Synthesis and luminescent properties of new molecular compounds of divalent lanthanides LnCl2·0.5H2O (Ln = Yb, Sm, Tm, and Eu)

Abstract

For the first time, in addition to the previously studied EuCl2·0.5H2O, new complexes of divalent lanthanides with the composition LnCl2·0.5H2O (Ln = Yb, Sm, Tm) were synthesized by reacting a suspension of LnCl3·6H2O in toluene with iBu3Al in an argon atmosphere. All complexes of Ln2+ were isolated in solid form and characterized using standard methods: chemical analysis for the determination of Cl– according to Volhard method, complexometry for Ln2+ and Al3+, and elemental analysis for C, H, O. The luminescent properties of Ln2+ complexes were studied: the excitation and photoluminescence (PL) spectra were measured, as well as the lifetimes (τ) of the excited states of the Ln2+* ions. When excited by UV–visible light, the Ln2+ complexes luminesce in the blue (Eu2+), green (Yb2+), red (Sm2+), and orange (Tm2+) region of the spectrum. The PL intensity decreases in the series Eu2+ > Sm2+ ≫ Tm2+ > Yb2+; the bright PL in the case of europium and samarium is visible to the naked eye. The PL of Ln2+ complexes is due to 4fn-15d1 → 4fn interconfigurational transitions and manifest itself as diffuse maxima in the PL spectra and nanosecond lifetimes of excited ions: Eu2+* (λem = 488 nm, τ = 322 ns), Yb2+* (λem = 548 nm, τ = 3.9 ns), Sm2+* (λem = 704 nm, τ = 3.6 ns), Tm2+* (λem = 596 nm, τ = 4.5 ns). Significantly higher PL intensity and lifetime of the Eu2+* ion compared with other Ln2+* are explained by the lower probability of nonradiative deactivation of Eu2+* due to the larger energy gap between the excited 5d1 and ground 4f7 states of the Eu2+ ion. Compounds LnCl2·0.5H2O are quite resistant to moisture and air. The PL spectra of the complexes remain unchanged for more than 6 months under conditions of limited access to oxygen and air moisture. Using the example of EuCl2·0.5H2O, for the first time for divalent lanthanides, the effect of enhancement of the PL intensity of the Ln2+ ion upon contact with water vapor was found.