The first registration of a green liquid-phase chemiluminescence of the divalent Eu2+* ion in interaction of β-diketonate complexes Eu(acac)3·H2O, Eu(dpm)3, Eu(fod)3 and Eu(CH3COO)3·6H2O with Bui2AlH in THF with the participation of oxygen

Abstract

The first liquid-phase green chemiluminescence (CL) of the divalent europium Eu²⁺* ion in a study of interactions in the systems EuL₃·(H₂O)_x–THF–Buⁱ₂AlH–O₂ (L = acac, dpm, fod, and CH₃COO; x = 0, 1, 6; Buⁱ = iso-C₄H₉) was discovered. The attack of Buⁱ₂AlH on the water of crystallization of complexes EuL₃·(H₂O)_x affords a luminoxane (Buⁱ₂Al)₂O, hydrogen, and isobutane. The reaction of Buⁱ₂AlH and EuL₃ moiety of the initial Eu³⁺ complexes leads to the not previously synthesized divalent europium complexes EuL₂·(THF)₂, which were isolated and characterized by elemental and spectral analyses in the present work. In the reaction solutions, these complexes are associated into bulky complexes with an excess of diisobutylaluminum hydride, EuL₂·(THF)₂–Buⁱ₂AlH. The measured CL and photoluminescence (PL) spectra of yellow-green reaction solutions in both cases consist of one broad band with λ_max = 555 ± 10 nm due to the emission of the electron-excited state of the Eu²⁺* ion (4f⁶5d¹ → 4f⁷ transition), a part of the bulky complex EuL₂·(THF)₂–Buⁱ₂AlH. The lifetime of the excited states, and PL and CL quantum yields of the Eu²⁺* ion in the EuL₂·(THF)₂–Buⁱ₂AlH complexes were determined. A mechanism (“indirect CL”) is proposed to explain the generation of the green CL. The primary emitter, triplet-excited isobutyric aldehyde ³R′C(H)O* (R′ = iso-C₃H₇), is formed in the disproportionation reaction of peroxyl radicals – intermediates of Buⁱ₂AlH oxidation by oxygen. The exited ³R′C(H)O* molecules transfer energy to Eu²⁺ ion converting it into the electronically exited state Eu²⁺*, which is then deactivated by emitting quanta of green light due to the 4f⁶5d¹ → 4f⁷ transition. It was found that for divalent Eu²⁺ ions, unlike trivalent Ln³⁺ ions, replacement of the inorganic anion-ligand Cl⁻ (complex EuCl₂·(THF)₂–Buⁱ₂AlH) with organic one L (complex EuL₂·(THF)₂–Buⁱ₂AlH) decreases the CL and PL intensity and leads to the green-shift of the luminescence maximum of Eu²⁺* from the blue (λ_max = 465 nm) to the green (λ_max = 555 nm) region. The high brightness and duration of the CL, visible to the naked eye in the system Eu(fod)₃–THF–Buⁱ₂AlH–O₂, make it promising as a chemical source of green light.