Drastic Photoluminescence Shift of an Organic Molecular Crystal Modulated with High Pressure

Pressure is evidenced to be an effective method to modulate the optical properties of organic solid-state light-emitting materials through enhancing intermolecular interactions and/or generating new polymorphs. Herein, we found the organic molecular crystal of diphenylfluorenone (DPFO) undergoes large-scale and continuous red shifts of UV-vis absorption and photoluminescence (PL) spectra upon the application of high pressure, which was across the entire visible light region. Remarkably, a red-shift of about 300 nm was observed for its PL peak when applying the pressure to 13.32 GPa, from greenish-yellow to the near-infrared (NIR) region. The in situ high-pressure IR spectra and synchrotron X-ray diffraction (XRD) analyses suggest a phase transiton from the - to the - phase (newly identified) at around 3 GPa, accompanied by the phenyl units of DPFO coverting from perpendicular to parallel packing. Theoretical calculations reveal that the enhanced - interactions and the overlap of the molecular oribitals in the -phase are responsible for such a tremendous red shift in the PL spectra. Our work highlights the large red-shift phenomenon in DPFO system under high pressure which may find potentical applications in photonic devices. Furthermore, the work reveals the clear structure-property correlation, which will provide deep insights into the tailor of the optical properties of organic molecular materials.