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Switching of Conductivity and Magnetism by Coupled Deuterium and Electron Transfer in a Purely Organic Conductor κ-D3(Cat-EDT-TTF)2

Mori Group

A hydrogen bond (H-bond) is one of the most fundamental and important non-covalent interactions in chemistry, biology, physics, and all other molecular sciences. In particular, the dynamics of a proton or a hydrogen atom in the H-bond has attracted increasing attention, because it plays a crucial role for (bio)chemical reactions and some physical properties, such as dielectricity and proton conductivity. Here we report unprecedented H-bond-dynamics-based π-electronic switching of conductivity and magnetism in an H-bonded purely organic conductor crystal, κ-D3(Cat-EDT-TTF)2 (abbreviated as κ-D) [1].

Fig. 1. Schematic drawing of thermal switching of electrical conductivity and magnetism in κ-D3(Cat-EDT-TTF)2. A purely organic material, κ-D3(Cat-EDT-TTF)2, is composed of a unit structure where Cat-EDT-TTF molecules (green colored, left) are connected by an [O···D···O] hydrogen bond. At 185 K (–88 °C), the deuterium and electron transfer occurs, to change the electric charge on the two Cat-EDT-TTF molecules [(+0.5 vs +0.5) ↔ (+0.06 vs +0.94)], which results in the switching of electrical conductivity (semiconductor ↔insulator) and magnetism (paramagnetic ↔ nonmagnetic).

Fig. 2. Temperature dependence of (a) electrical resistivity (on a single crystal) and (b) magnetic susceptibility (of a polycrystalline sample) of κ-D (blue and red circles represent the cooling and heating processes, respectively) and κ-H (black circles in the cooling process). The orange line represents the fitting curve for κ-D by the singlet-triplet dimer model with an antiferromagnetic coupling of 2J/kB ~ –600 K.

This novel crystal κ-D, a deuterated analogue of κ-H3(Cat-EDT-TTF)2 (abbreviated as κ-H [2,3]), is composed only of an H-bonded molecular unit, in which two crystallographically equivalent catechol-fused ethylenedithiotetrathiafulvalene (Cat-EDT-TTF) skeletons with a +0.5 charge are linked by a symmetric anionic [O···D···O]–1-type strong H-bond (Fig. 1). The deuterated and parent hydrogen systems, κ-D and κ-H, are isostructural and paramagnetic semiconductors with a dimer-Mott-type electronic structure in triangular lattice at room temperature. However, the ground states of κ-D and κ-H are totally different. As shown in Fig. 2(a), ρ of a single crystal of κ-D (blue circles) monotonically increased with decreasing temperature from 300 K to 182 K, showing typical semiconducting behavior similar to that of the parent κ-H (black circles). The room-temperature electrical conductivity σrt (= 1/ρrt) and activation energy Ea of κ-D (6.2 Scm–1, 0.08 eV) are, however, slightly higher and lower than those of κ-H (3.5 Scm–1, 0.11 eV), respectively, probably related to deuterium dynamics. Upon further cooling, κ-D showed a rapid increase in ρ at 182 K, to undergo a semiconductor-insulator-like phase transition, which is in sharp contrast to the continuous monotonic semiconducting behavior in κ-H. Such a remarkable H/D substitution effect is also observed in the temperature dependence of the magnetic susceptibility (χp) of κ-D and κ-H in the polycrystalline state in the temperature range of 2–300 K (Fig. 2(b)). With decreasing temperature, χp of κ-H (black circles) follows the Heisenberg model of triangular lattice with J/kB = 80 – 100 K and shows no magnetic order down to 50 mK, indicating quantum spin liquid state [4]. On the other hand, the deuterated analogue κ-D (blue circles) exhibits an abrupt drop of χp at 185 K after the monotonic increase similar to κ-H. After the abrupt drop, χp of κ-D gradually approaches ~ 0.0 emu mol−1 with decreasing temperature. This low-temperature magnetic behavior is reproduced by the singlet-triplet dimer model with an antiferromagnetic coupling of 2J/kB ~ –600 K (orange line in Fig. 2(b)). Therefore, a spin-singlet state is formed in κ-D by the magnetic transition from the high-temperature (HT) paramagnetic phase, which leads to a non-magnetic ground state, in sharp contrast to κ-H with a quantum spin liquid ground state. Such a surprisingly large increase in Tc by over 180 K, deuterium isotope effect, has not been reported, to our knowledge.

In order to gain structural insight into this deuteration-induced phase transition with the electronic switching, we have carried out X-ray diffraction studies on a single crystal of κ-D using synchrotron radiation in the temperature range of 50–270 K. The κ-D system at 270 K is isostructural to κ-H (vide infra). Upon the phase transition, the H-bonded deuterium transfers from the center of the two oxygen atoms towards one oxygen atom, to form an asymmetric [O–D•••O]–1 H-bond with short O–D (1.02(5) Å) and long O•••D (1.51(5) Å) distances (Fig. 1 right), instead of the two equivalent O•••D distances at 270 K (1.265(7) Å, Fig.1 left). As a result, the two crystallographically equivalent Cat-EDT-TTF+0.5 skeletons in the HT phase are also desymmetrized, to give a charge-poor Cat-EDT-TTF+0.06 with the short O–D distance and a charge-rich Cat-EDT-TTF+0.94 with the long O•••D distance, as estimated from the bond lengths of the TTF skeletons. Due to the intra-unit charge disproportionation through the H-bond, the overall electronic structure of κ-D in the LT phase is intrinsically different from that in the HT phase. Below 180 K, the charge-rich (blue-colored) and -poor (orange-colored) Cat-EDT-TTF skeletons are separately π-dimerized with their neighboring cofacial Cat-EDT-TTF skeletons having the same valence, to form two kinds of π-dimeric pairs, composed of the charge-rich skeletons or the charge-poor ones. These π-dimers are stacked two-dimensionally with maintaining the κ-type molecular arrangement, which results in a charge-ordered (CO) electronic structure (Fig. 1 right) from a dimer-Mott state in the HT phase (Fig. 1 left). In this CO state, since the charge-rich TTF+0.94 skeleton is expected to have nearly one electron spin (S = 1/2), a spin singlet should be formed within each charge-rich π-dimer, which rationalizes the paramagnetic–non-magnetic transition in magnetic susceptibility (Fig. 2(b)) as well as the semiconductor-insulator-like transition in resistivity (Fig. 2(a)).

In summary, we have discovered unprecedented H-bond-dynamics-based π-electronic properties switching in an purely organic conductor crystal: Deuteration of the [O•••H•••O]–1 H-bond in a catechol-fused TTF-based purely organic conductor crystal, κ-H3(Cat-EDT-TTF)2 or κ-H, gives rise to a phase transition at 185 K with significant switching of the π-electronic structure (dimer-Mott ↔ charge order), electrical conductivity (semiconducting ↔ insulating), and magnetism (paramagnetic ↔ non-magnetic), due to deuterium transfer or displacement within the H-bond accompanied by electron transfer between the H-bonded Cat-EDT-TTF π-systems. This result clearly demonstrates that the H-bonded deuterium dynamics and the TTF π-electron are cooperatively coupled in the present system. Further systematic tuning of proton and π-electron dynamics in this coupled system will afford novel chemical and physical functionalities beyond the framework of π-electronics so far.


References
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Authors
  • A. Ueda, S. Yamada, T. Isonoa, H. Kamo, A. Nakaob, R. Kumaic, H. Nakaoc, Y. Murakamic, K. Yamamotod, Y. Nishioe, and H. Mori
  • a National Institute for Materials Science
  • bComprehensive Research Organization for Science and Society (CROSS),
  • cCMRC and Photon Factory, Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK)
  • dOkayama University of Science
  • eToho University