Synthesis of a novel tetra-phenol π-extended phenazine and its application as an organo-photocatalyst.

In this paper, the synthesis of a novel tetra-phenol π-extended dihydrophenazine is reported. The obtained derivative presents marked reducing properties in the excited state and was exploited as an organo-photocatalyst in dehalogenation and C-C bond formation reactions. These results underline the great potential of functionalized π-extended dihydrophenazines as organo-photocatalysts.

Direct C2-H alkylation of indoles driven by the photochemical activity of halogen-bonded complexes.

A light-driven metal-free protocol for the synthesis of sulfone-containing indoles under mild conditions is reported. Specifically, the process is driven by the photochemical activity of halogen-bonded complexes formed upon complexation of a sacrificial donor, namely 1,4-diazabicyclo[2.2.2]octane (DABCO), with α-iodosulfones. The reaction provides a variety of densely functionalized products in good yields (up to 96% yield). Mechanistic investigations are reported. These studies provide convincing evidences for the photochemical formation of reactive open-shell species.

Photoinduced Cascade Reactions of 2-Allylphenol Derivatives toward the Production of 2,3-Dihydrobenzofurans.

A light-driven protocol for the synthesis of 2,3-dihydrobenzofurans under mild conditions is reported. Specifically, the cascade process is initiated by the photochemical activity of allyl-functionalized phenolate anions, generated in situ upon deprotonation of the corresponding phenols. The reaction proceeds rapidly with reaction times as low as 35 min, delivering a wide range of densely functionalized products (20 examples, yields up to 69%). Mechanistic studies have also been performed providing convincing evidence for the photochemical formation of carbon-centered radical species. A cascade reaction pathway involving a tandem atom transfer radical addition (ATRA) and an intramolecular nucleophilic substitution (SN) process is proposed to occur.

N,N-Diphenyl Dihydrophenazines: Using π-Extension to Access Dicationic Multifunctional Materials.

Dihydrophenazines are receiving increasing attention due to applications in numerous fields of chemistry, from light emission to organo-photocatalysis. Despite this growing interest and numerous works involving the preparation of radical cations based on this scaffold, the isolation and study of the aromatic dications obtained by 2 electron oxidation of dihydrophenazines is still mostly unexplored. From this point of view, along with the substitution at the N atoms generally used to tune dihydrophenazine properties, the π-extension of the phenazine core could play a crucial role in making dicationic states accessible. This could result in an extension of the knowledge on these elusive dications and in potentially highly interesting applications ranging from material science to molecular actuators.

Boron Nitride-Doped Polyphenylenic Organogels.

Herein, we describe the synthesis of the first boron nitride-doped polyphenylenic material obtained through a [4 + 2] cycloaddition reaction between a triethynyl borazine unit and a biscyclopentadienone derivative, which undergoes organogel formation in chlorinated solvents (the critical jellification concentration is 4% w/w in CHCl3). The polymer has been characterized extensively by Fourier-transform infrared spectroscopy, solid-state 13C NMR, solid-state 11B NMR, and by comparison with the isolated monomeric unit. Furthermore, the polymer gels formed in chlorinated solvents have been thoroughly characterized and studied, showing rheological properties comparable to those of polyacrylamide gels with a low crosslinker percentage. Given the thermal and chemical stability, the material was studied as a potential support for solid-state electrolytes. showing properties comparable to those of polyethylene glycol-based electrolytes, thus presenting great potential for the application of this new class of material in lithium-ion batteries.

Phenanthrene-Extended Phenazine Dication: An Electrochromic Conformational Switch Presenting Dual Reactivity.

The synthesis and isolation of one of the few examples of a π-extended diamagnetic phenazine dication have been achieved by oxidizing a phenanthrene-based dihydrophenazine precursor. The resulting dication was isolated and fully characterized, highlighting an aromatic distorted structure, generated by the conformational change upon the oxidation of the dihydrophenazine precursor, which is also correlated with a marked electrochromic change in the UV-vis spectrum. The aromaticity of the dication has also been investigated theoretically, proving that the species is aromatic based on all major criteria (structural, magnetic, and energetic). Moreover, the material presents an intriguing dual reactivity, resulting in ring contraction to a π-extended triarylimidazolinium and reduction to the dihydrophenazine precursor, depending on the nature of the nucleophile involved. This result helps shed light on the yet largely unexplored reactivity and properties of extended dicationic polycyclic aromatic hydrocarbons (PAHs). In particular, the fact that the molecule can undergo a reversible change in conformation upon oxidation and reduction opens potential applications for this class of derivatives as molecular switches and actuators.

BN-Doped Metal-Organic Frameworks: Tailoring 2D and 3D Porous Architectures through Molecular Editing of Borazines.

Building on the MOF approach to prepare porous materials, herein we report the engineering of porous BN-doped materials using tricarboxylic hexaarylborazine ligands, which are laterally decorated with functional groups at the full-carbon 'inner shell'. Whilst an open porous 3D entangled structure could be obtained from the double interpenetration of two identical metal frameworks derived from the methyl substituted borazine, the chlorine-functionalised linker undergoes formation of a porous layered 2D honeycomb structure, as shown by single-crystal X-ray diffraction analysis. In this architecture, the borazine cores are rotated by 60° in alternating layers, thus generating large rhombohedral channels running perpendicular to the planes of the networks. An analogous unsubstituted full-carbon metal framework was synthesised for comparison. The resulting MOF revealed a crystalline 3D entangled porous structure, composed by three mutually interpenetrating networks, hence denser than those obtained from the borazine linkers. Their microporosity and CO2 uptake were investigated, with the porous 3D BN-MOF entangled structure exhibiting a large apparent BET specific surface area (1091 m2 g-1 ) and significant CO2 reversible adsorption (3.31 mmol g-1 ) at 1 bar and 273 K.

Revealing the Impact of Heat Generation Using Nanographene-Based Light-Emitting Electrochemical Cells.

Self-heating in light-emitting electrochemical cells (LECs) has been long overlooked, while it has a significant impact on (i) device chromaticity by changing the electroluminescent band shape, (ii) device efficiency because of thermal quenching and exciton dissociation reducing the external quantum efficiency (EQE), and (iii) device stability because of thermal degradation of excitons and eliminate doped species, phase separation, and collapse of the intrinsic emitting zone. Herein, we reveal, for the first time, a direct relationship between self-heating and the early changes in the device chromaticity as well as the magnitude of the error comparing theoretical/experimental EQEs-that is, an overestimation error of ca. 35% at usual pixel working temperatures of around 50 °C. This has been realized in LECs using a benchmark nanographene-that is, a substituted hexa-peri-hexabenzocoronene-as an emerging class of emitters with outstanding device performance compared to the prior art of small-molecule LECs-for example, luminances of 345 cd/m2 and EQEs of 0.35%. As such, this work is a fundamental contribution highlighting how self-heating is a critical limitation toward the optimization and wide use of LECs.

Boron-Nitrogen-Doped Nanographenes: A Synthetic Tale from Borazine Precursors.

In this work, a comprehensive account of the authors' synthetic efforts to prepare borazino-doped hexabenzocoronenes by using the Friedel-Crafts-type electrophilic aromatic substitution is reported. Hexafluoro-functionalized aryl borazines, bearing an ortho fluoride leaving group on each of the N- and B-aryl rings, was shown to lead to cascade-type electrophilic aromatic substitution events in the stepwise C-C bond formation, giving higher yields of borazinocoronenes than those obtained with borazine precursors bearing fluoride leaving groups at the ortho positions of the B-aryl substituents. By using this pathway, an unprecedented boroxadizine-doped PAH featuring a gulf-type periphery could be isolated, and its structure proven by single-crystal X-ray diffraction analysis. Mechanistic studies on the stepwise Friedel-Crafts-type cyclization suggest that the mechanism of the planarization reaction proceeds through extension of the π system. To appraise the doping effect of the boroxadizine unit on the optoelectronic properties of topology-equivalent molecular graphenes, the all-carbon and pyrylium PAH analogues, all featuring a gulf-type periphery, were also prepared. As already shown for the borazino-doped hexabenzocoronene, the replacement of the central benzene ring by its B3 N2 O congener widens the HOMO-LUMO gap and dramatically enhances the fluorescence quantum yield.

Probing Peripheral H-Bonding Functionalities in BN-Doped Polycyclic Aromatic Hydrocarbons.

The replacement of carbon atoms at the zigzag periphery of a benzo[fg]tetracenyl derivative with an NBN atomic triad allows the formation of heteroatom-doped polycyclic aromatic hydrocarbon (PAH) isosteres, which expose BN mimics of the amidic NH functions. Their ability to form H-bonded complexes has never been touched so far. Herein, we report the first solution recognition studies of peripherally NBN-doped PAHs to form H-bonded DD·AA- and ADDA·DAAD-type complexes with suitable complementary H-bonding acceptor partners. The first determination of Ka in solution showed that the 1:1 association strength is around 27 ± 1 M-1 for the DD·AA complexes in C6D6, whereas it rises to 1820 ± 130 M-1 for the ADDA·DAAD array in CDCl3. Given the interest of BN-doped polyaromatic hydrocarbons in supramolecular and materials chemistry, it is expected that these findings will open new possibilities to design novel materials, where the H-bonding properties of peripheral NH hydrogens could serve as anchors to tailor the organizational properties of PAHs.

Structural Properties of Highly Doped Borazino Polyphenylenes Obtained through Condensation Reaction.

Here we describe the synthesis and spectroscopic and structural characterization of various borazine-doped polyphenylenes displaying high doping dosages (16-18%). Capitalizing on the condensation reaction approach, the desired products were formed using a mixture of p-phenylendiamine and aniline with BCl3, followed by the addition of an aryl lithium derivative. The use of mesityl lithium (MesLi) yields strained multiborazine derivatives, which proved to be unstable in the presence of moisture. However, when xylyl lithium (XylLi) was used, chemically stable multiborazines were obtained, with oligomers showing molecular weight up to 104, corresponding to 16-18 monomer units. While the dimer, trimer, and tetramer could be isolated as pure products and their structure characterized by mass and NMR analysis, higher oligomers could only be isolated as mixtures of B-hydroxy-substituted derivatives and characterized by gel permeation chromatography. The structures of the dimer and trimer derivatives were confirmed by X-ray analysis, which nicely showed the presence of the two and three borazine rings spaced by one and two 1,4-aryl bridges, respectively. Notably, the trimer forms a porous crystalline clathrate. The peripheral xylyl and phenyl moieties of each molecule intramolecularly embrace each other through C-H and π-π stacking interactions. Steady-state UV-vis absorption characterization suggested that the molecules are UV absorbers, with the extinction coefficient linearly scaling with the degree of oligomerization. On the other hand, low-emission quantum yields were obtained for all derivatives (<7%), suggesting that high BN-doping dosages dramatically affect the emission properties of the doped polyphenylenes.

Synthesis and Optoelectronic Properties of Hexa-peri-hexabenzoborazinocoronene.

The first rational synthesis of a BN-doped coronene derivative in which the central benzene ring has been replaced by a borazine core is described. This includes six C-C ring-closure steps that, through intramolecular Friedel-Crafts-type reactions, allow the stepwise planarization of the hexaarylborazine precursor. UV/Vis absorption, emission, and electrochemical investigations show that the introduction of the central BN core induces a dramatic widening of the HOMO-LUMO gap and an enhancement of the blue-shifted emissive properties with respect to its all-carbon congener.