Remarkable Differences in Spin Couplings for Various Self-Paired Dimers of Ring-Expansion-Radicalized Uracil: A Basis for the Design of Magnetically Anisotropic Assemblies

External Electric Field Effects on AFM-like Magnetism in Nitroxide-Functionalized C+:C Base Pair

In this work, we computationally designed a set of nitroxide diradical base pairs (rC⁺:rC) to propose promising magnetic building blocks for spintronic or magnetic molecular materials. C⁺:C12 is found to possess a considerably large antiferromagnetic-like (AFM-like) spin-coupling magnitude (J = -3286.681 cm^-1) and sensitive magnetic responses to the external electric field. Especially, the presence of the Y direction field that is oriented perpendicular to intermolecular hydrogen bonds has the greatest influence on the magnetic exchange interaction (J being from -2549.578 to -4231.286 cm^-1, ΔJ_Y = 1681.708 cm^-1), which could be understood by two simultaneously occurring effects. On the one hand, the external electric field in the -Y direction can regulate the charge polarization of negative and positive electrostatic potentials on C12 moiety and further facilitate the spin transport property. On the other hand, with increasing electric field strength on the -Y axis, the spin density on diradical sources diminishes and that on the coupler increases, which can lead to a homogenous spin-density distribution. The achieved understanding provides a new strategy for designing self-assembly magnetic nanomaterials or nanodevices and enhancing the AFM coupling through the assistance of external electric field.

Rational magnetic modification of N,N-dioxidized pyrazine ring expanded adenine and thymine: a diradical character induced by base pairing and double protonation

Rational modification of biomolecules especially DNA base pairs for the theoretical design of molecular magnets has attracted extensive interest. In this work, we report a modification strategy for adenine/thymine-based magnets through introducing a N,N-dioxidized pyrazine ring to either adenine or thymine to form ring-expanded bases (noA/noT) based on their experimentally synthesized derivatives. Further functionalization is conducted by double protonation and pairing with a normal complementary base (nohA-T/nohT-A), respectively. The diversity of protonation sites in noA generates totally six nohA-Ts, together with nohT-A forming seven two-step modified topic base pairs. DFT calculations are performed to characterize the magnetic properties and the diradical character, which indicate three diamagnetic (DM) nohA-Ts and three antiferromagnetic (AFM) nohA-Ts with extremely large magnetic coupling constants J ranging from -1279.7 to -2807.4 cm-1, while a relatively mild AFM nohT-A with a J of -194.6 cm-1. The electron separation effect induced by attraction of positive charges originating from protonation is proposed to explain the diradicalization, which is different from the traditional radical-coupler-radical coupling mode. In addition, atomic natural charges and spin densities, and H-bond and molecular orbital analyses are further discussed for verification and deep understanding of the observed unique phenomena. It should be noted that our designed seven topic base pairs have excellent characters including a good synthetic basis, a large scope of the |J| values, and the AFM-DM magnetic conversion or AFM strength modulation controlled by protonation/deprotonation, prototropic tautomerization, base pairing/dissociation, single proton transfer, and even the applied electric field. All these indicate the promising applications in the field of magnetic information storage or switch control. This work highlights the magnetic modification schemes and possible modulation methods of double positive charge doped DNA base pairs by utilizing their potential spin coupling modes.