Kazuo Mukai

Heat Capacity Study of the Doping Effect of Paramagnetic Radicals on the Organic Spin–Peierls System (p-CyDOV Radical Crystal)

The doping effect of paramagnetic organic radical impurities ( p -CyDTV) on the organic spin–Peiels (SP) system of p -CyDOV (3-(4-cyanophenyl)-1,5-dimethyl-6-oxoverdazyl; the SP transition temperature T SP =15.0 K) was studied by the heat capacity measurement down to 0.1 K. At low temperatures below 1 K, quite a sharp heat capacity peak was detected for each system with small impurity concentration ( x ) at the temperature T N ( x ); T N (0)=0.135 K (±0.002 K), T N (0.01)=0.290 K and T N (0.07)=0.164 K. The appearance of the peak for the system with x =0 is considered to be due to minute and inevitable impurities in the pure p -CyDOV. From the dependence of T N ( x ), T N (0)< T N (0.01), and the heat capacity behavior on x , the peaks for x =0 and 0.01 are explained to be accompanied with the three-dimensional (3D) antiferromagnetic (AFM) transition in the SP phase. While the decrease of T N ( x ) for higher impurities x =0.07 than that for x =0.01 may be interpreted as merely a 3D AFM transition from th...

Genuine organic magnetic semiconductors: electrical and magnetic properties of the TCNQ and iodide salts of methylpyridinium-substituted verdazyl radicals

Two kinds of iodide salts of 3-(4- and 3-methylpyridinium)-1,5-diphenylverdazyl radical cations ([p-MePyDV]+I− (1) and [m-MePyDV]+I− (2)) and four kinds of 1 ∶ 1 and 1 ∶ 2 salts of the same verdazyl radical cations with tetracyanoquinodimethanide (TCNQ) anion ([p-MePyDV]+[TCNQ]− (3), [m-MePyDV]+[TCNQ]− (4), [p-MePyDV]+[TCNQ]2− (5), and [m-MePyDV]+[TCNQ]2− (6)) have been prepared, and the magnetic susceptibilities (χM’s) have been measured between 1.8 and 300 K. Salts (1) and (2) behave as one-dimensional (1D) alternating and nonalternating Heisenberg antiferromagnets with exchange interactions of 2JI-II/kB = −171 ± 4 K (alternation parameters α = JI-III/JI-II = 0.5) and 2J/kB = −15.4 ± 0.9 K (α = 1), respectively. The χM of (3) follows the Curie–Weiss law with a Curie constant of 0.365 K emu mol−1 and a positive Weiss constant of +2.1 ± 0.2 K, indicating disappearance of a net magnetic moment due to the formation of strong spin pairs in TCNQ anion molecules. The χM of (4) shows a broad maximum at Tmax = 24 ± 1 K. The χM can be well reproduced by the sum of the contributions from i) a 1D Heisenberg antiferromagnetic alternating-chain system with 2J1/kB = −47 ± 2 K (α = J2/J1 = 0.9) and ii) a dimer system with 2J/kB = −636 ± 30 K. The susceptibilities of (5) and (6) showed anomalous magnetic behavior that cannot be explained by a simple model. The separate contributions of the verdazyl cation and TCNQ anion subsystems to the total magnetic susceptibility of the salts were evaluated from measurements of the temperature dependence of the g-value. The 1 ∶ 1 salts (3) and (4) are insulators. On the other hand, the pressed pellet conductivity (σ) of the 1 : 2 salts (5) and (6) at 20 °C was σ = 8.1 × 10−3 and 2.9 × 10−2 S cm−1 with activation energy of EA = 0.092 ± 0.005 and 0.091 ± 0.005 eV, respectively. The two 1 ∶ 2 salts (5) and (6) are new genuine organic magnetic semiconductors.

4 – Verdazyl-based Magnetic Systems

This chapter describes the magnetic properties obtained for verdazyl radicals. The verdazyl radicals are one of the representative stable-free radicals that can be isolated as solvent-free pure radicals in crystalline state. The magnetic susceptibility of the verdazyl radicals shows a broad maximum, and the temperature dependence of magnetic susceptibility is explained by one-dimensional (ID) antiferromagnetic (AFM) Heisenberg nonaltenating or alternating chain model. It is found that in these verdazyl radicals, the exchange interaction within the molecule is great enough to stabilize the a-SOMO. In other words, the spin correlation causes a large spin polarization effect in 6-oxo-verdazyl radicals. The FM interaction of the verdazyl radicals is considered to originate mainly in the P-SOMO-p-NHOMO charge transfer interaction. Weak ferromagnetism has also been reported for three kinds of verdazyl radicals:TPV, TOV, and p-CyDpTV. An antisymmetric spin coupling of D–M type may contribute to the spin canting in p -CyDpTV. The spin Peierls transition in verdazyl radical crystal is also elaborated in the chapter.