The synthesis of difluoro and dimethyl derivatives of 2,6- Bis(dicyanomethylene)-2,6-dihydro-4H-cyclopenta[2,1-b:3,4-b′] -dithiophen-4-one (CPDT-TCNQ) and the conducting properties of the metallic salts based on the dimethyl derivative

Article abstract of DOI:10.1246/bcsj.77.1487

In order to investigate how the sheet-like network along the side-by-side direction constructed by the strong inter-column S...N and O...H atom-atom contacts influences the stability of the metallic state of novel one-dimensional metallic anion radical salts of 2,6-bis(dicyanomethylene)-2,6- dihydro-4H-cyclopenta[2,1-b:3,4-b′]dithiophen-4-one (CPDT-TCNQ), the 3,5-difluoro (F₂CPDT-TCNQ) and 3,5-dimethyl (Me₂CPDT-TCNQ) derivatives of CPDT-TCNQ, not possessing hydrogen atoms at the 3,5-positions to construct the inter-column O...H contacts, have been synthesized. The anion radical salts, Me₄X(Me₂CPDT-TCNQ)₂ (X = P and As), showed a metallic temperature dependence of the resistivity from room temperature down to around 240 K and 200 K, respectively, whereas Me ₄X(Me₂CPDT-TCNQ)₂ (X = N and Sb) were semiconducting. The metal-insulator transition temperatures (T_M1) of Me₄X(Me₂CPDT-TCNQ)₂ (X = P and As) are significantly higher than those of MX₄(CPDT-TCNQ)₂ (X = P and As). Moreover, the extent of destabilization of the metallic state is much more significant in Me₄N(Me₂CPDT-TCNQ)₂, since MeN₄(CPDT-TCNQ)₂ is metallic down to 130 K. Although electric conduction can not occur along the side-by-side direction in either salt of CPDT-TCNQ and Me₂CPDT-TCNQ, these facts suggest that the two-dimensional sheet-like molecular network along the a-axis may contribute to stabilize the one-dimensional metallic states of the anion radical salts of CPDT-TCNQ.