Piezochromic phenomena of 9-fluorenone derivatives

Piezochromic luminescent materials with multi-color switching have received considerable attention in fields such as displays, sensors, and biomedicine. However, enhancing the sensitivity of piezochromic color change through rational molecular design remains a significant challenge. Herein, we report the design, synthesis and high-pressure study of two 9-fluorenone derivatives of DPA (diphenylamine)-FO and DMAcr (9,9-dimethylcarbazine)-FO, realizing pronounced piezochromic phenomena in both emission colors and crystal colors. DPA-FO features a classic donor–acceptor molecular architecture. Its emission wavelength is highly sensitive to the solvent polarity, and as polarity increases, the redshift continues, indicating the emission nature of intramolecular charge transfer (ICT) luminescence. Under pressure, the emission color gradually changes from yellow to reddish brown, and a pressure coefficient of the emission wavelength is 10.7 nm/GPa. To amplify the piezochromic response, the donor unit is strategically modified by replacing the DPA group with DMAcr, a donor with stronger electron-donating ability. The resulting compound, DMAcr-FO, exhibits a more pronounced ICT process, as evidenced by its higher sensitivity of luminescence to solvent polarity. Under pressure, its emission color gradually changes from yellow to deep red. Correspondingly, the pressure coefficient of the emission wavelength increases 17.5 nm/GPa. Pressure-dependent UV-Vis absorption spectra reveal a continuous redshift in the absorption edge of both derivatives, attributed to structural shrinkage caused by enhanced orbital coupling. Notably, DMAcr-FO exhibits more significant changes in absorption edge and Stokes shift, indicating more substantial structural deformation under pressure. In addition, compared with DPA-FO, the infrared (IR) modes of DMAcr-FO present higher shifting rates with the increase of pressure, which also supports the above conclusion. Meanwhile, with the increase of pressure, the considerable structural distortion is also one of the factors that make DMAcr-FO has a more significant piezochromic phenomenon. This study not only deepens the understanding of structure–property relationships in piezochromic materials but also offers a viable strategy for designing high-performance piezo-responsive luminophores through tailored molecular engineering.