Deciphering Spin-Governed Dual Emissions in TTM Diradicaloids: Unraveling the Interplay of Singlet and Triplet Fluorescence Pathways

The photophysical mechanisms governing luminescent diradicals, particularly the interplay between their singlet and triplet emissions, remain incompletely understood. Herein, we report three luminescent radicals, monoradical MTA and diradical DTA and DTA(t-Bu)₂, based on a tris(2,4,6-trichlorophenyl)methyl (TTM) scaffold. Experimental results and DFT calculations demonstrated the dual emission mechanism of two diradicaloids with distinct S₁ → S₀ (first excited singlet state to singlet ground state) and T₁ → T₀ (first excited triplet state to triplet ground state) fluorescent radiative pathways, while monoradical MTA shows only doublet emission with a D₁ → D₀ (first excited doublet state to doublet ground state) fluorescent radiative pathway. Varying-temperature electron spin resonance measurements revealed the singlet ground state and thermal accessible triplet ground state of DTA and DTA(t-Bu)₂, with ΔE_S-T values of -0.33 and -0.35 kcal/mol, respectively. Boltzmann population analysis based on the ΔE_S-T values reveals the coexistence of singlet and triplet states at 298 K, with the S₀ state predominating at 63.6% for DTA and 64.4% for DTA(t-Bu)₂, while the corresponding T₀ state populations are 36.4% and 35.6%, respectively. Moreover, two diradicaloids exhibit maximum magnetoluminescence phenomena near 100 K, reflecting the synergistic interplay of thermal energy and magnetic-field-induced S₀ → T₀ spin conversion. Notably, temperature-dependent photoluminescent experiments of two diradicaloids unveiled the dual emission behavior of two diradicaloids: cooling from 298 to 78 K progressively suppressed triplet fluorescence at ∼690 nm while it enhanced weak singlet fluorescence in the near-infrared region beyond 900 nm. External heavy-atom effects of two diradicaloids also demonstrated intramolecular intersystem crossing from T₁ to S₁. Furthermore, theoretical excited-state conformation analysis on two diradicaloids revealed their distinct structural preservation during T₁ → T₀ transitions compared to S₁ → S₀ transitions, which is also confirmed by the smaller reorganization energy, weaker high-frequency vibrations, and smaller radiative transition rate constant. Our work establishes a comprehensive understanding of spin-governed dual fluorescence in TTM diradicaloid systems, providing critical design principles for developing high-spin luminescent materials with tunable spin-controlled emission properties.