1 + 1 >> 2: dramatically enhancing the emission...

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1 + 1 >> 2: Dramatically Enhancing the Emission Efficiency of TPE-Based AIEgens but Keeping their Emission Color through Tailored Alkyl Linkages

Dongfeng Dang, Zijie Qiu, Ting Han, Yong Liu, Ming Chen, Ryan T. K. Kwok, Jacky W. Y. Lam, and Ben Zhong Tang*

Currently, the development of aggregation-induced emission (AIE) luminogens (AIEgens) has enabled us to “see” never before seen scenery. However, not all AIEgens exhibit the impressive emission efficiency in aggregated states. Moreover, the emission color of AIEgens can be seriously affected when their performance is improved. Therefore, to overcome this limitation, an efficient method is proposed here through the tailored alkyl linkages to greatly improve the emission efficiency of tetraphenylethene (TPE)-based AIEgens but retain their emission color. Encouragingly, significantly enhanced emission efficiency is achieved with the quantum yield up to 68.19% and 65.20% for BTPE-C4 and BTPE-C8, respectively, in contrast to that of TPE (25.32%), demonstrating the proverb that one plus one is much larger than two (1 + 1 >> 2). Interestingly, when alkyl linkages in skeletons are fine-tuned, self-assembled nanorods, nanosheets, and nanofibers are successfully achieved for BTPE-C1, BTPE-C4, and BTPE-C8 in tetrahydrofuran and water system. Also, these developed emissive AIEgens not only exhibit impressive response to the environmental stimuli of mechanical force, viscosity, temperature, and light, but can also be used to dynamically monitor and control the phase-separated morphology in polymeric blends.

DOI: 10.1002/adfm.201707210

and phase separation in polymer blends, is important for the fundamental under-standing and real-world applications.[1] Although differential scanning calorimetry (DSC) is well established and widely used currently for the analysis of phase transi-tion, the DSC results are ambiguous in some cases and inaccurate for some block copolymers.[2] On the other hand, the com-monly used scanning electron microscopy or transmission electron microscopy for phase separation analysis are expensive and usually time consuming, which may also change the samples irreversibly.[3] Therefore, new approaches with promising performance are urgently needed.

Generally, fluorescence-based methods with features of fast response, high sen-sitivity, and easy detection,[4] could gen-erate new avenues for different analysis and process monitoring applications in materials science. Also, any slight change in fluorescence properties, in response to the environmental stimuli of mechanical

force,[5] temperature,[6] light,[7] solvent polarity,[8] and viscosity,[9] could provide the possibility for practical applications, indi-cating the vast potential of fluorescence-based methods. Undoubtedly, luminescent materials play the key role in this above mentioned method[10] and among them, organic lumines-cent materials (OLMs) are significant for their good chemical

Aggregation-Induced Emission

Dr. D. Dang, Z. Qiu, T. Han, Dr. Y. Liu, Dr. M. Chen, Dr. R. T. K. Kwok, Dr. J. W. Y. Lam, Prof. B. Z. TangDepartment of ChemistryHong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration and ReconstructionDivision of Life ScienceState Key Laboratory of Molecular NeuroscienceInstitute for Advanced Study, Institute of Molecular Functional MaterialsDivision of Biomedical EngineeringThe Hong Kong University of Science and TechnologyClear Water Bay, Kowloon, Hong Kong 999077, ChinaE-mail: [email protected]

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.201707210.

Dr. D. DangSchool of ScienceXi’an JiaoTong UniversityXi’an 710049, ChinaProf. B. Z. TangGuangdong Provincial Key Laboratory of Brain ScienceDisease and Drug DevelopmentHKUST-Shenzhen Research InstituteNo.9 Yuexing 1st RD, South Area, Hi-tech Park, Nanshan Shenzhen 518057, ChinaProf. B. Z. TangGuangdong Innovative Research TeamSCUT-HKUST Joint Research LaboratoryState Key Laboratory of Luminescent Materials and DevicesSouth China University of TechnologyGuangzhou 510640, China

1. Introduction

Monitoring the changes in chemical compositions or physical characteristics of materials is essential in the fields of material science. In particular, the dynamic monitoring of polymeric phase variation, such as phase transition in single polymer

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stability and structural tailorability.[11] However, although tre-mendous efforts have been devoted to the development of OLMs, moderate emission efficiency remains a limiting factor in their further practical applications. On the other hand, it should be mentioned that although OLMs with high emission efficiency can be easily achieved in dilute solution, lumines-cent materials are usually used in aggregated states for the vast majority of applications, which could partially or completely quench their emission intensity due to the aggregation-caused quenching (ACQ) effect,[12] finally leading to poor emission effi-ciency. Therefore, developing approaches to overcome the prob-lematic ACQ effect and obtain highly emissive luminophores in aggregated states is now one of the main challenges for OLMs in both scientific and technological development.

In 2001, our group proposed an uncommon photophysical phenomenon of aggregation-induced emission (AIE), in which nonemissive luminogens are induced to emit intensively through restriction of intramolecular motions (RIM) by the for-mation of aggregates.[13] For instance, hexaphenylsilole is non-emissive when it is well-dissolved in tetrahydrofuran (THF), but bright emission can be achieved when water is added to afford the heavy aggregation.[14] This fantastic discovery has opened a new chapter in the exploration of novel OLMs for fur-ther applications. However, it is also worth noting that although the development of AIE luminogens (AIEgens) has enabled us to “see” never before seen scenery, not all the AIEgens exhibit impressive emission efficiency in the aggregated states. For example, tetraphenylethene (TPE), known as a prototypical AIEgen and famous for its easy synthesis,[15] just exhibited a moderate absolute quantum yield (QY) of 20%, indicating its huge margin for improvement in emission efficiency. Mean-while, although it has been reported that the direct linkage of aromatic rings to TPE building blocks could significantly enhance their emission efficiency, the emission wavelength can be easily shifted to the corresponding low energy region simul-taneously for its enlarged molecular conjugation.[16] Therefore, the strategy that can dramatically enhance the emission effi-ciency of organic luminophores, such as TPE, while retaining

their emission color, is needed to be explored. Li and co-workers controlled the conjugation effectively between two TPE rings by fine-tuning their linking mode to induce the twisted conforma-tions and obtained highly emissive AIEgens successfully with the similar emission color of TPE.[17] However, the complicated synthesis involved in this strategy is prohibitive and the poor solution-processibility for these developed AIEgens also limited their further application in some cases. Therefore, based on these considerations, a facile but efficient method is proposed in this work to greatly improve the emission efficiency of TPE derivatives but retain their emission color by linking two TPE building blocks together through tailored alkyl linkages. The introduced alkyl linkages could not only break the molecular conjugation or charge transfer, but also afford excellent solu-tion-processibility. In contrast to TPE (QY = 25.32%), although inferior emission efficiency was observed for BTPE-C1 and BTPE-C12, much higher emission efficiency was achieved with QY values up to 68.19% and 65.20% for BTPE-C4 and BTPE-C8, respectively, when the butyl or octyl chain was linked between two TPE building blocks (Figure 1). Interestingly, dif-ferent self-assembled morphologies of nanorods, nanosheets, and nanofibers were also observed in THF and water system when alkyl linkages in skeletons were fully tailored. Moreover, these developed AIEgens are found to be very “smart” as their luminescent properties could change obviously in response to the environmental stimuli of mechanical force, viscosity, tem-perature, and light. Finally, these as prepared highly emissive AIEgens were successfully used to dynamically monitor and analyze the phase separations in polymeric blends.

2. Results and Discussion

2.1. Synthesis and Thermal Properties

All the developed BTPE derivatives, including BTPE-C1, BTPE-C4, BTPE-C8, and BTPE-C12, were prepared using 4-(1,2,2-triphenylvinyl)phenol and their corresponding

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Figure 1. Schematic diagram of emission efficiency and molecular structures of TPE, BTPE-C4, and BTPE-C8. Color code: gray (C), Blue (H), and red (O).



dibromo-alkane derivatives under basic conditions according to Williamson ether synthesis.[18] Their molecular structures are also displayed (Scheme S1, Supporting Information). The chemical structures of BTPE-C1, BTPE-C4, BTPE-C8, and BTPE-C12 were also well characterized and confirmed by 1H NMR, 13C NMR, and TOF-MS. Additionally, all molecules here exhib-ited good solubility in common solvents of dichloromethane, chloroform, and THF for the alkyl linkages in molecular skel-etons, indicating their excellent solution-processibility for practical applications. For all these as-prepared molecules, high decomposition temperatures (Td) at 5% weight loss were observed, implying their good thermal stability (Figure S1a, Supporting Information). On the other hand, step-shape sig-nals were also observed for these developed AIEgens in DSC thermograms indicating the transition from hard glassy state to soft rubbery state (Figure S1b, Supporting Information), which is similar to the glass temperature (Tg) for polymeric materials.[19] The onset values from DSC curves for this tran-sition are also summarized (Table 1) and interestingly, when the alkyl linkage becomes longer, this observed temperature becomes lower.

2.2. Optical Properties

The UV–vis absorption spectra for BTPE-C1, BTPE-C4, BTPE-C8, and BTPE-C12 in THF solution and thin films were then inves-tigated. Similar absorption spectra in high energy region were observed for all molecules (Figure S2, Supporting Informa-tion), which were almost same to that of TPE.[20] After that, PL spectra of all prepared molecules in THF/water mixtures were checked to study their AIE properties (Figures S3 and S4, Supporting Information). It was found that, when water frac-tion increased, the PL intensity of all molecules increased grad-ually and significantly enhanced emission features were finally achieved with 90% fraction of water for the formation of aggre-gates, demonstrating their promising AIE characteristics. The PL spectra and their corresponding chromaticity coordinates for TPE, BTPE-C1, BTPE-C4, BTPE-C8, and BTPE-C12 in aggre-gated states are also compared (Figure 2a). As the introduced alkyl linkages could break the molecular conjugation between two TPE units effectively, similar emission spectra were suc-cessfully achieved for TPE and our developed BTPE derivatives here (Figure 2a and Table 1). Then, the relative PL intensity (I/I0) for TPE, BTPE-C1, BTPE-C4, BTPE-C8, and BTPE-C12

in THF with different fractions of water is also plotted here to further demonstrate their impressive AIE features (Figure 2b). It should also be mentioned that when TPE units were linked together to afford the BTPE derivatives, much larger I/I0 values can be achieved for the easily formed aggregation, probably leading to their high signal-to-noise ratios for various applications.

The absolute emission quantum yields for TPE, BTPE-C1, BTPE-C4, BTPE-C8, and BTPE-C12 in solids were also pre-sented in Figure 2c and Table 1. Interestingly, in contrast to the QY value of 25.32% for TPE, when the short methyl or long dodecyl groups were employed, decreased QY of 18.03% and 21.70% were observed for BTPE-C1 and BTPE-C12, respec-tively. However, significantly enhanced emission efficiency with QY up to 68.19% and 65.20% were achieved when the inserted alkyl linkages were optimized in BTPE-C4 and BTPE-C8, indicating that the linkage of two luminescent building blocks together through proper alkyl chain do greatly improve their emission efficiency without changing the emission color. To further understand the altered emission efficiency of TPE, BTPE-C1, BTPE-C4, BTPE-C8, and BTPE-C12, their transient decay spectra in solids are displayed in Figure 2d. The corre-sponding life-time (τ), radiative decay rates (kr), and nonradia-tive decay rates (knr) are summarized in Table 1.[21] To our sur-prise, comparable kr values were obtained for TPE, BTPE-C1, BTPE-C4, and BTPE-C8, implying that the large enhance-ment in emission efficiency for BTPE-C4 and BTPE-C8 was caused by the blocking of nonradiative channels due to the decreased knr values. This can also be easily understood for the inserted alkyl linkages could further restrict the molecular motions to enhance their emission properties according to the RIM mechanism for AIE systems. On the other hand, if the alkyl linkages in molecular skeletons are not proper, negative effects may also occur. For instance, when short methyl group was employed in BTPE-C1, more twisted molecular structure was obtained to decrease the effective intermolecular interac-tions, then leading to an increased knr value and poor emis-sion efficiency. Also, the long and flexible linkage in BTPE-C12 can easily consume the “excited” energy to afford decreased kr but increased knr value, finally resulting in the inferior emis-sion efficiency for BTPE-C12. On the other hand, BTPE-C16 with much longer flexible chain in molecular skeleton was also prepared (Scheme S2, Supporting Information) and the poor QY value of 13.42% for BTPE-C16 further confirmed the above mentioned hypothesis.

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Table 1. Thermal properties and photophysical properties of TPE, BTPE-C1, BTPE-C4, BTPE-C8, and BTPE-C12.

Compounds Thermal properties UV PL

Td [°C] Tga) [°C] Tg

b) [°C] λabsc) [nm] λabs

d) [nm] λeme) [nm] QYf) [%] τf) [ns] kr

g) [× 108 s−1] knrg) [× 108 s−1]

TPE – – – 314 325 472 25.32 1.42 1.78 5.26

BTPE-C1 348 79 74 311 320 468 18.03 0.90 2.00 9.11

BTPE-C4 368 73 71 313 324 471 68.19 3.78 1.80 0.84

BTPE-C8 370 55 52 314 325 472 65.20 3.68 1.77 0.94

BTPE-C12 391 45 42 315 325 474 21.70 2.10 1.01 3.41

a)Measured in DSC (Onset values); b)Measured in ADEtect; c)Measured in THF solution; d)Measured in thin films; e)Measured in THF/water mixtures with 90% fraction of water; f)Measured in solids; g)The radiative decay rate in solids, kr = QY/τ. The nonradiative decay rate in solids, knr = 1/τ − kr.



2.3. Self-Assembled Properties and Piezochromic Properties

In addition to emission efficiency of these developed AIE-gens, the fine-tuned alkyl linkages in BTPE-C1, BTPE-C4, BTPE-C8, and BTPE-C12 could also significantly affect their intermolecular interactions or molecular packing,[22] which could in turn influence their self-assembled properties. There-fore, the precipitated nanoaggregates in THF/water mixtures were thoroughly investigated by fluorescence microscope (Figure 3). Probably owing to the rigid molecular structure, nanorods were readily obtained for BTPE-C1. When butyl and octyl linkages were inserted to afford BTPE-C4 and BTPE-C8, self-assembled nanosheets and nanofibers were then observed, respectively. On the other hand, highly “flex-ible” BTPE-C12 with the longest alkyl chain between two TPE units exhibited amorphous nanoprecipitates. Interestingly, as observed in the dark filed of fluorescence microscope, the deep-blue nanostructured BTPE-C1, BTPE-C4, and BTPE-C8 as well as sky-blue nanoprecipitated BTPE-C12 were clearly displayed (Figure 3).

To further understand the different self-assembled properties, the PL spectra of BTPE-C1, BTPE-C4, BTPE-C8, and BTPE-C12 as pristine powders were measured (Figure 4). As shown, the deep-blue powders of BTPE-C1, BTPE-C4, and BTPE-C8 exhib-ited similar emission spectra located at 450 nm, whereas, a

significant bathochromic shift was observed for BTPE-C12 as sky-blue powder. These results are similar to those cor-responding precipitated nanoaggregates, indicating that the observed nanostructures for these developed AIEgens were caused by their own features. Therefore, X-ray diffractions (XRD) were then performed on the pristine BTPE-C1, BTPE-C4, BTPE-C8, and BTPE-C12. As observed, sharp peaks were obtained in the XRD patterns for BTPE-C1, BTPE-C4, and BTPE-C8, dem-onstrating their well-ordered crystalline structures. However, weak and broad diffraction signals were observed for the amor-phous BTPE-C12. The altered crystalline structures and molec-ular packing here partly explain why different self-assembled properties were observed for BTPE-C1, BTPE-C4, BTPE-C8, and BTPE-C12 by fluorescence microscopy, which also give reasons to their altered emission color for both pristine powders and observed nanostructures in THF/water system.[5,23] Then, the piezochromic properties of our developed AIEgens were inves-tigated. As displayed in Figure 4, obvious red-shift was observed for the ground samples of BTPE-C1, BTPE-C4, and BTPE-C8. Moreover, the sharp peaks in XRD patterns also changed and became broad diffraction peaks after the thorough grinding.[5,24] On the other hand, when the pristine powder of BTPE-C12 was ground for 10 min, very similar PL spectra and XRD diffraction patterns were observed, further demonstrating its amorphous nature.

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Figure 2. a) Normalized PL spectra of TPE, BTPE-C1, BTPE-C4, BTPE-C8, and BTPE-C12 in aggregates (The inset shows their corresponding chroma-ticity coordinates in aggregates). b) Variation of PL intensity (I/I0) for TPE, BTPE-C1, BTPE-C4, BTPE-C8, and BTPE-C12 in THF with different fractions of water. c) Quantum yields and d) Transient decay spectra of TPE, BTPE-C1, BTPE-C4, BTPE-C8, and BTPE-C12 in solids.



2.4. Viscochromic and Thermochromic Properties

The as-prepared AIEgens not only possessed the typical molecular “rotors,” but also contained flexible linkages, sug-gesting their potential to display smart response to solvent viscosity.[25] Therefore, PL spectra of BTPE-C1, BTPE-C4, BTPE-C8, and BTPE-C12 were measured in THF solution ([c] = 1 × 10−4 m) with different viscosities (Figure 5). As antici-pated, when ethylene glycol was added to increase the solvent viscosity, enhanced emission intensities were achieved for all molecules through the RIM mechanisms, implying their good viscochromic properties. On the other hand, the viscochromic property for TPE was also fully investigated and compared here. The corresponding PL spectra for TPE in THF solution ([c] = 1 × 10−4 m) with different fractions of ethylene glycol is displayed (Figure S5, Supporting Information). It is interesting that when different fractions of ethylene glycol were added to the TPE contained THF solution, weak and similar emis-sion features were observed, even the ratio between THF and ethylene glycol was 1:99 (Figure S5a, Supporting Informa-tion), which is totally different from the result for BTPE-C1,

BTPE-C4, BTPE-C8, and BTPE-C12. Additionally, after the con-centrated THF solution was employed ([c] = 1 × 10−3 m), the vis-cochromic property was then observed (Figure S5b, Supporting Information), indicating that in contrast to these as prepared AIEgens, TPE exhibited the inferior viscochromic property. On the other hand, it is encouraging that when longer and more flexible linkages were introduced in skeletons, such as dodecyl chain in BTPE-C12 and hexadecyl chain in BTPE-C16, the observed viscochromic properties become much more impres-sive and sensitive (Figure 5d; Figure S6, Supporting Infor-mation), which also provide a facile strategy to enhance the viscosity responsive properties of AIEgens by the incorporation of long and flexible linkages.

As displayed in DSC thermograms, the insets of alkyl link-ages in BTPE-C1, BTPE-C4, BTPE-C8, and BTPE-C12 also pro-vided a featured transition from hard glassy state to soft rubbery state, which could partly affect their luminescent properties to afford thermochromic properties.[20,26] Therefore, the emission intensity at different temperatures was plotted and their corre-sponding second derivative of plotted curves was also obtained to reveal the transition temperatures, which can be determined

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Figure 3. Fluorescence images of a,e) BTPE-C1, b,f) BTPE-C4, c,g) BTPE-C8, and d,h) BTPE-C12 in THF/water system under dark field (Left) and bright field (Right).



from the lowest point clearly.[26] Firstly, PL spectra of BTPE-C1, BTPE-C4, BTPE-C8, and BTPE-C12 in aggregated states were measured (Figure S7, Supporting Information). Although the emission intensity decreased as temperature increased gradually, the observed transition temperatures were not in accordance with the DSC results for the interference of solvent at high tempera-tures (Figure S8, Supporting Information). Then, the spin-coated thin films based on BTPE-C1, BTPE-C4, BTPE-C8, and BTPE-C12 were employed to further investigate their thermochromic proper-ties according to the developed detection method using AIEgens, denoted as ADEtect,[26] through which the transitions tempera-tures can be calculated as the lowest point in the second deriva-tive of “emission intensity” (grayscale) from fluorescent images by camera. Encouragingly, when the transitions occurred at the determined temperature, obvious change in emission intensity was observed and the so-called “Tg” values for these developed AIEgens can be easily obtained from the corresponding second derivative (Figure 6), which were also similar to the DSC results (Table 1), indicating their impressive thermochromic properties.

2.5. Photochromic Properties and 2D Photopattern

As demonstrated, all these as-prepared AIEgens exhibited good solubility in common solvents, indicating their potential to provide emissive thin films by spin-coating method, which also makes them promising candidates for photolithography

applications. Additionally, all these molecules possessed two parts in their skeletons: the rigid TPE cores and also the flexible alkyl linkages. Therefore, BTPE-C1, BTPE-C4, BTPE-C8, and BTPE-C12 were doped in both the “hard” polymer of polysty-rene (PS) and “soft” polymer of polybutadiene (PB) (w%, 1%). As anticipated, emissive PS and PB thin films were success-fully achieved (Figures S9 and S10, Supporting Information). On the other hand, it has been demonstrated that TPE deriva-tives can undergo photo-induced cyclization reactions easily,[27] which could also affect their luminescent properties and result in photochromic properties. Based on this consideration, the PS and PB thin films doped with our developed AIEgens were irradiated by high-power UV light (180 W). Unfortunately, the emissive PS films were quickly photobleached. However, bright blue emission was observed for the corresponding PB-based films caused by cyclization reactions and subse-quently, similar emission quenching occurred. Interestingly, in contrast to TPE-based PB film, when longer alkyl linkages were inserted in skeletons, more obvious blue emission was achieved (Figure S9, Supporting Information). To understand this observed photochromic phenomenon, BTPE-C1, BTPE-C4, BTPE-C8, BTPE-C12, and also TPE in toluene solution were irradiated. However, only the TPE solution exhibited blue emis-sion, while weak or no emission was observed for other four AIEgens (Figures S11 and S12, Supporting Information). These results indicated that photoinduced cyclization reactions easily occurred for TPE solution, but when the flexible linkages were

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Figure 4. Normalized PL spectra of a) BTPE-C1, b) BTPE-C4, c) BTPE-C8, and d) BTPE-C12 as pristine powders (black curves) and ground powders (red curves) (The insets show the corresponding fluorescence photos and XRD curves for pristine powders and ground powders).


1707210 (7 of 11) © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimAdv. Funct. Mater. 2018, 28, 1707210

Figure 5. PL spectra of a) BTPE-C1, b) BTPE-C4, c) BTPE-C8, and d) BTPE-C12 in THF solution ([c] = 1 × 10−4 m) with different fractions of ethylene glycol.

Figure 6. PL intensity of a) BTPE-C1, b) BTPE-C4, c) BTPE-C8, and d) BTPE-C12 based thin films at different temperatures (The insets show their photos and the corresponding second derivative of PL intensity at different temperatures).



introduced for BTPE-C1, BTPE-C4, BTPE-C8, BTPE-C12, the cyclization reactions and their corresponding photochromic properties can be achieved in PB thin films. It is proposed that the double bonds in PB can easily induce and produce radicals under UV irradiation, which could then initiate the cyclization reactions effectively. To demonstrate our hypothesis, BTPE-C8 in toluene, with and without the PB, was irradiated under the same conditions. As expected, the photochromic phenomenon induced by cyclization reactions was observed only when PB was also present in the BTPE-C8 system (Figure S13, Sup-porting Information). Therefore, it can be concluded that the flexible linkages in these developed AIEgens are beneficial to their good dispersion in the “soft” PB substrates to make the initiation of cyclization process much easier. This also explains why the insertion of longer alkyl chains resulted in brighter blue emissions.

Then, BTPE-C8 doped PB film was irradiated with UV light through a copper photomask to obtain 2D photopatterns (Figure 7).[28] The exposed area undergoes photoinduced cycli-zation reactions in the presence of UV irradiation to afford the blue emission (Figure 7a,b). With further irradiation, the light emission was completely quenched (Figure 7f). On the other hand, unexposed squares remained emissive, and by removing

the copper mask at an appropriate time, the corresponding well-resolved 2D fluorescent photopattern with different emission colors can be easily achieved (Figure 7c,g). Finally, the observed fluorescent photopattern can also undergo the process of photobleaching (Figure 7e,h). The results confirm that these developed AIEgens in this work have great potential in the facile preparation of high resolution photonic devices.

2.6. Morphology Monitoring and Control

Generally, polymer blends can display more desirable proper-ties than those of the individual components, but there is an inevitable phase separation process during their blending.[1c,29] Therefore, the as-prepared AIEgens were also used to monitor this phase separation process in the blended films of PS and PB. Initially, PS- and PB-based blended films (w/w, 1:4) doped with BTPE-C1, BTPE-C4, BTPE-C8, and BTPE-C12 (1%, w%) were prepared by spin-coating and as observed in the bright field of fluorescent microscope, it is hard to capture the phase separations even though the scope figures were enlarged. However, high-contrast phase-separated morphology with the distributions of highly emissive spherical “islands” as PS rich

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Figure 7. Photopatterns generated by photolithography of PB-based films doped with BTPE-C8 (1%, w%) through copper masks under UV irradiation. a) The prepared films with mask. b) Panel (a) under irradiation for 1 min. c) Panel (b) removing the mask. d) Panel (c) under irradiation for 1 min. e) Panel (d) under irradiation for 10 min. f) Panel (b) under irradiation for 10 min. g) Panel (f) removing the mask. h) Panel (g) under irradiation for 10 min.

Figure 8. Phase separations in PS and PB films (w/w, 1:4) doped with a,e) BTPE-C1, b,f) BTPE-C4, c,g) BTPE-C8, and d,h) BTPE-C12 (1%, w%) in dark (Top) and bright field (Bottom) (Insets show the enlarged phase separation images).



domains was observed when viewed under UV irradiation (Figure 8), indicating that all the as-prepared AIEgens enabled us to visualize previously unobserved phenomena.[30] Also, as wPB increased (w/w, 1:1), emissive “isolated islands” with larger diameter were observed for the blends doped with BTPE-C1 (Figure 9a). However, it is interesting to find that the isolated PS spheres tend to coalesce together to generate irregular domain structures (Figure 9b) for BTPE-C4 containing blends. Finally, bicontinuous interpenetrating networks were achieved for BTPE-C8- and BTPE-C12-based systems (Figure 9c,d). Then, the morphologies of PS- and PB-based blended films (w/w, 4:1) were also monitored and contrarily, the dark PB rich domains were surrounded by the continuous and emissive PS phase (Figure S14, Supporting Information). These results also demonstrated that our AIEgens can not only monitor the phase-separated morphology, but could also control the phase separations effectively through varying the linkages in molecular skeletons.

3. Conclusion

In this work, to greatly improve the emission efficiency of TPE-based AIEgens but keeping their emission color, series of BTPE derivatives were synthesized here through the linkage of lumi-nescent TPE units via tailored alkyl chains. Although inferior emission efficiency was observed for BTPE-C1 and BTPE-C12, significantly enhanced QY values up to 68.19% and 65.20% were achieved for BTPE-C4 and BTPE-C8 in contrast to that of TPE, indicating that the linkage of two luminescent building blocks together through proper alkyl chain do greatly improve their emission efficiency without changing the emission color. Also self-assembled nanorods, nanosheets, and nanofibers were suc-cessfully obtained for BTPE-C1, BTPE-C4, and BTPE-C8 via the variation of alkyl linkages in THF/water systems. Meanwhile, our developed “smart” AIEgens here exhibited impressive response to the slight environmental stimuli of mechanical force, viscosity, temperature, and light, which were also successfully used in the dynamic monitoring and analysis of polymeric phase separations.

4. Experimental SectionMaterials: All reagents and chemicals were purchased from commercial

sources (Aldrich, Acros and TCI) and used without further purification unless stated otherwise. 4-(1,2,2-triphenylvinyl)phenol was purchased from Zhengzhou Alfachem Co., Ltd and AIEgen Biotech Co. Ltd. The polystyrene, polybutadiene, and the corresponding dibromo-alkyls were purchased from J&K or Meryer.

Characterization and Measurement: 1H NMR and 13C NMR spectra were recorded on Bruker ARX 400 NMR using CDCl3 as solvent. Mass spectrometric measurements were performed on GCT premier CAB048 mass spectrometer operating in matrix-assisted laser desorption/ionization time of flight mass spectrometry mode. UV–vis spectra were measured on a Milton Ray Spectronic 3000 array spectrophotometer. PL spectra were taken on PerkinElmer LS 55 spectrophotometer. The absolute quantum yield was obtained on an Edinburgh Instrument FLS980 Integrating sphere. The fluorescence lifetime was measured using a Hamamatsu Compact Fluorescence Lifetime Spectrometer C11367. Thermo-gravimetric analyses (TGA) were recorded on TA TGA Q5000 analyzer and DSC was performed on a TA Instruments DSCQ1000 at a heating rate of 5 °C min−1 under a nitrogen atmosphere. The fluorescent images were recorded on a fluorescence optical microscope (Nikon Eclipse 80i) taken under UV light irradiation.

ADEtect Device and Calculation: AIEgen-doped PS and PB films were spin-coated onto the cleaned silicon wafers by their corresponding homogeneous solution. Then the fluorescent figures excited from a hand-held UV lamp (365 nm) were recorded on a Canon EOS 60D 18 MP CMOS digital SLR camera with Canon EF100 mm f/2.8L Macro IS USM lens controlled by CanonEOS Utility 2 software.[26] These photos were recorded under the same settings with a constant rate (1 photo per 10 s). Sample heating was performed on a Linkam temperature-controlled stage with a T95-PE system controller. The brightness of obtained fluorescent figures (G) can be calculated according to the equation: Grayscale = 0.2989 × Red + 0.5870 × Green + 0.1140 × Blue. Then, the PL intensity of each figure can be defined as G/G0, where G0 is the grayscale at time of 0 s. Finally, the second derivative of fitting curve to reveal the change in G/G0 (PL intensity) at different temperatures can be observed, in which the transition temperatures can be determined from the lowest point.[26]

Supporting InformationSupporting Information is available from the Wiley Online Library or from the author.

Adv. Funct. Mater. 2018, 28, 1707210

Figure 9. Phase separations in PS and PB (w/w, 1:1) films doped with a,e) BTPE-C1, b,f) BTPE-C4, c,g) BTPE-C8, and d,h) BTPE-C12 (1%, w%) in dark (Top) and bright field (Bottom).



AcknowledgementsThis work was supported by the National Basic Research Program of China (973 Program; Grant Nos. 2013CB834701 and 2013CB834702), the University Grants Committee of Hong Kong (Grant No. AoE/ P-03/08), the Research Grants Council of Hong Kong (Grant Nos. 16301614, 16305015, and N_HKUST604/14), Innovation and Technology Commission (Grant No. ITC-CNERC14SC01), the National Science Foundation of China (Grant Nos. 81372274, 81501591, 8141101080, and 51603165), the Science and Technology Planning Project of Guangdong Province (Grant Nos. 2014A030313033 and 2014A050503037), and the Shenzhen Science and Technology Program (Grant Nos. JCYJ20130402103240486 and JCYJ20160509170535223). B.Z.T. is also grateful for the support from the Guangdong Innovative Research Team Program of China (Grant No. 201101C0105067115). D.D. wants to thank Yanzi Xu and Ying Zhi in XJTU for the help in preparation and measurement of DTPE-C16.

Conflict of InterestThe authors declare no conflict of interest.

Keywordsaggregation-induced emission, alkyl linkages, emission efficiency, stimuli-responsive emitters, tetraphenylethene

Received: December 11, 2017Revised: January 22, 2018

Published online: February 21, 2018

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1 + 1 >> 2: dramatically enhancing the emission...

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