Molecular Paramagnetic Semiconductor: Crystal Structures and Magnetic and Conducting Properties of the Ni(dmit)2 Salts of 6-Oxoverdazyl Radical Cations (dmit = 1,3-Dithiol-2-thione-4,5-dithiolate)

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Abstract
Four kinds of 1:1 and 1:3 salts of 3-[4-(trimethylammonio)phenyl]-1,5-diphenyl-6-oxoverdazyl radical cation ([1]+) and its mono- and dimethyl derivatives ([2]+ and [3]+) with Ni(dmit)2 anions (dmit = 1,3-dithiol-2-thione-4,5-dithiolate) ([1]+[Ni(dmit)2]- (4), [2]+[Ni(dmit)2]- (5), [3]+[Ni(dmit)2]- (6), and [1]+[Ni(dmit)2]3- (7)) have been prepared, and the magnetic susceptibilities (χM) have been measured between 1.8 and 300 K. The χM values of salts 5 and 7 can be well reproduced by the sum of the contributions from (i) a Curie−Weiss system with a Curie constant of 0.376 (K emu)/mol and negative Weiss constants (ϑ) of −0.4 and −1.7 K and (ii) a dimer system with strong negative exchange interactions of 2J/kB = −354 and −258 K, respectively. The dimer formations in Ni(dmit)2 anions have been ascertained by the crystal structure analyses of salts 4−6. In salts 4 and 6, Ni(dmit)2 dimer molecules are sandwiched between two verdazyl cations, indicating the formation of a linear tetramer in 4 and 6. The magnetic susceptibility data for salts 4 and 6 have been fitted to a linear tetramer model using an end exchange interaction of 2J1/kB = −600 K and a central interaction of 2J2/kB = −280 K for 4 and 2J1/kB = −30 K and 2J2/kB = −580 K for 6, respectively. The results of the temperature dependence of the g(T) value in salts 4−6 obtained by ESR measurement also support the above analyses. The 1:1 salts 4 − 6 are insulators. On the other hand, the conductivity of the 1:3 salt 7 at 20 °C was σ = 0.10 S cm-1 with an activation energy EA = 0.099 eV, showing the semiconductor property. Salt 7 is a new molecular paramagnetic semiconductor.