Fluorescent Composites Prepared of Tb³⁺ and Sulfonated Sulfate Polymer Constructed through Post-Sulfonation Sulfur-Fluorine Exchange Polymerization by Symmetric Molecular

Abstract

Organic fluorescent materials are widely applied in different important fields, such as light-emitting display, explosive detection, molecular imprinting, and so on, because of their low cost, easy functionalization, and large-scale fabrication. In this work, we designed and synthesized a new kind of organic fluorescent polysulfate composite material through post-sulfonation sulfur-fluorine exchange polymerization (a new kind of click chemistry) by symmetric molecular. Sulfur-fluorine exchange polymerization: symmetrical structure SO2F−R1−SO2F molecular reacted with symmetrical OH−R2−OH molecular through nucleophilic reaction in the presence of inorganic base. The polysulfate material was further modified by ClSO₃H to get PSE−SO₃H materials. Tb³⁺ was highly dispersed on PSE−SO₃H to afford organic-inorganic hybrid fluorescent materials through the conventional coordination chemistry method. The emission wavelength of the organic-inorganic hybrid fluorescent polymer PSE−SO₃H−Tb³⁺ was between 475 and 685 nm, the quantum yield reached 1.18%, and fluorescence lifetime lasted for 730.168 us, with the pure green light emission and long light-emitting lifetime. The fluorescence film was prepared through phase transformation method by the fluorescent polymer material PSE−SO₃H−Tb³⁺. The film has the strong stability property in different pH conditions (pH 1~13). Thus, this kind of organic fluorescent polysulfate composite material may have certain prospects application in terms of detection and luminescence in extreme chemical environments in the future.

1. Introduction

The click reaction was discovered and reported by Sharpless’s group in 2001 [1]. With the great efforts made during several years of research, the click reaction has been studied and applied in synthetic chemistry, e.g., small molecule drug synthesis and polymers synthesis, because of its wide application areas, quantitative yields, inoffensive byproducts, and a variety of functional group tolerances [2,3,4]. The definition of a click reaction refers to the quick and efficient connection of several functional fragments and backbones together. The traditional click reaction includes CuII-catalysed azide–alkyne cycloaddition reaction [5], thiol-ene/yne reaction [6,7,8,9], azide–nitrile cycloaddition [10] etc. In 2014, Sharpless’s group reported a new kind of click chemistry, the sulfur-fluorine exchanged reaction, which has been called a new generation of click chemistry. Sulfur-fluorine exchanged reaction could be applicated to construct a series of fluorinated reagents which are widely used in biomedicine, polymer materials, etc. In fact, as early as the 1970s and 1980s, fluorosulfates were synthesized by sulfur-fluorine exchanged reaction [11]. Polysulfate materials, because of their strong chemical stability attracts many scientists interesting. However, polysulfate materials (organic molecule or polymers containing S⁶⁺ sulfate structural fragment −O−S(=O)₂−O−) prepared by using methylsilyl ether and sulfuryl fluoride reaction under organic bases exist [12,13,14]. By this way, not only are silane groups necessary for the reagent monomers, but small molecule byproducts of tert-butylfluorodimethylsilane (TBSF) are also produced after sulfur-fluorine exchanged reaction, which is a huge challenge in terms of polymerization economy for large scale industrial application. In 2019 [15], we found a new method to prepare polysulfate polymers through directly thermal stepwise polymerization between bisphenol monomers and sulfuryl fluoride monomers under inorganic bases existing. However, as a new kind of material, scientists are not only focusing on how to design and synthesize different chemical structures by series building blocks, but also investigating the chemical and physical properties of such materials. According to relevant literature research, apart from some reports on the polysulfate polymers stability and film-forming properties, there are few reports on studying other applications properties or how to design and synthesize functional composite materials. So, in this work, our purpose was to prepare functionalized polysulfate composite materials, especially organic-inorganic hybrid materials. It is well known that organic-inorganic hybrid fluorescent materials [16,17,18,19] are always applicated in luminescent film, explosive detection, bioluminescence probe etc. As a kind of new material with strong chemical stability, polysulfate material is a kind of polymer which has the potential to be used in extreme environments. At present, to the best of our knowledge, there is no paper reporting the fluorescent composite materials prepared by the polysulfate as the subject part in organic-inorganic hybrid fluorescent materials. When we talk about the object in the organic-inorganic hybrid composite fluorescent materials, rare earth elements may have the potential to be used as the light emitting object part because of their good monochromaticity, long lifetime, and large stokes shift [20,21]. In order to increase the hydrophilicity and coordination ability with metal ions of polysulfate, a sulfonic acid group was introduced to the sulfate polymer through post modification strategy. Why should we choose the post modification strategy? The reasons are as follows:

(1) Firstly, Polysulfate materials was synthesized through stepwise polycondensation between −SO₂F and −OH groups, in order to obtain a certain molecular weight for ensuring that the polymer has the film-forming and processability. The reaction monomers should adopt symmetric structures where the reactive sites have the same activity for the polymer to obtain a high molecular weight.

(2) Secondly, it is usually difficult to control the reaction conditions for the phenolic monomers or sulfuryl fluoride monomers sulfonated with chlorosulfonic acid to get the target monomers with symmetrical sulfonated structure. The synthesis of sulfonated monomers often brings purification problems which will be a great challenge to the economy of future industrialization.

In this work, the fluorescent composites comprised of Tb³⁺ and sulfonated sulfate polymer were constructed through post-sulfonation sulfur-fluorine exchange polymerization by symmetric molecular. The sulfate polymer was synthesized through step polymerizing (Figure 1) by 4,4’-dihydroxydiphenyl ether, bisphenol A, and bisphenol A sulfuryl fluoride monomers in the presence of sodium carbonate. Sulfonated polysulfate material was prepared with ClSO₃H and the sulfonation degree reached to 10–13%. The rare earth Tb³⁺ was introduced into the polymer through homogeneous mixed coordination. The strong luminescence organic-inorganic hybrid light-emitting polymers were prepared, and their structures and properties were characterized. Relying on the good film-forming properties of the materials, preparation adopted the phase inversion method. The light-emitting film has opened up new ideas regarding the light-emitting application for the new polysulfate materials.

2. Materials and Methods

Bisphenol A (BA), 4,4’-Dihydroxydiphenyl ether(DHE), Chlorosulfonic acid(CA), Sodium carbonate (Na₂CO₃) were purchased from Aladdin Chemical Co., Ltd. (Beijing, China), Propane-2,2-diylbis(4,1−phenylene) bis(sulfurofluoridateFerric) [BA− (SO₂F)₂] was purchased from baiyin TW company, China, Tb(NO₃)₃.6H₂O was purchased from Shanghai MackLin Biochemical Technology Co., Ltd., (Shanghai, China) DMF, Sulfolane, DMSO, MeOH, EtOH, THF were supplied by Sinopharm Chemical Reagent Co(Shanghai, China)., Ltd. All organic solvents used were of analytical grade and without further purification. Deionized water was prepared by Lanzhou Chemical Research Center (Lanzhou, China).

2.1. Preparation of PSE1 (Polysulfate-1)

Figure 2 shows that the PSE1 (Polysulfate−1 polymer) was synthesized following a literature procedure [15]. The solution of 4,4’−Dihydroxydiphenyl ether (0.1 mol, 20.2 g), BA− (SO₂F)₂ (0.1 mol, 39.2 g), and Na₂CO₃ (0.2 mol, 21.2 g) in 200 mL Sulfolane was replaced with nitrogen three times and stirred at room temperature for 15 min. Then, the temperature of the solution was gradually increased to 150 °C in 30 min. The solution was stirred at 150~170 °C for another 6–8 h. After the viscosity of the reaction mixture reached a certain degree, the mixture was cooled to 100 °C and poured into hot deionized water (60 °C) for precipitation to get white solid crude product. Then, the crude product was dissolved into THF, precipitated with ethanol for 3 times. Finally, the pure white powder was obtained from being extracted with hot deionized water and ethanol for another 24 h, the pure product was dried in vacuum for 5 h to obtain the white solid powder PSE1 (50.9g, yield 92%). ¹H−NMR (CDCl₃, 400 MHz) δH: 7.22~7.32 (m,12H), 7.0~7.7 (m,4H), 1.64~1.71 (m,6H). ¹³C−NMR (CDCl₃, 400 MHz) δ: 155.7, 149.6, 148.43, 145.97, 128.43, 122.78, 120.61, 120.06, 42.76, 30.78.

2.2. Preparation of (Polysulfate-2)

Figure 3 shows that the PSE2 (Polysulfate−2 polymer) was synthesized following a literature procedure [15]. The solution of 4,4’-Dihydroxydiphenyl ether (0.12 mol, 25 g), BA− (SO₂F)₂ (0.16 mol, 64.68 g), BA (0.045 mol, 10.26 g) and Na₂CO₃ (0.34 mol, 35.7 g) in 300 mL Sulfolane was replaced with nitrogen three times and stirred at room temperature for 15 min. Then the temperature of the solution was gradually increased to 150 °C in 30 min. The solution was stirred at 150~170 °C for another 6–8 h. After the viscosity of the reaction mixture reached a certain degree, the mixture was cooled to 100 °C and poured into hot deionized water (60 °C) for precipitation to get white solid crude product. Then, the crude product was dissolved into THF and precipitated with ethanol for 3 times. Finally, the white powder was obtained from being extracted with hot deionized water and ethanol for 24 h, the pure product was dried in vacuum for 5 h to obtain the white solid powder PSE2 (86.8 g, yield 93%), ¹H−NMR (CDCl₃, 400 MHz) δH: 7.19~6.94 (m, 10 H), 6.81~6.73 (m, 2 H), 1.59 (s, 6 H) (supporting information Figure S1). ¹³C−NMR (CDCl₃, 400 MHz) δ: 154.66, 148.47, 148.12, 147.39, 127.42, 127.37, 121.76, 119.6, 119.04, 41.70, 29.74.

2.3. Preparation of PSE-SO₃H

PSE solid powder (25 g) was dissolved in 300 mL dichloroethane at 80 °C, and the temperature of the solution was cooled to below 10 °C in an ice-water bath. Chlorosulfonic acid (70 mL) was added dropwise to the solution, and the rate of addition was controlled to ensure that the reaction temperature was kept below 10 °C. After the dropwise addition, the reaction mixture continued to be stirred for another 4–5 h. The reaction solution was slowly poured into cold n-hexane to precipitate the sulfonated polymer. The sulfonated polymer was washed with deionized water and methanol until the eluent was neutral. After vacuum drying, the sulfonated polymer was pulverized, and the obtained powder was washed with hot water for 3 times. The powder was then vacuum-dried to obtain the pure sulfonated polymer PSE−SO₃H.The sulfonation degree of the sulfonated polymer was 10–13% through titration test. After sulfonation, the hydrophilicity of the polymer is improved. In the supporting information Figure S2, the contact angle of the sulfonated PSE is also tested, where the average value (θc) of PSE1 is about 109.25°and PSE2 is about 100.67°. It also shows that the hydrophilicity of the polymer increases with the content of diphenyl ether segment structure in the PSE polymer increased.

2.4. Device Fabrication Process of PSE−SO₃H−Tb³⁺

PSE−SO₃H−Tb³⁺ 15 g was re-dissolved into 100 mL DMF to obtain a transparent viscous liquid.

(a) Fabrication Film: PSE−SO₃H−Tb³⁺ viscous liquid 2 mL was put on a glass plate (100 mm × 100 mm × 1.1 mm), and the viscous liquid was scraped with a 100 um wet film scraper. Then, the glass plate was placed on a desk for another 2 min and subsequently placed into a deionized water bath to afford a white film. The film was washed with deionized water several times and dried to obtain PSE-SO₃H-Tb³⁺ film (Figure 4).

(b) Fabrication line: PSE−SO₃H−Tb³⁺ viscous liquid 5 mL was extracted with a 10 mL medical syringe. The liquid was injected it slowly into 80 °C hot deionized water under stirring at a certain speed, a continuous polymer line was formed, and the resulting agglomerated coil was washed with deionized water several times and dried to obtain PSE−SO₃H−Tb³⁺ line (Figure 4).

(c) Fabrication spherical particles: PSE−SO3H−Tb³⁺ viscous liquid 5 mL was extracted with a 10 mL medical syringe. The viscous liquid was dropped into 100 mL hot deionized water slowly to form polymer spherical particles. The size of the particles can be controlled by the size of the syringe needle. The particles water mixture was hardened under constant stirring. Then, the particles were repeatedly washed with deionized water and dried to obtain PSE−SO₃H−Tb³⁺ spherical particles (Figure 4).

3. Results and Discussion

3.1. Structure Descripition

A new kind of linear polysulfate material was synthesized by sulfur-fluorine exchanged reaction using symmetric molecular bisphenol A skeleton and diphenyl ether skeleton and the polymers were coordinated with Tb (NO₃)₃.6H₂O. The chemical structure information of the material was determined by nuclear magnetic resonance (NMR) and IR in supporting information. According to the calculation of the chemical shift of the hydrogen spectrum peak and the peak area, for the PSE2 material, it can be seen that the quality of diphenyl ether contained in the polymer reached 22.3 wt%. Moreover, bisphenol A structure reached 77.6 wt%, and the data showed that the Sulfur-fluorine exchange stepwise polymerization can achieve complete polymerization with few by-products, consistent with the original feeding ratio and the yield (92−93 wt %) due to some small molecular fragments being lost with precipitation and washing steps. When the click reaction of the monomer was 1:1(PSE1), the reaction can only be carried out according to the stoichiometry, the molecular weight tested by GPC. The molecular weight of PSE1 was Mn = 3.052 × 10⁴ g/mol and the molecular weight distribution was 1.69, the molecular weight of PSE2 was Mn = 4.0925 × 10⁴ g/mol and the molecular weight distribution was PD = 1.49.

IR Figure 5 shows that the chemical structure of the polymer does not change before and after the rare earth metal ion Tb³⁺ was coordinated, while the DSC test Figure 6 shows that the glass transition temperature of the rare earth metal composite material increases, and the metal ions may form a weak three-dimensional structural connection with the polymer to enhance its stability. The black line represents the first heating curve of Sulfonated-PSE2, the blue line represents the first heating curve of Sulfonated-PSE2-Tb³⁺, the green line represents the cooling curve of Sulfonated-PSE2-Tb³⁺, and the red curve represents the cooling curve of Sulfonated-PSE2. It can be seen from the figure that the glass transition temperature of rare earth-coordinated sulfate polymer is about 9~5 °C higher than that of polysulfate without metal ion Tb³⁺ coordinated, so the figure shows that microphysical crosslinking could increase the glass transition temperature (Tg) of the polymer. For polymer synthesis design, materials with different toughness and strength can be obtained by adjusting the content of monomer diphenyl ether structure, among which PSE1 has better toughness compared with PSE2.

3.2. Luminescence

The luminescence of PSE-SO₃H and PSE-SO₃H-Tb³⁺ in solid state at room temperature was evaluated. As shown in Figure 7, under 254 nm UV-light, the rare earth ion Tb³⁺ composite and uncomplexed sulfate materials showed different luminescence characteristics. The PSE-SO₃H-Tb³⁺ materials, both PSE1 and PSE2, whether coils or films and spheres, exhibited bright green luminescence. Figure 7 shows PSE-SO₃H materials under 254 nm UV-light irradiation, where PSE-SO₃H displays dull color luminescence.

Figure 8 shows that PSE−SO₃H excited at 356 nm, where PSE−SO₃H exhibited emission at 428 nm. Figure 9 and supporting information Figures S3 and S4 shows that upon excitation at 307 nm, as expected, the luminescence spectrum of PSE−SO₃H−Tb₃₊ exhibited four distinct characteristic emission peaks at 490, 548, 582, and 623 nm, which correspond to the characteristic transitions (⁵D₄–⁷F₆, ⁵D₄–⁷F₅, ⁵D₄–⁷F₄ and ⁵D₄–⁷F₃) of ion Tb³⁺. Among them, ⁵D₄–⁷F₅ transition, as the strongest emission and the high-sensitive magnetic dipole transition, is usually used in spectroscopic studies. Furthermore, the photoluminescence quantum yield (Φ) reached to 1.18% and the photoluminescence lifetime reached 730.168 μs (double-exponential curve, Figure 10). The number of spectra to be emitted according to the imported CIE-1931 color coordinates, the corresponding CIE color coordinates, was calculated to be (0.2307, 0.4901) (Figure 11), which was located in the pure green area. As shown in Figure 12, X-ray photoelectronspectroscopy (XPS) was used to characterize the Tb³⁺. The Tb 4d is evidenced by a shoulder observed on the main peak at 149.9 eV (Figure 8b), which is assigned to Tb³⁺ species. The binding energy(BE) S 2p1 peaked at 166.5 eV. Tb and S elements are confirmed by XPS. This also proves that the polymer contains Tb and S elements, which is consistent with the original chemical structure design.

3.3. Chemical Stability

The polysulfate material showed good chemical stability in a previous report. Therefore, by soaking the prepared film in different chemical environments and testing its chemical stability properties after soaking for seven days (Figure 13), the test showed that the material has good chemical stability. The experimental results showed that the composite material has good chemical stability in concentrated sulfuric acid, concentrated nitric acid, and different pH liquids and its mass and molecular weight almost do not decay (Mn = 30,596 g/mol). It has a wide range of application prospects in the field of fluorescence detection in extreme environments in the future, and its good film-forming properties provide a good prospect for the preparation of fluorescent coatings and fluorescent pellets.

4. Conclusions and Outlook

In this work, symmetrical monomers were selected to prepare a certain film-forming and processable resin material through simple sulfur fluorine exchange reaction copolymerization, which is compounded with rare earth metals to form a highly stable fluorescent material with luminescent properties, providing a new idea for the preparation of luminescent materials that may realize industrial production in the future. This is of great significance in the context of research on the new application of polysulfate materials prepared by the sulfur fluorine exchange reaction. In the future, research on the synthesis methods and properties of this kind of ultra-high molecular weight material will also become an important topic in the pursuit of further industrialization.