title: New aspects of π-d interactions in magnetic molecular conductors.
abstract: The 2 : 1 cation radical salts of bent donor molecules of ethylenedithio-tetrathiafulvalenoquinone-1,3-dithiolemethide (EDT-TTFVO), ethylenedithio-diselenadithiafulvalenoquinone-1,3-dithiolemethide (EDT-DSDTFVO), ethylenedithio-diselenadithiafulvalenothioquinone-1,3-diselenolemethide (EDT-DSDTFVSDS), ethylenedioxy-tetrathiafulvalenoquinone-1,3-dithiolemethide (EDO-TTFVO) and ethylenedioxy-tetrathiafulvalenoquinone-1,3-diselenolemethide (EDO-TTFVODS) with FeX4-(X = Cl, Br) ions are prepared by electrocrystallization. The crystal structures of these salts are composed of alternately stacked donor molecule and magnetic anion layers. The band structures of the donor molecule layers are calculated using the overlap integrals between neighboring donor molecules and are compared with the observed electronic transport properties. The magnetic ordering of the Fe(III) d spins of FeX4-ions is determined from magnetization and heat capacity measurements. The magnetic ordering temperatures are estimated by considering a combination of a direct d-d interaction between the d spins and an indirect π-d interaction between the conduction π electron and the d spins, whose magnitudes are separately calculated from the crystal structures with an extended Hückel molecular orbital method. The occurrence of a π-d interaction is proved by the negative magnetoresistance, and the magnitude of magnetoresistance reflects the strength of the π-d interaction. The effect of pressure on the magnetoresistance is studied, and the result indicates that the magnitude of magnetoresistance increases, namely, the π-d interaction is enhanced with increasing pressure. From these experimental results it is shown that (EDT-TTFVO)2•FeBr4is a ferromagnetic semiconductor, (EDT-DSDTFVO)2•FeX4(X = Cl, Br) and (EDT-DSDTFVSDS)2•FeBr4are metals exhibiting antiferromagnetic ordering of the d spins, and (EDO-TTFVO)2•FeCl4and (EDO-TTFVODS)2•FeBr4•(DCE)0.5(DCE =-dichloroethane) are genuine antiferromagnetic metals. Among them, the (EDT-TTFVO)2•FeBr4salt is the first π-d molecular system where the d spins of FeBr4-ions are ferromagnetically ordered through antiferromagnetic interaction with the conduction π electrons. Corresponding to this ferromagnetic ordering, an anomalous dielectric slow-down phenomenon toward the ordering temperature is observed. The π-d interaction in (EDT-DSDTFVSDS)2•FeBr4is very large and comparable to that in λ-(BETS)2•FeCl4, which has the highest reported value so far, while the d-d interaction is fairly small. Concerning the ratio between the magnitudes of π-d and d-d interactions (Jπd/Jdd), this salt is currently the best π-d molecular system.
authors.affiliation: Department of Chemistry, Graduate School of Science, Osaka Prefecture University, Osaka 599-8570, Japan.
authors.forename: Toyonari
authors.initials: T
authors.lastname: Sugimoto
authors.affiliation: Department of Chemistry, Graduate School of Science, Osaka Prefecture University, Osaka 599-8570, Japan.
authors.forename: Hideki
authors.initials: H
authors.lastname: Fujiwara
authors.affiliation: Department of Physics and Electronics, Graduate School of Engineering, Osaka Prefecture University, Osaka 599-8531, Japan.
authors.forename: Satoru
authors.initials: S
authors.lastname: Noguchi
authors.affiliation: Department of Molecular Materials Science, Graduate School of Science, Osaka City University, Osaka 558-8585, Japan.
authors.forename: Keizo
authors.initials: K
authors.lastname: Murata
keywords: band structure; bent donor molecule; cation radical salt; crystal structure; ferro-/antiferro-magnetic ordering; magnetic FeX4− ion; magnetic molecular conductor; magnetoresistance; metallic conductivity; π–d/d–d interaction