High-Pressure Raman Study of [2.2]Paracyclophane

The high-pressure structural properties of PCP were investigated by using Raman scattering techniques in a diamond anvil cell up to 10 GPa at room temperature. Compression caused a relative flat xylyl fragment and twisted ethylene bridges in PCP. Raman analysis showed that more peaks related to vibrational modes of aliphatic CH2 emerged due to the transition to a low symmetric phase after compression pressure reached 3.9 GPa. Other blue-shifting peaks showed discontinuity in the frequency–pressure curves at pressure 3.9 GPa because of molecular relaxation by phase transition. Compression strengthened the conjugation of xylyl fragments. When releasing pressure, the normal vibrational modes could be recovered with a hysteresis of ∼1.3 GPa. The normal phase of PCP could be fully recovered under ambient conditions.