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

Magnetic coupling modulation in meta-nitroxide-functionalized isoalloxazine magnets with redox-active units as efficient side-modulators

Magnetic conversion can be accomplished in a variety of ways, as organic molecules with switchable magnetic characteristics offer numerous technological applications. It is crucial to find magnetism-switchable systems because, in the field of organic magnetic materials, the redox-induced magnetic reversal is very simple to achieve and shows significant applications. Herein, we computationally design isoalloxazine-based diradicals through oxidizing N10 and adding a nitroxide to C8 as the spin source (i.e. 8-nitroxide-isoalloxazine 10-oxide, an m-phenylene-like nitroxide diradical expanded with a redox unit as a side-modulator) and its N1/N5-hydrogenated/protonated diradical derivatives and introducing substituents (-OH, -NH₂, and -NO₂) to C6. We demonstrate that the basically modified structure exhibits ferromagnetic (FM) characteristics with a magnetic coupling constant (J) of 561.3 cm^-1 calculated at the B3LYP/6-311+G(d,p) level, obeying the meta-phenylene-mediated diradical character, and dihydrogenation can lead to an AFM diradical with considerably large J (-976.1 cm^-1). Surprisingly, protonation at N1 or N5 can lead to distinctly different magnetic variations (561.3 → -1602.9 cm^-1 at N1 versus 561.3 → 379.1 cm^-1 at N5). Analyses indicate that small singlet-triplet energy gaps and small energy gaps between the highest occupied and lowest unoccupied molecular orbitals (HOMO, LUMO) of the closed shell singlet state are the key features of these isoalloxazine diradicals, and aromaticity variations, significant spin delocalization from the π-conjugated structure and spin polarization from the non-Kekule structure induced by modification are responsible for the magnetic conversion. Furthermore, the spin alternation rule, the singly occupied molecular orbital (SOMO) effect, and the SOMO-SOMO energy splitting of the triplet state are used to analyze these distinct variations. This work provides a novel understanding of the structures and characteristics of modified isoalloxazine diradicals, as well as essential details for the intricate design and characterization of new isoalloxazine-based potential organic magnetic switches.

Ultrafast Nonadiabatic Photoisomerization Dynamics Mechanism for the UV Photoprotection of Stilbenoids in Grape Skin

Natural UV photoprotection plays a vital role in physiological protection. It has been reported that UVC radiation can make resveratrol (RSV) and piceatannol (PIC) accumulate in grape skin. In this work, we demonstrated that RSV and PIC could significantly absorb UVA and UVB, and confirmed their satisfactory photostability. Furthermore, we clarified the UV photoprotection mechanism of typical stilbenoids of RSV and PIC for the first time by using combined femtosecond transient absorption (FTA) spectroscopy and time-dependent density functional theory (TD-DFT) calculations. RSV and PIC can be photoexcited to the excited state after UVA and UVB absorption. Subsequently, the photoisomerized RSV and PIC quickly relax to the ground state via nonadiabatic transition from the S₁ state at a conical intersection (CI) position between potential energy surfaces (PESs) of S₁ and S₀ states. This ultrafast trans-cis photoisomerization will take place within a few tens of picoseconds. As a result, the UV energy absorbed by RSV and PIC could be dissipated by an ultrafast nonadiabatic photoisomerization process.