A thiadiazole-capped nanoribbon with 18 linearly fused rings

Molecular Graphene Nanoribbon Junctions

One of the challenges for the realization of molecular electronics is the design of nanoscale molecular wires displaying long-range charge transport. Graphene nanoribbons are an attractive platform for the development of molecular wires with long-range conductance owing to their unique electrical properties. Despite their potential, the charge transport properties of single nanoribbons remain underexplored. Herein, we report a synthetic approach to prepare N-doped pyrene-pyrazinoquinoxaline molecular graphene nanoribbons terminated with diamino anchoring groups at each end. These terminal groups allow for the formation of stable molecular graphene nanoribbon junctions between two metal electrodes that were investigated by scanning tunneling microscope-based break-junction measurements. The experimental and computational results provide evidence of long-range tunneling charge transport in these systems characterized by a shallow conductance length dependence and electron tunneling through >6 nm molecular backbone.

Modulating Strain in Twisted Pyrene-Fused Azaacenes

The design and synthesis of strained aromatics provide an additional insight into the relationship between structure and properties. In the last years, several approaches to twist pyrene-fused azaacenes have been developed that allow to introduce twists of different sizes. Herein, we describe the synthesis of a new set of twisted dibenzotetraazahexacenes constituted by fused pyrene and quinoxaline residues that have been distorted by introducing increasingly larger substituents on the quinoxaline residues. Their twisted structure has been demonstrated by single-crystal X-ray diffraction. Furthermore, absorption, fluorescence, electrochemical and theoretical studies shine light on the effects of the substituents and twists on the optoelectronic and redox properties.

The Radical Anion and Dianion of Benzo[3,4]cyclobuta[1,2-b]phenazine

We present the reduction of two azaacenes (a benzo-[3,4]cyclobuta[1,2-b]phenazine and a benzo[3,4]cyclobuta[1,2-b]naphtho[2,3-i]phenazine derivative), featuring a single cyclobutadiene unit, to their radical anions and dianions. The reduced species were produced using potassium naphthalenide in the presence of 18-crown-6 in THF. Crystal structures of the reduced representatives were obtained and their optoelectronic properties evaluated. Charging these 4n Hückel systems gives dianionic 4n + 2 π-electron systems with increased antiaromaticity, according to NICS(1.7)_zz calculations, featuring unusually red-shifted absorption spectra.

Acceleration Effect of Bowl-Shaped Structure in Aerobic Oxidation Reaction: Synthesis of Homosumanene ortho-Quinone and Azaacene-Fused Homosumanenes

An oxidation reaction of hydroxyhomosumanene on silica gel providing homosumanene ortho-quinone and its synthetic application for azaacene-fused homosumanenes is described. Hydroxyhomosumanene is photochemically oxidized by air, when it is coated on silica gel; this aerobic oxidation proceeds faster than that of planar analogues. The difference of such reactivity was attributed to the unusual keto-enol tautomerization due to structural difference between planar and curved π-system. The homosumanene ortho-quinone was used in the synthesis of several azaacene-fused homosumanenes, azaacenohomosumanenes. X-ray diffraction analysis of the single crystals revealed their columnar stacking structures due to the interactions between each bowl. Azaacenohomosumanenes exhibited high electron affinity due to the combination of buckybowl and electron-deficient azaacene moieties.

Doubling the Length of the Longest Pyrene-Pyrazinoquinoxaline Molecular Nanoribbons

Molecular nanoribbons are a class of atomically-precise nanomaterials for a broad range of applications. An iterative approach that allows doubling the length of the longest pyrene-pyrazinoquinoxaline molecular nanoribbons is described. The largest nanoribbon obtained through this approach-with a 60 linearly-fused ring backbone (14.9 nm) and a 324-atoms core (C276 N48 )-shows an extremely high molar absorptivity (values up to 1 198 074 M-1  cm-1 ) that also endows it with a high molar fluorescence brightness (8700 M-1  cm-1 ).

Planar and Helical Dinaphthophenazines

In this study, we report the synthesis of a series of planar and helical dinaphthophenazines by cyclocondensation reactions between the newly developed 9,10-bis((triisopropylsilyl)ethynyl)anthracene-1,2-dione and different diamines. Their optoelectronic and electrochemical properties are studied by ultraviolet-visible (UV-vis) spectroscopy, fluorescence spectroscopy, cyclic voltammetry, and density functional theory calculations.

Benzo[de]isoquinoline-1,3-dione condensed asymmetric azaacenes as strong acceptors

We have designed and synthesized three novel benzo[de]isoquinoline-1,3-dione (BQD) condensed asymmetric azaacenes, BQD-TZ, BQD-AP and BQD-PA, with different end groups of 1,2,5-thiadiazole, acenaphthylene and phenanthrene. The triisopropylsilylethynyl groups were grafted to increase the solubility and crystallinity of the three molecules. These molecules have high electron affinity values of 3.87, 3.69 and 3.74 eV for BQD-TZ, BQD-AP and BQD-PA, respectively as confirmed by cyclic voltammetry measurements. Single-crystal X-ray diffraction revealed that these molecules have strong π-π stacking with distances of 3.31-3.41 Å and different stacking arrangements.

Depositing Molecular Graphene Nanoribbons on Ag(111) by Electrospray Controlled Ion Beam Deposition: Self-Assembly and On-Surface Transformations

The chemical processing of low-dimensional carbon nanostructures is crucial for their integration in future devices. Here we apply a new methodology in atomically precise engineering by combining multistep solution synthesis of N-doped molecular graphene nanoribbons (GNRs) with mass-selected ultra-high vacuum electrospray controlled ion beam deposition on surfaces and real-space visualisation by scanning tunnelling microscopy. We demonstrate how this method yields solely a controllable amount of single, otherwise unsublimable, GNRs of 2.9 nm length on a planar Ag(111) surface. This methodology allows for further processing by employing on-surface synthesis protocols and exploiting the reactivity of the substrate. Following multiple chemical transformations, the GNRs provide reactive building blocks to form extended, metal-organic coordination polymers.

Inducing Single-Handed Helicity in a Twisted Molecular Nanoribbon

Molecular conformation has an important role in chemistry and materials science. Molecular nanoribbons can adopt chiral twisted helical conformations. However, the synthesis of single-handed helically twisted molecular nanoribbons still represents a considerable challenge. Herein, we describe an asymmetric approach to induce single-handed helicity with an excellent degree of conformational discrimination. The chiral induction is the result of the chiral strain generated by fusing two oversized chiral rings and of the propagation of that strain along the nanoribbon's backbone.

Recent Advances in Heterocyclic Nanographenes and Other Polycyclic Heteroaromatic Compounds

This review surveys recent progress in the chemistry of polycyclic heteroaromatic molecules with a focus on structural diversity and synthetic methodology. The article covers literature published during the period of 2016-2020, providing an update to our first review of this topic (Chem. Rev. 2017, 117 (4), 3479-3716).

5,11-Diazadibenzo[hi,qr]tetracene: Synthesis, Properties, and Reactivity toward Nucleophilic Reagents

5,11-Diazadibenzo[hi,qr]tetracene was synthesized as a new nitrogen-substituted polycyclic heteroaromatic compound by Pd-catalyzed cycloisomerization of an alkyne precursor followed by oxidative cyclization with bis(trifluoroacetoxy)iodobenzene. The substitution of imine-type nitrogen atoms significantly enhanced its electron-accepting character and facilitated the direct nucleophilic addition of arylamines under strongly basic conditions to afford the desired amino-substituted products. The introduction of amino groups induced a remarkable red-shift in their absorption spectra; the tetrasubstituted product exhibited intense near-infrared absorbing property. Furthermore, the π-electronic system, which includes a redox-active 1,4-diazabutadiene moiety, underwent reversible interconversion to its corresponding reduced form upon reduction with NaBH₄ and aerobic oxidation.

Twisted Molecular Nanoribbons with up to 53 Linearly-Fused Rings

The synthesis of three molecular nanoribbons with a twisted aromatic framework is described. The largest one shows a 53 linearly fused rings backbone (12.9 nm) and 322 conjugated atoms in its aromatic core (C₂₉₆N₂₄S₂). This new family of nanoribbons shows extremely high molar absorptivities, reaching 986 100 M^-1 cm^-1, and red-emitting properties.

Synthesis and Optoelectronic Properties of a Quinoxalino-Phenanthrophenazine (QPP) Extended Tribenzotriquinacene (TBTQ)

A six‐step synthesis towards a tribenzotriquinacene (TBTQ) bearing three quinoxalinophenanthrophenazine (QPP) units is presented. The optoelectronic properties are investigated and the effect of the three‐dimensional arrangement of the individual QPP planes is examined using optical spectroscopy, electrochemical analysis and quantum‐chemical calculations.

Triptycene End-Capping as Strategy in Materials Chemistry to Control Crystal Packing and Increase Solubility

In materials chemistry of polycyclic aromatic compounds (PACs) the kind of aggregation and the spatial arrangement of the π-planes are of utmost importance, e. g. for charge transport properties. Unfortunately, controlling these during crystallization is not trivial. In the past decade, we have introduced one-fold triptycene end-capping of quinoxalinophenanthrophenazines (QPPs) and other related structures to overcome this problem. When two instead of one triptycene end-caps are introduced, packing is largely suppressed, making typical PACs or pigments soluble in common organic solvents - which is another important property for such compounds to be processable from solution. In this account an overview of our research on using triptycene end-capping as dual strategy is given.

Increasing and dispersing strain in pyrene-fused azaacenes

A new strategy to obtain distorted pyrene-fused azaacenes is reported. The careful alignment and selection of substituents give rise to highly twisted pyrene-fused azaacenes. A combined global and local theoretical analysis shows how the strain is generated and dispersed across the aromatic backbone. Furthermore, simulation of the observed optoelectronic properties shines light on the structural factors that govern the properties of twisted pyrene-fused azaacenes.

Di- and Tetracyano-Substituted Pyrene-Fused Pyrazaacenes: Aggregation in the Solid State

Five di- and tetracyano-substituted pyrene-fused pyrazaacenes were synthesized and studied as potential electron acceptors in the solid state. Single crystals of all compounds were grown and the crystal packing studied by DFT calculations (transfer integrals and reorganization energies) to get insight into possible use for semiconducting charge transport.

Dibenzoanthradiquinone Building Blocks for the Synthesis of Nitrogenated Polycyclic Aromatic Hydrocarbons

A straightforward method for the synthesis of two dibenzo[a,h]anthracene-5,6,12,13-diquinone building blocks is reported. To showcase their usefulness, a series of dibenzo[a,h]anthracene nitrogenated derivatives have been synthesized that show different optoelectronic, redox, and charge transport properties, illustrating their potential as organic semiconductors.

Optimized synthesis of thermally stable axially modified pyrazine-acene nanoribbon with gelation properties

A comprehensive synthesis study to axially modify N-heteroacene nanoribbon and its outstanding thermal and one-dimensional assembly properties.

Dimesitylboryl-Decorated Azaarene: Synthesis, Enhanced Stability and Optoelectronic Property

The instability of large acenes and analogues usually limits their wide potentials in organic devices. Thus, effectively constructing large acenes or heteroacenes and examining their optoelectronic properties are of great interest. We herein describe the synthesis, optoelectronic behaviors and electroluminescent property of dimesitylboryl-decorated azaarene 5 and its homologue 7. The former emits strong green fluorescence in non-polar solvents but yellow light in polar solvents. The latter emits a blue light in various organic solvents. Moreover, their electrochemical behavior, photostability and electroluminescent property were further studied in a comparative manner, and the experimental findings suggest that the desired heteroarenes are appealing materials for fabricating electroluminescent devices.

N-Acenoacenes

The syntheses of new, fourfold alkynylated tetraazaacenoacenes (tetraazaanthracenoanthracene, tetraazatetracenotetracene and tetraazapentacenopentacene) are reported. This novel heteroacenoacene motif exhibits surprisingly strong electronic coupling between its constituting diazaacene units.

Facile and versatile access to substituted hexabenzoovalene derivatives: characterization and optoelectronic properties

We describe the design and modular synthesis of a library of substituted hexabenzoovalene derivatives (SHBO), along with the key precursor dinaphthopyrenes (3), highlighting the influence of a wide array of substituent variation on the photophysical properties via UV-vis absorption, fluorescence spectra and electrochemical methods. The results show that the cyclized hexabenzoovalenes present a stronger spectroscopic red-shift than the corresponding dinaphthopyrenes. X-ray diffraction analysis suggests that intermediate 3hx containing two nitro groups forms a trans-configuration with twisted structures. Our systematic investigation might provide a realistic design strategy to construct large one-dimensional and two-dimensional materials via bottom-up approaches.

N-Heteroacenes and N-Heteroarenes as N-Nanocarbon Segments

N-Heteroacenes and N-heteroarenes are the heterocyclic congeners of the acenes and arenes, in which one or several perimeter C-H bonds have been substituted by pyridine-type nitrogen atoms. They are formally segments out of N-doped nanographenes. Position and number of the nitrogens vary greatly, making N-heteroacenes and N-heteroarenes define a vast class of N-nanographene segments; they display modular electronic and structural properties. The nitrogen atoms in the perimeter lead to finely tunable frontier molecular orbital positions and therefore improved electron affinity and higher oxidative stability but conversely also require and allow different synthetic approaches than those reported for the synthesis of their hydrocarbon and nanographene analogues. The chemistry of N-heteroarenes, despite being known for more than a century, has made significant progress in the last years and established these materials both as powerful n-channel semiconductors in thin film transistors and as useful emitters in organic light emitting diodes (OLEDs) and in photovoltaic devices. The electronegative nitrogen atoms impart a deep LUMO into the azaacenes and azaarenes, improve electron injection, and enable powerful electron transport but also charge separation in bulk-heterojunction type organic photovoltaic (OPV) devices. At the same time, azaacenes and azaarenes are fundamentally exciting materials that push the limits of structure and stability, constantly displaying novel topologies and structures as variations of a simple leitmotif; we expect a bright future for esthetically pleasing yet highly functional N-heterocyclic species. Firstly, we discuss novel structures and structural elements that have evolved during the last years in N-heteroacene and N-heteroarene chemistry and delineate their properties. An important aspect is the oligomerization or better multimerization of azaacene and azaarene units into novel and surprising topologies, in which multiple azaarenes or azaacenes are stitched together. Examples are tetrahedral assemblies of tetraazapentacenes but also cyclic tetramers of different types of azaacenes and linearly bent, S-shaped, formally dimeric species. An exciting aspect of the exploration of the structural manifold of azaacenes is their electronic interaction in such assemblies and their solid-state microstructure. A further aspect of this work is the increase in size of the azaacenes and concepts that allow stabilization of the larger congeners. The attachment of four benzo units to the azaacene core is a powerful concept that stabilizes tetraazaheptacenes and should also be useful to achieve persistent tetraazanonacenes. Secondly, we describe the success of N-heteroacenes and N-heteroarenes in organic electronic devices; specifically, the use of symmetrical halogenated tetraazapentacenes as superb n-channel transistor materials with air stable and persistent radical anions as charge carriers; we discuss the structural reason for their success. Use of azaacenes and azaarenes is not restricted to transistors, but they are also applied in bulk heterojunction photovoltaic devices and in brightly emitting OLEDs. Azaacenes and azaarenes are attractive segments out of hetero-nanographenes and objects of study, starting from fundamental structural and topological questions, ranging to powerful applications in organic electronics. The general interest in azaacenes is witnessed by the constantly increasing number of groups who discover and work on these materials as novel functional and flexible species.