Structural change of trans-azobenzene crystal and powder under high pressure

Abstract Structural changes in the trans-azobenzene single crystal and powder upon compression realized by a diamond anvil cell (DAC) were disclosed via the analysis of Raman spectroscopy. Compared to the shearing force that destroys the crystal structure of trans-azobenzene, the hydrostatic pressure makes the crystal structure of trans-azobenzene stacking more closely and orderly, which are proved by the blue-shifts of Raman bands. During the compression process, the peaks at 1159, 1182 and 1593 cm−1, highly related to C–H in-plane bending, C–C stretching and in-plane bending vibrations of benzene ring, show more obvious blue-shifts Δ = 21, 35 and 21 cm−1, respectively. Two types of trans-azobenzene under hydrostatic pressure display different spectral behaviors, due to the strong intermolecular interactions in their crystal forms, but not in powder. When the pressure was gradually reduced to the ambient pressure, the initial forms of the Raman spectra of trans-azobenzene were entirely recovered, indicating that the pressure-induced structural changes are reversible in a certain pressure range. Our spectroscopic observations enable a deeper understanding of the molecular interactions of azobenzene compounds.