Research article Received: 10 June 2014,
Revised: 02 December 2014,
Accepted: 27 December 2014
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/bio.2860
Visible-near-infrared luminescent lanthanide ternary complexes based on beta-diketonate using visible-light excitation Lining Sun,a* Yannan Qiu,a Tao Liu,a Jing Feng,b Wei Denga and Liyi Shia* ABSTRACT: We used the synthesized dinaphthylmethane (Hdnm) ligand whose absorption extends to the visible-light wavelength, to prepare a family of ternary lanthanide complexes, named as [Ln(dnm)3phen] (Ln = Sm, Nd, Yb, Er, Tm, Pr). The properties of these complexes were investigated by Fourier transform infrared (FT-IR) spectroscopy, diffuse reflectance (DR) spectroscopy, thermogravimetric analyses, and excitation and emission spectroscopy. Generally, excitation with visible light is much more advantageous than UV excitation. Importantly, upon excitation with visible light (401–460 nm), the complexes show characteristic visible (Sm3+) as well as near-infrared (Sm3+, Nd3+, Yb3+, Er3+, Tm3+, Pr3+) luminescence of the corresponding lanthanide ions, attributed to the energy transfer from the ligands to the lanthanide ions, an antenna effect. Now, using these near-infrared luminescent lanthanide complexes, the luminescent spectral region from 800 to 1650 nm, can be covered completely, which is of particular interest for biomedical imaging applications, laser systems, and optical amplification applications. Copyright © 2015 John Wiley & Sons, Ltd. Additional supporting information may be found in the online version of this article at the publisher’s web site. Keywords: visible-light excitation; visible luminescence; NIR luminescence; lanthanide complex; energy transfer
Introduction Because of the excellent luminescent properties, the lanthanide ions have attracted much interest and have been widely investigated, and they show potential applications for flat display, biological fluorescent probes, lasers and optical telecommunications. Among them, the Eu3+ and Tb3+ ions have had most attention due to their excellent luminescent properties in the visible region of the spectrum (1–7). In contrast, Sm3+, Yb3+ , Nd3+, Er3+, Pr3+ and Tm3+ ions, with emissions in the visible and/or near-infrared (NIR) region, have been less investigated, but have attracted more and more attention (5,6,8). Among the lanthanide ions mentioned above, Sm3+, Yb3+, and Pr3+ ions possess emission bands around 1000 nm, which is in the transparent window for biology. The light absorption of biological tissue and blood is extremely low for this band, thus these lanthanide ions have great application value in optical bioimaging and biological diagnosis (9). There are two telecommunication windows for amplification used for long-distance communication, one at 1.3 μm using Nd3+ ion emission and the other at around 1.5 μm using Er3+, Tm3+ and Pr3+ ions emission (10). Therefore, the need to strengthen the research on the luminescence properties of these lanthanide ions for the development of biomedical imaging applications, laser systems, and optical amplification applications, etc. is of great importance. However, due to strong Laporte-forbidden 4f→4f transitions, the luminescence intensities of lanthanide ions are limited by their weak absorption. Since Weissman found that organic ligands could transfer absorbed energy to the lanthanide ions in the lanthanide complex, an antenna effect, there has been considerable effort devoted to designing ligands that optimize this energy transfer and thus give efficient lanthanide
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luminescence. Normally, most investigations of the ligands have the absorption in the ultraviolet (UV) range of the spectra (5), however, materials excited with visible light are advantageous over UV excitation. Thus, one of the growing challenges in the design of luminescent lanthanide complexes for application in bioanalyses and biomedical imaging is the extension of the excitation wavelength from the UV to the visible range (≥400 nm). β-Diketones are commonly used ligands for the coordination with lanthanide ions, and as antenna chromophores to sensitize these ions (11–13). Their advantages are: (i) β-diketones have strong absorption ability in a relatively wide wavelength range; and (ii) the carbonyl groups in β-diketones have strong coordination ability with lanthanide ions, and the formed chelate ring structure with lanthanide ions is very stable (14,15). In addition, non-charged ligands such as 1,10-phenanthroline (phen) or 2,2′-bipyridine (bipy) can serve as the synergistic agent, as they can prevent water molecules from binding to lanthanide ions and transfer energy to lanthanide ions more effectively (6).
* Correspondence to: Lining Sun and Liyi Shi, Research Center of Nano Science and Technology, Shanghai University, Shanghai 200444, People’s Republic of China. E-mail: [email protected]; [email protected] a
Research Center of Nano Science and Technology, Shanghai University, Shanghai 200444, People’s Republic of China
b
State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China Abbreviations: DR, diffuse reflectance; DSC, differential scanning calorimetry; EDTA, ethylene diamine tetraacetic acid; TGA, thermogravimetric analysis.
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L. Sun et al. In this paper, the dinaphthylmethane (Hdnm) ligand containing a naphthalene ring, which can extend the absorption to the visible-light wavelength, is synthesized and utilized as one of the sensitizers of the lanthanide ions (16). The lanthanide complexes, [Ln(dnm)3phen] (Ln = Sm, Nd, Yb, Er, Tm, Pr), were synthesized and their properties were investigated using Fourier transform infrared (FT-IR) spectra, diffuse reflectance (DR) spectra and thermogravimetric analyses. The excitation spectra of the [Ln(dnm)3phen] complexes can extend to more than 500 nm in the visible region and, following visible-light excitation, they show not only the characteristic visible emission (Sm3+), but also NIR luminescence of respective Ln3+ ions (Sm3+, Nd3+, Yb3+, Er3+, Tm3+, Pr3+).
Experimental
Characterization 1
HNMR spectra were measured by a Bruker AVANCE-500 spectrometer with tetramethylsilane (TMS) as internal reference (CDCl3 as solvent). Fourier transform infrared (FT-IR) spectra were measured in the range of 4000–400 cm 1 on an American Thermo Nicolet Corporation model AVATAR370 FT-IR infrared spectrophotometer with the KBr pellet technique. The DR spectra were performed with a Hitachi U-4100 spectrophotometer. The excitation and emission spectra were recorded by an Edinburgh FLS920 fluorescence spectrometer equipped with a 450 W Xe-lamp as an excitation source and a monochromatoriHR320 equipped with a liquid-nitrogen-cooled R5509–72 PMT as the detector. Thermogravimetry (TG) was performed on a Netzsch STA 449 at a heating rate of 10 °C·min 1 under an air atmosphere.
Materials 1,10-Phenanthroline mono-hydrate (phenH2O, 99%, Aladdin), ethyl-2-naphthoate (Aldrich), 2′-acetonaphthone (Aldrich), sodium hydride (NaH, 95%, Aldrich), hydrochloric acid, N,Ndimethylformamide (DMF) and ethanol were commercially available and used without purification. The solvent tetrahydrofuran (THF) was used after desiccation by refluxing. Ytterbium oxide (Yb2O3, 99.99%), samarium oxide (Sm2O3, 99.99%), neodymium oxide (Nd2O3, 99.99%), erbium oxide (Er2O3, 99.99%), praseodymium oxide (Pr2O3, 99.99%), and thulium oxide (Tm2O3, 99.99%) were purchased from Sigma-Aldrich.
Synthesis of the dinaphthylmethane (Hdnm) ligand Dinaphthylmethane was prepared according to published methods (16,17). The golden yellow product was recrystallized from ethyl acetate and a golden yellow crystal was obtained: [C23H16O2]: 1HNMR (CDCl3): δ 7.9–8.6 (14H, m, C12H7), 7.14 (1H, s, CH), 1.56 (1H, s, OH).
Results and discussion Fourier transform infrared (FT-IR) spectra The FT-IR spectra of [Sm(dnm)3phen] and [Yb(dnm)3phen] complexes are shown in Fig. S1 (ESI) as a representative, since the FT-IR spectra of [Ln(dnm)3phen] complex (Ln = Sm, Nd, Yb, Er, Tm, Pr) are similar. In the spectra of [Sm(dnm)3phen] and [Yb(dnm)3phen], a strong absorption band at 1586 cm 1 appears, which can be assigned to the superposition of stretching vibration of C=N bond from phen ligand and asymmetrical stretching vibration of C=O bond from dnm ligand. Peaks at 474 and 472 cm 1 in curves a and b are attributed to the antisymmetric vibration absorption of the Sm–O bond and Yb–O bond, respectively. In addition, an absorption band at 509 cm 1 can be observed in both curves, which can be attributed to the stretching vibration of the Sm–N (or Yb–N) bond. Overall, the FT-IR results suggest the successful coordination of lanthanide ions with the dnm and phen ligands.
Synthesis of lanthanide complexes [Ln(dnm)3phen] (Ln = Sm, Nd, Yb, Er, Tm, Pr) Hydrated LnCl3 salts (Ln = Sm, Nd, Yb, Er, Tm, Pr) were obtained by dissolving Ln2O3 (Ln = Sm, Nd, Er, Yb, Pr, Tm) in hydrochloric acid (HCl). The resulting solutions were evaporated with heating. The hydrated LnCl3 salts after drying were dissolved in anhydrous ethanol, transferred to a volumetric flask and diluted. The concentration of the lanthanide ion was measured by titration with a standard ethylenediaminetetraacetic acid (EDTA) aqueous solution. The [Ln(dnm)3phen] complex was prepared according to the following process: the ligands Hdnm and phen in stoichiometric molar ratio were dissolved in a suitable volume of anhydrous ethanol. Then, an appropriate amount of 1 M NaOH solution was added to the solution dropwise, to adjust the pH value to about 7. A stoichiometric amount of LnCl3 ethanol solution was then added to the solution under stirring. The molar ratio of Ln3+:Hdnm:phen was 1:3:1. The mixture was heated to reflux and kept at 80°C for 6 h, and then cooled to room temperature. The precipitates were collected after filtration and recrystallized from an acetone/ethanol mixture, then dried at 80°C under vacuum for 10 h. The total synthesis procedure for [Ln (dnm)3phen] is illustrated in Scheme 1.
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O
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Scheme 1. Synthesis procedure for the [Ln(dnm)3phen] (Ln = Sm, Nd, Yb, Er, Tm, Pr) complex. Reagents and experimental conditions: (a) NaH, anhydrous THF, reflux; (b) LnCl3, phen, NaOH, ethanol, reflux.
Copyright © 2015 John Wiley & Sons, Ltd.
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Vis-NIR luminescent lanthanide complexes upon visible-light excitation Diffuse reflectance (DR) spectra The DR spectra of [Ln(dnm)3phen] (Ln = Sm, Yb, Er, Tm, Pr) are shown in Fig. S2 (ESI). The spectra all exhibit broad absorption bands in the range of 200–410 nm, which can be assigned to the absorption of the organic ligands, likely corresponding to electronic transitions from the ground state (S0) to the excited state (S1) of the ligands. In the visible and NIR region of these spectra, each absorption band is attributed to the characteristic absorption transition of the corresponding lanthanide ions, that is, from the ground states to the excited states. The ground states of the Sm3+, Yb3+, Er3+, Tm3+, and Pr3+ ions are 6H5/2, 2 F7/2, 4I15/2, 3H6, and 3H4 levels, respectively (18). The excited states of each lanthanide ion are shown in the DR spectra, respectively (Fig. S2, ESI).
Antenna luminescence The excitation and emission spectra (visible and NIR regions) of [Sm(dnm)3phen] were measured and shown in Fig. 1. The excitation spectrum was obtained by monitoring the strongest emission band in the visible region of Sm3+ ion at 647 nm, which exhibits a broad band in the UV/visible region of 240–500 nm. This is due to the absorption by the dnm and phen ligands. The excitation spectrum monitored at the strongest emission band in NIR region at 948 nm was similar to that monitored at 647 nm (not given here). With photoexcitation with visible light 6
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at 401 nm, the [Sm(dnm)3phen] complex shows orange red light. As shown in Fig. 1(a), the visible emission spectrum consists of four emission bands at 565 nm, 609 nm, 647 nm, and 712 nm, attributed to 4G5/2 → 6H5/2, 4G5/2 → 6H7/2, 4G5/2 → 6H9/2, and 4 G5/2 → 6H11/2 transitions, respectively. The strongest transition in the visible region is the 4G5/2 → 6H9/2 transition. The transition located at 565 nm, 4G5/2 → 6H5/2, has a predominant magnetic dipole character (19). The intensity ratio of I(4G5/2 → 6H9/2)/I (4G5/2 → 6H5/2) can be used as a measure for the polarizability of the chemical environment of the Sm3+ ion (18,20) and, in this case, it was calculated to be 4.20. For the NIR emission spectrum of [Sm(dnm)3phen] complex (see Fig. 1b) excited with visible light, all the NIR emission bands also come from the 4G5/2 excited state, among which the strongest band at 948 nm is assigned to 4 G5/2 → 6F5/2 transition. The luminescence lifetime of [Sm(dnm) 3phen] was measured at room temperature by using an excitation wavelength of 401 nm and monitored around the strongest emission band at 647 nm. The luminescent decay curve was fitted by single exponential function and the lifetime of 4G5/2→6H9/2 transition was fitted to be 7.67 μs. Figure 2 shows the excitation and emission spectra of [Yb (dnm)3phen] complex. The excitation spectrum was measured by monitoring at the strongest emission band (977 nm). The key feature is that there is a broad band from 240 to 560 nm, assigned to the absorption of the dnm and phen ligands. Following the visible-light excitation on the [Yb(dnm)3phen] complex, the characteristic NIR emission of Yb3+ ion can be obtained. It shows a broad band in the range of 920–1100 nm, comprised of a sharp peak at 977 nm attributed to the 2F5/2 → 2F7/2 transition and broader vibronic components at the longer wavelength (8). It can be observed that the NIR emission of [Yb(dnm)3phen] complex is not a single sharp band but a broad band, which is caused by the splitting of the energy levels of the Yb3+ ion as a consequence of ligand field effects (21). Yb3+ ion is a more unusual case, which has only a single excited state, 2F5/2, 10200 cm 1 above the ground state 2F7/2. Therefore, the Yb3+ ion has some advantages for laser emission due to its very simple energy level scheme (22). The excitation and emission spectra of [Nd(dnm)3phen] complex were obtained and shown in Fig. 3. For the excitation spectrum (monitored at the strongest emission band of Nd3+ ion at 1059 nm), it shows a broad band ranging from 235 to 550 nm, which can be attributed to the π-π* electron transition of the
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Wavelength (nm) Figure 1. Excitation and emission spectra for the [Sm(dnm)3phen] complex in solid state: (a) excitation (λem = 646 nm) and visible emission (λex = 401 nm) spectra; (b) NIR emission spectrum (λex = 401 nm).
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Figure 2. Excitation (λem = 977 nm) and emission (λex = 401 nm) spectra for the [Yb(dnm)3phen] complex in solid state.
Copyright © 2015 John Wiley & Sons, Ltd.
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Wavelength (nm) Figure 3. Excitation (λem = 1059 nm) and emission (λex = 401 nm) spectra for the [Nd(dnm)3phen] complex in solid state.
ligands. After excitation at 401 nm, the emission spectrum of [Nd (dnm)3phen] complex clearly shows three NIR emission bands at 896, 1059 and 1332 nm, assigned to 4F3/2 → 4I9/2, 4F3/2 → 4I11/2 and 4F3/2 → 4I13/2 transitions, respectively, which is characteristic emission of Nd3+ ion (23). Among the three emission bands, the intensity of 1059 nm emission band is the strongest, and for a long time this center has been found to have potential application in laser systems. The 1332 nm band offers the good opportunity to develop new materials (containing Nd3+ ion) suitable for the optical amplification operating at 1.3 μm, one of the telecommunication windows (8). The fluorescent spectra of [Er(dnm)3phen] complex are shown in Fig. 4. In the excitation spectrum, it is dominated by a broad band extending to 500 nm in the visible region, attributed to the absorption of the dnm and phen ligands. In addition, there is a weak band at 488 nm in the excitation spectrum, originating from the characteristic absorption transition 4I15/2→ 4F7/2 of Er3+ ion, consistent with the DR spectrum (Fig. S2, ESI). Because this absorption transition is much weaker than the absorption of the ligands, sensitization the luminescence of Er3+ ion by exciting the ligands absorption is much more efficient than directly exciting the absorption of Er3+ ion. The longer absorption wavelength allows excitation by using the visible light. Following the visible-light excitation, the characteristic NIR emission of Er3+ ion
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can be obtained. The emission band centered at 1537 nm covers large spectral range extending from 1419 to 1652 nm, attributed to the typical 4I13/2 → 4I15/2 transition of Er3+ ion. Erbium-doped materials have been attracted much interest for many years, because the transition around 1540 nm is in the right position of the third telecommunication window (24). For the study on the NIR luminescent complexes, most reports focus on Yb3+, Nd3+ and Er3+ion, however, the research on the NIR luminescence of Pr3+ and Tm3+ complexes has been less reported, especially with visible-light excitation. Figure 5 shows the excitation and emission spectra of [Pr(dnm)3phen] complex. Compared with Sm3+, Yb3+, Er3+ and Nd3+ ions, the emission spectrum of Pr3+ ion is more complicated, since the Pr3+ ion can display emission bands from three levels (3P0, 1D2 and 1G4) after exciting the absorption of organic ligands (25). In the excitation spectrum of [Pr(dnm)3phen] complex, it shows a broad band ranging from 240 to 540 nm due to the absorption of ligands, most of which extends to the visible region. Upon excitation with the strongest absorption at 454 nm, the NIR emission spectrum of [Pr(dnm)3phen] complex can be obtained. There are three bands located at 890, 1067, and 1500 nm, which is attributed to the transition from the excited state (1D2) to the 3F2, 3 F4 and 1G4 levels of Pr3+ ion (26). Some crystal-field fine structure can be observed, which is an indication that the Pr3+ ion occupies well-defined crystallographic sites in the complex (27,28). Figure 6 presents the excitation and emission spectra of [Tm (dnm)3phen] complex. In the excitation spectrum (monitored at 803 nm), a broad band extending to 600 nm visible region is observed, which can be attributed to the absorption of the ligands. Excitation of the ligands absorption band with visible light (λex = 425 nm), the NIR emission band located at 803 nm can be obtained, which is attributed to the 3H4 → 3H6 transition of Tm3+ ion (6). Time-resolved measurement was carried out on [Tm(dnm)3phen] complex monitored at 803 nm. The decay curve is singly exponential, and the lifetime of Tm3+ ion excited state (3H4) is 1.78 μs. \As described above, under excitation and the absorption of ligands with the visible-light, the characteristic visible-NIR emission of Ln3+ ion was obtained in the corresponding [Ln(dnm)3phen] complex (Ln = Sm, Yb, Nd, Er, Tm, Pr). Since no direct absorption of the lanthanide ions occurred at the visible excitation wavelengths (401 nm for [Sm(dnm)3phen], 401 nm for [Yb (dnm)3phen], 401 nm for [Nd(dnm)3phen], 401 nm for [Er(dnm)
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Wavelength (nm) Figure 4. Excitation (λem = 1537 nm) and emission (λex = 401 nm) spectra for the [Er(dnm)3phen] complex in the solid state.
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Figure 5. Excitation (λem = 1065 nm) and emission (λex = 454 nm) spectra for the [Pr(dnm)3phen] complex in the solid state.
Copyright © 2015 John Wiley & Sons, Ltd.
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Vis-NIR luminescent lanthanide complexes upon visible-light excitation
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Acknowledgements
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We are grateful for the financial support from the National Natural Science Foundation of China (Grant Nos. 21231004, 21102088, 21174081), Innovation Program of Shanghai Municipal Education Commission (13ZZ073), the Science and Technology Commission of Shanghai Municipality (13NM1401100, 13NM1401101, 14520722200), Shanghai RisingStar Program (14QA1401800), and the project from State Key Laboratory of Rare Earth Resource Utilization (RERU2014012). We are also grateful to the Instrumental Analysis & Research Center of Shanghai University.
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Wavelength (nm) Figure 6. Excitation (λem = 803 nm) and emission (λex = 425 nm) spectra for the [Tm(dnm)3phen] complex in the solid state.
3phen],
454 nm for [Pr(dnm)3phen], and 425 nm for [Tm(dnm) the obtained visible-NIR emissions do only originate from the lanthanide ions sensitized by the organic ligands moiety. This result shows that: (i) the triplet states of the ligands could match well with the lanthanide ions; and (ii) the ligands are able to transfer the absorbed visible light to the lanthanide ions via the antenna effect (29). 3phen]),
Thermogravimetric analyses In order to investigate the thermal stability of the [Ln(dnm) 3phen] complex (Ln = Sm, Yb, Nd, Er, Tm, Pr), thermogravimetric analysis (TGA) was performed on them. Representative TG weight loss curves and differential scanning calorimetry (DSC) curves of [Yb(dnm)3phen] and [Sm(dnm)3phen] are shown (Fig. S3; ESI), as the TGA diagrams of all the [Ln(dnm)3phen] complexes are quite similar. For the TG curves of [Yb(dnm)3phen] and [Sm(dnm)3phen], they both show two distinct weight loss stages. The first weight loss corresponds to the decomposition of the dnm ligand, followed by a second weight loss ascribed to the loss of the phen ligand (30). The analyses show that the melting points of the [Yb(dnm)3phen] and [Sm(dnm)3phen] complexes are around 320 °C.
Conclusions Based on the ligand Hdnm, six new ternary lanthanide complexes [Ln(dnm)3phen] (Ln = Sm, Nd, Yb, Er, Tm, Pr) have been synthesized. Their FT-IR spectra, DR spectra, optical properties, and TG-DSC analyses have been reported. With the Hdnm ligand, extension of the excitation wavelength from the UV to the visible range was achieved. Upon excitation of the ligand absorption bands with the visible light (401–460 nm), the characteristic visible-NIR luminescence of the corresponding lanthanide complexes was demonstrated, known as an antenna effect. With these NIR luminescent lanthanide complexes, the luminescent spectral region from 800 to 1650 nm can be covered completely, which is of particular interest for biomedical imaging applications, laser systems, and optical amplification applications. Furthermore, the above results are pertinent to the application of these complexes in NIR electroluminescent devices.
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26. Foley TJ, Harrison BS, Knefely AS, et al. Facile preparation and photophysics of near-infrared luminescent lanthanide(III) monoporphyrinate complexes. Inorg Chem 2003; 42: 5023–32. 27. Sun L-N, Yu J-B, Zheng G-L, et al. Syntheses, Structures and near-ir luminescent studies on ternary lanthanide (ErIII, HoIII, YbIII, NdIII) complexes containing 4,4,5,5,6,6,6-heptafluoro-1-(2-thienyl)hexane1,3-dionate. Eur J Inorg Chem 2006; 3962–73. 28. Stouwdam JW, van Veggel FCJM. Near-infrared emission of 3+ 3+ 3+ redispersible Er , Nd , and Ho doped LaF3 nanoparticles. Nano Lett 2002; 2: 733–7. 29. Sun L-N, Zhang H-J, Yu J-B, Meng Q-G, Liu F-Y Peng C-Y. Performance of near-IR luminescent xerogel materials covalently bonded III III III with ternary lanthanide (Er , Nd , Yb ) complexes. J Photochem Photobiol A 2008; 193: 153–60. 30. Chen X, Zhang P, Wang T Li HR. The first europium(III) β-diketonate complex-functionalized polyhedral oligomeric silsesquioxane. Chem Eur J 2014; 20: 2551–22.
Supporting information Additional supporting information may be found in the online version of this article at the publisher’s web site.
Copyright © 2015 John Wiley & Sons, Ltd.
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Revised: 02 December 2014,
Accepted: 27 December 2014
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/bio.2860
Visible-near-infrared luminescent lanthanide ternary complexes based on beta-diketonate using visible-light excitation Lining Sun,a* Yannan Qiu,a Tao Liu,a Jing Feng,b Wei Denga and Liyi Shia* ABSTRACT: We used the synthesized dinaphthylmethane (Hdnm) ligand whose absorption extends to the visible-light wavelength, to prepare a family of ternary lanthanide complexes, named as [Ln(dnm)3phen] (Ln = Sm, Nd, Yb, Er, Tm, Pr). The properties of these complexes were investigated by Fourier transform infrared (FT-IR) spectroscopy, diffuse reflectance (DR) spectroscopy, thermogravimetric analyses, and excitation and emission spectroscopy. Generally, excitation with visible light is much more advantageous than UV excitation. Importantly, upon excitation with visible light (401–460 nm), the complexes show characteristic visible (Sm3+) as well as near-infrared (Sm3+, Nd3+, Yb3+, Er3+, Tm3+, Pr3+) luminescence of the corresponding lanthanide ions, attributed to the energy transfer from the ligands to the lanthanide ions, an antenna effect. Now, using these near-infrared luminescent lanthanide complexes, the luminescent spectral region from 800 to 1650 nm, can be covered completely, which is of particular interest for biomedical imaging applications, laser systems, and optical amplification applications. Copyright © 2015 John Wiley & Sons, Ltd. Additional supporting information may be found in the online version of this article at the publisher’s web site. Keywords: visible-light excitation; visible luminescence; NIR luminescence; lanthanide complex; energy transfer
Introduction Because of the excellent luminescent properties, the lanthanide ions have attracted much interest and have been widely investigated, and they show potential applications for flat display, biological fluorescent probes, lasers and optical telecommunications. Among them, the Eu3+ and Tb3+ ions have had most attention due to their excellent luminescent properties in the visible region of the spectrum (1–7). In contrast, Sm3+, Yb3+ , Nd3+, Er3+, Pr3+ and Tm3+ ions, with emissions in the visible and/or near-infrared (NIR) region, have been less investigated, but have attracted more and more attention (5,6,8). Among the lanthanide ions mentioned above, Sm3+, Yb3+, and Pr3+ ions possess emission bands around 1000 nm, which is in the transparent window for biology. The light absorption of biological tissue and blood is extremely low for this band, thus these lanthanide ions have great application value in optical bioimaging and biological diagnosis (9). There are two telecommunication windows for amplification used for long-distance communication, one at 1.3 μm using Nd3+ ion emission and the other at around 1.5 μm using Er3+, Tm3+ and Pr3+ ions emission (10). Therefore, the need to strengthen the research on the luminescence properties of these lanthanide ions for the development of biomedical imaging applications, laser systems, and optical amplification applications, etc. is of great importance. However, due to strong Laporte-forbidden 4f→4f transitions, the luminescence intensities of lanthanide ions are limited by their weak absorption. Since Weissman found that organic ligands could transfer absorbed energy to the lanthanide ions in the lanthanide complex, an antenna effect, there has been considerable effort devoted to designing ligands that optimize this energy transfer and thus give efficient lanthanide
Luminescence 2015
luminescence. Normally, most investigations of the ligands have the absorption in the ultraviolet (UV) range of the spectra (5), however, materials excited with visible light are advantageous over UV excitation. Thus, one of the growing challenges in the design of luminescent lanthanide complexes for application in bioanalyses and biomedical imaging is the extension of the excitation wavelength from the UV to the visible range (≥400 nm). β-Diketones are commonly used ligands for the coordination with lanthanide ions, and as antenna chromophores to sensitize these ions (11–13). Their advantages are: (i) β-diketones have strong absorption ability in a relatively wide wavelength range; and (ii) the carbonyl groups in β-diketones have strong coordination ability with lanthanide ions, and the formed chelate ring structure with lanthanide ions is very stable (14,15). In addition, non-charged ligands such as 1,10-phenanthroline (phen) or 2,2′-bipyridine (bipy) can serve as the synergistic agent, as they can prevent water molecules from binding to lanthanide ions and transfer energy to lanthanide ions more effectively (6).
* Correspondence to: Lining Sun and Liyi Shi, Research Center of Nano Science and Technology, Shanghai University, Shanghai 200444, People’s Republic of China. E-mail: [email protected]; [email protected] a
Research Center of Nano Science and Technology, Shanghai University, Shanghai 200444, People’s Republic of China
b
State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China Abbreviations: DR, diffuse reflectance; DSC, differential scanning calorimetry; EDTA, ethylene diamine tetraacetic acid; TGA, thermogravimetric analysis.
Copyright © 2015 John Wiley & Sons, Ltd.
L. Sun et al. In this paper, the dinaphthylmethane (Hdnm) ligand containing a naphthalene ring, which can extend the absorption to the visible-light wavelength, is synthesized and utilized as one of the sensitizers of the lanthanide ions (16). The lanthanide complexes, [Ln(dnm)3phen] (Ln = Sm, Nd, Yb, Er, Tm, Pr), were synthesized and their properties were investigated using Fourier transform infrared (FT-IR) spectra, diffuse reflectance (DR) spectra and thermogravimetric analyses. The excitation spectra of the [Ln(dnm)3phen] complexes can extend to more than 500 nm in the visible region and, following visible-light excitation, they show not only the characteristic visible emission (Sm3+), but also NIR luminescence of respective Ln3+ ions (Sm3+, Nd3+, Yb3+, Er3+, Tm3+, Pr3+).
Experimental
Characterization 1
HNMR spectra were measured by a Bruker AVANCE-500 spectrometer with tetramethylsilane (TMS) as internal reference (CDCl3 as solvent). Fourier transform infrared (FT-IR) spectra were measured in the range of 4000–400 cm 1 on an American Thermo Nicolet Corporation model AVATAR370 FT-IR infrared spectrophotometer with the KBr pellet technique. The DR spectra were performed with a Hitachi U-4100 spectrophotometer. The excitation and emission spectra were recorded by an Edinburgh FLS920 fluorescence spectrometer equipped with a 450 W Xe-lamp as an excitation source and a monochromatoriHR320 equipped with a liquid-nitrogen-cooled R5509–72 PMT as the detector. Thermogravimetry (TG) was performed on a Netzsch STA 449 at a heating rate of 10 °C·min 1 under an air atmosphere.
Materials 1,10-Phenanthroline mono-hydrate (phenH2O, 99%, Aladdin), ethyl-2-naphthoate (Aldrich), 2′-acetonaphthone (Aldrich), sodium hydride (NaH, 95%, Aldrich), hydrochloric acid, N,Ndimethylformamide (DMF) and ethanol were commercially available and used without purification. The solvent tetrahydrofuran (THF) was used after desiccation by refluxing. Ytterbium oxide (Yb2O3, 99.99%), samarium oxide (Sm2O3, 99.99%), neodymium oxide (Nd2O3, 99.99%), erbium oxide (Er2O3, 99.99%), praseodymium oxide (Pr2O3, 99.99%), and thulium oxide (Tm2O3, 99.99%) were purchased from Sigma-Aldrich.
Synthesis of the dinaphthylmethane (Hdnm) ligand Dinaphthylmethane was prepared according to published methods (16,17). The golden yellow product was recrystallized from ethyl acetate and a golden yellow crystal was obtained: [C23H16O2]: 1HNMR (CDCl3): δ 7.9–8.6 (14H, m, C12H7), 7.14 (1H, s, CH), 1.56 (1H, s, OH).
Results and discussion Fourier transform infrared (FT-IR) spectra The FT-IR spectra of [Sm(dnm)3phen] and [Yb(dnm)3phen] complexes are shown in Fig. S1 (ESI) as a representative, since the FT-IR spectra of [Ln(dnm)3phen] complex (Ln = Sm, Nd, Yb, Er, Tm, Pr) are similar. In the spectra of [Sm(dnm)3phen] and [Yb(dnm)3phen], a strong absorption band at 1586 cm 1 appears, which can be assigned to the superposition of stretching vibration of C=N bond from phen ligand and asymmetrical stretching vibration of C=O bond from dnm ligand. Peaks at 474 and 472 cm 1 in curves a and b are attributed to the antisymmetric vibration absorption of the Sm–O bond and Yb–O bond, respectively. In addition, an absorption band at 509 cm 1 can be observed in both curves, which can be attributed to the stretching vibration of the Sm–N (or Yb–N) bond. Overall, the FT-IR results suggest the successful coordination of lanthanide ions with the dnm and phen ligands.
Synthesis of lanthanide complexes [Ln(dnm)3phen] (Ln = Sm, Nd, Yb, Er, Tm, Pr) Hydrated LnCl3 salts (Ln = Sm, Nd, Yb, Er, Tm, Pr) were obtained by dissolving Ln2O3 (Ln = Sm, Nd, Er, Yb, Pr, Tm) in hydrochloric acid (HCl). The resulting solutions were evaporated with heating. The hydrated LnCl3 salts after drying were dissolved in anhydrous ethanol, transferred to a volumetric flask and diluted. The concentration of the lanthanide ion was measured by titration with a standard ethylenediaminetetraacetic acid (EDTA) aqueous solution. The [Ln(dnm)3phen] complex was prepared according to the following process: the ligands Hdnm and phen in stoichiometric molar ratio were dissolved in a suitable volume of anhydrous ethanol. Then, an appropriate amount of 1 M NaOH solution was added to the solution dropwise, to adjust the pH value to about 7. A stoichiometric amount of LnCl3 ethanol solution was then added to the solution under stirring. The molar ratio of Ln3+:Hdnm:phen was 1:3:1. The mixture was heated to reflux and kept at 80°C for 6 h, and then cooled to room temperature. The precipitates were collected after filtration and recrystallized from an acetone/ethanol mixture, then dried at 80°C under vacuum for 10 h. The total synthesis procedure for [Ln (dnm)3phen] is illustrated in Scheme 1.
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O
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Scheme 1. Synthesis procedure for the [Ln(dnm)3phen] (Ln = Sm, Nd, Yb, Er, Tm, Pr) complex. Reagents and experimental conditions: (a) NaH, anhydrous THF, reflux; (b) LnCl3, phen, NaOH, ethanol, reflux.
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Vis-NIR luminescent lanthanide complexes upon visible-light excitation Diffuse reflectance (DR) spectra The DR spectra of [Ln(dnm)3phen] (Ln = Sm, Yb, Er, Tm, Pr) are shown in Fig. S2 (ESI). The spectra all exhibit broad absorption bands in the range of 200–410 nm, which can be assigned to the absorption of the organic ligands, likely corresponding to electronic transitions from the ground state (S0) to the excited state (S1) of the ligands. In the visible and NIR region of these spectra, each absorption band is attributed to the characteristic absorption transition of the corresponding lanthanide ions, that is, from the ground states to the excited states. The ground states of the Sm3+, Yb3+, Er3+, Tm3+, and Pr3+ ions are 6H5/2, 2 F7/2, 4I15/2, 3H6, and 3H4 levels, respectively (18). The excited states of each lanthanide ion are shown in the DR spectra, respectively (Fig. S2, ESI).
Antenna luminescence The excitation and emission spectra (visible and NIR regions) of [Sm(dnm)3phen] were measured and shown in Fig. 1. The excitation spectrum was obtained by monitoring the strongest emission band in the visible region of Sm3+ ion at 647 nm, which exhibits a broad band in the UV/visible region of 240–500 nm. This is due to the absorption by the dnm and phen ligands. The excitation spectrum monitored at the strongest emission band in NIR region at 948 nm was similar to that monitored at 647 nm (not given here). With photoexcitation with visible light 6
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at 401 nm, the [Sm(dnm)3phen] complex shows orange red light. As shown in Fig. 1(a), the visible emission spectrum consists of four emission bands at 565 nm, 609 nm, 647 nm, and 712 nm, attributed to 4G5/2 → 6H5/2, 4G5/2 → 6H7/2, 4G5/2 → 6H9/2, and 4 G5/2 → 6H11/2 transitions, respectively. The strongest transition in the visible region is the 4G5/2 → 6H9/2 transition. The transition located at 565 nm, 4G5/2 → 6H5/2, has a predominant magnetic dipole character (19). The intensity ratio of I(4G5/2 → 6H9/2)/I (4G5/2 → 6H5/2) can be used as a measure for the polarizability of the chemical environment of the Sm3+ ion (18,20) and, in this case, it was calculated to be 4.20. For the NIR emission spectrum of [Sm(dnm)3phen] complex (see Fig. 1b) excited with visible light, all the NIR emission bands also come from the 4G5/2 excited state, among which the strongest band at 948 nm is assigned to 4 G5/2 → 6F5/2 transition. The luminescence lifetime of [Sm(dnm) 3phen] was measured at room temperature by using an excitation wavelength of 401 nm and monitored around the strongest emission band at 647 nm. The luminescent decay curve was fitted by single exponential function and the lifetime of 4G5/2→6H9/2 transition was fitted to be 7.67 μs. Figure 2 shows the excitation and emission spectra of [Yb (dnm)3phen] complex. The excitation spectrum was measured by monitoring at the strongest emission band (977 nm). The key feature is that there is a broad band from 240 to 560 nm, assigned to the absorption of the dnm and phen ligands. Following the visible-light excitation on the [Yb(dnm)3phen] complex, the characteristic NIR emission of Yb3+ ion can be obtained. It shows a broad band in the range of 920–1100 nm, comprised of a sharp peak at 977 nm attributed to the 2F5/2 → 2F7/2 transition and broader vibronic components at the longer wavelength (8). It can be observed that the NIR emission of [Yb(dnm)3phen] complex is not a single sharp band but a broad band, which is caused by the splitting of the energy levels of the Yb3+ ion as a consequence of ligand field effects (21). Yb3+ ion is a more unusual case, which has only a single excited state, 2F5/2, 10200 cm 1 above the ground state 2F7/2. Therefore, the Yb3+ ion has some advantages for laser emission due to its very simple energy level scheme (22). The excitation and emission spectra of [Nd(dnm)3phen] complex were obtained and shown in Fig. 3. For the excitation spectrum (monitored at the strongest emission band of Nd3+ ion at 1059 nm), it shows a broad band ranging from 235 to 550 nm, which can be attributed to the π-π* electron transition of the
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Wavelength (nm) Figure 1. Excitation and emission spectra for the [Sm(dnm)3phen] complex in solid state: (a) excitation (λem = 646 nm) and visible emission (λex = 401 nm) spectra; (b) NIR emission spectrum (λex = 401 nm).
Luminescence 2015
Figure 2. Excitation (λem = 977 nm) and emission (λex = 401 nm) spectra for the [Yb(dnm)3phen] complex in solid state.
Copyright © 2015 John Wiley & Sons, Ltd.
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L. Sun et al.
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ligands. After excitation at 401 nm, the emission spectrum of [Nd (dnm)3phen] complex clearly shows three NIR emission bands at 896, 1059 and 1332 nm, assigned to 4F3/2 → 4I9/2, 4F3/2 → 4I11/2 and 4F3/2 → 4I13/2 transitions, respectively, which is characteristic emission of Nd3+ ion (23). Among the three emission bands, the intensity of 1059 nm emission band is the strongest, and for a long time this center has been found to have potential application in laser systems. The 1332 nm band offers the good opportunity to develop new materials (containing Nd3+ ion) suitable for the optical amplification operating at 1.3 μm, one of the telecommunication windows (8). The fluorescent spectra of [Er(dnm)3phen] complex are shown in Fig. 4. In the excitation spectrum, it is dominated by a broad band extending to 500 nm in the visible region, attributed to the absorption of the dnm and phen ligands. In addition, there is a weak band at 488 nm in the excitation spectrum, originating from the characteristic absorption transition 4I15/2→ 4F7/2 of Er3+ ion, consistent with the DR spectrum (Fig. S2, ESI). Because this absorption transition is much weaker than the absorption of the ligands, sensitization the luminescence of Er3+ ion by exciting the ligands absorption is much more efficient than directly exciting the absorption of Er3+ ion. The longer absorption wavelength allows excitation by using the visible light. Following the visible-light excitation, the characteristic NIR emission of Er3+ ion
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can be obtained. The emission band centered at 1537 nm covers large spectral range extending from 1419 to 1652 nm, attributed to the typical 4I13/2 → 4I15/2 transition of Er3+ ion. Erbium-doped materials have been attracted much interest for many years, because the transition around 1540 nm is in the right position of the third telecommunication window (24). For the study on the NIR luminescent complexes, most reports focus on Yb3+, Nd3+ and Er3+ion, however, the research on the NIR luminescence of Pr3+ and Tm3+ complexes has been less reported, especially with visible-light excitation. Figure 5 shows the excitation and emission spectra of [Pr(dnm)3phen] complex. Compared with Sm3+, Yb3+, Er3+ and Nd3+ ions, the emission spectrum of Pr3+ ion is more complicated, since the Pr3+ ion can display emission bands from three levels (3P0, 1D2 and 1G4) after exciting the absorption of organic ligands (25). In the excitation spectrum of [Pr(dnm)3phen] complex, it shows a broad band ranging from 240 to 540 nm due to the absorption of ligands, most of which extends to the visible region. Upon excitation with the strongest absorption at 454 nm, the NIR emission spectrum of [Pr(dnm)3phen] complex can be obtained. There are three bands located at 890, 1067, and 1500 nm, which is attributed to the transition from the excited state (1D2) to the 3F2, 3 F4 and 1G4 levels of Pr3+ ion (26). Some crystal-field fine structure can be observed, which is an indication that the Pr3+ ion occupies well-defined crystallographic sites in the complex (27,28). Figure 6 presents the excitation and emission spectra of [Tm (dnm)3phen] complex. In the excitation spectrum (monitored at 803 nm), a broad band extending to 600 nm visible region is observed, which can be attributed to the absorption of the ligands. Excitation of the ligands absorption band with visible light (λex = 425 nm), the NIR emission band located at 803 nm can be obtained, which is attributed to the 3H4 → 3H6 transition of Tm3+ ion (6). Time-resolved measurement was carried out on [Tm(dnm)3phen] complex monitored at 803 nm. The decay curve is singly exponential, and the lifetime of Tm3+ ion excited state (3H4) is 1.78 μs. \As described above, under excitation and the absorption of ligands with the visible-light, the characteristic visible-NIR emission of Ln3+ ion was obtained in the corresponding [Ln(dnm)3phen] complex (Ln = Sm, Yb, Nd, Er, Tm, Pr). Since no direct absorption of the lanthanide ions occurred at the visible excitation wavelengths (401 nm for [Sm(dnm)3phen], 401 nm for [Yb (dnm)3phen], 401 nm for [Nd(dnm)3phen], 401 nm for [Er(dnm)
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Figure 5. Excitation (λem = 1065 nm) and emission (λex = 454 nm) spectra for the [Pr(dnm)3phen] complex in the solid state.
Copyright © 2015 John Wiley & Sons, Ltd.
Luminescence 2015
Vis-NIR luminescent lanthanide complexes upon visible-light excitation
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We are grateful for the financial support from the National Natural Science Foundation of China (Grant Nos. 21231004, 21102088, 21174081), Innovation Program of Shanghai Municipal Education Commission (13ZZ073), the Science and Technology Commission of Shanghai Municipality (13NM1401100, 13NM1401101, 14520722200), Shanghai RisingStar Program (14QA1401800), and the project from State Key Laboratory of Rare Earth Resource Utilization (RERU2014012). We are also grateful to the Instrumental Analysis & Research Center of Shanghai University.
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3phen],
454 nm for [Pr(dnm)3phen], and 425 nm for [Tm(dnm) the obtained visible-NIR emissions do only originate from the lanthanide ions sensitized by the organic ligands moiety. This result shows that: (i) the triplet states of the ligands could match well with the lanthanide ions; and (ii) the ligands are able to transfer the absorbed visible light to the lanthanide ions via the antenna effect (29). 3phen]),
Thermogravimetric analyses In order to investigate the thermal stability of the [Ln(dnm) 3phen] complex (Ln = Sm, Yb, Nd, Er, Tm, Pr), thermogravimetric analysis (TGA) was performed on them. Representative TG weight loss curves and differential scanning calorimetry (DSC) curves of [Yb(dnm)3phen] and [Sm(dnm)3phen] are shown (Fig. S3; ESI), as the TGA diagrams of all the [Ln(dnm)3phen] complexes are quite similar. For the TG curves of [Yb(dnm)3phen] and [Sm(dnm)3phen], they both show two distinct weight loss stages. The first weight loss corresponds to the decomposition of the dnm ligand, followed by a second weight loss ascribed to the loss of the phen ligand (30). The analyses show that the melting points of the [Yb(dnm)3phen] and [Sm(dnm)3phen] complexes are around 320 °C.
Conclusions Based on the ligand Hdnm, six new ternary lanthanide complexes [Ln(dnm)3phen] (Ln = Sm, Nd, Yb, Er, Tm, Pr) have been synthesized. Their FT-IR spectra, DR spectra, optical properties, and TG-DSC analyses have been reported. With the Hdnm ligand, extension of the excitation wavelength from the UV to the visible range was achieved. Upon excitation of the ligand absorption bands with the visible light (401–460 nm), the characteristic visible-NIR luminescence of the corresponding lanthanide complexes was demonstrated, known as an antenna effect. With these NIR luminescent lanthanide complexes, the luminescent spectral region from 800 to 1650 nm can be covered completely, which is of particular interest for biomedical imaging applications, laser systems, and optical amplification applications. Furthermore, the above results are pertinent to the application of these complexes in NIR electroluminescent devices.
Luminescence 2015
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Luminescence 2015
