Dual Excited-State Charge-Transfer Process in Luminescent Radicals with Spatially Folded Arrangement

The through-space charge transfer (TSCT) mechanism has attracted significant attention in closed-shell organic luminescent materials due to its strong environmental sensitivity, small singlet-triplet energy gap, and high exciton utilization efficiency. However, open-shell luminescent radicals mostly adopt D-A or D-π-A skeletons to overcome the dark-state limitations of radicals, relying solely on a through-bond charge transfer (TBCT) process. To date, TSCT excited states in radical systems remain largely unexplored. In this study, we report the coexistence of TSCT and TBCT mechanisms in radical molecules. Four luminescent radicals (TB-MR, TB-DR, TBS-MR, and TBS-DR) were systematically investigated using absorption and fluorescence spectroscopy combined with theoretical calculations. The results reveal that the spatially folded conformation between carbazole units and radical centers in the TBS series induces abnormal photophysical behaviors deviating from conventional paradigms. Further electron-hole analysis indicates that the TBS series exhibits a dual excited-state charge transfer (CT) mechanism, with TSCT and TBCT contributions of approximately 70% and 30%, respectively. This discovery not only elucidates the influence of multiple CT processes on the excited-state behaviors of luminescent radicals but also establishes a novel excited-state engineering strategy for tuning the luminescence of open-shell systems.