How the Energy Gaps between Partners Rule ns-Scale Sensitization and Back-Energy Transfer in Lanthanide(III) Complexes?

Abstract Molecular lanthanide complexes are widely utilized in optoelectronics and bioimaging due to their unique luminescence properties, which inherently rely on the light-harvesting antenna effect. While the ultrafast energy transfer (ET) dynamics of highly emissive ions like Eu3+ have been extensively investigated, mechanisms governed by uncommon, slower kinetic regimes remain largely unresolved. Here, we apply nanosecond transient absorption spectroscopy (ns-TAS) to investigate these ET processes within an isostructural series of lanthanide(III) complexes, with a primary focus on unraveling the singular sensitization kinetics of Yb3+. We provide direct kinetic evidence for Yb3+ sensitization, demonstrating a slow (∼40 ns) triplet-to-metal energy transfer. To contextualize this unusual kinetic regime, we contrast it with the corresponding Tb3+ complex. In the latter, the well-established reversible energy transfer (ET/BET) loop maintains a long-lived ligand triplet population that is highly susceptible to oxygen quenching, which consequently depletes the Tb3+ emissive state. In striking contrast, the irreversible nature of the Yb3+ energy transfer efficiently outcompetes intermolecular oxygen quenching despite its slow rate. These results elucidate the complex kinetic picture of lanthanide(III) sensitization, highlighting the energy gap as the fundamental parameter that governs both photosensitization kinetics and the subsequent photophysical pathways. This study establishes ns-TAS as a critical tool to unravel such long-lived excited-state dynamics.