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

Herein, we report synthesis, electrochemical and photophysical properties of aromatic hydrocarbons having one and two dimesitylborylethynyl peripherals 1 and 2 . The mono- ( 1 ) and di-boryl 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 steric hindrance between the bulky dimesitylboryl groups and the aromatic rings, leading to effective π -conjugation over the B - C ≡ C -Ar linkages as revealed by cyclic voltammetry. Despite the small structural differences, photophysical properties of the benzene, naphthalene, and anthracene derivatives were different. Virtually no emission was observed for the benzene derivatives, whereas the anthracene derivatives emitted with high quantum yields both in solution and solid states. Notably, the naphthalene derivatives showed aggregation-induced emission behavior. Unlike common sterically congested triarylborane derivatives reported so far, the anthracene derivatives showed π - π * type absorption and emission bands, which derived from the efficient intramolecular orbital interactions between the boron centers and the anthracene moieties as supported by the density functional calculations. As a result, the dimesitylborylethynyl substituents effectively lowers the LUMO levels of the aromatic hydrocarbon parts while the HOMO levels are almost unaffected, leading to materials with controllable HOMO-LUMO gaps.