Design of Thermally Activated Delayed Fluorescence Emitters for Organic Solid-State Microlasers

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 TADF 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 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. As in addition, the energetic spacing of ΔES1-T2 = 0.11eV 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, HLECT singlet provides larger oscillator strength and therefore larger SE cross-section than those of CT state. 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 provide a design of TADF gain materials.