Abstract
A family of homodinuclear Ln3+ (Ln3+ = Gd3+, Eu3+) luminescent complexes with the general formula [Ln2(β-diketonato)6(N-oxide)y] has been developed to study the effect of the β-diketonato and N-oxide ligands on their thermometric properties. The investigated complexes are [Ln2(tta)6(pyrzMO)2] (Ln = Eu (1·C7H8), Gd (5)), [Ln2(dbm)6(pyrzMO)2] (Ln = Eu (2), Gd (6)), [Ln2(bta)6(pyrzMO)2] (Ln = Eu (3), Gd (7)), [Ln2(hfac)6(pyrzMO)3] (Ln = Eu (4), Gd (8)) (pyrzMO = pyrazine N-oxide, Htta = thenoyltrifluoroacetone, Hdbm = dibenzoylmethane, Hbta = benzoyltrifluoroacetone, Hhfac = hexafluoroacetylacetone, C7H8 = toluene), and their 4,4′-bipyridine N-oxide (bipyMO) analogues. Europium complexes emit a bright red light under UV radiation at room temperature, whose intensity displays a strong temperature (T) dependence between 223 and 373 K. This remarkable variation is exploited to develop a series of luminescent thermometers by using the integrated intensity of the 5D0 → 7F2 europium transition as the thermometric parameter (Δ). The effect of different β-diketonato and N-oxide ligands is investigated with particular regard to the shape of thermometer calibration (Δ vs T) and relative thermal sensitivity curves: i.e.. the change in Δ per degree of temperature variation usually indicated as Sr (% K–1). The thermometric properties are determined by the presence of two nonradiative deactivation channels, back energy transfer (BEnT) from Eu3+ to the ligand triplet levels and ligand to metal charge transfer (LMCT). In the complexes bearing tta and dbm ligands, whose triplet energy is ca. 20000 cm–1, both deactivation channels are active in the same temperature range, and both contribute to determine the thermometric properties. Conversely, with bta and hfac ligands the response of the europium luminescence to temperature variation is ruled by LMCT channels since the high triplet energy (>21400 cm–1) makes BEnT ineffective in the investigated temperature range.
Highlights
During the past decade the increasing use of noncontact techniques for temperature measurements has led to many research efforts in the design of innovative lanthanide-based luminescent thermometers.[1−8] Lanthanide metal−organic frameworks (LOFs) and coordination polymers (CPs), in particular, due to their thermal stability[2,9−17] have been extensively explored as luminescent thermometers in a wide temperature range, from ∼600 K down to cryogenic temperatures (
Conditions to an image taken at a known reference condition.[36−38] Recently, we focused on lanthanide coordination chemistry with the heterotopic divergent N-oxide ligand 4,4′-bipyridine N-oxide.[39−41] The different affinities of 4f metal ions toward O- and N-donor ligands afford the synthesis of lanthanide dinuclear complexes with the composition [Ln2(β-diketonate)6(bipyMO)x] (x = 2, 3 depending on the β-diketonate) where the oxygen atom of bipyMO bridges two lanthanide ions and the nitrogen donor atom is not coordinated.[41−44] In this context, we developed a family (16 members) of homodinuclear Eu3+ and Gd3+ luminescent compounds to study the effect of the β-diketonato and N-oxide ligands on the thermometric properties of the complexes
We studied the correlations between temperature-dependent emission and chemical composition in a family of homodinuclear Eu3+ complexes of the general formula [Eu2(βdiketonato)6(pyrzMO)x] and [Eu2(β-diketonato)6(bipyMO)x] (x = 2 for β-diketonato = dbm, bta, and tta; and x = 3 for βdiketonato = hfac)
Summary
During the past decade the increasing use of noncontact techniques for temperature measurements has led to many research efforts in the design of innovative lanthanide-based luminescent thermometers.[1−8] Lanthanide metal−organic frameworks (LOFs) and coordination polymers (CPs), in particular, due to their thermal stability[2,9−17] have been extensively explored as luminescent thermometers in a wide temperature range, from ∼600 K down to cryogenic temperatures (
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