Unanticipated Strong Blue Photoluminescence from Fully Biobased

Research Article pubs.acs.org/journal/ascecg

Unanticipated Strong Blue Photoluminescence from Fully Biobased Aliphatic Hyperbranched Polyesters Yuqun Du, Hongxia Yan,* Wei Huang, Fu Chai, and Song Niu 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: Nonconventional fluorescent polymers without πaromatic 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 biobased aliphatic hyperbranched polyesters via a one-pot A2 + B3 esterification reaction. Intriguingly, the resultant hyperbranched polyesters carrying no conventional fluorescent units exhibited unexpected bright blue fluorescence under 365 nm UV light. It was found that the fluorescence intensity was enhanced with increasing solution concentrations and molecular weights of the polyesters. Moreover, the results suggested that the luminescence of polyesters could be controlled by solvents and metal ions. In particular, the fluorescence of the polyesters was extremely sensitive to Fe3+. More interesting is that the fluorescence of the polyesters showed an aggregation-induced enhanced emission in the mixture system. Notably, the resulting polyesters displayed a remarkably enhanced quantum yield (16.75%) as compared with that of other types of these polymers. Preliminary results demonstrate that 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 biobased luminescent materials to new application areas. KEYWORDS: Fluorescent hyperbranched polyester, Biomass materials, Blue light, Clustering-induced emission (CIE)

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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 increasing attention because it was found that these polymers can emit strong fluorescence under appropriate conditions.1 As compared with the conventional 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)s,9 poly(amido acid)s,10 and poly(amino ester)s.11,12 The luminescence of these polymers is closely related to the N-branched tertiary amine moiety, © 2017 American Chemical Society

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 siloxanepoly(amidoamine) (Si-PAMAM) emits 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 and hydroxyl and epoxy groups,17 could also emit bright blue light. Additionally, some linear luminescent polymers lacking traditional fluorescent units have been reported, such as dithiol/ amino-succinimides,18 sulfonated acetone-formaldehyde condensate,19 and sulfonated ethylenediamine-acetone-formaldeReceived: April 4, 2017 Revised: May 26, 2017 Published: June 12, 2017 6139

DOI: 10.1021/acssuschemeng.7b01019 ACS Sustainable Chem. Eng. 2017, 5, 6139−6147

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ACS Sustainable Chemistry & Engineering hyde condensate.20 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, exploration of the mechanism is still in its infancy. As a result, the design and synthesis of polymers without conventional fluorescent moieties are of crucial importance 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 biomedical applications, such as paints, adhesives, laminates, surface coatings, and inks. Recently, renewed interest has been given toward the hyperbranched polyester, owing to the fact that many of their monomeric building blocks can be obtained from renewable sources. Generally, diverse biomass multifunctional aliphatic alcohols, such as glycerol, pentaerythritol, xylitol, and erythritol, and multifunctional acids including sebacic, aconitic, succinic, adipic, citric, glutaric, and azelaic, have been studied in recent years.22−24 Among these sustainable materials, glycerol is a byproduct of the interesterification of vegetable oil and animal fats into biodiesel,25 and the byproduct glycerol will likely increase drastically with increasing biodiesel production in the coming decades.26 Thus, the effective utilization of glycerol is key to helping compensate for the expense of biodiesel production. To date, a few studies regarding biobased hyperbranched polyesters have been reported,27−33 but research on their luminescent properties has been rare. Therefore, the design and synthesis of novel biobased hyperbranched polyesters using renewable sources are intriguing and significant for providing new potential application fields. In this work, the fully biobased hyperbranched polyesters were synthesized by a step-growth polymerization reaction with succinic or adipic acid and glycerol through the A2 + B3 strategy. Surprisingly, fully biobased 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; here, we carefully investigated their fluorescence properties and corresponding emission mechanism for the first time.

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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. Fluorescence 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 Biobased Hyperbranched Polyesters. The synthetic procedure of biobased hyperbranched polyesters is as follows: First, 0.6 mol glycerol (54.7015 g), 0.48 mol succinic acid (56.6832 g), 0.15 mass % dibutyltin oxide, and 30 mL of toluene were added to a 250 mL four-necked round-bottomed flask fitted with mechanical stirring, a thermometer, a water segregator, and an N2 gas inlet at room temperature. Then, the mixture was heated slowly to ∼100 °C to completely dissolve the raw material. Thereafter, heating was used to continue increasing the mixture temperature to around 150 °C to further the reaction and remove the water. The mixture was kept at this temperature for 4 h. Then, the resulting polymer was discharged into a vial when the system temperature was cooled to ∼60 °C. Finally, the pale yellow biobased hyperbranched polyester was prepared and named SG-1. As the molar ratio of glycerol/succinic acid was changed to 1.5:1 and 1.8:1, the hyperbranched polyesters were synthesized and denoted SG-2 and SG-3, respectively, according to the experiment process described above. 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 of toluene on the basis of the synthesis procedure of SG-1. AG-1 and AG2 were synthesized based on the molar ratio of glycerol/adipic acid of 1.5:1 and 1.8:1, respectively. The synthesis route of biobased hyperbranched polyesters is illustrated in Scheme 1.

Scheme 1. Synthesis Route of Biobased Hyberbranched Polyester

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, dimethylformamide (DMF), N-methyl pyrrolidone (NMP), and 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 and 13 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 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 measurements were 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

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RESULTS AND DISCUSSION FTIR Study. The FTIR spectra of hyberbranched polyesters are displayed in Figure 1. The strong and broad absorption peak at ∼3408 cm−1 is assigned to the stretching vibrations of OH groups. A characteristic band at 1732 cm−1 belonged to the stretching vibrations of the aliphatic ester group (O−CO), which is produced by the polycondensation reaction between carboxylic and hydroxyl groups. The absorption bands at 2961 and 2864 cm−1 illustrate the presence of CH2 in these polymers. The peak at 1406 cm−1 is related to the stretching vibrations of C−O bands. The identical characteristic peaks are 6140


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those of an early report by Ankur et al.35 13NMR and 1NMR analyses of SG-1 are displayed in Figure S1. Optical Properties. The as-prepared biobased hyperbranched polyesters were dissolved in methanol, showing colorless and transparent under daylight, as exhibited in Figure S2. The UV−vis absorptions of SG-1 and AG-1 solutions were measured as shown in Figure 4 (a). Both the 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 a wavelength higher than 300 nm is observed in polyesters. Nevertheless, it is very intriguing that 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 that of the AG-1 solution under the same excitation wavelength (λex = 350 nm). Furthermore, SG-1 solution has wider emission peaks than those of AG-1 solutions but exhibit the same bright blue light. Clearly, 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 possibly multiple emission species exist in both the SG-1 and AG-1 solutions. The 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 is enhanced with increasing Mw (Figure S3) at the same concentration of 100 mg/mL. From the inset in Figure 4 (c), we can see that AG-1, which has a largest Mw (Mw = 4115) shows the strongest blue fluorescence, whereas the AG-3 (Mw = 3177) solution provides the lowest one, implying that the Mw of hyperbranched polyester has 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 biobased hyperbranched polyester solutions has Mw-dependent behavior. This fluorescence emission phenomenon was also detected in hyperbranched polyethers.37 Additionally, this result is distinct from the reported linear polyacrylonitrile, whose molecular weight has a very small impact on the photoluminescence 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 are maintained at the same location, suggesting that these polyesters have analogous emission mechanisms. Moreover, the presence of

Figure 1. FTIR spectra of polyesters.

also found in all polyesters, which indicate that various types of polyesters formed in all cases. 13 C NMR Study. Glycerol possesses two primary OH groups and one secondary OH group, which have very different reaction activities toward the esterification reaction. Thus, esterification of glycerol and diacid could produce five sorts of expected structures, as shown in Figure 2. Specifics 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. 13C NMR analysis in DMSO-d6 is utilized to explore the structures of the polyesters. A representative 13C NMR spectra of AG-1 shown in Figure 2 shows that the acid and ester carbonyl resonances are located at 174.4 and 172.8 ppm, respectively. 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 are displayed in the inset of Figure 2. 13C 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, and 69.2 ppm for D, which were assigned in accordance with previous literature.34 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 attributed to the methyne proton CH−O(CO) of D, L1,2, T1,3, and TG, respectively. Terminal unit methylene protons CH2−OH show a complex multiplet of signals in the region of 3.25−3.55 ppm. The assignments of protons of polyesters are consistent with

Figure 2. 13C NMR spectrum of AG-1. 6141


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Figure 3. 1H NMR spectrum of AG-1.

Figure 4. (a) UV−vis spectra of SG-1 and AG-1 solutions. (b) Fluorescence spectra of SG-1 and AG-1 solutions. (c) Fluorescence spectra of AG-1− 3 solutions. (d) Fluorescence spectra of SG-1−3 solutions. Concentration of 100 mg/mL. (inset) Photographs taken under 365 nm UV light; λex = 350 nm.

Figure 5. Fluorescence spectra of (a) AG-1 and (b) SG-1 solutions at different concentrations with λex = 350 nm.

three kinds of emission wavelengths indicates that multiple emission species may exist in these hyperbranched polyesters.

As exhibited in Figure S4, with an increase in the excitation wavelength from 340 to 400 nm, the emission bands show a 6142


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ACS Sustainable Chemistry & Engineering Scheme 2. Scheme of the Possible Fluorescent Mechanism of Hyperbranched Polyesters

Figure 6. Fluorescence spectra of the samples in a water−methanol mixture with different water fractions of (a) AG-1 and (b) SG-1 solutions at a concentration of 20 mg/mL with λex = 350 nm.

polymer chains are well spread, and it is 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 very weak or no luminescence. The polymer chains are readily close to each other and become entangled, which facilitates the formation of aggregated states in concentrated solutions. As a consequence, the clustering of carbonyl groups would occur. Furthermore, the carbonyl groups are in close proximity to each other in concentrated solutions, resulting in interactions and overlap between lone pair electrons and π electrons in the clusters to produce spatial conjugation, which further leads to enhancement of the electron delocalization and rigidity of the molecular configuration. At the same time, nonradiative relaxation is hindered to some extent. The result is that the polyesters are easy to excite 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 thus regarded as the precise fluorophores for the intriguing blue light emission from the polyesters. This emission behavior is called clustering-induced emission (CIE). The possible fluorescent mechanism toward the hyperbranched polyesters is described in Scheme 2. For further verifying the CIE mechanism, the linear polyester was prepared using 2-methyl-1,3-propanediol and succinic acid. There are many carbonyl groups in the linear polyester backbone, as shown in Scheme S1. Then, the prepared linear polyester was dissolved in DMF solvent, and the fluorescence spectra of solutions with different concentrations were tested, as displayed in Figure S6. Similarly, the fluorescence intensity of linear polyester solutions is enhanced with increasing concentration of the solutions. It is also because more carbonyl groups clusters form with increasing concentration of the linear

gradual bathochromic shift accompanied by a decreased in fluorescence intensity. This can be attributed to the existence of various aggregation states in the polyester solutions, which is similar to that of quantum dots. Effect of Concentration. It is observed that the dilute polyester solution (1 mg/mL) do not exhibit obvious blue emission upon irradiation. Visible weak blue emission is noted when the concentration is increased to a concentration of 20 mg/mL, as displayed in Figure S5. The intensity of blue light gradually brightens with increasing concentration of the solution. Moreover, the fluorescence spectra of both AG-1and SG-1 solutions at different concentrations are displayed in Figure 5. Interestingly, the fluorescence intensities were enhanced as both SG-1 and AG-1 solution concentrations were increased from 1 to 100 mg/mL, showing concentrationdependent behavior. Furthermore, this concentration enhancement emission phenomenon from the biobased hyperbranched polyesters is consistent with the previously reported PAMAM38 and Si-PAMAM.13 Moreover, only one emission band is detected at a concentration of 1 mg/mL, and the shoulder peak appears at a concentration of 10 mg/mL for both SG-1 and AG-1 solutions. The shape of emission bands became more obvious as the solution concentration increased. All of these results signify that certain polymer aggregates may be formed in the concentrated solution. Exploration of the Mechanism. Exploration of the exact fluorophores is key to deeper understanding of the amazing luminescence from the hyperbranched polyesters bearing no classic chromophores. It is well-known that there are hydroxyl, ester, and a few carboxyl groups in the structures of hyperbranched polyesters. Combining these 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 similar to that with Si-PAMAM13 and PMA.14 The 6143


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Influence of the Organic Solvent. As is known, environmental effects play an important role in luminescence behavior. Hence, the photophysical characteristics of the hyperbranched 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 intensities of both AG-1 and SG-1 solutions are successively enhanced in the order of methanol, THF, DMF, and NMP. Moreover, the fluorescence intensity of polyesters in NMP is much stronger than in other solvents, presenting solvent-dependent emission. In comparison with the fluorescence intensity of the polyesters in methanol solution, that of the polyesters in NMP solution is increased ∼5 times. Notably, when polyesters are dissolved in oxygenic solvents such as THF and methanol, the fluorescence intensity is less improved. When dissolved in NMP and DMF with electron-rich atoms, polyesters show enhanced fluorescence intensity, especially in NMP. As a consequence, we can deduce that more clusters of the carbonyl groups will form in solvent with electron-rich atoms, 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 leads to the generation of more clusters of carbonyl groups. Effect of the Metal Ions. For demonstrating the feasibility of utilizing biobased hyperbranched polyesters as a fluorescent probe for the detection of metal ions, the fluorescence spectra of the AG-1 solutions in the presence of varying metal ions were surveyed. All of the 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 considerably depending on the type of metal ion involved. The fluorescence intensity of the AG-1 solution barely changed after the addition of Al3+ ions. Furthermore, the decrease in fluorescence intensity is small for the Co2+, Ni2+, and Cu2+ ions but becomes more significant for Fe2+ and Fe3+, in particular, for Fe3+. Among these metal ions, Fe3+ can form the strongest interaction with AG-1 because it holds the largest charge/radius ratio;40 thus, it presents with the largest decrease in fluorescence intensity observed. Moreover, the influence of Fe3+ concentrations on the luminescence intensity of the polyester was also determined. The fluorescence intensity of the AG-1 solutions decreases drastically with increasing 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, which implies that the fluorescence of the 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+ reaches 1 × 10−2 mol/L.41 The fluorescence of hyperbranched polyesters is more sensitive to Fe3+ than that reported for SiPAMAM. Furthermore, the biobased hyperbranched polyester has many merits, including sustainable raw materials, easy preparation, and so forth. Therefore, hyperbranched polyester has significant potential development value for detecting Fe3+. QY and FL Study. The absolute quantum yield (QY) and fluorescence lifetime (FL) of pure SG-1 were measured, as presented in Figure 10. As seen in Figure 10 (a), emission

polyester solutions. This further confirms the proposed CIE mechanism. Fluorescence in the Water−Methanol System. The results presented above clearly demonstrate the critical role of the clustering of carbonyl groups to intrinsic emission of the hyperbranched polyester. For more information to be obtained on this proposed CIE mechanism, the fluorescence property of polyesters in the water−methanol mixture was investigated in detail. Water is adopted because it is a poor solvent for the polyesters, whereas methanol is a good solvent. As can be seen in Figure 6 (a) and (b), the fluorescence intensities of both AG1 and SG-1 sulutions increase as the volume fraction of water increases from 10 to 50%, demonstrating that the fluorescence intensity of polyesters exhibits aggregation-induced enhanced emission (AIEE). With increasing volume fractions of water, the polyesters are less soluble in the water−methanol system, but the solutions remain clear. During this process, the polyester becomes insoluble in the mixed system, thus entanglement and aggregation of the polyester chains occurs. Simultaneously, the clustering of carbonyl groups was also produced. In short, higher volume fractions of water result in more clusters of the carbonyl being generated, consequently providing much brighter emission. In addition, the fluorescence intensity of polyesters is enhanced until the water−methanol mixture becomes nontransparent. From Figure 6 (b), it is clearly observed that the emission peaks at different sites become visible as more water is added, whereas the SG-1 methanol solution exhibits a very wide emission peak. The same trend is also seen in the AG-1 solution. These also suggest that the clusters of carbonyls are formed in the water−methanol mixture, which help to rigidify the polyester conformation. Thus, intramolecular motion and rotation are suppressed; subsequently, nonradiative relaxation is deactivated. Hence, the emission energy is consumed by the radiative channel when excited by UV light. All of these results strongly support the proposed CIE mechanism for the photoluminescence of hyperbranched polyesters. Fluorescence of Raw Material. To gain insight into the fluorescence properties of the biobased hyperbranched polyesters, we also checked the fluorescence of the raw material with small molecules and further excluded disturbances 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.

Figure 7. (a) Fluorescence photographs of SA, AA, and G under a 365 nm UV lamp. (b) Fluorescence photographs of succinic acid aqueous solution under a 365 nm UV lamp.

Apparently, both the AA and G are nonemissive, whereas extremely weak fluorescence is emitted from SA. When pure SA is dissolved in water, the weak fluorescence disappears, as shown in Figure 7 (b). It is found that the fluorescence intensity of the SA solution is almost identical to that of ultrapure water (Figure S7). In terms of the reported thesis, this emission phenomena is in line with crystallization-induced phosphorescence (CIP).39 6144


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Figure 8. Fluorescence spectra of (a) AG-1 and (b) SG-1 dissolved in various solvents at a concentration of 20 mg/mL with λex = 350 nm.

Figure 9. Fluorescence spectra of AG-1 solution with (a) different metal ions and (b) Fe3+ at a concentration of 40 mg/mL with λex = 350 nm.

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

transient photoluminescence decay curve of the pure SG-1 is recorded at 383 nm after excitation at 346 nm (Figure 10 (b)). The decay curve is then fitted using double exponential functions R(t) based on a nonlinear least-squares analysis via

bands of pure SG-1 are separately centered at 383 and 402 nm excited at 346 nm (Figure S8). The fluorescence lifetime, a characteristic period, refers to the polyesters remaining in an excited state before returning to the ground state.42 The 6145



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ACKNOWLEDGMENTS This work is sponsored by the Innovation Foundation for Doctor Dissertation of Northwestern Polytechnical University (CX201719) and College students’ innovative experimental project of China (201610699226).

the equation R(t) = B1exp(−t/τ1) + B2exp(−t/τ2), where B1 and B2 are the fractional contributions of the time-resolved decay lifetime of τ1 and τ2, respectively. 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, and the decay curve of the pure AG-3 displays a single exponential with a lifetime of 3.05 ns (Figure S9). More impressively, the fluorescence quantum yield of the pure SG-1 is measured to be 16.75%. It is worth noting that the QY value is much higher than those of the reported hyperbranched polysiloxanes at 4.6143 and 3.68%15 and poly(amino esters) of 8.66%,12 even though the excitation wavelengths are different for these hyperbranched polymers.

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CONCLUSIONS In conclusion, we have designed and synthesized several types of novel, conventional fluorophore-free biobased 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 a 365 nm UV lamp. The fluorescence intensity of polyesters displays concentration- and molecular weight-dependent behaviors. Moreover, an AIEE phenomenon was observed in the water− methanol mixture. Importantly, organic solvents and metal ions can dramatically affect the photoluminescence properties. The biobased hyperbranched polyester could act as a fluorescent sensor for effectively detecting Fe3+. The fluorescence quantum yield of pure SG-1 is as high as 16.75%. This work provides a new perspective for deeply understanding the fluorescence properties of hyperbranched polyesters and for developing more novel CIE materials based on biomass. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01019. 13 C and 1H NMR spectra of SG-1, photographs of polyesters in methanol solutions, GPC curves of the polyesters, fluorescence spectra of AG-1 and SG-1 solutions, fluorescence 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 pure AG-1, and transient photoluminescence decay curve of pure AG-2 and AG-3 (PDF)

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Research Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. Tel: +8613720583261. ORCID

Yuqun Du: 0000-0002-5044-6540 Notes

The authors declare no competing financial interest. 6146


Research Article

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Unanticipated Strong Blue Photoluminescence from Fully Biobased

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