Protonation-Enhanced Antiferromagnetic Couplings in Azobenzene-Bridged Diradicals

Proton-induced magnetic enhancement in an organic diradical is an appealing phenomenon. Here, taking two nitroxide groups as spin sources, we predict the magnetic properties of the trans and cis forms of azobenzene (AB)-bridged diradicals in which the central −N═N– unit can undergo single protonation to convert to its protonated counterpart or vice versa. The calculated results for these two pairs of diradicals (protonated versus unprotonated trans and cis forms) indicate that the signs of their magnetic coupling constants J do not change, but the magnitudes remarkably increase after protonation from −716.4 to −1787.1 cm–1 for the trans form and from −388.1 to −1227.9 cm–1 for the cis form, respectively. Such noticeable magnetic enhancements induced by protonation are mainly attributed to the strong mediating role of the coupler AB between two radical groups through its lowest unoccupied molecular orbital (LUMO) with a lower energy level after protonation. The planar structure for the protonated trans dir...