Photoluminescence of LaI3 switched on and off by association and dissociation of non-luminescent tetrahydrofuran

Abstract

Although luminescent lanthanide-containing molecular complexes are well-known, it is not the case for La³⁺ with no 4f or 5d electrons, and the few luminescent complexes of La³⁺ contain luminescent ligands. Herein, we report an unexpected photoluminescence phenomenon of the LaI₃(THF)₄ complex that was easily prepared by immersing commercial anhydrous LaI₃ in THF. Strong blue emission was observed when LaI₃(THF)₄ was subjected to UV irradiation at 254 nm, which contrasts the non-luminescence character of either LaI₃ or THF. The absolute emission quantum yield was determined to be 50.5%. The observed luminescence of LaI₃(THF)₄ results from metal-to-ligand charge transition (MLCT) which differs from the known charge transition types for lanthanum complexes according to DFT/TDDFT calculations. The photoluminescence of LaI₃ can be switched on and off by reversible THF association and dissociation. THF concentrations as low as 0.1 vol% in n-hexane and 0.5 vol% in the gas phase can be detected, indicating the capability of anhydrous LaI₃ as a sensor for convenient and rapid THF detection.