Unexpected luminescent and thermal properties of novel tetrakis Eu3+- indandionate complex

Abstract

Luminescent coordination compounds based on trivalent europium ions (Eu3+) have found wide range of applications in new technologies, such as optoelectronics, molecular thermometers, and biomedical devices. Applications of this materials may be associated with narrow emission bands, which are arise from intra-configurational Laporte forbidden 4f – 4f transitions (5D0 → 7FJ). Furthermore, Eu3+ ion can act as a powerful spectroscopic probe and the 5D0 → 7F2 transition are strongly sensitive to small angular variations in the coordination polyhedron. However, the energy structures of the organic ligands in the complexes play the most important role on the Eu3+ luminescence sensitization process1. In this context, this work reports about theoretical, syntheses, characterization, and photoluminescence studies of a series of tetrakis complexes containing tetraethylammonium, Et4N+[Ln(L)4]– (Et4N+: tetraethylammonium cation, Ln: Gd and Eu, and L: 2-acyl-1,3-indandionate). The Judd–Ofelt intensity parameter (Ωλ), lifetime (τ), radiative (Arad) and non-radiative (Anrad) coefficients, and intrinsic quantum yield ΩEu/Eu) values were calculated for different temperatures (80 - 475 K) (Table 1). The Et4N+[Eu(isovind)4]– complex, where isovind: 2-isovaleryl-1,3-indandonate) shows an extraordinarily high value of the intensity parameter (Ω2 = 73.5×10-20 cm2, considering the index of refraction equal to 1.5) and radiative decay rate (Arad = 2.468×103 s-1) at 300 K. These optical results show an abnormally high emission intensity of the 5D0 → 7F2 transition (Figure 1), leading to the highest measured Ω2 value of the europium materials, to the best our knowledge, reported in the literature. In addition, this complex shows different solid phases at 367 and 460 K, leading significant changes in the band profile assigned to the 5D0 → 7F2 transition. The spectroscopic study of luminescent systems has shown very interesting and promising results for applications, such as OLED devices and luminescent thermometers based on Ln3+ ions.