How Plausible Is Getting Ferromagnetic Interactions by Coupling Blatter's Radical via Its Fused Benzene Ring?

With ongoing efforts to synthesize super-stable Blatter's diradicals having strong ferromagnetic exchange interactions, all the 10 possible isomers of di-Blatter diradical coupled through the fused benzene rings are investigated. A variety of electronic structure theory such as broken-symmetry methods in density functional theory (DFT), spin-constraint DFT (CDFT), and wave function-based multi-configurational methods, e.g., CASSCF/NEVPT2 are applied to compute the magnetic exchange interactions. Surprisingly, anti-ferromagnetic interactions are revealed for all the stable isomers of di-Blatter diradicals. Indeed, it is commensurate with the experimental observations for the only available synthesized isomer. However, the other nine isomeric diradicals in the series are yet to be synthesized. Despite a good match between theory and experiment, the anti-ferromagnetic exchange interactions could not be explained based on the spin alternation rule due to unique spin distributions in the triazinyl ring. Thus, we propose the zonal spin-alternation rule, which explains the observed ground spin-state for the conjugated di-Blatter diradicals quite accurately. Further, the fractional spin-moment localization on the N-atoms activates multiple exchange pathways and the dominating exchange interactions render anti-ferromagnetic interactions in the conjugated isomers. The study further reveals that, due to strong steric hindrance in certain coupled isomers, the exchange interaction switches from anti-ferromagnetic to weak ferromagnetic interactions with the cost of stabilization energy of the radicals. Thus, it questions the possibility of synthesizing ferromagnetic di-Blatter diradicals.