Influence of the nature of the halide and triiodide anion on absorption and photoluminescence spectra of EuX2 (X = Cl, Br, I) solutions in tetrahydrofuran

Abstract

The photoluminescence (PL) spectra of solutions of commercial dihalides EuCl2 (λem = 417 nm) and EuBr2 (λem = 482 nm) in tetrahydrofuran C4H8O (THF), caused by emission from the excited state of the Eu2+* ion (transition 4f65d1 → 4f7), were measured for the first time. The red shift (65 nm) of the EuCl2 PL band relative to the EuBr2 band is attributed to the nephelauxetic effect. The PL quantum yields of EuCl2 (φ = 0.07 %) and EuBr2 (φ = 0.12 %), as well as the lifetimes of Eu2+* ions (194–213 ns) were determined. As a result of comparing the absorption spectrum of a solution of EuI2 in THF with the absorption spectra of model THF solutions of iodide anion (NaI), molecular iodine (I2) and triiodide ion (I3¯) (NaI + I2), it was discovered for the first time that commercial samples of EuI2 contain an impurity of triiodide anion (I3¯) in the form of europium triiodide Eu2+(I3¯)2, the content of which, according to the results of iodometric titration, is 0.63 %. When EuI2 is dissolved in THF, this impurity goes into solution and appears in the form of absorption bands at 295 and 366 nm and a weak “iodine” color. When this solution is exposed to metallic Eu or hydride iBu2AlH (iBu = iso-C4H9), both triiodide bands at 295 and 366 nm and the “iodine” color disappear and the absorption band of the Eu2+ ion appears at 337 nm. The absorption spectra of THF solutions of EuCl2 (λabs = 248, 320 nm) and EuBr2 (λabs = 263, 394 nm), containing broad bands due to the 4f7 → 4f65d1 transition, were also measured. Unlike EuCl2 and EuBr2, the Eu2+ maximum does not appear in the absorption spectrum of a solution of commercial EuI2 in THF, because it is masked by the much stronger bands of the triiodide anion (I3¯) at 295 and 366 nm. It has been established that PL of a solution of commercial EuI2 in THF is not detected due to the quenching of its emitter Eu2+* by triiodide ion because of electron transfer from the excited Eu2+* ion to the triiodide anion with the formation of Eu3+ and iodide ion I¯. When metallic europium is added to a solution of commercial EuI2 in THF, PL of the Eu2+ ion appears at 429 nm. This occurs as a result of the consumption of the PL quencher triiodide anion I3¯ in the reaction with Eu to form iodide ion. A similar situation is observed when iBu2AlH is added to a solution of commercial EuI2 in THF.