A Thiocyanate Complex of Cerium Showing Bright Fluorescence in Ambient Air: Crystal Structure and Photophysical Properties

Despite rapid advances in the research on luminescent Ce 3+ complexes, general design principles for maximizing the fluorescence quantum yield of molecular complexes of Ce 3+ remain undefined. Herein, we report the single‐crystal structure of a crystalline thiocyanate Ce 3+ complex that shows bright cyan fluorescence with a quantum yield up to 93% in ambient air. Sub‐centimeter sized crystals with the formula of [(CH 3 ) 4 N] 5 [Ce(NCS) 8 ]•2CH 3 CN are prepared. Absorption and fluorescence band maxima are observed at 347 and 452 nm, respectively, in solvent, indicating a Stokes shift of 105 nm (0.83 eV). Density functional theory (DFT) calculations of the complex [Ce(NCS) 8 ] 5− suggest that the absorption and fluorescence can be ascribed to the 5d–4f transition of Ce 3+, which includes a contribution from ligand‐to‐metal charge transfer in the excited state. This study demonstrates that the coordination of thiocyanate ligands to the Ce 3+ center can effectively suppress nonradiative deactivation of the 5d excited state. Moreover, our results indicate that thiocyanate is a useful and readily accessible ligand motif for achieving bright luminescence in Ce 3+ complexes under ambient conditions, providing a practical complement to previously explored ligand systems such as tris‐guanidinate, bis(pyrazolyl)borate, and poly(mercaptoimidazolyl)borate.