Complexes (TBA+){(TMI-NPS)2·LnIIII4}− (Ln = Dy, Tb) with two axial photochromic spiropyran ligands: photoswitching and zero-field SIM behaviour with a high magnetization blocking barrier for the Dy complex

Abstract

Two photochromic spiropyran molecules (1,3,3-trimethylindolino-β-naphthopyrylospiran, TMI-NPS) react in the open form with Ln^III iodides (Ln = Dy, Tb) to form in the presence of TBAI crystalline (TBA⁺){(TMI-NPS)₂·Ln^IIII₄}⁻·0.5C₆H₄Cl₂ (1 and 2, respectively). Two TMI-NPS ligands are coordinated to lanthanides (Ln) by oxygen atoms at the axial position, and four iodide anions located at the equatorial position are weakly coordinated to Ln. Three types of anion complexes {(TMI-NPS)₂·Ln^IIII₄}⁻ with slightly different geometries were found. The Ln–O bond lengths are in the range 2.18–2.25 Å, while the Ln–I bond lengths are 3.06–3.09 Å. The geometry of the lanthanide complexes is favorable for the manifestation of single-ion magnetism (SIM). Compound 1 with Dy exhibits in zero magnetic field a slow magnetic relaxation with a high magnetization reversal barrier U_eff of 470 cm⁻¹. Magnetic hysteresis loops for 1 were recorded at 0.5–8 K, the divergence between zero-field and in-field cooling curves was also observed below 8 K, indicating a magnetization blocking temperature of 8 K. Ab initio calculations showed that the electronic structure of three types of Dy complexes is close and U_eff value of at least 760 cm⁻¹ is predicted for all of them, while the experimental value lies between the first (E₂ ≈ 300 cm⁻¹) and second (E₃ ≈ 600 cm⁻¹) excited Kramers doublets. The Tb compound 2 exhibits a SIM behavior only in an external magnetic field of 2000 Oe, magnetic hysteresis is not observed for it at 2 K. Ab initio calculations predicted significantly different energies of the lowest excited non-Kramers doublets and very small tunneling splitting of the ground doublet for the three types of Tb complexes, the latter is much smaller than their splitting in a 2000 Oe magnetic field. Photodissociation of the Dy complex was detected in solution under green light excitation (548 nm). The complex was recovered very slowly, but the irradiation with UV light at 365 nm significantly accelerates the complexation.