Photophysical properties and energy transfer mechanism of three novel lanthanide upconverting materials (UCMs)

Dedicated to Professor Jin-Shun Huang on the occasion of his 80th birthday.
https://doi.org/10.1016/j.jssc.2019.121160Get rights and content

Highlights

  • It reports three thermally stable lanthanide upconverting materials.

  • The emission bands result from the characteristic emissions of the lanthanide ions.

  • They have remarkable CIE chromaticity coordinates.

  • Energy transfer mechanism is explained by the energy level diagram.

  • They have wide optical band gaps.

Abstract

Three 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) have been synthesized through hydrothermal reactions and structurally characterized by single crystal X-ray diffraction technique. These materials are crystallographically isostructural and are characteristic of three-dimensional (3-D) frameworks. The photoluminescence measurements with solid-state samples reveal that compounds 13 exhibit upconversion photoluminescence emissions in red, yellow and green region, respectively. The photoluminescence emission bands could be assigned to the characteristic emission of the 4f electrons intrashell transition of the 4G5/2 ​→ ​6H7/2, 4G5/2 ​→ ​6H9/2 of the Sm3+ ions in 1, 4F9/2 ​→ ​6H17/2, 4F9/2 ​→ ​6H15/2, 4F9/2 ​→ ​6H13/2 of the Dy3+ ions in 2, and 5G65I8, 5S25I8 of the Ho3+ ions in 3. An energy transfer mechanism is explained by the energy level diagrams of the lanthanide ions and the 2,5-H2PA ligand. Solid-state UV/Vis diffuse reflectance spectra reveal that they are potential wide optical band gap semiconductors with the band gaps being of 3.73, 3.56 and 3.45 ​eV for 13, respectively. TG curves reveal that all of them are very thermal stable with the onset temperature being of 219 ​°C–322 ​°C. The photoluminescence quantum yields of compounds 13 were determined to be 14.1%, 18.3% and 19.6%, respectively.

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.

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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 13 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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    • Series of novel lanthanide complexes with a ladder-shaped 1-D double chain: Preparation, structures and photophysical properties

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      Citation Excerpt :

      The different band gaps of complexes 1 ~ 3 are ascribed to their different lanthanide ions. The band gap of complex 1 is smaller than that of the samarium complex (3.73 eV) in the reference [38]. The band gap of complex 2 is larger than that of the gadolinium complex (3.52 eV) in the reference [39].

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