DOI: 10.1002/chem.201402746
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& Luminescence
Polysiloxane-Based Luminescent Elastomers Prepared by Thiol– ene “Click” Chemistry Yujing Zuo,[a] Haifeng Lu,[a] Lei Xue,[a] Xianming Wang,[b] Lianfeng Wu,[b] and Shengyu Feng*[a]
and luminescence spectroscopy as well as by scanning electron microscopy, thermogravimetric analysis, and X-ray photoelectron spectroscopy. The europium elastomer luminophores exhibited intense red light at 617 nm under UV excitation at room temperature due to the 5D0 !7F2 transition in EuIII ions. The newly synthesized luminescent materials offer many advantages, including the desired mechanical flexibility. They cannot be dissolved or fused, and so they have potential for use in optical and electronic applications.
Abstract: Side-chain vinyl poly(dimethylsiloxane) has been modified with mercaptopropionic acid, methyl 3-mercaptopropionate, and mercaptosuccinic acid. Coordinative bonding of EuIII to the functionalized polysiloxanes was then carried out and crosslinked silicone elastomers were prepared by thiol–ene curing reactions of these composites. All these europium complexes could be cast to form transparent, uniform, thin elastomers with good flexibility and thermal stability. The networks were characterized by FTIR, NMR, UV/Vis,
Introduction
pium complexes. Good dispersion of the small particles leads to a higher transparency and larger interfacial interaction region. A good method for addressing this problem is to employ the coordination method.[12, 13] Polysiloxanes have received considerable attention in the field of polymers because they offer a number of advantageous physical properties. The SiOSi moiety is particularly flexible and commonly shows bond angles in the range of 135–1808. In addition, polysiloxanes exhibit excellent permeability as well as low surface energy.[14, 15] The combination of these properties makes polysiloxane an ideal candidate for use in luminescent materials. The formation of complexes between europium ions and a functionalized polysiloxane matrix improves the energy transfer in these materials and enhances the luminescent intensity of the composites.[16] In our previous work we have prepared many kinds of poly(dimethylsiloxane)–lanthanide-ion composites with excellent luminescent properties.[17–20] However, the simple coordination of lanthanide ions to polysiloxanes to form composites results in poor mechanical properties. Therefore a new method of preparing silicone luminescent composites should be developed. The method is expected to involve further crosslinking reactions, which, it is believed, would solve the problem of their poor mechanical properties. Radiation curing of polymers is an efficient way of converting suitable low-viscosity, reactive liquids into solids that exhibit good properties for applications in coatings, inks, adhesives, and dental materials.[21, 22] Thiol–ene “click” chemistry provides an advantage over similar approaches to the crosslinking of poly(dimethylsiloxane) (PDMS) because it does not require an elevated temperature or an expensive heavy-metal catalyst, and it does not produce potentially detrimental byproducts. It also meets the requirements for a general, simple, efficient, and catalyst-free approach to the preparation of
Europium complexes have received considerable attention over the past few decades because of their numerous potential photophysical applications ranging from organic light-emitting devices (OLEDs) to luminescent probes (protein labeling, immunoassays) and optical imaging.[1–3] Research interest is driven largely by the excellent luminescence characteristics of europium complexes, for example, high luminous intensity and narrow emission bands, which are effects of the unique 4f electron structure of rare-earth ions and strong energy transfer from organic ligands to the europium ions.[4, 5] However, the poor thermal stability and processibility of neat europium complexes hinder their further application. Incorporation of europium complexes into polymers to form composites has been shown to be a good method of addressing these problems. The resulting composites show improved luminescence, thermal stability, and mechanical properties over those of neat europium complexes.[6–9] Many europium complex composites have been prepared by physical blending, which is cheap and convenient.[4, 10, 11] However, this method does not produce composites with a high content or fine dispersion of the euro[a] Y. Zuo, H. Lu, L. Xue, Prof. S. Feng Key Laboratory of Special Functional Aggregated Materials and Key Laboratory of Colloid and Interface Chemistry Shandong University, Ministry of Education School of Chemistry and Chemical Engineering Jinan 250100 (P.R. China) Fax: (+ 86) 531-88564464 E-mail: [email protected] [b] X. Wang, L. Wu Marine Chemical Research Institute Company Limited and State Key Laboratory of Marine Coatings Qingdao 266071 (P.R. China) Chem. Eur. J. 2014, 20, 1 – 10
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Full Paper crosslinked polysiloxanes.[23–26] The PDMS-based hybrid organic/inorganic design strategy could overcome the shortcomings of traditional thiol–ene networks derived from flexible sulfide bonds. Thiol–ene-functionalized organic and inorganic materials can be synergistically combined to improve the properties of a material, for example, rubbery modulus, abrasion resistance, and thermal stability.[27] In our previous work we prepared silicone rubber by using thiol–ene “click” chemistry,[28] but extensive and diverse research is still needed to extend the thiol–ene reaction to practical applications. For example, silicone elastomers should exhibit superior luminescent performance. This paper presents an easy thiol–ene “click” chemistry method for the preparation of polysiloxane-based luminescent elastomers under mild conditions. EuIII–carbonyl complexes are highly functional compounds with outstanding optical, electrical, and magnetic properties, and they have widespread applications in various fields, for example, as luminophores.[29] Three kinds of carbonyl-functionalized polysiloxanes complexed to europium ions have been synthesized to combine the excellent fluorescence properties of europium ions and the reactivity of the double bond with thiol groups. This paper reports a new method for the preparation of luminescent silicone elastomers. A series of side-chain carbonylmodified polysiloxanes (PMP, PMPE, and PMS) were synthesized by grafting the carbonyl-containing monomers mercaptopropionic acid (MP), methyl 3-mercaptopropionate (MPE), and mercaptosuccinic acid (MS), respectively, onto vinyl-functionalized polysiloxane (P1) by using thiol–ene “click” chemistry (Scheme 1), and their properties were studied in detail by 1 H NMR spectroscopy and gel permeation chromatography (GPC). This newly designed thiol–ene protocol produced functionalized polysiloxanes under benign conditions compared with the traditional hydrosilylation method. The polysiloxanes could be partially functionalized by controlling the molar ratio of vinyl moieties in the functionalized polysiloxane and carbonyl-containing monomers. Excess vinyl groups could then be used as crosslinking sites to prepare polysiloxane-based elastomers. First, europium ions were coordinated to the side-chains of the functionalized polysiloxanes and then a two-step sequential thiol–ene “click” reaction was applied to produce silicone luminescent elastomers (Scheme 2).
Scheme 2. Scheme showing the crosslinking of the silicone elastomer networks.
Results and Discussion Synthesis of carbonyl-functionalized polysiloxanes The carbonyl-functionalized poly(dimethylsiloxanes) (poly(dimethylsiloxane-co-(1-thiopropanoic acid)ethylmethylsiloxaneco-methylvinylsiloxane) (PMP), poly(dimethylsiloxane-co-(1-thiomethylpropionate)ethylmethylsiloxane-co-methylvinylsiloxane) (PMPE), and poly(dimethylsiloxane-co-(1-thiomalic acid)ethylmethylsiloxane-co-methylvinylsiloxane) (PMS)) were obtained in quantitative yields within a short time. The full conversion of the thiol–ene reaction was confirmed by 1H NMR analysis by the decrease in the vinyl peaks (-Si-CH=CH2) in the range 5.0– 5.7 ppm and the appearance of signals arising from -Si-CH2(0.94 ppm) and -CH2-S-CH2- (2.61 and 2.76 ppm). The 1H NMR spectra and corresponding peak assignments of the vinyl-functionalized polysiloxane (P1) and PMPE are shown in Figure 1. The side-chain olefin protons of P1 are located between 5.0 and 5.7 ppm. On the basis of the integral ratio of the vinyl peaks, the actual graft density was found to be around 5 % after the thiol–ene reactions, which was a perfect match with the designed strategy. These results proved that thiol–ene “click” chemistry is an efficient method for grafting carbonyl groups onto polysiloxane chains. In addition this method makes possible the accurate control of graft densities by simply mixing the functionalized polysiloxane with monomers in the desired molar ratios. However, a small peak at around
Figure 1. 1H NMR spectra of P1 and PMPE in CDCl3.
Scheme 1. Synthesis of PMP, PMPE, and PMS
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Full Paper 1.3 ppm can be observed in the 1H NMR spectrum, which has been attributed to Markovnikov addition products. The thiol– ene reaction proceeded by free-radical step-growth addition and consequently the thiol radical reacts with different sites of the C=C bond, consistent with previous reports.[31, 32] From a calculation of the integral areas of the peaks arising from the two different addition patterns, the proportion of anti-Markovnikov and Markovnikov addition products was determined to be about 95:5. Thus, the results indicate the good regioselectivity of the anti-Markovnikov addition, and the a-addition product has little effect on the structures of the functionalized polysiloxanes. These systems undergo traditional thiol–ene reactions in which the vinyl group does not readily homopolymerize by a radical mechanism, indeed, step-growth is the dominant polymerization mechanism. In a typical polymerization procedure, this mechanism is restricted by oxygen inhibition.
Table 1. Hardness, crosslinking densities, and Mc of the elastomers when swollen at equilibrium. Sample
f
Crosslinking density ne [104 mol cm3]
Mc [g mol1]
Hardness (Shore A)
PMP-Eu3:1-SH PMP-Eu4:1-SH PMPE-Eu3:1-SH PMPE-Eu4:1-SH PMS-Eu3:1-SH PMS-Eu4:1-SH
0.35 0.36 0.40 0.38 0.39 0.33
2.66 2.64 2.17 2.45 2.40 2.89
3258 3369 3960 3257 3584 2994
32 27 25 18 37 34
The crosslinking density increased with increasing lanthanide ion ratio, and, correspondingly, the Shore A hardness also increased with increasing molar ratio of the europium ions. However, the hardness value of PMP-Eu3:1-SH is greater than that of PMPE-Eu3:1-SH, mainly due to the inherent crystalline nature of MP; physical crosslinking between PMP and the polysiloxane main chain is more likely to occur due to the presence of hydrogen bonds between molecular chains, and this further enhances its hardness. The regularity of the change in the crosslinking density is consistent with that of the mechanical properties, which means that the europium ion acts to some extent as a crosslinker in the network.
Crosslinking to form silicone elastomers
An illustration of elastomer crosslinking chemistry is shown in Scheme 2. The thiol–ene curing reaction is a radical-induced process. The polymerization process, which is based on an equimolar mixture of vinyl and multifunctional thiol, is a freeradical reaction that proceeds by a step-growth mechanism; this involves two main steps, namely free-radical addition followed by a chain-transfer reaction. The reaction proceeds by FTIR analysis the rapid formation of a uniformly crosslinked network with low shrinkage, reduced oxygen inhibition during curing, and Figure 2shows the FTIR spectra recorded at room temperature of PMP, PMP-Eu4:1, PMPE, PMPE-Eu4:1, PMS, and PMS-Eu4:1 in excellent adhesion. The thiol–ene crosslinking of europium-coordinated polysiloxanes was accomplished by preparing a mixthe region 2500–800 cm1. The spectra reveal the coordination ture containing crosslinkable polysiloxanes and the tetrathiol of the europium ion to PMP, PMPE, and PMS. The FTIR spectra crosslinking agent, pentaerythritol tetrakis(3-mercaptopropioof the europium complexes are similar because the PMP, nate). The mixture was then exposed to 365 nm UV light for PMPE, and PMS ligands are similar; keto–enol structures are 5 min. Crosslinking was confirmed qualitatively by rinsing the formed because of the coordination of the carbonyl group to cured films with THF, which is a good solvent for uncured subthe europium ion (Scheme 3). The successful attachment of the rare-earth ions to the polymer backbones is strongly supstrates; the thiol–ene crosslinked polymer elastomers were found to be insoluble in this solvent. Measurement of the tensile or tear strengths of the unfilled silicones was not possible because the elastomers easily cracked during measurement. The mechanical properties of the materials were therefore tested by identifying the Shore A hardness by using a Shore A durometer. The hardness of the resulting polymers has a direct correlation with the crosslinking density. The toluene swelling method was used to measure the crosslinking density (n) and the average molecular weights between two crosslinking points (Mc) of the silicone elastomers obtained. Figure 2. FTIR spectra of (a) PMP and PMP-Eu4:1, (b) PMPE and PMPE-Eu4:1, (c) PMS and PMS-Eu4:1, (d) enlargeAll the data are listed in Table 1. ment of the spectra of PMP and PMP-Eu4:1, and (e) enlargement of the spectra of PMS and PMS-Eu4:1. Chem. Eur. J. 2014, 20, 1 – 10
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Figure 3. UV spectra of PMP, PMPE, PMS, PMP-Eu4:1, PMPE-Eu4:1, and PMSEu4:1.
structure group, respectively. This result is consistent with previous conjecture. The spectra of both PMP and PMPE show differences at 277 nm after coordination. More pronounced shoulders appear at around 275 nm in the spectra of PMP–Eu and PMPE–Eu. These results indicate that europium ions successfully coordinate to PMP and PMPE. X-ray photoelectron spectroscopy Scheme 3. Structures of the possible modes of coordination of europium in PMP-Eu-SH, PMP-Eu-SH, and PMP-Eu-SH.
X-ray photoelectron spectroscopy (XPS) is an efficient tool for investigating the coordination process by analyzing changes in binding energy. X-ray photoelectron spectra were recorded to further confirm that coordination occurs between the carbonyl-functionalized polysiloxanes and the europium ions. Eu(NO3)3·6H2O, PMP-Eu4:1-SH, PMPE-Eu4:1-SH, and PMS-Eu4:1-SH were analyzed by XPS because of the similarity in coordination of these complexes. The binding energy curves of Eu 3d5/2 for Eu(NO3)3·6H2O, PMP-Eu4:1-SH, PMPE-Eu4:1-SH, and PMS-Eu4:1SH are shown in Figure 4a. The peak of the Eu 3d5/2 curve appears at 1141 eV in europium nitrate but is shifted to a lower binding energy of about 1138 eV (double-bond equivalent = 3.0 eV) in PMP-Eu4:1-SH. The peaks of PMPE-Eu4:1-SH and PMS-Eu4:1-SH are also shifted to a lower energy. This phenomenon is due to the coordination of carbonyl groups by europium ions in the silicone elastomer and the alteration of the previous coordination environment of lanthanide nitrate.[17] A pair of electrons are donated by the ligands to the metal ions, which act as electron acceptors in a s-bond-type coordination mode; this process strengthens the electron shielding and reduces the inner-electron binding energy in the metal ions. Hence the binding energy of the free metal ions is higher than that of the coordinated ions. The degree of shift for the three different ligands varies slightly with the coordinating ability of the different carbonyl groups. The negative charge density of the europium ions in PMPE-Eu-SH is greater than in the other complexes. The binding energy curves of C 1s for PMP-Eu4:1-SH and PMP-SH are illustrated in Figure 4b. Owing to their similar structure, we show the curve of PMPE-Eu4:1-SH as an example. The C 1s peaks of the carbonyl groups in this complex appear at 289.55 eV and are shifted to a higher energy by 0.8 eV in
ported by the vibration of C=C at around 1635 cm1, which appears as a new absorption peak in the spectra of the modified polymers.[33, 34] The bands at 1381 and 1301 cm1 are due to the OCO stretching frequency. These suggest that coordination bonds are formed between the europium ion and PMP, PMPE, and PMS. The coordination between NO3 and europium ions produces a band at 1500 cm1, which can be attributed to the N=O bond; this band was found difficult to eliminate.[13] The spectrum of PMPE-Eu 4:1 shows a band at 1744 cm1, which has been attributed to the C=O stretching frequency of the carboxy group; after coordinating to Eu3 + , it shifted to a lower wavenumber (1738 cm1), which indicates that PMPE coordinates to Eu3 + more effectively than PMP and PMS. These results suggest that coordination bonds form between Eu3 + and the carbonyl groups of the functionalized PDMS. UV/Vis analysis The UV/Vis absorption spectra of PMP, PMPE, and PMS and their europium complexes (4:1) in THF solution (1 105 mol L1) are shown in Figure 3. The UV/Vis absorption spectra of the europium complexes of PMP, PMPE, and PMS are similar, the absorption bands arising from the organic ligands because the lanthanide ions exhibit negligible absorption. The absorption spectra of PMS and PMS–Eu show a maximum absorption at 245 nm with a shoulder at 277 nm, which correspond to the p!p* electronic transition of the carbonyl group and the n!p* electronic transition of the keto–enol &
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Figure 5. Emission spectra (lex = 330 nm) of PMP-Eu4:1, PMPE-Eu4:1, and PMS-Eu4:1 before crosslinking in THF solution.
A prominent feature observed in these spectra is the very high intensity of the 5D0 !7F2 transition, which indicates the presence of a highly polarizable chemical environment around the europium ions of these complexes; this environment is responsible for the brilliant red emission.[35] As a result, strong coordinative interactions take place between the organic groups and the Eu3 + ions in the uncrosslinked complexes. In addition, efficient energy transfer from the ligands to the central europium ion in these complexes also occurs due to the coordination of the “antenna” ligands to the europium ions.[36] The photoluminescent (PL) properties of all the europiumcomplexed elastomers in the solid state were investigated in detail at room temperature. The spectral emissions are shown in Figure 6. The crosslinked and non-crosslinked composites exhibit similar spectral emission, with the crosslinked elastomers showing much more stable baselines than the linear polysiloxane–europium composites. The removal of small, noncrosslinked molecules by immersion in fresh THF affected the luminescent properties and enhanced the luminescent intensity. The very intense 5D0 !7F2 line at 617 nm clearly dominates the spectrum and is responsible for the red luminescence that is visible by the naked eye upon UV irradiation. Thus, crosslinking has been shown to be fully compatible with the fabrication of luminescent elastomers, which show better luminescent
Figure 4. XPS of (a) Eu 3d5/2 in Eu(NO3)3·6H2O, PMP-Eu4:1-SH, PMPE-Eu4:1SH, and PMS-Eu2:1-SH and (b) C 1s in PMPE-Eu4:1-SH and PMPE-SH.
comparison with the peak at 288.75 eV for PMPE-SH due to the formation of coordination bonds between the carbonyl groups and lanthanide ions; this brings about a transfer of electrons from the carbonyl groups to the lanthanide ions and weak electron shielding in the carbonyl groups. The observed shift of the binding energy peak for europium and carbon confirms the effective coordination of europium ions to the silicone matrix. Photoluminescence of PMP–Eu3 + , PMPE–Eu3 + , and PMS– Eu3 + The room-temperature emission spectra of PMP-Eu4:1, PMPEEu4:1, and PMS-Eu4:1 in THF solution are shown in Figure 5. These complexes exhibit the red emission characteristic of europium ions on excitation at 330 nm, which suggests that these complexes may be potential red fluorescent materials. All of the europium complexes exhibit the characteristic emission lines of Eu3 + , namely 5D0 !7F1 (591 nm) and 5D0 !7F2 (617 nm). The 5D0 !7F1 (591 nm) transition is a parity-allowed magnetic dipole transition and is insensitive to the local environment, whereas the 5D0 !7F2 (617 nm) transition is an induced electric dipole transition and is very sensitive to the environment. This luminescence line is responsible for the red luminescence color visible by the naked eye upon UV irradiation. Chem. Eur. J. 2014, 20, 1 – 10
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Figure 6. Emission spectra (lex = 330 nm) of (a) PMP-Eu-SH, (b) PMP-Eu-SH, and (c) PMP-Eu-SH with different europium ion ratios after crosslinking to form the in silicone elastomer.
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Full Paper performance than the non-crosslinked europium-coordinated polysiloxanes. Elastomers containing different molar ratios of europium ions and carbonyl groups were investigated to analyze the factors affecting the emission properties. The emission intensity increased with increasing europium content and reached a maximum at 4:1; a further increase in europium content resulted in quenching of the emission. When the amount of europium doped in the PDMS matrix is small, the probability of energy migration by diffusional collision of the europium complex should be small. Aggregates of the complex occurred in the solid matrix at high concentrations of the complex. Therefore the quenching phenomenon may be caused by the deactivation of the 5D0 or 5D1 state through electrostatic multipolar interactions or by excitation migration by the Fçrster dipole– dipole mechanism in solid complexes.[37]
nature. This observation is indicative of well-bonded interface adhesion induced by coordination, which is consistent with the previous discussion.
Contact angle measurement The surface energies of the crosslinked thiol–ene networks were determined through static contact angle measurements. The image of the drop was calculated from the shape of the drop (both left and right contact angles) with an accuracy of 0.18 by using an image analysis system. Distilled water was used as the test liquid and all measurements were performed at room temperature. The results of the goniometer experiments are shown in Figure 8. The contact angles of PMP-SH,
Morphology of crosslinked silicone elastomers Figure 8. Contact angles of the crosslinked elastomers: (a) PMP-Eu4:1-SH, (b) PMPE-Eu4:1-SH, and (c) PMS-Eu4:1-SH.
The morphologies of the films were characterized by SEM analysis. The micrographs of PMP-Eu4:1-SH, PMPE-Eu4:1-SH, and PMS-Eu4:1-SH are shown in Figure 7. No phase separation was observed in any of the micrographs, which indicates that the europium ions are well dispersed. The homogeneity between the organic and inorganic phases prevented aggregation of the ions; the europium salt melted and integrated well into the elastomeric matrix as a result of the coordination to the polymeric chain. The chemical bonding (coordination) of the Eu3 + ions to the silicone main chain also prevented them from forming aggregates. No crystalline domains were observed because the polymer and Eu3 + ions are both amorphous in
PMPE-SH, and PMS-SH are 85.3, 96.7, and 75.78, which compares with a value of 1098 for PDMS.[38] As expected, the contact angles are lower for the networks containing a side-chain and the europium ion. The structures of the carbonyl groups affect the observed material properties; as hydrophilic carbonyl-functionalized monomers are introduced, the contact angle decreases. The lower contact angle of PMP has been attributed to the greater hydrophilicity of the acid functionality. Therefore inclusion of the carbonyl side-chains can render the hydrophobic PDMS a more hydrophilic material without altering its transparency. Thermogravimetric analysis The thermal stability of the complexes is very important because their decomposition leads to a decrease in device performance. Therefore their thermal stabilities were measured by thermogravimetric analysis (TGA) at a heating rate of 10 8C min1 under N2. The thermogravimetric weight loss curves for all the complexes show that the decomposition of the complexes progresses similarly (Figure 9). The first weight loss observed in PMP-Eu4:1-SH, PMPE-Eu4:1-SH, and PMSEu4:1-SH has been attributed to the loss of coordinated water.[39] The second weight loss step may be due to the decomposition of the organic groups and the breaking of the polysiloxane main chain.[17] The TG curves indicate that these complexes decompose above 280 8C, which means that device fabrication by the vacuum evaporation method is more feasible.
Conclusion Polysiloxane-based luminescent elastomers complexed by europium ions have been prepared by thiol–ene “click” chemistry,
Figure 7. Scanning electron micrographs of (a) PMP-Eu4:1-SH, (b) PMPEEu4:1-SH, and (c) PMS-Eu4:1-SH.
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Full Paper and 2.5 nm, respectively. Thermogravimetric analysis (TGA) was performed by using a TA SDTQ600 instrument at a heating rate of 10 8C min1 from room temperature to 800 8C under N2. Contact angles were recorded by using a Dataphysics OCA-20 contact angle analyzer with distilled water as the test liquid. Scanning electronic microscopy (SEM) images were obtained by using an Hitachi S-4800 instrument (7 kV). Samples were cut and coated with a thin layer of gold before investigation. X-ray photoelectron spectroscopy was performed by using a Thermo Fisher Scientific Escalab 250 spectrometer with a monochromated AlKa X-ray source at a residual pressure of 107 Pa. Survey and high-resolution scans were performed with pass energies of 100 and 20 eV with steps of 1 and 0.05 eV, respectively. Hardness (Shore A) was measured according to the ASTMD 2240--2005 method by using a Shore durometer (Laizhou Huayin Research Instruments Co., China). Figure 9. TGA curves of PMP-Eu4:1-SH, PMPE-Eu4:1-SH, and PMS-Eu4:1-SH.
Synthesis of vinyl-functionalized polysiloxane P1 Poly(methylvinylsiloxane) P1 was synthesized by the classical procedure.[30] Yield: Yield: 90 %; 1H NMR (400 MHz, CDCl3): d = 0.06– 0.17 (m, -SiCH3), 5.79–6.03 ppm (m, -CH=CH2).
with a new type of luminescent hybrid material being produced by doping EuIII complexes into a PDMS-crosslinked network, which acts as an elastomer. The performance of the PDMS elastomers complexed by europium (III) ions has also been presented. However, the applicability of this system is not limited to europium (III) or to these types of ligands alone. The luminescence color of the flexible PDMS elastomers can be tuned by a suitable choice of lanthanide ions, for example, blue for Tm3 + and green for Tb3 + . White luminescent materials can also be designed by using this method. This study aimed to provide some useful information for the development of silicone elastomers.
Synthesis of side-chain carbonyl-functionalized polysiloxanes PMP, PMPE, and PMS The procedure used for the thiol–ene “click” chemistry is shown in Scheme 1 along with the chemical structures of the thiols. Vinylfunctionalized polysiloxanes P1 (4.05 g, 50 mmol), the corresponding thiol (0.05 equiv, with respect to the –SiO- group), DMPA (2 wt %, 0.04 g), and a certain amount of THF as solvent were placed in a glass vessel and then exposed to a UV light source (365 nm, 100 W) with stirring for 15 min at room temperature. The resulting polymers were purified by precipitation in MeOH to eliminate unreacted thiols and photoinitiator. The gel permeation chromatography (GPC) data for P1, PMP, PMPE, and PMS are presented in Table 2.
Experimental Section Materials Octamethylcyclotetrasiloxane (D4), tetramethyltetravinylcyclotetrasiloxane (D4Vi), and hexamethyldisiloxane (MM) were obtained as commercial products from Qufu Wanda Chemical Co., Ltd. and used directly. Methyl 3-mercaptopropionate (MPE), 3-mercaptopropionic acid (MP), and mercaptosuccinic acid (MS) were purchased from Aladdin Chemistry Co. Ltd. 2,2-Dimethoxy-2-phenylacetophenone (DMPA) was purchased from Aldrich and used as received. Pentaerythritol tetrakis(3-mercaptopropionate) (PTTMP) was purchased from China Energy Chemical Group and used as received. Europium nitrates were obtained from their corresponding oxides in strong nitric acid. THF was purified by a routine procedure and distilled over sodium before use.
Table 2. GPC data of the synthesized polymers.
The thiol–ene reaction was performed by irradiation using UV light from a Spectroline Model SB-100P/FA lamp (365 nm, 100 W). The UV intensity is 4500 mW cm2 at a distance of 38 cm. 1H NMR spectra were recorded on a Bruker AVANCE 400 spectrometer at 25 8C in CDCl3 as solvent and without tetramethylsilane (TMS) as interior label. FTIR spectra were recorded on a Bruker TENSOR27 infrared spectrophotometer in the range 4000–400 cm1 by using KBr pellets. UV absorption spectra were recorded in THF with a Beijing TU-1901 double beam UV/Vis spectrophotometer. Luminescence (excitation and emission) spectra were recorded with an Hitachi F4500 fluorescence spectrophotometer using a monochromated Xe lamp as the excitation source; excitation and emission slits were 5 www.chemeurj.org
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Mn [g mol1]
Mw [g mol1]
Mz [g mol1]
PDI
P1 PMP PMPE PMS
62 458 63 897 61 209 64 710
79 650 82 564 10 8513 81 235
10 2130 11 2358 18 8776 14 2356
1.27 1.29 1.77 1.25
PMP: 1H NMR (400 MHz, CDCl3): d = 0.01–0.30 (m, -SiCH3), 0.91–0.95 (m, -SiCH2CH2S-), 2.61–2.70 (m, -SiCH2CH2SCH2-), 2.80–2.84 (t, 13 5.78–6.02 ppm (m, -CH=CH2); C NMR -SCH2CH2COOH), (100.62 MHz, CDCl3): d = 1.01 (-SiCH3), 18.82 (-SiCH2CH2S-), 26.37 (-SiCH2CH2S-), 26.98 (-SCH2CH2COOH), 34.40 (-CH2COOH), 131.41 (-CH=CH2), 138.41 (-CH=CH2), 176.77 ppm (-COOH).
Characterization and measurements
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PMPE: 1H NMR (400 MHz, CDCl3): d = 0.01–0.25 (m, -SiCH3), 0.90– 0.95 (m, -SiCH2CH2S-), 2.60–2.65 (m, -SiCH2CH2SCH2-), 2.80–2.84 (t, -SCH2CH2COOCH3), 3.73 (s, -COOCH3), 5.74–6.06 ppm (m, -CH=CH2); 13 C NMR (100.62 MHz, CDCl3, ): d = 0.19 (-SiCH3), 18.93 (-SiCH2CH2S-), 26.70 (-SiCH2CH2S-), 26.73 (-SCH2CH2COOCH3), 34.44 (-CH2COOCH3), 50.98 (-COOCH3), 131.27 (-CH=CH2), 139.01 (-CH=CH2), 172.37 ppm (-COOCH3). PMS: 1H NMR (400 MHz, CDCl3): d = 0.01–0.25 (m, -SiCH3), 0.90–0.95 (m, -SiCH2CH2S-), 2.70–2.85 (m, -SiCH2CH2SCH2-), 3.59–3.75 (m, -CHCOOH), 5.58–5.94 ppm (m, -CH=CH2); 13C NMR (100.62 MHz,
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Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant no. 21274080 and 21204043) and the Key Natural Science Foundation of Shandong Province of China (Grant no. ZR2011BZ001).
Europium coordination A series of europium complexes (Table 3) were prepared by the following procedure. To take PMP-Eu4:1 as an example, Eu(NO3)3·6H2O (0.35 g, 0.77 mmol) and PMP (5.33 g) were dissolved in THF (30 mL) in a flask. The solution was heated at reflux at 70 8C for 4 h with continuous stirring. The solvent was then removed by evaporation under vacuum to yield PMP-Eu4:1. PMPE-Eu, and PMSEu were prepared by the same procedure.
Keywords: click chemistry luminescence · silicon
C=O/Ln3 + Polysiloxane [g] Ln(NO3)3·6H2O [g] PTTMP [g]
PMP-Eu3:1-SH PMP-Eu4:1-SH PMP-Eu6:1-SH PMPE-Eu3:1-SH PMPE-Eu4:1-SH PMPE-Eu6:1-SH PMS-Eu3:1-SH PMS-Eu4:1-SH PMS-Eu6:1-SH
3:1 4:1 6:1 3:1 4:1 6:1 3:1 4:1 6:1
5.33 5.33 5.33 5.37 5.37 5.37 5.47 5.47 5.47
0.46 0.35 0.23 0.46 0.35 0.23 0.46 0.35 0.23
0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02
General procedure for thiol–ene photocuring Crosslinking was achieved by using a solution of the europium-coordinated crosslinkable polymer (PMP–Eu, PMPE–Eu, and PMS–Eu) and the tetrafunctionalized crosslinker pentaerythritol tetrakis(3mercaptopropionate) (PTTMP). The corresponding quantities are shown in Table 3. Solutions were prepared at 10 mg mL1 polymer in THF. THF was then removed by evaporation before the crosslinking step. UV crosslinking was performed at room temperature for 5 min in air and under 365 nm light to form an insoluble crosslinked network (Scheme 2). All of the elastomers were swollen within the THF to interchange the substrates and remove the uncrosslinked molecules. They were then dried under vacuum three times to ensure that all uncrosslinked materials were removed from the network. Transparent crosslinked networks were finally obtained.
Measurement of crosslinking density The extent of crosslinking of samples was measured by swelling at equilibrium using toluene as solvent at room temperature and for an immersion period of 72 h. The crosslinking densities of the samples were calculated by using the Flory and French[25] equation (1) in which Mc is the average molecular weight between two crosslinking points, ue is the crosslinking density of the sample, f is the volume fraction of the rubber in the swollen sample, c is the rubber solvent interaction parameter, 1 is the density of the rubber before swelling, and v0 is the molar volume of the solvent.
ue ¼
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p ½lnð1 fÞ þ f þ cf2 ¼ Mc v0 f1=3
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[1] E. Brunet, O. Juanes, J. C. Rodriguez-Ubis, Curr. Chem. Biol. 2007, 1, 11 – 39. [2] N. N. Katia, A. Lecointre, M. N. Regueiro-Figueroa, C. Platas-Iglesias, L. c. J. Charbonnire, Inorg. Chem. 2011, 50, 1689 – 1697. [3] H. Tsukube, S. Shinoda, Chem. Rev. 2002, 102, 2389 – 2404. [4] L. N. Sun, H. J. Zhang, L. S. Fu, F. Y. Liu, Q. G. Meng, C. Y. Peng, J. B. Yu, Adv. Funct. Mater. 2005, 15, 1041 – 1048. [5] M. H. Werts, Sci. Prog. 2005, 88, 101 – 131. [6] K. Kuriki, Y. Koike, Y. Okamoto, Chem. Rev. 2002, 102, 2347 – 2356. [7] J. Boyer, N. Johnson, F. van Veggel, Chem. Mater. 2009, 21, 2010 – 2012. [8] J. Kai, M. C. Felinto, L. A. Nunes, O. L. Malta, H. F. Brito, J. Mater. Chem. 2011, 21, 3796 – 3802. [9] A. Gulino, F. Lupo, G. G. Condorelli, A. Motta, I. L. Fragal, J. Mater. Chem. 2009, 19, 3507 – 3511. [10] L. D. Carlos, R. S Ferreira, J. Rainho, V. de Zea Bermudez, Adv. Funct. Mater. 2002, 12, 819 – 823. [11] W. Q. Fan, J. Feng, S. Y. Song, Y. Q. Lei, G. L. Zheng, H. J. Zhang, Chem. Eur. J. 2010, 16, 1903 – 1910. [12] B. Yan, H.-F. Lu, Inorg. Chem. 2008, 47, 5601 – 5611. [13] H.-F. Lu, B. Yan, J.-L. Liu, Inorg. Chem. 2009, 48, 3966 – 3975. [14] F. Abbasi, H. Mirzadeh, A. A. Katbab, Polym. Int. 2001, 50, 1279 – 1287. [15] A. Morikawa, M. Kakimoto, Y. Imai, Macromolecules 1991, 24, 3469 – 3474. [16] L. Liu, Y. L. Lu, L. He, W. Zhang, C. Yang, Y. D. Liu, L. Q. Zhang, R. G. Jin, Adv. Funct. Mater. 2005, 15, 309 – 314. [17] Y. Yue, Y. Liang, H. Wang, L. Feng, S. Feng, H. Lu, Photochem. Photobiol. 2013, 89, 5 – 13. [18] Q. Lai, H. Lu, D. Wang, H. Wang, S. Feng, J. Zhang, Macromol. Chem. Phys. 2011, 212, 1435 – 1442. [19] L. Liu, H. Lu, H. Wang, Y. Bei, S. Feng, Appl. Organomet. Chem. 2009, 23, 429 – 433. [20] H. Lu, H. Wang, S. Feng, J. Photochem. Photobiol. A 2010, 210, 48 – 53. [21] A. F. Senyurt, H. Wei, B. Phillips, M. Cole, S. Nazarenko, C. E. Hoyle, S. G. Piland, T. E. Gould, Macromolecules 2006, 39, 6315 – 6317. [22] T. Y. Lee, T. M. Roper, E. S. Jonsson, C. Guymon, C. Hoyle, Macromolecules 2004, 37, 3606 – 3613. [23] C. E. Hoyle, C. N. Bowman, Angew. Chem. 2010, 122, 1584 – 1617; Angew. Chem. Int. Ed. 2010, 49, 1540 – 1573. [24] A. B. Lowe, C. E. Hoyle, C. N. Bowman, J. Mater. Chem. 2010, 20, 4745 – 4750. [25] R. Acosta Ortiz, A. Y. R. Martinez, A. E. Garca Valdez, M. L. Berlanga Duarte, Carbohydr. Polym. 2010, 82, 822 – 828. [26] J. W. Chan, C. E. Hoyle, A. B. Lowe, J. Am. Chem. Soc. 2009, 131, 5751 – 5753. [27] B. J. Sparks, T. J. Kuchera, M. J. Jungman, A. D. Richardson, D. A. Savin, S. Hait, J. Lichtenhan, M. F. Striegel, D. L. Patton, J. Mater. Chem. 2012, 22, 3817 – 3824. [28] L. Xue, Y. Zhang, Y. Zuo, S. Diao, J. Zhang, S. Feng, Mater. Lett. 2013, 106, 425 – 427. [29] Z. Hong, C. Liang, R. Li, W. Li, D. Zhao, D. Fan, D. Wang, B. Chu, F. Zang, L. S. Hong, Adv. Mater. 2001, 13, 1241 – 1245. [30] L. Xue, D. X. Wang, Z. Z. Yang, Y. Liang, J. Zhang, S. Y. Feng, Eur. Polym. J. 2013, 49, 1050 – 1056. [31] D. A. Shipp, C. W. McQuinn, B. G. Rutherglen, R. A. McBath, Chem. Commun. 2009, 6415 – 6417. [32] K. L. Killops, L. M. Campos, C. J. Hawker, J. Am. Chem. Soc. 2008, 130, 5062 – 5064.
Table 3. Amounts of reactants used for the synthesis of PDMS-Eu-SHs (PDMS = PMP, PMPE, PMS) complexes. Sample
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Full Paper [33] Z. Chen, Y. Wu, F. Huang, D. Gu, F. Gan, Spectrochim. Acta Part A 2007, 66, 1024 – 1029. [34] S. M. Bruno, R. A. S Ferreira, L. D. Carlos, M. Pillinger, P. Ribeiro-Claro, I. S. GonÅalves, Microporous Mesoporous Mater. 2008, 113, 453 – 462. [35] M. H. Werts, R. T. Jukes, J. W. Verhoeven, Phys. Chem. Chem. Phys. 2002, 4, 1542 – 1548. [36] D. A. Raj, B. Francis, M. Reddy, R. R. Butorac, V. M. Lynch, A. H. Cowley, Inorg. Chem. 2010, 49, 9055 – 9063. [37] L.-H. Wang, W. Wang, W.-G. Zhang, E.-T. Kang, W. Huang, Chem. Mater. 2000, 12, 2212 – 2218.
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[38] J. T. Han, D. H. Lee, C. Y. Ryu, K. Cho, J. Am. Chem. Soc. 2004, 126, 4796 – 4797. [39] Y.-Y. Li, B. Yan, L. Guo, Y.-J. Li, Microporous Mesoporous Mater. 2012, 148, 73 – 79.
Received: March 24, 2014 Revised: June 16, 2014 Published online on && &&, 0000
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FULL PAPER & Luminescence
Luminescent elastomers: The novel crosslinking in poly(dimethylsiloxane) networks prepared by easy thiol–ene “click” chemistry has been studied as well as their elastomeric character. The europium poly(dimethylsiloxane)-based elastomer luminophores exhibit intense red light at 617 nm under UV excitation at room temperature (see figure).
Y. Zuo, H. Lu, L. Xue, X. Wang, L. Wu, S. Feng* && – && Polysiloxane-Based Luminescent Elastomers Prepared by Thiol–ene “Click” Chemistry
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& Luminescence
Polysiloxane-Based Luminescent Elastomers Prepared by Thiol– ene “Click” Chemistry Yujing Zuo,[a] Haifeng Lu,[a] Lei Xue,[a] Xianming Wang,[b] Lianfeng Wu,[b] and Shengyu Feng*[a]
and luminescence spectroscopy as well as by scanning electron microscopy, thermogravimetric analysis, and X-ray photoelectron spectroscopy. The europium elastomer luminophores exhibited intense red light at 617 nm under UV excitation at room temperature due to the 5D0 !7F2 transition in EuIII ions. The newly synthesized luminescent materials offer many advantages, including the desired mechanical flexibility. They cannot be dissolved or fused, and so they have potential for use in optical and electronic applications.
Abstract: Side-chain vinyl poly(dimethylsiloxane) has been modified with mercaptopropionic acid, methyl 3-mercaptopropionate, and mercaptosuccinic acid. Coordinative bonding of EuIII to the functionalized polysiloxanes was then carried out and crosslinked silicone elastomers were prepared by thiol–ene curing reactions of these composites. All these europium complexes could be cast to form transparent, uniform, thin elastomers with good flexibility and thermal stability. The networks were characterized by FTIR, NMR, UV/Vis,
Introduction
pium complexes. Good dispersion of the small particles leads to a higher transparency and larger interfacial interaction region. A good method for addressing this problem is to employ the coordination method.[12, 13] Polysiloxanes have received considerable attention in the field of polymers because they offer a number of advantageous physical properties. The SiOSi moiety is particularly flexible and commonly shows bond angles in the range of 135–1808. In addition, polysiloxanes exhibit excellent permeability as well as low surface energy.[14, 15] The combination of these properties makes polysiloxane an ideal candidate for use in luminescent materials. The formation of complexes between europium ions and a functionalized polysiloxane matrix improves the energy transfer in these materials and enhances the luminescent intensity of the composites.[16] In our previous work we have prepared many kinds of poly(dimethylsiloxane)–lanthanide-ion composites with excellent luminescent properties.[17–20] However, the simple coordination of lanthanide ions to polysiloxanes to form composites results in poor mechanical properties. Therefore a new method of preparing silicone luminescent composites should be developed. The method is expected to involve further crosslinking reactions, which, it is believed, would solve the problem of their poor mechanical properties. Radiation curing of polymers is an efficient way of converting suitable low-viscosity, reactive liquids into solids that exhibit good properties for applications in coatings, inks, adhesives, and dental materials.[21, 22] Thiol–ene “click” chemistry provides an advantage over similar approaches to the crosslinking of poly(dimethylsiloxane) (PDMS) because it does not require an elevated temperature or an expensive heavy-metal catalyst, and it does not produce potentially detrimental byproducts. It also meets the requirements for a general, simple, efficient, and catalyst-free approach to the preparation of
Europium complexes have received considerable attention over the past few decades because of their numerous potential photophysical applications ranging from organic light-emitting devices (OLEDs) to luminescent probes (protein labeling, immunoassays) and optical imaging.[1–3] Research interest is driven largely by the excellent luminescence characteristics of europium complexes, for example, high luminous intensity and narrow emission bands, which are effects of the unique 4f electron structure of rare-earth ions and strong energy transfer from organic ligands to the europium ions.[4, 5] However, the poor thermal stability and processibility of neat europium complexes hinder their further application. Incorporation of europium complexes into polymers to form composites has been shown to be a good method of addressing these problems. The resulting composites show improved luminescence, thermal stability, and mechanical properties over those of neat europium complexes.[6–9] Many europium complex composites have been prepared by physical blending, which is cheap and convenient.[4, 10, 11] However, this method does not produce composites with a high content or fine dispersion of the euro[a] Y. Zuo, H. Lu, L. Xue, Prof. S. Feng Key Laboratory of Special Functional Aggregated Materials and Key Laboratory of Colloid and Interface Chemistry Shandong University, Ministry of Education School of Chemistry and Chemical Engineering Jinan 250100 (P.R. China) Fax: (+ 86) 531-88564464 E-mail: [email protected] [b] X. Wang, L. Wu Marine Chemical Research Institute Company Limited and State Key Laboratory of Marine Coatings Qingdao 266071 (P.R. China) Chem. Eur. J. 2014, 20, 1 – 10
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Full Paper crosslinked polysiloxanes.[23–26] The PDMS-based hybrid organic/inorganic design strategy could overcome the shortcomings of traditional thiol–ene networks derived from flexible sulfide bonds. Thiol–ene-functionalized organic and inorganic materials can be synergistically combined to improve the properties of a material, for example, rubbery modulus, abrasion resistance, and thermal stability.[27] In our previous work we prepared silicone rubber by using thiol–ene “click” chemistry,[28] but extensive and diverse research is still needed to extend the thiol–ene reaction to practical applications. For example, silicone elastomers should exhibit superior luminescent performance. This paper presents an easy thiol–ene “click” chemistry method for the preparation of polysiloxane-based luminescent elastomers under mild conditions. EuIII–carbonyl complexes are highly functional compounds with outstanding optical, electrical, and magnetic properties, and they have widespread applications in various fields, for example, as luminophores.[29] Three kinds of carbonyl-functionalized polysiloxanes complexed to europium ions have been synthesized to combine the excellent fluorescence properties of europium ions and the reactivity of the double bond with thiol groups. This paper reports a new method for the preparation of luminescent silicone elastomers. A series of side-chain carbonylmodified polysiloxanes (PMP, PMPE, and PMS) were synthesized by grafting the carbonyl-containing monomers mercaptopropionic acid (MP), methyl 3-mercaptopropionate (MPE), and mercaptosuccinic acid (MS), respectively, onto vinyl-functionalized polysiloxane (P1) by using thiol–ene “click” chemistry (Scheme 1), and their properties were studied in detail by 1 H NMR spectroscopy and gel permeation chromatography (GPC). This newly designed thiol–ene protocol produced functionalized polysiloxanes under benign conditions compared with the traditional hydrosilylation method. The polysiloxanes could be partially functionalized by controlling the molar ratio of vinyl moieties in the functionalized polysiloxane and carbonyl-containing monomers. Excess vinyl groups could then be used as crosslinking sites to prepare polysiloxane-based elastomers. First, europium ions were coordinated to the side-chains of the functionalized polysiloxanes and then a two-step sequential thiol–ene “click” reaction was applied to produce silicone luminescent elastomers (Scheme 2).
Scheme 2. Scheme showing the crosslinking of the silicone elastomer networks.
Results and Discussion Synthesis of carbonyl-functionalized polysiloxanes The carbonyl-functionalized poly(dimethylsiloxanes) (poly(dimethylsiloxane-co-(1-thiopropanoic acid)ethylmethylsiloxaneco-methylvinylsiloxane) (PMP), poly(dimethylsiloxane-co-(1-thiomethylpropionate)ethylmethylsiloxane-co-methylvinylsiloxane) (PMPE), and poly(dimethylsiloxane-co-(1-thiomalic acid)ethylmethylsiloxane-co-methylvinylsiloxane) (PMS)) were obtained in quantitative yields within a short time. The full conversion of the thiol–ene reaction was confirmed by 1H NMR analysis by the decrease in the vinyl peaks (-Si-CH=CH2) in the range 5.0– 5.7 ppm and the appearance of signals arising from -Si-CH2(0.94 ppm) and -CH2-S-CH2- (2.61 and 2.76 ppm). The 1H NMR spectra and corresponding peak assignments of the vinyl-functionalized polysiloxane (P1) and PMPE are shown in Figure 1. The side-chain olefin protons of P1 are located between 5.0 and 5.7 ppm. On the basis of the integral ratio of the vinyl peaks, the actual graft density was found to be around 5 % after the thiol–ene reactions, which was a perfect match with the designed strategy. These results proved that thiol–ene “click” chemistry is an efficient method for grafting carbonyl groups onto polysiloxane chains. In addition this method makes possible the accurate control of graft densities by simply mixing the functionalized polysiloxane with monomers in the desired molar ratios. However, a small peak at around
Figure 1. 1H NMR spectra of P1 and PMPE in CDCl3.
Scheme 1. Synthesis of PMP, PMPE, and PMS
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Full Paper 1.3 ppm can be observed in the 1H NMR spectrum, which has been attributed to Markovnikov addition products. The thiol– ene reaction proceeded by free-radical step-growth addition and consequently the thiol radical reacts with different sites of the C=C bond, consistent with previous reports.[31, 32] From a calculation of the integral areas of the peaks arising from the two different addition patterns, the proportion of anti-Markovnikov and Markovnikov addition products was determined to be about 95:5. Thus, the results indicate the good regioselectivity of the anti-Markovnikov addition, and the a-addition product has little effect on the structures of the functionalized polysiloxanes. These systems undergo traditional thiol–ene reactions in which the vinyl group does not readily homopolymerize by a radical mechanism, indeed, step-growth is the dominant polymerization mechanism. In a typical polymerization procedure, this mechanism is restricted by oxygen inhibition.
Table 1. Hardness, crosslinking densities, and Mc of the elastomers when swollen at equilibrium. Sample
f
Crosslinking density ne [104 mol cm3]
Mc [g mol1]
Hardness (Shore A)
PMP-Eu3:1-SH PMP-Eu4:1-SH PMPE-Eu3:1-SH PMPE-Eu4:1-SH PMS-Eu3:1-SH PMS-Eu4:1-SH
0.35 0.36 0.40 0.38 0.39 0.33
2.66 2.64 2.17 2.45 2.40 2.89
3258 3369 3960 3257 3584 2994
32 27 25 18 37 34
The crosslinking density increased with increasing lanthanide ion ratio, and, correspondingly, the Shore A hardness also increased with increasing molar ratio of the europium ions. However, the hardness value of PMP-Eu3:1-SH is greater than that of PMPE-Eu3:1-SH, mainly due to the inherent crystalline nature of MP; physical crosslinking between PMP and the polysiloxane main chain is more likely to occur due to the presence of hydrogen bonds between molecular chains, and this further enhances its hardness. The regularity of the change in the crosslinking density is consistent with that of the mechanical properties, which means that the europium ion acts to some extent as a crosslinker in the network.
Crosslinking to form silicone elastomers
An illustration of elastomer crosslinking chemistry is shown in Scheme 2. The thiol–ene curing reaction is a radical-induced process. The polymerization process, which is based on an equimolar mixture of vinyl and multifunctional thiol, is a freeradical reaction that proceeds by a step-growth mechanism; this involves two main steps, namely free-radical addition followed by a chain-transfer reaction. The reaction proceeds by FTIR analysis the rapid formation of a uniformly crosslinked network with low shrinkage, reduced oxygen inhibition during curing, and Figure 2shows the FTIR spectra recorded at room temperature of PMP, PMP-Eu4:1, PMPE, PMPE-Eu4:1, PMS, and PMS-Eu4:1 in excellent adhesion. The thiol–ene crosslinking of europium-coordinated polysiloxanes was accomplished by preparing a mixthe region 2500–800 cm1. The spectra reveal the coordination ture containing crosslinkable polysiloxanes and the tetrathiol of the europium ion to PMP, PMPE, and PMS. The FTIR spectra crosslinking agent, pentaerythritol tetrakis(3-mercaptopropioof the europium complexes are similar because the PMP, nate). The mixture was then exposed to 365 nm UV light for PMPE, and PMS ligands are similar; keto–enol structures are 5 min. Crosslinking was confirmed qualitatively by rinsing the formed because of the coordination of the carbonyl group to cured films with THF, which is a good solvent for uncured subthe europium ion (Scheme 3). The successful attachment of the rare-earth ions to the polymer backbones is strongly supstrates; the thiol–ene crosslinked polymer elastomers were found to be insoluble in this solvent. Measurement of the tensile or tear strengths of the unfilled silicones was not possible because the elastomers easily cracked during measurement. The mechanical properties of the materials were therefore tested by identifying the Shore A hardness by using a Shore A durometer. The hardness of the resulting polymers has a direct correlation with the crosslinking density. The toluene swelling method was used to measure the crosslinking density (n) and the average molecular weights between two crosslinking points (Mc) of the silicone elastomers obtained. Figure 2. FTIR spectra of (a) PMP and PMP-Eu4:1, (b) PMPE and PMPE-Eu4:1, (c) PMS and PMS-Eu4:1, (d) enlargeAll the data are listed in Table 1. ment of the spectra of PMP and PMP-Eu4:1, and (e) enlargement of the spectra of PMS and PMS-Eu4:1. Chem. Eur. J. 2014, 20, 1 – 10
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Figure 3. UV spectra of PMP, PMPE, PMS, PMP-Eu4:1, PMPE-Eu4:1, and PMSEu4:1.
structure group, respectively. This result is consistent with previous conjecture. The spectra of both PMP and PMPE show differences at 277 nm after coordination. More pronounced shoulders appear at around 275 nm in the spectra of PMP–Eu and PMPE–Eu. These results indicate that europium ions successfully coordinate to PMP and PMPE. X-ray photoelectron spectroscopy Scheme 3. Structures of the possible modes of coordination of europium in PMP-Eu-SH, PMP-Eu-SH, and PMP-Eu-SH.
X-ray photoelectron spectroscopy (XPS) is an efficient tool for investigating the coordination process by analyzing changes in binding energy. X-ray photoelectron spectra were recorded to further confirm that coordination occurs between the carbonyl-functionalized polysiloxanes and the europium ions. Eu(NO3)3·6H2O, PMP-Eu4:1-SH, PMPE-Eu4:1-SH, and PMS-Eu4:1-SH were analyzed by XPS because of the similarity in coordination of these complexes. The binding energy curves of Eu 3d5/2 for Eu(NO3)3·6H2O, PMP-Eu4:1-SH, PMPE-Eu4:1-SH, and PMS-Eu4:1SH are shown in Figure 4a. The peak of the Eu 3d5/2 curve appears at 1141 eV in europium nitrate but is shifted to a lower binding energy of about 1138 eV (double-bond equivalent = 3.0 eV) in PMP-Eu4:1-SH. The peaks of PMPE-Eu4:1-SH and PMS-Eu4:1-SH are also shifted to a lower energy. This phenomenon is due to the coordination of carbonyl groups by europium ions in the silicone elastomer and the alteration of the previous coordination environment of lanthanide nitrate.[17] A pair of electrons are donated by the ligands to the metal ions, which act as electron acceptors in a s-bond-type coordination mode; this process strengthens the electron shielding and reduces the inner-electron binding energy in the metal ions. Hence the binding energy of the free metal ions is higher than that of the coordinated ions. The degree of shift for the three different ligands varies slightly with the coordinating ability of the different carbonyl groups. The negative charge density of the europium ions in PMPE-Eu-SH is greater than in the other complexes. The binding energy curves of C 1s for PMP-Eu4:1-SH and PMP-SH are illustrated in Figure 4b. Owing to their similar structure, we show the curve of PMPE-Eu4:1-SH as an example. The C 1s peaks of the carbonyl groups in this complex appear at 289.55 eV and are shifted to a higher energy by 0.8 eV in
ported by the vibration of C=C at around 1635 cm1, which appears as a new absorption peak in the spectra of the modified polymers.[33, 34] The bands at 1381 and 1301 cm1 are due to the OCO stretching frequency. These suggest that coordination bonds are formed between the europium ion and PMP, PMPE, and PMS. The coordination between NO3 and europium ions produces a band at 1500 cm1, which can be attributed to the N=O bond; this band was found difficult to eliminate.[13] The spectrum of PMPE-Eu 4:1 shows a band at 1744 cm1, which has been attributed to the C=O stretching frequency of the carboxy group; after coordinating to Eu3 + , it shifted to a lower wavenumber (1738 cm1), which indicates that PMPE coordinates to Eu3 + more effectively than PMP and PMS. These results suggest that coordination bonds form between Eu3 + and the carbonyl groups of the functionalized PDMS. UV/Vis analysis The UV/Vis absorption spectra of PMP, PMPE, and PMS and their europium complexes (4:1) in THF solution (1 105 mol L1) are shown in Figure 3. The UV/Vis absorption spectra of the europium complexes of PMP, PMPE, and PMS are similar, the absorption bands arising from the organic ligands because the lanthanide ions exhibit negligible absorption. The absorption spectra of PMS and PMS–Eu show a maximum absorption at 245 nm with a shoulder at 277 nm, which correspond to the p!p* electronic transition of the carbonyl group and the n!p* electronic transition of the keto–enol &
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Figure 5. Emission spectra (lex = 330 nm) of PMP-Eu4:1, PMPE-Eu4:1, and PMS-Eu4:1 before crosslinking in THF solution.
A prominent feature observed in these spectra is the very high intensity of the 5D0 !7F2 transition, which indicates the presence of a highly polarizable chemical environment around the europium ions of these complexes; this environment is responsible for the brilliant red emission.[35] As a result, strong coordinative interactions take place between the organic groups and the Eu3 + ions in the uncrosslinked complexes. In addition, efficient energy transfer from the ligands to the central europium ion in these complexes also occurs due to the coordination of the “antenna” ligands to the europium ions.[36] The photoluminescent (PL) properties of all the europiumcomplexed elastomers in the solid state were investigated in detail at room temperature. The spectral emissions are shown in Figure 6. The crosslinked and non-crosslinked composites exhibit similar spectral emission, with the crosslinked elastomers showing much more stable baselines than the linear polysiloxane–europium composites. The removal of small, noncrosslinked molecules by immersion in fresh THF affected the luminescent properties and enhanced the luminescent intensity. The very intense 5D0 !7F2 line at 617 nm clearly dominates the spectrum and is responsible for the red luminescence that is visible by the naked eye upon UV irradiation. Thus, crosslinking has been shown to be fully compatible with the fabrication of luminescent elastomers, which show better luminescent
Figure 4. XPS of (a) Eu 3d5/2 in Eu(NO3)3·6H2O, PMP-Eu4:1-SH, PMPE-Eu4:1SH, and PMS-Eu2:1-SH and (b) C 1s in PMPE-Eu4:1-SH and PMPE-SH.
comparison with the peak at 288.75 eV for PMPE-SH due to the formation of coordination bonds between the carbonyl groups and lanthanide ions; this brings about a transfer of electrons from the carbonyl groups to the lanthanide ions and weak electron shielding in the carbonyl groups. The observed shift of the binding energy peak for europium and carbon confirms the effective coordination of europium ions to the silicone matrix. Photoluminescence of PMP–Eu3 + , PMPE–Eu3 + , and PMS– Eu3 + The room-temperature emission spectra of PMP-Eu4:1, PMPEEu4:1, and PMS-Eu4:1 in THF solution are shown in Figure 5. These complexes exhibit the red emission characteristic of europium ions on excitation at 330 nm, which suggests that these complexes may be potential red fluorescent materials. All of the europium complexes exhibit the characteristic emission lines of Eu3 + , namely 5D0 !7F1 (591 nm) and 5D0 !7F2 (617 nm). The 5D0 !7F1 (591 nm) transition is a parity-allowed magnetic dipole transition and is insensitive to the local environment, whereas the 5D0 !7F2 (617 nm) transition is an induced electric dipole transition and is very sensitive to the environment. This luminescence line is responsible for the red luminescence color visible by the naked eye upon UV irradiation. Chem. Eur. J. 2014, 20, 1 – 10
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Figure 6. Emission spectra (lex = 330 nm) of (a) PMP-Eu-SH, (b) PMP-Eu-SH, and (c) PMP-Eu-SH with different europium ion ratios after crosslinking to form the in silicone elastomer.
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Full Paper performance than the non-crosslinked europium-coordinated polysiloxanes. Elastomers containing different molar ratios of europium ions and carbonyl groups were investigated to analyze the factors affecting the emission properties. The emission intensity increased with increasing europium content and reached a maximum at 4:1; a further increase in europium content resulted in quenching of the emission. When the amount of europium doped in the PDMS matrix is small, the probability of energy migration by diffusional collision of the europium complex should be small. Aggregates of the complex occurred in the solid matrix at high concentrations of the complex. Therefore the quenching phenomenon may be caused by the deactivation of the 5D0 or 5D1 state through electrostatic multipolar interactions or by excitation migration by the Fçrster dipole– dipole mechanism in solid complexes.[37]
nature. This observation is indicative of well-bonded interface adhesion induced by coordination, which is consistent with the previous discussion.
Contact angle measurement The surface energies of the crosslinked thiol–ene networks were determined through static contact angle measurements. The image of the drop was calculated from the shape of the drop (both left and right contact angles) with an accuracy of 0.18 by using an image analysis system. Distilled water was used as the test liquid and all measurements were performed at room temperature. The results of the goniometer experiments are shown in Figure 8. The contact angles of PMP-SH,
Morphology of crosslinked silicone elastomers Figure 8. Contact angles of the crosslinked elastomers: (a) PMP-Eu4:1-SH, (b) PMPE-Eu4:1-SH, and (c) PMS-Eu4:1-SH.
The morphologies of the films were characterized by SEM analysis. The micrographs of PMP-Eu4:1-SH, PMPE-Eu4:1-SH, and PMS-Eu4:1-SH are shown in Figure 7. No phase separation was observed in any of the micrographs, which indicates that the europium ions are well dispersed. The homogeneity between the organic and inorganic phases prevented aggregation of the ions; the europium salt melted and integrated well into the elastomeric matrix as a result of the coordination to the polymeric chain. The chemical bonding (coordination) of the Eu3 + ions to the silicone main chain also prevented them from forming aggregates. No crystalline domains were observed because the polymer and Eu3 + ions are both amorphous in
PMPE-SH, and PMS-SH are 85.3, 96.7, and 75.78, which compares with a value of 1098 for PDMS.[38] As expected, the contact angles are lower for the networks containing a side-chain and the europium ion. The structures of the carbonyl groups affect the observed material properties; as hydrophilic carbonyl-functionalized monomers are introduced, the contact angle decreases. The lower contact angle of PMP has been attributed to the greater hydrophilicity of the acid functionality. Therefore inclusion of the carbonyl side-chains can render the hydrophobic PDMS a more hydrophilic material without altering its transparency. Thermogravimetric analysis The thermal stability of the complexes is very important because their decomposition leads to a decrease in device performance. Therefore their thermal stabilities were measured by thermogravimetric analysis (TGA) at a heating rate of 10 8C min1 under N2. The thermogravimetric weight loss curves for all the complexes show that the decomposition of the complexes progresses similarly (Figure 9). The first weight loss observed in PMP-Eu4:1-SH, PMPE-Eu4:1-SH, and PMSEu4:1-SH has been attributed to the loss of coordinated water.[39] The second weight loss step may be due to the decomposition of the organic groups and the breaking of the polysiloxane main chain.[17] The TG curves indicate that these complexes decompose above 280 8C, which means that device fabrication by the vacuum evaporation method is more feasible.
Conclusion Polysiloxane-based luminescent elastomers complexed by europium ions have been prepared by thiol–ene “click” chemistry,
Figure 7. Scanning electron micrographs of (a) PMP-Eu4:1-SH, (b) PMPEEu4:1-SH, and (c) PMS-Eu4:1-SH.
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Full Paper and 2.5 nm, respectively. Thermogravimetric analysis (TGA) was performed by using a TA SDTQ600 instrument at a heating rate of 10 8C min1 from room temperature to 800 8C under N2. Contact angles were recorded by using a Dataphysics OCA-20 contact angle analyzer with distilled water as the test liquid. Scanning electronic microscopy (SEM) images were obtained by using an Hitachi S-4800 instrument (7 kV). Samples were cut and coated with a thin layer of gold before investigation. X-ray photoelectron spectroscopy was performed by using a Thermo Fisher Scientific Escalab 250 spectrometer with a monochromated AlKa X-ray source at a residual pressure of 107 Pa. Survey and high-resolution scans were performed with pass energies of 100 and 20 eV with steps of 1 and 0.05 eV, respectively. Hardness (Shore A) was measured according to the ASTMD 2240--2005 method by using a Shore durometer (Laizhou Huayin Research Instruments Co., China). Figure 9. TGA curves of PMP-Eu4:1-SH, PMPE-Eu4:1-SH, and PMS-Eu4:1-SH.
Synthesis of vinyl-functionalized polysiloxane P1 Poly(methylvinylsiloxane) P1 was synthesized by the classical procedure.[30] Yield: Yield: 90 %; 1H NMR (400 MHz, CDCl3): d = 0.06– 0.17 (m, -SiCH3), 5.79–6.03 ppm (m, -CH=CH2).
with a new type of luminescent hybrid material being produced by doping EuIII complexes into a PDMS-crosslinked network, which acts as an elastomer. The performance of the PDMS elastomers complexed by europium (III) ions has also been presented. However, the applicability of this system is not limited to europium (III) or to these types of ligands alone. The luminescence color of the flexible PDMS elastomers can be tuned by a suitable choice of lanthanide ions, for example, blue for Tm3 + and green for Tb3 + . White luminescent materials can also be designed by using this method. This study aimed to provide some useful information for the development of silicone elastomers.
Synthesis of side-chain carbonyl-functionalized polysiloxanes PMP, PMPE, and PMS The procedure used for the thiol–ene “click” chemistry is shown in Scheme 1 along with the chemical structures of the thiols. Vinylfunctionalized polysiloxanes P1 (4.05 g, 50 mmol), the corresponding thiol (0.05 equiv, with respect to the –SiO- group), DMPA (2 wt %, 0.04 g), and a certain amount of THF as solvent were placed in a glass vessel and then exposed to a UV light source (365 nm, 100 W) with stirring for 15 min at room temperature. The resulting polymers were purified by precipitation in MeOH to eliminate unreacted thiols and photoinitiator. The gel permeation chromatography (GPC) data for P1, PMP, PMPE, and PMS are presented in Table 2.
Experimental Section Materials Octamethylcyclotetrasiloxane (D4), tetramethyltetravinylcyclotetrasiloxane (D4Vi), and hexamethyldisiloxane (MM) were obtained as commercial products from Qufu Wanda Chemical Co., Ltd. and used directly. Methyl 3-mercaptopropionate (MPE), 3-mercaptopropionic acid (MP), and mercaptosuccinic acid (MS) were purchased from Aladdin Chemistry Co. Ltd. 2,2-Dimethoxy-2-phenylacetophenone (DMPA) was purchased from Aldrich and used as received. Pentaerythritol tetrakis(3-mercaptopropionate) (PTTMP) was purchased from China Energy Chemical Group and used as received. Europium nitrates were obtained from their corresponding oxides in strong nitric acid. THF was purified by a routine procedure and distilled over sodium before use.
Table 2. GPC data of the synthesized polymers.
The thiol–ene reaction was performed by irradiation using UV light from a Spectroline Model SB-100P/FA lamp (365 nm, 100 W). The UV intensity is 4500 mW cm2 at a distance of 38 cm. 1H NMR spectra were recorded on a Bruker AVANCE 400 spectrometer at 25 8C in CDCl3 as solvent and without tetramethylsilane (TMS) as interior label. FTIR spectra were recorded on a Bruker TENSOR27 infrared spectrophotometer in the range 4000–400 cm1 by using KBr pellets. UV absorption spectra were recorded in THF with a Beijing TU-1901 double beam UV/Vis spectrophotometer. Luminescence (excitation and emission) spectra were recorded with an Hitachi F4500 fluorescence spectrophotometer using a monochromated Xe lamp as the excitation source; excitation and emission slits were 5 www.chemeurj.org
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Mn [g mol1]
Mw [g mol1]
Mz [g mol1]
PDI
P1 PMP PMPE PMS
62 458 63 897 61 209 64 710
79 650 82 564 10 8513 81 235
10 2130 11 2358 18 8776 14 2356
1.27 1.29 1.77 1.25
PMP: 1H NMR (400 MHz, CDCl3): d = 0.01–0.30 (m, -SiCH3), 0.91–0.95 (m, -SiCH2CH2S-), 2.61–2.70 (m, -SiCH2CH2SCH2-), 2.80–2.84 (t, 13 5.78–6.02 ppm (m, -CH=CH2); C NMR -SCH2CH2COOH), (100.62 MHz, CDCl3): d = 1.01 (-SiCH3), 18.82 (-SiCH2CH2S-), 26.37 (-SiCH2CH2S-), 26.98 (-SCH2CH2COOH), 34.40 (-CH2COOH), 131.41 (-CH=CH2), 138.41 (-CH=CH2), 176.77 ppm (-COOH).
Characterization and measurements
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PMPE: 1H NMR (400 MHz, CDCl3): d = 0.01–0.25 (m, -SiCH3), 0.90– 0.95 (m, -SiCH2CH2S-), 2.60–2.65 (m, -SiCH2CH2SCH2-), 2.80–2.84 (t, -SCH2CH2COOCH3), 3.73 (s, -COOCH3), 5.74–6.06 ppm (m, -CH=CH2); 13 C NMR (100.62 MHz, CDCl3, ): d = 0.19 (-SiCH3), 18.93 (-SiCH2CH2S-), 26.70 (-SiCH2CH2S-), 26.73 (-SCH2CH2COOCH3), 34.44 (-CH2COOCH3), 50.98 (-COOCH3), 131.27 (-CH=CH2), 139.01 (-CH=CH2), 172.37 ppm (-COOCH3). PMS: 1H NMR (400 MHz, CDCl3): d = 0.01–0.25 (m, -SiCH3), 0.90–0.95 (m, -SiCH2CH2S-), 2.70–2.85 (m, -SiCH2CH2SCH2-), 3.59–3.75 (m, -CHCOOH), 5.58–5.94 ppm (m, -CH=CH2); 13C NMR (100.62 MHz,
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Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant no. 21274080 and 21204043) and the Key Natural Science Foundation of Shandong Province of China (Grant no. ZR2011BZ001).
Europium coordination A series of europium complexes (Table 3) were prepared by the following procedure. To take PMP-Eu4:1 as an example, Eu(NO3)3·6H2O (0.35 g, 0.77 mmol) and PMP (5.33 g) were dissolved in THF (30 mL) in a flask. The solution was heated at reflux at 70 8C for 4 h with continuous stirring. The solvent was then removed by evaporation under vacuum to yield PMP-Eu4:1. PMPE-Eu, and PMSEu were prepared by the same procedure.
Keywords: click chemistry luminescence · silicon
C=O/Ln3 + Polysiloxane [g] Ln(NO3)3·6H2O [g] PTTMP [g]
PMP-Eu3:1-SH PMP-Eu4:1-SH PMP-Eu6:1-SH PMPE-Eu3:1-SH PMPE-Eu4:1-SH PMPE-Eu6:1-SH PMS-Eu3:1-SH PMS-Eu4:1-SH PMS-Eu6:1-SH
3:1 4:1 6:1 3:1 4:1 6:1 3:1 4:1 6:1
5.33 5.33 5.33 5.37 5.37 5.37 5.47 5.47 5.47
0.46 0.35 0.23 0.46 0.35 0.23 0.46 0.35 0.23
0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02
General procedure for thiol–ene photocuring Crosslinking was achieved by using a solution of the europium-coordinated crosslinkable polymer (PMP–Eu, PMPE–Eu, and PMS–Eu) and the tetrafunctionalized crosslinker pentaerythritol tetrakis(3mercaptopropionate) (PTTMP). The corresponding quantities are shown in Table 3. Solutions were prepared at 10 mg mL1 polymer in THF. THF was then removed by evaporation before the crosslinking step. UV crosslinking was performed at room temperature for 5 min in air and under 365 nm light to form an insoluble crosslinked network (Scheme 2). All of the elastomers were swollen within the THF to interchange the substrates and remove the uncrosslinked molecules. They were then dried under vacuum three times to ensure that all uncrosslinked materials were removed from the network. Transparent crosslinked networks were finally obtained.
Measurement of crosslinking density The extent of crosslinking of samples was measured by swelling at equilibrium using toluene as solvent at room temperature and for an immersion period of 72 h. The crosslinking densities of the samples were calculated by using the Flory and French[25] equation (1) in which Mc is the average molecular weight between two crosslinking points, ue is the crosslinking density of the sample, f is the volume fraction of the rubber in the swollen sample, c is the rubber solvent interaction parameter, 1 is the density of the rubber before swelling, and v0 is the molar volume of the solvent.
ue ¼
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p ½lnð1 fÞ þ f þ cf2 ¼ Mc v0 f1=3
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[1] E. Brunet, O. Juanes, J. C. Rodriguez-Ubis, Curr. Chem. Biol. 2007, 1, 11 – 39. [2] N. N. Katia, A. Lecointre, M. N. Regueiro-Figueroa, C. Platas-Iglesias, L. c. J. Charbonnire, Inorg. Chem. 2011, 50, 1689 – 1697. [3] H. Tsukube, S. Shinoda, Chem. Rev. 2002, 102, 2389 – 2404. [4] L. N. Sun, H. J. Zhang, L. S. Fu, F. Y. Liu, Q. G. Meng, C. Y. Peng, J. B. Yu, Adv. Funct. Mater. 2005, 15, 1041 – 1048. [5] M. H. Werts, Sci. Prog. 2005, 88, 101 – 131. [6] K. Kuriki, Y. Koike, Y. Okamoto, Chem. Rev. 2002, 102, 2347 – 2356. [7] J. Boyer, N. Johnson, F. van Veggel, Chem. Mater. 2009, 21, 2010 – 2012. [8] J. Kai, M. C. Felinto, L. A. Nunes, O. L. Malta, H. F. Brito, J. Mater. Chem. 2011, 21, 3796 – 3802. [9] A. Gulino, F. Lupo, G. G. Condorelli, A. Motta, I. L. Fragal, J. Mater. Chem. 2009, 19, 3507 – 3511. [10] L. D. Carlos, R. S Ferreira, J. Rainho, V. de Zea Bermudez, Adv. Funct. Mater. 2002, 12, 819 – 823. [11] W. Q. Fan, J. Feng, S. Y. Song, Y. Q. Lei, G. L. Zheng, H. J. Zhang, Chem. Eur. J. 2010, 16, 1903 – 1910. [12] B. Yan, H.-F. Lu, Inorg. Chem. 2008, 47, 5601 – 5611. [13] H.-F. Lu, B. Yan, J.-L. Liu, Inorg. Chem. 2009, 48, 3966 – 3975. [14] F. Abbasi, H. Mirzadeh, A. A. Katbab, Polym. Int. 2001, 50, 1279 – 1287. [15] A. Morikawa, M. Kakimoto, Y. Imai, Macromolecules 1991, 24, 3469 – 3474. [16] L. Liu, Y. L. Lu, L. He, W. Zhang, C. Yang, Y. D. Liu, L. Q. Zhang, R. G. Jin, Adv. Funct. Mater. 2005, 15, 309 – 314. [17] Y. Yue, Y. Liang, H. Wang, L. Feng, S. Feng, H. Lu, Photochem. Photobiol. 2013, 89, 5 – 13. [18] Q. Lai, H. Lu, D. Wang, H. Wang, S. Feng, J. Zhang, Macromol. Chem. Phys. 2011, 212, 1435 – 1442. [19] L. Liu, H. Lu, H. Wang, Y. Bei, S. Feng, Appl. Organomet. Chem. 2009, 23, 429 – 433. [20] H. Lu, H. Wang, S. Feng, J. Photochem. Photobiol. A 2010, 210, 48 – 53. [21] A. F. Senyurt, H. Wei, B. Phillips, M. Cole, S. Nazarenko, C. E. Hoyle, S. G. Piland, T. E. Gould, Macromolecules 2006, 39, 6315 – 6317. [22] T. Y. Lee, T. M. Roper, E. S. Jonsson, C. Guymon, C. Hoyle, Macromolecules 2004, 37, 3606 – 3613. [23] C. E. Hoyle, C. N. Bowman, Angew. Chem. 2010, 122, 1584 – 1617; Angew. Chem. Int. Ed. 2010, 49, 1540 – 1573. [24] A. B. Lowe, C. E. Hoyle, C. N. Bowman, J. Mater. Chem. 2010, 20, 4745 – 4750. [25] R. Acosta Ortiz, A. Y. R. Martinez, A. E. Garca Valdez, M. L. Berlanga Duarte, Carbohydr. Polym. 2010, 82, 822 – 828. [26] J. W. Chan, C. E. Hoyle, A. B. Lowe, J. Am. Chem. Soc. 2009, 131, 5751 – 5753. [27] B. J. Sparks, T. J. Kuchera, M. J. Jungman, A. D. Richardson, D. A. Savin, S. Hait, J. Lichtenhan, M. F. Striegel, D. L. Patton, J. Mater. Chem. 2012, 22, 3817 – 3824. [28] L. Xue, Y. Zhang, Y. Zuo, S. Diao, J. Zhang, S. Feng, Mater. Lett. 2013, 106, 425 – 427. [29] Z. Hong, C. Liang, R. Li, W. Li, D. Zhao, D. Fan, D. Wang, B. Chu, F. Zang, L. S. Hong, Adv. Mater. 2001, 13, 1241 – 1245. [30] L. Xue, D. X. Wang, Z. Z. Yang, Y. Liang, J. Zhang, S. Y. Feng, Eur. Polym. J. 2013, 49, 1050 – 1056. [31] D. A. Shipp, C. W. McQuinn, B. G. Rutherglen, R. A. McBath, Chem. Commun. 2009, 6415 – 6417. [32] K. L. Killops, L. M. Campos, C. J. Hawker, J. Am. Chem. Soc. 2008, 130, 5062 – 5064.
Table 3. Amounts of reactants used for the synthesis of PDMS-Eu-SHs (PDMS = PMP, PMPE, PMS) complexes. Sample
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Full Paper [33] Z. Chen, Y. Wu, F. Huang, D. Gu, F. Gan, Spectrochim. Acta Part A 2007, 66, 1024 – 1029. [34] S. M. Bruno, R. A. S Ferreira, L. D. Carlos, M. Pillinger, P. Ribeiro-Claro, I. S. GonÅalves, Microporous Mesoporous Mater. 2008, 113, 453 – 462. [35] M. H. Werts, R. T. Jukes, J. W. Verhoeven, Phys. Chem. Chem. Phys. 2002, 4, 1542 – 1548. [36] D. A. Raj, B. Francis, M. Reddy, R. R. Butorac, V. M. Lynch, A. H. Cowley, Inorg. Chem. 2010, 49, 9055 – 9063. [37] L.-H. Wang, W. Wang, W.-G. Zhang, E.-T. Kang, W. Huang, Chem. Mater. 2000, 12, 2212 – 2218.
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Received: March 24, 2014 Revised: June 16, 2014 Published online on && &&, 0000
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FULL PAPER & Luminescence
Luminescent elastomers: The novel crosslinking in poly(dimethylsiloxane) networks prepared by easy thiol–ene “click” chemistry has been studied as well as their elastomeric character. The europium poly(dimethylsiloxane)-based elastomer luminophores exhibit intense red light at 617 nm under UV excitation at room temperature (see figure).
Y. Zuo, H. Lu, L. Xue, X. Wang, L. Wu, S. Feng* && – && Polysiloxane-Based Luminescent Elastomers Prepared by Thiol–ene “Click” Chemistry
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