Molecular magnet is an interdisciplinary subject of chemistry and physics developed on the basis of coordination chemistry in recent 30 years. The discovery of molecular magnet is of great significance because it shows many novel quantum magnetic behaviors, such as quantum tunneling effect and slow magnetic relaxation effect. Compared with traditional magnets, its extremely high magnetic density and nano-size make it an excellent choice for the next generation of quantum devices. The research project of Masahiro Yamashita and Shen Yongbing of Tohoku University in Japan is about how to realize high magnetic density devices by using molecular magnets to overcome the shortcomings of current information storage. For example, the hard disks of computers and mobile phones we use every day generally have some shortcomings, such as small capacity, slow reading speed and high calorific value (molar limit), which greatly weakens the development of semiconductor industry. Molecular magnets bring light to these problems. The team used the nanometer nature of molecular magnets and the controllability of valence state to mix organic conductors. Molecular magnets and organic conductors crystallize in different nano-layers through electrochemical crystallization, forming nano-materials with molecular magnetic layers and conductive layers interlaced with each other. Based on this strategy, in recent years, they have synthesized a series of conductive monomolecular magnets with lanthanides (Dy3+) and transition metals (Co2+) as magnetic centers and tetrathiafulvalene and its derivatives as conductors. Many of these materials show the coexistence of single molecule magnetism and metal conductivity in the same temperature range for the first time, which is a key step to realize the interaction between electricity and magnetism. The team's research shows that this coexistence shows external physical properties such as magnetic sequence and metal-insulator transition. Through the test of magnetoresistance effect, they observed the negative magnetoresistance effect under this material for the first time, which laid a good theoretical foundation for the development of high magnetic density devices.

Recently, the team published a paper on j.am.chem.soc, focusing on the conductive phase (please refer to the paper for the magnetic part). They added a small amount of potassium chloride into lanthanide metal complex K4Dy(NCS)7 and organic conductor BEDT-TTF(ET), and obtained a new conductive molecular magnet of 4 f -π system through electrochemical crystallization. Interestingly, the added K+ ions coordinate with organic conductors to form a special conductive layer (Figure 1b). Due to the coordination between K+ ions and NCS, adjacent Dy(NCS)7 are connected by K+ ions, forming a one-dimensional magnetic link (figure 1a). The one-dimensional chain then forms a three-dimensional layered structure with alternating electricity and magnetism through the interaction between ET conductors (Figure 1c).

Figure 1. Crystal structure. (a) crystal structure along the c axis. (b) Structural patterns of endothelin molecules and KCl salts. (c) crystal accumulation on the c)ab plane.

The absorption spectrum shows that the compound has electrical conductivity. The temperature-dependent single crystal conductivity test shows the semiconductor behavior. Interestingly, the two phase transitions occurred at 250 K and 125 K respectively, corresponding to three different semiconductor regions (Figure 2a). Especially near 125 K, there is obvious short-term metal conduction behavior with hysteresis loop. In order to reveal these phase transition principles, the team first tested the single crystal structures at 300 K, 225 K, 170 K, 125 K and 97 K. The results show that at 125 K, the C axis of the unit cell increases obviously, which directly leads to the increase of the unit cell volume (Figure 2b). They calculated the oxidation state of each ET through the bond length of TTF, and found that the valence states of these five ETs changed obviously with the change of temperature. At 300 K, the four coordinated ETs show a valence close to+1, and the free ETs show a valence close to 0. With the temperature decreasing to 125 K, the valence of four coordinated Ets decreased significantly to +0.5, and the free Ets also increased to +0.3, indicating the formation of partial oxidation state. The change of valence state leads to the change of conductive behavior. When the temperature drops from 125 K to 97 K, the valence states of four coordinated Ets and free Ets return to+1 and 0 respectively. The author thinks that the instability of this valence state may be related to the embedding of K+.

Figure 2. Electrical conductivity. (a) The temperature dependence of σ in the temperature range of 75–300 K; (b) Temperature dependence of cell parameters. (c) The arrangement of c)ET molecules in the localized crystal structure.

DFT calculation verifies the experimental results. The research team calculated the energy band structure of these five temperature points, and the results show that it is a two-dimensional conductive material. As the temperature decreases from 300 K to 125 K, the valence band at the front edge changes from camel-like splitting to molten state, suggesting that the hole concentration is increasing, which explains the increase of conductivity near125 K. With the further decrease of temperature, the molten valence state splits into camel-like again, and the conductivity returns to the state of high temperature region (Figure 3).

Figure 3. Calculation of electronic structure at different temperature points. (a) Predictive state density (PDOS) based on density functional theory. Energy band structure calculation.

4π molecular hybridization shows rich conductive phases and slow magnetization relaxation.

Shen Yongbing *, Goulven Cosquer, (Zhang Haitao), Brian K. Breedlove, Cui Mengxing and Masahiro Yamashita*

J. speaking. Chemistry. Socialists, 202 1,143,9543–9550, doi:10.1021jacs.1c03748.

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Shen Yongbing, Masahiro Yamashita

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