Controlling energy transfer routes in dye-sensitized lanthanide-based nanoparticles for enhanced emission

Near-infrared luminescent lanthanide (Ln)-doped nanomaterials are currently attracting high interest in view of their sharp f-f emission peaks and long luminescence lifetimes, which establish a unique value for the development of optical amplifiers, lasers and biosensors. To improve the optical pumping of the weakly absorbing lanthanide ions (Ln³⁺), the doped nanoparticles are coupled with an organic dye sensitizer able to efficiently harvest light and subsequently transfer the absorbed energy to the emitter. However, this through-space "remote" sensitization is severely subjected to energy losses due to competitive energy migration or deactivation routes limiting the overall luminescence quantum yields. The implementation of the Förster's model of resonance energy transfer on the basis of advanced ultra-fast transient absorption and photoluminescence spectroscopy with the support of density functional theory calculations demonstrate that the sensitization efficiency from the dye to the doped nanoparticle is strictly regulated by the geometry and localization of the transition dipole moment of the dye molecule. Within the nanoparticle, the energy transfer pathways can be harnessed through the spatial confinement of â€energy bridges', accepting energy from the surface dyes and donating to core emitters. We show that the FITC (fluorescein-isothiocyanate) dye allows reaching exceptional sensitization efficiency close to unity for the NIR-emitting triad Nd³⁺, Er³⁺ and Yb³⁺.