A Study For π-π Stacking Molecules Under High Pressure

Pressure, temperature, composition are three basic parameters for substance. The change of temperature and composition is the usual method to study the characters and improve the performance. We can obtain new material, which cannot be prepared atmosphere by study the state of sample at high pressure. The essential effect of pressure is to reduce interatomic distance, which leads to modification in lattice constants of crystalline material as well as to change in atomic positions within crystallographic cells. In general, decrease of lattice spacing and interaction between atoms can induce modifications in the band structures of solids and thus in electronic properties and optical properties. Thus the physical and chemical characters of substances at high pressure are quite abundant.During the past few years, people have paid more and more concentration on putting the high pressure on the broader fields, some biological big molecules and organic molecules are used to do high pressure experiment. The organic molecule, azobenzene, biphenyl and naphthalene cause much interesting because it has special conformation.In this paper we studied the varieties of properties for trans- azobenzene, biphenyl and naphthalene. High pressure Raman experiment and synchronic radiation X-ray diffraction for trans-azobenzene were performed. Moreover, high pressure Raman experiment for biphenyl and naphthalene were also taken out, the following conclusions can be obtained:(1) High pressure Raman spectra for trans-azobenzene have been stuied. According to the earlier reports, nine vibration modes observed in the experiment can be assigned. All the nine vibration modes are intramolecular vibrations. It can be concluded that the value ofνP–ν0 increases continuously. The slope of the curve changes obviously at 2.3 GPa. Besides, at 4.8 GPa there is another distinct change of the slope. These phenomenon can be observed clearly forν18,ν15,ν10 andν8 modes. The second-order phase transition takes place at 2.3 GPa and 4.8 GPa, respectively.The in situ high-pressure x-ray diffraction (XRD) spectra also indicate that the first-order phase transition doesn't occur. The lattice distance d changes at 2.98 GPa and 4.55 GPa. This indicates that the molecular structure and crystal structure change at 2.98 GPa and 4.55 GPa. The result is in accord with the observed varieties for the Raman vibrations. So the results prove that the second-order phase transitions occur at 2.3 and 4.8 GPa. In order to deduces the origination for the two second-order phase transitions, a theory model suggested by Harada is taken account. Besides, trans-azobenzene crystal becomes amorphous under 10 GPa, and the process is inreversible.(2) High pressure Raman experiment for biphenyl have been performed. All the vibration peaks blue shift obviously. Five main peaks of biphenyl have been fitted, and the slopes of beelines in different pressure ranges have been obtained. The varieties indicate that a second-order phase transition occurs at about 2.3GPa.(3) High pressure Raman spectra for naphthalene have been studied. The results have been fitted to obtain the changes of Raman vibration modes with the increasing pressure. It indicates that the slopes for 1335.4 cm-1,1463.9 cm-1 change at 5.2 GPa. In the experimental pressure range, the naphthalene molecular crystal doesn't become amorphous.