Physical properties of charge transfer salt (EDO-TTFBr2)(2)AsF6 in Mott insulating state

We have investigated the physical properties of a new charge transfer salt based on an asymmetric donor 4,5-dibromo-4',5'-ethylenedioxy-tetrathiafulva lene (EDO-TTFBr2) whose crystal structure contains a weakly dimerized donor stacking structure. This salt is considered to be in a Mott insulating sta te in the vicinity of the metal-insulator boundary, taking account of the h ighly conductive semiconducting behavior and the feature of the susceptibil ity explained by the one-dimensional Heisenberg antiferromagnet model with an S = 1/2 localized spin per dimer and a large exchange interaction of abo ut -190 K. The large exchange interaction and the exceptionally small resis tivity (0.06 Omega cm at room temperature) originate from the strong inter- dimer transfer integral. The results of the ESR not only suggest an antifer romagnetic transition at 37 K, but also reflect the one-dimensional electro nic state giving a negligible contribution to the spin-lattice relaxation p rocess.