Influence of charge transfer states on lanthanide photophysics

The luminescence of lanthanide (Ln) ions underpins numerous domestic applications. However, its intriguing nature makes the design of Ln chromophores with tailored optical properties quite challenging. A key issue is the weak absorptivity of intraconfigurational f-f transitions which necessitates using the ion's surroundings for light harvesting and subsequent energy transfer on Ln excited states, a process termed antenna effect. Since its discovery in 1942, this concept has been extensively studied. Early simplistic models essentially involving one donor (e.g., triplet) state have evolved into more sophisticated descriptions proposed around the turn of the 21st century, incorporating multiple donor/acceptor states, mechanisms, and selection rules. Nevertheless, one critical class of electronic states — charge transfer (CT) states —, remains inadequately described, with the exception of well-characterized ligand-to-metal charge transfer (LMCT) states. Ligands have often a polar nature enabling intra-ligand charge transfer (ILCT) or/and ligand-to-ligand CT (LLCT) between adjacent ligands within the complex. More importantly many Ln chromophores possess intricate networks of weak interactions in their crystal structures. These interactions, though often overlooked, are structurally significant (e.g., in extended structures) and crucially modulate electronic structure, thereby influencing energy transfer processes and efficiency. The present critical review examines these underexplored CT states, highlighting their impact on luminescence tuning, and providing guidelines for their identification and classification. Following concise introductions to the antenna effect and LMCTs, the core of the review delves into ILCTs, LLCTs, and supramolecular CTs (stacking induced CTs, H-bond induced CTs, anion- and cation-π induced CTs) illustrating them with representative examples. Critical discussions shed light on the difficulties of identifying these states and a systematic road-map is proposed to address them.