Intramolecular B–N Coordination as a Scaffold for Electron-Transporting Materials: Synthesis and Properties of Boryl-Substituted Thienylthiazoles

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.

Spiroborate-Based Host Materials with High Triplet Energies and Ambipolar Charge-Transport Properties

Organic light-emitting diodes (OLEDs) receive considerable attention because of their commercial use in flat panel displays. Herein, highly efficient spiroborate-based host materials are reported for use in blue OLEDs. Our designed spiroborates (SBOX) were simple to synthesize and exhibited high triplet excitation energies, narrow S-T gaps, and balanced charge carrier mobilities. A blue OLED containing one of the designed spiroborates, SBON, as a host exhibited a high external quantum efficiency (27.6 %) and low turn-on voltage (3.7 V) compared to those observed using 3,3'-di(9H-carbazol-9-yl)-1,1'-biphenyl (17.6 % and 4.5 V, respectively), indicating their potential as host materials in OLEDs.

Carbon Dots-Based Delayed Fluorescent Materials: Mechanism, Structural Regulation and Application

Delayed fluorescent (DF) materials have high internal quantum efficiency because of the triplet excitons involved in the radiation transition, and the spin-forbidden transition can effectively improve their luminescent lifetime. Compared with traditional afterglow materials including metal-containing inorganic coordination compounds and organic compounds, the DF materials based on carbon dots (CDs) have drawn extensive attention because of their advantages of low toxicity, environmental friendliness, stable luminescence, easy preparation and low cost. Most CDs-based DF materials can be realized by embedding CDs in matrix with covalent bonds, hydrogen bonds or/and other supramolecular interactions. Recently, matrix-free self-protective CDs-based DF materials are emerging. This review systematically summarizes the DF mechanism and structural regulation strategies of CDs-based DF materials, and the applications of CDs-based DF materials in anti-counterfeiting, information encryption, temperature sensing and other fields are introduced. Finally, the existing problems and future potentials of CDs-based DF materials are proposed and prospected.

Boron-Locked Starazine - A Soluble and Fluorescent Analogue of Starphene

A starlike heterocyclic molecule containing an electron‐deficient nonaaza‐core structure and three peripheral isoquinolines locked by three tetracoordinate borons, namely isoquinoline‐nona‐starazine (QNSA), is synthesized by using readily available reactants through a rather straightforward approach. This new heteroatom‐rich QNSA possesses a quasi‐planar π‐backbone structure, and bears phenyl substituents on borons which protrude on both sides of the π‐backbones endowing it with good solubility in common organic solvents. Contrasting to its starphene analogue, QNSA shows intense fluorescence with a quantum yield (PLQY) of up to 62 % in dilute solution.

Synthesis, photophysical, electrochemical, and non-linear optical properties of triaryl pyrazole based B-N coordinated boron compounds

We report here a set of triaryl pyrazole based B-N coordinated boron compounds ( 11 - 17 ) synthesized by electrophilic aromatic borylation strategy. All the pyrazole boron compounds were thoroughly characterized using multinuclear NMR spectroscopy, LCMS, and single crystal X-ray diffraction analysis (for 12 - 17 ). The photoluminescence measurements of 11 - 17 revealed that the emission peak maxima were tuned based on the substitution on Nphenyl. The photophysical and electrochemical properties were further supported by theoretical calculations. Z-scan based investigations at 515 nm pump wavelength showed that B-N coordination led to enhancement of nonlinear absorption (two-photon absorption (TPA)) in these compounds if an electron deficient moiety is attached. It has also been observed that an appropriate choice of moiety allows to optimally maneuver the molecular polarizability of the π-system and consequently, assists in controlling the third-order nonlinear optical response.

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.

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.

Modular Synthesis of Organoboron Helically Chiral Compounds: Cutouts from Extended Helices

Two types of helically chiral compounds bearing one and two boron atoms were synthesized by a modular approach. Formation of the helical scaffolds was executed by the introduction of boron to flexible biaryl and triaryl derived from small achiral building blocks. All-ortho-fused azabora[7]helicenes feature exceptional configurational stability, blue or green fluorescence with quantum yields (Φfl ) of 18-24 % in solution, green or yellow solid-state emission (Φfl up to 23 %), and strong chiroptical response with large dissymmetry factors of up to 1.12×10-2 . Azabora[9]helicenes consisting of angularly and linearly fused rings are blue emitters exhibiting Φfl of up to 47 % in CH2 Cl2 and 25 % in the solid state. As revealed by the DFT calculations, their P-M interconversion pathway is more complex than that of H1. Single-crystal X-ray analysis shows clear differences in the packing arrangement of methyl and phenyl derivatives. These molecules are proposed as primary structures of extended helices.

Discovery of Functional Luminescence Properties Based on Flexible and Bendable Boron-Fused Azomethine/Azobenzene Complexes with O,N,O-Type Tridentate Ligands

Azomethine (C=N) and azo (N=N) scaffolds are a part of structural units in poly(p-phenylene azomethine) (PAM) and poly(p-phenylene azo) (PAZ), respectively. Poly(p-phenylene vinylene) (PPV) is known to be one of luminescent π-conjugated polymers, meanwhile PAM and PAZ, which are the aza-substituted PPV analogues, are regarded as weak or no emissive materials. However, by the boron complexation, intense emission can be induced. Furthermore, environment-sensitivity and stimuli-responsivity were also observed. In this review, we demonstrate unique and versatile luminescent properties based on "flexible and bendable" π-conjugated systems composed of the boron-fused azomethine and azobenzene complexes (BAm and BAz) with the O,N,O-type tridentate ligands. The "flexible and bendable" luminophores showed intriguing optical behaviors, such as thermosalient effect, aggregation-induced emission (AIE) and crystallized-induced emission (CIE). Moreover, highly efficient emissions both in solution and film states were observed from the polymers. We illustrate the results and mechanisms on these luminescent properties from the series of our recent studies with BAm and BAz complexes and polymers.

B←N-Incorporated Dibenzo-azaacene with Selective Near-Infrared Absorption and Visible Transparency

Organic compounds with selective near-infrared absorption and visible transparency are very desirable for fabrication of transparent/semitransparent optoelectronic devices. Herein, we develop a molecule with selective near-infrared absorption property, QBNA-O, in which four B←N units are incorporated to the core and two benzodioxin groups are introduced at the termini of the dibenzo-azaacene skeleton. QBNA-O exhibits a small optical gap of 1.39 eV due to the strong electron-donating benzodioxin groups and the strong electron-withdrawing B←N units. In toluene solution, QBNA-O shows a strong absorption peak at 856 nm with the full width at half maximum (FWHM) of only 41 nm as well as very weak absorption in the visible range from 380 nm to 760 nm. Thin films of QBNA-O exhibit the average visible transparency (AVT) of 78 % at the thickness of 205 nm and 90 % at the thickness of 45 nm. Solution-processed organic field-effect transistors (OFETs) of QBNA-O display ambipolar transporting behavior with the electron mobility of 0.52 cm²  V^-1  s^-1 and the hole mobility of 0.013 cm²  V^-1  s^-1 together with excellent air-stability. The selective NIR absorbing property and excellent charge transporting property imply that QBNA-O can be used to fabricate transparent organic optoelectronic devices.

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.

Arylisoquinoline-derived organoboron dyes with a triaryl skeleton show dual fluorescence

Four new dyes that derive from borylated arylisoquinolines were prepared, containing a third aryl residue (naphthyl, 4-methoxynaphthyl, pyrenyl or anthryl) that is linked via an additional stereogenic axis. The triaryl cores were synthesized by Suzuki couplings and then transformed into boronic acid esters by employing an Ir(I)-catalyzed reaction. The chromophores show dual emission behavior, where the long-wavelength emission band can reach maxima close to 600 nm in polar solvents. The fluorescence quantum yields of the dyes are generally in the range of 0.2-0.4, reaching in some cases values as high as 0.5-0.6. Laser-flash photolysis provided evidence for the existence of excited triplet states. The dyes form fluoroboronate complexes with fluoride anions, leading to the observation of the quenching of the long-wavelength emission band and ratiometric response by the build-up of a hypsochromically shifted emission signal.

8.78% Efficient All-Polymer Solar Cells Enabled by Polymer Acceptors Based on a B←N Embedded Electron-Deficient Unit

In the field of all-polymer solar cells (all-PSCs), all efficient polymer acceptors that exhibit efficiencies beyond 8% are based on either imide or dicyanoethylene. To boost the development of this promising solar cell type, creating novel electron-deficient units to build high-performance polymer acceptors is critical. A novel electron-deficient unit containing B←N bonds, namely, BNIDT, is synthesized. Systematic investigation of BNIDT reveals desirable properties including good coplanarity, favorable single-crystal structure, narrowed bandgap and downshifted energy levels, and extended absorption profiles. By copolymerizing BNIDT with thiophene and 3,4-difluorothiophene, two novel conjugated polymers named BN-T and BN-2fT are developed, respectively. It is shown that these polymers possess wide absorption spectra covering 350-800 nm, low-lying energy levels, and ambipolar film-transistor characteristics. Using PBDB-T as the donor and BN-2fT as the acceptor, all-PSCs afford an encouraging efficiency of 8.78%, which is the highest for all-PSCs excluding the devices based on imide and dicyanoethylene-type acceptors. Considering that the structure of BNIDT is totally different from these classical units, this work opens up a new class of electron-deficient unit for constructing efficient polymer acceptors that can realize efficiencies beyond 8% for the first time.

Expansion of the scope of alkylboryl-bridged N → B-ladder boranes: new substituents and alternative substrates

A series of new boranes capable of forming intramolecular N → B-heterocycles has been prepared and their properties have been studied by electrochemical methods and UV-vis-spectroscopy complemented by DFT calculations.

The Importance of Excited-State Energy Alignment for Efficient Exciplex Systems Based on a Study of Phenylpyridinato Boron Derivatives

Understanding excited-state dynamics is critical for improving the photoluminescence (PL) efficiency of exciplexes. A series of exciplexes based on conventional hole-transporting materials as donor and newly developed phenylpyridinato boron derivatives as acceptor were investigated. High PL efficiencies were achieved in only some combinations, and a large difference in performance among combinations provided insight into nonradiative processes in exciplex systems. Furthermore, the triplet local excited states (³ LE) of each donor and acceptor were found play an important role in triplet exciplex harvesting. Significant contributions from triplets were clearly observed when the charge-transfer excited states (¹ CT and ³ CT) and ³ LE were ideally aligned. We also demonstrated fine control of relative energy alignment via the concentration to improve the PL efficiency.

Effects of the Substituents of Boron Atoms on Conjugated Polymers Containing B←N Units

Organoboron chemistry is a new tool to tune the electronic structures and properties of conjugated polymers, which are important for applications in organic optoelectronic devices. To investigate the effects of substituents of boron atoms on conjugated polymers, we synthesized three conjugated polymers based on double B←N-bridged bipyridine (BNBP) with various substituents on the boron atoms. By changing the substituents from four phenyl groups and two phenyl groups/two fluorine atoms to four fluorine atoms, the BNBP-based polymers show blue-shifted absorption spectra, decreased LUMO/HOMO energy levels, and enhanced electron affinities, as well as increased electron mobilities. Moreover, these BNBP-based polymers can be used as electron acceptors for all-polymer solar cells. These results demonstrate that substituents of boron atoms can effectively modulate the electronic properties and applications of conjugated polymers.

n-Type Azaacenes Containing B←N Units

We disclose a novel strategy to design n-type acenes through the introduction of boron-nitrogen coordination bonds (B←N). We synthesized two azaacenes composed of two B←N units and six/eight linearly annelated rings. The B←N unit significantly perturbed the electronic structures of the azaacenes: Unique LUMOs delocalized over the entire acene skeletons and decreased aromaticity of the B←N-adjacent rings. Most importantly, these B←N-containing azaacenes exhibited low-lying LUMO energy levels and high electron affinities, thus leading to n-type character. The solution-processed organic field-effect transistor based on one such azaacene exhibited unipolar n-type characteristics with an electron mobility of 0.21 cm²  V^-1  s^-1 .

Electronic and structural properties of N → B-ladder boranes with high electron affinity

A series of electronically and structurally diverse N → B-ladder boranes has been prepared by hydroboration.

Cobalt-Catalyzed Cross-Dehydrogenative C(sp2 )-C(sp3 ) Coupling of Oxazole/Thiazole with Ether or Cycloalkane

Direct C5-alkylation of oxazole/thiazole with ether or cycloalkane has been achieved through a cobalt-catalyzed cross-dehydrogenative coupling (CDC) process in moderate to good yields. This transformation represents the first C(sp² )-C(sp³ ) cross-coupling at the C5-position of the oxazole/thiazole via double C-H bond cleavages. Various functional groups on oxazole/thiazole substrates, as well as water and air, are well-tolerated with this concise and practical protocol, constituting straightforward access to heterocycles with great medicinal significance. A preliminary mechanism involving a radical process has also been proposed.

4,7-Bis[3-(dimesitylboryl)thien-2-yl]benzothiadiazole: Solvato-, Thermo-, and Mechanochromism Based on the Reversible Formation of an Intramolecular B-N Bond

4,7-Bis-[3-(dimesitylboryl)thien-2-yl]benzothiadiazole (1) and monoborylated derivative 2 were synthesized and their chromic behavior was investigated. Photophysical measurements, single-crystal XRD analysis, and theoretical calculations revealed that an intramolecular B-N coordination bond formed reversibly. The equilibrium of this reversible bond formation depends on the solid-state structure, solvent, temperature, and mechanical forces, and leads to significant changes in the electronic structure and chromic behavior of these molecules. The responsiveness toward external stimuli, resulting in the reversible formation of open and closed forms of this system, is achieved through weak intramolecular B-N coordination bonds induced by the steric bulk of the mesityl groups on the boron centers.

Highly Emissive Far Red/Near-IR Fluorophores Based on Borylated Fluorene-Benzothiadiazole Donor-Acceptor Materials

Stille, Suzuki-Miyaura and Negishi cross-coupling reactions of bromine-functionalised borylated precursors enable the facile, high yielding, synthesis of borylated donor-acceptor materials that contain electron-rich aromatic units and/or extended effective conjugation lengths. These materials have large Stokes shifts, low LUMO energies, small band-gaps and significant fluorescence emission >700 nm in solution and when dispersed in a host polymer.

Polymer Acceptor Based on Double B←N Bridged Bipyridine (BNBP) Unit for High-Efficiency All-Polymer Solar Cells

A novel polymer acceptor based on the double B←N bridged bipyridine building block is reported. All-polymer solar cells based on the new polymer acceptor show a power conversion efficiency of as high as 6.26% at a photon energy loss of only 0.51 eV.