Pancake bonding plays a critical role in self-assembly of (partially-) oxidized tetrathiafulvalene (TTF) derivatives, magnetic and conducting materials of their salts and it was fruitfully used in design of molecular rotors and machines. Such bonding was examined via X-ray structural, UV-Vis-NIR spectral and magnetic susceptibility measurements of the salts comprising mono- and dicationic di- and trimeric associations of TTF and its analogues together with the computational analysis of such dimers and trimers. To minimize counter-ion effects, the cations-radical salts with various stoichiometry with weakly-coordinating anions were prepared. Solid-state (X-ray and EPR) measurements together with the variable-temperature UV-Vis studies clarified effects of structural variations in tetrathiafulvalene analogues on the π-dimer formation. Computational analysis demonstrated strong multicentre π-bonding within mono- and dicationic dimers and trimers. Bond critical points  and NCI analysis of the interaction between monomers in the trimeric association is illustrated in the Figure. Computational study also indicated that while a weakly-covalent component was critical to formation and properties of these complexes, the variation of their binding energies were related mainly to the interplay between electrostatic interaction between monomers and dispersion. Specifically, an electrostatic repulsion between cationic monomeric moieties in the dicationic trimers was close to that in the corresponding dimers, but the dispersion energy in the former was roughly twice that in the dimers. As a result, the trimers are thermodynamically stable in the gas and condensed phases – in contrast to the dicationic dimers, which are stable only if electrostatic repulsion is attenuated by the polar solvent and/or compensated by interaction with counter-ions in the solid state. Also, the weaker electrostatic repulsion between monomers in the monocationic trimers and dimers led to the higher stabilities of such complexes as compared to their dicationic analogues.

Multicenter π-bonding and spin equilibria in the dimeric and trimeric associations of the oxidized tetrathiafulvalene derivatives

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