Understanding and Controlling the Emission Brightness and Color of Molecular Cerium Luminophores

Molecular cerium complexes are a new class of tunable and energy-efficient visible- and UV-luminophores. Understanding and controlling the emission brightness and color are important for tailoring them for new and specialized applications. Herein, we describe the experimental and computational analyses for series of tris(guanidinate) (1-8, Ce{(R₂N)C(N ⁱPr)₂}₃, R = alkyl, silyl, or phenyl groups), guanidinate-amide [GA, A = N(SiMe₃)₂, G = (Me₃Si)₂NC(N ⁱPr)₂], and guanidinate-aryloxide (GOAr, OAr = 2,6-di- tert-butylphenoxide) cerium(III) complexes to understand and develop predictive capabilities for their optical properties. Structural studies performed on complexes 1-8 revealed marked differences in the steric encumbrance around the cerium center induced by various guanidinate ligand backbone substituents, a property that was correlated to photoluminescent quantum yield. Computational studies revealed that consecutive replacements of the amide and aryloxide ligands by guanidinate ligand led to less nonradiative relaxation of bright excited states and smaller Stokes shifts. The results establish a comprehensive structure-luminescence model for molecular cerium(III) luminophores in terms of both quantum yields and colors. The results provide a clear basis for the design of tunable, molecular, cerium-based, luminescent materials.