Comparison of two isostructural organic compounds, one metallic and the other insulating

Abstract

The organic donor hexamethylenetetrathiafulvalene (HMTTF) forms charge-transfer salts with both tetracyano-$p$-quinodimethane (TCNQ) and its tetrafluoro derivative (TCNQ${\mathrm{F}}_4$). We report here the preparation, structure, and conductivity of HMTTF-TCNQ${\mathrm{F}}_4$. This salt is found to be isostructural with the corresponding TCNQ salt, but with a conductivity at 300 K approximately seven orders of magnitude lower, similar to the behavior previously found for the analogous salts of the selenium derivative, hexamethylenetetraselenafulvalene (HMTSF). In order to determine the cause of this large difference in conductivity, we have thoroughly examined the molecular and solid-state differences between HMTTF-TCNQ${\mathrm{F}}_4$ and HMTTF-TCNQ. The hyperfine coupling constants, molecular-orbital energy levels, solution optical spectra, and electrochemical properties of TCNQ${\mathrm{F}}_4^{-}$ and TCN${\mathrm{Q}}^{-}$ are compared. For the HMTTF salts, we compare the structures, conductivities, Madelung energies, magnetic susceptibilities, and optical spectra. It is concluded that the large difference in conductivity is caused by a difference in the degree of charge transfer, and not by a difference in the value of the intramolecular Coulomb repulsion $U$ (as had been suggested). Thus, HMTTF-TCNQ is a mixed-valence metal with a degree of charge transfer of $\rho =0.72\mathrm{}$; the stronger electron affinity of TCNQ${\mathrm{F}}_4$ causes the charge transfer in HMTTF-TCNQ${\mathrm{F}}_4$ to be complete ($\rho =1\mathrm{}$), making it a Mott insulator. In addition, clear evidence is given that $U$ on the TCNQ${\mathrm{F}}_4$ stack is not screened by the excitonic polarizability of the HMTTF molecules.