Photophysical properties and energy transfer mechanism of three novel lanthanide upconverting materials (UCMs)
Graphical abstract
Three novel lanthanide upconverting materials were reported. They feature thermally stable 3-D frameworks. They exhibit upconversion photoluminescence emissions and remarkable CIE chromaticity coordinates. Energy transfer mechanism is explained. Their optical band gaps are 3.73, 3.56 and 3.45 eV.
Introduction
Because many lanthanide materials possess fascinating physicochemical properties, such as excellent photoluminescent and magnetic performances, in recent years lanthanide materials have gained increasing attention from chemists and material scientists [[1], [2], [3], [4], [5], [6], [7], [8]]. As far as know, up to date many scientists have completed a large number of studies about lanthanide materials, for the sake of finding out the potential applications as electrochemical displays, magnetic materials, luminescent probes, light-emitting diodes (LEDs) and so forth [[9], [10], [11], [12], [13], [14]]. The fascinating photoluminescence performances of a lanthanide material are dominantly resulted from the abundant 4f electrons and orbits of the lanthanide (LN) ions. Generally speaking, lanthanide materials could emit ideal photoluminescence emission bands, on the condition that the 4f electronic transitions of the lanthanide ions could efficiently take place. However, under most circumstances the 4f electrons are difficult to transfer between the orbits, because lanthanide ions usually possess very low absorption coefficient to light energy.
For the purpose of making lanthanide materials emitting ideal photoluminescence emission bands, scientists generally introduce organic molecules as coordinating ligands to synthesize novel lanthanide compounds [15,16]. These organic molecules better own a conjugated structure, for instance, heterocyclic derivatives, aromatic sulfonic acids or aromatic carboxylic acids, etc [17,18]. Scientists deem that such an organic molecule can probably absorb ultraviolet light and transmit the energy to lanthanide ions. This is antenna effect [19]. Many factors could affect the efficient photoluminescence emissions of the lanthanide ions, such as the intersystem crossing quantum yields of the antenna ligands, the distances between the antenna ligands and the central lanthanide, the energy match between the antenna’s triplet state energy levels and the resonant energy levels of the central lanthanide ions, and so on. Amongst these factors, the energy match is the most vital one. According to the energy match theory [20], an efficient photoluminescence emission can be attained if the energy levels can match well to each other. Based on such an energy transfer and energy match theory, scientists can possibly predict the photoluminescence properties of lanthanide compounds.
Up to date, There are several similar molecules of 2, 5 - pyridinedicarboxylic acid such as 2,6-pyridinedicarboxylic acid, 4-hydroxypyridine-2, 6-dicarboxylic acid, and so on. For example, Martin et al. reported a series of mixed tetravalent uranium and trivalent lanthanide complexes associated with the 2,6-pyridinedicarboxylic acid ligand [21], Zou and Du reported two lanthanide-based complexes with the 4-hydroxypyridine-2, 6-dicarboxylic acid ligand [22], Qian et al. reported several lanthanide coordination polymers based on 4-oxo-1,4-dihydro-2,6-pyridinedicarboxylic acid ligand [23], and so forth. However, investigations for these complexes are mainly focused on the photoluminescence, magnetism and other properties, while studies on the optical band gap properties are very rare. Moreover, for these complexes the investigations on the energy transfer mechanism are also rare.
Since many years ago, exploring photoluminescent, magnetic and semiconductive lanthanide compounds have attracted my attention, for the sake of getting new findings into their crystal structures, magnetic, photoluminescence and semiconductive properties. In present work the hydrothermal syntheses, structures, photoluminescence, energy transfer mechanism, CIE chromaticity coordinates, semiconductive performances and thermal stability of a series of novel lanthanide upconverting materials [LN(2,5-HPA)(2,5-PA)]n (LN = Sm, 1; Dy, 2; Ho, 3; 2,5-H2PA = 2,5-pyridinedicarboxylic acid) with 3-D frameworks are reported. The crystal structure of compound 1 has been reported [24], but no other properties (photoluminescence, CIE chromaticity coordinates, energy transfer mechanism, solid-state UV/Vis diffuse reflectance spectra, TG, PXRD in this work) of this compound were reported.
Section snippets
Materials and characterization
The chemicals and reagents used for the syntheses of compounds 1–3 were commercially obtained and directly applied. Powder X-ray diffraction (PXRD) patterns were conducted on a Bruker D8 Advance powder diffractometer. Photoluminescence experiments were performed on a F97XP photoluminescence spectrometer. Solid-state UV/Vis diffuse reflectance spectra were carried out on a TU1901 UV/Vis spectrometer. Thermogravimetry (TG) measurements were conducted on a NETZSCH TG 209F3 analyzer in nitrogen
Crystal structures
The title compounds are crystallographically isostructural and are crystallized in the space group of Pbcn in the orthorhombic system with four formula units in each cell, as uncovered by the single-crystal X-ray diffraction analyses. In this section, only compound 1 is chose as an example to describe their crystal structures. The samarium atom is surrounded by six oxygen atoms from six 2,5-PA ligands and two nitrogen atoms from two 2,5-PA ligands to yield a distorted square antiprismatic
Conclusion
In summary three novel lanthanide compounds have been synthesized via hydrothermal reactions and their crystal structures and photophysical properties are explored. As revealed by the single-crystal X-ray diffraction analyses, they are crystallographically isostructural and feature a 3-D framework. These compounds show highly thermal stability as found by TG measurements. Moreover, these compounds exhibit upconversion photoluminescence emissions. Their photoluminescence emission bands are
Supporting information
Crystallographic data for the structural analysis have been deposited with the Cambridge Crystallographic Data Centre, CCDC-1848286, 1848287 and 1848288 for compounds 1, 2 and 3. Copies of this information may be obtained free of charge from the Director, CCDC, 12 Union Road, Cambridge, CBZ 1EZ, UK (Fax: +44-1223-336033; email: [email protected] or www: http://www.ccdc.cam.ac.uk).
CRediT authorship contribution statement
Wen-Tong Chen: Supervision.
Acknowledgments
It is support by Jiangxi Department of Education’s Item of Science and Technology (GJJ170637).
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