Electrical and Optical Properties of the 5,6:11,12-Bis(epidithio)naphthacene-Bromine System (TTT–Brn). Formation of Organic Mixed-valence Compounds (TTT)(TTT+)Br− and (TTT+)(TTT2+)(Br−)3

Abstract

Abstract The electrical resistivity of the 5,6: 11,12-bis(epidithio)naphthacene-bromine system (TTT–Brn) was examined over the composition range from n=0.3 to 2.0 using polycrystalline compactions. The room-temperature resistivity shows a minimum of about 7 Ω cm near n=0.5, suggesting the formation of a mixed-valence compound, (TTT)(TTT+)Br−. The resistivity increases monotonously with n. No anomaly was found at the composition of (TTT+)Br−. The formation of another mixed-valence compound, the ideal composition of which may be expressed by the formula (TTT+)(TTT2+)(Br−)3, is indicated by an abrupt change of the resistivity from 100 to 4000 Ω cm around n=1.5. This change is accompanied by an increase of the activation energy for semiconduction by a factor of almost two and also by the change of sign in the variation of the Seebeck coefficient with the temperature from positive to negative. A further increase of the resistivity by a factor of ten was observed between n=1.5 and 2.0. The electronic and vibrational spectra support the coexistence of the TTT molecule and the TTT+ ion near n=0.5 and that of the TTT+ and TTP2+ ions near n=1.5.