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

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.铈离子（Ce3+）的4f-5d跃迁具备高灵敏度与量子产率等优异光学光谱特性，使其成为镧系激活磷光体中至关重要的掺杂剂。精准调控发光色彩的这类材料的设计核心，在于深入理解Ce3+的基态与激发态结构，以及能量转移动力学过程。此类数据对于直接通过光学光谱推导不同化学环境中Ce3+离子的配位化学信息同样至关重要。本研究结合4f-5d吸收与发光光谱技术，以及高精度量子化学电子结构计算方法，系统探究了水溶液与结晶水合物中Ce3+的局域结构、Ce与水配体及氯配体间的Ce-O/Cl相互作用强度，以及由此产生的吸收与发光光谱之间的内在关联。具有确定几何结构的[Ce(H₂O)₉]³+与[Ce(H₂O)₈]³+的实验与理论吸收光谱，为分析水溶液中这些物种随温度变化的平衡关系提供了有效手段（298K时平衡常数K=0.20±0.03）；而对不同水合氯配合物光谱的分析表明，高氯离子浓度下的溶液中存在八配位水合氯配合物。这类体系中一个引人关注的特征是观测到的大斯托克斯位移（5500~10100 cm⁻¹）。通过相对论多参考态计算探究激发态势能面，我们发现此类位移源于最低5d¹激发态中显著的几何弛豫过程。对于[*Ce(H₂O)₈]³+，该弛豫过程使Ce-O键长缩短，斯托克斯位移约为5500 cm⁻¹；而对于[*Ce(H₂O)₉]³+，其最低5d¹态会自发解离一个水分子，斯托克斯位移约为10100 cm⁻¹。上述发现对于理解与优化铈配合物的发光性能具有重要意义。