Cerium Photosensitizers: Structure–Function Relationships and Applications in Photocatalytic Aryl Coupling Reactions

Two complete mixed-ligand series of luminescent CeIII complexes with the general formulas [(Me3Si)2NC­(N i Pr)2] x CeIII[N­(SiMe3)2]3–x (x = 0, 1-N; x = 1, 2-N, x = 2, 3-N; x = 3, 4) and [(Me3Si)2NC­(N i Pr)2] x CeIII(OAr)3–x (x = 0, 1-OAr; x = 1, 2-OAr, x = 2, 3-OAr; x = 3, 4) were developed, featuring photoluminescence quantum yields up to 0.81(2) and lifetimes to 117(1) ns. Although the 4f → 5d absorptive transitions for these complexes were all found at ca. 420 nm, their emission bands exhibited large Stokes shifts with maxima occurring at 553 nm for 1-N, 518 nm for 2-N, 508 nm for 3-N, and 459 nm for 4, featuring yellow, lime-green, green, and blue light, respectively. Combined time-dependent density functional theory (TD-DFT) calculations and spectroscopic studies suggested that the long-lived 2D excited states of these complexes corresponded to singly occupied 5d z 2 orbitals. The observed difference in the Stokes shifts was attributed to the relaxation of excited states through vibrational processes facilitated by the ligands. The photochemistry of the sterically congested complex 4 was demonstrated by C–C bond forming reaction between 4-fluoroiodobenzene and benzene through an outer sphere electron transfer pathway, which expands the capabilities of cerium photosensitizers beyond our previous results that demonstrated inner sphere halogen atom abstraction reactivity by 1-N.