Enhancing the thermally activated delayed fluorescence of nido -carborane-appended triarylboranes by steric modification of the phenylene linker

An approach to enhance the thermally activated delayed fluorescence (TADF) of nido -carborane-appended triarylboranes is proposed. A methyl group is introduced into the 4-position of the phenylene ring bearing the nido -carborane (7,8-dicarba- nido -undecaborane) cage, i.e. , at the ortho position to the cage, in the PhBMes 2 acceptor moiety of nido -carborane–triarylborane dyads ( nido - 1–4 ). The crystal structure of 8-H substituted nido - 1 reveals the existence of steric congestion around the cage, resulting in a highly twisted connectivity between the cage plane and the Ph ring ( θ = 85.7°). All compounds with different 8-R groups (R = H, Me, i Pr) exhibit strong TADF with long emission lifetimes ( τ d = 4.8–9.7 μs in THF) in both THF and PMMA films, which is supported by a very small singlet–triplet energy splitting (Δ E ST < 0.03 eV). Attaching an additional Me group to the ortho position of the BMes 2 group further enhances the intensity of delayed fluorescence ( nido - 4 ). Theoretical studies show that although nido - 1–4 reach their local energy minima in a perpendicular arrangement between the cage plane and the Ph ring, there exist large energy barriers to cage rotation. In contrast, the reference compound lacking a 4-Me group, which exhibits very weak delayed fluorescence in solution, has low rotational energy barriers.