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 π-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 ${\mathrm{Cu}}^{2+}$ to ${\mathrm{Cu}}^{+}$ 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 KVV and Cu LVV 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.