Dimesitylborylethynylated Arenes: Unique Electronic and Photophysical Properties Caused by Ethynediyl (C≡C) Spacers

Herein, we report the synthesis and electrochemical and photophysical properties of aromatic hydrocarbons having one or two dimesitylborylethynyl peripherals. The mono- (1) and diboryl compounds (2), readily prepared by nucleophilic substitution reaction, are fairly stable to air and moisture in the solid state. The inserted ethynediyl (C≡C) spacer cancels the steric hindrance between the bulky dimesitylboryl groups and aromatic rings, leading to effective π conjugation over the B-C≡C-Ar linkages, as revealed by cyclic voltammetry. Despite the small structural differences, the photophysical properties of the benzene, naphthalene, and anthracene derivatives are different. Virtually no emission was observed from the benzene derivatives, whereas the anthracene derivatives emitted with high quantum yields both in solution and in the solid state. Notably, the naphthalene derivatives showed aggregation-induced emission behavior. Unlike the common sterically congested triarylborane derivatives reported so far, the anthracene derivatives showed π-π*-type absorption and emission bands, which derive from efficient intramolecular orbital interactions between the boron centers and anthracene moieties, as supported by DFT calculations. As a result, the dimesitylborylethynyl substituents effectively lower the LUMO levels of the aromatic hydrocarbon parts, whereas the HOMO levels are almost unaffected, thereby leading to materials with controllable HOMO-LUMO gaps.