A near-infrared luminescent organic radical with switchable emission and SOMO–HOMO inversion via a protonation/deprotonation process

We synthesized a novel NIR-emissive (785 nm) D-A˙-type triphenylmethyl radical TTM-EMICz using a rigidly structured and large-conjugated donor 6-butylindolo[3,2-b]carbazole (ICz) unit. Studies showed that the non-Aufbau electronic structure and PL spectra of the radical can be regulated by the protonation process, in which the PL spectra of the radical can be adjusted to the visible region. Furthermore, the protonated radical TTM-EMICz shows emission centered at 610 nm and 633 nm with PLQY of 4.5% and 14.9% in cyclohexane and toluene, and exhibits excited state CT properties. The photostability investigation indicated that the radical TTM-EMICz has 350 times higher photostability in cyclohexane compared to the well-known carbazole-capped triphenylmethyl radical TTM-1Cz. This is the first reported near-infrared organic radical with two-state-emissive characteristics (non-Aufbau and Aufbau electronic states) which can be adjusted by protonation/deprotonation processes.