English

• 0   Introduction

  Rare earth (RE) complexes are the most promising luminescent materials with appealing features, such as large stock shift, sharp emission bands, long lifetimes, and high quantum efficiency, etc., which render them potential applications in the fields of bio-imaging, laser systems, optical amplification, organic light-emitting diodes, and pressure/disorder sensors^[1-9], etc. Among rare earth complexes, the near infrared emitting complexes, such as Yb³⁺, Er³⁺, Nd³⁺, Ho³⁺, Pr³⁺ complexes, etc., have been intensely studied in the past decades^[10-15]. There are two particular driving forces for these researches: one is the rapid development of telecommunication networks; the other is the vivo sensing and imaging in bio-applications^[16]. Er³⁺-based materials play a special role in telecommunication technologies since they emit characteristic emission at ~1.55 μm (0.8 eV), thus covering a spectroscopic window in which glass optical fibers show high transparency (the so-called third-communication window)^[17]. The organic erbium complexes have also attracted ever-increasing intention in recent years due to their advantages of easy processing and flexible molecular design, etc., compared with their inorganic glass fibers counterparts^[18]. But for practical application in the fields of telecommunication, optical communication systems, or optical amplifiers, the NIR yield and efficiency of the organic RE complexes should still be improved enormously.

  Because of their small energy gaps between the emissive states and the ground states, the NIR luminescent Re³⁺ ions usually suffer the problem of quenching from many phonon-related nonradiative processes, especially the vibrations of the C-H, N-H and O-H bonds in the organic complexes^[19-20]. To achieve high yield of NIR emission, one of the strategies is to use heavy atoms to replace the H atom in the ligands, and fluorination is a beneficial method^[19-20]. Another drawback of the RE-based NIR emitting complexes is the pervasive energy mismatch between the ligands and the center ions. For most RE complexes, their absorption or excitation bands locate mainly in ultraviolet region, which means that the excited states energy levels of ligands are much higher than the RE NIR emitting levels, thus giving rise to inefficient energy transfer from the ligand to the center ion, and therefore the unsatisfactory NIR quantum efficiency^[21]. In the past decade, many groups have devoted to designing and synthesizing novel RE complexes with low exciting energy levels, such as large conjugated systems, heterometallic complexes, etc. Among them, d-f block complexes are feasible choices, in which long-lived ³MLCT states of d-f transition metals (e.g. Ru²⁺, Re⁺, Os²⁺, Au⁺, Pt²⁺, Ir³⁺) can be excited by visible light and transfer efficiently their energy onto the 4fⁿ manifolds, thus providing an efficient pathway for energy migration within heterometallic complexes^{[7, 21-23]}. Mixed RE complexes, which have Re³⁺/Re³⁺ couples and possible mutual energy transfer, are another useful approach to enhance the expected NIR yield^[24-27]. Yb³⁺ ion is usually a prime candidate of the sensitizer to be chosen due to its high luminescent efficiency and relatively simple electronic structure of two energy level manifolds: the ²F_7/2 ground state and ²F_5/2 excited state around at 10 000 cm^-1 in the NIR region, especially for the Er³⁺ complexes. Thanks to the energy match between the ⁴I_11/2 level of Er³⁺ and the ²F_5/2 level of Yb³⁺, the energy transfer between excited state Yb³⁺ ion and ground state Er³⁺ ion can occur resonantly. This process is very efficient since the absorption cross section of Yb³⁺ (about 10^-20 cm²) is one order of magnitude higher than that of Er³⁺ (about 10^-21 cm²)^[28]. In this case, the Yb³⁺ ion functions as the bridge between ligands and Er³⁺ ion, thus enhancing the emission intensity of Er³⁺.

  Herein we synthesized erbium and ytterbium complexes using fluorinated ligand TPB and large conjugated ligand Bath as the first and the second ligands, respectively. In addition, the mixed complexes Er_xYb_1-x(TFB)₃Bath (x=0.218, 0.799, 0.896, and 0.987, respectively) were also prepared aiming to improve the Er³⁺ emission intensity. Photoluminescence properties of the as-prepared complexes were investigated in detail, and the experimental results reveal that all the complexes exhibit the characteristic transition of the Er³⁺ and/or Yb³⁺ ion. In the Er³⁺-Yb³⁺ mixed systems, the emission intensity of Er³⁺ ion can be enhanced by modulating the n_Er/n_Yb to facilitate the energy transfer from Yb³⁺ to Er³⁺ ion.

  1   Experimental

  1.1   Materials and instruments

  Ytterbium(Ⅲ) nitrate pentahydrate (99.9%), and erbium(Ⅲ) nitrate pentahydrate (99.9%), 4, 4, 4-trifluoro-1-phenyl-1, 3-butanedione (99.9%), 4, 7-diphenyl-1, 10-phenanthroline (98%) were obtained commercially from Aladdin company and used as received without further purification. Excitation and emission spectra were measured with an Edinburgh FLSP 920 fluorescence spectrophotometer. The Er³⁺ and Yb³⁺ ion contents were measured by Thermo iCAP6300 ICP-OES.

  1.2   Preparation of Er(TPB)₃Bath

  0.648 g (3 mmol) TPB and 0.332 g (1 mmol) Bath were dissolved in 15 mL ethanol, and NaOH aqueous (1 mmol·L^-1) was added into it under stirring and heating to adjust the pH value to 6~8. Then, the solution of 0.449 g Er(NO₃)₃·5H₂O (1 mmol) in 10 mL ethanol was dropped into the above solution. A lot of precipitates appeared. The reaction was kept under reflux for 3 h. After that, the precipitates were filtered and washed by de-ionized water and ethanol for several times, respectively. Finally, the precipitates were dried and recrystallized with acetone/ethanol mixed solvent, and the resulted products were dried at 80 ℃ under vacuum. Yield: 0.78 g (68%).

  1.3   Preparation of Yb(TPB)₃Bath

  The same procedure as that of Er(TPB)₃Bath. Yield: 76%.

  1.4   Preparation of Er_xYb_1-x(TPB)₃Bath

  The same procedure as that of Er(TPB)₃Bath, but herein the Er(NO₃)₃/Yb(NO₃)₃ mixed solution with different n_Er/n_Yb, instead of pure Er(NO₃)₃ or Yb(NO₃)₃, was added.

  Er_0.218Yb_0.782(TPB)₃Bath: 0.2 mmol Er(NO₃)₃+0.8 mmol Yb(NO₃)₃ was added. Yield: 65%. Elemental analysis: Er: 28 540 μg·g^-1, Yb: 106 200 μg·g^-1.

  Er_0.896Yb_0.104(TPB)₃Bath: 0.5 mmol Er(NO₃)₃+0.5 mmol Yb(NO₃)₃ was added. Yield: 72%. Elemental analysis: Er: 88 020 μg·g^-1, Yb:11 300 μg·g^-1.

  Er_0.799Yb_0.201(TPB)₃Bath: 0.8 mmol Er(NO₃)₃+0.2 mmol Yb(NO₃)₃ was added. Yield: 70%. Elemental analysis: Er: 137 800 μg·g^-1, Yb: 2 036 μg·g^-1.

  Er_0.987Yb_0.013(TPB)₃Bath: 0.9 mmol Er(NO₃)₃+0.1 mmol Yb(NO₃)₃ was added. Yield: 66%. Elemental analysis: Er: 155 300 μg·g^-1, Yb:11 300 μg·g^-1.

  2   Results and discussion

  The preparation routes of these complexes are shown in Scheme 1.

  图Scheme1 Preparation routes of RE(TPB)₃Bath Scheme1. Preparation routes of RE(TPB)₃Bath

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  The mixed complexes Er_xYb_1-x(TPB)₃Bath were synthesized by using the Er(NO₃)₃/Yb(NO₃)₃ mixed solution with different n_Er/n_Yb as the rare earth sources. The actual Er³⁺ and Yb³⁺ content in the as-prepared complexes were detected by means of ICP. And the mixed Er_xYb_1-x(TPB)₃Bath complexes were denoted by the detected results, as shown in Table 1.

  Table 1.  Raw and detected n_Er/n_Yb for the mixed complexes Er_xYb_1-x(TPB)₃Bath

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  Raw nEr/nYb	Detected nEr/nYba			Resulted complexes
  	CEr/(μg·g-1)	CYb/(μg·g-1)	nEr/nYb
  2/8	28 540	106 200	0.218/0.782	Er0.218Yb0.782(TPB)3Bath
  5/5	88 020	11 300	0.896/0.104	Er0.896Yb0.104(TPB)3Bath
  8/2	137 800	35 860	0.799/0.201	Er0.799Yb0.201(TPB)3Bath
  9/1	155 300	2 036	0.987/0.013	Er0.987Yb0.013(TPB)3Bath
  aDetected nEr/nYb is resulted from: (CEr/MEr)/(CYb/MYb), where CEr and CEr denote the detected contents of Er3+ and Yb3+ ion, MEr and MYb is the atomic weight of the Er3+ and Yb3+ ion, respectively.

  The photoluminescence properties of the Er(TPB)₃Bath and Yb(TPB)₃Bath were investigated firstly. Fig. 1 shows the excitation and emission spectra of Er(TPB)₃Bath and Yb(TPB)₃Bath. There are three obvious bands in the excitation spectrum of Er(TPB)₃Bath, locating at 374, 463 and 520 nm, respectively. The strongest one at 374 nm can be ascribed to the π-π* transition of the ligands. And the band at 463 nm can be ascribed to the additive contribution from the ligands and the transition of ⁴I_15/2→⁴H_7/2 from Er³⁺ ion^[26]. The sharp peak at 520 nm can be contributed to the transition of ⁴I_15/2→²H_11/2^{[26, 29]}. As depicted in its emission spectrum, the Er(TPB)₃Bath complex emit its characteristic emission peaked at 1 530 nm, which is originated from the ⁴I_13/2→⁴I_15/2 transition of Er³⁺ ion.

  图1 Excitation (Ex) and emission (Em) spectra of Er(TPB)₃Bath and Yb(TPB)₃Bath Figure1. Excitation (Ex) and emission (Em) spectra of Er(TPB)₃Bath and Yb(TPB)₃Bath

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  The Yb(TPB)₃Bath exhibit similar excitation spectrum with that of Er(TPB)₃Bath, locating mainly at 378 nm with a shoulder band at about 445 nm, which can be ascribed to the contribution of ligands. However, the transition of the Yb³⁺ ion is not observed in the excitation spectrum, which is rational because the Yb³⁺ ion owns only one possible transition between its ground state and excited state locates in the NIR transition. As shown in Fig. 1, the Yb(TPB)₃Bath complex shows a emission band in NIR region, split into three peaks at 976, 1 005 and 1 031 nm, respectively, which is related with the coordination field effect of the ligands^[30-32].

  To enhance the emission yield of the Er³⁺ ion, we prepared a series of mixed Er_xYb_1-x(TPB)₃Bath (x=0.218, 0.799, 0.896, and 0.987, respectively) complexes because the presence of Yb³⁺ was expected to facilitate the energy transfer from ligands to Er³⁺ ions. The photoluminescence properties of these complexes were investigated. For better comparison, all the emission spectra were measured under the same conditions. Firstly, we investigate the effect of excitation source (374 or 378 nm) on the emission spectra, and the experimental results show that 378 nm excitation source causes slightly higher emission intensities of both Yb³⁺ and Er³⁺ ions than that from 374 nm source. Fig. 2 compares the emission spectra of complexes Er_0.896Yb_0.104(TPB)₃Bath and Er_0.218Yb_0.782(TPB)₃Bath under the excitation of 374 and 378 nm, respectively.

  图2 Emission spectra of (a) Er_0.896Yb_0.104(TPB)₃Bath and (b) Er_0.218Yb_0.782(TPB)₃Bath Figure2. Emission spectra of (a) Er_0.896Yb_0.104(TPB)₃Bath and (b) Er_0.218Yb_0.782(TPB)₃Bath

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  What should be noted is the possible energy transfer from Yb³⁺ to Er³⁺ ion. As mentioned above, the 378 nm excitation source corresponds to the maximum excitation of the obtained Yb³⁺ complex. Compared with Er³⁺ ion, the Yb³⁺ ion possesses wider absorption cross section, which is helpful in harvesting the energy from ligands. In this case, part excited energy of Yb³⁺ ion would be transferred to the Er³⁺ ion, thus enhancing the emission of Er³⁺ ion. So, the enhanced Er³⁺ emission can be interpreted as the improved energy transfer from Yb³⁺ to Er³⁺ ion.

  The energy transfer between Yb³⁺/Er³⁺ ions and the enhancement of Er³⁺ emission were found to be more obvious by comparing the emission spectra of the mixed Er_xYb_1-x(TPB)₃Bath (x=0.218, 0.799, 0.896, and 0.987, respectively) complexes with that of the pure Er(TPB)₃Bath complex. Under excitation at 378 nm, the emission intensity of the Er_xYb_1-x(TPB)₃Bath varied with different n_Er/n_Yb (Fig. 3). Fig. 3a depicts the emission spectra of the Er_xYb_1-x(TPB)₃Bath (x=0.218, 0.799, 0.896, and 0.987, respectively) complexes and Er(TPB)₃Bath. It could be found that the emission intensity of Yb³⁺ ion (Em-Yb) is dominated in the cases of x=0.218 and 0.799, while Em-Yb and the emission intensity of Er³⁺ ion (Em-Er) is comparable when x=0.896 (Fig. 2a).

  图3 Emission spectra of the Er_xYb_1-x(TPB)₃Bath (x=0.218, 0.799, 0.896, 0.987, 1, respectively) excited at 378 nm (a) whole emission spectra and (b) emission spectra of the Er³⁺ ion Figure3. Emission spectra of the Er_xYb_1-x(TPB)₃Bath (x=0.218, 0.799, 0.896, 0.987, 1, respectively) excited at 378 nm (a) whole emission spectra and (b) emission spectra of the Er³⁺ ion

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  As far as the Em-Er is concerned, it varies with the n_Er/n_Yb also, but the variation should be distinguished carefully. As shown in Fig. 3b, the intensity order of the Em-Er of the mixed complexes is Er_0.799Yb_0.201(TPB)₃Bath > Er_0.896Yb_0.104(TPB)₃Bath=Er_0.218Yb_0.782(TPB)₃Bath >Er(TPB)₃Bath > Er_0.987Yb_0.013(TPB)₃Bath. With the increasing ratio of Er³⁺ ion from x=0.218 to 0.896, the mixed complexes show enhanced Em-Er, which is higher than that of Er(TPB)₃Bath. This result confirm the presence of energy transfer from Yb³⁺ ion to Er³⁺ ion in these cases. Based on theory and experimental results, the energy processes involved in these mixed complexes was analyzed and depicted in Scheme 2. It is found that there is an optimal n_Er/n_Yb for the mixed complexes. The Em-Er is most desirable when x=0.799, but further increasing x to 0.896, it decreased to the level as that of the case of x=0.218, and further increasing x to 0.987 causes the rapid decrease of Em-Er to even lower than that of the Er(TPB)₃Bath complex. In this case, back energy transfer from Er³⁺ to Yb³⁺ ion was suggested to responsible for this phenomenon^[26]. Further investigation about this phenomenon should be performed in future.

  图Scheme2 Energy transfer between Yb³⁺/Er³⁺ ion, here the higher energy levels of Er³⁺ ion are omitted Scheme2. Energy transfer between Yb³⁺/Er³⁺ ion, here the higher energy levels of Er³⁺ ion are omitted

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  3   Conclusions

  In summary, a series of complexes based on Er_xYb_1-x(TPB)₃Bath (x=0, 0.218, 0.799, 0.896, 0.987, and 1, respectively) with TPB and Bath as the first and second ligands, respectively, were prepared. Their luminescence properties of these obtained complexes were investigated experimentally. All the mixed complexes exhibit the characteristic emission of Yb³⁺ and Er³⁺ ion. Interestingly, the emission intensity of Er³⁺ ion can be enhanced by appropriately modulating the n_Er/n_Yb. When x=0.799, the correspond mixed complexes realize the highest emission intensity of Er³⁺ ion attributed to the energy transfer from Yb³⁺ to Er³⁺ ion.

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• 

  Scheme1  Preparation routes of RE(TPB)₃Bath

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  Figure 1  Excitation (Ex) and emission (Em) spectra of Er(TPB)₃Bath and Yb(TPB)₃Bath

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  Figure 2  Emission spectra of (a) Er_0.896Yb_0.104(TPB)₃Bath and (b) Er_0.218Yb_0.782(TPB)₃Bath

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  Figure 3  Emission spectra of the Er_xYb_1-x(TPB)₃Bath (x=0.218, 0.799, 0.896, 0.987, 1, respectively) excited at 378 nm (a) whole emission spectra and (b) emission spectra of the Er³⁺ ion

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  Scheme2  Energy transfer between Yb³⁺/Er³⁺ ion, here the higher energy levels of Er³⁺ ion are omitted

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  Table 1.  Raw and detected n_Er/n_Yb for the mixed complexes Er_xYb_1-x(TPB)₃Bath

  Raw nEr/nYb	Detected nEr/nYba			Resulted complexes
  	CEr/(μg·g-1)	CYb/(μg·g-1)	nEr/nYb
  2/8	28 540	106 200	0.218/0.782	Er0.218Yb0.782(TPB)3Bath
  5/5	88 020	11 300	0.896/0.104	Er0.896Yb0.104(TPB)3Bath
  8/2	137 800	35 860	0.799/0.201	Er0.799Yb0.201(TPB)3Bath
  9/1	155 300	2 036	0.987/0.013	Er0.987Yb0.013(TPB)3Bath
  aDetected nEr/nYb is resulted from: (CEr/MEr)/(CYb/MYb), where CEr and CEr denote the detected contents of Er3+ and Yb3+ ion, MEr and MYb is the atomic weight of the Er3+ and Yb3+ ion, respectively.

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