LMCT facilitated room temperature phosphorescence and energy transfer in substituted thiophenolates of Gd and Yb

Abstract

To obtain luminescent lanthanide complexes with a low energy LMCT state the 2-(2′-mercaptophenyl)benzothiazolates, Ln(SSN)₃, and 2-(2′-mercaptophenyl)benzoxazolates, Ln₂(OSN)₆ (Ln = Gd, Yb), were synthesized by the reaction of amides Ln[N(SiMe₃)₂]₃ with respective thiophenols. Ytterbium complexes were structurally characterized by X-ray diffraction analysis. Cyclic voltammetry revealed that the deprotonated mercaptophenyl ligands have significantly lower oxidation potentials than their phenoxy analogues and some β-diketones. The photophysical properties of Gd and Yb compounds were studied both in solution and in the solid state. The fluorescence spectra of the compounds in solution display the bands of the keto and enol forms of the ligands. No energy transfer from the organic part to Yb³⁺ has been detected in solutions of both Yb complexes, whereas in solids an intense metal-centered emission in the near infrared region was observed. The solid Gd compounds exhibited room temperature phosphorescence caused by unusually efficient intersystem crossing facilitated by the essentially reducing properties of OSN and SSN ligands. To explain the sensitization process occurring in solids Yb₂(OSN)₆ and Yb(SSN)₃ a specific non-resonant energy transfer mechanism via a ligand to metal charge transfer state has been proposed. Based on the Yb derivatives, NIR-emitting OLEDs with 860 μW cm⁻² maximal irradiance were obtained. Their Gd counterparts showed bright electrophosphorescence (up to 1350 cd m⁻²) in the devices containing doped emission layers.