Role of the Intramolecular-Locking Strategy in the Construction of Organic Thermally Activated Delayed Fluorescence Emitters with Rotation-Restricted Acceptors

Constructing highly twisted and rigid donor-acceptor systems is important for obtaining highly efficient thermally activated delayed fluorescence (TADF) emitters. In the traditional framework of structurally twisted charge-transfer TADF molecular design, increasing the rigidity of the molecule through constructing rigid groups guarantees the minimized overlap integral between their highest occupied molecular orbitals and lowest unoccupied molecular orbitals for a small singlet and triplet energy gap, which ensures efficient reverse intersystem crossing. Increasing rigidity also prevents the molecule from deforming when in the excited state, effectively suppresses non-radiative energy dissipation, and improves the photoluminescence quantum yield. As one of the important fragments for TADF emitters, the development of acceptor units with rigid structure deserves in-depth understanding. Therefore, this review offers a comprehensive summary of the designs and developments of rigid TADF acceptors with "intramolecular locks" including covalent bonds, coordination bonds, and non-covalent bonds, via the intramolecular-locking strategy. The mechanism of TADF based on results from density functional theory is also described. Finally, the role of the intramolecular-locking strategy in the construction of pure organic TADF emitters with rotation-restricted acceptors and in improving the efficiency of all-organic TADF-based organic light-emitting diodes is discussed.