NBN/BNB-doped phenalenyl homo- and heterodyads: structural uniformity but optoelectronic diversity

Abstract

Phenalenyl anions and cations are fundamentally interesting but highly reactive complementary species. One way to tame this reactivity and prevent comproportionation as soon as the two species are allowed to interact is by switching to the isosteric NBN- or BNB-containing heterocycles. Herein, we show that the heteroatom-doped pair preserves to a certain extent the desirable complementarity of the original carbonaceous molecules. To this end, we synthesized a series of 1,4-phenylene-bridged (NBN)₂- or (BNB)₂-homodyads and (NBN,BNB)-heterodyads. The homodyads are accessible via treatment of a 1,4-diborylated benzene with a 1,8-diaminonaphthalene or of 1,4-diaminobenzene with 1,8-naphthalenediyl-bridged diborane(6), respectively. The heterodyads were prepared from the same diborane(6) and an NBN-phenalenyl with B-bonded p-NH₂-aryl substituent. All products were characterized by NMR spectroscopy, X-ray crystallography, cyclic voltammetry, UV/vis absorption and emission spectroscopy, as well as state-of-the-art quantum-chemical calculations. The heterodyads undergo both oxidation and reduction and thus qualify as ambipolar compounds. Any intramolecular NBN-to-BNB charge-transfer (CT) emission is negligible on conformational grounds. In contrast, an appreciable aggregation-induced green emission is achieved by adding H₂O to THF solutions of the heterodyads and is presumably caused by intermolecular CT between head-to-tail aligned molecules upon aggregation.