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

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Abstract
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 5 D 0 → 7 F 0 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.

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