Azulene‐Moiety‐Based Ligand for the Efficient Sensitization of Four Near‐Infrared Luminescent Lanthanide Cations: Nd3+, Er3+, Tm3+, and Yb3+

Abstract

The ML₄ complexes formed by reaction between the bidentate azulene-based ligand diethyl 2-hydroxyazulene-1,3-dicarboxylate (HAz) and several lanthanide cations (Pr³⁺, Nd³⁺, Gd³⁺, Ho³⁺, Er³⁺, Tm³⁺, Yb³⁺, and Lu³⁺) have been synthesized and characterized by elemental analysis, FT-IR vibrational spectroscopy and electrospray ionization mass spectroscopy. Spectrophotometric titrations have revealed that four Az⁻ ligands react with one lanthanide cation to form the ML₄ complex in solution. Studies of the luminescence properties of these ML₄ complexes demonstrated that Az⁻ is an efficient sensitizer for four different near-infrared emitting lanthanide cations (Nd³⁺, Er³⁺, Tm³⁺, and Yb³⁺); the resulting complexes have high quantum yield values in CH₃CN. The near-infrared emission arising from Tm³⁺ is especially interesting for biologic imaging and bioanalytical applications since biological systems have minimal interaction with photons at this wavelength. Hydration numbers, representing the number of water molecules bound to the lanthanide cations, were obtained through luminescence lifetime measurements and indicated that no molecules of water/solvent are bound to the lanthanide cation in the ML₄ complex in solution. The four coordinated ligands protect well the central luminescent lanthanide cation against non-radiative deactivation from solvent molecules.