Polymorphism in a π Stacked Blatter Radical: Structures and Magnetic Properties of 3-(Phenyl)-1-(pyrid-2-yl)-1,4-dihydrobenzo[e][1,2,4]triazin-4-yl
3-(Phenyl)-1-(pyrid-2-yl)-1,4-dihydrobenzo[e][1,2,4]triazin-4-yl (2) demonstrates the first example of polymorphism in the family of Blatter radicals. Two polymorphs, 2α and 2β, have been identified and characterized by single crystal X-ray diffractometry and magnetic susceptibility measurements to investigate their magnetism-structure correlations. Both polymorphs form one-dimensional (1D) π stacks of evenly spaced radicals with distinctly different π-π overlap modes. Within the 1D π stacks, radicals are located at evenly interplanar distances, 3.461 Å for 2α and 3.430 Å for 2β. Magnetic susceptibility studies indicate that both polymorphs exhibit antiferromagnetic interactions inside their 1D π stacks. The magnetic susceptibility data are best interpreted in terms of a regular chain model of antiferromagnetically coupled quantum spins (H=-2J∑iSi•Si+1) with exchange-interactions of J/kB = –37.2(5) K (–25.9(4) cm–1) for 2α and J/kB = –75(5) K (–52(4) cm–1) for 2β. For polymorph 2β, a crossover on the magnetic susceptibility around 20 K suggests the presence of a phase transition, which might be related to dimerization of the radicals along the chain. DFT calculations support the experimental structure-magnetism results and the antiferromagnetic nature of the local interactions between radicals within the 1D π stacks.