Nonconventional photoluminescence from sulfonated acetone–formaldehyde condensate with aggregation-enhanced emission

Wei Yu ab, Zhongyu Wang ab, Dongjie Yang ab, Xinping Ouyang ab, Xueqing Qiu *ab and Yuan Li *ab
aSchool of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, China. E-mail: celiy@scut.edu.cn; Fax: +86-20-87114033; Tel: +86-20-87114033
bState Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou, China. E-mail: cexqqiu@scut.edu; Fax: +86-20-87114722; Tel: +86-20-87114722

Received 9th January 2016 , Accepted 29th April 2016

First published on 9th May 2016


Abstract

The fluorescence of sulfonated acetone–formaldehyde condensate (SAF) with a nonconventional chromophore is reported for the firsttime. The fluorescence intensity of SAF can be enhanced by the introduction of phenols to obtain sulfonated phenol–acetone–formaldehyde (SPAF) and the emission color can be changed. Both of them show aggregation-enhanced emission (AEE) properties.


Water-soluble sulfonated acetone–formaldehyde (SAF) was a relatively low cost and high performance resin developed in the 1990s.1 Due to the structure of a long non-conjugated aliphatic chain with plentiful hydroxyl and sulfonic groups, SAF has been widely studied and used as a dispersant in the construction industry.2,3 Accidentally, we found that SAF had obvious fluorescent performance which had been ignored by previous researchers. Nonconventional luminescent polymers without traditional chromophores have rarely been reported and have attracted increasing attention.4–14 Cluster induced emission of carbonyl groups has been widely approved as the emission mechanism of these polymers.12–14 Pucci found that polymers (PIBSA and PIBSI) with a non-emissive chromophore showed AIE property caused by the cluster of carbonyl groups.12 As for SAF, the structure was analogous to PIB derivatives on a long aliphatic chain with carbonyl groups, and without an emissive chromophore. In many cases, aggregation causes partial or even complete quenching of emission called aggregation-caused quenching (ACQ) which greatly limits the application of luminescent materials.15 However, several luminescent systems conversely show aggregation-induced emission (AIE) or aggregation-enhanced emission (AEE) phenomenon, where aggregation facilitates light-emitting process.16–19 The mechanism has been widely recognized as the restriction of intramolecular rotation (RIR) which blocks the non-radiative pathway and opens up the radiative channel.20,21 It can be proposed that cluster of carbonyl groups induces the emission of SAF, and further aggregation caused its AEE behavior through RIR effect. The mechanism of these two emission processes were vividly displayed in Scheme 1.
image file: c6ra00718j-s1.tif
Scheme 1 (a) The chemical structures of SAF and SPAFs, SPAFs denotes NSPAF and t-SPAF with n-C9H19 and t-Bu, respectively; (b) schematic mechanism of emission induced by cluster of carbonyl groups and emission enhanced by aggregation.

In order to improve fluorescent intensity of SAF, we introduced rigid aromatic ring to its structure using nonylphenol and p-tert-butylphenol as another raw material, and successfully synthesized sulfonated phenol–acetone–formaldehyde polymers (NSPAF and t-SPAF, respectively). As a result, SPAFs possessed more strong fluorescence but relatively weaker AEE characteristic than that of SAF. The introduction of aromatic rings enhanced the rigidness of polymer backbone and changed the amphipathy which promoted the cluster effect.22 Fluorescent materials with AIE or AEE properties had been extensively studied in sensor and bioimaging fields because it can effectively eliminate the influence of ACQ.23–25 Compared with traditional AIE or AEE materials, there are several interesting aspects of SAF and SPAFs: (1) the raw materials are cheap and accessible, (2) the synthesis process is very simple, (3) the good water solubility can make an organic solvent-free route. In a word, it provides a powerful scaffold to synthesize water-soluble fluorescent materials with AEE characteristic which have a good application prospective. In addition, it is worth to further study the structure of SPAFs that can be modified by tailoring the phenol derivatives in the future.

Initially, it was an exciting result that we found the phenomenon of bright green photoluminescence (PL) on SAF. In order to study the structural features of SAF and SPAFs, fundamental characterization of molecular weight, 1H-NMR and FTIR spectra, element contents were measured and shown in ESI. Fig. S1 shows that the weight-average molecular weights (Mws) of all samples are higher than 6000 Da that effectively confirms their polymer structure. The Mws of SPAFs are smaller than that of SAF, indicating that the introduction of phenols inhibits the polymerization activity. Comparing with SAF, the 1H-NMR spectra of SPAFs have a new wide signal between 6.0 ppm and 8.0 ppm which comes from the efficient introduction of the phenols (see Fig. S2a). The wide proton signals between 0.50 ppm and 4 ppm are associated to the long aliphatic chains of these three samples. It is accordant with the Mw results that they are polymers. Moreover, the characteristic vibrations of S–O (A1043), C–O–C (A1187), C[double bond, length as m-dash]O (A1705) in the FTIR spectra of all samples reflect the existence of sulfonic groups, ether bonds and carbonyl groups (see Fig. S2b). Besides, the high element contents of S for all samples indicate the high sulfonation degree (shown in Table S1). These results effectively demonstrated the successful polycondensation process. Based on the results analyzed above, we proposed the structures and synthesis routes of SAF and SPAFs in Scheme S1.

In the following study, we conducted detailed investigation on fluorescence of these polymers. Their UV-Vis absorption and PL spectra were measured and shown in Fig. 1. There is a wide absorption between 190 nm and 500 nm in the UV spectra of all samples. It reveals the strong absorption ability of ultraviolet light and the existence of fluorescence as the PL spectra displayed. The UV spectra were adjusted at the similar absorbance in order to evaluate the PL intensity more effectively. The maximum emission wavelengths of SPAFs are around 475 nm which has a blue-shift (about 50 nm) from the 525 nm of SAF. More importantly, SPAFs possess much higher PL intensity than that of SAF, and NSPAF is the highest one. It is clear that SPAFs show more remarkable blue fluorescence than weak green emission of SAF at the same concentration of 0.5 g L−1 (from the inserted image). This result can be quantified by the quantum yield which had been calculated and showed in Table S1. NSPAF has the highest quantum yield of 8.3%, followed by 4.3% of t-SPAF. The quantum yields of SPAFs showed an obvious increase from 1.4% of SAF. Although the quantum yields of these three samples are all lower than 10%, it is reasonable for the particular AIE or AEE systems.26–28 The measurement of quantum yields in solution is commonly controlled at the UV absorbance of samples less than 0.06. Under this extremely low concentration, the fluorescence is weak for AIE or AEE materials. For instance, tetraphenylethene-a typical AIE micromolecule, has a bright green fluorescence in high concentration but very low quantum yield less than 0.6% in good solvent (THF).28


image file: c6ra00718j-f1.tif
Fig. 1 UV-Vis absorption and PL spectra of SAF, NSPAF and t-SPAF in aqueous, λex = 320 nm; inset: photograph of liquid samples taken under UV illumination at the concentration of 0.5 g L−1.

For uncovering this interesting phenomenon thoroughly, the effect of concentration on fluorescence properties of NSPAF and SAF was investigated shown in Fig. S3. It is similar both of these two polymers that PL intensity and fluorescent wavelength present significant changes. In order to judge this variation intuitively, the curves of PL intensity and maximum emission wavelength versus gradient concentrations in water were given. With the increase of concentration, PL intensity of these two samples increased at first and then decreased with a highest point, the wavelength had a continuous growth with red shift more than 100 nm. There was a sharply drop of PL intensity after the inflexion due to it had exceeded the critical aggregation concentration. For NSPAF, the increase of fluorescent wavelength was unconspicuous before its inflexion at concentration of 0.1 g L−1. But the increase trend of SAF in the same section was very remarkable. These results reflect the great aggregation effect of these polymers and SAF is stronger than others. Moreover, it also had been confirmed by the particle size distributions from dynamic light scattering (DLS) measurement and AFM image in Fig. 2. Obviously, SAF can be evenly dispersed as nanoparticles after drying on a substrate (insert image). The average particle sizes of SAF and SPAFs are around 500 nm and 300 nm, respectively. Therefore, all of these polymers possess conspicuous aggregation behavior that promotes the cluster of carbonyl groups, which is the origin of fluorescence.


image file: c6ra00718j-f2.tif
Fig. 2 Particle sizes distributions of SAF, NSPAF and t-SPAF in aqueous by DLS measurement. Inset: AFM image of SAF dried on mica plate with a concentration of 0.5 g L−1.

Inspired by the strong aggregation effects of these samples, we decided to study aggregation-enhanced emission (AEE) effects by a readily method. Under a certain concentration, we designed a gradient volume fractions of poor solvent to detect the change of PL intensity.16 Using ethanol and water as the poor and good solvent respectively and the experimental results were presented in Fig. 3. Fig. 3(a)–(c) shows that PL intensity of all three polymers are enhanced with the increase of ethanol fractions (fw). To obtain a more intuitionistic understanding of this phenomenon, the variation trends of I/I0 and maximum emission wavelength with different fw were shown in Fig. 3(d). The values of I/I0 increased continuously with the increase of fw for all samples, and the increase gradient can effectively reflect the intensity of AEE effect. Thus, all samples possess AEE property and SAF shows more remarkable behavior than SPAFs. Meanwhile, it should be pointed out that the maximum emission wavelengths show remarkable blue-shift in this process, especially SAF with around 100 nm. It is a typical phenomenon of AIE or AEE materials and its mechanism was proposed as the twisted intramolecular charge transfer (TICT) motion.29,30


image file: c6ra00718j-f3.tif
Fig. 3 PL spectra of the samples in water/ethanol mixtures with different ethanol fractions (fw); (a) SAF, (b) NSPAF, (c) t-SPAF, λex = 320 nm, [c] = 1.0 × 10−3 g L−1; (d) solid line: plot of I/I0 versus fw in the water/ethanol mixture (I are PL intensity in water/ethanol mixtures; I0 is PL intensity in pure water solution); dashed: the variation trends of maximum emission wavelength with different ethanol fractions.

Moreover, the corresponding UV-Vis absorption spectra in the same process with increasing ethanol fractions were also measured.16,31 Fig. S4 shows the variation in UV spectra of SAF and SPAFs with fw from 0 to 90%. On the whole, UV spectra of each sample exhibit similar profiles accompanied by a slight enhancement with the increase of fw, especially in SAF. In this process, the sample concentration stayed constant and baseline correction was made when updating the solvent ratio continuously during the UV measurement. It reflects the change of their aggregate state in solution with the addition of ethanol. To further identify the formation of nanoparticles in this process, morphology images of samples in water/ethanol mixtures were provided. Fig. 4 presents the AFM images of SAF and SPAFs with fw at 90% in the concentration of 0.1 g L−1 as 1.0 × 10−3 g L−1 is too dilute for morphology test. Nanoparticles can be clearly observed of all these three samples in the sizes around several hundred nanometers. This result manifests the formation of nanoparticles and it contributes to further fluorescent enhancement. Therefore, these polymers possess aggregation-enhanced emission (AEE) property.


image file: c6ra00718j-f4.tif
Fig. 4 AFM images of SAF, NSPAF and t-SPAF in water/ethanol mixtures with ethanol fractions (fw) of 90% dried on mica plate, (a) SAF, (b) NSPAF, (c) t-SPAF, [c] = 0.1 g L−1.

For the mechanism of AEE effect, Tang and his co-workers proposed that it is induced by restriction of intramolecular rotations effect (RIR) derived from the aggregation process.20,21 To demonstrate the existence of RIR effect, the effect of viscosity was tested using glycerin as the additive.32,33 The PL intensity of samples in water/glycerin mixtures with different glycerin ratios from 0 to 90% were measured. Fig. 5 shows that the intensity of SAF increased continuously, and there is a turning point of its increasing rate at the concentration of 60%. When the ration of glycerin was below 60%, the dominating factor of the emission enhancement was the viscosity, however, the aggregation became to be the main reason since the ration of glycerin overtook 60%. Meanwhile, in this process, the emission peak also showed a blue-shift of 100 nm. As same as SAF, the turning points also can be found in the PL intensity variations of SPAFs at ration of glycerin around 70% (see Fig. S5). Therefore, the RIR effect was proposed as the mechanism of AEE effect among these three samples and it is caused by aggregation, which is similar to our recent work on lignin fluorescence.33


image file: c6ra00718j-f5.tif
Fig. 5 The variation curves for PL intensity (blue line) and maximum emission wavelength (green line) of SAF in water/glycerin mixtures with glycerin fractions from 0 to 90%. λex = 320 nm, [c] = 1.0 × 10−3 g L−1.

Overall, according to the nonconventional emission mechanism of similar structures reported previously, cluster of carbonyl groups is proposed as the fluorescent origin of SAF and SPAFs. On account of the existence of hydroxy, carbonyl, and sulfonic groups in their structures, they tend to aggregate in aqueous solution. What's more, it must be noted that green emission has been rarely reported on this similar system, and a large number of sulfonic groups make these polymers have excellent water solubility. On the other hand, the remarkable AEE effect also can be detected in these three polymers by adding poor solvent to promote the aggregation of molecules. In this process, RIR mechanism of AEE active was proved by the glycerol experiment. In addition, SPAFs show stronger fluorescence but weaker AEE effect compared with SAF. The introduced aromatic rings enhance the rigidness of polymer backbone and change the amphipathy which promotes the cluster and restrained the aggregation of the polymers.34

Conclusions

We revealed the photoluminescence of SAF which was prepared from cheap and widely available raw materials. SPAFs with stronger fluorescence were also prepared by introducing aromatic ring to the backbone. It is a novel system of water-soluble fluorescent polymer with nonconventional chromophore and AEE properties. The cluster of carbonyl groups was demonstrated as the emission mechanism and RIR effect was proposed as the mechanism of AEE active during the aggregation process. Besides, we can readily obtain different derivations with diverse fluorescent properties by altering the phenols into other building blocks, which might readily adjust the color of emission and fluorescence quantum yield. Our result shows a potential fundamental research value in the area of water soluble fluorescent material.

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).

Notes and references

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Footnote

Electronic supplementary information (ESI) available: Experimental details and additional figures and analysis. See DOI: 10.1039/c6ra00718j

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