Highly emissive dinuclear europium(iii) complex with heteroaryl β-diketone and fluxidentate pyrazine: dual role as UV converters and semiconductors

Abstract

Single component red light emissive complexes with single emissive centers are emerging as promising color converters for light emitting diodes (LEDs). In this work, a thiophene-based β-diketone ligand, 4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedione (TTBD), featuring a trifluorobutane group at the opposite position, was used to develop a dinuclear Eu(III) complex, Eu₂(TTBD)₆pyz (EuD), where pyz refers to pyrazine. Additionally, its mononuclear counterpart (EuM) and a binary analogue (EuA) were also prepared for comparative studies. The complexes were thoroughly characterized using IR, NMR, UV-vis, photoluminescence (PL) spectroscopy and thermogravimetric analysis (TGA). Experimental results demonstrated that a dinuclear Eu(III) complex displays excellent thermal stability and broad, strong excitation bands spanning 200–350 nm, monitored at the characteristic 612 nm emission. Under near-UV light excitation, the complex shows intense red luminescence, attributed to f–f transitions of the central Eu(III) ions. Importantly, red emission intensity of the dinuclear complex is significantly higher than its mononuclear analogues, indicating a synergistic effect between the two Eu(III) centers. The lack of ligand based emission in all complexes suggests efficient energy transfer from the attached sensitizer to the Eu(III) ion. Based on the emission spectrum, the CIE chromaticity coordinates (x = 0.63, y = 0.34) confirm the suitability of the complex as an effective red phosphor for white LED applications. The fluorescence lifetime measurements, along with estimation of triplet energy (T₁) level of TTBD (20 600 cm⁻¹), which is higher than the ⁵D₀ excitation level of Eu(III), support a ligand sensitized luminescence mechanism i.e. antenna effect. This conclusion is further supported by theoretical (JOES) and computational (DFT) studies, which also provided insights into the electronic density distribution. Altogether, these findings validate the potential of the synthesized complexes, particularly the dinuclear system, as efficient red components for integration in near-UV pumped white LEDs.