Mechanochromic Luminescence of Aggregation-Induced Emission

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Mechanochromic Luminescence of Aggregation-Induced Emission Luminogens Yong Qiang Dong, Jacky Wing Yip Lam, and Ben Zhong Tang J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.5b01090 • Publication Date (Web): 13 Aug 2015 Downloaded from http://pubs.acs.org on August 18, 2015

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The Journal of Physical Chemistry Letters

Mechanochromic Luminescence of Aggregation-Induced Emission Luminogens Yong Qiang Dong,† Jacky W. Y. Lam*,‡,ξ and Ben Zhong Tang*,‡,ξ,§ †

Beijing Key Laboratory of Energy Conversion and Storage Materials, Department of Chemistry,

Beijing Normal University, Beijing, 100875, P. R. China ‡

Department of Chemistry, Division of Life Science, State Key Laboratory of Molecular

Neuroscience, Institute for Advanced Study, Institute of Molecular Functional Materials and Division of Biomedical Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China. ξ

HKUST-Shenzhen Research Institute, No. 9 Yuexing 1st RD, South Area, Hi-tech Park,

Nanshan, Shenzhen 518057, China. §

Guangdong Innovative Research Team, SCUT-HKUST Joint Research Laboratory, State Key

Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China.

ACS Paragon Plus Environment

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ABSTRACT: Mechanochromic (MC) luminogens have found promising applications in mechanosensors, security papers and optical storage for their change in emission behaviors in response to mechanical stimuli. Examples on MC luminescent materials are rare before the discovery of MC luminescence in aggregation-induced emission (AIE) luminogens. The twisted conformations of AIE luminogens (AIEgens) with appropriate crystallization capability afford loosely packing patterns, which facilitate their phase transformation in the solid state. The amorphous films of AIEgens exhibit enhanced emission intensity upon pressurization due to the increased molecular interactions, while crystals of AIEgens exhibit MC luminescence due to their amorphization by mechanical stimuli. AIEgens enrich the type of MC luminogens but those showing high emission contrast and multi-color emission switching, and working in a turn-on emission mode are seldom reported. Disclosure of the design strategy of high performance MC luminogens and exploration of their high-tech applications may be the future research directions for MC luminogens.

TOC GRAPHICS

Mechanochromic (MC) luminogens refer to a kind of smart materials which change their emission color or intensity in response to mechanical stimuli such as pressing, grinding, crushing


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or rubbing.1−5 MC luminogens have attracted considerable attention due to their promising applications in mechanosensors, security papers and optical storage.6−10 MC luminogens alter their emission by changing their molecular structure or aggregate morphology.1,11−15 Although the first one is a general way to tune the emission of a luminogen, limited examples of MC luminogens based on this mechanism have been reported due to the incomplete and irreversible chemical reactions in the solid state. Although each system has its own characteristic, the MC luminescence of most reported luminogens was achieved through modulation of their morphology by mechanical stimuli. These luminogens can transform from crystalline to amorphous states, from stable to metastable liquid crystalline phase, or between two different crystalline phases upon variation in intermolecular interactions, such as π–π interaction,16 dipole-dipole interaction,17 and hydrogen bonding11 before and after the application of mechanical stimuli. Some luminophore-doped polymer composites are also found to exhibit the phenomenon of mechanochromism.18−20 Although the mechanisms for mechanochromism are somewhat clear, few materials with MC luminescence had been reported before 2008, probably due to two reasons. First, there is still no clear design strategy for their syntheses. Second, the emission of many luminogens is totally or partly quenched upon aggregate formation due to the aggregation-caused quenching (ACQ) effect or traps or defects in the aggregates. Thus, the development of MC luminogens becomes a daunting task. Some molecules exhibit a phenomenon exactly opposite to the ACQ effect.21−28 These luminogens are non-emissive in solution but emit intensely in the aggregated state. Such phenomenon was coined as aggregation-induced emission (AIE) by Tang and his coworkers. Through a series of designed experiments and theoretical calculations, they hypothesized that the AIE phenomenon is attributed to the restriction of intramolecular motion (RIM).25 Based on the


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mechanistic understanding, an array of AIE luminogens (AIEgens) with emission color covering the entire visible spectral region and luminescence efficiency up to unity have been developed. During the investigation of their optical properties, Tang and coworkers found that the crystals of some AIEgens exhibited higher emission intensity and bluer emission color than their amorphous counterparts. Later, this crystallization-enhanced emission (CEE) effect or morphologydependent emission was observed in many AIEgens.29−31 AIEgens generally possess twist conformations and hence afford a more loosely packing pattern than traditional luminogens with a planar structure. This facilitates their transformation between different morphologies by heating, solvent fuming, mechanical stimuli, etc. Until now, many AIEgens exhibiting MC luminescence have been developed by Chi,1 Weder18, Park, 15 Xu,32,33 Tian34,35 and Tang,25 and their coworkers, and the relationship between the molecular structure and the MC luminescence has been evaluated. Those with high performances in emission contrast and multi-color emission switching, however, are rare. In this perspective, we will discuss only the MC luminescence of AIEgens due to the page limitation. We will also disclose the relationship between the molecular structure and the MC luminescence, highlight our recent work on high performance MC AIEgens, and point out the possible future research direction in this field.

CEE of AIEgens The CEE phenomenon was first noticed in crystals of hexaphenylsilole (1 or HPS; Chart 1) and its derivatives.31,36 Later, many AIEgens such as 2−4 have been found to be CEE-active,29,30, 37 details of which are summarized in a recent review.38 In all the reported CEE luminogens, they are generally propeller in shape due to the intramolecular steric hindrance. The propeller-shape


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of the CEE luminogens rules out any strong intermolecular interactions such as π-π stacking or H/J-aggregation that may weaken or quench their light emission in the solid state. On the other hand, such a molecular structure affords a loose packing pattern. This enables the peripheral aryl rings of the molecules to undergo rotation or vibration in some extent, and hence partly quenches their light emission in the amorphous state. In the crystal state, the molecules are in close proximity to each other and the existence of weak interactions such as C-H…π and C-H…O between molecules helps lock the motion of the aryl rings and hence rigidify the molecular conformation. The excitons can now undergo radiative relaxation, leading to increased PL intensity and efficiency. The bluer emission observed in the crystals of AIEgens may be attributed to the conformation twisting of the aryl rings of the molecules in order to fit into the crystalline lattices. Without such constraint, the molecules in the amorphous state may assume a more planar conformation and thus show a redder emission. Chart 1


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MC luminescence of CEE luminogens The CEE mechanism suggests that CEE luminogens may exhibit MC luminescence in the presence of mechanical stimulus. Indeed, when a hydrostatic pressure was applied to an amorphous film of HPS, its emission became stronger. The photoluminescence (PL) intensity increased swiftly up to 9% with increasing pressure (up to 104 atm) but started to decrease slowly when the film was further pressurized (Figure 1A). This result suggests that the amorphous film of HPS exhibits MC emission and constitute the first report of MC luminescence of AIEgen.39 The increased PL intensity at the low pressure region may be attributed to the stronger intermolecular interaction caused by increased pressure, which restricts the motion of the aromatic rings. This strengths the RIM process and thus boosts the emission. At high pressure (>104 atm), the silole molecules are brought to such a close distance that excimer formation becomes possible. This may explain why the emission drops upon further pressurization. The result from a control experiment on a solid film of AlQ3 shows that its PL intensity is monotonously weakened with increasing pressure, which is commonly observed in conventional luminophore systems (Figure 1B). In addition to MC photoluminescence, the electroluminescence of the HPS device is also enhanced by pressurization.


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Figure 1. (A) PL spectra of an HPS film obtained at different pressures and (B) effect of pressure on the PL intensity of HPS and AlQ3 films. Reproduced from reference 39 with permission from The Royal Society of Chemistry.

In addition to static pressure, shear stress may amorphize the crystals of AIEgens, and hence weaken and red-shift their emission. This is really the case and luminogen 3, a diphenyldibenzofulvene derivative (DPDBF), is a good example for demonstration.37 The crystals of 3 emit a bright green light, which transform into weakly orange emissive powders


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upon grinding (Figures 2a and 2b). The coincidence of the PL spectrum of the ground powders with that of the amorphous solids indicates that the crystals are amorphorized upon grinding. The amorphous powders of 3 crystallize slowly when standing at room temperature or quickly upon solvent fuming or heating (Figure 2e). Clearly, 3 exhibits reversible MC luminescence. Other DPDBFs are also found to exhibit MC luminescence. For example, the crystals of luminogen 4 emit a bluer light than those of 3, probably due to its more twisted conformation, which become non-fluorescent upon grinding (Figures 2f and 2g). Unlike 3, its ground powders are rather stable and remain at the emission “off” state when left at room temperature for 150 min. Like 3, the MC process of 4 is reversible (Figure 2i). These results suggest that we can endow MC fluorescent dyes with stability or self-recovering ability by fine tuning their chemical structures to satisfy various needs.

a)

b)

e) annealed

c)

d)

f)

g)

h)

Normalized PL Intensity

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420

510

ground

600

690

j)

pristine ground annealed amorphous

i)

400

480 560 Wavelength (nm)

640

Figure 2. (a−i) Photos of crystals of luminogens (a−d) 3 and (f−i) 4 taken at different conditions: (a and f) before and (b and g) after grinding, (c and h) after thermal annealing the powders in (b


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and g), and after grinding the samples in (c and h). (e) Normalized PL spectra of 3 obtained by consecutive grinding and annealing cycles. (j) Normalized PL spectra of pristine, ground, annealed and amorphous samples of luminogen 4. All the photos were taken under UV illumination. Annealing condition: 120 oC and 160 oC for 1 min for 3 and 4, respectively; excitation wavelength: 370 nm. Reproduced from reference 37 with permission from WileyVCH.

Except these two examples, many papers on the MC luminescence of AIEgens with different molecular structures have been published.1,25 Almost all the MC luminescence of AIEgens is attributed to the amorphization of their crystals in response to mechanical stimuli owing to their propeller shapes and loosely packing patterns. This enables their ease destruction by pressure or shear force and transformation from the crystalline to amorphous state. Similar to the CEE phenomenon, why do the amorphorized powders of AIEgens exhibit redder but weaker emission than their crystals? Some researchers suppose that the weakened PL intensity may be possibly attributed to the less and weakened intermolecular interaction and the planarization of the molecular conformation in amorphized AIEgens leads to their redder emission. Hence, if an AIEgen can form two kind of single crystals with different emission color and efficiency, it may help get further insight into the CEE feature as well as the MC phenomenon. We recently succeeded in obtaining single crystals of 3 with green (500 nm; 3GSC) and yellow (545 nm; 3YSC) emission and PL quantum yield of 82.1% and 56.2%, respectively. Single crystal X-ray analysis suggests that both crystals take twisted molecular conformation, which rules out the existence of any strong intermolecular interactions and demonstrates that the different emission color should be ascribed to the conformation difference. The torsion angles


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between phenyl rings in 3GSC (θ1 = 62.5o & 57.0o and θ2 = 51.1o & 51.4o; Chart 1) are larger than those in 3YSC (θ1 = 52.9o and θ2 = 47.8o), revealing that 3GSC takes a more twisted conformation. This imparts a lower conjugation and hence a bluer emission. Unexpectedly, 3GSC exhibits a higher emission efficiency than 3YSC. Close inspection reveals that both phenyl rings in 3GSC are locked by short interaction, while only one phenyl ring in 3YSC is locked. Thus, the further solidification in the molecular conformation of 3GSC affords its higher efficiency. Through the comparison between 3GSC and 3YSC, the redder and weaker emission in the amorphous ground powders of AIEgens is evidently caused by the more planar molecular conformation and lesser intermolecular interaction. Some AIEgens do not respond to mechanical stimuli. Understanding the relationship between the MC luminescence and the molecular structure is thus of fundamental importance. Tetraphenylethene (5; TPE; Chart 2), a well-known AIEgen, exhibits no emission change upon grinding. This is due to its inherent property of fast crystallization, which makes the transition from crystals to amorphous powders impossible. Locking two of its phenyl rings by an oxygen bridge destroys its symmetry and generates luminogen 6 with MC luminescence, whose emission can be switched between blue and yellow through repeated heating and grinding processes. Further locking the free phenyl rings of 6 gives luminogen 7 with a stable conformation, which exerts little change even when a strong mechanical force is applied, thus making it active in MC luminescence. These results suggest that a flexible structure is necessary but is not sufficient for MC luminescence.40 Another two AIEgens 8 and 9 also exhibit no MC luminescence due to their high tendency for crystallization.41 However, many MC luminogens have been developed by introducing substituents to the molecular structure of 5, 8 and 9.42−44 The attached substituents


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may break the molecular symmetry and weaken the intermolecular interaction and crystallization capability, thus affording luminogens such as 3 with MC luminescence. Chart 2

In addition to those AIEgens showing fast crystallization, some AIEgens exhibit no MC luminescence because they cannot form crystals, which is a prerequisite for a luminogen to exhibit mechanochromism6,45. For example, the as-synthesized 10E exhibits MC luminescence and its emission maximum shifts from 447 to 477 nm by grinding (Chart 3 and Figure 3). However, mechanical grinding of the as-synthesized solid of 10Z causes a little change in its emission color. As suggested by the results from powder X-ray diffraction (PXRD), the assynthesized 10E is crystalline and transforms to amorphous powders upon grinding, while the asprepared 10Z is amorphous due to its poor molecular packing and thus exhibits no morphology change upon grinding and MC luminescence. It seems that to achieve MC luminescence in AIEgens, their crystallization capability should be kept at an appropriate level.


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Chart 3 N N N (CH2 )6O

10E (MC-active)

O(CH 2) 6 N

N N

N N N (CH 2) 6O

10Z (MC-inactive)

N N

N (CH 2) 6O


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Figure 3. (A) Photographs of (left) as-prepared and (right) ground samples of 10E taken under (upper) room lighting and (lower) UV illumination. Abbreviation: G = grinding, H = heating at 120 °C for 1 min. (B) Mechano- and thermochromic processes of 10Z with photographs arranged in the same order as in panel A. (C) PL spectra of (left) 10E and 10Z before and after grinding. Excitation wavelength: 332 nm. Reprinted from reference 45.

Many MC luminogens have been developed through modification of AIEgens. However, those exhibiting high contrast in emission color or emission intensity are rare. We have developed some AIEgens with switchable “on-off” MC luminescence. As aforementioned, the emission of DPDBFs 3 and 4 can be switched off and on through repeated grinding-heating or grinding-fuming cycle. Through such discovery, a series of DPDBFs with such emission property have been developed.42 As stated before, the emission of DPDBF derivatives is nearly turned off by mechanical stimuli and many AIEgens, on the other hand, show a bathochromic shift in emission color and a decreased fluorescence intensity when exposed to mechanical force. Indeed, it is of interest to develop MC luminogens with turn-on fluorescence properties because it allows us to follow the MC process by naked eyes from the dark background and is less likely to generate false-positive signals. Jia and coworkers constructed a luminogen with dipeptide and pyrene, which exhibited unusual “turn-on” MC luminescence due to different excited states in the amorphous and crystalline states.46 Park and coworkers developed a series of MC luminogens operated in a “turn-on” mode based on a novel donor-acceptor-donor triad structure by using dicyanodistyrylbenzene unit as an acceptor and carbazole as a donor.47 The emission of the luminogens in the crystalline state was quenched by a non-luminescent charge transfer (CT)


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process. When exposed to mechanical stress, the crystals are amorphized. This slows down the CT process and hence leads to the recovery of the fluorescence. Zhang and coworkers developed a luminogen from benzophenone and arylamine. This molecule adopts a more twisted conformation and exhibits weaker emission in the crystal state. When amorphized by external stimuli, the molecule may relax to a more planar conformation and hence emit a strong fluorescence.48 Although such kind of luminogens is limited in number, during our search for new AIEgens, we obtained one with a molecular structure shown in Chart 4.49 Chart 4

Luminogen 11 is AIE-active. Different from most of the propeller-shaped AIEgens, the crystals of 11 adopt a planar conformation with strong intermolecular π-π stacking interactions and hence show very weak emission with a low quantum yield of 3.8%. As many crystals of AIEgens can be amorphized by external forces, it is anticipated that 11 may exhibit MC luminescence. Indeed, 11 emits a strong PL at 563 nm with a high quantum yield of 48.8% after disruption of its intermolecular interactions by grinding or hitting by a glass rod (Figure 4). PXRD analysis shows that the ground crystals exhibit fewer peaks with decreased intensity than the untreated ones. Clearly, the transformation of crystals to amorphous powders in the presence of mechanical stimulus is responsible for the “turn-on” MC luminescence process. Quantitative experiments are carried out by using a needle to apply a mechanical force on the crystal surface, through which a low detection limit of 0.1 Newtons is recorded. The MC luminescence of 11


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provides a possible design strategy for synthesizing MC luminogens with turn-on fluorescence properties. As emission of many panel-like luminogens is quenched by close π-π stacking, by structural modification of these luminogens to keep their π-π interaction at appropriate level and sensitive to external force, new luminogens with turn-on MC emission may be obtained.

Figure 4. (A) PL spectra of single crystals of 11 before and after grinding. Excitation wavelength: 445 nm. (B and C) XRD diffractograms of (B) intact and (C) ground single crystals of 11. (D) Photographs of single crystals of 11 before and after grinding/hitting taken under UV irradiation from a hand-held UV lamp. Reproduced from reference 49 with permission from The Royal Society of Chemistry.

Many AIEgens with efficient MC luminescence have been prepared. Their emission color contrast, however, is low in most cases. Recently, two MC luminogens with high contrast in


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emission color are prepared by attaching a benzothiazolium50 or a pyridinium unit51 to TPE through vinyl functionality (Chart 5). Both luminogens exhibit AIE and strong intramolecular charge transfer characteristics. Chart 5

The crystals of luminogen 12 emit intense yellow light at 565 nm (Figure 5). After gentle grinding, they transform to amorphous powders with red emission at 650 nm, giving a high emission contrast of ca. 85 nm. The emission of the ground powder changes to yellow after fumed with acetone vapor for 10 min. That is, the emission of 12 can be switched between yellow and red through repeated grinding-fuming process. The emission switching can also be achieved by repeated grinding-heating process. Similar to other MC luminogens, the MC luminescence of 12 was attributed to the morphological change from the crystalline to the amorphous state, as confirmed by the PXRD analysis.


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Figure 5. Switching the solid-state emission of 12 by repeated grinding–fuming and grinding– heating processes. The photographs were taken under 365 nm UV irradiation. Reproduced from reference 50 with permission from The Royal Society of Chemistry.

Like 12, the emission of luminogen 13 can switch between green (515 nm) and orange (600 nm) with a high contrast of 85 nm (Figure 6). Such results show that the synergy combination of the ICT and AIE attributes may lead to MC luminogens with a high emission contrast.

Figure 6. (A) Switching the solid-state emission of 13 by repeated grinding–fuming/heating process. (B) Fluorescent images of thin films of 13 on filter papers (a) without and (b) with letters of “AIE” being written by using a metal spatula. The photograph in (c) was obtained by fuming the film in (b) with acetone vapor for 10 min, while that in (d) was obtained by writing


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the letters of “TPE” in (c) by using a metal spatula. All the photos were taken under 365 nm UV irradiation. Reproduced from reference 51 with permission from The Royal Society of Chemistry.

Most of the reported MC luminogens exhibit emission between two states. Few of them show switchable multi-color emission although they are promising materials for multi-responsive sensors and optical data storage. Through introducing weak interactions such as C−H···π and C−H···O interaction to TPE, two TPE derivatives (14 and 15; Chart 6) exhibiting multicolor emission switching are obtained.52 Luminogen 14 affords two kinds of crystals with deep blue (14CA; 448 nm) and sky blue (14CB; 462 nm) emission colors (Figure 7). On the contrary, its amorphous solid (14Am) exhibits green emission at 491 nm. 14Am can transform only to 14CB upon thermal treatment and 14CA upon solvent fuming. 14CA can be converted to 14CB when annealed at 115 oC but the opposite process, however, is forbidden.

Chart 6


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Figure 7. Photos of ground powders from 14CA (a and d) before and (b, c and e) after annealing at (b) 90 °C and (c and e) 115 °C. (g) PL spectra of solid powders in (a)−(e). The photos were taken under UV illumination. Conditions: I (90 °C, 1 min); II (115 °C, 1 min); III (grinding). Reprinted from reference 52.

Luminogen 14CA exhibits MC luminescence and its emission color turns to green upon grinding due to the amorphization of its crystals. However, different from 14Am, the ground powders crystallize to 14CA (deep blue) and 14CB (sky blue) when annealed at 90 °C and 115 °C, respectively. The crystals of 14CA may not be absolutely amorphized by grinding. The residual small crystals may act as nuclei for the amorphous parts to crystallize when heated at low temperature. On the other hand, the ground powders from 14CB and 14Am behave similar to those of 14CA, converting to 14CA (deep blue) and 14CB (sky blue) when annealed at 90 °C


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and 115 °C, respectively. Evidently, the ground powders from 14CA are different from 14Am, and grinding, in one hand, may amorphize the crystals, but on the other hand, endow them to transform into special crystals. Such phenomenon is seldom reported. Thus, the emission of 14CA can be switched among three different colors (green, deep blue and sky blue) by mechanical and thermal stimuli. Luminogen 15 exhibits a similar property but its ground powders show an apparent spontaneous recovery property due to its loosely molecular packing caused by its long alkyl chains. The multi-color MC luminescence of 14 and 15 afford their potential application in optical recording. For example, luminogen 14 was ground in a piece of weighing paper. The resulting green film turned to sky blue (14CB) after annealing at 115 °C for 1 min (Figure 8). We wrote a letter “B” on the paper, which appeared green due to the amorphization of 14CB in the sheared area. When the paper was heated at 90 °C, the letter turned to deep blue (14CA) but was still visible from the sky-blue background by the naked eye. However, after heating at 115 °C, the letter nearly disappeared due to the complete transformation of 14CA in the “B” area to 14CB.

Figure 8. Repeated writing and erasing processes utilizing the multi-color MC luminescence of 14. The photoluminescent images were taken on a weighing paper under 365 nm UV irradiation. Reprinted from reference 52.


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In addition to our work, Jia and coworkers constructed one MC luminogen by linking pyrene and rhodamine B by using tetraphenylalanine as a spacer.53 The luminogen exhibit multi-color emission switching due to the force-induced transformation of assembled structure coupled with chemical reactivity. Ma and coworkers developed a donor–acceptor-type MC luminogen containing twisted diphenylacrylonitrile and triphenylamine units.54 The luminogen reveals a multi-color fluorescence switching upon mechanical grinding or applying a hydrostatic pressure due to the alteration of two different excited states.

Conclusion and Outlook MC luminescent materials have drawn much attention due to their potential applications in mechanosensors, security papers and optical storage. Examples of MC luminescent materials are rare before the discovery of the MC luminescence in AIEgens. Many AIEgens exhibit CIEE effect and many MC luminescent materials have been developed through modification of their structures. The twisted conformations of AIEgens afford loosely packing patterns, which facilitate their phase transformation in the solid state. The amorphous films of AIEgens exhibit enhanced emission intensity upon static pressure due to the increased molecular interactions, while crystals of AIEgens exhibit MC luminescence due to their amorphization upon exposure to external mechanical stimulus. Appropriate crystallization capability is essential for the MC luminescence of AIEgens. Although it is easy to obtain normal MC luminescent materials through modification of AIEgens, there is still no clear design strategy to generate such luminophores with high contrast in emission color or emission intensity, and “turn-on” emission and multi-color emission switching properties. Thus, the disclosure of the design strategy of high


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performance MC luminogens, development of sensitive MC luminescent sensor for quantifying stress and for biological use may be the possible future research directions for MC luminogens.

AUTHOR INFORMATION Biographies Dr. Yong Qiang Dong received his BS and MS degrees from Beijing University of Chemical Technology in 1998 and 2001, and PhD degree from Peking University in 2005 under the supervision of Prof. Xinde Feng and Prof. Ben Zhong Tang. He did his postdoctoral research at Zhejiang University in 2005–2007. He joined Beijing Normal University in 2007. His research interests focus on stimuli responsive luminescent materials. Dr. Jacky W. Y. Lam received his PhD degree from HKUST in 2003 under the supervision of Prof. Tang. In 2003–2007, he carried out his postdoctoral work on novel polymers with linear and hyperbranched structures and advanced functional properties in Tang’s group. He is currently a research assistant professor in the Department of Chemistry at HKUST. Prof. Ben Zhong Tang received his PhD degree from Kyoto University and conducted his postdoctoral work at the University of Toronto. He is Chair Professor in the Department of Chemistry and Division of Biomedical Engineering, Stephen K. C. Cheong Professor of Science at HKUST, and also honorary professor at SCUT. He was elected to the Chinese Academy of Sciences in 2009. His research interest lies in the creation of new molecules with novel structures and unique properties with implications for high-tech applications. He is currently an Associate Editor of Polymer Chemistry and is on the editorial board of a dozen journals.

Corresponding Author


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*Email: [email protected] or [email protected]. Web-page: http://webhost1.ust.hk/~tangbz/ Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was partially supported by the National Science Foundation of China (51173018, 21490570 and 21490574), the Fundamental Research Funds for the Central Universities, Program for Changjiang Scholars and Innovative Research Team in University, the National Basic Research Program of China (973 Program, 2013CB834701), the Research Grants Council of Hong Kong (16305015, 16301614 and N_HKUST604/14) and the University Grants Committee of Hong Kong (AoE/P-03/08).

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Quotes 1. The twisted conformation of an AIEgen afford its loose packing pattern and thus its MC luminescence. 2. Appropriate crystallization capability is essential for the MC luminescence of AIEgens.


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Mechanochromic Luminescence of Aggregation-Induced Emission

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