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1、4.8 Organic conductors and superconductors Materials based on weak intermolecular interactionsInsulator: 10-15 Scm-1Semiconductor: 10-15 100 Scm-1 Conductor (metal): 100 105 Scm-1Superconductor: = The vast majority of organic solids are insulators because(1) completely occupied HOMO and significant
2、energy difference between HOMO and LUMO for most organic molecules(2) organic solids usually molecular very less interaction between MOs of individual molecules narrow and completely occupied bands, and large band gap, no overlapThree approaches for obtaining organic conductors:Decreasing the energy
3、 difference between HOMO and LUMO of organic moleculemolecular engineeringextensive -bonding, including heteroatoms with long pair electrons, etc., e.g., graphiteIncreasing the interactions of MOs of individual molecules in solidcrystal engineeringModulating the occupied status of energy bands and m
4、aking zero band gap in solidcrystal engineering1911, conductivity predicted1954, perylene complexed with Br, organic conduction1964, superconductivity predicted1973, (TTF)(TCNQ) salt, first organic conductor1980, (TMTSF)2X salts (Bechgaard salts), organic superconductor1, Charge transfer solids: DA
5、salts2, -electron interactions of individual moleculesPacking of TTF and TCNQ in the (TTF)(TCNQ) crystal. Segregated stacks of donors and acceptorsInterplane distance: TTF column, 3.42 , TCNQ column, 3.17 For TTF and TCNQ crystals, 3.62 and 3.74 54 K R.T.: metallicbelow 54K: insulatordue to “Peierls
6、 transition”i.e., M-I transition, appearance of CDW or super-lattice Electron or charge-transfer between donor and acceptor Three critical factors for organic conductivity:(1)Packing segregated stack formation of suitable band structure(2)Amount of charge transfer fractional unoccupied band(3)Higher
7、 dimensionFractional charge-transferIntegral charge-transferTypical 2D stacking styles of ET, determined by anionsRelationship between packing and conductivity : & : conductors & : semiconductorsTc=11.6 K at ambient pressureInteractions involved in DA saltsvdW interactions, large for TTF type donors
8、Hydrogen bonds, C-Hanion type, for exampleIonic interactions between D and AMetallic bonding between individual D, small and governed by the three interactions above, but determining the conductivity of the materials CE of organic conductors and superconductorsstill trial-and-error methodsCase study
9、: ”-(ET)2SF5CHRCF2SO3R=H: pure organic superconductorR=F: superconductivity suppresseddisorder of anion below 150K, non-metallic ground statesubtle change of band structure due to small distortion of ET donor networkC-HFno C-HFsemiconductorThe anion layer dictates the packing of cationic donorparall
10、el,flipped,44Weak interactions are sensitive to external pressure, thus superconductivity is effected by the external pressure or the similar factorthe anions size.Science, 2001, 291, 285TTF fragmentNi(dmit)2 fragmentMetallic behavior from R.T. to 0.6 KCombining ionic and electronic conductivity as
11、well as magnetismmultifunctional materials?P. Day et al., Chem. Comm., 2001, 14624.9 Molecular Magnetic MaterialsMMMMagnetismMagnetism is a cumulative effect based on the coupling or interaction of the spins or internal angular momentum of unpaired electrons in various molecules throughout an entire
12、 solid cooperative property at supramolecular levelThree key properties of magnet:critical temperature Tclimiting or saturation magnetism Mscoercive field HcTc above Tc, No interaction, independentbelow Tc, Ferromagnetic or Antiferromagnetic orderMs saturation magnetism, intrinsic and important prop
13、erty, depends on the number of unpaired eHc magnetic field needed to orient and coalesce local regions of strong spin coupling into one large region“hard” magnet (high Hc100 Oe ): store information, disk, tape“soft” magnet (low Hc10 Oe): switch polarity rapidly, alternative current motor, magnetic s
14、hieldingDomain, hysteresis loop, andremnant magnetization Mrem Types of magnets or magnetismDiamagnetic Molecule has 2e paired in each orbitalParamagnetic Molecule has unpaired eEs-c Es-c-bs-c spin-coupling,s-c-b spin-coupling breaking by thermal energy, spins do not coupleEs-cEs-c-b spins coupling(
15、a) parallel coupling ferromagnetic(b) antiparallel coupling antiferromagneticnot property of one molecule, it is a cooperative property seen only in the solid state, i.e. at supramolecular level.(c) more up spins than down spins ferrimagnetic (d) metamagnetic antiferromagnetic to ferromagnetic under
16、 external field(e) canted ferromagnetic (weak ferromagnet) spins coupling ferromagnetically but only partially aligned(f) spin glass, spin cluster, etc.Schematic illustration of spin coupling behaviorsSpin glasssaturation magnetizationwhen all spins are alignedwhen spins couple, magnetization ferro magnetization antiferro Mechanism of spin coupling: Exchange through bridging atoms (inter-molecular)Configuration InteractionDipole-dipole interaction (inter-molecular) For MMM, multi-mechanisms are
