Naphthalene diimide-Annulated Heterocyclic Acenes: Synthesis, Electrochemical and Semiconductor Properties and their Multifaceted Applications

Acenes and Naphthalene Diimides (NDIs) stand as distinguished classes of organic compounds, each possessing unique and intriguing properties that have garnered significant attention across various scientific disciplines. Acenes, characterized by linearly fused aromatic rings, have captivated researchers due to their diverse electronic structures and promising applications in materials science. On the other hand, NDIs, known for their distinctive electron-accepting properties, exhibit remarkable versatility in fields ranging from organic electronics, supramolecular to spin chemistry. In this review, we navigate through the fascinating realms of both acenes and NDIs before converging our focus on the highly diverse and distinctive subgroup of NDI-annulated heterocyclic acenes. This potentially important subgroup, has emerged as a subject of intense investigation, encapsulating their fascinating synthesis, optical and electrochemical characteristics, and multifaceted applications that span the realms of chemistry, physics, and biology. Through the exploration of their synthetic strategies, unique properties, and diverse applications, this review aims to offer a comprehensive understanding of the pivotal role played by NDI-based heterocyclic acenes in contemporary multidisciplinary research and technological innovation.

Isolation, Structural and Physical Characterization as well as Reactivity of Persistent Acenium Radical Cation Salts

The unsubstituted acenium radical cations (ARCs) are extremely sensitive and were hitherto only studied in situ, i.e. in the gas phase, as dilute solutions in strong acids or by matrix isolation spectroscopy at about 10 K. In this study, we prepared, structurally, electro-chemical-ly and spectroscopically characterized room temperature stable ARC salts with the weakly coordi-nating anion [F{Al(ORF)3}2]- (ORF = -OC(CF3)3) supported by the weakly coordi-nating sol-vent 1,2,3,4-tetrafluorobenzene (TFB). Reaction of the neutral acenes with Ag+[F{Al(ORF)3}2]- led, non-innocent, to intermediate [Ag2(acene)2]2+ complexes, which decompose over time to Ag0 and the corresponding (impure) ARC salts. By contrast, direct deelectronation with the recently developed innocent deelectronator radical cation salt [anthraceneHal]+∙[F{Al(ORF)3}2]- led to phase-pure products [acene]+∙[F{Al(ORF)3}2]- (anthraceneHal = 9,10-dichlorooctafluoro-anthracene; acene = anthra-, tetra-, pentacene). For the first time, a homogenous set of spectroscopic data on analytically pure ARC salts was obtained. In addition, cyclo-voltammetric measure-ments of the acenes connected the potentials in solution with those in the gas-phase. Hence, the data complement the existing isolated gas-phase, strong acid or matrix isolation studies. A first entry to follow-up chemistry of the acenium radical cations as ligand forming oxidizers was demonstrated by reaction with ½ Co2(CO)8 giving [Co(anthracene)(CO)2]+.

Pyridone-Doped Acenes with Improved Stability

Besides the peripheral modification, the introduction of heteroatoms to modulate the property of longer acenes with improved chemical stability has been extensively studied for their potential applications in organic electronics. However, the utilization of 4-pyridone, a common unit in the air- and photo-stable acridone and quinacridone, to decorate higher acenes with increased stability has not been realized yet. Here we present the synthesis of a series of monopyridone-doped acenes up to heptacene via the palladium-catalyzed Buchwald-Hartwig amination of aniline and dibromo-ketone. The effect of pyridone on the property of doped acenes were investigated experimentally and computationally. With the π-extension of doped acenes, the pyridone ring shows the weakened conjugation and gradual loss of aromaticity. The doped acenes demonstrate enhanced stability in solution and maintain the electron communication between the acene planes.

Diazadiboraacenes: Synthesis, Spectroscopy and Computations

The incorporation of heteroatoms into hydrocarbon compounds greatly expands the chemical space of molecular materials. In this context, B-N doping takes a center stage due to its isosterism with a C=C-bond. Herein, we present a new and modular synthetic concept to access novel diazadiborabenzo[b]triphenylenes 7 a-h using the B-N doped biradical 16 as intermediate. Characterization of the photophysical properties revealed the emission spectra of the diazadibora benzo[b]triphenylenes 7 a-h can conveniently be tuned by small changes of the substitution on the boron-atom. All of the diazadibora compounds show a short life-time phosphorescence. Additionally, we were able to rationalize the excited-state relaxation of the diazadiboraacene 7 a via intersystem crossing by quantum chemical calculations. The new synthetic strategy provides an elegant route to various novel B-N doped acenes with great potential for applications in molecular materials.

Closed Aromatic Tubes-Capsularenes

In this study, we describe a synthetic method for incorporating arenes into closed tubes that we name capsularenes. First, we prepared vase-shaped molecular baskets 4-7. The baskets comprise a benzene base fused to three bicycle[2.2.1]heptane rings that extend into phthalimide (4), naphthalimide (6), and anthraceneimide sides (7), each carrying a dimethoxyethane acetal group. In the presence of catalytic trifluoroacetic acid (TFA), the acetals at top of 4, 6 and 7 change into aliphatic aldehydes followed by their intramolecular cyclization into 1,3,5-trioxane (¹ H NMR spectroscopy). Such ring closure is nearly a quantitative process that furnishes differently sized capsularenes 1 (0.7×0.9 nm), 8 (0.7×1.1 nm;) and 9 (0.7×1.4 nm;) characterized by X-Ray crystallography, microcrystal electron diffraction, UV/Vis, fluorescence, cyclic voltammetry, and thermogravimetry. With exceptional rigidity, unique topology, great thermal stability, and perhaps tuneable optoelectronic characteristics, capsularenes hold promise for the construction of novel organic electronic devices.

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.

Bending versus Twisting Acenes - A Computational Study

Polycyclic aromatic hydrocarbons (PAHs) are widely used in organic electronic devices. The electronic, magnetic, and optical properties of PAHs can be tuned by structural modifications to the aromatic backbone to introduce an inherent distortion from planarity, such as bending or twisting. However, it remains difficult to isolate and control the effects of such distortions. Here, we sought to understand how backbone twisting and bending affect the electronic properties of acenes, as models for larger PAHs. We found that, even when highly distorted from planarity (30° per ring), acenes maintain their aromatic character and π orbital delocalization with minor mixing of the σ and π orbitals. In addition, the energy gap between the HOMO and LUMO decreases with increasing twist, while the gap is hardly affected by bending, since the energy of both orbitals increase to a similar extent. For bent acenes in the triplet state, the spin becomes more localized with increasing bend, whereas twisting produces an evenly distributed spin delocalization. These findings can guide the synthesis of PAHs with tailored properties.

Ultrasound-assisted synthesis of RO- and RS-substituted (hetero)acenes via oxo- and thio-Friedel-Crafts/Bradsher reactions

Two heteroatom-variants of the Friedel-Crafts/Bradsher cyclization of o-acetalaryl(aryl)methyl ethers and o-dithioacetalaryl(aryl)methyl thioethers, have been realized with the ultrasound assistance. The environmentally friendly "oxo-variant" (Oxo-F-C/B), proceeding in a medium containing mineral acid and a large amount of water (HCl_aq/CH₃CN) led to a very efficient formation of RO-substituted (hetero)acenes in less than 5 min. In the "thio-variant" (Thio-F-C/B), o-dithioacetalaryl(aryl)methyl thioethers underwent ultrasound-assisted cyclization in nonaqueous medium (FeCl₃/KI/EtOH) in less than 25 min., in lower yields than in the "oxygen variant" to give RS-substituted (hetero)acenes. The RO-(hetero)acenes cyclized at 25-60 °C in aqueous media but did not cyclize in organic solvents while the RS-(hetero)acenes required higher temperatures 55-60 °C and cyclized in organic solvents but did not react in aqueous media. The acceleration of the ultrasound-assisted reactions compared to the reactions carried out under silent conditions exceeded 7500 times in the most effective example of the oxo-variant and on average 2 times for the thio-variant. The plausible reaction mechanisms under ultrasound and silent conditions have been proposed. The ultrasonic mechanism involves disturbing of solvation layers and formation of the reactive ("naked") carbocations upon operation of the shock wave produced by the bubble collapse. The o-acetalaryl(aryl)methyl ethers underwent a selective ultrasound-assisted deacetalization to give o-formylaryl(aryl)methyl ethers, without subsequent cyclization under the acidic reaction conditions.

Strategies for the Synthesis of Higher Acenes

The outstanding performance of pentacene-based molecules in molecular electronics, as well as the predicted enhanced semiconducting properties of extended acenes, have stimulated the development of new synthetic methods and functionalization strategies for the preparation of stable and soluble acenes larger than tetracene with the aim of obtaining improved functional materials.

Effect of Diels-Alder Reaction in C60-Tetracene Photovoltaic Devices

Developing organic photovoltaic materials systems requires a detailed understanding of the heterojunction interface, as it is the foundation for photovoltaic device performance. The bilayer fullerene/acene system is one of the most studied models for testing our understanding of this interface. We demonstrate that the fullerene and acene molecules chemically react at the heterojunction interface, creating a partial monolayer of a Diels-Alder cycloadduct species. Furthermore, we show that the reaction occurs during standard deposition conditions and that thermal annealing increases the concentration of the cycloadduct. The cycloaddition reaction reduces the number of sites available at the interface for charge transfer exciton recombination and decreases the charge transfer state reorganization energy, increasing the open circuit voltage. The submonolayer quantity of the cycloadduct renders it difficult to identify with conventional characterization techniques; we use atom probe tomography to overcome this limitation while also measuring the spatial distribution of each chemical species.