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Preparation and Photoluminescence Property of PolymerRare Earth Complexes Constituted by Bidentate Schiff Base Ligand-Functionalized Polysulfone and Eu(#) Ion Baojiao Gao, Dandan Zhang, and Tintin Don J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 23 Jun 2015 Downloaded from http://pubs.acs.org on June 24, 2015

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The Journal of Physical Chemistry

Preparation and Photoluminescence Property of Polymer-Rare Earth Complexes Constituted by Bidentate Schiff Base Ligand-Functionalized Polysulfone and Eu(Ⅲ Ⅲ) Ion

Baojiao Gao﹡, Dandan Zhang, Tintin Don Department of Chemical engineering, North University of China, Taiyuan 030051, People' s Republic of China

﹡To whom correspondence should be addressed Tel: 86-351-3924795 Fax: 86-351-3922118 E-mail: [email protected]

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Abstract A bidentate Schiff base (SB) ligand was synthesized and bonded on the side chains of polysulfone (PSF) via several polymer reactions by molecular design, and the bidentate Schiff base ligand-functionalized PSF, PSF-SB, was gained. On this basis, a new type of polymer-rare earth complexes, binary complex PSF-(SB)3-Eu(Ⅲ) and ternary complex PSF-(SB)3-Eu(Ⅲ)-(Phen)1, was prepared via the coordination reactions between PSF-SB as macromolecular ligand and 1,10-phenanthroline (Phen) as the second small-molecule ligand and Eu(Ⅲ) ion, respectively. The functional polymer PSF-SB was fully characterized by FTIR, 1H-NMR and chemical analysis method, and the complexes were also fully characterized by FTIR and UV absorption spectroscopy. The fluorescence emission characteristics and luminescent mechanism of the complexes were investigated in depth. The experimental results show that the macromolecular ligand PSF-SB itself emits strong fluorescence centered at 415 nm. However, after coordinating to Eu(Ⅲ) ion, the fluorescence emission of PSF-SB itself weakens remarkably, and it should be attributed to the intramolecular energy transfer. Both the binary and ternary complexes exhibit the strong characteristic fluorescence emission of Eu(Ⅲ) ion. This fact demonstrates that the bidentate Schiff base ligand SB bonded on the side chains of PSF can effectively produce the intramolecular energy transfer, namely it can strongly sensitize the fluorescence emission of Eu(Ⅲ) ion, indicating that the triplet state energy of the bonded ligand SB is well matched with the resonant level energy of Eu(III) ion. The fluorescence emission intensity of the ternary complex PSF-(SB)3-Eu(Ⅲ)-(Phen)1 is stronger than that of the binary complex PSF-(SB)3-Eu(Ⅲ), and it associates with the synergism coordination of the second ligand Phen with the bond ligand SB as well as its effect of substituting the coordinated water molecules around Eu(Ⅲ) ion.

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1. Introduction Luminescent rare earth complexes with small molecular organic ligands have attracted considerable attentions due to their very narrow emission bands, long excited-state lifetimes and large Stokes shifts, and they have wide applications in electroluminescent devices, laser technology, photoluminescence devices, display application, fluoroimmunoassays, sensor sand so on.

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However, in general, these luminescent

small molecular rare earth complexes are used by doping them into the matrixes. Not only there is the drawback of uneven dispersion owing to the poor compatibility of the small molecular complexes with the matrixes, but also they have other serious defects such as instability, insolubility in organic solvent and hard machinability. These drawbacks limit their applications greatly. By contrast, luminescent rare earth-containing polymers, namely luminescent polymer-rare earth complexes, in whose structure, the ligands bonded to the macromolecular backbone coordinate directly to the rare earth ions, have been attracted significant attention duo to their excellent mechanical flexibility, homogeneity and good processibility, 6-8 especially fine film-forming property. These advantages will accelerate their applications greatly in optical and electronic fields. 9,10 Therefore, luminescent polymer-rare earth complexes are a class of advanced materials, and should be vigorously developed although such materials that have been reported are still rare. 11,12 In the reported luminescent polymer-rare earth complexes, the major ligands bonded to the macromolecular backbone are fatty carboxyl groups. 11,13-15 Under the circumstances, only after the synergy ligands such as 1,10-phenanthroline (Phen) and 2,2′-dipyridyl are added, can the luminescent complex materials be formed, leading weaker luminescent intensity. The reason for this is that the bonded fatty carboxyl groups only play a role of coordination chelating, and they have no sensitization towards the central rare earth ions. Based on this drawback, our group proposed a new concept or a new route for 2



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preparing luminescent polymer-rare earth complexes with high performance: only by bonding the ligands with the dual functions of coordination chelating and sensitization to rare earth ions onto macromolecular backbones via molecular design, can the luminescent polymer-rare earth complexes with high performance including good stability, excellent comprehensive performance and especially very strong luminescent property be obtained after their coordination with earth ions. In our previous studies, aromatic carboxylic acids (benzoic acid, its derivatives and naphthoic acid) as ligands were chemically introduced on the side chains of polystyrene (PS) and polysulfone (PSF) that is one of polymer materials with higher comprehensive properties, and various luminescent polymer-rare earth complexes were prepared. 16-19 The bonded aromatic carboxylic acid ligands possess the dual functions described above. They not only can coordinate to rare earth ions to form stable chelates, but also can strongly sensitize the fluorescence emission of rare earth ions, greatly enhancing the luminescence property of polymer-rare earth complexes. Schiff bases are considered as a very important class of organic compounds, and they are able to form stable complexes with many different transition metal and rare-earth metal ions via N and O atoms. Bidentate Schiff base compounds as well as polydentate Schiff base compounds, between which and rare earth ions chelating coordination can occur, can be obtained through organic synthesis, and furthermore, in their molecules larger conjugated aromatic rings often exist, leading to strong UV absorption and possible energy transfer. Therefore, like as aromatic carboxylic acids, bidentate Schiff base compounds also have dual functions of coordination chelating and sensitization toward rare earth ions. In fact, some smallmolecule rare earth complex with Schiff base compounds as ligands have been researched. 20,21 If bidentate Schiff base compounds as ligands are chemically attached to the macromolecular backbones, a new type of luminescent polymer-rare earth materials will be obtained. Exactly based on such consideration, in our present work, we synthesized and bonded a bidentate Schiff base ligand (denoted as SB), in whose 3


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structure a larger conjugated aromatic ring is contained, onto the side chain of PSF, and a bidentate Schiff base ligand-functionalized polysulfone, PSF-SB, was successfully prepared through 3-step macromolecular reactions. Finally, the binary and ternary polymer-rare earth (Eu(Ⅲ) and Tb(Ⅲ)) complexes were obtained, and their photoluminescence properties and luminescent mechanism were researched. The study result showed that the bidentate Schiff base ligand bonded to the side chains of PSF, SB, can strongly sensitize the fluorescence emission of Eu(Ⅲ) ion because of the better energy level matching. To our knowledge, the luminescent bidentate Schiff base-type polymer-rare earth complexes have not been reported in the literature. The study result in this work is significant in developing photoluminescence polymer-rare earth complex materials. 2. Experimental 2.1. Materials and instrument Polysulfone (PSF, M r = 67000, Shuguang Chemical Plant of Shanghai Plastic Industry Associated Company, Shanghai, China) was of industrial grade. Chloromethylated polysulfone (CMPSF) was self-synthesized with 1,4-bis (chloromethoxy) butane (BCMB) as chloromethylation reagent, which was without carcinogenic toxicity. p-Hydroxy benzaldehyde (HBA) was purchased from Tianjin Kemio Chemical Reagent Development Center (China). 3-Aminopyridine (AP) was supplied by Hefei Kaihua Chemical Engineering Co., Ltd. (China). 1, 10-Phenanthroline (Phen) was obtained from Tianjin Damao Chemical Reagent Factory. N,N-dimethyl formamide (DMF) and N,N-dimethylacetamide (DMAC) was purchased from Tianjin Bodi Chemical Engineering Co., Ltd. (China). Europium oxide (Eu2O3) and tetraterbium heptaoxide (Tb4O7) were obtained from Beijing Sinopharm Chemical Reagent Beijing Co., Ltd. (China). Other chemicals were all commercial reagents with analytical pure grade and were purchased from Chinese companies. 4



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The instruments used in this study were as follows: Perkin-Elmer 1700 infrared spectrometer (FTIR, Perkin-Elmer Company, USA), Unic UV/Vis-2602 spectrophotometer (Unic Company, USA), DRX300 nuclear magnetic resonance spectrometer (Bruker Company, Switzerland), HITACHI F-2500 fluophotometer (HITACHI Company, Japan) and ZCT-A thermogravimetric analyzer (Beijing Precision Instrument Gaoke Instrument Co., Ltd., China) 2.2. Preparation and characterization of bidentate Schiff base ligand-functionalized PSF 2.2.1 Preparation of CMPSF According to the procedure described in Ref.,

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PSF was chloromethylated first with BCMB as

reagent, and the main process is described as follows. PSF was dissolved in dichloromethane in a four-necked flask equipped with a mechanical agitator and a dropping funnel. After adding Lewis acid catalyst, SnCl4, the chloromethylation reagent BCMB was added by dripping slowly from a dropping funnel, and the chloromethylation reaction was allowed to be carried out at room temperature for 8 h. After finishing the reaction, the solution in the bottle was treated with diluted hydrochloric acid, and then the product polymer was precipitated with ethanol as precipitator. By fully washing and drying, the chloromethylated PSF, CMPSF, was obtained, and its chlorine content was 1.75mmol/g that was determined with oxygen bomb-burning/Volhard method. CMPSF was characterized by FTIR and 1H NMR spectroscopy. 16 2.2.2 Bonding benzaldehyde on side chain of PSF DMAC (20 mL) and CMPSF (0.5 g) were added into a four-necked flask equipped with a mechanical agitator, a reflux condenser and a thermometer, and CMPSF was allowed to be fully dissolved into the solvent. Subsequently, 30 mL of DMAC solution, in which 0.11 g of HBA was dissolved, was added, and 0.1 Na2CO3 as acid-binding agent was also added. The nucleophilic substitution reaction 5


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between the chloromethyl group of CMPSF and the hydroxyl group of HBA was allowed to be conducted at 90 ℃ for 12 h with stirring. After finishing the reaction, the polymer in the solution was precipitated with 150 mL of ethanol as precipitator. At that time, there was a supernatant of 200 mL, in which 50 mL of DMAC and 150 mL of ethanol were contained. The sample of the supernatant with a certain volume was taken in order to determine the conversion of the chlomethyl group of CMPSF and to evaluate the reaction degree of the substitution reaction. The product polymer was separated out by filtering, and was washed alternately with ethanol and distilled water. By drying under vacuum, the resultant polymer was the modified polysulfone, PSF-BA, on whose side chain benzaldehyde as a group was bonded. The concentration of unreacted HBA in the supernatant sample was determined by UV spectrophotometry at 284 nm, so that the amount of HBA that had taken part in the reaction in the total reaction system was calculated. Based on this calculation result and the used amount of CMPSF, the bonding amount of BA on PSF for PSF-BA was calculated, and further, the conversion of the chloromethyl group of CMPSF was gotten. The determination result indicated that CMPSF as the precursor of the modified polysulfone PSF-BA, had a chloromethyl group conversion of 94% under the given reaction conditions as described above. 2.2.3. Synthesizing and bonding bidentate Schiff base ligand on side chain of PSF DMF (50 mL) and PSF-BA (0.5 g) were added into a four-necked flask, and PSF-BA was allowed to be fully dissolved into the solvent, followed by adding 0.08 g of AP. The Schiff base reaction between the aldehyde group of PSF-BA and the primary amino group of AP was allowed to be conducted at 60 ℃ for 4 h with stirring. After finishing the reaction, the resultant polymer was precipitated with 150 mL of distilled water as precipitator. At that time, the supernatant of 200 mL was composed of 50 mL of DMF and 150 mL of distilled water. The sample of the supernatant was taken and remained to be analysed. The 6



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product polymer was separated out by filtering, and was washed alternately with ethanol and distilled water. By drying under vacuum to a constant weight, the functional polymer PSF-SB, on whose side chain bidentate Schiff base ligand SB had been synchronously synthesized and bonded, was gained. 2.2.4. Characterization of PSF-SB The functional polymer PSF-SB was fully characterized by different means. (1) The FTIR spectrum of PSF-SB was determined with KBr pellet method. (2) 1H-NMR spectrum of PSF-SB was records with deuterated chloroform as solvent. The main data are as follows. 1H-NMR: 4.565 (2H, -CH2-), 8.019 (1H, -CH=N-) and 9.827-9.808 (2H, Py-H). (3) The bonding amount (mmol/g) of SB of PSF-SB was determined indirectly by UV spectrophotometry, and the procedure was follows. The supernatant sample (1 mL) was diluted to fixed volume with a mixed solvent of DMF and distilled water with a volume ratio of 1:3, and the content of AP was determined by UV spectrophotometry at 294 nm, so that the amount of AP unreacted in the total supernatant was gained. Further, the amount of AP that had taken part in the reaction was calculated, and it was namely the amount of bidentate Schiff base ligand BA synthesized and bonded on the side chains of the functional macromolecule PSF-SB. The determination result indicated that the bonding amount of the ligand BA on the side chains of PSF-SB was 1.54mmol/g. 2.3. Preparation and characterization of polymer-rare earth complexes 2.3.1. Preparation of binary polymer-rare earth complexes Eu2O3 was dissolved in HCl solution, and the solution was concentrated by heating, obtaining the crystal EuCl3·6H2O. The functional polymer PSF-SB (0.392 g), in which 0.604 mmol of SB ligand was contained, was dissolved in 50 mL of DMF, followed by adding 0.074 g of crystal EuCl3·6H2O (0.201 mmol). The coordination reaction between PSF-SB and Eu3+ ion was conducted for 8 h at 50 ℃ with stirring. It was various that in the coordination reaction system, the molar ratio of the ligand BA of PSF-SB 7


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to Eu3+ ion was equal to 3:1. After finishing the reaction, the polymer was precipitated out with ethanol as precipitator, washed with ethanol and distilled water, and dried under vacuum. The resultant polymer was namely the binary polymer-rare earth complex PSF-(SB)3-Eu(Ⅲ). Tb4O7 was dissolved in 50mL of HCl solution, and then hydrogen peroxide solution was added. The content was heated and concentrated, and the crystals of TbCl3·6H2O was gotten.

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With the same

procedure as that for the preparation of PSF-(SB)3-Eu(Ⅲ), the binary polymer-rare earth complex PSF-(SB)3-Tb(Ⅲ) was also obtained. 2.3.2. Preparation of ternary polymer-rare earth complex Phen (0.0394 g, 0.201 mmol) was dissolved in 25 mL of DMF, and then 0.074 g of crystal EuCl3·6H2O (0.201 mmol) was added. The coordination reaction between Phen and Eu (Ⅲ) ion was conducted for 4 h at 50 ℃ with magnetic stirring. The functional polymer PSF-SB (0.392 g), in which 0.604 mmol of SB ligand was contained, was dissolved in 25 mL of DMF, and this solution was mixed with the previous one. The coordination reaction was again carried out at 50 ℃ for 8 h. It is obvious that in the final coordination reaction system, the molar ratio of reactants (BA of PSF-SB: Phen: Eu (Ⅲ) ion) was 3:1:1. After the reaction finished, the product polymer was precipitated out with ethanol as precipitator, washed with ethanol and distilled water, and dried under vacuum. The resultant product was the ternary polymer-rare earth complex, PSF-(SB)3-Eu(Ⅲ)-(Phen)1. 2.3.3. Characterization of complexes The FTIR spectra of the polymer-rare earth complexes were determined with KBr pellet method to confirm their chemical structures. The solutions of the polymer-rare earth complexes were prepared, and their UV absorption spectra in the solution were determined. The thermo-gravimetric curves of the polymer-rare earth complexes were also recorded (air atmosphere, a heating rate of 10 ℃/ min-1) to confirm 8



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the existing of the coordinated water around the central ion in these complex structures, especially in the binary polymer-rare earth complex structures. 2.4. Determination of florescence emission spectra The DMF solutions of binary and ternary complexes as well as that of EuCl3 and TbCl3 were prepared, respectively, and their excitation spectra were firstly monitored with the emissions of the center ions, at 620 nm for Eu3+ ion and at 545 nm for Tb3+ ion, so that their optimal excitation peaks were obtained. And then their fluorescence emission spectra were determined with the corresponding optimal excitation peak (for Eu (Ⅲ) species, at about 310 nm; for Tb (Ⅲ) species, at 275 nm). Besides, in this work, the films of the polymer-rare earth complexes were prepared with casting method, and the fluorescence emission spectra of the complex films were determined. The complexes were first dissolved in chloroform, respectively, and then these solutions were poured into several glass plates, followed by dried in oven at 40 ℃ to remove the solvent, obtaining solid complex films with about a thickness of 60µm. Finally, the fluorescence emission spectra of the complexes films were also determined. 3. Results and discussion 3.1. Chemical process to prepare bidentate Schiff base ligand-functionalized PSF and chemical structures of binary and ternary polymer-rare earth complexes Bidentate Schiff base ligand-functionalized PSF, PSF-SB, was prepared through 3-step polymer reactions. CMPSF was first prepared via chloromethylation reaction of PSF with BCMB as reagent. On this basis, the nucleophilic substitution reaction between the chloromethyl group of CMPSF and the hydroxyl group of HBA was conducted, resulting in the modified PSF, PSF-BA, in whose structure benzaldehyde (BA) as a group was bonded onto the side chains of PSF. Finally, by using AP as reaction reagent, the Shiff base reaction between the aldehyde group of PSF-BA and the primary amino group of AP was allowed to 9


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occur, and the bidentate Schiff base ligand SB is formed on the side chains of PSF, realizing the synchronous synthesis and bonding of ligand SB on the side chain of PSF. In the structure of the ligand SB, there are two coordination atoms, one N atom of azomethine bond C=N of Schiff base and one N atom of pyridine ring, and so it is called as bidentate Schiff base ligand and resultant polymer PSF-SB is called bidentate schiff base ligand-functionalized PSF. The chemical process to prepare PSF-SB can schematically expressed in Scheme 1. Through the subsequent coordination reactions of the functional polymer PSF-SB with Eu3+ ion in DMF solutions, the binary polymer-rare earth complex PSF-(SB)3-Eu( Ⅲ ) was obtained. In the coordination reaction system, by using Phen as small-molecule ligand in proportion, the ternary complex PSF-(SB)3-Eu(Ⅲ)-(Phen)1 was also gained. The structures of the two complexes are presented in Scheme 2. It is obvious that around the central rare earth ion, several stable five-membered chelating rings are formed. In regard to the chemical structures of these polymer-rare earth complexes, the following concepts need to be pointed out. The obtained polymer-rare earth complexes in this study are prepared in the diluted solution, and then are precipitated from the solution with precipitator ethanol. Therefore, these complexes are soluble in organic solvents, and they belong to a kind of intramolecular or intrachain complexes,

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namely, the complexes are formed by the coordination of the ligands SB on the same

macromolecular chain with Eu (Ⅲ) ion. Otherwise, the closslinking process between the macromolecules will be occur with Eu (Ⅲ) ions as closslinking bridges, and the crosslinked production will not dissolve in the solution.

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Therefore, the polymer-rare earth complexes prepared in this study should have the

explicit structures as shown in Scheme 2.

Scheme 1 10



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Scheme 2

3.2. Characterization of functional polymer PSF-SB and complexes 3.2.1 Infrared spectra The infrared spectra of CMPSF, PSF-BA and PSF-SB are presented in Fig. 1, and that of the binary complex PSF-(SB)3-Eu(III) and ternary complex PSF-(SB)3-Eu(III)-(Phen)1 are presented in Fig. 2.

Fig. 1

Fig. 2

In the spectrum of CMPSF, except for displaying all of characteristic absorption bands of PSF, two absorption bands of chloromethyl group appear at 1440 and 670 cm-1. The former is attributed to the in-plane bending vibration absorption of C-H bond in the chloromethyl group -CH2Cl, and the latter is assigned to stretching vibration of C-Cl bond in the group -CH2Cl. Besides, the characteristic absorption of benzene ring after triple substitution at 1, 2 and 4 positions appears at 880 cm-1. The above spectrum data indicates that PSF has been chloromethylated and CMPSF has been formed. In the spectrum of PSF-BA as modified PSF, the absorption bands of the chloromethyl group -CH2Cl at 1440 and 670 cm-1 have weakened remarkably or disappeared basically. At the same time, three new bands appear at 1690, 2730 and 1212 cm-1. The first is attributed to the characteristic absorption of the carbonyl group of aldehyde group, the second is ascribed to the stretching vibration absorption of C-H bond 11


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of aldehyde group, and the last is corresponding to the stretching vibration absorption of aromatic ether bond. These spectrum data indicate that the nucleophilic substitution reaction between CMPSF and HBA has been produced, and the modified PSF, PSF-BA, on whose side chains benzaldehyde as a group is bonded, has been formed. In the spectrum of the functional PSF, PSF-SB, the characteristic absorption band at 1690 and 2730 cm-1 have disappeared, and the characteristic absorption of azomethine bond C=N bond of Shiff base has been appeared at 1662 cm-1. The absorption band of azomethine bond C=N of pyridine ring also appears near this band. The above spectrum data confirm that the Shiff base reaction between PSF-SB and AP has been take placed, and bidentate Schiff base ligand-functionalized PSF, PSF-SB, has been gained. In the spectrum of the binary complex PSF-(SB)3-Eu(III), the absorption band of C=N bond of Schiff base group and pyridine ring has red-shifted from to 1662 to 1658 cm-1, indicating that both the N atom of azomethine bond C=N of Schiff base and N atom of pyridine ring have coordinated to Eu(III) ion, namely the coordination chelating action of the bidentate Schiff base ligand SB for Eu(III) ion has been produced, forming the binary complex PSF-(SB)3-Eu(III). In the spectrum of the binary complex PSF-(SB)3-Eu(III)-(Phen)1, the red shift degree of the absorption band of azomethine bond C=N of the Shiff base and pyridine ring as well as the absorption band of C=N bond of Phen (it also overlays with the absorption band of azomethine bond C=N of the Shiff base and pyridine ring ) becomes greater (from1662 cm-1 to 1656 cm-1). The reason for this is that the adding of small-molecule ligand Phen increases the ligand number around Eu(III) ion and causes the decrease of the vibration energy. This fact demonstrates that all of the three kinds of N atoms, the N atoms of Shiff base, pyridine ring and Phen, have coordinate to Eu(III) ion, forming the ternary complex PSF-(SB)3-Eu(III) –(Phen)1. 12



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3.2.2. 1H NMR spectra Fig. 3 and Fig. 4 give the 1H NMR spectra of CMPSF and PSF-SB, respectively.

Fig. 3

Fig. 4

In the spectrum of CMPSF, a group of peaks at a range of 6.831-7.883（b-h）ppm are corresponding to the various protons on the benzene ring of PSF, and these various protons are denoted in the chemical structure formula of CMPSF. It is more important that the resonance absorption peak of the protons of the methyl group in the bisphenol A unit appears at 1.734 (a) ppm, and the resonance absorption peak of the protons in chloromethyl group -CH2Cl appears at 4.541(i) ppm. These data further confirm the formation of CMPSF. In the spectrum of PSF-SB, the resonance absorption peaks (b-h) of the protons on the benzene ring of PSF and the resonance absorption peaks (k and l) of the protons on the benzene ring of the bonded BA group as well as that (m and n) of the protons on pyridine ring all overlap each other, and form a group of peaks (b-h and k-n). The peak at 4.565 (i) ppm is attributed the resonance absorption of the protons in the methoxy group -CH2-O- that acts as a bridge links PSF skeleton with SB ligand. It is more important that there appear some new absorption peaks. The resonance absorption peak of the proton of azomethine group -CH=N- appears at 8.019 (j) ppm, and the peaks at 9.827 (o) and 9.808 (p) ppm are ascribed to the resonance absorptions of other two protons on pyridine ring. The above 1H-NMR data further prove that the bidentate Schiff base ligand SB has been synthesized and bonded on the side chains of PSF through 13


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nucleophilic substitution reaction and Shiff base reaction, and the functional PSF, PSF-SB, has been gained. 3.3. UV absorption spectra of PSF-SB and complexes Fig. 5 presents the UV absorption spectra of PSF-SB, Phen, the binary complex PSF-(SB)3-Eu(Ⅲ) and the ternary complex PSF-(SB)3-Eu(Ⅲ)-(Phen)1 (10−4M in DMF).

Fig. 5

The following facts can be observed in Fig. 5. PSF-SB has strong UV absorption centered at 283 nm in a range of 260-360 nm. It can be seen that in the structure of the bonded ligand SB, A greater π bond conjugated system is constituted by two aromatic rings trough the linking of the azomethine bond of the Shiff base as a bridge, and it is the greater π bond conjugated system that makes PSF-SB to produce strong UV absorption. The absorption peak at 283 nm is ascribed to the π-π* electron transition of this greater π bond conjugated system. The absorption peak at 348nm is attributed to the n-π* electron transition of the N atom in the azomethine group -CH=N-. In the spectrum of the small-molecule ligand Phen, there are two absorption peaks at 286 and 323 nm, and they are assigned to the π-π* electron transition and n-π* electron transition, respectively. It can be observed from Fig. 5 that the binary complex PSF-(SB)3-Eu(Ⅲ) also have strong UV absorption. The spectral profile is very similar to that of PSF-SB, and only the characteristic absorption peaks slightly red-shift. The absorption peak caused by π-π* electron transition red-shifts from 283nm to 289nm, and the absorption peak caused by n-π* electron transition red-shifts from 248 nm to 350 nm. The red shifts of the absorption peaks of the binary complex PSF-(SB)3-Eu(Ⅲ) implies that the ligand SB of PSF-SB has coordinated to Eu3+ ion and the binary polymer-rare earth complex PSF-(SB)3-Eu(Ⅲ) has 14



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been formed. It is important that lanthanides ion do not nearly contribute to the UV absorption spectra of their complexes since f-f transitions are Laporte-forbidden, and the UV absorption of Eu3+ ion is very weak (extinction coefficients ε < 1 M−1 cm−1). Therefore, the strong UV absorption of PSF-(SB)3-Eu(Ⅲ) comes from the macromolecular ligand PSF-SB and it is unrelated to the central ion, Eu3+ ion. It also shows that the luminescence of the binary complex PSF-(SB)3-Eu(Ⅲ) is the result of energy transfer from the bonded ligand SB to Eu(III) ion, namely the sensitization effect of SB towards Eu(III) ion (see below). In the spectrum of the ternary complex PSF-(SB)3-Eu(Ⅲ)-(Phen)1, there is a strong absorption peak at about 292 nm. The spectral band of the ternary complex is the result of the overlap of the absorption band of PSF-SB and the partial absorption band of Phen, which is centered at 286 nm. The second absorption peak 325 nm is attributed to the second chatacteristic absorption peak of Phen, and it also has obvious red-shift (from 323 nm to 325nm). The third absorption peak at 353 nm should be assigned to the n-π* electron transition of N atom of azomethine bond of PSF-SB with an obvious red shift (from 348 nm to 353nm). After coordinating of the two N atoms of SB and N atom of Phen to Eu3+ ion, multiple chelating rings around the center ion are formed, and it makes the conjugation system to be enlarged and makes the density of π electron cloud around the central ion to be increased. As a consequence, the electron delocalization degree increases and the electron transition energy decreases, resulting in the red shift of the absorption bands. The above spectrum data fully demonstrate that the coordination bonds between Eu3+ ion and two ligands, macromolecular ligand PSF-SB and small-molecule ligand Phen, have formed, forming the ternary complex PSF-(SB)3-Eu( Ⅲ )-(Phen)1. At the same time, the above spectrum data also demonstrate that the UV absorption of the ternary complex comes from the synergy absorption of the macromolecular ligand PSF-SB and the small-molecule co-ligand Phen. Similarly, the fluorescence emission of the ternary complex is also unrelated to the central Eu (Ⅲ) ion, and it is only the result of the 15


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energy transfer from two ligands to Eu3+ ion as explained in the following sections. 3.4. Fluorescence emission spectra of complexes 3.4.1. Fluorescence emission spectra of binary complexes in solution By exciting of the optimal excitation peak at 310 nm, the fluorescence emission spectrum of the binary complex PSF-(SB)3-Eu(Ⅲ) in DMF solution was recorded. At the same time, the fluorescence emission spectrum of EuCl3 in DMF solution was also recorded. In the two solutions, the concentrations of Eu3+ ion were identical, and they were 4.0×10-4 mol. These recorded fluorescence emission spectra as well as the excitation spectra are presented in Fig. 6.

Fig. 6

In Fig. 6, the following facts can be found and the corresponding theoretical explanations are given below. (1) PSF-SB itself exhibits broad fluorescence band centered at 415 nm attributed to π-π* transition. (2) After coordination to Eu(III) ion and forming the binary complex PSF-(SB)3-Eu(III), the fluorescence emission of PSF-SB itself is weakened remarkably, and it is very similar to the small-molecule Shiff base-rare earth complexes.

23,24

This phenomenon is caused by incomplete energy transfer. (3) The binary

complex PSF-(SB)3-Eu(III) exhibits the characteristic emission of Eu(III) ion. In the spectrum, three main emission peaks at 580, 593 and 620 nm are displayed, and they are assigned to the transitions of 5D0 →7F0, 5

D0→7F1 and 5D0 → 7F2, respectively. Among these bands, the 5D0→7F2 at 620 nm is the strongest

emission as shown in Fig. 6, and it exhibits the red luminescence (see below). (4) The emission intensity of PSF-(SB)3-Eu(III) at 620 nm is enhanced by 15 times in comparison with that of EuCl3. The above facts full demonstrate the following several points. (1) The bonded bidentate Schiff base 16



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ligand SB is a good chelating organic chromophore, and can strongly sensitize the fluorescence emission of Eu3+ ion, namely an apparent “Antenna Effect” has been produced, leading to the strong characteristic fluorescence emissions of Eu3+ ion. That is, SB of PSF-SB is first excited to the excited singlet state (S1) by UV absorption, and then the excited electrons partially relax to the triplet state (T1) via intersystem crossing. Subsequently, the energy is nonradiatively transferred from the triplet state level of SB of PSF-SB to the resonance state level of the coordinated Eu3+ ion. Finally, the Eu3+ ion emits characteristic fluorescence in the visible region by a multiphoton relaxation from the exited state to the ground state of Eu3+ ion. At the same time, the fluorescence emission of PSF-SB itself reduces greatly due to the energy transfer. (2) The triplet state energy of the bonded ligand SB is well matched with the resonant level energies of Eu(III) ion. According to the Antenna effect theory, for the fluorescent intensity of rare earth complexes, except for the ability of optical absorption of the ligands in the UV region, intramolecular energy transfer efficiency is strongly dependent on the energy matching degree between the triplet state energies of organic ligands and the resonant level energies of rare earth trivalent ions. 25,26 According to the calculation result in Ref., 27 for Eu3+ ion, as the energy gap between the triplet state energies of organic ligands and the resonant level energy of Eu3+ ion is about 3000 cm−1, the energy transfer efficiency is the highest. The lowest resonant level energy of Eu3+ ion is lower (17,277 cm-1

28

), and therefore, It can be judged that the triplet state

energy of the ligand SB is also lower. It remains to be determined by low temperature phosphorescence. (3) The better energy level matching and effective intramolecular energy transfer make the fluorescence of PSF-SB itself is much more subdued. However, it can be seen from Fig. 6 as well as from following Fig. 9 that the integrated intensity of π-π* photoluminescence is much stronger than the Eu(III) ion emission. This reflects that the energy transfer or the energy level matching degree is affected negatively by the structure of the ligand SB. It can 17


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be guessed that for the luminescent Schiff base-type polymer-rare earth complexes, the energy level matching degree can be improved via designing and changing the structure of the bidentate Schiff base ligand SB so that the π-π* photoluminescence of the ligand SB almost completely disappears and the Eu(III) ion emission is strengthened further. For comparison, in this study, the bidentate Schiff base ligand-functionalized PSF, PSF-SB, also was made to coordinate to Tb(III) ion, preparing another binary polymer-rare earth complex PSF-(SB)3-Tb(III). The DMF solutions of PSF-(SB)3-Tb(Ⅲ) and TbCl3 were prepared. By exciting of the optimal excitation peak at 275 nm, the fluorescence emission spectrum of the binary complex PSF-(SB)3-Tb(Ⅲ) in DMF solution as well as that of TbCl3 was recorded. In the two solutions, the concentrations of Tb3+ ion were identical, and they were 4.0×10-4 mol. These recorded fluorescence emission spectra as well as the excitation spectra are presented in Fig. 7.

Fig. 7

The fluorescence emission status of PSF-(SB)3-Tb(III) is completely different from that of PSF-(SB)3-Eu(III). The following facts can be seen from Fig.7. (1) After the binary complex between PSF-SB and Tb(III) ion is formed, the weakening degree of the fluorescence emission of PSF-SB itself is very smaller, implying that there nearly is no intramolecular energy transfer from PSF-SB to the exited state to Tb(III) ion, indicating that the triplet state energy of the bonded ligand SB is severely unmatched with the resonant level energy of Tb(III) ion. (2) The fluorescence emission intensity of PSF-(SB)3-Eu(III) is even lower than that of TbCl3, displaying a energy back transfer from the excited state level of Tb(III) ion to the triplet state level of the bonded SB probably occurs. 29,30 As previously mentioned, in the 18



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structure of the bonded SB, there is a larger π bond conjugated system. The larger conjugated system of the ligand not only will strengthen the UV absorption ability of the ligand, but also will decrease the energies of the excited singlet state and the triplet state levels.

31

Therefore, the triplet state energy of the bonded

ligand SB is probably lower as pointed out above. However, the resonant energy level (5D4) of Tb3+ ion reaches up to 20500 cm-1.

32

It can be speculated that the resonant energy level (5D4) of Tb3+ ion is even

higher than the triplet state level of the bonded ligand SB. The bonded ligand SB not only can not sensitize the fluorescence emission of Tb3+ ion, but also the energy back transfers from the excited state levels of Tb(III) ion, 5D4 (20500cm-1) or 5D3 (23,666 cm-1) 32, to the triplet state level of the bonded SB (5D4→T1 or 5

D3→T1) probably occur, resulting in the lower fluorescence emission intensity of PSF-(SB)3-Tb (III) than

that of TbCl3. By contrast, the resonant energy level (5D0) of Eu3+ ion is only 17,277 cm-1, and there is probably a better matching degree between the triplet state energy level of the bonded ligand SB and the resonant energy level of Eu3+ ion, resulting in a stronger fluorescence emission of PSF-(SB)3-Eu(III)). The actual images of PSF-(SB)3-Eu(III) and PSF-(SB)3-Tb (III) solutions under UV radiation are presented in Fig. 8. By comparison, the PSF-(SB)3-Eu(III) solution (A) displays red luminescence, exhibiting good photoluminescence property, whereas the PSF-(SB)3-Tb (III) solution (B) does not display any color, namely it is colourless, implying that for the complex PSF-(SB)3-Tb (III), there is no photoluminescence phenomenon to occur.

Fig. 8

3.4.2. Fluorescence emission spectrum of ternary complex in solution 19


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By using Phen as small-molecule co-ligands, the ternary complex PSF-(SB)3-Eu(III)-(Phen)1 was prepared. The DMF solution of PSF-(SB)3-Eu(III)-(Phen)1 with a concentration of 4.0×10-4 mol/L of Eu3+ ion was prepared, and the fluorescence emission spectrum was determined. The determination result is shown in Fig. 9. For comparison, the fluorescence emission spectrum of the binary complex PSF-(SB)3-Eu(III) is also displayed in Fig. 9.

Fig. 9

The following facts can be found in Fig. 9. (1) After PSF-SB and Phen coordinate together to Eu(III) ion to form the ternary complex PSF-(SB)3-Eu(III)-(Phen)1, the weakening extent of the fluorescence of PSF-SB itself becomes greater, implying a greater intramolecular energy transfer. (2) The ternary complex PSF-(SB)3-Eu(III)-(Phen)1 also exhibits the characteristic emission of Eu(III) ion. Moreover, the luminescence intensity is enhanced about 20 times than that of EuCl3, displaying a stronger sensitization towards Eu(III) ion in the ternary complex than that in the binary complex PSF-(SB)3-Eu(III). (3) The fluorescence emission intensity of the ternary complex PSF-(SB)3-Eu(III)-(Phen)1 is stronger than that the binary complex PSF-(SB)3-Eu(III). The reason for this is explained as follows. In the ternary complex, the second small-molecule ligand plays two roles. On the one hand, the addition of the second ligand makes the ligand number around rare earth ions to be increased and the conjugated system around the central ions to be enlarged. The synergistic coordination of the two ligands will strengthen the intramolecular energy transfer. 33,34 On the other hand, the second ligand can effectively substitute the coordinated water molecules around the central ion as shown in Scheme 2, and avoid the fluorescence quenching caused by the vibration of the hydroxyl groups of water molecules. 20


31,34

It is the


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combining result of the two factors that makes the ternary complexes to have stronger fluorescence emission than the binary complex. In order to confirm the substitution effect of the coordinated water molecules around the central ion for the second ligand Phen, the thermal gravimetric measurements of the binary and ternary complexes were done, and their TGA curves are given in Fig. 10.

Fig. 10

Fig. 10 shows that for the binary complex PSF-(SB)3-Eu(III), there is a greater weightless degree in the temperature range of 0-130℃, and it is caused by the volatilization of water, meaning greater coordinated water molecules are contained in the original binary complex. However, for the ternary complex PSF-(SB)3-Eu(III)-(Phen)1, there is only slight weightlessness in the temperature range of 0-90 ℃, indicating less coordinated water molecules are contained in the original ternary complex. The result of the thermal gravimetric experiment shows that the second small-molecule ligand indeed plays a role of substituting the coordinated water molecules around the central ion, and this is an important factor for enhancing the luminescence intensity of the ternary polymer-rare earth complex. 3.4.3. Quantum yield of photoluminescence of complexex The photoluminescence quantum yields of the binary polymer-rare earth complex PSF-(SB)3-Eu(III) and ternary polymer-rare earth complex PSF-(SB)3-Eu(III)-(Phen)1 were measured by relative method using the quinine sulfate as the standard (0.546 in 0.5 mol/L H2SO4) 35. The quantum yield was calculated from the following equation:

F A n  Φs = Φr s r  r  Fr As  ns 

2

21


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In the above expression, Φ s is the fluorescent quantum yield, F is the integration of the emission intensities, n is the index of refraction of the solution, and A is the absorbance of the solution at the exciting wavelength. The subscripts r and s denote the reference and unknown samples, respectively. The values of quantum yields of the binary complex PSF-(SB)3-Eu(III) and ternary complex PSF-(SB)3-Eu(III)-(Phen)1 were 41.12％and 52.75％, in DMF, respectively. The following facts can be found from the determined 3+

data. (1) The polymer-rare earth complexes prepared in this work give strong Eu

emission with high

fluorescent quantum yields. (2) The quantum yields probably are affected by the florescence emission of the ligand PSF-SB itself, or else the quantum yields might be higher than such values. (3) The quantum yield of the ternary complex PSF-(SB)3-Eu(III)-(Phen)1 is higher than that of the binary complex PSF-(SB)3-Eu(III), and it shows that the synergistic coordination of the two ligands is conducive to the enhancement of the quantum yield. 3.5. Fluorescence emission spectra of solid films of complexes It is well known that PSF has excellent film-forming property, and it leads to that the polymer-rare earth complexes with PSF as polymer skeleton are also easy to form thin film. This advantage provides much convenience for the applications of these polymer-rare earth complexes. The solid films of the binary and ternary complexes were prepared by using casting method. Fig. 11 gives the fluorescence emission spectra of PSF-(SB)3-Eu(III) and PSF-(SB)3-Eu(III)-(Phen)1. Fig. 11 displays that the solid films of these complexes also emit the stronger characteristic fluorescence of Eu3+ ion, exhibiting strong sensitization actions, and furthermore, it also shows the fluorescence emission intensities of the ternary complex film is higher than that of the binary complex film as like the status in the solution.

Fig. 11 22



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4. Conclusions A bidentate Schiff base ligand-functionalized polysulfone, PSF-SB, was prepared through polymer reactions by molecular design. The novel polymer-rare earth complexes, binary polymer-rare earth complex PSF-(SB)3-Eu(Ⅲ) and ternary polymer-rare earth complex PSF-(SB)3-Eu(Ⅲ)-(Phen)1, were obtained via the coordination reactions between PSF-SB as macromolecular ligand and Phen as the second small-molecule ligand and Eu(Ⅲ) ion. On the basis of characterizing PSF-SB and complexes, the fluorescence emission characteristics and luminescent mechanism of the complexes were mainly investigated. The macromolecular ligand PSF-SB itself emits strong fluorescence. However, after coordinating to Eu(Ⅲ) ion to form polymer-rare earth complexes, its fluorescence emission weakens remarkably, and it should be attributed to the intramolecular energy transfer. Both the binary and ternary complexes exhibit the strong characteristic fluorescence of Eu(Ⅲ) ion, indicating that the bond bidentate Schiff base ligand SB has stronger sensitization towards Eu(Ⅲ) ion. The fluorescence emission intensity of the ternary complex PSF-(SB)3-Eu(Ⅲ)-(Phen)1 is stronger than that of the binary complex PSF-(SB)3-Eu(Ⅲ) owing to the synergism coordination of the second ligand Phen and its role of replacing the coordinated water molecules around Eu(Ⅲ) ion. Besides, the binary complex of PSF-(SB)3-Tb(Ⅲ) does not exhibit the characteristic fluorescence of Tb(Ⅲ) ion, demonstrating that the bonded bidentate Schiff base ligand SB has no sensitization for Tb(Ⅲ) ion. This fact indicates the triplet state energy of the bonded ligand SB is matched with the resonant energy level of Eu(III) ion and it is unmatched with that of Tb(Ⅲ) ion. The polymer-rare earth complex with the bonded bidentate Schiff base group as ligand will have potential applications in science and technology areas.

23


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For Table of Contents Only

700 600

PSF-SB

PSF-(SB)3-Eu(III)-(Phen)1

500 Intensity(a.u)

Page 29 of 43

PSF-(SB)3-Eu(III)

400 300

EuCl3

200 100 0 350

400

450

500

550

600

650

700

λ/nm

28



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure Captions Scheme 1 Schematic expression of chemical process to preparing PSF-SB Scheme 2 Schematic expressions of structures of binary and ternary complexes Fig. 1 FTIR spectra of CMPSF, PSF-BA and PSF-SB Fig. 2 FTIR spectra of PSF-(SB)3-Eu(Ⅲ) and PSF-(SB)3-Eu(Ⅲ)-(Phen)1 Fig. 3 1H-NMR spectrum of CMPSF Fig. 4 1H-NMR spectrum of PSF-SB Fig. 5 UV absorption spectra of PSF-SB, Phen, binary complex PSF-(SB)3- Eu(Ⅲ) and ternary complex PSF-(SB)3- Eu(Ⅲ)-(Phen)1 Solvent: DMF Fig. 6 Fluorescence spectra of binary complex PSF-(SB)3- Eu(Ⅲ) and EuCl3 Solvent: DMF; Concentration of Eu3+ ion: 4.0×10-4 mol/L Fig. 7 Fluorescence spectra of PSF-SB, binary complex PSF-(SB)3-Tb(Ⅲ) and TbCl3 Solvent: DMF; Concentration of Tb3+ ion: 4.0×10-4 mol/L Fig. 8 Images of PSF-(SB)3-Eu(III) and PSF-(SB)3-Tb (III) solutions under UV radiation Fig. 9 Fluorescence emission spectrum of PSF-SB and ternary complex PSF-(SB)3-Eu(Ⅲ)-(Phen)1 Solvent: DMF; Concentration of Eu3+ ion: 4.0×10-4 mol/L Fig. 10 TGA curves of binary and ternary complexes Fig. 11 Film fluorescence spectra of complexes

29


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Scheme 1

30



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Scheme 2

31


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Page 33 of 43

670

1440

PSF-BA

880

CMPSF

1212

1662

PSF-SB

1690

T/% 2730

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4000 3500 3000 2500 2000 1500 1000

500

-1

Wavenumber/cm

Fig. 1

32



PSF-(SB)3-Eu(III) T/% PSF-(SB)3-Eu(III)-(Phen)1

1658

1662

PSF-SB

1656

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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4000

3500

3000

2500

2000

1500

1000

-1

Wavenumber/cm

Fig. 2

33


500

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f

CH3 C

e d

g

O

CH3

a

O S

b c O

O

h H2C Cl

i

Fig. 3

34


n


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

f g

CH3 C

e d

O

O

CH3

a

S

O n

O

h

m

H2C O

i

b c

n

CH N

j

k l

p N

o

Fig. 4

35


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π-π*

1.6

PSF-(SB)3-Eu(III)-(Phen)1 Phen

PSF-SB

1.2

PSF-(SB)3-Eu(III)

A

0.8

n-π*

0.4 0.0 200

250

300

350

λ/nm

Fig. 5

36


400


700

Emission

600

PSF-SB 500

Intensity(a.u)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

PSF-(SB)3-Eu(III)

400 Excitation 300 200

EuCl3

100 0 300

400

500

600

λ/nm

Fig. 6

37


700

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Page 39 of 43

700

Emission

600

PSF-SB

500

Intensity(a.u)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


PSF-(SB)3-Tb(III)

400 300 200

TbCl3

Excitation 100 0 200

300

400

500

600

λ/nm

Fig. 7

38


700


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

A

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B

Fig. 8

39


Page 41 of 43

700

Emission 600 Excitation


500 Intensity(a.u)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


PSF-(SB)3-Eu(III)

400 300 200

EuCl3

100 0 200

300

400

500

600

λ/nm

Fig. 9

40


700


20

0-90℃


0 -20 Weight loss/%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 42 of 43

-40

0-130℃

-60

PSF-(SB)3-Eu(III)

-80 -100 0

200

400

600

T/℃

Fig. 10

41


800

Page 43 of 43

Emission

700


600 500

Intensity(a.u)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


PSF-(SB)3-Eu(III)

400 300 200 100 0 550

600

650

λ/nm

Fig. 11

42


700

Preparation and Photoluminescence Properties of ... - ACS Publications

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