Pressure-Induced Monotonic Phosphorescence Enhancement in Organic Crystals: Spin-Orbit Coupling and Molecular Packing Strategies.

Nonmonotonic pressure-dependent luminescent efficiency is commonly observed in inorganic systems. In contrast, organic room-temperature phosphorescence (RTP) materials exhibiting monotonic efficiency enhancement remain scarce, with the underlying mechanisms being poorly understood. Herein, we present a comprehensive theoretical investigation of pressure-induced RTP dynamics in organic crystals, which not only advances the fundamental understanding of excited-state processes but also paves the way for high-precision pressure-sensing applications. We systematically elucidate the intricate relationship among hydrostatic pressure, molecular packing, and photophysical properties by combining thermal vibration correlation function (TVCF) theory with quantum mechanics/molecular mechanics (QM/MM) simulations. In PFP-F crystalline aggregates, pressure-induced densification results in a slight blue shift accompanied by monotonic enhancements in the RTP efficiency and lifetime. These phenomena are attributed to accelerated radiative decay rates (kr) due to the pressure-enhanced spin-orbit coupling (SOC) effect, suppressed nonradiative transitions through closer π-π stacking, restricted geometric changes, and reduced root-mean-square displacement (RMSD) values. The pressure-tunable π-stacking geometry in PFP-F crystals enables precise control over SOC constants and energy levels, facilitating monotonic efficiency optimization. Furthermore, strengthened intermolecular interactions under compression provide efficient channels for the excited-state energy consumption process, significantly minimizing the nonradiative decay rate. These findings offer profound mechanistic insights into pressure-induced RTP properties and establish design principles for developing piezochromic luminescent materials with tailored optoelectronic properties.