Hybrid Edge Results in Narrowed Band Gap: Bottom-up Liquid-Phase Synthesis of Bent N = 6/8 Armchair Graphene Nanoribbons.

Scalable fabrication of graphene nanoribbons with narrow band gaps has been a nontrivial challenge. Here, we have developed a simple approach to access narrow band gaps using hybrid edge structures. Bottom-up liquid-phase synthesis of bent N = 6/8 armchair graphene nanoribbons (AGNRs) has been achieved in high efficiency through copolymerization between an o-terphenyl monomer and a naphthalene-based monomer, followed by Scholl oxidation. An unexpected 1,2-aryl migration has been discovered, which is responsible for introducing kinked structures into the GNR backbones. The N = 6/8 AGNRs have been fully characterized to support the proposed structure and show a narrow band gap and a relatively high electrical conductivity. In addition, their application in efficient gas sensing has also been demonstrated.

Backbone Engineering of Monodisperse Conjugated Polymers via Integrated Iterative Binomial Synthesis.

Synthesis of sequence-defined monodisperse π-conjugated polymers with versatile backbones remains a substantial challenge. Here we report the development of an integrated iterative binomial synthesis (IIBS) strategy to enable backbone engineering of conjugated polymers with precisely controlled lengths and sequences as well as high molecular weights. The IIBS strategy capitalizes on the use of phenol as a surrogate for aryl bromide and represents the merge between protecting-group-aided iterative synthesis (PAIS) and iterative binomial synthesis (IBS). Long and monodisperse conjugated polymers with diverse irregular backbones, which are inaccessible by conventional polymerizations, can be efficiently prepared by IIBS. In addition, topology-dependent and chain-length-dependent properties have been investigated.

Programmable Fabrication of Monodisperse Graphene Nanoribbons via Deterministic Iterative Synthesis.

While enormous progress has been achieved in synthesizing atomically precise graphene nanoribbons (GNRs), the preparation of GNRs with a fully predetermined length and monomer sequence remains an unmet challenge. Here, we report a fabrication method that provides access to structurally diverse and monodisperse "designer" GNRs through utilization of an iterative synthesis strategy, in which a single monomer is incorporated into an oligomer chain during each chemical cycle. Surface-assisted cyclodehydrogenation is subsequently employed to generate the final nanoribbons, and bond-resolved scanning tunneling microscopy is utilized to characterize them.

Acyl radical to rhodacycle addition and cyclization relay to access butterfly flavylium fluorophores.

Transition metal-catalyzed C-H activation and radical reactions are two versatile strategies to construct diverse organic skeletons. Here we show the construction of a class of flavylium fluorophores via the merge of radical chemistry and C-H activation starting from (hetero)aryl ketones and alkynes. This protocol is not only applicable to aryl ketones but also to heteroaryl ketones such as thiophene, benzothiophene and benzofuran, thus leading to structural diversity. Mechanism studies, including control experiments, intermediate separation, radical trapping, EPR and ESI-HRMS experiments, demonstrate that the key step lies in the addition of the acyl radical generated by the copper-catalyzed C-C bond cleavage of aryl ketone to the rhodacycle formed via the C-H activation of aryl ketone. The flavylium fluorophores feature butterfly symmetrical configuration, nearly planar skeleton and delocalized positive charge, and exhibit intriguing photophysical properties, such as tunable absorption and emission wavelengths and high quantum yields.

An unusual [4 + 2] fusion strategy to forge meso-N/O-heteroarene-fused (quinoidal) porphyrins with intense near-infrared Q-bands.

Here we present a divergent synthesis of brand-new types of meso-N/O-heteroarene-fused (quinoidal) porphyrins through Rh-catalyzed ß-C-H activation/annulation of 5,15-dioxoporphyrins and dioxime derivatives with alkynes, in which the synthetic disconnections are difficult to access through the commonly used intramolecular cyclization strategy. Using the O-methyl oxime as a traceless oxidizing directing group, the meso-N-embedded pyridine-fused anti-quinoidal porphyrin 3 and pyridinium-fused cation 4 are formed with controllable chemoselectivity and complete anti-selectivity. Replacing the exocyclic oxime with a carbonyl group delivers the pyran-fused porphyrin 5, achieving structural conversion from a quinoidal conformation to a stable porphyrin macrocycle. Further oxidation of the expanded dimer 5ea gives the oxonium 6, which exhibits intense near-infrared (NIR) Q-bands up to 1300 nm. Theoretical studies demonstrate that the incorporation of a heteroatom at the meso-position enables more effective π-extension, resulting in a 22π aromatic (vs. 18π aromatic) character of pyran-fused porphyrins (syn/anti-5aa). Compared with the commercially available methylene blue (MB), syn-5al exhibits a better ability (Φ Δ = 0.61) to sensitize singlet oxygen (1O2) when irradiated with a 680 nm laser beam, and has potential as a photodynamic therapy (PDT) photosensitizer in the body's therapeutic window (650-900 nm).

Double ortho-C-H Activation/Annulation of Benzamides with Aryl Alkynes: A Route to Double-Helical Polycyclic Heteroaromatics.

It remains a challenge to achieve N,O-double annulations of primary benzamides with aryl alkynes due to competitive N,N-double annulations. Herein, we employed sterically hindered 1-methylcyclohexane-1-carboxylic acid to address this challenge, the double ortho-C-H activation of benzamides and subsequent N,O-double annulations with aryl alkynes have been accomplished for the first time. The resulting product can be further transformed into a double-helical extended π-conjugated polycyclic heteroarene via Scholl oxidation, which exhibits blue emission with high fluorescence quantum yields.

Concise Synthesis of Polysubstituted Carbohelicenes by a C-H Activation/Radical Reaction/C-H Activation Sequence.

Disclosed herein is the merging of C-H activation and radical chemistry, enabling rapid access to a structurally diverse family of fused carbohelicenes through the fusion of α-acetylnaphthalenes with alkynes under oxidative conditions. This cascade process exhibits exquisite chemoselectivity and regioselectivity. The reaction pathway was analyzed by intermediate separations, control experiments, radical trapping, EPR, MALDI-TOF-MS, and ESI-HRMS experiments, and shown to involve a C2-H activation/radical reaction/C8-H activation relay.

Multicomponent Reactions of Pyridines To Give Ring-Fused Pyridiniums: In Situ Activation Strategy Using 1,2-Dichloroethane as a Vinyl Equivalent.

Reported herein is a rhodium(III)-catalyzed three-component annulation reaction of simple pyridines, alkynes, and 1,2-dichloroethane (DCE), affording a streamlined pathway to diverse ring-fused pyridiniums. DCE not only serves as a vinyl equivalent but also as an in situ activating agent for pyridine C2-H activation. A cationic five-membered rhodacycle complex has been isolated and proposed as a possible intermediate. This strategy can be extended to other N-containing heteroarenes for the synthesis of multiring-fused pyridiniums. These multicomponent reactions exhibit excellent regioselectivity for 1,3-diynes, paving a path to the cascade cyclization of 3-fluoropyridine or N-methylpyridin-3-amine with 1,3-diynes for the construction of brand-new tricyclic-fused pyrano- or hydropyridoquinolizinium salts. These ionic fluorophores have been investigated as potential biomarkers.

Annulation cascade of arylnitriles with alkynes to stable delocalized PAH carbocations via intramolecular rhodium migration.

Herein, we propose the conception of heteroatom-promoted delocalization of the positive charge of an oxonium ion and thus develop a highly efficient rhodium(iii)-catalyzed hydration and three fold C-H activation/annulation cascade of arylnitriles with alkynes, affording a structurally diverse family of delocalized polycyclic aromatic hydrocarbon (PAH) carbocations. DFT calculations demonstrate that the positive charge mostly locates around the C1 atom and is partly delocalized by ambient N, O1 and C5 atoms. A mechanistic study indicates that the hydration of the arylnitrile and three fold insertion of the alkyne is a successive process rather than a step by step process, wherein a unique intramolecular rhodium migration is probably involved. These novel carbeniums show tunable fluorescence emission, low cytotoxicity and the ability to specifically target lysosomes.

Manganese/cobalt-catalyzed oxidative C(sp3)-H/C(sp3)-H coupling: a route to α-tertiary ß-arylethylamines.

Reported herein is an oxidative coupling reaction of an α-C(sp3)-H bond of amine with a benzylic C(sp3)-H bond through Mn or Co catalysis to provide diverse collections of α-tertiary ß-arylethylamines. This protocol features an easily installed and removable coordinating activation group, a wide scope of substrates, low-cost metal catalysts, easily available starting materials and synthetic simplicity.

Synthesis of Phenalenyl-Fused Pyrylium Cations: Divergent C-H Activation/Annulation Reaction Sequence of Naphthalene Aldehydes with Alkynes.

Described herein is the synthesis of stable oxonium-doped polycyclic aromatic hydrocarbons (PAHs) by the rhodium-catalyzed C-H activation/annulations of naphthalene-type aldehydes with internal alkynes. This protocol provides four divergent reaction types, including two unexpected annulations with an oxygen transposition process, which lead to diverse types of phenalenyl-fused pyrylium cations comprising a four-, five-, or six-ring-fused π-conjugated core. The annulations exhibit an exquisite regioselectivity and a high tolerance of sensitive functional groups. These PAHs feature intriguing photophysical properties such as full-color tunable fluorescence emission, high quantum yield, and positively charged core, and can be reduced easily to the phenalenyl radicals.

Iridium-Catalyzed Direct Regioselective C4-Amidation of Indoles under Mild Conditions.

An efficient Ir-catalyzed amidation of indoles with sulfonyl azides is disclosed, affording diverse C4-amidated indoles exclusively under mild conditions. In this protocol, a variety of indoles with commonly occurring functional groups such as formyl, acetyl, carboxyl, amide, and ester at the C3 position are well tolerated.