Ferromagnetic coupling between 4f- and delocalized π-radical spins in mixed (phthalocyaninato)(porphyrinato) rare earth double-decker SMMs

Abstract

Mixed (phthalocyaninato)(porphyrinato) rare earth double-decker complexes M^III(Pc)(TNP) (M = Tb (1), Dy (2), and Y (3); Pc = phthalocyaninate; TNP = tetranaphthylporphyrinate) were synthesized and characterized by elemental analysis and a range of spectroscopic methods. They represent the first examples of double-decker rare earth complexes containing a tetranaphthylporphyrin ligand. The neutral radical state of 1–3 was revealed by electronic absorption measurements, as indicated by the characteristics involving the semi-occupied molecular orbitals (SOMOs). The IR spectra show a marker band of a phthalocyanine radical anion (Pc˙⁻) at 1312–1314 cm⁻¹, suggesting the high population of a π-radical electron on the Pc ring in the double-decker complexes 1–3. The magnetic behavior of both 1 and 2 features a slow relaxation of magnetization at low temperature under a zero direct current field, demonstrating the SMM nature of such lanthanide–radical systems. In particular for Dy^III(Pc)(TNP) (2), the quantum tunneling of magnetization (QTM) was suppressed at an applied direct current (dc) field of 2000 Oe, leading to the extraction of an effective energy barrier of U_eff = 15.5 cm⁻¹ (22.3 K) and τ₀ = 7.74 × 10⁻⁷ s. The magnetic exchange couplings were analysed by means of high-frequency electron paramagnetic resonance (HF-EPR) measurements. The results clearly show the ferromagnetic exchange coupling between 4f- and delocalized π-radical spins in Tb^III(Pc)(TNP) (1) and Dy^III(Pc)(TNP) (2), with exchange coupling constants (J_Ln-rad) of 3.43(10) and 2.74(10) cm⁻¹, respectively.