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.