Effect of Methyl Substitution on Conformation and Molecular Arrangement of BEDT-TTF Derivatives in the Crystalline Environment

Abstract

Abstract Two methylated bis(ethylenedithio)tetrathiafulvalene (ET) derivatives, Me2ET and Me4ET were stereoselectively synthesized to examine the effect of methylation on conformations of dihydrodithiin rings and molecular arrangements in the crystalline state. Since the donating ability of Me2ET and Me4ET are similar to that of ET, the methylated ET derivatives are considered to be appropriate to investigate the “lattice pressure” effect on ET radical salts by changing the volume of donor molecules. The upper limit of an activation energy for the ring inversion of the dimethylated dihydrodithiin in solution was estimated to be 32 kJ mol−1 by 13C NMR spectroscopy. The X-ray structure analyses revealed that orientations of methyl groups are fixed to axial in Me2ET and to equatorial in Me4ET, accompanied by the change of molecular stacking. The “volume of a methyl group” was evaluated by comparing the molecular volumes of Me2ET and Me4ET with that of ET, and the effective volume for the axial methyl group turns out to be 15% larger than that of the equatorial. The solid state 13C NMR (CP/MAS) spectra of ET and its derivatives showed that the chemical shifts of resonance lines reflect the conformations of dihydrodithiin rings in crystals.