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(NiPr)2]xCeIII[N­(SiMe3)2]3–x (x = 0, 1-N; x = 1, 2-N, x = 2, 3-N; x = 3, 4) and [(Me3Si)2NC­(NiPr)2]xCeIII(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 5dz2 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.