Achieving Blue-Shifted and Optimal Room Temperature Phosphorescence Emission via Hydrostatic Pressure Effect: A...

Hydrostatic pressure provides a powerful external stimulus to modulate the excited-state properties of organic room-temperature phosphorescent (RTP) materials, yet the microscopic origin of pressure-induced spectral evolution remains insufficiently understood. Herein, we present a comprehensive theoretical investigation into the origin of blue-shifted RTP emission in IH-MPT under hydrostatic pressure. The calculations reveal that increasing pressure induces a pronounced blue shift in the phosphorescence spectrum, which originates from a continuous upward shift of the lowest triplet excited state (T₁) energy level rather than aggregation or excimer effects. Structural analyses show that pressure progressively enhances molecular rigidity through packing densification and intramolecular planarization, effectively suppressing excited-state geometric relaxation. Consistently, the excited-state wave function becomes increasingly localized, accompanied by reduced vibronic coupling, decreased Huang-Rhys factors, and lower reorganization energies. Dimer calculations and Hirshfeld surface analyses further confirm that intermolecular electronic coupling is negligible and that the emission modulation is dominated by intrinsic monomer behavior reinforced by pressure-enhanced intermolecular constraints. Notably, an optimal pressure of approximately 16 GPa is identified, at which IH-MPT exhibits the bluest emission and simultaneously maximizes intersystem crossing and radiative decay rates. This optimal behavior arises from a synergistic balance between molecular rigidification and favorable energetic matching in the excited-state relaxation process, whereas excessive compression disrupts this balance and reduces emission efficiency. These results elucidate the mechanistic origin of pressure-regulated RTP emission and provide valuable insights into the rational control of phosphorescence through external pressure.