Various Tetraphenylethene-Based AIEgens with Four Functional

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Various Tetraphenylethen-based AIEgens with Four Functional Polymer Arms: Versatile Synthetic Approach and Photophysical Properties Xiaolin Guan, Donghai Zhang, Li Meng, Yang Zhang, Tianming Jia, Qijun Jin, Qiangbing Wei, Dedai Lu, and Hengchang Ma Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b03780 • Publication Date (Web): 23 Dec 2016 Downloaded from http://pubs.acs.org on December 28, 2016

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Industrial & Engineering Chemistry Research

Various Tetraphenylethen-based AIEgens with Four Functional Polymer Arms: Versatile Synthetic Approach and Photophysical Properties Xiaolin Guan, * Donghai Zhang, Li Meng, Yang Zhang, Tianming Jia, Qijun Jin, Qiangbing Wei, Dedai Lu and Hengchang Ma Key Laboratory of Eco-Environment-Related Polymer Materials Ministry of Education, Key Laboratory of Polymer Materials Ministry of Gansu Province, College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou, Gansu 730070, P.R. China * E-mail: [email protected]

Abstract: There are a lot of demands to develop abundant superior properties of aggregation-induced emission (AIE) polymers used in aggregation state. In this work, we reported a practical and versatile approach of preparation of AIE polymers by conventional free radical polymerization. As an initiator, TPE-AZO has excellent ability to initiate various kinds of vinyl monomers to obtain AIE functional materials. It was found that TPE-polymers emit multiple colors from 370 to 482 nm, and they

also

exhibited

the

advantages of combining the AIE characteristic with unique properties of polymers. The fluorescence properties of temperature-sensitive TPE-PNIPAM, pH-sensitive TPE-PMAA

and counterions-sensitive

TPE-PMETAC was investigated

ACS Paragon Plus Environment


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respectively. The results indicated that all the responsive behaviours of three TPE-polymers were related to the change of fluorescence. Our versatile approach would provide a platform to fabricate AIE polymers with various superior properties by AZO-based AIE molecular initiator under mild conditions.

1 Introduction Since a unique photophysical process of aggregation-induced emission (AIE) was discovered by Tang’s group, 1 the AIE fluorescent organic molecules have been one of the hottest topics in the field of lighting devices, fluorescent sensors and biological probes.2-7 A typical AIEgen, tetraphenylethene (TPE) consisting of a central olefin stator surrounded by four peripheral aromatic rotors (phenyl rings), has been widely investigated.8-14 Mechanistic studies suggest that the rotations of the peripheral phenyl rings against the olefinic stator around the single-bond axes consume the excited state energy lead to the isolated TPE molecules in a dilute solution is almost nonemissive. When the molecules are aggregated, the excited state energy consumed by non-radiative pathways is blocked, resulting from the effect of the restricted intramolecular rotation (RIR), and vitalized strong emission.15 Based on the principle of the RIR, a growing number of TPE small molecular derivatives have been designed.16-19 Jiang and coauthors

16

reported a novel sensitive and selective

fluorescence turn-on sensor for Zn2+ based on the coordination between tetraphenylethylene derivatives and Zn2+. Moreover, the corresponding ester


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precursor was successfully used for intracellular Zn2+ imaging. Liang and coauthors17 demonstrated the RIR characteristics of TPE derivatives of TR4 and successfully utilized it as a fluorescence bioprobe for tracking cell membranes. In order to restrict the molecular rotations easily, researchers are committed to modify the TPE molecule using polymers.20-28 As a matter of fact, AIE polymers materials have a number of advantages over small molecular materials. New types of attractive AIE materials will be generated by combining the polymer unique properties with AIE characteristic, and it will also give emission response towards external stimulus and possess potential application for the fluorescent sensors and imaging.29, 30 Wang and coworkers25 reported a novel fluorescence turn-on probe by attaching a large number of tetraphenylethene (TPE) labels to a chitosan (CS) chain, which can trace live cells over a long period of time. Li and coauthors26 constructed biocompatible cross-linked fluorescent polymeric nanoparticles (FPNs) based on hydrophilic PEG and hydrophobic TPE moieties, which are successfully used for cell imaging. Studies have shown that though introducing a smaller proportion of TPE moieties in a polymer chains, it will also be able to give the AIE characteristic throughout the polymer. There is no doubt that this work provides more choices to the researchers on designing diverse AIE functional materials, which combine AIE characteristic with excellent properties of polymer and can be better catered to the specific applications.15 With the unique properties endowed by the polymer, it is an exciting area to develop numerous AIE polymers and



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apply in different areas. Recently, a few papers have reported a fascinating way to generate AIE-active polymers by using the AIEgen-derived initiators.26-28 In order to meet the demands of wide applications of AIE luminogens in many fields, it is required to explore a novel TPE-derived initiator and develop a versatile method to covalently grafting different specific properties polymer on TPE molecules. In this work, a novel TPE-AZO initiator and a series of TPEpolymers owning specific properties were successfully synthesized by a practical and versatile approach, which have easy processability.31 These TPE-polymers successfully combined the AIE characteristic with excellent properties of polymer, and gave emission response towards aqueous with different pH, temperature and counterions. This work could provide a new versatile method to synthesize AIE functional materials possessing excellent properties. Obviously, the versatility of this polymerization method for the preparation of diverse AIE polymer is helpful to the design of AIE functional materials and makes them suitable candidates for optical devices, fluorescent sensors and bioimaging probes.

2 Experimental Section 2.1 Materials and Methods 4, 4-dihydroxybenzophenone (98%), titanium tetrachloride (TiCl4, 99%), zinc (Zn, AR), tetrahydrofuran (THF, AR), N, N-dicyclohexylcarbodiimide (DCC, 98%), 4, 4-Azobis(4-cyanovaleric acid) (ACVA, 98%), N-Vinyl-2-


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pyrrolidone (VP, 98%), N-isopropylacrylamide (NIPAM, 98%), N-Vinyl carbazole (VK, 98%), methyl acrylic acid (MAA, 99%), methyl methacrylate (MMA,

99%),

styrene

(S,

98%),

methyl

acrylate

(MA,

95%),

methacryloxyethyl-trimethyl ammonium chloride (METAC, 80% aqueous solution), Poly(ethylene glycol) methyl ether methacrylate (MPEGMA, average Mn 300) were purchased from Energy Chemical. The tetrahydrofuran was purified by distillation. 2.2 Characterizations 1

H NMR,

15

N NMR and

13

C NMR spectroscopic measurements were performed on

MERCURY spectrometer. MS measurements were carried out on Full-scan positive electrospray ionization-mass spectrometer. Fourier Transform infrared spectroscopy (FTIR) measured by Nicolet AVATAR 360 FT-IR infrared spectrometer. The Gel Permeation Chromatography (GPC) measurements were performed at a GPCV2000 Gel Permeation Chromatography (Waters, America). The fluorescence measurements were carried out on F97 Pro fluorometry at room temperature using a monochromator Xe lamp as an excitation source. The excitation and emission slits were both set at 5 nm. 2.3 Synthesis of the 1, 1, 2, 2-tetrakis (4-hydroxyphenyl) ethylene (TPE-OH) [6] A suspension of 4, 4’-dihydroxybenzophenone (10.0 g, 0.05 mol), Zn dust (12.2 g, 0.19 mol) and TiCl4 (10.0 mL, 0.09 mol) in 200 mL of dry tetrahydrofuran (THF) was refluxed under argon condition. After stirred for 24 h, the reaction mixture was cooled to room temperature and poured into



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K2CO3 aqueous solution (10%, 200 mL), then vigorous stirring for 10 min and filtered. The organic layer was separated and the organic fractions were evaporated, the resulting crude product was purified by a silica gel column using ethyl acetate-petroleun ether (1:1, v/v) as eluent. TPE-OH was obtained as slight yellow powder of 30% yield (3.1 g). 1H NMR (600 MHz, DMSO-d6, δ), 9.18 (s, 4H), 6.68 (d, J = 6.9 Hz, 8H), 6.45 (d, J = 8.0 Hz, 8H).

13

C NMR

(151 MHz, DMSO-d6, δ), 155.78, 138.12, 135.49, 132.37, 114.91. ESI-MS Calcd for C26H20O4: m/z 396.14. Found m/z 396.09. 2.4 Synthesis of the TPE-AZO 1.68 g (6.0 mmol) 4, 4-Azobis (4-cyanovaleric acid) (ACVA), 0.5 g (1.3 mmol) TPE-OH and 0.15 g (0.72 mmol) 4-(dimethylamino) pyridine-p-toluenesulfonate (DPTS) 32 was dissolved in 50 mL dry THF. A solution of degassed 1.48 g (7.2 mmol) of N, N-dicyclohexylcarbodiimide (DCC) in 20.0 mL THF was added into the co-solvent mixture solution drop by drop and stirred for 24 h. The resultant solution was filtered and the filtrate was evaporated. The resulting TPE-AZO was purified by ethyl acetate-ether (1:3, v/v) in three repeated cycles and obtained yellow powder of 0.4g. 1H NMR (400 MHz, DMSO-d6, δ), 7.36-7.28 (d, Ar-H), 7.16-7.10 (d, Ar-H), 3.54-3.50 (-CH2-C=O), 1.90-1.79 (-CH2-CH2-C=O), 1.24-1.22(-CH3). 15N NMR (61 MHz, DMSO-d6, δ), 134.42 (-N=N-), -119.90(-C≡N). IR (KBr): 2933 cm-1 (νas , -CH2-), 2853 cm-1 (νs , -CH2-), 1740 cm-1 (νas ,-C=O), 1628 cm-1 (ν,-C=C- of benzene ring), 1508 cm-1 (ν,-C=C- of benzene ring), 1458 cm-1(ν,-C=C- of benzene ring), 1382 cm-1 (νs, -CH3),1201 cm-1 (νas, Ar-O-C ), 1150 cm-1 (νas, C-O-C), 837 cm-1 (ϒ,


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-C-H of benzene ring). 2.5 Synthesis of the TPE-Polymers The samples of TPE-polymers were synthesized by radical polymerization using TPE-AZO as initiator. Details for the polymerization of TPE-METAC are as follows. Under argon condition, METAC (5.0 g) and TPE-AZO (0.05 g) in 30 mL of dry THF was stirred for 24 h at 75 ºC. The reaction mixture was cooled to room temperature and precipitated in 100 mL ethanol, the resulting white flocculent precipitate was collected by centrifugalization, and purified by water and ethanol (V: V =1:3) in three repeated cycles to give the product (3.4 g). The other polymerization reactions were similar to TPE-PMETAC. TPE-PMMA and TPE-PVK were precipitated in ethanol, TPE-PMAA, TPE-PNIPAM and TPE-PMPEGMA were precipitated in ether, TPE-PMA was precipitated in deionized water, TPE-PS was precipitated in methanol, TPE-PVP was precipitated in acetone, respectively.

3 Results and Discussion 3.1. Synthesis and Characterization of TPE-AZO and TPE-polymers As a versatile initiator for free radical polymerization, TPE-AZO was synthesized by DCC condensation reaction between hydroxyl group (-OH) in TPE-OH and carboxyl group (-COOH) in ACVA using DPTS as catalyst, and subsequently utilized it as a versatile initiator synthesized TPE-polymers by free radical polymerizations (Scheme 1). The TPE-AZO was characterized by 1H NMR (Figure S1),

15

N NMR

(Figure S2), FTIR (Figure S3) and GPC (Table S1). FTIR spectra of the TPE-AZO which exhibited a new methylene (-CH2-) peak at 2852 cm-1 and strong peak at 1757



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cm-1 was both ascribed to the carbonyl (C=O) of ester groups (Figure S3). Additionally, the peak of 1, 4-subtituted benzene showed in the finger-print region at 837 cm-1 indicated that the TPE-AZO was successfully prepared. The number-average and weight-average molecule weights of TPE-AZO determined by gel permeation chromatography (GPC) was 3.9×103 and 5.8×103 with polydispersity index (PDI) of 1.487 (Table S1). TPE-AZO could be identified as a hyperbranched polymer because the Mn and Mw are much higher than the molecular weight of TPE-AZO monomer (1445). Furthermore, the structure of TPE-OH was confirmed by 1H NMR, 13C NMR and MS spectroscopy (Figure S4, S5 and S6).

Scheme 1. Schematic diagram of the synthetic routes of TPE-polymers. The TPE-polymers was characterized by 1H NMR (Figure S7-S15), gel permeation chromatography (GPC) (Table S1) and FT-IR (Figure S3). Besides the peak of 1,


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4-subtituted benzene at 837 cm-1 and the enhancement of methylene peak at 2852 cm-1, the IR spectra of TPE-polymers shows the characteristic absorption peak of vinyl monomers (Figure 1). For instance, TPE-PNIPAM shows an imino (-NH-) peak at 3310 cm-1 and carbonyl (C=O) peak of an amide groups at 1651 cm-1, TPE-PS shows the C-H stretching vibration and out-of-plane bending vibration of phenyl rings at 3026 cm-1 and 698 cm-1, TPE-PMETAC, TPE-PMA, TPE-PMMA and TPE-PMPEGMA exhibit carbonyl (C=O)

peak of ester groups

at 1728 cm-1, TPE-PVP exhibits ketone (C=O) peak groups at 1670 cm-1, TPE-PMAA shows hydroxyl group (-OH ) peak at 3502 cm-1 and carbonyl (C=O) peak of carboxyl group at 1700 cm-1, TPE-PVK exhibits C-H stretching vibration and out-of-plane bending vibration of o-disubstituted phenyl at 3046 cm-1 and 745 cm-1, respectively. These results suggested the TPE-polymers were successfully synthesized.



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Figure 1 FTIR spectrum of TPE-AZO and TPE-polymers. 3.2. Fluorescence of TPE-polymers

It is important to investigate the fluorescence characteristic of TPE-polymers in solution. As can be seen in Figure 2, the fluorescence emission wavelength of TPE-polymers exhibited obvious spectral shift compared with the corresponding peaks of TPE-AZO at 461 nm. Such as the fluorescence emission peaks of TPE-polymers are 370 nm (TPE-PMA), 419 nm (TPE-PS), 482 nm (TPE-PMMA) and 405 nm (TPE-PVK) in THF; 382 nm (TPE-PMPEGMA), 398 nm (TPE-PMETAC), 400 nm (TPE-PMAA), 415 nm (TPE-PNIPAM) and 471 nm (TPE-PVP) in aqueous solution, respectively, which is ascribed to the effective polymer modification of TPE molecule. To the best of our knowledge, hydrophilic TPE-polymers tend to form core-shell structure in aqueous with TPE moieties as the core and polymer chains as the shell, which caused the aggregation of TPE moieties


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and vitalized strong emission. TPE-polymers possessed conjugate groups have strong chromophore interactions (e.g., π−π stacking interaction), which lead to the enhancement of fluorescence and spectral shift of fluorescence emission wavelength.

Figure 2 (a) Fluorescence spectra of TPE-AZO and TPE-polymers at high concentration (5 mg/mL); (b) its photographs under 365 nm UV light irradiation. In order to further investigate the fluorescence characteristic of TPE-polymers in solution, we studied the AIE properties of typical TPE-PNIPAM. Obviously, the fluorescence intensity of TPE-PNIPAM was increased in a nonlinear fashion with the volume fraction of water increases (Figure 3). As we all know, fluorescence decay certainly occur when AIE molecules were dissolved in the miscible solvents. However, when large amounts of good solvent water (fw > 80 vol%) was added, TPE-PNIPAM gave a bright blue emission with maxima at ~468 nm. To our mind, this phenomenon was derived from self-assembly of TPE-PNIPAM in water/THF mixture solvent because of its amphiphilic properties, which lead to the hydrophobic



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TPE moieties aggregated in the core and the hydrophilic PNIPAM extended into water. Therefore, AIE happened resulting from hydrophobic effect, which promoted TPE to aggregate in well-organized manner. The experimental result indicates that the AIE feature of TPE-PNIPAM is different from conventional AIE molecules, which is originated from the advantages of combining the polymer unique properties with AIE characteristic. Therefore, we further investigated the fluorescence properties of temperature-sensitive TPE-PNIPAM, counterions-sensitive TPEPMETAC and pH-sensitive TPE-PMAA.

Figure 3 (a) Fluorescent spectra of 1mg/mL of TPE-PNIPAM in water/THF mixture solvent with different water fractions (λex=365 nm); (b) The plot of fluorescence intensity changes of TPE-PNIPAM in water/THF mixture solvent with different water fraction. 3.3. Temperature-sensitive properties of TPE-PNIPAM

PNIPAM is a well-known temperature-sensitive polymer, which would undergo a transition from hydrated coil to dehydrated granule in aqueous with lower critical solution temperature (LCST) at 32 °C. To investigate the effect of temperature on the optical property of our TPE-PNIPAM, we tested the fluorescence change of


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TPE-PNIPAM in aqueous with different temperature from 25 °C to 45 °C. As shown in Figure 4a, the fluorescence intensity decreased with increase of temperature from 25 °C to 33 °C, but increased rapidly with increase in temperature from 34 °C to 39 °C, and gradually decreased later. With the temperature increasing, the molecular motions and intramolecular rotations are quickened and resulted in the decrease of fluorescent intensity. However, the aggregation of TPE moieties occur at the responsive temperature of 34 °C, which caused the fluorescent intensity increased. Figure 4b exhibits that TPE-PNIPAM changes from the transparent solution to turbidity solution when the temperature above the LCST. It is interesting that the phase transfer of TPE-PNIPAM originated from the characteristic of PNIPAM is able to lead the fluorescent response. The results also indicated that there are more opportunities to create new materials through combining the polymer unique properties with AIE characteristic.

Figure 4 (a) The plot of fluorescence intensity changes of TPE-PNIPAM (1 mg/mL) against varied temperature from 25 °C to 45 °C; (b) Photographs of TPE-PNIPAM solutions at (left) 25 °C and (right) 40 °C under visible light and UV light. 3.4. pH-sensitive properties of TPE-MAA



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We then investigated the effect of pH on the optical property of TPE-PMAA. As shown in Figure 5, at strong acidic condition with pH ranging from 1 to 3, the fluorescence intensities of 1mg/mL TPE-PMAA aqueous at 363 nm decreased gradually. With the increasing of pH from 4 to 5, the fluorescence intensities also decreased with the fluorescence emission wavelength shift to 404 nm. When the pH increased from 6 to 13, the fluorescence intensities of TPE-PMAA at 404 nm firstly increased to maximum at pH 8, and gradually decreased later. To our mind, when pH was below the pKa of MAA at 4.28, TPE-PMAA was on contraction state in aqueous due to decrease of the hydrophility of carboxyl groups (-COOH). On the contrary, when pH was above the pKa of MAA, TPE-PMAA was in swelling state because of carbonyl exhibiting as dissociated state in aqueous. In this transformation process, the aggregation of TPE moieties changed from contraction state to hydrophobic caused aggregate state, and along with the change of fluorescence intensities and emission wavelength. It should be noted that the electrostatic repulsion resulting from carboxylic groups (-COO-) in polymer chains prevent the TPE moieties from approaching each other, which caused the fluorescence of TPE-PMAA decreased with increase of pH from 8 to 13.


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Figure 5 (a) Fluorescent spectra of TPE-MAA (1 mg/mL) aqueous with different pH value. (λex=300 nm); (b) The fluorescence intensity of TPE-PMA under different pH value. 3.5. Counterions-sensitive properties of TPE-PMETAC

As one of a typical cationic polyelectrolyte, PMETAC possessing many ionization ion groups in side chain has been widely researched due to its response to counterions. Figure 6 shows the change of fluorescence of 1 mg/mL TPE-PMETAC aqueous when adding ClO4-. Obviously, with the concentrations of ClO4- increased, the fluorescence intensities increased in a nonlinear fashion along with the fluorescence emission wavelengths slightly red-shifted. As a matter of fact, the polymer chains of TPE-PMETAC existed with a fully stretched conformation because of the electrostatic repulsion force of the positive charges on polymer chains resulting from the ion pair of TPE-PMETAC dissociated in water. 33 After ClO4- was added, the hydrophilic Cl- was exchanged by hydrophobic ClO4-, which caused collapse transition exclusion of polymer chains and turned to more favorable to tiled collapsed conformation. As a result, TPE moities aggregated and then caused the emission of TPE-PMETAC.



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Figure 6 (a) Fluorescent spectra TPE-PMETAC (1 mg/mL) with different concentrations of ClO4-; (b) The plot of fluorescence intensity changes of TPE-PMETAC against varied concentrations of ClO4-.

4 Conclusions In summary, we developed a versatile approach for the preparation of TPE-polymers by conventional free radical polymerizations using TPE-AZO as initiator. The obtained polymers include TPE-PS and TPE-PVK with conjugate group; hydrophilic TPE-PNIPAM, TPE-PMETAC, TPE-PMAA, TPE-PMPEGMA, TPE-PVP and hydrophobic TPE-PMMA, TPE-PMA. TPE-polymers exhibited different emission from 370 nm to 482 nm due to the impact of amphiphilic and chromophore interactions. It is important that our versatile approach has many advantages, such as mild reaction conditions, simple and efficient synthetic procedures, multiple choices for monomers and easy processability. Moreover, the synthesized TPE-polymers successfully combined the AIE characteristic with excellent properties of polymer, such as the TPE-PNIPAM displayed LCST at 34 °C and its fluorescence increased rapidly when the temperature above the LCST with the TPE-PMETAC aqueous solution changed from transparent to turbidity; the TPE-PMMA exhibited the changing of fluorescence wavelength and intensity due to the different aggregation state of TPE-PMAA at different pH; the TPE-PMATAC gave emission response to counterions (ClO4-) because of the swellingcollapse transition of PMETAC caused the aggregation of TPE moieties. So，we believe that this research would provide a good vantage point to create novel


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AIE functional materials and give them potential applications for the fluorescent sensors and imaging.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: GPC data and 1H NMR spectrum of TPE-polymers and TPE-AZO,

15

N NMR of

TPE-AZO, FTIR spectrum of TPE-OH, ACVA and TPE-AZO, as well as 1H NMR, 13

C NMR and MS spectrum of TPE-OH.

Acknowledgements: This work is supported by the National Natural Science Foundation of China (51363019) and the National Natural Science Foundation of China (21504070).

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For Table of Contents Only

We developed a practical and versatile approach for preparation of AIE polymers by conventional free radical polymerizations.


Various Tetraphenylethene-Based AIEgens with Four Functional

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