Unexpected Strong Blue Photoluminescence Produced from the

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Unexpected Strong Blue Photoluminescence Produced from the Aggregation of Unconventional Chromophores in Novel Siloxane− Poly(amidoamine) Dendrimers Hang Lu, Linglong Feng, Shusheng Li, Jie Zhang, Haifeng Lu,* and Shengyu Feng* Key Laboratory of Special Functional Aggregated Materials & Key Laboratory of Colloid and Interface Chemistry (Shandong University), Ministry of Education; School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, P. R. China ABSTRACT: Poly(amidoamine) (PAMAM) dendrimers and hyperbranched poly(amidoamine)s are the first reported and most investigated luminescent polymers containing unconventional chromophores. The luminescence of these compounds is associated with the N-branched tertiary amine moiety, and the oxidation of the tertiary amine is assigned to the emitting source. However, in this paper, a series of novel siloxane− poly(amidoamine) (Si-PAMAM) dendrimers were synthesized by aza-Micheal reaction, and strong blue photoluminescence was observed even with the naked eye when these compounds were excited by a UV lamp. All of these compounds were not oxidated at all. Studies on the molecular structure showed that N → Si coordination bonds existed in these compounds, and those N → Si bonds caused the aggregation of carbonyl groups which show the strong luminescence. been reported that the poly(propyleneimine) dendrimer,14 which contains only alkyl amine units, can also emit blue fluorescence. These results suggest that oxidized aliphatic tertiary amine is the key to fluorescence emission. The XPS analysis about PAMAM before and after oxidation assumed that the chromophore is oxygen−amine “contact” donor−acceptor complex which forms during oxidation of aliphatic tertiary amines.9 Further study suggested N → O bonds formation based on the IR and 1H NMR spectra of hyperbranched poly(amine−ester) and its oxidized products. The luminescence of these compounds is associated with the N-branched tertiary amine moiety, and the oxidation of the tertiary amine is assigned to the emitting source. Meanwhile, some distinct kinds of polymers which containing carbonyl groups rather than aliphatic tertiary amines also show blue photoluminescence.1,16−18 These polymers do not show fluorescence properties in solution. However, in solids or viscous liquids, a strong photoluminescence could be observed. The photoluminescence is attributed to the aggregation and interaction of multiple carbonyl groups which was called the aggregation-induced emission (AIE).19 For example, it was reported that an unusual AIE phenomenon took place in polyisobutene succinic anhydrides and imides17 and poly[(maleic anhydride)-alt-(vinyl acetate)].18 Many series of silicone-containing luminescent macromolecules have been synthesized, and their luminescent properties have been investigated in our group.20 The introductions of Si

1. INTRODUCTION Luminescent polymers have numerous applications, including tunable lasers, displays, medical diagnostics, solar energy conversion, and amplifiers for optical communication. Luminescent polymers are often referred to these polymers which are constructed by conjugated main chain or composed by πaromatic building blocks, like benzene ring, thiophene, and fluorine, functioning as emitting units. Recently, a few kinds of polymers containing unconventional chromophores have been paid increasing attention because it was found that those polymers can demonstrate strong luminescence under proper conditions.1 Those unconventional chromophores include aliphatic tertiary amine, carbonyl, ester, and amide. Compared with the conventional luminescent polymers, the luminescent polymers containing unconventional chromophores are more environmentally friendly, easy to be prepared, and hydrophilic. Poly(amidoamine) (PAMAM) dendrimers and hyperbranched poly(amidoamine)s are the first reported and most investigated luminescent polymers containing unconventional chromophores.2,3 Lately, this kind of polymer has been extended to hyperbranched poly(amino−ester)s, hyperbranched poly(ether−amide)s, polyurea dendrimer, and supramolecular hyperbranched polymer containing tertiary amine moieties.4−8 The mechanism of photoluminescence in these polymers containing unconventional chromophores is still under investigation.7−15 Many studies show that aliphatic tertiary amines play a vital role in photoluminescence. It was found that the emission and fluorescence intensity of fourth-generation PAMAM with different terminal groups are nearly identical under the same condition. After treatment with (NH4)2S2O8, the fluorescence intensity of PAMAM was increased.3 It has also © 2015 American Chemical Society

Received: November 21, 2014 Revised: January 7, 2015 Published: January 28, 2015 476

DOI: 10.1021/ma502352x Macromolecules 2015, 48, 476−482

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Macromolecules Scheme 1. Synthetic Routes of Si-PAMAM Dendrimers

PAMAM. Strong blue photoluminescence can be observed even with the naked eye from low generation of Si-PAMAM without addition of extra oxidizing reagent. It is interesting that as the generation increased, the fluorescence emission intensity of the Si-PAMAM increased rapidly. Then the mechanism of fluorescence phenomena in Si-PAMAM was investigated. Results imply that the relevant mechanism is different from those described in previous works,4−7 which suggests that tertiary amines play a key role in photoluminescence. What is more, aggregation-induced enhanced emission phenomenon was observed in water−methanol system. Further studies suggest that fluorescence is closely related to the N → Si coordination bonds. In the Si-PAMAM, N → Si coordination bonds result in the aggregations of carbonyl groups, which cause strong fluorescence. This research aims to provide deeper under-

atom or siloxane group not only influence the stability of the dendrimer but also change the interspaces configuration which affect the luminescence subsequently. Based on the research about siloxane−poly(amidoamine) (Si-PAMAM) dendrimers, a proposal was suggested that if the interspaces configuration of the group in dendrimers could be adjusted by the introduction of the silicon atoms, the luminescence of dendrimers could be altered. In the present study, several generations of Si-PAMAM dendrimers were synthesized in high yield by alternant azaMicheal reaction and amidation reaction. Those dendrimers take 1,3-bis(3-aminopropyl)tetramethyldisiloxane(G0) as core and branch point. Their structures were confirmed by FTIR, 1H NMR, 29Si NMR, and LC/MS. Further investigation found that N → Si coordination bonds exist between Si and N atoms in Si477

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Macromolecules standing on the fluorescence of PAMAM dendrimer containing unconventional chromophores.

(ppm) 0.04 (s, 156H, SiCH3), 0.44−0.53 (m, 52H, SiCH2CH2CH2N), 1.42−1.51 (m, 52H, SiCH2CH2CH2N), 2.38 (t, 56H, CH2CO), 2.43 (t, 28H, SiCH2CH2CH2N), 2.78 (t, 56H, NCH2CH2CO), 3.18 (t, 24H, SiCH2CH2CH2NHCO). The real molecular weight of G2.0 was slightly lower than the calculated one because of the steric hindrance. As the product generation increased, steric hindrance increased and terminal groups were unable to react completely. 2.2.5. Preparation of Fluorescent Elastomer. A fluorescent elastomer was prepared via aza-Micheal reaction. 0.98 g of G2 and 0.4011 g of 1,4-butylene diacrylate were dissolved in methanol and stirred at 50 °C for about 3 days. The dissolution was placed into a PTFE-mold, and then the solvent was slowly volatilized under room temperature. To investigate the influence of the Si−O−Si in the PAMAM, hexamethylenediamine-based PAMAM (C-PAMAM) dendrimers (g0.5−g1.5) were synthesized according to the literature.21

2. EXPERIMENTAL SECTION 2.1. Materials and Methods. 1,3-Bis(3-aminopropyl)tetramethyldisiloxane (G0) was obtained as a commercial product and used directly. Methyl acrylate (MA), methanol, and petroleum ether were purchased from Tianjin Fuyu Chemical Co., Ltd. 1,4-Butylene diacrylate was provided by Sigma-Aldrich. All materials were used as received. Proton nuclear magnetic resonance (1H NMR) spectra and silicon nuclear magnetic resonance (29Si NMR) spectra were recorded on a Bruker AVANCE 400 spectrometer at 25 °C using CDCl3 as the solvent and without tetramethylsilane as an interior label. Molecular weights were determined by Aglient HP1100 LC-Applied Biosystems API 4000 TQ mass spectrometer (LC/MS). Ultraviolet absorption (UV) spectra in methanol solution were detected using a Beijing TU-1901 doublebeam UV−vis spectrophotometer. Fourier transform infrared spectra (FT-IR) 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 determined with a Hitachi F-7000 fluorescence spectrophotometer using a monochromated Xe lamp as an excitation source. The excitation and emission slits were both set at 5 nm. X-ray photoelectron spectroscopy was performed using a Thermo Fisher Scientific Escalab 250 spectrometer with a monochromated Al Kα X-ray source at a residual pressure of 10−7 Pa. The survey and high-resolution scans were generated at 100 eV pass energy with a 1 eV step and 20 eV pass energy with a 0.05 eV step, respectively. 2.2. Synthesis. Si-PAMAM dendrimers were synthesized in high yield by alternant aza-Micheal reaction and amidation reaction. The synthetic routes are shown in Scheme 1. 2.2.1. Synthesis of Si-PAMAM Dendrimers (G0.5). 2.61 g (10.52 mmol) of G0 and 5.39 g (62.67 mmol) of MA were added to 25 mL of methanol and then stirred at 50 °C for about 2 days. After the reaction was completed, the excess MA and methanol were distilled under reduced pressure at 50 °C. 5.99 g (10.13 mmol) of G0.5 was obtained (yield: 96%). 1H NMR (CDCl3): δ (ppm) 0.03 (s, 12H, SiCH3), 0.42 (t, 4H, SiCH2CH2CH2N), 1.38−1.44 (m, 4H, SiCH2CH2CH2N), 2.36− 2.45 (m, 12H, SiCH2CH2CH2NCH2), 2.76 (t, 8H, NCH2CH2CO), 3.64 (s, 12H, OCH3). 2.2.2. Synthesis of Si-PAMAM Dendrimers (G1.0). 2.76 g (4.66 mmol) of G0.5 and 10.19 g (41.09 mmol) of G0 were added to 25 mL of methanol and stirred at 50 °C for about 7 days. After the reaction was completed, the excess G0 was extracted three times by petroleum ether. Then, the solvent was distilled under reduced pressure at 50 °C. 5.36 g (3.68 mmol) of G1.0 was obtained (yield: 80%). 1H NMR (CDCl3): δ (ppm) 0.01 (s, 60H, SiCH3), 0.45 (t, 20H, SiCH2CH2CH2N), 1.34− 1.44 (m, 20H, SiCH2CH2CH2N), 2.30 (t, 8H, CH2CONH), 2.42 (t, 4H, SiCH2CH2CH2N), 2.65 (t, 8H, SiCH2CH2CH2NH2), 2.74 (t, 8H, NCH2CH2CONH), 3.15 (t, 8H, SiCH2CH2CH2NHCO). LC/MS (C62H148N10O9Si10): m/z 1458.9 [M]+ (calcd 1458.7). 2.2.3. Synthesis of Si-PAMAM Dendrimers (G1.5). 2.35 g (1.61 mmol) of G1.0 and 4.14 g (48.1 mmol) of MA were added to 25 mL of methanol and then stirred at 50 °C for about 3 days. After the reaction was completed, the excess MA and methanol were distilled under reduced pressure at 50 °C. 3.39 g (1.58 mmol) of G1.5 was obtained (yield: 98%). 1H NMR (CDCl3): δ (ppm) 0.04 (s, 60H, SiCH3), 0.45− 0.51 (m, 20H, SiCH 2 CH 2 CH 2 N), 1.38−1.50 (m, 20H, SiCH2CH2CH2N), 2.34−2.40 (m, 24H, CH2CO), 2.43 (t, 12H, SiCH2CH2CH2N), 2.74−2.78 (m, 24H, NCH 2), 3.17 (t, 8H, SiCH 2 CH 2 CH 2 NHCO), 3.46 (s, 24H, OCH 3 ). LC/MS (C94H196N10O25Si10): m/z 2148.3 [M]+ (calcd 2147.5). 2.2.4. Synthesis of Si-PAMAM Dendrimers (G2.0). 2.40 g (1.12 mmol) of G1.5 and 10.0 g (40.3 mmol) of G0 were added to 25 mL of methanol and stirred at 50 °C for about 7 days. After the reaction was completed, the excess G0 was extracted three times by petroleum ether. Then the solvent was distilled under reduced pressure at 50 °C. 3.51 g (0.91 mmol) of G2.0 was obtained (yield: 81%). 1H NMR (CDCl3): δ

3. RESULTS AND DISCUSSION Figure 1 shows the FTIR spectra of G0.5−G2.0. A characteristic band at 1100 cm−1, corresponding to Si−O bond and intense

Figure 1. IR spectra of Si-PAMAM dendrimers: (a) G0.5, (b) G1.0, (c) G1.5, and (d) G2.0.

bands near 1250 cm−1 attributed to Si-CH3, are observed. In G0.5 and G1.5, the band at 1735 cm−1 suggests the presence of ester groups. In G1.0 and G2.0, bands at 3292 and 3366 cm−1 indicate that primary amine groups are introduced to the dendrimers. The presence of the band at 1680 cm−1 and disappearance of the band at 1735 cm−1 in G1.0 and G2.0 imply that ester groups are converted into amide groups in the process of amidation from G0.5 to G1.0 and from G1.5 to G2.0. IR spectra show that compounds are successfully synthesized. The 29Si NMR spectra of those Si-PAMAM dendrimers are shown in Figure 2. The signal of G0 is located at 6.94 ppm, and the signal of G0.5 is found at 7.21 ppm. The shift from 6.94 to

Figure 2. 29Si NMR spectra of Si-PAMAM dendrimer (G0−G1.0). 478

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7.21 ppm suggests that the environments of silicon atoms are changed after the aza-Micheal reaction. A more complicated situation was observed in the 29Si NMR spectrum of G1.0. Three different signals were located at 7.28, 7.05, and 6.96 ppm. Three different signals mean that there are three different environments of silicon atoms in G1.0 when the ester bonds are turned into amido bonds. However, silicon atoms are not involved in all of the reactions. Thus, it is the variations of noncovalent bond which changed the environment of silicon atoms. According to the previous research,22 for example, in silatrane, silicon atom could form N → Si coordination bonds with nitrogen atom of tertiary amines by accepting unpaired p electrons of nitrogen atom. Compared with the structures of the silatrane, it is presumed that silicon atom and nitrogen atom could also form N → Si coordination bonds in Si-PAMAM because they all could form a pentacircle of coordination structure. The structure of N → Si coordination bond in G0.5 is shown in Scheme 2.

Si NMR spectra support the hypothesis above. The signal at 7.28 ppm represents the N → Si bonds formed by tertiary amine; the signal at 7.05 ppm represents the N → Si bonds formed by secondary amine; and the signal at 6.96 ppm represents the N → Si bonds formed by primary amine. When forming the N → Si coordination bond, the unpaired p electrons of nitrogen atom occupy the empty 3d orbital of silicon atom. The electron density and nucleophilicity of tertiary amine are higher than that of primary amine. Thus, the N → Si bonds formed by primary amine were weaker than the N → Si bonds formed by tertiary amine. So the Si signal shift to higher field (from 7.28 to 6.96 ppm). In order to investigate the existence of N → Si coordination bonds clearly, an elastomer cross-linked between G2 and 1,4butylene diacrylate was synthesized and then examined by XPS. Figure 3 shows the XPS survey spectrum of the elastomer. The peaks of Si 2p and N 1s are located at 102.3 and 399.3 eV, respectively. The peak of N 1s is asymmetric, which suggests the presence of different N atoms in the G2. This peak could be divided into subpeaks of 399.7 and 399.3 eV. According to the literature,21 the binding energy of Si 2p is 102.3 eV and the binding energy of N 1s is 399.7 eV when forming N → Si bond in CH3Si(OCH2CH2)3N. The binding energies of Si 2p and N 1s were similar to those described in the literature,21 which confirms the presence of N → Si bonds in G2.0. In the broad O 1s spectrum of the G2 elastomer, the O 1s peak is observed at 532.0 eV. Distinguishing the peaks of Si−O−Si O 1s or hydroxyl groups O 1s was difficult. The PAMAM dendrimers were dissolved in methanol, and the luminescent properties of the solutions were measured. The UV−vis absorption spectrum of the G2.0 dendrimer (Figure 4) shows two absorption bands centered at 209 and 279 nm,

Scheme 2. Structure of N → Si Coordination Bond in SiPAMAM

In the Si-PAMAM, the ratio of tertiary amine, secondary amine, and primary amine was 1:2:2, and each nitrogen atom can form a coordination bond with a silicon atom. The three different Si signals in G1.0 were corresponding to three kinds of N → Si coordination bonds. The ratio of Si signals at 7.28, 7.05, and 6.96 ppm was 1:2:2, which was the same to the ratio of amines. The

Figure 3. XPS spectra of the elastomer: (a) elastomer XPS survey spectrum, (b) XPS of the Si 2p spectral region, (c) XPS of the N 1s spectral region, and (d) XPS of the O 1s spectral region. 479

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Macromolecules

generations of Si-PAMAM dendrimers are highly different. In order to analysis the fluorescence more thoroughly, C-PAMAM dendrimers (g0.5−g1.5) were synthesized and their fluorescence was monitored. Figure 6 demonstrates the emission fluorescence spectra of SiPAMAM dendrimers (G0.5−G2.0) and C-PAMAM dendrimers

Figure 4. UV−vis spectrum of G2.0 Si-PAMAM dendrimer.

respectively. The absorption peak at 235 nm is contributed to π−π* transitions of carbonyl groups, and the absorption peak at 279 nm is contributed to n−π* transitions of carbonyl groups. All of the dendrimers have similar UV−vis absorption bands. Excited by UV light, the Si-PAMAM showed two different emissions. Figure 5 illustrates the excitation and emission

Figure 6. Fluorescence spectra of Si-PAMAM dendrimers (G0.5−G2.0) and C-PAMAM dendrimers (g0.5−g1.5) excited at 353 nm: (a) G0.5, (b) G1.0, (c) G1.5, (d) G2.0, (e) g0.5, (f) g1.0, and (g) g1.5.

(g0.5−g1.5) excited at the same conditions. It can be noticed that the luminescent intensities of Si-PAMAM dendrimers boosted up with the increase of generations, while C-PAMAM dendrimers hardly had any luminescence. Low generation of C-PAMAM dendrimers did not show strong photoluminescence in previous research. After the oxidization of aliphatic tertiary amines, high generation of PAMAM dendrimers show blue photoluminescence which was caused by peroxy radicals or excimers (the oxygen−amine “contact” donor−acceptor complex).9−12 However, in this paper, low generations of Si-PAMAM dendrimers have strong photoluminescence without extra oxidizing agents. Because of the absence of the oxidizing agents, aliphatic tertiary amines do not form oxygen−amine “contact” donor−acceptor complexes. So the luminescence is not caused by the oxidization of aliphatic tertiary amine and should be caused by the other unconventional chromophores, namely, carbonyl groups. Polymers containing carbonyl groups without aliphatic tertiary amines have been observed to show blue photoluminescence.1,16−18 The aggregation of carbonyl groups is supposed to cause the photoluminescence. According to the literature,17 the emission band occurs at about 430 nm and the excitation band occurs at about 360 nm in carbonyl-aggregation polymer systems. In the fluorescence spectra of Si-PAMAM dendrimers, strong blue photoluminescence located at 435 nm was observed by excitation at 363 nm. So a conclusion could be drawn that the aggregation of carbonyl groups took placed in the Si-PAMAM dendrimers and the aggregation caused the photoluminescence. Compared with C-PAMAM, the aggregation in Si-PAMAM is confirmed to be caused by the N → Si coordination bonds. Figure 6 illustrates that the fluorescence intensity of Si-PAMAM dendrimers is much stronger than that of C-PAMAM dendrimers. The amount of carbonyl groups in the C-PAMAM should be the same as those in Si-PAMAM when the generation is same. The reason for different fluorescence intensities between two kinds of PAMAM is attributing to different aggregations of

Figure 5. Fluorescence spectra of the G2.0 Si-PAMAM dendrimer: (a) Excitation spectrum emitted at 338 nm and emission spectrum excited at 285 nm. (b) Excitation spectrum emitted at 435 nm and emission spectrum excited at 363 nm.

fluorescence spectra of G2.0 Si-PAMAM dendrimers. G2.0 SiPAMAM dendrimer emits fluorescence in the spectral region of 338 nm under the excitation at 285 nm, while it emitted blue fluorescence in the spectral region of 435 nm under the excitation at 363 nm. All of the Si-PAMAM dendrimers showed similar fluorescence spectra. But the fluorescence intensities of different 480

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Macromolecules Scheme 3. Scheme of the Luminous Mechanism

carbonyl groups in them. The structure of Si-PAMAM molecules was flexible owing to the Si−O−Si group, and the fluorescence took placed because N → Si bonds enhanced the aggregation of carbonyl groups. Nevertheless, the structure of C-PAMAM is rigid and the aggregation of carbonyl groups could hardly take place without additional force. In Scheme 3, it is clearly that the aggregation of carbonyl groups took placed when N → Si coordination bonds formed. Figure 6 illustrates that the fluorescence intensity increases quickly as the Si-PAMAM generation increases. As the SiPAMAM generation increased, the amount of carbonyl groups increased quickly. These dendrimers have taken a densely packed globular configuration, and the carbonyl groups were much more aggregated, which caused the enhancement of the fluorescence intensity. This phenomenon reasonably confirms our hypothesis. In order to further confirm the hypothesis mentioned above, two experiments were designed to analyze the fluorescence intensity by changing the extent of carbonyl groups’ aggregation in the Si-PAMAM solution. One experiment was planned to change the concentration of the Si-PAMAM dendrimer. The fluorescence spectra of Si-PAMAM dendrimer (G2.0) at different concentration are shown in Figure 7. Figure 7a shows the fluorescence from Si-PAMAM dendrimer excited at 385 nm. Figure 7b shows the fluorescence from SiPAMAM dendrimer excited at 285 nm. Figure 7a illustrates that the fluorescence intensity of the spectral region at 337 nm decreases and the fluorescence intensity of the spectral region at 430 nm increases as the PAMAM concentration increases. Figure 7b illustrates that the fluorescence intensity increases as the PAMAM concentration increases. Our prior empirical data suggest the n−π* transitions of substituted carbonyl groups contribute to the spectral region of 338 nm and the aggregation of carbonyl groups contribute to the spectral region of 435 nm. The PAMAM dendrimers and carbonyl groups become more aggregated as the PAMAM concentration increase. It is easy to explain that the contribution from the “free” carbonyl groups to the fluorescent (emission at 337 nm) is less and less as the carbonyl groups become more aggregated. And the fluorescence intensity contributed by aggregation of carbonyl groups is more and more strong. As a result, the fluorescence intensity of the spectral region at 337 nm

Figure 7. Fluorescence spectra of Si-PAMAM dendrimer (G2.0) at different concentrations: (a) excited at 285 nm; (b) excited at 363 nm.

excited at 285 nm decreases and the fluorescence intensity of the spectral region at 430 nm excited at 285 and 363 nm increases as the PAMAM concentration increases. Another experiment is arranged to add poor solvent (water) in to Si-PAMAM dendrimer solution. Fluorescence spectra of SiPAMAM dendrimers (G2.0) in methanol−water solution are shown in Figure 8. Figure 8 illustrates that the fluorescence intensity increases quickly as the volume fraction of water increases (from 1% to 481

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AUTHOR INFORMATION

Corresponding Authors

*Tel +86 531 88364866; e-mail [email protected] (S.F.). *E-mail [email protected] (H.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (No. 21274080 and No. 21204043), the Key Natural Science Foundation of Shandong Province of China (No. ZR2011BZ001 and No. ZR2009BZ006), and Shandong Special Fund for Independent Innovation and Achievements transformation (No. 2014ZZCX01101).

Figure 8. Fluorescence spectra of Si-PAMAM dendrimers (G2) solution.

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REFERENCES

(1) Huang, T.; Wang, Z.; Qin, A.; Zhi, S. J.; Zhong, T. B. Acta Chim. Sin. 2013, 71 (7), 979. (2) Lee, W. I.; Bae, Y.; Bard, A. J. J. Am. Chem. Soc. 2004, 126 (27), 8358−9. (3) Wang, D.; Imae, T. J. Am. Chem. Soc. 2004, 126 (41), 13204−5. (4) Yang, W.; Pan, C.-Y. Macromol. Rapid Commun. 2009, 30, 2096. (5) Yang, W.; Pan, C.-Y.; Luo, M.-D.; Zhang, H.-B. Biomacromolecules 2010, 11, 1840. (6) Chen, Y.; Zhou, L.; Pang, Y.; Huang, W.; Qiu, F.; Jiang, X. Y.; Zhu, X. Y.; Yan, D.; Chen, Q. Bioconjugate Chem. 2011, 22, 1162. (7) Wu, D.; Liu, Y.; He, C.; Goh, S. H. Macromolecules 2005, 38 (24), 9906−9. (8) Li, W. Y.; Qu, J. L.; Du, J. W.; Ren, K. F.; Wang, Y. X.; Sun, J. Z.; Hu, Q. L. Chem. Commun. 2014, 50, 9584−9587. (9) Saravanan, G.; Imae, T. J. Nanosci. Nanotechnol. 2011, 11 (6), 4838−45. (10) Chu, C. C.; Imae, T. Macromol. Rapid Commun. 2009, 30 (2), 89− 93. (11) Yang, W.; Pan, C. Y. Macromol. Rapid Commun. 2009, 30 (24), 2096−101. (12) Sun, M.; Hong, C.-Y.; Pan, C.-Y. J. Am. Chem. Soc. 2012, 134 (51), 20581−4. (13) Lin, Y.; Gao, J.-W.; Liu, H.-W.; Li, Y.-S. Macromolecules 2009, 42 (9), 3237−46. (14) Wang, D.; Imae, T.; Miki, M. J. Colloid Interface Sci. 2007, 306 (2), 222−7. (15) Lin, S. Y.; Wu, T. H.; Jao, Y. C.; Liu, C. P.; Lin, H. Y.; Lo, L. W.; et al. Chem.Eur. J. 2011, 17 (26), 7158−61. (16) Chen, Y.; Spiering, A.; Karthikeyan, S.; Peters, G. W.; Meijer, E.; Sijbesma, R. P. Nat. Chem. 2012, 4 (7), 559−62. (17) Pucci, A.; Rausa, R.; Ciardelli, F. Macromol. Chem. Phys. 2008, 209 (9), 900−6. (18) Qin, A.; Lam, J. W.; Tang, B. Z. Prog. Polym. Sci. 2012, 37 (1), 182−209. (19) Hong, Y.; Lam, J. W.; Tang, B. Z. Chem. Soc. Rev. 2011, 40 (11), 5361−88. (20) Niu, Y.; Lu, H.; Wang, D.; Yue, Y.; Feng, S. J. Organomet. Chem. 2011, 696 (2), 544−50. (21) Wang, J.; Di, X.; Li, C.; Hu, F. Chem. Res. Appl. 2008, 20 (2), 117− 21. (22) Gray, R. C.; Hercules, D. M. Inorg. Chem. 1977, 16 (6), 1426−7.

20%), which is similar to the aggregation-induced enhanced emission (AIEE). The Si-PAMAM dendrimers are insoluble in water and soluble in methanol. As the volume fraction of water increased, the Si-PAMAM dendrimers are less soluble in the water−methanol system, but the solution still hyaline and clear. The Si-PAMAM dendrimers and the carbonyl groups are more aggregated as the volume fraction of water increased. The aggregation of carbonyl groups restrains relaxation and the selfquenching, thereby causing enhancement of the fluorescence intensity of the Si-PAMAM dendrimers. The experiment results suggested that the fluorescence intensity increased as the aggregation extent of carbonyl groups was increased. The same phenomenon was observed in the two experiments. The fluorescence intensity increased when carbonyl groups getting more aggregated. The above two experiments confirm the mechanism of the intense fluorescence presumed above; namely, the strong blue photoluminescence from Si-PAMAM was caused by aggregation of carbonyl groups and the aggregation was caused by N → Si coordination bonds in the Si-PAMAM.

4. CONCLUSIONS In this paper, a series of luminescent Si-PAMAM dendrimers were synthesized successfully, and the luminescence can be controlled. These Si-PAMAM dendrimers take 1,3-bis(3aminopropyl)tetramethyldisiloxane (G0) as core and branch point. They were synthesized in high yield by alternant azaMicheal reaction and amidation reaction. Compared with similar C-PAMAM dendrimer, those low generations of Si-PAMAM dendrimers could emit strong blue luminescence without any extra oxidizing reagent. It is interesting that as the generation increased, the fluorescence emission intensity of the Si-PAMAM increased rapidly. What is more, an aggregation-induced enhanced emission phenomenon was observed in the water− methanol system. Investigation of the mechanism of fluorescence of Si-PAMAM implied that the aggregations of carbonyl groups caused strong fluorescence and N → Si coordination bonds enhanced the aggregations. The follow-up experiments suggested that the fluorescence intensity could be further toned up when the aggregation extent of carbonyl groups was further increased. This research is aimed at providing a deeper understanding on the fluorescence of PAMAM dendrimer containing unconventional chromophores. 482

DOI: 10.1021/ma502352x Macromolecules 2015, 48, 476−482