Phase transition-driven abnormal fluorescence response in pyrene under high pressure

In this study, in situ high-pressure fluorescence, absorption, and Raman spectroscopy techniques were employed to systematically elucidate the spectral evolution laws and microscopic mechanisms of pyrene over the pressure range of 0–10.2 GPa. Spectroscopic observations revealed a two-stage fluorescence enhancement of pyrene, which is directly associated with its crystalline phase transitions. At 0.2 GPa, pyrene undergoes a phase transition from phase I to phase II. The pressure-induced structural rearrangement and strengthened intermolecular interactions in pyrene work synergistically to achieve luminescence enhancement and fluorescence red-shift at this pressure. Within the pressure range of 2.0–3.0 GPa, the II → IV phase transition takes place, giving rise to the second stage of fluorescence enhancement. A key finding is that an anomalous phenomenon of simultaneous fluorescence enhancement and blue-shift is observed in the fluorescence spectrum during the II → IV phase transition; multi-spectral correlation analysis demonstrates that this phenomenon originates from the synergistic effect of reduced crystal symmetry and weakened π–π orbital overlap, which significantly enhances radiative recombination efficiency, and thus breaks the traditional understanding of spectral “red-shift-quenching” under high pressure. Beyond 7.2 GPa, the IV → V phase transition of pyrene induces molecular bending and electronic structure reconstruction, leading to accelerated fluorescence red-shift and complete quenching. This work constructs a comprehensive “pressure-structure-spectra” correlation system for pyrene under high pressure, and provides direct spectroscopic evidence for understanding the high-pressure photophysical behaviors of conjugated molecules.