Azobenzene-bridged diradical janus nucleobases with photo-converted magnetic properties between antiferromagnetic and ferromagnetic couplings

We computationally design a series of azobenzene (AB)-bridged double radicalized nucleobases, a novel kind of diradical Janus-type nucleobases, and explore their spin coupling characteristics. Calculations prove that such diradical Janus-bases not only normally match with their complementary bases, but also exhibit well-defined diradical character with photo-convertible intramolecular magnetic couplings (antiferromagnetic vs. ferromagnetic). Combination of four radical nucleobases (rG, rA, rC, rT) and photoswitch AB can yield 10 diradical Janus-bases with different magnetic characteristics in which AB functions a bridge to mediate the spin coupling between two radical bases. The trans-form supports mild antiferromagnetic couplings with the spin coupling constants (J) ranging from -153.6 cm-1 to -50.91 cm-1 while the cis-form has weak magnetic couplings with ferromagnetic (0.22-8.50 cm-1 ) for most of them or antiferromagnetic (-0.77, -1.73, -3.30 cm-1 ) properties for only three. Further structural examination and frontier molecular orbital analyses indicate that the extended π conjugation for better spin polarization provides an effective through-π-bond pathway to mediate the spin coupling in the trans conformation while nonplanarity of the cis conformation weakens the through-bond coupling and causes a competitive through-space pathway and as an overall result inhibits the spin coupling between two spin moieties. Meanwhile, we also find that the J values of the cis conformation vary with their angle between the radical base and its linked phenylene. Furthermore, the magnetic properties of the diradical Janus-bases can be significantly increased by interacting with metal ions. They also maintain a good UV absorption characteristics and there is a clear redshift compared with AB. This work provides a promising strategy for the rational design of photo-convertible Janus-base magnets as the magnetism-tunable DNA building blocks. © 2018 Wiley Periodicals, Inc.