Rational Design of Magnetic DNA Motifs with Diradical Character: Nitroxide Functionalization of Nucleobases

In the current work, the nitroxide radical groups are utilized to functionalize the nucleobases, obtaining the nucleobase diradical building blocks for magnetic DNA with significant ferromagnetic or antiferromagnetic coupling characteristics. The nitroxide functionalization strategies include introduction of nitroxide radical group to the carbon site and oxidization of the amino group in nucleobases, and the diradical-functionalized nucleobases are denoted by ^2NOX, where X = A, G, T, and C bases. The density functional theory calculations reveal that these nitroxide diradicalized nucleobases are stable and have large magnetic spin coupling magnitudes. Almost all of them possess antiferromagnetic-like spin coupling characteristics with considerably large spin coupling constants [J = −671.7 (^2NOA1), −463.3 (^2NOA3), −370.5 (^2NOG), −494.9 (^2NOC1), −3265.5 (^2NOT), and −2445.5 cm^–1 (^2NOC3)] expect for ^2NOC2 and ^2NOA2 which have the ferromagnetic-like spin coupling characteristics (J = 149.1 and 440.7 cm^–1), respectively. The spin alternation rule works well for these nitroxide-diradicalized nucleobases in interpreting the magnetic spin coupling characters although such heterocyclic nucleobases (purine and pyrimidine) are as the couplers, and the spin coupling constants present good linear relationships with the highest occupied molecular orbital–lowest unoccupied molecular orbital energy gaps and the energy gaps between the closed-shell singlet and triplet state of these nucleobase diradicals. Besides, their magnetic coupling properties are also analyzed by the shape of the singly occupied molecular orbitals (SOMOs) and SOMO–SOMO energy splitting of the triplet state, the H-bonding with their complementary nucleobases, and the nitroxide radical group orientations. Clearly, this work provides a novel strategy for the rational design of the magnetic DNA motifs with well-defined diradical characters and also provides insights into the spin coupling interactions in these nucleobase-based magnet building blocks of the magnetic DNA nanowires.