Synthesis, structures, and properties of a series of double-bridged tetrathiafulvalenophanes as novel electron donors for conductive radical cation salts

A series of double-bridged tetrathiafulvalenophanes (TTF phanes) with different alkylenedithio (methylenedithio, ethylenedithio, trimethylenedithio, and tetramethylenedithio) bridges was synthesized as a model of interactive dimeric TTFs. They were found to exist as a mixture of three stereoisomers of cis/cis, cis/trans, and trans/trans (twist) types, which were separated by fractional crystallization, and the unique stacking structures of the three isomers were elucidated by X-ray crystallographic analyses and NMR spectroscopy. Their cyclic voltammograms demonstrated multi-redox waves split by intramolecular through-space electronic interactions depending on the bridge lengths. The principal redox-active species was estimated to be of the trans/trans (eclipse) type in the low oxidation states, which was not detected in the neutral state, and to be of the cis/cis type in the high oxidation states. Electrocrystallization gave various radical cation salts, which were mostly semiconductive and characterized by segregated stacking type of crystal structures exclusively including the cationic species of the trans/trans (eclipse) type. An exception to this was the I₃^- salt of the ethylenedithio-bridged TTF phane, which appeared in dimorphorous shapes of plate and needle, and both included the cationic species of the cis/cis type. The plate crystal showed a very high room-temperature conductivity of 28 S cm^-1 with metallic temperature behavior, which turned out to be induced by a network of marked intermolecular S···S contacts of the donor species.