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

Abstract

The reaction of the β-triketonate ligands tris(4-methylbenzoyl)methanide and tribenzoylmethanide with the trivalent lanthanoids Eu, Er, and Yb in the presence of Cs afforded polymeric structures where the repeating units are represented by bimetallic tetranuclear assemblies of formulation {[Ln(Cs)(β-triketonate)]}. The only exception is the structure formed by the reaction of tris(4-methylbenzoyl)methanide, Yb, and Cs, which yielded a polymeric assembly where the repeating units are mononuclear Yb complexes bridged by Cs cations. Photophysical measurements on the obtained materials confirmed efficient sensitization from the ligand excited states to the 4f* excited states of the three lanthanoids. According to transient absorption data, Er and Yb are sensitized via energy transfer from the triplet state of the β-triketonate ligands. On the other hand, energy transfer to Eu seems to occur via an alternative pathway, possibly directly via the singlet state or through ligand to metal charge transfer states. The emission measurements confirm efficient sensitization for all three lanthanoids and bright near-infrared emission for Er and Yb, a characteristic that seems to be linked to the specific chemical structure of the β-triketonate ligands.

Energy transfer between Eu3+ and Nd3+ in near-infrared emitting β-triketonate coordination polymers

“…However, only the cesium-containing coordination polymer [Cs(mtbm)] n was deposited (ESI †). 20 When the same procedure was followed for the hydrated NdCl 3 and tbmH with RbOH or CsOH, the formation of assemblies with formula [Nd(Rb)(tbm) 4 ] 2 and [Nd(Cs·2HOEt) (dbm) 4 ] n was found, respectively. The [Nd(Rb)(tbm) 4 ] 2 (6) complex presents a similar structure to the previously reported tetranuclear assemblies.…”

“…The tbmH and mtbmH molecules were synthesised according to the previously reported methodology. 20,39 Following a similar procedure to that previously reported for the preparation of {[Ln(Cs)(tbm) 4 ] 2 } (Ln 3+ = Eu, Er, Yb) and {[Ln(Cs) (mtbm) 4 ] 2 } n (Ln 3+ = Eu, Er), 20 one equivalent of hydrated LnCl 3 (Ln 3+ = Eu, Nd) was made to react with four equivalents of mtbmH and four equivalents of RbOH in ethanol. Slow evaporation of the solvent resulted in the formation of suitable crystals for X-ray diffraction, revealing the formation of coordination polymers with formula {[Ln(Rb)(mtbm) 4 ] 2 } n where Ln 3+ = Eu(1), Nd (2).…”

“…The resulting solids are isolated with variable degrees of solvation, which has been found previously for these Ln 3+ /Ae + based complexes. 19,20 The Eu 3+ /Nd 3+ mixed assemblies were synthesised in a similar fashion to the {[Ln(Rb)(mtbm) 4 ] 2 } n , except for the use of mixtures of hydrated EuCl 3 and NdCl 3 in molar ratios of Nd 3+ of 0.25 (3), 0.50 (4) and 0.75 (5).…”

“…The mdbmH precursor was synthesised following a previously reported procedure. 20 Lanthanoid assemblies. RbOH (21 mg, 0.02 mmol) was added to a mixture containing mtbmH (74 mg, 0.20 mmol) and hydrated LnCl 3 (18 mg, 0.05 mmol) in ethanol (10 mL).…”

“…These ligands have been found to support the formation of unique assemblies that display particularly enhanced ytterbium and erbium emission properties. Our previous studies with tris-benzoylmethane (tbmH) and tris(4-methylbenzoyl)methane (mtbmH) resulted in the isolation of tetranuclear assemblies and polymeric structures of formulation [Ln(Ae·HOEt)(tbm) 4 ] 2 (Ln 3+ = Eu 3+ , Er 3+ , Yb 3+ /Ae + = Na + , K + , Rb + ) 18,19 and {[Ln(Cs)(mtbm) 4 ] 2 } n (Ln 3+ = Eu 3+ and Er 3+ ), 20 respectively. In contrast, our initial attempts to isolate the corresponding neodymium analogues were not successful and their photophysical properties remained unknown.…”