A novel holmium–mercury complex: Structure,green photoluminescence,energy transfer mechanism and semiconducting properties

Abstract

The complex (Ho(IA)₃(H₃O)(H₂O)) _n (0.5nHg₂I₆) (HIA = isonicotinic acid) has been synthesized via a hydrothermal reaction and has a one-dimensional (1D) chain-like structure. It has an emission band in the green region of 542 nm, which could originate from the ⁵S₂ → ⁵I₈ transition of the Ho³⁺ ion. The compound possesses Commission Internationale de I’Éclairage chromaticity coordinates of (0.2065, 0.4969) and has a wide optical band gap of 2.32 eV, as found by solid-state UV-Vis diffuse reflectance measurements.

A novel holmium-mercury complex was reported. It features a 1-D chain-like structure. The solid-state photoluminescence measurement reveals that it has a green emission peak which is originated from the ⁵S₂ → ⁵I₈ transition of the Ho³⁺ ion. The energy transfer mechanism is explained by using the energy level figure of Ho³⁺ ions and isonicotinic acid. It displays remarkable CIE chromaticity coordinates. It is a potential candidate for green light emitting materials. It possesses a wide semiconductor band gap of 2.32 eV.

Introduction

Due to their fascinating photoluminescence, magnetic, semiconductive and other properties, lanthanide compounds have received more and more interest in exploring the potential application of these compounds in the domain of light-emitting diodes (LEDs), luminescent probes, electrochemical displays, magnets and so on.^1–3 To our knowledge, in comparison with the great number of explorations on the photoluminescence and magnetic properties of the lanthanide compounds, investigation on the semiconductor properties of these compounds is still underdeveloped and needs more study. ¹

Mercury has attracted interest in its coordination geometries; photoelectric, semiconductive and photoluminescence behaviour; and so on.^2,3 Recently, we have been exploring lanthanide–mercury compounds to gather new findings on their crystal structures, photoluminescence and semiconductive performance. In this work, we report the preparation, crystal structure, photoluminescence, energy transfer mechanism and semiconductor properties of a holmium–mercury complex (Ho(IA)₃(H₃O)(H₂O)) _n (0.5nHg₂I₆) (HIA = isonicotinic acid) (1) with a one-dimensional (1D) chain-like structure. It is noteworthy that complex 1 is the first example of the lanthanide complexes with a ${\mathrm{Hg}}_2{\mathrm{I}}_6^{2-}$ anion.

Experimental

Reagents were used as purchased for the synthesis of the title complex. Elemental microanalyses of carbon, hydrogen and nitrogen were carried out on an Elementar Vario EL elemental analyzer. Photoluminescence measurements were conducted on a F97XP photoluminescence spectrometer. Solid-state UV-Vis diffuse reflectance spectra were performed on a TU1901 UV-Vis spectrometer and infrared (IR) measurements on a Perkin Elmer Spectrum-One Fourier-transform infrared (FT-IR) spectrophotometer using KBr pellets.

Synthesis and characterization of 1

Ho(NO₃)₃·6H₂O (1 mmol, 459 mg), HgI₂ (1 mmol, 455 mg), isonicotinic acid (3 mmol, 369 mg) and distilled water (10 mL) were mixed into a 25 mL Teflon-lined stainless steel vessel. The vessel was heated to 473 K and kept at that temperature for 7 days. Once the vessel had slowly cooled down to room temperature, yellow crystals were obtained. The yield was 30% based on Ho(NO₃)₃·6H₂O. Anal. calcd for C₁₈H₁₇HgHoI₃N₃O₈: C, 18.81; H, 1.49; N, 3.66; found: C, 18.74; H, 1.45; N, 3.61%. IR peaks (cm⁻¹): 3445(vs), 3081(w), 1600(m), 1547(m), 1495(w), 1385(s), 1271(w), 1236(s), 1196(vs), 1140(w), 1096(s), 1043(s), 907(m), 832(m), 767(m), 714(w), 662(s), 623(s), 570(w), 543(w), 482(w) and 443(w).

X-ray crystallographic study

A single crystal was adhered to the tip of a glass fibre and mounted on a SuperNova charge-coupled device (CCD) diffractometer with X-ray source being graphite monochromated Mo-Kα radiation. The crystal data set was obtained with an ω scan mode. CrystalClear software ⁴ was used for data reduction and the empirical absorption correction. The crystal structure was solved with direct methods and the final structure was refined on F² with full-matrix least-squares by virtue of Siemens SHELXTL^TM V5 crystallographic software. ⁵ All non-hydrogen atoms were found based on difference Fourier maps and refined anisotropically. Crystal data as well as the details of data collection and refinement are presented in Table 1; selected bond lengths and bond angles are shown in Table 2. Crystallographic data for the structural analysis have been deposited with the Cambridge Crystallographic Data Centre (CCDC No. 1872603). 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] http://www.ccdc.cam.ac.uk).

Table 1.

Crystal data and structural refinements.

Formula	C18H17HgHoI3N3O8
Mr	1149.57
Colour	Yellow
Crystal size (mm3)	0.26 0.04 0.04
Crystal system	Triclinic
Space group	Pī
a (Å)	9.5148(3)
b (Å)	11.6144(4)
c (Å)	14.4120(5)
α (°)	102.227(3)
β (°)	95.890(3)
γ (°)	111.981(3)
V (Å3)	1414.01(8)
Z	2
2θmax (º)	50
Reflections collected	15,975
Independent, observed reflections (Rint)	4977, 4593 (0.0318)
dcalcd (g cm−3)	2.700
μ (mm−1)	11.517
F(000)	1032
T (K)	293(2)
R1, wR2	0.0388, 0.0860
S	1.049
Largest and mean Δ/σ	0.005, 0
Δρ (max, min) (e Å−3)	1.835, −2.004

Table 2.

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

Bond lengths	(Å)	Bond angles	(°)
Hg(1)–I(1)	2.7677(6)	I(1)–Hg(1)–I(2)	109.73(1)
Hg(1)–I(2)	2.7442(5)	I(1)–Hg(1)–I(3)	113.12(2)
Hg(1)–I(3)	2.7390(6)	I(2)–Hg(1)–I(3)	129.42(2)
Ho(1)–O(1)#1	2.371(2)	O(5)#1–Ho(1)–O(4)	145.39(7)
Ho(1)–O(2)	2.331(2)	O(4)–Ho(1)–O(2)	141.11(7)
Ho(1)–O(3)#2	2.334(2)	O(4)–Ho(1)–O(6)	79.93(6)
Ho(1)–O(4)	2.284(1)	O(4)–Ho(1)–O(1W)	71.55(5)
Ho(1)–O(5)#1	2.283(2)	O(2)–Ho(1)–O(1W)	71.22(5)
Ho(1)–O(6)	2.356(2)	O(6)–Ho(1)–O(1W)	72.10(6)
Ho(1)–O(1W)	2.442(1)	O(4)–Ho(1)–O(2W)	71.77(6)
Ho(1)–O(2W)	2.493(1)	O(1)#1–Ho(1)–O(2W)	70.15(6)

Symmetry codes: #1 −x, −y + 1, −z + 3; #2 −x − 1, −y + 1, −z + 3.

Results and discussion

The crystal structure of complex 1 is presented in Figure 1. All crystallographically independent atoms are located on the general sites. The structure of complex 1 consists of one ${[{\mathrm{HO}(\mathrm{IA})}_3({\mathrm{H}}_3\mathrm{O})({\mathrm{H}}_2\mathrm{O})]}_n^{n+}$ cationic chain and a half-isolated ${\mathrm{Hg}}_2{\mathrm{I}}_6^{2-}$ anion. The mercury ion is surrounded by four iodide anions to give a distorted tetrahedron. Two tetrahedra connect together via edge-sharing two iodide anions to yield an isolated ${\mathrm{Hg}}_2{\mathrm{I}}_6^{2-}$ anion. The Hg–I bond lengths are in the range of 2.7390(6)–2.7677(6) Å, which is normal and comparable with those documented previously.^6–8 The I–Hg–I bond angles are in the range of 109.73(1)º–129.42(2)º. The Ho³⁺ ion is in an eight coordination environment and coordinated by two oxygen atoms of two coordination water molecules and six oxygen atoms of six isonicotinic acid molecules to form a slightly contorted square anti-prismatic geometry. The top and bottom planes of this anti-prism are made by O(2W), O(4), O(1W), O(3)(−x − 1, −y + 1, −z + 3) and O(1)(−x, −y + 1, −z + 3), O(6), O(2), O(5)(−x, −y + 1, −z + 3) atoms, respectively. The bond lengths of Ho–O_water are 2.442(1) and 2.493(1) Å, while those of Ho–O_{isonicotinc acid} are in the range of 2.283(2)–2.371(2) Å. These Ho–O bond lengths are in the normal range and comparable with those found in the literature.^9–12 The O–Ho–O bond angles locate in a very wide range of 70.15(6)º–145.39(7)º. The neighbouring Ho³⁺ ions are interlinked by the isonicotinic acid molecules with the number of the isonicotinic acid molecules being of -2-4-2-4- to yield a 1D infinite chain running along the a direction, as shown in Figure 2. The distances between the neighbouring Ho³⁺ ions are 5.0924(2) and 4.4282(2) Å. The electrostatic interaction and van der Waals interaction between the ${[{\mathrm{HO}(\mathrm{IA})}_3({\mathrm{H}}_3\mathrm{O})({\mathrm{H}}_2\mathrm{O})]}_n^{n+}$ cationic chains and the isolated ${\mathrm{Hg}}_2{\mathrm{I}}_6^{2-}$ anions solidifies the crystal packing structure of complex 1 (Figure 3).

Figure 1.

The crystal structure of 1. Symmetry codes: #1 −x, −y + 1, −z + 3; #2 −x − 1, −y + 1, −z + 3.

Figure 2.

The 1D chain in 1. Green: HoO₈ polyhedra.

Figure 3.

The packing diagram of 1.

The photoluminescence spectrum of 1 was measured in the solid state at room temperature. Figure 4 shows the result of the photoluminescence measurement. When 1 is excited by the wavelength of 286 nm, its photoluminescence emission spectrum exhibits a peak at 542 nm, which is located in the green region. The green light emission of compound 1 is in good agreement with its Commission Internationale de I’Éclairage (CIE) chromaticity coordinates (0.2065, 0.4969), as presented in Figure 5. As a result, compound 1 is a potential candidate for green light emission materials. This emission band of compound 1 can be ascribed to the characteristic emission of the 4f electron intrashell transitions of the Ho³⁺ ions. More concretely, the photoluminescence emission band at 542 nm could originate from the ⁵S₂ → ⁵I₈ transition of the Ho³⁺ ion.^13–15

Figure 4.

The solid-state photoluminescence emission spectrum of 1. Inset: excitation spectrum.

Figure 5.

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

In order to reveal the nature of the photoluminescence emission spectrum of 1, the phosphorescence emission spectrum of isonicotinic acid was performed at 77 K, as depicted in Figure 6. The onset of the emission diagram of isonicotinic acid is about 434 nm. As a result, the lowest triplet state energy of isonicotinic acid can be calculated as 23,041 cm⁻¹. The energy difference between the lowest triplet state of isonicotinic acid and the resonant energy level of Ho³⁺ ions (⁵S₂, 18,300 cm⁻¹) is accordingly 4741 cm^-1 in compound 1 (Figure 7). According to established intramolecular energy transfer theory,^16,17 an intramolecular energy transfer efficiency is mainly dependent on two processes, i.e. energy transfer from the lowest triplet energy level of the ligands to the resonant energy level of lanthanide ions via a Dexter’s resonant exchange interaction, and a reverse energy transfer process from lanthanide ions to ligands via a thermal deactivation process. A reverse energy transfer process could possibly occur easily if the energy difference is small enough. These energy transfer processes are clearly contrary, in that they depend on the energy gap between the ligand’s lowest triplet energy level and the lanthanide ion’s resonant energy level. An optimal energy gap could be about 3000 ± 500 cm⁻¹, as depicted by intramolecular energy transfer theory. ¹⁸ Either a smaller or a larger energy gap could result in weakening the photoluminescence properties of a material.

Figure 6.

Phosphorescence spectrum of isonicotinic acid measured at 77 K.

Figure 7.

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

As discussed above, the energy gap between the isonicotinic acid’s lowest triplet energy level and the Ho³⁺ ion’s resonant energy level is 4741 cm⁻¹ in compound 1, a value far larger than 3000 ± 500 cm⁻¹. As a result, compound 1 is probably not expected to display good photoluminescence performance. Such a prediction is in accord with the photoluminescence emission diagram of compound 1 (Figure 4), which shows that the emission band is not fine enough. This suggests that the isonicotinic acid is not a suitable ligand for exciting the Ho³⁺ ion in compound 1, that is, not a fine ‘antenna’.

Some mercury-containing compounds are famous semiconductor materials, for instance, the MCT (Hg_xCd_1-xTe) which is a very useful IR detector in the military area. Compound 1 contains mercury, so it is expected to possess semiconductor behaviour. We conducted its solid-state UV-Vis diffuse reflectance spectra by using a powder sample at room temperature. Its diffuse reflectance spectrum was treated by using the Kubelka–Munk function, that is, α/S = (1 − R)²/2R. In this function, α means the absorption coefficient, S is the scattering coefficient, while R refers to the reflectance. Its semiconductor band gap can be determined by extrapolating from the linear part of the absorption edges in the α/S versus energy figure, as presented in Figure 8. With regard to compound 1, the solid-state UV-Vis diffuse reflectance spectrum shows that it has a wide semiconductor band gap of 2.32 eV. Therefore, compound 1 is possibly a candidate for wide optical band gap semiconductors. Furthermore, the solid-state UV-Vis diffuse reflectance spectrum of compound 1 shows a slow slope of the optical absorption edge; this indicates an indirect transition. ¹⁹

Figure 8.

Solid-state diffuse reflectance spectrum for 1.

In summary, a novel holmium–mercury complex was obtained via a hydrothermal reaction and characterized by single-crystal X-ray diffraction. Compound 1 features a 1D chain-like structure. The solid-state photoluminescence measurement reveals that it has a green emission peak which is originated from the ⁵S₂ → ⁵I₈ transition of the Ho³⁺ ion. The energy transfer mechanism is explained by using the energy level figure of Ho³⁺ ions and isonicotinic acid. Compound 1 displays remarkable CIE chromaticity coordinates of (0.2065, 0.4969). Thus, compound 1 is a potential candidate for green light-emitting materials. The solid-state UV-Vis diffuse reflectance measurement reveals that compound 1 possesses a wide optical band gap of 2.32 eV.