(BMI)3LnCl6 Crystals as Models for the Coordination Environment of LnCl3 (Ln = Sm, Eu, Dy, Er, Yb) in 1‑Butyl-3-methylimidazolium Chloride Ionic-Liquid Solution

A series of (BMI)3LnCl6 (Ln = Sm, Eu, Dy, Er, Yb) crystals was prepared from solutions of LnCl3 dissolved in the ionic liquid, 1-butyl-3-methylimidazolium chloride (BMICl). Crystals with Ln = 5% Sm + 95% Gd and with Ln = 5% Dy + 95% Gd were also grown to assess the importance of cross-relaxation in the Sm and Dy samples. The crystals are isostructural, with monoclinic space group P21/c and four formula units per unit cell. The first coordination sphere of Ln3+ consists of six Cl– anions forming a slightly distorted octahedral LnCl63– center. The second coordination sphere is composed of nine BMI+ cations. The emission spectra and luminescence lifetimes of both (BMI)3LnCl6 crystals and LnCl3 in BMICl solution were measured. The spectroscopic similarities suggest that crystalline (BMI)3LnCl6 provides a good model of the Ln3+ coordination environment in BMICl solution.