ELECTRONIC-STRUCTURE OF THE QUASI-ONE-DIMENSIONAL ORGANIC CONDUCTORS DCNQI (N,N'-DICYANOQUINONEDIIMINE)-CU SALTS

A comparative study of the electronic structure of in situ synthesized quasi-one-dimensional organic conductors (DMe-DCNQI)(2)Cu and (MeBr-DCNQI)(2)Cu has been carried out using various techniques of electron spectroscopy, where DMe-DCNQI and MeBr-DCNQI are 2,5-dimethyl-N,N'-dicyanoquinonediimine and 2,5-methylbromine-N,N'-dicyanoquinonediimine, respectively. From the photon-energy dependence of the valence-band photoemission spectra obtained using synchrotron radiation, the origins of each observed feature are unambiguously characterized. While the feature at the Fermi level is primarily derived from pi-bonded C and N 2p states, the contribution of Cu 3d states at the Fermi level is larger in the (MeBr-DCNQI)(2)Cu compared to the (DMe-DCNQI)(2)Cu. X-ray photoemission spectra of the valence band imply extensive hybridization of the Cu 3d states with C and N 2p states near the Fermi level. Line-shape analyses of the Cu 2p core-level spectra show that the ratio of CU2+ to Cu+, is higher in (MeBr-DCNQI)(2)Cu compared to (DMe-DCNQI)(2)Cu, with the ratio being closer to 1:2 for (MeBr-DCNQI)(2)Cu. From a comparison of C KW and Cu LW Auger spectra with the self-convolution of the valence-band spectra, it is found that the effective on-site Coulomb correlation energies between the valence electrons are high on C sites as well as Cu sites in both salts, with U(pp)=6.5 eV and U(dd)=8.0 eV, respectively. In conjunction with core-level spectra, the spectra indicate that the on-site Coulomb correlation, the hybridization strength, and the charge-transfer energy between the Cu 3d and N 2p ligands are very similar in the two salts. The metal-insulator transition in (MeBr-DCNQI)(2)Cu at 160 K is then facilitated by the proximity of the Cu2+-to-Cu+ ratio to 1:2 supporting charge disproportion, while deviation from it stabilizes the metallic phase in (DMe-DCNQI)(2)Cu down to very low temperatures.