Hydrogen-Bonding-Assisted Substituent Engineering for Modulating Magnetic Spin Couplings and Switching in m-Phenylene Nitroxide Diradicals

Rational modification of the coupler for the theoretical design of molecular magnets has attracted extensive interest. Substituent insertion is a widely used strategy for adjusting molecular properties, but its effect and modulation on magnetic spin couplings have been less investigated. In this work, we predict the magnetic properties of the design m-phenylene nitroxide (NO) diradicals regulated by introducing substituents. The calculated results for those two pairs of diradicals indicate that the signs of their magnetic coupling constants J do not change, but the magnitudes remarkably change after substituent regulation in the range from 253 to 730 cm^–1. Such noticeable magnetic changes induced by introducing subsituents are mainly attributed to different electronic effects of substituents, assisted by the proximity of two NO groups, good planarity, conjugation, and an intramolecular hydrogen bond. In particular, the insertion of intramolecular H-bonds not only indicates an electronic effect but also has greatly changed the spin density distribution. Further aromaticity of the coupler ring, spin densities, and molecular orbitals and energetics was evaluated to gain a better understanding of magnetic regulation. Interestingly, further protonation of some substituents (e.g., −NO₂ and −CO₂) can noticeably turn the spin coupling from ferromagnetic to antiferromagnetic, showing manipulable magnetic switching. This work provides a promising strategy based on substituent engineering for magnetic spin coupling modulation, not only turning the coupling magnitude but also enabling the magnetic switching, thus providing insights into molecular magnetic manipulation for spintronics applications.