Multiple Stimuli Responses of Stereo-Isomers of AIE-Active

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Multiple Stimuli Responses of Stereo-Isomers of AIE-Active Ethynylene-Bridged and Pyridyl-Modified Tetraphenylethene Zhaoyang Wang, Xiao Cheng, Anjun Qin, Haoke Zhang, Jing Zhi Sun, and Ben Zhong Tang J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b10929 • Publication Date (Web): 15 Jan 2018 Downloaded from http://pubs.acs.org on January 15, 2018

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The Journal of Physical Chemistry

Multiple Stimuli Responses of Stereo-Isomers of AIE-Active Ethynylene-Bridged and Pyridyl-Modified Tetraphenylethene Zhaoyang Wang†, Xiao Cheng†, Anjun Qin‡, Haoke Zhang§, Jing Zhi Sun*,†, Ben Zhong Tang*,†,‡,§ †

MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China

‡

Guangdong Innovative Research Team, SCUT‐HKUST Joint Research Laboratory, State Key Laboratory of Lumi‐ nescent Materials and Devices, South China University of Technology, Guangzhou, Guangdong 510640, China

§

Department of Chemistry, Hong Kong Branch of Chinese National Engineering Research Center for Tissue Resto‐ ration and Reconstruction, Division of Life Science, State Key Laboratory of Molecular Neuroscience, Institute for Advanced Study, Institute of Molecular Functional Materials, Division of Biomedical Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China

ABSTRACT: Luminescent molecules with aggregation‐induced emission (AIE) property, or AIE‐gens are typical stimuli‐ responsive materials and many AIE‐gens have shown luminescent responses to mechano‐, thermo‐, electro‐, vapo‐ and/or solvato‐stimulus, but the detailed structure‐property relationship has been addressed for only few of them. Here, we re‐ port a tetraphenylethene (TPE) derivative with pyridyl modifiers and ethynylene bridges. The (Z)‐ and (E)‐isomers are clearly purified and both of them are AIE‐active and demonstrate multiple luminescent responses to external stimuli. Dis‐ tinct from other reported TPE‐derivatives, the two isomers show negative solvatochromism due to the large dipole in the ground electronic state. By correlating with the single‐crystal structures, the subtle differences in quantum efficiency and emission peak wavelength of the solids of the (Z)‐ and (E)‐isomers are rationally explained. Moreover, the ground powder of the (E)‐isomer can recover its emission color from green to blue in the air at room temperature but the (Z)‐isomer can‐ not. This difference is interpreted by a mechanism of water‐triggered conformational variation, which depends on the hydrogen bond formation between pyridyl moieties and water molecules in the air. In addition to the reversible emission color changes by cyclic grinding‐fuming treatments, both of the isomers exhibit reversible luminescent response to acid‐ base treatments by switching the emission color between green (basic) and yellow (acid), owing to the incorporation of pyridyl units into the molecule. The unprecedented multiple stimuli‐responsive behaviors and clear mechanism explana‐ tions allow this kind of AIE‐gens to be promising smart materials.

INTRODUCTION In recent years, luminescent stimuli‐responsive materials have attracted wide attention due to the intrinsic ad‐ vantages such as high sensitivity, color tuning ability, and visualization capacity, thus they have found applications in many areas including sensing, optical recording, dis‐ playing and lighting materials and devices.1-6 Classical luminescent molecules are planar conjugated compounds and they emit brightly as dispersed in the inert matrix at the single‐molecular level or dissolved in dilute solution, but their emission is evidently weakened or even totally quenched in aggregates. This is the notorious aggrega‐ tion‐caused quenching (ACQ) phenomenon.7,8 In 2001, Tang et al. reported a brand‐new luminogen exhibiting intriguing aggregation‐induced emission (AIE) property, where the naturally spontaneous process of aggregation

played a helpful role in fluorescence.9 The AIE concept, since then, leads a new way towards the design and syn‐ thesis of luminescent molecules and macromolecules that work efficiently in solid and aggregation states. The underlying mechanisms for the AIE phenomena have been systematically addressed in recent reviews.10,11 Take tetraphenylethene (TPE)12 as an example, as depict‐ ed in Scheme 1, it is composed of a central junction (vinyl) and four peripheral aromatic units (phenyl rings). The central double bond can be excited to a single‐bonding state (Figure S1) and in dilute solutions the exciton energy can be non‐radiatively exhausted by intramolecular rota‐ tions and vibrations. In aggregates, the emission is in‐ duced by the restricted intramolecular motion (RIM) as well as the heavily twisted conformation which hinders intermolecular π−π stacking.

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The propeller shape, twisted conformation and high degree of intramolecular rotation and vibration freedoms make TPE and its derivatives quite soft, thus they are sen‐ sitive to external stimuli. Moreover, the responses from the AIE‐active materials can be embodied by the variation of emission features such as efficiency (), wavelength (), and life time ().10‐20 Mechano‐luminochromism (MLC), piezoluminochromism (PLC), vapoluminochromism (VLC) and solvatoluminochromism (SLC) have been frequently observed for the AIE‐active luminogens. The earliest re‐ ported stimuli‐responsive AIE molecule could be dated from 2005. 1,1,2,3,4,5‐Hexaphenylsilole, which evidently changed its emission intensity by fuming the solid with solvent vapors (VLC).16 For TPE derivatives, the first dis‐ covery of VLC phenomenon was reported by Dong and coworkers in 2007.17 The first record of TPE derivative showing MLC behavior was reported by Zhang and Chi et al. in 2011.18 Since then, a large number of TPE derivatives have been found to possess MLC property and a compre‐ hensive review on this specific topic was given by Chi, and Weder and Kato and colleagues.1, 19,20 Misra and colleagues reported a series of TPE derivatives functionalized by phenanthroimidazoles, acenapthene‐quinoxalines, phe‐ nanthrene‐quinoxalines as well as benzothiadiazoles and all these molecules show significant AIE‐activity and MLC behavior with good color contrast.21‐24

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substituted TPE (TPE‐2OXE). (Z)‐TPE‐2OXE isomer solid showed bathochromic emission with a quantum yield 5 times higher than that of (E)‐TPE‐2OXE.27 These differ‐ ences in PL behavior are derived from distinct excited states. More recently, Peng et al. reported the synthesis and characterization of a pair of pure isomers of ureidopyrimidinone‐functionalized TPE ((Z)‐TPE‐UPy and (E)‐TPE‐UPy).28 The isomers showed distinct fluores‐ cence in the aggregate state: (Z)‐ and (E)‐TPE‐UPy exhib‐ ited green and blue emission, respectively. In addition, the functional groups on the TPE core also play crucial role in regulating the stimuli‐response behav‐ iors of the AIE‐active materials. For example, the tenta‐ tive structure‐property relationship for the MLC behav‐ iors have been summarized in some reviews and book chaptors.1, 18‐20 Modifying a TPE core with electron donor (D) and electron acceptor (A) groups could offer the TPE derivatives with pronounced SLC effect and red‐ shiftedMLC behavior. Moreover, introducing strong in‐ termolecular interactions will recede the fluorescent re‐ sponse to applied mechanical stimulus, as has been con‐ firmed by experimental results observed for the pyridini‐ um‐modified TPE molecules.29 Reasonable modification of the TPE core with reactive groups allows the TPE‐ derivatives to be used as fluorescent sensors, based on the chemical reaction‐induced variation of emission features. These TPE‐derivatives fluorescently respond to the chem‐ ical stimulations from external species such as metal cati‐ ons, hazardous anions and biological analytes. For exam‐ ple, (Z)‐TPE‐UPy exhibited highly sensitive detection of Hg2+ using the cavity formed by the two UPy moieties; while for (E)‐TPE‐UPy, the oppositely extended two UPy moieties allowed to form high‐molecular‐weight poly‐ mers by self‐assembling and to fabricate highly fluores‐ cent fibers.24 Scheme 2. Synthetic route to the (Z)‐ and (E)‐isomers of ethynylene‐pyridyl substituted TPE derivative.

As shown in Scheme 1, an intrinsic problem of the well‐ established McMurry coupling route is that it always pro‐ duces a pair of cis‐(Z)/trans‐(E) isomers when products are asymmetrically substituted. In many cases, they are impossible to be neatly purified when TPE‐derivatives are used as AIE‐gens (AIE‐active luminogens = AIE‐gens). However, a subtle structural difference may result in non‐ trivial responses to external stimuli. In our previous works, the pure (E)‐isomer of 1,2‐bis{4‐[1‐(6‐phenoxyhexyl)‐4‐ (1,2,3triazol)yl]phenyl}‐1,2‐diphenylethene demonstrated stronger MLC and SLC effects, better self‐organization ability to form ordered microstructure, and higher ther‐ mal stability than the (Z)‐isomer.25 Liu and colleagues reported the tautomerization‐induced isomerization of a hydroxy‐substituted TPE‐derivative.26 After replacing the proton of the hydroxy with alkyl groups, pure (E)‐ and (Z)‐isomers were obtained and they emitted red and yel‐ low fluorescence in DMSO at −20 °C, respectively. Wu et al. reported the pure stereoisomers of an oxetane‐

Here, we report the facile synthesis, clear purification, structural characterization, AIE‐activity and fluorescent responses to external stimuli including mechanical force, solvent polarity, and acidic/basic vapors of a pair of (E)‐ and (Z)‐ isomers of a TPE derivative functionalized by two ethynylene‐pyridyl groups (see Scheme 2). The rationale of the molecular design is to enlarge the differences in molecular shape and dipole moment.21,25 In the present work, the TPE core ensures the derivatives to show AIE

Scheme 1. McMurry coupling route towards tetra‐ phenylethene (TPE) and its derivatives. R1

R1

O

R1

TiCl4 /Zn, THF

R2 McMurry Coupling

R2 R1 = R2 = H tetraphenylethene (TPE)

+

reflux

R2

R1

R2 cis(Z)-isomer

R2

R1

trans(E)-isomer mixture


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activity, the ethynylene‐pyridyl groups bestow the (E)‐ /(Z)‐ isomers with pronounced differences in shape and dipole moment. Furthermore, the pyridyl group functions as an electron‐withdrawing group (acceptor), a proton acceptor (alkaline), a hydrogen‐bond formation site (N‐ atom), and a polar group (hydrophilicity). Moreover, the asymmetric substitution of the isomers leads to different molecular packing modes in the single crystals and dis‐ tinct responses to external stimuli.

RESULTS AND DISCUSSION Synthesis and purification of the stereo-isomers. According to the above‐mentioned design rationale, we synthesized 1,2‐diphenyl‐1,2‐bis(4‐(pyridin‐4‐yl‐ethynyl)‐ phenyl)‐ethene following the typical procedure of the palladium‐catalyzed Sonogashira coupling (see Scheme 2), which is frequently used in the synthesis of conjugated molecules for its high yield with hardly any by‐products. The reaction went on smoothly and the gross yield of the resultant mixture was as high as 86%. The synthetic pro‐ cedures and structure characterization data are described in Experimental Section and Electronic Supplementary Information (ESI, Figure S2 to S9), and the results indi‐ cate that target resultants have been obtained. More im‐ portantly, the strong electron‐withdrawing nature of the pyridine functionality renders significant difference in the polarity between the (E)‐ and (Z)‐isomers. At the same time, the substitution of 4‐pyridin‐ethynyl enlarges the geographical asymmetry of the two isomers. These struc‐ tural factors make them separate well with each other on the thin layer chromatography (TLC) plate. As a result, g a b

c

f

d

e

N

N

* (a) 1 a

b c

e,g

d f

*

(b) 2

d c a

8.60 8.55

b

f e,g

7.35 7.30 7.25 7.20 7.15 7.10 7.05 7.00 Chemical Shift (ppm)

Figure 1. 1H NMR spectra of components 1 and 2 (in CDCl3, 400 MHz). The solvent peaks are marked as asterisks.

yield of 34% for component 1 [named as 1 in below, Rf = 0.44, PE (petroleum ether): THF (tetrahydrofuran) = 3:1] and 44% for component 2 (named as 2 in below, Rf = 0.54, PE: THF = 3:1), respectively. The variation in Rf value helps with the attribution of the (Z)‐/(E)‐configurations to these two components. The component with higher polarity has stronger interaction with silica gel, and will run more slowly in a TLC experi‐ ment, ending up with a smaller Rf value. The (Z)‐isomer, with its two ethynylene‐pyridyl groups pointing to the same side, tends to have a higher polarity than the (E)‐ isomer, in which the direction of its ethynylene‐pyridyl groups is almost opposites to each other. Therefore, we tentatively assigned 1 and 2 to the (Z)‐ and (E)‐isomers, respectively. Both components were characterized with 1H NMR spectroscope and similar profiles were observed. However, a careful inspection reveals that chemical shifts of Hc, Hd for 1 are downfield‐shifted, while He, Hf, and Hg are up‐ field‐shifted from its counterpart 2. The coupling con‐ stants of Hb for 1 and 2 are comparative to each other, and so for Hc. Considering that the pyridine moiety has an electron‐withdrawing effect, the non‐functionalized phe‐ nyl should be more electron‐rich and causes an up‐field shift in the chemical shifts of the vicinal phenyl rings, and thus 1 can be assigned to a cis‐configuration, in accord‐ ance with our assignment based on TLC experiments.

Crystallographic data analysis. To confirm our tentative assignment on the basis of the TLC experiment and 1H NMR measurement, we tried to grow single crystals of these two isomers and both were obtained by the slow evaporation of hexane into the di‐ chloromethane solution of the products. As displayed in Figures 2(a) and 2(b), the single‐crystal X‐ray diffraction (XRD) data provide unambiguous proofs that 1 and 2 are the (Z)‐ and (E)‐isomers, respectively. It’s worth noting that their crystal structures for 1 and 2 are obviously different. Firstly, 1 and 2 belong to different crystal systems. The single crystal of the (Z)‐isomer 1 is an orthorhombic system in a pccn space group, and there are 8 molecules in a unit cell while the single crystal of 2 be‐ longs to a triclinic system in a p‐1 space group, and only two molecules occupy a unit cell (Figure 2(c) and 2(d)). Secondly, the intermolecular interactions in the single crystal of 1 are more complicated than those found in the single crystal of 2. Interactions of CH…(phenyl ring), CH…(C≡C) and CH…N (hydrogen bond) can be found in the single crystal of 1, while in that of 2, only CH…(phenyl ring) and CH…(C≡C) are observed. As displayed in Figures 2(e) and 2(g), a pair of CH…N interaction modes can be found and the nearest distance between the N and H atoms on the two pyridyl groups of the adjacent two (Z)‐isomer molecules is 2.618 Å and the other is 3.098 Å, indicating that the CH…N interactions are quite strong. Meanwhile, a pair of CH…(C≡C) interactions are observed between the H atoms on the labelled phenyl ring and the center of the labelled C≡C



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Figure 2. Single crystal structure of (Z)‐isomer and (E)‐isomer. (a) and (b) Conformation of an individual molecule in crystals 1 and 2 respectively. The aromatic rings colored in blue and red indicate the two groups of phenyl and pyridyl rings correlated with ethynylene bridges, and the data correspond to the dihedral angles between the correlated phenyl and pyridyl rings. (c) and (d) Molecular packing modes in the unit cells of crystals 1 and 2, the axes of a, b, c are shown as red, green and blue lines, respectively. (e) and (f) showing the intermolecular interactions in crystals 1 and 2, the inter‐ atom distances are highlighted in dotted green lines. (g) A highlight for the strong CH…N hydrogen bonding interac‐ tions existing in the crystal of (Z)‐isomer. (h) Cell parameters of crystals 1 and 2. In the stick model, C, H, and N atoms are colored in grey, white and light blue respectively. The solvent molecules are eliminated for showing the skeletons clearly.

bond and the distances are 2.805 Å and 3.290 Å, implying the co‐existence of a strong and a weak interaction. There also exist a pair of strong CH…(phenyl ring) interac‐ tion between the adjacent TPE units with an identical distance of 2.906 Å. In contrast, the strongest CH…N interaction has not been found in the single crystal struc‐ ture of (E)‐isomer (2). A pair of CH…(C≡C) interac‐ tions with the same distance of 3.015 Å are observed, which is weaker than that observed in the crystal of 1 (2.805 Å). The corresponding CH…(phenyl ring) inter‐ action distance is 2.881 Å, a little shorter than that found in the crystal of 1 (2.906 Å) (Figure 2(f)). In general, the intermolecular interactions in the crystal of (Z)‐isomer are substantially stronger than that in crystal of (E)‐ isomer, which may have subtle impacts on the stimuli‐ responsive properties of the isomers. Thirdly, the conformation of the TPE core in the single crystals of 1 and 2 has small difference, despite the dra‐ matic difference in molecular shape. Taking the C=C double bond as the basic plane, for the (Z)‐isomer, the dihedral angles between this plane and the four phenyl rings of 1, 2, 3 and 4 are 51.69o, 48.75o, 37.68o and 58.59o, respectively. The corresponding dihedral angles for the (E)‐isomer are 48.18o, 57.13o, 50.88o and 50.08o respectively, as illustrated in Figure S10 and the data in Table S1. In

contrast, more twisted conformations are observed for the two pyridyl rings against the ethynylene‐linked phenyl rings. In the single crystal of 1, pyridyl ring A is nearly perpendicular to phenyl ring 2 (dihedral angle of 85.30o), while the other is almost coplanar with its ethynylene‐ linked phenyl ring 4 (dihedral angle of 13.30o). But in the single crystal of 2, both pyridyl rings A and B have large dihedral angles to the ethynylene‐linked phenyl rings 2 and 4, which are 55.27o and 71.42o, respectively. These structural differences will manifest effects on the stimuli‐ responsive properties of this couple of isomers. Lastly, it should be pointed out that the hexane mole‐ cules are eliminated for showing the skeletons clearly. In fact, both of 1 and 2 contain hexane molecules inside the crystals. According to the single crystal analysis results, the molecular ratio between the (Z)‐isomer and hexane is 1:1, and this ratio is in good agreement with the 1H NMR measurement data (Figure S5). As indicated by the rhom‐ bus frame in Figures 2(e) and 2(g), the hexane molecules localize in the one‐dimensional channels formed by pairs of two adjacent (Z)‐isomers, as displayed in Figures S11 and S12. This is further confirmed by thermal gravimetric analysis (TGA) measurement results (Figure S13), the ~10% weight loss at 170 0C is coincident with the 4:1 molecular ratio, as calculated out according to their relative molecu‐


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lar weights. For the single crystal of the (E)‐isomer, the 1H NMR measurement (Figure S2) disclosed that certain amount of hexane solvent existed in the single crystal sample, despite of the ratio between solvent and (E)‐ iso‐ mer molecules is not an integrity. The TGA experiment results confirmed the releasing of solvent component by the weight loss at around 144 oC and 260 oC (Figure S13).

Aggregation-induced emission. The TPE unit is a representative AIE‐gen, as mentioned above, and a variety of TPE‐derivatives have shown to be AIE‐active molecules. It is rational to examine the photo‐ physical properties of the (Z)‐ and (E)‐isomers to identify whether they are AIE‐active or not. We measured the UV‐ vis absorption spectra of these two isomers in their dilute ethanol solutions (1×10‐5 mol L‐1) and the spectral profiles exhibit three bands, 205, 281, 347 nm for (Z)‐isomer and 204, 289, 345 nm for (E)‐isomer (Figure S14). These data indicate that these two stereo‐isomers have nearly identi‐ cal ground electronic state. Optically excited with 345 nm UV‐light (ex), we exam‐ ined the photoluminescence (PL) behaviors of (Z)‐ and (E)‐isomers. In dilute ethanol solution (1×10‐5 mol L‐1), only faint fluorescence can be detected for (Z)‐isomer. The PL intensity begins to increase when the volume frac‐ tion of the non‐solvent water (fW) increases to 65%. When fW goes up to 70%, the PL becomes intense, a clear emis‐ sion profile peaked at 480 nm can be recorded. The strongest PL intensity is observed when fW is at 80%, a 63‐ fold enhancement as compared to the single‐molecule state (AIE). A slight drop in PL intensity occurs when goes further to 90%, which can be associated with the formation of precipitates at higher fW conditions. Similar phenomena were observed for (E)‐isomer in water/ etha‐ nol solvent systems, except for its higher threshold of water fraction (fW = 70%) and comparatively lower PL enhancement (AIE = 47) (Figure S15(b)). According to the results demonstrated in Figures 3 and S15, both (Z)‐ and (E)‐isomers are typical AIE‐active molecules. The PL spectra of the solids of (Z)‐ and (E)‐isomers are shown in Figure 4. The powders of (Z)‐ and (E)‐isomers are brightly emissive and the emission peaks (em) appear at 470 and 464 nm, respectively. The longer em of the (Z)‐ isomer relative to that of (E)‐isomer can be correlated with the crystallographic data, the coplanar of the pyridyl ring B and phenyl ring 4 (Figure 2(a) and Table S1) allows better conjugation between the pyridyl ring and the TPE core, which enlarges the conjugation and thereby red‐ shifts the emission spectrum. In addition, pyridine is an electron‐deficient group with a lone electron pair, it re‐ sults in the *←n transition, or other proximity electronic effects.30,31 Thus the conjugation between the pyridine and TPE moieties would reduce the PL efficiency. In fact, the absolute PL quantum yields for (Z)‐ and (E)‐isomers in solid state are 37. 6% and 43.8%, respectively.

Figure 3. Normalized absorption (a) and PL (b) spectra of (Z)‐isomer in different solvents with variation of polarity. Insets of (b) display the fluorescence images in CH (right) and THF (left) taken under UV‐light (ex = 365 nm). CH = cyclohexane, DMF = N,N‐dimethylformamide, EA = ethyl acetate, MCH = methylcyclohexane, THF = tetrahydrofuran. [M] = 10‐5 mol L‐1; ex = 345 nm.

Negative solvatoluminochromism. When a luminogen dissolved in solvents of varying polari‐ ty manifests evident changes in emission wavelength (em) and intensity (I), the phenomenon is named as solvato‐ luminochromism (SLC). The bathochromic and hypso‐ chromic shift with increasing solvent polarity are called positive and negative SLC, respectively. SLC phenomenon has been observed for many AIE‐gens modified with elec‐ tron donor (D), acceptor (A) or simultaneously with D and A groups.32‐37 Pyridyl group is considered as an A unit, thus both of the (Z)‐ and (E)‐isomers are expected to ex‐ hibit SLC behavior. We measured the UV‐vis and PL spec‐ tra of the two isomers in a series of solvents, from non‐ polar solvent pentane to high polar solvent DMF. For (Z)‐ isomer, the absorption peak with the lowest transition energy in these solvents varies slightly from 342 to 346 nm (Figure 3(a), Table S2). However, the emission peak shifts pronouncedly. In non‐polar solvents such as pentane and hexane, the PL spectrum of (Z)‐isomer peaks at 530 nm. When the polarity of the solvent goes higher, e.g., in di‐ ethyl ether, the emission peak appears at 505 nm. In DMF, the highest polar solvent in the detection list, the emis‐ sion peak shifts to 499 nm (Figure 3(b)). The change in emission color was recorded by photographs displaying in Figures 4(b) and S16, faint blue‐greenish and weak yellow fluorescence can be observed for polar and non‐polar sol‐ vents respectively. Similar absorption and emission be‐ haviors were also observed when it comes to (E)‐isomer, as displayed in Figure S18. The phenomenon observed for (Z)‐ and (E)‐isomers is a typical negative SLC, which has not been previously ob‐ served for AIE‐gens. The negative SLC is believed to orig‐ inate from the decrease in di‐polarity of the luminogen after photo‐excitation, and this phenomenon usually oc‐ curs to luminogens with polarity‐dependent tautomeric structures (e.g., keto‐enol transformation) and mero‐ cyanine dyes (containing C=N or N=N units).38‐42 To give a specific and better understanding of the negative SLC behavior of (Z)‐ and (E)‐isomers, we simulated the



electronic structure of them based on the single crystal data, and calculation results of the HOMO and LUMO are displayed in Figures S19 and S20. In the ground state (HOMO as the representative), the two pyridyl units con‐ jugate with the TPE core inefficiently, and they play the role of electron acceptor. In excited state (LUMO as the representative), in contrast, the two pyridyl units fully conjugate with the central TPE core, the lone electron pairs on the two N‐atoms participate into the LUMO. As a result, the pyridyl units largely lose their electron‐ acceptor function but become conjugation moieties. Therefore, the dipole strength of the excited state is lower than that of the ground state and a negative SLC phe‐ nomenon comes into sight. In the (Z)‐ and (E)‐isomers, the ethynyl bridges also play an important role of partially blocking the conjugation between the pyridyl and TPE‐ core. Accordingly, the negative SLC can be explained in the theoretical frame of the fragment model proposed by Benson and Murrell,41 and later summarized by Buncel and Rajagopal.42

(Figure 4(b)). The powder sample of (Z)‐isomer also un‐ derwent a spectral red‐shift induced by grounding treat‐ ment (em, from 474 to 506 nm). Similar to its (E)‐ counterpart, the green fluorescence of the ground powder could be switched to blue by fuming with DCM vapor, and the process is also reversible and repeatable in con‐ tinuous grinding‐fuming treatments (Figure S21). To elucidate the mechanism of the MLC phenomena, we conducted X‐ray diffraction (XRD) measurements for the as‐prepared, ground and fumed powders of both iso‐ mers. As shown in Figure 5(a1), the XRD pattern of as‐ prepared (Z)‐isomer powder features a series of sharp diffraction peaks. The pattern suggests that it is in a dif‐ ferent state in comparison with the single crystal simula‐ tion data (Figure 5 (d1)). The mismatch between them can be ascribed to the existence of hexane molecules inside the crystal of (Z)‐isomer (Figure S12), which are partially or totally absent in the ground powder. The grinding pro‐ cess led to crystal smash, part of the crystal was trans‐ formed into amorphous solid, and the other part became

Moisture-sensitive mechanoluminochromism. Mechanoluminochromism (MLC) is one of the character‐ istics of TPE‐derivatives.1,10‐21,43‐49 Linking two ethynyl‐ pyridyl groups to the TPE core extends the molecular size and enlarges the molecular asymmetry. Accordingly, it is expected that both the (Z)‐ and (E)‐isomers possess MLC property. Indeed, the experiment results shown in Figures 4 and S19 reveal that the solid samples of the (Z)‐ and (E)‐ isomers can fluorescently respond to mechanical force, and the (E)‐isomer exhibits a larger red‐shift in em as compared with the (Z)‐isomer. The as‐prepared solid of (E)‐isomer emitted strong blue light with em at 464 nm. After it was ground with a pestle, the em red‐shifted to 504 nm. The emission color changed from blue to green (inset of Figure 4(a)). The blue emission was restored immediately after it was fumed by dichloromethane (DCM) vapor. The green blue emission color change is reversible by grinding‐fuming treatment for several cycles

(Z)-isomer (a1) as-prepared (b1) ground (c1) fumed

(d1) simulated 5

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(E)-isomer (a2) as-prepared (b2) ground (c2) fumed

(d2) simulated 5

10

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25

2(degree)

Figure 4. (a) Normalized PL spectra of the solid sample of the (E)‐isomer under different treatments. Inset: photo‐ graphs of the fluorescence images for as‐prepared, ground, and fumed powder sample taken under UV‐light (ex = 365 nm). (b) Switching of the solid‐state emission of the (E)‐ isomer between blue and green by repeated grinding‐fuming process. ex = 345 nm.

30

35

Figure 5. X‐ray diffraction patterns for (Z)‐ and (E)‐isomers measured in different states; (a1) and (a2) as‐prepared solid powder, (b1) and (b2) after grinding treatment, (c1) and (c2) after fuming treatment on the ground sample with DCM vapor, and (d1) and (d2) simulated diffraction pattern based on single crystal data. The red dashed‐dot lines indicate the corresponding diffraction peaks for different treatments.


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H H

H H H

H (a) Free rotations very flexible

R

H H

H H R

H H R

(b) Partially free rotations (c) Reduced free rotation flexible less flexible

Figure 6. An illustration for pyridyl‐functionalized TPE de‐ rivatives with different bridging groups (a) ethynylene bridge, (b) vinylene bridge and (c) single‐bond bridge.

microcrystal, as manifested by the weakened and broad‐ ened diffraction peaks and the blunt bump around 2 of 20o (Figure 5(b1)), such as the peaks standing for the 210 , 310 , 421 and 042 lattice plane. The amor‐ phous solid contributes to the green fluorescence. After fuming by DCM vapor, the amorphous solid was trans‐ formed into crystal again, the XRD pattern was largely recovered, as compared the XRD patterns in Figure 5 (a1) and (c1). Similar variations can be observed for the grind‐ ing‐fuming treatments to the as‐prepared powder of the (E)‐isomer (Figure 5(a2)‐(c2)). While the peaks for the ground powder of the (E)‐isomer are still discernible, such as the ones for the 101 , 111 , 120 and 030 plane, they are severely weakened and broadened. It can be found that the diffraction angles (2) have on‐ ly subtle shifts in the as‐prepared solid, ground powder and solvent‐fumed samples, if the attention was paid to the details of XRD patterns in Figure 5, as indicated by the red dash‐dot lines. These observations are quite dif‐ ferent from that observed for many TPE‐derivatives show‐ ing evident MLC behavior. Since the mechanism for the MLC phenomenon is ascribed to the mechanical‐force induced changes in molecular packing modes, evident changes in XRD patterns have been usually detected in those reported MLC‐active TPE‐derivatives.11‐15,25,46,50‐54 The “abnormal” observations in the present case can be tentatively explained as below. Based on the single crystal data, the crystals of the (Z)‐ and (E)‐isomers belong to different systems, but the conformations of the TPE core are similar to each other. The molecular packing modes in the unit cell, as well as the emission peaks depend mainly on the ethynylene‐pyridyl moieties. The small dihedral angle and the better conjugation between the pyridyl ring B and the phenyl ring 4 (Figure 2(a)) render the crystal of (Z)‐isomer (em = 474 nm) emits longer wavelength than the crystal of (E)‐isomer (em = 464 nm). In the literature‐ reported TPE derivatives,25 the TPE‐core are linked to the modifiers with C=C double bonds and/or phenyl rings as schematically illustrated in Figure 6 (b) and (c), the or‐ tho‐positioned CH bonds (colored in blue and green) hinder the free rotations of the phenyl rings on TPE core against the C=C double bonds and/or phenyl rings. As a result, the whole molecule should adjust its confor‐ mations to respond to the applied mechanical stimulus

(grinding or pressing). Distinctly, in the present case of (Z)‐ and (E)‐isomers, the ethynylene bridge has a cylin‐ drical shape but not any CH bonds to hinder the free rotations of the phenyl and pyridyl rings (Figure 6 (a)). Consequently, the molecules can respond to the applied mechanical stress by flexibly adjusting the conformations of pyridyl rings against the phenyl rings on TPE‐core, ra‐ ther than the whole molecule. The rotational freedom was supported by single crystal analysis result. As shown in Figure S22, the pyridyl ring took multiple conformations even in crystalline state. In experiments, only small changes in XRD patterns and narrow spectral shifts can be found (474 to 506 nm for the (Z)‐ isomer and 464 to 504 nm for the (E)‐isomer). In addition, the quick restoration of the ground solid to crystalline and the green to blue emission by solvent fum‐ ing may be associated with the porous structure of the solids and the increased molecular flexibility. An unex‐ pected observation is that the ground powder of (E)‐ isomer recovered to blue from green fluorescence sponta‐ neously when the sample was kept untouched in air at room temperature (RT,  25 oC) for hours. The fluores‐ cence images for the as‐ground and the aged powder (for 24 h, in air) are shown in Figure 7(a) and the PL spectra were displayed in Figures 4(a) (green line, em = 503 nm) and 9(b) (blue line, em = 474 nm). We firstly associated this observation with the flexibility stemming from the ethynylene‐linkages, which may allow the molecules of the (E)‐isomer in the ground powder to undergo a phase transition at RT. To confirm this assumption, we cooled the ground sample to ‐20 oC and recorded the PL spec‐ trum and fluorescence image. The emission color gradual‐ ly changed from green to blue‐greenish (left arrow, Figure 7(a)) and the em blue‐shiftedfrom 503 to 496 nm (dark yellow line, Figure 7(b)), but did not recover completely in 24 h. These results cannot fully support our assumption, since the emission color variation occurred at such a low temperature. Accordingly, we examined the thermal tran‐ sition of the ground powders of the (E)‐isomer and the differential scanning calorimetry (DSC) curves are depict‐ ed in Figure 7(e). In the first heating run, these is a sharp endothermic transition at 261 oC, but it disappears in the first cooling run at a proper temperature. In the second heating run, the endothermic peak is absent from 10 oC to 300 oC, but a step appears at around 98 oC, which is tenta‐ tively ascribed to the glass transition. The DSC data sug‐ gest that the color variation of ground powder of the (E)‐ isomer is unlikely accompanied with a phase transition, because no transitions have been recorded below 98 oC. At the same time, the absence of the exothermic peak in the 1st cooling run and the endothermic peak in the 2nd heating run implies that the (E)‐isomer might undergo certain unexpected processes in the heating process, for example, thermally‐induced evaporation of the solvent molecules absorbed in the powder sample, or oxygen‐



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Figure 7. The Mechanoluminochromism (MLC) behaviors of (Z)‐ and (E)‐isomers. (a) The fluorescent images showing the color changes of the ground powder of (E)‐isomer upon different treatments (cooling, cooling + aging for 24 hours at RT or ‐20 oC, aging in N2 for 24 hours at RT and ‐20 oC; and in dry air for 24 hours). (b) PL spectra recorded for the ground powder of the (E)‐ isomer after different treatments (aging in ambient air, cooling in air at ‐20 oC, aging in N2 at RT or ‐20 oC, for 24 hours). (c) The fluorescent images showing the color changes of the ground powder of (Z)‐isomer upon different treatments for comparison with (a). (d) PL spectra recorded for the ground powder of the (Z)‐isomer after different treatments. All of the fluorescent imag‐ es were taken under 365 nm UV‐light, and ex for the PL spectra was 345 nm. (e) Differential scanning calorimetry (DSC) curves of the ground powders of the (E)‐isomer in the 1st and 2nd heating and 1st cooling runs at a scanning rate of 10 oC/min.

induced reactions. To exclude the role of oxygen, we ex‐ amined the emission behaviors of the ground powders in N2 atmosphere at RT and ‐20 oC and the results are dis‐ played in Figure 7(a) (right hand arrows) and 9(b) (light and dark green lines). The emission color change from green to blue‐greenish was observed again in N2 atmos‐ phere and cooling treatments. Similar to that observed in the case of cooling in air, the emission color could not completely recover to blue fluorescence as recorded for the crystal of the (E)‐isomer, the emission peaks for the samples, keeping untouched for a day at RT and ‐20 oC under N2 protection, blue‐shifted from 503 to 495 and 493 nm, respectively. The independent of oxygen (exotic species) and incomplete recovery at low temperature hint that the emission color change should be correlated with the internal structure factors. Then, we turned our attention to a special structure factor, i.e. the pyridyl moieties. The data shown in Figure 2(e) and 2(f) indicate that the intermolecular interactions between the pyridyl moieties are quite different for (E)‐ and (Z)‐isomers. Thus we compared the emission behav‐ iors of its stereo‐isomer, or the (Z)‐counterpart under the same experimental conditions. Interestingly, the ground powder of (Z)‐isomer cannot automatically recover its

emission color from green to blue, no matter in air or in N2, at RT or at ‐20 oC, as demonstrated by the photo‐ graphs (Figure 7(c)) and PL spectra (Figure 7(d)). These dramatically different observations suggest us to associate the differences between the ground powder of the (Z)‐ and (E)‐isomers with their distinct intermolecular inter‐ actions. As highlighted in Figure 2(g), there exist a pair of strong CHN interactions between the pyridyl units on the adjacent molecules for the (Z)‐isomer, whereas there are no such interactions in the crystal of (E)‐isomer. The distance of 2.618 Å is much shorter than the critical dis‐ tance of hydrogen bond (3.60 Å). It means that this CHN interaction is a strong hydrogen bond and the other (3.098 Å, Figure 2(g)) is relatively weak. Based on the above experimental data and analyses, the formation of hydrogen bonds comes into the center of the scenario. Without intermolecular hydrogen bonding, the pyridyl groups on (E)‐isomer can form hydrogen bonds with wa‐ ter molecules absorbed from the air. The chemically ab‐ sorbed water molecules play the role of solvent, and the hydroscopic process is a solvent fuming process. At low temperature (‐20 oC) and in N2, water content in the at‐ mosphere is evidently lower than that in ambient air, thus


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the color recovery from green to blue becomes slow, the process cannot finish even after aging for 24 hours (Figure 7 (a) and (b)). Rationally, it can be expected that the color recovery would be totally restricted in fully dried air. To validate the above deduction, we monitored the emission of the ground powder of the (E)‐isomer in fully dried air and the result indicates that the green emission had no change by aging for 24 h (rightmost photographs in Figure 7(a)). In contrast, due to the occupied pyridyl groups by intermolecular hydrogen bonding, the ground powder of the (Z)‐isomer cannot effectively react with water molecules. As a result, no emission color recovery of the ground (Z)‐isomer powders in air was observed, as shown in Figure 7(c) and (d). Reasonably, the ground powder of the (E)‐isomer can be used as a fluorescent reporter to monitor the trace amount moisture in air or other gas mixtures, in the circumstances that the mois‐ ture content needs to be detected. Meanwhile, other kinds of vapors that can form hydrogen bond with the (E)‐isomer may be also fluorescently detected and the quantitative researches are going on in our laboratory.

Reversible Fluorescent Response to Acid/Base. In our molecular design, the pyridyl groups are the func‐ tional units intentionally introduced into the TPE‐ derivative, in order to offer the AIE‐active molecules with fluorescent response to acidic/basic stimuli. AIE‐gens responding to different kinds of acids/bases (such as ami‐ no acids, nuclear acids, picric acid (an explosive) and base pairs) have been also widely studied in recent years.55‐64 We expect our design rationale of combing the proton‐ acceptor property of pyridyl group and the flexibility of the ethynylene bridge enables the isomers to respond to acids more quickly. As indicated by the molecular packing modes in Figure 2, the propeller‐like structure of the TPE core results in the loose molecular packing, and there are free spaces to accommodate the solvent molecules. In long range, the molecules of the (E)‐isomer form a lamella structure, while for the (Z)‐isomer, porous structure can be ob‐ served (Figure S23). These structural features allow suffi‐ cient adsorption and efficient diffusion of the chemical absorbents (solvent and/or gas molecules) in the powder samples. To investigate the potential application of these two isomers as active component of chemical sensors, their thin films were prepared by drop‐casting method. The as‐ prepared solid film of the (E)‐isomer emits blue‐greenish light peaked at 495 nm, suggesting a mixture containing amorphous solid and microcrystalline. When exposed to HCl vapor, its blue‐greenish fluorescence faded out and yellow fluorescence peaked at approximate 537 nm turned on (Figure 8(a)). The blue‐greenish fluorescence restored when the film was fumed by triethylamine (TEA). The variation in emission wavelength in response to the HCl and TEA vapors can be repeated for several times on this very film (Figure 8(b)), and similar cycles can also be conducted for the cast film of the (E)‐isomer (Figure S24).

Figure 8. (a) Normalized PL spectra and variation of the em of (E)‐isomer film fumed by HCl and NEt3 in turn (ex = 345 nm). (b) Reversible change of emission peak wavelength by cyclic treatment of the solid film of the (E)‐isomer with HCl and triethylamine vapors. (c) Chemical structure of com‐ pound 3. Inset images show a piece of filter paper soaked acetone solution of compound 3 taken under daylight (1) and UV‐light (2), respectively; image (3) displays writing letters of AIE on the paper with triethylamine dilute solution under UV‐light. (d) PL spectra of compound 3 in solid film in its original from and fumed by NH3. Inset: Fluorescent images of films 3 in its original state (A), fumed by NH3 (B) and heated after NH3 fuming (C).

These structural essences bestow the solids of the (Z)‐ and (E)‐isomers with swift responses to stimuli. In addi‐ tion, the bright emission in solid states allows the re‐ sponses of AIE‐active isomers to be highly contrastive. The primary success in sensing acid and base vapors by the fluorescent response of the (Z)‐ and (E)‐isomers en‐ couraged us to take a step further. We synthesized com‐ pound 3 by reacting 2 with hexafluorophosphoric acid, a less‐volatile inorganic acid (Figure 8(c)), and the details are described in the Experiment Section (Scheme S1). Compound 3 can dissolve in organic solvents such as eth‐ anol, acetone and THF. As demonstrated by the inset im‐ ages of Figure 8(c), when a filter paper was soaked with acetone solution of 3 and dried, it looked untouched un‐ der daylight, but under UV‐light the paper emitted strong yellow fluorescence. Using triethylamine acetone solution as ink, three letters of “AIE” were written on the paper, the acid‐base neutralization reaction led to the formation of compound 2. As a result, blue emission “AIE” letters were observed under UV‐light illumination. Compound 3 can be fabricated into amorphous film on glass slides with drop‐cast method. The original film of 3 emitted yellow light with an emission peak at 548 nm.



The emission color changed sharply from yellow to green and the em blue‐shifted from 548 to 511 nm when it was fumed by ammonia vapor (Figure 8(d)). Considering that NH3 is a dangerous gaseous industrial exhaust, which not only cause irreversible damage to the natural environ‐ ment, but also do harm to human health, the sharp re‐ sponse of the solid films made of compound 3 provides a proof of concept example of fluorescently detecting NH3 and other gaseous basic pollutes.

EXPERIMENTAL SECTION Materials and Methods. All commercially available chemicals were purchased from AIE‐gen Biotech, J&K chemistry or Sinopharm Chemical Reagent Co., Ltd and used as received. Tetrahydrofuran (THF) was distilled from sodium benzophenone ketyl under nitrogen imme‐ diately prior to use. All 1H and 13C NMR spectra were measured with a Bruker AVANCE III 400 spectrometer using CDCl3 or acetone‐d6 as the solvent, and tetrame‐ thylsilane (TMS) was used as the internal reference. High resolution mass spectra (HRMS) were recorded using a Waters GCT premier mass spectrometer operated in GC‐ TOF mode. UV‐visible absorption spectra were measured with Shimadzu UV‐1800 Spectrophotometer. PL spectra were measured with a RF‐5301 PC spectrofluorometer. The absolute  values were obtained with a Hamamatsu Quantaurus‐QY C11347 spectrometer. Single crystal X‐ray diffraction analysis was conducted on a Gemini A Ultra diffractometer at 293K. DSC curves were measured on the DSCQ 1000 (TA, USA) calorimeter. The electron cloud distributions of HOMO and LUMO of the two isomers were calculated by the B3LYP/6‐31G(d) program. Synthetic procedures of molecules 1 and 2. The starting reagent 1,2‐bis(4‐ethynylphenyl)‐1,2‐diphenyl‐ ethene (BETPE) was obtained following reported proce‐ dure.49 Into a 250 mL two‐necked flask were added 1.14 g (3 mmol) of BETPE and 0.92 g (4.5 mmol) of 4‐iodo‐ pyridine. The flask was then transferred into the glove box. After that, 84 mg (0.12 mmol) of PdCl2(PPh3)2, 46 mg (0.24 mmol) of CuI and 126 mg (0.48 mmol) of PPh3 were added to the system. Then the flask was transferred out of the glove box and 20 mL of NEt3 and 40 mL of dry THF were injected into the system. The mixture was then al‐ lowed to stir at 50 oC overnight. The mixture was filtered after cooled down to room temperature. The filtrate was then evaporated under reduced pressure to remove the solvent to get the crude, which then passed through col‐ umn chromatography with PE:THF = 3:1 as the eluent. (Z)‐1,2‐di‐phenyl‐1,2‐bis(4‐(pyridin‐4‐ylethynyl) phenyl)‐ ethene (1). Yellow powder, yield: 34%. 1H NMR (400 MHz, CDCl3) δ (TMS, ppm) 8.59 (dd, J = 4.6, 1.4 Hz, 4H), 7.35 (dd, J = 4.6, 1.5 Hz, 4H), 7.32 (d, J = 8.3 Hz, 4H), 7.19 – 7.09 (m, 6H), 7.09 – 7.04 (m, 4H), 7.03 – 6.99 (m, 4H). 13C NMR (100 MHz, CDCl3) δ (TMS, ppm) 149.62, 144.62, 142.82, 141.12, 131.46, 127.90, 127.00, 125.54, 120.23, 94.21, 87.12. IR (KBr pellet): 2218 cm‐1 (‐C≡C‐ stretching), 1591

Page 10 of 14

cm‐1 (‐N=C‐ stretching). HRMS (GC‐TOF): m/z 534.2099 ([M]+), calcd. for C40H26N2 534.2096) (Figures S2‐4 and S8). (E)‐1,2‐diphenyl‐1,2‐bis(4‐(pyridin‐4‐ylethynyl)‐ phenyl) ethene (2). Yellow powder, yield: 44%. 1H NMR (400 MHz, CDCl3), δ (TMS, ppm) 8.59 (d, J = 5.5 Hz, 4H), 7.35 (d, J = 5.9 Hz, 4H), 7.30 (d, J = 8.3 Hz, 4H), 7.20 – 7.11 (m, 6H), 7.07 – 7.00 (m, 8H). 13C NMR (100 MHz, CDCl3) δ (TMS, ppm) 149.48, 144.69, 142.79, 141.14, 131.77, 131.36, 128.06, 127.11, 125.57, 120.06, 94.45, 86.99. IR (KBr pellet): 2219 cm‐1 (‐C≡C‐ stretching), 1589 cm‐1 (‐N=C‐ stretching). HRMS (GC‐TOF): m/z 534.2098 ([M]+), calcd. for C40H26N2 534.2096) (Figures S5‐7 and S9). Synthesis of (E)‐4,4'‐(((1,2‐diphenylethene‐1,2‐diyl)‐ bis(4,1‐phenylene))bis(ethyne‐2,1‐diyl))bis‐(pyridin‐1‐ium) hexafluorophosphate (3). The synthetic route is shown as Scheme S1, and the experimental procedures are de‐ scribed below. 50 mg of 2 was dissolved in metha‐ nol/DCM mixture (1:5 in volume). 3 mL of 65% hexafluor‐ ophosphoric acid was added. The reaction mixture was the allowed to stir for half an hour at room temperature. Then the solvent was evaporated and a suspension was obtained, which was then filtered to collect the solid as the product. 1H NMR (400 MHz, acetone‐d6) δ (TMS, ppm) 9.09 (d, J = 6.7 Hz, 4H), 8.44 (s, 2H), 8.30 (d, J = 6.7 Hz, 4H), 7.53 (d, J = 8.3 Hz, 4H), 7.27 – 7.16 (m, 10H), 7.12 (dd, J = 6.6, 3.0 Hz, 4H). 13C NMR (100 MHz, acetone‐d6) δ (TMS, ppm) 147.36, 143.36, 142.94, 142.76, 142.54, 133.00, 132.54, 132.02, 129.93, 129.07, 128.24, 119.40, 104.14, 86.64. 19F NMR (376 MHz, acetone‐d ) δ (TMS, ppm) 27.27 (d, J = 6 23.3 Hz), 26.32 (d, J = 19.9 Hz) (Figures S25‐27).

CONCLUSIONS Novel TPE derivatives have been derived in high yield of 86% through Sonogashira coupling reaction and the (Z)‐ and (E)‐isomers can be purified. The availability of the pure stereo‐isomers and single crystal data allow us to clearly address the relationship between the chemical structure and photophysical properties. Both of the two isomers are AIE‐active molecules, or AIE‐gens. Owing to the two pyridyl moieties, the two isomers show negative solvatoluminochromism. These two isomers show re‐ versible and repeatable responsiveness to the stimulus of protonic acids. More importantly, the solid of (E)‐isomer exhibits unprecedented “auto‐recovery” MLC property, due to the absence of the strong CHN interactions. The ethynylene linker between the TPE core and pyridyl unit also plays an important role. The linear shape and cylindrical symmetry provide large rotational free‐ dom for the molecules. Thus, the two isomers have mod‐ erate αAIE values, which is a good annotation of the RIR mechanism. Meanwhile, it offers better flexibility to the molecules so that the they respond to the applied me‐ chanical stimulus by flexibly adjusting the conformations of pyridyl rings against the TPE‐core, as confirmed by relatively narrow spectral shifts and small changes in XRD patterns. In addition, the ethynylene bridges play the role of rigid sticks and help to support the porous structure of


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the solids, which enables the fast responses of the thin films to the fuming treatment with solvent and acid/base vapors. In summary, the conjugation of TPE core with pyridyl units via ethynylene bridges not only derive two new ste‐ reo‐pure AIE‐gens, but also provide us with a unique chance to understand more details about the structure‐ property relationship for the multiple stimuli‐responsive behaviors for TPE‐derivatives, which will help us to de‐ sign and prepare more and better AIE‐gens as promising candidates for advanced materials to find high‐tech appli‐ cations such as chemical sensors, security writings and smart optical displays. ASSOCIATED CONTENT Supporting Information. Supporting figures include mechanistic illustration for AIE of TPE isomers (Figure S1); characterization data for compounds 1, 2 and 3 (Figures S2 to S9, S25‐S27); supplementary crystal data for (Z)‐ and (E) –isomers (Figures S11, S12, S22 and S23); absorption and emission spectra for compounds 1 and 2 in different states (Figures S14 to S21, and S24); TGA data for compound 1 (Figure S13); Scheme S1 for the synthesis of compound 3; crystallographic data and photophysical properties for compounds 1 and 2 (Tables S1 and S2, respectively).

AUTHOR INFORMATION Corresponding Authors *[email protected] *[email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We are grateful for financial support from the National Natu‐ ral Scientific Foundation of China (51573158), the National Basic Research Program of China (2013CB834704), the Uni‐ versity Grants Committee of Hong Kong (AoE/P‐03/08), the Research Grants Council of Hong Kong (16301614, 16305015, C2014‐15G), and the Innovation and Technology Commission (ITCCNERC14SC01). B.Z.T. is also grateful for support from the Guangdong Innovative Research Team Program of China (201101C0105067115) and the Science & Technology Plan of Shenzhen (JCYJ20160229205601482).

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