Hydroquinone clathrates by temperature-dependent terahertz time-domain spectroscopy

Using Terahertz Time-Domain Spectroscopy (THz-TDS), we measured the structural transition from hydroquinone (HQ) clathrates having the storage properties to crystalline a-form hydroquinone as a function of temperatures. We found that all three HQ clathrates, CO₂-, CH₄-, and CO₂/CH₄-loaded HQ clathrates, have the different vibrational lines and transform into the α-form HQ at around 102±7°C. The resonance peak of the CO₂-loaded HQ clathrate at 2.15 THz decreases with increasing temperature, indicating that CO₂ guest molecules are readily released from the host framework prior to the structural transformation. This reveals that the HQ clathrates may transform into the stable a-form HQ via the metastable form of guest-free clathrate, which depends on guest molecules enclathrated in the cages of the host frameworks.