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

ABSTRACT

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 S ₁ 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.