Trifluoromethyl-substituted tetrathiafulvalenes

A series of tetrathiafulvalenes functionalized with one or two trifluoromethyl electron-withdrawing groups (EWG) is obtained by phosphite coupling involving CF(3)-substituted 1,3-dithiole-2-one derivatives. The relative effects of the EWG such as CF(3), CO(2)Me and CN on the TTF core were investigated from a combination of structural, electrochemical, spectrochemical and theoretical investigations. Electrochemical data confirm the good correlations between the first oxidation potential of the TTF derivatives and the σ(meta) Hammet parameter, thus in the order CO(2)Me < CF(3) < CN, indicating that, in any case, the mesomeric effect of the substituents is limited. Besides, crystal structure determinations show that the deformation of the unsymmetrically substituted dithiole rings, when bearing one, or two different EWG, and attributed to the mesomeric effect of ester or nitrile groups, is not notably modified or counter-balanced by the introduction of a neighboring trifluoromethyl group. DFT calculations confirm these observations and also show that the low energy HOMO–LUMO absorption band found in nitrile or ester-substituted TTFs is not found in TTF-CF(3), where, as in TTF itself, the low energy absorption band is essentially attributable to a HOMO→LUMO + 1 transition. Despite relatively high oxidation potentials, these donor molecules with CF(3) EWG can be involved in charge transfer complexes or cation radical salts, as reported here for the CF(3)-subsituted EDT-TTF donor molecule. A neutral charge transfer complex with TCNQ, (EDT-TTF-CF(3))(2)(TCNQ) was isolated and characterized through alternated stacks of EDT-TTF-CF(3) dimers and TCNQ in the solid state. A radical cation salt of EDT-TTF-CF(3) is also obtained upon electrocrystallisation in the presence of the FeCl(4)(−) anion. In this salt, formulated as (EDT-TTF-CF(3))(FeCl(4)), the (EDT-TTF-CF(3))(+•) radical cations are associated two-by-two into centrosymmetric dyads with a strong pairing of the radical species in a singlet state.