The Scholl Reaction as a Powerful Tool for Synthesis of Curved Polycyclic Aromatics.

The past decade has witnessed remarkable success in the synthesis of curved polycyclic aromatics through Scholl reactions which enable oxidative aryl-aryl coupling even in company with the introduction of significant steric strain. These curved polycyclic aromatics are not only unique objects of structural organic chemistry in relation to the nature of aromaticity but also play an important role in bottom-up approaches to precise synthesis of nanocarbons of unique topology. Moreover, they have received considerable attention in the fields of supramolecular chemistry and organic functional materials because of their interesting properties and promising applications. Despite the great success of Scholl reactions in synthesis of curved polycyclic aromatics, the outcome of a newly designed substrate in the Scholl reaction still cannot be predicted in a generic and precise manner largely due to limited understanding on the reaction mechanism and possible rearrangement processes. This review provides an overview of Scholl reactions with a focus on their applications in synthesis of curved polycyclic aromatics with interesting structures and properties and aims to shed light on the key factors that affect Scholl reactions in synthesizing sterically strained polycyclic aromatics.

Dipole Moment Modulation of Terminal Groups Enables Asymmetric Acceptors Featuring Medium Bandgap for Efficient and Stable Ternary Organic Solar Cells.

This study puts forth a novel terminal group design to develop medium-bandgap Y-series acceptors beyond conventional side-chain engineering. We focused on the strategical integration of an electron-donating methoxy group and an electron-withdrawing halogen atom at benzene-fused terminal groups. This combination precisely modulated the dipole moment and electron density of terminal groups, effectively attenuating intramolecular charge transfer effect, and widening the bandgap of acceptors. The incorporation of these terminal groups yielded two asymmetric acceptors, named BTP-2FClO and BTP-2FBrO, both of which exhibited open-circuit voltage (VOC) as high as 0.96 V in binary devices, representing the highest VOCs among the asymmetric Y-series small molecule acceptors. More importantly, both BTP-2FClO and BTP-2FBrO exhibit modest aggregation behaviors and molecular crystallinity, making them suitable as a third component to mitigate excess aggregation of the PM6: BTP-eC9 blend and optimize the devices' morphology. As a result, the optimized BTP-2FClO-based ternary organic solar cells (OSCs) achieved a remarkable power conversion efficiency (PCE) of 19.34%, positioning it among the highest-performing OSCs. Our study highlights the molecular design importance on manipulating dipole moments and electron density in developing medium-bandgap acceptors, and offers a highly efficient third component for high-performance ternary OSCs.

Robust Radical Cations of Hexabenzoperylene Exhibiting High Conductivity and Enabling an Organic Nonvolatile Optoelectronic Memory.

Herein, we report robust π-conjugated radical cations resulting from the oxidation of hexabenzoperylene (HBP) derivatives, HBP-B and HBP-H, which have butyl and hexyl groups, respectively, attached to the same twisted double helicene π-backbone. The radical cation of HBP-B was successfully crystallized in the form of hexafluorophosphate, which exhibited conductivity as high as 1.32 ± 0.04 S cm-1. Photochemical oxidation of HBP-H by molecular oxygen led to the formation of its radical cation in the solid state, as found with different techniques. This allowed the organic field effect transistor of HBP-H to function as a nonvolatile optoelectronic memory, with the memory switching contrast above 103 and long-term stability without using a floating gate, an electret layer, or photochromic molecules.

A Near-Infrared Absorbing and Emissive Quadruple Helicene Enabled by the Scholl Reaction of Perylene.

Herein, we report the synthesis, structural analysis, optical and chiroptical properties of a novel quadruple helicene, which has two [6] and two [7]helicene moieties fused in a contorted framework of 92 sp2 carbon atoms. It was synthesized by the Scholl reaction of a perylene-containing substrate with the formation of eight carbon-carbon bonds on the perylene unit in a single synthetic operation. Chemical oxidation of the quadruple helicene with tris(4-bromophenyl)ammoniumyl hexachloroantimonate resulted in an air-stable dication, which exhibits the same helicity in its four helicene moieties as unambiguously identified by single-crystal X-ray crystallography. The quadruple helicene exhibits unusual near-infrared absorption and emission with absorption and emission maxima at 848 nm and 977 nm, respectively, and its isolated enantiomers exhibit electronic circular dichroism in the near-infrared and visible-light regions.

Charging a Negatively Curved Nanographene and Its Covalent Network.

This study explores a bottom-up approach toward negatively curved carbon allotropes from octabenzo[8]circulene, a negatively curved nanographene. Stepwise chemical reduction reactions of octabenzo[8]circulene with alkali metals lead to a unique highly reduced hydrocarbon pentaanion, which is revealed by X-ray crystallography suggesting a local view for the reduction and alkali metal intercalation processes of negatively curved carbon allotropes. Polymerization of the tetrabromo derivative of octabenzo[8]circulene by the nickel-mediated Yamamoto coupling reaction results in a new type of porous carbon-rich material, which consists of a covalent network of negatively curved nanographenes. It has a specific surface area of 732 m2 g-1 and functions as anode material for lithium ion batteries exhibiting a maximum capacity of 830 mAh·g-1 at a current density of 100 mA·g-1. These results indicate that this covalent network presents the key structural and functional features of negatively curved carbon allotropes.

Synthesis of Zigzag Carbon Nanobelts through Scholl Reactions.

Zigzag carbon nanobelts are a long-sought-after target for organic synthesis. Herein we report new strategies for designing and synthesizing unprecedented zigzag carbon nanobelts, which present a wave-like arrangement of hexagons in the unrolled lattice of (n,0) single wall carbon nanotubes (n=16 or 24). The precursors of these zigzag carbon nanobelts are hybrid cyclic arylene oligomers consisting of meta-phenylene and 2,6-naphthalenylene as well as para-phenylene units. The Scholl reactions of these cyclic arylene oligomers form multiple carbon-carbon bonds selectively at the α-positions of naphthalene units resulting in the corresponding zigzag carbon nanobelts. As monitored with fluorescence spectroscopy, one of these nanobelts binds C60 with an association constant as high as (6.6±1.1)×106  M-1 in the solution in toluene. Computational studies combining linear regression analysis and hypothetical homodesmotic reactions reveal that these zigzag nanobelts have strain in the range of 67.5 to 69.6 kcal mol-1 , and the ladderization step through Scholl reactions is accompanied by increase of strain as large as 69.6 kcal mol-1 . The successful synthesis of these nanobelts demonstrates the powerfulness and efficiency of Scholl reactions in synthesizing strained polycyclic aromatics.

Carbazole-Fused Polycyclic Aromatics Enabled by Regioselective Scholl Reactions.

The synthesis of new carbazole-fused polycyclic aromatics with interesting geometry and useful properties was explored using Scholl reactions. As found from the Scholl reactions of substrates having two carbazole units linked at different positions through o-phenylene, oxidative coupling of carbazole units occurred in a regioselective manner with new carbon-carbon bonds preferably formed at C3 and C4 in N-alkyl carbazoles. A new N-containing aromatic bowl was characterized by single-crystal X-ray crystallography, and new p-type organic semiconductors exhibited field effect mobility of up to 0.070 cm2 V-1 s-1 in solution-processed thin-film transistors.

Synthesis, Structures, and Properties of Heptabenzo[7]circulene and Octabenzo[8]circulene.

This study puts forth two new members of fully ortho-benzannulated [ n]circulenes, heptabenzo[7]circulene and octabenzo[8]circulene, which are new negatively curved nanographenes and also represent unprecedented structures of septuple [4]helicene and octuple [4]helicene, respectively. The successful synthesis of them through Scholl reaction in good to excellent yields takes advantage of the reactivity of naphthalene. Quantum chemistry calculations reveal that heptabenzo[7]circulene and octabenzo[8]circulene are both flexible π-molecules and adopt saddle-shaped geometry of C2 and D2 d symmetry, respectively, at the global energy minimum in agreement with the single-crystal structures. A serendipitous discovery from this study is that tetra( tert-butyl) octabenzo[8]circulene in the single crystals self-assemble into a supramolecular nanosheet with an unprecedented motif of π-π stacking. Such a new molecular packing mode, in combination with the demonstrated semiconducting property of octabenzo[8]circulene, suggests a new supramolecular two-dimensional material.

Toward Negatively Curved Carbons.

Negatively curved carbons are theoretical carbon allotropes as proposed by embedding heptagons or octagons in a graphitic lattice. Unlike five-membered rings in fullerenes, which induce positive curvature, the seven- or eight-membered rings induce negative curvature, giving rise to a variety of esthetic carbon nanostructures known as Mackay crystals or carbon schwarzites. In addition, hypothetical toroidal carbon nanotubes consisting of five-, six-, and seven-membered rings present positive curvature on the outside and negative curvature on the inside of the torus. These carbon allotropes with negative curvature are predicted to have interesting properties and potential applications on the basis of computational studies but are yet to be synthesized. A promising bottom-up approach to these intriguing but still imaginary carbon structures is organic synthesis of negatively curved polycyclic arenes, which are also known as negatively curved nanographenes. They not only are segments of negatively curved carbon allotropes containing important structural information but also can in principle be used as templates or monomer units for the synthesis of carbon schwarzites and toroidal carbon nanotubes. This Account describes research on the design, synthesis, structure, stereochemical dynamics, and properties of negatively curved nanographenes, with emphasis on our efforts in this field. In our designs of negatively curved nanographenes, a few heptagon- or octagon-embedded π systems were employed as basic structural units, including [7]circulene, heptagon-embedded hexa- peri-benzocoronene, tetrabenzodipleiadiene, and [8]circulene. They present a saddle-shaped geometry and consist of a relatively small number of sp2 carbon atoms. By expanding or connecting these structural units, we designed and synthesized larger negatively curved nanographenes consisting of up to 96 sp2 carbon atoms. A method of key importance in the synthesis of negatively curved nanographenes is the Scholl reaction, which enables the formation of multiple carbon-carbon bonds in a single step by intramolecular oxidative cyclodehydrogenation. The unique structures of negatively curved nanographenes were studied by experimental and computational methods. In particular, X-ray crystallography of single crystals revealed remarkably curved π faces accompanied by severe out-of-plane deformation of benzenoid rings, which sheds light on the limit of π bonds and the aromaticity of polycycles. As found mainly from calculations, the flexible polycyclic frameworks of negatively curved nanographenes are associated with stereochemical dynamics that is not available for planar polycyclic aromatics. In addition, some negatively curved nanographenes have been found to function as organic semiconductors in the solid state. We envision that the study of negatively curved nanographenes will serve as an important initial step toward the eventual synthesis of new carbon allotropes with negative curvature and new frontiers of nanocarbon materials.

A Dipleiadiene-Embedded Aromatic Saddle Consisting of 86 Carbon Atoms.

This study presents a new type of negatively curved nanographene (C86 H32 ) that contains an unprecedented pattern of heptagons. A tert-butylated derivative of C86 H32 was successfully synthesized using tetrabenzodipleiadiene as a key building block. This synthesis involved a ring expansion reaction as a key step to form the seven-membered rings in the framework of tetrabenzodipleiadiene. The single-crystal structure reveals a saddle-shaped molecule with a highly bent naphthalene moiety at the center of the polycyclic backbone. As found from the DFT calculations, this aromatic saddle is flexible at room temperature and has a saddle-shaped geometry as the dominant conformation. The DFT calculations along with experimental results show that the attachment of t-butyl groups to the central tetrabenzodipleiadiene moiety of nanographene C86 H32 can stabilize the saddle conformation and make this nanographene less flexible.

A new armchair carbon nanobelt synthesized by tuning the regioselectivity of the Scholl reaction of quinquephenyl.

A new armchair carbon nanobelt is successfully synthesized by tuning the regioselectivity of the Scholl reaction of 1,1':2',1'':4'',1''':2''',1''''-quinquephenyl. This nanobelt exhibits a preferential binding affinity towards C70 over C60 as found from photoluminescence titration experiments.

Precisely Controlling Polymer Acceptors with Weak Intramolecular Charge Transfer Effect and Superior Coplanarity for Efficient Indoor All-Polymer Solar Cells with over 27% Efficiency.

Indoor photovoltaics (IPVs) are garnering increasing attention from both the academic and industrial communities due to the pressing demand of the ecosystem of Internet-of-Things. All-polymer solar cells (all-PSCs), emerging as a sub-type of organic photovoltaics, with the merits of great film-forming properties, remarkable morphological and light stability, hold great promise to simultaneously achieve high efficiency and long-term operation in IPV's application. However, the dearth of polymer acceptors with medium-bandgap has impeded the rapid development of indoor all-PSCs. Herein, a highly efficient medium-bandgap polymer acceptor (PYFO-V) is reported through the synergistic effects of side chain engineering and linkage modulation and applied for indoor all-PSCs operation. As a result, the PM6:PYFO-V-based indoor all-PSC yields the highest efficiency of 27.1% under LED light condition, marking the highest value for reported binary indoor all-PSCs to date. More importantly, the blade-coated devices using non-halogenated solvent (o-xylene) maintain an efficiency of over 23%, demonstrating the potential for industry-scale fabrication. This work not only highlights the importance of fine-tuning intramolecular charge transfer effect and intrachain coplanarity in developing high-performance medium-bandgap polymer acceptors but also provides a highly efficient strategy for indoor all-PSC application.