Near-Infrared Luminescence from Sol−Gel Materials Doped with

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J. Phys. Chem. C 2008, 112, 13240–13247

Near-Infrared Luminescence from Sol-Gel Materials Doped with Holmium(III) and Thulium(III) Complexes Song Dang,†,‡ Li-Ning Sun,†,‡ Hong-Jie Zhang,*,† Xian-Min Guo,†,‡ Zhe-Feng Li,†,‡ Jing Feng,†,‡ Hua-Dong Guo,†,‡ and Zhi-Yong Guo†,‡ State Key Laboratory of Rare Earth Resource Utilizations, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China, and Graduate School of the Chinese Academy of Sciences, Beijing, P. R. China ReceiVed: May 11, 2008; ReVised Manuscript ReceiVed: June 15, 2008

A series of ternary Ln(tta)3L complexes (Ln ) Ho, Tm; Htta ) 2-thenoyltrifluoroacetone; L ) 1,10phenanthroline, 2,2′-bipyridine, or triphenyl phosphate oxide) and their corresponding sol-gel hybrid materials formed via the in situ synthesis process (designated as Ln-T-L gel) were reported. The complexes and the gels were studied in detail, which suggest the complexes have been successfully synthesized in the corresponding gels. After excitation at the maximum absorption of the ligands, the complexes and the gels all show the characteristic near-infrared (NIR) luminescence of the corresponding Ln3+ ions, which is due to efficient energy transfer from the ligands to the central Ln3+ ions via an antenna effect. The radiative properties of the Ho3+ ion in the Ho-T-B gel are discussed by Judd-Ofelt analysis, indicating that the 5F5 f 5I6 transition of the Ho3+ ion in the Ho-T-B gel could be a possible candidate for optoelectronics. 1. Introduction The luminescence of lanthanide(III) complexes is unique in their high color purity, long lifetimes, and insensitivity to environmental quenching,1-9 which makes them of particular value in sensors and displays, and also is employed for applications such as fluoroimmunoassay and fluorescence microscopy.10-15 Lanthanide β-diketonate complexes exhibit intense emission lines when irradiated, due to the effective intramolecular energy transfer from the coordinating ligands to the central lanthanide ions, which in turn undergo the corresponding radiative emitting process, known as the antenna effect.16 Another beneficial effect of the organic ligands is to expel water from the first coordination sphere. It is well-known that water molecules in the first coordination sphere of lanthanide complexes can deactivate the excited vibration manifold, which can lead to luminescence quenching.17 Recently, there was developed interest in the photophysical properties of lanthanide complexes that are luminescent in the near-infrared (NIR) region, such as Er(III), Yb(III), and Nd(III),18-20 but the sensitization of other NIR-luminescent lanthanide ions, such as Ho(III) or Tm(III),21,22 is of high interest as supplementary emission wavelengths, which would then also be available. However, the lanthanide complexes have some disadvantages for practical applications, such as their low chemical, optical, and thermal stabilities. These problems can be expected to be solved by doping the lanthanide complexes into suitable hosts, which can be sol-gel glasses, inorganic-organic hybrid materials, polymers, or liquid crystals.23-27 The sol-gel technique has proven to be an excellent approach for the preparation of inorganic-organic hybrid materials. The obtained materials incorporated with the lanthanide complexes not only can provide some optical properties, such as spectral narrowing, sensitive * Author to whom correspondence should be addressed. Tel: +86-43185262127; Fax: +86-431-85698041. E-mail: [email protected]. † Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. ‡ Graduate School of the Chinese Academy of Sciences.

temperature dependence of fluorescence, and so on, but also have improved thermal stability and mechanical strength.28 Nevertheless, by using the conventional sol-gel approach, the sol-gel materials doped with the complexes have low solubility of lanthanide complexes in the sol-gel matrix.29 A preferred approach is the in situ synthesis of lanthanide complexes during the sol-gel process. The ligands and the lanthanide ions are doped into sol-gel precursor solutions, and therefore a homogeneous solution is made in the sol-gel precursor solution; finally the lanthanide complex with a higher lanthanide concentration is formed during the sol-gel process.30 Furthermore, the in situ sol-gel technique can essentially be directed toward the reduction or virtual suppression of groups responsible for luminescence quenching [e.g., silanol (Si-OH) groups and residual solvent]. Therefore, the in situ approach was selected to synthesize the Ho and Tm complexes in the corresponding sol-gel materials. In this paper, we investigate the synthesis of a series of NIRluminescent Ho(tta)3L [Htta ) 2-thenoyltrifluoroacetone; L ) 2,2′-bipyridine (bipy), or triphenyl phosphate oxide (TPPO)] and Tm(tta)3L [L ) 1,10-phenanthroline (phen), bipy, or TPPO] complexes and their corresponding sol-gel materials via the in situ synthesis process (designated as Ln-T-L gel). At the same time, the properties of the Ln(tta)3L complexes (Ln ) Ho, Tm) and Ln-T-L gels were investigated by infrared (IR), diffuse reflectance (DR), scanning electron micrographs (SEM), and luminescent spectra. The optical properties of the Ho(III) ion in the Ho-T-B gel are discussed by using the Judd-Ofelt theory.31,32 2. Experimental Section 2.1. Materials. Holmium oxide (Ho2O3, 99.99%) and thulium oxide (Tm2O3, 99.99%) were purchased from Yue Long Chemical Plant (Shanghai, China). 1,10-Phenanthroline monohydrate (phen · H2O, 99%, A. R.), 2,2′-bipyridine (g99.5%, A. R.), triphenyl phosphate oxide (99%, A. R.), 2-thenoyltri-

10.1021/jp8041632 CCC: $40.75  2008 American Chemical Society Published on Web 08/05/2008

NIR Luminescence of Ho-/Tm-Doped Sol-Gel Materials fluoroacetone (Htta, Fluka), and tetraethoxysilane (TEOS, Aldrich) were commercially available and used without purification. 2.2. Synthesis of Ho(tta)3L (L ) bipy, or TPPO) and Tm(tta)3L [L ) phen, bipy, or TPPO] Complexes. The LnCl3 · 6H2O (Ln ) Ho, Tm) were prepared from Ln2O3 by dissolution in the hydrochloric acid, respectively. Ho(tta)3bipy is prepared as follows: 3 mmol Htta and 1 mmol bipy were dissolved in anhydrous ethanol at room temperature, then 1 M sodium hydroxide solution was added dropwise to the solution to adjust the pH to about 7. One millimole HoCl3 · 6H2O in ethanol solution was then added to the mixture under stirring. The mixture was heated to reflux for 6 h. The precipitates were collected after filtration and dried at 70 °C under vaccum for 4 h. Yield: 70% (based on Ho3+ ion). The synthesis of other complexes was similar to that of Ho(tta)3bipy, except the molar ratio of Ln3+/Htta/ TPPO was 1:3:2 for the synthesis of Ln(tta)3(TPPO)2 complexes. Elemental Analysis: For Ho(tta)3bipy, Calcd: C, 41.35%; H, 2.35%; N, 2.84%. Found: C, 41.23%; H, 2.47%; N, 2.75%. For Ho(tta)3(TPPO)2, Calcd: C, 51.94%; H, 3.27%. Found: C, 51.82%; H, 3.29%. For Tm(tta)3phen, Calcd: C, 41.83%; H, 2.24%; N, 2.71%. Found: C, 41.92%; H, 2.18%; N, 2.67%. For Tm(tta)3bipy, Calcd: C, 41.18%; H, 2.34%; N, 2.83%. Found: C, 41.10%; H, 2.40%; N, 2.92%. For Tm(tta)3(TPPO)2, Calcd: C, 51.79%; H, 3.26%. Found: C, 51.71%; H, 3.30%. 2.3. In Situ Synthesis of Lanthanide Complexes via a Sol-Gel Process (Ln-T-L gel; Ln ) Ho, Tm; L ) phen, bipy, or TPPO). Ho-T-B gel was prepared as follows. The molar ratio of TEOS/ethanol/deionized water in the starting solution was 1:4:4. The deionized water was acidified with HCl. After stirring for 3 h, Htta, bipy, and HoCl3 · 6H2O ethanol solution were added into the starting solution. The molar ratio of Ho3+/ Htta/bipy was 1:3:1, and Ho3+/Si ) 1 mol %. The mixed solution was stirred for 4 h at room temperature to ensure homogeneous mixing and complete hydrolysis, and then placed in a sealed plastic container, which was kept at 45 °C until the precursor solution was converted into a transparent monolithic gel. The lanthanide complexes were supposed to be in situ synthesized during the conversion from sol to monolithic xerogel, which was accompanied by the evaporation of HCl. The synthesis of other gels was similar to that of Ho-T-B gel, except the molar ratio of Ln3+/Htta/ TPPO was 1:3:2 for the synthesis of the Ln-T-T gels. 2.4. Characterization. The CHN elemental analyses were carried out on a VarioEL analyzer. Fourier transform infrared (FTIR) spectra were measured within a 4000-400 cm-1 region on an American BIO-RAD Company model FTS135 infrared spectrophotometer with the KBr pellet technique. The UV-vis absorption spectra and DR measurements were recorded at ambient temperature at a SHIMADZU UV3600 spectrophotometer. The concentration of the complexes in acetone (or ethanol) was 5 × 10-5 M. Scanning electron micrographs were obtained using Hitachi S4800 microscope at an accelerating voltage of 15 kV. The fluorescence spectra were measured on a Horiba Jobin Yvon Fluorolog-3 fluorescence spectrophotometer, equipped with a 450 W Xe-lamp as the excitation source and a monochromator iHR320 equipped with a liquid-nitrogencooled R5509-72 PMT as detector. The time-resolved measurements, were done by using the third harmonic (355 nm) of a Spectra-physics Nd:YAG laser with a 5 ns pulse width and 5 mJ of energy per pulse as the source, and the NIR emission lines were dispersed by a HORIBA Jobin Yvon emission monochromator iHR320 equipped with liquid-nitrogen-cooled

J. Phys. Chem. C, Vol. 112, No. 34, 2008 13241

Figure 1. FTIR spectra of (a) Ho(tta)3bipy and (b) Ho-T-B gel.

R5509-72 PMT, and the data were analyzed with a LeCroy WaveRunner 6100 1 GHz oscilloscope. The luminescence lifetime was calculated by the Origin 7.5 software package. The spectra optical density D(λ)dλ of the Ho-T-B gel was recorded at a SHIMADZU UV3600 spectrophotometer at room temperature. The corresponding concentration of the Ho3+ ion in the Ho-T-B gel is calculated to be 1.1065 × 1020 Ho/cm3, using the formula NHo ) (nN)/[π(D/2)2L], where n is the molar number of the Ho3+ ion, N is 6.023 × 1023, L is the sample thickness (L ) 0.245 cm), and D is the diameter. 3. Results and Discussion 3.1. IR and DR Spectra. The Ho-T-P gel was not obtained in our experiment brcause of the precipitation, even when the concentration of Ho3+ ion dopant was decreased to 0.8 mol%, which can be attributed to the strong π-π interactions between the phenanthroline ligands of neighboring molecular complexes in the matrix, but the phenanthroline complexes often have a poor solubility in organic solvents. Therefore, the Ho(tta)3phen complex and Ho-T-P gel will not be investigated in the following discussion. The FTIR spectra of the Ln(tta)3L complexes and Ln-T-L gels were obtained, and only the Ho(tta)3bipy and the Ho-T-B gel will be described in detail as an example (Figure 1). For other complexes and gels, the FTIR spectra are quite similar to those of Ho(tta)3bipy and Ho-T-B gel, respectively, which will be present in Figure S1 to Figure S4 in the Supporting Information. For the Ho(tta)3bipy complex, peaks in the range of 400-438 cm-1 corresponding to νHo-O vibrations and the peak at 515 cm-1 associated with νHo-N vibrations can be observed. This suggests the formation of coordination bonds between Ho3+ ion and Htta, and Ho3+ ion and bipy, respectively.33,34 With regard to the FTIR spectrum of Ho-T-B gel, the formation of a Si-O-Si framework is evidenced by the peak located at 1083 cm-1 allocated to the Si-O-Si symmetric stretching vibrations and the band at 452 cm-1 attributed to O-Si-O bending vibrations.35 The bending vibration frequency of CdN has red-shifted to 1622 cm-1 in the Ho-T-B gel, suggesting the coordination of a Ho-N bond. A weak peak at 418 cm-1, which can be assigned to νHo-O stretching vibration,34,36 probably can be attributed to the low concentration of Ho3+ ion in the Ho-T-B gel (Ln3+/Si ) 1 mol%). These results can suggest the possible in situ formation of the lanthanide complexes in the corresponding Ln-T-L gels (Ln ) Ho, Tm). The DR spectra of Ho(tta)3bipy complex and Ho-T-B gel and Tm(tta)3bipy complex and Tm-T-B gel are shown in Figure 2

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Figure 2. DR spectra of (a) Ho(tta)3bipy and (b) Ho-T-B gel. The inset shows the energy level diagrams of Ho3+ ion. The transitions of the bands (nm) are shown using arrows.

Figure 4. Scanning electron micrograph images of the Ho-T-B gel (a) with a magnification of 40 000 and (b) with a magnification of 120 000.

Figure 3. DR spectra of (a) Tm(tta)3bipy and (b) Tm-T-B gel. The inset shows the energy level diagrams of Tm3+ ion. The transitions of the bands (nm) are shown using arrows.

and Figure 3, respectively. The other complexes and gels are quite similar to those of Ho(tta)3bipy and Tm(tta)3bipy complexes and Ho-T-B and Tm-T-B gels, respectively. Thus they are only present in Figure S5 to Figure S7 in the Supporting Information. The spectra all exhibit broad absorption bands in the range from 200 to 400 nm, which could correspond to electronic transitions from the ground-state level (π) S0 to the excited level (π*) S1 of the organic ligands. In the region above 400 nm in these curves, each absorption band can be assigned to the characteristic transition between two spin-orbit coupling levels of Ho3+ ion and Tm3+ ion (see the insets of Figure 2 and Figure 3, respectively). From the energy level scheme of the lanthanide ions, it is indicated that the band can be assigned to the transitions from the ground-state levels 5I8 and 3H6 to the excited levels of the Ho3+ ion and Tm3+ ion for Ho(tta)3bipy complex and Ho-T-B gel and Tm(tta)3bipy complex and TmT-B gel, respectively.37-39 3.2. SEM. The scanning electron micrographs of the LnT-L gels are quite similar, and the Ho-T-B gel will be displayed as the representative patterns, as shown in Figure 4. From the SEM images, it is quite clear that the material seems homoge-

neous, and no any segregation phenomena was observed, even when the magnification was increased to 120 000. This can be attributed to the in situ synthesis technique, by which all the compositions are mixed at a molecular level.30 Therefore, the hybrid organic-inorganic materials we obtained are quite uniformly composed. 3.3. Luminescent Properties. Lanthanide complexes have been well-known for their bright emissions under UV irradiation because of the effective energy transfer from ligands to central ions called the “antenna effect”, which exhibits in spectrum as the overlaps between the excitation spectrum of complex and the absorption spectra of its corresponding ligands.40,41 We will take the Ho(tta)3bipy complex as an example to discuss the antenna effect. The excitation spectrum of the Ho(tta)3bipy complex and the absorption spectra of the ligands (Htta and bipy) are shown together in Figure 4. The overlaps between the excitation band of Ho(tta)3bipy and the absorption bands of the ligands Htta and bipy can been observed clearly, which suggest the typical sensitization of the central Ho3+ ion by the ligands (Htta and bipy), an antenna effect. In the Fo¨rster model,42,43 the overlap between the emission spectrum of the donor and the absorption spectrum of the acceptor is essential to energy transfer phenomena. From Figure 5 we can observe the overlaps between the absorption spectrum of HoCl3 · 6H2O in ethanol and the emission spectra of ligands Htta and bipy, which can be concluded that both Htta and bipy ligands can sensitize the luminescence of the central Ho3+ ion.25 Meanwhile, it is also clear that the overlap between the absorption band of ligand Htta and the excitation band of the complex is much larger than that between the absorption band of ligand bipy and the excitation band of the complex, suggesting that Htta is a more efficient sensitizer than bipy for the luminescence of the central Ho3+ ion. Therefore, we can come to the conclusion that the intramolecular energy transfer in Ho(tta)3bipy occurs mainly between the ligand Htta and the central Ho3+ ion.44

NIR Luminescence of Ho-/Tm-Doped Sol-Gel Materials

Figure 5. The excitation spectrum for Ho(tta)3bipy as a solid state (a, monitored at 979 nm), and absorption spectra of Htta (b) and bipy (c) at 5 × 10-4 M in acetone. All spectra are normalized to a constant intensity at the maximum.

In the excitation spectrum of the Ho(tta)3bipy complex (Figure 4a), a broadband ranging from 250 to 500 nm is observed, which corresponds to the absorption of the ligands. It is also observed that some excitation bands come from the characteristic absorption transitions of the Ho3+ ion in the excitation band. These f-f transitions can be assigned to 5I8 f 5G5 (422 nm) and 5I8 f 5F1 + 5G6 (452 nm) of the Ho3+ ion, which is consistent with those of the DR spectrum (Figure 2a). Since these absorption transitions are weaker than those of the ligands, it is easily concluded that luminescence sensitized by excitation of the ligands is much more efficient than direct excitation of the Ho3+ ion. UV-vis absorption spectra of the Ln(tta)3L complexes are shown in Figure S8 in the Supporting Information. The curves of the five complexes are quite similar. Broad bands ranging from 290 to 390 nm are observed, corresponding to the π-π* transition of the ligands in the complexes. Upon excitation at the maximum absorption of the ligands (λex ) 386 and 362 nm, respectively), the NIR emission spectra of the Ho(tta)3bipy complex and the Ho-T-B gel were obtained, as shown in Figure 6. Three bands are observed for the Ho3+ ions centered at 978 nm, 1185 nm, and 1480 nm, corresponding to the 5F5f5I7, 5I6f5I8, and 5F5f5I6 transitions, respectively.21 The 978 nm emission is observed as the most prominent one, while the emissions at 1185 and 1480 nm are weaker. Together with the conclusion from the IR and DR spectra, this result can prove that the Ho complex has been in situ synthesized in the Ho-T-B gel. In particular, the emission broad at 1500 nm could be a suitable candidate for optoelectronics.45 The NIR emissions of the Ho(tta)3(TPPO)2 complex and Ho-T-T gel, shown in Figure 7 (λex ) 382 and 360 nm, respectively), are quite similar to those of the Ho(tta)3bipy complex and the Ho-T-B gel, and thus those will not be further discussed. Figure 8 displays the emission spectra of the Tm(tta)3phen complex and Tm-T-P gel, by irradiation at 380 and 360 nm, respectively. Both the spectra exhibit a sharp peak at 801 nm and a broadband emission of 1470 nm, which can be assigned to 3H4 f 3H6 and 3H4 f 3F4 transitions of Tm3+ ions, respectively. It is also noted that the 1470 nm band emission consists of three peaks, which are ascribed to the Stark splitting of 4f electronic levels.22 It confirms that the Tm complex has been in situ synthesized in the Tm-T-P gel, which is in accord

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Figure 6. Emission spectra of bipy (a, excited at 322 nm) and Htta (b, excited at 360 nm) at 5 × 10-4 M in ethanol. Absorption spectrum of HoCl3 · 6H2O at 5 × 10-4 M in ethanol (c). All spectra are normalized to a constant intensity at the maximum.

Figure 7. Emission spectra of the Ho(tta)3bipy complex (λex ) 386 nm) and Ho-T-B gel (λex ) 362 nm).

with the preliminary conclusion obtained from the IR and DR spectra. It is worth noting that the Tm3+ ion luminescence is less reported. Yet the luminescence is especially appealing for bioimaging, since its main NIR emission band is located around 800 nm, which corresponds to an absorption minimum for water and tissues.46 The NIR emission of other Tm complexes and their gels, shown in Figure 9 (λex ) 384 nm and 370 nm, respectively) and Figure 10 (λex ) 379 nm and 367 nm, respectively), are quite similar to those of the Tm(tta)3phen complex and the Tm-T-P gel, and thus those will not be discussed further. The luminescence lifetimes have been recorded in order to explore the coordination environment around the lanthanide ions in these complexes and gels. The luminescence decays were measured upon excitation at 355 nm and monitored around the most intense emission lines (at 980 nm for the Ho3+ ion and at 801 nm for the Tm3+ ion). The results fit well with monoexponential decays (Table 1), indicating that a unique and consistent coordination environment is present around the lanthanide ions. However, the lifetimes in the gels are lower than that in the complexes, which is possibly due to the quenching by O-H or silanol groups in the hybrid materials.25

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Figure 8. Emission spectra of the Ho(tta)3(TPPO)2 complex (λex ) 382 nm) and Ho-T-T gel (λex ) 360 nm).

Figure 9. Emission spectra of the Tm(tta)3phen complex (λex ) 380 nm) and Tm-T-P gel (λex ) 360 nm).

Figure 10. Emission spectra of the Tm(tta)3bipy complex (λex ) 384 nm) and Tm-T-B gel (λex ) 370 nm).

3.4. Intramolecular Energy Transfer. The lanthanide ions are characterized by a very low absorption coefficient due to the strictly parity forbidden f-f transitions in the lanthanide

Dang et al. ions, which makes direct excitation impossible. It has to form lanthanide complexes with organic ligands which strongly absorb light in the UV region and transfer the excitation energy from the triplet states of the ligands to the resonance levels of the 4fn configuration of the lanthanide ions. In our case, we obtain the characteristic Ln3+ ion emissions upon excitation at the maximum absorption of the organic ligands in both complexes and gels. The NIR luminescence obtained indicates that formation of the complexes between lanthanide ions and certain organic ligands has a double beneficial effect, not only protecting the central ions from their surroundings but also efficiently transferring the absorbed energy to the Ln3+ ions, an antenna effect. Therefore, a model is proposed to represent the indirect excitation process (see Scheme 1). First, the ligands absorb the energy and are excited from the singlet S0 groundstate to the singlet S1 excited state. Then the energy of the S1 state is transferred to the triplet excited-state of the ligands via the intersystem-crossing. Finally, the excited energy is transferred to the 4f levels of the Ln3+ ions, resulting in the emission of the sensitized Ln3+ ions. Scheme 1 depicts a detailed scheme of the energy transfer process of the Ln3+ (Ln ) Ho, Tm) ions. For the Ho(tta)3L complexes and the Ho-T-L gels (L ) bipy and TPPO, respectively), shown in Figure 6 and Figure 7, the emission bands are centered at 978 nm, 1185 nm, and 1480 nm, corresponding to the 5F5 f 5I7, 5I6 f 5I8, and 5F5 f 5I6 transitions, respectively. It can be concluded that the excitation energy is transferred from the ligands to the 4f levels of the Ho3+ ions via intramolecular energy transfer process, followed by the relaxation from the upper 4f levels to 5F5 and 5I6 first excited states of the Ho3+ ions, and then decays to 5I6, 5I7, and 5I with emissions of 978 nm, 1185 nm, and 1480 nm, 8 respectively. The Tm(tta)3L complexes and the Tm-T-L gels (L ) phen, bipy, TPPO, respectively) displayed in Figure 8, Figure 9, and Figure 10, respectively, are similar. The obtained band at 801 and 1470 nm can be assigned to 3H4 f 3H6 and 3H f 3F transitions of Tm3+ ions, respectively. First, the 4 4 ligands absorb the energy and pass it to the 4f levels of the Tm3+ ions through an intramolecular energy. Then a relaxation to the 3H4 level happens, followed by a decay to 3F4 and 3H6 exhibiting emissions at 801 and 1470 nm, respectively. According to Dexter’s theory,47 the energy gap between the lanthanide ions and the ligands should be just right, neither too big nor too small. If the energy gap is too big, the overlap between the donor and the accepter will diminish, and finally the energy transfer rate constant would decrease sharply. In contrast, if the energy gap is too small, there will be an energy back-transfer from the Ln3+ ions to the resonance level of the triplet states of the ligands. In our case, Htta, whose triplet state is 20400 cm-1,48 can match well with the 4f levels of the Ho3+ ion and the Tm3+ ion, and finally obtain an efficient NIRluminescence of the corresponding lanthanide ion. Because of the strong absorption within a large wavelength range, the β-diketones are one of the most popular ligands for lanthanide ions and consequently have been targeted to sensitize the lanthanide luminescence.49,50 It is also known that the water molecules in the first coordination sphere of the complexes can deactivate the excited vibration manifold. Therefore, the second ligands, such as phen, bipy, and TPPO, were introduced to the complexes, which can effectively saturate the first coordination sphere of lanthanide ions and protect the central ions against the water molecules that may quench the luminescence. Also, the second ligands can absorb excitation energy and transfer the energy to the excited states of the central ions.

NIR Luminescence of Ho-/Tm-Doped Sol-Gel Materials

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TABLE 1: Luminescence Lifetimes of the Complexes and Gels As Solid States complexes

Ho(tta)3bipy

Ho(tta)3(TPPO)2

Tm(tta)3phen

Tm(tta)3bipy

Tm(tta)3(TPPO)2

lifetimes (ns)

14.8

24.26

26.55

25.29

43.27

gels

Ho-T-B

Ho-T-T

Tm-T-P

Tm-T-B

Tm-T-T

lifetimes (ns)

9.72

18.26

14.61

18.84

25.37

SCHEME 1: Model for the Main Intramolecular Energy Transfer Process (Left) and Energy Diagram of the 4f Levels of the Ho3+ and Tm3+ Ions (Right)

TABLE 2: Measured and Calculated Line Strengths of the Ho3+ Ion in the Ho-T-B Gela level 5

F1 G6 5 F3 5 F4 5 F5 5 I6

λ (nm)

Sexp ( × 10-20 cm2)

Scal ( × 10-20 cm2)

452

6.3004

6.3010

486 538 642 1183

0.1494 0.8613 0.8282 0.8441

0.3435 0.8564 0.8390 0.7430

5

a

All transitions are from the 5I8 state.

TABLE 3: Spectra Parameters for the 5F5 f 5IJ Transition of the Ho3+ Ion in the Ho-T-B Gel A λ transition (nm) Scal ( × 10-20 cm2) (S1-) 5F 5 5F 5

3.5. Judd-Ofelt Analysis. The Judd-Ofelt theory is widely used to calculate the optical properties of rare earth ions in various hosts. The radiative properties of rare earth organic complexes, which are essential to study the potential of rareearth-doped materials further, can be predicted from optical absorption measurements by using the Judd-Ofelt theory. The absorption spectrum of the Ho-T-B gel is shown in Figure 11. Five Ho3+ ion absorption bands in the absorption spectrum were chosen to determine the phenomenological oscillator strength parameters. According to the Judd-Ofelt theory, the data of absorption spectra can be used to predict the radiative lifetime of the 5F5 excited J manifold and the branching ratios of the fluorescence transitions to the lower-lying 5IJ manifold.

f 5I6 f 5I7

1480 978

β

0.6140 36.79 0.198 0.7159 148.66 0.802 τR ) 5.39 ms

σe( × 10-20 cm2) 0.149 0.222

The band positions, together with the assignments in the absorption spectrum, are shown in Table 2. The measured absorption line strength Sexp for transitions from the ground-state to the excited J′ level can be obtained using the following expression:51,52 3 2

8π e λ 1 L ∫ D(λ)dλ ) NJ 2.303 3hc(2J + 1) n

[

]

(n2 + 2)2 Sexp (1) 9

where D(λ)dλ is the measured optical density as a function of the wavelength, NJ is the Ho3+ concentration (NHo ) 1.1065 × 1020 cm-3 Ho/cm3), h is Planck’s constant, e is the electron charge, λj is the mean wavelength of the absorption band, c is the vacuum speed of light, n is the refractive index of the gel (n ) 1.5), J is the total angular momentum of the ground state (J ) 8 for Ho3+ ion), and L is the sample thickness (L ) 0.245 cm). According to the Judd-Ofelt theory, the experimental oscillator strength for an electric-dipole transition between the initial J manifold |(S,L,)J〉 and terminal J′ manifold |(S′,L′)J′〉 can be expressed in the formula53

Scal(J f J ′ ) )

∑

Ωt|〈(S, L)J|U(t) |(S ′ , L′)J′〉|2

(2)

t)2,4,6

Figure 11. Emission spectra of the Tm(tta)3(TPPO)2 complex (λex ) 379 nm) and Tm-T-T gel (λex ) 367 nm).

where U(t) (t ) 2, 4, 6) are the matrix elements of the unit tensor calculated by Carnall et al.,54 since the reduced matrix elements exhibit only small variations with the host lattice.55 The oscillator strength parameters Ωt (t ) 2,4,6), which are independent of electronic quantum numbers with the ground 4fn configuration of the Ho3+ ion, may be regarded as phenomenological parameters that characterize the radiative transition probabilities within the ground configuration. Using eq 1, the experimental oscillator strength Sexp for the electric-dipole transition can be obtained. After a least-squares fitting of Sexp to Scal, the three Judd-Ofelt intensity parameters Ω2 ) 3.70 × 10-20 cm2, Ω4 ) 0.65 × 10-20 cm2, and Ω6 ) 0.99 × 10-20 cm2 were obtained. It is known that the parameter of Ω2 is usually related to the degree of covalency in the lanthanide-first coordination shell interaction.56 The higher the value, the stronger the covalence

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characteristics of the gel. The relatively large value of Ω2 suggests the weaker ionic characteristics of the gel. To justify our fitting quality, we use these parameters and eq 2 to recalculate the oscillator strength Scal of the five absorption bands, which are listed in Table 2. The root-mean-square (rms) deviation between the experimental and calculated strengths was defined by

rms∆S )



N

∑ (Sexp - Scal)2/(N - M)

(3)

i)1

where N is the number of the spectral bands, and M is the number of parameters determined; N ) 5 and M ) 3 in this case. A measurement of the relative error of the fitting is given by rms error ) (rms∆S/rmsS) × 100%, where rmsS ) [(∑Scal2)/ (N - M)]1/2. The rms error is 3.39%, which confirms a good consistency of our fitting between the experimental and calculated line strengths. The Judd-Ofelt intensity parameters were applied to eq 4 to calculate the line strengths. The expression for the radiative transition rate A from the J′ manifold |(S′,L′)J′〉 to terminal initial j ,)Jj 〉 can be determined by the following J manifold |(Sj, L expression:

A[5F5 ;(S, L)J] ) n(n2 + 2)2 64π4e2 Ωt|〈5F5|U(t) |(S, L)J〉|2 (4) × 3 9 3h(2J ′ + 1)λ t)2,4,6

∑

where J′ ) 5, and λj is the mean wavelength of the emission j )Jj 〉|2 is given in transition shown in Figure 6. |〈5F5|U(t)|(Sj, L 3+ ref 57 for Ho ion. Then the radiative lifetime τR for the excited 5F state of the Ho3+ ion is determined from the radiative 5 j )Jj ] using eq 5: transition rate A[5F5;(Sj, L

τR )

1

∑ A[ F5;(S, L)J] 5

(5)

S,L,J

The fluorescence branching ratio β can be determined from the radiative transition rate by

β[5F5, (S, L)J] )

A[5F5, (S, L)J]

∑ A[5F5, (S, L)J]

(6)

S,L,J

The value of the radiative transition rates A, the fluorescence branching ratio β, and the radiative lifetime τR are presented in Table 3. In our calculations we have not considered the terms involving magnetic dipole line strength since most of the sharp bands arising from the f-f transitions are essentially of the electric dipole type. The radiative life of the 5F5 state has been calculated to be 5.39 ms. The experimental lifetime of the 5F5 state is much lower than the value calculated from the theory. This is owing to the nonradiative relaxation proceeding in the Ho-T-B gel in the presence of the O-H and C-H groups in the sol-gel-derived materials, which can lead to the great difference between the calculated lifetime and the experimental value. With the corresponding emission spectrum, for a Lorentz line, the stimulated emission cross-section σe can be calculated by

σe )

Aλ2 4π2n2∆ν

(7)

where λ is the wavelength of the emission peak, and ∆ν is the frequency full width at half-maximum. The values of σe, which

Figure 12. Absorption spectrum of Ho-T-B Gel.

are also present in Table 3, can be compared with those obtained for the same transitions in inorganic hosts. From the magnitudes of the fluorescence branching ratio, calculated radiative lifetime, and stimulated emission cross section, it is therefore concluded that the 5F5 f 5I6 transition of Ho3+ ion in sol-gel is favorable for laser action.57,58 4. Conclusions A series of Ho and Tm complexes and their corresponding in situ sol-gel-derived transparent homogeneous hybrid materials were successfully synthesized. The properties of Ln-T-L gels were investigated and compared with the corresponding complexes. All the evidence showed that the complexes have been successfully in situ synthesized in the gels. The NIR-luminescent properties of the complexes and their gels were studied, and an indirect excitation model was proposed. Furthermore, the Judd-Ofelt theory was used to discuss the radiative properties of Ho3+ ion in the Ho-T-B gel. On the basis of the data obtained and Judd-Ofelt analysis, we come to the conclusion that the broad emission at 1500 nm of Ho can be considered as a possible candidate for optoelectronics. Acknowledgment. The authors are grateful to the financial aid from the National Natural Science Foundation of China (Grant No. 206301040) and the MOST of China (Grant No. 2006CB601103). Supporting Information Available: FTIR spectra of Ho(tta)3(TPPO)2, Tm(tta)3phen, Tm(tta)3bipy, Tm(tta)3(TPPO)2, and their corresponding gels. DR spectra of Ho(tta)3(TPPO)2, Tm(tta)3phen, Tm (tta)3(TPPO)2, and their corresponding gels, and the absorption spectra of all the complexes as solid state. This information is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Forsberg, J. H. In Gmelin Handbook of Inorganic Chemistry: Sc, Y, La-Lu Rare Earth Elements; Springer-Verlag: Berlin, 1981; Vol. D3, p 65. (2) Melby, L. R.; Rose, N. J.; Abramson, E.; Caris, J. C. J. Am. Chem. Soc. 1964, 86, 5117. (3) de Sa´, G. F.; Malta, O. L.; Donega, C. D.; Simas, A. M.; Longo, R. L.; Santa-Cruz, P. A.; da Silva, E. F. Coord. Chem. ReV. 2000, 196, 165. (4) Batista, H. J.; de Andrade, A. V. M.; Longo, R. L.; Simas, A. M.; de Sa´, G. F.; Ito, N. K.; Thompson, L. C. Inorg. Chem. 1998, 37, 3542.

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