Sulfonated ethylenediamine–acetone–formaldehyde condensate: preparation, unconventional photoluminescence and aggregation enhanced emission

Abstract

The unconventional photoluminescence of sulfonated acetone–formaldehyde (SAF) and acetone–formaldehyde (AF) condensates are uncovered and studied in this work. Their fluorescence had been ignored although they were developed and commercialized as water reducing agents in the concrete industry decades ago. More importantly, based on the traditional mechanism of the Mannich reaction, sulfonated ethylenediamine–acetone–formaldehyde (SEAF) was successfully synthesized by introducing imino groups to SAF. It shows highly enhanced fluorescence both in solution and the solid state. The emission mechanism of these polymers is proposed to be via the cluster of carbonyl groups within the long linear nonconjugated chain. Hydroxy, sulfonic and amino groups in SEAF can produce strong ionic and hydrogen bonds, which contribute to its fluorescent enhancement. Furthermore, it is more interesting that SAF and SEAF possess an aggregation-enhanced emission (AEE) effect, while AF shows aggregation-caused quenching (ACQ). This result confirms the previous report that hydrogen bonds can induce AEE effects. Our study provides a novel perspective for the design of water-soluble luminescent materials with green photoluminescence and AEE property.

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Introduction

Highly luminescent organic materials including conjugated polymers and small molecules have been extensively studied for their application in solid organic electronics.^1,2 Many conjugated molecules showed weak or even no emission in solution, but extremely enhanced emission in the solid state. This phenomenon is named aggregation-induced emission (AIE) or aggregation-enhanced emission (AEE) and it was firstly reported by Tang and his co-workers in 2001.³ In many cases, aggregation often causes partial or even complete quenching of the luminogens, called aggregation-caused quenching (ACQ). However, with regard to the AIE or AEE materials, the advantage of AIE/AEE makes them promising candidates as solid luminescent materials and sensors because they can overcome the notorious application obstacle of ACQ. Hereafter, researchers, especially in the Tang group, have developed multitudinous materials possessing AIE or AEE characteristics.^4–8 However, polymers with unconventional chromophores possessing AIE or AEE effects have rarely been reported, to date.^9–14

Sulfonated acetone–formaldehyde (SAF), a kind of high performance resin, is known as the superplasticizer in the construction industry. It was developed earlier than the 1990s and then got extensive research and application due to its good water solubility and superior dispersing performance.^15,16 Fortunately, we observed an interesting property of SAF that it shows obvious photoluminescence (PL) phenomenon. It is well known that SAF is a polymer containing non-conjugated long linear aliphatic chain with carbonyl and ether bonds.¹⁷ In order to investigate the intrinsic mechanism of its emission, we studied the fluorescence of acetone–formaldehyde (AF) polymer, which is the precursor of SAF. AF also possesses PL property in organic solution such as ethanol and THF. For the view of molecular structures of AF and SAF, both of them have carbonyl groups in their polymer backbone without any conventional chromophores. Hence, we proposed the photoluminescence of these polymers might come from a special mechanism. Based on the search of literature, we found that Tang and Feng had reported the cluster induced emission (CIE) of carbonyl groups.^9,10 This type of luminogens have attracted more and more attentions in recent years, however, water soluble fluorescent polymer based on CIE has been rarely reported previously. More interestingly, compared with the ACQ effect of AF, SAF possessed AEE behavior. The AEE property of SAF is a more meaningful discovery and it motivates our interest to further study this type of polymers. Furthermore, in order to enhance the cluster effect of SAF molecule, sulfonated ethylenediamine–acetone–formaldehyde (SEAF) condensate was designed by introducing imino groups into SAF considering the strong ionic and hydrogen bonds interaction of amidogens. SEAF was successfully prepared through a proven technique of Mannich reaction using acetone, formaldehyde and ethylenediamine as raw materials.^18,19 As a result, SEAF exhibited highly enhanced photoluminescence in solution, especially in solid state comparing with AF and SAF. Moreover, it also showed an obvious AEE effect.

It is worth mentioning that there are several highlights of these polymers. Firstly, it is a new kind of fluorescent polymer originated from CIE effect of the carbonyl groups. Secondly, it provides a new method to design water soluble fluorescent polymer with AEE property. Thirdly, due to the cheap raw materials and simple synthetic route of the polymer, it provides a readily available method of luminescent materials. In this paper, the preparation, characterization, photoluminescence of AF, SAF and SEAF, aggregation-enhanced emission of SAF and SEAF, and their mechanism were studied and discussed in details.

Results and discussion

Structural characterization

The synthesis routes and chemical structures of AF, SAF and SEAF are given in Scheme 1. AF and SAF were prepared according to the previously reported method of our group.¹⁷ As for SEAF, we introduced ethylenediamine into the backbone of SAF based on Mannich reaction. Formaldehyde can react with both of ethylenediamine and acetone, respectively. As a result, we successfully obtained the water-soluble multi-unit polymer SEAF. In order to identify the polymer structure of these three samples, their weight-average molecular weight (M_w) distributions were measured by gel permeation chromatography (GPC). The intensity–retention time spectra of samples were shown in Fig. 1. According to the theory of exclusion chromatography, the peak of larger M_w polymer will emerge in shorter retention time. Thus, the M_ws of SAF and SEAF are obviously higher than that of AF. Moreover, their numeric M_ws were also calculated through comparing with the standard curve. The M_ws of AF, SAF and SEAF are 1751 Da, 7471 Da and 5277 Da, respectively (see Table 1). This result indicated that the introduction of sulfonic groups enhanced the polymerization degree of products due to their improved solubility in water reaction condition. Meanwhile, the lower M_w of SEAF than that of SAF shows the weaker reactivity of amidogens which limited the polymerization. The polymer structure of AF, SAF and SEAF can be exactly confirmed from the M_w results.

Scheme 1 Synthesis routes and chemical structures of AF, SAF and SEAF. Parameters of s, m, n, x, y and z denote the polymerization degree of each unit, and they are different from each other.

Fig. 1 M_w distributions spectra of AF, SAF and SEAF (intensity–time curves); AF was dissolved in THF, SAF and SEAF were dissolved in water.

Table 1 Molecular weights, element contents (C, H, N, S) and optical properties of AF, SAF and SEAF^a

Samples	Mw (Da)	λab (nm)	λem (nm)	Eg (eV)	QY (%)	τ [ns]		Element content (%)
						Solution	Powder	C	H	N	S
a Abbreviation: Mw = molecular weight, λab = absorption wavelength, λem = maximum emission wavelength, Eg = energy gap, QY = quantum yield (solution), τ = fluorescence lifetime.
AF	1751	215, 300	455	2.48	3.3	9.31	4.51	66.26	6.58	—	—
SAF	7471	208, 250, 320	551	2.25	1.8	6.98	3.70	32.98	4.56	—	9.91
SEAF	5277	200, 250, 318	534	1.91	7.3	10.33	4.62	35.84	5.59	1.53	8.37

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To further study the fundamental structural characteristics and their differences of the three polymers, Fourier transform infrared spectroscopy (FTIR) and hydrogen nuclear magnetic resonance (¹H-NMR) spectra were measured and shown in Fig. 2 and S1,† respectively. On account of their different solubility in deuterated solvent. The ¹H-NMR spectra of AF (dissolved in DMSO), SAF and SEAF (dissolved in D₂O) with the same amount were tested (see Fig. S1†). The broad peaks between 1.0 ppm and 4.0 ppm are ascribed to the long alkyl chain of the three samples. Compared with AF, the chemical shift of the main peaks have a lightly shift of SAF and SEAF due to the introduction of sulfonic groups (and imino groups of SEAF). Particularly, in terms of SEAF, there is a new peak at 4.5 ppm which can be assigned to the methylene introduced by ethylenediamine. Furthermore, Fig. 2 presents the FTIR spectra of AF, SAF and SEAF. Obviously, there are considerable enhancements of SAF and SEAF at the peak of 1043 cm⁻¹ (A₁₀₄₃) which is corresponded to S–O stretching vibrations. In contrast, vibrations of SAF and SEAF at 1705 cm⁻¹ (A₁₇₀₅), which are attributed to the C=O carbonyl stretching, are weaker than that of AF. Besides, the proportion (A₃₄₄₄/A₂₉₃₂) of hydroxyl (A₃₄₄₄) relative to methylene (A₂₉₃₂) significantly increases from AF to SAF. It reflects that a plenty of sulfonic groups have been introduced by reacting with segmental carbonyl groups and changing them into hydroxyl groups. That is also the origin of the excellent water-solubility of SAF and SEAF rather than AF. As for SEAF, the unique peaks at 1511 cm⁻¹ (A₁₅₁₁) and 3015 cm⁻¹ (A₃₀₁₅) are assigned to the C–N stretching mode and the integration of the imino groups and methylene vibration, respectively. It indicated the existence of imino groups which were introduced by ethylenediamine. It is worth mentioning that the peak of imino group is usually emerged at around 3300 cm⁻¹.²⁰ This peak shift is caused by the cumulated effect from ionic and hydrogen bonds of imino groups due to the existence of hydroxyl, imino and sulfonic groups.^21–23

Fig. 2 FTIR spectra of AF, SAF and SEAF.

In addition, elemental analysis further confirmed the structure properties and the results were tabulated in Table 1. The S contents of SAF and SEAF are 9.91% and 8.37% respectively originated from the sulfonation. Meanwhile, the contents of carbon inversely decreased about 2-fold from AF to SAF and SEAF due to the increase of oxygen content within the introduced sulfonic groups. As for SEAF, the content of N is 1.53% demonstrating the existence of imino groups. Elemental analysis commendably verified the above results of ¹H-NMR and FTIR spectra. All these results manifested the successful polymerization of ethylenediamine, acetone and formaldehyde. The synthesis routes and structures of these three polymers had been demonstrated comprehensively and displayed in Scheme 1.

Characterization of photoluminescence

The UV-Vis absorption and PL spectra of AF, SAF and SEAF were measured as shown in Fig. 3, and their detailed optical properties were displayed in Table 1. AF was tested in ethanol (CH₃CH₂OH) because it is not soluble in water, however, both SAF and SEAF are extremely soluble in water. All of these samples show an obvious UV absorption between 200 nm and 500 nm, and the band gaps of SAF and SEAF decrease comparing with that of AF (see Table 1). More interestingly, under the same concentration of 0.5 g L⁻¹, SEAF possesses the highest PL intensity and AF shows the lowest one. The maximum emission wavelength of SAF and SEAF are around 550 nm which show a huge redshift around 100 nm from that of AF. What's more, their fluorescence can be intuitively seen from the insert picture likewise. Under UV lamp, SEAF shows a bright green fluorescence and it is the brightest one among these three samples, and that of SAF is brighter than AF. Besides, both of SEAF and SAF present a green color, but AF exhibits a weak sky-blue light. These results demonstrated that the introduction of sulfonic or imino groups enhanced the emission and adjusted PL wavelength.

Fig. 3 UV-Vis absorption with normalized absorbance and PL spectra at 0.5 g L⁻¹ of AF (in ethanol), SAF and SEAF (both in H₂O), λ_ex (AF) is 320 nm, λ_ex (SAF, SEAF) is 400 nm. Inset: image of liquid samples taken under UV illumination at the same concentration, [c] = 0.5 g L⁻¹.

Significantly, the enhanced fluorescence of SEAF can be embodied more remarkably in solid state. Fig. 4 presents the comparison photographs under sunlight and UV lamp of solid samples. All samples display alike tawny color under sunlight, however, only SEAF presents bright yellow fluorescence under UV lamp. It indicated that the introduction of amino groups effectively enhanced the photoluminescence, especially in solid state. In fact, it is an important feature of AIE or AEE fluorescent materials.

Fig. 4 Comparison photographs under sunlight and UV lamp of solid AF, SAF and SEAF, (a) samples under sunlight, (b) samples under UV lamp.

Furthermore, to identify their specific photoluminescence, the quantum yields (QY) in solution and fluorescence lifetimes (τ) in solution and powder were measured and shown in Table 1. Unexpectedly, the quantum yields of all these three samples are less than 10%, but it is reasonable for AIE or AEE materials.^24,25 SEAF shows the highest QY of 7.3%, and that of SAF (1.8%) is the lowest one even lower than that of AF (3.3%). In fact, SAF shows brighter emission under a higher concentration at 0.5 g L⁻¹ than that of AF (see insert picture in Fig. 2). Meanwhile, the fluorescence lifetimes (τ) of these samples range from 6 to 11 ns in solution, and it is lower in powder around 4 ns. Above all, SEAF shows the longest lifetime among these three samples both in solution and powder states (see Table 1 and Fig. S2†).

Cluster induced emission of carbonyl groups

Based on the totally non-conjugated molecular structure of these polymers, we proposed the cluster induced emission (CIE) of carbonyl groups as the luminous mechanism which had been reported previously. Lu and his co-works reported the strong blue photoluminescence of PAMAM dendrimer caused by the aggregation of carbonyl groups.⁹ Zhao reported a pure oxygenic nonconjugated macromolecule (PMV) with emission generated from the clustering of the locked carbonyl groups. PMV exhibits bright blue and green emission in THF and DMSO or DMF, respectively.¹⁰ Interestingly, it is very similar that AF displays blue emission while SAF and SEAF show green fluorescence. Thus, as for these three samples, cluster of carbonyl groups are proposed to be the emission mechanism of their unconventional photoluminescence. Moreover, the strong hydrogen bonds within the polymers play a key role to their fluorescence as it promotes the clustering of carbonyl groups.^21,22 As for SAF and SEAF, the introduced sulfonic groups and increased hydroxyl together improved their hydrogen bond effects. The effects of hydrogen bonds can effectively enhance the aggregation of intermolecular and cluster of carbonyl groups. It is the reason why SAF and SEAF show higher PL intensity than that of AF at the same concentration. Particularly, the imino groups of SEAF further enhanced its hydrogen bond and originated ionic bonds with sulfonic groups. As we know, there is a strong electrostatic interaction between amino and sulfonic group.²⁶ In other words, for SEAF, the electrostatic interaction between sulfonic and amino groups contributes to its remarkable fluorescence enhancement.

To confirm the aggregation behavior of these polymers, their particle sizes distributions were measured by DLS and shown in Fig. 5. The samples possess relatively narrow distributions of particle sizes, indicating that they exist as inerratic aggregate state with nanoparticles. It is mentionable that the average particle size of SEAF (around 1000 nm) is about one fold larger than that of AF and SAF (both are around 500 nm). It illustrates that the aggregation behavior of SEAF is the most notable one among the three polymers. That is why the solid SEAF uniquely presents bright fluorescence under UV lamp than AF and SAF. Furthermore, their morphology images were also detected by atomic force microscope (AFM) with samples dried on mica plates at the same concentration and shown in Fig. 6. Apparently, all these three samples display the evenly dispersed nanoparticles. On account of the different solvents, the higher solvent evaporation rate of AF (in ethanol) makes the aggregates more homogeneous and compact than that of SAF and SEAF (in water). Moreover, the nanoparticles size of SEAF is obviously higher than that of SAF, which is consistent with the results of their particle sizes distributions.

Fig. 5 Particle size distributions of AF, SAF and SEAF with concentration of 0.5 g L⁻¹. AF in ethanol, SAF and SEAF in water.

Fig. 6 AFM images of AF, SAF and SEAF spread out on mica plates at the concentration of 0.5 g L⁻¹. Their sizes are 5 × 5 μm, (a) AF in ethanol, (b) SAF and (c) SEAF in water.

More importantly, this result motivated us to study the effect of aggregation on the fluorescence enhancement as the following works.

Aggregation-enhanced emission (AEE) property

Based on the results above, we tried to explore whether they possess AEE effect. Contrary to SAF and SEAF, ethanol and water respectively are the good and poor solvents of AF. We prepared a series of solutions with poor and good solvent mixtures with different ratios under a certain sample (AF, SAF and SEAF) concentration. PL spectra of them in water/ethanol mixtures with different poor solvent fractions were shown in Fig. 7. AF shows ACQ effect as the PL intensity decreased continuously with the increase of water fraction from 0 to 90% (see Fig. 7a). However, opposite phenomenon is found in SAF and SEAF that the PL intensity increased continuously with the increase of ethanol fraction. It indicates that both of SAF and SEAF possess AEE effect. These results can obviously be obtained from the plot of I/I₀ versus poor solvent fractions in the water/ethanol mixture (see Fig. 7d). The variation trend of I/I₀ with the increase of poor solvent fractions more intuitively revealed the change of PL intensity. It is clear that SAF and SEAF are AEE material, but AF is not. Moreover, the corresponding UV absorption spectra of these three samples during adding poor solvent process were also measured and shown in Fig. S3.† With the increase of poor solvent, all samples show similar profiles with a slight variation in different mixture proportions. Particularly, the UV spectra of SEAF show a significant change in the wavelength of 250 nm (see Fig. S3c†). After the ethanol concentration reached 40%, the peak shape at 250 nm become more evident. It might be caused by the formation of nanoparticles, which reflects the aggregation behavior induced by adding ethanol.²⁷

Fig. 7 PL spectra of samples in water/ethanol mixtures with different poor solvent fractions (f_w or f_e). (a) AF, water is the poor solvent. (b) SAF, ethanol is the poor solvent. (c) SEAF, ethanol is the poor solvent. λ_ex = 320 nm, [c] = 1.0 × 10⁻³ g L⁻¹. (d) Plot of I/I₀ versus poor solvent fraction in the water/ethanol mixture (I is PL intensity in water/ethanol mixtures with different poor solvent fractions; I₀ is PL intensity in pure good solvent).

The mechanism of AIE and AEE has been widely recognized as effects of the restriction of intramolecular rotations (RIR) which blocks the non-radiative pathway and opens up the radiative channel.⁴ Moreover, it has been approved that hydrogen bonds within specific luminogens can effectively rigidify their molecular structures and activate their RIR processes.^28,29 That is also the reason why SAF and SEAF possess AEE property but AF do not. To further identify the existence of RIR effect for SAF and SEAF, the glycerin experiments were conducted. Using glycerin as the additive to test viscosity effect of AIE or AEE materials is an effective method to detect RIR effect.³⁰ As shown in Fig. 8, the PL intensity of both SAF and SEAF increase continuously with a similar variation tendency. Through the linear fitting, the obvious turning points of their increasing trend can be found at glycerin concentration around 70%. The dominated factor of the emission enhancement is viscosity when glycerin ration is below 70%, while aggregation became the major factor after glycerin ration exceeded 70%. Hence, the RIR effect caused by aggregation is proposed as the mechanism of AEE effect for SAF and SEAF.^31,32

Fig. 8 The variation trends for PL intensity of SAF and SEAF in water/glycerin mixtures with glycerin fractions from 0 to 90%. λ_ex = 320 nm, [c] = 1.0 × 10⁻³ g L⁻¹.

Based on the discussions above, we vividly described the proposed mechanism of the unconventional fluorescence and AEE properties of SEAF in Scheme 2. In addition, these polymers deserve to be attached importance because they would provide a new scaffold for the design of water-soluble luminescent materials with application potential in the fields of bioimaging and biosensor.⁸ What's more, the synthesis method of SEAF based on the SAF structure can be applied to prepare highly efficient luminogens in the future and the relative work is under progress in our lab.

Scheme 2 Proposed mechanism of the photoluminescence and AEE property of SEAF, (a) the chemical structure of SEAF; (b) schematic mechanism of emission and AEE processes; (c) schematic molecular interaction for SEAF cluster.

Conclusions

We reported the unrevealed photoluminescence of acetone–formaldehyde (AF) and sulfonated acetone–formaldehyde (SAF) condensate. Moreover, sulfonated ethylenediamine–acetone–formaldehyde (SEAF) was prepared by introducing ethylene-diamine to the SAF backbone and it exhibited highly enhanced fluorescence both in solution and solid state. Cluster induced emission of carbonyl groups caused by hydrogen and ionic bond was proposed as the mechanism of their unconventional photoluminescence. More importantly, AF showed ACQ behavior, while SAF and SEAF possessed AEE effects due to the RIR mechanism during the aggregation process. Our research provided a novel perspective for preparing water-soluble polymers with efficient photoluminescence and AEE property.

Acknowledgements

The authors would like to acknowledge the financial support of National Natural Science Foundation of China (21402054, 21436004), International S&T Cooperation Program of China (2013DFA41670), Guangdong Innovative Research Team Program of China (201101C0105067115).

Notes and references

1. R. H. Friend, R. W. Gymer, A. B. Holmes, J. H. Burroughes, R. N. Marks, C. Taliani, D. D. C. Bradley, D. A. Dos Santos, J. L. Brédas, M. Lögdlund and W. R. Salaneck, Nature, 1999, 397, 121–128.
2. S. Günes, H. Neugebauer and N. S. Sariciftci, Chem. Rev., 2007, 107, 1324–1338.
3. J. D. Luo, Z. L. Xie, J. W. Lam, L. Cheng, H. Y. Chen, C. F. Qiu, H. S. Kwok, X. W. Zhan, Y. Q. Liu, D. B. Zhu and B. Z. Tang, Chem. Commun., 2001, 18, 1740–1741.
4. J. Mei, N. L. Leung, R. T. Kwok, J. W. Lam and B. Z. Tang, Chem. Rev., 2015, 115, 11718–11940.
5. Y. N. Hong, J. W. Y. Lam and B. Z. Tang, Chem. Soc. Rev., 2011, 40, 5361–5388.
6. R. R. Hu, N. L. C. Leung and B. Z. Tang, Chem. Soc. Rev., 2014, 43, 4494–4562.
7. D. Ding, K. Li, B. Liu and B. Z. Tang, Acc. Chem. Res., 2013, 46, 2441–2453.
8. Y. Y. Yuan, C. J. Zhang, S. D. Xu and B. Liu, Chem. Sci., 2016, 7, 1862–1866.
9. H. Lu, L. L. Feng, S. S. Li, J. Zhang, H. F. Lu and S. Y. Feng, Macromolecules, 2015, 48, 476–482.
10. E. G. Zhao, J. W. Lam, L. M. Meng, Y. N. Hong, H. Q. Deng, G. X. Bai, X. H. Huang, J. H. Hao and B. Z. Tang, Macromolecules, 2014, 48, 64–71.
11. Y. Ruff, E. Buhler, S. J. Candau, E. Kesselman, Y. Talmon and J. M. Lehn, J. Am. Chem. Soc., 2010, 132, 2573–2584.
12. S. J. Zhu, Y. B. Song, J. R. Shao, X. H. Zhao and B. Yang, Angew. Chem., Int. Ed., 2015, 54, 14626–14637.
13. S. J. Zhu, L. Wang, N. Zhou, X. H. Zhao, Y. B. Song, S. Maharjan, J. H. Zhang, L. J. Lu, H. Y. Wang and B. Yang, Chem. Commun., 2014, 50, 13845–13848.
14. S. Niu, H. X. Yan, Z. Y. Chen, L. X. Yuan, T. Y. Liu and C. Liu, Macromol. Rapid Commun., 2016, 37, 136–142.
15. M. S. Pei, Y. Q. Yang, X. Z. Zhang, J. Zhang and Y. B. Li, J. Appl. Polym. Sci., 2004, 91, 3248–3250.
16. H. M. Lou, K. B. Ji, H. K. Lin, Y. X. Pang, Y. H. Deng, X. Q. Qiu, H. B. Zhang and Z. G. Xie, Cem. Concr. Res., 2012, 42, 1043–1048.
17. X. Qiu, M. Zhou, D. Yang, H. Lou, X. Ouyang and Y. Pang, Fuel, 2007, 86, 1439–1445.
18. B. List, P. Pojarliev, W. T. Biller and H. J. Martin, J. Am. Chem. Soc., 2002, 124, 827–833.
19. A. Córdova, Acc. Chem. Res., 2004, 37, 102–112.
20. C. Zhou, T. Q. Xie, R. Zhou, C. O. Trindle, Y. Tikman, X. Y. Zhang and G. Q. Zhang, ACS Appl. Mater. Interfaces, 2015, 7, 17209–17216.
21. M. G. Mohamed, F. H. Lu, J. L. Hong and S. W. Kuo, Polym. Chem., 2015, 6, 6340–6350.
22. K. Y. Shih, Y. C. Lin, T. S. Hsiao, S. L. Deng, S. W. Kuo and J. L. Hong, Polym. Chem., 2014, 5, 5765–5774.
23. Y. Q. Sun, P. Wang, J. Liu, J. Y. Zhang and W. Guo, Analyst, 2012, 137, 3430–3433.
24. J. Q. Shi, N. Chang, C. H. Li, J. Mei, C. M. Deng, X. L. Luo, Z. P. Liu, Z. S. Bo, Y. Q. Dong and B. Z. Tang, Chem. Commun., 2012, 48, 10675–10677.
25. H. Tong, Y. N. Hong, Y. Q. Dong, M. Häussler, Z. Li, J. W. Lam, Y. P. Dong, H. H. Y. Sung, I. D. Williams and B. Z. Tang, J. Phys. Chem. B, 2007, 111, 11817–11823.
26. K. Dhara, Y. Hori, R. Baba and K. Kikuchi, Chem. Commun., 2012, 48, 11534–11536.
27. J. W. Chen, Z. L. Xie, J. W. Lam, C. C. W. Law and B. Z. Tang, Macromolecules, 2003, 36, 1108–1117.
28. C. Y. Bao, R. Lu, M. Jin, P. C. Xue, C. H. Tan, G. F. Liu and Y. Y. Zhao, Org. Biomol. Chem., 2005, 3, 2508–2512.
29. H. Zhou, Q. Ye, X. Y. Wu, J. Song, C. M. Cho, Y. Zong, B. Z. Tang, T. S. Andy Hor, E. K. L. Yeow and J. W. Xu, J. Mater. Chem. C, 2015, 3, 11874–11880.
30. J. W. Chen, C. C. W. Law, J. W. Y. Lam, Y. P. Deng, S. M. F Lo, L. D. Williams, D. B. Zhu and B. Z. Tang, Chem. Mater., 2003, 15, 1535–1546.
31. Y. Y. Xue, X. Q. Qiu, Y. Wu, Y. Qian, M. S. Zhou, Y. H. Deng and Y. Li, Polym. Chem., 2016, 7, 3502–3508.
32. W. Yu, Z. Y. Wang, D. J. Yang, X. P. Ouyang, X. Q. Qiu and Y. Li, RSC Adv., 2016, 6, 47632–47636.

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Footnote

† Electronic supplementary information (ESI) available: Experimental details including materials, synthetic procedures and characterization methods; supplemental figures of ¹H-NMR, fluorescence decay and UV absorption spectra. See DOI: 10.1039/c6ra06227j

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