Synthesis and Delayed Fluorescent Properties of p-Nido-Carborane-Triarylborane Conjugates with a Methyl-Substituted Phenylene Linker

Abstract

A series of p?nido?carborane?triarylborane conjugates (nido?1?3) in which a methyl group is introduced at the ortho?position to the carborane cage in the phenylene linker was prepared and characterized. All compounds exhibit broad low?energy absorptions (λabs = ca. 350?400?nm) attributable to the intramolecular charge transfer transition from the nido?carborane donor to the (MePh)BMes2 acceptor. Electrochemical studies confirm that oxidation occurs at the nido?carborane while the boryl moieties are responsible for the reduction. All nido?compounds show broad green emissions in tetrahydrofuran (THF) with good photoluminescence (PL) quantum yields (ΦPL = 24%?78%). In particular, different from the almost non?thermally activated delayed fluorescence (TADF) properties of unsubstituted para?conjugates, the transient PL decay curves of nido?1?3 show the existence of weak TADF (τd = 0.9?1.4 μs in THF). The TADF properties are further supported by the very small singlet?triplet energy splitting below 0.15?eV and are also observed in poly(methyl methacrylate) (PMMA) films doped with nido?1?3.