Boron-bridged bis(tetrathiafulvalene) zwitterionic neutral radical conductors: substituent effects on intra- and intermolecular electronic interactions and physical properties

Abstract We have recently proposed a new molecular design for neutral radical molecular conductors, based on the chemical composition of D2X-type charge-transfer salts (D = π-electron donor molecule, X = monovalent anion). Namely, we connected two tetrathiafulvalene (TTF) -based π-electron skeletons having a charge of +0. 5 via a boron anion B–, to obtain a purely organic zwitterionic neutral radical (PDT-TTF-Cat) 2+B–• that shows unique electronic states and properties in the crystal due to the intramolecular and intermolecular interactions of the +0. 5-charged π-skeletons. In order to further explore the structural and electronic features of this kind of zwitterionic neutral radical conductor, in this study, we have examined chemical modifications to (PDT-TTF-Cat) 2+B–•. As a result, an analog molecule (BMT-TTF-Cat) 2+B–•, in which propylenedithio (PDT) groups on the TTF terminals in (PDT-TTF-Cat) 2+B–• are replaced by bis (methylthio) ones, was successfully synthesized as air-stable single crystals. Interestingly, this chemical modification causes intramolecular charge disproportionation between the two TTF-based π-skeletons (i. e. the monovalent cation radical TTF+• and the neutral TTF0) coupled to the modulation of the intermolecular distances and interactions between these π-skeletons, leading to electrical and magnetic properties significantly different from those of the crystal of (PDT-TTF-Cat) 2+B–•.