Photophysical properties and energy transfer mechanism of a praseodymium compound with a two-dimensional layer

Abstract

A hydrothermal reaction results in the formation of a novel [Pr₂(2,5-PA)₂(2,5-HPA)₂(H₂O)₄]_n·2nH₂O complex (2,5-H₂PA = 2,5-pyridinedicarboxylic acid). The complex is structurally characterized by single-crystal X-ray diffraction and crystallizes in the space group P2₁ of the monoclinic system with two formula units in one cell. This praseodymium complex is characterized by a two-dimensional layered structure. A solid-state photoluminescence experiment reveals that the praseodymium complex shows an emission in the red region. The complex has Commission Internationale de I’Éclairage chromaticity coordinates of 0.5495 and 0.4492. The photoluminescence emission bands could be assigned to the characteristic emission of the 4f electron intrashell transition of the ³P₀ → ³H₅, ¹D₂ → ³H₄, ³P₀ → ³H₆, ³P₀ → ³F₂, and ³P₁ → ³F₃ of the Pr³⁺ ions. The energy transfer mechanism is explained by the energy level diagrams of the praseodymium ions and the 2,5-H₂PA ligand. A solid-state diffuse reflectance measurement shows that the complex possesses a wide optical band gap of 3.48 eV.

Keywords

Commission Internationale de I’Éclairage crystal lanthanide photoluminescence praseodymium

A novel praseodymium complex is prepared and characterized. This praseodymium complex is characterized by a two-dimensional layered structure. It shows an emission in the red region, and it has Commission Internationale de I’Éclairage chromaticity coordinates of 0.5495 and 0.4492. The emission bands are resulted from the characteristic emission of the ³P₀ → ³H₅, ¹D₂ → ³H₄, ³P₀ → ³H₆, ³P₀ → ³F₂, and ³P₁ → ³F₃ of the Pr³⁺ ions. It possesses a wide optical band gap of 3.48 eV.

Introduction

In recent years, lanthanide compounds have been a research hotspot in materials and chemical sciences.^1–9 This is mainly because lanthanide compounds are widely used in sensors, luminous materials, magnetic materials, cell imaging, electroluminescent devices, luminous probes, catalysts, and many other fields.^10–19 These broad applications have resulted from the versatile properties of lanthanide compounds. Among these properties, photoluminescence is particularly attractive. The photoluminescence of lanthanide compounds is mainly caused by lanthanide ions because lanthanide ions are rich in 4f electrons. If 4f electrons can be transferred effectively, lanthanide compounds usually emit ideal photoluminescence. However, due to the low absorption coefficient of lanthanide ions, 4f electrons cannot be transferred effectively. Therefore, to promote the transition of 4f electrons of lanthanide ions and improve their absorption coefficient, many kinds of organic molecules with conjugated structures have been used as coordination ligands. Aromatic carboxylic acids and heterocyclic derivatives are such organic compounds. It is considered that these organic compounds can absorb light, then transfer light energy to the lanthanide ions, and promote the transition of 4f electrons, that is, the so-called “antenna effect.”^20,21 Compared with a large number of structures of photoluminescence properties, there are few studies on the properties of lanthanide semiconductors, which need to be further explored.²²

The 2,5-H₂PA is an aromatic carboxylic acid and a heterocyclic derivative. It is a linear molecule with two carboxyl groups at both ends and a nitrogen atom on the heterocycle. Therefore, 2,5-H₂PA has five coordination atoms, which enable it to bind to multiple metal ions to form different coordination configurations or extended structures. It is considered that lanthanide compounds containing 2,5-H₂PA ligands may have interesting extended structures and new physicochemical behavior. Recently, the crystal engineering of lanthanide compounds with 2,5-H₂PA ligand is studied in my laboratory. In this paper, the hydrothermal synthesis, crystal structure, photoluminescence, energy transfer mechanism, Commission Internationale de I’Éclairage (CIE), and solid UV-Vis diffusion reflectance of a new praseodymium compound [Pr₂(2,5-PA)₂(2,5-HPA)₂(H₂O)₄]_n·2nH₂O (1) are reported.

Results and discussion

The molecular structure of 1 is given in Figure 1. Single-crystal X-ray diffraction analysis unveiled that all the crystallographically independent atoms are at general positions. The crystal structure of 1 is composed of electronically neutral two-dimensional (2D) [Pr₂(2,5-PA)₂(2,5-HPA)₂(H₂O)₄]_n layers and lattice water molecules. There are two crystallographically independent praseodymium ions, and both are coordinated by two nitrogen atoms of two 2,5-PA ligands and seven oxygen atoms, of which two are from two coordinating water molecules and five are from five 2,5-pyridinecarboxylic acid anion (2,5-PA) ligands, yielding a mono-capped square anti-prism. The bond lengths of Pr–O are in the range of 2.441(5)–2.610(4) Å. The bond lengths of Pr–O are in the normal range and are comparable with those documented previously.^23,24 The bond lengths of Pr–N are in the range of 2.628(6)–2.751(6) Å. The bond angles of O–Pr–O are in the range of 66.12(16)°–145.46(16)°. The bond angles of O–Pr–N are in the range of 61.13(18)°–145.39(17)°. The bond angles of N(1)-Pr(1)-N(2) and N(3)-Pr(2)-N(4) are 71.17(18)° and 71.05(18)°, respectively.

Figure 1.

A molecular structure of 1 with hydrogen atoms and lattice water molecules being omitted for clarity.

In compound 1, four crystallographically independent 2,5-PA ligands can be grouped into two types. Two (N2- or N4-containing) are μ₂-bridging ligands, using nitrogen and one oxygen atom to coordinate to one praseodymium ion. Differently, the other two (N1- or N3-containing) are μ₄-bridging ligands, using nitrogen and three oxygen atoms to coordinate to four praseodymium ions. The praseodymium ions are connected by two μ₄-bridging 2,5-PA ligands to form a one-dimensional (1D) chain running along the c-axis, as shown in Figure 2. The chains are further interconnected by the 2,5-PA ligands to give a 2D layer extending along the ac plane (Figure 3). The layers are interlinked via van der Waals interactions to construct a crystal packing diagram, as presented in Figure 4.

Figure 2.

A 1D chain runs along the c direction.

Figure 3.

A 2D layer viewed from different directions: (a) Viewed from the a-axis and (b) from the b-axis.

Figure 4.

A packing diagram of 1.

To our knowledge, praseodymium compounds^25,26 generally show photoluminescence emission. Based on this consideration and with efforts directed to discover the potential photoluminescent performance, solid-state photoluminescent measurements were conducted on complex 1 at room temperature. The experimental result of the photoluminescent measurement is given in Figure 5. When the sample was excited at a wavelength of 267 nm, the emission spectrum exhibited five bands in the region of 550–700 nm, locating at 552, 600, 610, 639, and 684 nm. The maximum emission is at 610 nm. These emission bands can be attributed to the ³P₀ → ³H₅, ¹D₂ → ³H₄, ³P₀ → ³H₆, ³P₀ → ³F₂, and ³P₁ → ³F₃ of the Pr³⁺ ions. The title complex has CIE chromaticity coordinates of 0.5495 and 0.4492 (Figure 6).

Figure 5.

Solid-state photoluminescent emission spectra of 1 measured at room temperature with λ_ex = 267 nm.

Figure 6.

The CIE chromaticity diagram and chromaticity coordinates of the photoluminescence emission spectrum of 1.

To reveal the energy transfer mechanism of the photoluminescence emission of the title compound, the phosphorescence emission spectrum of 2,5-H₂PA at 77 K was measured and is given in Figure 7. The onset of the emission spectrum of 2,5-H₂PA is around 525 nm, while the lowest triplet state energy of 2,5-H₂PA is 19,048 cm⁻¹. The energy difference between the lowest triplet state of 2,5-H₂PA and the resonant energy level of Pr³⁺ ions (³P₁, 21,100 cm⁻¹) is −2052 cm⁻¹, as presented in Figure 8. According to the intramolecular energy transfer theory proposed by Dexter²⁷ and Sato and Wada,²⁸ the intramolecular energy transfer efficiency mainly depends on two processes. One process is to transfer from the lowest triplet energy level of the ligand to the resonance energy level of the lanthanide ion through the resonance exchange interaction of Dexter. Another process is the reverse energy transfer from the lanthanide ion to the ligand by thermal inactivation. When the energy difference is too small, the reverse energy transfer process may be more likely to occur. These two energy transfer processes are obviously opposite, although they depend on the energy gap between the lowest triplet energy level of the ligand and the resonance energy level of the lanthanide ion. The intramolecular energy transfer theory points out that the assumed optimal energy gap should be about 2500–3500 cm⁻¹.²⁹ A small or large energy gap may weaken the photoluminescence properties of the material.

Figure 7.

Phosphorescence spectrum of 2,5-H₂PA carried out at 77 K with λ_ex = 357 nm.

Figure 8.

Schematic and partial energy level diagrams of the main energy absorption transfer and phosphorescence processes in 1 and the 2,5-H₂PA ligand.

Based on the above discussion, for compound 1, the energy gap is −2052 cm⁻¹ which is much smaller than 2500–3500 cm⁻¹. Therefore, compound 1 probably cannot exhibit a good photoluminescence performance. This is in good agreement with the photoluminescence emission spectrum of compound 1, as presented in Figure 5. As depicted in Figure 5, the five emission bands are wide and obtuse, that is, not well shaped. Such an observation is indicative of the conclusion that 2,5-H₂PA is not an ideal ligand to excite Pr³⁺ ions in compound 1, namely, not a suitable “antenna.” Nevertheless, 2,5-H₂PA is still an “antenna” because compound 1 actually shows the characteristic emission of the internal transition of 4f electrons of Pr³⁺ ions.

The solid-state UV-Vis diffuse reflection spectra of complex 1 were obtained by using powder samples at room temperature. The Kubelka–Munk function α/S = (1 − R)²/2R is used to process the solid diffuse reflection spectral data, where the parameter α refers to the absorption coefficient, S is the scattering coefficient which is actually independent of the wavelength when the particle size is larger than 5 µm, and R is the reflectivity. By extrapolating the linear part of the α/S absorption edge with the energy graph, the value of the optical band gap can be obtained. Solid UV-Vis diffuse reflection spectra show that complex 1 has a wide optical band gap of 3.48 eV, as shown in Figure 9, so complex 1 may be a candidate material for broadband gap semiconductors. The steep slope of the light absorption edge indicates that complex 1 would undergo a direct transition.³⁰

Figure 9.

Solid-state UV-Vis diffuse reflectance spectrum of 1.

Conclusion

In summary, a novel praseodymium compound with a 2D layer has been synthesized and characterized by single-crystal X-ray diffraction. A solid-state photoluminescence experiment revealed that complex 1 shows an emission in the red region. In addition, it has remarkable CIE chromaticity coordinates of (0.5495 and 0.4492). A solid-state diffuse reflectance measurement revealed that it possesses a wide optical band gap of 3.48 eV.

Experimental

Elemental microanalyses of carbon, hydrogen, and nitrogen were carried out on an Elementar Vario EL elemental analyzer. The infrared spectrum was measured on a PE Spectrum-One Fourier-transform infrared (FTIR) spectrophotometer over the frequency range 4000–400 cm⁻¹ by using the KBr pellet technique. A solid-state photoluminescence experiment was performed on an F97XP photoluminescence spectrometer. Solid-state UV-Vis measurements were conducted on a TU-1901 UV-Vis spectrometer. Powder X-ray diffraction (PXRD) patterns were carried out on a Bruker D8 Advance powder diffractometer using Cu-Kα (λ = 1.54056 Å) with a step size of 0.02°. The simulated powder pattern was calculated using single-crystal X-ray diffraction data and processed by the free Mercury v1.4 program provided by the Cambridge Crystallographic Data Centre, as shown in Figure 10.

Figure 10.

PXRD patterns for 1.

Synthesis and characterization of [Pr₂(2,5-PA)₂(2,5-HPA)₂(H₂O)₄]_n·2nH₂O (1)

All reagents are analytical grade and commercially available and were used without further purification. Complex 1 was synthesized by mixing Pr(NO₃)₃·6H₂O (1 mmol, 435 mg), 2,5-pyridinedicarboxylic acid (2 mmol, 334 mg), and distilled water (10 mL) in a 25-mL Teflon-lined stainless steel autoclave and heated at 413 K in an oven for 7 days. After cooling to room temperature, yellowish block-like crystals were obtained. The yield was 62% based on Pr(NO₃)₃·6H₂O. Anal. calcd for C₂₈H₂₆N₄O₂₂Pr₂: C, 31.96; H, 2.49; N, 5.32; found: C, 32.04; H, 2.52; N, 5.38. IR peaks (cm⁻¹): 3430(vs), 3121(w), 3100(w), 2773(w), 2679(w), 2625(w), 2512(m), 2361(w), 2340(w), 1973(m), 1704(s), 1657(s), 1608(vs), 1587(vs), 1552(s), 1487(m), 1400(vs), 1348(vs), 1291(s), 1174(m), 1140(w), 1118(w), 1040(s), 948(w), 892(w), 865(m), 805(s), 770(m), 745(m), 702(m), 652(m), 613(w), 565(w), 517(s), 452(w), and 413(w). The purity of the bulky sample was confirmed by PXRD (Figure 10).

X-ray crystallographic study

The X-ray diffraction dataset was measured on a SuperNova charge-coupled device (CCD) X-ray diffractometer by using a carefully selected single crystal of complex 1. The X-ray source is graphite monochromated Mo-Kα radiation. The diffraction data reduction and empirical absorption correction were performed with CrystalClear software.³¹ The crystal structure was solved by direct methods using Siemens SHELXTL^TM V5 software³² and refined with a full-matrix least-squares refinement on F². All non-hydrogen atoms were found based on the difference Fourier maps and anisotropically refined, while hydrogen atoms were theoretically generated and included in the structural factor calculations with assigned isotropic thermal parameters. The details of the crystal data collection and refinement are listed in Table 1, while selected bond lengths and bond angles are presented in Table 2. Crystallographic data for the structural analysis have been deposited with the Cambridge Crystallographic Data Centre, CCDC No. 1945135. 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: deposit@ccdc.cam.ac.uk or http://www.ccdc.cam.ac.uk).

Table 1.

Crystal data and structure refinement details.

Formula	C₂₈H₂₆N₄O₂₂Pr₂
Mr	1052.35
Color	Yellow
Crystal size (mm³)	0.10, 0.06, 0.05
Crystal system	Monoclinic
Space group	P2₁
a (Å)	9.1352(2)
b (Å)	19.6990(5)
c (Å)	9.2801(2)
β (°)	101.481(2)
V (Å³)	1636.58(7)
Z	2
2θ_max (°)	50
Reflections collected/unique (R_int)	9985/5566 (0.0284)
d_calcd (g cm⁻³)	2.136
μ (mm⁻¹)	3.047
F(000)	1032
T (K)	200.00(10)
R¹, wR²	0.0254, 0.0467
S	1.036
Δρ (max, min) (e Å⁻³)	0.642, −0.425

Table 2.

Selected bond lengths (Å) and bond angles (°).

Pr(1)-O(1W)
Pr(1)-O(2W)
Pr(1)-O(11)^a
Pr(1)-O(1)^b
Pr(1)-O(7)
Pr(1)-O(9)
Pr(1)-O(4)
Pr(1)-N(1)
Pr(1)-N(2)
Pr(2)-O(3W)
Pr(2)-O(4W)
Pr(2)-O(12)^c
Pr(2)-O(2)^d
Pr(2)-O(13)
Pr(2)-O(4)
Pr(2)-O(9)
Pr(2)-N(3)
Pr(2)-N(4)

2.459(6)
2.557(5)
2.497(4)
2.505(5)
2.441(5)
2.560(4)
2.598(4)
2.628(6)
2.742(6)
2.472(6)
2.506(6)
2.499(5)
2.500(4)
2.460(5)
2.564(4)
2.610(4)
2.648(6)
2.751(6)

O(11)^a-Pr(1)-O(9)
O(2W)-Pr(1)-O(9)
O(7)-Pr(1)-O(4)
O(7)-Pr(1)-N(1)
O(1W)-Pr(1)-N(1)
O(1)^b-Pr(1)-N(1)
O(7)-Pr(1)-N(2)
O(4)-Pr(1)-N(2)
N(1)-Pr(1)-N(2)
O(2)^d-Pr(2)-O(4)
O(4W)-Pr(2)-O(4)
O(13)-Pr(2)-O(9)
O(3W)-Pr(2)-N(3)
O(13)-Pr(2)-N(4)
O(3W)-Pr(2)-N(4)
O(12)^c-Pr(2)-N(4)
O(9)-Pr(2)-N(4)
N(3)-Pr(2)-N(4)

144.78(15)
76.80(15)
66.12(16)
95.53(18)
73.41(19)
145.39(17)
61.39(18)
103.14(17)
71.17(18)
145.46(16)
77.39(16)
66.31(15)
72.03(19)
61.13(18)
140.44(19)
73.33(18)
103.15(16)
71.05(18)

Symmetry transformations used to generate equivalent atoms: ^ax − 1, y, z − 1; ^bx + 1, y, z; ^cx − 1, y, z; ^dx + 1, y, z + 1.

Footnotes

Author’s note

This study is dedicated to Professor Jin-Shun Huang on the occasion of his 80th birthday.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The study was funded by Jiangxi Provincial Department of Education’s Item of Science and Technology (GJJ170637) and the Open Foundation (20180008) of the State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, CAS.

ORCID iD

Wen-Tong Chen

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Photophysical properties and energy transfer mechanism of a praseodymium compound with a two-dimensional layer

Abstract

Keywords

Introduction

Results and discussion

Conclusion

Experimental

Synthesis and characterization of [Pr2(2,5-PA)2(2,5-HPA)2(H2O)4] n ·2nH2O (1)

X-ray crystallographic study

Footnotes

Author’s note

Declaration of conflicting interests

Funding

ORCID iD

References

Synthesis and characterization of [Pr₂(2,5-PA)₂(2,5-HPA)₂(H₂O)₄]_n·2nH₂O (1)