Novel oxygen-containing pi-electron donors for organic metals: 2-(1,3-dithiol-2-ylidene)-5-(pyran-4-ylidene)-1,3,4,6-tetrathiapentalenes

A bis-fused pi-electron donor incorporating a pyran-4-ylidene moiety, 2-(1, 3-dithiol-2-ylidene)-5-(pyran-4-ylidene)-1,3,4,6-tetrathiapentalene (PDT-TT P, 1a), and its derivatives (1b-d) have been synthesized. The cyclic voltam mograms of PDT-TTPs are composed of three pairs of redox waves. Comparison of the first oxidation potentials (E-1) suggests PDT-TTP (+0.42 V vs SCE in benzonitrile) is a weaker donor than TTF (+0.35 V). The present donors hav e produced a large number of highly conducting (sigma(rt) = 10(0)- 10(2) S cm(-1)) molecular complexes. Among them, conducting salts based on the ET-P DT (1d) are metallic down to liquid helium temperature, The metallic PF6- s alt of ET-PDT has a composition of(ET-PDT)(4)PF6(cn), where cn is 1-chloron aphthalene, in which two crystallographically independent ET-PDT molecules A and B form a face-to-face stack with a four-folded period as A'ABB'. In t he present salt, the head-to-tail overlap of the unsymmetrical pi-electron framework of PDT-TTP prevents an effective intrastack overlap and produces a strongly dimerized electronic structure along the stack direction. Tight- binding band calculations indicate that it has opened Fermi surfaces charac teristic of quasi-one-dimensional metals.