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.