Substituent and Hydrostatic Pressure Regulation of Triplet Exciton Dynamics in Organic Room-Temperature Phosphorescent Crystals

Abstract

Organic room temperature phosphorescence (RTP) materials rely on efficient utilization of triplet excitons, yet the microscopic relationship among molecular distortion, crystal packing, and triplet decay dynamics remains insufficiently understood. Herein, we employ multiscale excited-state calculations combined with crystal-level simulations to elucidate how molecular substitution and hydrostatic pressure regulate RTP properties in three phenothiazine-based molecular crystals (MM-HPT, MH-MPT, and MM-MPT). We demonstrate that methyl substitution induces progressive conformational distortion, which reconstructs intermolecular packing and frontier orbital compositions, thereby controlling the competition between radiative and non-radiative triplet decay pathways. Although MH-MPT exhibits the strongest spin–orbit coupling and the largest phosphorescence radiative rate due to enhanced n-π orbital mixing and localized triplet excitons, its flexible excited-state geometry leads to severe structural relaxation and accelerated non-radiative decay. In contrast, MM-MPT achieves more favorable triplet utilization by rigidifying the excited-state structure through strong intermolecular confinement, resulting in suppressed non-radiative losses. Furthermore, hydrostatic compression of MM-HPT continuously reduces excited-state structural relaxation and enlarges the T₁-S₀ energy gap, producing a pronounced phosphorescence hypsochromic shift from 528 to 471 nm. Pressure-induced orbital reconstruction and weakened excitonic coupling further reveal the microscopic origin of the emission evolution. These findings establish a molecular-level design principle that efficient RTP emission requires balancing triplet generation, radiative transition, and structural relaxation suppression rather than simply enhancing spin–orbit coupling, providing new insights into the rational design of stimulus-responsive organic phosphorescent materials.