Studies On Phase Transitions And Optical Properties Of Several Organic Molecular Crystals Under High Pressures

Organic molecular crystals are soft, highly compressible solids, composed of dispersive molecules hold together by noncovalent interactions such as van der Waals forces, charge transfer and hydrogen bonding. This combination of strong intramolecular covalent bonds with weak intermolecular forces confers on organic molecular crystals unique properties that are often markedly different from those of covalent or ionic crystals. Pressure is an ideal parameter suited to the study of organic molecular crystals, as small variation of applied forces typically results in large changes in intermolecular separations, which often trigger dramatic reorganizations of crystal packing. Hydrogen bonding under pressure is one of the most important intermolecular interactions in organic molecular crystals. The shortening of the distances between atoms and molecules cause the changes of the structure of substance (crystal structure, molecular structure and the rearrangement of bonds). Furthermore the properties influenced by electronic structure will also be changed. The variations of the overlap degree of electronic cloud result in the arrangement change of molecular orbits. The optical and electronic properties of organic molecular crystals attribute directly to the changes of the density of electronic clouds. The distance between organic molecular crystals decrease with increasing pressure, and the repulsive intermolecular and intramolecular interactions increase. The unsaturated covalent bonds become instability, and can be broken under high pressures, activating the molecules. High pressure chemical reaction may take place in order to reach low inner energy and to stabilize the system. Organic chemical reaction occurs in crystal phase without catalyst and solvents with increasing pressure, leading to derive materials who property is different from that of normal products. In this thesis, we have investigated Raman and fluorescence spectra of several organic molecular crystals made of, such as, benzoic acid, urea and thiourea, new crystal structure and luminescent materials obtained from high pressure. A non-fluorescent material can be transformed into a strong luminescent material at certain high pressure, and the luminescent property can be maintained after releasing pressure. The thesis is made of following nine chapters.In Chapter One, we introduce the history of development of high pressure technique with emphasis on the technique of diamond anvil cell and the related to pressure transmitting medium and scaler of pressure calibration. It is given a review on the study about organic...