Shaping the emission bandwidth of multi-resonance emitters

Multi-resonance thermally activated delayed fluorescence (MR-TADF) has attracted significant attention due to its exceptional photoelectric properties, including high emission efficiency and narrow emission profiles, making it a prominent focus in organic electroluminescence research. The frontier molecular orbitals distribution in MR-TADF systems is notably distinctive. Specifically, the highest occupied molecular orbital electron density is primarily concentrated on the electron-donating (D) atoms and the adjacent para-carbon atoms of the benzene ring, while the lowest unoccupied molecular orbital electron density is localized on the electron-withdrawing (A) atoms and their corresponding para-carbon atoms, with alternating distributions across the central aromatic core. Upon excitation to the singlet state or radiative transition back to the ground state, short-range charge transfer (SRCT) occurs between neighboring atoms. This SRCT mechanism is distinct from the twisted intramolecular charge transfer excited state observed in donor–acceptor (D–A) TADF systems. It resembles a locally excited (LE) state, preserving a high degree of electron–hole overlap and minimizing vibrational coupling, which leads to higher photoluminescence quantum yield and faster radiative transition rates. The MR effect not only enables the narrow full-width at half maximum emission but also mitigates spectral fine structures caused by LE emissions under high conjugation to the maximum extent. However, there remains limited exploration of the mechanisms underlying the narrowband emission in MR-TADF materials. This review seeks to elucidate the fundamental principles behind the narrowband emission of MR-TADF materials, providing valuable insights for the design and development of next-generation MR-TADF materials.