Design of Molecular Lanthanide‐Based Quantum Light Sources for On‐Chip Integration

Lanthanide-based optical emitters are emerging as promising materials for quantum applications because of the long optical coherent lifetimes associated with their narrow emission lines. In this work, we investigate a series of lanthanide molecular complexes with a highly rigid tetrapodal benzimidazolic ligand (L) as potential highly coherent quantum light sources suitable for on-chip integration. The LnL complexes show sensitized Ln-centered emission in the visible and near-infrared spectral ranges with a well-resolved fine structure of the J sublevels. Remarkably, the EuL complex exhibits a single line related to the purely electric dipole 5D0→7F0 transition, of particular interest in quantum photonics. Notably, this formally forbidden line likely originates from an LMCT state, rather than from low symmetry or strong crystal-field effects. This mechanism enables the appearance of the band while maintaining a limited electric inhomogeneity of the ligand system, as supported by Judd–Ofelt analysis. These features contribute to the significant reduction of the room-temperature inhomogeneous linewidth (∼500 GHz) compared to typical zero-phonon lines of polyaromatic molecules in polymers. Importantly, these favorable properties persist in doped silica-based SiCO and PMMA films. Additionally, the doped SiCO film provides high excitation selectivity, minimal host autofluorescence, and broad color tunability.