Luminescence of lanthanide complexes in mesoporous silica matrices: a textbook example of tris-dipicolinate europium complex adsorbed in SBA-15

Incorporation of luminescent lanthanide complexes in inorganic matrices opens appealing possibilities in the rational upstream conception of new luminescent materials with easier recyclability. In this regard, the influence of SBA-15 mesoporous silica, as a host matrix, on the photophysical properties of Europium-trisdipicolinate cesium salt was investigated. The lanthanide complex can be completely and reversibly adsorbed on the silica, using incipient wetness impregnation (IWI) in water, without the need of anchoring groups or covalent bonding on the silica surface. This specific procedure allows the homogeneous dispersion of the lanthanide complex into the host silica matrix. Appropriately assessing the photophysical properties of the targeted luminescent material proved remarkably challenging, demanding utmost caution, particularly due to the strong scattering of the mesoporous matrices. Additionally, while our observations confirm an important increase of luminescence lifetimes of lanthanide complexes upon integration into these mediums, we attribute this variation to the differing refractive indexes and not to specific surface interactions or confinement effect. Indeed, it appeared that previous literature precedents depicted erroneous exhalations of the complexes intrinsic quantum yield due to the repeated use of erroneous refractive index values. In the meantime, the luminance of these materials under UV irradiation improves drastically in comparison to what is obtained with the physical dispersion of micro-crystals of the lanthanide complex onto silica. We demonstrate that this improvement is due to a reduced inner filter effect in the adsorbed samples. In this regards, IWI of lanthanide complexes in mesoporous matrices method could be of interest in the development of recyclable emitting layers, less demanding in terms of emitter quantity.