First-principles Studies On The Diradical Molecules And Sandwich Clusters Magnetic Materials

Organic magnetic materials and their magnetic formation mechanism have always been important research topics in physics,chemistry and materials science.In recent years,with the rise of computational physics,the theoretical exploration of the structure and magnetic properties of diradical and organometallic sandwich clusters has attracted extensive attention.In this thesis,we first introduce the research background and the first-principles calculation method based on the density functional theory.Then,we systematically study the magnetic effects of different couplers on diradical and the effects of different ligands on the structural and magnetic properties of organometallic sandwich clusters.The works of this thesis include:Different poly-p-phenylene and it’s derivatives were selected as couplers to study their effects on the magnetic properties of nitronyl nitroxide diradicals.The calculation results show that when the diradicals are connected in para-position and meta-position at both ends of the coupler,the diradicals show antiferromagnetic and ferromagnetic ground states,respectively,and the magnetic property of the molecules gradually weakens with the increase of the coupler chain length.The calculation of dihedral angles between benzene rings shows that there is a linear relationship between the value of the magnetic coupling coefficient J and the square of the cosine of the dihedral angle.When some carbon atoms are replaced by nitrogen atoms of poly-p-phenylene,the antiferromagnetism of most of the diradicals that are connected in para-position is enhanced,but there are still some diradicals changing from antiferromagnetic state to ferromagnetic state.In addition,we also found that the hydrogenation of poly-p-phenylene significantly weakens the magnetic property of the diradicals,indicating that hydrogenation is not conducive to the formation of intramolecular magnetic coupling.The geometrical structure and magnetic properties of vanadium-benzene sandwich clusters and its derivatives have been systematically studied.When benzene ligands are replaced by 1,3,5-triazine molecules,the calculated results show that the ground state of sandwich clusters changes from ferromagnetic state to antiferromagnetic state,and the average vanadium-vanadium spacing of the clusters increases compared with that of vanadium-benzene sandwich clusters.The stability of the clusters changes little with the increase of molecular layers,and the stability of the clusters is higher than that of the vanadium-benzene sandwich clusters.Subsequently,we use-CN and-NH₂functional groups to polarize benzene molecules,and then use polar benzene molecules4-cyananiline and 4,5-diamino-phthalonitrile as ligands to form sandwich clusters with vanadium atoms.The calculated results show that the polar benzene molecules in the sandwich cluster are arranged in the form of antiferroelectric,and the average vanadium-vanadium spacing decreases significantly with the increase of molecular layers.The stability of the cluster decreases gradually with the increase of molecular layer number,but tends to be stable rapidly.Clusters composed of polar benzene molecules show semi-metallic or metallic property,and their magnetic ground states and their stability strongly depend on the number of molecular layers,which is very different from that of benzene molecules.The research works in this thesis not only contribute to the understanding of the magnetic properties of diradical molecules,but also provide a new way to search for organometallic complexes with magneto-electric properties.