Stable organic radicals are promising building blocks for multifunctional molecular materials owing to their unique physical properties. These materials combine optical, transport and magnetic properties which can be fine-tuned by modifying their molecular structure using organic synthesis. Control of their solid-state packing is essential to custom tailor their macroscopic properties. This requires the development of structure-activity relationships. We have developed a variety of synthetic routes to air- and moisture-stable benzo[e][1,2,4]triazinyl radicals (aka Blatter radicals) in an effort to identify structure-magnetism correlations. Herein, we present a magneto-structural study of 6,7-dicyano-1,3-diphenyl-1,4-dihydro-1,2,4-benzotriazin-4-yl. This phthalonitrile Blatter radical is electron deficient and highly delocalized. It forms 1D linear chains wherein the radicals are evenly spaced. The interplanar distances along the chain are comparatively short for this family of radicals. Nevertheless, these 1D chains are stable regarding Peierls’ distortion which usually renders the material diamagnetic. The macroscopic magnetic susceptibility was fitted using the Bonner-Fisher model for 1D Heisenberg antiferromagnetic regular chain of S = 1/2 spins. A good fitting was obtained when a temperature-dependence parameter for the exchange interaction was introduced into the model. The radical follows the exchange elasticity phenomenon where the antiferromagnetic exchange interaction decreases with thermal lattice expansion. The First-Principles Bottom-Up approach established and verified the magneto-structural correlation.

Temperature-dependent AFM exchange along 1D linear regular chains of phthalonitrile Blatter radical

ACS Spring 2022

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