Organic Metals. Systematic Molecular Modifications of Hexamethylenetetraheterofulvalene Donors

Two synthetic approaches for modifying hexamethylenetetraheterofulvalene donors are described for the purpose of perturbing in a systematic way the interesting solid state properties of the TCNQ salts of the parent systems. The first approach consists of a steric modification in which a methyl group is introduced into the outer five-membered rings of the parent molecules to give di(3-methylcyclopenteno)-[1,2-b;1',2'-h]-1,4,5,8-tetrathiafulvalene (6a, α-MeHMTTF), di(4-methylcyclopenteno)-[1,2-b; 1' 2'-h]-l,4,5,8-tetrathiafulvalene (6b, β-MeHMTTF), di(4-methylcyclopenteno)-[l,2-b;1',2'-h]-1,5- (and/or 1,8-)diselena-4,8- (and/or 4,5-)dithiafulvalene (11, β-MeHMDSeDTF) and di(4-methylcyclopenteno)-[1,2-b-1',2'-h]-1,4,5,8-tetraselenafulvalene (9, β-MeHMTSeF). Donor 6b forms both insulating and highly conducting (σRT=80/ohm·cm, single crystal) charge transfer complexes with TCNQ, whereas the selenium-containing analogues, 11 and 9, form only insulating TCNQ compounds. Donor 6a gives a moderately conducting TCNQ complex (σRT=10-1/ohm·cm). The second type of modification involves an electronic perturbation in which the outer five-membered alkyl rings are replaced with fused thiophene derivatives to give cis-/trans-dithieno[2,3-b;2',3'-h]-1,4,5,8-tetrathiafulvalene (26, α-TTTF) and the separable cis and trans isomers of di(4,5-dihydrothieno)-[2,3-b;2',3'-h]-l,4,5,8-tetrathiafulvalene (24a, cis-, and 24b, trans-α-DTTTF). Both 24a and 24b form highly conducting TCNQ complexes (σRT ≈ 20/ohm·cm, powder) whereas 26 forms a semiconducting TCNQ complex. © 1978, American Chemical Society. All rights reserved.