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, 2 alpha and 2 beta, 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) pi stacks of evenly spaced radicals with distinctly different pi-pi overlap modes. Within the 1D pi stacks, radicals are located at evenly interplanar distances, 3.461 angstrom for 2 alpha and 3.430 angstrom for 2 beta. Magnetic susceptibility studies indicate that both polymorphs exhibit antiferromagnetic interactions inside their 1D pi stacks. The magnetic susceptibility data are best interpreted in terms of a regular chain model of antiferromagnetically coupled quantum spins H = - 2J Sigma(i)(S-i) over right arrow.(Si+1) over right arrow) with exchange-interactions of J/k(B) = -36.7(3) K (-25.5(2) cm(-1)) for 2 alpha and J/k(B) = -72(3) K (-50(2) cm(-1)) for 2 beta. For polymorph 2 beta, 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 pi stacks.