Chiral diethyl-EDT-TTF and tetraethyl-BEDT-TTF: synthesis, structural characterization, radical cation salt and charge transfer complexes

Abstract

The chiral electroactive precursors diethyl-ethylenedithio-tetrathiafulvalene (DE-EDT-TTF 1) and tetraethyl-bis(ethylenedithio)-tetrathiafulvalene (TE-BEDT-TTF 2) have been synthesized in the racemic form and separated in pure enantiomers, together with the meso form of 2, by chiral HPLC. All the neutral forms (rac)-, (S)- and (R)-1, (rac)-, (S)-, (R)- and (meso)-2 have been structurally characterized by single crystal X-ray diffraction analysis. In all the structures the ethyl substituents are in axial positions, in agreement with the results of the DFT calculations, which indicate that the axial conformation of the ethyl substituents of 1 is more stable by 0.95 kcal mol⁻¹ than the equatorial conformation in the gas phase. Circular dichroism measurements combined with TD-DFT calculations are also in favour of the predominance of the axial conformers in solution. The crystalline radical cation salt [(S)-1]PF₆ and charge transfer complexes [(R)-1]TCNQF₄ and [(meso)-2]TCNQF₄ have been structurally characterized. A segregated donor–acceptor structure is observed for [(R)-1]TCNQF₄, while in [(meso)-2]TCNQF₄ donors and acceptors alternate in the stacks.