Direct Control of Spin Distribution and Anisotropy in Cu-Dithiolene Complex Anions by Light

Abstract

Electrical and magnetic properties are dominated by the (de)localization and the anisotropy in the distribution of unpaired electrons in solids. In molecular materials, these properties have been indirectly controlled through crystal structures using various chemical modifications to affect molecular structures and arrangements. In the molecular crystals, since the energy band structures can be semi-quantitatively known using band calculations and solid state spectra, one can anticipate the (de)localization of unpaired electrons in particular bands/levels, as well as interactions with other electrons. Thus, direct control of anisotropy and localization of unpaired electrons by locating them in selected energy bands/levels would realize more efficient control of electrical and magnetic properties. In this work, it has been found that the unpaired electrons on Cu(II)-complex anions can be optically controlled to behave as anisotropically-delocalized electrons (under dark) or isotropically-localized electrons like free electrons (under UV), the latter of which has hardly been observed in the ground states of Cu(II)-complexes by any chemical modifications. Although the compounds examined in this work did not switch between conductors and magnets, these findings indicate that optical excitation in the [Cu(dmit) 2 ] 2´s alts should be an effective method to control spin distribution and anisotropy.

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

“…Both resistivity and magnetic properties indicate that there are practically no unpaired electrons in the ground states. The observed properties are consistent with the crystal (Figure 67) and calculated band structures, 244 indicating that they should have band gaps. The resistivity under UV (250 450 nm)-irradiation upon (2) slightly decreased compared with that under dark conditions, yet E a remained unchanged independently of UV-irradiation ( Figure 69).…”

“…244 Still, interestingly, the spin distributions with different structures do not differ from each other; spin densities are concentrated around the CuS 4 cores with partial delocalization over the entire molecules ( Figure 68). This can be explained as follows.…”

“…Under dark conditions, they exhibited insulating (1 and 3; immeasurably high resistivity for both) or semiconducting [2; RT resistivity μ RT of (6.5 « 3.5) © 10 5 ohm¢cm and an activation energy E a of 0.15 « 0.02 eV] properties. 244 All of them exhibited diamagnetism at 2300 K without any anomaly or hysteresis. Both resistivity and magnetic properties indicate that there are practically no unpaired electrons in the ground states.…”

“…243,244 All these salts possess S = 1/2 on the [Cu(dmit) 2 ] 2¹ anions, originating from the Cu(II) ions with d 9 configuration. Thus the degree of ) along with »¤ dark (black) for comparison.…”

Dye-Sensitized Molecular Charge Transfer Complexes: Magnetic and Conduction Properties in the Photoexcited States of Ni(dmit)2 Salts Containing Photosensitive Dyes

“…This situation is clearly different from dark conductivity, where carriers can be discussed based on a single kind of relaxation time characteristic of their mean free path. Since all of these salts absorb UV-Vis-NIR light (~10 15 Hz), τ UV should be on the order of 10 −15 s, and succeeding processes are considered to differ from each other to result in different relaxation times of photoexcited carriers on the anions and localized spins on the cations. Based on the discussion thus far, the overall relaxation time of photoexcited spins on the cations and anions is unusually prolonged to produce the observed photoresponse in ESR and conduction.…”