Step-wise assembly of a dysprosium–viologen framework with photoswitchable slow magnetic relaxation under a zero dc field

Abstract

Lanthanide molecular nanomagnets featuring reversible photoresponsive characteristics and photomediated magnetic behavior are highly appealing for developing next-generation optics and memory devices though the rational assembly of such materials remains elusive. Herein, we demonstrate a step-wise approach, which employs a photochromic dysprosium–viologen compound {[Dy(ipbp)₂(H₂O)₃]·NO₃·2H₂O}_n (1) (H₂ipbp·Cl = (1-(3,5-dicarboxyphenyl)-4,4′-bipyridinium chloride)) as the building block and 2,5-pyrazinedicarboxylate dianions (2,5-pzdc²⁻) as bridges, giving rise to a new framework {[Dy(ipbp)₂(2,5-pzdc)_0.5(H₂O)]·5H₂O}_n (2) exhibiting a photochromic effect and slow magnetic relaxation under a zero dc field. This finding is notable and exciting, as the starting compound 1 shows no magnetic relaxation on its own. The ab initio calculations demonstrate that by replacing water molecules at the equatorial plane in 1 with the 2,5-pzdc²⁻ bridge, the longer Dy–N bond length and the less charged pyrazine nitrogen atom result in a weaker equatorial field strength, and the coordination geometry around the Dy³⁺ ion in 2 is thus closer to the ideal triangular dodecahedral geometry than in 1, leading to stronger axial magnetic anisotropy of the Dy³⁺ ion and switch-on of slow magnetic relaxation under a zero dc field. Further magnetic analysis of compound 2 before and after UV light irradiation reveals the weak ferromagnetic coupling between the photogenerated radicals and the Dy³⁺ ions, a slight inhibition of quantum tunnelling of magnetization, as well as a modest enhancement of the energy barrier for the slow magnetic relaxation, highlighting the photomodulation of the dynamic magnetic behavior.