BN-Patterning of Metallic Substrates through Metal Coordination of Decoupled Borazines

Borazatruxenes as precursors for hybrid C-BN 2D molecular networks

Synthesizing atomically thin, crystalline two-dimensional (2D) molecular materials which combine carbon with other elements is an emerging field requiring both custom-designed molecular precursors and their ability to organize into networks (hydrogen-bonded or covalent). Hybrid carbon-boron nitride (C-BN) networks face the additional challenge of needing hydrolytically-stable BN-containing molecular precursors. Here, we show that borazatruxenes (truxene-like molecules with a borazine core) and their halogenated derivatives are highly stable precursors suitable for on-surface assembly. Using scanning tunneling microscopy (STM) and density functional theory (DFT) simulations we demonstrate hierarchical H-bonded assembly based on chiral homodimers of tribromo-borazatruxenes (3Br-borazatruxenes) as building blocks for both 1D chains and 2D networks. A low-symmetry, H-bonded chiral 2D lattice forms on Au(111) from the C₃-symmetric 3Br-borazatruxenes, leading to large enantiomorphic domains that are molecularly homochiral. Such homochiral segregation is a necessary condition if chiral C-BN covalent networks are to be obtained via subsequent on-surface reactions. We show via DFT that up to two Na atoms can be trapped within the small pores of this dense lattice, while further Na atoms can adsorb on preferred network sites; this leads to hybrid Na-molecular network electronic bands with anisotropic dispersion and significant (up to hundreds of meV) bandwidths, as well as significant doping, that can engender anisotropic transport through the network. Finally, electronic structure comparisons (combining both experiment and computation) between borazatruxene, its tri-brominated derivative and truxene show that the borazine core controls the band gap increase, while also inducing C-B p_z-p_z electron delocalization that facilitates a continuous electron path across the molecule. Furthermore, as shown by DFT, the borazine core drives inter-layer B-N polar interactions that promote adsorption of BN containing molecules in a staggered configuration, a mechanism to be exploited in layer-by-layer supra-molecular assembly of novel hybrid C-BN materials.

BN-Doped Metal-Organic Frameworks: Tailoring 2D and 3D Porous Architectures through Molecular Editing of Borazines

Building on the MOF approach to prepare porous materials, herein we report the engineering of porous BN-doped materials using tricarboxylic hexaarylborazine ligands, which are laterally decorated with functional groups at the full-carbon 'inner shell'. Whilst an open porous 3D entangled structure could be obtained from the double interpenetration of two identical metal frameworks derived from the methyl substituted borazine, the chlorine-functionalised linker undergoes formation of a porous layered 2D honeycomb structure, as shown by single-crystal X-ray diffraction analysis. In this architecture, the borazine cores are rotated by 60° in alternating layers, thus generating large rhombohedral channels running perpendicular to the planes of the networks. An analogous unsubstituted full-carbon metal framework was synthesised for comparison. The resulting MOF revealed a crystalline 3D entangled porous structure, composed by three mutually interpenetrating networks, hence denser than those obtained from the borazine linkers. Their microporosity and CO2 uptake were investigated, with the porous 3D BN-MOF entangled structure exhibiting a large apparent BET specific surface area (1091 m2  g-1 ) and significant CO2 reversible adsorption (3.31 mmol g-1 ) at 1 bar and 273 K.

Structural Properties of Highly Doped Borazino Polyphenylenes Obtained through Condensation Reaction

Here we describe the synthesis and spectroscopic and structural characterization of various borazine-doped polyphenylenes displaying high doping dosages (16-18%). Capitalizing on the condensation reaction approach, the desired products were formed using a mixture of p-phenylendiamine and aniline with BCl₃, followed by the addition of an aryl lithium derivative. The use of mesityl lithium (MesLi) yields strained multiborazine derivatives, which proved to be unstable in the presence of moisture. However, when xylyl lithium (XylLi) was used, chemically stable multiborazines were obtained, with oligomers showing molecular weight up to 10⁴, corresponding to 16-18 monomer units. While the dimer, trimer, and tetramer could be isolated as pure products and their structure characterized by mass and NMR analysis, higher oligomers could only be isolated as mixtures of B-hydroxy-substituted derivatives and characterized by gel permeation chromatography. The structures of the dimer and trimer derivatives were confirmed by X-ray analysis, which nicely showed the presence of the two and three borazine rings spaced by one and two 1,4-aryl bridges, respectively. Notably, the trimer forms a porous crystalline clathrate. The peripheral xylyl and phenyl moieties of each molecule intramolecularly embrace each other through C-H and π-π stacking interactions. Steady-state UV-vis absorption characterization suggested that the molecules are UV absorbers, with the extinction coefficient linearly scaling with the degree of oligomerization. On the other hand, low-emission quantum yields were obtained for all derivatives (<7%), suggesting that high BN-doping dosages dramatically affect the emission properties of the doped polyphenylenes.