Eu³⁺‐tetrakis β‐diketonate complexes for solid‐state lighting application

Abstract

Eu 3+ -β-diketonate complexes are used, for example, in solid-state lighting (SSL) or light-converting molecular devices. However, their low emission quantum efficiency due to water molecules coordinated to Eu 3+ and low photostability are still problems to be addressed. To overcome such challenges, we synthesized Eu 3+ tetrakis complexes based on [Q][Eu(tfaa) 4 ] and [Q][Eu(dbm) 4 ] (Q1 = C 26 H 56 N + , Q2 = C 19 H 42 N + , and Q3 = C 17 H 38 N + ), replacing the water molecules in the tris stoichiometry. The tetrakis β-diketonates showed desirable thermal stability for SSL and, under excitation at 390 nm, they displayed the characteristic Eu 3+ emission in the red spectral region. The quantum efficiencies of the dbm complexes achieved values as high as 51%, while the tfaa complexes exhibited lower quantum efficiencies (28-33%), but which were superior to those reported for the tris complexes. The structures were evaluated using the Sparkle/PM7 model and comparing the theoretical and the experimental Judd-Ofelt parameters. [Q1][Eu(dbm) 4 ] was used to coat a near-UV light-emitting diode (LED), producing a red-emitting LED prototype that featured the characteristic emission spectrum of [Q1][Eu(dbm) 4 ]. The emission intensity of this prototype decreased only 7% after 30 h, confirming its high photostability, which is a notable result considering Eu 3+ complexes, making it a potential candidate for SSL. KEYWORDS dbm, energy transfer, light-emitting diodes, Sparkle/PM7 model, tfaa 1 | INTRODUCTION Lanthanide luminescent complexes are found in numerous applications, for example in luminescent sensors, solid-state lighting (SSL)and light-converting molecular devices (LCMD). [1][2][3] Focusing our attention on SSL, near-UV-to-red downshifting systems based on Eu 3+ complexes and applied as a coat to near-UV LEDs are an exciting alternative to solve the lack of commercial white light-emitting diodes (WLEDs) matching desirable correlated colour temperature (~4500 K) and colour rendering index (> 90) for indoor lighting. [4] However, the main challenges concerning the application of Eu 3+ complexes in SSL are: (i) choice of suitable ligands that exhibit high-energy transfer efficiency between their triplet states and the Ln 3+ emitting level; and (ii) how to overcome the low photo and thermal stability of the complexes. [5] To surpass these challenges, β-diketones such as 1,1,1-trifluoro-2,4-pentanedione (Htfaa) [6] and 1,3-diphenyl-1,3-propanedione (Hdbm) [7] are prominent due to their desirable thermal stability (200°C) and strong π-conjugated molecular moieties that enclose the energy of their triplet state to the Eu 3+ emitting level, enhancing the ion emission quantum efficiency. [8,9] Therefore, Eu 3+ emission quantum efficiency in such complexes is directly influenced by ligand absorption, energy transfer rate from ligand to Eu 3+ , as well as nonradiative losses from the emitting state. [10] /journal/bio 877 d [EuCl 3 ] = 0.054 mol L −1 SCHEME 1 Scheme of the complex syntheses and structural formulae of the rea...

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