Increasing Chemical Diversity of B2 N2 Anthracene Derivatives by Introducing Continuous Multiple Boron-Nitrogen Units

Increasing the chemical diversity of organic semiconductors is essential to develop efficient electronic devices. In particular, the replacement of carbon-carbon (C-C) bonds with isoelectronic boron-nitrogen (B-N) bonds allows precise modulation of the electronic properties of semiconductors without significant structural changes. Although some researchers have reported the preparation of B2 N2 anthracene derivatives with two B-N bonds, no compounds with continuous multiple BN units have been prepared yet. Herein, we report the synthesis and characterization of a B2 N2 anthracene derivative with a BNBN unit formed by converting the BOBN unit at the zigzag edge. Compared to the all-carbon analogue 2-phenylanthracene, BNBN anthracene exhibits significant variations in the C-C bond length and a larger highest occupied molecular orbital-lowest unoccupied molecular orbital energy gap. The experimentally determined bond lengths and electronic properties of BNBN anthracene are confirmed through theoretical calculations. The BOBN anthracene organic light-emitting diode, used as a blue host, exhibits a low driving voltage. The findings of this study may facilitate the development of larger acenes with multiple BN units and potential applications in organic electronics.

Mixed-Valence BN-Doped Corannulene Trimer Radical Cations

Mixed-valence (MV) dimers have been extensively investigated, however, the structure and properties of purely organic MV trimers based on open-shell polycyclic aromatic hydrocarbons remain elusive. Herein, unprecedented MV BN-doped corannulene radical cations [BN-Cor1]3 ⋅⋅2+  ⋅ 2[BArylF 4 ]- and [BN-Cor2]3 ⋅⋅2+  ⋅ 2[BArylF 4 ]- were synthesized via chemical oxidation, and their structures were unambiguously confirmed by single-crystal X-ray diffraction. These uncommon radical cations consist of three corannulene cores and two [BArylF 4 ]- anions, and three corannulene motifs [BN-Cor1]3 ⋅⋅2+ and [BN-Cor2]3 ⋅⋅2+ in the unit cell exhibit a trimer structure with a slipped π-stacking configuration. Detailed structural analyses further revealed that the corannulene cores exhibit an infinite layered self-assembly configuration, allowing their potential applications as building blocks for molecular conductors. The detection of a forbidden transition (Δms =±2) by electron paramagnetic resonance (EPR) spectroscopy further confirmed the existence of two unpaired electrons in the π-trimers and the MV characteristic of these two species. Variable-temperature EPR and conductivity measurements suggested that the BN-doped π-trimers exhibited antiferromagnetic coupling and conductivity properties.

Two-Step Synthesis of B2 N2 -Doped Polycyclic Aromatic Hydrocarbon Containing Pentagonal and Heptagonal Rings with Long-Lived Delayed Fluorescence

Pentagon-heptagon embedded polycyclic aromatic hydrocarbons (PAHs) have aroused increasing attention in recent years due to their unique physicochemical properties. Here, for the first time, this report demonstrates a facile method for the synthesis of a novel B2 N2 -doped PAH (BN-2) containing two pairs of pentagonal and heptagonal rings in only two steps. In the solid state of BN-2, two different conformations, including saddle-shaped and up-down geometries, are observed. Through a combined spectroscopic and calculation study, the excited-state dynamics of BN-2 is well-investigated in this current work. The resultant pentagon-heptagon embedded B2 N2 -doped BN-2 displays both prompt fluorescence and long-lived delayed fluorescence components at room temperature, with the triplet excited-state lifetime in the microsecond time region (τ = 19 µs). The triplet-triplet annihilation is assigned as the mechanism for the observed long-lived delayed fluorescence. Computational analyses attributed this observation to the small energy separation between the singlet and triplet excited states, facilitating the intersystem crossing (ISC) process which is further validated by the ultrafast spectroscopic measurements.

Wide-range color-tunable polycyclo-heteraborin multi-resonance emitters containing B-N covalent bonds

Boron- and nitrogen (BN)-fused polycyclic aromatic frameworks with amine-directed formation of B-N covalent bonds have the potential to form a new family of facile-synthesis multi-resonance luminophores, which, however, still face imperative challenges in diversifying the molecular design to narrow the emission bandwidth and tune the emission colors. Here, we demonstrate a strategic implementation of B-N bond containing polycyclo-heteraborin multi-resonance emitters with wide-range colors from deep-blue to yellow-green (442-552 nm), small full-width at half-maxima of only 19-28 nm and high photoluminescence efficiencies, by stepwise modifying the basic para B-π-B structures with heteroatoms. The corresponding electroluminescent devices show superior maximum external quantum efficiencies with an exceptional low-efficiency roll-off, retaining 21.0%, 23.6% and 22.1% for the sky-blue, green and yellow-green devices at a high luminance of 5000 cd m^-2, respectively.

Synthesis and Late-Stage Diversification of BN-Embedded Dibenzocorannulenes as Efficient Fluorescence Organic Light-Emitting Diode Emitters

We report the synthesis and late-stage diversification of a new class of hetero-buckybowl, BN-embedded dibenzocorannulenes (B₂ N₂ -DBCs). The synthesis is achieved via one-shot halogenative borylation, comprising the nitrogen-directed haloboration of alkyne and an intramolecular bora-Friedel-Crafts reaction, which provides BN-embedded dibenzocorannulene possessing two bromo substituents (B₂ N₂ -DBC-Br). B₂ N₂ -DBC-Br undergoes diversification via coupling reactions to provide a variety of arylated derivatives (B₂ N₂ -DBC-R), exhibiting strong blue fluorescence. An organic light-emitting diode (OLED) employing one of the derivatives as an emitter exhibited a high external quantum efficiency of 6.6 % and long operational lifetime of 907 h at an initial luminance of 1000 cd m^-2 , indicating the significant potential for the development of efficient and stable hetero-buckybowl-based OLED materials.

Angularly fused diaza-dinaphthopyrenes: regio-selective synthesis, crystal structures and isomer-dependent mechanochromic fluorescent properties

We report a one-pot synthesis of a series of unprecedented angular-fused diaza-dinaphthopyrene isomers (1,8-DNPy and 1,6-DNPy) in high yields, which are enabled by regio-selective Bischler-Napieralski cyclization to fuse two quinolone rings either on the same or opposite faces of a pyrene core. Benefiting from the high reactivity of the 1- and 8-positions of the pyrene ring, steric effect from substitution and remarkably different dipole moments, high ring closure selectivity for the 1,8-form vs. the 1,6-form up to 6 : 1 is achieved with ease of separation. With differentiated molecular symmetry, conformation, intermolecular interactions and aromaticity, the two kinds of regio-isomers exhibit distinct single-crystal structures and optoelectronic properties. Impressively, isomer-dependent mechanochromic fluorescent properties of these 2D-azaacenes are identified, which are unique in their turn-on fluorescence feature and contrasting spectral shifts. These findings allow facile and modular access to regio-specific 2D-N-heteroarenes, which provide a way to create innovative optical sensors with improved sensitivity and fruitful fluorescent properties.

Access to tetracoordinate boron-doped polycyclic aromatic hydrocarbons with delayed fluorescence and aggregation-induced emission under mild conditions

Boron-doped polycyclic aromatic hydrocarbons (PAHs) have attracted ongoing attention in the field of optoelectronic materials due to their unique optical and redox properties. To investigate the effect of tetracoordinate boron in PAHs bearing N-heterocycles (indole and carbazole), a facile approach to four-coordinate boron-doped PAHs was developed, which does not require elevated temperature and pre-synthesized functionalized boron reactants. Five tetracoordinate boron-doped PAHs (NBNN-1–NBNN-5) were synthesized with different functional groups. Two of them (NBNN-1 and NBNN-2) could further undergo oxidative coupling reactions to form fused off-plane tetracoordinate boron-doped PAHs NBNN-1f and NBNN-2f. The investigation of photophysical properties showed that the UV/vis absorption and fluorescence emission are significantly red-shifted compared to those of the three-coordinate boron-doped counterparts. In addition, the emission of NBNN-1–NBNN-3 consisted of prompt fluorescence and delayed fluorescence. The compounds NBNN-1f and NBNN-2f showed aggregation-induced emission.

A series of tetracoordinate boron-doped polycyclic aromatic hydrocarbons have been synthesized under mild conditions, featuring delayed fluorescence and aggregation-induced emission.

Synthesis, Structure, and Photophysical Properties of BN-Embedded Analogue of Coronene

Two BN-embedded benzo[ghi]perylene (Bzp) and coronene derivatives (BN-Bzp and BN-Cor) have been successfully synthesized from binaphthyl precursors by new efficient one-pot-multibond routes, and their single crystal structures were analyzed. Both experimental spectra and DFT theoretical calculations indicated that the absorption and emission of these BN-embedded polycyclic aromatic hydrocarbons are significantly enhanced comparing with those of their all carbon analogues. Especially, the fluorescence quantum yield of BN-Cor is nearly 20 times higher than that of ordinary coronene.

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).

Going beyond the borders: pyrrolo[3,2-b]pyrroles with deep red emission

A two-step route to strongly absorbing and efficiently orange to deep red fluorescent, doubly B/N-doped, ladder-type pyrrolo[3,2-b]pyrroles has been developed. We synthesize and study a series of derivatives of these four-coordinate boron-containing, nominally quadrupolar materials, which mostly exhibit one-photon absorption in the 500-600 nm range with the peak molar extinction coefficients reaching 150 000, and emission in the 520-670 nm range with the fluorescence quantum yields reaching 0.90. Within the family of these ultrastable dyes even small structural changes lead to significant variations of the photophysical properties, in some cases attributed to reversal of energy ordering of alternate-parity excited electronic states. Effective preservation of ground-state inversion symmetry was evidenced by very weak two-photon absorption (2PA) at excitation wavelengths corresponding to the lowest-energy, strongly one-photon allowed purely electronic transition. π-Expanded derivatives and those possessing electron-donating groups showed the most red-shifted absorption- and emission spectra, while displaying remarkably high peak 2PA cross-section (σ _2PA) values reaching ∼2400 GM at around 760 nm, corresponding to a two-photon allowed higher-energy excited state. At the same time, derivatives lacking π-expansion were found to have a relatively weak 2PA peak centered at ca. 800-900 nm with the maximum σ _2PA ∼50-250 GM. Our findings are augmented by theoretical calculations performed using TD-DFT method, which reproduce the main experimental trends, including the 2PA, in a nearly quantitative manner. Electrochemical studies revealed that the HOMO of the new dyes is located at ca. -5.35 eV making them relatively electron rich in spite of the presence of two B^--N⁺ dative bonds. These dyes undergo a fully reversible first oxidation, located on the diphenylpyrrolo[3,2-b]pyrrole core, directly to the di(radical cation) stage.

BN-Embedded Perylenes

Transition metal catalyzed coupling reaction strategy has been utilized in the synthesis of two novel BN-perylenes starting from halogenated BN-naphthalene derivatives. The molecular structures and packing modes of BN-perylenes were confirmed by NMR spectroscopy and X-ray single-crystal diffraction experiments. Their photophysical properties were further investigated using UV-vis and fluorescence spectroscopy and DFT calculations. Interestingly, the isosteric BN-insertion in perylene system resulted in stronger π-π stacking interaction both in solid and solution phases. The synthesized BN-perylenes are proved to be highly stable and thus provide a new valuable platform for novel organic materials applications which is otherwise inaccessible to date.

PN-Doped tetraphenylnaphthalene: a straightforward synthetic strategy analogous to BN-annulation

Compared to BN heterocycles, few studies on PN heterocycles have been reported to date. Herein, we developed an efficient synthetic strategy analogous to BN-annulation to simultaneously incorporate a PN bond and a halogen group into the naphthalene core. Subsequently, we prepared PN-containing tetraphenylnaphthalene using this method, followed by palladium-catalyzed cross-coupling and reduction reactions. The prepared molecule was characterized via X-ray crystallography, NMR spectroscopy, UV-vis spectroscopy, and cyclic voltammetry.

Parent B2 N2 -Perylenes with Different BN Orientations

Introducing BN units into polycyclic aromatic hydrocarbons expands the chemical space of conjugated materials with novel properties. However, it is challenging to achieve accurate synthesis of BN-PAHs with specific BN positions and orientations. Here, three new parent B₂ N₂ -perylenes with different BN orientations are synthesized with BN-naphthalene as the building block, providing systematic insight into the effects of BN incorporation with different orientations on the structure, (anti)aromaticity, crystal packing and photophysical properties. The intermolecular dipole-dipole interaction shortens the π-π stacking distance. The crystal structure, (anti)aromaticity, and photophysical properties vary with the change of BN orientation. The revealed BN doping effects may provide a guideline for the synthesis of BN-PAHs with specific stacking structures, and the synthetic strategy employed here can be extended toward the synthesis of larger BN-embedded PAHs with adjustable BN patterns.

BN/BO-Ullazines and Bis-BO-Ullazines: Effect of BO Doping on Aromaticity and Optoelectronic Properties

We have achieved substitutional doping of ullazine with either two BO units or with one BO unit and one BN unit. The synthesis of these B-doped ullazines is straightforward, using demethylation and borylative cyclization as the key steps. Ullazine cores of both BN/BO-ullazines (2) and bis-BO-ullazines (3) are very close to being planar. Their electronic and photophysical properties were investigated by ultraviolet-visible, fluorescence spectroscopy, cyclic voltammetry, and density functional theory calculations.

Heteroatom Cycloaddition at the (BN)2 Bay Region of Dibenzoperylene

Cycloaddition-dehydration involving a BNBN-butadiene analogue at the bay region of a dibenzoperylene and a non-enolizable aldehyde provides a novel strategy for incorporation of the oxadiazadiborinane (B₂ N₂ CO) ring into the scaffold of a polycyclic aromatic hydrocarbon resulting in highly emissive compounds.

Pyrrolic Type N Directed Borylation Route to BN-PAHs: Tuning the Photophysical Properties by Varying the Conjugation Shape and Size

Two series of BN-cyclopenta[a]phenalenes have been synthesized through an indole/pyrrole oriented borylation reaction. A total of five compounds are obtained and fully characterized; one of them is unambiguously confirmed by single X-ray crystal structure. Their photophysical properties could be finely tuned through varying the conjugation size and shape of the bottom PAHs applied. Moreover, their response toward fluoride anions was also investigated.

One-Pot Synthesis of Boron-Doped Polycyclic Aromatic Hydrocarbons via 1,4-Boron Migration

Herein, we demonstrate a novel one-pot synthetic method towards a series of boron-doped polycyclic aromatic hydrocarbons (B-PAHs, 1 a-1 o), including hitherto unknown B-doped zethrene derivatives, from ortho-aryl substituted diarylalkynes with high atom efficiency and broad substrate scopes. A reaction mechanism is proposed based on the experimental investigation together with the theoretical calculations, which involves a unique 1,4-boron migration process. The resultant benchtop-stable B-PAHs are thoroughly investigated by X-ray crystallography, cyclic voltammetry, UV/Vis absorption, and fluorescence spectroscopies. The blue and green organic light-emitting diode (OLED) devices based on 1 f and 1 k are further fabricated, demonstrating the promising application potential of B-PAHs in organic optoelectronics.