Pressure-Dependent Luminescence and Absorption in 3,3′-Diamino-4,4′-azoxyfurazan: Secondary Bonding Interaction in Molecular Crystals

3,3′-Diamino-4,4′-azoxyfurazan (DAAF) is a new energetic material and has potential for application in many fields. Few works about its luminescence under high pressure have been reported so far. To probe its behavior of emission and electronic structure evolution, fluorescence, absorption, Raman, and Fourier transform infrared spectroscopies were performed under high pressure. Time-dependent density functional theory was carried out to reveal the luminescence mechanism of DAAF under ambient conditions. The yellow-green emission of DAAF mainly originated from the π* → π transition. The absorption edge shifted to red, and the color changed from yellow to dark with the increasing pressure. Two electronic structure changes were observed at ∼3 and ∼12 GPa because of the variation of hydrogen bond network and distortion of the molecular structure. The strengthened secondary bonding interactions and a slightly enlarged π-conjugation system resulted in the evident enhancement in the fluorescence intensity and mo...