Pressure-Activated and Retained Blue Emission in Melamine Through Hydrogen-Bond and Packing Regulation

Pressure provides a direct way to regulate molecular packing and excited-state relaxation in organic crystals, but retaining pressure-induced emission after decompression remains challenging. Here, we report pressure-activated and retained blue emission in melamine (MA), a simple nitrogen-rich molecular crystal assembled by N-H···N hydrogen bonds. MA shows weak blue emission at ambient conditions, whereas compression markedly enhances its photoluminescence and pressure release produces an even brighter state, with an approximately 35-fold increase in integrated emission intensity. Time-resolved photoluminescence and UV-vis absorption measurements indicate that the retained enhancement is not mainly governed by lifetime prolongation or optical-absorption changes. In situ Raman, FTIR, and powder X-ray diffraction reveal pressure-induced changes in molecular and lattice vibrations, the N─H-related hydrogen-bonding environment, and anisotropic lattice compression, together with increasing structural disorder at higher pressures and incomplete recovery after decompression. TD-DFT calculations show that N─H···N shortening slightly increases the S₁ oscillator strength and modifies the local hole-electron distribution, qualitatively supporting the influence of hydrogen-bond modulation on the excited-state properties. Together, these results associate the retained emission with an incompletely recovered pressure-modified intermolecular and lattice environment, highlighting hydrogen-bonded molecular crystals for pressure-activated and retained luminescence.