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

Abstract

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 (χ_M’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 2J_I-II/k_B = −171 ± 4 K (alternation parameters α = J_I-III/J_I-II = 0.5) and 2J/k_B = −15.4 ± 0.9 K (α = 1), respectively. The χ_M 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 χ_M of (4) shows a broad maximum at T_max = 24 ± 1 K. The χ_M can be well reproduced by the sum of the contributions from i) a 1D Heisenberg antiferromagnetic alternating-chain system with 2J₁/k_B = −47 ± 2 K (α = J₂/J₁ = 0.9) and ii) a dimer system with 2J/k_B = −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 E_A = 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.