Crystal structure and physical properties of radical cation salt based on 4,5-ethylenedioxy-4′-iodotetrathiafulvalene (EDO-TTF-I) with iodine bonding ability

Abstract

Single crystals of 4,5-ethylenedioxy-4′-iodotetrathiafulvalene (EDO-TTF-I) with iodine bonding ability were obtained by recrystallization from acetonitrile and crystallized in the orthorhombic system with the space group Pbca. Bent EDO-TTF-I molecules formed the dimer structure, and a short I⋯O contact was observed between dimers. Electrocrystallization of EDO-TTF-I in absolute ethanol containing tetrabutylammonium hexafluorophosphate, (TBA)PF₆, afforded single crystals of (EDO-TTF-I)₂PF₆. An X-ray structural analysis revealed that EDO-TTF-I molecules form dimerized stacking columns in a head-to-tail manner, which is referred to as a β′-type arrangement. Iodine bonds between EDO-TTF-I and PF₆ anions and short I⋯S contacts between EDO-TTF-I molecules were observed. Tight-binding band calculations indicated that the strong molecular dimerization splits the energy band into upper and lower bands with a small energy gap, where the upper band is effectively half-filled. It was found that (EDO-TTF-I)₂PF₆ is semiconducting (σ_RT = 5.3 S cm⁻¹ and E_a = 0.17 eV) and exhibits magnetic properties reminiscent of a low-dimensional localized spin system. The magnetic data were analyzed by the Bonner–Fisher model incorporating interchain interactions: |J|/k_B = 67.1 K and zJ′/k_B = −61.1 K. No splitting of the CC stretching modes was observed in the range of 5–300 K by Raman spectroscopy, indicating a homogeneous charge distribution of EDO-TTF-I^0.5+. Therefore, (EDO-TTF-I)₂PF₆ is considered as a dimerized Mott insulator.