Hydrosoluble aliphatic tertiary amine-containing hyperbranched polysiloxanes with bright blue photoluminescence

Abstract

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.

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1 Introduction

Photoluminescent polymers have many applications, such as medical diagnostics, tunable lasers, solar energy conversion, displays, amplifiers for optical communication, metallic ion detection, and cellular imaging,^1,2 and they often carry conventional π–π conjugated systems, such as thiophene, benzene rings, silole, and anthracene,^3,4 which function as light-emitting centers. In recent years, inherent fluorescence from linear, dendritic, or hyperbranched polymers lacking any types of well-known conventional fluorogens has been observed and widely investigated.^5–10 Unlike conventional chromogens, unconventional luminophores are not composed of π-aromatic systems, and they often consist of aliphatic tertiary amines, amides, carbonyls, C=N, N=O, N=N, or C=S in the molecular frameworks.¹¹ Compared to conventional fluorescent polymers, luminescent macromolecules bearing unconventional emissive units are endowed with more remarkable features, such as environmental friendliness, easy preparation, and hydrophilicity.¹² Therefore, these types of polymers have become increasingly eye-catching.

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.

2 Experimental

2.1 Materials

Tetraethoxysilane (TEOS, 99%), diethylene glycol (DEG, 99%) and t-butyl acetoacetate (t-BAA) were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. N-methyldiethanolamine (NMDEA, 98%) and triethanolamine (TEA, 98%) were obtained from Shanghai Macklin Biochemical Co., Ltd. All the materials were used without further purification unless otherwise stated.

2.2 Synthesis of hyperbranched polysiloxanes

In these reactions, –OH is in excess compared with –OC₂H₅, and the two TAHPSi were synthesized from various polyols, as shown below.

Polymer S1. The molar ratio of TEOS, TEA, and NMDEA is 3 : 3 : 5, so the molar ratio of –OH and –OC₂H₅ should be 1.9 : 1.2. The detailed synthesis process is described as follows: first, 74.4476 g TEOS (0.3538 mol), 53.8577 g TEA (0.3538 mol), and 71.6947 g NMDEA (0.5896 mol) were charged into a 250 mL four-necked flask coupled with a N₂ gas inlet, a thermometer, a stirrer, a distilling setup, and a water condenser. Then, the mixture in the flask was heated to ∼110 °C, and held at this temperature until some of the byproduct was distilled off. Subsequently, the temperature was continuously raised to a maximum of ∼160 °C and the distillate temperature was kept at 78 °C below. The reaction system was sustained at 160 °C until the distillate temperature was lower than 55 °C. Finally, the hydrosoluble aliphatic hyperbranched polysiloxane, denoted as S1, was fabricated. In theory, the weight of the distillate is 65.19 g when the –OC₂H₅ is fully depleted. However, in practice, the amount of the gathered distillate was 61.47 g due to the fact that the –OC₂H₅ group cannot be consumed by the excess –OH as a result of the increasing steric hindrance.

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.

Polymer S2. The molar ratio of TEOS, TEA, and DEG is 3 : 3 : 5; namely, the molar ratio of –OH and –OC₂H₅ is 1.9 : 1.2. S2 was synthesized using 77.7483 g TEOS (0.3695 mol), 56.2455 g TEA (0.3695 mol), and 66.0062 g DEG (0.6158 mol) based on the synthesis process for S1. The weight of the distillate in theory is 68.05 g. However, in practice, the amount of the gathered distillate was 62.51 g.

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.

2.3 Measurements

The molecular weights and their distributions within the medium were determined with an Agilent 1260 Gel Permeation Chromatograph (GPC) connected to an Agilent RID using a column system. Tetrahydrofuran (THF) acted as an eluent, and the measurement was executed at a flow rate of 1 mL min⁻¹.

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).

3 Results and discussion

3.1 Synthesis and characterization of TAHPSi

In this work, deionized water has not been used during the reactions. The HHPTa can be facilely synthesized by a transesterification reaction using the ethoxyl (–OC₂H₅) group in TEOS, which can serve as a “carboxyl group”, and the hydroxyl group (–OH) of polyols under catalyst-free conditions; meanwhile, the byproduct, ethanol, is synchronously distilled off from the reactor. The synthesis route to HHPTa is presented in Scheme 1. In this polycondensation reaction, –OH is in excess compared with –OC₂H₅. The transesterification reaction of TEOS with polyols like TEA, NMDEA, and DEG was gradually conducted from 110 to 160 °C, and the distillation temperature was contemporaneously kept at 77 ± 2 °C. Then, the reaction was maintained at 160 °C until the distillation temperature dropped to 55 °C below. Eventually, the two liquids, HHPTa S1 and S2, were prepared based on the same molar ratio of raw materials, as seen in Fig. 1 and Table 1. Significantly, the two polymers carry different molecular weights (M_w), and the M_w of S1 synthesized using TEOS, TEA, and NMDEA is a little smaller than that of S2 prepared from TEOS, TEA and DEG, although they are obtained using the same molar ratio of –OH and –OC₂H₅. This may be ascribed to the fact that the steric hindrance of NMDEA, containing a methyl group, is greater compared to that of DEG. However, needless to say, the polycondensation reaction has been successfully carried out. In addition, the calculated hydroxyl values (OHV) and amine values (AMV) are also collected in Table 1.

Scheme 1 Synthesis of HHPTa.

Fig. 1 GPC curves of S1 and S2.

Table 1 Characterization data of the two polymers S1 and S2

Polymer	Molar ratio	Mn	Mw	PDI	OHV^a (mgKOH per g)	AMV^b (mgKOH per g)
a The calculated –OH value.b The calculated amine value.
S1	3 : 3 : 5 (TEOS/TEA/NMDEA)	3838	22 871	5.9	343.53	392.61
S2	3 : 3 : 5 (TEOS/TEA/DEG)	4177	23 341	5.6	366.62	157.12

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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. 2 The ¹H NMR spectrum of S1.

Fig. 3 The ¹³C NMR spectrum of S1.

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. 4 The ¹H NMR spectrum of S2.

Fig. 5 The ¹³C NMR spectrum of S2.

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.

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.

Fig. 7 The ²⁹Si NMR spectra of TEOS, S1, and S2 predicted by the software MestReNova v10.0.2.

3.2 Optical properties of TAHPSi

We are surprised that the resulting TAHPSi (S1 and S2), without containing conventional chromophores, such as conjugated C=C bonds, can display significant UV absorption and emit bright blue fluorescence. The optical behaviors of the as-prepared polymers were then investigated by UV-vis and photoluminescence spectroscopy. Fig. 8A and B show the UV-vis absorption spectra of S1 and S2 in water solution at various concentrations, respectively. We can find that both the obtained polymers exhibit prominent absorption bands centered at around 230 nm, which are associated with the n → σ* electron transitions from C–OH and C–NR₂ of the two polymers. In addition, their UV absorption intensities gradually increase, and significant red-shifts of the maximum absorption positions are also observed; meanwhile, the absorption bands become much wider with increasing concentration. This is probably due to the fact that an expanded “electron delocalization system”³⁴ is formed, triggering an easier electron transition, when the C–OH and C–NR₂ groups become more and more compact with concentration, and accordingly the UV absorption energy decreases gradually, inducing prominent red-shifts for the maximum absorption positions.

Fig. 8 UV-vis absorption spectra of (A) S1 and (B) S2 in water solution at various concentrations.

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

Fig. 9 (A) Photo images of S1 (left) and S2 (right) in aqueous solutions at various concentrations: (a and e) 20 mg mL⁻¹; (b and f) 40 mg mL⁻¹; (c and g) 60 mg mL⁻¹, and (d and h) 100% solid content under irradiation with 365 nm UV light and white light; (B₁ and B₂) fluorescence spectra of S1 and S2 in water solution at varying concentrations: excitation spectra (monitored at λ_em = 400 nm for S1 and 374 nm for S2) and emission spectra (excited at λ_ex = 349 nm for S1 and 329 nm for S2) measured at room temperature; (C₁ and C₂) excitation-dependent photoluminescence 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 (insets: normalized emission intensity.) and (D₁ and D₂) CIE chromaticity coordinates calculated from the emission spectra 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.

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) = B₁ exp(−t/τ₁) + B₂ exp(−t/τ₂) + B₃ exp(−t/τ₃) + B₄ exp(−t/τ₄), where B_n (n = 1, 2, 3, 4) is the fractional contribution of the decay lifetime τ_n. 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.

Fig. 10 (A₁ and A₂) Fluorescence spectra of pure S1 and S2: excitation spectra (monitored at λ_em = 398 nm for S1 and 450 nm for S2) and emission spectra (excited at λ_ex = 262 nm for S1 and 393 nm for S2) measured at room temperature; (B₁ and B₂) the transient photoluminescence decay curve of S1 at 398 nm after excitation at 263 nm and S2 at 450 nm after excitation at 393 nm; (C₁ and C₂) the absolute fluorescence quantum yield of S1 excited at 263 nm and S2 excited at 393 nm and (D) CIE chromaticity coordinates (x, y) calculated from the emission spectra of pure S1 and S2.

3.3 pH-dependent photoluminescence

Both polymers S1 and S2 in water solutions at a concentration of 20 mg mL⁻¹ show prominent pH-dependent photoluminescence properties in the pH range from 1 to 10 as a result of adding hydrochloric acid (as shown in Fig. 11A), indicating that pH has a significant effect on the conformation and fluorescence of the obtained polymers.⁵¹ The pH values for S1 and S2, with no acidification, are around 9.98 and 9.20, respectively. As the pH increased from 1 to 7, there was a gradual increase in the excitation and emission intensities; however, these suddenly decreased when the pH was increased further (Fig. 11B). The case in the present work is very different from the previously reported pH-dependent fluorescence of NH₂-terminated PAMAM dendrimers¹⁵ and CO₂H-terminated hyperbranched poly(amido acids) (HBPAAs),⁵² which showed maximum emissions at pH values around 2.5 and 9, respectively.² When the pH was at around 7, the N of tertiary amino groups was almost protonated completely with the addition of H⁺. The intense charge–charge repulsion within the macromolecular interior made the molecular configuration of TAHPSi much tighter and the molecular structures more rigid, which gives rise to lower solubility of the polymer molecules and more compact aggregation of the peripheral hydroxyl groups. Therefore, the fluorescence of the TAHPSi at pH = 7 could be significantly enhanced. In addition, the precipitates of S1 and S2 in neutral solutions are formed because of the lower solubility, and they can be obviously observed, as shown in Fig. 11C. By contrast, acidic and alkaline conditions are not conductive to aggregation of TAHPSi, and thus relatively weaker emission is observed.

Fig. 11 (A₁ and A₂) Effects of different pH values on the photoluminescence properties of 20 mg mL⁻¹ S1 and S2 water solutions; (B₁ and B₂) relationship between pH value and fluorescence intensity and (C) photographs of S1 and S2 with different pH values under irradiation with 365 nm UV light and white light, respectively.

3.4 Effects of metal ions on photoluminescence

It has been reported that PAMAM dendrimers containing a large number of oxygen and nitrogen atoms, which provide lone pair electrons, can form complexes with many metal ions.¹² The optical performances of the obtained HHPTa may be influenced because of the formation of HHPTa/cation complexes.

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.

Fig. 12 (A) Effects of metal ions (1 × 10⁻⁴ M) on fluorescence spectra of the 20 mg mL⁻¹ S1 water solution; (B) the influence of various metal ions on the fluorescence intensity of the 20 mg mL⁻¹ S1 water solution; (C) the fluorescence quenching rate (ΔI/I₀) of 20 mg mL⁻¹ S1 water solution with different metal ions (1 × 10⁻⁴ M) and (D) photographs of S1 and S2 with different metal ions under irradiation with 365 nm UV light and white light, respectively.

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⁺.

Fig. 13 (A₁ and A₂) Quenching effects of Fe³⁺ with different ion concentrations on the 20 mg mL⁻¹ S1 and S2 water solutions; (B₁ and B₂) effects of Fe³⁺ with different ion concentrations on the excitation and emission intensities of the 20 mg mL⁻¹ S1 and S2 water solutions.

4 Conclusions

Water-soluble hydroxyl-group-terminated hyperbranched polysiloxanes containing aliphatic tertiary amines have been successfully synthesized by one-pot transesterification reaction of TEOS with excess polyols. The chromophore-free polymers, without any treatment, show obvious blue photoluminescence, both in water solution and the bulk state, and the fluorescence intensities in water solution are enhanced with increasing concentration. A primary investigation reveals that the hydroxyl group, rather than the aliphatic tertiary amine, plays an important role in the formation of a fluorescent center. The protonation of nitrogen significantly changes the fluorescence intensity, and prominent molecular aggregation and particularly intense photoluminescence can be observed at pH = 7.0. Additionally, the fluorescence of the obtained polymers manifests high selective quenching toward Fe³⁺, and therefore they are also promising as biological Fe³⁺ probes.

Acknowledgements

This work is sponsored by Innovation Foundation for Doctor Dissertation of Northwestern Polytechnical University (CX201626) and Specialized Research Fund for the Doctoral Program of Higher Education of China (20136102110049). We acknowledge the UV-vis and fluorescence measurement support of Dr Qing Li and Dr Xiangzhi Dong from Xi'an Polytechnic University.

Notes and references

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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

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