Distance-Independent Efficiency of Triplet Energy Transfer from π-Conjugated Organic Ligands to Lanthanide-Doped Nanoparticles

Lanthanide-doped nanoparticles (LnNPs) possess unique optical properties and are employed in various optoelectronic and bioimaging applications. One fundamental limitation of LnNPs is their low absorption cross-section. This hurdle can be overcome through surface modification with organic chromophores with large absorption cross-sections. Controlling energy transfer from organic molecules to LnNPs is crucial for creating optically bright systems, yet the mechanisms are not well understood. Using pump-probe spectroscopy, we follow singlet energy transfer (SET) and triplet energy transfer (TET) in systems comprising different length 9,10-bis(phenylethynyl)anthracene (BPEA) derivatives coordinated onto ytterbium and neodymium-doped nanoparticles. Photoexcitation of the ligands forms singlet excitons, some of which convert to triplet excitons via intersystem crossing when coordinated to the LnNPs. The triplet generation rate and yield are strongly distance-dependent. Following their generation, TET occurs from the ligands to the LnNPs, exhibiting an exponential distance dependence, independent of solvent polarity, suggesting a concerted Dexter-type process with a damping coefficient of 0.60 Å^-1. Nevertheless, TET occurs with near-unity efficiency for all BPEA derivatives due to the lack of other triplet deactivation pathways and long intrinsic triplet lifetimes. Thus, we find that close coupling is primarily important to ensure efficient triplet generation rather than efficient TET. Although SET is faster, we find its efficiency to be lower and more strongly distance-dependent than the TET efficiency. Our results present the first direct distance-dependent energy transfer measurements in LnNP@organic nanohybrids and establish the advantage of using the triplet manifold to achieve the most efficient energy transfer and best sensitization of LnNPs with π-conjugated ligands.