New NIR-emissive tetranuclear Er(iii) complexes with 4-hydroxo-2,1,3-benzothiadiazolate and dibenzoylmethanide ligands: synthesis and characterization

Abstract

New tetranuclear heteroleptic complexes [Er₄(dbm)₆(O-btd)₄(OH)₂] (1) and [Er₄(dbm)₄(O-btd)₆(OH)₂] (2) (O-btd = 4-hydroxo-2,1,3-benzothiadiazolate and dbm = dibenzoylmethanide) and their solvates with toluene, THF and CH₂Cl₂ were prepared using two synthetic approaches. The structures of the products were confirmed by single-crystal X-ray diffraction. Magnetic properties of 1 and 2 are in good agreement with X-ray data. The effective magnetic moment (μ_eff) values at 300 K for 1 and 2 corresponds to a system of 4 non-interacting Er(III) ions in the ground state ⁴J_15/2 with g = 6/5. At ambient temperature and upon excitation with λ_exc = 450 nm, complexes 1 and 2 exhibit luminescence at ∼1530 nm, i.e. in the near infra-red (NIR) region. The luminescence intensity grows with increasing the number of the (O-btd)⁻ ligands in the complexes. This observation suggests (O-btd)⁻ as a new efficient antenna ligand for the lanthanide-based NIR luminescence.