Design of thermally activated delayed fluorescent emitters for organic solid-state microlasers†
A small energy gap between charge transfer (CT) singlet and triplet states enables thermally activated delayed fluorescence (TADF). Nevertheless, the small oscillator strength associated with CT states and their long exciton lifetimes are detrimental to establishing a population inversion for stimulated emission (SE), hindering the application of a TADF material in organic lasers. Here, we demonstrated that a TADF molecule of sulfide-substituted difluoroboron derivatives can achieve stimulated emission in microcrystals by employing a new molecular design, in which an ultrafast reverse intersystem crossing (RISC) process was achieved between a hybrid locally excited CT (HLECT) singlet S1 and a high-lying triplet T2 (3HLECT) state. Femtosecond transient abaorption and time-reolved PL spectra reveal that the two states of S1 and T2 equilibrate within a time of 180 ps. In addition, the energetic spacing of ΔES1–T2 = 0.11 eV enables delayed fluorescence involving the T2 state at room temperature. Besides, the extremely fast exciton lifetime (0.31 μs) that decreases the probability of carrier annihilation, the HLECT singlet provides larger oscillator strength and therefore larger SE cross-section than those of the CT state. A multimode TADF laser was realized based on the good optical feedback (cavity quality factor Q ≈ 2000) provided by Fabry Perot (FP) microcrystal microcavity. Our results not only confirm that the high-lying Tn state plays a key role in the RISC process of TADF, but also provides a design of TADF gain materials.