Fig. 3: Luminescent R-A mechanism and optical readout.

a, R-A energy level diagram showing rapid quartet state generation following light absorption by the radical. At room temperature, activation to the ²[D₁S₀] state is efficient and only radical PL is seen. b, Molecular orbitals of TTM-1Cz-An obtained with MCSCF calculations showing highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO) and singly occupied molecular orbital (SOMO) from radical and acene components (H_R, radical HOMO; S_R, radical SOMO; H_A, acene HOMO; L_A, acene LUMO). c, Summary scheme of dominant orbital contributions during quartet formation and radical emission. d, Vectorial depiction of the evolution of state mixing during reversible quartet formation. Character of the photoexcited doublet state evolves from majority ²[D₁S₀] to majority ²[D₀T₁] at ultrafast timescales. If energetically accessible, the ²CT state may be additionally involved in doublet state mixing and thus assist the ISC toward a pure quartet state. e, ODMR spectra (9.4 GHz) of dilute frozen toluene solution of TTM-1Cz-PhAn under 532 nm excitation at 10 K, showing participation of the quartet state in the full-field and half-field region. Inset, trODMR of TTM-1Cz-PhAn at 10 K at 337.5 mT showing PL reduction in resonance. f, ODMR spectra of dilute PMMA films of TTM-1Cz-PhAn under 532 nm excitation at 293 K.