Dual-Channel Energy Transfer from Ligand-Centered Singlet and Triplet States to Trivalent Lanthanide Ions

Efficient light harvesting and subsequent energy transfer (EnT) to trivalent lanthanide ions (LnIII) are critical for achieving high-performance luminescent LnIII complexes. However, because of the complexity of the underlying dynamic processes, in-depth investigations of their sensitization mechanisms are still limited. In this work, we systematically studied three neutral LnIII complexes (GdIIIL, TbIIIL, and EuIIIL), all featuring the same carbostyril ligand, using steady-state and transient absorption and emission spectroscopy. Our study revealed efficient intersystem crossing (ISC) from the ligand-centered lowest singlet excited state (S1) to the ligand-centered lowest triplet excited state (T1) in GdIIIL, as well as dual-channel EnT from both ligand-centered S1 and T1 to the metal-centered excited state (LnIII*) in TbIIIL and EuIIIL. More importantly, the competition between ISC and EnT from the ligand-centered S1 determines the predominant sensitization pathway, resulting in the main EnT from T1 in TbIIIL, while EnT is from S1 in EuIIIL. These findings provide deeper insights into the sensitization mechanisms of LnIII complexes and offer valuable guidance for the design of new LnIII-based luminescent materials.