Unraveling the Dual-Emission Mechanisms and Pressure-Induced Response in Organic Polymorphic Room-Temperature Phosphorescence.

The rational design of stimuli-responsive organic room-temperature phosphorescence (RTP) materials is often hindered by an incomplete understanding of the intricate interplay between molecular structure, crystal packing, and excited-state dynamics, particularly in polymorphic systems. Clarifying how subtle structural variations govern photophysical properties is crucial for advancing tunable luminescent materials. Herein, we systematically investigate the dual-emission mechanism and pressure-responsive behavior of a polymorphic RTP material, BrTA-F, in its two crystalline phases (Cry-A and Cry-B), using density functional theory (DFT) and time-dependent density functional theory (TDDFT) combined with quantum mechanics and molecular mechanics methods (QM/MM) and thermal vibration correlation function (TVCF) methods. The results reveal that the distinct spatial distribution of fluorine (F) atoms modulates intermolecular interactions and molecular planarity, leading to different hydrogen bond strengths and excited-state characteristics between the two polymorphs. The dual-RTP emission in Cry-B is attributed to competitive radiative decay from the monomeric first (T1) and second (T2) triplet excited state, which is facilitated by enhanced spin orbit coupling (SOC) resulting from variations in n-π*/ππ* transition proportions. Furthermore, Cry-A demonstrates high sensitivity to hydrostatic pressure, which tunes the emission wavelength and decay rates by compressing the lattice and altering intermolecular force balances. This work provides fundamental insights into the structure-property relationships in polymorphic RTP systems and offers guidance for designing stimuli-responsive luminescent materials.