High Fluorescence Quantum Yield Based on the Through-Space

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Cite This: Macromolecules XXXX, XXX, XXX−XXX

High Fluorescence Quantum Yield Based on the Through-Space Conjugation of Hyperbranched Polysiloxane Yuanbo Feng,† Tian Bai,† Hongxia Yan,* Fan Ding, Lihua Bai, and Weixu Feng*

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MOE Key Laboratory of Material Physics and Chemistry under Extraordinary Conditions, Ministry of Education, and Key Laboratory of Polymer Science and Technology, Shaanxi Province, School of Science, Northwestern Polytechnical University, Xi’an 710129, China S Supporting Information *

ABSTRACT: Unorthodox luminogenic polymers without aromatic luminogens have attracted great interest in recent years; however, the low fluorescence efficiency is still a big drawback. In this paper, we synthesized a fluorescent hyperbranched polysiloxane with both carbonyl and vinyl groups (P1). Surprisingly, it exhibited nontraditional intrinsic luminescence with the highest quantum yield up to 43.9% among the reported silica-containing hyperbranched fluorescent polymers to date. Reference oligomers P2 and P3, theoretical calculations, and transmission electron microscopy were employed to explore the fluorescence mechanism. The high fluorescence quantum yield is ascribed to the synergism of vinyl and carbonyl groups as well as the Si−O grouppromoted through-space conjugation. Thus, the supramolecular hyperbranched polysiloxane was assembled by conjugation to increase the oscillator strength and decrease the band gap. Moreover, the solvent effect and pH dependency properties of P1 and its application as an Fe3+ probe were also studied.

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INTRODUCTION Nontraditional intrinsic luminescence (NTIL) polymers, which do not contain traditional conjugated chromophores, have attracted widespread attention in recent years.1−4 The advantages of easy preparation, excellent biocompatibility, and low cytotoxicity5,6 make them ideal as emissive materials for application in sensing, anticounterfeiting, biological or medical imaging, and so on.7−9 Since the first report of unexpected luminescence from archetypal poly(amidoamine)s,10 more and more unorthodox fluorescent polymers have been prepared, such as poly(amido amines), polyurea, polyethylenimine, poly(amido acid)s, poly (amino ester)s, polyacrylonitrile, and hyperbranched polysiloxanes (HBPSi).11−17 Our previous experience in unorthodox fluorescent polymers concerns the relationship between the functional groups and the fluorescence performance of hyperbranched polysiloxanes. Various kinds of intriguing HBPSi with different functional groups were skillfully elaborated and fabricated, such as hydroxy, carbon−carbon double bonds, or primary aminecontaining hyperbranched polymers;18−22 according to the earlier study, the fluorescence efficiency is highly related to the terminal groups of the polymers; in addition, the Si−O group can be used to further enhance the fluorescence by the siliconinduced aggregation.23 However, compared with the extensive progress in traditional conjugated luminescent materials,24 hyperbranched polysiloxane still suffers from a low fluorescent intensity and efficiency. Actually, this is a general problem for unorthodox luminogenic polymers; especially the quantum yield for the reported unconventional luminescent polymers is © XXXX American Chemical Society

much lower compared with those of traditional luminescent materials,25 and also their emission mechanism remains under debate.26 Although the aggregation-enhanced emission (AEE) and clustering-triggered emission mechanisms27 can be used to rationalize the luminescent behaviors of unorthodox luminogenic polymers, the interactions between the Si−O group and other groups are normally ignored; thus, there is still an urgent, but significantly challenging, need to gain a deeper understanding about the mechanism of enhancement of the fluorescence quantum yield. In this paper, we synthesized a novel HBPSi (P1). Surprisingly, P1 exhibited blue NTIL with an outstanding quantum yield of 43.9%, which is the highest among the reported silica-containing hyperbranched fluorescent polymers and even comparable to that of traditional conjugated fluorescent polymers. To explore the fluorescence mechanism, the reference oligomers P2 and P3, which contain either carbonyl groups or vinyl groups, were also synthesized for a comparison. Then, we carefully deciphered the fluorescence mechanism of P1. In addition, the solvent effect and pH dependency properties of P1 as well as its application as an Fe3+ probe have also been examined. Received: February 7, 2019 Revised: March 28, 2019

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DOI: 10.1021/acs.macromol.9b00263 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Synthetic Route of P1−P3 in a One-Step Reaction

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For P2: 1H NMR (400 MHz, chloroform-d) δ 3.88 (dt, J = 14.0, 5.3 Hz, 15H), 3.72 (q, J = 7.0 Hz, 2H), 1.31−1.18 (m, 26H), 0.97 (s, 1H). 13C NMR (101 MHz, chloroform-d) δ 172.06, 58.36, 35.56, 18.28, 13.99. For P3: 1H NMR (400 MHz, chloroform-d) δ 6.20−5.78 (m, 7H), 4.01−3.61 (m, 12H), 2.00−1.71 (m, 6H), 1.22 (ddd, J = 11.0, 7.5, 5.9 Hz, 16H). 13C NMR (101 MHz, chloroform-d) δ 137.42, 129.08, 64.73, 58.59, 58.50, 58.15, 31.21, 18.29, 18.15.

EXPERIMENTAL SECTION

Materials and Methods. All chemicals were purchased from Sigma-Aldrich and used directly without further purification. Molecular weight and distribution within the medium were measured using an ultimate 3000 UHPLC System (Dionex, Sunnyvale, CA). Tetrahydrofuran (THF) was used as the mobile phase, and the measurement was performed at a flow rate of 1 mL/min. 13C NMR (101 MHz) and 1H NMR (400 MHz) spectra were recorded on a Bruker Avance 400 MHz spectrometer in CDCl3 or dimethyl sulfoxide (DMSO)-d6 solvent, using TMS as an internal standard. Fourier transform infrared (FTIR) spectra of the oligomers and distillates were collected using a NICOLET 5700 FTIR spectrometer ranging from 4000 to 400 cm−1. UV−vis absorption spectra for EtOH solutions of P1, P2, and P3 were measured by a Shimadzu UV-2500 spectrophotometer. Fluorescent excitation/emission spectra for EtOH solutions of P1, P2, and P3 were measured on a Hitachi F-4600 fluorescence spectrophotometer. Fluorescence lifetimes and absolute quantum yields for pure P1−P3 were measured on a steady/transientstate fluorescence spectrometer coupled with an integrating sphere (FLS980, Edinburgh Instruments). Transmission electron microscopy (TEM) images were obtained via an FEI Tecnai G2 F20 microscope. Theoretical calculations were performed by using the density functional theory (DFT) or time-dependent density functional theory (TD-DFT) with B3LYP/6-31G(d). Synthesis. A mixture of malonic acid (0.2 mol, 20.81 g for P1 and P2) or 1,3-propanediol (0.2 mol, 15.22 g for P3) and A-151 (0.25 mol, 47.57 g for P1 and P3) or tetraethyl orthosilicate (TEOS) (0.1875 mol, 39.06 g for P2) was stirred in a 250 mL four-necked flask equipped with a mechanical stirrer, a thermograph, an N2 gas inlet, and a condenser under normal ambient conditions. Then, the system was slowly heated to 100 °C and maintained until clear and transparent. Meanwhile, some distillate was distilled off. Thereafter, the distillation temperature was maintained at about 60 °C to promote the reaction. The mixture was heated up to 160 °C until the distillation temperature dropped to 35 °C and no more distillate came over. Soon after that, the preproduct was poured in a vial and was dialyzed in ethanol to remove the products with a low molecular weight. Finally, the solution was rotary evaporated at 45 °C and dried at 60 °C in vacuum for 6 h, which then yielded the target oligomer. For P1: 1H NMR (400 MHz, DMSO-d6): δ 6.01−5.84 (m, 1H), 3.84−3.69 (m, 2H), 3.75 (s, 11H), 3.64 (s, 2H), 3.51−3.38 (m, 1H), 1.22−1.09 (m, 16H), 1.06 (t, J = 7.0 Hz, 1H), 1.02 (s, 5H). 13C NMR (101 MHz, chloroform-d): δ 167.03, 137.48, 136.44, 136.23, 135.98, 130.56, 130.48, 128.39, 58.54, 58.48, 58.40, 58.28, 30.87, 18.35, 18.11, 18.04.

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RESULTS AND DISCUSSION FTIR Study. As shown in Scheme 1, P1 was synthesized by a one-step catalyst-free polymerization reaction. To get more

Figure 1. FTIR spectra of P1, malonic acid, and A-151.

Figure 2. 1H NMR spectra of P1, malonic acid, and A-151.

insights into the mechanism of the fluorescence, we also synthesized the reference hyperbranched oligomers P2 and P3. The FTIR spectra of malonic acid, A-151, and P1−P3 are B

DOI: 10.1021/acs.macromol.9b00263 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Table 1. Photophysical Properties of Pure P1−P3

a

oligomer

λem/nm (intensity)

ϕ (%)a

τ (ns)b

P1 P2 P3

385 nm (316 159) 400 nm (63 309) 388 nm (153 665)

43.9 16.3 10.5

0.85 4.29 4.16

Fluorescence quantum yield. bFluorescence lifetime.

absorption peaks of −OH from the −COOH and −C−OH groups also disappear in P2 and P3. The spectra of the distillate from the reaction of P1, P2, and P3 are almost identical to that of pure ethanol. Thus, it can be essentially proved that the distillate is ethanol. This is another important evidence to indicate the successfully completed nucleophilic substitution polycondensation reaction. 1 H NMR Study. 1H NMR spectra of malonic acid, A-151, and P1 are illustrated in Figure 2. As shown in Figure 2b, the proton peaks marked by 1&3 and 2 associated with the carboxy group (HOOC−CH2−COOH) and the methylene group (HOOC−CH2−COOH) can be observed at 12.56 and 3.23 ppm, respectively. In Figure 2c, the proton peaks marked by 1&6&8 at 1.02−1.23, 2&5&7 at 3.76 ppm, or 3 at 6.11 ppm, 4 at 5.81−6.01 ppm are associated with Si−O−CH2−CH3, Si−O−CH2−CH3 or Si−CHCH2 respectively. In Figure 2a, it can be seen that the proton peaks associated with the primary carbon Si−O−CH2−CH3 and secondary carbon Si−O−CH2−CH3 at 1.09−1.22 and 3.69− 3.84 ppm are corresponding to H1 and H2, respectively; the proton peaks associated with Si−CHCH2 at 6.01−5.84 ppm are corresponding to H3 and H4. The proton peaks associated with the secondary carbon Si−OOC−CH2−COO−Si at 3.51− 3.38 ppm are corresponding to H5. It is worth noting that the proton peaks at 1.09−1.22 and 3.69−3.84 ppm respectively correlated to the methyl group (Si−O−CH2−CH3) and the methylene group (Si−O−CH2−CH3) of P1 are much weaker than those of A-151 (Figure 2c). Another important

Figure 3. 13C NMR spectra of P1, malonic acid, and A-151.

Figure 4. Fluorescence of P1 in ethanol with different concentrations.

shown in Figures 1 and S1. As we can see in Figure 1, it is clear that the absorption peaks ranging from 3100 to 2900 cm−1 of the malonic acid classified to the −OH from the −COOH group disappeared in P1. Meanwhile, the absorption peak of −CO from the −COOH group at 1720 cm−1 for malonic acid moved to 1745 cm−1 in the IR spectrum of P1. A-151 and P1 both have the major peaks at 1600 and 960 cm−1, which can be respectively attributed to CC and Si−O bonds. In short, the FTIR spectra can be an important evidence to indicate that the target oligomer P1 has been successfully synthesized. Similarly, as shown in Figure S1A−C, the

Figure 5. (A) UV−vis absorption spectra of P1 in ethanol at various concentrations; (B) emission spectra of P1 in ethanol with the concentration of 200 mg/mL by different excitations and the normalized emission intensity; (C) PL spectra of P1 in ethanol solution at different concentrations; (D) excitation and emission spectra of pure P1 (the inset photograph of P1 is taken at RT under a 365 nm UV lamp). C

DOI: 10.1021/acs.macromol.9b00263 Macromolecules XXXX, XXX, XXX−XXX

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Figure 8. TEM micrograph of the P1 self-assembly morphology in ethanol at the concentrations of 5 mg/mL (A, C) and 50 mg/mL (B, D).

Figure 6. (A) Schematic diagram of interactions between different groups of P1; (B) schematic diagram of through-space conjugation of P1 (first generation).

Table 2. TD-DFT Calculation Results of Oscillator Strengths for the First-Generation P1, P2, and P3 Molecules oligomer

P1

P2

P3

excitation energy (nm) oscillator strengths

200.27 0.0162

200.86 0.0046

208.30 0.0078

Figure 9. Fluorescence change (ΔI/I0) of P1 ethanol solution (10 mg/mL) with different metal ions (1 × 10−3 mol/L), and the relationship between ΔI/I0 and the concentration of Fe3+.

groups (HO−CH2−CH2−CH2−OH) from malonic acid (Figure S2A) and 1,3-propanediol (Figure S2B) also disappeared in P2 and P3. The above results are strong evidence to prove that the target oligomers P1, P2, and P3 were successfully synthesized. 13 C NMR Study. Figure 3 shows the 13C NMR spectra of malonic acid, A-151, and P1. In Figure 3b, the peak of C1, C6, and C8 related to the primary carbon CH3−CH2−O−Si can be observed at 18.52 ppm, whereas the peak of C2, C5, and C7 related to CH3−CH2−O−Si can be found at 58.31 ppm. The peaks of the carbon related to the vinyl group Si−CHCH2 at 137.18 and 130.18 ppm are separately corresponding to C4 and C3. As illustrated in Figure 3c, it can be seen that the carbon peaks marked by 1&2 and 3 at 168.88 and 42.36 ppm are respectively corresponding to the carboxy group (HOOC− CH2−COOH) and the methylene group (HOOC−CH2− COOH). Finally, in Figure 3a, the peaks related to the primary carbon CH3−CH2−O−Si at 18.04−18.35 ppm are corresponding to C1, the peak of carbon related to CH3−CH2−O− Si at 58.48 ppm is observed toward C2, the carbon peaks

Figure 7. Schematic diagram of the fluorescence mechanism for P1, P2, and P3.

phenomenon is that the proton peak at 12.56 ppm pertaining to the carboxy group (HOOC−CH2−COOH) of malonic acid (Figure 2b) disappeared in P1. In addition, the proton peaks of the carboxy group (HOOC−CH2−COOH) and the hydroxy D

DOI: 10.1021/acs.macromol.9b00263 Macromolecules XXXX, XXX, XXX−XXX

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Figure 10. (A) Emission spectra of P1 in various solvents at the concentration of 20 mg/mL; (B) emission spectra of P1 in the water−ethanol solution (10 mg/mL) at different pHs.

related to CH2CH−Si−O at 136.44 and 130.48 ppm are corresponding to C4 & C3, the peak of carbon related to the secondary carbon OOC−CH2−COO at 30.87 ppm is corresponding to C5, and the carbon peak related to OOC− CH2−COO at 167.03 ppm can be assigned to C6. It is interesting to note that the peaks related to the ester group (OOC−CH2−COO) and the methylene group (OOC−CH2− COO) at 167.03 and 30.87 ppm of P1 are weaker than those of malonic acid. Similarly, we also checked the 13C NMR spectra of TEOS, malonic acid, and P2 (Figure S3A) and of A151, 1,3-propanediol, and P3 (Figure S3B). Likewise, the carbon peaks of the ester group (OOC−CH2−COO) and the methylene group (OOC−CH2−COO) of P2 are weaker than those of malonic acid, and also the carbon peaks related to (O−CH2−CH2−CH2−O) and (O−CH2−CH2−CH2−O) of P3 are weaker than those of 1,3-propanediol. These are also important evidences to prove that the target oligomers have been successfully synthesized. GPC Study. The GPC data for P1−P3 are shown in Figure S4 and Table S1. It is obvious that all of the oligomers exhibit a broad molecular distribution. The polydispersities of the three oligomers are 6.42, 9.74, and 12.56 for P1−P3, respectively, whereas the weight average molecular weights (Mw) of the three oligomers are 11 600 for P1, 13 200 for P2, and 16 100 for P3, and the number average weights (Mn) are 1800, 1300, and 1200 for P1−P3, respectively, which indicated that the reactions have been smoothly accomplished. Optical Properties. The optical properties of P1 were analyzed in depth. As illustrated in Figure 4, the fluorescence intensity of P1 shows a concentration-enhanced feature from 2 to 200 mg/mL under the excitation of a UV lamp at 365 nm, which implies an aggregation-enhanced emission (AEE).28 Then, the UV−vis absorption spectra of P1 at various concentrations were investigated. As presented in Figure 5A, there is only one strong absorption peak at 215 nm (peak 1) for P1 at the concentration of 2 mg/mL, which can be presumably attributed to the n → π* electronic transitions between the carbonyl or oxy groups from the P1 backbone. When the concentration of P1 increases, peak 2 from the π → π* electronic transitions between the ester, carbonyl, or vinyl groups gradually appears, while the absorption intensity enhances and peak 1 is clearly red-shifted (peak 1 → peak 1*); this can be attributed to the progressively amassed carbonyl or vinyl groups and the newly formed coordination bonds between silicon−oxygen and heteroatoms, the so-called “electronic delocalization system”,29 which lead to easier electronic transition and lower absorption energy consumption.

To further study the photophysical properties of P1, we analyzed the emission spectra of P1 in ethanol solution at different concentrations and excitation wavelengths. As depicted in Figure 5B, the emission spectra of P1 show an excitation-dependent behavior, which progressively red-shifted with the gradual increase of the excitation wavelengths from 300 to 380 nm. This can be attributed to the huge heterogeneity and broad molecular distribution of the oligomer.30 In addition, the excitation (Ex) and emission (Em) spectra of P1 in ethanol solution were also detected at different concentrations ranging from 2 to 200 mg/mL, as shown in Figure 5C; the excitation bands red shift for about 90 nm along with the increase of the concentration. This trend is consistent with the UV−vis absorption spectra (Figure 5A); meanwhile, all the emission bands stay at about 430 nm and their intensity evidently enhances along with the increase of the concentration. It shows an apparent aggregation-enhanced emission identical to that seen in Figure 4, which is in contrast to the ACQ effect normally encountered in conventional luminescent materials. The reason for this phenomenon could be that the higher the concentration, the tighter the aggregation of functional groups (such as carbonyl groups). What’s more, we checked the photophysical properties of pure P1; as we can see in Figure 5D, the emission intensity is much higher compared with that in dilute solution. The Ex band for pure P1 centered at 350 nm with a shoulder peak at 330 nm (λem = 385 nm), whereas the Em band centered at 385 nm (λex = 350 nm). The fluorescence lifetime and the absolute fluorescence quantum yield for pure P1 were also tested by a steady-/transient-state fluorescence spectrometer tied to an integrating sphere. As shown in Table 1, the fluorescence lifetime is 0.85 ns for pure P1. To our surprise, the absolute fluorescence quantum yield for P1 excited at 348 nm is 43.9%, which is the highest among silica-containing hyperbranched fluorescent polymers reported to date to the best of our knowledge,31 and even comparable to that of traditional conjugated luminescent materials. Subsequently, to get more insights into the relationship between the functional groups and the luminescence of P1, we investigated the photophysical properties of the reference oligomers P2 (with carbonyl groups only) and P3 (with vinyl groups only). As shown in Figure S5A, the emissions for P2− P3 are much weaker compared with those of P1 under a UV lamp at 365 nm, which can also be proved by the PL and UV− vis absorption spectra (Figure S5B,C); it is worth noting that the absorption wavelength for P1 is longer than those for P2 and P3, which clearly suggests that P1 has the strongest conjugation among the three oligomers. The fluorescence E

DOI: 10.1021/acs.macromol.9b00263 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules lifetimes and the absolute fluorescence quantum yields for P2− P3 were also checked for a comparison, as we can see in Table 1, which are 16.3% and 4.29 ns for P2 and 10.5% and 4.16 ns for P3, respectively; these results show that the synergism of vinyl and carbonyl groups could be the reason to the high quantum yield of P1. Theoretical Calculations. To theoretically verify the fluorescence mechanism of P1, we used the density functional theory (DFT) to determine the energy levels of P1−P3 in optimized conformations. To simplify the calculation of P1, we only examined four models, in which the first-generation molecules increased from 1 to 4. Their structures are listed in Figure S7. Obviously, the optimized structure shows that the molecules are clustered because of the strong intermolecular H···O interactions (2.415, 2.686, 2.733, 2.760, 2.789, and 2.862 Å) (Figure S8), the interactions between O → Si molecules,32 and carbonyl and carbon−carbon double-bond groups (Figure 6A). Thus, there is a strong through-space conjugation33−35 between the oligomers (Figure 6B). The highest unoccupied molecular orbital−lowest unoccupied molecular orbital energy levels of various molecules with different conformations of P1 were also calculated, and the results are given in Table S2. Obviously, the band gap decreases with the increase of molecules. Therefore, the oligomers will be easier to be brought to the excited state and most likely to return to the ground state in the form of fluorescence due to their rigid conformations (Figure S9). Finally, we made a comparison between P1−P3, and the results showed that the aggregation of P2 and P3 is looser and they have a higher band gap than P1 (Figure S10 and Table S3), which are in accordance with the lower quantum yield we detected for the two oligomers. Then, we used the TD-DFT calculations based on the optimized ground geometries to investigate the oscillator strength of the first-generation oligomers of P1−P3 at the excited states (Sn). As shown in Figure S11, the trend of the calculated UV spectra agrees well with the experimental data (Figure S5C) despite only the first-generation models being used. Furthermore, the oscillator strength for P1 is the strongest among the three oligomers (Table 2), which means that, after being excited, P1 consumes the least excitation energy through the nonradiative channel and thus has the highest fluorescence intensity and quantum yield (Figure 7). These results, together with the results presented above, show that the synergism of vinyl and carbonyl groups as well as the Si−O group promotes the through-space conjugation, and the formation of supramolecular hyperbranched polysiloxane plays an important role in the strong photoluminescence and high quantum yield of P1. TEM Study. TEM was used to verify the proposed fluorescence mechanism of P1; we analyzed the microstructure morphology of P1 at different concentrations in ethanol solution. As shown in Figure 8A,C, P1 assembles into loose flocculent amorphous structures at a low concentration of 5 mg/mL, and the chains of the oligomer could move freely; it would be difficult to generate the through-space conjugation and most of the excited energy will be consumed by nonradiative transition at this concentration. However, when the concentration increases to 50 mg/mL, the oligomers gather to form tighter supramolecular polymers such as the spherical particles shown in Figure 8B,D, having an average diameter of 200−400 nm, and then, the strong through-space conjugation and hardening conformation is generated owing to their

intermolecular hydrogen bonds; thus, most of the excited energy will be converted into fluorescence by the radiative transition. This phenomenon well verified the calculations we performed before. Ion Probe. It has been reported that the fluorescence of polycarbonate, polyester, and their derivatives can be quenched by metal ions.17,36 These researches inspired us to explore the potential application of P1 as an ion probe. Therefore, the fluorescence responses of P1 (10 mg/mL, 8:2 water−ethanol solution) to Ba2+, Na+, Ca2+, Hg2+, Cd2+, Al3+, Fe3+, Cu2+, Zn2+, Co2+, and Fe2+ ions (1 × 10−3 mol/L) were detected. The results clearly signified that the Fe3+ ion quenched the fluorescence the most compared with the blank sample (Figures 9 and S12; ΔI = I0 − I; I0 is the emission intensity of P1 in water−ethanol solution without any metal Ions; I is the emission intensity of P1 in water−ethanol solution with different ions). The mechanism for this phenomenon could be that the HBPSi−Fe3+ complex is more stable and the intermolecular charge transfer from Fe3+ to HBPSi occurs due to the large charge radius ratio of Fe3+, which leads to the fluorescence quenching (Scheme S1). Then, we analyzed the relationship between the fluorescence of P1 and the concentration of Fe3+ by titration experiments, which showed that the emission intensity of P1 decreased linearly with the increase of the concentration of Fe3+ ion (1 × 10−5 to 5 × 10−4 mol/L, Figures 9 and S13); thus, P1 could be a promising probe for the detection of Fe3+. To further study the fluorescence “turn-on” sensing strategy of P1, we put a strong metal-complexing agent Na2EDTA (1 × 10−3 mol/L) into the P1−Fe3+ solution, which cooperated with Fe3+ and destroyed the HBPSi−Fe3+ complex, as we can see in Figure S14, and the fluorescence intensity of P1 is clearly restored. Solvent Effect and pH Dependency. The effects of the solvent and pH on the fluorescence of P1 were also analyzed. P1 was dissolved in different solvents (20 mg/mL) including N,N-dimethylformamide, ethanol, tetrahydrofuran (THF), Nmethyl-2-pyrrolidone (NMP), and dimethyl sulfoxide (DMSO). As shown in Figure 10A, the fluorescence intensity increases with the decrease of the solvent polarity, which is in accordance with the fluorescence quantum yield of P1 (QY = 1.61% in NMP and QY = 1.05% in DMSO; Figure S15). The mechanism for the solvent effect is that the lower the polarity, the closer the P1 molecules gather together. Then, the free movement between molecules is limited, and the energy will be more dissipated in the form of fluorescence. As seen in Figure 10B, the fluorescence intensity of P1 in the water−ethanol solution (10 mg/mL) is the highest under neutral condition (pH = 6.8) and lower under acidic (pH = 2.5, 4.0) or alkaline (pH = 8.8, 9.9) conditions. Similarly, we also checked the fluorescence quantum yield of P1 at different pHs (pH = 6.8, QY = 1.29%; pH = 2.5, QY = 0.98%; Figure S15). The pH dependency property could be attributed to the existence of H+ and OH−, which changes the intermolecular force and aggregation structure of P1, and thus affects the fluorescence property; what needs to be pointed out is that the emission peak of P1 in different solvents and pHs are slightly different, which can be attributed to the different sizes of P1 in different environments.

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CONCLUSIONS In summary, a new NTIL hyperbranched polysiloxane (P1) with a high absolute fluorescence quantum yield (43.9%) has been successfully synthesized. The comparison between the F

DOI: 10.1021/acs.macromol.9b00263 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

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photophysical properties of P1 (carbonyl and vinyl groups) and the reference oligomers P2 (carbonyl groups only) or P3 (vinyl groups only) showed that the remarkable properties can undoubtedly be attributed to the mutual stacking of the π-track on carbonyl and vinyl groups, the n−π effect between oxy and carbonyl groups, and the electronic conjugation between Si−O and other groups. These interactions played an important role in the through-space conjugation, which promoted the formation of supramolecular hyperbranched polysiloxane, increased the oscillator strength, and decreased the band gap of P1, and thus led to its high fluorescence quantum yield. In addition, the selective quenching to Fe3+ among different metal ions endows P1 with the potential for application as an Fe3+ ion probe. Besides, P1 also exhibits solvent effect and pH dependency properties, with a higher quantum yield in lowpolarity solvents or under neutral condition. This study provides new insights into the design of high-absolutequantum-yield unconventional fluorescent polymers.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.9b00263.

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Experimental details, FTIR spectra, NMR spectra, UV− vis spectra, Em spectra, absolute fluorescence quantum yields, GPC data, optimized molecular conformations, fluorescence mechanism, and DFT calculations (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.Y.). *E-mail: [email protected] (W.F.). ORCID

Hongxia Yan: 0000-0001-7432-2385 Author Contributions †

Y.F. and T.B. contributed equally to this article.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by the National Natural Science Foundation (21875188), the Natural Science Basic Research Plan in Shaanxi Province of China (2018JM2024), China Postdoctoral Science Foundation (2018M633564), and the Seed Foundation of Innovation and Creation for Graduate Students in Northwestern Polytechnical University (ZZ2019223).

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DOI: 10.1021/acs.macromol.9b00263 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.9b00263 Macromolecules XXXX, XXX, XXX−XXX