Effect of Structural Similarity Between Dopants and Matrices in Organic Room‐Temperature Phosphorescence Materials: “Like Dissolves Like”

Host-guest doping has emerged as a facile and efficient strategy for the fabrication of organic room-temperature phosphorescence (RTP) materials. Nonetheless, how the chemical structures of dopants and matrices systematically influence the triplet exciton dynamics of solid-state materials remains elusive. Although investigations have been focusing on electronic structures and energy levels, here we show that doped phenothiazine model systems largely follow the well-known principle of "like dissolves like" and exhibit phase-separation-controlled photophysical state switching between RTP and triplet-triplet annihilation delayed fluorescence (TTA-DF), evidenced by scanning electron microscopy, confocal microscopy, and microregion spectrometry. In the structurally similar host-guest combination, the miscible solid solution exhibits strong guest RTP emission; in the thermodynamically favored microphase-separated state, the RTP is severely outcompeted by TTA-DF of the guest aggregate, which has significantly faster decay kinetics. Microscopy results show that simple mechanical force by gentle grinding is sufficient to partially amorphize the nanophase-separated crystalline sample and promote intense host-sensitized guest RTP. This work demonstrates that, in addition to quantum mechanical considerations, thermodynamic rules provide the crucial missing link to predictably govern structural morphology and macroscopic luminescence in doped organic crystals, laying a more complete foundation for the design of dynamically tunable, stimuli-responsive RTP materials.