Intriguing light-emission features of ketoprofen-based Eu(III) adduct due to a strong electron–phonon coupling

Lanthanide-based coordination compounds of molecular formula [Ln(keto)₃(H₂O)], where Ln=Gd, Eu and keto=ketoprofen, were synthesized and their photoluminescence features were detailed in the 12–300 K interval. The emission spectra of [Eu(keto)₃(H₂O)] reveal a strong dependence with the temperature, well-illustrated by an unusual large blue-shift (∼17 cm⁻¹) of the ⁵D₀→⁷F₀ transition as the temperature is raised. The arithmetic difference between the absorption spectrum of [Eu(keto)₃(H₂O)] with that of [Gd(keto)₃(H₂O)] pointed out the presence of a ligand-to-metal charge transfer (LMCT) band (320–370 nm). From the [Gd(keto)₃(H₂O)] phosphorescence the energy of the keto-related triplet states was determined showing a large overlap with the intra-4f⁶ levels and a strong electron–phonon coupling. The high-resonance between the LMCT and triplet states and the Eu³⁺ levels leads to a strong dependence of the ⁵D₀ lifetime value, typical of the presence of operative energy transfer processes. - Highlights: • Luminescence characterization of [Eu(keto)₃(H₂O)] and [Gd(keto)₃(H₂O)]. • Unusual large blue-shift (∼17 cm⁻¹) of the ⁵D₀→⁷F₀ transition, 12-300 K. • High-resonance between the LMCT and triplet states and the Eu³⁺ levels. • Strong electron–phonon coupling and strong dependence of the ⁵D₀ lifetime value.