Photochemical modification of magnetic properties in organic low-dimensional conductors

Abstract

Magnetic properties of organic charge transfer salts Ag(DX) 2 (DX = 2,5-dihalogeno-N,N'-dicyanoquinonediimine; X = Cl, Br, I) were modified by UV irradiation from paramagnetism to diamagnetism in an irreversible way. The temperature dependence of susceptibility revealed that such change in magnetic behavior could be continuously controlled by the duration of irradiation. The observation with scanning electron microprobe revealed that the original appearance of samples, e.g. black well-defined needle-shaped shiny single crystals, remained after irradiation irrespective of the irradiation conditions and the duration. Thermochemical analysis and X-ray diffraction study demonstrated that the change in the physical properties were due to (partial) decomposition of Ag(DX) 2 to AgX, which was incorporated in the original Ag(DX) 2 lattices. Because the physical properties of  Corresponding author: tnaito@sci.hokudai.ac.jp 2 low-dimensional organic conductors are very sensitive to lattice defects, even a small amount of AgX could effectively modify the electronic properties of Ag(DX) 2 without making the original crystalline appearance collapse.

Development of a Control Method for Conduction and Magnetism in Molecular Crystals

“…Similarly to the photochemical transformation of α to γ, Ag(DX) 2 (X = Cl, Br, I) changed to nonmagnetism during UVirradiation ( Figure 27), 91 while before irradiation, they exhibited Pauli paramagnetism, which is the characteristic behavior of metallic substances. Consistent changes were observed in their IR reflectance spectra measured on single crystals (Figure 28), indicating that irradiation altered the conduction properties from that of metal to that of non-metal.…”

“…Consistent changes were observed in their IR reflectance spectra measured on single crystals (Figure 28), indicating that irradiation altered the conduction properties from that of metal to that of non-metal. 91 The phenomena occurring during irradiation have turned out different between Ag(DX) 2 and Ag(DM) 2 (α-to-γ), though the initial and final magnetic properties are qualitatively the same. The differences in phenomena, in its turn, originated from the different chemistry of substituents, which will be discussed later.…”

“…Change in magnetic susceptibility during UVirradiation upon Ag(DCl) 2 . 91 During irradiation, the magnetism gradually changes from paramagnetism to diamagnetism. The pristine sample exhibits the magnetic transition of Pauli paramagnetism to diamagnetism around 60 K. The upturn of » observed at low temperature (¯2030 K) is ascribed to lattice defects, i.e., Curie tail, which once increases from 0 to 3 h, then decreases from 3 to 48 h. Adapted with permission from ref 91. some differences after irradiation since the physical properties qualitatively differed from what they were before irradiation.…”

“…Polarized (a) reflectance and (b) conductivity spectra measured on pristine and UV-irradiated (6 h) single crystals of Ag(DI) 2 . 91 The polarization is (a) parallel or perpendicular, and (b) parallel to the c axis, i.e., the highest conducting direction. Both spectra clearly show that Ag(DI) 2 is metallic before irradiation, but that it is nonmetallic with much lower conductivity after 6-h irradiation.…”

“…Adapted with permission from ref 91. . 91 Reprinted with permission from ref 91. single crystal. In order to clarify which is the case, the irradiated crystals were viewed by transmission electron microscopy (TEM) in order to directly observe the inner structure of the irradiated crystals ( Figure 33).…”