Highly luminescent Tb(iii) cluster for the sensitization of Eu(iii) through f → f energy transfer for LED applications

Abstract

A new class of Na–Ln nanoclusters was synthesized with the general formula [Ln₂L′₂(hfac)₄(tfa)₂]·Na₂L₂, where Ln = Tb (TbC), Eu (EuC) or Gd (GdC). The molecular structure was confirmed using the X-ray diffraction method. The steady-state luminescence and decay profiles for the Ln(III) clusters were investigated in detail. The TbC cluster displayed prominent, intense green emissions with a long excitation lifetime of 1.01 ms. The doping of Tb(III) and Eu(III) into a series of Tb_1−xEu_x clusters (x = 0.04–0.10) was investigated. The ET efficiency (η_T) and energy-transfer rate (k_ET) confirmed the energy-transfer process of Tb → Eu. The luminescence intensity and lifetimes for Eu(III) increased from 0.22 ms in pure EuC to 1.39 ms in the co-doped Tb_0.90Eu_0.10 cluster. Furthermore, the intrinsic quantum yield, ϕ_Eu, improved 6 times to 79% in the Tb_0.90Eu_0.10 cluster. This behavior was attributed to (i) the short intermetallic Tb⋯Eu distance of ∼4 Å, where the coordination spheres of Tb(III) and Eu(III) overlapped. (ii) Tb(III) played a vital role as a bridge to transfer energy from the ligand's triplet-state energy to the excited states of Eu(III), consequently avoiding any non-radiative deactivation pathways. Nevertheless, the co-doped compounds exhibited significant fine-tuning of colors and promising applications as colored phosphors for the fabrication of LED devices.