N-Aryl- and N-Thienylcarbazoles with Dimesitylboryl and 1,3,2-Benzodiazaborolyl Functions

Synthesis and Stereodynamic and Emission Properties of Dissymmetric Bis-Aryl Carbazole Boranes and Identification of a CPL-Active B-C Atropisomeric Compound

We synthesized bis-aryl carbazole borane derivatives having emissive properties and axial chirality. The resolution of a thermally stable atropisomeric pair (compound 1b), due to a B-C chiral axis, was achieved by chiral stationary-phase high-performance liquid chromatography (CSP-HPLC). Complete photophysical properties of all compounds were measured and simulated by time-dependent density functional theory (TD-DFT) calculations of the complete fluorescence cycle, and circularly polarized luminescence spectra were obtained for the atropisomers of compound 1b, whose absolute configuration was derived using a TD-DFT simulation of the electronic circular dichroism (ECD) spectra.

Crystal structure of bis-(mesit-yl)(pyrrol-1-yl)borane

In the crystal structure of the title compound, C₂₂H₂₆BN, the B atom acts to reduce the delocalization of the nitro-gen lone-pair electron density into the pyrrole ring, so that the two N-C bonds increase in length to 1.4005 (14) and 1.3981 (14) Å. The N-B bond length is 1.4425 (15) Å, which is longer than a typical N-B bond because the nitro-gen lone pair is not fully available to participate in the bond.

Electronic and Photophysical Properties of 9,10-Anthrylene-Bridged B-π-N Donor-Acceptor Molecules with Solid-State Emission in the Yellow to Red Region

9,10-Anthrylene-bridged triarylborane-triarylamine donor-acceptor compounds were prepared to examine the influence of the bulky π-bridge on the electronic and photophysical properties of the compounds, with the aim of realizing their solid-state emission. The intramolecular charge-transfer (ICT) absorption and emission between the vacant p orbital on the boron center, p(B), and occupied p orbital on the nitrogen center, p(N), through the π orbital of the anthrylene, π(anthrylene), were observed, and it was demonstrated that the HOMO-LUMO gap decreased with increasing number of introduced anthrylene units because of the effective lowering of LUMO originating from the p(B)-π(anthrylene) orbital interaction. The compounds exhibited solid-state emission with emission maxima at 560 nm and 643 nm, respectively, in the yellow to red region, with the corresponding absolute solid-state quantum yield of 18 % and 31 %, as a result of the combination of the highly congested structure originating from the anthrylene π-bridge and the introduction of bulky tert-butyl protecting groups.

Preparation and Characterization of a π-Conjugated Donor-Acceptor System Containing the Strongly Electron-Accepting Tetraphenylborolyl Unit

A novel thiophene-bridged donor-acceptor system was synthesized with a carbazole as donor and a borole as acceptor unit. The borole group was successfully installed via the tin-boron exchange reaction of 1,1-dimethyl-2,3,4,5-tetraphenylstannole with 9-(5-(dibromoboryl)thiophen-2-yl)carbazole. The effect of the borole on the optoelectronic properties of the donor-acceptor system was explored by spectroscopic (UV/Vis and fluorescence spectroscopy), electrochemical (cyclic voltammetry) and theoretical (TD-DFT) methods as well as by modifying its structure. The corresponding donor-acceptor compound bearing the widely employed dimesitylboryl acceptor group was also synthesized for comparison.

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

We highlight recent developments in the synthesis, optical and electronic properties of 3-coordinate boron compounds and their applications in materials.

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.

A Boryl-Substituted Diphosphene: Synthesis, Structure, and Reaction with n-Butyllithium To Form a Stabilized Adduct by pπ-pπ Interaction

A boryl-substituted diphosphene was synthesized through the nucleophilic borylation of PCl3 with a borylzinc reagent, followed by a reduction with Mg. A combined analysis of the resulting diboryldiphosphene by single-crystal X-ray diffraction, DFT calculations, and UV/Vis spectroscopy revealed a σ-electron-donating effect for the boryl substituent that was slightly weaker than that of the 2,4,6-tri-tert-butylphenyl (Mes*) ligand. The reaction of this diboryldiphosphene with (n) BuLi afforded a boryl-substituted phosphinophosphide that was, in comparison with the thermally unstable Mes*-substituted diaryldiphosphene, stabilized by a π-electron-accepting effect of the boryl substituent.

Lowering the reduction potential of a boron compound by means of the substituent effect of the boryl group: one-electron reduction of an unsymmetrical diborane(4)

We have clarified and observed the high electron affinity of pinB-BMes2 (1; Mes = mesityl, pin = pinacolato). By using electrochemistry, it was shown that 1 has a higher electron affinity than those of B2pin2 and Mes3B. One-electron reduction of 1 gave the corresponding radical anion. The ESR spectroscopy and DFT calculation revealed the unsymmetrical distribution of electron density over the B-B bond. UV/Vis spectroscopy showed that the SOMO-related absorption supports the deep purple color of the radical anion. DFT studies on the torsion angle dependency of the LUMO levels and relative energies revealed the reason why 1 has high electron affinity as a result of the substituent effect of the Bpin group.

Palladium-catalysed cross-coupling reaction of ultra-stabilised 2-aryl-1,3-dihydro-1H-benzo[d]1,3,2-diazaborole compounds with aryl bromides: A direct protocol for the preparation of unsymmetrical biaryls

There has been a significant interest in organoboron compounds such as arylboronic acids, arylboronate esters and potassium aryltrifluoroborate salts because they are versatile coupling partners in metal-catalysed cross-coupling reactions. On the other hand, their nitrogen analogues, namely, 1,3,2-benzodiazaborole-type compounds have been studied extensively for their intriguing absorption and fluorescence characteristics. Here we describe the first palladium-catalysed Suzuki–Miyaura cross-coupling reaction of easily accessible and ultra-stabilised 2-aryl-1,3-dihydro-1H-benzo[d]1,3,2-diazaborole derivatives with various aryl bromides. Aryl bromides bearing electron-withdrawing, electron-neutral and electron-donating substituents are reacted under the catalytic system furnishing unsymmetrical biaryl products in isolated yields of up to 96% in only 10 minutes.

Luminescence properties of C-diazaborolyl-ortho-carboranes as donor-acceptor systems

Seven derivatives of 1,2-dicarbadodecaborane (ortho-carborane, 1,2-C(2)B(10)H(12)) with a 1,3-diethyl- or 1,3-diphenyl-1,3,2-benzodiazaborolyl group on one cage carbon atom were synthesized and structurally characterized. Six of these compounds showed remarkable low-energy fluorescence emissions with large Stokes shifts of 15100-20260 cm(-1) and quantum yields (Φ(F)) of up to 65% in the solid state. The low-energy fluorescence emission, which was assigned to a charge-transfer (CT) transition between the cage and the heterocyclic unit, depended on the orientation (torsion angle, ψ) of the diazaborolyl group with respect to the cage C-C bond. In cyclohexane, two compounds exhibited very weak dual fluorescence emissions with Stokes shifts of 15660-18090 cm(-1) for the CT bands and 1960-5540 cm(-1) for the high-energy bands, which were assigned to local transitions within the benzodiazaborole units (local excitation, LE), whereas four compounds showed only CT bands with Φ(F) values between 8-32%. Two distinct excited singlet-state (S(1)) geometries, denoted S(1)(LE) and S(1)(CT), were observed computationally for the benzodiazaborolyl-ortho-carboranes, the population of which depended on their orientation (ψ). TD-DFT calculations on these excited state geometries were in accord with their CT and LE emissions. These C-diazaborolyl-ortho-carboranes were viewed as donor-acceptor systems with the diazaborolyl group as the donor and the ortho-carboranyl group as the acceptor.

9-(4-Bromophenyl)-3,6-di-tert-butyl-9H-carbazole

The asymmetric unit of the title compound, C₂₆H₂₈BrN, contains two independent mol­ecules in which the carbazole rings are almost planar, with r.m.s. deviations of 0.0212 (1) and 0.0229 (1) Å. The dihedral angles between the carbazole ring system and the pendent benzene ring are 60.5 (1) and 56.3 (1)° in the two mol­ecules. In the crystal, mol­ecules are linked into chains along the b axis by C—H⋯π inter­actions.