Abstract
An isostructural series of anion radical salts, β'-(EtxMe4−xZ)[Pd(dmit)2]2 (x = 0–2, Z = P, As, Sb), with a quasi-triangular lattice comprising the dimer unit [Pd(dmit)2]2− belong to a strongly correlated electron system with geometrical frustration. Intra and interdimer transfer integrals between the frontier molecular orbitals, which characterize the strength of electron correlation and degree of frustration, can be tuned by selection of the counter cation. We have systematically analyzed the crystal structure with X-ray diffraction method and intermolecular transfer integrals using extended Hückel molecular orbital calculations based on structural data. The variation in the cation affects the unit cell in a manner equivalent to an anisotropic pressure. Increasing the covalent radius of the central atom Z and the number of ethyl groups (x) in the cation leads to slight arching of the Pd(dmit)2 molecule. This arch-shaped distortion of the Pd(dmit)2 molecule modifies the interdimer transfer integrals in formation of the regular triangular dimer lattice. On the other hand, the intradimer transfer integral, which is correlated with the effective on-site Coulomb interaction of the dimer, is weakly dependent on the type of cation.
Highlights
Molecular conductors are notable for their simple and clear electronic structures
The conduction process in molecular conductors is governed by electron transfer between the frontier molecular orbitals; that is, the highest occupied molecular orbital (HOMO) or the lowest unoccupied molecular orbital (LUMO), and the transfer integrals between HOMOs or LUMOs determine the electronic structure
The crystal structures of β'-type Pd(dmit)2 salts corresponding to a two-dimensional Mott system with a quasi triangular lattice were investigated systematically, and the effects of the counter cation on intermolecular transfer integrals revealed
Summary
Molecular conductors are notable for their simple and clear electronic structures. This means that simple tight-binding band calculations based on the extended Hückel method are quite useful for describing the energy bands in the vicinity of the Fermi level [1]. The transfer integrals are generally sensitive to the molecular arrangement and orientation, because the component molecules belong to conjugated π systems with highly anisotropic molecular shapes. Another notable attribute of molecular conductors is the variety of chemical modifications that are possible, leading to a diversity of electronic properties and enabling their fine tuning. The strength of the electron correlation and degree of frustration are characterized by the intra- and interdimer transfer integrals between the frontier molecular orbitals [2,15] These transfer integrals can be systematically tuned by selecting counter cations, EtxMe4−xZ+, with different x (number of ethyl groups) and Z (central pnictogen atom). We report on the cation dependence of the crystal and molecular structures of β'-type Pd(dmit) salts and discuss their relation in terms of intermolecular transfer integrals
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