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Monochromatic light-emitting copolymer of methyl methacrylate and Eu-complexed 5-acrylamido-1,10-phenanthroline
Dyes and Pigments 98 (2013) 493e498
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Monochromatic light-emitting copolymer of methyl methacrylate and Eu-complexed 5-acrylamido-1,10-phenanthroline Cun-Jin Xu a, Jin-Tao Wan b, c, Bo-Geng Li b, * a
College of Material, Chemistry and Chemical Engineering, Hangzhou Normal University, Hangzhou 310036, China The State Key Laboratory of Chemical Engineering, Department of Chemical and Biochemical Engineering, Zhejiang University, Hangzhou 310027, China c Zhejiang Jiamin Plastics Co. Ltd., Jiaxing 314027, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 January 2013 Received in revised form 24 March 2013 Accepted 3 April 2013 Available online 17 April 2013
We present an effective approach to prepare Eu-containing polymer with excellent photoluminescence properties through the copolymerization of highly luminescent Eu-complex monomer featuring 2thenoyltrifluoroacetone (HTTA) and 5-acrylamido-1,10-phenanthroline with methyl methacrylate (MMA). The copolymer was characterized by FT-IR, UVeVis, 1H NMR, GPC, TGA, and DSC. The results reveal that the copolymer is readily soluble, and possesses excellent thermal stability, with high glass transition and decomposition temperatures of 125 and 335 C, respectively. Intense red emission peaked at 612 nm, corresponding to the 5D0 / 7F2 transition of Eu(III) ions is recorded under UV excitation. The luminescence lifetimes and efficiencies of some Eu-containing monomers could be greatly enhanced upon copolymerization. The special microenvironment, in which the Eu-complex units are uniformly bonded to and surrounded by the polymer chain, appears to be responsible for the improvement of the luminescence properties for the copolymer. Moreover, the applicability of the well-known JuddeOfelt theory to the luminescence properties of Eu3þ complexes is also investigated. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Luminescence Copolymer 1,10-Phenanthroline 2-Thenoyltrifluoroacetone JuddeOfelt theory Synthesis
1. Introduction Over the past decades, considerable attention has been devoted to the polymer-based rare earth luminescent materials [1e3], because they have the advantages of polymers, including mechanical strength, flexibility, and processability and the luminescence characteristics of rare earth complexes, such as long luminescence lifetimes, sharp emission bands, large Stockes shift, and high quantum yields [4,5]. Seminal work in this area has primarily focused on polymer systems doped or blended with lanthanide complexes [6e11]. However, blending can also cause some side-effects, such as poor dispersion and compatibility of the rare earth complexes in a polymer matrix, resulting in weak emission of the polymer phosphors [12]. Recently, many researchers have synthesized some polymers containing coordination groups and studied the luminescence properties of the complexes [13e17]. There are, however, problems associated with this approach: first, the polymer complexes prepared by direct reaction of the polymer ligands with rare earth ions usually have an ion aggregation nature, giving rise to weak luminescence of
* Corresponding author. Tel.: þ86 571 87952623; fax: þ86 571 87951612. E-mail address: [email protected] (B.-G. Li). 0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.dyepig.2013.04.001
polymer luminophores [18,19]; second, the microenvironment of rare earth ions is very complicated, which makes the elucidation of the luminescence properties difficult [20]. Polymers with the lanthanide complexes covalently bound in the main chain would be excellent candidates to solve the above-mentioned problems. Furthermore, energy transfer (ET) in such covalently bound system is anticipated to be more efficient due to the close proximity of the two components [21]. Therefore, the design and synthesis of highly luminescent complex monomers have become an important theme in supramolecular and coordination chemistry [22,23]. In previous work, we designed and synthesized a luminescent europium complex monomer Eu(TTA)2(amq) (HTTA ¼ 2thenoyltrifluoroacetone, Hamq ¼ 5-acryloxyethoxymethyl-8hydroxyquinoline), and its copolymers with methyl methacrylate (MMA) [24,25]. The luminescence property of monomer Eu(TTA)2(amq) is better than that of Eu(TTA)2(AA) (HAA ¼ acrylic acid) [26], but worse than that of Eu(TTA)3(phen) (phen ¼ 1,10phenanthroline) [27]. This may be due to the longer conjugation length in phen group and the more antenna chromophores in Eu(TTA)3(phen) complex unit. It therefore occurred to us that it should be possible to design a functional ligand for europium that bearing a polymerizable C¼C double bond for the copolymerization with other vinyl monomers and a phen fragment coordinating to the Ln and effectively acting as photon antenna for the transfer
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energy to Eu3þ ion. To accomplish this goal, we synthesized a functional ligand, 5-acrylamido-1,10-phenanthroline (Aphen), as well as a highly luminescent complex monomer Eu(TTA)3(Aphen) (1) [28]. In this paper, 1 was further copolymerized with MMA to prepare Eu-containing copolymer PMMA-Eu(TTA)3(Aphen) (2) (Scheme 1), and the luminescence property of the copolymer was investigated. 2. Experimental 2.1. Reagents and instruments Azoisobutyronitrile (AIBN) was purified by twice re-crystallizing in methanol. Tetrahydrofuran (THF) and MMA were purified according to standard procedures. The complex monomer Eu(TTA)3(Aphen) was prepared as reported previously [28]. Other chemicals were analytical grade and used as received. Element analysis (CHN) was performed with a Flash-EA1112 elemental analyzer, and lanthanide ions were analyzed by complexometric titration with EDTA. 1H NMR spectra were collected on a Bruker-400 spectrometer. Infrared spectra of KBr pellets were carried out at room temperature by using a Bruker Tensor 27 FT-IR spectrophotometer. UV spectra were obtained by a Unico UV-2102PC spectrophotometer. The excitation and emission spectra were recorded in a Hitachi F-4500 spectrofluorometer. Luminescence lifetimes were measured on an Edinburgh FLS 920 fluorescence spectrograph using a microsecond flashlamp as the excitation source with the pulse width of 2 ms. Thermogravimetric analyses (TGA) were conducted on a Perkin Elmer Pyris 1 TGA thermogravimetric analyzer at a heating rate of 10 C min1 in air. Differential scanning calorimetry (DSC) measurements were run on a Perkin Elmer DSC 7 system under nitrogen atmosphere at a heating rate of 10 C min1. The glass transition temperatures were taken as the midpoint of the change in slope of the baseline. Molecular weights and molecular weight distributions were determined by GPC (Waters 1525) with three Waters Styragel columns (HR 4, 3, 2). Molecular weights were derived from a calibration curve based on polystyrene standards. The eluent was THF with a flow rate of 1.0 mL min1. 2.2. Preparation of PMMA-Eu(TTA)3(Aphen) (2) A mixture of MMA (0.30 g, 3.0 mmol), Eu(TTA)3(Aphen) (0.0532 g, 0.05 mmol) and 4.0 mg of the AIBN initiator was dissolved in 3 mL of dry THF in a glass polymerization tube. The homogeneous solution was purged with argon for 5 min, then sealed and heated in water bath at 60 C for 48 h. The viscous solution was diluted with 5 mL of THF and poured into 70 mL of methanol under vigorous stirring. The resulting precipitate was redissolved in THF and reprecipitated in methanol twice and finally dried under vacuum at 50 C for 24 h to afford powder polymer. PMMA was prepared using the same polymerization conditions for comparison. The composition of 2 was determined by element analysis, x ¼ 0.014, y ¼ 0.986. The content of Eu-complex in copolymer is about 13.12 wt%. The copolymer is fully soluble in common organic
solvents such as chloroform and THF, and can be easily cast into transparent, uniform, thin films with good thermal and moisture stable properties, which is important in optical application. The copolymer 2 exhibits a polydispersity (Mw/Mn) of 1.29 and has the Mn value of 10,000, which is not as high as that in general vinyl polymerization due to the lower polymerization reactivity of Eucomplex monomer [26]. IR (KBr pellet, cm1): 2951, 1732, 1603, 1537, 1485, 1243, 1190, 1148, 749, and 581. 1H NMR (400 MHz, DMSO-d6, d, ppm): 9.10 (d, J ¼ 41.7 Hz, 2H), 8.44 (m, 3H), 7.81 (d, J ¼ 55.0 Hz, 2H), 7.55 (d, J ¼ 10.7 Hz, 4H), 6.40 (d, J ¼ 69.6 Hz, 9H), 3.54 (s, 210H), 2.20e0.40 (m, 353H). 3. Results and discussion 3.1. IR spectra Apart from some weak characteristic absorption peaks from the complex unit including the peaks at 1603, 1537 and 1306 cme1 due to the C¼O, C¼C and CeF stretching vibrations of TTA, respectively, the main peaks of 2 are in accordance with those of PMMA as the content of the complex moiety in the copolymer is low. The C¼C stretching vibrations of the MMA (1639 cme1) and 1 (1629 cme1) are absent in 2, indicating that the monomers have been successfully copolymerized (Fig. 1). This is also verified by the 1H NMR spectrum of 2 (Fig. 2) in which the proton peaks assigned to terminal vinyl group of 1 and methylene group of MMA have completely disappeared and all the peaks become rather broad. 3.2. UV spectra Fig. 3 displays the absorption spectra of 1, 2, and PMMA in THF solution. Two absorption peaks at 238 and 275 nm are observed for pure PMMA, which are ascribed to the pep* and nep* transitions of the C¼O group, respectively. It is noticed that the absorption characteristics of 1 and pure PMMA are observed simultaneously from the spectrum of 2, revealing that the complex moieties have been bonded to the polymer backbone and that the dissociation of ligands is negligible during the copolymerization and purification processes. 3.3. Luminescence properties Fig. 4 shows the photoluminescence spectra of complex 1 and copolymer 2 in solid state at room temperature. The excitation spectra of 1 and 2 were obtained by monitoring the emission wavelength of the Eu(III) ions at 612 nm. In complex 1, there was a broad excitation band ranging from 250 to 420 nm, which was assigned to the pep* electron transition of the ligands. Intriguingly, such an excitation for the copolymer 2 was relatively narrower and split into two bands centered at 288 and 348 nm, respectively. This suggested that the site symmetry of Eu3þ ions became lower in the polymer due to the influences of the neighboring chain segments of PMMA [29,30].
Scheme 1. Synthetic route for the copolymer 2.
C.-J. Xu et al. / Dyes and Pigments 98 (2013) 493e498
495
Fig. 1. The FT-IR spectra of 1, 2, and PMMA. Fig. 3. Absorption spectra of 1, 2, and PMMA in THF at room temperature.
The emission spectra of 1 and 2 excited at 385 and 348 nm, respectively, do not show significant differences and all display characteristic sharp peaks associated with the 5D0 / 7FJ transitions of the Eu(III) ion between 575 and 720 nm, suggesting that the light-emitting sites in copolymer 2 are the same as those in complex 1. Therefore, the luminescence of 2 came from the corresponding Eu-complex, and the polymer matrix just functionalized as the skeleton causing no damage to the Eu-complex. As shown in Fig. 4, five expected emission peaks of different intensities at 579.0, 591.6, 611.8, 650.0 and 700.0 nm are well resolved, which belong to the transitions of 5D0 / 7F0, 5D0 / 7F1, 5D0 / 7F2, 5D0 / 7F3 and 5 D0 / 7F4 of Eu(III) ion, respectively. The hypersensitive 5D0 / 7F2 transition is very intense, suggesting a highly polarizable chemical environment around the Eu(III) ion, and is responsible for the brilliant red emission color of the two samples. Moreover, the emission spectrum shows only one peak for the 5D0 / 7F0 transition, indicating the presence of a single chemical environment around the Eu(III) ion and also showing that the Eu(III) ion is at a site with Cn, CnV or Cs symmetry [31]. The luminescence properties such as the emission intensity (I), lifetime (s), and quantum efficiency (V) of some selected Eu(III) complexes are given in Table 1. As shown in Table 1, the copolymer 2 exhibits excellent luminescence properties because of its strongly luminescent complex monomer 1. It is also observed that the copolymer PMMA-Eu(TTA)2(AA) gives relatively shorter s, V, and weaker I, which is attributed to its weakly luminescent complex monomer Eu(TTA)2(AA) whose active ligand AA makes little or no contribution to the luminescence of the complex. The luminescence decay curves of the Eu(III) 5D0 / 7F2 transition for 2 was
Fig. 2. 1H NMR spectra of the monomer 1 (top) and the copolymer 2 (bottom), for both spectra: 400 MHz, DMSO-d6, 298 K.
found to be a single exponential (Fig. 5), and is consistent with the presence of one major luminescent species. The lifetime of 2 is slightly higher than that reported recently for luminescent europium b-diketonate complexes covalently linked to other types of polymers or to a silica matrix [32e34]. 3.4. JuddeOfelt analysis The JuddeOfelt theory is a useful tool for analyzing fef inner shell electronic transitions [35]. Interaction parameters of ligand fields are given by the JuddeOfelt parameters Ul (l ¼ 2, 4, and 6), in which U2 is more sensitive to the symmetry and sequence of ligand fields [36]. The experimental U2 and U4 intensity parameters were determined from the 5D0 / 7F2 and 5D0 / 7F4 electronic transitions, respectively, using the magnetic dipole transition of 5D0 / 7F1 as the reference [37,38]. The detailed principles and specific calculations were provided in the Supporting Information, and the obtained U2 and U4 intensity parameters for some selected Eu(III) complexes were listed in Table 1. It was evident that the Eu-containing copolymers had higher U2 values than their corresponding complex monomers, suggesting an increase of the covalence degree in the first coordination shell of Eu3þ ions and an enhancement of the 5 D0 / 7F2 hypersensitive transition [39]. This may be also due to the change of the chemical environment surrounding Eu3þ ions, which was induced by the intra- and intermolecular interactions between Eu-complex unit and neighboring soft chain consisting of MMA
Fig. 4. Excitation (EX, lem ¼ 612 nm) and emission (EM) spectra for 1 (red line, lex ¼ 385 nm), 2 (blue line, lex ¼ 348 nm) in solid state at room temperature.
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Table 1 The relative intensity of the 5D0 / 7F2 transition (I), experimental intensity parameters (U2,4), radiative (Arad) and nonradiative (Anr) decay rates, lifetime (s), and emission quantum efficiency (F) for some selected Eu(III) complexes in the solid state at 293 K. Complex
I (a.u.)
U2 (1020 cm2)
U4 (1020 cm2)
Arad (s1)
Anr (s1)
s (ms)
F (%)
1 [28] Eu(TTA)2(amq) [24] Eu(TTA)2(AA)a PMMA-Eu(TTA)2(AA)b PMMA-Eu(TTA)2(amq)b 2b
687 [28] 289 [24] 100 [28] 311 597 883
21.07 22.08 13.86 22.05 23.53 21.51
2.24 2.34 1.92 2.49 2.48 2.51
722.99 754.96 658.37 711.84 680.33 740.06
971.92 1815.73 10330.64 1167.86 959.01 854.84
590 [28] 389 [24] 91 [28] 532 610 627
43 29 6 38 42 46
a b
Prepared by Wang’s method [26]. Prepared by the same procedures as in the case of 2.
units. The higher values of U4 for the Eu-containing polymers as compared with that of their corresponding complex monomers indicated a perturbation on the coordination effect of the bidentate TTA by the steric factors from the surrounding PMMA [40]. The luminescent quantum efficiency (F) for the 5D0 / 7F0e4 transitions of Eu3þ ions basically determines the luminescent properties of the Eu(III) complex, which is defined as Arad/ (Arad þ Anr) [41], where Arad is the total radiative transition rate of the 5D0 / 7F0e4 transitions and Anr is the nonradiative transition rate. The Arad, Anr, and F values of some selected Eu(III) complexes were calculated based upon the JuddeOfelt theory, and listed in Table 1. The detailed calculations were provided in the Supporting Information. It is somewhat surprising that the copolymers, especially PMMA-Eu(TTA)2(AA) and PMMA-Eu(TTA)2(amq), show higher F values when compared with their corresponding Eu-containing monomers because the main flexible framework groups (PMMA) have an adverse effect on the F value, as has been reported in the literature [42]. Now we should consider the case where the Eucontaining monomer has been grafted on the PMMA chains. Since the Eu-containing monomer has a lower reactivity ratio [43], the resulting copolymer may be a block copolymer consisting of several successive MMA moieties and a Eu-complex moiety. That is, the Eucomplex units are uniformly dispersed in the polymer backbone. As a result, every Eu-complex unit should be surrounded by the soft chain consisting of MMA units, like being enclosed in cages. Thus, the interaction between the Eu-complex units is very weak, and therefore the probability of emission quenching is reduced under this conformation. In addition, the coordination stability of the Eucomplex units such as Eu(TTA)2(AA) and Eu(TTA)2(amq) in the copolymers is enhanced because the coordination degree of the unsaturated Eu-complex unit is satisfied through the intra- and intermolecular coordination with the carbonyl oxygen of the MMA
unit, which leads to the decrease in the nonradiative decay rate of the excited levels of Eu(III) ion and the increase in the luminescence lifetime. A combination of the above factors gives rise to the enhancement of the F values of the Eu-containing copolymers.
Fig. 5. Excited-state lifetime of 2 in the solid state at room temperature. Curve fit to a single exponential. The excited-state lifetime of 2 is 627 ms with an overall c2 of 0.9991.
Fig. 6. (a) DSC of 2 and PMMA measured in nitrogen at a heating rate of 10 C min1; (b) TGA (solid) and DTG (dash) of 2 measured in air at a heating rate of 10 C min1.
3.5. Thermal property The thermal property of the copolymer 2 was investigated by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) under air or nitrogen atmosphere. The glass transition temperature (Tg) and the onset temperature of thermal decomposition of 2 are 125 and 335 C (Fig. 6), respectively, which are higher than those of PMMA [25]. This result indicates that the thermal stability and the Tg of the copolymer can be enhanced upon the introduction of the Eu-complex moiety into the polymer chain. Since the Eu-complex is quite bulky, the mobility of the polymer chain is restricted due to the steric hindrance, leading to an increased Tg value [44]. For PMMA, there is no residue left above 430 C in air. For 2, however, a weight-loss behavior was observed between 430 and 565 C, which is caused by the degradation of the Eu-complex moiety. Above 600 C, some residues remain in the
C.-J. Xu et al. / Dyes and Pigments 98 (2013) 493e498
form of white powders, which are identified as Eu2O3. The percentage of the Eu2O3 residue is in fairly good agreement with the Eu content of the copolymer, which also gives a further proof that the Eu-complex moiety has been bonded to the polymer backbone as an integrated unit. 4. Conclusion We synthesized a novel Eu-containing copolymer 2 by the direct copolymerization of Eu-complex monomer Eu(TTA)3(Aphen) with MMA. The copolymer exhibits good solubility, and high thermal stability, with glass transition and decomposition temperatures of 125 and 335 C, respectively. Luminescence studies revealed that the luminescence properties such as lifetime (s) and quantum efficiency (F) of Eu-containing monomers could be enhanced upon copolymerization. Spectroscopic parameters (U2, U4, Arad, and Anr) of the Eu3þ ions were then calculated based on the JuddeOfelt theory. The chemical microenvironments of the Eu-complex units in the polymer and the factors that resulted in the enhancement of luminescent efficiency were discussed. This study provided new guidance in the design and synthesis of highly luminescent Eucontaining copolymers whose excellent photoluminescence properties may enable them to be used as pure red-emitting materials. Acknowledgments We acknowledge the valuable comments and suggestions of the anonymous reviewers and of the editor M. Wainwright. We thank Dr. Zujin Zhao for his assistance. This work was partially supported by the National Natural Science Foundation of China (no. 21172049 and 21074028). C.J.X. acknowledges grants from Special Funds for Key Innovation Team of Zhejing Province (no. 2010R50017), the Opening Foundation of Zhejing Provincial Top Key Discipline (no. 20121118) and the Youth Training Project of Hangzhou Normal University (no. 2011XJPY01). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.dyepig.2013.04.001. References [1] Binnemans K. Lanthanide-based luminescent hybrid materials. Chem Rev 2009;109:4283e374. [2] Smirnov VA, Philippova OE, Sukhadolski GA, Khokhlov AR. Multiplets in polymer gels. Rare earth metal ions luminescence study. Macromolecules 1998;31:1162e7. [3] Yang C, Xu J, Ma J, Zhu D, Zhang Y, Liang L, et al. An efficient long fluorescence lifetime polymer-based sensor based on europium complex as chromophore for the specific detection of Fe, CH3COOe, and H2PO-4. Polym Chem 2012;3: 2640e8. [4] Li M, Selvin PR. Luminescent polyaminocarboxylate chelates of terbium and europium: the effect of chelate structure. J Am Chem Soc 1995;117:8132e8. [5] Piguet C, Bünzli JCG, Bernardinelli G, Hopfgartner G, Williams AF. Self-assembly and photophysical properties of lanthanide dinuclear triple-helical complexes. J Am Chem Soc 1993;115:8197e206. [6] De Bettencourt-Dias A. Lanthanide-based emitting materials in light-emitting diodes. Dalton Trans 2007:2229e41. [7] Zhang T, Xu Z, Qian L, Tao DL, Teng F, Gao X, et al. The condition for electroplex emission from an europium complex doped poly(N-vinylcarbazole). Chem Phys Lett 2005;415:30e3. [8] Kang TS, Harrison BS, Foley TJ, Knefely AS, Boncella JM, Reynolds JR, et al. Near-infrared electroluminescence from lanthanide tetraphenylporphyrin: polystyrene blends. Adv Mater 2003;15:1093e7. [9] McGehee MD, Bergstedt T, Zhang C, Saab AP, O’Regan MB, Bazan GC, et al. Narrow bandwidth luminescence from blends with energy transfer from semiconducting conjugated polymers to europium complexes. Adv Mater 1999;11:1349e54. [10] Eliseevaa SV, Bünzli JCG. Lanthanide luminescence for functional materials and bio-sciences. Chem Soc Rev 2010;39:189e227.
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Monochromatic light-emitting copolymer of methyl methacrylate and Eu-complexed 5-acrylamido-1,10-phenanthroline Cun-Jin Xu a, Jin-Tao Wan b, c, Bo-Geng Li b, * a
College of Material, Chemistry and Chemical Engineering, Hangzhou Normal University, Hangzhou 310036, China The State Key Laboratory of Chemical Engineering, Department of Chemical and Biochemical Engineering, Zhejiang University, Hangzhou 310027, China c Zhejiang Jiamin Plastics Co. Ltd., Jiaxing 314027, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 January 2013 Received in revised form 24 March 2013 Accepted 3 April 2013 Available online 17 April 2013
We present an effective approach to prepare Eu-containing polymer with excellent photoluminescence properties through the copolymerization of highly luminescent Eu-complex monomer featuring 2thenoyltrifluoroacetone (HTTA) and 5-acrylamido-1,10-phenanthroline with methyl methacrylate (MMA). The copolymer was characterized by FT-IR, UVeVis, 1H NMR, GPC, TGA, and DSC. The results reveal that the copolymer is readily soluble, and possesses excellent thermal stability, with high glass transition and decomposition temperatures of 125 and 335 C, respectively. Intense red emission peaked at 612 nm, corresponding to the 5D0 / 7F2 transition of Eu(III) ions is recorded under UV excitation. The luminescence lifetimes and efficiencies of some Eu-containing monomers could be greatly enhanced upon copolymerization. The special microenvironment, in which the Eu-complex units are uniformly bonded to and surrounded by the polymer chain, appears to be responsible for the improvement of the luminescence properties for the copolymer. Moreover, the applicability of the well-known JuddeOfelt theory to the luminescence properties of Eu3þ complexes is also investigated. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Luminescence Copolymer 1,10-Phenanthroline 2-Thenoyltrifluoroacetone JuddeOfelt theory Synthesis
1. Introduction Over the past decades, considerable attention has been devoted to the polymer-based rare earth luminescent materials [1e3], because they have the advantages of polymers, including mechanical strength, flexibility, and processability and the luminescence characteristics of rare earth complexes, such as long luminescence lifetimes, sharp emission bands, large Stockes shift, and high quantum yields [4,5]. Seminal work in this area has primarily focused on polymer systems doped or blended with lanthanide complexes [6e11]. However, blending can also cause some side-effects, such as poor dispersion and compatibility of the rare earth complexes in a polymer matrix, resulting in weak emission of the polymer phosphors [12]. Recently, many researchers have synthesized some polymers containing coordination groups and studied the luminescence properties of the complexes [13e17]. There are, however, problems associated with this approach: first, the polymer complexes prepared by direct reaction of the polymer ligands with rare earth ions usually have an ion aggregation nature, giving rise to weak luminescence of
* Corresponding author. Tel.: þ86 571 87952623; fax: þ86 571 87951612. E-mail address: [email protected] (B.-G. Li). 0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.dyepig.2013.04.001
polymer luminophores [18,19]; second, the microenvironment of rare earth ions is very complicated, which makes the elucidation of the luminescence properties difficult [20]. Polymers with the lanthanide complexes covalently bound in the main chain would be excellent candidates to solve the above-mentioned problems. Furthermore, energy transfer (ET) in such covalently bound system is anticipated to be more efficient due to the close proximity of the two components [21]. Therefore, the design and synthesis of highly luminescent complex monomers have become an important theme in supramolecular and coordination chemistry [22,23]. In previous work, we designed and synthesized a luminescent europium complex monomer Eu(TTA)2(amq) (HTTA ¼ 2thenoyltrifluoroacetone, Hamq ¼ 5-acryloxyethoxymethyl-8hydroxyquinoline), and its copolymers with methyl methacrylate (MMA) [24,25]. The luminescence property of monomer Eu(TTA)2(amq) is better than that of Eu(TTA)2(AA) (HAA ¼ acrylic acid) [26], but worse than that of Eu(TTA)3(phen) (phen ¼ 1,10phenanthroline) [27]. This may be due to the longer conjugation length in phen group and the more antenna chromophores in Eu(TTA)3(phen) complex unit. It therefore occurred to us that it should be possible to design a functional ligand for europium that bearing a polymerizable C¼C double bond for the copolymerization with other vinyl monomers and a phen fragment coordinating to the Ln and effectively acting as photon antenna for the transfer
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energy to Eu3þ ion. To accomplish this goal, we synthesized a functional ligand, 5-acrylamido-1,10-phenanthroline (Aphen), as well as a highly luminescent complex monomer Eu(TTA)3(Aphen) (1) [28]. In this paper, 1 was further copolymerized with MMA to prepare Eu-containing copolymer PMMA-Eu(TTA)3(Aphen) (2) (Scheme 1), and the luminescence property of the copolymer was investigated. 2. Experimental 2.1. Reagents and instruments Azoisobutyronitrile (AIBN) was purified by twice re-crystallizing in methanol. Tetrahydrofuran (THF) and MMA were purified according to standard procedures. The complex monomer Eu(TTA)3(Aphen) was prepared as reported previously [28]. Other chemicals were analytical grade and used as received. Element analysis (CHN) was performed with a Flash-EA1112 elemental analyzer, and lanthanide ions were analyzed by complexometric titration with EDTA. 1H NMR spectra were collected on a Bruker-400 spectrometer. Infrared spectra of KBr pellets were carried out at room temperature by using a Bruker Tensor 27 FT-IR spectrophotometer. UV spectra were obtained by a Unico UV-2102PC spectrophotometer. The excitation and emission spectra were recorded in a Hitachi F-4500 spectrofluorometer. Luminescence lifetimes were measured on an Edinburgh FLS 920 fluorescence spectrograph using a microsecond flashlamp as the excitation source with the pulse width of 2 ms. Thermogravimetric analyses (TGA) were conducted on a Perkin Elmer Pyris 1 TGA thermogravimetric analyzer at a heating rate of 10 C min1 in air. Differential scanning calorimetry (DSC) measurements were run on a Perkin Elmer DSC 7 system under nitrogen atmosphere at a heating rate of 10 C min1. The glass transition temperatures were taken as the midpoint of the change in slope of the baseline. Molecular weights and molecular weight distributions were determined by GPC (Waters 1525) with three Waters Styragel columns (HR 4, 3, 2). Molecular weights were derived from a calibration curve based on polystyrene standards. The eluent was THF with a flow rate of 1.0 mL min1. 2.2. Preparation of PMMA-Eu(TTA)3(Aphen) (2) A mixture of MMA (0.30 g, 3.0 mmol), Eu(TTA)3(Aphen) (0.0532 g, 0.05 mmol) and 4.0 mg of the AIBN initiator was dissolved in 3 mL of dry THF in a glass polymerization tube. The homogeneous solution was purged with argon for 5 min, then sealed and heated in water bath at 60 C for 48 h. The viscous solution was diluted with 5 mL of THF and poured into 70 mL of methanol under vigorous stirring. The resulting precipitate was redissolved in THF and reprecipitated in methanol twice and finally dried under vacuum at 50 C for 24 h to afford powder polymer. PMMA was prepared using the same polymerization conditions for comparison. The composition of 2 was determined by element analysis, x ¼ 0.014, y ¼ 0.986. The content of Eu-complex in copolymer is about 13.12 wt%. The copolymer is fully soluble in common organic
solvents such as chloroform and THF, and can be easily cast into transparent, uniform, thin films with good thermal and moisture stable properties, which is important in optical application. The copolymer 2 exhibits a polydispersity (Mw/Mn) of 1.29 and has the Mn value of 10,000, which is not as high as that in general vinyl polymerization due to the lower polymerization reactivity of Eucomplex monomer [26]. IR (KBr pellet, cm1): 2951, 1732, 1603, 1537, 1485, 1243, 1190, 1148, 749, and 581. 1H NMR (400 MHz, DMSO-d6, d, ppm): 9.10 (d, J ¼ 41.7 Hz, 2H), 8.44 (m, 3H), 7.81 (d, J ¼ 55.0 Hz, 2H), 7.55 (d, J ¼ 10.7 Hz, 4H), 6.40 (d, J ¼ 69.6 Hz, 9H), 3.54 (s, 210H), 2.20e0.40 (m, 353H). 3. Results and discussion 3.1. IR spectra Apart from some weak characteristic absorption peaks from the complex unit including the peaks at 1603, 1537 and 1306 cme1 due to the C¼O, C¼C and CeF stretching vibrations of TTA, respectively, the main peaks of 2 are in accordance with those of PMMA as the content of the complex moiety in the copolymer is low. The C¼C stretching vibrations of the MMA (1639 cme1) and 1 (1629 cme1) are absent in 2, indicating that the monomers have been successfully copolymerized (Fig. 1). This is also verified by the 1H NMR spectrum of 2 (Fig. 2) in which the proton peaks assigned to terminal vinyl group of 1 and methylene group of MMA have completely disappeared and all the peaks become rather broad. 3.2. UV spectra Fig. 3 displays the absorption spectra of 1, 2, and PMMA in THF solution. Two absorption peaks at 238 and 275 nm are observed for pure PMMA, which are ascribed to the pep* and nep* transitions of the C¼O group, respectively. It is noticed that the absorption characteristics of 1 and pure PMMA are observed simultaneously from the spectrum of 2, revealing that the complex moieties have been bonded to the polymer backbone and that the dissociation of ligands is negligible during the copolymerization and purification processes. 3.3. Luminescence properties Fig. 4 shows the photoluminescence spectra of complex 1 and copolymer 2 in solid state at room temperature. The excitation spectra of 1 and 2 were obtained by monitoring the emission wavelength of the Eu(III) ions at 612 nm. In complex 1, there was a broad excitation band ranging from 250 to 420 nm, which was assigned to the pep* electron transition of the ligands. Intriguingly, such an excitation for the copolymer 2 was relatively narrower and split into two bands centered at 288 and 348 nm, respectively. This suggested that the site symmetry of Eu3þ ions became lower in the polymer due to the influences of the neighboring chain segments of PMMA [29,30].
Scheme 1. Synthetic route for the copolymer 2.
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Fig. 1. The FT-IR spectra of 1, 2, and PMMA. Fig. 3. Absorption spectra of 1, 2, and PMMA in THF at room temperature.
The emission spectra of 1 and 2 excited at 385 and 348 nm, respectively, do not show significant differences and all display characteristic sharp peaks associated with the 5D0 / 7FJ transitions of the Eu(III) ion between 575 and 720 nm, suggesting that the light-emitting sites in copolymer 2 are the same as those in complex 1. Therefore, the luminescence of 2 came from the corresponding Eu-complex, and the polymer matrix just functionalized as the skeleton causing no damage to the Eu-complex. As shown in Fig. 4, five expected emission peaks of different intensities at 579.0, 591.6, 611.8, 650.0 and 700.0 nm are well resolved, which belong to the transitions of 5D0 / 7F0, 5D0 / 7F1, 5D0 / 7F2, 5D0 / 7F3 and 5 D0 / 7F4 of Eu(III) ion, respectively. The hypersensitive 5D0 / 7F2 transition is very intense, suggesting a highly polarizable chemical environment around the Eu(III) ion, and is responsible for the brilliant red emission color of the two samples. Moreover, the emission spectrum shows only one peak for the 5D0 / 7F0 transition, indicating the presence of a single chemical environment around the Eu(III) ion and also showing that the Eu(III) ion is at a site with Cn, CnV or Cs symmetry [31]. The luminescence properties such as the emission intensity (I), lifetime (s), and quantum efficiency (V) of some selected Eu(III) complexes are given in Table 1. As shown in Table 1, the copolymer 2 exhibits excellent luminescence properties because of its strongly luminescent complex monomer 1. It is also observed that the copolymer PMMA-Eu(TTA)2(AA) gives relatively shorter s, V, and weaker I, which is attributed to its weakly luminescent complex monomer Eu(TTA)2(AA) whose active ligand AA makes little or no contribution to the luminescence of the complex. The luminescence decay curves of the Eu(III) 5D0 / 7F2 transition for 2 was
Fig. 2. 1H NMR spectra of the monomer 1 (top) and the copolymer 2 (bottom), for both spectra: 400 MHz, DMSO-d6, 298 K.
found to be a single exponential (Fig. 5), and is consistent with the presence of one major luminescent species. The lifetime of 2 is slightly higher than that reported recently for luminescent europium b-diketonate complexes covalently linked to other types of polymers or to a silica matrix [32e34]. 3.4. JuddeOfelt analysis The JuddeOfelt theory is a useful tool for analyzing fef inner shell electronic transitions [35]. Interaction parameters of ligand fields are given by the JuddeOfelt parameters Ul (l ¼ 2, 4, and 6), in which U2 is more sensitive to the symmetry and sequence of ligand fields [36]. The experimental U2 and U4 intensity parameters were determined from the 5D0 / 7F2 and 5D0 / 7F4 electronic transitions, respectively, using the magnetic dipole transition of 5D0 / 7F1 as the reference [37,38]. The detailed principles and specific calculations were provided in the Supporting Information, and the obtained U2 and U4 intensity parameters for some selected Eu(III) complexes were listed in Table 1. It was evident that the Eu-containing copolymers had higher U2 values than their corresponding complex monomers, suggesting an increase of the covalence degree in the first coordination shell of Eu3þ ions and an enhancement of the 5 D0 / 7F2 hypersensitive transition [39]. This may be also due to the change of the chemical environment surrounding Eu3þ ions, which was induced by the intra- and intermolecular interactions between Eu-complex unit and neighboring soft chain consisting of MMA
Fig. 4. Excitation (EX, lem ¼ 612 nm) and emission (EM) spectra for 1 (red line, lex ¼ 385 nm), 2 (blue line, lex ¼ 348 nm) in solid state at room temperature.
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Table 1 The relative intensity of the 5D0 / 7F2 transition (I), experimental intensity parameters (U2,4), radiative (Arad) and nonradiative (Anr) decay rates, lifetime (s), and emission quantum efficiency (F) for some selected Eu(III) complexes in the solid state at 293 K. Complex
I (a.u.)
U2 (1020 cm2)
U4 (1020 cm2)
Arad (s1)
Anr (s1)
s (ms)
F (%)
1 [28] Eu(TTA)2(amq) [24] Eu(TTA)2(AA)a PMMA-Eu(TTA)2(AA)b PMMA-Eu(TTA)2(amq)b 2b
687 [28] 289 [24] 100 [28] 311 597 883
21.07 22.08 13.86 22.05 23.53 21.51
2.24 2.34 1.92 2.49 2.48 2.51
722.99 754.96 658.37 711.84 680.33 740.06
971.92 1815.73 10330.64 1167.86 959.01 854.84
590 [28] 389 [24] 91 [28] 532 610 627
43 29 6 38 42 46
a b
Prepared by Wang’s method [26]. Prepared by the same procedures as in the case of 2.
units. The higher values of U4 for the Eu-containing polymers as compared with that of their corresponding complex monomers indicated a perturbation on the coordination effect of the bidentate TTA by the steric factors from the surrounding PMMA [40]. The luminescent quantum efficiency (F) for the 5D0 / 7F0e4 transitions of Eu3þ ions basically determines the luminescent properties of the Eu(III) complex, which is defined as Arad/ (Arad þ Anr) [41], where Arad is the total radiative transition rate of the 5D0 / 7F0e4 transitions and Anr is the nonradiative transition rate. The Arad, Anr, and F values of some selected Eu(III) complexes were calculated based upon the JuddeOfelt theory, and listed in Table 1. The detailed calculations were provided in the Supporting Information. It is somewhat surprising that the copolymers, especially PMMA-Eu(TTA)2(AA) and PMMA-Eu(TTA)2(amq), show higher F values when compared with their corresponding Eu-containing monomers because the main flexible framework groups (PMMA) have an adverse effect on the F value, as has been reported in the literature [42]. Now we should consider the case where the Eucontaining monomer has been grafted on the PMMA chains. Since the Eu-containing monomer has a lower reactivity ratio [43], the resulting copolymer may be a block copolymer consisting of several successive MMA moieties and a Eu-complex moiety. That is, the Eucomplex units are uniformly dispersed in the polymer backbone. As a result, every Eu-complex unit should be surrounded by the soft chain consisting of MMA units, like being enclosed in cages. Thus, the interaction between the Eu-complex units is very weak, and therefore the probability of emission quenching is reduced under this conformation. In addition, the coordination stability of the Eucomplex units such as Eu(TTA)2(AA) and Eu(TTA)2(amq) in the copolymers is enhanced because the coordination degree of the unsaturated Eu-complex unit is satisfied through the intra- and intermolecular coordination with the carbonyl oxygen of the MMA
unit, which leads to the decrease in the nonradiative decay rate of the excited levels of Eu(III) ion and the increase in the luminescence lifetime. A combination of the above factors gives rise to the enhancement of the F values of the Eu-containing copolymers.
Fig. 5. Excited-state lifetime of 2 in the solid state at room temperature. Curve fit to a single exponential. The excited-state lifetime of 2 is 627 ms with an overall c2 of 0.9991.
Fig. 6. (a) DSC of 2 and PMMA measured in nitrogen at a heating rate of 10 C min1; (b) TGA (solid) and DTG (dash) of 2 measured in air at a heating rate of 10 C min1.
3.5. Thermal property The thermal property of the copolymer 2 was investigated by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) under air or nitrogen atmosphere. The glass transition temperature (Tg) and the onset temperature of thermal decomposition of 2 are 125 and 335 C (Fig. 6), respectively, which are higher than those of PMMA [25]. This result indicates that the thermal stability and the Tg of the copolymer can be enhanced upon the introduction of the Eu-complex moiety into the polymer chain. Since the Eu-complex is quite bulky, the mobility of the polymer chain is restricted due to the steric hindrance, leading to an increased Tg value [44]. For PMMA, there is no residue left above 430 C in air. For 2, however, a weight-loss behavior was observed between 430 and 565 C, which is caused by the degradation of the Eu-complex moiety. Above 600 C, some residues remain in the
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form of white powders, which are identified as Eu2O3. The percentage of the Eu2O3 residue is in fairly good agreement with the Eu content of the copolymer, which also gives a further proof that the Eu-complex moiety has been bonded to the polymer backbone as an integrated unit. 4. Conclusion We synthesized a novel Eu-containing copolymer 2 by the direct copolymerization of Eu-complex monomer Eu(TTA)3(Aphen) with MMA. The copolymer exhibits good solubility, and high thermal stability, with glass transition and decomposition temperatures of 125 and 335 C, respectively. Luminescence studies revealed that the luminescence properties such as lifetime (s) and quantum efficiency (F) of Eu-containing monomers could be enhanced upon copolymerization. Spectroscopic parameters (U2, U4, Arad, and Anr) of the Eu3þ ions were then calculated based on the JuddeOfelt theory. The chemical microenvironments of the Eu-complex units in the polymer and the factors that resulted in the enhancement of luminescent efficiency were discussed. This study provided new guidance in the design and synthesis of highly luminescent Eucontaining copolymers whose excellent photoluminescence properties may enable them to be used as pure red-emitting materials. Acknowledgments We acknowledge the valuable comments and suggestions of the anonymous reviewers and of the editor M. Wainwright. We thank Dr. Zujin Zhao for his assistance. This work was partially supported by the National Natural Science Foundation of China (no. 21172049 and 21074028). C.J.X. acknowledges grants from Special Funds for Key Innovation Team of Zhejing Province (no. 2010R50017), the Opening Foundation of Zhejing Provincial Top Key Discipline (no. 20121118) and the Youth Training Project of Hangzhou Normal University (no. 2011XJPY01). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.dyepig.2013.04.001. References [1] Binnemans K. Lanthanide-based luminescent hybrid materials. Chem Rev 2009;109:4283e374. [2] Smirnov VA, Philippova OE, Sukhadolski GA, Khokhlov AR. Multiplets in polymer gels. Rare earth metal ions luminescence study. Macromolecules 1998;31:1162e7. [3] Yang C, Xu J, Ma J, Zhu D, Zhang Y, Liang L, et al. An efficient long fluorescence lifetime polymer-based sensor based on europium complex as chromophore for the specific detection of Fe, CH3COOe, and H2PO-4. Polym Chem 2012;3: 2640e8. [4] Li M, Selvin PR. Luminescent polyaminocarboxylate chelates of terbium and europium: the effect of chelate structure. J Am Chem Soc 1995;117:8132e8. [5] Piguet C, Bünzli JCG, Bernardinelli G, Hopfgartner G, Williams AF. Self-assembly and photophysical properties of lanthanide dinuclear triple-helical complexes. J Am Chem Soc 1993;115:8197e206. [6] De Bettencourt-Dias A. Lanthanide-based emitting materials in light-emitting diodes. Dalton Trans 2007:2229e41. [7] Zhang T, Xu Z, Qian L, Tao DL, Teng F, Gao X, et al. The condition for electroplex emission from an europium complex doped poly(N-vinylcarbazole). Chem Phys Lett 2005;415:30e3. [8] Kang TS, Harrison BS, Foley TJ, Knefely AS, Boncella JM, Reynolds JR, et al. Near-infrared electroluminescence from lanthanide tetraphenylporphyrin: polystyrene blends. Adv Mater 2003;15:1093e7. [9] McGehee MD, Bergstedt T, Zhang C, Saab AP, O’Regan MB, Bazan GC, et al. Narrow bandwidth luminescence from blends with energy transfer from semiconducting conjugated polymers to europium complexes. Adv Mater 1999;11:1349e54. [10] Eliseevaa SV, Bünzli JCG. Lanthanide luminescence for functional materials and bio-sciences. Chem Soc Rev 2010;39:189e227.
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