Abstract:The synthesis of a hexa-peri-hexabenzocoronene (HBC) with a central borazine core is described. The solid state structure of this BN doped HBC (BN-HBC) is isotypic with that of parent HBC. Scanning tunneling microscopy shows that BN-HBC is lying flat on Au(111) and is two-dimensional.

We report on the bottom-up fabrication of BN-substituted heteroaromatic networks achieved by surface-assisted polymerization and subsequent cyclodehydrogenation of specifically designed BN-substituted precursor monomers based on a borazine core structural element. To get insight into the cyclodehydrogenation pathway and the influence of molecular flexibility on network quality, two closely related precursor monomers with different degrees of internal cyclodehydrogenation have been employed. Scanning tunneling microscopy shows that, for both monomers, surface-assisted cyclodehydrogenation allows for complete monomer cyclization and the formation of covalently interlinked BN-substituted polyaromatic hydrocarbon networks on the Ag(111) surface. In agreement with experimental observations, density functional theory calculations reveal a significantly lower energy barrier for the cyclodehydrogenation of the conformationally more rigid precursor monomer, which is also reflected in a higher degree of long-range order of the obtained heteroaromatic network. Our proof-of-concept study will allow for the fabrication of atomically precise substitution patterns within BNC heterostructures.

Formal removal of two bonding partners from boranes, BR, yields borylenes, RB, which have been inferred as reactive intermediates in a number of reactions. Phenylborylene (R = CH; 1) is accessible from phenyldiazidoborane by photochemical extrusion of dinitrogen under matrix isolation conditions. Concomitantly, the nitrene PhNBN is formed via phenyl rearrangement. Here we used a combination of UV/vis, IR, and ESR spectroscopy under cryogenic matrix isolation conditions to investigate the properties and reactivity of phenylborylene. We detected an absorption band of phenylborylene at 375 nm (S → S) and tentatively assigned the S → S transition to a very weak band at 518 nm. We also show for the first time that an electrophilic borylene such as 1 can react with N reversibly and with CO irreversibly under photochemical conditions. The corresponding photoproducts PhBNN and PhBCO have triplet electronic ground states. Their small E values are in agreement with the linear arrangements Ph-B-N-N and Ph-B-C-O obtained by density functional theory computations. The D values decrease in the series PhNBN > PhBNN > PhBCO and approach the value for phenylcarbene (PhCH). Indeed, the boron center in PhBCO is isoelectronic with the carbene center in PhCH. The compounds are the first examples of boron analogues of diazoalkanes (RCNN) and ketenes (RCCO), and their formation may serve as a demonstration of the high reactivity of phenylborylene.

We investigate the electronic structure of a quasi-free-standing single layer of a B3N3-doped nanographene molecule deposited on Au(111) single crystals. The single layer shows very high orientational order, as well as high chemical and vacuum stability. We demonstrate that the borazine doping is an alternative way to design a material having electronic properties similar to doped graphene/h-BN. Specifically, borazine doping of nanographenes leads to tuning the gap in the same energy range of carbon-doped h-BN, in agreement with the expected doping effect of graphene quantum dots in h-BN. We support our experimental findings by first-principles calculations.

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