Highly twisted arenes by Scholl cyclizations with unexpected regioselectivity

Let's twist! The Scholl reaction with quinquephenyl derivatives has been shown to have an unexpectedly strong preference for forming twisted, helicene aromatic polycycles, instead of their flat counterparts. This tendency is so strong that it will overcome even severe steric hindrance, and the procedure can be used in the efficient synthesis of hexa-tert-butylhexabenzotriphenylene from a simple biaryl starting material (see scheme).     

Recent Progress in Chemistry of Multiple Helicenes

The last decade has witnessed multiple helicenes arising as an interesting class of nonplanar polycyclic aromatics of inherent multihelicity. These molecules present esthetic structures and interesting properties not available to helicenes of single helicity. Herein an overview of multiple helicenes with respect to structures, stereochemical dynamics, synthesis, and applications is provided. Recently reported multiple helicenes are surveyed with an emphasis on molecular structures and stereochemistry of multiple carbohelicenes. After this survey, the synthesis of multiple helicenes through the Scholl reaction is discussed and recent applications of multiple helicenes in organic electronics are summarized. On the basis of these discussions, conclusions are reached on the current status of multiple helicenes and an outlook for this field is provided.     

Extension of N‐Heteroacenes through a Four‐Membered Ring

The synthesis of novel π‐extended N‐heteroacenes, which have a large tetraazaacene subunit and a quinoxaline subunit connected through a four‐membered ring, is reported. They were studied with experimental and computational methods in comparison to the corresponding tetraazaacenes. As found from the DFT calculation, the four‐membered ring is a better linker than a five‐membered ring or a C?C single bond to extend N‐heteroacenes for a new design of n‐type semiconductors in terms of the spatial delocalization and energy level of LUMO as well as the reorganization energy. In solution‐processed thin film transistors, the π‐extended N‐heteroacenes are found to function as n‐type semiconductors with field effect mobility of up to 0.02 cm² V^?1 s^?1 under ambient conditions.     

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.     

从含有八元环的稠环芳烃到具有负曲率的碳纳米结构:进展与展望

完全由sp²杂化的碳原子所构成的同素异形体呈现出或平坦或弯曲的面,该表面的曲率反映了碳纳米结构的整体几何特性。完全由六元环构成的石墨烯的曲率为零;由六元环和五元环共同构成的富勒烯的曲率为正;向碳原子的六边形网格中引入七元环或八元环则产生形如马鞍的面,其曲率为负。具有负曲率的三维周期性碳结构被命名为马凯晶体或碳施瓦茨体,是碳纳米科学研究长期追寻的目标,然而至今仍未被确定无疑地合成出来。为精确合成具有负曲率的碳纳米结构,一种至下而上的策略是先合成具有负曲率的稠环芳烃,再以其为模板或单体来制备更大的碳纳米结构。具有负曲率的稠环芳烃可以通过向稠环骨架中引入七元或八元环来设计、合成,表现出一些平面稠环芳烃所不具有的结构特征与性质。本文以包含八元环的稠环芳烃为例,介绍具有负曲率的稠环芳烃的设计、合成、立体动力学及其他特征,并展望具有负曲率的碳纳米结构的新研究方向。     

Twisted Polycyclic Aromatic Systems Prepared by Annulation of Bis(arylethynyl)arenes with Biphenylboronic Acids

Twisted polycyclic aromatic hydrocarbons (PAHs) were prepared by the successive rhodium‐catalyzed annulation and dehydrogenative cyclization of bis(arylethynyl)arenes with di‐tert‐butylbiphenyl‐2‐ylboronic acid. The molecular structures of the PAHs were determined by single‐crystal XRD analysis. The PAHs showed up to four fjord regions, and the twisting angle was 46.7°. The nonplanarity (NP) and harmonic oscillator model of aromaticity (HOMA) were calculated by using the structural data obtained from XRD analysis. The PAHs derived from dialkynyl naphthalenes showed low planarity and HOMA of the central ring. The optical properties of the PAHs were investigated by UV/Vis absorption and fluorescence spectroscopy analyses. The absorption and emission maxima of the PAHs with a larger planar region appeared at a longer wavelength. DFT calculations support that the absorption band at λ≈450 nm can be mainly attributed to the HOMO–LUMO transition.     

Tertiary Amines Differentiated from Primary and Secondary Amines by Active Ester‐Functionalized Hexabenzoperylene in Field Effect Transistors

Herein, we report two novel derivatives of hexabenzoperylene (HBP) that are functionalized with ester groups. Methyl acetate functionalized HBP ( 1 ) in single crystals self‐assembles into a supramolecular nanosheet, which has a two‐dimensional π‐stack of HBP sandwiched between two layers of ester groups. With the same self‐assembly motif, active ester‐functionalized HBP ( 2 ) in field effect transistors has enabled differentiation of tertiary amines from primary and secondary amines, in agreement with the fact that active ester reacts with primary and secondary amines but not with tertiary amines to form amides.     

Comparison of Oxidative Aromatic Coupling and the Scholl Reaction

Does the dehydrogenative coupling of aromatic compounds mediated by AlCl₃ at high temperatures and also by FeCl₃, MoCl₅, PIFA, or K₃[Fe(CN)₆] at room temperature proceed by the same mechanism in all cases? With the growing importance of the synthesis of aromatic compounds by double C? H activation to give various biaryl structures, this question becomes pressing. Since some of these reactions proceed only in the presence of non‐oxidizing Lewis acids and some only in the presence of certain oxidants, the authors venture the hypothesis that, depending on the electronic structure of the substrates and the nature of the “catalyst”, two different mechanisms can operate. One involves the intermediacy of a radical cation and the other the formation of a sigma complex between the acid and the substrate. The goal of this Review is to encourage further mechanistic studies hopefully leading to an in‐depth understanding of this phenomenon.     

First synthesis of new polycyclic systems from ortho-di(heteroaryl)-substituted furazanopyrazine derivatives by the Scholl reaction

A facile synthetic protocol to hard-to-get polycyclic (hetero)-aromatic compounds annulated to [1,2,4]oxadiazolo[3,4-b]-pyrazine scaffold via the FeCl₃-mediated intramolecular Scholl cross-coupling has been developed. Based on the electrochemical and photophysical measurements, the synthesized polycyclic systems may be regarded as narrow band-gap (from 1.42 to 1.89 eV) n-type organic semiconductors whose energy levels are comparable to those of the commercially available n-type semiconductors     

Controlling the Regioselectivity of the Hydrosilylation Reaction in Carbon Nanoreactors

Hollow graphitized carbon nanofibres (GNF) are employed as nanoscale reaction vessels for the hydrosilylation of alkynes. The effects of confinement in GNF on the regioselectivity of addition to triple carbon–carbon bonds are explored. A systematic comparison of the catalytic activities of Rh and RhPt nanoparticles embedded in a nanoreactor with free‐standing and surface‐adsorbed nanoparticles reveals key mechanisms governing the regioselectivity. Directions of reactions inside GNF are largely controlled by the non‐covalent interactions between reactant molecules and the nanofibre channel. The specific π–π interactions increase the local concentration of the aromatic reactant and thus promote the formation of the E isomer of the β‐addition product. In contrast, the presence of aromatic groups on both reactants (silane and alkyne) reverses the effect of confinement and favours the formation of the Z isomer due to enhanced interactions between aromatic groups in the cis‐orientation with the internal graphitic step‐edges of GNF. The importance of π–π interactions is confirmed by studying transformations of aliphatic reactants that show no measurable changes in regioselectivity upon confinement in carbon nanoreactors.     

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.     

Phase Transition and Band Gap Regulation by Halogen Substituents on the Organic Cation in Organic–Inorganic Hybrid Perovskite Semiconductors

In the last decade, hybrid materials have received widespread attention. In particular, hybrid lead halide perovskite-type semiconductors are very attractive owing to their great flexibility in band gap engineering. Here, by using precise molecular modifications, three one-dimensional perovskite-type semiconductor materials are designed and obtained: [Me₃PCH₂X][PbBr₃] (X=H, F, and Cl for compounds 1 , 2 , and 3 , respectively). The introduction of a heavier halogen atom (F or Cl) to [Me₄P]⁺ increases the potential energy barrier required for the tumbling motion of the cation, hence achieving the transformation of the phase transition temperature from low temperature (192 K) to room temperature (285 K) and high temperature (402.3 K). Moreover, the optical band gaps reveal a broadening trend with 3.176 eV, 3.215 eV, and 3.376 eV along the H→F→Cl series, which is attributed to the formation of the structural distortion.     

Reviewing the Polyolefin Cyclization Reaction of the C35 Polyprene Catalyzed by Squalene‐Hopene Cyclase

A review of the polycyclization reaction of the C₃₅ polyprenoid by squalene‐hopene cyclase: Surprisingly, our results completely disagree with a previous publication in which it was reported that a hexacyclic skeleton was constructed as the single product. In our work many tri‐ and tetracyclic scaffolds were isolated, but no penta‐ or hexacycles. The reasons for the different results and the mechanism of the polycyclization reaction are discussed (see figure).