First-Principle Design of Blatter's Diradicals with Strong Ferromagnetic Exchange Interactions

The stable organic diradicals that exhibit strong intramolecular ferromagnetic exchange interactions are suitable building blocks for organic magnetic materials (OMMs). Based on the ab initio calculations, here, we report the electronic and magnetic properties of 1,2,4-benzotriazinyl-based mono- and diradicals (known as Blatters radicals). The quantum mechanical calculations based on the density functional theory (DFT) reveal the merostability of the superstable Blatters radicals. The stability could further be enhanced by tuning the spin densities on the radical centers via the extended pi-conjugation. The magnetic exchange interactions (2J) have been investigated for Blatters radical coupled to the nitronyl nitroxide radical (i.e., Bl-NN) as the prototypical system that has recently been synthesized by Rajca et al.. The broken-symmetry (BS) approach within the standard DFT and constraint spin-density DFT (CDFT) methods are applied to compute the exchange interactions, while for wave function-based multireference methods, the spin symmetry-adopted (e.g., CASSCF/NEVPT2) approach is applied. It is observed that the CBS-DFT provides much better 2J values as compared to the standard BS-DFT. The multireference calculations based on the minimal active space [i.e., CAS(2,2)] incorporating the delocalized magnetic orbitals provide quite reliable exchange interactions. After validating the applied computational methods, a number of ferromagnetically coupled hybrid diradicals are modeled by coupling Blatters monoradical with various known stable organic radicals. A few of them turned out to be quite promising candidates for the building block of OMMs.