Effects of structural variations on the π-dimer formation: long-distance multicenter bonding of cation-radicals of tetrathiafulvalene analogues
The multicenter (pancake) bonding between cation-radicals of tetramethyltetraselenafulvalene, TMTSF+•, tetramethyltetrathiafulvalene, TMTTF+•, and bis(ethylenedithio)-tetrathiafulvalene, ET+• was compared to that of the tetrathiafulvalene, TTF+•. To minimize counter-ion effects, the cations-radical salts with weakly-coordinating anions (WCA), tetrakis(3,5-trifluoromethylphenyl)borate, dodecamethycarborane and hexabromocarborane were prepared. Solid-state (X-ray and EPR) measurements revealed diamagnetic π-dimers in the TMTSF and ET salts and the separate monomers in the TTF salts with all WCAs, while TMTTF existed as dimers in one and monomers in two salts. The variable-temperature UV-Vis studies of these salts showed that thermodynamics of formation of the π-bonded dimers in solutions of TMTTF+• was close to that of the TTF+•, while TMTSF+• and ET+•showed higher propensity to π-dimerization. These data indicated that replacement of sulfur with heavier selenium or insertion of ethylenedithia-substituents into TTF core increase the π-dimers stability. Computational analysis indicated that weakly-covalent component of π-bonding decreases in the order TTF > TMTTF > TMTSF > ET. The higher stability of the π-dimers of TMTSF+• and ET+• cation-radicals was related to decrease in the electrostatic repulsion between cationic counter-parts and increase of dispersion components in these associations.