Max Martin

The transformation of A ₄‐symmetric nickel porphyrins to the respective π‐extended derivatives via a Scholl oxidative route is presented. Different meso‐aryl substituents were successfully fused to the porphyrin core using standard Scholl conditions. The photophysical characteristics of fused‐porphyrins were significantly different to the non‐fused derivatives. Insight into the solid‐state properties was obtained by X‐ray crystallography.

The extension of the aromatic π‐system of porphyrins is a powerful method to alter their optoelectronic properties. Herein, aryl substituents were fused to porphyrin cores by Scholl oxidation reactions that selectively produced mono‐ and doubly‐fused porphyrins in yields of up to 69 %. Several different aryl substituents attached to the porphyrin were investigated with respect to their reactivity under Scholl conditions. The fused products were fully characterized, i.e., by UV/Vis absorption spectroscopy, which showed drastic changes in the electronic features. Insight into the solid‐state behavior was obtained by X‐ray crystallography. Our approach represents a novel option for the late‐stage functionalization of porphyrin‐based compounds.

Switching! The synthesis of several norbornadiene (NBD)–fullerene adducts is herein reported. Within these new hybrid systems, the photochemical isomerization of NBD is combined with the electron affinity of C₆₀. This enables the selective light‐induced isomerization of the NBD derivative to its corresponding quadricyclane (QC) counterpart and the back conversion of the latter mediated by the photochemically excited fullerene core.

Abstract

The synthesis and properties of various norbornadiene/quadricyclane (NBD/QC) fullerene hybrids are reported. By cyclopropanation of C₆₀ with malonates carrying the NBD scaffold a small library of NBD–fullerene monoadducts and NBD–fullerene hexakisadducts was established. The substitution pattern of the NBD scaffold, as well as the electron affinity of the fullerene core within these hybrid systems, has a pronounced impact on the properties of the corresponding energy rich QC derivatives. Based on this, the first direct photoisomerization of NBD–fullerene hybrids to their QC derivatives was achieved. Furthermore, it was possible to use the redox‐active fullerene core of a QC–fullerene monoadduct to enable the back reaction to form the corresponding NBD–fullerene monoadduct. Combining these two processes enables switching between NBD and QC simply by changing the irradiation wavelength between 310 and 400 nm. Therefore, turning this usually photo/thermal switch into a pure photoswitch. This not only simplifies the investigation of the underlying processes of the NBD–QC interconversion within the system, but also renders such hybrids interesting for applications as molecular switches.

The valence isomerization of norbornadiene (NBD) and quadricyclane (QC) has been widely studied as a potential molecular solar thermal (MOST) energy storage and release system. By combining this interconversion couple with C₆₀ fullerene, a hybrid system with novel properties was created. Within this system, light‐induced switching between NBD and QC in both directions is possible, thereby transforming the former photo/thermal system into a pure photoswitch. More information can be found in the Full Paper by P. Lorenz and A. H. Hirsch on https://doi.org/10.1002/chem.201904679page 5220.

Shell‐by‐shell : This Review describes the formation of highly ordered organic–inorganic nanoparticle hybrids generated by the shell‐by‐shell (SbS)‐coating method. Starting by covalent functionalization of the nanoparticles, initial stability and properties are achieved that can be further exploited in a noncovalent functionalization based on solvophobic interactions. This second functionalization step results in an orthogonally inverted dispersibility behavior of the nanoparticle hybrids and acts as a cornerstone for the application of these nanoparticle hybrids in water purification and biomedical treatments.

Abstract

The current state of the hierarchical chemical functionalization of inorganic nanoparticles (NPs) by shell‐by‐shell (SbS)‐assembly of organic layers around the NP cores is summarized. This supramolecular functionalization concept is based on two steps: 1) the covalent grafting of a first ligand–shell consisting of, for example, long chain phosphonic acids and 2) the noncovalent interdigitation of amphiphiles forming the second ligand shell. The latter process is guaranteed predominantly by solvophobic interactions. These highly order organic–inorganic hybrid architectures are currently an emerging field at the interface of synthetic chemistry, nanotechnology, and materials science. The doubly functionalized NPs display tunable materials properties, such a controlled dispersibility and stability in various solvents, highly efficient trapping of guest molecules in between the ligand shells (water cleaning) as well as compartmentalization and modification of electronic interactions between photoactive components integrated in such complex nano‐architectures. Such SbS‐functionalized NPs have a high potential as water‐cleaning materials and also some first prototype applications as biomedicinal therapeutics have been presented.

Dynamic helicenes: Double helicenes with a coronene core were synthesized. Temperature‐dependent NMR experiments and DFT calculations shine light on the inversion process of the helicenes and revealed a two‐step isomerization pathway. The isolated enantiomers exhibit remarkable optical and chiroptical properties.

Abstract

The synthesis of a new type of chiral and dynamic nonplanar aromatics containing a combination of fused perylene‐based coronenes and helicenes is reported. Either one or two helicene moieties were fused to the bay regions of an extended perylene core. The target compounds contain either identical or two different helicene building blocks. The combination with two helicene units leads to six different isomers, including two pairs of enantiomers and two meso forms. The experimental determination of the isomerization barriers the corresponding double [5]‐helicenes revealed activation energies of E _a=24.81 and 25.38 kcal mol⁻¹, which is slightly above the barrier of the parent [5]‐helicene. Resolution of all possible regio‐ and stereoisomers allowed for the systematic investigation of the chiroptical properties. They revealed remarkable dissymmetry factors Ig _abs I of up to 1.2×10⁻², which mirror the synergy between the strong absorbing perylenes and the inherent chirality of helicenes.

Adaptable porphyrin‐based macrocycles: Macrocycles containing aromatic porphyrin and quinoidal bithiophene units were synthesized by a template‐free method. The molecules adopted a distorted conformation with local aromaticity in the neutral state, but a more planar geometry with global aromaticity in the dication state (see picture).

Abstract

We report the template‐free synthesis and characterization of a new type of porphyrin/quinoidal‐bithiophene‐based conjugated macrocycle. X‐ray crystallographic analysis of the dimer (2MC) revealed a cyclophane‐like geometry with large dihedral angles between the porphyrin and the neighboring thiophene rings, and NMR measurements and theoretical calculations confirmed a localized aromatic character of the porphyrin/thiophene rings and quinoidal character of the bithiophene linkers. Restricted rotation of the thiophene rings linked to the porphyrin unit was observed by variable‐temperature NMR measurements. The dication (2MC²⁺ ) adopts a chair‐shaped conformation to facilitate π‐electron delocalization around the whole macrocycle. As a result, the molecule is globally aromatic, with a dominant 54 π conjugation pathway. The trimer (3MC) also shows localized aromatic character of porphyrin rings and conformational flexibility, but its dication (3MC²⁺ ) is rigid and globally aromatic with a dominant 82 π conjugation pathway.

Nature Chemistry, Published online: 20 January 2020; doi:10.1038/s41557-019-0398-3

Does aromaticity have a size limit? Evidence is presented for global aromaticity in porphyrin nanorings with circuits of up to 162 π-electrons. The conformation of the nanoring can be altered by changing the template, which in turn controls the aromaticity. Whenever a ring current is observed, its direction is correctly predicted by Hückel’s rule.

Porphyrinoids for pros: Polycyclic aromatic hydrocarbon (PAH)‐/heterocycle‐embedded porphyrinoids are porphyrinoids in which one or two pyrrole rings are replaced with polycyclic aromatic hydrocarbons/heterocycle moiety. These macrocycles possess the structural features of both polycyclic aromatic hydrocarbons/heterocycles and cyclic polypyrrolic porphyrinoids. The presence of a PAH moiety in the porphyrin core changes the electronic properties and PAH‐embedded porphyrinoids may behave as nonaromatic/aromatic/antiaromatic systems.

Abstract

Porphyrinoids in which one or two pyrrole rings are replaced with polycyclic aromatic hydrocarbons/heterocycles such as naphthalene, phenanthrene, biphenyl, bipyridine, or phenanthroline are a new class of porphyrinoids represented as polycyclic aromatic hydrocarbon (PAH)‐/heterocycle‐embedded porphyrinoids. These macrocycles possess the structural features of both polycyclic aromatic hydrocarbons/heterocycles and cyclic polypyrrolic porphyrinoids. The PAH‐embedded porphyrinoids have drawn significant interest in recent times because of the following: (1) these macrocycles exhibit different π‐conjugation pathways unlike porphyrinoids, which exhibit circular conjugation pathways in their macrocyclic rings with various molecular structures; (2) the PAHs embedded macrocycles show variable degree of aromaticity which varies from weak aromaticity to antiaromaticity and (3) the PAHs embedded porphyrinoids provide unique ligand environment to form stable coordination and organometallic complexes in which metals may show uncommon oxidation states and unusual reactivity. All these above‐listed features depend on polycyclic aromatic hydrocarbon/heterocycle moiety present as a part of porphyrinoid macrocyclic framework. This Minireview focuses on the synthesis of different polycyclic aromatic hydrocarbon‐/heterocycle‐embedded porphyrins, contracted porphyrins and expanded porphyrins and briefly discusses their structural, spectral, aromatic and coordination properties.

A stimulus‐responsive receptor was prepared to control the ligand‐binding ability of three active sites with one effector. A stable supramolecular complex of the receptor and the effector was produced by the cooperative formation of multiple hydrogen and coordination bonds. As a result, the binding of a ligand to the active sites was inhibited in a competitive and allosteric mechanism.

Abstract

A stimulus‐responsive receptor 1 was designed and prepared to control the ligand‐binding ability of three active sites, two zinc tetraphenylporphyrin units (P1) and one zinc diethynyldiphenylporphyrin unit (P2), with one effector molecule 2. Bulky hexarylbenzene units were incorporated as shielding panels in the middle of the flexible side arms of 1. Spectroscopic titrations indicated that a stable supramolecular complex 1⋅2 (K _1⋅2 =6.7×10⁶  m ⁻¹) was produced by the cooperative formation of multiple hydrogen and coordination bonds. As a result, the binding of a ligand to P1 was inhibited by 2 in a competitive manner. Additionally, the formation of 1⋅2 brought about conformational restriction of the side arms to cover both faces of P2 with the shielding panels. The binding constant of 4‐phenylpyridine with P2 in 1⋅2 decreased to 8.9 % of that in 1. Namely, the ligand‐binding ability of P2 was inhibited according to an allosteric mechanism.

Polycyclic aromatic hydrocarbons with hexagons/penta‐gons or hexagons/heptagons have been intensively investigated in recent years, but those with simultaneous presence of hexagons, pentagons and heptagons remain rare. In this paper, we report dicyclohepta[ijkl,uvwx]rubicene (DHR), a non‐benzenoid ­isomer of dibenzo[bc,kl]coronene with two pentagons and two heptagons. We developed an efficient and scalable synthetic method for DHR by using Scholl reaction and dehydrogenation. Crystal structure of DHR shows that the benzenoid rings, two pentagons and two heptagons are coplanar. The bond lengths analysis and the ICSS(1)zz and LOL‐π calculations indicate that the incorporation of two formal azulene moieties has an effect on the conjugated structure. The π‐electrons of benzenoid and pentagon rings are more delocalized. Cyclic voltammetry studies indicate that DHR shows multiple oxidation and reduction potentials. Interestingly, DHR exhibits unusual S0 to S2 absorption and abnormal anti‐Kasha S2 to S0 emission. Moreover, crystals of DHR exhibit semiconducting behaviour with hole mobility up to 0.082 cm2V‐1s‐1.

An odd contribution: Azulene‐embedded helical nanographenes with global aromaticity are synthesized by a Scholl‐type cyclization. X‐ray crystallographic analysis clearly reveals the formation of an azulene unit in the helical π‐system. The embedded azulene core adopts a highly twisted conformation and is less aromatic than pristine azulene.

Abstract

Three unprecedented helical nanographenes (1, 2, and 3) containing an azulene unit are synthesized. The resultant helical structures are unambiguously confirmed by X‐ray crystallographic analysis. The embedded azulene unit in 2 possesses a record‐high twisting degree (16.1°) as a result of the contiguous steric repulsion at the helical inner rim. Structural analysis in combination with theoretical calculations reveals that these helical nanographenes manifest a global aromatic structure, while the inner azulene unit exhibits weak antiaromatic character. Furthermore, UV/Vis‐spectral measurements reveal that superhelicenes 2 and 3 possess narrow energy gaps (2: 1.88 eV; 3: 2.03 eV), as corroborated by cyclic voltammetry and supported by density functional theory (DFT) calculations. The stable oxidized and reduced states of 2 and 3 are characterized by in‐situ EPR/Vis–NIR spectroelectrochemistry. Our study provides a novel synthetic strategy for helical nanographenes containing azulene units as well as their associated structures and physical properties.

The selective on‐surface synthesis of a porphyrin–graphene nanoribbon hybrid is reported by S. Decurtins, G. Bottari, R. Fasel, T. Torres, and co‐workers in their Research Article on https://doi.org/10.1002/anie.201913024page 1334. The atomically precise structure of the hybrid has been unambiguously characterized by bond‐resolved scanning tunneling microscopy and noncontact atomic force microscopy. The electronic properties of the hybrid have been investigated by scanning tunneling spectroscopy (STS) in combination with DFT calculations. Cover designed by Roland Wilkins.

Made on the surface: A porphyrin–graphene nanoribbon hybrid is prepared by selective on‐surface synthesis. The atomically precise structure of the hybrid is characterized by bond‐resolved scanning tunneling microscopy and noncontact atomic force microscopy. Scanning tunneling spectroscopy (STS), in combination with DFT calculations, reveals a low electronic gap of 0.4 eV.

Abstract

On‐surface synthesis offers a versatile approach to prepare novel carbon‐based nanostructures that cannot be obtained by conventional solution chemistry. Graphene nanoribbons (GNRs) have potential for a variety of applications. A key issue for their application in molecular electronics is in the fine‐tuning of their electronic properties through structural modifications, such as heteroatom doping or the incorporation of non‐benzenoid rings. In this context, the covalent fusion of GNRs and porphyrins (Pors) is a highly appealing strategy. Herein we present the selective on‐surface synthesis of a Por–GNR hybrid, which consists of two Pors connected by a short GNR segment. The atomically precise structure of the Por–GNR hybrid has been characterized by bond‐resolved scanning tunneling microscopy (STM) and noncontact atomic force microscopy (nc‐AFM). The electronic properties have been investigated by scanning tunneling spectroscopy (STS), in combination with DFT calculations, which reveals a low electronic gap of 0.4 eV.

He loves me… he loves me not: In the cover image, porphyrin‐hexaphenylbenzenes are drawn as daisies, that are used to play the game “he loves me … he loves me not”. These multiple‐porphyrin substituted hexaphenylbenzenes were used as precursors for the preparation of the respective hexabenzocoronene derivatives, for example the flower with orange petals. Given that the transformation, commonly known as Scholl reaction, did not always work well, the game “Scholl loves me … Scholl loves me not” is shown in the cover image. More information can be found in the Full Paper by N. Jux et al. on https://doi.org/10.1002/chem.201903113page 15083.

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.

Robustly coherent spin centers that can be integrated into devices are a key ingredient of quantum technologies. Vacancies in semiconductors are excellent candidates, and theory predicts that defects in conjugated carbon materials should also display long coherence times. However, the quantum performance of carbon nanostructures has remained stunted by an inability to alter the sp²-carbon lattice with atomic precision. Here, we demonstrate that topological tailoring leads to superior quantum performance in molecular graphene nanostructures. We unravel the decoherence mechanisms, quantify nuclear and environmental effects, and observe spin-coherence times that outclass most nanomaterials. These results validate long-standing assumptions on the coherent behavior of topological defects in graphene and open up the possibility of introducing controlled quantum-coherent centers in the upcoming generation of carbon-based optoelectronic, electronic, and bioactive systems.

Switching! The synthesis of several norbornadiene (NBD)–fullerene adducts is herein reported. Within these new hybrid systems, the photochemical isomerization of NBD is combined with the electron affinity of C₆₀. This enables the selective light‐induced isomerization of the NBD derivative to its corresponding quadricyclane (QC) counterpart and the back conversion of the latter mediated by the photochemically excited fullerene core.

Abstract

The synthesis and properties of various norbornadiene/quadricyclane (NBD/QC) fullerene hybrids are reported. By cyclopropanation of C₆₀ with malonates carrying the NBD scaffold a small library of NBD–fullerene monoadducts and NBD–fullerene hexakisadducts was established. The substitution pattern of the NBD scaffold, as well as the electron affinity of the fullerene core within these hybrid systems, has a pronounced impact on the properties of the corresponding energy rich QC derivatives. Based on this, the first direct photoisomerization of NBD–fullerene hybrids to their QC derivatives was achieved. Furthermore, it was possible to use the redox‐active fullerene core of a QC–fullerene monoadduct to enable the back reaction to form the corresponding NBD–fullerene monoadduct. Combining these two processes enables switching between NBD and QC simply by changing the irradiation wavelength between 310 and 400 nm. Therefore, turning this usually photo/thermal switch into a pure photoswitch. This not only simplifies the investigation of the underlying processes of the NBD–QC interconversion within the system, but also renders such hybrids interesting for applications as molecular switches.

Made on the surface: A porphyrin–graphene nanoribbon hybrid is prepared by selective on‐surface synthesis. The atomically precise structure of the hybrid is characterized by bond‐resolved scanning tunneling microscopy and noncontact atomic force microscopy. Scanning tunneling spectroscopy (STS), in combination with DFT calculations, reveals a low electronic gap of 0.4 eV.

Abstract

On‐surface synthesis offers a versatile approach to prepare novel carbon‐based nanostructures that cannot be obtained by conventional solution chemistry. Graphene nanoribbons (GNRs) have potential for a variety of applications. A key issue for their application in molecular electronics is in the fine‐tuning of their electronic properties through structural modifications, such as heteroatom doping or the incorporation of non‐benzenoid rings. In this context, the covalent fusion of GNRs and porphyrins (Pors) is a highly appealing strategy. Herein we present the selective on‐surface synthesis of a Por–GNR hybrid, which consists of two Pors connected by a short GNR segment. The atomically precise structure of the Por–GNR hybrid has been characterized by bond‐resolved scanning tunneling microscopy (STM) and noncontact atomic force microscopy (nc‐AFM). The electronic properties have been investigated by scanning tunneling spectroscopy (STS), in combination with DFT calculations, which reveals a low electronic gap of 0.4 eV.

Nature, Published online: 23 October 2019; doi:10.1038/s41586-019-1666-5

Quantum supremacy is demonstrated using a programmable superconducting processor known as Sycamore, taking approximately 200 seconds to sample one instance of a quantum circuit a million times, which would take a state-of-the-art supercomputer around ten thousand years to compute.

Nature, Published online: 23 October 2019; doi:10.1038/s41586-019-1661-x

The construction of a self-assembled nanocage composed of four metal ions and six antiaromatic walls is demonstrated, and the effect of antiaromaticity on the host–guest properties is investigated.

He loves me… he loves me not: In the cover image, porphyrin‐hexaphenylbenzenes are drawn as daisies, that are used to play the game “he loves me … he loves me not”. These multiple‐porphyrin substituted hexaphenylbenzenes were used as precursors for the preparation of the respective hexabenzocoronene derivatives, for example the flower with orange petals. Given that the transformation, commonly known as Scholl reaction, did not always work well, the game “Scholl loves me … Scholl loves me not” is shown in the cover image. More information can be found in the Full Paper by N. Jux et al. on https://doi.org/10.1002/chem.201903113page 15083.

Superbenzene! Reported herein is the stepwise dearomatization of a superbenzene molecule to give a series of superbenzoquinones with two, four, and six ketone groups. They have an intrinsic tendency to recover aromaticity and thus show open‐shell di‐/tetra‐/hexaradical characters. They also exhibit geometry‐ and molecular‐symmetry‐dependent electronic properties.

Abstract

Reported here is the step‐by‐step dearomatization of a highly aromatic polycyclic aromatic hydrocarbon (PAH), the hexa‐peri‐hexabenzocoronene (also called as “superbenzene”), to give a series of superbenzoquinones containing two, four, and six ketone groups. Different from traditional PAH‐based quinones, these superbenzoquinones show open‐shell multiradical character by rearomatization in the open‐shell forms as experimentally validated by X‐ray crystallographic analysis, NMR and ESR spectroscopy, and FT‐IR measurements, as well as theoretically supported by restricted active space spin‐flip calculations. These compounds exhibit structure‐ and molecular‐symmetry‐dependent optical, electrochemical, and magnetic properties.

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.

Polyaromatic compounds are currently of great interest as potential semiconducting materials for organic field‐effect transistors, LEDs and solar panels. We aimed to cover the recent progress in the application of the Diels–Alder reaction in PACs synthesis with the emphasis on the structures of dienophiles. Examples of tetra‐ and hexadehydro Diels–Alder reactions are also described.

Polycyclic aromatic compounds (PACs) represent a class of molecules consisting of two or more condensed aromatic rings. They are currently of great interest as potential semiconducting materials for organic field‐effect transistors, light‐emitting diodes, and solar panels. Substituted PACs are not always readily available and may require complicated multi‐step synthetic procedures to be obtained. One of the promising methods of the synthesis of functionalized PACs is the assembly of polycyclic backbones from substituted building blocks via cycloaddition reactions, in particular, the Diels–Alder reaction. In this minireview we aimed to cover the recent progress in the application of the Diels–Alder reaction in PACs synthesis with the emphasis on the structures of dienophiles. While quinones and arynes still act most frequently as the dienophilic components, other dienophiles with activated double and triple carbon–carbon bonds attract attention due to the ability of insertion of functional groups into polycyclic structures. In the recent decade variants of Dehydro Diels–Alder reaction, especially HexaDehydro Diels–Alder reaction (HDDA) attract much attention leading to the formation of polycyclic structures in a single step. The present review covers mostly papers published during the past decade; older works are referenced only when it is necessary for the understanding of the results discussed.