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

An approach to enhance the thermally activated delayed fluorescence (TADF) ofnido-carborane-appended triarylboranes is proposed. A methyl group is introduced into the 4-position of the phenylene ring bearing thenido-carborane (7,8-dicarba-nido-undecaborane) cage,i.e., at theorthoposition to the cage, in the PhBMes(2)acceptor moiety ofnido-carborane-triarylborane dyads (nido-1-4). The crystal structure of 8-H substitutednido-1reveals the existence of steric congestion around the cage, resulting in a highly twisted connectivity between the cage plane and the Ph ring (theta= 85.7 degrees). All compounds with different 8-R groups (R = H, Me,Pr-i) exhibit strong TADF with long emission lifetimes (tau(d)= 4.8-9.7 mu s in THF) in both THF and PMMA films, which is supported by a very small singlet-triplet energy splitting (Delta E-ST< 0.03 eV). Attaching an additional Me group to theorthoposition of the BMes(2)group further enhances the intensity of delayed fluorescence (nido-4). Theoretical studies show that althoughnido-1-4reach 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.