Molecular Size Matters: Ultrafast Dye Singlet Sensitization Pathways to Bright Nanoparticle Emission

Abstract Dye‐sensitized luminescent lanthanide (Ln)‐based nanoparticles enable broad applications spanning from fluorescent microscopy to biological therapy. However, the limited understanding of the dye→Ln³⁺ sensitization process still leaves ample room for the improvement of its efficiency. In this work, a unique combination of photoluminescence and transient absorption spectroscopy is employed to reveal the hereto hidden dye→Ln³⁺ or dye→Ln₁³⁺→Ln₂³⁺ energy transfer pathways in the ultrafast time scale. Steady‐state and time‐resolved data, supported by density functional theory calculations, demonstrate that Ln³⁺ sensitization is realized directly from the singlet excited state of dye molecules and is strictly regulated by a distance‐dependent regime overcoming the role of the donor–acceptor spectral overlap for the size and geometry of dye molecules. It is shown that exceptionally high efficiency is achieved by judiciously selecting small‐sized dye molecules with localized molecular orbitals sitting close (<0.5 nm) to the nanoparticle surface. This new understanding will enable a rational design of dye‐sensitized Ln nanoparticles allowing for a dramatic improvement of the emission efficiency in a variety of nanomaterials for light conversion.