Deciphering the doublet luminescence mechanism in neutral organic radicals: spin-exchange coupling, reversed-quartet mechanism, excited-state dynamics.

Neutral organic radical molecules have recently attracted considerable attention as promising luminescent and quantum-information materials. However, the presence of a radical often shortens their excited-state lifetime and results in fluorescence quenching due to enhanced intersystem crossing (EISC). Recently, an experimental report introduced an efficient luminescent radical molecule, tris(2,4,6-trichlorophenyl)methyl-carbazole-anthracene (TTM-1Cz-An). In this study, we systematically performed quantum theoretical calculations combined with the path integral approach to quantitatively calculate the excited-state dynamics processes and spectral characteristics. Our theoretical findings suggest that the sing-doublet D state, originating from the anthracene excited singlet state, is quickly converted to the doublet (trip-doublet) state EISC, facilitated by a significant nonequivalence exchange interaction, with Δ = 0.174 cm. The formation of the quartet state (Q, trip-quartet) was predominantly dependent on the exchange coupling 3/2 = 0.086 cm between the triplet spin electrons of anthracene and the TTM-1Cz radical. Direct spin-orbit coupling ISC to the Q state was minimal due to the nearly identical spatial wavefunctions of the and Q levels. The effective occurrence of reverse intersystem crossing (RISC) from the Q to D state is a critical step in controlling the luminescence of TTM-1Cz-An. The calculated RISC rate , including the Herzberg-Teller effect, was 3.64 × 10 s at 298 K, significantly exceeding the phosphorescence and nonradiative rates of the Q state, thus enabling the D repopulation. Subsequently, a strong electronic coupling of 37.4 meV was observed between the D and D states, along with a dense manifold of doublet states near the D state energy, resulting in a larger reverse internal conversion rate of 9.26 × 10 s. Distributed to the D state, the obtained emission rate of = 2.98-3.18 × 10 s was in quite good agreement with the experimental value of 1.28 × 10 s, and its temperature effect was not remarkable. Our study not only provides strong support for the experimental findings but also offers valuable insights for the molecular design of high-efficiency radical emitters.