Stimuli-Responsive Fluorescence of AIE Elastomer Based on PDMS

Article pubs.acs.org/Macromolecules

Stimuli-Responsive Fluorescence of AIE Elastomer Based on PDMS and Tetraphenylethene Ryosuke Taniguchi,† Taihei Yamada,† Kazuki Sada,*,†,‡ and Kenta Kokado*,†,‡ †

Graduate School of Chemical Sciences and Engineering and ‡Faculty of Science, Hokkaido University, Kita 10 Nishi 8, Kita-ku, Sapporo, Hokkaido 060-0810, Japan S Supporting Information *

ABSTRACT: We synthesized a tetravinyl AIE luminogen based on tetraphenylethene (TPE-CL), followed by its reaction with H-terminated PDMS via hydrosilylation to construct AIE elastomers. NMR and IR spectroscopy studies showed facile progress of the preparative reaction. The obtained sample strips represented typical elastomeric behavior revealed by tensile test, while the mechanical properties were varied by the chain length of employed PDMS. The homogeneous distribution of TPE-CL in an elastomer was confirmed by UV−vis absorption spectra variation upon increase of TPE-CL content. The elastomers exhibited stimuli-sensitive fluorescence against organic solvents and temperature, and the responsiveness was found to be reversible. These characteristics are clearly derived from AIE property of TPE-CL, which is sensitive to intramolecular rotation.

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INTRODUCTION Fluorescent organic molecules have been vigorously investigated in the past two decades due to the interesting photophysical properties accompanied by their intrinsic softness and lightness. Many researches have focused on their applications for such as organic light-emitting diodes (OLED), semiconductor laser, and fluorescent sensors.1−7 In general, they present these prominent fluorescence in a solution, not in a solid state, while the practical uses of them are based on their solid state such as film. However, fundamental molecular design of organic molecules exhibiting highly efficient solid-state luminescence is still ambiguous, since the emission from organic fluorophore is prone to aggregation-caused quenching (ACQ) due to aggregate formation. In recent years, aggregation-induced emission (AIE) has been one of the hottest topics in the research field of fluorescent organic molecules.8−15 This phenomenon is an exact opposite phenomenon of ACQ; thus, aggregate formation of a compound which is nonluminescent in solution drastically enhances its fluorescence intensity, resulting in emissive in aggregated state. Since the pioneering work by Tang et al., a plethora of papers about AIE have been published, which include applications of AIE luminogens in such as fluorescent sensors, biological probes, and lighting devices.9−11 On the © 2014 American Chemical Society

basis of the series of AIE studies, restriction of intramolecular rotation (RIR) of the luminogens was found to be primarily responsible for AIE phenomenon. In other words, the reduction of void or free volume around the AIE luminogens restricts the molecular motions causing nonradiative decay and, as a result, enhances the emission intensity. From this viewpoint, we also constructed AIE hydrogel system demonstrating water-sensitive fluorescence of hydrogel consisting of poly(γ-glutamic acid) and AIE luminogen.16,17 Therein, the hydrogel networks effectively suppressed the intramolecular rotation of AIE luminogen, while water absorption immediately triggered fluorescence quenching due to revival of intramolecular rotation. Although several researches have reported AIE luminogen involved in a physical gel such as organogelators with hydrogen bonding or electrostatic interaction,18−26 investigation of them in chemically cross-linked gel is extremely limited. In this context, we focus on elastomers to achieve stimulisensitive AIE properties. Elastomers are known to be rubbery material consisting of soft and flexible polymer chains and Received: June 10, 2014 Revised: August 26, 2014 Published: September 10, 2014 6382

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mL). Then it was dried by sodium sulfate, and the solvent was removed in vacuo. The residue was purified by silica gel column chromatography (eluent, chloroform) to obtain Ph-CL as a pale yellow oil (345 mg, 1.19 mmol, 14%). 1H NMR (500 MHz, CDCl3) 6.08 (s, 3H), 5.92−5.84 (m, 3H), 5.17−5.08 (m, 6H), 3.95 (t, 6H, J = 6.8 Hz), 2.51 (q, 6H, J = 6.7 Hz). 13C NMR (126 MHz, CDCl3) δ (ppm) 160.8, 134.4, 117.0, 94.2, 67.2, 33.6. HRMS (EI) Calcd for C18H24O3 [M]+: m/z 288.172 54. Found: m/z 288.171 31. Preparation of PDMS Gel. In a 4 mL snap vial, PDMSx (x = 1− 3) and the cross-linker (TPE-CL and/or Ph-Cl) were dissolved in dry toluene, and Karstedt’s catalyst (90 mM xylene solution) was added to the mixture. Unless stated otherwise, the amount of PDMSx and the cross-linker was determined by the molar ratio of hydrosilyl and vinyl group, 1.0. The reaction mixture was poured into a Teflon mold (H20 × W5 × D1 mm) and allowed to still stand for 24 h. The set elastomer was immersed in THF for 24 h and dried in vacuo.

cross-linking points; thus, the chain length between crosslinking points seriously affects the elastomeric property.27−30 In addition, the swelling behavior of elastomers is substantially influenced by employed solvents. Among various reported elastomers, we selected poly(dimethylsiloxane) (PDMS), an elastomer with well-established chemistry, as the platform polymer in our research. In this article, we designed a tetravinyl AIE luminogen based on tetraphenylethene (TPE), and its reaction with H-terminated PDMS via hydrosilylation was examined to construct AIE elastomers, which present stimulisensitive fluorescence.

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EXPERIMENTAL SECTION

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Materials. H-terminated polysiloxanes (MW = 400−500 (PDMS1), 6000 (PDMS2), and 17 200 (PDMS3)) were purchased from Gelest Inc. Unless stated otherwise, all other reagents were obtained from commercial sources and used without further purification. Tetra(4-hydroxyphenyl)ethene was synthesized and characterized according to the literature.31 Measurements. 1H (500 MHz) and 13C (126 MHz) NMR measurements were recorded on a Bruker Biospin AVANCE DRX500 instrument, using 0.05% tetramethylsilane (TMS) as an internal standard. UV−vis spectra were recorded on a JASCO V-570 spectrophotometer at room temperature. Emission spectra were observed with a JASCO FP-6600 spectrofluorometer. The sample temperature was controlled by a JASCO ETC-273 Peltier-type temperature controller. The absolute luminescence quantum yield (ΦF) was measured by a Hamamatsu C9920-02 absolute photoluminescence quantum yield measurement system equipped with an integrating sphere apparatus and a 150 W continuous-wave xenon light source. Fourier transform infrared (FTIR) spectra were observed with a JASCO FTIR-4100 SK spectrometer with a ZnSe prism kit PKSZNSE for ATR technique. Tensile test was carried out by an Instron 5965 5kN dual column tabletop universal testing system with tensile rate of 10 mm/min. Differential scanning calorimetry (DSC) was conducted by a METLLER TOLEDO DSC1 Star System with heating rate of 10 °C/min under a nitrogen atmosphere. Synthesis of Tetra(4-(1-butenyl)oxyphenyl)ethene (TPE-CL). This reaction was carried out under a nitrogen atmosphere. In a twonecked round-bottom flask, tetra(4-hydroxyphenyl)ethene (498 mg, 1.26 mmol) and pulverized potassium carbonate (1.74 g, 12.6 mmol, 10 equiv) were placed, and dry acetonitrile (20 mL) was added to the flask. The reaction mixture was stirred at room temperature for 1 h, and then 4-bromo-1-butene (1.58 mL, 15.6 mmol, 12 equiv) was injected to the mixture. It was refluxed for 20 h and cooled to room temperature. The precipitate was filtered off, and the filtrate was evaporated to dryness in vacuo. The residue was dissolved in dichloromethane, and the solution was washed by distilled water twice. The collected organic layer was dried by sodium sulfate, and the solvent was removed in vacuo. Purification by silica gel column chromatography (eluent, hexane/chloroform = 3/7 (v/v)) afforded TPE-CL as a white powder (297 mg, 0.49 mmol, 39%). 1H NMR (500 MHz, CDCl3) δ (ppm) 6.91 (d, 8H, J = 8.8 Hz), 6.62 (d, 8H, J = 8.8 Hz), 5.93−5.84 (m, 4H), 5.17−5.07 (m, 8H), 3.94 (t, 8H, J = 6.8 Hz), 2.50 (q, 8H, J = 6.7 Hz). 13C NMR (126 MHz, CDCl3) δ (ppm) 157.1, 138.4, 136.9, 134.5, 132.5, 116.9, 113.6, 67.0, 33.7. HRMS (EI) Calcd for C42H44O4 [M]+: m/z 612.323 96. Found: m/z 612.323 44. Synthesis of 1,3,5-Tri-1′-butenyloxybenzene (Ph-CL). This reaction was carried out under a nitrogen atmosphere. In a two-necked round-bottom flask, phloroglucinol (1.08 g, 8.56 mmol) and pulverized potassium carbonate (13.1 g, 94.8 mmol, 11 equiv) were placed, and dry DMF (30 mL) was added to the flask. The reaction mixture was heated at 70 °C for 2 h, and then 4-bromo-1-butene (1.58 mL, 15.6 mmol, 12 equiv) was injected to the mixture. The reaction mixture was heated at 70 °C for 11 h and cooled to room temperature. Diethyl ether (100 mL) and distilled water (100 mL) were added to the flask, the aqueous layer was extracted by diethyl ether (100 mL × 2), and the collected organic layer was washed by distilled water (100

RESULTS AND DISCUSSION The AIE cross-linker TPE-CL was synthesized as shown in Scheme 1. The tetraphenylethylene (TPE) skeleton, which Scheme 1. Synthetic Route for the Cross-Linkers TPE-CL and Ph-CLa

a Reagents and conditions: (a) 4-bromo-1-butene, K2CO3, CH3CN, reflux, 20 h; (b) 4-bromo-1-butene, K2CO3, DMF, 70 °C, 11 h.

exhibits AIE property, was constructed via McMurry coupling of 4,4′-dimethoxybenzophenone, as previously reported.31 Demethylation of tetra(4-methoxyphenyl)ethene and following Williamson ether synthetic reaction at the four points with 4bromo-1-butene afforded TPE-CL in modest yield. As a control compound, non-AIE cross-linker Ph-CL was synthesized via similar Williamson reaction using phloroglucinol as the starting compound. The synthesized compounds were purified by silica gel column chromatography and characterized by 1H NMR, IR, and high-resolution mass spectroscopies. The spectral data were in good agreement with their chemical structures. In 1H NMR spectra, both of the cross-linkers showed characteristic peaks at around 6.00−5.00 ppm derived from the butenyloxy group, which is essential for the following hydrosilylation polymerization, i.e., the cross-linking reaction. To elucidate the optical properties of TPE-CL, UV−vis absorption and fluorescence spectra were measured as shown in Figure 1. In the UV−vis absorption spectra (10 μM acetonitrile solution, bold line), the absorption maximum (λmax) appeared at 318 nm with high molar extinction coefficient (ε = 17 500 M−1 cm−1), derived from π → π* transition of TPE unit.32,33 In a poor solvent (acetonitrile/H2O = 1/99 (v/v), thin line), the absorption maximum was shifted to longer wavelength region (λmax = 335 nm), probably due to the aggregation, whereas the peak shape retained its original feature observed in a good solvent. The diminution in absorbance presumably resulted 6383

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detail). The amount of polymer and cross-linker was arranged according to equal molar ratio of the hydrosilyl and vinyl group (Table 1). After immersion in THF for removal of the reactants Table 1. Preparation Condition of Elastomersa sample PDMS1-TPE PDMS2-TPE PDMS3-TPE PDMS3-Ph

cross-linker PDMS (μmol of Si−H) (μmol of vinyl) 178 50 25 25

164 51 25 26

Karstedt’s catalystb (fmol) 680 210 110 110

a The reaction was carried out at room temperature for 24 h. 150 μL of toluene was used as the solvent. bStocked in xylene solution (90 mM).

and reagents, the sample strip was dried under vacuum for 24 h. In FT-IR spectra of PDMS1, TPE-CL, and PDMS1-TPE (Figure S1), the peaks at 2128 cm−1 (PDMS1) and 1641 cm−1 (TPE-CL), assigned to ν Si−H and ν CC , respectively, completely disappeared in the spectrum of PDMS1-TPE after the cross-linking reaction, while the other peaks were retained. Additionally, the observed absorption ratio of νC−H (2962 cm−1)/νAr (1508 cm−1) vs feed ratio of methyl group of PDMS and aromatic rings in TPE-CL showed linear correlation (Figure S2), meaning similar reactivity of PDMSx (x = 1−3) in this reaction condition. These data indicated that the crosslinking reaction successfully proceeded without damaging the structures. Indeed, TPE-CL readily underwent the hydrosilylation reaction with 1,1,1,3,3-pentamethyldisiloxane and gave a tetra-substituted TPE compound (see Supporting Information). To obtain a deep insight of cross-linking in these elastomers, differential scanning calorimetry (DSC) was carried out on both H-terminated polymers PDMSx and elastomeric samples PDMSx-TPE. In the literature, a DSC chart of PDMS shows several peaks, i.e., a glass transition, a cold crystallization peak, and two melting points at around −125, −85, and −40 °C, respectively.34,35 For PDMS1 and PDMS1-TPE, we could not observed apparent peaks, presumably due to its too small molecular weight (Mn = 400−500). For PDMS2 and PDMS3, a glass transition was observed at around −125 °C, and an exothermic peak and two endothermic peaks were detected, which is assigned to the cold crystallization and melting point,

Figure 1. UV−vis and fluorescence spectra of TPE-CL in acetonitrile (thin line) and mixed solvent of acetonitrile/H2O (20/80 = standard line, 1/99 = bold line, 10 μM, λex = 350 nm).

from phase separation by aggregation, followed by adhesion of the aggregate onto quartz cell, causing decrease of concentration. In the fluorescence measurement, TPE-CL exhibited an archetypal AIE property. In dissolved state in a good solvent, it showed a slight emission at around 480 nm. On the contrary, the aggregation caused by addition of H2O clearly augmented the fluorescence intensity (solid line, acetonitrile/H2O = 20/ 80) with its maximum at 492 nm. In a mixed solvent of acetonitrile/H2O = 1/99, the fluorescence intensity turned out to be more than 200 times compared to that in dissolved state. From this result, we judged that TPE-CL can act as a crosslinker in hydrosilylation polymerization, which has the AIE property. The hydrosilylation polymerization between H-terminated polysiloxanes (PDMSx: x = 1−3) and cross-linkers (TPE-CL and/or Ph-CL) was carried out by the aid of Karstedt’s catalyst as shown in Scheme 2. At first, we examined the reaction between TPE-CL and PDMSx to obtain PDMSx-TPE. The polymer and cross-linker were dissolved in toluene, and the catalyst was homogeneously mixed. Then, the precursor solution was poured into Teflon mold and kept standing at room temperature for 24 h (see Experimental Section for

Scheme 2. Preparation of Elastomers PDMSx-TPE and PDMSx-Ph (x = 1−3)

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Figure 2. DSC charts of polysiloxanes PDMS2 and PDMS3 and elastomers PDMS2-TPE and PDMS3-TPE. The measurement was performed under a N2 atmosphere with 10 °C/min heating rate.

respectively (Table 2). Generally, the cross-linking of PDMS provides coalescence of melting points and disappearance of

Figure 3. Stress−strain curves measured at room temperature for elastomers PDMS1-TPE, PDMS2-TPE, PDMS3-TPE, and PDMS3Ph.

Table 2. Thermal Transition Temperatures Obtained by DSCa

Table 3. Mechanical Properties of the Obtained Elastomer Samplesa

sample

Tgb (°C)

Tcc (°C)

Tmd (°C)

PDMS2 PDMS2-TPE PDMS3 PDMS3-TPE

−129.4 −123.6 −127.5 −125.5

−90.3

−46.2, −32.4

−86.8

−45.8, −33.8 −43.3

Heating rate of DSC was 10 °C/min, measured under a N2 atmosphere. bGlass transition temperature. cCrystallization temperature. dMelting point. a

a

sample

max strain (mm/mm)

max stress (MPa)

Young’s modulus E (MPa)

cross-linking density ve (μmol/cm3)

PDMS1-TPE PDMS2-TPE PDMS3-TPE PDMS3-Ph

0.36 0.69 0.97 1.02

0.24 0.088 0.041 0.19

0.73 0.16 0.060 0.30

98 22 8.1 40

Measured at room temperature. The tension rate was 10 mm/min.

E = 3veRT

where E is Young’s modulus, ve is molar density of polymer chain, R is the gas constant, and T is temperature.37 From the equation, ve of these samples was calculated as shown in Table 3. These data suggested that our strategy will sufficiently function as a preparative system for elastomers having AIE properties. Next, we scrutinized the optical property of the obtained elastomer samples, especially their AIE characteristics. To prepare elastomeric samples for optical measurements, we adopted PDMS3 as the precursor polymer. At first, we prepared samples cross-linked by both TPE-CL and Ph-CL, in which the molar ratio of TPE-CL is less than 10%; thus, that of Ph-CL is more than 90% (Figure 4a, y is molar ratio of TPECL). The samples were measured after dried in vacuo. From the UV−vis spectra, as the TPE-CL content increased, a proportional hyperchromic effect was observed at 327 nm (Figure 4a inset). Although this measurement was performed in a solid state, the concentration-dependent behavior appears to virtually obey Lambert−Beer’s law, which usually holds in dilute solution state. In fact, in PDMS3-TPE7.8-Ph, the sample contains 0.34 μmol of TPE-CL in 200 mm3 of PDMS strip, roughly calculated concentration of TPE units in the sample was 1.7 mM, which is small enough to be a dilute solution. With more than 10% of TPE-CL, the absorbance at the maxima saturated, meaning a limitation of the measurement due to too much concentration in the sample. Analogous to the UV−vis

cold crystallization because cross-links interfere with the crystalline lamellae growing.31 This is also valid for PDMS3TPE which showed glass transition at −125.5 °C and a melting point at −43.3 °C, respectively, whereas PDMS2-TPE showed only glass-transition at −123.6 °C. These data, substantially different from those of the starting polymers, also supported the successful process of cross-linking reaction, resulting in appropriate anchoring of the employed polymer chains. The elucidation for mechanical properties of the elastomeric samples, including strain−stress curve and Young’s modulus (E), was conducted by tension experiment at room temperature. For the test, we prepared a dog-bone-shaped specimen. With increasing molecular weight of employed PDMS (PDMS1-TPE → PDMS3-TPE), the maximum stress decreased (0.24 MPa → 0.041 MPa), while the maximum strain drastically rose (0.36 → 0.97). Variation of molecular weight between the cross-liking points caused this phenomenon. For PDMS3-Ph, employing cross-linker Ph-CL with three butenyloxy groups, enhancement of both stress and strain maxima was observed. Young’s moduli were assessed from the curves between 0.05 and 0.15 of strain (Table 3). These values are in good agreement with those of the reported PDMS elastomers.36 According to the affine network model, the relationship between Young’s modulus and cross-linking density is dictated as follows 6385

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Figure 4. (a) UV−vis and (b) fluorescence spectra (λex = 350 nm) of elastomers PDMS3-TPEy-Ph (y = 0−7.8).

Figure 5. Solvent responsive fluorescence of PDMS3-TPE (λex = 350 nm). (a) Time-course fluorescence intensity of PDMS3-TPE in various solvents. (b) Fluorescence switching of PDMS3-TPE in between THF (circle) and methanol (square).

spectra, fluorescence spectra also showed concentration dependent increase of intensity (Figure 4b). The maximum in fluorescence spectra was observed at 492 nm, which is identical to that in mixed solvent system of acetonitrile/H2O (Figure 1). These data demonstrated that the cross-linkers homogeneously dispersed in the elastomeric samples without forming macroscopic aggregates or domains, which is similar to that in dilute solution. We also evaluated the fluorescence response of the obtained elastomer samples against organic solvents. The elastomers showed high compatibility with THF, hexane, and chloroform, while poor compatibility was observed for methanol. In this experiment, we used PDMS3-TPE, a typical sample. Immersed in organic solvents which can swell the elastomer, such as THF, chloroform, and hexane, the fluorescence of PDMS3-TPE was readily quenched without any fluorescence maximum shift, and the intensity decreased to less than 0.2 times in 60 min compared to that in dried state (Figure 5a). On the other hand, immersed in organic solvent without swelling ability such as methanol, the fluorescence intensity remained a constant value. We consider that these features are derived from molecular motions around the AIE cross-linker inside; thus, organic solvents with swelling ability effectively liberate the AIE cross-

linker from confinement by PDMS chains. We have also observed similar behavior for poly(γ-glutamic acid) hydrogel immersed in water.16 In this research, we found solventselective fluorescence response of PDMS elastomer dependent on its swelling behavior. Additionally, by altering the immersing solvent from swelling to collapsing or vice versa, the fluorescence intensity drastically changed depending on the swelling degree. The compatibility relates to swelling degree (Q) Q = (Wwet − Wdry )/Wdry

where Wwet and Wdry represent weight of elastomer in wet and dried state, respectively. As shown in Figure 5b, the reversible fluorescent cycles of PDMS3-TPE were observed between immersion in THF (swollen, Q = 4−5) and methanol (collapsed, Q ∼ 1) with the change of swelling degree. The optical switching of fluorescence can be repeated three times without any apparent fatigue. Independent from the change of swelling degree, the elastomer PDMS3-TPE exhibited fluorescence enhancement upon cooling, in other words, the temperature change. At −196 °C, the fluorescence intensity rose 8-fold (absolute fluorescence quantum yield (ΦF) = 7.8% (r.t.) → 59% (−196 °C) with 40 6386

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (K.K.). *E-mail [email protected] (K.S.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support from JSPS Grant-in-Aid for Young Scientists (B) (23750117). The authors greatly appreciate Prof. Dr. S. Takeda, Prof. Dr. Y. Kageyama, and Mr. J. Okamoto for the DSC measurement, Prof. Dr. J. P. Gong and Prof. Dr. T. Nakajima for the tensile test, and Prof. Dr. M. Kato, Prof. Dr. A. Kobayashi, and Mr. H. Ohara for the temperature-dependent fluorescence spectroscopy and fluorescence quantum yield measurement.

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Figure 6. Temperature-dependent fluorescence of PDMS3-TPE at room temperature and −196 °C (λex = 350 nm).

nm hypsochromic shift of the emission maximum (492 nm → 450 nm). An analogous hypsochromic shift and enhancement of fluorescence were observed upon freezing TPE-CL solution in 2-MeTHF (Figure S3), which can form a glassy matrix under its Tm (−136 °C). As we presented the DSC thermogram of PDMS3-TPE in Figure 2, it has melting point (Tm) at around −50 °C (vide supra), indicating that the freezing of matrix surrounding TPE moiety is responsible for the hypsochromic shift. From these observations, we conclude the obtained elastomer can present solvent- or temperature-responsive fluorescence.

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CONCLUSION In this research, we demonstrated a synthesis of a tetravinyl AIE luminogen based on tetraphenylethene (TPE-CL) and following hydrosilylation reaction of TPE-CL and H-terminated PDMS was examined to construct AIE elastomers. Facile progress of the preparative reaction was confirmed by NMR and IR spectroscopy studies. A tensile test of the obtained sample strips showed archetypal elastomeric behavior, while the chain length of employed PDMS substantially affected on their mechanical properties such as Young’s modulus and maximum strain. UV−vis absorption measurement upon increase of TPECL content in the elastomeric samples represented the homogeneous distribution of TPE-CL in them, virtually obeying Lambert−Beer’s law. Fluorescence of the elastomers was found to be stimuli-sensitive against employed organic solvents and temperature, and reversibility of these responsiveness was also confirmed. The AIE characteristics are clearly responsible for these features, which is susceptible for intramolecular rotation. The combination of elastomeric polymer network and AIE luminogen will open a new horizon for material chemistry, such as sensing materials for volatile organic compounds (VOCs).

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ASSOCIATED CONTENT

S Supporting Information *

Summary of model reaction and FT IR spectra. This material is available free of charge via the Internet at http://pubs.acs.org. 6387

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