Lanthanoid/Alkali Metal β‐Triketonate Assemblies: A Robust Platform for Efficient NIR Emitters

Abstract

The reaction of hydrated lanthanoid chlorides with tribenzoylmethane and an alkali metal hydroxide consistently resulted in the crystallization of neutral tetranuclear assemblies with the general formula [Ln(Ae⋅HOEt)(L)4 ]2 (Ln=Eu(3+) , Er(3+) , Yb(3+) ; Ae=Na(+) , K(+) , Rb(+) ). Analysis of the crystal structures of these species revealed a coordination geometry that varied from a slightly distorted square antiprism to a slightly distorted triangular dodecahedron, with the specific geometrical shape being dependent on the degree of lattice solvation and identity of the alkali metal. The near-infrared (NIR)-emitting assemblies of Yb(3+) and Er(3+) showed remarkably efficient emission, characterized by significantly longer excited-state lifetimes (τobs ≈37-47 μs for Yb(3+) and τobs ≈4-6 μs for Er(3+) ) when compared with the broader family of lanthanoid β-diketonate species, even in the case of perfluorination of the ligands. The Eu(3+) assemblies show bright red emission and a luminescence performance (τobs ≈0.5 ms, ${{\Phi}{{{\rm L}\hfill \atop {\rm Ln}\hfill}}}$≈35-37 %, ηsens ≈68-70 %) more akin to the β-diketonate species. The results highlight that the β-triketonate ligand offers a tunable and facile system for the preparation of efficient NIR emitters without the need for more complicated perfluorination or deuteration synthetic strategies.

Visible and Near-Infrared Emission from Lanthanoid β-Triketonate Assemblies Incorporating Cesium Cations

“…Our initial results highlighted the serendipitous discovery of discrete bimetallic tetranuclear assemblies of formulation [Ln(Ae• HOEt)(tbm) 4 ] 2 , where Ae + is an alkali-metal cation and tbm − is the tribenzoylmethanide ligand. 29 Our studies have shown that this assembly is consistently obtained upon variation of the lanthanoid cation (Eu 3+ , Er + , and Yb 3+ ), alkali-metal cation (Na + , K + , or Rb + ), and alcoholic solvent (ethanol or n-butanol). Despite the structural novelty of these species, the most remarkable discovery occurred on investigating the photophysical properties of these lanthanoid assemblies.…”

The Impact of the Alkali Metal Ion on the Crystal Structure and (Mechano)luminescence of Terbium(III) Tetrakis(β‐diketonates)

“…This tendency agrees with the reported decreasing of luminescence intensity in [Ln(M · EtOH)(β‐tridiketonate) 4 ], when the geometry of [LnO 8 ] core has changed from TDD‐8 (for M = Cs, Rb) to SAPR‐8 (for M = Na). [7a] Authors attributed the luminescence quenching in [Ln(Na · EtOH)(β‐triketonate) 4 ] to the closer proximity of the solvent ethanol molecule coordinated with alkali metal ion to the Ln center. [7a] In our case, alkali metal coligands (H 2 O, MeOH) cannot have such effect because lanthanide ion is well shielded by both trifluoromethyl and branched dialkoxy substituents of diketonate anions.…”

“…[7a] Authors attributed the luminescence quenching in [Ln(Na · EtOH)(β‐triketonate) 4 ] to the closer proximity of the solvent ethanol molecule coordinated with alkali metal ion to the Ln center. [7a] In our case, alkali metal coligands (H 2 O, MeOH) cannot have such effect because lanthanide ion is well shielded by both trifluoromethyl and branched dialkoxy substituents of diketonate anions. Comparison of the photophysical properties of complex 4 and [(TbL 3 )(LiL)(H 2 O)][11b] supports this argument (Table ).…”

“…The lanthanide(III) tetrakis(β‐diketonates) with the general formula (cation)[Ln(β‐diketonate) 4 ] have been extensively studied over the past decades . In their structure, four β‐diketonate ligands form the saturated coordination sphere around Ln III ion.…”

Recent Trends Concerning Upconversion Nanoparticles and Near-IR Emissive Lanthanide Materials in the Context of Forensic Applications

“…[19] Research in this area has expanded to b-triketonates in an effort to boost the NIR efficiency of Er 3þ and Yb 3þ complexes. [22][23][24] This has been effective in boosting the NIR emissive output, an outcome justified as stemming from the removal of methylene C-H oscillators in close proximity to the Ln centre. [22] Furthermore, these complexes exhibit very long luminescent lifetimes relative to that of their b-diketonate counterparts.…”