Synthesis, Aggregation-Induced Emission, and Liquid Crystalline

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Synthesis, Aggregation-Induced Emission, and Liquid Crystalline Structure of Tetraphenylethylene–Surfactant Complex via Ionic Self-Assembly Hui Jing, Lin Lu, Yakai Feng, Jun-Feng Zheng, Liandong Deng, Er-Qiang Chen, and Xiang-Kui Ren J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b09901 • Publication Date (Web): 18 Nov 2016 Downloaded from http://pubs.acs.org on November 27, 2016

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The Journal of Physical Chemistry

Synthesis, Aggregation-Induced Emission, and Liquid Crystalline Structure of Tetraphenylethylene–Surfactant Complex via Ionic SelfAssembly †

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Hui Jing, Lin Lu, Yakai Feng, Jun-Feng Zheng,*, Liandong Deng, Er-Qiang Chen*, and Xiang†

Kui Ren*,

†

School of Chemical Engineering and Technology, Tianjin University, Tianjin 300350, P. R. China.

‡

College of Chemical Engineering, Nanjing Forestry University, Nanjing 210037, P. R. China.

§

Beijing National Laboratory for Molecular Sciences, College of Chemistry, Peking University, Beijing

100871, P. R. China.

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ABSTRACT:

A novel tetraphenylethylene (TPE) material with liquid crystalline (LC) helical structure and aggregation-induced emission (AIE) property was prepared by ionic self-assembly (ISA). The AIE activity, phase behavior, self-assembly structure and molecular packing behavior of the complex were then elucidated via a combination of different experimental techniques such as UV-vis absorption spectra, photoluminescence spectra, differential scanning calorimetry, polarized optical microscopy, one- and two-dimensional X-ray diffraction, and Fourier transform infrared spectroscopy. The experimental results reveal that the ISA complex possesses high efficiency luminescent property with quantum yield as high as 46% in solid state. Meanwhile, the complex could self-assemble into different interesting structures which are sensitive to peripheral chain motions. During heating, the complex takes a low ordered helical supramolecular structure at ambient temperature, and then forms another LC phase with high ordered helical molecular stacking. These ordered hierarchical structures, in combination with the liquid crystallinity and excellent AIE property of the ISA complex, make it a promising material for fabrication of luminescent devices.


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1. INTRODUCTION Because of their unique combination of optophysical and anisotropic organization features within a liquid crystalline phase, luminescent liquid crystals (LCs) have attracted great attentions for optoelectronic applications such as organic light-emitting diodes,1-3 imaging systems,4-5 light-emitting LC displays,6-7 and organic semiconductors.8 However, most conventional luminogens are highly emissive in solution but will be weakly or be quenched in the aggregated state due to the aggregationcaused quenching (ACQ) effect,9-11 which ultimately limit the use of luminophores in optoelectronic devices. To overcome this shortcoming, various molecular strategies are used to avoid the quenching configurations in solid state.12-17 For example, we and other laboratories have demonstrated that bulky substituents can disturb the stacking of conjugated luminophores and maintain light emission even in solid state.15-17 Nevertheless, the light emission is often enhanced at the expense of long-range ordered supramolecular structure, thus making the synthesis of mesomorphic materials with efficient exciton emission still a daunting task. In 2001, Tang's group presented the fascinating concept of aggregation-induced emission (AIE),18 which provides another powerful mean for the daunting ACQ problem. Since then, researchers have reported abundant AIE19-27 or aggregation-induced emission enhancement (AIEE)28–30 materials, which are no luminescence or weak in solution but exhibit strong fluorescence in aggregation or solid state. Among all the AIE luminogens, tetraphenylethylene (TPE) is usually employed due to its high solidsate emission efficiency and versatile functionalization approache.31,32 The AIE effect provides the possibility to fabricate novel AIE-active LCs by introducing peripheral mesogens or long flexible groups to the AIE cores.33-39 Recently, the research on AIE-active LCs is truly an area with tantalizing prospect due to the real-world applications, especially in the fabrication of light-emitting LC devices.7,27 For example, a group of such LC systems have been built with TPE core decorated with either flexible alkyl/alkoxy tails or mesogenic units.27 However, most synthesis methods for these AIE LCs require complex and harsh conditions, and often the yield is relatively low.


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It has been demonstrated that ionic self-assembly (ISA) is a facile and efficient mechanism to construct novel functional materials by binding oppositely charged tectonic units together via electrostatic interactions.40-42 This technique has attracted much attention now as a powerful tool to create highly ordered materials for its advantages of easiness, cheapness, universality, high yield and environmentally-friendly properties. However, there are few reports on the creation of ISA materials with AIE property.43 In particular, to the best of our knowledge, the fabrication of TPE LCs via ISA strategy has still not been reported. It is of great interest to know whether it is possible to fabricate high luminescent TPE LCs with novel hierarchical nanostructure by the ISA process. In this publication, we report the synthesis of an AIE-active TPE LC (TPE-DOAB) with hierarchical supramolecular structure and excellent luminescent property by ISA (Scheme 1). The AIE activity of the LC complex was determined by UV-vis absorption and photoluminescence (PL) experiments, which show a quantum yield as high as 46% in solid state. The phase behavior, self-assembly structure and molecular packing behavior of the complex were then studied using differential scanning calorimetry (DSC), polarized optical microscopy (POM), one- and two-dimensional (1D and 2D) X-ray diffraction (XRD) experiments. At room temperature, the complex can self-assemble into low ordered helical columns as the precursor. Upon heating, it enters another LC phase with high ordered helical molecular stacking. Moreover, Fourier transform infrared spectroscopy (FT-IR) experiment indicates that this thermally-induced phase transition may be attributed to the variation in the degree of peripheral chain motions. Notably, the strategy and principle outlined in this work should be applicable to design and fabricate other types of high efficiency luminescent mesomorphic materials by ISA.


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Scheme 1. The synthetic route towards TPE-DOAB. 2. EXPERIMENTAL SECTION 2.1 Materials and Sample Preparation 4,4'-Dimethoxybenzophenone, Tataniumtetrachloride (TiCl4), Boron tribromide (BBr3), Methyl 4(bromomethyl)benzoate, Tetrabutylammonium bromide (TBAB) and Dimethyldioctadecylammonium bromide (DOAB) were purchased from Heowns Biochemical Technology Co. Ltd. (Tianjin, China). All the other chemicals were bought from YuanLi Chemical Reagents Co. Ltd. (Tianjin, China). Deionized water was used in the experiments. Tetrahydrofuran (THF) and dichloromethane (DCM) was distilled from calcium hydroxide prior to use. Other chemicals were of analytical grade and used without further ACS Paragon Plus Environment

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purification. For 2D XRD experiment, the oriented sample was prepared by mechanically shearing the TPE-DOAB at 70 °C, followed by fast cooling to room temperature. 2.2 Measurement and Characterization 1

H NMR and

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C NMR spectra were recorded on a Bruker Avance 400 MHz and 100 MHz

spectrometer at room temperature using deuterated chloroform (CDCl3) or dimethyl sulfoxide (DMSOd6) as solvents and tetramethylsilane (TMS) as the internal standard. To detect the thermal stability and phase behaviors of the sample, thermogravimetric analysis (TGA) and DSC were carried out on TA Q100 and TA SDTQ-600 controllers at a scan rate of 10 °C/min under nitrogen at a flow rate of 50 mL/min. FT-IR were recorded with a Perkin-Elmer FTIR-100 spectrometer equipped with a Linkman FTIR 600 hot stage. The TPE-DOAB film for polarized optical microscopy (POM) experiment was prepared by slow evaporation from solution with a concentration of 5 mg/mL. 1D XRD experiments were performed with a high-flux small-angle X-ray scatting instrument (SAXSess, Anton Paar) equipped with a Kratky block-collimation system. Using an imaging plate as the detector, it can simultaneously measure small-angle and wide-angle X-ray scattering of the sample, covering the q-range from 0.06 to 29 nm−1 (q = 4πsin θ/λ, where the λ is the wavelength of 0.1542 nm and 2θ is the scattering angle). A temperature control unit of Anton Paar TCS300 was used to control the sample temperature during measurement. 2D XRD experiments of the oriented sample were recorded on a Bruker D8 Discover diffractometer with a Vantec500 as the 2D detector. The oriented sample prepared by mechanical shearing was mounted on the sample stage equipped with a temperature control unit. The 2D diffraction patterns were recorded in a transmission mode with the point-focused X-ray beam aligned perpendicular to the shear direction. The background scattering was recorded and subtracted from the sample pattern. For both the 1D and 2D XRD, the calibration was conducted using silicon powder and silver behenate. Reconstruction of relative electron density map was performed based on the XRD results. UV-vis absorption spectra were recorded using a Perkin Elmer Lambda 20 UV/Vis spectrophotometer with sample in solution and a quartz glass cuvette. Fluorescence spectra were obtained on a Hitachi FLACS Paragon Plus Environment

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7000 luminescence spectrometer. The solvents for spectroscopic studies were spectroscopic grade and used as received. The quantum yield in N,N-dimethylformamide (DMF) solution (ΦF,s) was estimated using quinine sulfate (ΦF = 54.6% in H2SO4) as standard. The absolute photoluminescence quantum yield in the solid-state film (ΦF,f) was measured on an Edinburg FLS 920 fluorescence spectrometer with a calibrated integrating sphere. A 365-nm CW laser was used as the excitation source. 2.3 Synthesis The chemical structure and synthetic route of the target complex TPE-DOAB are illustrated in Scheme 1. The synthetic method for Tetrakis(4-hydroxyphenyl)ethane (3) is the same as reported earlier.44 The synthetic procedure in detail is as follows. 2.3.1 Synthesis of 1,1,2,2-tetrakis(4-methoxyphenyl)ethane (2). Under a nitrogen atmosphere, a 1000 mL two-necked flask equipped with a magnetic stirrer and a condenser was charged with 400 mL dried THF and stirred for 5 min at room temperature. Then zinc powder (64.46 g, 0.99 mol) was added. The mixture was cooled to -5 – 0 °C, and TiCl4 (55 mL) was slowly added by a syringe. Afterwards, the suspending mixture was warmed to room temperature and stirred for 0.5 h, then heated at reflux for 1.5 h. The mixture was again cooled to -5 – 0 °C, and 4, 4'-dimethoxybenzophenone (200 g, 0.83 mol) was added. After addition, the reaction mixture was heated at reflux until the carbonyl compounds were consumed (monitored by TLC). The reaction was quenched with 10% K2CO3 aqueous solution. The precipitate was filtrated and the filtrate was evaporated under reduced pressure. After solvent evaporation, the crude product was recrystallized in CH2Cl2/CH3OH (v/v = 1/2) to yield a white crystalline product 2 (261 g, 70%). 1H NMR (CDCl3, 400 MHz), δ (TMS, ppm): 6.96 (d, 8H, phenyl’s H), 6.67 (d, 8H, phenyl’s H), 3.77 (s, 12H, 4OCH3). 13C NMR (CDCl3, 100 MHz), δ (TMS, ppm): 55.3, 113.26, 132.76, 137.13, 138.62, 158.02. 2.3.2 Synthesis of tetrakis(4-hydroxyphenyl)ethane (3). To a cooled solution of tetrakis(4methoxyphenyl)ethane (5 g, 11.06 mmol) in dried CH2Cl2 (100 mL) was added BBr3 dropwise (4.2 mL, 44.25 mmol) by a syringe at -10 – 0 °C. The resulting deep red mixture was stirred for 2 h at -10 – 0 °C. ACS Paragon Plus Environment

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After removal of the cooling bath, the mixture was stirred for 16 h at RT and then hydrolyzed under icecooling by dropwise addition of H2O. The precipitate was isolated by filtration, washed with H2O. Recrystallization from CH3CH2OH/H2O (v/v = 1/1) to yield a pink solid 3 (4.03 g, 92%). 1H NMR (DMSO-d6, 400 MHz), δ (TMS, ppm): 9.24 (s, 4H, 4OH), 6.70 (d, 8H, phenyl’s H), 6.48 (d, 8H, phenyl’s H). 13C NMR (DMSO-d6, 100 MHz), δ (TMS, ppm): 114.48, 131.94, 135.07, 137.72, 155.33. 2.3.3

Synthesis

of

tetramethyl

4,4',4'',4'''-(((ethene-1,1,2,2-tetrayltetrakis(benzene-4,1-

diyl))tetrakis(oxy))tetrakis(methylene))tetrabenzoate (4). A mixture of 3 (3 g, 7.57 mmol), K2CO3 (4.2 g, 30.43 mmol), methyl 4-(bromomethyl)benzoate (8.7 g, 37.99 mmol), tetrabutylammonium bromide (0.75 g, 2.33 mmol) in acetone (100 mL) was heated for 24 h at 80 °C. After cooling to room temperature, the solvent was removed under reduced pressure at 40 °C. The residue was dissolved in DCM and washed with water three times. The organic part was dried over magnesium sulfate and concentrated under reduced pressure. The crude product was recrystallized from CH2Cl2/CH3OH (v/v = 1/2) to get a white product 4 (6.45 g, 86%). 1H NMR (CDCl3, 400 MHz), δ (TMS, ppm): 8.07 (d, 8H， phenyl’s H), 7.50 (d, 8H, phenyl’s H), 6.96 (d, 8H, phenyl’s H), 6.73(d, 8H, phenyl’s H), 5.07(s, 8H, 4OCH2), 3.93(s, 12H, 4COOCH3). 13C NMR (CDCl3, 100 MHz), δ (TMS, ppm): 167.01, 157.08, 142.48, 138.77, 137.46, 132.85, 130.04, 127.24, 114.26, 69.42, 52.33. 2.3.4

Synthesis

of

4,4',4'',4'''-(((ethene-1,1,2,2-tetrayltetrakis(benzene-4,1-

diyl))tetrakis(oxy))tetrakis(methylene))tetrabenzoic acid (5). 4 (3 g, 3.03 mmol) and potassium hydroxide (2.04 g, 36.43 mmol) were added into a mixture of THF, ethanol and water (v/v/v = 3:3:1). The mixture was refluxed and gradually became turbid. By addition of water, the solution became clear, then the mixture was refluxed for more than 4 h. After removing the organic solvent, the aqueous phase was acidified with 6 M HCl to yield white precipitate, which was filtered, then washed with water, and dried under vacuum to get a white product 5 (2.69 g, 95%). 1H NMR (DMSO-d6, 400MHz), δ (TMS, ppm): 13.00 (s, 4H, 4COOH), 7.97 (d, 8H, phenyl’s H), 7.54 (d, 8H, phenyl’s H), 6.88 (d, 8H, phenyl’s H), 6.79 (d, 8H, phenyl’s H), 5.10 (s, 8H, 4OCH2). 13C NMR (DMSO-d6, 100 MHz), δ (TMS, ppm): 167.75, 157.17, 142.71, 138.83, 137.25, 132.72, 130.87, 130.11, 128.10, 114.76. IR (KBr, cm-1): 3430, ACS Paragon Plus Environment

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3032, 2877, 2671, 2548, 1695, 1610, 1502, 1422, 1287, 1240, 1174, 1113, 1033, 1014, 929, 883, 827, 756. 2.3.5 Synthesis of TPE-DOAB. 5 (0.068 g, 0.07 mmol) was at first dissolved in 20 mL KOH aqueous solution (18.3 mM) to yield the potassium salt (6). The DOAB solution was prepared by dissolving DOAB (0.19 g, 0.29 mmol) in 24 mL binary solvent of water/ethanol (v/v = 1/3). Then solution of 6 was added into the DOAB mixture at 48 °C and immediately a charge stoichiometric complex was precipitated from solution. The precipitate was isolated by filtration, and the solid was washed with a mixture of water and ethanol (v/v = 1/3) several times to remove any remaining inorganic salt or free surfactants. The complex was dried under vacuum at room temperature, resulting in a cyan powder TPE-DOAB (0.22 g, 97.5%). 1H NMR (CDCl3, 400MHz), δ (TMS, ppm): 8.07 (d, 8H, phenyl’s H),7.35 (d, 8H, phenyl’s H), 6.92 (d,8H, phenyl’s H), 6.70 (d, 8H, phenyl’s H), 5.01 (s, 8H, 4OCH2), 3.35 (m, 16H, 8CH2), 3.29 (s, 24H, 8CH3), 1.61 (m, 16H, 8CH2), 1.28 (m, 240H, 120CH2), 0.89 (t, 24H, 8CH3). 13C NMR (CDCl3, 100 MHz), δ (TMS, ppm): 171.64, 157.40, 139.06, 138.82, 138.49, 137.17, 132.71, 129.85, 126.80, 114.16, 69.97, 63.67, 51.59, 32.13, 29.93, 29.90, 29.88, 29.86, 29.73, 29.61, 29.57, 29.37, 26.41, 22.89, 14.32. IR (KBr, cm-1): 3027, 2955, 2918, 2850, 1599, 1550, 1508, 1470, 1400, 1241, 1175, 1010, 1037, 1017, 830, 771, 715. 3. RESULTS AND DISCUSSION 3.1 Synthesis The target molecule was prepared by simple stoichiometric amounts of 6 and DOAB in solution. As an important precursor, 5 was firstly dissolved in KOH aqueous solution. DOAB solution was prepared by dissolving the DOAB in the water/ethanol (v/v = 1/3) mixture. And then mixed them together at a molar ratio of 1:4 to obtain the precipitation, which was isolated by filtration. The solid was washed with a mixture of water/ethanol (v/v = 1/3) and dried under vacuum to get TPE-DOAB in a yield of 97.5%. Compound 2-5 were verified by 1H NMR,

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C NMR (Figure S1-S8). The target complex was

characterized via 1H NMR, 13C NMR and FT-IR spectroscopy (Figure S9-S11). 1H NMR measurement in Figure S9 indicated the proton signals were accordance with the chemical structure of TPE-DOAB. In ACS Paragon Plus Environment

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FT-IR, the bending vibration band of N+O- was found in 3027 cm-1, which further confirmed the successful attachment of DOAB to the tetraphenylethylene backbone. 3.2 Aggregation-Induced Emission Performance Contrary to existing TPE derivatives, TPE-DOAB consists of a TPE core and eight long alkyl chains attached via ionic bonds. Thus, its solubility in common organic solvents depends on a balance of nonpolar alkyl chains and polar ionic bonds. The solubility of the complex was tested qualitatively in various solvents. TPE-DOAB is able to dissolve in chloroform, dichloromethane, methanol and DMF, but cannot be dissolved in water. The UV-vis absorption spectra of TPE-DOAB and 5 in DMF are depicted in Figure S12. As expected, the complex shows almost identical absorption profile to that of 5 with two typical peaks around 268 and 323 nm, which corresponds to the π–π transition of phenyl rings and TPE segments, respectively. The absence of obvious electronic transitions beyond 330 nm suggests the highly twisted TPE configurations therein and accordingly, the DMF solution does not emit any visible light under UV illumination. Strong light of TPE-DOAB can be generated from its solid film, indicating the AIE-active property of the complex. To verify the visual observation, PL spectra of TPE-DOAB in DMF and DMF/water mixtures were measured with water as the poor solvent of the ISA complex. As shown in Figure 1a, when water fraction (fw) is inferior to 20%, the solution has virtually no emission because the molecules are genuinely dissolved. However, higher fw than 20% lead to severe aggregation of the complex and significant enhancement of the PL intensity which reached the maximum value at 80%. The formation of aggregate is speculated from the level-off tail in the UV-vis spectra (Figure S13), which is attributed to Mie or light-scattering effect caused by nanosized particles.45 When fw is higher than 80%, the PL intensity decreases to some extent, which is due to that aggregates form and precipitate quickly at high water content. Moreover, similar trends are observed in the peak intensities (first increase to maxima, then decrease) at 480 nm in aqueous mixtures (Figure 1b). From the well-dissolved molecules in DMF to the nanosuspension in 80% aqueous mixture, the emission intensities of TPE-DOAB at 480 nm are increased by about 65 times. All of these phenomena showed that ISA complex TPE-DOAB is a typical ACS Paragon Plus Environment

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AIE material, which can also be seen from the insert photographs in Figure 1. In addition, the emission property of the solid-state film was also studied. As shown in Figure S14, the complex exhibited strong cyan emission at 480 nm at room temperature. To get more quantitative insight into the AIE activity of the ISA complex, the fluorescence quantum yield of TPE-DOAB is measured both in solution and in solid state. While that in DMF is as low as 0.68%, the ΦF,f value of the solid film is improved to 46% upon aggregate formation, resulting in an AIE factor (αAIE = ΦF,f/ΦF,s) as high as 67.6. The αAIE value is close to the highest level of luminescent LCs reported in the solid state.34, 46

Figure 1. (a) PL spectra of TPE-DOAB in DMF and DMF/water mixture. The inset graph is TPEDOAB film taken under UV illumination. (b) Plot of I/I0 versus water fraction of DMF/water mixture of TPE-DOAB, where I0 denotes the emission intensity in pure DMF solution. Concentration = 1.0×10-5 M. Excitation wavelength = 365 nm. The inset graph is TPE-DOAB in DMF and 10/90 DMF/water mixture taken under UV illumination.

3.3 Thermal Property and Structural Evolution The complex was subjected to thermal analysis to determine its stability and phase behavior, TGA result shows that TPE-DOAB has a comparably good thermal stability (Figure S15). Figure 2 depicts the DSC curves obtained during cooling and subsequent heating at a rate of 10 °C/min, respectively. Two thermal transitions, which are quite broad, can be clearly observed before the onset of degradation. The onset temperatures of the two transitions during cooling and heating processes are rather close, suggesting the existence of a LC-to-LC transition. The liquid crystalline properties of TPE-DOAB were further characterized by POM. As shown in Figure 3, the anisotropic mesomorphic textures were clearly observed, which are both ascribed to columnar mesophases. Together with its excellent AIE activity, it


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can be concluded that TPE-DOAB complex is an AIE LC, thus implying the possibility of a new strategy toward high efficient TPE LC by ISA.

Figure 2. DSC traces of the TPE-DOAB recorded upon cooling and subsequent heating at a rate of 10 °C/min.

Figure 3. Typical textures of the TPE-DOAB film as observed in the polarized optical microscope at (a) 30 °C and (b) 70 °C. Although DSC experiment is sensitive to heat absorption and release events, this technique cannot provide direct information about structural changes and molecular arrangements. Therefore, thermal 1D XRD experiment was utilized to identify the corresponding structural evolution. Figure 4 shows a set of 1D XRD powder patterns during heating from room temperature at a rate of 10 °C/min for TPE-DOAB. Structures with two different length scales, i.e., one on the nanometer scale and another on the subnanometer scale, can be identified. Moreover, a very broad phase transition process can be detected in Figure 4, which agrees well with the observation in DSC heating diagram (Figure 2). At temperature of 30 °C, there are at least six diffractions showing in the low-q region, which cannot be indexed easily. In


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the high-q region, several diffractions overlapped with an amorphous halo can be detected, although they are not very sharp, which may be associated with the ordered arrangement of the peripheral alkyl chains. While the low-q diffractions remain largely unchanged, heating the sample to 40 °C leads to the disappearance of the high-q diffractions. With a further increase of temperature, the original high-q diffractions vanish entirely. Meanwhile, two relatively broad diffractions at q = 13.5 (d-spacing of 0.47 nm) and 14.6 nm-1 (d-spacing of 0.43 nm) appear and evolve, which may originate from the rearrangements of the TPE cores and peripheral chains, respectively. At temperatures above 60 °C, only the diffraction of 0.47 nm overlapped with an amorphous halo can be detected, implying the ordered arrangement of TPE cores and the liquid-like disordered packing of alkyl chains.

Figure 4. Set of 1D XRD profiles of TPE-DOAB recorded at various temperatures upon heating. From the DSC and 1D XRD results, we can assign two different phases of TPE-DOAB as loose ordered LC (at room temperature) and high ordered LC (at high temperature above 60 °C) denoted as ΦLl and ΦLh, respectively. Note that the low-q diffractions do not show distinct change, it is speculated that the difference between the two LC phases may come from the rearrangement of the alkyl chains. However, the 1D XRD patterns lack lattice dimensionality of the ordered structure and thus, are not sufficient for a complete structure determination. To obtain detailed structural information, we focused on the 2D WAXD results of the oriented sample, which will be discussed in detail below. 3.4 Structure Identification and Molecular Packing Arrangement


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Figure 5 presents the 2D WAXD uniaxial pattern of the oriented ΦLl phase of TPE-DOAB. The pattern is recorded at room temperature and exhibits diffractions not only on the equator and meridian but also in the quadrants. The relatively weak diffractions in the high-angle quadrants are not very sharp, which agree well with 1D XRD results and reflect the ordered arrangement of the alkyl chains. According to the reciprocal lattice principle, the (00l) diffractions are on the meridian and the (hk0) diffractions are along the equator. On the meridian, there is a pair of very diffuse arcs (indexed by 1 in Figure 5) in the high-angle region. Considering their corresponding d-spacing of 0.47 nm is the typical spacing between the stacked TPE cores,34 this set of broad arcs seems to suggest that TPE moieties are stacked parallel to each other. More importantly, another set of culminant diffraction arcs (indexed by 2 in Figure 5) near the beam stop should not be overlooked. This pair of low-angle arcs indicates that the ΦLl phase of TPEDOAB possesses a large c parameter, identifying that the molecules are rotationally stacked and helically assembled in a column. It should be noted that since the high-angle culminant arcs (indexed by 1 in Figure 5) are so diffuse and so weak, the molecules should be packed loosely in the helical column. In addition, the (hk0) diffractions are on the equator, indicating that the axes of column are well aligned along the shear direction.

Figure 5. 2D WAXD pattern of sheared TPE-DOAB recorded at 30 ºC. The shear direction is on the meridian and the X-ray beam was perpendicular to the shear direction.


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When the oriented TPE-DOAB was heated to 70 °C, the 2D WAXD pattern exhibits different diffractions and another LC phase ΦLh is observed (Figure 6a). As indicated in this figure, the a*- and b*- axes are assigned to the equator, and the c*- axis is along the meridian. Following the standard procedure of determining the crystal lattice,15, 17, 47, 48 an oblique lattice of the (hk0) diffractions on the equator can be constructed as: a = 6.71 nm, b = 6.06 nm, γ = 109.83º. Figure 6c provides a relative electron density map of TPE-DOAB calculated based on the (hk0) diffractions, illustrating the molecular arrangement in the ab plane. It is reasonable to assign the red area with the highest electron density to TPE moieties, while the surrounding areas are packed with disordered alkyl tails.

Figure 6. (a) 2D WAXD pattern of sheared TPE-DOAB recorded at 70 ºC. (b) A zoom-in view at center part of (a). (c) Relative electron density map of TPE-DOAB calculated based on the diffracted intensity of (hk0) shown in (a). In the quadrants, the original weak diffractions at high-angle region disappear, indicating the ordered arrangement of the alkyl chains has been destroyed in ΦLh phase. Meanwhile, some new distinct diffractions emerge not only in the quadrants but also on the equator, suggesting the birth of a more ordered LC structure. Moreover, it is surprising that the weak arc of 0.47 nm on the meridian finally convert to a strong arc which corresponds to the high-q diffraction in 1D XRD pattern. In fact, this pair of diffractions possesses the strongest intensity among all of the meridianal diffractions. One may speculate that this significant intensity change represents an evolution from a loose packing to a high ACS Paragon Plus Environment

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ordered packing arrangement of TPE cores. This result is also supported by the solid photoluminescence spectra of TPE-DOAB. As shown in Figure S14, the fluorescence peak red-shifts about 10 nm from 480 nm to 490 nm with temperature increasing, indicating a higher ordered packing arrangement of TPE cores in the ΦLh phase. In addition, the culminant arcs near the beam stop remain there, indicating that the helically assembled structure still maintains in ΦLh phase. Note that the culminant arc of 0.47 nm is in fact located on the twenty-first layer of the diffractions, and thus should be indexed as the (0021), while the culminant arc near beam stop should be indexed as (002). In this case, we presume that the TPE-DOAB molecules are helically assembled with more ordered packing of TPE cores in the column, giving a 21-fold helix (Figure 7). It should be noted that since TPE-DOAB does not have a chiral center to guide the helical formation, the helical sense could be either right-handed or left-handed, depending on the initial stacking nucleus scheme. Then, these helical columns further pack into a lattice irrespective of the helical sense and construct a unique supramolecular structure shown in Figure 7. As a result, the circular dichroism spectra of the TPE-DOAB assemblies in mixed solvents and the TPEDOAB film showed no Cotton effect at the absorption band of the complex. Using the standard refinement procedure,15, 17, 47, 48 the unit cell of the supramolecular structure is finally determined to be monoclinic one with dimensions of a = 6.71 nm, b = 6.06 nm, c = 9.87 nm, α = β = 90º, γ = 109.83º. The experimental and calculated diffraction angles and d-spacing values of ΦLh phase are listed in Table 1.

Figure 7. (a) Schematic diagram of the TPE-DOAB molecule. (b) Model of the helical column in the monoclinic lattice (ΦLh) of TPE-DOAB. For clarity, peripheral alkyl chains are not shown in the lattice and the four benzene rings of TPE core are drawn in different colors. ACS Paragon Plus Environment

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Table 1 Crystallographic parameters of the LC phase (ΦLh) of TPE-DOAB

(hkl) -110 002 200 103 020 006 400 -116 410 430 360 4112 0021

2θ (deg) expta calcdb 1.70 1.70 1.79 1.79 2.80 2.80 3.05 3.03 3.10 3.10 5.25 5.37 5.49 5.60 5.55 5.63 6.30 6.30 8.40 8.41 11.50 11.45 12.20 12.47 18.90 18.88

d-spacing (nm) expta calcdb 5.20 5.20 4.94 4.94 3.16 3.16 2.90 2.92 2.85 2.85 1.68 1.65 1.61 1.58 1.59 1.57 1.40 1.40 1.05 1.05 0.77 0.77 0.73 0.71 0.47 0.47

Intensity exptc m w vs s vs m vw m m m m w s

a

Experimental values observed in Figure 5. b Calculated values based on the monoclinic unit cell of a = 6.71 nm, b = 6.06 nm, c = 9.87 nm, α = β = 90º, γ = 109.83º. c The experimental intensities in Figure 5 are semiquantitatively estimated via amicrodesitometer and classified as very strong (vs), strong (s), medium (m), weak (w), and very weak (vw). 3.5 Correlation between Phase Structure and Peripheral Chain Motion Based on the infrared spectroscopic experiments, it has been demonstrated that the phase transition of LC materials may be associated with the thermally-induced conformation change of methylene units.4951

Considering the 1D and 2D XRD results of TPE-DOAB, we presume that the thermally-induced

phase transition from ΦLl to ΦLh is led by the variation in the degree of TPE stacking order which is influenced by peripheral chain motions. To get quantitative insight into the conformation variation of the peripheral chains, the temperature-dependent FT-IR spectra in methylene bending modes were performed. As shown in Figure 8a, the two bands at around 1470 and 1458 cm−1 are associated with the trans- and gauche-dominated conformation, respectively.52 Upon heating, the absorbance at 1458 cm−1 increases while that at 1470 cm−1 decreases gradually. This indicates that the methylene units adopt more and more gauche conformation with the increase of temperature, in accordance with the XRD results. Furthermore, the ln(A1470/A1458) is plotted against the reciprocal of temperature (1/T) in Figure 8b. Interestingly, there are two linear lines with different slopes in this figure. It should be noted that the ACS Paragon Plus Environment

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slope changes at about 50 °C, which agrees well with the transition temperature of ΦLl–to-ΦLh detected by DSC and XRD. As a result, it is reasonable to infer that the variation of peripheral chain motion is the main reason for the phase transition of ΦLl–to-ΦLh.

Figure 8. (a) FT-IR spectra of TPE-DOAB recorded at various temperatures during heating. (b) Plot of ln (A1470/A1458) against 1/T. A1470 and A1458 are the peak areas of the two bands at 1470 and 1458 cm−1.

According to the previous research studies, both the mesogen unit and its peripheral chains are crucial for the phase behavior and molecular packing in supramolecular structure. On the one hand, for the synthesis of functional materials by complexing mesogens and surfactants via ISA, the number and length of the surfactants' alkyl chains should be enough to obtain the targeted precipitate. Therefore, the variation of alkyl chain motion in ISA complex tends to be sensitive to different thermal condition. On the other hand, in contrast to conventional “flat” discogens, the AIE-active mesogens with propeller-like structure usually exhibit weak intermesogenic interactions. Therefore, the stacking order of AIE-active mesogens tends to be more sensitive to the variation of flexible chain motions. Consequently, we presume that most of the AIE-active ISA LCs may have similar hierarchical supramolecular structure and thermal-sensitive phase behavior. The principle outlined herein should be applicable to design and obtain other types of supramolecular LCs with hierarchical structure and excellent luminescent property by ISA.


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4. CONCLUSIONS In summary, we have synthesized a kind of AIE-active TPE LC with helical supramolecular structure and excellent luminescent property by ISA process. The targeted LC complex exhibits obvious AIE properties with quantum yield as high as 46% in solid-state film. The DSC and XRD results reveal that this ISA complex can self-assemble to form low ordered helical columns at ambient temperature, which then enters another LC phase with high ordered molecular stacking upon heating. Based on the FT-IR experiments, it is presumed that this thermally-induced phase transition can be attributed to the variation in the degree of peripheral chain motions. These ordered hierarchical structures, in combination with the liquid crystallinity and excellent AIE property of the ISA complex, make it a promising material for fabrication of LC luminescent devices. Further investigation on the LC emitters of this ISA complex is ongoing in our group.

ASSOCIATED CONTENT Supporting Information The supporting information is available free of charge on the ACS Publications website at DOI: 1

H NMR and 13C NMR (Figure S1-S10), FT-IR spectra (Figure S11), UV-vis absorption spectra and

emission spectra of thin film of TPE-DOAB (Figure S12-S14), TGA analysis of TPE-DOAB (Figure S15). See DOI:

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].

Notes The authors declare to competing financial interest.


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ACKNOWLEDGEMENTS The authors thank Prof. Stephen Z. D. Cheng at the University of Akron for helpful and insightful discussions. This work was supported by the National Natural Science Foundation of China (Grant Nos. 21304069 and 21304048), the Natural Science Foundation of Tianjin and Youth Science Foundation (14JCQNJC02900), and the Foundation of Beijing National Laboratory for Molecular Sciences.

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