Chiral excited-state dynamics drive circularly polarized luminescence in a europium complex

Circularly polarized luminescence (CPL) from chiral molecules and materials underpins emerging applications in display technology, optical sensing, and quantum information science. However, achieving a high degree of circular polarization is an open challenge, partly because the chirality of electronic excited states remains difficult to resolve directly during their evolution. Here, ultrafast circular dichroism spectroscopy maps the complete electronic and chiral structural evolution of a prototypical CPL emitter, CsEu((+,-)-hfbc)4 (hfbc = 3-heptafluorobutyryl camphorate). Its exceptional degree of circular polarization results from the electronic coupling of the Eu(III)-centered luminescent transitions to the chiral environment of the helically arranged sensitizing ligands. Combined with transient absorption measurements and electronic structure calculations, we unravel the intra-molecular energy transfer to the metal-ion, and extract the chiral spectroscopic signature of all involved excited states through their ligand-centered circular dichroism. We find that excitation of the luminescent Eu(III)-ion increases the helical twist of the ligand sphere by 3.0±1.4◦ toward an ideal square-antiprismatic geometry, where the CPL performance is most favorable. More generally, the results show that our approach may resolve previously inaccessible chiral excited-state dynamics in emerging chiral optical materials.