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

Abstract

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 PhBMes₂ 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, ⁱPr) 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 (ΔE_ST < 0.03 eV). Attaching an additional Me group to the ortho position of the BMes₂ 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.