Ru-Catalyzed Benzannulation Leads to Luminescent Boron-Containing Polycyclic Aromatic Hydrocarbons

The Olympicenyl Radical and Its Derivatives

The olympicenyl radical (OR) has long been a fascinating spin doublet hydrocarbon radical that evoked theoretical and experimental research interests, but the chemistry of olympicenyl was limited by its inherent instability. Recently, this field was revived by the advent of stable, multi-substituted ORs and the isolation of them in the crystalline phase. In this minireview, we summarize the early studies on the pristine OR, as well as the recent advances on the substituted OR derivatives, heteroatom-containing OR derivatives, and OR-based diradicals and polyradicals. The synthetic chemistry, stabilization strategies, self-association behaviors, reactivities, and applications in the biological field of the abovementioned compounds were discussed.

A Combination of B- and N-Doped π-Systems Enabling Systematic Tuning of Electronic Structures and Properties

Doping heteroatoms into polycyclic aromatic hydrocarbons (PAHs) may alter their structures and thereby physical properties. This study reports the construction of B/N-codoped PAHs via combining the B- and N-doped π-systems. Two π-extended B/N-codoped PAHs were synthesized through the Mallory photoreaction. Both feature a C48 BN2 π-skeleton, which is assembled by linearly fusing three substructures including B-doped and sp2 -hybridized N-doped π-moieties and one pyrene unit. In comparison to the pristine B-doped analog, their intramolecular charge transfer (ICT) states are distinctly modulated by the fused N-doped π-system and the further incorporated cyano group, leading to their tunable optical properties, as revealed by detailed theoretical and experimental analysis. Furthermore, these three molecules have sufficient Lewis acidity and can coordinate with Lewis base to form Lewis acid-base adducts, and notably, such intermolecular complexation can further dynamically modulate their ICT transitions and thereby photophysical properties, such as producing blue, green and red fluorescence.

Triarylborane-triphenylamine based luminophore for the mitochondria targeted live cell imaging and colorimetric detection of aqueous fluoride

Bioimaging of subcellular organelles such as mitochondria is crucial for detecting physiological abnormalities induced by fluctuations in the levels of various analytes. Herein, we report the design and synthesis of two novel water-soluble cationic Lewis acid triarylborane-triarylamine conjugates 1 and 2. The optical characteristics of 1 and 2 and their precursor compounds BTPA-NMe2 and BTPA-2NMe2 were evaluated, which show similar absorption and fluorescence spectra, with 1 and 2 exhibiting higher quantum yields of 0.73 and 0.64, respectively, than those of the precursors BTPA-NMe2 and BTPA-2NMe2, indicating the partial disruption of the ICT process and the activation of alternative emission bands in 1 and 2. The live cell imaging ability of compound 2 was examined in HeLa cells using a confocal microscope. Moreover, mitochondrial internalisation using compound 2 was effective and it was found to have high photostability under UV light conditions. Furthermore, compound 2 demonstrated an evident colorimetric response with a colour change to dark yellow in aqueous environments, indicating that it could be used for anion sensing. The spectral changes were observed in UV-visible and fluorescence titration experiments, which were strongly supported by DFT calculations. In short, compound 2 synthesized by us can be exclusively utilized for the selective localization of mitochondria with less cytotoxicity and shows excellent colorimetric response to aqueous inorganic fluoride at levels as low as 0.1 ppm with high selectivity.

Recent Advances in Heterocyclic Nanographenes and Other Polycyclic Heteroaromatic Compounds

This review surveys recent progress in the chemistry of polycyclic heteroaromatic molecules with a focus on structural diversity and synthetic methodology. The article covers literature published during the period of 2016-2020, providing an update to our first review of this topic (Chem. Rev. 2017, 117 (4), 3479-3716).

Highly Electron-Deficient Dicyanomethylene-Functionalized Triarylboranes with Low-Lying LUMO and Strong Lewis Acidity

A series of dicyanomethylene-functionalized triarylboranes is reported in this work, with low-lying LUMO energy levels at ca. -3.66 eV for ^FMesB-ACN. The single-crystal structures of the mono- and dianion of Mes*B-ACN were obtained via chemical reduction, which revealed a conversion from a quinoidal to an aromatic structure. The strong Lewis acidity of these compounds is reflected in a fluoride-anion binding experiment. This work introduces a facile strategy for modulating the electron deficiency of boron-containing compounds.

BNB-doped phenalenyls - aromaticity switch upon one-electron reduction

Fully aromatic, luminescent, and highly robust BNB-doped phenalenyls have been prepared, which are isoelectronic to the phenalenyl cation. B-Fluoromesityl-substitution leads to fluorescence in an extremely narrow range and significant increase in the reduction potential. Detailed theoretical investigations revealed an intramolecular aromaticity switch upon one-electron reduction.

Thiophene-based twisted bistricyclic aromatic ene with tricoordinate boron: a new n-type semiconductor

The incorporation of tricoordinate boron into conjugated systems is of current interest in the field of organic electronics. In this study, a tricoordinate boron-embedded thiophene-based bistricyclic aromatic ene (BAE) was synthesized as a new boron-containing conjugated system. The combination of tricoordinate boron and fused thiophene rings imposed the twisted conformation in the BAE structure, resulting in the narrow energy absorption with the low-lying LUMO. Preliminary studies on the application of the highly electron-deficient boron-embedded BAE to organic field-effect transistors (OFETs) were also performed, revealing its moderately high electron mobility.

BNB-Doped Phenalenyls: Modular Synthesis, Optoelectronic Properties, and One-Electron Reduction

A highly modular synthesis of BNB- and BOB-doped phenalenyls is presented. Treatment of the 1,8-naphthalenediyl-bridged boronic acid anhydride 1 with LiAlH₄/Me₃SiCl afforded the corresponding 1,8-naphthalenediyl-supported diborane(6) 2, which served as the starting material for all subsequent transformations. Upon addition of MesMgBr/Me₃SiCl, 2 was readily converted to the tetraorganyl diborane(6) 5. The further heteroatoms were finally introduced through the reaction of 2 with (Me₃Si)₂NR' or 5 with H₂NR' or H₂O (R' = H, Me, p-Tol). A helically twisted, fully BNB-embedded PAH 11 was prepared by combining 2 with a dibrominated m-terphenylamine, followed by a Grignard-mediated double ring-closure reaction. All compounds devoid of B-H bonds show favorable optoelectronic properties, such as luminescence and reversible reduction behavior. In the case of the BNB-phenalenyl 7 (BMes, NMe), the radical-anion salt K[7^•] was generated through chemical reduction with K metal and characterized by EPR spectroscopy. K[7^•] is not long-term stable in a THF/c-hexane solution, but abstracts an H atom with formation of the diamagnetic BNB-doped 1H-phenalene K[7H].

Palladium-Catalyzed Synthesis of Benzophenanthrosilines by C-H/C-H Coupling through 1,4-Palladium Migration/Alkene Stereoisomerization

A new and efficient synthesis of 8H-benzo[e]phenanthro[1,10-bc]silines from 2-((2-(arylethynyl)aryl)silyl)aryl triflates under palladium catalysis has been developed. The reaction mechanism was experimentally investigated and a catalytic cycle involving C-H/C-H coupling through a new mode of 1,4-palladium migration with concomitant alkene stereoisomerization is proposed.

Selective access to either a doubly boron-doped tetrabenzopentacene or an oxadiborepin from the same precursor

Depending on the solvent, a brominated arylborane gave the multiple helicene B₂-TBPA (pyridine) or the oxadiborepin ODBE (THF) after intramolecular Yamamoto coupling.

The well-known red emitter tetrabenzo[de,hi,op,st]pentacene (TBPA) has been transformed into a bright blue emitter (B₂-TBPA; λ_em = 472 nm; c-hexane) via substitutional doping with two boron atoms. In contrast to the electron-rich TBPA, which forms endo-peroxides with O₂ under daylight, the benchtop-stable B₂-TBPA is a good electron acceptor and undergoes reversible reduction at a moderate half-wave potential of E_1/2 = –1.73 V (vs. FcH/FcH⁺; THF). Although the size of B₂-TBPA falls within the nanoscale, the helically twisted compound readily dissolves in c-hexane and does not require solubilizing substituents. The synthesis of B₂-TBPA is based on the nickel-mediated Yamamoto-type dehalogenation of tetrabrominated 9,10-di(naphth-1-yl)-9,10-dihydro-9,10-diboraanthracene. This intramolecular C–C heterocoupling reaction shows a remarkable solvent dependence: B₂-TBPA forms only in pyridine (79% yield), whereas an oxadiborepin is obtained from THF solutions (ODBE, 81%; the reaction mixture is quenched with air in both cases). Insight into the corresponding reaction mechanism was gained from the isolation of intermediates and an investigation of their chemical properties. ODBE is an interesting blue emitter in its own right. Furthermore, it can be ring-opened with excess BBr₃ at the B–O–B moiety to afford a dimeric borabenzo[de]anthracene.

Tunable Low-LUMO Boron-Doped Polycyclic Aromatic Hydrocarbons by General One-Pot C-H Borylations

Boron-doping has long been recognized as a promising LUMO energy-lowering modification of graphene and related polycyclic aromatic hydrocarbons (PAHs). Unfortunately, synthetic difficulties have been a significant bottleneck for the understanding, optimization, and application of precisely boron-doped PAHs for optoelectronic purposes. Herein, a facile one-pot hydroboration electrophilic borylation cascade/dehydrogenation approach from simple alkene precursors is coupled with postsynthetic B-substitution to give access to ten ambient-stable core- and periphery-tuned boron-doped PAHs. These include large hitherto unknown doubly boron-doped analogues of anthanthrene and triangulene. Crystallographic, optical, electrochemical, and computational studies were performed to clarify the effect of boron-doped PAH shape, size, and structure on optoelectronic properties. Our molecular tuning allowed the synthesis of molecules exhibiting visible-range absorption, near-unity fluorescence quantum yields, and, to our knowledge, the most facile electrochemical reductions of any reported ambient-stable boron-doped PAHs (corresponding to LUMO energy levels as low as fullerenes). Finally, our study describes the first implementation of a precise three-coordinate boron-substituted PAH as an acceptor material in organic solar cells with power conversion efficiencies (PCEs) of up to 3%.

Simultaneous expansion of 9,10 boron-doped anthracene in longitudinal and lateral directions

Doubly boron-doped anthracenes and pentacenes have been longitudinally and laterally expanded through annulation of thiophene or benzene rings. The obtained series of closely related compounds allowed an assessment of key structure-property relationships with a focus on optoelectronic characteristics. Most of the products are benchtop-stable blue emitters and capable of accepting two electrons in a reversible manner. The syntheses involved late-stage modifications through photocyclization or stepwise oxidative C-C coupling (DDQ/BF₃·Et₂O) as well as cyclocondensation of ortho-disilylated or -diborylated aryl building blocks.

Nanographene and Graphene Nanoribbon Synthesis via Alkyne Benzannulations

The extension of π-conjugation of polycyclic aromatic hydrocarbons (PAHs) via alkyne benzannulation reactions has become an increasingly utilized tool over the past few years. This short review will highlight recent work of alkyne benzannulations in the context of large nanographene as well as graphene nanoribbon synthesis along with a brief discussion of the interesting physical properties these molecules display.

C-Halogenated 9,10-Diboraanthracenes: How the Halogen Load and Distribution Influences Key Optoelectronic Properties

9,10-Dihydro-9,10-diboraanthracenes (DBAs) have low-energy LUMOs and narrow HOMO-LUMO gaps and are thus attractive electron-transporting and light-emitting materials in optoelectronic devices. A systematic series of ten C-halogenated 9,10-(Mes)₂ -DBAs was synthesized and studied by cyclic voltammetry, UV/Vis absorption and emission spectroscopy, and quantum-chemical calculations (Mes=mesityl). We probed the influence of the nature of the halogen atoms and the halogen substitution patterns on key optoelectronic properties of the DBAs. All 9,10-(Mes)₂ derivatives can be reversibly reduced at the DBA cores and at electrode potentials between E_1/2 ^Red1 =-1.84 and -1.26 V (vs. FcH/FcH⁺ ). The most bathochromic UV/Vis absorption and the fluorescence emission of each DBA correspond to an ICT transition between the Mes rings and the DBA core. Br substituents lower the DBA LUMO energy and narrow the energy gap to the highest degree along the series F<Cl<Br. Halogen atoms located at 1,4,5,8-positions are more influential than those at 2,3,6,7-positions. An increasing fluorine load continuously decreases the LUMO levels and HOMO-LUMO gaps. Colorless octafluoro- and tetrafluoro-DBAs form colored and fluorescent π-stacked hetero dimers with anthracene in C₆ H₆ solutions and in the solid state. The sterically congested 1,5-(ThMe)₂ -9,10-(Mes)₂ -DBA was prepared in 74 % yield from 1,5-(Br)₂ -9,10-(Mes)₂ -DBA and Me₃ Sn-ThMe through a Stille-type C-C coupling reaction (ThMe=5-Me-2-thienyl).

A modular route to boron doped PAHs by combining borylative cyclisation and electrophilic C-H borylation

Heteroatom doping into polyaromatic hydrocarbons (PAHs) is a powerful approach for modifying key physical properties, however, there are extremely few modular routes that enable facile formation of B-, B2- and B,N-(specifically not containing direct B-N bonds) doped PAHs despite the growing importance of these materials. Sequential, one pot borylative cyclisation/intramolecular electrophilic C-H borylation of naphthyl-alkynes provides a simple new route to access novel B-, B,N- and B2-doped (PAHs). The initial products, dihydronaphthalene/dihydroquinoline B-mesityl PAHs, were reacted with [Ph3C][BF4]/pyridyl base to form the oxidised B-, and B,N-doped PAHs. However, for B-triisopropylphenyl (Trip) PAH congeners oxidation has to be performed prior to Trip installation due to preferential oxidation of an isopropylaryl moiety to the styrene. This alternative sequence enables access to Trip-B-PAHs and to structurally constrained B and B2-PAHs. Analysis of the solid state structures and optoelectronic properties of these PAHs confirm that frontier orbital energies, extended packing structures, Stokes shift and quantum yields all can be rationally modified using this methodology. The simplicity of this synthetic approach makes it a powerful tool for rapidly generating novel bench stable boron doped PAHs, which is important for facilitating further structure-property relationship studies and the wider utilisation of these materials in optoelectronic applications.

Facile Route to Quadruply Annulated Borepins

A two-step synthesis sequence furnishes quadruply annulated borepins in high yields. The first step involves a nucleophilic substitution reaction between aryl-BF₃K salts (aryl = mesityl, phenyl) and lithiated bromonapthalene derivatives LiNaph^Br,R (HNaph^Br,R = 8-bromonaphthalene (a), 5-bromoacenaphthene (b), 5-bromoacenaphthylene (c)). In the second step, the resulting heteroleptic triarylboranes aryl-B(Naph^Br,R)₂ (3a-c) are subjected to an intramolecular Ni-mediated Yamamoto reaction to close the seven-membered rings and create the borepins 4a-c. Only in the case of 3b is the Yamamoto reaction accompanied by a C-H activation reaction furnishing the 7-hydro-7-borabenzo[de]anthracene derivative 5. The product ratio 4b/5 can be influenced by control of the local Ni(0) concentration. The borepins 4a-c are benchtop stable and highly soluble even in hexane. Compounds 4a-c undergo reversible one-electron reduction; 4c is also able to accept a second electron in a reversible manner and already at moderate potential values (E_1/2 = -1.49 V and -1.84 V (vs FcH/FcH⁺)). 4a, 4b, and 5 show photoluminescence in the blue-green region of the spectrum, while 4c is nonfluorescent, which is likely attributable to an intramolecular charge-transfer transition.

Recent developments in and perspectives on three-coordinate boron materials: a bright future

The empty p z -orbital of a three-coordinate organoboron compound leads to its electron-deficient properties, which make it an excellent π-acceptor in conjugated organic chromophores. The empty p-orbital in such Lewis acids can be attacked by nucleophiles, so bulky groups are often employed to provide air-stable materials. However, many of these can still bind fluoride and cyanide anions leading to applications as anion-selective sensors. One electron reduction generates radical anions. The π-acceptor strength can be easily tuned by varying the organic substituents. Many of these compounds show strong two-photon absorption (TPA) and two-photon excited fluorescence (TPEF) behaviour, which can be applied for e.g. biological imaging. Furthermore, these chromophores can be used as emitters and electron transporters in OLEDs, and examples have recently been found to exhibit efficient thermally activated delayed fluorescence (TADF). The three-coordinate organoboron unit can also be incorporated into polycyclic aromatic hydrocarbons. Such boron-doped compounds exhibit very interesting properties, distinct from their all-carbon analogues. Significant developments have been made in all of these areas in recent years and new applications are rapidly emerging for this class of boron compounds.

Tetraarylethylenes from Diarylmethanones and Hexachlorodisilane: The "Sila-McMurry" Reaction

Hexachlorodisilane reacts with diarylmethanones (aryl=C₆ H₅ , 4-MeC₆ H₄ , 4-tBuC₆ H₄ , 4-ClC₆ H₄ , 4-BrC₆ H₄ ) to furnish the corresponding tetraarylethylenes in good yields. The reductive conversion requires temperatures of about 160 °C and reaction times of 60-72 h. In the initial step, the Lewis-basic carbonyl groups likely generate low-valent [SiCl₂ ] as an analogue of [TiCl₂ ] in the classical McMurry reaction. The coupling sequence further proceeds via benzopinacolones, which have been isolated as key intermediates.

1,3-Dien-5-ynes: Versatile Building Blocks for the Synthesis of Carbo- and Heterocycles

1,3-Dien-5-ynes have been extensively used as starting materials for the synthesis of a wide number of different carbo- and heterocycles. The aim of this review is to give an overview of their utility in organic synthesis, highlighting the variety of compounds that can be directly accessed from single reactions over these systems. Thus, cycloaromatization processes are initially commented, followed by reactions directed toward the syntheses of five-membered rings, other carbocycles and, finally, heterocycles. The diverse methodologies that have been developed for the synthesis of each of these types of compounds from 1,3-dien-5-ynes are presented, emphasizing the influence of the reaction conditions and the use of additional reagents in the outcome of the transformations.