Nido-Carboranes: Donors for Thermally Activated Delayed Fluorescence

An approach to the design of nido-carborane-based luminescent compounds that can exhibit thermally activated delayed fluorescence (TADF) is proposed. 7,8-Dicarba-nido-undecaboranes (nido-carboranes) having various 8-R groups (R=H, Me, i-Pr, Ph) are appended to the meta or para position of the phenyl ring of the dimesitylphenylborane (PhBMes₂) acceptor, forming donor–acceptor compounds (nido-m1–m4 and nido-p1–p4). The bulky 8-R group and meta substitution of the nido-carborane are essential to attain a highly twisted arrangement between the donor and acceptor moieties, leading to a very small energy splitting between the singlet and triplet excited states (ΔE_ST <0.05 eV for nido-m2, -m3, and -p3). These compounds exhibit efficient TADF with microsecond-range lifetimes. In particular, nido-m2 and -m3 display aggregation-induced emission (AIE) with TADF properties.