Room‐Temperature Phosphorescence Enhanced by Narrowing Down ΔEST through Tuning Excited States Energy Levels

Abstract In this work, two spiro‐type compounds are investigated with similar molecular structure and stacking structure. By simply altering the acceptor's heteroatoms, a strategy to enhance room temperature phosphorescence (RTP) performance is presented. The intersystem crossing process can effectively be accelerated by reducing the energy gap between the singlet and triplet states (Δ E ST ) through adjusting the molecular energy level. Besides, thermally activated delayed fluorescence (TADF) is emerging in dilute solution due to such small Δ E ST . This is helpful for deeply understanding the mechanism of competition in different radiative decay pathways (RTP and TADF).