Novel Oxygen-Containing π-Electron Donors for Organic Metals: 2-(1,3-Dithiol-2-ylidene)-5-(pyran-4-ylidene)- 1,3,4,6-tetrathiapentalenes

A bis-fused π-electron donor incorporating a pyran-4-ylidene moiety, 2-(1,3-dithiol-2-ylidene)-5-(pyran-4-ylidene)-1,3,4,6-tetrathiapentalene (PDT−TTP, 1a ), and its derivatives ( 1b − d ) have been synthesized. The cyclic voltammograms 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 have produced a large number of highly conducting (σ rt = 10 0 −10 2 S cm - 1 ) molecular complexes. Among them, conducting salts based on the ET−PDT ( 1d ) are metallic down to liquid helium temperature. The metallic PF 6 - salt of ET−PDT has a composition of (ET−PDT) 4 PF 6 (cn), where cn is 1-chloronaphthalene, 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 the present salt, the head-to-tail overlap of the unsymmetrical π-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 characteristic of quasi-one-dimensional metals.