In Situ Solid-State Generation of (BN)2 -Pyrenes and Electroluminescent Devices

Designing an Efficient Singlet Fission Material with B-N Substitution in Pyrene: A Model Exact Study

The electronic structure of boron (B)-nitrogen (N)-substituted pyrene molecules is the center of attraction in designing an efficient intermolecular singlet fission (x-SF) material. Thermodynamic energy criteria required for x-SF are obtained by captodative substitution with B and N in pristine pyrene to increase the lowest singlet-triplet energy gap. We computed low-lying excited states of BN-embedded pyrene molecules by exactly solving the Pariser-Parr-Pople (PPP) model Hamiltonian and compared these results with the TDDFT and EOM-CCSD values. Exact diagonalization of the PPP model Hamiltonian suggests that pristine pyrene, which is endothermic for x-SF, becomes isoergic with certain (BN)2 substitution. The low-lying excited state energies calculated using the model Hamiltonian match very well with experimental values over EOM-CCSD and TDDFT. Moreover, the low value of the spin-orbit coupling constant calculated for BN-substituted pyrene strengthens its applicability as an SF material.

Synthesis of phenanthrylboroles and formal nitrene insertion to access azaborapyrenes

Extended conjugation has been introduced into boroles but only on the 2,3- or 4,5-positions of the central BC4 core. In this work we synthesize phenanthrylboroles that also have conjugation on the 3,4-positions and demonstrate their insertion reactivity with azides to furnish B,N-analogues of pyrene.

Tetrazole-Functionalized Organoboranes Exhibiting Dynamic Intramolecular N→B-Coordination and Cyanide-Selective Anion Binding

Starting from two different cyano-functionalized organoboranes, we demonstrate that 1,3-dipolar [3+2] azide-nitrile cycloaddition can serve to generate libraries of alkyl-tetrazole-functionalized compounds capable of intramolecular N→B-Lewis adduct formation. Due to the relatively low basicity of tetrazoles, structures can be generated that exhibit weak and labile N→B-coordination. The reaction furnishes 1- and 2-alkylated regio-isomers that exhibit different effective Lewis-acidities at the boron centers, and vary in their optical absorption and fluorescence properties. Indeed, we identified derivatives capable of selectively binding cyanide over fluoride, as confirmed by 11B NMR. This finding demonstrates the potentialities of this synthetic strategy to systematically fine-tune the properties of lead structures that are of interest as chemical sensors.

Assessing the Role of BN-Embedding Position in B2N2-Perylenes

Incorporating heteroatoms can effectively modulate the molecular optoelectronic properties. However, the fundamental understanding of BN doping effects in BN-embedded polycyclic aromatic hydrocarbons (PAHs) is underexplored, lacking rational guidelines to modulate the electronic structures through BN units for advanced materials. Herein, a concise synthesis of novel B2N2-perylenes with BN doped at the bay area is achieved to systematically explore the doping effect of BN position on the photophysical properties of PAHs. The shift of BN position in B2N2-perylenes alters the π electron conjugation, aromaticity and molecular rigidness significantly, achieving substantially higher electron transition abilities than those with BN doped in the nodal plane. It is further clarified that BN position dominates the photophysical properties over BN orientation. The revealed guideline here may apply generally to novel BN-PAHs, and aid the advancement of BN-PAHs with highly-emissive performance.

Solvatochromic and Aggregation-Induced Emission Active Nitrophenyl-Substituted Pyrrolidinone-Fused-1,2-Azaborine with a Pre-Twisted Molecular Geometry

Boron-nitrogen-containing heterocycles with extended conjugated π-systems such as polycyclic aromatic 1,2-azaborines, hold the fascination of organic chemists due to their unique optoelectronic properties. However, the majority of polycyclic aromatic 1,2-azaborines aggregate at high concentrations or in the solid-state, resulting in aggregation-caused quenching (ACQ) of emission. This practical limitation poses significant challenges for polycyclic aromatic 1,2-azaborines' use in many applications. Additionally, only a few solvatochromic polycyclic aromatic 1,2-azaborines have been reported and they all display minimal solvatochromism. Therefore, the scope of available polycyclic 1,2-azaborines needs to be expanded to include those displaying fluorescence at high concentration and in the solid-state as well as those that exhibit significant changes in emission intensity in various solvents due to different polarities. To address the ACQ issue, we evaluate the effect of a pre-twisted molecular geometry on the optoelectronic properties of polycyclic aromatic 1,2-azaborines. Specifically, three phenyl-substituted pyrrolidinone-fused 1,2-azaborines (PFAs) with similar structures and functionalized with diverse electronic moieties (-H, -NO2, -CN, referred to as PFA 1, 2, and 3, respectively) were experimentally and computationally studied. Interestingly, PFA 2 displays two distinct emission properties: 1) solvatochromism, in which its emission and quantum yields are tunable with respect to solvent polarity, and 2) fluorescence that can be completely "turned off" and "turned on" via aggregation-induced emission (AIE). This report provides the first example of a polycyclic aromatic 1,2-azaborine that displays both AIE and solvatochromism properties in a single BN-substituted backbone. According to time-dependent density function theory (TD-DFT) calculations, the fluorescence properties of PFA 2 can be explained by the presence of a low-lying n-π* charge transfer state inaccessible to PFA 1 or PFA 3. These findings will help in the design of future polycyclic aromatic 1,2-azaborines that are solvatochromic and AIE-active as well as in understanding how molecular geometry affects these compounds' optoelectronic 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.

Sequential-Stimuli Induced Stepwise-Response of Pyridylpyrenes

Stimuli-responsive materials, especially multi-stimuli-responsive materials, can sense external stimuli such as light, heat, and force, have shown great potential in drug delivery, data storage, encryption, energy-harvesting, and artificial intelligence. Conventional multi-stimuli-responsive materials are sensitive to each independent stimulus, causing losses in the diversity and accuracy of the identification for practical application. Herein, a unique phenomenon of sequential-stimuli induced stepwise-response generated from elaborately designed single-component organic materials is reported, which shows large bathochromic shifts up to 5800 cm-1 under sequential stimuli of force and light. In contrast to multi-stimuli-responsive materials, the response of these materials strictly relies on the sequence of stimuli, allowing logicality, rigidity, and accuracy to be integrated into one single-component material. The molecular keypad lock is built based on these materials, pointing promising to a future for this logical response in significant practical applications. This breakthrough gives a new drive to classical stimuli-responsiveness and provides a fundamental design strategy for new generations of high-performance stimuli-responsive materials.

Diradical B/N-Doped Polycyclic Hydrocarbons

Heterocyclic diradicaloids with atom-precise control over open-shell nature are promising materials for organic electronics and spintronics. Herein, we disclose quinoidal π-extension of a B/N-heterocycle for generating B/N-type organic diradicaloids. Two quinoidal π-extended B/N-doped polycyclic hydrocarbons that feature fusion of the B/N-heterocycle motif with the antiaromatic s-indacene or dicyclopenta[b,g]naphthalene core were synthesized. This quinoidal π-extension and B/N-heterocycle leads to their open-shell electronic nature, which stands in contrast to the multiple-resonance effect of conventional B/N-type emitters. These B/N-type diradicaloids have modulated (anti)aromaticity and enhanced diradical characters comparing with the all-carbon analogues, as well as intriguing properties, such as magnetic activities, narrow energy gaps and highly red-shifted absorptions. This study thus opens the new space for both of B/N-doped polycyclic π-systems and heterocyclic diradicaloids.

Synthesis and isolation of dinuclear N,C-chelate organoboron compounds bridged by neutral, anionic, and dianionic 4,4'-bipyridine via reductive coupling of pyridines

The reaction of Bppy(Mes)2 (BN1; ppy = 2-phenylpyridine) and BCH₂ppy(Mes)₂ (BN3) with the reducing reagent KC₈ resulted in C-C bond formation via intermolecular radical coupling to generate the 4,4'-bipyridyl ligand compounds BN2 and BN4. Adding 1 equivalent of KC₈ to a THF solution of BN2 and BN4 generated the 4,4'-bipyridyl radical anions BN2K and BN4K. The dianion species BN2K2 and BN4K2 could be obtained by adding 2 equivalents of KC₈ to the THF solution of BN2 and BN4. In the presence of 2,2,2-cryptand or 18-crown-6, the radical anion salt BN2K(crypt) and the dianion salt BN2K2(18c6)2 were isolated for single-crystal X-ray diffraction analysis. Structural, spectroscopic, and computational studies were performed on the three species of BN2 derivatives (neutral, radical anion, and dianion species). BN2 and BN4 were stable and did not undergo photoisomerization or photoelimination under UV light irradiation.

Synthesis, Characterization, and Properties of BN-Fluoranthenes

Boron/nitrogen-doped fluoranthenes, a new class of BN-doped cyclopenta-fused polycyclic aromatic hydrocarbons, were synthesized via pyrrolic-type nitrogen directed C-H borylation. Regioselective bromination of BN-fluoranthene (3a) gave mono- and dibrominated BN-fluoranthenes. The halogenated BN-fluoranthene (3b) can undergo various of further cross-coupling reactions to deliver a series of BN-fluoranthenes. Moreover, incorporating BN unit in to fluoranthene resulted in a wider HOMO-LUMO energy gaps. The aromaticities of the BN-fluoranthene (3a) were quantified by experimental and computational studies.

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.

Concise Synthesis of BN-Dibenzo[f,k]tetraphenes with Different BN Substitution Positions and Direct Comparison with Their Carbonaceous Analogue

Two types of "parental" BN-dibenzo[f,k]tetraphenes (BNDBT-1 and BNDBT-2) have been synthesized via a transition-metal-catalyzed tandem cross-coupling reaction as key steps. Both BNDBT-1 and BNDBT-2 are fully characterized; one of them is unambiguously confirmed by a single X-ray crystal structure. Compared to its all-carbon analogue DBT, BNDBT-1 and BNDBT-2 exhibit a higher highest occupied molecular orbital (HOMO) and lower lowest unoccupied molecular orbital (LUMO) energy, while the BN doping position slightly influences the HOMO and LUMO energies of BNDBT-1 and BNDBT-2. Both BNDBT-1 and BNDBT-2 exhibit red-shifted absorption and emission spectra and higher emission efficiencies, as compared to their carbonaceous analogue DBT. Moreover, organic light emitting diodes were fabricated using BNDBT-1 and BNDBT-2 as emitters, demonstrating their potential applications.

Aggregation caused quenching to aggregation induced emission transformation: a precise tuning based on BN-doped polycyclic aromatic hydrocarbons toward subcellular organelle specific imaging

Polycyclic aromatic hydrocarbons (PAHs) with boron–nitrogen (BN) moieties have attracted tremendous interest due to their intriguing electronic and optoelectronic properties. However, most of the BN-fused π-systems reported to date are difficult to modify and exhibit traditional aggregation-caused quenching (ACQ) characteristics. This phenomenon greatly limits their scope of application. Thus, continuing efforts to seek novel, structurally distinct and functionally diverse structures are highly desirable. Herein, we proposed a one-stone-two-birds strategy including simultaneous exploration of reactivity and tuning of the optical and electronic properties for BN-containing π-skeletons through flexible regioselective functionalization engineering. In this way, three novel functionalized BN luminogens (DPA-BN-BFT, MeO-DPA-BN-BFT and DMA-DPA-BN-BFT) with similar structures were obtained. Intriguingly, DPA-BN-BFT, MeO-DPA-BN-BFT and DMA-DPA-BN-BFT exhibit completely different emission behaviors. Fluorogens DPA-BN-BFT and MeO-DPA-BN-BFT exhibit a typical ACQ effect; in sharp contrast, DMA-DPA-BN-BFT possesses a prominent aggregation induced emission (AIE) effect. To the best of our knowledge, this is the first report to integrate ACQ and AIE properties into one BN aromatic backbone with subtle modified structures. Comprehensive analysis of the crystal structure and theoretical calculations reveal that relatively large twisting angles, multiple intermolecular interactions and tight crystal packing modes endow DMA-DPA-BN-BFT with strong AIE behavior. More importantly, cell imaging demonstrated that luminescent materials DPA-BN-BFT and DMA-DPA-BN-BFT can highly selectively and sensitively detect lipid droplets (LDs) in living MCF-7 cells. Overall, this work provides a new viewpoint of the rational design and synthesis of advanced BN–polycyclic aromatics with AIE features and triggers the discovery of new functions and properties of azaborine chemistry.

A one-stone-two-birds strategy including simultaneous exploration of reactivity and tuning of the optical and electronic properties for BN-fused polycyclic aromatics through flexible regioselective functionalization engineering is presented.

BN-Substitution in Dithienylpyrenes Prevents Excimer Formation in Solution and in the Solid State

Boron-nitrogen substitutions in polycyclic aromatic hydrocarbons (PAHs) have a strong impact on the optical properties of the molecules due to a significantly more heterogeneous electron distribution. However, besides these single-molecule properties, the observed optical properties of PAHs critically depend on the degree of intermolecular interactions such as π-π-stacking, dipolar interactions, or the formation of dimers in the excited state. Pyrene is the most prominent example showing the latter as it exhibits a broadened and strongly bathochromically shifted emission band at high concentrations in solution compared to the respective monomers. In the solid state, the impact of intermolecular interactions is even higher as it determines the crystal packing crucially. In this work, a thiophene-flanked BN-pyrene (BNP) was synthesized and compared with its all-carbon analogue (CCP) in solution and in the solid state by means of crystallography, NMR spectroscopy, UV-vis spectroscopy, and photoluminescence (PL) spectroscopy. In solution, PL spectroscopy revealed the solvent-dependent presence of excimers of CCP at high concentrations. In contrast, no excimers were found in BNP. Clear differences were also observed in the single-crystal packing motifs. While CCP revealed overlapped pyrene planes with centroid distances in the range of classical π-stacking interactions, the BNP scaffolds were displaced and significantly more spatially separated.

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.

The rapid construction of bis-BN dipyrrolyl[a,j]anthracenes and a direct comparison with a carbonaceous analogue

A series of bis-BN dipyrrolyl[a,j]anthracenes and a representative carbonaceous analogue have been synthesized. We studied the optical properties and OLED applications of these BN-PAHs and compared them with the carbonaceous counterpart.

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.

Preparation of Structurally and Electronically Diverse N → B-Ladder Boranes by [2 + 2 + 2] Cycloaddition

We report the synthesis of a series of eight N → B-ladder boranes through cobalt-mediated cyclotrimerization of (2-cyanophenyl)-dimesitylborane with different dialkynes. The resulting tetracoordinate boranes show variable electrochemical and optical properties depending on the substitution pattern in the backbone of the coordinating pyridine-derivatives. While boranes containing alkyl-substituted pyridines show lower electron affinities than the known parent compound, boranes featuring π-extended pyridine derivatives show higher electron affinities in the range of acceptor substituted triarylboranes. All derivatives show larger Stokes shifts (8790-6920 cm^-1) compared to the N → B-ladder borane coordinated by an unsubstituted pyridine.

Aggregation effects on photophysical properties of NBN-doped polycyclic aromatic hydrocarbons: a theoretical study

To realize the precise manipulation of the optoelectrical properties of boron-nitrogen (B-N) unit-doped polycyclic aromatic hydrocarbons (PAHs), unraveling the structure-property relationship behind them is of vital importance. In this work, we take two representative NBN-doped PAHs with similar structures (NBN-5 and NBN-6) as examples and explore the influence of molecular packing on their photophysical properties in different environments (including dilute solution, amorphous aggregate and crystal) using a theoretical protocol that combines molecular dynamics (MD) simulations and thermal vibration correlation function coupled quantum mechanics/molecular mechanics (QM/MM) calculations. We found that the different symmetries of the transition orbitals in NBN-5 and NBN-6 lead to their distinct distributions of transition orbitals, with NBN-5 delocalizing on the whole backbone and NBN-6 bearing clear intramolecular charge transfer. Therefore, the S₁ state of NBN-6 demonstrates redshifted emission spectra in contrast to those of NBN-5. We confirmed that NBN-6 in dilute solution is more flexible than NBN-6 in aggregated states. After aggregation, the fluorescent quantum efficiency (FQE) of NBN-6 increases significantly with the nonradiative decay constants decreasing by 2-4 orders of magnitude, because the dense molecular packing in the aggregated state could effectively suppress the out-of-plane rotation and distortion of the phenyl-ring at the boron position, indicating its AIE feature. While the photophysical properties of conjugated NBN-5 with high rigidity are independent of the environment, showing bright emission in both solution and solid states, consistent with the experimental results. Our theoretical protocol is general and applicable to other doped PAHs, thus laying a solid foundation for the rational design of advanced materials.

Formation of an air-stable diborane via a stepwise BH3 addition of pyrido[1,2-a]isoindole with H2 evolution

A novel and air-stable organo(hydro)diborane featuring a five-membered aryl ring supported bridging B-C-B three-centre-two-electron (3c-2e) bond has been reported. Pyrido[1,2-a]isoindole was found to undergo a stepwise BH₃ addition reaction, during which a mono-BH₃ adduct was formed from a electrophilic addition at the C^γ in pyrido[1,2-a]isoindole. A molecule of hydrogen was eliminated throughout the second step of addition reaction. DFT calculations indicate that the H₂ evolution is concerted to the second BH₃ addition rather than forming BC before the second BH₃ attack.

One tool to bring them all: Au-catalyzed synthesis of B,O- and B,N-doped PAHs from boronic and borinic acids

The isoelectronic replacement of C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C bonds with ⁻BN⁺ bonds in polycyclic aromatic hydrocarbons (PAHs) is a widely used tool to prepare novel optoelectronic materials. Far less well explored are corresponding B,O-doped PAHs, although they have a similarly high application potential. We herein report on the modular synthesis of B,N- and B,O-doped PAHs through the [Au(PPh₃)NTf₂]-catalyzed 6-endo-dig cyclization of BN–H and BO–H bonds across suitably positioned C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C bonds in the key step. Readily available, easy-to-handle o-alkynylaryl boronic and borinic acids serve as starting materials, which are either cyclized directly or first converted into the corresponding aminoboranes and then cyclized. The reaction even tolerates bulky mesityl substituents on boron, which later kinetically protect the formed B,N/O-PAHs from hydrolysis or oxidation. Our approach is also applicable for the synthesis of rare doubly B,N/O-doped PAHs. Specifically, we prepared 1,2-B,E-naphthalenes and -anthracenes, 1,5-B₂-2,6-E₂-anthracenes (E = N, O) as well as B,O₂-containing and unprecedented B,N,O-containing phenalenyls. Selected examples of these compounds have been structurally characterized by X-ray crystallography; their optoelectronic properties have been studied by cyclic voltammetry, electron spectroscopy, and quantum-chemical calculations. Using a new unsubstituted (B,O)₂-perylene as the substrate for late-stage functionalization, we finally show that the introduction of two pinacolatoboryl (Bpin) substituents is possible in high yield and with perfect regioselectivity via an Ir-catalyzed C–H borylation approach.

Singly and doubly B,E-doped PAHs were synthesized using a protocol that starts from easy-to-handle boronic and borinic acids and offers the possibility to choose between the preparation of B,O- and B,N-PAHs in the final reaction step.

Sequential and Diverse Synthesis of BN-Heterocycles and Investigation of Their Photoreactivity

Substituents modification of BN-heterocycles on the boron atom has proven important to the photoreactivity and optoelectronic properties of BN-heterocycles. We developed a sequential and diverse synthetic strategy toward BN-heterocycles, in which the boron building block can be introduced with fully pre-functionalized substituents (Route A) or the substituents can be partially (Route B) or fully (Route C) modified after borylation. These three routes are complementary to provide more diverse BN-heterocycles, which will find broad applications in manipulating/controlling molecular transformations and the development of new photoresponsive materials.

Divergent and Multi-Stage Photoisomerization of Four-Coordinated Boron Compounds with a Naphthyl-Pyridyl/Thiazolyl Backbone

Examination of the photoreactivity of a new class of N,C-chelate organoboron compounds, including a series of unsymmetrically substituted boron molecules, B(naph-pyridyl)(Ar¹ )(Ar² ) and B(naph-thiazolyl)(Ar¹ )(Ar² ), led to the discovery of new and divergent photothermal isomerization phenomena. These include the clean and regioselective photoisomerization by unsymmetrical boron, forming borepin isomers, some of which further isomerize to the corresponding boratanorcaradiene diastereomer pairs as a result of the generation of two chiral centers. Significantly, the boratanorcaradienes involving a 3-thienyl substituent on boron were found to thermally convert to BN-fluoranthene annulated borapentalene via an unprecedented reversible boratacyclopropane-boratacyclopentene rearrangement. Changing the pyridyl donor to a thiazolyl donor on the boron was found to provide the B(naph-thiazolyl)(Mes)₂ compounds with a distinct new photoisomerization pathway-instead of borepin, forming new blue fluorescent polycyclic azaborinine species. This work illustrates the richness and complexity of boron photochemistry.

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].

Turn-on fluorescence sensors based on dynamic intramolecular N→B-coordination

A series of ten aryl-triazole-functionalized boranes bearing BMes₂-groups and capable of forming intramolecular five-membered N→B-coordinated heterocycles, has been prepared by 1,3-dipolar cycloaddition.

Embedding a boroxazine ring into a nanographene scaffold by a concise bottom-up synthetic strategy

The stepwise synthesis of a polycyclic aromatic hydrocarbon with an internalized boroxazine ring provides an unusual heterocycle with emitter properties.

Syntheses and structures of benzo-bis(1,3,2-diazaboroles) and acenaphtho-1,3,2-diazaboroles

Colorless crystalline 2,6-dibromo-4,8-dimethyl-1,3,5,7-tetraphenylbenzobis(diazaborole) 4 resulted from the cyclocondensation of 3,6-dimethyl-1,2,4,5-tetraphenylaminobenzene 3d with two equivalents of boron tribromide in the presence of calcium hydride. Synthesis of the dark-red crystalline 2-bromo-N,N'-bis(diisopropylphenyl)acenaphtho-1,3,2-diazaborole 7 was effected by the cyclocondensation of 1,2-bis(N-2',6'-diisopropylphenylimino)acenaphthene (5) and boron tribromide with subsequent sodium amalgam reduction of the initially formed burgundy red diazaborolium salt 6. Compounds 4, 6 and 7 are characterised by elemental analyses, ¹H, ¹¹B and ¹³C NMR spectroscopy, as well as by single X-ray diffraction studies. The electronic structures of 4, 6 and 7 are subject to DFT calculations.

Rapid Construction of Fold-Line-Shaped BN-Embedded Polycyclic Aromatic Compounds through Diels-Alder Reaction

The Diels-Alder reaction strategy that can rapidly extend the conjugated backbone was applied to facilely synthesize fold-line, coplanar BN-embedded polycyclic aromatic hydrocarbons from simple small BN compounds. The molecular structures and packing modes of these BN-embedded acenes were confirmed by single-crystal X-ray diffraction. Their electronic and photophysical properties were studied by using UV-vis, fluorescence spectroscopy, electrochemical cyclic voltammetry, and density functional theory calculations. These results demonstrate the efficiency and feasibility of this synthetic strategy.

Regioselective Functionalization of Stable BN-Modified Luminescent Tetraphenes for High-Resolution Fingerprint Imaging

A series of novel BN tetraphene derivatives have been prepared successfully for the first time via a post-functionalization strategy. The optical and electronic properties of these derivatives could be tuned systematically by the incorporation of different substituents on the main skeleton. The functionalized BN-containing luminogens have been explored for the detection of latent fingerprints (LFPs) on different substrates, including glass, aluminum foil, plastic, and ironware. This strategy provides great versatility in LFP imaging and good potential in elucidating the chemical information within LFPs, making the strategy valuable in forensic investigations.

Aromatic Metamorphosis of Indoles into 1,2-Benzazaborins

Among the plethora of aromatic compounds, indoles represent a privileged class of substructures that is ubiquitous in natural products and pharmaceuticals. While numerous exocyclic functionalizations of indoles have provided access to a variety of useful derivatives, endocyclic transformations involving the cleavage of the C2-N bond remain challenging due to the high aromaticity and strength of this bond in indoles. Herein, we report the "aromatic metamorphosis" of indoles into 1,2-benzazaborins via the insertion of boron into the C2-N bond. This endocyclic insertion consists of a reductive ring-opening using lithium metal and a subsequent trapping of the resulting dianionic species with organoboronic esters. Considering that 1,2-azaborins have attracted increasing academic and industrial attention as BN isosteres of benzene, the counterintuitive aromatic metamorphosis presented herein can feasibly be expected to substantially advance the promising chemistry of 1,2-azaborins.

The opposite and amplifying effect of B ← N coordination on photophysical properties of regioisomers with an unsymmetrical backbone

1,3-Dipolar cycloaddition of pyrido[2,1-a]isoindole with internal alkynes functionalized by a BMes2ph and an N-aromatic heterocycle leads to the formation of two types of regioisomers (major a and minor b) that have distinct physical and photophysical properties. Examination on 5 pairs of regioisomers unveils that the major isomers consistently have a smaller optical energy gap and emission energy than the corresponding minor isomers, which is greatly amplified by the formation of an internal B ← N bond. The regioisomers with a B ← N bond display contrasting temperature-dependent structural dynamics and response to fluoride ions, owing to an entropy-driven or fluoride initiated B ← N bond rupture/ring-opening process and the different B ← N bond strength. The opposite inductive effect and the Lewis pair properties of the dichotomic substituent units are responsible for the contrasting properties of the regioisomers in this system.

Identifying (BN)2-pyrenes as a New Class of Singlet Fission Chromophores: Significance of Azaborine Substitution

Singlet fission converts one photoexcited singlet state to two triplet excited states and raises photoelectric conversion efficiency in photovoltaic devices. However, only a handful of chromophores have been known to undergo this process, which greatly limits the application of singlet fission in photovoltaics. We hereby identify a recently synthesized diazadiborine-pyrene ((BN)₂-pyrene) as a singlet fission chromophore. Theoretical calculations indicate that it satisfies the thermodynamics criteria for singlet fission. More importantly, the calculations provide a physical chemistry insight into how the BN substitution makes this happen. Both calculation and transient absorption spectroscopy experiments indicate that the chromophore has a better absorption than pentacene. The convenient synthesis pathway of the (BN)₂-pyrene suggests an in situ chromophore generation in photovoltaic devices. Two more (BN)₂-pyrene isomers are proposed as singlet fission chromophores. This study sets a step forward in the cross-link of singlet fission and azaborine chemistry.