Molecular size matters : ultrafast dye singlet sensitization pathways to bright nanoparticle emission

Dye-sensitized luminescent lanthanide (Ln)-based nanoparticles enable broad applications spanning from fluorescent microscopy to biological therapy. However, the limited understanding of the dye -> Ln(3+) 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 -> Ln(3+) or dye -> Ln(1)(3+)-> Ln(2)(3+) energy transfer pathways in the ultrafast time scale. Steady-state and time-resolved data, supported by density functional theory calculations, demonstrate that Ln(3+) 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 (