Near-infrared luminescent copolymerized hybrid materials built from tin nanoclusters and PMMA

Abstract

Novel near-infrared (NIR) luminescent copolymerized hybrid materials were prepared by covalently grafting and physically doping Ln complexes (Ln = Er, Sm, Yb, and Nd) into a copolymer matrix built from nanobuilding blocks. The structures of the obtained hybrid materials were investigated by Fourier transform infrared (FTIR) spectra, nuclear magnetic resonance (NMR), gel permeation chromatography (GPC), and thermogravimetric analysis (TGA). In the photoluminescence studies, the hybrid materials showed characteristic NIR luminescence of corresponding Ln³⁺ ions through intramolecular energy transfer from ligands to Ln³⁺ ions. Transparent films of these materials can be easily prepared through spin-coating on indium tin oxide (ITO) glasses taking advantage of the matrix nature.

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Introduction

While increasing studies have been performed on the utilization of the NIR emission of rare earths in bioimaging,¹ optical electroluminescent,² and laser materials,³ it remains an interesting pursuit for people to synthesize new kinds of rare earth optical materials with wide applications. Among all the rare earth ions, Er(III), Sm(III), Yb(III), and Nd(III) are commonly popular ions used in preparing materials with NIR luminescence, which is attributed to both their wide wavelength range (varying from 900 nm to 1600 nm) and their relative strong emitting.⁴ However, since the Laporte-forbidden 4f → 4f transitions can prevent direct excitation of the photoluminescence and make the absorption coefficients of the lanthanide ions very low,⁵ separate lanthanide ions are rarely chosen in the synthesis of optical hybrid materials due to their unsatisfactory luminescent efficiency. Interestingly, lanthanide complexes with organic ligands have more efficient absorption coefficients and energy transfer between ligands and lanthanide ions, which have found wide applications in optical hybrid materials.⁶ Considering their relative low stability and plasticity, lanthanide complexes were usually applied through introducing them into matrixes. Some kinds of stable organic and inorganic materials have been recommended as matrixes for doping (or grafting) lanthanide complexes inside, and corresponding hybrid materials with excellent optical properties have been reported.⁷ But, as we all know, lanthanide complexes are usually hydrophobic, thus making them difficult to dope in some inorganic matrixes especially through physically doping. So, it is still a topic of long-term interest for people to explore new matrix materials suitable for doping lanthanide complexes without destroying their structure and reducing their optical properties.

Organic-inorganic hybrid materials built from nanobuilding blocks have attracted intensive attention in the field of class II hybrid materials due to their multiple properties from both organic and inorganic components.⁸ Since the oxo-hydroxo butyltin cluster {(BuSn)₁₂O₁₄(OH)₆}(OH)₂ has been reported in several papers, it has become challenges to design inorganic/organic hybrid materials from butyltin cluster nanobuilding blocks.⁹ L. Angiolini and F. Ribot, et al. have reported some kinds of inorganic/organic hybrid materials prepared from functionalized {(BuSn)₁₂O₁₄(OH)₆}²⁺ clusters,¹⁰ and also doped the hybrid materials with photochromic dyes as functional hybrid materials.¹¹ In our recent report,¹² we successfully designed europium complex-doped polymer materials built from oxo-hydroxo organotin nanobuilding blocks (Sn₁₂clusters) and MMA. The europium complexes were used as optical probes, and visible optical property of the hybrid materials were investigated. We found that this kind of the matrix could be easily grafted with europium complexes, and also has a high capacity for physical doping for europium complexes, which could be attributed to their special structure where the PMMA chains are bridged with nanobuilding blocks endowing the material with micro-space structure. This flexible polymer structure also makes these kinds of luminescent hybrid materials have shape controllability. So, it is considered as a novel matrix for lanthanide (Ln) complexes. Otherwise, the Ln complexes have been studied in the optical field for a long time, and their optical properties are mainly classified into two parts (visible and near-infrared emitting), due to their remarkably different wavelength and applications in optics. Therefore, we further prepared NIR luminescent hybrid materials built from tin nanoclusters in order to extend the application of this matrix in NIR optical area.

In this paper, we introduced NIR lanthanide complexes into the copolymer based on Sn₁₂clusters. Two kinds of the NIR lanthanide complexes were prepared: Ln(TTA)₃phen (Ln = Er, Sm, Yb, and Nd) were used for physically doping inside the copolymer, while LnAA(TTA)₂phen for grafting with PMMA (TTA = 2-thenoyltrifluoroacetone, phen = phenanthroline and AA = acrylic acid). The obtained hybrid materials exhibited characteristic NIR luminescence of corresponding Ln³⁺ ions. Furthermore, the structure and properties of these hybrid materials were characterized by GPC, TGA, SEM, ¹H NMR, ¹¹⁹Sn NMR, FTIR, diffuse reflectance (DR) spectra, and photoluminescence (PL) spectra respectively. Transparent films are also prepared from PMMA-co-Sn₁₂Cluster/Ln(TTA)₃phen and PMMA-co-Sn₁₂Cluster-co-LnAA(TTA)₂phen by spin-coating.

Results and discussion

2.1 Structural characterizations of PMMA-co-Sn₁₂Cluster/Ln(TTA)₃phen and PMMA-co-Sn₁₂Cluster-co-LnAA(TTA)₂phen

The proposed mechanisms for the formation of PMMA-co-Sn₁₂Cluster-co-LnAA(TTA)₂phen (step a) and PMMA-co-Sn₁₂Cluster/Ln(TTA)₃phen (step b) hybrid materials are described in Scheme 1.

Scheme 1 The proposed mechanisms for the formation of PMMA-co-Sn₁₂Cluster/Ln(TTA)₃phen and PMMA-co-Sn₁₂Cluster-co-LnAA(TTA)₂phen hybrid materials (Ln = Er, Yb, Nd, and Sm).

In steps a and b, {(BuSn)₁₂O₁₄(OH)₆}₂(MA)₂ was introduced as nanobuilding blocks into the polymers through covalent polymerization with MMA, and the MMA forms the PMMA chains bridged with Sn₁₂ clusters due to the two methacrylate groups on the opposite sides of the Sn₁₂ clusters. Thus, the soft PMMA chains were supported by rigid inorganic clusters and the structure with micro-spaces formed. The copolymerization was verified by IR spectra (Fig. 1), the four sharp bands (672 cm⁻¹, 623 cm⁻¹, 536 cm⁻¹, and 430 cm⁻¹) due to the vibrations related to the Sn–O–Sn framework were maintained in {(BuSn)₁₂O₁₄(OH)₆}₂(MA)₂ and products (PMMA-co-Sn₁₂Cluster, PMMA-co-Sn₁₂Cluster/Er(TTA)₃phen, and PMMA-co-Sn₁₂Cluster-co-ErAA(TTA)₂phen),¹³ which could approximately indicate that the Sn₁₂ cluster framework were preserved during the reaction process. Moreover, two bands at 3650 cm⁻¹ and 1647 cm⁻¹ in {(BuSn)₁₂O₁₄(OH)₆}(MA)₂ are attributed to the typical ν(O–H) of Sn–OH around and ν_as(C=C) of the MA, and the copolymerization between MA and MMA could be easily explained from the disappearance of ν_as(C=C) in the copolymers. The 1581 and 1417 cm⁻¹ (COO–) were found in the ErAA(TTA)₂phen complexes, which suggested that the AA ligand was bonded with the Er³⁺ ions. the ErAA(TTA)₂phen complexes with active CH=CH₂ group could subsequently be copolymerized with MMA and {(BuSn)₁₂O₁₄(OH)₆}(MA)₂ to form PMMA-co-Sn₁₂Cluster-co-ErAA(TTA)₂phen hybrid materials. The IR data of the other samples (such as Sm, Yb, and Nd) were similar to the Er sample.

Fig. 1 IR spectra for {(BuSn)₁₂O₁₄(OH)₆}₂(OH)₂ (a), {(BuSn)₁₂O₁₄(OH)₆} (MA)₂ (b), PMMA-co-Sn₁₂Cluster (c), PMMA-co-Sn₁₂Cluster/Er(TTA)₃phen (d), PMMA-co-Sn₁₂Cluster-co-ErAA(TTA)₂phen (e).

As previously reported, the Sn₁₂ cluster is composed of six tin atoms in the five-coordinate sites and six atoms in the six-coordinate sites respectively, and the Sn₁₂ cluster fine structure in the products could be studied by ¹¹⁹Sn NMR (CD₂Cl₂) through testing the tin atoms in the two sites.¹⁴ As see in Fig. 2, the ¹¹⁹Sn NMR spectra of the hybrid materials (PMMA-co-Sn₁₂Cluster, PMMA-co-Sn₁₂Cluster/Er(TTA)₃phen, and PMMA-co-Sn₁₂Cluster-co-ErAA(TTA)₂phen) all show the two characteristic resonances at −282 and −454 ppm, which correspond to the five- and six-coordinate tin atoms in the Sn₁₂ cluster respectively.¹¹ The ratio of the two resonances is almost 1 : 1. Therefore, we consider that the structure of the Sn₁₂ cluster in the hybrid materials was preserved, and Er complexes doped or grafted into the polymer system hardly affected the existence of Sn₁₂ cluster.

Fig. 2 ¹¹⁹Sn NMR spectra for PMMA-co-Sn₁₂Cluster (a), PMMA-co- Sn₁₂Cluster/Er(TTA)₃phen (b), and PMMA-co-Sn₁₂Cluster-co-ErAA(TTA)₂phen (c).

During the copolymerization process, though the feeding ratios of the Sn₁₂ cluster and the MMA are all 1 : 30, the ratios would be changed in the obtained products due to incomplete polymerization. The molar ratios of the Sn₁₂ cluster and the MMA in the products could be calculated according to the TGA data. The molar ratios altered between 40 : 1 and 64 : 1, which are lower than the feeding ratio. In Table 1, the number-average molecular weight (M_n), and polydispersity indexes (M_w/M_n) of final products are in the range of 8000–11 000 and 1.2–1.3, respectively, the low molecular weight would enhance the solubility of the hybrid materials and facilitate the formation of films. The Er/Sn molar ratio determined by ICP in the PMMA-co-Sn₁₂Cluster-co-ErAA(TTA)₂phen is just about 2% (the feeding ratios is 5%).

Table 1 GPC and TGA data of the samples

	SnO2 content (wt%)	MMA/Sn12Cluster^a	M n/g mol^−1	M w/Mn
a Experimental calculated MMA/Sn12Cluster molar ratio in hybrid materials.
PMMA-co-Sn12Cluster	26.6	42/1	10347	1.3
PMMA-co-Sn12Cluster-co-ErAA(TTA)2phen (5%)	23.5	51/1	8186	1.2
PMMA-co-Sn12Cluster-co-SmAA(TTA)2phen (5%)	21.6	57/1	8278	1.2
PMMA-co-Sn12Cluster-co-YbAA(TTA)2phen (5%)	21.0	60/1	8071	1.2
PMMA-co-Sn12Cluster-co-NbAA(TTA)2phen (5%)	20.1	64/1	8372	1.2

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2.2. Optical property studies

2.2.1. DR Spectra. Improvement of the luminescence intensity of the lanthanide complexes is typically achieved through resonant energy transfer from the lowest triplet states of ligands to the lanthanide ions and then gives the luminescence through f–f transitions.¹⁵ So, the absorption bands of the ligands play an important role in studying the optical property of the lanthanide complexes,¹⁶ and here we demonstrate the DR spectra of the lanthanide complexes and corresponding hybrid materials in order to learn about their absorption position and assist in research of the excitation bands of the following PL spectra. In Fig. 3, the DR spectra of LnAA(TTA)₂phen and Ln(TTA)₃phen exhibit both broad absorption (below 400 nm) from the ligands and also absorption bands of the lanthanide ions. Among these four kinds of lanthanide ions, the ground state to the excited states of the Er³⁺ ions are both observed in the Er(TTA)₃phen and ErAA(TTA)₂phen, where the bands at 488, 522, and 652 nm are assigned to ⁴I_15/2 → ⁴F_7/2, ⁴I_15/2 → ²H_11/2, and ⁴I_15/2 → ⁴F_9/2 respectively,¹⁷ while the bands at 527, 580, and 683 nm correspond to the f–f transitions of the Nd³⁺ ions: ⁴I_9/2 → ²K_13/2 + ⁴G_7/2, ⁴I_9/2 → ²G_7/2, and ⁴I_9/2 → ⁴F_9/2.¹⁸ Contrarily, the absorption bands of the Yb³⁺ and Sm³⁺ ions in the corresponding samples were not discerned, because the main absorption of these two ions commonly appears above 800 nm. Basically, the hybrid materials doped and grafted with lanthanide complexes showed absorption of ligands and absorption of lanthanide ions. But, the absorption intensities were relatively weak, which could be attributed to the low concentration of lanthanide complexes in the polymer matrix. However, to some extent, we could suggest that the lanthanide complexes have been embedded into the polymer matrix by both ways according to the DR spectra.

Fig. 3 DR spectra for Ln(TTA)₃phen (a), LnAA(TTA)₂phen (b), PMMA-co-Sn₁₂Cluster/Ln(TTA)₃phen (c), PMMA-co-Sn₁₂Cluster-co-LnAA(TTA)₂phen (d) (Ln = Er, Sm, Yb, and Nd).

2.2.2. PL spectra. Lanthanide complexes were widely used in the hybrid materials due to their excellent luminescence properties, because the lanthanide luminescence can be remarkably improved through an intramolecular energy transfer process, which is called an “antenna effect”. Otherwise, due to the strong coordination capacity between the ligands and lanthanide ions, the lanthanide complexes could keep their structures and still possess outstanding an optical nature in the matrix materials.^5a Then, to provide a clear understanding of the photoluminescence properties of these hybrid materials with different lanthanide ions, the excitation and emission spectra of each lanthanide hybrid materials were characterized and discussed below.

The excitation and emission spectra of Er complexes and hybrid materials (PMMA-co-Sn₁₂Cluster/Er(TTA)₃phen, and PMMA-co-Sn₁₂Cluster-co-ErAA(TTA)₂phen). As is shown in Fig. 5, the excitation spectra of the Er samples were obtained by monitoring the characteristic emission of the Er³⁺ ions at 1535 nm. In the excitation spectra, there are broad bands ranging from 250 to 450 nm for Er complexes and broad bands from 250 to 400 nm for Er hybrid materials, which are also in agreement with the DR spectra mentioned above. We then compared the excitation spectra with the absorption spectra of corresponding ligands in order to further study the broad bands in the excitation spectra. In Fig. 4, the absorption bands of the TTA and phen are overlapped with the excitation spectra, especially the bands of the TTA are almost consistent with the excitation spectra, which indicate that the excitation spectra mainly came from the organic ligands and there are energy transfers between ligands and Er³⁺ ions. Otherwise, the excitation bands of Er hybrid materials are relatively narrower, which may be attributed to the change of the environment surrounding the Er complexes in polymer matrix materials.¹⁹ In the excitation spectra, we also could see the f–f absorption transitions originating from the Er³⁺ ions above 400 nm. The bands at 450 nm and 486 nm can be attributed to the energy transitions ⁴I_15/2 → ⁴F_5/2 and ⁴I_15/2 → ⁴F_7/2. Based on the excitation spectra, excitation bands are mainly contributed by the ligands, which indicated that the ligands still coordinated to the Er³⁺ ions and then the ligands transferred the energy absorbed from the ultraviolet (UV) light to the Er³⁺ ions. Moreover, it also gives a further proof that the Er complexes still keep their structure in the polymer matrix materials whether physically doping or grafting.

Fig. 4 UV-vis absorption spectra of phen (a) and TTA (b) in ethanol, excitation spectra for PMMA-co-Sn₁₂Cluster/Er(TTA)₃phen (c) and PMMA-co-Sn₁₂Cluster-co-ErAA(TTA)₂phen (d).

Fig. 5 Excitation (EX, λ_em = 1535 nm) and emission spectra for Er(TTA)₃phen (red solid line, λ_ex = 381 nm), ErAA(TTA)₂phen (blue solid line, λ_ex = 381 nm), PMMA-co-Sn₁₂Cluster/Er(TTA)₃phen (red dot line, λ_ex = 365 nm), PMMA-co-Sn₁₂Cluster-co-ErAA(TTA)₂phen (blue dot line, λ_ex = 361 nm).

The emission spectra are shown in Fig. 5; all the Er samples were excited at a wavelength that maximises the excitation broad band according to excitation spectra respectively, the emission maxima all located at 1535 nm which is attributed to the transition from the first excited state (⁴I_13/2) to the ground state (⁴I_15/2) of Er³⁺ ions. The emission bands cover large spectral ranges extending from 1445 to 1640 nm, and all the samples have almost the same spectral width, it may suggest that the light emitting sites in hybrid materials are same as the Er complexes. Therefore, the luminescence of hybrid materials came from the corresponding Er complexes, and the polymer matrix just functionalized as the skeleton without damage to the Er complexes. In addition, PL intensity of the ErAA(TTA)₂phen is lower than the Er(TTA)₃phen which can be attributed to the molecular vibration of the AA, however, the low intensity of the PMMA-co-Sn₁₂Cluster-co-ErAA(TTA)₂phen compared with the PMMA-co-Sn₁₂Cluster/Er(TTA)₃phen are mainly because of the lower ratios of the ErAA(TTA)₂phen and Sn₁₂ cluster than the feeding ratio during the grafting process according to the ICP data.

The transition ⁴I_13/2 → ⁴I_15/2 of Er³⁺ ion is in the right position for the third telecommunication window. Therefore, these Er hybrid materials with 1.5 μm emission of Er³⁺ ions have attracted considerable attention.²⁰ The full width at half maximum (fwhm) of the erbium is an important parameter for potential application in the telecommunication area, due to the urgent demand for optical amplifiers with a wide and flat gain spectrum in the telecommunication window because of the rapid increase of information capacity recently. However, the erbium-doped inorganic materials commonly show relatively small fwhm, so erbium organic compounds, endowed with the nature of broad-band emission at 1.5 μm have been intensively studied.^17,21 The Er samples we prepared here have a relative broad band, the fwhm of the ⁴I_13/2 → ⁴I_15/2 transition for PMMA-co-Sn₁₂Cluster-co-ErAA(TTA)₂phen and PMMA-co-Sn₁₂Cluster/Er(TTA)₃phen are 65 and 60 nm respectively. Such relative broad spectra may enable a wide gain bandwidth for optical amplification.

Similarly, the PL spectra of the other three Ln (Sm, Yb, and Nb) hybrid materials were obtained, and the emission spectra showed strong and characteristic luminescence of the three kinds of Ln³⁺ ions.

In Fig. 6, the strong bands at 955 nm are observed in the emission spectrum of the Sm samples and are assigned to the transitions ⁴G_5/2 → ⁶F_5/2 of the Sm³⁺ ions, and two other weak bands (1034 nm and 1185 nm) could also be observed in the longer wavelength, which came from the transitions of ⁴G_5/2 → ⁶F_7/2 and ⁴G_5/2 → ⁶F_9/2.

Fig. 6 Excitation (EX, λ_em = 955 nm) and emission spectra for Sm(TTA)₃phen (red solid line, λ_ex = 381 nm), SmAA(TTA)₂phen (blue solid line, λ_ex = 381 nm), PMMA-co-Sn₁₂Cluster/Sm(TTA)₃phen (red dot line, λ_ex = 365 nm), PMMA-co-Sn₁₂Cluster-co-SmAA(TTA)₂phen (blue dot line, λ_ex = 361 nm).

For the emission spectra of Yb complexes, the ²F_5/2 → ²F_7/2 transitions of the Yb³⁺ ions were split into three bands (978, 1009, and 1038 nm) in Fig. 7, which may be because of the ligand field effects.²² However, when the Yb complexes are physically doped or grafted in the polymer matrix, the last two bands almost merged into a broad band, which is probably due to the polarity of polymer matrix.¹¹ In the field of the laser materials, materials with Yb attract much attention due to the ²F_5/2 → ²F_7/2 transition of the Yb³⁺ ions, for its very simple energy levels and relatively broad absorption which will be well-suitable for laser diode and pumping in this range, and the smaller Stokes shift between absorption and emission reduces the thermal loading of the material during laser operation.²³

Fig. 7 Excitation (EX, λ_em = 980 nm) and emission spectra for Yb(TTA)₃phen (red solid line, λ_ex = 381 nm), YbAA(TTA)₂phen (blue solid line, λ_ex = 380 nm), PMMA-co-Sn₁₂Cluster/Yb(TTA)₃phen (red dot line, λ_ex = 364 nm), PMMA-co-Sn₁₂Cluster-co-YbAA(TTA)₂phen (blue dot line, λ_ex = 358 nm).

Besides the application within laser systems, the Nd-containing materials also have potential application in the optical amplification due to the characteristic emitting wavelength of the Nd³⁺ ions. Because two telecommunication windows for amplification are commonly used for long distance communication, except for 1.5 μm, the second one at 1.3 μm is suitable for Nd emission.²⁴ In Fig. 8, excited at UV light, the four Nd samples exhibited three bands: 885 nm (⁴F_3/2 → ⁴I_9/2), 1065 nm (⁴F_3/2 → ⁴I_11/2), and 1336 nm (⁴F_3/2 → ⁴I_13/2), respectively. The bands at 1065 nm and 1336 nm of the Nd hybrid materials will offer the opportunity to develop new materials for application in laser and amplifiers.

Fig. 8 Excitation (EX, λ_em = 1061 nm) and emission spectra for Nd(TTA)₃phen (red solid line, λ_ex = 382 nm), NdAA(TTA)₂phen (blue solid line, λ_ex = 382 nm), PMMA-co-Sn₁₂Cluster/Nd(TTA)₃phen (red dot line, λ_ex = 365 nm), PMMA-co-Sn₁₂Cluster-co-NdAA(TTA)₂phen (blue dot line, λ_ex = 360 nm).

2.2.3. Luminescence lifetimes. The luminescence lifetimes of Ln complexes and hybrid materials (Ln= Er, Sm, Nd, and Yb) were measured using an excitation wavelength of 355 nm and monitored around the strongest emission line of the corresponding Ln³⁺ ions (Fig. S1, ESI†). The luminescence lifetimes of Ln complexes are all fitted well by a single-exponential function and basically in agreement with the previous reports,^22a which suggest that the Ln³⁺ ions occupy the same average local environment.²⁵ Otherwise, some of the Ln hybrid materials are also fitted by double-exponential function, and the lifetimes of these samples (such as PMMA-co-Sn₁₂Cluster-co-SmAA(TTA)₂phen, PMMA-co-Sn₁₂Cluster-co-YbAA(TTA)₂phen, and PMMA-co-Sn₁₂Cluster/Yb(TTA)₃phen) were calculated as average lifetimes. All the lifetime data are listed in the Table 2. The luminescence lifetimes of the Ln complexes lowered after being doped into the matrix; we consider that the molecular high-energy vibration of the main flexible framework groups (PMMA) in hybrid materials might have strong effect on nonradiative transitions, which would play a disadvantageous role in the luminescence lifetimes.

Table 2 Lifetime data for samples^a

Sample	Lifetime/μs	Sample	Lifetime/μs
a Ln doped materials (PMMA-co-Sn12Cluster/Ln(TTA)3phen). Ln grafted materials (PMMA-co- Sn12Cluster-co-LnAA(TTA)2phen). b The lifetime fitted by a single-exponential function: I = Aexp(−t/τ). c The lifetime fitted double-exponential functions: I = A1exp(−t/τ1) + A2exp(−t/τ2) and the lifetime was calculated as average lifetime.
Er(TTA)3phen	1.82^b	ErAA(TTA)2phen	1.81^b
Er doped material	1.80^b	Er grafted material	1.81^b
Sm(TTA)3phen	76.03^b	SmAA(TTA)2phen	77.73^b
Sm doped material	54.25^b	Sm grafted material	74.22^c
Yb(TTA)3phen	11.29^b	YbAA(TTA)2phen	12.37^b
Yb doped material	9.80^c	Yb grafted material	10.89^c
Nd(TTA)3phen	1.17^b	NdAA(TTA)2phen	1.14^b
Nd doped material	1.11^b	Nd grafted material	0.65^b

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2.3. Films made from PMMA-co-Sn₁₂Cluster/Ln(TTA)₃phen and PMMA-co-Sn₁₂Cluster-co-LnAA(TTA)₂phen

We chose Er hybrid materials as an example to spin-coat on indium tin oxide (ITO) glass, and dried them in air. The obtained films on the ITO glass are transparent as we can see in Fig. 9a and 9b. The morphology of films were further studied by SEM images, as shown in Fig. 9c and 9d; the films are homogeneous and flat coated on the ITO glasses. We consider that the feasible formation of film from these hybrid materials is mainly due to the soft PMMA chains in the matrix. Therefore, this kind of hybrid material is very suitable for film forming, which will make it conducive to designing NIR optical devices.

Fig. 9 Photographs for the PMMA-co-Sn₁₂Cluster/Er(TTA)₃phen film (a), PMMA-co-Sn₁₂Cluster-co-ErAA(TTA)₂phen film (b). SEM images for PMMA-co-Sn₁₂Cluster/Er(TTA)₃phen film (c), PMMA-co-Sn₁₂Cluster-co-ErAA(TTA)₂phen film (d).

Conclusions

Two kinds of lanthanide complexes were introduced into the PMMA-co-Sn₁₂Cluster matrix. The LnAA(TTA)₂phen (Ln = Er, Sm, Nd, and Yb) complexes with vinyl group were grafted on the PMMA chains of the PMMA-co-Sn₁₂Cluster matrix, while the Ln(TTA)₃phen without vinyl group were just physically doped in the matrix. The obtained hybrid materials all showed similar NIR photoluminescence properties compared with the pure NIR lanthanide complexes, which indicated that the NIR lanthanide complexes keep their structures inside the matrix. Due to their good solubility in organic solvents and capacity to forming films, we also prepared transparent films of hybrid materials on ITO glasses through spin-coating. Incorporating NIR lanthanide complexes into the matrix built from butyltin cluster nanobuilding blocks, which combine the NIR luminescence properties and soft structure bridged inorganic clusters into a single material, would extend the applications of the rare earth in NIR optical fields.

Experimental

Solid reagents were of analytical grade and used as received without further purification. Liquid reagents were used after desiccation.

4.1. Synthesis of polymer PMMA-co-Sn₁₂Cluster

{(BuSn)₁₂O₁₄(OH)₆}₂(OH)₂ [Sn₁₂Cluster(OH)₂], {(BuSn)₁₂O₁₄(OH)₆}(MA)₂, and copolymer were prepared according to ref. 9 and 10 10 wt% solution of {(BuSn)₁₂O₁₄(OH)₆}(MA)₂ in THF was mixed with methyl methacrylate (MMA) (MMA : cluster = 30 : 1). The reaction mixture was heated at 70 °C for 60 h using 2,2′-azobis (isobutyronitrile) (AIBN) (5% molar ratio of the methacrylic group) as radical initiator at N₂ atmosphere. Finally, the polymer was obtained by precipitation of the product in diethyl ether. {(BuSn)₁₂O₁₄(OH)₆}(MA)₂, ¹H NMR (400 MHz, CDCl₃): δ = 5.87, 5.21 (ss, 4 H, (CH₃)C=CH₂), 1.90 (s, 6H, (CH₃)C=CH₂), 1.16 (m, 24H, CH₂CH₂CH₂CH₃), 1.72, 1.57 (m, 24H, CH₂CH₂CH₂CH₃), 1.32 (m, 24 H, CH₂CH₂CH₂CH₃), 0.94, 0.87 (tt, J = 7.2 Hz, 36 H, CH₂CH₂CH₂CH₃).

4.2. PMMA-co-Sn₁₂Cluster physically doped with the Er(TTA)₃phen complex [PMMA-co-Sn₁₂Cluster/Er(TTA)₃phen]

The Er(TTA)₃phen complex was synthesized according to a typical procedure.²⁶ Er(TTA)₃phen complex was dissolved in the tetrahydrofuran (THF), then added dropwise into a THF solution of PMMA-co-Sn₁₂Cluster, with the Er/Sn₁₂Cluster molar ratio of 5%. The solution was stirred until totally dissolved, and then dried in air.

4.3. Synthesis of PMMA-co-Sn₁₂Cluster polymer grafted with ErAA(TTA)₂phen [PMMA-co-Sn₁₂Cluster-co-ErAA(TTA)₂phen]

ErAA(TTA)₂phen was prepared similarly to the previously report.²⁷ The synthesis procedure for PMMA-co-Sn₁₂Cluster-co-ErAA(TTA)₂phen was similar to that of PMMA-co-Sn₁₂Cluster except that ErAA(TTA)₂phen (Er/cluster molar ratio = 5%) was added into the reaction mixture. ErAA(TTA)₂phen (IR, cm⁻¹): 1581 and 1417 (COO–), 1520 (phen C=C), 1600 (coordinated C=O), 416 (Er–O).

The Sm(III), Yb(III), and Nd(III) hybrid materials were synthesized using the same procedures as Er described above.

4.4. Preparation of the films from PMMA-co-Sn₁₂Cluster/Er(TTA)₃phen, and PMMA-co-Sn₁₂Cluster-co-ErAA(TTA)₂phen

The transparent films were easily prepared by first dissolving the samples in the THF, then spin-coating on the ITO glass, and finally drying in air.

4.5. Characterization

Field-emission scanning electron microscopy (FE-SEM) images were obtained with a XL30 ESEM FEG microscope. The luminescence excitation and emission spectra were recorded on a HORIBA Jobin Yvon FluoroLog-3 spectrofluorometer equipped with a 450 W Xe-lamp as an excitation source and a liquid-nitrogen-cooled R5509-72 PMT as a detector. The time-resolved measurements were performed 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 a source, and the NIR emission lines were dispersed by the emission monochromator of the HORIBA Jobin Yvon FluoroLog-3 equipped with a liquid-nitrogen-cooled R5509-72 PMT, and the data were analysed with a LeCroy WaveRunner 6100 1GHz Oscilloscope. The optical measurements were performed at room temperature. FT-IR spectra were measured within a 4000–400 cm⁻¹ region on an American BIO-RAD Company model FTS135 infrared spectrophotometer using the KBr pellet technique. The diffuse reflectance (DR) measurements were recorded on a SHIMADZU UV-3600 spectrophotometer. ¹H and ¹¹⁹Sn NMR spectra were recorded on Bruker AV-400 spectrometer, ¹¹⁹Sn NMR spectra (CD₂Cl₂) were obtained under proton decoupling and chemical shifts are relative to external tetramethyltin. TGA measurements were performed on a Perkin–Elmer Thermal Analysis Pyris Diamond TG/DTA. The samples were heated under air from 40 to 800 °C at a heat rate of 10 °C min⁻¹. The content of Er³⁺ ions was obtained by inductively coupled plasma-atomic emission spectroscopy (ICP-AES-MS) with a TJA-POEMS spectrometer. The molecular weights and molecular weight distributions of the samples were determined in THF with a Gel Permeation Chromatography (GPC) instrument using WATERS-410GPC equipped with WATO44219 HT6E.

Acknowledgements

The authors are grateful for financial aid from the National Natural Science Foundation of China (Grant No. 20631040, and 20771099) and the MOST of China (Grant No. 2006CB601103, 2006DFA42610).

Notes and references

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Footnote

† Electronic Supplementary Information (ESI) available: Figure S1. See DOI: 10.1039c0nr00233j

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