High red luminescence intensity under sunlight exposure of a PMMA polymer doped with a tetrakis Eu3+ β-diketonate complex containing a benzimidazolium counterion†

New tetrakis Eu³⁺ and Gd³⁺ β-diketonate complexes containing benzimidazolium (Bzim) as the counterion were synthesized by the one-pot method. The Bzim[Eu(tta)₄]·H₂O complex was further incorporated into a poly(methyl methacrylate) matrix (PMMA) at 1, 5, and 10% (w/w), which revealed highly desirable photonic features. The Eu³⁺ and Gd³⁺ complexes were characterized by elemental and thermal analyses, in addition to ESI-MS spectrometry, FTIR, and Raman spectroscopy. Single-crystal X-ray diffraction studies of the tetrakis Bzim[Eu(tta)₄]·EtOH complex revealed that the Bzim⁺ counteraction and EtOH molecules exhibited several intermolecular interactions with very short hydrogen bond distances between two [Eu(tta)₄]⁻ anion units. The PMMA:(1%) Bzim[Eu(tta)₄]-doped material was thermally stable up to 120 °C, which was close to the values found for the Eu³⁺-complex. Regarding the photoluminescence properties, either the Bzim[Eu(tta)₄]·H₂O or the doped films showed intense emission arising from the metal ion over a wide range of excitation wavelengths comprising UVA, UVB, and UVC regions. In addition, when the polymer films were exposed to sunlight radiation in an open external environment, the materials revealed a high Eu³⁺-centered red emission arising from the ⁵D₀ → ⁷F_J transition. The Bzim[Eu(tta)₄]·H₂O and Bzim[Eu(tta)₄]·EtOH complexes showed high absolute quantum yields (Q^L_Eu) of 56% and 70%, respectively, whereas the doped polymer films displayed only ∼38%. All materials exhibited a highly red monochromatic emission characteristic. We believe that such luminescent systems could be promising photonic materials with a wide excitation range, including UVA, UVB, UVC, and sunlight, acting as efficient light-converting molecular devices (LCMDs).