Sensitization of a europium complex by hydroxybenzoic acid and benzophenone: optical and solvatochromic insights

This study reports the synthesis, characterization, and behavior of a novel europium(III) complex, based on 3,5-dihydroxybenzoic acid (dic) and 2-hydroxy-4-methoxy-benzophenone (bzf) ligands. Fourier Transform Infrared Spectroscopy (FTIR) analysis confirmed Eu³⁺ coordination via the ligands carbonyl groups, forming chelate rings. The proposed composition, [Eu(dic)₂(bzf)(DMF)(H₂O)], was supported by Eu³⁺ titration and elemental analysis. Density Functional Theory (DFT) and Time-Dependent Density Functional Theory (TD/DFT) calculations provided theoretical support for the structural and electronic features observed experimentally. The emission spectrum revealed characteristic Eu³⁺ f-f transitions (⁵D₀→⁷F₀,₄), with the hypersensitive ⁵D₀→⁷F₂ transition dominating over ⁵D₀→⁷F₁. Judd-Ofelt analysis yielded Ω₂ and Ω₄ parameters of 20.23 × 10⁻²⁰ cm² and 2.46 × 10⁻²⁰ cm², respectively, indicating both asymmetry and partial covalency in the Eu–ligand bonding. The luminescence lifetime (0.20 ms) further confirms efficient photophysical properties. The triplet energy level (20,451 cm⁻¹) was determined using an isostructural gadolinium(III) complex, confirming that bzf ligand plays a central role in sensitizing Eu³⁺ emission. To further explore the spectroscopic characteristic of the complex, a solvatochromic study was performed in solvents of varying polarities. Analysis using the Lippert–Mataga approach revelead a strong linear correlation (R² = 0.94) between pseudo-Stokes shift and solvent polarity, suggesting dominant dielectric solute–solvent interactions. These findings highlight, the complex’s potential for optical sensing applications in diverse solvent environments.