Intramolecular mixed-valence state through silicon or germanium double bridges in rigid bis(tetrathiafulvalenes)

The synthesis and characterization of two ortho-dimethyltetrathia-fulvalene (o-DMTTF)-based rigid dimers containing dimethylsilicon (Me2Si) or dimethylgermanium (Me2Ge) linkers are described. Single-crystal X-ray analysis reveals planar geometry for the central 1,4-disilicon or 1,4-digermanium six-membered rings. DFT calculations provide optimized conformations in agreement with the experimental ones, and also emphasize the role of the heteroatomic linkers in the conjugation between the two redox active units. Cyclic voltammetry measurements show sequential oxidation into radical cation, and then dication species. Solution EPR measurements on the radical-cation species indicate full delocalization of the unpaired electron over both electroactive TTF units, with an associated coupling of 0.42 G with twelve equivalent protons. DFT calculations afford fully planar geometry for the radical-cation species and confirm the experimental isotropic coupling constant. Single-crystal X-ray analyses of two charge-transfer compounds obtained upon chemical oxidation, formulated as [(Me2Si)(2)(o-DMTTF)2]center dot 1/2[TCNQ]center dot 1/2[TCNQF(4)] and [(Me2Ge)(2)(o-DMTTF)(2)]center dot[TCNQ], demonstrate the occurrence of genuine mixed-valence radical-cation species, as well as a three-dimensional network of short S center dot center dot center dot S intermolecular contacts. Temperature-dependent conductivity measurements demonstrate semiconducting behavior for both charge-transfer compounds, with an increase of the absolute value of the conductivity upon applying external pressure. Band structure calculations reveal peculiar pseudo-two-dimensional electronic structures, also confirming electronic interactions through SiMe2 and GeMe2 bridges.