Comparative Study on Properties, Structural Changes, and Isomerization of Cis/Trans-Stilbene under High Pressure

The comparison of different stereoisomeric organic compounds under high pressure has been less investigated. Here, we chose different stereochemical configurations of cis/trans-stilbene to study the luminescence properties, polymerization reaction, and structural changes at 0-20 GPa by spectroscopy and XRD. No fluorescence enhancement occurred in cis-stilbene due to pi-pi stacking. At 16 GPa, the IR, UV-vis, and sample color changes show that it undergoes an irreversible polymerization, that C(sp2)-H changes to C(sp(2) + sp(3))-H. However, trans-stilbene undergoes fluorescence enhancement at 0-4 GPa due to the reduction of the torsion angle of the benzene ring and the C=C bond leading to the formation of rigid planar molecules, which is further confirmed by the IR and XRD results. At 8 GPa, the new peaks in UV-vis and XRD results show the formation of new substances by structural change. However, the structure of transstilbene is more stable, which leads to the return to the raw state after releasing the pressure, and a reversible transformation occurs at high pressure. The cis-trans isomerization under high pressure was also briefly investigated by combining heating and laser irradiation. The cis -> trans-stilbene transition can only happen under a fixed-range light irradiation, and the trans -> cis-stilbene transition could not happen even under irradiation with a 360 nm laser, which may provide a new idea for synthesizing trans isomers with a higher purity.