The inorganic mimic of benzene, borazine, first reported by Stock in 1926 possesses a wide energy gap of ~6 eV, although displaying similarity to benzene in both geometry and in formal topology of the π-molecular orbitals. The UV emissive properties of borazine and of its potential oligomeric derivatives make such molecular substrates attractive candidates to be implemented as active layer in UV light-emitting devices (LEDs) where high photon energy is necessary for either processing or sensing/characterization purposes. Thus, we turned our attention to borazine units as substituting unit to replace aromatic six-member rings [Chem. Commun., 2015, 51, 5222]. However, the susceptibility of the BN bonds to undergo hydrolysis in the presence of moisture is one of the major deterrent toward the use of this substrate, and attempts to significantly prevent the hydrolytic decomposition of borazines by introduction of various substituents were not successful until bulky substituents were placed in the ortho positions of B-aryl groups [Chem. Eur. J., 2013, 19, 7771].
With this approach, different borazine derivatives could be prepared, laterally exposing different functional groups. The electroluminescent properties of borazine-containing LEDs and LECs devices (in coll. with Prof. F. Cacialli, UCL London, United Kingdom) showed for the first time an UV-electroluminescent behavior. One patents also cover these works [WO 2012/126842]. Exploiting the decarbonilative [4+2] cycloaddition route, we have very recently tackled the first synthesis of borazine-based dendritic structures. Upon increasing the number of the borazine cores, a dramatic enhancement the UV-centered emission quantum yield has been observed, up to 60% for the heptaborazino-doped derivative [J. Am. Chem. Soc., 2017, 139, 503]. Also, we have prepared hydroxyl- and methyl-pentaaryl borazine molecules and studied their self-assembly behavior on a metal surface to prepare unprecedented bottom-up borazine-based supramolecular architectures [Angew. Chem. Int. Ed., 2013, 52, 7410; Chem. Eur. J., 2014, 20, 11856].
By means of the STM technique (Prof. G. Costantini, University of Warwick, United Kingdom and Prof. A. De Vita King’s College, United Kingdom) it has been found that hydroxyl-pentaaryl borazine molecules assemble in magic clusters of 7, 10, 11, 12, and 13 monomers on Cu(111). When peripheral ethynyl-phenyl substituents are added, porous networks are obtained with the borazine essentially decoupled from the metal surface (Cu or Au) [Chem. Eur. J., 2014, 20, 11856].Angew. Chem. Int. Ed., 2016, 55, 5947]. This allowed us to develop the first family of O-doped dyes, the type of O-annulation revealed to dictate the HOMO-LUMO gap and its chromatic properties.
Descending in the chalcogenic group, we also tackled the synthesis of Se- and Te-doped scaffolding, in particular the benzo-1,3-chalcogenazoles. Exceptionally, these aromatic heterocycles proved to be very stable and thus very handy to form controlled solid-state organizations in which wire-like polymeric structures are formed through secondary N…Y bonding interactions engaging the chalcogen (Y = Se or Te) and nitrogen atoms. In particular, it has been shown that the recognition properties of the chalcogen center at the solid state could be programmed by selectively barring one of its σ-holes through a combination of electronic and steric effects exerted by the substituent at the 2-position.
As predicted by the electrostatic potential surfaces calculated by quantum chemical modeling, the pyridyl groups revealed to be the stronger chalcogen bonding acceptors, and thus the best ligand candidate for programming the molecular organization at the solid state. The weaker chalcogen donor properties of the Se-analogues trigger the formation of feeble N…Se contacts, which are manifested in similar solid-state polymers featuring longer nitrogen-chalcogen distances [Chem. Eur. J., 2015, 21, 15377].