CONFORMATIONAL-ANALYSIS OF TETRATHIAFULVALENE ISOMERS AND THE BAND-STRUCTURE OF THEIR ONE-DIMENSIONAL POLYMERS

The electronic structure of tetrathiafulvalene (TTF) and its three con formational isomers, 2,5-dimethylene-1,3-dithiolo[4,5-d] -1.3-dithiol (DDD), 1,4,5,8-tetrathianaphthalene (TTN), and bis(4-methylene-1,3-dit hietan-2-ylidene) (BMDY), are studied using ab initio and PRDDO molecu lar orbital calculations and extended Huckel band calculations. The on e-dimensional expansion of DDD is identical to that of TTF. TTN has tw o conformations, corresponding to a boat and chair form. Our calculati ons show that TTF is the most stable structure among the monomers stud ied. The stability of the polymers arising from these monomers is esti mated by oligomer calculations extrapolated to infinite chain length. The bandwidths of their highest occupied band are rather narrow (0.54- 1.03 eV); however, the three highest valence bands of each isomer are separated from each other by 0.1-0.4 eV. The calculated band gaps (2.7 -2.9 eV) of all the isomers are larger than that of polythiophene (1.7 5 eV), whereas the ionization potentials (9.7-10.1 eV) are smaller tha n that of polythiophene (11.0 eV). Thus, it should be easier to make a p-type conductor by doping the systems with an electron-accepting mol ecule.