Tetraphenylethene Cross-Linked Thermosensitive Microgels via

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

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Tetraphenylethene Cross-Linked Thermosensitive Microgels via Acylhydrazone Bonds: Aggregation-Induced Emission in Nanoconfined Environments and the Cononsolvency Effect Jinqiao Xue,† Wei Bai,† Hanyi Duan,† Jingjing Nie,‡ Binyang Du,*,† Jing Zhi Sun,*,† and Ben Zhong Tang*,§ †

MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science & Engineering, and Department of Chemistry, Zhejiang University, Hangzhou 310027, China § Department of Chemistry, Division of Life Science, Division of Biomedical Engineering, Institute for Advanced Study, and Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration and Reconstruction, The Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong, China

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‡

S Supporting Information *

ABSTRACT: We studied the aggregation-induced emission (AIE) phenomenon in a nanoconfined environment, where the AIE-active molecule, namely, 1,1,2,2-tetrakis(4-methanoylphenyl)ethane (TPE-4ALD), was held in space via four acylhydrazone bonds within the thermosensitive microgel networks. The thermosensitive microgels, namely N-AH-TPE, were synthesized via the copolymerization of N-isopropylacrylamide (NIPAM) and 4-acylhydrazine-(2-hydroxy-3-(methacryloxypropyl)pyridine hydrochloride (AH monomer) with TPE-4ALD as cross-linker via surfactant free emulsion polymerization (SFEP) in aqueous solution at 70 °C. Acylhydrazone-bonded tetraphenylethene (TPE-4AH) moieties were thus constructed and worked as the fluorophore in N-AH-TPE microgels. The aqueous suspensions of N-AH-TPE microgels exhibit strongly bluish-green fluorescence under ultraviolet excitation because the four arms of TPE-4AH moieties were held and their intramolecular motions are strongly restricted. It is estimated that there is one TPE-4AH moiety per about 394 nm3 for the swollen N-AH-TPE microgels. The fluorescent properties of N-AH-TPE microgels can be modulated via the change of hydrophilic and hydrophobic environments of TPE-4AH moieties exerted by external stimuli, like addition of various good solvents for TPE-based structures, i.e., N,N-dimethylformamide (DMF), methanol, ethanol, tetrahydrofuran (THF), and N,Ndimethyl sulfoxide (DMSO), varying the solution temperature as well as the counteranions of the microgels. An unusual enhancement in the fluorescent intensity is observed when specific amounts of organic solvent are added into the aqueous suspensions of N-AH-TPE microgels, which can be attributed to the cononsolvency of the polyNIPAM network chains. The shrinkage of N-AH-TPE microgels caused by the cononsolvency effect further strengthens the confinement of TPE-4AH moieties and hence enhances the fluorescent emission of the microgels even though the organic solvents added are good solvents for TPE-4AH. Increasing the solution temperature of N-AH-TPE microgels or introducing hydrophobic counteranions into the microgels also significantly enhances the fluorescent emission of the microgels.

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INTRODUCTION

conventional luminogens which undergo emission weakening in

Since Tang et al. first discovered and proposed the aggregationinduced emission (AIE) effect in 2001,1 massive organic/ polymeric materials containing AIEgens have been synthesized and applied in fields like bioimaging, chemosensing, lightemitting devices, solar concentrators, and solar cells.2−20 Unlike © XXXX American Chemical Society

aggregation state due to the aggregation-caused quenching Received: May 25, 2018 Revised: July 2, 2018

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DOI: 10.1021/acs.macromol.8b01100 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Synthetic Route of (A) AH Monomer, (B) TPE-4ALD, and (C) N-AH-TPE Microgels

(ACQ) effect, AIEgens exhibit enhanced fluorescence emission in solid states or aggregates formed in poor solvents. This unusual phenomenon was later attributed mainly to the restriction of intramolecular rotations (RIR) after conducting plenty of control experiments like increasing the solvent viscosity, decreasing temperature, and applying pressure.21−25 The relevant mechanisms have been expanded to the restriction of intramolecular motions (RIM) up to date by the efforts of researchers.26,27 Nevertheless, detailed information on the structure−property relationships for AIE need to be further elaborated. One of the most sensible strategies here is to trigger the RIM process by covalent bond fixation of AIE-active fluorogens at the molecular level, thus resulting in AIE activity. Besides, more evidence will be offered if the RIM process could be modulated by the internal or external variations. Microgels are three-dimensional cross-linked colloidal nanoparticles, which can swell in good solvents.28−32 Because of the tiny micro- or nanoscale sizes, microgels respond quickly to external stimuli such as temperature, pH, light, magnetic field, and so on, which endowed them with broad application prospects in catalysis, drug delivery systems, biosensors, chemical sensors, etc.33−43 In view of the above, we thus consider microgel as a potential candidate for the investigation of AIE in nanoconfined environments. Poly(N-isopropylacrylamide) (PNIPAM), which exhibits a typical lower critical solution temperature in aqueous solution, was chosen as the backbone of the microgels. A monomer bearing a hydrazide side group, namely 4-acylhydrazine-(2-hydroxy-3-(methacryloxypropyl))pyridine hydrochloride (AH monomer), was synthesized and used as the comonomer. A four-aldehydesubstituted tetraphenylethene (TPE) derivative, namely 1,1,2,2tetrakis(4-methanoylphenyl)ethene (TPE-4ALD), was synthesized and used as the cross-linker. The thermosensitive microgels, namely N-AH-TPE, were then prepared via surface

free emulsion polymerization (SFEP) of NIPAM and AH monomer in the presence of TPE-4ALD at 70 °C in aqueous solution. The cross-linking network was formed by acylhydrazone bonds via the Schiff base reaction of hydrazide and aldehyde groups under the catalysis of acetic acid. The newly formed acylhydrazone-bonded TPE (TPE-4AH) moieties were then held in space via four acylhydrazone bonds within the microgel networks, resulting in the restriction of intramolecular motions. The obtained N-AH-TPE microgels hence exhibit typical AIE properties in water and several organic solvent/water mixtures, i.e., DMF/water, methanol/water, ethanol/water, THF/water, and DMSO/water. Interestingly, the unique cononsolvency phenomenon of the N-AH-TPE microgel was clearly observed in all of the mixed solvents studied, which decreased the size of microgels and eventually gave rise to the enhancement of the fluorescence intensity. The enhanced confinement caused by the cononsolvency effect overwhelms the solvation of good solvent for TPE-4AH moieties. Besides, the fluorescent properties of N-AH-TPE microgels can be also regulated via the solution temperature and counteranions of the microgels, imposing the variation of confinement.

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EXPERIMENTAL SECTION

Materials. Isonicotinic acid hydrazide and N-isopropylacrylamide (NIPAM) were purchased from Acros. 3-Chloro-2-hydroxypropyl methacrylate was purchased from TCI. 2,2′-Azobis(2-methylpropionamidine) dihydrochloride (AIBA) was purchased from Aldrich. Titanium tetrachloride and zinc powder were purchased from Aladdin. 4,4′-Dibromobenzophenone (98%), n-butyllithium (n-BuLi, 2.4 M in n-hexane), potassium bromide (KBr), potassium tetrafluoroborate (KBF4), potassium trifluoromethanesulfonate (KTFS), potassium hexafluorophosphate (KPF 6 ), and potassium trifluoromethanesulfonimide (KTFSI) were purchased from J&K. N,NDimethylformamide (DMF, anhydrous), tetrahydrofuran (THF, for liquid chromatography), and all the other organic solvents used in the B


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Macromolecules

Figure 1. (A) Representative TEM image and (B) hydrodynamic diameter of N-AH-TPE microgels. (C) FT-IR spectra of (a) TPE-4ALD cross-linker, (b) AH comonomer, (c) NIPAM, and (d) N-AH-TPE microgels. (D) Normalized fluorescence emission spectra of TPE-4ALD powder and N-AHTPE microgel dispersed in deionized water. λex = 340 nm. tests of fluorescence spectra were purchased from J&K. Anhydrous THF was collected by distilling THF (chemical pure) under normal pressure with nitrogen and sodium to remove oxygen and water. All other chemicals and solvents were used as received without further purification. Synthesis of 4-Acylhydrazine-(2-hydroxy-3(methacryloxypropyl)pyridine Hydrochloride. The comonomer, 4-acylhydrazine-(2-hydroxy-3-(methacryloxypropyl)pyridine hydrochloride, was synthesized via the quaternization reaction of isonicotinic acid hydrazide and 3-chloro-2-hydroxypropyl methacrylate, as shown in Scheme 1A. Briefly, isonicotinic acid hydrazide (2.000 g, 14.584 mmol) and 3-chloro-2-hydroxypropyl methacrylate (1.309 g, 7.329 mmol) were added into a round-bottom flask followed by addition of 25 mL of ethanol, and the reaction mixture was stirred at 80 °C for 72 h with reflux. Afterward, the reaction mixture was placed in the refrigerator overnight and then filtered to remove the precipitated unreacted isonicotinic acid hydrazide. The slightly yellowish liquid phase was collected, rotary evaporated, and vacuum-dried to get the target product with yield of 57% (yellow viscous solid, 1.330 g), which was named as AH monomer for short. The chemical structure of the AH monomer was confirmed by 1H NMR and ESI-MS measurements (see Figures S1 and S2 in the Supporting Information). Synthesis of Tetraphenylethylene Cross-Linker (TPE-4ALD). The tetraphenylethene cross-linker, namely 1,1,2,2-tetrakis(4methanoylphenyl)ethene, or TPE-4ALD for short, was synthesized via a two-step procedure as shown in Scheme 1B. In the first step, 1,1,2,2-tetrakis(4-bromophenyl)ethene (2) was synthesized. Briefly, 4,4′-dibromobenzophenone (1) (3.400 g, 10 mmol) and zinc powder (1.961 g, 30 mmol) were placed into a 500 mL two-necked roundbottom flask with a reflux condenser. The flask was evacuated under

vacuum and flushed with dry nitrogen three times. Anhydrous THF (120 mL) was then added into the reaction system. The mixture was cooled down to 0 °C, and titanium tetrachloride (1.7 mL, 15 mmol) was then added dropwise. The ice bath was taken out, and the mixture was stirred until it was warmed to room temperature. Afterward, the mixture was refluxed overnight and then cooled to room temperature, followed by quenching with dilute HCl solution (1 M, 50 mL). The mixture was filtered, and the liquid phase was extracted by dichloromethane (DCM) three times. The organic phase was combined and dried over MgSO4. The organic phase was then filtered, and the solvent was removed with a rotary evaporator under reduced pressure. The crude product was purified by silica column chromatography, using the petroleum ether (PE)/DCM mixture (10/1 by volume) as eluent. 1,1,2,2-Tetrakis(4-bromophenyl)ethene (2) (white solid, 1.974 g) was obtained with a yield of 61%. In the second step, compound 2 (2 mmol, 1.296 g) was placed into a 250 mL two-necked round-bottom flask, which was evacuated under vacuum and flushed with dry nitrogen three times. Anhydrous THF (100 mL) was then injected into the flask, and n-BuLi (4.2 mL, 2.4 M, 10 mmol) was added dropwise at −78 °C. The mixture was stirred at −78 °C for 2 h. Anhydrous DMF (1 mL) was then added dropwise. The mixture was further reacted at −78 °C for 4 h. Afterward, the reaction was quenched by adding dilute HCl solution (1 M, 30 mL). The mixture was extracted with DCM, and the organic phase was combined and dried over MgSO4. The organic phase was then filtered, and the solvent was removed by rotary evaporation. The crude product was purified by silica column chromatography with PE/ethyl acetate (EA) mixture (2/1 by volume) as eluent. The AIE-active cross-linker, TPE4ALD, was obtained with yield of 56% (yellow solid powder, 0.494 g). C


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Macromolecules The chemical structure of TPE-4ALD was confirmed by 1H NMR and HRMS measurements (see Figures S3 and S4). Synthesis of Tetraphenylethylene Cross-Linked Thermosensitive Microgels. As shown in Scheme 1C, tetraphenylethylene crosslinked thermosensitive microgels, named as N-AH-TPE microgels, were prepared via SFEP by using NIPAM as main monomer, AH monomer as the comonomer, and TPE-4ALD as the cross-linker, according to a similar procedure reported previously.44−46 Briefly, 200 mg (1.768 mmol) of NIPAM was dissolved in 49 mL of deionized water at 70 °C under vigorous stirring. Four drops of acetic acid were then added into the aqueous solution to adjust the pH to about 3. Oxygen was eliminated by bubbling nitrogen for 30 min. 1 mL of AIBA aqueous solution (25 mg, 0.092 mmol) was injected into the solution to initiate the polymerization. After 10 min, 25 mg (0.079 mmol) of AH monomer in 2 mL of DMF was injected into the reaction mixture. After 30 min, 16 mg (0.036 mmol) of TPE-4ALD predissolved in 2 mL of DMF were added into the reaction mixture. The reaction was continued at 70 °C for 6 h. The obtained N-AH-TPE microgels were purified by dialysis against DMF and deionized water successively with molecular weight cutoff (MWCO) of 14000 for 3 days. Solvent Effects on the Fluorescent Properties of N-AH-TPE Microgels. Before each fluorescence test, a specified quantity (75 or 100 μL) of N-AH-TPE microgel mother liquor was first added into the Petri dish. Precalculated amounts of deionized water and organic solvent (DMF, methanol, ethanol, THF, or DMSO) were then added into the Petri dish successively to give a final volume of 3 mL with water molar fractions ranging from 1 to 0.1. For the DLS measurements of N-AH-TPE microgels in the solvent mixtures with various water molar fractions, the viscosities of the mixed solvents were first measured with an Ubbelohde viscometer at 25 °C and calculated by taking the outflow time of deionized water as a reference (detailed data of the viscosities are listed in Table S1 of the Supporting Information). Anion Exchange Reaction of N-AH-TPE Microgels. To investigate the effect of counteranions on the properties of the obtained N-AH-TPE microgels, five kinds of potassium salts, namely potassium bromide (KBr), potassium tetrafluoroborate (KBF4), potassium trifluoromethanesulfonate (KTFS), potassium hexafluorophosphate (KPF6), and potassium trifluoromethanesulfonimide (KTFSI), were used to perform the anion exchange reaction of N-AH-TPE microgels. Various amounts of potassium salts were added into the aqueous suspensions of N-AH-TPE microgels. The mixtures were then shaken gently at 25 °C on a shaker overnight. Characterization. 1H NMR spectra were recorded at room temperature on a Bruker 400 MHz Avance III spectrometer by using tetramethylsilane as the internal standard and DMSO-d6 as solvent. Electrospray ionization−tandem mass spectrometry (ESI-MS) analysis was performed on a Varian 500 mass spectrometer using methanol as solvent. The high-resolution mass spectrum (HRMS) was measured on a GCT Premier CAB 048 mass spectrometer operated in MALDI-TOF mode. Hydrodynamic diameters (Dh), size distributions, and thermosensitive behaviors of N-AH-TPE microgels were measured by dynamic light scattering (DLS) at scattering angle θ of 90° using a 90 Plus particle size analyzer (Brookhaven Instruments Corp). The wavelength of laser light λ was 657 nm. The microgel suspensions were equilibrated at each measured temperature for 10−15 min before measurements. The morphology of N-AH-TPE microgel was observed by transmission electron microscopy (TEM) on a HT-7700 electron microscope operated at an acceleration voltage of 100 kV. The TEM samples were prepared by dip-coating with Formvar-coated copper grids into the microgel suspension. The grids were allowed to dry in air at room temperature before observation. The microgels were also imaged under a confocal laser scanning microscope (CLSM 780, Zeiss LSM) with an excitation wavelength (λex) of 405 nm. Fourier transform infrared (FT-IR) spectra were recorded on a Bruker Vector 22 spectrometer with KBr pellets. UV−vis spectra were recorded on a Cary 300 instrument (Varian Australia Pty Ltd.). Fluorescence emission spectra were recorded on a Shimadzu RF-

5301PC spectrophotometer at room temperature or at various temperatures with a temperature control.

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RESULTS AND DISCUSSION Synthesis of N-AH-TPE Microgels. N-AH-TPE microgels were successfully obtained via SFEP of NIPAM and AH monomer with TPE-4ALD as the cross-linker at 70 °C in aqueous solution. Figure 1A shows the typical TEM morphology of N-AH-TPE microgels, which are spherical in shape with the average diameter of about 95 ± 19 nm. DLS measurement also showed that N-AH-TPE microgels have a relatively narrow particle size distribution with polydispersity index (PDI) of 0.086 and an average hydrodynamic diameter of about 302 nm at 25 °C (Figure 1B). The hydrodynamic diameter of N-AHTPE microgels in aqueous solution was much larger than the diameter of N-AH-TPE microgels calculated from the corresponding TEM images, indicating the microgels dispersed in water were in the swollen state. At the reaction temperature of 70 °C, which is well above the lower critical solution temperature (LCST) of polyNIPAM, i.e. 32 °C, NIPAm copolymerized with AH monomer and began to form hydrophobic nanoparticles. The later added cross-linker TPE4ALD was then adsorbed into the hydrophobic nanoparticles because TPE-4ALD is hydrophobic in nature. In the presence of the catalyst (acetic acid), the condensation of acylhydrazine group of AH monomer and the four aldehyde groups of TPE4ALD within the hydrophobic nanoparticles resulted in the formation of four acylhydrazone bonds,47−50 which cross-linked the nanoparticles and kept the shape and size of the hydrophobic nanoparticles at 70 °C, leading to the formation of N-AH-TPE microgels. Sivakumaran et al.47 reported the synthesis of covalently cross-linked, colloidally stable PNIPAM microgels via the formation of acylhydrazone bond at 50, 60, and 70 °C by simple mixing hydrazide and aldehyde functionalized PNIPAM precursors. Deng et al.49 reported that the polymer gels can be fabricated via the formation of acylhydrazone bond in DMF, ethanol, DMSO, etc. After cooling down to room temperature, the N-AH-TPE microgels swell in aqueous solution because polyNIPAM chains are hydrophilic at temperature below its LCST. The aqueous suspension of obtained N-AH-TPE microgels is stable for months. These results indicated that the acylhydrazone bond is stable in these organic solvents and aqueous solution at various temperatures. It is worthy to note that no microgels could be obtained without addition of TPE4ALD. Gao and Frisken51,52 have reported that cross-linked PNIPAM microgels can be formed via sufficient self-crosslinking of PNIPAM without addition of a cross-linker in some specific conditions, which require the suitable reaction temperatures (52−78 °C), high initial NIPAM monomer concentration CNIPAM (5−10 g/L), and initial KPS initiator concentration CKPS with CKPS/CNIPAM ratio less than 0.06. In our case, the initial NIPAM monomer concentration CNIPAM was about 4 g/L, which does not lie in the effective range of 5−10 g/L. Besides, the ratio of CAIBA to CNIPAM in the synthesis of N-AH-TPE microgels was about 0.125, which is much larger than 0.06. Under such conditions, the polymer chains become highly charged and hydrophilic and lie in insufficient self-cross-linking regions of PNIPAM as reported by Gao and Frisken.51,52 In fact, the DLS measurement could hardly give any signal of scattering intensity of the reaction mixture after polymerization without addition of TPE-4ALD, which suggested that there are not any microgels formed in the reaction mixture without addition of TPE-4ALD. D


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Macromolecules Figure 1C shows the FTIR spectra of NIPAM, AH monomer, TPE-4ALD, and N-AH-TPE microgels, which confirmed the occurrence and completion of the copolymerization of NIPAM and AH monomer. The characteristic absorption peaks at 1637 cm−1 of the AH monomer and 1618 cm−1 of NIPAM, which indicated the existence of CC stretching vibration, disappear in the spectrum of the resultant N-AH-TPE microgels. The weak −COO− stretching vibration absorption peak at 1733 cm−1 in the spectrum of N-AH-TPE microgel indicates the successful incorporation of AH monomer into the microgel network. The absorption from aldehyde groups of TPE-4ALD at 1696 cm−1 becomes invisible on the curve of the N-AH-TPE microgels, indicating the complete consumption of aldehyde groups of TPE-4ALD. Although the absorption peak accounting for the formation of acylhydrazone bond, which would likely appear at around 1633 cm−1, is overlapped by the characteristic peak of amide groups at 1657 cm−1, the absorption peak at 839 cm−1 clearly indicates the presence of 1,4-substituted benzene in the N-AH-TPE microgels. Figure S5A shows the UV−vis spectra of AH monomer, TPE4ALD, and the as-prepared N-AH-TPE microgels in DMF. The absorption peaks of the microgels at 340 and 267 nm represent the characteristic adsorption peaks of TPE and pyridine moieties, respectively. The content of TPE-4AH moieties in the N-AH-TPE microgels can be then determined by the UV− vis spectrum of the microgels redispersed in DMF by referring to the standard calibration curve of the precursor TPE-4ALD in DMF (Figure S5B). The swollen N-AH-TPE microgels in DMF further confirm that the microgels are chemically cross-linked, and the acylhydrazone bond is stable in DMF. For the N-AHTPE microgels with concentration of 0.6 mg/mL, the molar concentrations of TPE-4AH moieties are determined to be 72.9 μM. By taking the density of the NIPAM monomer, ρ = 1.115 g/ cm3,53 and the TEM diameter of 95 nm as the density and size of dried N-AH-TPE microgels, respectively, the content of TPE4AH moieties in each N-AH-TPE microgel is estimated to be 6.079 × 10−20 mol, which corresponds to about 36596 TPE4AH moieties in one N-AH-TPE microgel. Because the four arms of TPE-4AH moieties were held via the acylhydrazone bonds within the cross-linked network of N-AHTPE microgels, the intramolecular motions of TPE, including the intramolecular rotation and intramolecular vibration, are strongly restricted. The fluorescence emission of TPE-4AH moieties within the N-AH-TPE microgels is then highly expected. Indeed, the aqueous suspensions of N-AH-TPE microgels exhibit strong bluish-green fluorescence under ultraviolet excitation, as shown in Figure 1D. The maximum emission wavelength of the N-AH-TPE microgels in aqueous suspension is located at 505 nm. For the TPE-4ALD aggregates suspended in THF/water mixture (1/9 by volume), its maximum emission wavelength at 491 nm is observed under the same excitation. It means that a red-shift of 14 nm on emission is observed for TPE moieties within the N-AH-TPE microgels, which can be addressed to the elongation of conjugation of the TPE-based structures after the formation of acylhydrazone bonds in microgels. Furthermore, because of the strong fluorescence emission, the N-AH-TPE microgels can be observed by confocal laser scanning microscope (CLSM), as shown in Figure S6. Such microgels with AIE-active fluorogens might have potential application for bioimaging.54−56 By considering the hydrodynamic diameter of 302 nm for N-AHTPE microgels in aqueous suspension at 25 °C, it can be estimated that there is one TPE-4AH moiety per about 394 nm3

for the swollen N-AH-TPE microgels. Because the TPE-4AH moieties were constrained in the cross-linked network of N-AHTPE microgels and the hydrodynamic size of N-AH-TPE microgels can be well tuned by exerting external stimuli, the NAH-TPE microgels serve here as an excellent candidate for the investigation of aggregation or restriction induced emission phenomena in nanoconfined environments. Solvent Effects on the Fluorescent Properties of N-AHTPE Microgels. It is well documented that TPE is a typical AIEgen, which displays limited emission when it is dissolved in a good solvent but emits strongly when aggregation is induced by adding a poor solvent.27 In solvated state, the excited-state energy of TPE is thought to be dissipated through the rotations of its four phenyl rings.27 Herein, the TPE-4AH moieties are held via four acylhydrazone bonds within the cross-linked network of N-AH-TPE microgels. In fact, the TPE-4ALD molecules behave as the cross-linkers, and the TPE-4AH moieties are located exactly at the cross-linking points of the resultant microgel network. Furthermore, the TPE-4AH moieties are basically hydrophobic in nature. In the aqueous suspension of N-AH-TPE microgels, they are surrounded by hydrophilic polyNIPAM chain segments and water molecules. In such circumstance, the intramolecular motions of TPE are strongly restricted, resulting in the suppression of the nonirradiative decay pathways from the excited state to the ground state after the excitation and hence strong emission (cf. Figure 1D). Since the TPE-4AH moieties are held within the volume of N-AH-TPE microgels and cannot diffuse freely in the solvent, even good solvent, it will be very interesting to know how the good solvents of hydrophobic TPE will eventually affect the fluorescent properties of N-AH-TPE microgels. Therefore, five organic solvents, namely N,N-dimethylformamide (DMF), tetrahydrofuran (THF), N,N-dimethyl sulfoxide (DMSO), ethanol, and methanol, which are good solvents for TPE and TPE-4ALD and miscible with water, were investigated. A given amount of organic solvents was added into the aqueous suspensions of N-AH-TPE microgels, and the fluorescence spectra of the microgels in the mixed solvents were then recorded. The thermal and chemical stability of TPE-4ALD in mixed solvent was first investigated. The TPE-4ALD was first dispersed in THF/H2O (1/9 by volume) mixture and subjected to temperature jumps. The TPE-4ALD THF/H2O solution was heated immediately to 50 °C, maintained at 50 °C for 10 min, and then cooled quickly to the room temperature. The fluorescence emission (FL) spectrum of the TPE-4ALD in a THF/H2O mixture was recorded at 50 °C and room temperature. Figure S7a shows the relative FL intensities at maximum emission wavelengths of TPE-4ALD as a function of temperature in several temperature jumps. It can be seen that the FL intensities of TPE-4ALD exhibited a good thermal stability upon several temperature jumps, except for an irreversible decline in FL intensity appeared in the first jump of heating− cooling process. Furthermore, the FL spectrum of TPE-4ALD in THF/H2O mixture with the presence of various salts with concentration of 1 mM, namely KBr, KBF4, KPF6, KTFS, and KTFSI, is also measured (Figure S7b), which is similar to that shown in Figure 1D. TPE-4ALD was also dispersed in the mixed solvent of methanol, ethanol, DMF, DMSO, THF, and water. Note that the volume fraction of each organic solvent was 5% and the volume fraction of water was 75%. Figure S7b shows that the FL spectrum of TPE-4ALD in such mixed solvent is also similar to that shown in Figure 1D. These results indicate that E


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Figure 2. (A) Fluorescence (FL) spectra of N-AH-TPE microgels in water/DMF mixtures with various water molar fractions. (B) Plots of emission intensities at maximum emission wavelengths of the N-AH-TPE microgels in water/ethanol, water/methanol, water/DMF, water/DMSO, and water/ THF mixtures as a function of water molar fraction. Plots of relative fluorescent intensities at maximum emission wavelengths and relative hydrodynamic diameters of N-AH-TPE microgels in (C) water/DMF and (D) water/methanol mixtures as a function of water molar fraction. The emission intensities and hydrodynamic diameters of N-AH-TPE microgels in water were taken as the reference states. The data were all collected at 25 °C. λex = 340 nm. [TPE-4AH] = 2.43 μM.

accordance with the decrease of χwater. It seems that the enhancement of FL intensity is accompanied by the blue-shift of emission peak, whereas the decrease of FL intensity is along with the red-shift. Here it should be mentioned that when the N-AHTPE microgels were first redispersed in DMF and then different quantities of water were added into the microgel suspension, an almost identical plot of maximum FL intensities was observed (as shown in Figure S9), indicating that the emission behaviors of N-AH-TPE microgels are independent of the order of adding solvents. Interestingly, more obvious trends are observed when ethanol, methanol, THF, or DMSO was added, as shown in Figure S10. When adding small amounts of these organic solvents into the aqueous suspensions of N-AH-TPE microgels, the corresponding FL intensities rose rapidly, reaching maximum values of 1.2−1.4 times of the original intensity with χwater of 0.96, 0.92, 0.86, and 0.86 for THF, ethanol, DMSO, and methanol, respectively. After that, the fluorescence intensities decrease along with the increase of the contents of organic solvents (Figure 2B). Plots of the variations of the wavelength of maximum FL intensity of N-AH-TPE microgels in these water/organic mixtures also show similar relation between the intensity and wavelength as that found for N-AHTPE microgels in water/DMF mixtures (cf. Figure 2C and Figure S11).

TPE-4ALD is stable in various environmental conditions studied here. Figure 2A shows the fluorescence (FL) spectra of N-AH-TPE microgels in water/DMF mixtures with various water molar fractions. By and large, the FL intensity of N-AH-TPE microgels basically decreased along with the addition of DMF, which indicated a typical AIE activity of the obtained microgels originated from the incorporation of TPE molecule. However, a nonmonotonic decrease of the FL intensity emerges simultaneously beyond our expectations. The corresponding variation of relative emission intensities at maximum emission wavelengths of the N-AH-TPE microgels is shown in Figure 2B. We can see that when the molar fraction of DMF in the water/DMF mixture is less than 0.1, corresponding to χwater of 0.9, the fluorescent intensity of the N-AH-TPE microgels falls quickly below half of the origin, indicating the presence of good solvent, i.e. DMF, in the vicinity of the TPE molecules might activate the intramolecular rotation and vibration of phenyl rings, leading to the decrease of FL intensity. However, when more DMF is added into the N-AH-TPE microgel suspension, the fluorescent intensity increases again and then decreases with the maximum value appearing at χwater of 0.7. Meanwhile, the wavelength of maximum FL intensity of N-AH-TPE microgels in the water/ DMF mixture (see Figure S8) red-shifts from 505 nm for χwater of 1 to 524 nm for χwater of 0.86 and then fluctuates a little in F


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Figure 3. (A) Relative hydrodynamic diameters of N-AH-TPE microgels as a function of temperature (red curve) and relative FL intensities at maximum emission wavelengths of N-AH-TPE microgels as a function of temperature (green curve). The solid and open symbols are data points in the heating and cooling processes, respectively. (B) Maximum emission wavelengths of N-AH-TPE microgel aqueous suspensions at various temperatures. λex = 340 nm. [TPE-4AH] = 2.43 μM.

(acrylamide).58,60 The maximum increases of FL intensity in the deswelling region of N-AH-TPE microgels are observed at χwater = 0.7 and χwater = 0.86 for the microgels in water/DMF and water/methanol mixtures, respectively. In these cases, the mixed solvents are the bad solvents for polyNIPAM network chains. The deswelling of microgels might result in the enhanced restriction of intramolecular motions of TPE-4AH moieties, leading to the increase of FL intensity. The increase of FL intensity is much stronger for N-AH-TPE microgels in water/ methanol mixtures than that in water/DMF mixtures. The emission of the microgels depends on the microenvironment where the TPE moieties localize. According to the well-accepted mechanism of restricted intramolecular motion (RIM) for AIEactive molecules, the maximum emission should happen in the case that the solvent mixture shows the strongest cononsolvency effect on TPE moieties; this point may be different from the critical point of PNIPAM because TPE moieties are more “hydrophobic” than PNIPAM chains at room temperature. In other words, the maximum collapsing state of the microgel corresponds to the polymer chains but is not exactly coincident with the maximal restriction of the intramolecular motions of TPE moieties; as a result, the maximum emission has been observed at a certain χ value for a specific solvent pair. Furthermore, cononsolvency is not the same as nonsolvent. Generally, the strongest cononsocency effect occurs at a certain ratio of water/organic solvent. It implies that the cononsolvency may be relatively weak when a small amount of organic solvent is taken up by the microgels at a larger ratio of water/organic solvent, which might lead to the increase of Rh observed from Figure 2C when a small amount of DMF was added. In the region of reentrant swelling, the N-AH-TPE microgels in water/ DMF mixtures exhibit much lower FL intensities even with similar hydrodynamic diameter to those in water/methanol mixtures. Because the TPE moieties locate exactly at the crosslinking points of the microgels and the number of TPE moieties within the microgels does not change, the microgels with similar hydrodynamic diameter mean that the steric hindrance and stress exerted on TPE moieties by the network chains might be similar. These results indicated that the solvation ability of good solvents (DMF or methanol) present in the vicinity of TPE-4AH moieties plays a dominant role in the FL emission when reentrant swelling of N-AH-TPE microgels occurs. However, in the deswelling region of N-AH-TPE microgels, the enhanced

An intriguing behavior for poly(N-isopropylacrylamide) (polyNIPAM) in water/alcohol mixtures, such as water/ethanol or water/methanol, called cononsolvency, has been well documented.57−59 For example, water and ethanol are individually good solvents for polyNIPAM. However, polyNIPAM will precipitate in water/ethanol mixtures with certain compositions. This intriguing behavior of cononsolvency had been qualitatively attributed to the perturbation of solvent− solvent interaction parameter, the complexation between water and alcohol, and the competitive hydrogen bonding by water and alcohol molecules onto the polymer chain.57−59 However, the exact mechanism of cononsolvency remains an interesting topic subjected to experimental, simulation, and theoretical studies. Similarly, cononsolvency was also observed for polyNIPAM microgels in mixed water/methanol solvents.60 In fact, the cononsolvency of polyNIPAM is not limited to water/ alcohol mixtures; other water miscible polar solvents, including THF, DMF, DMSO, etc., can as well induce phenomena analogous to those observed in water/alcohol mixtures.61−65 We thus thought that the above observed unusual fluorescent emission of N-AH-TPE microgels in water/DMF, water/ ethanol, water/methanol, water/THF, and water/DMSO mixtures might be attributed to the cononsolvency of the microgels in these mixed solvents. The hydrodynamic diameters of N-AH-TPE microgels in the five mixed solvents with various χwaters were then measured by the DLS technique. Figure 2C shows that the hydrodynamic diameter of N-AH-TPE microgels in water/DMF mixed solvents is significantly influenced by the addition of DMF. When the molar ratio of water is 0.96, the hydrodynamic diameter of the microgels increases to be about 1.05 times its original size. Keeping addition of DMF, the hydrodynamic diameter of N-AH-TPE microgels first decreases and then increases again, indicating the deswelling and reswelling of microgels. Furthermore, the relative FL intensity performs opposite change to the hydrodynamic diameter of NAH-TPE microgels with various χwaters. Similar results are obtained for N-AH-TPE microgels in the water/methanol mixtures (Figure 2D) and three other water/organic solvent systems (Figure S11). The reentrant swelling-to-deswelling-toswelling transition of N-AH-TPE microgels in these five water/ organic solvent systems is clearly observed, which is consistent with those observed for linear polyNIPAM chains and polyNIPAM microgels cross-linked with N,N-methylenebisG


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Figure 4. (A) Relative FL intensity and (B) relative hydrodynamic diameter of N-AH-TPE microgel aqueous suspension with various concentrations of counteranions. λex = 340 nm. [TPE-4AH] = 7.29 μM.

microgels dominates the acceleration of intramolecular motions when increasing the solution temperature, leading to the higher FL intensity at higher temperature. When reducing the solution temperature, the FL intensity declines again, indicating a reversible transformation of the confined environment for the TPE-4AH moieties within N-AH-TPE microgels. Besides, a 13 nm blue-shift and a 5 nm red-shift of the maximum emission wavelengths of N-AH-TPE microgel are observed for the heating and cooling cycles, respectively, as shown in Figure 3B, which are consistent with those observed from Figure 2. This spectral fluctuation may be addressed to the twisting of the conformation and the reversed conformational change of TPE4AH moieties during the heating and cooling process, corresponding to the phase transition of PNIPAM chains. The variations of fluorescence intensity of N-AH-TPE microgel aqueous solution as a function of temperature were further measured in two successive heating−cooling cycles. Figure S14 shows the relative FL intensities at maximum emission wavelengths of N-AH-TPE microgel as a function of temperature in two successive heating−cooling cycles. For the first heating−cooling cycle, it was observed that the fluorescence of N-AH-TPE microgel cannot completely recover to the original value at a given temperature during the cooling process, which is similar to the results shown in Figure 3A. For a given temperature, the relative FL intensity is smaller in the cooling process than that in the heating process. However, change of fluorescent intensity of N-AH-TPE microgel in the second heating−cooling cycle seems to display better reversibility as comparing to that in the first heating−cooling cycle. Counteranion Effects on the Fluorescent Properties of N-AH-TPE Microgels. Since the fluorescence emission behavior of N-AH-TPE microgels could be regulated by adding good solvents and/or changing temperature, other factors that can alter the microenvironment around the TPE-4AH moieties are also considered to make a difference in the emission intensity. Our previous work has found that the sizes of thermosensitive ionic microgels could be well tuned by the type of counteranions.45 Therefore, transforming the chlorine ion in N-AH-TPE microgels to other hydrophobic counterions is supposed to affect its fluorescence emission behavior as well. Here, five kinds of potassium salts were chosen as the sources of the more hydrophobic counteranions; they are KBr, KBF4, KPF6, KTFS, and KTFSI. After anion exchange reaction, the fluorescence emission spectra of the N-AH-TPE microgel aqueous suspensions were recorded.

confinement of TPE-4AH moieties imposed by the shrinkage of polyNIPAM network chains overwhelms the solvation effect of the good solvents, leading to the increase of FL intensities. Thermoresponsive Fluorescent Properties of N-AHTPE Microgels. Thanks to the use of NIPAM, N-AH-TPE microgels exhibit thermosensitive character. The thermosensitive behavior of N-AH-TPE microgels in aqueous suspension is first examined by DLS measurements. Figure 3A shows that the hydrodynamic diameter of N-AH-TPE microgels varies reversibly with solution temperature. The hydrodynamic diameter of N-AH-TPE microgels decreases with increasing the solution temperature. The minimum hydrodynamic diameter of N-AH-TPE microgels is observed at 50 °C, which is about 85% of the original size. This is because polyNIPAM network chains gradually become hydrophobic and insoluble in water when raising the solution temperature, leading to the collapse of N-AH-TPE microgels. Upon cooling, the hydrodynamic diameter of N-AH-TPE microgels increases again along a similar track. We further investigated the thermosensitive fluorescence behavior of N-AH-TPE microgels. The green curve in Figure 3A shows the FL intensity at maximum emission wavelength of NAH-TPE microgel aqueous suspensions as a function of solution temperature. It is clear that when raising the temperature, the FL intensity at maximum emission wavelength of N-AH-TPE microgels increases. The FL emission spectra of N-AH-TPE microgels in the heating and cooling processes are shown in Figure S13. It is understandable that the shrinkage and collapse of PNIPAM network chains of N-AH-TPE microgels at elevated temperature may force the TPE-4AH moieties to adopt a more twisted conformation in order to adjust to the surroundings, and the intramolecular motions are restricted in the collapsed polymer networks, thus forbidding the nonirradiative decay pathways of the excited state energy to some extent, which correspondingly results in higher FL intensities. It resembles the circumstances of microgels dispersed in the water/organic solvents; the restriction of the intramolecular motions of TPE4AH moieties located at the cross-linking points of N-AH-TPE microgels is hypersensitive to change in adjacent domains, which is related to solvent types and contents, temperature variations, and other stimuli, leading to the change in the FL intensities. Here we should mention that although raising the temperature may largely accelerate the intramolecular motions, resulting in the decrease of FL intensities,55 such an effect was not observed in our cases. Possibly, the enhanced confinement of TPE-4AH moieties imposed by the collapse of N-AH-TPE H


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attributed to the cononsolvency of polyNIPAM network chains, which strengthens the confinement of TPE-4AH moieties and hence enhances the fluorescent emission of N-AH-TPE microgels even though the organic solvents added are good solvents for TPE-based structures. The trend of the change in FL intensity with the molar ratio between water and an organic solvent is consistent with the relative hydrodynamic diameter of the microgels, indicating that the unique emission behaviors of AIEgens can be used in the understanding of cononsovency. Furthermore, increasing the solution temperature of N-AH-TPE microgels or introducing hydrophobic counteranions into the microgels, which results in the decrease of hydrodynamic size of the microgels, also significantly imposes the confinement to TPE-4AH moieties and enhances the fluorescent emission of the microgels. The effect of hydrophobic counteranions is found to be more pronounced than that of increasing temperature.

Figure 4A shows the effects of types of counteranions and their concentration on the relative FL intensity of N-AH-TPE microgel aqueous suspensions. Obviously, addition of these five kinds of counteranions leads to the enhancement of fluorescent emission intensity. The fluorescent emission spectra are shown in Figure S15. Basically, along with the increase of salt concentration, the FL intensity of N-AH-TPE microgels first increases and then reaches a plateau value, indicating the totally exchange of the chlorine anions into the corresponding anions. Besides, the increasing extent of FL intensity follows the sequence of KTFSI > KPF6 > KTFS > KBF4 > KBr, which is consistent with the corresponding hydrophobicity of their counteranions, which is in the order of TFSI− > PF6− > TFS− > BF4− > Br−. The introduction of more hydrophobic counteranions into N-AH-TPE microgels greatly alters the hydrophobicity of cross-linked network, resulting in regional shrinkage or even global deswelling of networks, which leads to the enhancement of confinement for TPE-4AH moieties and hence the increase of FL intensity. Hydrodynamic diameters of N-AH-TPE microgels were recorded after the anion exchange reactions, as shown in Figure 4B. It can be seen that the apparent hydrodynamic diameters presents a slight decrease of less than 10% of the original size with increasing the amounts of hydrophobic counteranions up to 0.1 mM. However, it is remarkable that this tiny change in the size of N-AH-TPE microgels by adding hydrophobic counteranions causes more notably enhanced FL intensity of the microgels as compared to those observed above when varying the solvent composition or solution temperature. For instance, though the hydrodynamic diameter of microgels is only reduced by less than 2% with addition of KBr, the equilibrium FL intensity of N-AH-TPE microgels can be enhanced up to 1.2 times of its origin. This phenomenon is even more striking when adding other kinds of counteranions. For the case of most hydrophobic counteranion TFSI− studied here, the enhancement of FL intensity can be up to 1.7 times of its origin along with the reduction of less than 8% in microgel size. Possibly, the increase of hydrophobic microenvironment associated with the hydrophobic counteranions for TPE-4AH moieties within N-AH-TPE microgels significantly imposes the restriction of their intramolecular motions, leading to the pronounced enhancement of FL intensity.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b01100. 1 H NMR and HRMS spectra of AH monomer and TPE4ALD, viscosities of the water/organic mixtures with various χwaters, the standard calibration UV−vis curve of TPE-4ALD in DMF, CLSM image of N-AH-TPE microgels, additional fluorescent emission spectra of TPE-4ALD in various solvent mixtures with or without adding salts, additional fluorescent emission spectra of NAH-TPE microgels in various solvent mixtures or counteranions (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (B.D.). *E-mail: [email protected] (J.Z.S.). *E-mail: [email protected] (B.Z.T.). ORCID

Binyang Du: 0000-0002-5693-0325 Jing Zhi Sun: 0000-0001-5478-5841 Ben Zhong Tang: 0000-0002-0293-964X

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Author Contributions

CONCLUSIONS Thermosensitive N-AH-TPE microgels with tunable fluorescent properties are successfully synthesized by SFEP of NIPAM and AH monomer with TPE-4ALD as the cross-linker in aqueous solution at 70 °C. The cross-linking networks of N-AH-TPE microgels are formed via the acylhydrazone bonds resulting from the condensation of the acylhydrazine group of AH monomer and aldehyde groups of TPE-4ALD. The intramolecular motions of as-prepared TPE-4AH moieties in the nanoconfined environment of N-AH-TPE microgels are then strongly restricted so that the aqueous suspensions of the microgels exhibit strong fluorescent emission under ultravioletlight excitation. The fluorescent properties of N-AH-TPE microgels are responsive to the external stimuli, which can change both the extent of confinement and hydrophobic microenvironments of TPE-4AH moieties. An unusual enhancement in the fluorescent intensity has been observed when specific amounts of organic solvents, namely DMF, methanol, ethanol, THF, and DMSO, are added into the aqueous suspensions of N-AH-TPE microgels. Such a phenomenon is

J.X. and W.B. contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the National Natural Science Foundation of China (No. 21674097), the second level of 2016 Zhejiang Province 151 Talent Project, and Open Research Fund of State Key Laboratory of Polymer Physics and Chemistry (201601), Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, for financial support.

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REFERENCES

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