Doublet Charge Transfer Emission of Cerium(III) Complexes with Strong Electron-Accepting Ligands

Trivalent cerium (Ce(III)) complexes with doublet emission are promising materials for application in displays and sensor technologies. However, the requirements of strong electron-donating ligands for efficient doublet emission cause design limitations of the complexes. In this study, we propose a strategy enabling doublet charge transfer (2CT) emission from Ce(III) using strong electron-accepting ligands. Eight-coordinated Ce(III) complexes comprising electron-accepting anionic ligands (hexafluoroacetylacetonate (hfa)) and neutral ligands: water (Ce(III)-hfa-H2O) and triphenylphosphine oxide (Ce(III)-hfa-tppo) were synthesized. Their emission properties are investigated by using time-resolved emission spectroscopy, revealing characteristic green emission bands for both complexes. Comparative studies with the trivalent gadolinium (Gd(III)) and the tetravalent cerium (Ce(IV)) complexes, supported by quantum chemical calculations, confirm that the green emission originates from a CT transition from π* to 4f orbitals. These findings provide valuable insights into molecular doublet luminescence science and open new avenues for designing efficient Ce(III)-based emitters.