Antiaromaticity Relief-Mediated Intramolecular Charge Transfer Enables Efficient and Stable Radical Emission in Polar...

Triphenylmethyl (trityl) radicals have shown potential for use in organic optoelectronic and spintronic applications. However, due to the poor luminescence of alternant symmetry hydrocarbons, the design of practical trityl structures has been limited to donor-radical intramolecular charge-transfer systems to break the alternant symmetry, which results in poor emission and photostability in polar solvents. Here, we synthesized a series of donor-acceptor (D-A^•) neutral radicals based on the tris(2,4,6-trichlorophenyl)methyl (TTM) radical moiety by introducing the 9H-tribenzo[b,d,f]azepine (TBA) ring with 20 π electrons as a donor group. In contrast to other TTM-based D-A^• radicals, these TBA radicals exhibit unique and superior properties for luminescent efficiency (56% in acetonitrile) and photostability (t_1/2= 4.3 × 10⁵ s, with estimated half-lives reaching up to several days) in polar solvents. Transient absorption (TA) spectra reveal the occurrence of an efficient transformation from the Franck-Condon (FC) state to the zwitterionic resonance intramolecular charge-transfer (ICT) state with a time constant of a few picoseconds. Due to the zwitterionic resonance ICT character, nonradiative decay channels, which are dominated by high-frequency modes (over 1000 cm^-1) in TBA radicals, can be efficiently suppressed. The present study highlights the promotion of antiaromaticity relief in luminescent radicals and offers a valuable viewpoint for understanding the ICT excited-state dynamics of D-A-type dyads in polar solvents.