3 years ago

Protonation-Enhanced Antiferromagnetic Couplings in Azobenzene-Bridged Diradicals

Protonation-Enhanced Antiferromagnetic Couplings in Azobenzene-Bridged Diradicals
Yuxiang Bu, Xinyu Song, Fengying Zhang
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 diradical and two reduced CCNN torsional angles due to protonation for the cis one are responsible for the significant magnetic enhancements. Protonation not only supports the development of π conjugation among the spin groups and coupler but also creates a very favorable condition for spin transmission through the coupler AB LUMO by lowering the LUMO energy level and improving spin polarization and charge delocalization and thus enhances the spin coupling effectively. In addition, different spin sources and linking modes of the radical groups are also considered to confirm our conclusions, and the possibilities of protonation of such diradical systems are further discussed. The studied diradicals could be the promising candidates for the rational design of magnetic molecular switches.

Publisher URL: http://dx.doi.org/10.1021/acs.jpcc.7b06429

DOI: 10.1021/acs.jpcc.7b06429

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