Unanticipated Strong Blue Photoluminescence from Fully Biobased

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Unanticipated strong blue photoluminescence from fully bio-based aliphatic hyperbranched polyesters Yuqun Du, Hongxia Yan, Wei Huang, Fu Chai, and Song Niu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b01019 • Publication Date (Web): 12 Jun 2017 Downloaded from http://pubs.acs.org on June 15, 2017

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ACS Sustainable Chemistry & Engineering

Unanticipated strong blue photoluminescence from fully bio-based aliphatic hyperbranched polyesters Yuqun Dua, Hongxia Yana*, Wei Huanga, Fu Chaia, Song Niua a

MOE Key Laboratory of Material Physics and Chemistry under Extraordinary

Conditions, Ministry of Education, School of Science, Northwestern Polytechnical University, Xi’an 710129, China a

Key Laboratory of Polymer Science and Technology, Shaanxi Province, School of

Science, Northwestern Polytechnical University, Xi’an 710129, China Tel: +8613720583261 *

Corresponding author, e-mail: [email protected]; [email protected].

Mailing addressa: School of Science, Northwestern Polytechnical University, Dong Xiang Road 1#, Changan District, Xi’an 710129, Shaanxi Province, China.

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ACS Paragon Plus Environment


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Abstract Non-conventional fluorescent polymers without π-aromatic structure have attracted extensive interest in recent years. Hyperbranched polyesters are generally known because of their industrial applications, however, the luminescent properties of the polyester has not been reported. Herein, we synthesized a series of fully bio-based aliphatic hyperbranched polyesters via one-pot A2+B3 esterification reaction. Intriguingly, the resultant hyperbranched polyesters carrying no conventional fluorescent units exhibit unexpected bright blue fluorescence under 365 nm UV light. It was found that the fluorescence intensity enhances as the increase of the solution concentrations and molecular weights of polyesters. Moreover, results suggested that the luminescence of polyesters could be controlled by solvent and metal ions. Especially, the fluorescence of polyester is extremely sensitive to the Fe3+. It is more interesting that the fluorescence of polyesters show an aggregation-induced enhanced emission (AIEE) in the mixture system. More notably, the resulting polyesters display a remarkably enhanced quantum yield (16.75%) as compared with that of other these types of polymers. Preliminary results demonstrate that the clustering of the carbonyl groups is responsible for the unusual fluorescence in the aliphatic hyperbranched polyesters, namely clustering-induced emission (CIE). This study provides a novel perspective for the design of bio-based luminescent materials to new application areas. Keywords: Fluorescent hyperbranched polyester, Biomass materials, Blue light, Clustering-induced emission (CIE) Introduction Organic photoluminescent materials have attracted considerable attention due to their superior photophysical properties and broad application fields, such as biosensor, cellular imaging, drug delivery, and so on. Conventional polymers are normally constructed by π-aromatic building blocks or composed of conjugated main chains serving as emitting units. Apart from the typical conjugated light-emitting polymers, recently, unorthodox luminogens carrying no classic chromophores have received more and more attention because it was found that these polymers can emit strong fluorescence under appropriate condition.1 As compared with the conventional 2


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polymers, unorthodox luminescent polymers have more advantages, including environmental friendliness, facile preparation, and excellent biocompatibility. Most of the existing research on heterodox luminescent polymers are dendritic and hyperbranched polymers bearing aliphatic tertiary amine moieties, for instance, poly(amidoamines)(PAMAM),2-3 polyethylenimine (PEI),4 poly(N-vinylpyrrolidone) (PVP),5 poly(propyl ether imine) (PPEI),6 polyurea (PU),7 hyperbranched poly(amido amine)s,8 poly(ether amide)s9, poly(amido acids)10 and poly(amino ester)s11-12. The luminescence of these polymers is closely related to the N-branched tertiary amine moiety, whose oxidation product by extra oxidizing reagent is considered to be ascribed to the emission center. On the side, some other polymers containing only ester and carbonyl groups rather than tertiary amine are also reported to be fluorescent. For example, Feng et al. reported that the siloxane-poly(amidoamine) (Si-PAMAM) emit strong blue photoluminescence, which is produced from the aggregation of carbonyl groups.13 Zhao et al. reported poly-[(maleic anhydride)-alt-(vinyl acetate)] (PMV), a pure oxygenic nonconjugated polymer. The emission of PMV is attributed to the clustering of the locked carbonyl groups.14 Furthermore, the hyperbranched polysiloxanes which contain different terminal groups, i.e. carbon–carbon double bonds and hydroxyl groups,15 hydroxyl and primary amine groups,16 hydroxyl and epoxy groups,

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could also emit bright blue light. In addition, some linear

luminescent polymers, lacking of traditional fluorescent units, have been reported, such as dithiol/amino-succinimides,18 sulfonated acetone-formaldehyde condensate19 and sulfonated ethylenediamine-acetone-formaldehyde condensate20. Very recently, Zhou et al. reported the luminescent properties of nonconjugated polyacrylonitrile, and found that the cyano clusters are conducive to generating light.21 Although a myriad of theories on the photoluminescent mechanism have been reported, the exploration of mechanism is still in its infant stage. As a result, the design and synthesis of polymers without conventional fluorescent moieties are of crucial important for exploring its mechanism. Because of its versatile properties and comparatively low cost, hyperbranched polyester, an important category of polymer, is extensively used in industrial and 3



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biomedical applications, such as paints, adhesives, laminates, surface coatings, and inks. Recently, renewed interest has been given towards the hyperbranched polyester, owing to many of their monomeric building blocks can be obtained from renewable sources. Generally, diverse biomass multifunctional aliphatic alcohols, such as glycerol, pentaerythritol, xylitol, erythritoland and multifunctional acids including sebacic, aconitic, succinic, adipic, citric, glutaric and azelaic, have been studied in recent years.22-24 Among these sustainable materials, the glycerol is a byproduct of the interesterification of vegetable oil and animal fats into biodiesel.25 And the byproduct glycerol would increase drastically with the increase of biodiesel production in the coming decades.26 Thus, the effective utilization of glycerol is a key to helping compensate the expense of biodiesel production. Up to now, a few of literatures about bio-based hyperbranched polyester have been reported.27-33 The luminescent property of bio-based hyperbranched polyester, however, has rarely been explored. Therefore, the design and synthesis of novel bio-based hyperbranched polyesters based on the renewable sources are intriguing and significant for opening new potential application fields. In this work, the fully bio-based hyperbranched polyesters were synthesized by a step-growth polymerization reaction with succinic or adipic acid and glycerol through A2+B3 strategy. Surprisingly, fully bio-based aliphatic hyperbranched polyesters can emit intense blue fluorescence under 365 nm UV light. To the best of our knowledge, the photoluminescence of hyperbranched polyesters has scarcely been reported. Herein, the fluorescence properties and corresponding emission mechanism of hyperbranched polyesters were carefully investigated for the first time. Experimental section Materials. Glycerol (G), succinic acid (SA) and adipic acid (AA) were purchased from Aladdin Chemistry Co. Ltd (Shanghai, China). FeCl3•6H2O and FeCl2•4H2O were purchased from Tianjin Fuchen Chemical Reagents Factory (Tianjin, China). CoCl2•6H2O, CuCl2•2H2O, Al(NO3)3•9H2O and Ni(NO3)2•6H2O were purchased from Sinopharm Chemical Reagent Co., Ltd (Xian, China). Toluene, methanol, dimethyl formamide (DMF), N-methyl pyrrolidone

(NMP) and 4


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tetrahydrofuran (THF) were supplied by Duangdong Guanghua Sci-Tech Co., Ltd (Duangdong, China). The chemicals were used as received without further purification. Characterization. Proton nuclear magnetic resonance (1H NMR,

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C NMR)

spectra were recorded on a Bruker Advance 400 spectrometer at 25 °C using DMSO-d6 as the solvent. Molecular weights were determined by a waters 1515 Gel Permeation Chromatography (GPC) equipped with a Waters 2414 Refractive Index Detector using a column system (styragel HT 3, 7.8×300 mm, Ireland). THF was used as an eluent, and the measurement was carried out at a flow rate of 1 mL min-1. UV-vis absorption spectra in methanol solution were detected using a SHIMADZU UV-2550 UV-Visible spectrophotometer. Fourier transform infrared spectra (FTIR) were recorded on a Bruker Tensor 27 infrared spectrophotometer using the KBr pellet technique within the 4000-400 cm-1 region. The luminescence (excitation and emission) spectra of the samples were collected with a Hitachi F-4600 fluorescence spectrophotometer using a monochromated Xe lamp as an excitation source. The slits of both excitation and emission were set at 5 nm, and the scan wavelength speed was 1200 nm min-1. Fluorescent excitation/emission spectra, fluorescence lifetime (FL) and absolute quantum yield (QY) of the pure hyperbranched polyesters were performed on a steady/transient-state fluorescence spectrometer equipped with an integrating sphere (FLS980, Edinburgh Instruments). Synthesis of Bio-based Hyperbranched polyesters. The synthetic procedure of bio-based hyperbranched polyesters is as follows: Firstly, 0.6 mol glycerol (54.7015 g), 0.48 mol succinic acid (56.6832 g), 0.15 mass% dibutyltin oxide and 30 mL toluene were added into a 250 mL four-necked round-bottomed flask fitted with a mechanical stirring, a thermometer, a water segregator and a N2 gas inlet at room temperature. Then, the mixture was heated slowly to about 100 °C to dissolve the raw material completely. Thereafter the heating was continued to raise the mixture temperature to around 150 °C to further reaction and remove the water. The mixture was kept at this temperature for four hours. Then, the resulting polymer was discharged into a vial when the system temperature was cooled to about 60 °C. Finally, 5



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the bio-based hyperbranched polyester with pale yellow was prepared and named as SG-1. As the molar ratio of glycerol/succinic acid was 1.5:1 and 1.8:1, the hyperbranched polyesters were synthesized, respectively denoted SG-2 and SG-3 according to the above experiment process. AG-1 was prepared using 0.6 mol glycerol (54.7015 g), 0.48 mol adipic acid (70.1472 g), 0.15 mass% dibutyltin oxide and 30 mL toluene on the basis of the synthesis procedure of SG-1. The AG-1 and AG-2 was synthesized based on the molar ratio of glycerol/ adipic acid was 1.5:1 and 1.8:1. The synthesis route of bio-based hyperbranched polyesters is illustrated in Scheme 1.

Scheme 1. Synthesis route of bio-based hyberbranched polyester Results and discuss FTIR study. The FTIR spectra of hyberbranched polyesters is displayed in Figure 1. The strong and broad absorption peak at about 3408 cm-1 is assigned to the stretching vibrations of OH groups. While a characteristic band at 1732 cm-1 belonged to the stretching vibrations of aliphatic ester groups (O-C=O), which is produced by the polycondensation reaction between carboxylic and hydroxyl groups. The absorption bands at 2961 cm-1 and 2864 cm-1 illustrate the presence of CH2 in the these polymers. The peak at 1406 cm-1 is related to the stretching vibrations of C-O bands. The identical characteristic peaks are also found in all polyesters, which indicate that various types of polyester formed in all cases.

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Figure 1. FTIR spectra of polyesters 13

C NMR study. Glycerol possesses two primary OH groups and one secondary

OH group, which have very different reaction activities to esterification reaction. So, esterification of glycerol and diacid could produce the five sorts of expected structures, as shown in Figure 2. The specific structures of the five expected structures are given below: (1) the primary monoester, which is a terminal unit, labeled TG; (2) the secondary monoester labeled T1,3, which is also a terminal unit, (3) the primary-primary diester linear unit is identified by L1,3; (4) the primary-secondary diester linear labeled L1,2; and (5) the trisubstituted dendritic labeled D. The 13C NMR analysis in DMSO-d6 is utilized to explore the structure of polyesters. A representative 13

C NMR spectra of AG-1 shown in Figure 2, displays the acid and ester carbonyl

resonances are respectively located at 174.4 and 172.8 ppm. The signals at δ=24.2 and 33.5 ppm are assigned to the methyne resonances of adipic acid units. The methyne peaks from the glycerol units, displayed in the inset of Figure 2. The

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C chemical

shifts are observed at 75.5 ppm for T1,3, 66.6 ppm for L1,3, 72.4 ppm for L1,2, 69.7, ppm for TG, 69.2 ppm for D, which were assigned in accordance with the previous literature34.

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Figure 2. 13C NMR spectra of the AG-1 1

H NMR study. As illustrated in Figure 3, the signals at 5.28 (i), 4.96 (r), 4.74 (n)

and 3.66 ppm (l) are respectively attribute to the methyne proton CH-O(C=O) of D, L1,2, T1,3 and TG. Terminal unit methylene protons CH2-OH shows a complex multiplet of signals in the region of 3.25-3.55 ppm. The assignments of protons of polyesters are consistent with early report by Ankur et al.35 The

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NMR and 1NMR

analysis of the SG-1 are displayed in Figure S1 of the Supporting Information.

Figure 3. 1H NMR spectra of the AG-1 Optical properties. The as-prepared bio-based hyperbranched polyesters were dissolved in methanol, showing colorless and transparence under daylight, as exhibited in Figure S2 of the Supporting Information. The UV-Vis absorption of SG-1 and AG-1 solutions was measured as shown in Figure 4 (a). Both of SG-1 and AG-1 solutions exhibit an absorption peak at around 284 nm, which is ascribed to the n-π* transition of the carbonyl groups.36 In addition, no absorption band at wavelength 8


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higher than 300 nm is observed in polyesters. Nevertheless, it is very intriguing that a bright blue luminescence is observed from both SG-1 and AG-1 solutions when they are excited by a 365 nm UV lamp. As displayed in Figure 4 (b), the fluorescence intensity of SG-1 solution is slightly stronger than AG-1 solution under same excitation wavelength (λex=350 nm). Again, SG-1 solution has more wide emission peak than AG-1 solutions, but they exhibit same bright blue light. Clearly, there are two emission bands at 386 and 404 nm and a shoulder peak at 430 nm are surveyed in both SG-1 and AG-1 solutions. This result indicates that the possible multiple emission species exist in the both SG-1 and AG-1 solutions. Effect of molecular weight (Mw) on the fluorescence intensity of the hyperbranched polyesters is presented in Figure 4 (c) and (d). It is observed that the fluorescence intensity of hyperbranched polyesters enhances as the increase of Mw (Figure S3 of the Supporting Information) under the same concentration of 100 mg/mL. From the inset of Figure 4 (c), we can see that the AG-1 which has a biggest Mw (Mw=4115) shows the strongest blue fluorescence, while AG-3 (Mw=3177) solution proves the lowest one, implying that the Mw of hyperbranched polyester have a large effect on its fluorescence intensity. The same tendency of the fluorescence enhancement also exists in glycerol-succinic acid series of polyesters, shown in Figure 4 (d). These results duly suggest that the fluorescence intensity of bio-based hyperbranched polyesters solutions has Mw-dependent behavior. This fluorescence emission phenomenon was also detected in hyperbranched polyether.37 Additionally, this result is distinct from the reported linear polyacrylonitrile, whose molecular weight has a very small impact on the photoluminescent emission.(21) This could be the result of the unique branched structure of the hyperbranched polyesters. It is noteworthy that the emission wavelengths of all the hyperbranched polyesters maintain at the same location, suggesting that these polyesters have analogous emission mechanisms. Moreover, the presence of three kinds of emission wavelength indicates that multiple emission species may exist in these hyperbranched polyesters. As exhibited in Figure S4 of the Supporting Information, with an increase in the excitation wavelength from 340 to 400 nm, the emission bands show a gradual bathochromic shifts accompanied by the 9



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decreased in fluorescence intensity. This can be attributed to the existence of various aggregation states in the polyester solutions, which is similar to the quantum dots.

Figure 4．．(a) UV-Vis spectra of SG-1 and AG-1 solutions. (b) Fluorescence spectra of SG-1 and SG-1 solutions. (c) Fluorescence spectra of AG-1, AG-2 and AG-3 solutions. (d) Fluorescence spectra of SG-1, SG-2 and SG-3 solutions. (at a concentration of 100 mg/mL; inset: Photographs taken under 365 nm UV light; λex=350 nm) Effect of concentration. It is observed that the dilute polyester solutions (1 mg/mL) do not exhibit obvious blue emission upon irradiation. The visible weak blue emission is noticed when the concentration is increased to the concentration of 20 mg/mL, as displayed in Figure S5 of the Supporting Information. The intensity of blue light is gradually bright with the increase of the solution concentration. Besides, the fluorescence spectra of both AG-1and SG-1 solutions at different concentrations are displayed in Figure 5. More interestingly, the fluorescence intensity enhanced as both SG-1 and AG-1 solutions concentration increase from 1 mg/mL to 100 mg/mL, showing concentration-dependent behavior. And this concentration enhancement emission phenomenon from the bio-based hyperbranched polyesters is consistent with 10


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the previous reported PAMAM38 and Si-PAMAM(13). Besides, only one emission band is detected at the concentration of 1 mg/mL, and the shoulder peak appears at the concentration of 10 mg/mL for both SG-1 and AG-1 solutions. The shape of emission bands became more obvious as the increase of the solution concentrations. All these results signify that the certain polymer aggregates may be formed in the concentrated solution.

Figure 5. Fluorescence spectra of AG-1 solutions (a) and SG-1 solutions (b) at different concentrations (λex=350 nm). Exploration of the mechanism. Exploration of the exact fluorophores is key to deeper understanding the amazing luminescence from the hyperbranched polyesters bearing no classic chromophores. It is well known that there are hydroxyl, ester groups and a few carboxyl groups in the structures of hyperbranched polyesters. Combining above mentioned results with the structure of the hyperbranched polyesters, it is reasonable to ascribe the intrinsic emission of polyesters to the aggregation of carbonyl groups in ester groups. The emission mechanism is very like with the Si-PAMAM(13) and PMA(14). The polymer chains are well spread and hard to generate aggregates in the dilute solutions. When being excited, the exciton energy will be efficiently wasted through the active intramolecular motion, resulting in the very weak or no luminescence. The polymer chains are readily close to each other and entangle which facilitate the formation of aggregated states in the concentrated solutions. As a consequence, the clustering of carbonyl groups would occur. Furthermore, the carbonyl groups, which are close proximity to each other in concentrated solutions, rendering interaction and overlap between lone pair electrons 11



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and π electrons in the clusters. So, a spatial conjugation is produced, which further leads to enhancing of the electron delocalization and rigidifying the molecular configuration. At the same time, the nonradiative relaxation is hindered to some extent. The result is that the polyesters are easy to be excited to generate much brighter light emission upon irradiation. Thereby, weak light is emitted from the dilute solutions, whereas the concentrated solutions can be highly emissive. In brief, the clustering of carbonyl groups in the aggregate states is regarded as the exact fluorophores for the intriguing blue light emission from the polyesters. This emission behavior is called as the clustering-induced emission (CIE). The possible fluorescent mechanism towards the hyperbranched polyesters is described in Scheme 2.

Concentrated solution

Scheme 2. Scheme of the possible fluorescent mechanism of hyperbranched polyesters In order to further verify the CIE mechanism, the linear polyester was prepared by using 2-methyl-1,3-propanediol and succinic acid. There are lots of carbonyl groups in linear polyester backbone, as shown in the Supporting Information Scheme S1. Then the prepared linear polyester was dissolved in DMF solution, and the fluorescence spectra of solutions with different concentration were tested, as displayed in the Supporting Information Figure S6. Similarly, the fluorescence intensity of linear polyester solutions enhances with the increase of solution concentration. It is also due to the more clustering of carbonyl groups form with the concentration of the linear polyester solution increasing. This is further confirmed the proposed CIE mechanism. Fluorescence in the water-methanol system. Above results clearly demonstrate that the critical role of the clustering of carbonyl groups to intrinsic emission of the 12


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hyperbranched polyester. In order to gain more information on this proposed CIE mechanism, the fluorescent property of polyesters in water-ethanol mixture was discreetly investigated. Water is adopted because it is a poor solvent for the polyesters, and methanol is a good solvent. As can be seen in Figure 6 (a) and (b), the fluorescence intensity of both AG-1 and SG-1 sulutions increases as the volume fraction of water increases from 10% to 50%, demonstrating that the fluorescence intensity of polyesters exhibits the aggregation-induced enhanced emission (AIEE). With increasing of the volume fractions of water, the polyesters are less soluble in the water-methanol system, but the solutions remain clarity. During this process, the polyester become insoluble in the mixed system, thus tangle and aggregation of polyester chains occurred. Simultaneously, the clustering of carbonyl was also produced. In short, the higher the volume fraction of water is, the more clusters of carbonyl are generated, consequently providing much brighter emission. In addition, the fluorescence intensity of polyesters enhances until the water-methanol mixture become non-transparent. From the Figure 6 (b), it is clearly observed that the emission peaks at different site are visible as the more water is added, whereas the SG-1 methanol solution exhibited a very wide emission peak. The same trend is also seen in the AG-1 solution. These also suggest that the clusters of carbonyl formed in the water-ethanol mixture, which give rise to rigidifying the polyester conformation. The intramolecular motion and rotation are suppressed, subsequently, non-radiative relaxation deactivates. Hence, the emission energy is consumed by the radiative channel when excited by UV light. All these results strongly support the proposed CIE mechanism for the photoluminescence of hyperbranched polyesters.

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Figure 6. Fluorescence spectra of the samples in water-methanol mixture with different water fraction: (a) AG-1 solution and (b) SG-1 solution (at the concentration of 20 mg/mL, λex=350 nm). Fluorescence of raw material. To insight into the fluorescence properties of the bio-based hyperbranched polyesters, we also checked the fluorescence of the raw material with small molecules and further excluded disturbance derived from the small molecules. All of the pure SA, AA and G were illuminated via 365 nm UV light, as shown in Figure 7. Apparently, both the AA and G is non-emission, while extremely weak fluorescence emits from the SA. When the pure SA dissolved into the water, the badly weak fluorescence disappeared, shown in Figure 7 (b). It is found that the fluorescence intensity of the SA solution is almost identical to that of the ultrapure water (Figure S7 of the Supporting Information). In terms of the reported thesis, this emission phenomena is in

well keeping with the crystallization-induced

phosphorescence (CIP).39 a

b SA

AA

G

Figure 7. (a) Fluorescent photographs of SA, AA and G under 365 nm UV lamp. (b) Fluorescent photographs of succinic acid aqueous solution under 365 nm UV lamp. Influence of the organic solvent. As is known, environmental effect plays an important role in the luminescence behavior. Hence, the photophysical characteristics of the hyperbrancched polyesters were tested in various organic solvents. Both AG-1 and SG-1 can be soluble in organic solvents including NMP, DMF, THF as well as methanol, as depicted in Figure 8 (a) and (b). The photoluminescence intensity of hyperbranched polyesters is greatly influenced by the solvent. It can be observed that the fluorescence intensity of both AG-1 and SG-1 solutions successively enhanced in the order of methanol, THF, DMF and NMP. Besides, the fluorescence intensity of polyesters in NMP is highly stronger than in other solvents, presenting 14


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solvent-dependent emission. In compare with the fluorescence intensity of the polyesters in methanol solution, that of the polyesters in NMP solution ascends to about 5 times. Notably, when polyesters are dissolved in oxygenic solvents such as THF and methanol, the fluorescence intensity is less improved. While dissolved in NMP and DMF with electron-rich atoms, polyesters show enhanced fluorescence intensity, especially in NMP. As a consequence, we can deduce that the more clusters of carbonyl groups will form in the electron-rich atoms solvent. And it reveals that the electron-rich atoms in solvent are involved in the aggregation of polymer chains, that is to say, electron-rich atoms promote the occurrence of aggregation of polymer chains, which lead to generate more clustering of carbonyl groups.

Figure 8. Fluorescence spectra of AG-1 (a) and SG-1 (b) dissolved in various solvent (at a concentration of 20 mg/mL, λex = 350 nm). Effect of the metal ions. In order to demonstrate the feasibility of utilizing bio-based hyperbranched polyesters as a fluorescent probe for the detection of the metal ions, the fluorescent spectra of the AG-1 solutions in the presence of varying metal ions was surveyed. All of metal ions were dissolved in methanol. The effect of metal ions on the fluorescence property of AG-1 is shown in Figure 9 (a). It is observed that the fluorescence intensity of the AG-1 solutions decreases in all cases, but the decrease varies remarkably with the kinds of metal ions. The fluorescence intensity of the AG-1 solutions barely changed after the addition of the Al3+ ion. And the decrease in fluorescence intensity is small for the Co2+, Ni2+, Cu2+, but become more significant for Fe2+ and Fe3+, in particular for the Fe3+. Among the above metal ions, Fe3+ can form the strongest interaction with AG-1 because it holds the largest 15



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charge/radius ratio,40 so the most decrease of fluorescence intensity is observed. Moreover, the influence of Fe3+ concentrations on the luminescence intensity of polyester was also checked. The fluorescence intensity of the AG-1 solution decreases drastically as the increase of Fe3+ concentration. An evident decrease is found at a concentration of 1×10-4 mol/L. The fluorescence of polyester completely disappears when the concentration of Fe3+ is 1×10-3 mol/L. It is imply that the fluorescence of hyperbranched polyester solution is sensitive to Fe3+ ions. The same fluorescence decrease phenomenon was also studied in Si-PAMAM, whose fluorescence is quenched as the concentration of Fe3+ reached 1×10-2 mol/L.41 The fluorescence of hyperbranched polyesters is more sensitivity for Fe3+ compare to the reported Si-PAMAM. What’s more, the bio-based hyperbranched polyester has a lot of merits, such as sustainable raw materials and easy preparation etc. Therefore, hyperbranched polyester has a very potential development value in detecting Fe3+.

Figure 9. (a) Fluorescence spectra of AG-1 solution with different metal ions; (b) Fluorescent spectra of AG-1 solution with Fe3+ (at a concentration of 40 mg/mL, λex = 350 nm). QY and FL study. The absolute quantum yield (QY) and fluorescence lifetimes (FL) of the pure SG-1 were measured, as presented in Figure 10. As seen in Figure 10 (a), emission bands of the pure SG-1 are separately centered at 383 and 402 nm under excited by 346 nm (Figure S8 of the Supporting Information). Fluorescence lifetime, a characteristic period, refers to the polyesters remain in its excited state before returning to its ground state.42 The transient photoluminescence decay curve of the pure SG-1 is recorded at 383 nm after excitation at 346 nm (Figure 10 (b)). The decay 16


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curve is then fitted using double exponential functions R(t) based on a non-linear least squares analysis via the following equation: R(t) = B1exp(-t/τ1)+B2exp(-t/τ2), where B1 and B2 are the fractional contributions of time-resolved decay lifetime of τ1 and τ2. Subsequently, the average fluorescence lifetime (τavg) of the hyperbranched polyesters was calculated by τavg = (B1τ12+B2τ22)/(B1τ1+B2τ2). The τavg of both pure AG-1 and SG-1 are calculated to be 6.49 and 6.88 ns, respectively. Moreover, the τavg of pure AG-2 reaches 5.60 ns while the decay curve of the pure AG-3 displays single exponential with a lifetime of 3.05 ns (Figure S9 of the Supporting Information). More impressively, the fluorescence quantum yield of the pure SG-1 is measured to be 16.75 %. It is worth noted that the QY value is much higher than the reported hyperbranched polysiloxanes with 4.61%43, 3.68%(15) and poly(amino esters) of 8.66%,(12) even though the excitation wavelengths are different for these hyperbranched polymers.

Figure 10. (a) Excitation spectra of the pure SG-1. The transient photoluminescence decay curve of the pure AG-1 (b) and SG-1 (c) at 383 nm after excitation at 346 nm. (d) The absolute fluorescence quantum yield of the pure SG-1 excited at 346 nm. 17



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Conclusions In conclusion, we have designed and synthesized several kinds of novel conventional fluorophore-free bio-based aliphatic hyberbranched polyesters through employing biomass materials including glycerol, succinic acid and adipic acid. Strong blue luminescence can be observed even with the naked eye from the synthetic polyesters when they are irradiated by 365 nm UV lamp. The fluorescence intensity of polyesters displays concentration and molecular weight-dependent behaviors. Besides, an AIEE phenomenon was observed in the water-methanol mixture. More importantly, organic solvent and metal ions can dramatically tune the photoluminescence properties. The bio-based hyperbranched polyester could act as a fluorescent sensor for effectively detecting the Fe3+. The fluorescence quantum yield of the pure SG-1 is as high as 16.75%. This work provides a new perspective to deeply understand the fluorescence properties on hyperbranched polyesters, and develop more novel CIE materials based on the biomass. Supporting information 13

C NMR spectra and 1H NMR spectra of the SG-1, photographs of polyesters

in methanol solutions, GPC curves of the polyesters, fluorescence spectra of AG-1 and SG-1 solutions, fluorescent images of SG-1 solutions with different concentrations, synthesis of linear polyester, fluorescence spectra of linear polyester in DMF solution, fluorescence spectra of deionized water and succinic acid aqueous solution, excitation spectra of the pure AG-1, transient photoluminescence decay curve of the pure AG-2 and AG-3 (PDF). Acknowledgement This work is sponsored by Innovation Foundation for Doctor Dissertation of Northwestern Polytechnical University (CX201719) and College students' innovative experimental project of China (201610699226). References (1) Yuan, W. Z.; Zhang, Y. M. Nonconventional macromolecular luminogens with aggregation-induced emission characteristics. J. Polym. Sci., Part A, Polym. Chem. 2017, 55, 560-574. 18


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For Table of Contents Use Only Synopsis The synthesized bio-based hyperbranched polyesters show an unexpected bright blue luminescence, which is closely related to the clustering of the carbonyl groups.

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Unanticipated Strong Blue Photoluminescence from Fully Biobased

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