Borazine materials for organic optoelectronic applications

Synthesis and Properties of B4 N4 -Heteropentalenes Fused with Polycyclic Hydrocarbons

Hybrid molecules of π-conjugated carbon rings and BN-heterocyclic rings (h-CBNs) fused with each other have been a rare class of compounds due to the limited availability of their synthetic methods. Here we report the synthesis of new h-CBNs featuring a B₄ N₄ -heteropentalene core and polycyclic aromatic hydrocarbon wings. Using 1,2-azaborinine derivatives as a building block, we developed a rational synthetic protocol that allows the formation of a B₄ N₄ ring in a stepwise manner, resulting in the fully fused ABA-type triblock molecules. Thus, three derivatives of 1 bearing naphthalene (1_Naph ), anthracene (1_Anth ), or phenanthrene (1_Phen ) wings fused with the B₄ N₄ core were synthesized and characterized. Among them, 1_Phen , which displays the highest triplet-state energy, was found to serve a host material for phosphorescent OLED devices, for which a maximum external quantum efficiency of 13.7 % was recorded. These findings may promote the synthesis of various types of h-CBNs aiming at new properties arising from the synergy of two different π-electronic systems.

Benzene and Borazine, so Different, yet so Similar: Insight from Experimental Charge Density Analysis

Although benzene and borazine are isoelectronic and isostructural, they have very different electronic structures, mainly due to the polar nature of the B-N bond. Herein, we present an experimental study of the charge density distribution obtained from the multipole model formalism and Hirshfeld atom refinement (HAR) based on high-resolution X-ray diffraction data of borazine B₃N₃H₆ (1) and B,B',B″-trichloroborazine (2) crystals. These data are compared to those obtained from HAR for benzene (4) and 1,3,5-trichlorobenzene (5) and further compared with values obtained from density functional theory calculations in the gas phase, where N,N',N″-trichloroborazine (3) was also included. The results confirm that, unlike benzene, borazines are only weakly aromatic with an island-like electronic delocalization within the B₃N₃ ring involving only the nitrogen atoms. Furthermore, delocalization indices and interacting quantum atom energy for bonded and non-bonded atoms were found to be highly suitable indicators capable of describing the origin of the discrepancies observed when the degree of aromaticity in 2 and 3 is evaluated using common aromaticity indices. Additionally, analysis of intermolecular interactions in the crystals brings further evidence of a weakly aromatic character of the borazines as it reveals surprising similarities between the crystal packing of borazine and benzene and also between B,B',B″-trichloroborazine and 1,3,5-trichlorobenzene.

An Air- and Water-Stable B4 N4 -Heteropentalene Serving as a Host Material for a Phosphorescent OLED

Replacement of the carbon-carbon bonds of antiaromatic compounds with polar boron-nitrogen bonds often provides isoelectronic BN compounds with excellent thermodynamic stability and interesting photophysical properties. By this element-substitution strategy, we synthesized a new B₄ N₄ -heteropentalene derivative, 1, which is fully substituted with mesityl groups. Owing to kinetic protection by the sterically bulky substituents, 1 is remarkably stable toward air and even water. Single-crystal X-ray analysis of 1 revealed the bonding characteristics of the B₄ N₄ -heteropentalene structure. In a glassy matrix, 1 emitted short-wavelength phosphorescence with an onset at 350 nm, indicating that the triplet energy is substantially high. DFT calculations reasonably explained the ground- and excited-state electronic structures of 1 as well as its emission properties. Motivated by the high-energy triplet state of 1, we used it as a host material to fabricate a phosphorescent organic light-emitting diode with an external quantum efficiency of 15 %.

Superalkali-doped borazine and lithiated borazine complexes: diffuse excess electron and large first-hyperpolarizability

A number of superalkali (M₃O / M₃S; M = Li, Na, K)-doped borazine and hexalithio borazine complexes are considered for the theoretical study of their electronic structure and quadratic polarizability. Electron-rich O/S atom of superalkali species remains very close to one boron atom of the ring through non-covalent interaction. The first-hyperpolarizability increases rather significantly upon superalkali doping. The chosen complexes possess diffuse excess electron which is located on the superpalkali moiety of borazine complexes and at the ring site of lithiated borazines. First-hyperpolarizability of M₃O(S)@B₃N₃Li₆ complexes are significantly larger than that of the corresponding M₃O(S)@B₃N₃H₆ complexes. The magnitude of first-hyperpolarizability of Li₃S@B₃N₃Li₆ is larger than that of Li₃S@B₃N₃H₆ by about three orders of magnitude.

Stacking interactions of borazine: important stacking at large horizontal displacements and dihydrogen bonding governed by electrostatic potentials of borazine

Potential energy surfaces of borazine-benzene and borazine-borazine stacking interactions were studied by performing DFT, CCSD(T)/CBS and SAPT calculations. The strongest borazine-benzene stacking was found in a parallel-displaced geometry, with a CCSD(T)/CBS interaction energy of -3.46 kcal mol-1. The strongest borazine-borazine stacking has a sandwich geometry, with a CCSD(T)/CBS interaction energy of -3.57 kcal mol-1. The study showed that borazine forms significant stacking interactions at large horizontal displacements (over 4.5 Å), with energies of -2.20 kcal mol-1 for the borazine-benzene and -1.96 kcal mol-1 for the borazine-borazine system. The strength of interactions and their geometrical preferences can be rationalized by observing the electrostatic potentials of borazine and benzene, which is in agreement with SAPT analysis showing that electrostatics is the most important energy component for borazine stacking. All the interactions found in crystal structures of borazine and related compounds were identified either as potential curve minima or the geometries obtained from their optimizations. We also report a new dihydrogen bonding dimer with a CCSD(T)/CBS interaction energy of -2.37 kcal mol-1, which is encountered in the borazine crystal structures and enables the formation of additional simultaneous interactions that contribute to the overall stability of the crystals.

Noble gas inserted compounds of borazine and its derivative B3N3R6: structures and bonding

Quantum chemistry computations were performed at the MP2 and B3LYP levels of theory using the basis sets aug-cc-pVDZ and def2-TZVPPD to study the noble gas (Ng) compounds formed by insertion of a Ng atom (Kr, Xe, Rn) into the B-H/F and N-H/F bonds of inorganic benzene B₃N₃H₆ and its fluorine derivative B₃N₃F₆. The geometrical structures were optimized and vibrational analysis was carried out to demonstrate these structures being local minima on the potential energy surface. The thermodynamic properties of the formation process of Ng compounds were calculated. A series of theoretical methods based on the wavefunction analysis, including NBO, AIM and ELF methods and energy decomposition analysis, was used to investigate the bonding nature of the noble gas atoms and the properties of the Ng compounds. The N-Ng bond was found to be stronger than the B-Ng bond, but the B-Ng bond is of typical covalent character and σ-donation from the Ng atom to the ring B atom makes the predominant contribution towards stability of the B-Ng bond. NICS calculation shows that these Ng-containing compounds are of weak π-aromaticity.

The borazine dimer: the case of a dihydrogen bond competing with a classical hydrogen bond

Dimers of borazine were studied using matrix isolation infrared spectroscopy and ab initio quantum chemical calculations. Computations were performed at the MP2 and M06-2X levels of theory using the 6-311++G(d,p) and aug-cc-pVDZ basis sets for the various homodimers. At both levels of theory, an aligned stacked structure was found to be the global minimum, which was nearly isoenergetic to a parallel displaced structure. A T-shaped structure, where the N-H of one borazine pointed towards the N of the second borazine, was found to be a local minimum. In addition to these structures, a dihydrogen bonded structure, where the hydrogen attached to the nitrogen of borazine interacted with the hydrogen attached to the boron atom of another borazine, was also indicated. Experimentally, we observed the T-shaped dimer and the dihydrogen bonded dimer. This is one of the rare examples of experimental evidence for a dihydrogen bond, in a system other than in a metal hydride. These results for the borazine dimer were clearly different from the benzene dimer where the parallel displaced structure was found to be the global minimum followed by the T-shaped structure at the MP2/aug-cc-pVDZ level of theory. AIM, EDA and NBO analyses were carried out for all the structures to explore the nature of interactions.

A highly emissive and stable zinc(ii) metal–organic framework as a host–guest chemopalette for approaching white-light-emission

A new adenine-containing metal–organic framework (MOF), [Zn₄O(adenine)₄(benzene-1,3-dicarboxylate)₄Zn₂] (named as ZnBDCA), was synthesized solvothermally.

Borazine-based inorganic–organic hybrid cyclomatrix microspheres by silicon/boron exchange precipitation polycondensation

Borazine-based inorganic–organic hybrid cyclomatrix microspheres with a mean diameter of about 900 nm have been obtained via a novel silicon/boron exchange precipitation polycondensation approach.

Boron-nitrogen doped carbon scaffolding: organic chemistry, self-assembly and materials applications of borazine and its derivatives

Discovered by Stock and Pohland in 1926, borazine is the isoelectronic and isostructural inorganic analogue of benzene, where the C[double bond, length as m-dash]C bonds are substituted by B-N bonds. The strong polarity of such heteroatomic bonds widens the HOMO-LUMO gap of the molecule, imparting strong UV-emitting/absorption and electrical insulating properties. These properties make borazine and its derivatives valuable molecular scaffolds to be inserted as doping units in graphitic-based carbon materials to tailor their optoelectronic characteristics, and specifically their semiconducting properties. By guiding the reader through the most significant examples in the field, in this feature paper we describe the past and recent developments in the organic synthesis and functionalisation of borazine and its derivatives. These boosted the production of a large variety of tailored derivatives, broadening their use in optoelectronics, H2 storage and supramolecular functional architectures, to name a few.

Self-assembly of decoupled borazines on metal surfaces: the role of the peripheral groups

Two borazine derivatives have been synthesised to investigate their self-assembly behaviour on Au(111) and Cu(111) surfaces by scanning tunnelling microscopy (STM) and theoretical simulations. Both borazines form extended 2D networks upon adsorption on both substrates at room temperature. Whereas the more compact triphenyl borazine 1 arranges into close-packed ordered molecular islands with an extremely low density of defects on both substrates, the tris(phenyl-4-phenylethynyl) derivative 2 assembles into porous molecular networks due to its longer lateral substituents. For both species, the steric hindrance between the phenyl and mesityl substituents results in an effective decoupling of the central borazine core from the surface. For borazine 1, this is enough to weaken the molecule-substrate interaction, so that the assemblies are only driven by attractive van der Waals intermolecular forces. For the longer and more flexible borazine 2, a stronger molecule-substrate interaction becomes possible through its peripheral substituents on the more reactive copper surface.

Polymorphism, fluorescence, and optoelectronic properties of a borazine derivative

We have prepared a new borazine derivative that bears mesityl substituents at the boron centers and displays exceptional chemical stability. Detailed crystallographic and solid-state fluorescence characterizations revealed the existence of several polymorphs, each of which showed different emission profiles. In particular, a bathochromic shift is observed when going from the lower- to the higher-density crystal. Computational investigations of the conformational dynamics of borazine 1 in both the gas phase and in the solid state using molecular dynamics (MD) simulations showed that the conformation of the peripheral aryl groups significantly varies when going from an isolated molecule (in which the rings are able to flip over the 90° barrier at RT) to the crystals (in which the rotation is locked by packing effects), thus generating specific nonsymmetric intermolecular interactions in the different polymorphs. To investigate the optoelectronic properties of these materials by fabrication and characterization of light-emitting diodes (LEDs) and light-emitting electrochemical cells (LECs), borazine 1 was incorporated as the active material in the emissive layer. The current and radiance versus voltage characteristics, as well as the electroluminescence spectra reported here for the first time are encouraging prospects for the engineering of future borazine-based devices.

Application of cycloaddition reactions to the syntheses of novel boron compounds

This review covers the application of cycloaddition reactions in forming the boron-containing compounds such as symmetric star-shaped boron-enriched dendritic molecules, nano-structured boron materials and aromatic boronic esters. The resulting boron compounds are potentially important reagents for both materials science and medical applications such as in boron neutron capture therapy (BNCT) in cancer treatment and as drug delivery agents and synthetic intermediates for carbon-carbon cross-coupling reactions. In addition, the use of boron cage compounds in a number of cycloaddition reactions to synthesize unique aromatic species will be reviewed briefly.