Trifluoromethyl-substituted tetrathiafulvalenes
Résumé
A series of tetrathiafulvalenes functionalized with one or two trifluoromethyl electron-withdrawing groups (EWG) is obtained by phosphite coupling involving CF3-substituted 1,3-dithiole-2-one derivs. The relative effects of the EWG such as CF3, CO2Me and CN on the TTF core were investigated from a combination of structural, electrochem., spectrochem. and theor. investigations. Electrochem. data confirm the good correlations between the first oxidn. potential of the TTF derivs. and the σmeta Hammet parameter, thus in the order CO2Me \textless CF3 \textless CN, indicating that, in any case, the mesomeric effect of the substituents is limited. Besides, crystal structure detns. show that the deformation of the unsym. 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 calcns. 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-CF3, where, as in TTF itself, the low energy absorption band is essentially attributable to a HOMO → LUMO + 1 transition. Despite relatively high oxidn. potentials, these donor mols. with CF3 EWG can be involved in charge transfer complexes or cation radical salts, as reported here for the CF3-subsituted EDT-TTF donor mol. A neutral charge transfer complex with TCNQ, (EDT-TTF-CF3)2(TCNQ) was isolated and characterized through alternated stacks of EDT-TTF-CF3 dimers and TCNQ in the solid state. A radical cation salt of EDT-TTF-CF3 is also obtained upon electrocrystn. in the presence of the FeCl4- anion. In this salt, formulated as (EDT-TTF-CF3)(FeCl4), the (EDT-TTF-CF3)+· radical cations are assocd. two-by-two into centrosym. dyads with a strong pairing of the radical species in a singlet state.