Nido‐Carboranes: Donors for Thermally Activated Delayed Fluorescence

Abstract 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 (PhBMes 2 ) 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 (Δ E 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.