Estimation of πd-interactions in magnetic molecular conductors

Conducting Coronene Cation Radical Salt Containing Magnetic Metal Ions

Coronene is the smallest homologue of benzene and is the smallest fragment of graphene among 6-fold symmetric polycyclic aromatic hydrocarbons. In this study, we obtained the first coronene cation radical solid containing magnetic counterions by an electrochemical method. Coronene monocations in the 1:1 salt, (coronene^•+)(FeBr₄^-), assemble in a stacking manner via π-π interactions, which lead to a rather high room-temperature conductivity of 0.6 S cm^-1. The salt shows semiconducting behavior as expected from the calculated band structure, and activation energies were estimated to be 0.25 eV at T ≥ 220 K and 0.18 eV at T ≤ 220 K. The magnetic susceptibility follows the Curie-Weiss law down to about 30 K, with a Curie constant (4.47 emu K mol^-1) expected for S = 5/2 spins of iron(III) ions and a high Weiss temperature (-32.2 K). Upon further cooling, the salt exhibits a susceptibility kink at 16.2 K followed by the loss of a significant fraction of the susceptibility due to long-range antiferromagnetic ordering. Theoretical calculations predicted that the indirect π-d magnetic exchange interaction through C-H···Br hydrogen bonds is equal to J_πd = -3.10 K. Although the absolute value is lower than that of the direct d-d magnetic exchange interaction between the FeBr₄^- anions (J_dd = -13.35 K), it is evident that the π-d interactions play a certain role in determining the magnetic behavior. Considering that an isomorphous salt, (coronene^•+)(GaBr₄^-) involving a nonmagnetic counterpart GaBr₄^-, exhibits singlet-triplet magnetic behavior with a spin gap of 1.44 × 10³ K, it is most likely that in (coronene)(FeBr₄) the nonmagnetic π-electrons serve as mediators of the magnetic ordering of d-spins through the π-d interactions.

Electronic Structure and Magnetic Interactions in the Radical Salt [BEDT-TTF]2[CuCl4]

The magnetic behavior and electric properties of the hybrid radical salt [BEDT-TTF]₂[CuCl₄] have been revisited through extended experimental analyses and DDCI and periodic DFT plane waves calculations. Single crystal X-ray diffraction data have been collected at different temperatures, discovering a phase transition occurring in the 250-300 K range. The calculations indicate the presence of intradimer, interdimer, and organic-inorganic π-d interactions in the crystal, a magnetic pattern much more complex than the Bleaney-Bowers model initially assigned to this material. Although this simple model was good enough to reproduce the magnetic susceptibility data, our calculations demonstrate that the actual magnetic structure is significantly more intricate, with alternating antiferromagnetic 1D chains of the organic BEDT-TTF⁺ radical, connected through weak antiferromagnetic interactions with the CuCl₄^2- ions. Combination of experiment and theory allowed us to unambiguously determine and quantify the leading magnetic interactions in the system. The density-of-states curves confirm the semiconductor nature of the system and the dominant organic contribution of the valence and conduction band edges. This general and combined approach appears to be fundamental in order to properly understand the magnetic structure of these complex materials, where experimental data can actually be fitted from a variety of models and parameters.

Evaluation of the Giant Ferromagnetic π-d Interaction in Iron-Phthalocyanine Molecule

The interaction between itinerant π and localized d electrons in metal-phthalocyanines, namely, J_πd interaction, is considered as responsible for the giant negative magnetoresistance observed in several phthalocyanine-based conductors, among many other important physical properties. Despite the fundamental and technological importance of this on-site intramolecular interaction, its giant ferromagnetic nature has been only recently demonstrated by the experiments conducted by Murakawa et al. in the neutral radical [Fe(Pc)(CN)₂]·2CHCl₃ ( Phys. Rev. B 2015 , 92 , 054429 ). In this article, we present the theoretical evaluation of this interaction combining wave function-based electronic calculations on isolated Fe(Pc)(CN)₂ molecules and density functional theory-based periodic calculations on the crystal. Our calculations confirm the ferromagnetic nature of the π-d interaction, with a coupling constant as large as J_πd/k_B = 570 K, in excellent agreement with the experiments, and the presence of intermolecular antiferromagnetic interactions driven by the π-π overlap of neighboring phthalocyaninato molecules. The analysis of the wave function of the ground state of the Fe(Pc)(CN)₂ molecule provides the clues of the origin of this giant ferromagnetic π-d interaction.

New aspects of π-d interactions in magnetic molecular conductors

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•FeBr4 is a ferromagnetic semiconductor, (EDT-DSDTFVO)2•FeX4 (X = Cl, Br) and (EDT-DSDTFVSDS)2•FeBr4 are metals exhibiting antiferromagnetic ordering of the d spins, and (EDO-TTFVO)2•FeCl4 and (EDO-TTFVODS)2•FeBr4•(DCE)0.5 (DCE =-dichloroethane) are genuine antiferromagnetic metals. Among them, the (EDT-TTFVO)2•FeBr4 salt 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•FeBr4 is 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.

Antiferromagnetic d-electron exchange via a spin-singlet pi-electron ground state in an organic conductor

Electron spin resonance reveals the spin behavior of conduction (pi) and localized (d) electrons in beta-(BDA-TTP)2MCl4 (M=Fe, Ga). Both the Ga3+(S=0) and Fe3+(S=5/2) compounds exhibit a metal-insulator transition at 113 K with the simultaneous formation of a spin-singlet ground state in the pi electron system of the donor molecules. The behavior is consistent with charge ordering in beta-(BDA-TTP)2MCl4 at the metal-insulator transition. At 5 K, the Fe3+ compound orders antiferromagnetically, even though the pi electrons, which normally would facilitate magnetic exchange, are localized nonmagnetic singlets.