Heteronuclear Eu2Pt2 luminescent arrays: composition–thermometric properties correlations

Abstract

Lanthanide dinuclear complexes [Ln₂(tta)₆(μ-pyrzMO)₂] (Ln³⁺ = Eu³⁺ and Gd³⁺; Htta = thenoyltrifluoroacetone; pyrzMO = pyrazine-N oxide) can behave as ligands for the synthesis of molecular heterometallic complexes. Fast reactions at room temperature occur with [Pt(μ-Cl)Cl(EPh₃)]₂ (E = P, As) affording as a single product [Ln₂(tta)₆(μ-pyrzMOPtCl₂EPh₃)₂] (E = P, Ln = Eu, 1; Ln = Gd, 2); (E = As, Ln = Eu, 3; Ln = Gd, 4). Similarly, the new platinum compound [PtClppy(pyrzMO)], 5, obtained reacting [Pt(μ-Cl)ppy]₂ (ppy = 2-phenylpyridine) with pyrzMO and structurally characterized through XRD, has been used to prepare [Ln₂(tta)₆(μ-pyrzMOPtClppy)₂] (Ln = Eu, 6; Ln = Gd, 7) starting from the anhydrous formally unsaturated [Ln(tta)₃] fragments. XRD studies on all heterometallic compounds have established a tetranuclear Ln₂Pt₂ molecular structure with the platinum fragment coordinated to the nitrogen atom of the pyrazine while the N-oxide donor functionality is bridging two lanthanide centres as in the dinuclear lanthanide precursors. PL and thermometric properties of the Eu³⁺ complexes are strongly correlated with the nature of the ligand coordinated to Pt that is PPh₃, AsPh₃ and ppy. Upon UV excitation, 1 and 3 exhibit intense red emission, typical of Eu³⁺ ions, with a luminance of ∼10 cd m⁻² significantly higher than that of 6 (∼0.3 cd m⁻²). Temperature-induced variations in emission intensity have been exploited to develop a series of luminescent thermometers. The integrated intensity of the ⁵D₀ → ⁷F₂ europium transition can be used as the thermometric parameter (Δ). 1 and 3 reach S_r = 1 at around 200 K, while 6 exhibits a steeper decay in Δ, achieving S_r = 1 at approximately 100 K. For 1 and 3, the temperature sensitivity is related to non-radiative deactivation channels involving the platinum-containing fragment and tta ligands. For 6, instead, the interactions are limited only to the Pt-containing fragment.