Synthesis,structure,photophysical properties,and energy transfer mechanism of an erbium–mercury compound

Abstract

A hydrothermal reaction leads to the formation of a novel erbium–mercury compound [Er(IA)₃(H₃O)(H₂O)] _n (0.5nHg₂I₆) (1) (HIA = isonicotinic acid). The compound has been characterized by single-crystal X-ray diffraction. It is characteristic of a one-dimensional chain-like structure and a two-dimensional supramolecular layer. A solid-state photoluminescence experiment reveals that this compound displays upconversion green photoluminescence. The photoluminescence emission peaks can be attributed to the ⁴G_11/2 → ⁴I_15/2, ⁴F_7/2 → ⁴I_15/2, and ²H_11/2 → ⁴I_15/2 of the Er³⁺ ions. The energy transfer mechanism is consistent with the energy-level diagrams of the erbium ions and isonicotinic acid ligand. This compound possesses Commission Internationale de I'Éclairage chromaticity coordinates of 0.1755 and 0.5213. A solid-state diffuse reflectance measurement reveals that this compound features a narrow optical band gap of 1.97 eV.

A novel erbium–mercury compound is prepared and characterized. It is characteristic of a one-dimensional chain-like structure and a two-dimensional supramolecular layer. It displays upconversion green photoluminescence with the emission peaks being attributed to the ⁴G_11/2 → ⁴I_15/2, ⁴F_7/2 → ⁴I_15/2 and ²H_11/2 → ⁴I_15/2 of the Er³⁺ ions. An energy transfer mechanism is explained by the energy-level diagrams of the erbium ions and isonicotinic acid ligand. It possesses Commission Internationale de I’Éclairage chromaticity coordinates of 0.1755 and 0.5213. It features a narrow optical band gap of 1.97 eV.

Introduction

It is well known that lanthanide metals possess more complicated electronic conformations than those of transition metals and so lanthanide metal compounds generally exhibit attractive physicochemical performances such as photoluminescence, sensors, magnetism, medicine, and catalysis.^1–9 Therefore, lanthanide metal compounds have attracted increasing interest from materials and chemical scientists. Up to date, these scientists have carried out many studies on lanthanide metal compounds with different organic ligands such as aromatic carboxylic acids and heterocyclic compounds.^10–18 It is supposed that these organic ligands can absorb light and transfer the light energy to the lanthanide ions so facilitating the transition of 4f electrons. This phenomenon is the so-called “antenna effect.”^19,20

Being a heterocyclic derivative, isonicotinic acid is a very attractive building unit because it has the two oxygen atoms of a carboxylic group at one end and one nitrogen atom at the other end. Therefore, isonicotinic acid can coordinate to several metal ions and form extended structures. In my research experience, lanthanide ions prefer to coordinate to oxygen atoms, while transition metal ions tend to bind to nitrogen atoms. So isonicotinic acid could coordinate to lanthanide ions via the oxygen atoms of the carboxylic group and bind to transition metal ions through its nitrogen atom leading to extended structures. Under such considerations, the crystal engineering of lanthanide-transition metal compounds with isonicotinic acid as a ligand has become one of my research topics. In this work, the synthesis, crystal structure, solid-state photoluminescence, energy transfer mechanism, CIE (Commission Internationale de I'Éclairage), and solid-state diffuse reflectance of a novel lanthanide-transition metal compound, [Er(IA)₃(H₃O)(H₂O)] _n (0.5nHg₂I₆) (1) (HIA = isonicotinic acid) are reported. This compound has a two-dimensional (2D) supramolecular layered structure.

Results and discussion

The coordination structure of compound 1 is presented in Figure 1. Single-crystal X-ray structural analysis results reveal that all of the crystallographically independent atoms reside at general positions. Compound 1 crystallizes in the triclinic space group of Pī with two formula molecules in each unit. The asymmetric unit of compound 1 consists of one erbium(III) ion, three isonicotinate anions, two coordination water molecules, one mercury(II) ion, and three iodine anions. The formula molecules of 1 are composed of one-dimensional (1D) [Er(IA)₃(H₃O)(H₂O)] _n ⁿ⁺ cationic chains and isolated Hg₂I₆²⁻ anions. The erbium(III) ion is coordinated by the two oxygen atoms of two coordination water molecules and six oxygen atoms of six isonicotinate anions, yielding the square anti-prismatic geometry of ErO₈. The bond lengths of Er–O_IA reside in the range of 2.283(6)–2.364(6) Å, while those of Er–O_water are 2.419(6) and 2.491(5) Å. The bond lengths of Er–O_IA are obviously shorter than those of Er–O_water suggesting that the erbium(III) ion shows a stronger affinity to O_IA atoms than to O_water atoms. These bond lengths of Er–O locate in the normal range and are comparable with those reported previously.^21–23 The bond angles of O–Er–O reside in the span of 69.85(19)°–145.4(2)°. The mercury ion is bound by two terminal and two μ₂-bridging iodine ions to yield a tetrahedral HgI₄ geometry. Two HgI₄ moieties are connected together via two μ₂-bridging iodine ions to generate one Hg₂I₆²⁻ anion. The bond lengths of Hg–I are in the span of 2.6909(13)–3.231(4) Å, which are comparable with those documented in the literature.^24–26 The bond angles of I–Hg–I locate in the range of 110.67(4)°–129.92(6)°.

Figure 1.

A molecular structure of 1 with hydrogen atoms being omitted for clarity.

The ErO₈ moieties are interconnected by four or two IA ligands to form a 1D [Er(IA)₃(H₃O)(H₂O)] _n ⁿ⁺ cationic chain running along the a direction, as shown in Figure 2. The chains are further interlinked by hydrogen bonding interactions of C5-H5A . . . I1(–x, –y + 1, –z + 3) to yield a 2D supramolecular layer extending along the ac plane, as presented in Figure 3. In compound 1, the crystal packing structure is consolidated by electrostatic interactions, Van der Waals interactions and hydrogen bonding interactions between the [Er(IA)₃(H₃O)(H₂O)] _n ⁿ⁺ cationic chains and isolated Hg₂I₆²⁻ anions (Figure 4).

Figure 2.

A 1D chain runs along the a direction.

Figure 3.

A 2D supramolecular layer viewed from different directions. Green dashed lines are hydrogen bonding interactions: C5-H5A . . . I1(–x, –y + 1, –z + 3) 3.911(13) Å, 178°. (a) Viewed from the a direction and (b) Viewed from the b direction.

Figure 4.

A packing diagram of 1.

It is well known that erbium compounds usually exhibit photoluminescence behavior.^27,28 Therefore, it was expected that the title compound would also display photoluminescence performance. The solid-state photoluminescence of compound 1 was studied at room temperature and the experimental results are presented in Figure 5. The effective energy absorption range of compound 1 locates at 580–610 nm, and the maximum absorption peak resides at 593 nm. When the solid-state powder sample was excited at 593 nm in the red region, the emission spectrum showed three peaks in the range of 350–550 nm, locating at 385, 468, and 530 nm. The maximum emission peak resides at 530 nm in the green region. These three emission peaks can be attributed to the ⁴G_11/2 → ⁴I_15/2, ⁴F_7/2 → ⁴I_15/2 and ²H_11/2 → ⁴I_15/2 transmissions of the Er³⁺ ion, respectively. Therefore, it is proposed that an upconversion photoluminescence behavior is probable for compound 1. It should also be noticed that the title compound possesses CIE chromaticity coordinates of 0.1755 and 0.5213 in the green region, as presented in Figure 6. As a result, the title compound is probably a candidate for upconversion green photoluminescence materials.

Figure 5.

Solid-state photoluminescent spectra of 1 measured at room temperature. Green: emission; red: excitation.

Figure 6.

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

To establish the energy transfer mechanism of the photoluminescence behavior of compound 1, the phosphorescence emission spectrum of isonicotinic acid was measured at 77 K and is depicted in Figure 7. The onset of the emission diagram of isonicotinic acid is 434 nm, so the lowest triplet state energy of isonicotinic acid is at 23,041 cm⁻¹. Therefore, the energy difference between the lowest triplet state of isonicotinic acid and the resonant energy level of the Er³⁺ ion (⁴G_11/2, 26,800 cm⁻¹) is 3759 cm⁻¹, as given in Figure 8. Based on the intramolecular energy transfer theory established by Dexter and Sato et al.,^29,30 the intramolecular energy transfer efficiency mainly relies on two processes. One is the energy transfer from the lowest triplet state of a ligand to the resonance energy level of a lanthanide ion via a resonance exchange interaction. Another is the contrary energy transfer from a lanthanide ion to a ligand through thermal inactivation. The contrary energy transfer happens if the energy difference is small enough. Both processes are clearly in opposite directions. This energy transfer theory proposes that an assumed optimal energy gap resides in the range of 2500–3500 cm⁻¹. ³¹ Either too small or too large an energy gap may weaken the photoluminescence performance of a material.

Figure 7.

Phosphorescence spectrum of isonicotinic acid carried out at 77 K. Green: excitation; red: emission.

Figure 8.

Schematic and partial energy-level diagrams of the main energy absorption transfer and phosphorescence processes in 1 and the isonicotinic acid ligand.

According to the above discussion, with regard to compound 1, the energy gap is 3759 cm⁻¹ which is larger than 3500 cm⁻¹. As a result, compound 1 probably cannot show ideal photoluminescence properties. This is in agreement with the photoluminescence emission diagram of compound 1, as depicted in Figure 5. In Figure 5, two of the three emission peaks are wide and obtuse, namely not well-shaped. Such a phenomenon suggests that isonicotinic acid is not an ideal ligand for the excitation of Er³⁺ ion in compound 1, that is, not a suitable “antenna.” However, isonicotinic acid still acts as an “antenna” because the title compound actually exhibits the characteristic emissions of the internal transitions of the 4f electrons of the Er³⁺ ion.

As a mercury-containing compound, the title compound is expected to have semiconductive performance. Therefore, the solid-state UV/visible diffuse reflection spectrum of compound 1 was carried out using a powder sample at room temperature. The well-known Kubelka–Monk function α/S = (1 − R)²/2R was applied to treat the solid diffuse reflection spectral data. In this function, the parameter α is the absorption coefficient, S means the scattering coefficient which is actually independent of the wavelength if the particle size is not smaller than 5 µm, and R refers to the reflectivity. By extrapolating the linear part of the α/S absorption edge versus the energy diagram, the optical band gap value can be determined. The solid-state UV/visible diffuse reflection spectrum revealed that compound 1 possesses a narrow optical band gap of 1.97 eV, as presented in Figure 9. As a result, compound 1 may be a candidate for narrow optical band gap semiconductive materials. The slope of the light absorption edge is not steep, which suggests that compound 1 undergoes an indirect transition process. ³²

Figure 9.

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

Conclusion

In summary, a novel erbium–mercury compound with a 1D chain-like structure has been prepared and characterized by single-crystal X-ray diffraction. The 1D [Er(IA)₃(H₃O)(H₂O)] _n ⁿ⁺ cationic chains and isolated Hg₂I₆²⁻ anions are interlinked by hydrogen bonding interactions to yield a 2D supramolecular layer. A solid-state photoluminescence measurement showed that this compound exhibits upconversion green photoluminescence. In addition, it displays remarkable CIE chromaticity coordinates of 0.1755 and 0.5213. A solid-state diffuse reflectance experiment unveiled that it has a narrow optical band gap of 1.97 eV.

Experiment

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 FT-IR spectrophotometer over the frequency range 4000–400 cm⁻¹ using the KBr pellet technique. The solid-state photoluminescence experiment was performed on the F97XP photoluminescence spectrometer. Solid-state UV/Vis measurements were conducted on a TU1901 UV/Vis spectrometer.

Synthesis and characterization of [Er(IA)₃(H₃O)(H₂O)]_n(0.5nHg₂I₆) ( 1 ): All reagents are analytical grade, commercially available, and were used without further purification. Complex 1 was synthesized by mixing Er(NO₃)₃·6H₂O (1 mmol, 460 mg), HgI₂ (2 mmol, 910 mg), isonicotinic acid (3 mmol, 369 mg), and 10 mL distilled water in a 25-mL Teflon-lined stainless-steel autoclave that was heated at 473 K in an oven for 10 days. After cooling to room temperature, yellowish block-like crystals were obtained. The yield was 47% based on Er(NO₃)₃·6H₂O. Anal. calcd for C₁₈H₁₇ErHgI₃N₃O₈: C, 18.77; H, 1.49; N, 3.65; found: C, 18.86; H, 1.53; N, 3.72. IR peaks (cm⁻¹): 3444(vs), 3078(w), 3020(vw), 2357(w), 1599(s), 1541(m), 1498(w), 1383(s), 1270(m), 1234(s), 1197(vs), 1144(w), 1102(s), 1043(s), 906(m), 831(m), 762(w), 667(s), 625(m), 572(w), 546 (m), 482(m) and 450(w).

X-ray crystallographic study: The X-ray diffraction data set was measured on a SuperNova CCD X-ray diffractometer using a carefully selected single crystal of complex 1. The X-ray source was graphite monochromated Mo-Kα radiation. The diffraction data reduction and empirical absorption correction were performed with the CrystalClear software. The crystal structure was solved with the direct method using the Siemens SHELXTL™ V5 software 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 presented in Table 1, while selected bond lengths and bond angles are given in Table 2. Crystallographic data for the structural analysis have been deposited with the Cambridge Crystallographic Data Centre, CCDC No. 1974806. Copies of this information may be obtained free of charge from the Director, CCDC, 12 Union Road, Cambridge, CBZ 1EZ, UK (Fax: [phone]; email: [e-mail] or http://www.ccdc.cam.ac.uk).

Table 1.

Crystal Data and Structure Refinement Details.

Formula	C18H17ErHgI3N3O8
Mr	1151.90
Color	Yellow
Crystal size (mm3)	0.15 0.05 0.04
Crystal system	Triclinic
Space group	Pī
a (Å)	9.4993 (5)
b (Å)	11.5907 (7)
c (Å)	14.4127 (8)
α (°)	102.312 (5)
β (°)	95.852 (5)
γ (°)	111.772 (5)
V (Å3)	1410.84 (14)
Z	2
2θmax (°)	50
Reflections collected/unique (Rint)	15,044/4815 (0.0538)
dcalcd (g/cm3)	2.712
μ (mm−1)	11.713
F(000)	1034
T (K)	293 (2)
R1, wR2	0.0519, 0.0939
S	1.024
Δρ (max, min) (e/Å3)	1.823, −1.950

Table 2.

Selected Bond Lengths (Å) and Bond Angles (°).

Er(1)-O(1W)	2.419(6)	I(2)-Hg(1)-I(1)	110.67(4)
Er(1)-O(2W)	2.491(5)	I(2)-Hg(1)-I(3)	129.92(6)
Er(1)-O(1)#1	2.364(6)	I(1)-Hg(1)-I(3)	113.49(7)
Er(1)-O(2)	2.314(6)	O(4)-Er(1)-O(5)#1	145.4(2)
Er(1)-O(3)#2	2.327(6)	O(4)-Er(1)-O(2)	141.6(2)
Er(1)-O(4)	2.283(6)	O(5)#1-Er(1)-O(2)	72.8(2)
Er(1)-O(5)#1	2.285(6)	O(4)-Er(1)-O(3)#2	99.6(2)
Er(1)-O(6)	2.342(6)	O(2)-Er(1)-O(1W)	71.3(2)
Hg(1)-I(1)	2.7285(14)	O(3)#2-Er(1)-O(1W)	74.0(2)
Hg(1)-I(2)	2.6909(13)	O(6)-Er(1)-O(1W)	71.9(2)
Hg(1)-I(3)	2.745(2)	O(1)#1-Er(1)-O(2W)	69.85(19)
Hg(1)-I(3)#3	3.231(4)	O(4)-Er(1)-O(2W)	72.0(2)

Symmetry transformations used to generate equivalent atoms: #1 −x, −y + 1, −z + 3; #2 −x − 1, −y + 1, −z + 3; #3 1 − x, 1 − y, 2 − z.