Highly Efficient Luminescent Liquid Crystal with Aggregation-Induced

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A Highly Efficient Luminescent Liquid Crystal with Aggregation Induced Energy Transfer Yi Liu, Li Hong You, Fa Xu Lin, Kuo Fu, Wang Zhang Yuan, Er-Qiang Chen, Zhen-Qiang Yu, and Ben Zhong Tang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b14575 • Publication Date (Web): 28 Dec 2018 Downloaded from http://pubs.acs.org on January 1, 2019

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ACS Applied Materials & Interfaces

A Highly Efficient Luminescent Liquid Crystal with Aggregation Induced Energy Transfer Yi Liu,†,‡ Li Hong You,† Fa Xu Lin,† Kuo Fu,† Wang Zhang Yuan,*,§ Er-Qiang Chen,# Zhen-Qiang Yu,*,†,‡ and Ben Zhong Tang*,⊥ †

School of Chemistry and Environmental Engineering, Shenzhen University, Shenzhen 518060,

China, ‡

Centre for AIE Research, School of Material Science and Engineering, Shenzhen University,

Shenzhen 518060, China, §

School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240,

China, #

Beijing National Laboratory for Molecular Sciences, Key Laboratory of Polymer Chemistry and

Physics of Ministry of Education, College of Chemistry, Peking University, Beijing 100871, China, ⊥

HKUST Shenzhen Research Institute, No. 9 Yuexing 1st RD, South Area, Hitech Park Nanshan,

Shenzhen 518057, China.

Keywords: luminescent liquid crystals, aggregation induced emission, aggregation induced energy transfer, hexagonal columnar phase, tetraphenylethene, tolane

ABSTRACT: A luminescent liquid crystal molecule (TPEMes) with efficient solid-state emission is rationally constructed via the chemically conjugation of blue-emitting tetraphenylethene cores and 1

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luminescent mesogenic tolane moieties, which are both featured with aggregation-induced emission properties. As for this fluorophore, aggregation induced energy transfer from the emissive tolane mesogens to the lighting-up tetraphenylethene units endows the molecule pure blue emission in the suspension and bulk state. Combining DSC, POM and 1D XRD experiments, TPEMes is deduced to adapt thermodynamically more stable layered crystalline phase, and can be “frozen” into a monotropic smectic mesophase due to kinetic reasons. As a result of more densely packing of TPEMes in the crystalline phase indicated by 1D XRD, the luminescence of TPEMes in crystalline phase has blue-shifted with 17 nm relative to the metastable mesophase.

1. Introduction

Liquid crystals (LCs), featured with both order and mobility on the molecular and macroscopic levels, have attracted numerous attentions from both the industry fields and scientific communities. 1-4 Along with the well-exploited application of LCs in optoelectronic displays, LCs have also found promising application in a wide variety of fields, like semiconductors,5,6 elastomer actuators7,8 and sensors,9,10 owing to their intrinsic ability of self-organizing into well-ordered mesophases and sensitivity to external environment. Among them, luminescent liquid crystals (LLCs) endowed with the unique combination of light-emitting ability with anisotropic organization have gained tremendous interest due to their promising application in optoelectronic devices in the past decade.11-13 However, the natural molecular self-organization and aggregates formation in the mesophases usually lead to quenched fluorescent emission for the conventional dyes, which are well known as aggregation caused quenching (ACQ) effects.14-16 In the attempts to achieve LLCs with high-efficient light emission, lots of efforts have been devoted to tuning the mesophase structure and 2


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intermolecular assembling of the luminescent mesogens in the solid state. Parallel with these efforts, another solution to this thorny conflict between molecular aggregation and emission efficiency has been proposed with the advent of a fantastic phenomenon, namely aggregation-induced emission (AIE).17-19

Contrast to the conventional luminogens, AIE-active dyes usually exhibit bright emission in aggregates or solid state, which are attributed to the restriction of intramolecular motion (RIM), including both rotation and vibration. However, the typical AIE luminogens, like silole and tetraphenylethene (TPE), are usually structurally featured with sterically congested and non-planar geometry, which makes these pristine AIE luminogens unfavorable for mesophase formation via π-stacking.20 Therefore, additional mesogenic motifs, such as biphenyl units and long aliphatic chains, are required to be conjugated onto the pristine AIE-active cores for constructing AIE-active LLCs.21-24 Following this protocol, a series of AIE-active LLCs have been synthesized by diverse combinations of mesogenic substitutions and archetypal AIE molecules.25-27 The ultimate phase transition behaviors and light-emitting performance of these LLCs are pre-dominated by both the AIE fluorophores and mesogenic units, which still require lots of efforts for further exploration and optimization toward efficient LLCs.

In a previous work, we have synthesized a TPE based LLC compound (TPE4Mes) with biaxially oriented mesomorphic structure and high emission efficiency in solid state.27 However, the high molar ratio of emissive mesogenic tolane moieties in compound TPE4Mes (four tolane peripheries per one TPE core) has remarkably broaden the emission line width of the compound. This has also significantly degraded the color purity and color tunability of TPE4Mes due to the existence of two

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distinct emission peaks from tolane substitutions and TPE cores, respectively. In order to improve their emission performance, it is of great significance to understand the structure-property relationship of the mesogens-incorporated AIE fluorophores on the molecular level. This can be investigated through delicately tuning the molecular structure, e.g. the composition of mesogens and fluorophores, and the length of aliphatic linkages. Herein, we report a novel LLC molecular by chemically linking a tolane substitution onto TPE fluorophore via long aliphatic chains. It is found to exhibit pure blue emission in bulk state, which is ascribed to the aggregation induced energy transfer (AIET) from the mesogenic tolanes assemblies to the TPE fluorophores in nano-aggregates, mesophase or crystalline phase.

2. Experimental Sections

2.1. Materials and Chracterizations. All reagents were all purchased from Aldrich and used as received. Solvents were purified according to standard laboratory methods. 1H and 13C NMR spectra were measured on a Bruker ARX 600 spectrometer using chloroform-d as solvent and tetramethylsilane (TMS, δ = 0) as internal standard. Matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) high-resolution mass spectra (HRMS) were recorded on a GCT premier CAB048 mass spectrometer. Absorption spectra were taken on a Milton Roy Spectronic 3000 Array spectrometer. Emission spectra were taken on a Perkin-Elmer spectrofluorometer LS 55. The thermal stability of the resulting compound TPEMes was evaluated on a Perkin-Elmer TGA 7 under nitrogen at a heating rate of 20 °C min-1. A Perkin-Elmer DSC 7 was employed to measure the phase transition thermograms. An Olympus BX 60 polarized optical microscope (POM) equipped with a Linkam TMS 92 hot stage was used to observe the anisotropic optical textures.

4


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One-dimensional X-ray diffraction (1D-XRD) experiments were performed on Ganesha system(SAXSLAB, U.S) equipped with a multilayer focused Cu Kα radiation as the X-ray source (Genix3D Cu ULD) and a semiconductor detector (Pilatus 100 K, DECTRIS, Swiss). The diffraction patterns were recorded on an imaging plate covering the q range from 0.06 to 29 nm-1 (q = 4π sinθ/λ, where λ is the X-ray wavelength of 0.1542 nm and 2θ is the scattering angle). The scattering peak positions were calibrated with LaB6 for wide-angle region and silver behenate for small-angle region, respectively. A Linkam TST350 hotstage was utilized to study the structural evolution as a function of temperature.

2.2. Synthesis of 4-(1,2,2-triphenylvinyl)phenol (3). This compound was synthesized according to the literature published.28 1H NMR (400 MHz, CDCl3, δ): 7.10, 7.09, 7.08, 7.07, 7.05, 7.04, 7.03, 7.02 (m, 15H), 6.87 (d, 2H), 6.55 (d, 2H), 4.96 (s, 1H, –OH).

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C NMR (100 MHz, CDCl3, δ):

154.09, 143.98, 143.88, 140.42, 140.12, 136.25, 132.69, 131.34, 131.31, 131.30, 128.49, 127.67, 126.52, 126.34, 126.22, 114.56.

2.3. Synthesis of TPEMes. This compound TPEMes were synthesized following the procedure reported in the literature.27 White solid, yield 86.9%. 1H NMR (600 MHz, CDCl3, δ): 7.42 (dd, 4H), 7.17 (d, 2H), 7.16, 7.12, 7.10, 7.08, 7.07, 7.05, 7.04, 7.03, 7.01, 6.99 (m, 15H), 6.91 (d, 2H), 6.85 (d, 2H), 6.62 (d, 2H), 3.96 (t, 2H), 3.86 (t, 2H), 2.46 (t, 1H), 1.87 (br, d, 4H), 1.78, 1.72 (m, 4H), 1.24~1.54 (m, 27H), 1.04 (m, 2H), 0.90 (t, 3H, –CH3).

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C NMR (150 MHz, CDCl3, δ): 159.04,

157.66, 147.91, 144.05, 144.00, 140.57, 139.93, 135.84, 132.93, 132.47, 131.38, 131.36, 131.34, 131.31, 127.67, 127.55, 126.83, 126.29, 126.17, 120.89, 114.47, 113.53, 88.70, 88.14, 68.04, 67.79,

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44.55, 37.36, 37.28, 34.15, 33.52, 32.21, 29.55, 29.41, 29.37, 29.30, 29.19, 26.64, 26.06, 26.01, 22.72, 14.13. HRMS (MALDI-TOF) m/z: [M]+ calcd. for C63H72O2: 860.553. Found: 860.470.

3. Results and Discussion

3.1. Synthesis of luminescent liquid crystal molecule. Herein, a molecule TPEMes consisting of a TPE blue-emitting luminogen and one single mesogenic tolane substituent is designed and prepared according to the synthetic routes shown in Scheme 1. We hope to obtain a high-quality blue-emitting LLC based on the possible energy transfer from mesogenic tolane units to the TPE luminogens with consideration of the spectral overlap between the tolane moieties and TPE cores. The SN2 reaction between 4-(1,2,2-triphenylvinyl)phenol (3) and previously synthesized tolane derivative 427 (RBr, shown in Scheme 1) readily gives rise to the targeted compound TPEMes in a high yield of 86.9% as a colorless solid. The matrix-assisted laser ionization time-of-flight (MALDI-TOF) mass spectrum of TPEMes (Figure S1) indicates the presence of a species with m/z = 860.47, consistent with the desired molar mass of 861.24 g/mol. And the chemical identity of TPEMes is also well characterized by 1H and 13C NMR spectroscopy (Figure S2 and S3).

Scheme 1. Synthetic routes toward compound TPEMes. Conditions: i) Zn, TiCl4, THF, refluxing; ii) 4 (RBr), K2CO3, acetone, refluxing, 86.9%. 6


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3.2. Aggregation-Induced Emission and Energy Transfer Behavior of TPEMes. To investigate the photophysical properties of the mesogen-incoporated TPE luminogen, the UV/Vis absorption spectra of both TPEMes and the mesogen precursor 4 are measured in THF (Figure S4). TPEMes exhibits two absorption peaks at 295nm and 313 nm which are identical to that of tolane derivative 4, and another broad shoulder peak aournd 330 nm ascribed to the existence of the TPE luminogen is also observed. In addition, the UV/Vis absorption and photoluminescence (PL) spectra of TPEMes in THF and THF/water mixtures are also measured to investigate its AIE behavior, in which water is used as a non-solvent of TPEMes. (Figure S6, and Figure 1)

Figure 1. (a) Emission spectra of TPEMes in THF and THF/water mixtures; (b) Plots of the mission intensities of TPEMes in THF and THF/water mixtures at 478 and 368nm versus water fractions (fw). Inset: images of TPEMes under UV lamp in THF and THF/water mixture (10/90 by volumn). Concentration = 20 M. Excitation wavelength = 330 nm.

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As previously reported, both the TPE cores and the mesogenic tolane moieties are AIE-active luminophores,29-32 which endow the compound TPEMes multiple fluorescence evolution behavior in THF/water mixture. When fw < 50 %, PL signals of the mixture is quite weak, because the TPEMes molecules are dissolved in the solvent mixture. Upon further addition of water (50% < fw < 70 %), the PL intensity at 368 nm corresponding to the emission of mesogenic tolane moieties is dramatically blasted (approximate 20 folds) with the appearance of a subtle shoulder emission at 460 nm (Figure 1). Compared to the more sterically twisted TPE skeleton, the tolane moieties with planar structure are more inclined to stack with each other due to π-π stackings and hydrophobic interactions in the aqueous mixture. Hence, the mesogenic tolane segments in TPEMes will aggregate in advance in the solvent mixture, leading to enhanced emission at 368 nm as a result of the RIM mechanism. However, the TPE units anchored onto the mesogenic tolane aggregates are still left soluble in the solvent, which furnish the weak emission at 476 nm under fw = 70 %. Parallel with further increasing water fractions (70% < fw < 90 %), the PL intensity at 368 nm exhibits an obvious decrease. By contrast, the emission at 476 nm ascribed to the TPE luminogens is significantly boosted due to the aggregation of TPE segments. This remarkable difference on fw-dependent PL spectra for these two AIE-active fluorophores (tolane and TPE) probably originated from the fluorescent resonance energy transfer (FRET) from the emissive tolane moieties to the relatively red-shifted TPE luminophore.33,34 When fw is above 70%, the anchored TPE units further collapse on the pre-assemblied tolane aggregates, and thus the emission from interior tolane aggregates is probably attenuated by efficient energy transfer to the neighboring lighting-up TPE luminogens (Figure 2). Consequently, the PL intensity at 476 nm increases with the sacrifice of the PL intensity at 368 nm. Furthermore, the emission line width of 8


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compound TPEMes is sharpened, and the color purity of the blue-emitting luminophore is improved as anticipated. By contrast, although the emission from tolane in compound TPE4Mes has also been attenuated when fw is above 60%, its PL peak at 380 nm is still overwhleming in the whole spectrum which leads to broad line width and poor color purity.27 The molar ratio between donors (tolane) and acceptor (TPE) in these two LLC analogues is thought to be the key for their distinction on AIE and AIET behavior, while the emission from donor cannot be completely diminished by energy trnasfer due to its high molar ratio in TPE4Mes.

Figure 2. Proposed aggregation model of TPEMes in THF/water mixture with various water fractions (fw) and possible energy transfer from emissive tolane moieties to lighting-up TPE units. From the solution in THF to the nanosuspension in 90 % aqueous mixture, the PL intensity of TPEMes at 476 nm is increased dramatically by approximate 220 folds (Figure 1), which can be directly observed from the remarkable fluorescence contrast of the images under UV lamp in Figure 1b. Additionally, the fluorescence quantum yield (ФF,s) of TPEMes in THF solution is below 0.99 %,

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and the fluorescence quantum yield (ФF,f) of the solid film is recorded to be 55.4 %, indicating an AIE factor (αAIE = ФF,f /ФF,s) as high as 55.4. Therefore, TPEMes is evidently an AIE-active fluorophore which shows efficient solid-state blue emission.

3.3. Thermal Transitions and Optical Textures. In order to study the potential of TPEMes as an efficient LLC, the thermotropic behavior of TPEMes has been investigated by means of differential scanning calorimetry (DSC) and polarized optical microscopy (POM). Figure 3a shows a set of DSC cooling diagrams at different scanning rates for TPEMes. Two exothermic peaks (see the inset of Figure 3a) can be observed for all cooling rate. And both transition temperatures are slightly dependent on cooling rate. At a cooling rate of 20 °C min-1, an exothermic peak at 99.5 °C and a shoulder peak at 101 °C are observed (ΔH = 17.2 J g-1). Upon further decreasing the cooling rates to 10, and 5.0 °C min-1, the major exothermic peak slightly increases to 101 °C, while the shoulder peak developes obviously much stronger and shifts to 102 °C. The subsequent DSC heating diagrams with the same rate as the prior cooling experiment (Figure 3a), are presented in Figure 3b. Regardless of the heating rate, the endothermic peaks in these DSC traces appear always at 137 °C, which probably originates from the crystal melting. However, another exothermic process in the DSC curves between 63 °C and 75 °C is highly dependent on the heating rate. The exothermic transition at 75 °C gradually shifts to 63 °C when the heating rates decrease from 20 to 5.0 °C min-1. This heating-rate dependency of the exothermic peaks suggests that this transition involves a crystallization phase transformation from the less-ordered mesophase. The peak temperature of the exothermic peak (around 101 °C) observed during cooling is

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approximately 36 °C lower than that of the endothermic transition (137 °C) in the DSC curves, indicating that the ordering process of TPEMes requires large energy barrier.

Figure 3. DSC traces of TPEMes during cooling (a) and subsequent heating cycles (b) at different scan rates. (c) Proposed phase diagrams of the free energy G for TPEMes on cooling from an isotropic melt (i-ii) and subsequent heating to melt (iii-v). On the basis of these observations, we hypothesized a phase diagram (Figure 3c) plotted between Gibbs free energy (G) and temperature (T) to illustrate the irreversible phase transitions shown in DSC cooling and heating diagrams (Figure 3a and 3b).35,36 The corresponding DSC curves scanned at 10 °C min-1 are also included for better clarification. During cooling from the isotropic phase (i to ii, Figure 3c), the sample bypasses the thermodynamically more stable crystal phase due to the kinetic reason, and directly enters the metastable mesophase.The mesophase formation 11



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requires a lower Gibbs free energy barrier compared with the crystallization process, which needs a much higher nucleation barrier. During heating, the monotropic mesophase transfers to the more stable crystalline phase (iii), and further goes to the melting of the crystalline phase into isotropic phase (iv to v).

Figure 4. (a) Pseudo focal-conic fan-shaped texture of TPEMes observed by POM via cooling from 180 to 40 °C with a rate of 2.0 °C min-1. (b) Spherulitic texture of TPEMes observed by POM via heating the sample from 40 to 100 °C with a rate of 1.0 °C min-1. In addition, a pseudo focal-conic fan texture is recorded on POM via cooling the sample from the isotropic state to 40 °C at a rate of 2.0 °C min-1, which is characteristic of a hexagonal columnar phase or smectic phase (Figure 4a).37-40 Subsequent heating process is also recorded after heating the sample from 40 to 100 °C with a rate of 1.0 °C min-1. The observed spherulitic texture clearly verifies the existence of the crystalline phase with high anisotropy and long-range self-assembly (Figure 4b),41 which agrees well with the DSC curves and proposed G-T curve.

3.4. Identification of the Mesophase and Crystalline Phase Structure. Although the DSC experiments combined with POM results can help identify the corresponding phase at different 12


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temperatures, however, the above results cannot provide direct informations about structural changes. Therefore, 1D XRD experiments at different temperatures are utilized to investigate the structural evolutions of TPEMes in solid state. 1D XRD experiments of TPEMes during slowly cooling at the rate of 1.0 °C min-1 are shown in Figure 5a. Above 110 °C, only one broad halo at q = 11~16 nm-1 appears at the wide-region, implies an average distance of approximate 0.47 nm which can be attributed to the short correlation length of the molten alkyl chains. Upon further decreasing the temperature to 105 °C, five strong reflections are observed at the q of 1.30, 2.57, 3.87, 5.17, and 6.56 nm-1, respectively. At the same time, the high-angle amorphous halo at q = 11~16 nm-1 is replaced by a series of sharp reflections, indicating the formation of a crystalline phase. These results reveal that the isotropic melts is directly entering into the more stable crystalline phase with slowly cooling, which is consistent with the G-T curve prposed above. When the sample is further slowly cooled to room temperature, those sharp diffractions remains intact suggesting the stability of the crystalline phase. For the crystalline phase, the corresponding distances for five strong reflections observed in the small angle region are 4.84 , 2.42 nm, 1.62 nm, 1.22 and 0.96 nm, which agrees well with the ratio of 1 : 1/2 : 1/3 : 1/4: 1/5, indicating the d001, d002, d003, d004, and d005 reflections of a lamellae packing for TPEMes in the crystaline phase. The observation of obvious (004) and (005) peaks also refelcts the highly ordered layer structure in the crystalline phase. The first refelction at 4.84 nm, indicating the distances between neighbouring layers in the crystalline phase, is also well consistent with the extended length of TPEMes (~4.9 nm) calculated by density functional theory (DFT) using functional B3LYP with the 6-31G(d) basis set (Figure S8). This result suggests that the soft dodecyl

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linkage and 4-pentylcyclohexyl tails in TPEMes are fully extended wth all-trans conformation, and TPEMes is probably anti-paraller aligned in each layer. These sharp peaks at the q of 11.8, 13.9, 15.3, and 16.9 nm-1 which probably originate from the packing of alkyl tails and the core–core distance between mesogen and TPE moieties, also implies the presence of intralayer order.

Figure 5. In-situ temperature-dependent 1D XRD profiles of TPEMes sample with different history: (a) slowly cooling (1.0 °C min-1) the sample to room temperature from the isotropic melt; (b) fast cooling the same sample to room temperature from the melt and then heating to isotropic state. To further investigate the monotropic mesophase, the sample is quickly cooled from isotropic melt to room temperature in order to “freeze” the metastable mesophase. Afterward, the 1D XRD

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results of TPEMes are recorded at various temperatures via heating the sample to isotropic state (Figure 5b). It is interesting to note that there are two diffractions at q of 1.05 and 2.11 nm-1 in the 1D XRD profiles of TPEMes at 30 and 50 °C. With the q ratio of 1:2, these two diffractions suggest a layer structure existing in mesophase of TPEMes. The interlayer distance of smectic mesophase is calculated to be 5.96 nm, which is longer than the estimated molecular length of 4.9 nm. In addition, a sharp peak at the high angle with q of 14.3 nm-1 (d = 0.44 nm) also appears in the large angle region, which can be attributed to the average intermolecular distance. Upon subsequent heating, the reflections at q of 1.05 and 2.11 nm-1 gradually vanishes with the appearance of a series of sharp diffraction at the q of 1.30, 2.56, 3.86, 5.17, and 6.57 nm-1. And the diffraction at the q of 14.3 nm-1 is replaced with a diversity of sharp diffrections at the q of 12.0, 13.7, 15.3 nm-1. Indicated by the exothermic peaks around 60 °C in the DSC heating curves, this structural evolution is the transformation from the metastable smectic mesophase to a thermodynamically more stable layered crystalline phase. When the temperature reaches 150 °C, the crystalline phase melts and a 1D XRD pattern of the isotropic state appears instead, which possesses one broaden halo around q of 13.2 nm-1. The possible molecular stacking models for mesophase and crystalline phase of TPEMes is proposed in Figure 6. In the semectic mesophase of TPEMes, the molecules are aligned anti-parallelly with the planar tolane mesogens stacking in line via π-π interaction, and peripheral TPE moieties are in interdigitated with each other. Therefore, the interlayer distance d001 is larger than the length of TPEMes. By contrast, the molecules are densly packed antiparallely while tolane mesogens and TPE luminogens are interdigitated with each other in a single layer, leading to a layer

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spacing similar to the extended length of TPEMes. Combining all the results from DSC, POM and 1D-XRD experiments, it is deduced that TPEMes can form an highly ordered layered crystalline phase, and a loosely packed smectic metastable mesophase.

Figure 6. Proposed molecular packing models for the smectic mesophase and layered crystalline phase of TPEMes. Inset: optimized geometry of TPEMes using functional B3LYP with the 6-31G(d) basis set and its chemical structure.

3.5. Luminescence in Mesophase and Crystalline Phase. In the attempt to further explore the potential of TPEMes as luminescent liquid crystals, the PL behavior of TPEMes in bulk state, when dispersed in the PMMA matrix or casted as a film, are both investigated. (Figure 7) The PL spectrum of TPEMes dispersed in PMMA matrix (10% w/w of TPEMes relative to PMMA) clearly manifests one emission peaks at 455 nm, which comes from the emission of TPE luminogens. A subtle peak around 368 nm corresponding to mesogenic tolane moieties can also be observed.

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By comparing the excitation and emission spectrum of TPEMes, we can clearly observe one spectral overlap between the emission spectrum of tolane mesogens and excitation curve for TPE luminogens (Figure 7a). This spectral overlap directly supports the existence of energy transfer within the case of TPEMes, leading to the diminished violet light-emission from mesogenic tolane segments in suspension and PMMA matrix.

Figure 7. (a) Normalized spectral overlap between absorption spectrum of TPEMes in THF (20 μM), excitation spectrum and emissive spectrum of TPEMes in THF/water (fw = 90%) mixture (20 μM). (b) PL spectra of TPEMes dispersed in PMMA matrix (10% w/w of TPEMes relative to PMMA), and drop-coated film after slow cooling and fast cooling from the isotropic state. In addition, the luminescent behaviors of the metastable mesophase and crystalline phase of TPEMes are also studied via cooling the casted film on quartz plate from isotropic melts to room temperature with different cooling rate. (Figure 7b) After fast cooling to room temperature, the sample is “frozen“ into a metastable smectic mesophase due to kinetic reason, which is validated by 17



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the previous DSC and 1D XRD experiments. One emission peak at 478 nm is clearly shown in the PL spectrum, while the emission coming from the tolane units completely disappears due to energy transfer. In contrast, the emission of the crystalline film after slow cooling from the melts is centered at 461 nm, and blue-shifted by 17 nm relative to the mesophase. This blueshift in emission wavelength of TPEMes is probably due to the packing modes and structural conformations of the TPEMes molecules in the solid state. 1D XRD experiments imply that the interlayer distance has decreased from 5.96 nm in mesophase to 4.84 nm in crystalline phase. In this respect, the TPE segments in the metastable mesophase probably adopts a more planar conformation, while the TPE skeletons are enfored to a more twisted conformation as a result of more densely packing in the crystalline phase. Accordingly, the TPE derivatives exhibts obvious blueshift after transformation from mesophase into crystalline phase. This phenomenon has been also observed in a series of TPE-based fluorescent molecules with mechanofluochromic characteristics, which is caused by the transformation between amorphous and crystallized states.42,43

4. Conclusion

In summary, a luminescent liquid crystal molecule TPEMes composed by a TPE luminogen and an emissive mesogenic tolane moiety is synthesized. PL spectra of TPEMes under different THF/water mixtures indicate its abnormal AIET property, while the ultraviolet emission from the mesogenic tolane units is diminished by internal energy transfer to the blue-emitting TPE moieties upon aggregation. Based on the DSC, POM and 1D XRD experiments, a monotropic smectic mesophase is uncovered for TPEMes in the cooling cycle ascribed to kinetic reasons, which can be transformed into a thermodynamically more stable layered crystalline phase upon subsequent heating.

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The luminescence of TPEMes in mesophase and crystalline phase is also studied, and a blue-shift of 17 nm in the emission wavelength is clearly observed upon transition from metastable mesophase into crystalline state as a result of more densely packing of TPE moieties in the lamellae layer. These results pave a new approach for the construction of functional liquid crystalline materials, and suggest the possibility of modulation the emission behavior via phase modulation and AIET process.

ASSOCIATED CONTENT

Supporting information

The Supporting Information is available free of charge on the ACS Publications website at DOI: .

NMR and MALDI-TOF MS spectra, UV-Vis absorption spectra, 1D XRD results and optimized geometry of TPEMes

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Z. Q. Yu.). *E-mail: [email protected] (W. Z. Yuan). *E-mail: [email protected] (B. Z. Tang).

ORCID Yi Liu: 0000-0001-9510-9559 Wang Zhang Yuan: 0000-0001-6340-3497 Er-Qiang Chen: 0000-0002-0408-5326 19



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Zhen-Qiang Yu: 0000-0002-0862-9415 Ben Zhong Tang: 0000-0002-0293-964X Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work is partially supported by the National Natural Science Foundation of China (21674065, 21704065), the Innovation Research Foundation of Shenzhen (JCYJ20170817100547775, JCYJ20170302150014024) and the Natural Science Foundation of Guangdong Province (2016A030312002). REFERENCES (1) Kato, T.; Mizoshita, N.; Kishimoto, K. Functional Liquid‐Crystalline Assemblies: Self‐Organized Soft Materials. Angew. Chem. Int. Ed. 2006, 45, 38-68. (2) Bisoyi, H. K.; Kumar, S. Liquid-Crystal Nanoscience: An Emerging Avenue of Soft Self-Assembly. Chem Soc Rev 2011, 40, 306-319. (3) Tschierske, C. Development of Structural Complexity by Liquid‐Crystal Self‐Assembly. Angew. Chem. Int. Ed. 2013, 52, 8828-8878. (4) Kato, T.; Uchida, J.; Ichikawa, T.; Sakamoto, T. Functional Liquid Crystals Towards the Next Generation of Materials. Angew. Chem. Int. Ed. 2018, 57, 4355-4371. (5) McCulloch, I.; Heeney, M.; Bailey, C.; Genevicius, K.; MacDonald, I.; Shkunov, M.; Sparrowe, D.; Tierney, S.; Wagner, R.; Zhang, W.; Chabinyc, M. L.; Kline, R. J.; McGehee, M. D.; Toney, M. F. Liquid-Crystalline Semiconducting Polymers with High Charge-Carrier Mobility. Nat. Mater. 2006,

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Highly Efficient Luminescent Liquid Crystal with Aggregation-Induced

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