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.
Graphene is one of the leading materials in today's science, but the lack of a band gap limits its application to replace semiconductors in optoelectronic devices. To overcome this limitation, the replacement of C=C bonds by isostructural and isoelectronic bonds is emerging as an effective strategy to open a band gap in monoatomic graphene layers. First prepared by Stock and Pohland in 1926, borazine is the isoelectronic and isostructural inorganic analogue of benzene, where the C=C bonds are replaced by B-N couples. The strong polarity of the BN bonds widens the molecular HOMO-LUMO gap, imparting strong UV-emission/absorption and electrical insulating properties. These properties make borazine a valuable molecular scaffold to be inserted as doping units in graphitic-based carbon materials to tailor a relevant band gap. It is with this objective that we became interested in the development of new synthetic organic methodologies to gain access to functionalized borazine derivatives. In particular, we have described the synthesis of borazine derivatives that, featuring aryl substituents at the B-centers bearing ortho-functionalities, are exceptionally stable against hydrolysis. Building on these structural motifs, we prepared hybrid BN-doped polyphenylene nanostructures featuring controlled doping patterns, both as dosage and orientation. Finally, exploiting the Friedel-Craft electrophilic aromatic substitution, we could develop the first rational synthesis of the first soluble hexa-peri-hexabenzoborazinocoronene and measured its optoelectronic properties, showing a widening of its gap compared to its full-carbon congener.
The replacement of carbon atoms at the zigzag periphery of a benzo[fg]tetracenyl derivative with an NBN atomic triad allows the formation of heteroatom-doped PAHs isosteres, which expose BN mimics of the amidic NH functions. Their ability to form H-bonded complexes has never been touched so far. Herein we report the first solution recognition studies of peripherally NBN-doped PAHs to form doubly H-bonded DD•AA and ADDA•DAAD-type complexes with suitable complementary H-bonding acceptor partners. The first determination of the Ka in solution showed that the 1:1 association strength is around 27 ± 1 M-1 for the DD•AA complexes in C6D6, whereas it rises to 1820 ± 130 M-1 for the ADDA•DAAD array in CDCl3. Given the interest of BNdoped polyaromatic hydrocarbons in supramolecular and materials chemistry, it is expected that these findings will open new possibilities to design novel materials, where the H-bonding properties of peripheral NH hydrogens could serve as anchors to tailor the organizational properties of PAHs. INTRODUCTION Following the vigorous synthetic developments of polycyclic aromatic hydrocarbons (PAHs), 1 the substitution (i.e., doping) of sp 2-carbon atoms with isoelectronic and isostructural BN couples is re-emerging as a versatile approach to tune the optoelectronic properties of these materials. 2-7 In particular, borazines 8,9 and BN-doped PAHs (e.g., azaborines, 10,11 borazapyrenes, 12,13 borazaphenanthrenes 12,14 borazanaphthalenes, 15,16 borazaanthracene 17,18 and boraazaperylene 19) are now increasingly attracting the attention of the physical and chemical community for their use in a broad spectrum of optoelectronic applications. 20-22 When used to decorate a periphery, nonsubstituted BN couples terminate with NH functions that, being more acidic than the CH analogues, could engage into H-bonding interactions as observed with boronic acids. 23 For instance, Liu and co-workers showed in a seminal report that 1,2-dihydro-1,2-azaborine can act as an H-bonding donor in the solid state and engage into a H-bond with the C=O
92 refs. + subrefs.]. -(BONIFAZI*, D.; FASANO, F.; LORENZO-GARCIA, M. M.; MARINELLI, D.; OUBAHA, H.; TASSEROUL, J.; Chem. Commun. (Cambridge) 51 (2015) 83, 15222-15236, http://dx.
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