 130 K, lower than that of 160 K observed with the related compound [Fe(bipy){H₂B(pyz)₂}₂] (2), which contains a conventional bipy ligand. A computational study of 1 and 2 reveals that the curvature of the nanohoop leads to the different SCO properties, suggesting that the SCO behaviour of iron(II) can be tuned by varying the size and diameter of the nanohoop.

Organic Materials 2021; 03: 051-059
DOI: 10.1055/s-0041-1722848

Supramolecular interactions between molecules of the same or different nature determine to a great extent the degree of their applicability in many fields of science. To this regard, planar polycyclic aromatic hydrocarbons (PAHs) and their nanometric congeners, nanographenes (NGs), as well as positively curved ones, as for instance corannulene, have been extensively explored. However, negatively curved saddle-shaped NGs have remained a curiosity to date within this field. Therefore, here we communicate the first systematic study on the supramolecular behavior of heptagon-containing hexa-peri-hexabenzocoronene analogues. Thus, their self-association and host–guest complexation processes with both flat and curved PAHs, and fullerenes have been studied by means of 1H and 13C NMR titrations in solution, identifying C70 as one of the guests with the highest association constant among all the ones tested.
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Georg Thieme Verlag KG Rüdigerstraße 14, 70469 Stuttgart, Germany

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The introduction of a 1,2‐azaborine ring into a CPP scaffold enables new chemical reactivity associated with the BN heterocycle that cannot be accomplished with carbonaceous CPP compounds.

Abstract

The first example of a BN‐doped cycloparaphenylene BN‐[10]CPP was synthesized and characterized. Its reactivity and photophysical properties were evaluated in direct comparison to its carbonaceous analogues Mes‐[10]CPP and [10]CPP. While the photophysical properties of BN‐[10]CPP remains similar to its carbonaceous analogues, the electronic structure changes associated with the introduction of a 1,2‐azaborine BN heterocycle into a CPP scaffold enables facile and selective late‐stage functionalizations that cannot be accomplished with carbonaceous CPPs. Specifically, Ir‐catalyzed hydrogenation of BN‐[10]CPP selectively reduces the BN heterocyclic ring, which upon hydrolysis produces a rare example of a macrocyclic paraphenylene 6 incorporating the versatile ketone functionality within the macrocyclic ring.

Inspired by natural rhodopsin, a natural chromophore retinal guest with visible‐light isomerization was introduced into a solid‐state membrane channel. A novel ethyl‐urea‐derived pillar[6]arene (Urea‐P6) host, capable of selectively binding Cl⁻, was self‐assembled with the retinal guest. Based on the visible‐light‐responsiveness of the host–guest system, Cl⁻ transport can be regulated by visible light between ON and OFF states.

Abstract

Inspired by the light‐regulating capabilities of naturally occurring rhodopsin, we have constructed a visible‐light‐regulated Cl⁻‐transport membrane channel based on a supramolecular host–guest interaction. A natural retinal chromophore, capable of a visible‐light response, is used as the guest and grafted into the artificial channel. Upon introduction of an ethyl‐urea‐derived pillar[6]arene (Urea‐P6) host, threading or de‐threading of the retinal and selective bonding of Cl⁻ can be utilized to regulate ion transport. Based on the visible‐light responsiveness of the host–guest interaction, Cl⁻ transport can be regulated by visible light between ON and OFF states. Visible‐light‐regulated Cl⁻ transport as a chemical model permits to understand comparable biological ion‐selective transport behaviors. Furthermore, this result also supplies a smart visible‐light‐responsive Cl⁻ transporter, which may have applications in natural photoelectric conversion and photo‐controlled delivery systems.

By incorporating azobenzene photochromic units in the double functionalization of [60]fullerene, the regiochemistry of the double cyclopropanation reaction could be controlled by using either the E or the Z isomeric forms of the photoswitch. Once covalently linked to the C₆₀ sphere, the azobenzene unit changes its light‐ and thermally‐induced isomerization properties.

Abstract

Multi‐functionalization and isomer‐purity of fullerenes are crucial tasks for the development of their chemistry in various fields. In both current main approaches—tether‐directed covalent functionalization and supramolecular masks—the control of regioselectivity requires multi‐step synthetic procedures to prepare the desired tether or mask. Herein, we describe light‐responsive tethers, containing an azobenzene photoswitch and two malonate groups, in the double cyclopropanation of [60]fullerene. The formation of the bis‐adducts and their spectroscopic and photochemical properties, as well as the effect of azobenzene photoswitching on the regiochemistry of the bis‐addition, have been studied. The behavior of the tethers depends on the geometry of the connection between the photoactive core and the malonate moieties. One tether lead to a strikingly different adduct distribution for the E and Z isomers, indicating that the covalent bis‐functionalization of C₆₀ can be controlled by light.

This work reports the first double π‐extended undecabenzo[7]helicene 1 , which is a large chiral nanographene, composed of 65 fused rings and 186 conjugated carbon atoms. The molecular identity of 1 has been confirmed by single crystal X‐ray diffraction. A wine coloured solution of 1 in dichloromethane absorbs light from ultraviolet to the near infrared, featuring an extremely large molar absorption coefficient of 844000 M ‐1 cm ‐1 at 573 nm. Optically pure 1 shows a record high electronic circular dichroism intensity in the visible spectral range (|Δε| = 1375 M −1 cm −1 at 430 nm) known for any discrete polycyclic aromatic hydrocarbon. These unusual photophysical properties of 1 contrast sharply with those of a mono‐undecabenzo[7]helicene derivative 2 .

Organic Materials 2020; 02: 330-335
DOI: 10.1055/s-0040-1721101

Hydrocarbon nanobelts have recently attracted tremendous interest. Herein, we report our recent progress towards the synthesis of a newly designed hydrocarbon nanobelt tetrabenzo[10]cyclacene, which can be regarded as a sidewall fragment of the (10,0) carbon nanotube. The structures of both key intermediates — “U”-shape diepoxy building block and tetraepoxy nanobelt — were confirmed by single-crystal X-ray diffraction. Our preliminary reductive aromatization reactions revealed that a tetrahydrogenated species instead of tetrabenzo[10]cyclacene was formed during this process. Computational results further revealed that hydrogenation can lead to significant strain release of the backbone structure of tetrabenzo[10]cyclacene, which may attribute to the absence of the target compound during the reductive aromatization.
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Georg Thieme Verlag KG Rüdigerstraße 14, 70469 Stuttgart, Germany

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Azobenzene (azo)‐based macrocycles are highly fascinating in supramolecular chemistry because of their light‐responsiveness. In this work, a series of azo‐based macrocyclic arenes 1 , 2 , 3 and 4 distinguished by the substituted positions of azo groups are rationally designed and synthesized via a fragment cyclization method. From the crystal and computed structures of 1 , 2 and 3 , we observe that the cavity size of these azo‐macrocycles decreases gradually upon E → Z photoisomerization. Moreover, light‐controlled host–guest complexations between azo‐macrocycle 1 and guest molecules (7,7,8,8‐tetracyanoquinodimethane, terephthalonitrile) are successfully achieved. This work provides a simple and effective method to prepare azo‐macrocycles, and the light‐responsive molecular encapsulation systems in this work may further advance the design and applications of novel photo‐responsive host–guest systems.

Organic Materials 2022;
DOI: 10.1055/s-0040-1721082

The preparation of cycloparaphenylenes ([n]CPPs) with their bent π-system poses a long-standing challenge in organic synthesis. In the current multi-step approaches to access CPPs, pre-angulated precursors were combined using transition metal-catalysed or mediated coupling reactions. Therefore, there is a long way to the realisation of the idea of an ‘ideal synthesis’. An easy and efficient synthesis of different [n]CPPs would represent a breakthrough, also pushing their incorporation into organic materials. By combining multiple steps in a one-pot approach, the overall time and workload can be drastically shortened. Herein, we present the application of this concept for the preparation of [6] and [9]CPP as a simple and fast alternative to current methods. By tuning the reaction conditions the selective synthesis of both [6] and [9]CPP was demonstrated.
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Georg Thieme Verlag KG Rüdigerstraße 14, 70469 Stuttgart, Germany

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Mixtures of poor solvents are always not poor. Water and amphiphilic organic solvents (poor solvents), that individually promote the supramolecular polymerization of cationic π‐systems, readily depolymerize them into monomers in their mixtures (good solvent). Segregation of water and organic solvents around the molecular surface of the π‐system is the key for this surprising phenomenon.

Abstract

Solvents are fundamentally essential for the synthesis and processing of soft materials. Supramolecular polymers (SPs), an emerging class of soft materials, are usually stable in single and mixtures of poor solvents. In contrast to these preconceived notions, here we report the depolymerization of SPs in the mixture of two poor solvents. This surprising behavior was observed for well‐known cationic perylene diimides ( c PDIs) in the mixtures of water and amphiphilic organic solvents such as isopropanol (IPA). c PDIs form stable SPs in water and IPA but readily depolymerize into monomers in 50–70 vol% IPA containing water. This is due to the selective solvation of the π‐surface of c PDIs by alkyl chains of IPA and ionic side chains by water, as evidenced by molecular dynamic simulations. Moreover, by systematically changing the ratio between water and amphiphilic organic solvent, we could achieve an unprecedented supramolecular polymerization both by increasing and decreasing the solvent polarity.

The adsorption of two metal–organic coordination capsules on alumina is quantified and the cages are shown to retain their ability to bind and release a molecular cargo following immobilization. The adsorbed cages are then used to stabilize the reagents of a Diels–Alder reaction prior to their subsequent release and reaction.

Abstract

Coordination cages encapsulate a wide variety of guests in the solution state. This ability renders them useful for applications such as catalysis and the sequestration of precious materials. A simple and general method for the immobilization of coordination cages on alumina is reported. Cage loadings are quantified via adsorption isotherms and guest displacement assays demonstrate that the adsorbed cages retain the ability to encapsulate and separate guest and non‐guest molecules. Finally, a system of two cages, adsorbed on to different regions of alumina, stabilizes and separates a pair of Diels–Alder reagents. The addition of a single competitive guest results in the controlled release of the reagents, thus triggering their reaction. This method of coordination cage immobilization on solid phases is envisaged to be applicable to the extensive library of reported cages, enabling new applications based upon selective solid‐phase molecular encapsulation.

The isomeric complexity of a common organic fullerene electron acceptor is resolved, giving ten enantiomeric pairs across six bond types with different chiroptical response. Single‐enantiomer fullerene transistor devices discriminate circularly polarized light with a fast light response and very high photocurrent dissymmetry factor (up to g _ph = 1.2 ± 0.06).

Abstract

Solubilized fullerene derivatives have revolutionized the development of organic photovoltaic devices, acting as excellent electron acceptors. The addition of solubilizing addends to the fullerene cage results in a large number of isomers, which are generally employed as isomeric mixtures. Moreover, a significant number of these isomers are chiral, which further adds to the isomeric complexity. The opportunities presented by single‐isomer, and particularly single‐enantiomer, fullerenes in organic electronic materials and devices are poorly understood however. Here, ten pairs of enantiomers are separated from the 19 structural isomers of bis[60]phenyl‐C61‐butyric acid methyl ester, using them to elucidate important chiroptical relationships and demonstrating their application to a circularly polarized light (CPL)‐detecting device. Larger chiroptical responses are found, occurring through the inherent chirality of the fullerene. When used in a single‐enantiomer organic field‐effect transistor, the potential to discriminate CPL with a fast light response time and with a very high photocurrent dissymmetry factor (g _ph = 1.27 ± 0.06) is demonstrated. This study thus provides key strategies to design fullerenes with large chiroptical responses for use as chiral components of organic electronic devices. It is anticipated that this data will position chiral fullerenes as an exciting material class for the growing field of chiral electronic technologies.