Homoleptic
Lanthanide 1,2,3-Triazolates <sub>∞</sub><sup>2–3</sup>[Ln(Tz*)<sub>3</sub>] and Their Diversified
Photoluminescence Properties
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Abstract
The series of homoleptic lanthanide
1,2,3-triazolates <sub>∞</sub><sup>3</sup>[Ln(Tz*)<sub>3</sub>] (Ln<sup>3+</sup> = lanthanide
cation, Tz*<sup>–</sup> = 1,2,3-triazolate anion, C<sub>2</sub>H<sub>2</sub>N<sub>3</sub><sup>–</sup>) is completed by synthesis
of the three-dimensional
(3D) frameworks with Ln = La, Ce, Pr, Nd, and Sm, and characterization
by X-ray powder diffraction, differential thermal analysis-thermogravimetry
(DTA/TG) investigations and molecular vibration analysis. In addition,
α-<sub>∞</sub><sup>2</sup>[Sm(Tz*)<sub>3</sub>], a two-dimensional polymorph of 3D β-<sub>∞</sub><sup>3</sup>[Sm(Tz*)<sub>3</sub>], is presented including the single crystal structure. The
3D lanthanide triazolates form an isotypic series of the formula <sub>∞</sub><sup>3</sup>[Ln(Tz*)<sub>3</sub>] ranging from La to Lu, with the exception of Eu, which forms
a mixed valent metal organic framework (MOF) of different structure
and the constitution <sub>∞</sub><sup>3</sup>[Eu(Tz*)<sub>6+<i>x</i></sub>(Tz*H)<sub>2–<i>x</i></sub>]. The main focus of this work is
put on the investigation of the photoluminescence behavior of lanthanide
1,2,3-triazolates <sub>∞</sub><sup>3</sup>[Ln(Tz*)<sub>3</sub>] and illuminates that
six different luminescence
phenomena can be found for one series of isotypic compounds. The luminescence
behavior of the majority of these compounds is based on the photoluminescence
properties of the organic linker molecules. Differing properties are
observed for <sub>∞</sub><sup>3</sup>[Yb(Tz*)<sub>3</sub>], which exhibits luminescence properties
based on charge transfer transitions between the linker and Yb<sup>3+</sup> ions, and for <sub>∞</sub><sup>3</sup>[Ce(Tz*)<sub>3</sub>] and <sub>∞</sub><sup>3</sup>[Tb(Tz*)<sub>3</sub>],
in which the luminescence properties are a combination of the ligand
and the lanthanide metal. In addition, strong inner-filter effects
are found in the ligand emission bands that are attributed to reabsorption
of the emitted light by the trivalent lanthanide ions. Antenna effects
of varying efficiency are present indicated by the energy being transferred
to the lanthanide ions subsequent to excitation of the ligand. <sub>∞</sub><sup>3</sup>[Ce(Tz*)<sub>3</sub>] shows a 5d-4f induced intense blue emission upon excitation
with UV light, while <sub>∞</sub><sup>3</sup>[Tb(Tz*)<sub>3</sub>] shows emission in the
green region of the visible spectrum, which can be identified with
4f-4f-transitions typical for Tb<sup>3+</sup> ions