Room-temperature luminescent lanthanide ionic liquids for optical tagging and anti-counterfeiting.

The development of robust, scalable materials for next-generation anti-counterfeiting technologies requires the integration of unique optical signatures with practical processability. Here, we report a modular platform of room-temperature luminescent ionic liquids based on trihexyl(tetradecyl)phosphonium salts of lanthanide tris(dipicolinate) complexes (Ln = Eu, Tb, Dy, Sm). These materials combine the sharp, line-like f-f emission characteristics of lanthanides with the inherent advantages of ionic liquids, including negligible vapor pressure, high miscibility, and facile formulation. Crucially, the liquid nature enables direct volumetric mixing of discrete emitters, allowing rapid generation of binary, ternary, and quaternary systems with dense, information-rich emission spectra. The intrinsic emission signatures of each lanthanide are retained in the liquid state, while mixture systems exhibit enhanced spectral complexity without requiring additional synthetic steps. Thermal and calorimetric analyses confirm the preservation of stable room-temperature liquid behaviour, supporting compatibility with established processing and printing technologies. This approach decouples spectral complexity from synthetic complexity, establishing a scalable route to tailorable photonic security materials that offer significant potential for advanced optical tagging, secure identification, and functional soft-material design.