Circularly Polarized Light-induced Photocatalytic Asymmetric Synthesis And Oxidation Of Chiral Molecules On A ZnS Surface

Chiral molecules are chemicals owning two enantiomeric and mirror-symmetric forms which are not superimposable on each other. Because of the specific stereocheimcal structure of a chiral molecule, different chiral component has unique physical and chemical properties. It has been found that circularly polarized light(CPL) can induce asymmetric synthesis or photolysis of chiral molecules. Spin polarized electron in the bulk of CPL-irradiated Fe-Ni alloy and Ga As can also make the dissymmetric decomposition of chiral molecules. Accorrdingly, this thesis at the first time puts forward a new hypothesis as follows: “circularly polarized light �' semiconductor carrier spin polarization �' molecular asymmetric form”.In order to test the above-mentioned hypothesis, a series of experiments, including photocatalytic asymmetric synthesis of chiral molecules and oxidation of racemic chiral molecules on semiconductor surfaces, were carried out. Pristine Zn S and Fe, Ni, Cu-doped Zn S particles were prepared and used as the photocatalysts. A mercury-xenon lamp and left or right-handed circularly polarized light(266 nm) modulated by the femtosecond laser system in the laboratory were employed as natural and CPL light sources, respectively. In the unpolarized natural light experiments, the results show that the photocatalytic efficiency of Fe and Ni doped Zn S is less than that of pristine Zn S in photocatalytic reduction of fumaric acid to form succinic acid under the irradiation of un-polarized natural light. On the contrary, Cu doping can significantly enhance the photocatalytic efficiency of Zn S, and the optimal doping concentration of Cu is 0.3 atom%. Subsequently, the thesis investigated the effect of polarized lights on the synthesis and oxidation of two types of chiral molecules, i.e., lactic acid and amino acids, by using pristine Zn S as the photocatalyst. The data show that left-handed CPL is conducive to the synthesis and oxidation of L--chiral molecules, right-handed CPL is conducive to the synthesis and oxidation of D--chiral molecules, while linearly polarized light has weak effect on the chirality of the products. Based on a review of literature combined with the above results, it would entertain a notion that the asymmetric synthesis and oxidation of lactic acid and amino acids is possibly related to circularly polarized light-induced spin polarization of the electrons and holes in the Zn S samples.The thesis is a tentative exploration on the spin-induced chirality selectivity effect. It is not only an extension of the study of spintronics, but also a bridge between spintronics and asymmetric organic synthesis. Meanwhile, in view of the fact that chiral molecules are of great value in organic synthesis, especially of drugs and some other chemical products, the theoretical and experimental model presented in this thesis may also have important potential practical applications.