Contorted Polycyclic Aromatic Hydrocarbons with Two Embedded Azulene Units

Synthesis, Structure, and Properties of a Nitrogen-Boron-Nitrogen-Embedded Polycyclic π-System Containing a Pleiaheptalene Framework

A novel polycyclic π-system (1) featuring both a pleiaheptalene framework (a three-fused heptagon system) and nitrogen-boron-nitrogen (NBN) unit was constructed by electrophilic borylation. A combined experimental and computational study demonstrated that 1 has a highly twisted π-backbone with approximate C2 symmetry, which can undergo conformational isomerization at room temperature in contrast to pleiaheptalene. It was also found that 1 can bind the fluoride ion in the solution, which induces changes in the absorption and emission spectra.

Ring-Expanding Rearrangement of Benzo-Fused Tris-Cycloheptenylenes towards Nonplanar Polycyclic Aromatic Hydrocarbons

A strongly twisted benzo-fused tris-cycloheptenylene, containing three dibenzosuberenone units fused to a common benzene ring, was subjected to Ramirez olefination and subsequent palladium-catalyzed Suzuki-Miyaura cross-coupling with 4-substituted phenylboronic acids. The high steric demand within the overcrowded, benzene-rich benzo-fused tris-cycloheptenylenes enforced an unprecedented 1,2-rearrangement upon π-extension during the Suzuki coupling reaction. According to crystal structure analysis, the resulting negatively curved polycyclic aromatic hydrocarbons consist of two heptagons and one octagon surrounding a central benzene ring as a result of strain release. In the solid state, the materials exhibit a blue to blue-green fluorescence with increased quantum yields and a hypsochromic shift of the emission maxima compared to their respective solutions.

Synthesis of a Dicyclohepta[a,g]heptalene-Containing Polycyclic Conjugated Hydrocarbon and the Impact of Non-Alternant Topologies

Incorporating non-hexagonal rings into polycyclic conjugated hydrocarbons (PCHs) can significantly affect their electronic and optoelectronic properties and chemical reactivities. Here, we report the first bottom-up synthesis of a dicyclohepta[a,g]heptalene-embedded PCH (1) with four continuous heptagons, which are arranged in a "Z" shape. Compared with its structural isomer bischrysene 1 R with only hexagonal rings, compound 1 presents a distinct antiaromatic character, especially the inner heptalene core, which possesses clear antiaromatic nature. In addition, PCH 1 exhibits a narrower highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) energy gap than its benzenoid contrast 1 R, as verified by experimental measurements and theoretical calculations. Our work reported herein not only provides a new way to synthesize novel PCHs with non-alternant topologies but also offers the possibility to tune their electronic and optical properties.

Near-Infrared-Responsive Hydrocarbons Designed by π-Extension of Indeno[1,2,3,4-pgra]perylene at the 1,2,12-Positions

The relationship between the overall electronic structure of π-conjugated molecules and the arrangement of their constituent elements is of fundamental importance. Establishing rational design guidelines for conjugated hydrocarbons with narrow HOMO-LUMO gaps is useful to develop near-infrared (NIR) responsive dyes and redox-active materials. This study describes the synthesis and properties of three conjugated hydrocarbons, i. e., an indenonaphthoperylene, an indenoterrylene, and a diindenoterrylene. These molecules exhibit NIR absorption despite the absence of significant antiaromaticity and diradical character. Notably, the indenonaphthoperylene exhibits red-to-NIR emission in the 620-850 nm region. The indenoterrylene and the diindenoterrylene exhibit NIR absorption tailing to 870 and 940 nm, respectively. Moreover, the effect of the π-extension of indenoperylene is disclosed in order to propose guidelines for achieving a narrow HOMO-LUMO gap with negligible antiaromaticity and diradical character.

π-Extended Pleiadienes by [5+2] Annulation of 1-Boraphenalenes and ortho-Dihaloarenes

Palladium-catalyzed [5+2] annulation of 1-boraphenalenes with ortho-dihaloarenes afforded negatively curved π-extended pleiadienes. Two benzo[1,2-i:4,5-i']dipleiadienes (BDPs) featuring a seven-six-seven-membered ring arrangement were synthesized and investigated. Their crystal structure revealed a unique packing arrangement and theoretical calculations were employed to shed light onto the dynamic behavior of the BDP moiety and its aromaticity. Further, a naphthalene-fused pleiadiene was stitched together by oxidative cyclodehydrogenation to yield an additional five-membered ring. This formal azulene moiety led to distinct changes in optical and redox properties and increased perturbation of the aromatic system.

Synthesis of Defective Nanographenes Containing Joined Pentagons and Heptagons

Defective nanographenes containing joined pentagons and heptagons exhibit striking physicochemical properties from both experimental and theoretical perspectives compared with their pure hexagonal counterparts. Thus, the synthesis and characterization of these unique polyarenes with well-defined defective topologies have attracted increasing attention. Despite extensive research on nonalternant molecules since the last century, most studies focused on the corresponding mutagenic and carcinogenic activities. Recently, researchers have realized that the defective domain induces geometric bending and causes electronic perturbation, thus leading to significant alteration of the photophysical properties. This review discusses the synthesis and characterization of small nonalternant polycyclic hydrocarbons in the early stage and recent developments in embedding pentagon-heptagon (5-7) pairs into large carbon skeletons through in-solution chemistry.

Benzo-Extended Cyclohepta[def]fluorene Derivatives with Very Low-Lying Triplet States

Open-shell non-alternant polycyclic hydrocarbons (PHs) are attracting increasing attention due to their promising applications in organic spintronics and quantum computing. Herein we report the synthesis of three cyclohepta[def]fluorene-based diradicaloids (1-3), by fusion of benzo rings on its periphery for the thermodynamic stabilization, as evidenced by multiple characterization techniques. Remarkably, all of them display a very narrow optical energy gap (Eg opt =0.52-0.69 eV) and persistent stability under ambient conditions (t1/2 =11.7-33.3 h). More importantly, this new type of diradicaloids possess a low-lying triplet state with an extremely small singlet-triplet energy gap, as low as 0.002 kcal mol-1 , with a clear dependence on the molecular size. This family of compounds thus offers a new route to create non-alternant open-shell PHs with high-spin ground states, and opens up novel possibilities and insights into understanding the structure-property relationships.

Transformation of Crowded Oligoarylene into Perylene-Cored Chiral Nanographene by Sequential Oxidative Cyclization and 1,2-Phenyl Migration

Synthetic innovation for constructing sophisticated nanographenes is of fundamental significance for a variety of advanced applications. Herein, we report a distinctive method to prepare π-extended chiral nanographenes with 29 benzenoid rings and two helical breaches from a highly crowded perylene-cored oligoarylene precursor. Under Scholl's conditions, the reaction predominantly involves the regioselective and sequential cyclization in the peri- and bay regions of the perylene core, and the complanation of the 1-phenyl[5]helicene intermediate module via 1,2-phenyl migration. The resulting chiral nanographenes are configurationally stable at 180 °C due to the high diastereomerization barriers of ca. 45 kcal mol^-1 . These molecules also possess globally delocalized π-systems with low HOMO/LUMO gaps, leading to nearly panchromatic absorption, intensive electronic circular dichroism signals and deep-red circularly polarized luminescence.

Indeno[1,2,3,4-pqra]Perylene: A Medium-Sized Aromatic Hydrocarbon Exhibiting Full-Range Visible-Light Absorption

We report the synthesis and properties of indeno[1,2,3,4-pqra]perylene, which was prepared by the fusion of one anthracene unit with one naphthalene unit via three carbon-carbon bonds. The synthetic route through two-fold C-H arylation enabled not only the synthesis of unsubstituted indenoperylene, but also rapid access to its arylated derivatives on the gram scale. Indenoperylene is a medium-sized aromatic hydrocarbon with the composition C₂₄ H₁₂ that is isomeric to coronene. Nevertheless, its absorption covers the entire visible region owing to its small HOMO-LUMO gap. Furthermore, indenoperylene exhibits high stability despite the absence of peripheral substituents. We propose that the unique electronic structure of indenoperylene originates from the coexistence of an electron-withdrawing subunit (benzoaceanthrylene) and an electron-donating subunit (perylene). The electronic properties of indenoperylene were modulated via post-functionalization through regioselective bromination. The current research demonstrates that indenoperylene is a promising candidate as a main skeleton for near-infrared-responsive and redox-active materials.

Construction of Heptagon-Containing Molecular Nanocarbons

Molecular nanocarbons containing heptagonal rings have attracted increasing interest due to their dynamic behavior, electronic properties, aromaticity, and solid-state packing. Heptagon incorporation can not only induce negative curvature within nanocarbon scaffolds, but also confer significantly altered properties through interaction with adjacent non-hexagonal rings. Despite the disclosure of several beautiful examples in recent years, synthetic strategies toward heptagon-embedded molecular nanocarbons remain relatively limited due to the intrinsic challenges of heptagon formation and incorporation into polyarene frameworks. In this Review, recent advances in solution-mediated and surface-assisted synthesis of heptagon-containing molecular nanocarbons, as well as the intriguing properties of these frameworks, will be discussed.

Chiral Distorted Hexa-peri-hexabenzocoronenes Bearing a Nonagon-Embedded Carbohelicene

A new family of chiral saddle-helix hybrid nanographenes is reported. The first hexa-peri-hexabenzocoronene (HBC) analogues bearing a nine-membered carbocycle are presented. Furthermore, for the first time, π-extended carbo[n]helicenes containing a nine-membered ring as part of the helical moiety have been synthesized. The combination of a [5]helicene moiety and a nonagon ring in a single chiral motif induces a tremendous distortion from planarity into the nanographenic structures compared to other saddle-helix hybrids such as heptagon- and octagon-containing π-extended carbo[5]helicenes. In fact, the interplanar angle of the two terminal rings reaches the largest angle (134.8°) of a carbohelicene reported to date, thus being by far the most twisted helicene yet prepared. Photophysical properties evaluation showed improved absorption dissymmetry factors (|gabs |=4.2×10-3 ) in the new family of nonagon-containing π-extended carbo[5]helicenes.

Contorted Heteroannulated Tetraareno[a,d,j,m]coronenes

Fused polycyclic aromatic compounds are interesting materials for organic electronics applications. To fine-tune photophysical or electrochemical properties, either various substituents can be attached or heteroatoms (such as N or S) can be incorporated into the fused aromatic backbone. Coronenes and heterocoronenes are promising compounds in this respect. Up until now, the possibilities for varying the attached fused heteroaromatics at the coronene core were quite limited, and realizing both electron-withdrawing and -donating rings at the same time was very difficult. Here, a series of pyridine, anisole and thiophene annulated tetraareno[a,d,j,m]coronenes has been synthesized by a facile two-step route that is a combination of Suzuki-Miyaura cross-coupling and a following cyclization step, starting from three different diarenoperylene dibromides. The contorted molecular π-planes of the obtained cata-condensed tetraarenocoronenes were analyzed by single-crystal X-ray crystallography, and the photophysical and electrochemical properties were systematically investigated by UV/Vis spectroscopy and cyclovoltammetry.

Scholl Reaction of Perylene-Based Polyphenylene Precursors under Different Conditions: Formation of Hexagon or Octagon?

A planar dibenzo-peri-hexacene derivative (2) was synthesized via FeCl₃ -mediated Scholl reaction from a cyclopenta-fused perylene (CP) based polyphenylene precursor (1). However, an unexpected octagon-containing, negatively curved molecule (3) was obtained in nearly quantitative yield when 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) and methanesulfonic acid (MeSO₃ H) were used. Similar results were observed when two smaller-sized precursors containing one (4) or two CP units (5) were tested. X-ray crystallographic analysis also revealed that there is no close π-π stacking between neighboring π-conjugated skeletons. DFT calculations suggest a radical cation mechanism in the presence of FeCl₃ while an arenium ion pathway for the DDQ/MeSO₃ H mediated Scholl reaction, which can well explain the selective formation of hexagons and octagons under different conditions. The obtained compounds showed tunable optical and electrochemical properties.

Benzo-Fused Perylene Oligomers with up to 13 Linearly Annulated Rings

The longer acenes with more than six linearly fused six-membered rings are still fascinating chemists and physicists because of their unique photophysical properties and their high potential for organic electronics applications. Unfortunately, with increasing size (seven and more rings) these compounds rapidly lose chemical stability. Besides kinetic and chemical stabilization approaches introducing either bulky or electron-withdrawing groups or both, such systems also have been stabilized by peri-annulation. Although strictly spoken, these peri-annulated compounds are no longer real acenes, they have fascinating properties as well. Herein, we describe the first synthesis of a new series of peri-annulated acenes with up to 13 linearly fused rings, which is unprecedented till date. Furthermore, this new series contains perylene units connected through benzene rings along their [b,k]edges, responsible for unique absorption and emission properties.

Modular Synthesis of Organoboron Helically Chiral Compounds: Cutouts from Extended Helices

Two types of helically chiral compounds bearing one and two boron atoms were synthesized by a modular approach. Formation of the helical scaffolds was executed by the introduction of boron to flexible biaryl and triaryl derived from small achiral building blocks. All-ortho-fused azabora[7]helicenes feature exceptional configurational stability, blue or green fluorescence with quantum yields (Φfl ) of 18-24 % in solution, green or yellow solid-state emission (Φfl up to 23 %), and strong chiroptical response with large dissymmetry factors of up to 1.12×10-2 . Azabora[9]helicenes consisting of angularly and linearly fused rings are blue emitters exhibiting Φfl of up to 47 % in CH2 Cl2 and 25 % in the solid state. As revealed by the DFT calculations, their P-M interconversion pathway is more complex than that of H1. Single-crystal X-ray analysis shows clear differences in the packing arrangement of methyl and phenyl derivatives. These molecules are proposed as primary structures of extended helices.

Octagon-Embedded Carbohelicene as a Chiral Motif for Circularly Polarized Luminescence Emission of Saddle-Helix Nanographenes

We report a new family of hexa-peri-hexabenzocoronene (HBC)-based helical nanographenes incorporating π-extended carbo[5]helicenes bearing an octagonal carbocycle. This family represents a new kind of highly distorted saddle-helix hybrid nanographenes. For the first time, the eight-membered ring becomes a constituent of both a carbo[5]helicene and a HBC and thus, the negative curvature is responsible for twisting both units. This novel chiral motif, namely, oct-[5]helicene results in the largest torsion angle recorded so far for a carbo[5]helicene (θ=79.5°), as it has been suggested by DFT-calculations and confirmed by X-ray crystallography. Consequently, the barriers of isomerization become exceptionally high for a [5]helicene unsubstituted in the fjord region since neither racemization nor decomposition were observed at 200 °C for 1 or 3 during 5 h. Therefore, racemic resolutions allowed subsequent chiroptical studies showing the ECD and CPL responses of this novel family of chiral nanographenes.

Edge-Functionalized Nanographenes

Nanographenes (NGs) have recently emerged as new carbon materials. Their nanoscale size results in a size-dependent quantum confinement effect, opening the band gap by a few eV. This energy gap allows NGs to be applied as optical materials. This property has attracted researchers across multiple scientific fields. The photophysical properties of NGs can be manipulated by introducing organic groups onto their basal planes and/or into their edges. In addition, the integration of organic functional groups into NGs results in NG-based hybrid materials. These features make the post-synthetic modification of NGs an active research area. As obtainable information on chemically functionalized NGs is limited owing to their nonstoichiometry and structural uncertainty, their structural characterization requires a combination of multiple spectroscopic methods. Therefore, information on the characterization procedures of recently published chemically functionalized NGs is of value for advancing the field of NG-based hybrid materials. The present review focuses on the structural characterization of chemically functionalized NGs. It is hoped that this review will help to advance this field.

Conformation and Aromaticity Switching in a Curved Non-Alternant sp2 Carbon Scaffold

A curved sp2 carbon scaffold containing fused pentagon and heptagon units (1) was synthesized by Pd-catalyzed [5+2] annulation from a 3,9-diboraperylene precursor and shows two reversible oxidation processes at low redox potential, accompanied by a butterfly-like motion. Stepwise oxidation produced radical cation 1.+ and dication 12+ . In the crystal structure, 1 exhibits a chiral cisoid conformation and partial π-overlap between the enantiomers. For the radical cation 1.+ , a less curved cisoid conformation is observed with a π-dimer-type arrangement. 12+ adopts a more planar structure with transoid conformation and slip-stacked π-overlap with closest neighbors. We also observed an intermolecular mixed-valence complex of 1⋅(1.+ )3 that has a huge trigonal unit cell [(1)72 (SbF6 )54 ⋅(hexane)101 ] and hexagonal columnar stacks. In addition to the conformational change, the aromaticity of 1 changes from localized to delocalized, as demonstrated by AICD and NICS(1)zz calculations.

An Isosteric Triaza Analogue of a Polycyclic Aromatic Hydrocarbon Monkey Saddle

Since a few years, the interest in negatively-curved fused polycyclic aromatic hydrocarbons (PAHs) has significantly increased. Recently, the first chiral negatively-curved PAH with the topology of a monkey saddle was introduced. Herein the synthesis of its triaza congener is reported. The influence of this CH↔N exchange on photophysical and electrochemical properties is studied as well as the isomerization process of the enantiomers. The aza analogue has a significantly higher inversion barrier, which makes it easier to handle at room temperature. All experimental results are underpinned by theoretical DFT calculations.

NIR-Absorbing π-Extended Azulene: Non-Alternant Isomer of Terrylene Bisimide

The first planar π-extended azulene that retains aromaticity of odd-membered rings was synthesized by [3+3] peri-annulation of two naphthalene imides at both long-edge sides of azulene. Using bromination and subsequent nucleophilic substitution by methoxide and morpholine, selective functionalization of the π-extended azulene was achieved. Whilst these new azulenes can be regarded as isomers of terrylene bisimide they exhibit entirely different properties, which include very narrow optical and electrochemical gaps. DFT, TD-DFT, as well as nucleus-independent chemical shift calculations were applied to explain the structural and functional properties of these new π scaffolds. Furthermore, X-ray crystallography confirmed the planarity of the reported π-scaffolds and aromaticity of their azulene moiety.

2,7,11,16-Tetra-tert-Butyl Tetraindenopyrene Revisited by an "Inverse" Synthetic Approach

A new synthetic route to tetraindenopyrene (TIP)—a bowl‐shaped cut‐out structure of C₇₀—is reported. The key step in this approach is a fourfold palladium‐catalyzed C−H activation that increases the yield more than 50 times in comparison to the approach originally described by Scott and co‐workers. Besides examination of its optoelectronic properties and study of its aggregation in solution, TIP was also re‐investigated by dispersion‐corrected DFT methods, which showed that dispersion interactions significantly increase the bowl‐to‐bowl inversion barrier. Furthermore, TIP was used as a semiconductor in p‐channel thin‐film transistors (TFTs).

The Macrocycle versus Chain Competition in On-Surface Polymerization: Insights from Reactions of 1,3-Dibromoazulene on Cu(111)

Ring/chain competition in oligomerization reactions represents a long‐standing topic of synthetic chemistry and was treated extensively for solution reactions but is not well‐understood for the two‐dimensional confinement of surface reactions. Here, the kinetic and thermodynamic principles of ring/chain competition in on‐surface synthesis are addressed by scanning tunneling microscopy, X‐ray photoelectron spectroscopy, and Monte Carlo simulations applied to azulene‐based organometallic oligomers on Cu(111). Analysis of experiments and simulations reveals how the ring/chain ratio can be controlled through variation of coverage and temperature. At room temperature, non‐equilibrium conditions prevail and kinetic control leads to preferential formation of the entropically favored chains. In contrast, high‐temperature equilibrium conditions are associated with thermodynamic control, resulting in increased yields of the energetically favored rings. The optimum conditions for ring formation include the lowest possible temperature within the regime of thermodynamic control and a low coverage. The general implications are discussed and compared to the solution case.

Helical Nanographenes Containing an Azulene Unit: Synthesis, Crystal Structures, and Properties

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.