Color-Tunable and Stimulus-Responsive Luminescent Liquid

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Cite This: ACS Appl. Mater. Interfaces 2019, 11, 15051−15059

Color-Tunable and Stimulus-Responsive Luminescent Liquid Crystalline Polymers Fabricated by Hydrogen Bonding Lei Tao,†,∥ Ming-Li Li,†,∥ Kai-Peng Yang,† Yan Guan,‡ Ping Wang,*,† Zhihao Shen,‡ and He-Lou Xie*,†,§

ACS Appl. Mater. Interfaces 2019.11:15051-15059. Downloaded from pubs.acs.org by UNIV OF SUSSEX on 04/27/19. For personal use only.

†

Key Laboratory of Environment-Friendly Chemistry and Application in Ministry of Education, and Key Laboratory of Advanced Functional Polymer Materials of Colleges, Universities of Hunan Province and College of Chemistry, Xiangtan University, Xiangtan, Hunan 411105, China ‡ Beijing National Laboratory for Molecular Sciences, Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, Center for Soft Matter Science and Engineering, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China § Key Laboratory of Theoretical Organic Chemistry and Functional Molecule of Ministry of Education, Hunan Provincial Key Laboratory of Controllable Preparation and Functional Application of Fine Polymers, Hunan University of Science and Technology, Xiangtan, Hunan 411201, China S Supporting Information *

ABSTRACT: Luminescent liquid crystalline polymers (LLCPs) show extensive application potentials, such as liquid crystal displays and circularly polarized luminescence. In this work, we employ a hydrogen-bonding strategy different from the traditional covalent-bonding method to fabricate LLCPs. First, the acceptor and donor of hydrogen bonding, (4,4′-dibutanoxy tetraphenylethylene)-1-pyridine (PTPEC4) and poly(2-vinyl terephthalic acid) (PPA), respectively, are successfully synthesized. Then, mixtures with different molar ratios (x’s) of PTPEC4 to PPA are used to prepare a series of LLCPs [denoted as PPA(PTPEC4)x]. The resultant LLCPs show a smectic A phase (x ≥ 0.8), a columnar nematic phase (0.6 ≤ x ≤ 0.05), and an amorphous state (x = 0.025), depending on the x value. Meanwhile, all polymers exhibit typical aggregation-induced emission behavior. More interestingly, with the variation of the PTPEC4 content, the series of LLCPs show different colors, that is, the emission peak red shifts from 510 nm (x = 1.0) to 551 nm (x = 0.025). Furthermore, because of the reversible protonation effect of the N atom of pyridine in PTPEC4 by the strong proton acid, PPA(PTPEC4)x shows reversible color transformation. This work provides a new method to construct LLCPs with different emission colors and reversible color transformation. KEYWORDS: luminescent liquid crystal, hydrogen bonding, “Jacketing” effect, aggregation-induced emission, color tunable, fluorescent switch

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INTRODUCTION

On the contrary, for the aggregation-induced emission (AIE) phenomenon discovered by Tang et al.,22,23 the active intramolecular motions of molecules in good solvents present weak or even nonluminous light, but in poor solvents or solids, the molecules strongly emit. Currently, many AIE luminogens with different chemical structures have been successfully obtained.24−27 Meanwhile, this discovery also provides an effective approach to construct LLCs. The general method to prepare LLCs with LC ordering and high emission in the solid state is to modify the periphery of fluorescence molecules using conventional mesogens.28−36 Among the AIE luminogens, tetraphenylethylene (TPE) with a distorted structure has been widely used to construct LLCs because of its high solid-state

Fluorescent materials as elegant and essential functional materials have inspired extensive research interests owing to their wide practical applications in optoelectronic devices, sensors, bioimaging, optical information storage, and so on.1−8 Luminescent liquid crystals (LLCs) with liquid crystalline (LC) ordering and photoluminescence can deliver linear or circularly polarized light after radiation.9−13 Some devices fabricated by LLCs have been successfully applied in some commercial products, such as organic light-emitting diodes and liquid crystal displays.14−16 Regular packing is the prerequisite for the formation of LLCs in the condensed state, which generally leads to fluorescent quenching, also known as the aggregationcaused quenching phenomenon.17−21 Thus, how to reasonably design and prepare LLCs is still a very important and challenging question. © 2019 American Chemical Society

Received: January 23, 2019 Accepted: April 3, 2019 Published: April 3, 2019 15051

DOI: 10.1021/acsami.9b01476 ACS Appl. Mater. Interfaces 2019, 11, 15051−15059

Research Article

ACS Applied Materials & Interfaces Scheme 1. Chemical Structure of the Hydrogen-Bonded Complex PPA(PTPEC4)x

Scheme 2. Synthetic Route of the Hydrogen-Bonded Complex PPA(PTPEC4)x

luminescent efficiency.37−39 For instance, incorporating rodshaped LC molecules into the periphery of TPE can achieve excellent LLC materials with a high solid-state efficiency. The resultant LLCs show a unique biaxial orientation, whereas the tetragonal columnar packing formed by TPE cores is inserted into the smectic matrix formed by the rod-shaped mesogens.37 Similarly, luminescent liquid crystalline polymers (LLCPs) can also be achieved by copolymerizing monomers with the mesogens and AIE luminogens.40,41 Recently, we proposed a new method for the fabrication of LLCPs combining AIE and the “Jacketing” effect.42 TPE in the side chain is linked to the 2 and 5 positions of the phenyl ring of polystyrene using different spacers. The bulky side group induces a powerful steric hindrance to constrain the main chain to adopt a strong stretching conformation, which leads to similar properties as main-chain LC polymers (MCLCPs).43,44 The results reveal that the solid-state quantum yield and the phase structure are significantly dependent on the spacer length. Different from the traditional methods using covalent bonds to prepare LCPs, the pathway to fabricate LCPs by hydrogen bonding is more convenient and simpler. This method does not need the complicated synthetic process. Just blending the donor and the acceptor can prepare the target product; moreover, the various ratios of the donor to the acceptor can result in different compounds with controlled properties. This approach can also achieve LCPs with different structures, such as MCLCPs, sidechain LCPs, and network supramolecular LCPs.45−50 The resultant LCPs also form versatile supramolecular struc-

tures.51−54 The most important feature of the hydrogen bond is its reversible breakage-formation, which offers an opportunity to produce functional materials with some special properties, such as self-healing property and stimuli-responsive behavior. On this basis, some LC functional materials have been successfully prepared. For example, LC elastomers fabricated by hydrogen bonds show self-healing property as well as multiple processing property.55,56 In addition, nanoporous materials with selective absorbance could be very easily prepared after the removal of the hydrogen-bonded core.47 In this work, we propose a hydrogen-bonding strategy to fabricate LLCPs. Poly(2-vinyl terephthalic acid) (PPA) as the donor and (4,4′-dibutanoxy tetraphenylethylene)-1-pyridine (PTPEC4) as the acceptor were successfully prepared. As shown in Scheme 1, the two AIE luminogens (PTPEC4) are directly attached to the main chain via hydrogen bonding. The resultant polymers show different colors and different phases depending on the complexing ratio (x) of the two components. Moreover, the protonation effect can render reversible color transformation in this system.

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EXPERIMENTAL SECTION

Materials. 4,4′-Dihydroxybenzophenone (97%, Energy Chemical), 4-bromobenzophenone (98%, Energy Chemical), titanium tetrachloride (99%, Energy Chemical), zinc (98%, Energy Chemical), 1bromobutane (98%, Energy Chemical), pyridine-4-boronic acid (98%, Energy Chemical), trifluoroacetic acid (99%, Energy Chemical), tert-butanol (99.5%, Energy Chemical), tetrakis(triphenylphosphine)

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Figure 1. FT-IR spectra of PPA(PTPEC4)x with different molar ratios between PPA and PTPEC4 (a) and the VT FT-IR spectra of PPA(PTPEC4)1.0 during heating (b) and cooling (c) processes. 1 H NMR (CDCl3): δ (ppm) 7.22 (d, 2H, Ar-H), 7 (d, 7H, Ar-H), 6.88 (m, 4H, Ar-H), 6.63 (m, 4H, Ar-H), 3.89 (m, 4H, −OCH2−), 1.73 (s, 4H, −CH2−), 1.46 (s, 4H, −CH2−), 0.98 (s, 6H, −CH3). Synthesis of PTPEC4. (4,4′-Dibutanoxy tetraphenylethylene)-1bromine (3.80 g, 7.60 mmol), tetrakis (triphenylphosphine) palladium (0.50 g, 0.380 mmol), pyridine-4-boronic acid (1.00 g, 8.40 mmol), and K2CO3 (6.90 g, 50.0 mmol) were put into a round-bottomed flask under a N2 atmosphere. The reagents were dissolved in a mixture solvent of methylbenzene/methanol/H2O (2:1:1, v/v/v) and reflexed for 24 h. After being cooled, the resultant mixture was filtered, and the filtrate was washed with water several times. Then, the solvent was removed under vacuum by a rotary evaporator. The crude product was further purified by silica gel column chromatography with a mixture solvent of ethyl acetate/n-hexane (6:1, v/v) as the eluent. Yield: 70.8%. 1 H NMR (CDCl3): δ (ppm) 8.61 (d, 2H, pyridine-H), 7.48 (d, 2H, pyridine-H), 7.42 (m, 2H, Ar-H), 7.14 (m, 5H, Ar-H), 7.05 (m, 2H, ArH), 6.95 (m, 4H, Ar-H), 6.63 (m, 4H, Ar-H), 3.88 (m, 4H, −OCH2−), 1.73 (s, 4H, −CH2−), 1.45 (s, 4H, −CH2−), 0.96 (s, 6H, −CH3). 13 C NMR (CDCl3): δ (ppm) 157.83 (TPE C−O), 150.11 (pyridine C), 147.92 (pyridine C-TPE), 145.66 (TPE C-pyridine), 144.11 (TPE C−CC−), 141.20 (TPE-CC−), 138.07, 135.91, 135.13, 132.61, 127.83, 126.16 (TPE C), 121.27 (pyridine C−C-TPE), 113.68 (TPE C−C−O), 67.54 (TPE-OCH 2 −), 31.38 (−OCH 2 −), 19.72 (−OCH2CH2−), 13.90 (−CH2CH3). MALDI-TOF MS (m/z): [M + H]+ calcd for C39H39NO2, 553.30; found, 554.36. Synthesis of Hydrogen-Bonded Complex of PPA(PTPEC4)0.8. PPA and PTPEC4 were completely dissolved in pyridine with a molar ratio of pyridine moieties to −COOH moieties at 0.8, and then the mixture was stirred at room temperature for 24 h. Afterward, the mixture was treated at 65 °C for about 7 days to evaporate pyridine, and it was then heated at 130 °C in vacuum for 2 days to completely remove the solvent. Finally, PPA(PTPEC4)0.8 was obtained.

palladium (99%, Energy Chemical), and pyridine (AR, Tianjin Chemical Co.) were directly used without any treatment. Synthesis. PPA was prepared using the procedure in our previous work,53 and the detailed synthesis information is presented in the Supporting Information (Scheme S1). The synthesis of a series of hydrogen-bonded complexes PPA(PTPEC4)x is shown in Scheme 2. Herein, this synthetic process is elucidated in detail with PPA(PTPEC4)0.8 as an example. Synthesis of (4,4′-Dihydroxy tetraphenylethylene)-1-bromine. The synthesis of (4,4′-dihydroxy tetraphenylethylene)-1bromine was according to the procedure in ref 42. 4,4′-Dihydroxybenzophenone (10.0 g, 0.0470 mol), 4-bromobenzophenone (14.7 g, 0.0560 mol), zinc powder (13.4 g, 0.200 mol), and 200 mL of refined tetrahydrofuran (THF) were put into a round-bottomed flask. After the mixture was stirred in ice−water bath for 1 h, TiCl4 (11.4 mL, 0.1 mol) was added to the flask drop by drop in a N2 atmosphere. After further stirring of 2 h, the mixture was refluxed overnight. When the mixture was cooled, 200 mL of 10% aqueous K2CO3 solution was added, followed by vigorous stirring of 30 min. The precipitate was filtered off and extracted with CH2Cl2 several times. The solvent was removed by using a rotary evaporator under vacuum. The crude product was further purified by silica gel column chromatography, with a mixture solvent of n-hexane/CH2Cl2 (3:1, v/v) as the eluent to obtain a white solid (4,4′dihydroxy tetraphenylethylene)-1-bromine. Yield: 54.2%. 1 H NMR (DMSO-d6): δ (ppm) 9.68 (s, 2H, −OH), 7.2 (d, 2H, ArH), 7 (d, 7H, Ar-H), 6.85 (m, 4H, Ar-H), 6.55 (m, 4H, Ar-H). Synthesis of (4,4′-Dibutanoxy tetraphenylethylene)-1-bromine. (4,4′-Dihydroxy tetraphenylethylene)-1-bromine (5.00 g, 9.60 mmol), 1-bromobutane (5.30 g, 38.0 mmol), K2CO3 (8.00 g, 580 mmol), and 150 mL of dry acetone were put into a round-bottomed flask. Then, the mixture was refluxed overnight. After the reaction was complete, the mixture was filtered, and the filtrate was removed by using a rotary evaporator under vacuum. The crude product was further purified by silica gel column chromatography by using a mixture solvent of CH2Cl2/n-hexane (1:3, v/v) as the eluent to obtain (4,4′-dibutanoxy tetraphenylethylene)-1-bromine as a white solid. Yield: 90.2%.

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RESULTS AND DISCUSSION Preparation and Characterization of HydrogenBonded Complexes PPA(PTPEC4)x. The hydrogen-bonding 15053


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Figure 2. DSC curves of PPA(PTPEC4)x during the first cooling process (a) and the second heating process (b) with a rate at 10 °C/min.

Figure 3. POM images of PPA(PTPEC4)x during the cooling process with x = 0.025 (a), 0.05 (b), 0.1 (c), 0.2 (d), 0.4 (e), 0.6 (f), 0.8 (g), and 1.0 (h). The scale bar is 50 μm.

intensities (Figure 1c). Further quantitative analysis was carried out through integrating the characteristic absorption peaks, as shown in Figure S2. Clearly, the variational tendency of the integrating area was in good agreement with the change in the intensity of the characteristic absorption peaks. All results indicated that the hydrogen bond was partially interrupted at high temperatures and formed again at low temperatures. LC Structures of Hydrogen-Bonded Complexes PPA(PTPEC4)x. The decomposition temperature (5% weight-loss) of PPA and PTPEC4 all exceeded 300 °C (Figure S3). On the basis of the VT FT-IR results, all subsequent VT measurements of PPA(PTPEC4)x were carried out below 150 °C to avoid breaking the hydrogen bond. Differential scanning calorimetry (DSC) was used to investigate the phase transitions of PPA(PTPEC4)x. The DSC traces of PPA(PTPEC4)x during the first cooling and the second heating were recorded with a heating rate of 10 °C/min under a N2 atmosphere. As shown in Figure 2, PPA(PTPEC4)x exhibited only one glass transition temperature (Tg) in the whole experimental temperature range, which was similar to other traditional mesogen-jacketed LCPs (MJLCPs) fabricated by covalent bond.44 On the other hand, with the increase of x, Tg gradually decreased, which could be attributed to the plasticization effect of the small molecule.50 Furthermore, the lowest Tg was 44 °C at x = 1.0. The DSC results of the complexes were different from that of PTPEC4, which only showed a melting peak at 27 °C (Figure S4). This result indicated the formation of hydrogen-bonded complexes.

acceptor PTPEC4 was successfully synthesized according to Scheme 2, and the chemical structure was analyzed and confirmed by the combined technology of matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) and nuclear magnetic resonance (NMR). The hydrogen-bonding donor PPA was prepared through hydrolysis of poly{2,5-bis[(tert-butyl) oxycarbonyl]styrene} (PPE), and the detailed synthetic routes and results are shown in Supporting Information (Scheme S1). Fourier transform infrared (FT-IR) was employed to confirm the structure of PPA(PTPEC4)x. Figure 1a describes the normalized FT-IR spectra of the compounds fabricated by hydrogen bonding with different molar ratios of PTPEC4 to PPA. For the pure PPA, the sharp peak at 1682 cm−1 and the broad absorbances at 2500 and 3000 cm−1 indicated the existence of −COOH groups. With the increase of the molar ratio of PTPEC4 to PPA, the intensities increase for the bands at 1910, 2510, and about 3000 cm−1, corresponding to the three typical broad bands of the hydrogen bond between the carboxylic acid group and the pyridine derivative. Meanwhile, the absorption band at 1682 cm−1 gradually shifted to 1695 cm−1. These results proved that the hydrogen bond was formed between PPA and PTPEC4. Additionally, variable-temperature (VT) FT-IR experiments were employed to explore the temperature dependence of the hydrogen bond of PPA(PTPEC4)x. PPA(PTPEC4)1.0 was used as an example. With increasing temperature, the intensities of the peaks at 1910 and 2510 cm−1 decreased (Figure 1b); and decreasing temperature leads to a gradual increase of their 15054


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at 2θ = 2.13° and 4.27° and one diffuse halo at 2θ = 20° in the low- and high-angle regions, respectively (Figure 5a). This indicated the formation of a layered structure with a layer spacing of 4.14 nm. During the cooling process, the diffraction peaks remained unchanged (Figure 5b), indicating the stability of the smectic phase in PPA(PTPEC4)1.0 during the whole temperature range, once it was formed. Presuming the alkyl tails connected to TPE were in an all-trans conformation, the calculated molecular size of the side chain of PPA(PTPEC4)1.0 was 4.80 nm, which was a little larger than the layer spacing. According to our previous work,42,57 we speculated that PPA(PTPEC4)1.0 formed a smectic A (SmA) phase with interdigitated alkyl tails. PPA(PTPEC4)0.8 formed a similar stable SmA phase in the whole temperature range (Figure S5). For PPA(PTPEC4)0.6, only one sharp diffraction in the lowangle region can be observed, and the diffraction peak is retained and unchanged in the heating and cooling processes as shown in Figure 5c,d, indicating that PPA(PTPEC4)0.6 forms a stable columnar nematic phase during the whole temperature range. Other four samples (x = 0.4, 0.2, 0.1, and 0.05) also formed the stable columnar nematic phase, with detailed VT 1D WAXD data shown in Figure S6. Photophysical Property and Color Dependence. The photophysical property of PTPEC4 was investigated first. The absorption spectra of PTPEC4 in THF solution with a concentration of 5 × 10−5 mol/mL are described in Figure S7a. The maximal absorption of PTPEC4 in THF solution appeared at 262 and 344 nm. On comparison with the TPE molecule, the incorporation of the pyridine ring leads to a little red shift. The THF solution of PTPEC4 showed a weak emission as depicted in Figure 6a, which could be ascribed to the planar structure of the pyridine ring. However, when the water content was gradually increased, the fluorescence of the TPE moiety began to turn on when the water content reached 90%, and the intensity of the maximum emission at 502 nm of PTPEC4 drastically increased. Meantime, PTPEC4 exhibited intense green emission in powder and thin film with the maximum at 508 nm (Figure 6b). We further investigated the thin-film photophysical properties of PPA(PTPEC4)x in detail. PPA(PTPEC4)x was dissolved into N,N-dimethylformamide (10 mg/mL), and then the solution was spin-coated onto a freshly cleaned quartz plate. The absorption spectra of PPA(PTPEC4)x were similar to that of PTPEC4 in the thin film. When the content of PTPEC4 gradually decreased (x from 1.0 to 0.025), the intensity of the maximum absorbance at 360 nm decreased (Figure S7b). Figure 7a depicts the AIE behaviors of PPA(PTPEC4)x in thin films. Compared with PTPEC4, PPA(PTPEC4)1.0 showed a slight red shift and presented maximum emission at 510 nm. With the decrease in the content of PTPEC4 (x from 1.0 to 0.025), the maximum emission shifted from 510 to 551 nm (Figure 7b). PTPEC4 and PPA(PTPEC4)x showed good similarity in the thin-film ultraviolet−visible (UV−vis) spectra, which indicated that PPA(PTPEC4)x formed a similar effective conjugation or presented a similar electronic structure in the ground state compared with PTPEC4. The marked red shift in terms of the solid-state emission wavelength from PTPEC4 to the complex with x = 0.025 was possibly because of the higher packing density of the PTPEC4 powder than the dispersed PAA(PTPEC4)x complex. Lower packing density led to larger reorganization energy and thus resulted in the red shift emission from the pure substance to dispersed complex.58,59 The presence of multiple hydrogen bonds restricted the active motion. With

Polarized optical microscopy (POM) was used to investigate the LC textures of PPA(PTPEC4)x. No birefringence was observed for all samples without annealing at room temperature, but some different phenomena could be observed by POM with the variation of the ratio of PTPEC4 to PAA after annealing. No birefringence appeared for PPA(PTPEC4)0.025, and only very weak birefringence was observed for PPA(PTPEC4)0.05 until the temperature reached 150 °C. However, for PPA(PTPEC4)x (x = 0.1, 0.2, 0.4, 0.6, 0.8, 1.0), distinct birefringence appeared as the temperature was above Tg, and no change occurred during the cooling process, which was typical for MJLCPs (Figure 3). From the structural point of view, PPA(PTPEC4)x showed a typical structural feature of MJLCPs.44 The bulky PTPEC4 was directly attached to the PPA main chain via hydrogen bonding, making the whole polymer chain extend along the main-chain direction. Further packing of the polymer chains results in liquid crystallinity. For PPA(PTPEC4)x with a lower value of x, the side groups were not crowded enough to give a strong enough “Jacketing” effect, which was not beneficial for the polymer chains to pack into an LC phase. One-dimensional (1D) wide-angle X-ray diffraction (WAXD) was carried out to further investigate the LC structure of PPA(PTPEC4)x. Figure 4 depicts the 1D WAXD patterns of

Figure 4. 1D WAXD profiles of PPA(PTPEC4)x at room temperature.

PPA(PTPEC4)x obtained at an ambient temperature after 2 h of annealing at 150 °C. The results were consistent with those of the POM experiments. PPA(PTPEC4)x with different contents of PTPEC4 showed rather different diffraction patterns. For x ≥ 0.8, the profile of PPA(PTPEC4)x showed two distinct diffraction peaks in the low-angle region with a q-ratio of 1:2 and one broad halo in the high-angle region, which indicated that a smectic structure was formed. With decreasing x (x ≤ 0.6), only one diffraction peak and an amorphous halo were observed in the low- and high-angle regions, respectively. Considering the special molecular packing of MJLCPs, a columnar nematic structure was formed.53 When the content of PTPEC4 was lower than 0.05, only two diffuse scattering halos were formed in both low- and high-angle regions, revealing the amorphous state. Furthermore, we used VT 1D WAXD to investigate the temperature dependence of the phase structure. Figure 5 describes the diffraction patterns of PPA(PTPEC4)1.0 and PPA(PTPEC4)0.6 at different temperatures. The profile of PPA(PTPEC4)1.0 during the heating process showed two peaks 15055


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Figure 5. VT 1D WAXD patterns of PPA(PTPEC4)1.0 (top) and PPA(PTPEC4)0.6 (bottom) recorded upon heating (a,c) and cooling (b,d).

Figure 6. Emission spectra of PTPEC4 in THF/H2O with different water contents (a) and in the thin film (b). The inset is the photo of PTPEC4 powder under 365 nm UV light.

increasing content of PTPEC4, the number of hydrogen bonds increased, leading to a more restricted conformation. Therefore, the reorganization energy would decrease with increasing content of PTPEC4, which resulted in the blue shift of the emission wavelength. The increase of reorganization energy could generally lead to the decrease of the quantum yields (ΦF’s). The integrating sphere results showed that the pure powder of PTPEC4 had a ΦF’s value of 34.94%, whereas the ΦF’s

value of PPA(PTPEC4)x decreased gradually with the decrease of the PTPEC4 content in the complex (Figure S8). Compared to previous LLCPs prepared by covalent bonding,42 the series of LLCPs could show adjustable colors depending on the content of the AIE molecule PTPEC4. More importantly, these LLCPs showed controlled colors, which offered an opportunity to fabricate functional emissive polymers with different colors through simply blending different components. 15056


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Figure 7. Emission spectra of PPA(PTPEC4)x in the thin film (a) and the relationship between the emission peak and x for PPA(PTPEC4)x (b). The insets are images of PPA(PTPEC4)x powder under illumination with a 365 nm UV light.

Figure 8. Emission variation of PPA(PTPEC4)1.0 (a) and PPA(PTPEC4)0.4 (b) under a 365 nm UV light by using phenol vapor. Corresponding FT-IR curves of PPA(PTPEC4)1.0 (c).

Reversible Fluorescence Switching under Proton Acid Vapor. Interestingly, PPA(PTPEC4)x showed reversible fluorescence switching with or without a strong proton acid vapor. As shown in Figure 8a, PPA(PTPEC4)1.0 without any treatment showed a strongly yellow-green emission under UV light illumination. After being fumed with phenol vapor, PPA(PTPEC4)1.0 showed a totally different orange-yellow emission. After the phenol vapor was completely evaporated, PPA(PTPEC4)1.0 recovered the yellow-green emission again. This reversible stimulus response could be repeated many times. A similar phenomenon was observed in other samples (Figure S9). Moreover, in addition to phenol, other strong proton acid vapors such as acetic acid could also induce the reversible fluorescence switching phenomenon of PPA(PTPEC4)x (Figure S10). FT-IR was further employed to investigate the mechanism of reversible switching. Figure 8c depicts the FT-IR curves of PPA(PTPEC4)1.0 under different environments. Compared to the initial state, the spectrum of PPA(PTPEC4)1.0 treated with phenol vapor showed two new peaks at 2710 and 2605 cm−1 (a,b) in addition to the initial hydrogen-bonding peaks at 1910 and 2510 cm−1 (c,d), which indicated the protonation of the N atom of pyridine in PTPEC4 by phenol and the extremely stable hydrogen bond. After complete removal of phenol by thermal annealing, the FT-IR curve recovered to the original one, which implied that the protonation effect had been eliminated. As a comparison, 1,4-dicarboxybenzene as a donor and PTPEC4 as an acceptor were used to construct a luminescent molecule [PTA(PTPEC4)1.0] through hydrogen bonding. As shown in Figure S11, similar reversible fluorescence switching and FT-IR

results under phenol vapor were observed. Therefore, the protonation effect was the important influencing factor to result in the reversible emission color change.

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CONCLUSIONS In conclusion, we successfully synthesized a series of LLCPs, PPA(PTPEC4)x, via hydrogen bonding with different molar ratios of PTPEC4 to PPA. PPA(PTPEC4)x exhibited a distinct solid-state emission. The phase and luminescent behaviors of PPA(PTPEC4)x were strongly dependent on the content of PTPEC4 in the side chain of the complex. With decreasing x, PPA(PTPEC4)x underwent a phase structure change from SmA (x ≥ 0.8) to the columnar nematic phase (0.6 ≤ x ≤ 0.05) and to the amorphous state (x = 0.025). Moreover, PPA(PTPEC4)x exhibited typical AIE behavior with tunable colors, with the emission peak at 510 nm (x = 1.0) gradually red-shifted to 551 nm (x = 0.025). Furthermore, PPA(PTPEC4)x showed reversible fluorescence switching under strong proton acid vapor owing to the protonation effect.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b01476. Instruments and measurements; synthesis of PPA;1 H NMR spectra of PPE (top) and PPA; integrating area of the characteristic absorption peaks 1910 and 2510 cm−1 of FT-IR for PPA(PTPEC4)1.0; thermogravimetric analysis and DSC of PTPEC4 and PPA; VT 1D WAXD 15057


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(10) Kumar, J.; Nakashima, T.; Kawai, T. Circularly Polarized Luminescence in Chiral Molecules and Supramolecular Assemblies. J. Phys. Chem. Lett. 2015, 6, 3445−3452. (11) Wang, Y.; Shi, J.; Chen, J.; Zhu, W.; Baranoff, E. Recent Progress in Luminescent Liquid Crystal Materials: Design, Properties and Application for Linearly Polarised Emission. J. Mater. Chem. C 2015, 3, 7993−8005. (12) Sha, J.; Lu, H.; Zhou, M.; Xia, G.; Fang, Y.; Zhang, G.; Qiu, L.; Yang, J.; Ding, Y. Highly Polarized Luminescence From an AIEE-Active Luminescent Liquid Crystalline Film. Org. Electron. 2017, 50, 177−183. (13) Yoshida, J.; Tamura, S.; Yuge, H.; Watanabe, G. Left- and RightCircularly Polarized Light-Sensing Based on Colored and MechanoResponsive Chiral Nematic Liquid Crystals. Soft Matter 2018, 14, 27− 30. (14) Chen, H.-W.; Lee, J.-H.; Lin, B.-Y.; Chen, S.; Wu, S.-T. Liquid Crystal Display and Organic Light-Emitting Diode Display: Present Status and Future Perspectives. Light: Sci. Appl. 2018, 7, 17168. (15) Zhao, D.; He, H.; Gu, X.; Guo, L.; Wong, K. S.; Lam, J. W. Y.; Tang, B. Z. Circularly Polarized Luminescence and a Reflective Photoluminescent Chiral Nematic Liquid Crystal Display Based on an Aggregation-Induced Emission Luminogen. Adv. Opt. Mater. 2016, 4, 534−539. (16) Han, J.; Guo, S.; Lu, H.; Liu, S.; Zhao, Q.; Huang, W. Recent Progress on Circularly Polarized Luminescent Materials for Organic Optoelectronic Devices. Adv. Opt. Mater. 2018, 6, 1800538. (17) Wang, P.; Collison, C. J.; Rothberg, L. J. Origins of Aggregation Quenching in Luminescent Phenylenevinylene Polymers. J. Photochem. Photobiol., A 2001, 144, 63−68. (18) Jakubiak, R.; Collison, C. J.; Wan, W. C.; Rothberg, L. J.; Hsieh, B. R. Aggregation Quenching of Luminescence in Electroluminescent Conjugated Polymers. J. Phys. Chem. A 1999, 103, 2394−2398. (19) Jakubiak, R.; Bao, Z.; Rothberg, L. Dendritic Sidegroups as Three-Dimensional Barriers to Aggregation Quenching of Conjugated Polymer Fluorescence. Synth. Met. 2000, 114, 61−64. (20) Ruseckas, A.; Namdas, E. B.; Theander, M.; Svensson, M.; Yartsev, A.; Zigmantas, D.; Andersson, M. R.; Inganäs, O.; Sundström, V. Luminescence Quenching by Inter-Chain Aggregates in Substituted Polythiophenes. J. Photochem. Photobiol., A 2001, 144, 3−12. (21) Gaylord, B. S.; Wang, S.; Heeger, A. J.; Bazan, G. C. WaterSoluble Conjugated Oligomers: Effect of Chain Length and Aggregation on Photoluminescence-Quenching Efficiencies. J. Am. Chem. Soc. 2001, 123, 6417−6418. (22) Luo, J.; Xie, Z.; Lam, J. W. Y.; Cheng, L.; Tang, B. Z.; Chen, H.; Qiu, C.; Kwok, H. S.; Zhan, X.; Liu, Y.; Zhu, D. Aggregation-induced Emission of 1-Methyl-1,2,3,4,5-Pentaphenylsilole. Chem. Commun. 2001, 1740−1741. (23) Hong, Y.; Lam, J. W. Y.; Tang, B. Z. Aggregation-induced Emission: Phenomenon, Mechanism and Applications. Chem. Commun. 2009, 4332−4353. (24) Chen, Z.; Bouffard, J.; Kooi, S. E.; Swager, T. M. Highly Emissive Iptycene−Fluorene Conjugated Copolymers: Synthesis and Photophysical Properties. Macromolecules 2008, 41, 6672−6676. (25) Yen, H.-J.; Chen, C.-J.; Liou, G.-S. Novel High-Efficiency PL Polyimide Nanofiber Containing Aggregation-induced Emission (AIE)-Active Cyanotriphenylamine Luminogen. Chem. Commun. 2013, 49, 630−632. (26) Tsujimoto, H.; Ha, D.-G.; Markopoulos, G.; Chae, H. S.; Baldo, M. A.; Swager, T. M. Thermally Activated Delayed Fluorescence and Aggregation Induced Emission with Through-Space Charge Transfer. J. Am. Chem. Soc. 2017, 139, 4894−4900. (27) An, B.-K.; Kwon, S.-K.; Jung, S.-D.; Park, S. Y. Enhanced Emission and Its Switching in Fluorescent Organic Nanoparticles. J. Am. Chem. Soc. 2002, 124, 14410−14415. (28) Yoon, S.-J.; Kim, J. H.; Kim, K. S.; Chung, J. W.; Heinrich, B.; Mathevet, F.; Kim, P.; Donnio, B.; Attias, A.-J.; Kim, D.; Park, S. Y. Mesomorphic Organization and Thermochromic Luminescence of Dicyanodistyrylbenzene-Based Phasmidic Molecular Disks: Uniaxially Aligned Hexagonal Columnar Liquid Crystals at Room Temperature

profiles of PPA(PTPEC4)x; UV−vis absorption spectra of PTPEC4 and PPA(PTPEC4)x; quantum yield of PPA(PTPEC4)x; emission variation of PPA(PTPEC4)x under phenol vapor and acetic acid vapor; and the emission variation of PTA(PTPEC4)1.0 (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.-L.X.). *E-mail: [email protected] (P.W.). ORCID

Zhihao Shen: 0000-0003-2858-555X He-Lou Xie: 0000-0003-4103-2634 Author Contributions ∥

L.T. and M.-L.L. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (NNSFS 21674088, 21374092, and 51503174), the Beijing National Laboratory for Molecular Sciences (BNMLS201815), the Hunan 2011 Collaborative Innovation Center of Chemical Engineering & Technology with Environmental Benignity and Effective Resource Utilization, and the Hunan graduate scientific research innovation project (CX2017B299). The authors thank Prof. Ben Zhong Tang and Dr. Ting Han at the Hong Kong University of Science & Technology for helpful discussions and suggestions.

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REFERENCES

(1) Furue, R.; Nishimoto, T.; Park, I. S.; Lee, J.; Yasuda, T. Aggregation-Induced Delayed Fluorescence Based on Donor/Acceptor-Tethered Janus Carborane Triads: Unique Photophysical Properties of Nondoped OLEDs. Angew. Chem., Int. Ed. 2016, 55, 7171−7175. (2) Yuan, L.; Lin, W.; Zheng, K.; Zhu, S. FRET-Based Small-Molecule Fluorescent Probes: Rational Design and Bioimaging Applications. Acc. Chem. Res. 2013, 46, 1462−1473. (3) Vetrone, F.; Naccache, R.; Zamarrón, A.; Juarranz de la Fuente, A.; Sanz-Rodríguez, F.; Martinez Maestro, L.; Martín Rodriguez, E.; Jaque, D.; García Solé, J.; Capobianco, J. A. Temperature Sensing Using Fluorescent Nanothermometers. ACS Nano 2010, 4, 3254−3258. (4) Guo, Z.; Nam, S.; Park, S.; Yoon, J. A highly Selective Ratiometric Near-infrared Fluorescent Cyanine Sensor for Cysteine with Remarkable Shift and Its Application in Bioimaging. Chem. Sci. 2012, 3, 2760− 2765. (5) Gu, X.; Kwok, R. T. K.; Lam, J. W. Y.; Tang, B. Z. AIEgens for Biological Process Monitoring and Disease Theranostics. Biomaterials 2017, 146, 115−135. (6) Yu, M.; Zhang, P.; Krishnan, B. P.; Wang, H.; Gao, Y.; Chen, S.; Zeng, R.; Cui, J.; Chen, J. From a Molecular Toolbox to a Toolbox for Photoswitchable Fluorescent Polymeric Nanoparticles. Adv. Funct. Mater. 2018, 28, 1804759. (7) Gopikrishna, P.; Meher, N.; Iyer, P. K. Functional 1,8Naphthalimide AIE/AIEEgens: Recent Advances and Prospects. ACS Appl. Mater. Interfaces 2018, 10, 12081−12111. (8) La, D. D.; Bhosale, S. V.; Jones, L. A.; Bhosale, S. V. Tetraphenylethylene-Based AIE-Active Probes for Sensing Applications. ACS Appl. Mater. Interfaces 2018, 10, 12189−12216. (9) San Jose, B. A.; Matsushita, S.; Akagi, K. Lyotropic Chiral Nematic Liquid Crystalline Aliphatic Conjugated Polymers Based on Disubstituted Polyacetylene Derivatives That Exhibit High Dissymmetry Factors in Circularly Polarized Luminescence. J. Am. Chem. Soc. 2012, 134, 19795−19807. 15058


Research Article

ACS Applied Materials & Interfaces with Enhanced Fluorescence Emission and Semiconductivity. Adv. Funct. Mater. 2012, 22, 61−69. (29) Sagara, Y.; Kato, T. Brightly Tricolored Mechanochromic Luminescence From a Single-Luminophore Liquid Crystal: Reversible Writing and Erasing of Images. Angew. Chem. 2011, 123, 9294−9298. (30) Sagara, Y.; Kato, T. Stimuli-Responsive Luminescent Liquid Crystals: Change of Photoluminescent Colors Triggered by a ShearInduced Phase Transition. Angew. Chem., Int. Ed. 2008, 47, 5175−5178. (31) Yamane, S.; Sagara, Y.; Mutai, T.; Araki, K.; Kato, T. Mechanochromic Luminescent Liquid Crystals Based on a Bianthryl Moiety. J. Mater. Chem. C 2013, 1, 2648−2656. (32) Gupta, M.; Pal, S. K. Triphenylene-Based Room-Temperature Discotic Liquid Crystals: A New Class of Blue-Light-Emitting Materials with Long-Range Columnar Self-Assembly. Langmuir 2016, 32, 1120− 1126. (33) Chen, Y.; Lin, J.; Yuan, W.; Yu, Z.; Lam, J. W.; Tang, B. Z. 1-((12Bromododecyl)oxy)-4-((4-(4-pentylcyclohexyl)phenyl)ethynyl) benzene: Liquid crystal with aggregation-induced emission characteristics. Sci. China: Chem. 2013, 56, 1191−1196. (34) Sagara, Y.; Yamane, S.; Mitani, M.; Weder, C.; Kato, T. Mechanoresponsive Luminescent Molecular Assemblies: An Emerging Class of Materials. Adv. Mater. 2016, 28, 1073−1095. (35) Sagara, Y.; Kato, T. Mechanically Induced Luminescence Changes in Molecular Assemblies. Nat. Chem. 2009, 1, 605−610. (36) Park, J. W.; Nagano, S.; Yoon, S.-J.; Dohi, T.; Seo, J.; Seki, T.; Park, S. Y. High Contrast Fluorescence Patterning in CyanostilbeneBased Crystalline Thin Films: Crystallization-Induced Mass Flow Via a Photo-Triggered Phase Transition. Adv. Mater. 2014, 26, 1354−1359. (37) Yuan, W. Z.; Yu, Z.-Q.; Lu, P.; Deng, C.; Lam, J. W. Y.; Wang, Z.; Chen, E.-Q.; Ma, Y.; Tang, B. Z. High Efficiency Luminescent Liquid Crystal: Aggregation-Induced Emission Strategy and Biaxially Oriented Mesomorphic Structure. J. Mater. Chem. 2012, 22, 3323−3326. (38) Jing, H.; Lu, L.; Feng, Y.; Zheng, J.-F.; Deng, L.; Chen, E.-Q.; Ren, X.-K. Synthesis, Aggregation-Induced Emission, and Liquid Crystalline Structure of Tetraphenylethylene-Surfactant Complex via Ionic Self-Assembly. J. Phys. Chem. C 2016, 120, 27577−27586. (39) Liu, Y.; You, L. H.; Lin, F. X.; Fu, K.; Yuan, W. Z.; Chen, E.-Q.; Yu, Z.-Q.; Tang, B. Z. Highly Efficient Luminescent Liquid Crystal with Aggregation-Induced Energy Transfer. ACS Appl. Mater. Interfaces 2019, 11, 3516−3523. (40) Yuan, W. Z.; Yu, Z.-Q.; Tang, Y.; Lam, J. W. Y.; Xie, N.; Lu, P.; Chen, E.-Q.; Tang, B. Z. High Solid-State Efficiency Fluorescent Main Chain Liquid Crystalline Polytriazoles with Aggregation-Induced Emission Characteristics. Macromolecules 2011, 44, 9618−9628. (41) Zabulica, A.; Perju, E.; Bruma, M.; Marin, L. Novel Luminescent Liquid Crystalline Polyazomethines. Synthesis and Study of Thermotropic and Photoluminescent Properties. Liq. Cryst. 2014, 41, 252−262. (42) Guo, Y.; Shi, D.; Luo, Z.-W.; Xu, J.-R.; Li, M.-L.; Yang, L.-H.; Yu, Z.-Q.; Chen, E.-Q.; Xie, H.-L. High Efficiency Luminescent Liquid Crystalline Polymers Based on Aggregation-Induced Emission and ″Jacketing″ Effect: Design, Synthesis, Photophysical Property, and Phase Structure. Macromolecules 2017, 50, 9607−9616. (43) Zhou, Q.; Li, Z.; Zhang, Z.; Pei, X. Studies on the PostPolymerization of a Liquid-Crystal Polymer with X-Shaped Mesogens. Macromolecules 1989, 22, 3821−3823. (44) Chen, X.-F.; Shen, Z.; Wan, X.-H.; Fan, X.-H.; Chen, E.-Q.; Ma, Y.; Zhou, Q.-F. Mesogen-Jacketed Liquid Crystalline Polymers. Chem. Soc. Rev. 2010, 39, 3072−3101. (45) Kato, T.; Kihara, H.; Uryu, T.; Fujishima, A.; Frechet, J. M. J. Molecular Self-Assembly of Liquid Crystalline Side-Chain Polymers Through Intermolecular Hydrogen Bonding. Polymeric Complexes Built From a Polyacrylate and Stilbazoles. Macromolecules 1992, 25, 6836−6841. (46) Alexander, C.; Jariwala, C. P.; Lee, C. M.; Griffin, A. C. Selfassembly of Main Chain Liquid Crystalline Polymers via Heteromeric Hydrogen Bonding. Macromol. Symp. 1994, 77, 283−294. (47) Gracia, I.; Romero, P.; Serrano, J. L.; Barberá, J.; Omenat, A. Templated Nanoporous Membranes Based on Hierarchically SelfAssembled Materials. J. Mater. Chem. C 2017, 5, 2033−2042.

(48) Kato, T.; Kihara, H.; Kumar, U.; Uryu, T.; Fréchet, J. M. J. A Liquid-Crystalline Polymer Network Built by Molecular Self-Assembly through Intermolecular Hydrogen Bonding. Angew. Chem., Int. Ed. Engl. 1994, 33, 1644−1645. (49) Yoon, H.-H.; Kim, D.-Y.; Jeong, K.-U.; Ahn, S.-k. Surface Aligned Main-Chain Liquid Crystalline Elastomers: Tailored Properties by the Choice of Amine Chain Extenders. Macromolecules 2018, 51, 1141− 1149. (50) Zhu, Y.; Zheng, M.; Tu, Y.; Chen, X.-F. Supramolecular Fluorescent Polymers Containing α-Cyanostilbene-Based Stereoisomers: Z/E-Isomerization Induced Multiple Reversible Switching. Macromolecules 2018, 51, 3487−3496. (51) Giese, M.; Krappitz, T.; Dong, R. Y.; Michal, C. A.; Hamad, W. Y.; Patrick, B. O.; MacLachlan, M. J. Tuning the Photonic Properties of Chiral Nematic Mesoporous Organosilica with Hydrogen-Bonded Liquid-Crystalline Assemblies. J. Mater. Chem. C 2015, 3, 1537−1545. (52) Pan, H.; Zhang, W.; Xiao, A.; Lyu, X.; Shen, Z.; Fan, X. Persistent Formation of Self-Assembled Cylindrical Structure in a Liquid Crystalline Block Copolymer Constructed by Hydrogen Bonding. Macromolecules 2018, 51, 5676−5684. (53) Xu, Y.; Qu, W.; Yang, Q.; Zheng, J.; Shen, Z.; Fan, X.; Zhou, Q. Synthesis and Characterization of Mesogen-Jacketed Liquid Crystalline Polymers through Hydrogen-Bonding. Macromolecules 2012, 45, 2682−2689. (54) Li, X.; Fang, L.; Hou, L.; Zhu, L.; Zhang, Y.; Zhang, B.; Zhang, H. Photoresponsive Side-Chain Liquid Crystalline Polymers with Amide Group-Substituted Azobenzene Mesogens: Effects of Hydrogen Bonding, Flexible Spacers, and Terminal Tails. Soft Matter 2012, 8, 5532−5542. (55) Yan, M.; Tang, J.; Xie, H.-L.; Ni, B.; Zhang, H.-L.; Chen, E.-Q. Self-Healing and Phase Behavior of Liquid Crystalline Elastomer Based on a Block Copolymer Constituted of a Side-Chain Liquid Crystalline Polymer and a Hydrogen Bonding Block. J. Mater. Chem. C 2015, 3, 8526−8534. (56) Ni, B.; Xie, H.-L.; Tang, J.; Zhang, H.-L.; Chen, E.-Q. A SelfHealing Photoinduced-Deformable Material Fabricated by Liquid Crystalline Elastomers Using Multivalent Hydrogen Bonds as CrossLinkers. Chem. Commun. 2016, 52, 10257−10260. (57) Yan, J.-J.; Xie, H.-L.; Weng, L.; Yang, S.; Zhang, H.-L. Manipulating Ordered Structure of Ionic Liquid Crystalline Polymers through Tuning the Alkyl Spacer Length. Polymer 2014, 55, 6504− 6512. (58) Zheng, X.; Peng, Q.; Zhu, L.; Xie, Y.; Huang, X.; Shuai, Z. Unraveling the Aggregation Effect on Amorphous Phase AIE Luminogens: A Computational Study. Nanoscale 2016, 8, 15173− 15180. (59) Zhao, Z.; Gao, S.; Zheng, X.; Zhang, P.; Wu, W.; Kwok, R. T. K.; Xiong, Y.; Leung, N. L. C.; Chen, Y.; Gao, X.; Lam, J. W. Y.; Tang, B. Z. Rational Design of Perylenediimide-Substituted Triphenylethylene to Electron Transporting Aggregation-Induced Emission Luminogens (AIEgens) with High Mobility and Near-Infrared Emission. Adv. Funct. Mater. 2018, 28, 1705609.

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Color-Tunable and Stimulus-Responsive Luminescent Liquid

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