Vibrational studies on electronic structures in metallic and insulating phases of the Cu complexes of substituted dicyanoquinonediimines (DCNQI). A comparison with the cases of the Li and Ba complexes

Abstract

Electronic structures in metallic and insulating phases of the Li, Cu, and Ba complexes of 2,5-R1,R2-DCNQI [R1=R2=Br (abbreviated as DBr) or R1=R2=CH3 (abbreviated as DMe); DCNQI=N,N′-dicyanoquinonediimine; 2,5- is usually omitted] have been studied by observing temperature dependencies of their infrared absorption bands between 295 and 23 K. At room temperature, the wave numbers (ν̃i) of infrared absorption bands of R1,R2-DCNQI and its Li and Ba complexes are linearly correlated with the degrees of charge transfer (ρ) (ρ=−0.5 and −1.0e for the Li and Ba complexes, respectively). The ν̃i–ρ relationships indicate that the ρ value for the Cu complexes is −0.67e. This result is consistent with the previously established view that the Cu cations in the Cu complexes at room temperature are in a mixed-valence state of Cu1.33+. In the infrared spectrum of Cu(DBr–DCNQI)2 at room temperature, no electron–molecular vibration (EMV) coupling bands are observed. Below the metal–insulator (M–I) transition temperature (TMI), EMV bands grow continuously and the ordinary infrared bands observed at room temperature gradually split into three bands with decreasing temperature. Similarly, the infrared bands of Li(DBr–DCNQI)2 split into two bands. These splittings are due to an inhomogeneous charge distribution in the DCNQI columns produced by the freezing of charge-density wave (CDW). The peak-to-peak amplitudes of CDWs in the DCNQI columns estimated by use of the ν̃i–ρ relationships are 0.08±0.04 and 0.40±0.04e, respectively, for the Li and Cu complexes of DBr–DCNQI. The state of the frozen CDW is inferred from the number of split bands. Based on the observed continuous change of the infrared spectra of Cu(DBr–DCNQI)2 and the discontinuous changes of other quantities such as x-ray satellite reflections, lattice parameters, and magnetic susceptibilities, the M–I transition in Cu(DBr–DCNQI)2 may be described as follows: (1) above TMI the charges on Cu cations (two Cu1+’s: one Cu2+) are dynamically averaged to +1.33e through the Cu...N≡C bridge. (2) At TMI the charges abruptly localize in the order of (Cu1+...Cu2+...Cu1+...)n. At the same time, the CDWs begin to be frozen in the DCNQI columns. (3) As temperature decreases below TMI, the order of the frozen CDW develops gradually. In contrast to these changes in Cu(DBr–DCNQI)2, neither EMV bands nor band splittings are observed in the infrared spectra of Cu(DMe–DCNQI)2 at low temperatures. Instead, almost all bands show negative absorption lobes on their low-wave number sides and become asymmetric. This asymmetrization is due to interactions between the vibrational levels and low-lying continuous electronic levels responsible for a broad band observed in the 1600–800 cm−1 region.