Spirofluorene-Bridged Through-Space Charge-Transfer Radicals with 1-Phenyl-1H-indole Donor

Organic luminescent radicals with through-space charge-transfer (TSCT) excited states are attractive for optoelectronic applications, yet donor-dependent structure–property relationships remain underexplored. Here we report a new spirofluorene-bridged TSCT radical, PID-FR-TTM, employing 1-phenyl-1H-indole (PID) as the donor. Single-crystal X-ray diffraction confirms a carbon-centered TTM radical and a less bulky, more planar five-membered N-heterocycle in the donor region. PID-FR-TTM shows TSCT-type absorption and an emission at 609 nm with a photoluminescence quantum yield (PLQY) of 23.1% and a 90.1 ns emission lifetime in cyclohexane. Calculations indicate a TSCT-dominated excited state and a pronounced singly occupied molecular orbital–highest occupied molecular orbital (SOMO–HOMO) inversion. Notably, PID-FR-TTM exhibits markedly improved stability, including high decomposition temperatures (≈340 °C), excellent electrochemical stability, and enhanced photostability. These results provide donor-structure insights for designing high-performance TSCT radical emitters.