Polymorph-Dependent Reconstruction of Excitonic Coupling under Pressure Regulates Room Temperature Phosphorescence

Abstract

Pressure-responsive organic room-temperature phosphorescence (RTP) offers a promising platform for optical sensing, information encryption, and anticounterfeiting. Understanding how external mechanical perturbations regulate triplet exciton dynamics in organic crystals remains a fundamental challenge in solid-state photochemistry. Here, density functional theory (DFT) and quantum mechanics/molecular mechanics (QM/MM) excited-state calculations are combined to compare the pressure-dependent RTP behavior of ToMoT-α and ToMoT-β. Despite nearly identical emission at ambient pressure, ToMoT-α exhibits a strongly nonmonotonic spectral excursion of 102 nm (449–551 nm), whereas ToMoT-β varies by only 10 nm. This divergence originates from their contrasting anisotropic compressibility and pressure adaptability. Under compression, ToMoT-α undergoes directional lattice relaxation and molecular reorganization, reconstructing intermolecular excitonic coupling and modulating triplet-state decay kinetics, whereas ToMoT-β largely preserves its structure and photophysical properties. These complementary responses support a conceptual dual-polymorph platform for self-referenced pressure readout and recovery-state discrimination. Ambient-pressure reoptimization further suggests that the relative crystal-energy difference can serve as a qualitative descriptor of structural recovery tendency. This work establishes a structure-packing and excited-state relationship for pressure-responsive organic phosphorescence and identifies polymorphism as a strategy for programming RTP responses.