Mechano-Responsive AIE Luminogens - ACS Symposium Series

Chapter 12

Mechano-Responsive AIE Luminogens

Downloaded by KANSAS STATE UNIV on September 30, 2016 | http://pubs.acs.org Publication Date (Web): September 27, 2016 | doi: 10.1021/bk-2016-1226.ch012

Zhiyong Yang, Zhu Mao, Tao Yu, Yi Zhang,* Siwei Liu, Jiarui Xu,* and Zhenguo Chi* PCFM Lab, GD HPPC Lab, Guangdong Engineering Technology Research Center for High-performance Organic and Polymer Photoelectric Functional Films, State Key Laboratory of Optoelectronic Material and Technologies, School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, China *E-mails: [email protected] (Y.Z.); [email protected] (J.X.); [email protected] (Z.C.)

In this chapter, recent progress in the area of mechanoresponsive aggregation-induced emission (AIE) luminogens is summarized, and majority of them are discussed, along with their derived structure-property relationships. The existence of a structural relationship between AIE luminogens and the common nature of mechanochromism is recognized, which guides researchers in identifying and synthesizing more mechano-responsive AIE luminogens.

1. Introduction Mechano-responsive luminogens are a class of smart luminescent molecules whose emission colors or strengths change under external forces including mechanical stimuli such as pressing, grinding, crushing, or rubbing. They exhibit emissive responses including mechanoluminochromism (MLC, if the luminescence is fluorescence, it is called mechanofluorochromism, MFC), mechano-switching (turn-on or turn-off of the photoluminescence), and mechanoluminescence (directly emitting under external force without excitation source), which have attracted considerable attention due to their promising applications in mechano-sensors, security papers, optical storage, miniature © 2016 American Chemical Society Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


photonic device and logic gate (1–3). Mechano-responsive luminogens that are dependent on changes in physical molecular packing modes change their luminescence usually accompanied by mutual inter-conversions, e.g. crystal-crystal, crystal-amorphous, or crystal-liquid crystal may be involved. Their emission generally can be recoved through simple treatments of thermal annealing and/or solvent vapor fuming to promote a recrystallization, resuted in a reversible process. Thus, controlling the mode of molecular packing (i.e. the aggregation states) as well as molecular conformation to achieve highly efficient, obviously contrastive and reversible mechano-responsive luminogens is more attractive for both fundamental research and practical applications, however, they are still rare. This rarity may be attributed to two major reasons (3). First, there is still no clear design strategy for their molecular structure. Second, the emission of many luminogens is totally or partly quenched upon aggregate formation due to the aggregation-caused quenching (ACQ) effect in the aggregates. Consequently, observing the mechano-responsive luminescent phenomenon becomes quite difficult. In 2001, Tang et al. (4) reported some “aggregation-induced emission” (AIE) molecules that are an important class of anti-ACQ luminogens that emit more efficiently when they are in the aggregated state than in the diluted solution state. Since then, AIE luminogens have attracted larger research attentions for their potential applications in various fields, such as organic light-emitting devices and chemo-sensors (5). And recently, a number of AIE luminogens have been found to possess mechano-responsive luminescence properties. Hence, utilizing AIE moieties to construct new molecules becomes an important strategy in the synthesis of various mechano-responsive AIE luminogens. Tang et al. (6–8) reported that several AIE luminogens possess bright-dark switching properties between crystalline and amorphous states. To prove the AIE mechanism of the restriction of intramolecular rotations (RIR), they applied a hydrostatic pressure to an amorphous film of siloles and found its emission became stronger and its 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, which is the first report of mechano-responsive luminescence of the AIE molecule (9). In 2010, Park et al. (10) reported the mechanofluorochromism of a cyano-distyrylbenzene derivative that was an AIE compound. However, at that time, it was not be well recognized that there existed a relationship in the molecular structures between the AIE molecule and the mechanofluorochromic nature. Almost within the same period, Chi and Xu’s group (11) synthesized and reported a number of new mechanofluorochromic compounds with AIE nature and pointed out that mechanochromism should be the common property for most of AIE luminogens. Since then, new mechano-responsive AIE luminogens have popping up like mushrooms after a spring rain. We have published a book summarizing all of the mechano-responsive AIE luminogens reported at that time (12). Due to space limitation, in this chapter, we will briefly classify the latest developments of new mechano-responsive AIE luminogens not including in the previous book.

222 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

2. Mechanochromic AIE Luminogens


2.1. Diarylvinylanthracene Derivatives The diarylvinylanthracene unit is an important AIE structural unit, whose AIE nature was first reported by Tian’s group (13), and the machanofluorochromic properties of the diarylvinylanthracene derivatives have been reported by Chi and Xu’s group (14). Some recent studies have demonstrated that the length and the substituted position of alkyl chain had complicated effects on their mechanofluorochromic (MFC) properties. Yang et al. reported a series of diarylvinylanthracene derivatives with different alkyl lengths, including 1 (PT-Cn) (15) with alkyl chains, 2 (F-Cn) (16) with N-alkyl chains and 3 (IACn) (17) with alkyl chains (Scheme 1). The results show that the fluorescence emissions of both the pressed and annealed PT-Cn are gradually blue-shifted, but the blue-shifted amplitudes of annealed states are more remarkable with the increase of alkyl length, leading to that longer alkyl-containing PT-Cn show larger MFC spectral shifts (Δλ). Similar MFC behavious are found in F-Cn, whose spectral shifts is 23–54 nm. However, IACn exhibit strong and negative alkyl length-dependent spectral shifts of 27–65 nm, i.e. the shorter the N-alkyl chain, the more remarkable the grindingor pressing-induced spectral shifts.

Scheme 1. Molecular structures of compounds 1-6. Wei et al. (18) reported similar positive length-denpendent spectral shifts in three diarylvinylanthracene derivatives (4, Scheme 1) with different propyl, hexyl, and dodecyl side chains (AnPh3, AnPh6, and AnPh12). Their MFC spectral shifts are 33, 40 and 45 nm, respectively, showing Δλ increase with alkyl length. Lu et al. (19) designed a series of bianthracene derivatives (5, PVBAn, Scheme 1) with different lengths of N-alkyl chains and found larger fluorescence contrasts after grinding with longer alkyl chains as well. Moreover, the fluorescence emission of ground solid PVBA16 can recover at room temperature which is ascribed to the low cold-crystallization temperature. Isomers of 9,10-bis(butoxystyryl)anthracene with the same alkyl chains at ortho or para positions (6, DSA4, Scheme 1), including n-butyl, i-butyl and tbutyl, were reported by Chen et al. (20). The DSA4 derivatives exhibit obviously different pressing-induced spectral shifts (Δλ = 3–53 nm), demonstrating that the isomerization of alkyl chains and attached positions also have a remarkable effect on MFC behaviors. 223 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Scheme 2. Molecular structures of compounds 7-10. On the other hand, Yang et al. (21) synthesized three N-phenylcarbazolecapped 9,10-divinylanthrane isomers by changing the linking positions of N-phenylcarbazole to examine isomeric effects on MFC properties. Luminogens 7, 8, and 9 (Scheme 2) are all MFC materials and exhibit reversible stimulus-response behavior (from 562 to 600 nm for 7, from 552 to 593 nm for 8, and from 528 to 565 nm for 9, after grinding). It is found that the linking position influences the emission wavelength more but takes little effect on their MFC spectral shifts.

Scheme 3. Molecular structures of compounds 11-15. Besides, Xu and Tian’s group (22) continued to report a relative study based on luminogen 10 (Scheme 2), which possesses multi-stimuli responsive fluorescent properties. High pressure-PL experiments confirm that the different polymorphs of 10 can be used to explain its MFC properties. Yang et al. synthesized a series of anthracene-centered cruciforms, 11 (23) with tetra-donor, 12 (24) with di-donor and 13 (25) with di-donor and di-acceptor (Scheme 3). Both of them show reversible emission changes upon pressing and annealing. Ouyang et al. (26) synthesized two luminogens, 14 and 15 (Scheme 3). Among them, 14 exhibits reversible chromism properties, including mechano- and thermos-chromism (from 525 to 573 nm after grinding). Different from 14, non-heteroatom-assisted 15 has no clear chromic processes. 2.2. Cyanoethylene Derivatives The cyanoethylene derivatives found to have AIE properties is only following the silole derivatives, thus, it is an important class of AIE materials and also named aggregation-induced emission enhacement (AIEE) materials by Park et al. (27). The MFC properties of cyanoethyene derivatives have been expanded to liquidcrystal, dual-stimuli responsive, multi-color switching, and nonlinear optical MFC properties. Huo et al. (28) reported two MFC luminogens 16 and 17 (Scheme 4), which are based on cyano oligo(p-phenylene vinylenes). On application of pressure, 16 with the cyano groups farther away from the central aromatic ring exhibits a more obvious color change (~ 53 nm) than those of 17. However, 17 is AIE-active, but 16 does not exhibit any AIE nature. 224 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Yang et al. (29) reported a novel luminescent liquid crystal (18, Scheme 4), which showed a mesomorphic organization by directly attaching the terminal dodecyloxy fragments to the dicyanodistylbenzene core. Luminogen 18 shows green, yellow and orange colors depending on the self-assembled structure. The luminescence of 18 can vary between those three different colors invoked by mechanical shearing and thermal annealing.

Scheme 4. Molecular structures of compounds 16-22. Wei et al. (30) synthesized a new cyano-substituted diarylethene derivative (19, Scheme 4) with reversible far-red MFC property (from 660 to 684 nm after grinding). The MFC mechanism was ascribed to the destruction of the crystalline structure, causing the changes of C-H out-of-plane bending vibrations in aryl group of the compound and the obvious increasement of fluorescence lifetime. Park et al. (31) reported an MFC material (20, Scheme 4) with an acid stimulus response based on a D–A–D molecular triad, which showed dual-stimuli responsive properties, i.e. acidochromic and mechanochromic behaviors (emission on switching). Zhang et al. (32) reported a D–A cruciform conjugated luminophore 22. Luminogen 22 exhibits MFC properties with a remarkable emission peak shift of 87 nm, much bigger than those of the linear molecule 21 (Scheme 4). Five more cruciform compounds (23, 24, 25, 26 and 27, Scheme 5) have been prepared from torsional cruciform skeletons consisted of donor and acceptor axes by Zhang et al. (33). These cross-conjugated geometries show spatially separated HOMO and LUMO located on the donor and acceptor axis respectively, revealing an extraordinary charge transfer process from one axis to the other in the excited state. Thus, effective MFC behaviors with λem red-shifted by 29–87 nm upon grinding are observed.

Scheme 5. Molecular structures of compounds 23-30. Wei et al. (34) reported three benzothiadiazolecored cyano-substituted diphenylethene derivatives (28, 29, and 30, Scheme 5) with different end groups, which endowed them with different D–π–A effects and gave birth to unique 225 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


and diverse MFC properties. After grinding, 28 and 29 showed a red-shift of emission peak of 22 nm (from 556 to 578 nm) and 12 nm (from 544 to 556 nm), respectively, while on the contrary, 30 exhibited a blue-shift of 15 nm (from 566 to 551 nm).

Scheme 6. Molecular structures of compounds 31-37. Park et al. (35) proposed a new strategy of supramolecular Föster resonance energy transfer (FRET) control to achieve complete red–green–blue (RGB) color switching with a bicomponent mixture system consisting of a B-to-G luminescence switching molecule with self-assembly capability (32, Scheme 6) and an off-to-on red luminescence switching molecule operating under different switching stimuli (31, Scheme 6). Switching of FRET between 31 and 32 by supramolecular structure control was a key idea of this system to suppress the crosstalk and to achieve high-contrast RGB switching. Reversible and independent RGB luminescence switching (λem = 594, 527, and 458 nm with ΦPL = 0.17, 0.26, and 0.23 for R, G, and B, respectively) were implemented by different external stimuli such as heat, solvent vapor exposure, and mechanical force. Zhang et al. (36) reported two structural-simple 3-aryl-2-cyano acrylamide derivatives (33 and 34, Scheme 6) and found that both of them exhibited obvious MFC properties with spectral red-shifted of 32 nm and 40 nm after grinding, respectively. Zhang et al. (37) continued to report a analogous molecule of 35 (Scheme 6) containing a twisted triphenylamine and diphenylacetonitrile (36, Scheme 6). The hydrostatic pressure in a diamond anvil cell was applied on its crystals and it was found that the fluorescence color changed from 530 to 665 nm with a significant red-shifted of 135 nm. Recently, Zhang et al. (38) developed another analogous D–π–A–π–A-type luminogen (37, Scheme 6). Upon mechanical grinding or hydrostatic pressure, the solid-state fluorophore reveals a very large spectral shift of 111 nm (from 507 to 618 nm). The evident color switching, accompanied by the tunable molecular stacking mode and molecular conformation, is mainly relative to two different kinds of excited state alteration: from the local excited (LE)-state into the charge-transfer (CT)-state.

Scheme 7. Molecular structures of compounds 38-46. 226 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Yuan et al. (39) reported two luminogens (38 and 39, Scheme 7) consisting of triphenylacrylonitrile (TPAN) units. They exhibit obvious MFC: upon grinding, the emission of the powders are red-shifted (from 559 to 600 nm for 38 and from 580 to 605 nm for 39). Yuan et al. (40) reported another D−A conjugate (40, Scheme 7) based on TPAN unit, which exhibits similar results (from 524 to 554 nm after grinding). Zhang et al. (41) also reported two AIE-active TPAN derivatives, 41 and 42 (Scheme 7). When 41 and 42 were ground, their colors and intensity turned red-shifted from saffron (549 nm, ΦPL=2.0%) to orange (557 nm, ΦPL=12.0%) and from jade green (506 nm, ΦPL=44.6%) to croci (545 nm, ΦPL=16.8%), respectively. Another TPAN derivative (43, Scheme 7) prepared by Zhang et al. (42) exhibits tunable fluorescence in the aggregate state depends on the polymorphic forms with different molecular conformations: three crystalline forms BCrys, SCrys and YCrys exhibit bright blue (458 nm), sky-blue (474 nm) and yellow emission (539 nm), respectively. Interestingly, the solvent vapour and heating stimuli can trigger a crystal-to-crystal transformation between SCrys form and YCrys form. D–π–A type phenothiazine modified TPAN derivatives 44, 45 and 46 (Scheme 7) have been synthesized by Lu et al. (43). The spectral shifts of as-synthesized crystals of 44, 45 and 46 upon grinding are 72, 26 and 54 nm respectively. A twisted TPAN derivative (47, Scheme 7) containing 9,9′-bianthracene group was designed and studied by Liu et al. (44). Three crystals of 47 can be grown and exhibit different emission properties upon grinding. One of these crystals shows “turn on” green emission from dark state. The second one exhibits blue to green emission transfer, while the last one has almost no emission color change. Such unique property can be explained by the location of radiative excited state alternates between Module I and Module II in the molecular structure of 47. Zhou et al. (45) reported four luminophores (48-51, Scheme 8) that generated from triphenylamine (TPA) with simple modification. In spite of very simple molecular structures, all compounds except 50 showed significant MFC properties. The aromatic rings of one molecule in crystal of 50 are all restricted by the weak interactions or hydrogen bonds, which provide more twisted and compressible space than that of 49. Thus, a redder shift of the emission wavelength (nearly 50 nm) can be observed in contrast to 49 (nearly 20 nm) after grinding.

Scheme 8. Molecular structures of compounds 48-56. Anthony et al. (46) reported a TPA based AIEE material (52, Scheme 8), which exhibits reversible MFC via controllable phase change and polymorphism, defection, topochemical and nanofabrication induced fluorescence tuning. 52 crystals grown form CH3CN (52-a) and CH3OH (52-b) exhibited fluorescence at 588 nm and 538 nm, respectively. Upon grinding, the fluorescence of 52-a and 52-b was gradually shifted to 562 nm, which is the emission of amorphous state. 227 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Roncali et al. (47) reported on the optical properties of a small push–pull molecule (53). Compared to the already known compounds that exhibit AIE and/or MFC behavior, the new molecule differs by the extreme simplicity of the structure and by the fact that, it is the first example of material showing nonlinear optical mechanochromic properties. Film casting from solutions produces a metastable deep-red amorphous material which rapidly evolves toward an almost colorless stable crystalline form. Mechanical stimulation by writing, rubbing, or smearing restores the initial deep-red form which spontaneously returns to the crystalline state. After 20–30 min storage in ambient conditions the films of compound 54 remain unchanged, while the films of compound 53 underwent discoloration (Scheme 8). An MFC luminescent D–π–A–π–A compound, 55, has been constructed by Zhang et al. (48). The much stronger MFC feature of 55 (Δλ >80 nm), compared with 56, implies that the 2,2-dicyanovinyl group plays an important role in realizing the strong emission-colour response towards grinding (Scheme 8). A series of dicyanomethylenated acridone derivatives 57-60 (Scheme 9) are synthesized by Wang et al. (49). They are highly luminescent in crystalline state but non-emissive in amorphous state, showing crystallization induced emission (CIE) behavior. The molecular packing of 57-59 in crystals is easily regulated by modifying the length of alkyl chains, resulting in the tunable emission colors from green to red. This report presents a mechano-responsive emission on−off switching system with various emission colors (560 to 707 nm).

Scheme 9. Molecular structures of compounds 57-66. 2.3. Tetraphenylethylene Derivatives Tetraphenylethylene (TPE) derivatives are hotly investigated in recent years for their AIE properties. The recent studies show that some TPE derivatives possess solid state MFC performances (50). Some simple AIE molecules with high crystallinty, such as phenyl-substituted tetraphenylethylene (51) and TPE (52), are considered as no MFC due to their high crystalinity. Their crystal structures are too fragile under shearing force to be detected by their color changes or their crystalline structures recover too fast to be observed. However, the high-pressure studies on TPE using diamond anvil cell technique with associated spectroscopic measurements reveal that TPE shows MFC based on its conformation planarization. The mechanism of conformation planarization has been confirmed by Zou et al. (53). During the compression process, the λem gradually red shifted from 448 to 467 nm at 5.3 GPa and eventually to 488 nm at 10 GPa. Xu and Tian’s group (54) investigated the MFC and polymorphism-dependent emission of one TPE derivative 62 (Scheme 9). It was found that the covalently linking dimethylamino groups into TPE brought the intermolecular interactions 228 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


(such as C−H···πand C−H···Ninteractions), and these relative “soft interactions” are easily broken after grinding or under pressure. Then the resulting change of packing patterns or intramolecular conformation finally makes MFC be realized. Chi and Xu’s group (55) reported a novel TPE derivative (63, Scheme 9) with AIE and CIE activity (ΦPL up to 85%). 63 has an exceptionally large two-photon absorption cross section of 5548 GM, and exhibits striking multi-stimuli-responsive single- and two-photon fluorescence switching with excellent reversibility in the solid state (from 469 to 513 nm after grinding). Tang et al. (56) reported that 1,1,2,2-tetrakis(4-ethynylphenyl) ethane (61, Scheme 9) is AIE-active and MFC (from 477 to 505 nm after grinding). Carbazole and TPA-substituted ethenes (64, 65 and 66, Scheme 9) with high solid-state ΦPL (up to 97.6%) are also synthesized by Tang et al. (57). They exhibit MFC properties: their emissions can be repeatedly switched between blue and green colors by simple grinding–fuming and grinding–heating processes (from 455, 454 and 429 nm to 465, 490 and 500 nm after grinding, respectively). Tang et al. (58) continued to characterize a series of luminogens (67, 68, 69 and 70, Scheme 9) comprised of TPE plus spirobifluorene or 9,9-diphenylfluorene. Reversible MFC feature is observed from their solids (for example, from 445 to 503 nm for 67 after grinding).

Scheme 10. Molecular structures of compounds 67-72. Two novel AIE compounds (71 and 72, Scheme 10) derived from TPE and gallic acid were reported by Chi and Xu’s group (59). Both of them possessed mesomorphic properties and exhibited the thermal-induced mesomorphic transition from metastable to stable phases accompanied by a change of the luminescent color. Compound 71 had no MFC property, however, a significant red shift of about 20 nm (from 452 to 472 nm) was observed in 72 after pressing. During synthesis of near-planar aromatic hydrocarbons by the twisted TPEbased oligomers, Wang et al. (60) found that some of the intermediate molecules (73-78, Scheme 10) possessed the MFC properties. After a careful examination, three of the compounds (73, 76, and 77) exhibited obvious MFC behavior with spectral shifts of 50, 40 and 44 nm, respectively.

Scheme 11. Molecular structures of compounds 73-82. Zhang et al. (61) reported a TPE-based phosphine (79), which was used as a ligand to synthesize metal organic framework (MOF). The ligand shows AIE and MFC (from 468 to 499 nm after grinding). 229 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Tang et al. (62) presented the synthesis of three butterfly-like derivatives, 80, 81 and 82 (Scheme 11), with different substituents on the periphery phenyl rings. Compound 82 shows a multicolor luminescence switching between three colors (blue crystal 82-b, green cystal 82-g, and yellow amorphous solid 82-am). Similarly, 81 exhibits reversible MFC between blue crystal (425 nm) and yellow amorphous solid (535 nm). The loose molecular packing with noncovalent intermolecular interactions, the extent of conformational twisting, and the packing density of the luminogens, as well as freedom of intermolecular motion in the excited state, are stemmed for their reversible polymorphism dependent emission behaviors. Zhu et al. (63) reported a new series of geminal-substituted tetraarylethene (TAE) chromophores (83-91, Scheme 12) with AIE properties, which were probed with respect to steric and electronic effects. In comparison to the solvent-free 85 crystal, the solvated 85 with embedded methanol or dichloromethane leads to some non-negligible conformational and packing alterations, which accounts for its distinct fluorescence properties. As an example, they investigated the MFC property of 86 (from 455 to 480 nm after grinding).

Scheme 12. Molecular structures of compounds 83-91. Misra et al. (64) attached TPE unit on the pyrazabole and explored its AIE and MFC properties. Compound 92 (Scheme 13) exhibits strong blue colored emission upon aggregation, and highly reversible MFC feature (from 453 to 497 nm after grinding).

Scheme 13. Molecular structures of compounds 92-99. Three luminogens based on N-phenylcarbazol-substituted TAE 93, 94 and 95 (Scheme 13), were synthesized by Dai et al. (65). All of the luminogens show AIE characteristics with high solid-state ΦPL of up to 83%. Only 95 reveals obvious MFC property: from 441 to 505 nm after grinding. This proved that by introducing a methoxy group into one of the phenyl rings at the para position, MFC materials can be easily obtained. Yuan et al. (66) reported that rational bridging of four TPA units by an ethylene group affords 96 (Scheme 13) with AIE characteristics (ΦPL up to unity) and reversible MFC property (from 501 to 530 nm after grinding). Three new D−π−A−π−D type quinoxalines modified with TPEs 97, 98 and 99 (Scheme 13) were studied by Lu et al. (67). They show obvious MFC properties: from 466 and 491 nm to 500 and 507 nm after grinding, respectively. 230 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Ma et al. (68) reported a tricolored switchable MFC single crystal 100 (Scheme 14), which was synthesized by combining two different luminophores, a TPE unit and a rhodamine B (RhB) moiety together. The obtained single crystal of 100 switched sequentially from deep-blue (441 nm) to green (468 nm) and to a reddish (576 nm) color. The reddish color could be returned to bluish-green at around 465 nm by heating the colored powder at 150 °C for ten minutes. However, the bluish-green powder could not be fully returned to the original deep-blue at 441 nm either by heating or by solvent treatment. Accordingly, compound 101 (Scheme 14) without a boron atom in the structure was an amorphous powder and could not be cultured to a single crystal. More importantly, 101 exhibited only two-color switching from green to reddish upon grinding. It is certain that the boron is critical in the crystallization of 100 with original deep-blue color, which turns the mechanochromic fluorescent emission from two colors to three colors. Chi and Xu’s group (69) developed an AIE-active luminophore 102 (Scheme 14) with remarkable four-colored switching based on the mechano- and protonation-deprotonation control. TPE substituted phenanthroimidazoles 103 and 104 (Scheme 14) were synthesized by Misra et al. (70). They show reversible MFC behaviors with contrast colors between sky-blue and yellow green (from 460 and 450 nm to 509 and 508 nm after grinding, respectively).

Scheme 14. Molecular structures of compounds 92-99. Tang et al. (71) reported a TPE derivative substituted with the electron-acceptor 1,3-indandione (IND) group. The targeted IND-TPE (105, Scheme 14) solids show an evident reversible MFC process in multiple grinding–thermal annealing and grinding–solvent–fuming cycles (from 515 to 570 nm after grinding).

Scheme 15. Molecular structures of compounds 106-113. Shan et al. (72) characterized three pyridine-azole-based AIE materials modified by TPE unit, i.e. 106, 107 and 108 (Scheme 15). Their crystalline aggregates exhibit effective MFC properties with high contrast in both emission color and intensity: from 429 to 460 nm for 106, from 446 to 464 nm for 107 and from 430 to 455 nm for 108 after grinding. Importantly, directly visualized, the ground samples show much stronger emission than those of the as-synthesized and annealed ones. 231 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


The D–A type benzothiazole (BT) substituted TPEs (BT-TPEs) 109-111 (Scheme 15) were prepaired by Misra et al. (73) and the results showed that their photophysical, AIE and MFC properties are dependent on the linkage between the BT and the TPE unit (ortho, meta, and para). The meta isomer 110 shows the highest grinding induced spectral shift (51 nm, from 432 to 483 nm) whereas the ortho isomer 109 shows the lowest spectral shift (9 nm, from 478 to 487 nm). Misra et al. (74) synthesized pyrene-based solid state emitters 112 and 113 (Scheme 15) by substituting the TPE and TPAN units on pyrenoimidazole, respectively. 112 and 113 exhibit strong solid-state fluorescence and drastic reversible MFC between blue and green (from 461 to 499 nm and from 473 to 510 nm, respectively). Misra et al. (75) synthesized TPE substituted unsymmetrical D–A benzothiadiazoles (BTDs) 114, and 115 (Scheme 16). The results show that the cyano-group containing BTD 115 exhibits reversible MFC behavior between green (526 nm) and yellow (~565 nm, ground form), whereas the 114 do not show MFC. Tao et al. (76) developed a series of AIE-active fluorenyl-containing tetra-substituted ethylenes (116-118, Scheme 16). The emission color before and after grinding demonstrates high contrast: for 116, the PL blue-shifted about 58 nm to 517 nm from 459 nm of the crystals; for 117 and 118 the fluorescence red-shifted 34 nm from 473 to 507 nm, and 52 nm from 464 to 516 nm, respectively. Different substituents proved to have a clear effect on the optical and MFC behaviors. Tao et al. (77) continued to investigate their relationship between the molecular conformations and the MFC behavior, especially the role of mechano-stimuli on the thermal annealing crystallization process. Through paired comparisons, they disclosed that the mechano-stimuli could not only destroy the crystallinity of crystalline materials but also bring a significant effect on the amorphous-to-crystalline transition of amorphous materials. That is, only when an amorphous material undergoes mechano-stimuli can it crystallize by thermal annealing to recover its emission. To clearly understand the solid-state amorphous to crystalline transformation, Tao et al. (78) developed an in situ and realtime imaging procedure to record the interface evolution in a solid-state crystallization of molecular amorphous particles. The details disclosed in this observation will deepen the understanding for a series of solid-state crystallization.

Scheme 16. Molecular structures of compounds 114-120. Tang et al. (79) reported their attempt to enhance the robustness of the MFC feature by introducing more ionic species into an AIE-active molecule, 120, which is a homolog of 119 (Scheme 16). This compound demonstrates typical AIEE and MFC behaviors. But the transition between the highly efficient yellow emission (~560 nm) of the crystalline and the moderate red emission (605 nm) of the amorphous 120 solid becomes irreversible by simple treatments of thermal annealing and/or solvent vapor fuming, whereas the ground sample can recover 232 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


its emission only by recrystallization from dissolving the amorphous solid in suitable solvents. The emission peak exhibits a trend of a monotonous blue-shift in grinding–fuming cycles. These behaviours are evidently distinct from the phenomena observed for 119 and other uncharged TPE derivatives. Zhang et al. (80) proposed a new single-arm extension strategy on traditional TPE and successfully developed a new series (121, Scheme 17) of full-color (from ~450 to ~740 nm) tunable MFC materials. These materials exhibit efficient solidstate emission (ΦPL > 10%) and high MFC contrast (wavelength shift from ~50 to ~100 nm) (Figure 1).

Scheme 17. Molecular structures of compounds 121.

Figure 1. (a) The relationship between the MFC contrast (emission peak shift) and molecular long-to-short axis ratio of TPE and its group I derivatives. (b) The relationship between the MFC contrast (emission peak shift) and the molecular dipole moment of group II TPE derivatives (black dots), the molecular long-to-short axis ratio is also indicated (red dots). Adapted with permission from ref. (80). Copyright 2015 The Royal Society of Chemistry. Dong et al. (81) changed the substituted groups and obtained three compounds derived from the TABD (122) molecule, 123, 124, and 125 for the systematic and comparative study of the structural effect on MFC performance (Scheme 18). All of these TABD derivatives are found to possess AIEE features and MFC properties. The results show that the MFC performance (spectal shift) follows the sequence of 125 > 123 > 124. This order can be attributed to the distinctions in the molecular polarity of the three compounds, as indicated by exploration of their solvatochromic properties and through theoretical calculations. Based on AIE-active TPE, a group of diethylamino (DEA) functionalized analogues, i.e. 126, 127 and 128 (Scheme 18) were prepared by Yuan et al. (82). Ground 126 and 127 solids demonstrate rapid self-recovery without any exental 233 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


treatment within a few minutes or even several seconds, respectively. It is found that the ground part with dim yellow (490 nm) light emission mostly returns to the original cyan light (481) in 2-3 min, and is fully restored in 5 min. The slower self-recovery rate of 127 compared to 126 should be ascribed to its stronger dipole–dipole interactions, which slow down the molecular motions. When further aromatic building block with stronger electron-accepting capacity is introduced, 128 shows MFC (from 588 to 598 nm after grinding) without self-restoration within a short time.

Scheme 18. Molecular structures of compounds 122-128. Wang et al. (83) reported two rigid snowflake-shaped luminophores 129 and 130 (Scheme 19) based on six TPE units as peripheries and benzene as a core group. The emission spectrum of the ground 129 powder exhibited a large red-shift of 30 nm (from 467 to 497 nm), revealing that 129 has MFC behavior. However, the MFC behavior did not appear when 130 was ground. These results, combined with its less defined PXRD patterns of the pristine and ground state, can be probably explained by the conformation of 130, which has already undergone planarization and compaction in its pristine powder state due to the extended structures, in comparison with the more twisted conformation of 129.

Scheme 19. Molecular structures of compounds 129-138. Bhosale et al. (84) described a rigid star-shaped luminogen (131, Scheme 19) of cyclohexanehexone bearing six TPE moieties, which exhibited strong AIE activity and reversible MFC behaviour (from 469 to 500 nm after grinding). Zhu et al. (85) reported that the bisanthracene modified dibenzofulvene (132) exhibits efficient MFC properties with the emission reversibly altered between 536 nm (ΦPL = 63%) and 620 nm (ΦPL = 11%). With respects to 133 and 134, both of them exhibit MFC with changes in the λem of about 20 nm (Scheme 19). Zhou et al. (86) constructed a metal−organic framework (MOF) denoted as PCN-128W, starting from chromophoric TPE-based linker (135, Scheme 19) and zirconium salt. PCN-128W exhibits interesting MCF behavior, the color reversibly changes from white to yellow and so does the emission from blue to green (470 to 538 nm). The process is fully reversible by treating PCN-128Y with trifluoroacetic acid (TFAA) in DMF at elevated temperature (Figure 2). It 234 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


indicates that PCN-128W can be considered as a microscissor lift. This work illustrates a very rare example of reversible 3D mechanofluorochromic MOF.

Figure 2. Simplified schematic diagram illustrating the reversible motion of the microscissor lift. Reproduced with permission from ref. (86). Copyright 2015 American Chemical Society. To study the effects of donor and acceptor substitutions, 136-138 (Scheme 19) containing multiple AIE units were synthesized by Tang et al. (87). Luminogen 136 film displays efficient green fluorescence (494 nm, ΦPL = 100%), evident AIE characteristic (αAIE = 154), and reversible MFC (from 472 to 505 nm after grinding). Replacing two phenyls by two cyano (A) groups derives 137, whose film shows efficient orange fluorescence (575 nm, ΦPL = 100%) and evident AIE feature (αAIE = 13). The MFC behavior of 137 (from 541 to 563 nm after grinding) is reversible. Further decoration of 137 with N,N-diethyamino (D) groups results in 138. Due to the cooperative effects of D and A groups, 138 shows dramatic red-shifted emission (713 nm), and reversible MFC behavior. Three tetraphenylvinyl-capped ethane derivatives with 0, 1, and 2 cyano groups at the ethane moiety 139, 140, and 141 (Scheme 20), respectively, were synthesized and characterized by Chi and Xu’s group (88). The results indicate that the compounds possess reversible MFC properties. The introduction of cyano groups to the molecular structures significantly enhanced their MFC activity.

Scheme 20. Molecular structures of compounds 139-151. 2.4. Other Typical AIE Luminogens Recent years, some AIE luminogens have been reported but they do not contain any common AIE units, such as diarylvinylanthracene, TPE, triphenylethylene, cyanoethylene or silole structure. Moreover, some of them have MFC properties. Nakano et al. (89) have reported that 142 (Scheme 20) exhibited MFC (from 465 to 490 nm after grinding). However, the amorphous state of 142 was not so 235 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


stable due to rapid re-crystallization and hence the fluorescent color returned to the original soon after grinding. It is expected that the increase in glass-transition temperature (Tg) of the material makes the amorphous state more stable to prevent crystallization at room temperature. Therefore, Nakano and Mizuguchi (90) focused on 143 (Scheme 20), whose Tg is (86 °C) higher than that of 142 (8 °C). By grinding the crystalline sample, the light blue emission (473 nm) of 143 was changed to greenish yellow (495 nm), and was quite stable at room temperature as intended. In the present study, Nakano et al. (91) found that 142 and 143 were AIE active. In addition, the fluorescence of the resulting 142 particles was found to change in the suspension through vigorous stirring upon heating. Zhou et al. (92) synthesized a new family of TPA-based Schiff bases (144151, Scheme 20) that exhibit different AIE or ACQ behavior in THF/water and as solids. Compounds 146, 147, 149, and 151 show good AIE characteristics due to the existence of J-aggregates or multiple intra- and inter-molecular interactions restricting the intra-molecular vibration and rotation. In addition, emission colors change from 576 to 583 nm and from 520 to 551 nm, respectively, for 147 and 149, after grinding. Liu et al. (93) reported a series of novel, simple, and colorful Salen ligands. Most of the Salen ligands have no MFC and AIE properties. However, 152 (Scheme 21) is an MFC material showing turn-on strong green fluorescence.

Scheme 21. Molecular structures of compounds 152-163. Anthony et al. (94) reported that aryl-ether amine based simple Schiff base molecules (153-157, Scheme 21) showed AIEE effect in the solid state and rare stimuli responsive fluorescence off–on switching. The grinding of 153 resulted in irreversible fluorescence blue shift from greenish-yellow (520 nm, ΦPL = 40%) to green (508 nm, ΦPL = 43%). Heating or solvent exposure did not result in any fluorescence reversibility. Interestingly, the grinding of 154-157 led to the quenching of the solid state fluorescence and heating/solvent exposure produced clear bright fluorescence. It is noted that the turn-on fluorescence of 154-157 was slightly blue shifted compared to the initial solids. Zhou et al. (95) reported three new anthryl Schiff base derivatives containing a similar molecular structure. Among these, 158 displayed an AIE feature, 159 exhibited AIEE active, while 160 showed ACQ behavior. 159 exhibited an MFC characteristic with a 15 nm spectral blue-shifted after grinding (Scheme 21). Taking into account the balance of steric constraints, hydrogen bonding, and π–π stacking interactions, Han et al. (96) synthesized a trigonal azobenzene derivative (161, Scheme 21), a new AIEE-active azobenzene chromophore, in which three phenyl rings are connected to a central 1,3,5-trihydroxybenzene core via azo groups. The compound can self-assembly form red fluorescent 236 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


1D structures. The enhanced red fluorescence of the fibrous structures can be switched off by pressing, rubbing, or annealing. These findings may be applicable to the development of stimuli-responsive luminescent materials that range from optoelectronic devices and sensors to fluorescent polarizers combined with unidirectional molecular arrangements in polymeric objects. Liu et al. (97) studied solid emission properties of 162 (Scheme 21). After grinding, the solid powder of 162 with strong yellow emission (543 nm ΦPL = 31.5%) was converted to a green luminescent powder (530 nm, ΦPL = 20.2 %).

Figure 3. Influence of the ultrasonic power on the morphology and fluorescence color of 163 suspensions in the THF-H2O mixtures (90% content of water) with a frequency of 40 kHz. (a) Non-ultrasonic; (b) 80 W; (c) 120 W; (d) 160 W; (e) 200 W. Reproduced with permission from ref. (98). Copyright 2014 The Royal Society of Chemistry.

Compared with the reported external stimuli, ultrasonication has overriding advantages such as high energy efficiency and quantitative controlling effects. Thus, ultrasound is likely to become a convenient, highly efficient and controllable external stimulus applied in MFC materials. Zhang and Xu’s group (98) reported a novel ultrasonic-sensitive MFC AIE-compound (163, Scheme 21). The fluorescent properties of the 163 suspensions were greatly affected by the ultrasonic treatment and extremely sensitive to its power, which show remarkable blue-shifting and enhanced emission. The aggregation morphologies were found to be greatly affected by the ultrasonic treatment and extremely sensitive to its power (Figure 3). In other words, the luminescent properties are tunable through controlling of the molecular packing mode. Naka et al. (99) reported an AIE-active maleimide luminogen 164 (Scheme 22) with MFC. The yellow emission (553 nm) of 164 crystal changed to green emission (525 nm) after grinding, whereas the emission colors of 165-168 (Scheme 22) were not changed by grinding. Naka et al. (100) continued to report AIE-activoty N-alkyl aminomaleimide luminogens with various kinds of N-alkyl groups. The luminogens exhibited different emission behaviors depending on the chemical structure of the N-alkyl group. Furthermore, it was found that propyl-substitued luminogen 169c (Scheme 22) displayed MFC (from 502 to 489 nm after grinding). 237 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Scheme 22. Molecular structures of compounds 164-169.

Cheng et al. (101) synthesized three D−π−A−π−A indene-1,3dionemethylene-1,4-dihydropyridine derivatives (170-172, Scheme 23) with TPA end groups. These target compounds with highly twisted conformations showed AIEE properties. It is found that they show MFC properties (170: from 610 to 681 nm, Δλ = 70 nm; 171: from 624 to 688 nm, Δλ = 64 nm; 172: from 638 to 683 nm, Δλ = 45 nm). A fluorescence color change similar to the MFC property could also be achieved by a simple dissolution−desolvation process in different solvent systems, which could be considered a solvent-induced emission change.


Yan et al. (102) reported a mechano-induced and solvent stimuli-responsive luminescent change by the assembly of a typical AIE molecule, niflumic acid (173, Scheme 23), into the interlayer region of Zn−allayered double hydroxides (LDHs) with heptanesulfonate (HPS) as a cointercalation guest. The 5%-173-HPS/LDH sample exhibits the most obvious MFC with a 16 nm blue-shift (from 439 to 423 nm) with increase in the intensity after grinding, while the pristine 173 solid shows little to no MFC behavior. Wang et al. (103) reported that AIE fluorenone derivatives 174 and 175 (Scheme 23) display reversible stimuli-responsive solid-state luminescence switching. 174 transforms between red (601 nm) and yellow (551 nm, crystals) under the stimuli of temperature, pressure, or solvent vapor. Similarly, 175 exhibits MFC behavior with luminescence switching between orange (571 nm) and yellow (557 nm). Single-crystal structures indicate that the variable solid-state luminescence is also attributed to the formation of different excimers in different solid phases (Figure 4). Additionally, the stimuli-responsive reversible phase transformations of 174 and 175 involve a structural transition between π−π stacking-directed packing and hydrogen bond-directed packing which result in a metastable solid/crystalline state luminescence system. 238 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Figure 4. Schematic diagram for the excimer formation and emission of molecule 174. Reproduced with permission from ref. (103). Copyright 2014 American Chemical Society. Reversible MFC is known for difluoroboron β-diketonates (BF2bdks) (104). Fraser et al. (105) continued to report the MFC for the methoxy-substituted dinaphthoylmethane (176, Scheme 24) ligand even without coordination to boron. In the as spun state, films of 176 showed faint blue emission (475 nm, ΦPL = 3.3%). After annealed at 140 °C, emission spectra showed a broad peak at 440 nm (ΦPL = 3.6%). Some fine structure emerged in the blue-shifted portion (385–480 nm) of the spectrum, however a shoulder peak was observed near 500 nm. In contrast, the smeared state of 176 was blue-green in color (503 nm, ΦPL = 10.6%), which was broader than observed for as spun and thermally annealed states.

Scheme 24. Molecular structures of compounds 176-184. Feng et al. (106) reported a new dual-boron-cored luminogen (177, Scheme 24) ligated with a nitrogen-containing multidentate ligand and four bulky phenyl rings. The unique molecular structure endows this BN-containing luminogen with rich photophysical properties. The sterically congested structure of compound 177 which plays a key role in its AIE activity, may render it responsive to mechanical stimuli (from ~501 to ~521 nm after grinding). Based on this consideration that the luminescent colors of anthracene derivatives in the solid state can be modified by varying their assemblies, Chujo et al. (107) synthesized an o-carborane-based anthracene (178, Scheme 24), in which o-carboranes are substituted at the 9- and 10-positions of anthracene. Its single crystals, with incorporated solvent molecules, were obtained from the CHCl3, CH2Cl2, and C6H6 solutions. Scratching the crystals dramatically decreases the ΦPL value. For example, the ΦPL value of 178·CH2Cl2 changed from 0.66 to 0.08. In addition, the PL and excitation spectra for the scratched solid were bathochromically shifted (for 178·CH2Cl2 from 594 to 640 nm). Lu et al. (108) synthesized three carbazole-based terephthalate derivatives (179, 180 and 181, Scheme 24), in which carbazole and ethoxylcarbonyl groups are used as electron-donating and -accepting moieties, respectively. Application 239 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

of mechanical grinding to the crystals resulted in red-shifts of emissive wavelength of 179 and 180 with a spectral shift of 25 and 15 nm, respectively. However, no MFC behavior was found for the 181 crystals.


2.5. AIE Complex In 2011, Chi and Xu’s group (109) had published a zinc ion complex which is the first mechano-responsive AIE complex. Three multifunctional cationic iridium(III)-based materials with AIE and MFC behavior have been synthesized by Su et al. (110). All complexes contain the same cyclometalated ligand with functionalized ancillary ligands. Complexes 182 and 183 (Scheme 24) undergo remarkable and reversible MLC in the solid state (from 461 and 462 nm to 482 and 478 nm after grinding, respectively). While complex 184 (Scheme 24), an amorphous material, which only displays AIE activity and on MLC property. More importantly, with the merits of reversible MLC and AIE properties of 183, the rare multi-channel color change and temperature-dependent emission behavior of the iridium(III) complex have been observed.


Zhu et al. (111) described two new dinuclear cationic Ir(III) complexes, 185 and 186 (Scheme 25) with Schiff base bridging ligands. The results demonstrate that both complexes 185 and 186 are AIE-active and simultaneously show MLC. Grinding both 185 and 186 on quartz plates induced a red-shift of the emission by ca. 20 nm to 635 and 648 nm from 612 and 627 nm, respectively. Liu et al. (112) reported a series of diisocyano-based dinuclear gold(I) complexes differing only in the bridge linking the two (identical) arms. The gold(I) complexes 187, 188 and 189 (Scheme 25) all exhibit AIE characteristics and MLC behavior: their phosphorescence properties show reversible switchable off–on green luminescence (from 485 nm, ΦPL = 1% to 500 nm, ΦPL = 67.5%). Liu et al. (113) synthesized three trinuclear gold(I) complexes, which exhibit AIE characteristics and show irreversible off–on green luminescence in response to mechanical grinding. Upon grinding of solid power 190, 191 and 192 (Scheme 26), a new emission band at 496 nm was observed and the corresponding emission was converted into strong green luminescence. The changing of weak multiple intermolecular C–H···For π-π interactions, or the formation of aurophilic interactions are possibly responsible for their MLC phenomena. 240 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.



Liu et al. (114) synthesized a series of constitutional isomers containing dinuclear gold(I) units. Complexes 193-195 (Scheme 26) exhibited significant AIE phenomena. The ortho-isomer 193 exhibited reversible MLC (from 430 to 502 nm after grinding), whereas meta-isomer 194 showed switchable mechanical force-induced luminescence enhancement behavior. No MLC behavior was observed for para-isomer 195. Liu et al. (115) continued to report two gold(I) complexes 196 and 197 (Scheme 27) with AIE active. The solid-state luminescence of complexes 196 and 197 can be significantly increased by grinding. Liu et al. (116) reported another dinuclear gold(I) complex with a fluorene-based skeleton (198, Scheme 27). Complex 198 is AIE-active and exhibits reversible MLC: from two emission bands at 490 and 523 nm to 559 nm after grinding. In addition, it shows crystallization-induced emission enhancement behavior. Šket et al. (117) reported a BF2 complex (199, Scheme 27), a molecule with two methoxy groups in one of the phenyl rings at meta positions. Compound 199 exists as two polymorphs having different mutual orientations of the two methoxy groups: in polymorph A away from each other (termed anti), while in polymorph B one methoxy group is oriented toward the other (syn−anti). It was observed that solid A but not solid B exhibited MLC and a striking CIEE effect. Solid A emitted strongly in the crystalline phase (490 nm) but only faintly in the amorphous phase (526 nm). The well ground powder of solid A on dropwise treatment with solvent (CH2Cl2) or on heating (thermochromism) reverted back to the initial blue emissive crystal state, as well as partially returning to the original emission color spontaneously at room temperature (chronochromism).



Chujo et al. (118) synthesized a variety of boron ketoiminates (200-205, Scheme 27) with AIE and MLC behavior to investigate the effect of the substituents on the optical properties by altering the end groups in the compounds.

3. Mechano-Switching Based on Emission Strength


Tang et al. (119) reported a new AIEE-active luminogen, 206 (Scheme 28), a diaminomaleonitrile-functionalized Schiff base. Its crystal are nonemissive. The defect areas of the crystal, however, are highly emissive at 563 nm. Interestingly, the pressure caused it turn-on is very small (about 0.1 Newton).


Zhang et al. (120) reported a bis(2′-hydroxychalcone)beryllium complex 207 (Scheme 28) that displays yellow fluorescence (557 nm; ΦPL = 0.10) in solution. Notably, the solution of this complex produces a non-emissive amorphous thin film (ACQ effect; fluorescent “OFF” state) but brightly emissive crystalline powders (AIEE-active; fluorescent “ON” state) with deep red (678 nm; ΦPL = 0.27) or near infrared (700 nm; ΦPL = 0.20) emission colors. The fluorescent “ON” and “OFF” states can be smoothly transformed into each other by simple mechanical grinding and solvent fuming. Zhang et al. (121) reported another class of beryllium complexes 208-211 (Scheme 28), which display CIE and exhibit morphology-dependent dark and bright red/NIR fluorescence. They show bright red/near-infrared (NIR) emission in the crystalline form (λem: 635–700 nm; ΦPL: 27%–40%) and faint emission in the amorphous state. Their emission can be smoothly switched “ON” and “OFF” by simple grinding/ solvent annealing processes. The two ligands of each complex are almost planar and perpendicularly fused by a beryllium atom with a dihedral angle of about 90°. The produced cross-shape structure facilitates molecules to arrange in a “#” manner with high molecular rigidity as well as packing stability confirmed by crystal structure analyses of 207 and 210 (Figure 5). The “#”-typed molecular packing feature may effectively restrict the photoinduced molecular distortions and eliminate the intermolecular π-electron interactions which is beneficial to the emission. After grinding, the molecules transfer into a random packing mode in which the intermolecular π–π interactions may dominate and the molecules can rotate freely, resulting in a fluorescence “OFF” state.



Figure 5. Molecular packing modes in the ground and fumed samples. Adapted with permission from ref. (121). Copyright 2014 The Royal Society of Chemistry.

Zhang et al. (122) continued to report a novel family of organoboron compounds 212-215 (Scheme 29) with bright NIR emissions in the crystalline state. They show CIE property and a morphology-dependent emission “ON” and “OFF” feature. The ΦPL of compound 213 powder (34%) is very high for the NIR emissive luminogen with an emission peak beyond 730 nm. Compounds 213-215 show similar solid-state NIR fluorescence “ON/OFF” switching, reflecting the generality of this elegant luminescent behavior. The Ifumed/Iground ratios of emission intensity are 12, 13, and 15 for 213, 214, and 215, respectively.


Xu and Tian’s group (123) systematically studied the intriguing turn-on and color-tuned luminescence of the molecular crystals of acridonyl (AD)-tetraphenylethene (TPE) (216, Scheme 28) in response to mechanical grinding and hydrostatic compression. The almost orthogonal conformation between TPE and AD fully separates the electronic distribution and inhibits the ICT process, leading to the emission from LE state in the D-phase of the molecular crystals. The twisted conformation can be changed by the force perturbation when the molecule is under the mechanical stimuli, resulting in an overlap of the frontier orbitals between donor and acceptor and the formation of ICT state. Thus, the switching of excited state characteristics by the mechanical stimuli induced the change in luminescence from the non-emission D-phase to the bright cyan emission B-phase. The concept of the mechanical switching of the excited state will inspire the development of a new class of MLC materials with high-contrast ratio, and further provide an important insight into the solid state luminescent properties of the twisted D−A molecules.



4. Mechanochromism Based on Dual-Emission The switching range of emission of organic MLC materials is seriously impeded by only one kind of emission (either a fluorescent or phosphorescent peak) in the spectrum of single organic compounds. Chi and Xu’s group (124) presented a design strategy for pure organic compounds with excellent room-temperature fluorescent–phosphorescent dual-emission (rFPDE) properties, which combines the effective factors of dipenylsulfone group, crystalline state, and heavy atom effect. The phosphorescent peak of luminogen 217 is very weak, however, after introduction of iodine atom to obtain luminogen 218, its phosphorescent peak becomes very strong (Scheme 30). Following the principle of color mixing, myriad emission colors with a wide range from orange to purple and across white zone in a straight line in the chromaticity diagram of the Commission Internationale de l’Eclairage (CIE) can be obtained by simply mechanical grinding the compound. The change of emission colors is realized by the ratio of fluorescent and phosphorescent peaks, where phosphorescent peak is strongly dependent on the crystallinity which is affected by the grinding treatment (Figure 6). The unique properties could be concentrated on a pure organic compound through this design strategy, which provides a new efficient channel for the discovery of efficient mechano- responsive organic materials. Chi and Xu’s group (125) reported a novel white-light-emitting organic molecule, which consists of carbazolyl- and phenothiazinyl-substituted benzophenone (220, Scheme 30) and exhibits aggregation-induced emissiondelayed fluorescence (AIE-DF) and MFC properties. The CIE color coordinates of 220 were directly measured with a non-doped powder, which presented white-emission coordinates (0.33, 0.33) at 244 K to 252 K and (0.35, 0.35) at 298 K. The asymmetric donor–acceptor–donor′ (D−A−D′) type of 220 exhibits an accurate inherited relationship from dicarbazolyl-substituted benzophenone (219, D−A−D, Scheme 30) and diphenothiazinyl-substituted benzophenone (221, D′−A−D′, Scheme 30). By purposefully selecting the two parent molecules, that is, 219 (blue) and 221 (yellow), the white-light emission of 220 can be achieved in a single molecule. This finding provides a feasible molecular strategy to design new AIE-DF white-light-emitting organic molecules. The MFC properties of 220 were studied by PL and PXRD. The results indicated that the original 220 showed a dual emission of 456 and 554 nm and the crystalline component was gradually obliterated with increasing grinding time, whereas the corresponding PL intensity of blue emission simultaneously declined. The blue emission of 456 nm was ascribed to a crystallographic state and the yellow emission of 554 nm was attributed to an amorphous state.

Scheme 30. Molecular structures of compounds 217-221. 244 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Figure 6. (A) PL spectra of luminogen 218 at different ground time. Excitation wavelength: 350 nm. (B) Corresponding CIE chromaticity coordinates in CIE-1931 chromaticity diagram of the 218 samples at different ground time. Adapted with permission from ref. (124). Copyright 2015Wiley-VCH.

Based on the above molecular design strategy through the molecular heredity principle for white-light emission molecules, Chi and Xu’s group (126) obtained another white-light emission molecule 224 (Scheme 31) with high contrast MFC and thermally activated delayed fluorescence (TADF). The symmetric compounds 222 and 223 (Scheme 31) are the parent molecules. 222 is a luminogen with impressive deep blue emission, whereas 223 has been reported to yield greenish-yellow TADF (127). The results reveal that both of 222 and 223 are MFC luminophores. Their offspring, 224, exhibits remarkable and linearly tunable MFC and bright white-light emission with TADF by fully inheriting the photophysical properties of the parent molecules 222 and 223. The deep blue and the yellow dual-emission of 224 can be assigned to two independent radiative decays of the excited 1CT states for the carbazole and phenothiazine moieties, respectively. In addition, it is proposed that the mechanism of luminochromism for 224 driven by the mechanical force correlates with the conformational planarization of the phenylcarbazole moiety. Such unusual observations have once again demonstrated that creating asymmetric molecules following the principle of molecular heredity holds promise as a strategy for the development of high performance functional materials.




5. Mechanoluminescence Organic materials exhibiting mechanoluminescence (ML) are promising for usage in displays, lighting and sensing. However, the mechanism for ML generation remains unclear, and the light-emitting performance of organic ML materials in the solid state has been severely limited by an ACQ effect. Chi and Xu’s group (128) reported two strongly photoluminescent polymorphs (i.e., Cg and Cb) with distinctly different ML activities based on a TPE derivative 225 (Scheme 31). As an AIE emitter, 225 perfectly surmounted the ACQ, making the resultant block-like crystals in the Cg phase exhibit brilliant green ML under daylight at room temperature (Figure 7). The ML-inactive prism-like crystals Cb can also have their ML turned on by transitioning toward Cg with the aid of dichloromethane vapor. Moreover, the Cg polymorph shows ML and mechanochromism (Cg from 498 to 523 nm, Cb from 476 nm to 523 nm, after grinding) simultaneously and respectively without and with UV irradiation under a force stimulus. It was point out that the different ML and mechanochromism behaviors of Cg and Cb are originated from the difference in conformation, electron distribution, dipole moment and energy level. Chi and Xu’s group (127) reported a series of diphenylsulfone derivativeds end-capped with carbazole and/or phenothiazine. It was found the molecules containing phenothiazine moiety exhibit AIE-active. Luminogen 226 is a normal TADF molecule, however, luminogens 223 and 227 are AIE-TADF molecules (Scheme 31). The asymmetry molecule (227) shows a high photoluminescence quantum yield of 93.3%, which is the highest quantum yield for long-lifetime non-doped emitters. Simultaneously, the compound with asymmetric molecular structure exhibited strong ML without pretreatment in the solid state, thus exploiting a design and synthetic strategy to integrate the features of TADF, AIE, and ML into one compound.

Figure 7. (a) The image of capital letters ‘AITL’ shown through ML of 225 in the dark under the pressure stimulus at room temperature. (b) ML images of 225 in the dark (left) and under daylight (right) at room temperature. (c) Writable mechanochromic fluorescence of 225 demonstrated by capital letters ‘PAIE’ generated with a metal rod. Reproduced with permission from ref. (128). Copyright 2015 The Royal Society of Chemistry. 246 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

6. Other Systems


6.1. Doping System To develop such mechano-responsive luminescent materials, incorporating fluorophores with mesogens, which have good self-assembly ability, via covalent bonds has been proven to be an effective strategy. While synthesis of these fluorescent molecules is usually difficult and tedious, physically doping a small amount of fluorophores into mesomorphic media can be a more convenient way to prepare MLC materials. Chen et al. (129) reported a mechano-responsive luminescent system composed of a crown ether derivative (molecule 228) doped with AIE luminogen 229 (Scheme 32). This mixture exhibits luminescent intensity response upon mechanical stimuli, which takes advantage of the metastable melt to stable crystalline phase transition of 228 by shearing. In comparison to the usual way of incorporating fluorophores with mesogens via covalent bonds and the method of taking advantage of assemblies or chemical reaction of fluorescent molecules, this investigation focuses on the change in local “viscosity” around fluorophores doped in a mesomorphic matrix. This study provides a facile way for developing mechano-responsive luminescent materials with processable and reproducible advantage.


6.2. Mechano-Memory Chromism Hu et al. (130) chose 1,6-hexamethylene diisocyanate (HDI) and 1,4-butanediol (BDO) as diisocyanate and chain extender to form hard segment (HSC=25%). The TPE-diol (230, Scheme 32) was directly connected to the polymer backbone (other than by physically mix) to increase the luminogen/matrix compatibility and the reliability of the performance in case the solvent extraction of the TPE unit is out of polymer film. This material is made of shape memory polyurethane with TPE units (0.1 wt%) covalently connected to the soft-segments (PCL, Mw = 54000). The material displays biocompatibility, shape fixity of 88–93%, and almost 100% shape recovery as well as reversible mechanochromic, solvatochromic, and thermochromic shape memory effect (Figure 8). 247 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Figure 8. A model to illustrate the molecular mechanism during, stretch-recovery process (a), heating-cooling process (b), solvent-dry process (c), and solvent induced shape recovery process (d). Reproduced with permission from ref. (130). Copyright 2014 Wiley-VCH.

7. Conclusion Lots of new mechano-responsive AIE luminogens have been designed and synthesized after the general feature of mechano-responsive emission was recognized for most AIE luminogens in 2011. In this chapter, we will briefly review the latest progress after 2012 on such rapidly developing field due to the limitation. The emission variation of these fuctional materials respond to external forces are comprehensive and systemic summaried. And the emphasis is focused on the relationship between the molecular structure and mechano-responsive luminescence property. We hope such short chapter may provide a clear panorama of these novel functional materials for different people and a guidance on design mechano-responsive AIE luminogens with various characteristic.

Acknowledgments This work was financial supported from NSFC (51473185), 863 Program (SS2015AA031701), Science and Technology Planning Project of Guangdong (2015B090913003 and 2015B090915003), the Fundamental Research Funds for the Central Universities and CSC.

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