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 1 state, originating from the anthracene excited singlet state, is quickly converted to the doublet (trip-doublet) state via EISC, facilitated by a significant nonequivalence exchange interaction, with Δ J ST = 0.174 cm -1 . The formation of the quartet state (Q 1 , trip-quartet) was predominantly dependent on the exchange coupling 3/2 J TR = 0.086 cm -1 between the triplet spin electrons of anthracene and the TTM-1Cz radical. Direct spin-orbit coupling ISC to the Q 1 state was minimal due to the nearly identical spatial wavefunctions of the and Q 1 levels. The effective occurrence of reverse intersystem crossing (RISC) from the Q 1 to D 1 state is a critical step in controlling the luminescence of TTM-1Cz-An. The calculated RISC rate k RISC , including the Herzberg-Teller effect, was 3.64 × 10 5 s -1 at 298 K, significantly exceeding the phosphorescence and nonradiative rates of the Q 1 state, thus enabling the D 1 repopulation. Subsequently, a strong electronic coupling of 37.4 meV was observed between the D 1 and D 2 states, along with a dense manifold of doublet states near the D 1 state energy, resulting in a larger reverse internal conversion rate k RIC of 9.26 × 10 10 s -1 . Distributed to the D 2 state, the obtained emission rate of k f = 2.98-3.18 × 10 7 s -1 was in quite good agreement with the experimental value of 1.28 × 10 7 s -1 , 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.