Luminescence Efficiency Enhanced by Simple Substitutions on Donor and Acceptor in Radicals with Donor–Acceptor Structure

Simple substitutions on the donor or acceptor units in radicals is an effective method to improve luminescent properties. However, the luminescence efficiency of radicals has not yet reached satisfactory levels through simple molecular structure modification. In this study, two [4-(N-Carbazolyl)-2,6-dichlorophenyl] bis(2,4,6-trichlorophenyl)methyl (Cz-TTM) radical derivatives (Mes2Cz-TTM, Mes2Cz-Mes2TTM) were synthesized and characterized by modifying the carbazole (donor) and tris-2,4,6-trichlorophenylmethyl radical (acceptor) units with 2,4,6-trimethylphenyl groups. The different substitutions showed varying influences on photoluminescence quantum efficiency (PLQE) compared to the Cz-TTM parent radical. The donor-only substitution suppressed the PLQE (39%) in Mes2Cz-TTM. In contrast, Mes2Cz-Mes2TTM exhibited a significantly higher PLQE of 92.6%, compared to the 68% PLQE of the Cz-TTM parent radical in toluene. Additionally, thermostability and photostability were improved with both donor and acceptor substitutions. The photophysical properties, molecular orbitals, and electrochemical behaviors were also systematically explored. This strategy provides a feasible approach to achieve high luminescence efficiency in radicals through simple substitutions on donor and acceptor units.