Temperature-Dependent Antiferromagnetic Exchange along 1D Linear Regular Chains of the Phthalonitrile Blatter Radical

1,3-Diphenyl-1,4-dihydrobenzo[e][1,2,4]triazin-4-yl-6,7-dicarbonitrile is an exceptionally stable electron-deficient organic radical with promising potential to be used as a building block in a range of electronic and spintronic materials. The radical has a fully reversible one-electron redox and is highly delocalized, with some spin density reaching as far as the nitrile groups. Two polymorphs, alpha and beta, were identified and characterized by single-crystal X-ray diffractometry. Both polymorphs form one-dimensional (1D) pi stacks. However, while in polymorph alpha radicals are located at evenly interplane distances (3.366 & Aring;), in polymorph beta radicals are located at alternate interplane distances (3.182 and 3.318 & Aring;). Magnetic susceptibility measurements for polymorph alpha indicate strong antiferromagnetic interactions along the 1D regular chain. Magnetic susceptibility data cannot be fully fitted to the Bonner and Fischer model for the 2-300 K temperature range. The steeper rise in paramagnetism above 80 K was rationalized by temperature-dependent antiferromagnetic exchange interactions between radicals within the 1D pi stacks, which is indeed supported by Density Functional Theory (DFT) calculations. A microscopic study of the magnetic topology of polymorph alpha together with the interpretation of its magnetic experimental data was pursued by using a First-Principles Bottom-Up approach. Minuscule changes in crystal packing upon changing the temperature significantly affect the magnetic interaction between spin-containing moieties. Temperature, therefore, is the key player in rationalizing the magnetism in polymorph alpha.