Z [m (dmit) <sub> 2 </ Sub>] <sub> 2 </ Sub> Molecular Conductor Band Structure Study

Molecular conductor is the abbreviation for the electrically conducting molecular crystal. Molecular conductor is different from the traditional molecular crystal, atomic crystal, ionic crystal and metal crystal. The conducting fragment of molecular conductor is molecule, such as small neutral molecule, molecular radical, cation or anion, and there exists strong intermolecular interaction in the molecular conductors. In the energy band structure of molecular conductors the energy gap is narrow and even crosses with each other, implying the semiconductivity, conductivity, and in certain conditions, even superconductivity of the molecular conductor.Molecular conductors receive extensive concern and investigation during the past decades because of their excellent electrical conductivity. One of the large families of the molecular conductor is the complex-type molecular conductor, the most representative one is Z[M(dmit)2]2-type (where Z is cation or cation radical, M is transition metal ions such as nickel and palladium, and dmit=(C₃S₅)²-) molecular conductors. Up to now, the Cambridge Structural Database (the version of November 2003) has collected more than 100 molecular conductors of such type, including 8 superconductors. Since the importance of the Z[M(dmit)2]2-type molecular conductors is very clear, the present thesis will calculate the molecular orbital overlap integrals and energy band structures of several Z[M(dmit)2]2-type molecular conductors and apply the energy band theory to explore the influence of crystal structures to the electrical conductivity performance. Detailed description is as follows:1. The crystal band theory and an approximate method for crystal band structure calculation-Extended Huckel Molecular Orbital-Tight Binding is summarized.2. The structure and packing characteristic of the conducting component M(dmit)2 is described. Influence of different packing modes and intermolecular layer distance to the HOMO orbital overlap integrals is calculated and the evolutionrules of overlap integrals to molecular dislocation is found, as shown in Fig. 1.(a) (b) (c)(a) Dislocation of the conducting component along the long axis (b) Dislocation of the conducting component along the short axis (c) Dislocation of the conducting component along the normal axis of the molecular planeFigure 1 Overlap integral scheme of M(dmit)2(M=Pd) molecularorbita]Overlap integral shakes upon the dislocation of the conducting component M(dmit>2 along the long axis. When the absolute value of the overlap integral is in the point of extremum, the molecular dislocation is just that structural ring of one molecule is very over the structural ring of another molecule. Thus the face-to-face structure of the molecular ring promotes the interaction of the conducting components.The absolute value of the overlap integral decreases upon the dislocation of the conducting component M(dmit)2 along the short axis. Consequently, the dislocationalong the short axis can not promote the interaction of the conducting components.When the conducting component M(dmit)2 dislocates along the normal axis of the molecular plane, the overlap integral curves is similar to that of intermolecular interaction. Molecular plane distance ranges from 0.33 nm to 0.37 nm in the real crystals. Small molecular plane distance will promote the the interaction of the conducting components.3. Using the EHMO tight binding method, the molecular orbital overlap integral of the conducting components in the 2-D conducting layers of the 3 molecular conductors and their energy band structures, as shown in Fig,2, are calculatedbased on their crystal structures of (PyH)[Ni(dmit)2]2> (PyH)[Pd(dmit)2]^ and (Et4N)[Pd(dmit)2]2.(PyH)[Ni(dmit)2]2 (PyH)[Pd(dmit)2]2 (Et4N)[Pd(dmit)2]2Figure 2 Energy band structures of 3 molecular conductorsThe calculated results of overlap integral and energy band structure indicates that the stronger interaction is between the conducting components in a 2-D conducting layer, the wider the conduction band is, the narrower the energy gap is, and thus the better conductivity of the crystal is. The conductivity of molecular conductor is essentially determined by the conducting components and crystal structures. Molecular packing modes in the crystal are crucial for the conductivity when the conducting components are established. Uniformization of the molecular stacks (including dislocation along the molecular axis direction as small as possible and interlayer distance uniformization) as well as small intermolecular plan distance is the structural factor that promotes the conductivity of the molecular conductors.