Song Niuab,
Hongxia Yan*ab,
Zhengyan Chenab,
Yuqun Duab,
Wei Huangab,
Lihua Baiab and
Qing Lvab
aKey Laboratory of Polymer Science and Technology, School of Natural and Applied Sciences, Northwestern Polytechnical University, Xi'an 710129, Shaanxi Province, People's Republic of China. E-mail: hongxiayan@nwpu.edu.cn
bKey Laboratory of Space Applied Physics and Chemistry, Ministry of Education, School of Natural and Applied Sciences, Northwestern Polytechnical University, Xi'an 710129, People's Republic of China
First published on 25th October 2016
Dendritic and hyperbranched poly(amidoamines) are the first covered and most investigated photoluminescent polymers carrying unconventional chromogens, and studies reveal that the oxidation or acidification of the N-branched tertiary amine in these compounds is responsible for the light emission. Here, we report the synthesis and photoluminescence properties of novel hydrosoluble aliphatic tertiary amine-containing hyperbranched polysiloxanes (TAHPSi). The as-prepared polymers are fabricated by a mild one-step transesterification reaction using raw materials, including tetraethoxysilane (TEOS), triethanolamine (TEA), N-methyldiethanolamine (NMDEA) and diethylene glycol (DEG). Amusingly, the resultant products, without any treatment, such as oxidation or acidification, can still produce bright blue fluorescence under a UV light, and our rudimentary study manifests that the terminal hydroxyl group plays a key role in forming blue-fluorescent species. In addition, the significant concentration and pH-dependent profiles of luminescence intensity are also observed. Moreover, the TAHPSi can be used to detect metal ions, and there is strong selective quenching in respect of Fe3+. Therefore, the TAHPSi show promise as Fe3+ probes.
Polymers incorporating unconventional chromophores have been designed and investigated widely. Dendritic and hyperbranched polyamidoamines (PAMAM) are the first covered and the most investigated photoluminescent polymers carrying unconventional chromogens.13–16 Currently, this type of polymer has been expanded to hyperbranched poly(amino ester) (PAE),1 hyperbranched poly(ether amide) (PEA),6 dendritic polyurea (PURE),9 linear poly(ethyleneimine) (PEI),6 poly(propyleneimine) (PPI) dendrimer,17 linear poly(N-vinylpyrrolidone) (PVP),2 and other hyperbranched polymers loading tertiary amine moieties.18–20 It is reasonable that the aliphatic tertiary amine in these polymers bearing unconventional chromophores plays a key role in the formation of fluorescent species, whose structures are still unidentified,21 and its oxidation or acidification should be responsible for the strong fluorescence.18,22,23 However, the deeper luminous mechanism of these compounds is still under investigation.5,24
Meanwhile, some seemingly ordinary polymers consisting of carbonyl groups, instead of aliphatic tertiary amines, can also emit bright blue photoluminescence, such as polyisobutene succinic anhydrides and imides (PIBSAs and PIBSIs),25 poly[(maleic anhydride)-alt-(vinyl acetate)] (PMV),11 sulfonated acetone–formaldehyde condensate (SAF),26 and sulfonated ethylenediamine–acetone–formaldehyde (SEAF).27 These polymers are almost non-emissive in their dilute solutions, but become highly emissive in condensed solutions or solids. The fluorescence is ascribed to the aggregation of carbonyl groups, and this type of light-emitting behavior is in accordance with the aggregation-induced emission (AIE) phenomenon,28 which was first found by Tang's group in 2001.29 It is also reported that hyperbranched (3-hydroxyphenyl) phosphate (HHPP) can exhibit unexpected bright fluorescence, and two obvious emission peaks can be discovered in its photoluminescence spectrum. The results show that the fluorescence peak at around 407 nm is related to the triphenyl phosphate structure, which leads to an extended π system in HHPP, and the peak at about 550 nm is concerned with the terminal phenolic groups.30
More recently, a series of silicone-containing photoluminescent polymers has been designed and prepared, and their fluorescence properties have been extensively investigated.31–35 In particular, siloxane–poly(amidoamine) (Si–PAMAM) dendrimers, without any oxidation or acidification, can still emit strong blue fluorescence, and the introduction of Si not only affects the molecular stability but also changes the spatial conformation, which can subsequently influence the light emission. Studies on the molecular structure of these Si–PAMAMs show that the N → Si coordination bond is present in these molecules, and the strong luminescence is due to the aggregation of carbonyl groups induced by N → Si coordination bonds.12,36,37 In addition, sulfur-containing carbosiloxane dendrimers consisting of unconjugated carbon–carbon double bonds can emit bright blue photoluminescence. It is speculated by the authors that the introduction of a sulfur atom is beneficial to the occurrence of luminescence, while more experimental data is required to determine the mechanism for this.38
From the above, we can see that the oxidation or acidification of aliphatic tertiary amines, the aggregation of carbonyl groups, and the presence of some other heteroatoms, such as P and S, favor photoluminescence. However, we also wonder whether it is possible to design and synthesize distinct molecular structures that do not contain P atoms, S atoms, or carbonyl groups, without treatment such as oxidation or acidification of aliphatic tertiary amines. In the present research, inspired by the transesterification reaction of ethoxyl from tetraethoxysilane with the hydroxyl group of polyols we studied previously,39,40 we have synthesized two water-soluble aliphatic tertiary amine-containing hyperbranched polysiloxanes (TAHPSi), using raw materials including tetraethoxysilane (TEOS), triethanolamine (TEA), N-methyldiethanolamine (NMDEA), and diethylene glycol (DEG). The two synthesized polymers are water dispersible due to the presence of many tertiary amine groups in their molecular backbones, which can generate many hydrogen bonds with water. Surprisingly, the resulting carbonyl-group-free TAHPSi, without any treatment, can emit bright blue luminescence. It is revealed by our primary study that the terminal hydroxyl group plays a vital role in producing blue-fluorescent centers. Significant concentration and pH-dependent profiles of luminescence intensity are also observed. In addition, the TAHPSi can be used to detect metal ions, and there is strong selective quenching in respect of Fe3+. Therefore, the TAHPSi show promise as Fe3+ probes.
1H NMR (400 MHz, chloroform-d) δ 4.41 (s, 1H), 3.63–3.49 (m, 3H), 3.51–3.26 (m, 5H), 2.67 (dt, J = 10.1, 5.8 Hz, 2H). 13C NMR (101 MHz, chloroform-d) δ 72.26, 72.21, 72.09, 62.15, 61.43, 61.21, 61.20, 59.40, 59.37, 57.37, 57.34, 57.13, 50.76, 50.66, 18.00.
1H NMR (400 MHz, chloroform-d) δ 4.11 (s, 0H), 3.56 (dt, J = 28.7, 6.2 Hz, 1H), 3.37 (dt, J = 24.5, 5.5 Hz, 1H), 2.66 (q, J = 6.1 Hz, 1H), 2.35 (dt, J = 11.0, 5.9 Hz, 1H), 2.07 (d, J = 3.6 Hz, 1H). 13C NMR (101 MHz, chloroform-d) δ 60.50, 60.33, 59.73, 59.43, 59.34, 59.20, 58.91, 58.65, 57.54, 57.40, 57.33, 57.30, 57.14, 50.84, 50.71, 42.51, 42.11, 18.14.
1H NMR (400 MHz) and 13C NMR (101 MHz) spectra were obtained in CDCl3 solvent using a Bruker Avance 400 MHz NMR spectrometer.
29Si NMR spectra of the synthesized polymers were predicted using the software MestReNova v10.0.2.
Fourier transform infrared (FTIR) spectra of the prepared polymers were recorded with a NICOLET 5700 FTIR spectrometer ranging from 4000 to 400 cm−1.
UV-vis absorption spectra of liquid samples in water were determined using a Shimadzu UV-2450 spectrophotometer.
Fluorescence excitation/emission spectra of liquid samples in water were collected with a Shimadzu RF-5301 fluorescence spectrometer with a 5 nm slit width using a 4 mm path quartz cell under xenon discharge lamp excitation. The fluorescence spectra of solution samples were determined without degassing. Fluorescence excitation/emission spectra, fluorescence lifetimes, and absolute quantum yields for solid samples were measured on a steady/transient-state fluorescence spectrometer coupled with an integrating sphere (FLS980, Edinburgh Instruments).
The molecular structures of the raw materials (Fig. S1 and S2, ESI†) and the resultant polymer S1 (Fig. 2 and 3) were proven using 1H NMR and 13C NMR. As seen in Fig. 2, the peak at 4.11 ppm corresponds to a hydroxyl group (H4). The proton signals of methylene (–CH2–) at 3.61–3.45, 3.42–3.38, 3.35–3.32, 2.68–2.65, and 2.38–2.32 ppm are attributed to Si–O–CH2– (H1), HO–CH2–CH2–N(–CH2–)2 (H3), HO–CH2–CH2–N–CH3 (H6), N(–CH2–)3 (H2), and CH3–N(–CH2–)2 (H5), respectively. The proton signal of the methyl group linked to a nitrogen atom (N–CH3) (H7) is observed at 2.07–2.04 ppm. The peak of the residual –O–CH2–CH3 (H8) cannot be found, owing to the overlap by the H1 signal. The faint signal at 0.98–0.91 ppm should be assigned to the residual –O–CH2–CH3 (H9) of the TEOS moiety, suggesting that the ethoxyl group could not be completely consumed by the excess hydroxyl groups due to the increasing steric effect as the molecular weight increased. Nevertheless, needless to say, the polycondensation reaction has been smoothly performed, and the expected polymer S1 can be synthesized according to this approach.
Fig. 3 presents the 13C NMR spectrum of S1. As seen from Fig. 3, the signals of the secondary carbons (–CH2–) at 60.50, 59.43, 59.34, 59.20, 58.91, 58.65, 57.40, and 50.84 ppm correspond, respectively, to Si–O–CH2–CH2–N(–CH2–)2 (C1), HO–CH2–CH2–N(–CH2–)2 (C4), HO–CH2–CH2–N–CH3 (C8), HO–CH2–CH2–N–CH3 (C9), HO–CH2–CH2–N(–CH2–)2 (C3), HO–CH2–CH2–N(–CH2–)2 (C2), Si–O–CH2–CH2–N–CH3 (C5), and Si–O–CH2–CH2–N–CH3, (C6). The carbon signals at 42.51 ppm for C10 and at 42.11 ppm for C7 should be assigned to N–CH3. In theory, the carbon signals of the Si–OCH2CH3 carbons (C11 and C12) in S2 should completely disappear; however, the very faint signals at 59.73 ppm for C11 and 18.14 ppm for C12 of the TEOS moiety are still visible in practice, indicating that the –OCH2CH3 of TEOS cannot be completely replaced by excess –OH during the reaction because of the increasing steric effect. However, the anticipated polymer can be successfully fabricated by the transesterification reaction.
The chemical configurations of the raw materials (Fig. S3 and S4†) and the as-prepared polymer S2 (Fig. 4 and 5) were then characterized by 1H NMR and 13C NMR. As seen in Fig. 4, the peaks relating to hydroxyl groups at 4.41–4.29 and 2.33 ppm are, respectively, ascribed to HO–CH2–CH2–N (H4) and HO–CH2–CH2–O (H7). The proton signals of methylene (–CH2–) at 3.61–3.53, 3.51–3.41, 3.38–3.29, and 2.70–2.64 ppm are ascribed to Si–O–CH2– & HO–CH2–CH2–O (H1, 6), –CH2–O–CH2– (H5), N–CH2–CH2–OH (H3), and N–CH2– (H2), respectively. The peak of the residual –O–CH2–CH3 (H8) cannot be observed due to the overlap by the H1, 6 signal. The weak signal at 0.97–0.90 ppm should be assigned to the residual –O–CH2–CH3 (H9) of the TEOS moiety, suggesting that the ethoxyl group could not be completely consumed by the excess hydroxyl groups owing to the steric effect increasing with the molecular weight. Nevertheless, needless to say, the polycondensation reaction has been smoothly performed, and the expected polymer S2 can be synthesized according to this approach.
Fig. 5 presents the 13C NMR spectrum of S2. As seen in Fig. 5, the signals of the secondary carbons (–CH2–) at 72.21, 62.15, 61.20, 59.37, 57.34, and 50.76 ppm correspond, respectively, to –CH2–O–CH2– (C5), Si–O–CH2–CH2–N (C1), HO–CH2–CH2–O (C3), HO–CH2–CH2–N (C6), Si–O–CH2–CH2–O– (C4), and N–CH2– (C2). In theory, the carbon signals of the Si–OCH2CH3 carbons (C7 and C8) in S1 should completely disappear; however, in fact, the very faint signals at 61.43 ppm for C7 and 18.00 ppm for C8 of the TEOS moiety are still visible, reflecting that the –OCH2CH3 of TEOS cannot be completely superseded by the excess –OH of DEG and TEA during the reaction because of the increasing steric effect. It is understood that the anticipated polymer has been successfully constructed by the transesterification reaction.
The FTIR spectra of all the raw materials and polymers are plotted in Fig. 6. Fig. 6A shows the spectra of NMDEA (a), TEA (b), TEOS (c), and S1 (d). In Fig. 6A(a), the significant absorption peak centered at around 3381 cm−1 for –OH can be observed. Fig. 6A(b) shows the characteristic peak relative to –OH at 3380 cm−1. In Fig. 6A(c), we can see that the distinct absorption peaks at 1107 and 1084 cm−1 should be ascribed to the Si–O–C bond. Particularly, the typical peak for –OH of the polymer S1 in Fig. 6A(d) is centered at around 3400 cm−1, which is obviously lower than those of NMDEA and TEA, probably due to the decreased hydrogen-bonded association caused by the steric effect of the macromolecular morphology. Fig. 6B presents the spectra of DEG (a), TEA (b), TEOS (c) and S2 (d). Likewise, the absorption band for –OH (3413 cm−1) of S2 moves to a higher frequency compared to DEG (3355 cm−1) and TEA (3380 cm−1) owing to the reason discussed above.
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Fig. 6 (A) FTIR spectra of (a) NMDEA, (b) TEA, (c) TEOS and (d) S1; (B) FTIR spectra of (a) DEG, (b) TEA, (c) TEOS and (d) S2. |
The characterization of hyperbranched polymers commonly involves the calculation of the degree of branching (DB) by identifying and quantifying dendritic (D), linear (L) and terminal (T) units.41–43 Compared to the 13C NMR and 1H NMR spectra, the changes in the Si chemical environment may provide more information in theory to calculate the DB. We start with a prediction on the 29Si NMR spectrum of TEOS (a), S1 (b) and S2 (c) using the software MestReNova v10.0.2, as displayed in Fig. 7 and S5.† We can clearly see that the Si atoms in TEOS and the two polymers are all centered at −82.00 ppm, suggesting that there are no changes in the Si chemical environment after the production of hyperbranched structures from TEOS and polyols. Therefore, further work on the DB calculation for these types of hyperbranched polysiloxanes needs to be carried out.
As we know, the photophysical properties of the N-branched dendrimers and hyperbranched polymers, such as poly(amidoamine) (PAMAM), poly(propyleneimine) (PPI), and poly(ethyleneimine) (PEI), have been widely reported in recent years. It is reasonable to assume that the aliphatic tertiary amine plays a key role in the formation of the fluorescent species, whose structures are still unidentified,21 and its oxidation or acidification is probably responsible for the strong fluorescence. It is also reported that the Si–PAMAM dendrimers bearing an aliphatic tertiary amine, without any oxidation or acidification, are highly emissive, and the authors confirm and conclude that this photoluminescence behavior is because of the aggregation of carbonyl groups induced by the N → Si coordination bonds.37 Similarly, in this work the two polymers, without any treatment (such as oxidation or acidification), can still emit bright blue luminescence. We speculate that the terminal hydroxyl group, rather than the aliphatic tertiary amine, plays a critical role in the generation of fluorescent species.
Subsequently, to confirm the above speculation and gain an insight into the cause of the photoluminescence, we synthesized two model polymers containing no aliphatic tertiary amines, based on our previous research,39,40 and these are hyperbranched polysiloxanes carrying only a terminal hydroxyl group (M1, Fig. S6A†) and an acetoacetyl group (M2, Fig. S6B†). Unexpectedly, the polymer M1 exhibits bright blue fluorescence (Fig. S6C†), and shows remarkable UV absorption centered at 205 and 216 nm (Fig. S6D†), and photoluminescence centered at 406 and 430 nm (Fig. S6E†). After the terminal –OH group of M1 was end-capped by the acetoacetyl group of t-BAA, the new as-prepared product M2 still presents prominent UV absorption (Fig. S6D†) contributed by the acetoacetyl group; however, it is almost non-emissive (Fig. S6E†). It can be concluded that the presence of the acetoacetyl group negatively affects the light emission, probably owing to intersystem crossing, and the existence of the –OH favors the luminescence.
Furthermore, the photoluminescence properties of the two polymers, S1 and S2, in water solution at varying concentrations were investigated. Fig. 9A shows photographs of S1 and S2 at different concentrations in water under irradiation with 365 nm UV light and white light. From the image, we can see that the fluorescence intensities of the two polymer solutions dramatically increase as their concentrations are increased, and the brightest photoluminescence can be observed even for the 100% solid content, showing typical AIE characterization. As shown in Fig. 9B, we can observe that both the maximum excitation and emission intensities progressively increase, and no emission quenching emerges as concentrations are boosted. In our case, the TAHPSi fluorescence behavior is very similar to the AIE phenomenon;28,29 namely, they are almost nonradiative in dilute solutions but highly emissive in concentrated solutions or the solid state, and this has been successfully adopted to explain the enhanced fluorescence phenomenon of some chromophore-free hyperbranched polymers.37,44 For polymer S1, the maximum excitation and emission bands are, respectively, centered at around 340 and 420 nm; for the polymer S2, they are observed at around 331 and 347 nm, respectively. Evidently, the two polymers in water solution exhibit different excitation and emission bands, possibly owing to the difference in the two molecular morphologies constructed by the varying diols, such as NMDEA containing a nitrogen atom (N) and DEG bearing an ether linkage (–O–), and some interactions between –OH and N as well as –OH and –O– may be possible.43,45–47
As seen in Fig. 9C, as the excitation wavelengths increase, the emission wavelengths of the two polymers, S1 and S2, present a gradual red-shift in 20 mg mL−1 water solutions. This excitation-dependent photoluminescence behavior has also been recognized as a generic feature of various types of hyperbranched polymers;18 meanwhile, this behavior is possibly due to the high structural heterogeneity and the wide molecular weight distributions of the TAHPSi,48 and the critical distribution of emission trap sites on the molecule possibly represents a deeper cause.49,50 Subsequently, we calculated the CIE (Commission Internationale d'00C9;clairage) coordinates of S1 and S2 at a concentration of 20 mg mL−1 in water solution after excitation at varying excitation wavelengths with 10 nm increments, based on Fig. 9C (as shown in Fig. 9D and Table S1†), and a deeper color for both the polymer solutions can be significantly observed with longer excitation wavelengths.
Moreover, we obtained the photoluminescence spectra, fluorescence lifetimes, and absolute fluorescence quantum yields (QY) of the pure TAHPSi using a steady/transient-state fluorescence spectrometer combing an integrating sphere (FLS980, Edinburgh Instruments), as shown in Fig. 10. Fig. 10A displays the fluorescence spectra of pure S1 and S2. For polymer S1, its excitation and emission bands are, respectively, centered at 263 and 396 nm wavelengths; for polymer S2, the excitation and emission bands are mainly centered at 393 and 450 nm wavelengths, respectively. Obviously, their spectral characteristics and peak positions differ from those in water solutions exhibited in Fig. 9B, probably due to the solvent effect. Fig. 10B and S7† show the fluorescence decay curves of pure S1 at 398 nm after excitation at 263 nm and S2 at 450 nm after excitation at 393 nm. The decay traces were then fitted on the basis of a non-linear least squares analysis according to the following equation: R(t) = B1exp(−t/τ1) + B2
exp(−t/τ2) + B3
exp(−t/τ3) + B4
exp(−t/τ4), where Bn (n = 1, 2, 3, 4) is the fractional contribution of the decay lifetime τn. The final measured fluorescence lifetimes (τ) for S1 and S2 are around 1.02 and 1.57 ns, respectively. Fig. 10C and S8† indicate the absolute QYs of pure S1 and S2, respectively, with values of 5.79% and 11.99% under excitation at 263 and 393 nm via multi scans. In addition, we can see that the fluorescence lifetime and absolute QY for polymer S2 are higher than those for S1 due to their different molecular structures. However, the deeper cause is still unclear, and this needs to be further analyzed in future work. The CIE coordinates (x, y) of both S1 (0.1439, 0.0497) and S2 (0.2324, 0.3053) were also obtained (Fig. 10D), and these are mainly concentrated in the blue area; however, an obvious red-shift for S2 can be seen compared to S1.
To evaluate whether the resulting TAHPSi can be employed to detect metal cations, the fluorescence response of the 20 mg mL−1 S1 water solution to various metal ions (1 × 10−4 M), including Na+, K+, Mg2+, Fe3+, Zn2+, Ca2+, Co2+, Cu2+, Ni2+, and Al3+, was measured, as shown in Fig. 12A. It can be seen that the fluorescence intensity varies significantly with the metal ion types. As seen in Fig. 12B, the stimuli response of fluorescence towards Na+, K+, Mg2+, Fe3+, Zn2+, Co2+, Cu2+, and Ni2+ is very significant, and the fluorescence intensities with these metal ions all decrease compared to the blank control. By contrast, the fluorescence intensity remains almost stable after the addition of Ca2+ and Al3+. In addition, the fluorescence change, defined as ΔI/I0 (ΔI = I0 − I), where I0 is the fluorescence intensity in the absence of metal ions and I is that in the presence of metal ions,53 is adopted as a parameter for evaluating the effect of these metal ions on the fluorescence behavior of S1. Fig. 12C indicates the calculated ΔI/I0 values for these metal cations at a concentration of 1 × 10−4 M. Obviously, the fluorescence quenching rate (ΔI/I0) is most significant in the presence of Fe3+, suggesting that Fe3+ is a strong fluorescence quencher in the in the TAHPSi water solutions. Additionally, we can visibly see from Fig. 12D that the fluorescence changes as the type of metal ion is varied.
It is revealed by existing reports that the stability of the metal complexes is improved as the metal cation charge/radius ratio increases.2,31,54 Among the above metal cations, Fe3+ possesses the largest charge/radius ratio,2 and thus it forms the strongest interaction with TAHPSi, giving rise to a distinct quenching effect. Besides, an obvious decrease in the fluorescence intensities of S1 and S2 can be found as the Fe3+ concentration increases (Fig. 13). Moreover, it is noteworthy that Fe3+ is a typical cation, since it plays an important role in many physiological processes of organisms. Therefore, our polymers are also promising as potential biological probes for detecting Fe3+.
Footnote |
† Electronic supplementary information (ESI) available: 1H NMR and 13C NMR spectra of raw materials, model polymers, the print screen of transient photoluminescence decay curves and absolute fluorescence quantum yields of polymers. See DOI: 10.1039/c6ra22916f |
This journal is © The Royal Society of Chemistry 2016 |