The Second Excited Triplet‐State Facilitates TADF and Triplet–Triplet Annihilation Photon Upconversion via a Thermally Activated Reverse Internal Conversion

Abstract Recent years have seen a surge in organic emitters that exhibit thermally activated delayed fluorescence (TADF) behavior owing to their high exciton utilization efficiency, low biological toxicity, etc. However, the existing TADF channel is insufficiently well developed as to design high‐performance materials especially in the high‐lying triplet state mediated reverse intersystem crossing process. A thermally activated reverse internal conversion (TARIC) pathway is reported here, which can achieve effective population transfer from T 1 to T 2 states via conical intersection point with the barrier of 3.81 kcal mol −1 . On this basis, the mediated T 2 state facilitates the TADF and triplet–triplet annihilation photon upconversion (TTA–UC) channel. Furthermore, benefited from the TARIC pathway, the designed 2′,7′‐dichlorofluorescein (DCF) derivative DCF−MPYM−Me photosensitizer presents an excellent upconversion efficiency of 13.6%. The high upconversion efficiency is the best performance in purely organic TADF photosensitizers without heavy atoms in TTA−UC systems.