Magnetic semiconductors: Molecular materials based on alkyl-pyridinium-substituted verdazyl radical cations and Ni(dmit)2 anion

Five kinds of (1:1) and (1:3) salts of 3-(4- and 3-alkyl-pyridinium)-1,5-diphenylverdazyl (alkyl = ethyl and methyl) radical cations with Ni(dmit)2 anion ([p-EtPyDV]+[Ni(dmit)2]− (1), [m-EtPyDV]+[Ni(dmit)2]− (2), [p-MePyDV]+[Ni(dmit)2]− (3), [m-MePyDV]+[Ni(dmit)2]− (4), and [p-MePyDV]+[Ni(dmit)2]3-(5)) (dmit = 1,3-dithiol-2-thione-4,5-dithiolate) have been prepared, and the magnetic susceptibilities (χM’s) have been measured between 1.8 and 300 K. The χM of salt 1 can be well explained by a four-spin linear tetramer model ([p-EtPyDV]+–[Ni(dmit)2]−–[Ni(dmit)2]−–[p-EtPyDV]+) with an end exchange interaction 2J1/kB = −780 K and a central interaction 2J2/kB = −200 K. The formation of magnetic tetramer in salt 1 has been ascertained by the crystal structure analysis of 1. The χM of salt 2 was explained by the sum of the contributions from: (i) a dimer system (2J/kB = −320 K) due to Ni(dmit)2 anions and (ii) the one-dimensional (1D) Heisenberg ferromagnetic linear-chain system (2J kB = +2.5 K) due to verdazyl cations. The χM of salt 3 is similar to that of salt 1, suggesting a formation of magnetic tetramer in salt 3. The χM of salt 4 shows a broad maximum at Tmax = 100 ± 2 K, and can be explained by a 1D Heisenberg antiferromagnetic alternating-chain model with 2J1/kB = −165 K (alternation parameter α = J2/J1 = 0.6). The conductivity (σ) of salts 1, 2, 3 and 4 at 293 K was σ = 2.3 × 10−5, 1.1 × 10−3, 1.4 × 10−5 and 5.4 × 10−6 S cm−1 with an activation energy of EA = 0.28, 0.52, 0.21 and 0.33 eV, respectively. The (1:3) salt 5 (σ = 1.0 S cm−1, EA = 0.048 eV) showed three to five orders of magnitude higher conductivity than those of the (1:1) salts 1–4 at room temperature. All the (1:1) and (1:3) salts 1–5 are new molecular paramagnetic semiconductors.

Five kinds of (1:1) and (1:3) salts of 3-(3- and 4-alkyl-pyridinium)-1,5-diphenylverdazyl (alkyl = ethyl and methyl) radical cations with Ni(dmit)2 anion have been prepared, and found to be new paramagnetic semiconductors.Figure optionsDownload as PowerPoint slide