Recent Progress in Polycyclic Aromatic Hydrocarbon‐Based Organic Co‐Crystals

Recently, the development of polycyclic aromatic hydrocarbon ( PAH )‐based organic co‐crystals has attracted increasing interest due to their unique packing modes, optic‐electronic properties and various potential applications in electronic, optic‐electronic and magnetic devices. In this account, we mainly discuss the definition, classification, packing patterns, preparation methods, and applications of PAH‐based co‐crystals. Specifically, the main categories of PAH‐based organic co‐crystals, the frequent methods to prepare them, three main packing patterns, their optical and electrical properties, and their potential applications will be presented. Finally, an outlook of this field is provided.     

Non‐linear,cata‐Condensed,Polycyclic Aromatic Hydrocarbon Materials: A Generic Approach and Physical Properties

A generic approach to the regiospecific synthesis of halogenated polycyclic aromatics is made possible by the one‐ or two‐directional benzannulation reactions of readily available (ortho‐allylaryl)trichloroacetates (the “BHQ” reaction). Palladium‐catalysed cross‐coupling reactions of the so‐formed haloaromatics enable the synthesis of functionalised polycyclic aromatic hydrocarbons (PAHs) with surgical precision. Overall, this new methodology enables the facile mining of chemical space in search of new electronic functional materials.     

Perylene‐Fused,Aggregation‐Free Polycyclic Aromatic Hydrocarbons for Solution‐Processed Distributed Feedback Lasers

Perylene‐fused, aggregation‐free polycyclic aromatic hydrocarbons with partial zigzag periphery ( ZY‐01 , ZY‐02 , and ZY‐03 ) were synthesized. X‐ray crystallographic analysis reveals that there is no intermolecular π–π stacking in any of the three molecules, and as a result, they show moderate‐to‐high photoluminescence quantum yield in both solution and in the solid state. They also display the characteristic absorption and emission spectra of perylene dyes. ZY‐01 and ZY‐02 with a nearly planar π‐conjugated skeleton exhibit amplified spontaneous emission (ASE) when dispersed in polystyrene thin films. Solution‐processed distributed feedback lasers have been fabricated using ZY‐01 and ZY‐02 as active gain materials, both showing narrow emission linewidth (<0.4 nm) at wavelengths around 515 and 570 nm, respectively. In contrast, ZY‐03 did not show ASE and lasing, presumably due to its highly twisted backbone, which facilitates nonradiative internal conversion and intersystem crossing.     

Synthesis and Characterization of Polycyclic Aromatic Hydrocarbons with Different Spatial Constructions Based on Hexaphenylbenzene Derivatives

In recent years, low‐bandgap polymers have attracted much attention in a wide range of fields. The synthesis of these compounds has been focused on three factors according to the Roncali bandgap theory: 1) the degree of bond‐length alternation (E ^δr), 2) the aromatic resonance energy of the cycle (E ^Res), and 3) the substituted groups (E ^Sub). Herein, we have designed and prepared low‐bandgap polymers in a different way by using the factors E ^θ (the deviation from planarity of the polymer chain) and E ^Int (the interaction of the molecular chains in the solid state). Thus, three polycyclic aromatic hydrocarbons with different spatial constructions, based on hexaphenylbenzene derivatives, were prepared in this work: linear ( P1‐OX ), V ( P2‐OX ), and zigzag ( P3‐OX ) types. These well‐defined polymers exhibited interesting optical and electrochemistry behavior due to their different extents of planarity. Matrix‐assisted laser desorption ionization time‐of‐flight mass spectrometry gave the incremental orderly molecular weight distributions of P1 , P2 , and P3 , the weight‐average molecular weights ( ) of which were 9000, 5500, and 69 000, respectively. Their lamellar layer structures and π–π intermolecular stacking were demonstrated by using two‐dimensional grazing‐incidence X‐ray diffraction, which revealed the edge‐on chain conformation. Finally, the materials were perfectly adapted to fabricate high‐performance organic field‐effect transistor devices, which revealed that these compounds could have great prospects as semiconductors.     

A Luminescent Nitrogen‐Containing Polycyclic Aromatic Hydrocarbon Synthesized by Photocyclodehydrogenation with Unprecedented Regioselectivity

We present a nitrogen‐containing polycyclic aromatic hydrocarbon (N‐PAH), namely 12‐methoxy‐9‐(4‐methoxyphenyl)‐5,8‐diphenyl‐4‐(pyridin‐4‐yl)pyreno[1,10,9‐h,i,j]isoquinoline (c‐TPE‐ON), which exhibits high quantum‐yield emission both in solution (blue) and in the solid state (yellow). This molecule was unexpectedly obtained by a three‐fold, highly regioselective photocyclodehydrogenation of a tetraphenylethylene‐derived AIEgen. Based on manifold approaches involving UV/Vis, photoluminescence, and NMR spectroscopy as well as HRMS, we propose a reasonable mechanism for the formation of the disk‐like N‐PAH that is supported by density functional theory calculations. In contrast to most PAHs that are commonly used, our system does not suffer from entire fluorescence quenching in the solid state due to the peripheral aromatic rings preventing π–π stacking interactions, as evidenced by single‐crystal X‐ray analysis. Moreover, its rod‐like microcrystals exhibit excellent optical waveguide properties. Hence, c‐TPE‐ON comprises a N‐PAH with unprecedented luminescent properties and as such is a promising candidate for fabricating organic optoelectronic devices. Our design and synthetic strategy might lead to a more general approach to the preparation of solution‐ and solid‐state luminescent PAHs.     

Leveraging Fluorescent Emission to Unitary Yield: Dimerization of Polycyclic Aromatic Hydrocarbons

We report on the synthesis and characterization of novel substituted 1,1′‐biperylene‐2,2′‐diols in which the dihedral angle between the two polycyclic aromatic hydrocarbon (PAH) units is tailored from ca. 60° to ca. 90° in the solid state by introduction of cyclo‐etheric straps or sterically hindered groups such as the triisopropylsilyl (TIPS) group. Depending on the type of substitution, we lock the dihedral angle between the perylenyl moieties enabling fine‐tuning of the molecular optoelectronic properties, with the molecules displaying the smallest angles acting as exceptionally strong emitters with unitary quantum yields.     

Postfunctionalization of BN‐Embedded Polycyclic Aromatic Compounds for Fine‐Tuning of Their Molecular Properties

New BN‐embedded, thiophene‐fused, polycyclic aromatic compounds with planar geometry were designed and synthesized. The molecules showed excellent stability and chemical robustness. Postfunctionalization on this skeleton was demonstrated with a series of electrophilic bromination, palladium‐catalyzed cross‐coupling, and Knoevenagel condensation reactions. The π skeleton remained intact during these late‐stage transformations. The optical and electronic properties have been well tuned through incorporation of electron‐rich and ‐deficient groups on the backbone. This work shows the great advantage of the postfunctionalization strategy on BN‐containing polycyclic aromatic compounds for fast diversification and materials screening.     

溴代稠环芳烃的氧化溴化法合成

以HBr/H2O2为溴化体系,采用氧化溴化法合成了6种稠环芳烃的溴化物。 合成反应无需催化剂,通过控制反应温度和溴化试剂用量,可以在稠环芳烃化合物的活性位选择性地单溴化或双溴化,产率可达51.1%～94.2%。 产品经熔点和1H NMR法确认,合成操作简单安全,环境污染少,有工业应用前景。     

Boron‐Containing Polycyclic Aromatic Hydrocarbons: Facile Synthesis of Stable,Redox‐Active Luminophores

Herein we show that replacing the two meso carbon atoms of the polycyclic aromatic hydrocarbon (PAH) bisanthene by boron atoms transforms a near‐infrared dye into an efficient blue luminophore. This observation impressively illustrates the impact of boron doping on the frontier orbitals of PAHs. To take full advantage of this tool for the targeted design of organic electronic materials, the underlying structure–property relationships need to be further elucidated. We therefore developed a modular synthesis sequence based on a Peterson olefination, a stilbene‐type photocyclization, and an Si–B exchange reaction to substantially broaden the palette of accessible polycyclic aromatic organoboranes and to permit a direct comparison with their PAH congeners.     

Perylene-Fused,Aggregation-Free Polycyclic Aromatic Hydrocarbons for Solution-Processed Distributed Feedback Lasers

Perylene-fused, aggregation-free polycyclic aromatic hydrocarbons with partial zigzag periphery ( ZY-01 , ZY-02 , and ZY-03 ) were synthesized. X-ray crystallographic analysis reveals that there is no intermolecular π–π stacking in any of the three molecules, and as a result, they show moderate-to-high photoluminescence quantum yield in both solution and in the solid state. They also display the characteristic absorption and emission spectra of perylene dyes. ZY-01 and ZY-02 with a nearly planar π-conjugated skeleton exhibit amplified spontaneous emission (ASE) when dispersed in polystyrene thin films. Solution-processed distributed feedback lasers have been fabricated using ZY-01 and ZY-02 as active gain materials, both showing narrow emission linewidth (<0.4 nm) at wavelengths around 515 and 570 nm, respectively. In contrast, ZY-03 did not show ASE and lasing, presumably due to its highly twisted backbone, which facilitates nonradiative internal conversion and intersystem crossing.     

BN‐Embedded Polycyclic Aromatic Hydrocarbon Oligomers: Synthesis,Aromaticity, and Reactivity

BN‐embedded oligomers with different pairs of BN units were synthesized by electrophilic borylation. Up to four pairs of BN units were incorporated in the large polycyclic aromatic hydrocarbons (PAHs). Their geometric, photophysical, electrochemical, and Lewis acidic properties were investigated by X‐ray crystallography, optical spectroscopy, and cyclic voltammetry. The B?N bonds show delocalized double‐bond characteristics and the conjugation can be extended through the trans‐orientated aromatic azaborine units. Calculations reveal the relatively lower aromaticity for the inner azaborine rings in the BN‐embedded PAH oligomers. The frontier orbitals of the longer oligomers are delocalized over the inner aromatic rings. Consequently, the inner moieties of the BN‐embedded PAH oligomers are more active than the outer parts. This is confirmed by a simple oxidation reaction, which has significant effects on the aromaticity and the intramolecular charge‐transfer interactions.     

Synthesis of Polycyclic Aromatic Hydrocarbons Decorated by Fluorinated Carbon Acids/Carbanions

The carboarylation reaction of biphenyl-alkynes was successfully triggered by electrophilic attack of 1,1-bis(triflyl)ethylene on the alkyne moiety to give polycyclic aromatic hydrocarbons (PAHs) decorated by superacidic carbon acid functionality. Neutralisation of thus obtained acids with NaHCO₃ yielded the corresponding sodium salts, which showed improved solubility in both aqueous and organic solvents.     

Understanding the Reactivity of Planar Polycyclic Aromatic Hydrocarbons: Towards the Graphene Limit

The Diels–Alder reactivity of maleic anhydride towards the bay regions of planar polycyclic aromatic hydrocarbons was explored computationally in the DFT framework. The process becomes more and more exothermic and the associated activation barriers become lower and lower when the size of the system increases. This enhanced reactivity follows an exponential behavior that reaches its maximum for systems having 18–20 benzenoid rings in their structures. This peculiar behavior was analyzed in detail by using the activation strain model of reactivity in combination with energy decomposition analysis. The influence of the change in the aromaticity of the polycyclic compound during the process on the respective activation barriers was also studied.     

π‐Extended Star‐Shaped Polycyclic Aromatic Hydrocarbons based on Fused Truxenes: Synthesis,Self‐Assembly,and Facilely Tunable Emission Properties

A new set of star‐shaped polycyclic aromatic hydrocarbons (PAHs) based on naphthalene‐fused truxenes, TrNaCn (n=1–4), were synthesized and characterized. The synthesis involved a microwave‐assisted six‐fold Suzuki coupling reaction, followed by oxidative cyclodehydrogenation. Multiple dehydrocyclization products could be effectively isolated in a single reaction, thus suggesting that the oxidative cyclodehydrogenation reaction involved a stepwise ring‐closing process. The thermal, optical, and electrochemical properties and the self‐assembly behavior of the resulting oxidized samples were investigated to understand the impact of the ring‐fusing process on the properties of the star‐shaped PAHs. Distinct bathochromic shift of the absorption maxima (λ_max) revealed that the molecular conjugation extended with the stepwise ring‐closing reactions. The optical band‐gap energy of these PAHs varied significantly on increasing the number of fused rings, thereby resulting in readily tunable emissive properties of the resultant star‐shaped PAHs. Interestingly, the generation of rigid “arms” by using perylene analogues caused TrNaC2 and TrNaC3 to show significantly enhanced photoluminescence quantum yields (PLQYs) in solution (η=0.65 and 0.66, respectively) in comparison with those of TrNa and TrNaC1 (η=0.08 and 0.16, respectively). Owing to strong intermolecular interactions, the TrNa precursor was able to self‐assemble into rod‐like microcrystals, which could be facilely identified by the naked eye, whilst TrNaC1 self‐assembled into nanosheets once the naphthalene rings had fused. This study offers a unique platform to gain further insight into—and a better understanding of—the photophysical and self‐assembly properties of π‐extended star‐shaped PAHs.     

A DFT‐Based Investigation of Hydrogen Abstraction Reactions from Methylated Polycyclic Aromatic Hydrocarbons

The growth of polycyclic aromatic hydrocarbons (PAHs) is in many areas of combustion and pyrolysis of hydrocarbons an inconvenient side effect that warrants an extensive investigation of the underlying reaction mechanism, which is known to be a cascade of radical reactions. Herein, the focus lies on one of the key reaction classes within the coke formation process: hydrogen abstraction reactions induced by a methyl radical from methylated benzenoid species. It has been shown previously that hydrogen abstractions determine the global PAH formation rate. In particular, the influence of the polyaromatic environment on the thermodynamic and kinetic properties is the subject of a thorough exploration. Reaction enthalpies at 298 K, reaction barriers at 0 K, rate constants, and kinetic parameters (within the temperature interval 700–1100 K) are calculated by using B3LYP/6‐31+G(d,p) geometries and BMK/6‐311+G(3df,2p) single‐point energies. This level of theory has been validated with available experimental data for the abstraction at toluene. The enhanced stability of the product benzylic radicals and its influence on the reaction enthalpies is highlighted. Corrections for tunneling effects and hindered (or free) rotations of the methyl group are taken into account. The largest spreading in thermochemical and kinetic data is observed in the series of linear acenes, and a normal reactivity–enthalpy relationship is obtained. The abstraction of a methyl hydrogen atom at one of the center rings of large methylated acenes is largely preferred. Geometrical and electronic aspects lie at the basis of this striking feature. Comparison with hydrogen abstractions leading to arylic radicals is also made.     

Epoxide‐Opening Cascades in the Synthesis of Polycyclic Polyether Natural Products

The structural features of polycyclic polyether natural products can, in some cases, be traced to their biosynthetic origin. However in case that are less well understood, only biosynthetic pathways that feature dramatic, yet speculative, epoxide‐opening cascades are proposed. We summarize how such epoxide‐opening cascade reactions have been used in the synthesis of polycyclic polyethers (see scheme) and related natural products.

     

Infrared Cavity Ringdown Spectroscopy of Jet‐Cooled Polycyclic Aromatic Hydrocarbons

Infrared absorption spectra of the CH stretching region were observed for naphthalene, anthracene, phenanthrene, pyrene, and perylene using a heated, supersonic, slit‐jet source and cavity ringdown spectroscopy. Band positions and intensities recorded with 0.2‐cm^?1 resolution were compared with previous gas‐phase and argon matrix isolation experiments, as well as theoretical calculations. The largest matrix shift in the absorption maximum (‐7.4 cm^?1) was observed for anthracene, with all others shifted by 3.0 cm^?1 or less. Spectral features in the supersonic jet spectrum were generally narrower than those observed in the Ar matrix, with the largest matrix broadening found for the perylene (80 % increase). Low number densities observed for the larger polycyclic aromatic hydrocarbons (PAHs) suggest that the lower vapor pressure of PAHs with catacondensed four‐membered rings and with five‐membered rings other than perylene will not be detectable using our current configuration.     

Eaton’s Reagent‐Mediated Domino π‐Cationic Arylations of Aromatic Carboxylic Acids to Iasi‐Red Polymethoxylated Polycyclic Aromatic Hydrocarbons: Products with Unprecedented Biological Activities as Tubulin Polymerization Inhibitors

A rapid domino π‐cationic arylation of aromatic carboxylic acids, mediated by Eaton’s reagent, has been developed for the synthesis of Iasi‐red polymethoxylated polycyclic aromatic hydrocarbons (PAHs). This route is currently the easiest method to obtain such popular PAH compounds, which bear in addition numerous methoxy groups. The domino process was generalized, the structure of the obtained red products and the mechanism of their formations were elucidated, and some of their photophysical properties were determined. Newly synthesized polymethoxylated‐PAHs were tested for their interaction with tubulin polymerization as well as for their cytotoxicity on a panel of NCI‐60 human cancer cell lines. Interestingly, one of these rubicene derivatives exhibited remarkable cytotoxicity in vitro, including inhibition of leukemia, colon, melanoma, CNS, and ovarian cancer cell lines with GI₅₀ values in the low nanomolar range (GI₅₀<10 nM ).