Proton-transfer regulated magnetic coupling characteristics in Blatter-based diradicals

Abstract

Stable organic diradicals that show robust intramolecular magnetic coupling interactions are pertinent building blocks for organic magnetic materials and the design of structure-controllable organic diradicals has great interest for practical application. Here we explore the magnetic coupling interactions of Blatter-based diradicals within the density functional theory regime by considering nitroxide (NO) and nitronyl nitroxide (NN) radical groups as spin sources attaching to different sites of the Blatter-radical core and –OH group as a proton source for protonating the Blatter radical core through intramolecular proton transfer (PT). Comparison of these two types of diradicals (with and without PT) reveals that PT has a considerable impact on magnetic spin coupling, as evidenced by J = −1510.9 to −1099.4 cm⁻¹ and J = 563.67 to 369.18 cm⁻¹, showing strong antiferromagnetic (AFM) and ferromagnetic (FM) exchange couplings, respectively, also indicating that PT can modulate the magnetic spin coupling interactions. The calculated results verify that the signs of magnetic coupling constants do not change, but their magnitudes significantly decrease after PT. In the structure, the better conjugation of the unprotonated NO-modified diradical and the reduced NO twist angle create a more effective spin transport, supporting the spin polarization, thus leading to strong spin coupling. Result analyses agree well with the spin alternation rule. Besides, by altering the linking position of radical groups, we not only find a distinctly modified magnetic coupling magnitude of Blatter/BlatterH⁺-based diradicals but also obtain considerable enhancement of the singlet–triplet energy gaps, ΔE_ST = 1.77–3.36 kcal mol⁻¹, with robust stability. Moreover, to endorse our conclusions, we contemplate different stable organic radical groups as spin sources. Furthermore, SOMO–SOMO splitting and spin polarization are also discussed to elucidate the magnetic coupling behaviors of these diradicals. This work contains invaluable information for the rational design of stable organic magnetic materials.