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Keywords:

  • Organic template;
  • Borates;
  • Heat treatment;
  • Variable temperature luminescence

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section

Templated pentaborates have potential application as LEDs. The organically templated pentaborate [C2H8N][B5O6(OH)4] (A) was synthesized by the reaction of dimethylamine with boric acid in pyridine solution and the compound was characterized by elemental analysis, FT-IR and photoluminescence spectroscopy as well as TG-DTA analysis and powder X-ray diffraction. The crystallographic structure of A was determined by single-crystal X-ray diffraction. It crystallizes in the monoclinic system, space group Pequation image with the parameters of a = 8.893(7) (Å), b = 8.902(7) (Å), c = 16.967(16) (Å), α = 102.620(12)°,β = 102.817(11)°, γ = 99.107(5)°, V = 1247.5(18) Å3 and Z = 4. The luminescent properties of the compound were studied, and a purple-pink luminescence occurs with an emission maximum at 473 nm. The photoluminescence of A can be changed from purple-pink to white by means of a simple heat-treatment process. The white-light-emission of the as-synthesized sample G makes the pentaborate a good candidate for display and lighting applications in white LEDs.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section

With properties of low power consumption and high light efficiency, light-emitting diodes (LED) are highly promising for display and lighting technology.13 The research on phosphors with strong absorbance at the wavelengths of LEDs that emit suitable light has attracted considerable attentions. Based on the theories of luminescence and colorimetry, there are three ways to produce white light LEDs: combining of a blue LED with yellow phosphors, exciting multi-phosphors by a UV LED, and mixing multi-component LEDs.46 White-light-emitting devices with multiple emitting components are mainly used.7 However, drawbacks of expensiveness, complication, poor stability, and white-emitting color changes with input power are existing. Single-emitting-component (SEC) phosphors would be a method to solve these problems.8 Thus far, the research on SEC phosphors has been basically focused on various materials, such as organic molecules, inorganic materials doped with rare-earth ions, nanomaterials and organic-inorganic hybrid materials.924 The organic-inorganic composites are particularly attractive, because the composites can preserve the advantages of both, for example, hardness, chemical resistance, optical function of inorganics, and flexibility, lightness, and processability of organics.25

Phosphor with borates as luminescent hosts have been receiving intense interests.26-28 The borate phosphor is synthesized by the way of doping rare earth ions as luminescent centers. The first reported organic-inorganic borate phosphor composite was (H2en)2(Hen)2B16O27 (en = ethylenediamine), which can emit blue light under the excitation of near-UV light at room temperature, and shows a temperature-variable emitting property from blue to white.8 So far, there is only a small amount of borate phosphor reported,[1, 5,7, 21, 25] including (H2en)2(Hen)2B16O27 (en = ethylenediamine), [C6N4H20]0.5[B5O6(OH)4], [C4H12N][B5O6(OH)4], [C10N2H9][B5O6(OH)4]·H3BO3·H2O, B3O4(OH)·0.5(C4H10N2), [C6N2H18]0.5[B5O6(OH)4], [C6N4H20]0.5[B5O6(OH)4], and [C6NH16][B5O6(OH)4].

In this work, we report an organically template pentaborate [C2H8N][B5O6(OH)4] (A) synthesized by reactions of dimethylamine with excess boric acid in pyridine solution. The photoluminescence of A can be modified from purple-pink to white by means of a simple heat treatment process. When sample G is illuminated with near-UV light, it shows a white-light emission. Therefore, this pentaborate could be a promising candidate in the white LED.

Results and Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section

Crystal Structure

[C2H8N][B5O6(OH)4] (A) crystallizes in the space group Pequation image with unit parameters of a = 8.893(7) (Å), b = 8.902(7) (Å), c = 16.967(16) (Å), α = 102.620(12)o, β = 102.817(11)o, γ = 99.107(5)°. Single X-ray structure determination reveals that it is a supramolecular network of hydrogen bonded isolated [B5O6(OH)4] anions, with quaternary ammonium occupying cavities within the network. The asymmetric unit contains one [B5O6(OH)4] anion and one [C2H8N]+ cation (Figure 1). The [B5O6(OH)4] anion is composed of one BO4 tetrahedron and four BO3 triangles and, which form two [B3O3] cycles, is linked by the common BO4 tetrahedron. The B–O distances for the trigonal boron atoms vary from 1.328(7) Å to 1.393(8) Å, and those of the tetrahedral boron atoms vary from 1.462(7) Å to 1.495(7) Å. The O–B–O angles at the trigonal boron atoms range from 115.8(5) to 124.0(5)° and angles at the tetrahedral boron atoms range from107.8(5) to 111.5(4)°. The [B5O6(OH)4] anion is of remarkably regular arrangement and can donate four hydrogen atoms, whereas the hydrogen atoms are accepted by any of the ten oxygen atoms to form hydrogen bonds. According to Schubert nomenclature of the available oxygen sites in pentaborate anions (see Figure 1), different hydrogen bond interactions between neighboring pentaborate anions can be classified.29 As shown in Figure 2, the [B5O6(OH)4] anion formatted the molecular skeleton structure thought the hydrogen bonds, and the [C2H8N]+ cation is embedded in the network. The cation as structure directing agent could control the balance of electric charge in structure.

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Figure 1. Asymmetric unit of [C2H8N][B5O6(OH)4].

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Figure 2. Packing structure of [C2H8N][B5O6(OH)4].

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X-ray Diffraction Patterns

The phase purity of [C2H8N][B5O6(OH)4] was detected by X-ray powder diffraction. As is shown in Figure 3, the XRD pattern of A is in good agreement with the pattern based on single-crystal X-ray solution in position, indicating the phase purity of the as-synthesized samples of the title compound. After heat-treated below 250 °C, the structure of compounds B, C, D, and E does almost not change, while the structure of G collapses after heat treatment at 360 °C.

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Figure 3. Experimental and simulated powder XRD patterns of samples AG.

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Infrared (IR) Spectroscopy

The IR spectrum of [C2H8N][B5O6(OH)4] is shown in Figure 4. The stretching vibrations of the O–H and C–H bands are observed at 3317 and 2821 cm–1. The weak band at 1645 cm–1 is related to the bending vibrations of N–H. The strong bands at 1427, 1319, and 925cm–1 are the asymmetric and symmetric stretching modes of B–O in BO3, while the bands at 1157, 1053, 1032, and 775 cm–1 are the characteristic peaks of the asymmetric and symmetric stretching modes of B–O in BO4,3032 and the stretching vibrations of the C–N band are also located around 1176 cm–1. The band at 528 cm–1 is the characteristic absorption peak of [B5O6(OH)4].

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Figure 4. IR spectra of samples AG.

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As shown in Figure 4, the bands at 1427 and 1319 cm–1 amalgamate with rise of the heat treatment temperature, which reveals that there is a dehydration polymerization between units of BO2(OH). After heat treatment at 360 °C, the band at 1176 cm–1 disappears and a new band at 1194 cm–1 appears. In addition, the band at 3317 cm–1 splits to two bands. Above results indicate that the structure of the compound collapses.

Thermal Properties

The TG curve of [C2H8N][B5O6(OH)4] shows that the compound is stable up to about 183 °C (see Figure 5). A weight loss of about 36 % is present in the range of 183 °C to 700 °C, which corresponds to the decomposition of the organic template and the removal of four water molecules by the dehydration of hydroxyls (theoretic: 34 %).

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Figure 5. TG curve of sample [C2H8N][B5O6(OH)4].

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Variable Temperature Luminescent Properties

Heat treatment of A yields B at 100 °C, C at 150 °C, D at 200 °C, E at 250 °C, F at 300 °C, and G at 360 °C. Interestingly, solid samples AG display purple-pink, purple-pink, purple-pink, purple-pink, purple-pink, purple-indigo, and white photoluminescence, respectively, when illuminated with near-UV light (Figure 6a). Figure 6b shows the emission and excitation spectra of solid samples AG. Compound A can be excited by light with a wavelength of 399 nm, a purple-pink photoluminescence phenomenon occurs with an emission maximum at 473 nm. It is noted that the emission spectra of B and C are similar to that of A expect the intensity. The results are in good agreement with that of the PXRD patterns, the structures of compounds B and C did not collapse, compared to that of A. The emission spectrum of D has 29 nm red-shift compared with that of A, while the emission spectra of E and F are similar with that of A expect the intensity. Interestingly, an obvious red-shift with 105 nm of G is found when compared with that of A, and a white photoluminescence phenomenon occurs with an emission maximum at 542 nm. When heat-treatment temperature rises to 360 °C, the ammonium ions removed and the framework has collapsed in G, so we did not observe the luminescence when illuminated with near-UV light.

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Figure 6. Emission spectra of samples AG.

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Conclusions

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section

[C2H8N][B5O6(OH)4] (A) is prepared with TETA molecules as template. The photoluminescence of A can be modified from purple-pink to white by means of a simple heat-treatment process. Compound A can be used as an intrinsic white SEC phosphor in the borate system.

Experimental Section

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section

Synthesis and Heat Treatment: In a typical procedure, H3BO3 (0.435 g) was added to pyridine (3 mL) whilst stirring at room temperature, followed by adding of dimethylamine (1 mL). After completely mixing, the mixture was transferred into a Teflon-lined stainless steel autoclave and heated in an oven at 180 °C for 1 week. The solid products were filtered and washed repeatedly with distilled water and ethanol, and dried at 60 °C overnight. Elemental analysis: calcd. C 12.33, H 4.83, N 9.59 %; found: C 12.45, H 4.79, N 9.62 %. The X-ray powder diffraction pattern for the product is in good agreement with the pattern based on single-crystal X-ray solution in position, indicating the phase purity of the as-synthesized samples of the title compound.

A powder sample of compound A was placed in an Al2O3 crucible, heated to certain temperature followed by a 1 h isothermal hold, finally cooled to room temperature to yield B, C, D, E, F, and G, which was treated at 100 °C, 150 °C, 200 °C, 250 °C, 300 °C, and 360 °C, respectively.

Crystallographic Studies: A suitable single crystal of A with the dimensions of 0.10 × 0.25 × 0.25 mm3 was carefully selected under an optical microscope and glued to thin glass fiber with epoxy resin. The intensities of the crystal data were collected with a Bruker SMART APEX CCD diffractometer with graphite-monochromated Mo-Kα (λ = 0.071073 nm) using the SMART and SAINT programs.33 All structure solutions were performed with direct methods using SHELXS-9734 and the structure refinement was done against F2 using SHELXL-97.35 All non-hydrogen atoms were found in the final difference Fourier map and refined with anisotropic thermal displacement coefficients except C3. All hydrogen atoms were fixed geometrically at calculated distances and allowed to ride on the parent non-hydrogen atoms. Some refinement details and crystal data of compound are summarized in Table 1. Selected bond lengths and bond angles are listed in Table 2.

Table 1. Crystallographic data of [C2H8N][B5O6(OH)4].
FormulaC2H12B5NO10
Formula weight264.18
Temperature /K293(2)
Wavelength /Å0.71073
Crystal systemtriclinic
Space groupPequation image
a8.893(7)
b8.902(7)
c16.967(16)
α102.620(12)
β102.817(11)
γ99.107(5)
Volume /Å31247.5(18)
Z4
ρcalcd. /g·cm–31.407
μ(Mo-Kα) /mm–10.131
F(000)544
Limiting indices–11 ≤ h ≤ 11
 –10 ≤ k ≤ 11
 –18 ≤ l ≤ 22
Weighting schemew = 1/[σ2(Fo2)+(0.1000P)2] where P = (Fo2 + 2Fc2)/3
Reflections collected9852
Independent reflections5677
Independent reflections with3493
Completeness to Θ= 24.9999.0 %
Refinement methodFull-matrix least-squares on F2
Goodness-of-fit on F20.841
FinalRindices [I > 2σ(I)]R1 = 0.0459, wR2 = 0.1224
R indices (all data)R1 = 0.0804, wR2 = 0.1518
Table 2. Selected bond lengths /Å and angles /° for compound A.
O8–B41.347(7)O9–B41.375(8)
O8–B31.462(7)O9–B51.388(7)
O10–B51.391(5)O4–B51.328(7)
O10–B31.495(7)O3–B41.349(8)
O6–B21.356(7)O5–B21.374(8)
O6–B31.466(7)O5–B11.393(8)
O7–B11.342(7)O2–B21.345(7)
O7–B31.468(7)O1–B11.346(8)
B4–O8–B3123.8(5)O4–B5–O9123.6(4)
B5–O10–B3123.1(5)O4–B5–O10117.2(5)
B2–O6–B3123.2(5)O9–B5–O10119.3(5)
B1–O7–B3124.0(5)O8–B4–O3122.0(6)
B4–O9–B5119.9(5)O8–B4–O9122.1(7)
B2–O5–B1119.8(5)O3–B4–O9115.8(5)
O8–B3–O6109.9(5)O2–B2–O6121.8(6)
O8–B3–O7109.3(5)O2–B2–O5117.1(5)
O6–B3–O7111.5(4)O6–B2–O5121.1(6)
O8–B3–O10110.1(4)O7–B1–O1121.3(6)
O6–B3–O10107.8(5)O7–B1–O5120.4(6)
O7–B3–O10108.2(5)O1–B1–O5118.2(5)