Achieving Nearly 100% Photoluminescence Quantum Efficiency in Organic Radical Emitters by Fine‐Tuning the Effective Donor‐Acceptor Distance

Abstract Donor‐acceptor (D–A • ) type luminescent organic radicals have received widespread attention as efficient doublet emitters. However, their generally low photoluminescence quantum efficiency (PLQE) and limited photostability restrict their various applications. Since unraveling the relationship between structure and properties of D–A • type luminescent radicals remains a challenge, here, a series of tri(2,4,6‐trichlorophenyl)methyl (TTM) radical derivatives, which differ by the location of their ring fusion sites and nature of their heteroatoms, is synthesized. The PLQE of isomers varies by ten times as a function of ring fusion sites. In particular, the PLQE of a radical undergoing ring fusion at the carbazole 3,4‐position is as high as 98.0%. Quantum‐chemical calculations show that in the case of overlapping holes and electrons, by increasing the effective distance between the D and A moieties, the radiative transition rates of the radicals increase. Also, decreasing the electronic coupling between the charge‐transfer and local‐excited states and avoiding large geometrical distortions between the ground state (D 0 )_and the first excited state (D 1 ) can significantly reduce the nonradiative transition rates. This work offers a design strategy to obtain efficient and stable luminescent radicals by modifying the sites of ring fusion, which allows control of the radiative and nonradiative transition rates.