Azaarenes: 13 Rings in a Row by Cyclopentannulation

Cyclopentannulation was explored as a strategy to access large, stable azaarenes. Buchwald-Hartwig coupling of previously reported di- and tetrabrominated cyclopentannulated N,N'-dihydrotetraazapentacenes furnished stable azaarenes with up to 13 six-membered rings in a row and a length of 3.1 nm. Their optoelectronic and semi-conducting properties as well as their aromaticity were investigated.

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 (C₂₇₆N₄₈)—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).

Persistent Ambipolar Heptacenes and Their Redox Species

Sixfold TIPS‐ethynylation combined with fourfold bromination of the armchair edges furnishes a long‐lived, soluble heptacene; π‐extension via Stille coupling accesses a persistent tetrabenzononacene. Both types of acenes were stabilized best by double TIPS‐ethynylation on every other benzene ring. Tetrabromoheptacene is an ambipolar transistor material (up to 0.036 cm² V⁻¹ s⁻¹ n‐channel), which was corroborated by generation of its monoanion and monocation.

Pushing the Length Limit of Dihydrodiboraacenes: Synthesis and Characterizations of Boron-Embedded Heptacene and Nonacene

Boron-embedded heteroacenes (boraacenes) have attracted enormous interest in organic chemistry and materials science. However, extending the skeleton of boraacenes to higher acenes (N≥6) is synthetically challenging because of their limited stability under ambient conditions. Herein, we report the synthesis of boron-embedded heptacene (DBH) and nonacene (DBN) as the hitherto longest boraacenes. The former is highly stable (even after 240 h in tetrahydrofuran), while the latter is air-sensitive with the half-life (t_1/2 ) of 11.8 min. The structures of both compounds are verified by single-crystal X-ray diffraction, revealing a linear backbone with an antiaromatic C₄ B₂ core. Photophysical characterizations associated with theoretical calculations indicate that both compounds exhibit highly efficient anti-Kasha emissions. Remarkably, the air-stable DBH manifests an ultrahigh photoluminescence quantum yield (PLQY) of 98±2 % and can be chemically reduced to its radical anion and dianion states, implying the value of boron-doped higher acenes as novel functional materials.

Electron-Deficient Heteroacenes that Contain Two Boron Atoms: Near-Infrared Fluorescence Based on a Push-Pull Effect*

Electron-deficient heteroacenes that contain two tricoordinate boron atoms in their acene skeletons and planarized phenyl ether moieties at their periphery were synthesized via the borylation of silicon-bridged precursors. X-ray crystallographic analysis revealed quinoidal structures, which give rise to two-step reversible redox processes for both the reduction and oxidation. These compounds exhibit intense absorption and sharp fluorescence bands with vibronic structures in the near-infrared (NIR) region. These properties originate from the push-pull effect along the long axis of the molecule derived from the electron-donating ether moieties and the electron-accepting boron moieties. Of particular note is the NIR emission of the thienothiophene-centered heteroacene, which has a maximum at 952 nm with a narrow band width of 309 cm^-1 in cyclohexane. A Franck-Condon analysis revealed the crucial role of the sterically less-hindered thienothiophene spacer in achieving this sharp emission band.

Persistent peri-Heptacene: Synthesis and In Situ Characterization

n‐peri‐Acenes (n‐PAs) have gained interest as model systems of zigzag‐edged graphene nanoribbons for potential applications in nanoelectronics and spintronics. However, the synthesis of n‐PAs larger than peri‐tetracene remains challenging because of their intrinsic open‐shell character and high reactivity. Presented here is the synthesis of a hitherto unknown n‐PA, that is, peri‐heptacene (7‐PA), in which the reactive zigzag edges are kinetically protected with eight 4‐tBu‐C₆H₄ groups. The formation of 7‐PA is validated by high‐resolution mass spectrometry and in situ FT‐Raman spectroscopy. 7‐PA displays a narrow optical energy gap of 1.01 eV and exhibits persistent stability (t_1/2≈25 min) under inert conditions. Moreover, electron‐spin resonance measurements and theoretical studies reveal that 7‐PA exhibits an open‐shell feature and a significant tetraradical character. This strategy could be considered a modular approach for the construction of next‐generation (3 N+1)‐PAs (where N≥3).

Rules of Nucleophilic Additions to Zigzag Nanographene Diones*

Nucleophilic addition of carbon-centered nucleophiles to nanographene ketones represents a valuable late-stage method for the functionalization of zigzag nanographenes, but its use is rare in the chemical literature. Using two model systems, non-Kekulé triangulene-4,8-dione and Kekulé anthanthrone, we identify unexpected regioselectivities and uncover the rules that govern these reactions. Considering the large number of nanographene ketones that have been reported since the pioneering work of Eric Clar, this method enables synthesis and exploration of hitherto unknown functionalized nanographenes.

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.

Pushing the Limits of Acene Chemistry: The Recent Surge of Large Acenes

Acenes, consisting of linearly fused benzene rings, are an important fundamental class of organic compounds with various applications. Hexacene is the largest acene that was synthesized and isolated in the 20th century. The next largest member of the acene family, heptacene, was observed in 2007 and since then significant progress in preparing acenes has been reported. Significantly larger acenes, up to undecacene, could be studied by means of low-temperature matrix isolation spectroscopy with in situ photolytic generation, and up to dodecacene by means of on-surface synthesis employing innovative precursors and highly defined crystalline metal surfaces under ultrahigh vacuum conditions. The review summarizes recent experimental and theoretical advances in the area of acenes that give a significantly deeper insight into the fundamental properties and nature of the electronic structure of this fascinating class of organic compounds.

(Aza)Pentacenes Clipped into a Ring: Stabilization of Large (Aza)Acenes

A doubly alkylene bridged 6,13‐diphenylpentacene and analogously bridged azapentacenes were prepared; they are persistent. The doubly bridged azapentacenes display superior photochemical, oxidative and thermal stabilities compared to azapentacenes protected by bis(TIPS‐ethynyl)‐substituents—clipping an azaacene into a large ring is a viable complement in stabilization.

The substituent effect on the photophysical and charge transport properties of non-planar dibenzo[a,m]rubicenes

This paper will provide some guidelines on developing new non-planar organic semiconductors with high charge carrier mobilities applied in optoelectronic devices.

Synthesis of a Sidewall Fragment of a (12,0) Carbon Nanotube

Synthesis of a carbon nanobelt (CNB) is a very challenging task in organic chemistry. Herein, we report the successful synthesis of an octabenzo[12]cyclacene based CNB (6), which can be regarded as a sidewall fragment of a (12,0) carbon nanotube. The key intermediate compound, a tetraepoxy nanobelt (5), was first synthesized by Diels-Alder reaction, and subsequent reductive aromatization gave the fully conjugated CNB 6. X-ray crystallographic analysis unambiguously confirmed the belt-shaped structure of 6. ¹ H NMR spectrum and theoretical calculations (ACID, NICS, and 2D/3D ICSS) revealed localized aromaticity and stronger shielding chemical environment in the inner region of the belt. The optical properties (absorption and emission) of 6 were studied and correlated to its electronic structure. Strain analysis indicates that the phenyl substituents at the zigzag edges are crucial to the successful synthesis of 6. This report presents a new strategy towards highly strained CNBs.

Nickel(II) Bisporphyrin-Fused Pentacenes Exhibiting Abnormal High Stability

A series of largely π-extended multichromophoric molecules including cross-conjugated, half cross-conjugated, conjugation-interrupted and linearly conjugated systems were synthesized and characterized. These multichromophoric molecular systems revealed interesting structural-property relationships. Bisporphyrin-fused pentacenes Pen-1 b and Pen-2 a showed rich redox chemistry with 7 and 8 observable redox states, respectively. The linearly-conjugated bisporphyrin-fused pentacenes (Pen-1 b and Pen-2 a) possess much narrower HOMO-LUMO gaps (1.65 and 1.42 eV redox, respectively) and higher HOMO energy levels than those of their pentacene analogues (2.23 and 2.01 eV redox, respectively), similar to those of much less stable hexacenes and heptacenes. An estimated half-life of >945 h was obtained for bisporphyrin-fused pentacene Pen-2 a, which is much longer than that of its pentacene analogue (BPE-P, half-life, 33 h).

Tetrabenzononacene: "Butterfly Wings" Stabilize the Core

In combination with bulky substituents at the core, fourfold benzannulation at the cata‐positions stabilizes a nonacene sufficiently to allow its isolation and characterization by ¹H NMR and X‐ray analysis. The four benzo units blueshift the absorption spectrum in comparison to a solely linear nonacene, but significantly increase the stability in the solid state.

Fused Pyrazine- and Carbazole-Containing Azaacenes: Synthesis and Properties

A new family of azaacenes has been designed and synthesized by incorporating the electron-withdrawing sp² -hybridized nitrogen of pyrazine and electron-donating nitrogen of carbazole in a molecular skeleton. Two different conjugated lengths of 8-ring aza-nonacene and 10-ring aza-undecene have been achieved by an efficient condensation reaction. The unique optoelectronic properties of these molecules were investigated using both experimental and theoretical techniques. The azaacenes show visible-region absorption and near-infrared (NIR) fluorescence. These compounds can serve as hole-transport semiconductors for solution-processed organic field-effect transistors (OFETs). Single-crystal transistor devices of one of the aza-nonacenes exhibit hole charge transport behavior with a hole mobility of 0.07 cm² /Vs and an on/off current ratio of 1.3x10⁶ .

X-Shaped Polycyclic Aromatic Hydrocarbons: Optical Properties and Tunable Assembly Ability

Although a number of synthetic methodologies have been developed to prepare stable polycyclic aromatic hydrocarbons (PAHs), much less research has been devoted to functionalizing the peripheries of molecules to tune the self-assembly ability or introduce functional groups without altering their photophysical properties. Herein, we report twisted "X"-shaped molecules prepared through annulation of hexacene with benzoanthracene on the zigzag edge, and an investigation of their photophysical properties and self-assembly properties. The shape-complementary "X"-shaped molecules prefer to dimerize, while the π-extension would lead to one-dimensional π-stacking. Our findings give some insights into the design of stable PAHs without disturbing the electronic structures.

Diazuleno-s-indacene Diradicaloids: Syntheses, Properties, and Local (anti)Aromaticity Shift from Neutral to Dicationic State

Non-alternant, non-benzenoid π-conjugated polycyclic hydrocarbons (PHs) are expected to exhibit very different electronic properties from all-benzenoid PHs. Reported herein are the syntheses and physical properties of four derivatives of two azulene-fused s-indacene isomers, the diazuleno[2,1-a:2',1'-g]-s-indacene (DAI-1) and diazuleno[2,1-a:1',2'-h]-s-indacene (DAI-2). The backbone of both isomers contains 28π electrons and is a 7-5-5-6-5-5-7 fused ring system. X-ray crystallographic analysis, NMR spectra, and theoretical calculations (ACID, NICS) reveal a structure with two aromatic azulene units fused with a central anti-aromatic s-indacene moiety. All compounds exhibit open-shell diradical character and are magnetically active, but the derivatives of DAI-2 show larger radical character than the respective ones of DAI-1. Their dications were isolated in crystalline form and all experimental and theoretical analyses disclose a shift of local (anti)aromaticity along the backbone, with two aromatic tropylium rings at the termini.

Higher Acenes by On-Surface Dehydrogenation: From Heptacene to Undecacene

A unified approach to the synthesis of the series of higher acenes up to previously unreported undecacene has been developed through the on-surface dehydrogenation of partially saturated precursors. These molecules could be converted into the parent acenes by both atomic manipulation with the tip of a scanning tunneling and atomic force microscope (STM/AFM) as well as by on-surface annealing. The structure of the generated acenes has been visualized by high-resolution non-contact AFM imaging and the evolution of the transport gap with the increase of the number of fused benzene rings has been determined on the basis of scanning tunneling spectroscopy (STS) measurements.

Synthesis, Crystal Analysis, and Optoelectronic Properties of Diazole-Functionalized Acenes and Azaacenes

Doping heteroatoms into the skeletons of parent acenes can provide more opportunities to construct novel thermally and photostable organic π-conjugated semiconductors. Herein, a family of diazole-decorated acenes (APyS and APySe) and azaacenes (PyP, PyTh, PyPy, PyPh, and PyAP) have been successfully synthesized through the classical reactions. Single-crystal X-ray analyses showed that these as-formed diazole-modified derivatives adopted a twisted topology configuration, whereas the azaacenes display reclining-chair architectures, besides a twisted structure. All these compounds displayed yellow or red light in solution. Moreover, their electrochemical behaviors were also examined. We also found that the azaacenes exhibited a positive spectroscopic response to acid.

n-Type Azaacenes Containing B←N Units

We disclose a novel strategy to design n-type acenes through the introduction of boron-nitrogen coordination bonds (B←N). We synthesized two azaacenes composed of two B←N units and six/eight linearly annelated rings. The B←N unit significantly perturbed the electronic structures of the azaacenes: Unique LUMOs delocalized over the entire acene skeletons and decreased aromaticity of the B←N-adjacent rings. Most importantly, these B←N-containing azaacenes exhibited low-lying LUMO energy levels and high electron affinities, thus leading to n-type character. The solution-processed organic field-effect transistor based on one such azaacene exhibited unipolar n-type characteristics with an electron mobility of 0.21 cm²  V^-1  s^-1 .