Max Martin

Hexabenzocoronenes (HBCs), functionalized with up to six porphyrins, were prepared and studied. UV/Vis absorption spectroscopy showed variations in the spectral characteristics depending on the number of porphyrins as well as their respective substitution pattern. Insight into solid‐state behavior was obtained by X‐ray crystal structures.

Abstract

Porphyrin–hexabenzocoronene architectures serve as good model compounds to study light‐harvesting systems. Herein, the synthesis of porphyrin functionalized hexa‐peri‐hexabenzocoronenes (HBCs), in which one or more porphyrins are covalently linked to a central HBC core, is presented. A series of hexaphenylbenzenes (HPBs) was prepared and reacted under oxidative coupling conditions. The transformation to the respective HBC derivatives worked well with mono‐ and tri‐porphyrin‐substituted HPBs. However, if more porphyrins are attached to the HPB core, Scholl oxidations are hampered or completely suppressed. Hence, a change of the synthetic strategy was necessary to first preform the HBC core, followed by the introduction of the porphyrins. All products were fully characterized, including, if possible, single‐crystal XRD. UV/Vis absorption spectra of porphyrin‐HBCs showed, depending on the number of porphyrins as well as with respect to the substitution pattern, variations in their spectral features with strong distortions of the porphyrins’ B‐band.

Nature, Published online: 15 August 2019; doi:10.1038/d41586-019-02473-z

Nature Communications, Published online: 08 August 2019; doi:10.1038/s41467-019-11467-4

Snowball effect: The rolling‐up of nanographenes by cyclodehydrofluorination on alumina resembles a snowball rolling down a slope. As the snowball accelerates and becomes bigger after each turn, the polycyclic aromatic hydrocarbon (PAH) core becomes more prone to undergo indenoannulation as its size grows after each cyclization. In contrast to the vast majority of reactions dealing with strain energies inherent to synthesis of non‐alternant PAHs, alumina‐mediated C−F bond activation turns out to be more sensitive to electronic effects rather than steric. This fact opens up new possibilities for effective synthesis of non‐alternant nanographenes. The artwork was created by the artist Polina Amsharov specially for this publication (material: acryl, software: Pixelmator). More information can be found in the Communication by K. Amsharov et al. on https://doi.org/10.1002/chem.201902586page 11609.

Cards on the table: Cyclopenta[hi]aceanthrylenes (CPAs) functionalized at two of the peripheral positions with trimethylsilylethynyl, as well as with electron‐donating 4‐ethynyl‐N,N‐dimethylaniline, ethynyl Zn^II phthalocyanine, and ethynyl Zn^II porphyrin units have been prepared and investigated by means of photophysical, electrochemical, and spectro‐electrochemical techniques.

Abstract

Cyclopenta[hi]aceanthrylenes (CPAs) have been functionalized at two of the peripheral positions with electronically inert trimethylsilylethynyl (1), as well as with electron‐donating 4‐ethynyl‐N,N‐dimethylaniline (2), ethynyl Zn^IIphthalocyanine (3), and ethynyl Zn^IIporphyrin (4) units. Consistent with X‐ray crystal structures of 2 and 4, analyses of absorption and fluorescence of 2–4 point to strong electronic communication between the CPA and the peripheral units, affording quadrupolar electron donor‐acceptor‐donor charge‐transfer conjugates. By virtue of their quadrupolar/dipolar charge‐transfer characters in the excited state, 2–4 exhibit fluoro‐solvatochromism. Transient absorption spectroscopy confirmed delocalized quadrupolar ground states and formation of weakly solvent stabilized quadrupolar singlet excited states. The latter transform into strongly stabilized dipolar excited states before deactivating to the ground state in 2 and give rise to a fully charge separated state in 3 and 4.

Coupling up: The dehydrogenative coupling of arenes is an ultimate example of C−H activation. This Review highlights the most exciting examples of the Scholl reaction, ranging from the synthesis of biphenols through natural products to nanographenes and on‐surface dehydrogenation reported since 2013.

Abstract

Oxidative aromatic coupling occupies a fundamental place in the modern chemistry of aromatic compounds. It is a method of choice for the assembly of large and bewildering architectures. Considerable effort was also devoted to applications of the Scholl reaction for the synthesis of chiral biphenols and natural products. The ability to form biaryl linkages without any prefunctionalization provides an efficient pathway to many complex structures. Although the chemistry of this process is only now becoming fully understood, this reaction continues to both fascinate and challenge researchers. This is especially true for heterocoupling, that is, oxidative aromatic coupling with the chemoselective formation of a C−C bond between two different arenes. Analysis of the progress achieved in this field since 2013 reveals that many groups have contributed by pushing the boundary of structural possibilities, expanding into surface‐assisted (cyclo)dehydrogenation, and developing new reagents.

Not to be underestimated: A tetrameric pentacene (see structure) was designed as a model system to explore the influence of exciton delocalization on singlet fission (SF) and subsequent triplet decorrelation by exciton diffusion following intramolecular SF. Experiment and theory showed the delocalization of singlet and triplet excitons among the four pentacenes and highlighted spin catalysis as a substantial obstacle to quantitative harvesting of SF products.

Abstract

A tetrameric pentacene, PT, has been used to explore the effects of exciton delocalization on singlet fission (SF). For the first time, triplet decorrelation through intramolecular triplet diffusion was observed following SF. Transient absorption spectroscopy was used to examine different decorrelation mechanisms (triplet diffusion versus structural changes) for PT and its dimeric equivalent PD on the basis of the rate and activation barrier of the decorrelation step. Charge‐separation experiments using tetracyano‐p‐quinodimethane (TCNQ) to quench triplet excitons formed through SF demonstrate that enhanced intersystem crossing, that is, spin catalysis, is a widely underestimated obstacle to quantitative harvesting of the SF products. The importance of spatial separation of the decorrelated triplet states is emphasized, and independent proof that the decorrelated triplet pair state consists of two (T₁) states per molecule is provided.

The generation of topologically complex nanocarbons can spur developments in science and technology. However, conventional synthetic routes to interlocked molecules require heteroatoms. We report the synthesis of catenanes and a molecular trefoil knot consisting solely of para-connected benzene rings. Characteristic fluorescence of a heterocatenane associated with fast energy transfer between two rings was observed, and the topological chirality of the all-benzene knot was confirmed by enantiomer separation and circular dichroism spectroscopy. The seemingly rigid all-benzene knot has rapid vortex-like motion in solution even at –95°C, resulting in averaged nuclear magnetic resonance signals for all hydrogen atoms. This interesting dynamic behavior of the knot was theoretically predicted and could stimulate deeper understanding and applications of these previously untapped classes of topological molecular nanocarbons.

Thermogravimetric and Raman analysis of aryl‐functionalized SWCNTs via diazonium compounds allows for precise quantification of product and side products, even physisorbed ones. Logarithmic concentration‐dependence of the diazonium reactant enables precise adjustment of the degree of functionalization up to 1.0 %.

Abstract

We present an in‐depth qualitative and quantitative analysis of a reaction between 4‐iodobenzenediazonium tetrafluoroborate and single‐walled carbon nanotubes (SWCNTs) via thermogravimetric analysis coupled with mass spectrometry (TG‐MS) or a gas chromatography and mass spectrometry (TG‐GC‐MS) as well as Raman spectroscopy. We propose a method for precise determination of the degree of functionalization and quantification of physisorbed aromates, detaching around their boiling point, alongside covalently bonded ones (cleavage over 200 °C). While the presence of some side products like phenol‐ or biphenyl species could be excluded, residual surfactant and minor amounts of benzene could be identified. A concentration‐dependent experiment shows that the degree of functionalization increases with the logarithm of the concentration of applied diazonium salt, which can be exploited to precisely adjust the amount of aryl addends on the nanotube sidewall, up to 1 moiety per 100 carbon atoms.

Large nonalternant PAHs can be easily accessed by this alumina‐mediated C−F bond activation protocol. The unique nature of the reaction leads to a rather counter‐intuitive outcome, since this strategy works better with the increase of the size system, which is typically vice versa in case of other conventional methods.

Abstract

Large polycyclic aromatic hydrocarbons (PAHs) containing pentagons represent an important class of compounds that are considered to be superior materials in future nano‐electronic applications. From this perspective, the development of synthetic approaches to large PAHs and nanographenes (NGs) is a matter of great importance. In this context indenoannulation appears to be the most practical way to introduce pentagons into NGs. Here we report that alumina‐mediated C−F bond activation is an attractive tool for the synthesis of non‐alternant NGs bearing several pentagons. The unique nature of the reaction leads to a rather counter‐intuitive outcome and allows considering each previous aryl–aryl coupling as a promoter of the following one, despite the continuous increase in the strain energy. Thus, the presented strategy combines both facile synthesis and significant yields for large nonalternant PAHs and NGs.

Nature, Published online: 26 June 2019; doi:10.1038/s41586-019-1331-z

A first series of examples for confined space interactions of electron‐rich and electron‐poor molecules organized in an internal corona of shell‐by‐shell (SbS)‐structured Al2O3‐NP‐hybrids is reported. The assembly concept of the corresponding hierarchical architectures relies on both, covalent grafting of phosphonic acids on the NPs surface (SAMs formation), and exohedral interdigitation of orthogonal amphiphiles as second ligand layer driven by solvophobic interactions. The electronic communication between the chromophores of different electron demand, such as pyrenes, PDIs (with and without pyridinium‐bromide headgroups) and fullerenes was promoted at the layer interface. We have demonstrated that the efficient construction principle of the bilayer hybrids assembled around the electronically “innocent” Al2O3 core is robust enough to achieve control over electronic communication between electron‐donors and –acceptors in the interlayer region. The electronic interactions between the electron‐accepting and electron‐donating moieties approaching each other at the layer interface were monitored by fluorescence measurements.

Since 1996, a growing number of strained macrocycles, comprising only sp2‐ or sp‐hybridized carbon atoms within the ring, have become synthetically accessible, with the [n]cycloparaphenyleneacetylenes (CPPAs) and the [n]cycloparaphenylenes (CPPs) being the most prominent examples. Now that robust and relatively general synthetic routes toward a diverse range of nanohoop structures are available, the research focus is beginning to shift towards the exploration of their properties and applications. From a perspective of supramolecular chemistry, these macrocycles offer unique opportunities due to their near‐perfect circular shape, the unusually high degree of shape‐persistence and the presence of both convex and concave π‐faces. In this minireview, we give an overview on the use of strained carbon‐rich nanohoops in host‐guest chemistry, the preparation of mechanically interlocked architectures and crystal engineering.

Clear communication: Hexabenzocoronene was utilized as a model system for nanographenes and probed spectroscopically through peripherally attached porphyrins that served as quasi optical electrodes. Nanographenes functionalized selectively in the ortho‐, meta‐, and para‐positions showed differences in the electronic communication between the porphyrins, which is reflected by a distortion of the B‐band.

Abstract

Single‐molecule electronic components (SMECs) are envisioned as next‐generation building blocks in quantum circuit systems. However, challenges such as the reproducibility of the electrode attachment to the individual molecules hamper their fundamental investigation. For our purpose, we introduced quasi optoelectronic electrodes (QOEs) that allow for rapid investigations of the properties and suitability of compounds for molecular electronic devices. In particular, we probed hexa‐peri‐hexabenzocoronene (HBC) as a model system for D _6h ‐symmetrical nanographenes, with porphyrins as QOEs attached to the periphery. We prepared selectively bis‐porphyrin‐functionalized HBCs with ortho‐, meta‐ and para‐substitution and studied their communication properties, in correlation to the geometrical alignment and size of the system, by electrochemistry and optical spectroscopy. Further insights into structure–property relationships were gained by DFT calculations and X‐ray diffraction analysis.

Curvy rings: Recent progress in strained nanohoops as well as their host–guest chemistry, mechanical interlocking, and self‐assembly is summarized in this Minireview.

Abstract

Since 1996, a growing number of strained macrocycles, comprising only sp²‐ or sp‐hybridized carbon atoms within the ring, have become synthetically accessible, with the [n]cycloparaphenyleneacetylenes (CPPAs) and the [n]cycloparaphenylenes (CPPs) being the most prominent examples. Now that robust and relatively general synthetic routes toward a diverse range of nanohoop structures have become available, the research focus is beginning to shift towards the exploration of their properties and applications. From a supramolecular chemistry perspective, these macrocycles offer unique opportunities as a result of their near‐perfect circular shape, the unusually high degree of shape‐persistence, and the presence of both convex and concave π‐faces. In this Minireview, we give an overview on the use of strained carbon‐rich nanohoops in host–guest chemistry, the preparation of mechanically interlocked architectures, and crystal engineering.

Donor–acceptor systems: Study of electronic communication in a series of meso–meso ethene‐bridged unsymmetrical diporphyrins is reported. Detailed physicochemical investigation revealed the impact of open‐shell nickel(II) on the electronic communication in ethene‐bridged heterobimetallic diporphyrins (see figure).

Abstract

At the focal point of this work is the photophysical characterization of three meso–meso ethene‐bridged diporphyrins featuring a diverse metallation pattern. Detailed investigations by means of cyclic voltammetry, absorption, fluorescence, and femto‐/nanosecond transient absorption spectroscopy revealed the impact of open‐shell nickel(II) on the electronic communication in ethene‐bridged heterobimetallic diporphyrins.

The first synthesis of tetrabenzoporphyrin phosphorus(V) complexes has been developed. All phosphorus(V) compounds showed the Soret and Q bands beyond 450 and 700 nm, respectively. The effects of fused benzo‐groups and peripheral substituted phenyl groups were rationalized by theoretical calculations.

A series of free‐base and phosphorus(V) complexes of tetrabenzoporphyrin (TBP) and some phosphorus(V) porphyrins containing four or eight phenyl groups on the periphery have been synthesized and characterized by X‐ray crystallography, electronic absorption, and magnetic circular dichroism (MCD) spectroscopy, together with quantum chemical calculations. All phosphorus TBP and meso‐tetraphenyl porphyrin complexes adapted ruffled conformations due to the small P(V) ion, although the Zn(II)TBPs containing eight phenyl (Φ) groups at the so‐called β and α positions showed planar and saddled structures in the solid state, due to, respectively, marginal or severe steric hindrance between the neighboring Φ groups. All P(V)TBPs showed the Soret and Q bands beyond 450 and 700 nm, respectively, which are some of the longest wavelengths exhibited by metallated TBPs reported to date. Of the eight Φ group‐substituted P(V)TBPs, those substituted at α positions always showed absorption bands at longer wavelengths than those at β positions, which was reproduced by calculated absorption spectra. The Soret band positions of meso‐tetraphenylated P(V) species without fused benzo‐groups (ca. 430–440 nm) were also some of the longest among metalloporphyrins of this type.

Clear communication: Hexabenzocoronene was utilized as a model system for nanographenes and probed spectroscopically through peripherally attached porphyrins that served as quasi optical electrodes. Nanographenes functionalized selectively in the ortho‐, meta‐, and para‐positions showed differences in the electronic communication between the porphyrins, which is reflected by a distortion of the B‐band.

Abstract

Single‐molecule electronic components (SMECs) are envisioned as next‐generation building blocks in quantum circuit systems. However, challenges such as the reproducibility of the electrode attachment to the individual molecules hamper their fundamental investigation. For our purpose, we introduced quasi optoelectronic electrodes (QOEs) that allow for rapid investigations of the properties and suitability of compounds for molecular electronic devices. In particular, we probed hexa‐peri‐hexabenzocoronene (HBC) as a model system for D _6h ‐symmetrical nanographenes, with porphyrins as QOEs attached to the periphery. We prepared selectively bis‐porphyrin‐functionalized HBCs with ortho‐, meta‐ and para‐substitution and studied their communication properties, in correlation to the geometrical alignment and size of the system, by electrochemistry and optical spectroscopy. Further insights into structure–property relationships were gained by DFT calculations and X‐ray diffraction analysis.

Five C−C bonds in one step! The feasibility of the alumina‐mediated C−F bond activation approach for multi‐assembly is demonstrated on the example of a fundamental bowl‐shaped polycyclic aromatic hydrocarbon (diindenochrysene) through the formation of all five C−C bonds in a single synthetic step. Insights into the reaction mechanism and the design of the precursors are reported.

Abstract

The unique nature of the alumina‐mediated cyclodehydrofluorination gives the opportunity to execute the preprogrammed algorithm of the C−C couplings rationally built into a precursor. Such multi‐assemblies facilitate the construction of the carbon‐skeleton, superseding the conventional step‐by‐step by the one‐pot intramolecular assembly. In this work, the feasibility of the alumina‐mediated C−F bond activation approach for multi‐assembly is demonstrated on the example of a fundamental bowl‐shaped polycyclic aromatic hydrocarbon (diindenochrysene) through the formation of all “missing” C−C bonds at the last step. Beside valuable insights into the reaction mechanism and the design of the precursors, a facile pathway enabling the two‐step synthesis of diindenochrysene was elaborated, in which five C−C bonds form in a single synthetic step. It is shown that the relative positions of fluorine atoms play a crucial role in the outcome of the assembly and that governing the substituent positions enables the design of effective precursor molecules “programmed” for the consecutive C−C bond formations. In general, these findings push the state of the field towards the facile synthesis of sophisticated bowl‐shaped carbon‐based nanostructures through multi‐assembly of fluoroarenes.

The coupling of two or more molecules of dinitrogen (N₂) occurs naturally under the radiative conditions present in the ionosphere and may be achieved synthetically under ultrahigh pressure or plasma conditions. However, the comparatively low N–N single-bond enthalpy generally renders the catenation of the strongly triple-bonded N₂ diatomic unfavorable and the decomposition of nitrogen chains a common reaction motif. Here, we report the surprising organoboron-mediated catenation of two N₂ molecules under near-ambient conditions to form a complex in which a [N₄]^2– chain bridges two boron centers. The reaction entails reductive coupling of two hypovalent-boron-bound N₂ units in a single step. Both this complex and a derivative protonated at both ends of the chain were characterized crystallographically.