NOVEL ELECTRON-ACCEPTORS BEARING A HETEROQUINONOID SYSTEM .4. SYNTHESES, PROPERTIES, AND CHARGE-TRANSFER COMPLEXES OF 2,7-BIS(DICYANOMETHYLENE)-2,7-DIHYDROBENZO[2,1-B-3,4-B']DITHIOPHENE, 2,7-BIS(DICYANOMETHYLENE)-2,7-DIHYDROBENZO[1,2-B-4,3-B']DITHIOPHENE, AND 2,6-BIS(DICYANOMETHYLENE)-2,6-DIHYDROBENZO[1,2-B-4,5-B']DITHIOPHENE

The three title compounds were prepared as novel hetero-TCNQ electron accepters comprising the quinonoid skeletons of different benzodithiophene types between two dicyanomethylene groups. On the basis of cyclic voltammetry, they have electron affinities comparable to or stronger than that of TCNQ. In addition, the extensive building blocks serve to induce the effective reduction of on-site coulombic repulsion. They thus behaved as superior electron accepters for forming electrically conductive charge-transfer complexes. In particular, 2,7-bis(dicyanomethylene)-2,7-dihydrobenzo-[2,1-b:3,4-b']dithiophene was the most tractable in terms of stability and solubility, giving a variety of conductive complexes with typical electron donors of TTT, TTF,and TPBP types. Most of these complexes showed a characteristic broad electronic transition in the infrared region of 2.9-4.0 kcm(-1), supporting the idea that their crystal structures assume a stacking form of segregated donor and acceptor columns with mixed valence states.