Photoluminescent ionic metal halides based on s2 typed ions and aprotic ionic liquid cations

Photoluminescence (PL) plays an important role in our daily life as it could be applied to bioimaging, sensing, illumination, display, etc. Inorganic-organic hybrid materials that contain both organic and inorganic parts have widely emerged owing to the achievable dual functions from both organic and inorganic parts. Ionic liquids (ILs) have been widely used as a “green” solvent in the synthesis of functional materials. Especially, relatively chemically stable aprotic ILs have been widely emphasized owing to no proton-transfer and no (or low) volatility. s2 typed metal ions (e.g., Ge2+, Pb2+, Sn2+, Sb3+, Bi3+, Te4+) are potential emissive ions. They are easily coordinated with halogen ions to form s2 typed halometallates. Diverse assemblies of the aprotic ionic liquid cations (AILCs) with the s2 typed halometallate anions could result in a new class of inorganic–organic hybrid ionic metal halides (IOMHs) with excellent and tunable PL properties. This review summarizes the recent progress on the syntheses, structures, and PL properties of IOMHs with the formulas of [AILC]a[MbXcLd] (M = s2 typed metal ion, X = halogen, L = N or O-donor organic ligand). Firstly, two types of halometallate anions based on s2 typed metal ions are structurally classified, namely the pure inorganic halometallate anions and the one with coordinating N or O-donor organic ligands; the latter mainly are haloantimonates and halobismuthates. Furthermore, the structural characteristics of IOMHs consisting of s2 typed halometallates and AILCs are provided, followed by the detailed interpretations of their syntheses, structures and PL properties, the PL-structure relationships and luminescent mechanisms. Finally, an outlook about the main challenges and opportunities in this topic are proposed.