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Unraveling the Ground State and Excited State Structures and Dynamics of Hydrated Ce3+ Ions by Experiment and Theory

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posted on 09.08.2018, 14:18 by Patric Lindqvist-Reis, Florent Réal, Rafał Janicki, Valérie Vallet
The 4f-5d transition of Ce3+ provides favorable optical spectroscopic properties such as high sensitivity and quantum yield, making it a most important dopant for lanthanide-activated phosphors. A key for the design of these materials with fine-tuned color emission is a fundamental understanding of the Ce3+ ground state and excited state structures and the dynamics of energy transfer. Such data is also crucial for deriving coordination chemistry information on Ce3+ ions in different chemical environments directly from their optical spectra. Here, by combining 4f-5d absorption and luminescence spectroscopy and highly accurate quantum chemical electronic structure calculations, we study the interplay between the local structure of Ce3+ in aqueous solutions and in crystalline hydrates, the strengths of Ce–O/Cl interactions with aqua and chloride ligands, and the resulting absorption and luminescence spectra. Experimental and theoretical absorption spectra of [Ce­(H2O)9]3+ and [Ce­(H2O)8]3+ with defined geometries provide a means for analyzing the equilibrium between these species in aqueous solution as a function of temperature (K(298) = 0.20 ± 0.03), while analyses of spectra of different aqua-chloro complexes reveal that eight-coordinate aqua-chloro complexes are present in solution at high chloride concentration. An intriguing feature in these systems concerns the large observed Stokes shifts, 5500–10 100 cm–1. By exploring the excited state potential energy surfaces with relativistic multireference calculations, we show that these shifts result from significant geometrical relaxation processes in the lowest 5d1 excited state. For [*Ce­(H2O)8]3+ the relaxation gives shorter Ce–O bonds and a Stokes shift of ∼5500 cm–1, while for [*Ce­(H2O)9]3+ the lowest 5d1 state results in a spontaneous dissociation of a water molecule and a Stokes shift of ∼10 100 cm–1. These findings are important for the understanding and optimization of luminescence properties of cerium complexes.