Multiple yet Controllable Photoswitching in a Single AIEgen System

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Multiple yet Controllable Photoswitching in a Single AIEgen System Peifa Wei, Jing-Xuan Zhang, Zheng Zhao, Yuncong Chen, Xuewen He, Ming Chen, Junyi Gong, Herman H.-Y. Sung, Ian D. Williams, Jacky W. Y. Lam, and Ben Zhong Tang J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b13364 • Publication Date (Web): 14 Jan 2018 Downloaded from http://pubs.acs.org on January 14, 2018

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Journal of the American Chemical Society

Multiple yet Controllable Photoswitching in a Single AIEgen System Peifa Wei,†,‡ Jing-Xuan Zhang,‡ Zheng Zhao,†,‡ Yuncong Chen,†,‡ Xuewen He,†,‡ Ming Chen,†,‡ Junyi Gong,†,‡ Herman H.-Y. Sung,‡ Ian D. Williams,‡ Jacky W. Y. Lam,†,‡ and Ben Zhong Tang*,†,‡,§ †HKUST-Shenzhen

Research Institute, No. 9 Yuexing 1st RD, South Area, Hi-tech Park, Nanshan, Shenzhen 518057, China of Chemistry, Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration and Reconstruction, Institute of Molecular Functional Materials, Institute for Advanced Study, Division of Biomedical Engineering and Division of Life Science, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China. §NSFC Center for Luminescence from Molecular Aggregates, SCUT-HKUST Joint Research Institute, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China. ‡Department

Supporting Information ABSTRACT: Seeking new methods to obtain elaborate artificial on-demand photoswitching with multiple functionalities remains challenging. Most of the systems reported so far possess only one specific function and their non-emissive nature in the aggregated state inevitably limit their applications. Herein, a tailored cyanostilbene-based molecule with aggregation-induced emission characteristic was synthesized and was found to exhibit efficient, multiple and controllable photoresponsive behaviors under different conditions. Specifically, three different reactions were involved, they were: (i) reversible Z/E isomerization under room light and thermal treatment in CH3CN, (ii) UV-induced photocyclization with a concomitant dramatic fluorescence enhancement, and (iii) regio- and stereo-selective photodimerization in aqueous medium with microcrystal formation. Experimental and theoretical analyses gave visible insights and detailed mechanisms of the photoreaction processes. Fluorescent 2D photopattern with enhanced signal-to-background ratio was fabricated based on the controllable “turnon” and “turnoff” photo-behavior in different states. The present study thus paves an easy yet efficient way to construct smart multiphotochromes for unique applications.

INTRODUCTION Nature exhibits many elegant examples of multiple functional organisms that respond to external stimuli by manipulating specialized changes to carry out various biological functions. For example, Chameleons have the ability to change colors to disguise themselves or to convey particular messages.1 Motivated by this, chemists are always seeking new methods to obtain elaborate artificial molecular systems with multiple on-demand functionalities.2,3 Light is an attractive method to realize this goal considering its high spatiotemporal resolution for precise investigation in a non-invasive manner.412 Although many light responsive units, such as azobenzenes,1316 spiropyrans,17,18 anthracenes,1922 coumarins2326 and diarylethenes,2732 have been utilized to control the microscopic structures and relevant macroscopic properties, most of the systems own only one specific function to limit definitely their applications. Developing photoswitching with multiple functionalities is thus not only of academic interest but also of practical implication. Then a question arises: how to encode multiple functionalities in a single photoresponsive system. The idea of installing several distinct types of photoresponsive units in an intermolecular manner by chemical tethering33,34 or through supramolecular assembly35,36 is

in principle feasible but is difficult to execute. Specifically, tedious and time-consuming organic synthesis and purification is needed. Wavelength-selective may introduce additional uncertainties to the systems to influence further their manipulation.37 The stability of the assemblies may be another issue for the supramolecular approach. Last but also the most important one is that whether all the units can work in a synergistic rather than antagonistic way in such a complicated system considering annoying interference between different independent photoresponsive units. Thus, mitigating interference and combing multiplicity and controllability in one single photoresponsive system remains a big challenge. Three different pathways are widely used in developing photoresponsive systems, they are: (i) Z/E isomerization,3845 (ii) photocyclization4648 and (iii) photodimerization.4958 Some elegant works have been reported to realize the mechanism/process/functionality control based on the three reactions and each reaction exhibits its unique charm in different systems.5964 For example, based on Z/E isomerization of stiff stilbene unit, Yang et al. constructed a photoresponsive quadruple hydrogenbonded supramolecular polymer, in which the polymerization mechanism and polymer property depend strongly on the isomeric state of the chromophore.42 Irie’s group discovered a new class of diarylethenes, which could

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undergo thermally irreversible and fatigue resistant photochromic reactions.63 Shimizu’s group reported the application of self-assembling bisurea macrocycles to control the stereoselectivity of the photodimerization of coumarin.25 Although the abovementioned systems enjoy high controllability, they suffered the drawbacks of “one turnip, one hole”. Thus, there is an urgent need to install the three independent photoreactions into one system to impart multiple specific functionalities while maintaining the controllability. Considering that almost all the photoreactions can twist or break or extend the molecular conjugation, they are often accompanied with color and also fluorescence change.65 However, a problem associated with the previous works is that most of the controllable photoswitches are constructed from traditional luminophores with aggregation-caused quenching (ACQ) effect in the solid state. Thus, it makes it difficult to follow the reaction process by photoluminescence (PL) measurement because these systems can offer only weak emission.1824 Recently, we and other groups discovered a phenomenon of aggregation-induced emission (AIE) in some propeller and shell-like molecules. AIE luminogens (AIEgens) are nonemissive or weakly emissive in solution but emit efficiently in the aggregated state.40,47,6674 Thus, AIE is exact opposite of ACQ and it is anticipated that if a system is both AIE-active and photosensitive in the solution and aggregated state, its photoreactions can not only be visualized but also trackable. It may exhibit high signal-to-noise ratio due to its emission turn-on character, which is beneficial for bio-imaging and information storage and reading.39,60,75,76 How to construct a single AIE luminogen with multiple and controllable photoresponsive abilities? Cyanostilbene is one of the well-studied AIEgen and is used to address the above question because of the following three reasons. First, cyanostilbene shows Z/E isomerization.75,7779 Second, if the two phenyl groups of cyanostilbene are located on the same side of the double bond, it may undergo photocyclization.46,48 Third, under certain conditions, the double bonds of the adjacent molecules may take a favorable orientation and distance for photodimerization.53,55,58,80 Another design principle is to red-shift the light emission of luminophore to enlarge the emission contrast before and after the destruction of conjugation. Incorporation of electron-donating and accepting groups to endow the system with twisted intramolecular charge-transfer (TICT) is an effective way. Further experiments indeed confirm the feasibility of these strategies: multiple yet controllable photoswitching in a single AIEgen was realized in this work! RESULTS AND DISCUSSION Synthesis and AIE property

To fulfill the requirements discussed above, electrondonating methoxynaphthalene unit and cationic, elec-

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tron-withdrawing 1-methylpyridinium group were introduced to the cyanostilbene core. The chemical structure and synthetic route of the newly designed AIEgen named MPPMNAN were shown in Scheme 1. Detailed procedures were provided in Scheme S1 in the Supporting Information. All the intermediates and final product were characterized by NMR and high-resolution mass spectroscopies with satisfactory results (Figure S114). The geometry concern often exists in cyanostilbenebased molecules. The 1H NMR and 13C NMR spectra of pristine MPPMNAN exhibited multiple but wellassigned resonance peaks, indicating that it adopted only one isomeric form instead of a mixture (Figure S12 and S13). 2D NMR spectroscopy was used to provide more detailed insight into the exact structure of MPPMNAN. By using 1H-1H COSY and 1H-13C HSQC spectroscopies, correct assignment of the phenyl proton resonances could be made (Figure S15 and S16). On the other hand, it is anticipated that the E-isomer should show nuclear overhauser effect (NOE) between the phenyl proton Hd and naphthalene proton Hf or Hg because they are located at the same side of the central double bond and are close to each other. As shown in Figure S17, no NOE signals were observed in MPPMNAN, indicative of the Z-isomeric nature of the synthesized compound. ZMPPMNAN was soluble in acetonitrile and acetone but was insoluble in chloroform and water. It showed good thermal stability, losing only merely 5% of its weight at  325 C (Figure S18). Scheme 1. Synthesis of Z-MPPMNAN and Its Multiple yet Controllable Photoreactions under Different Conditions. ZE: Z/E Isomerization; PC: Photocyclization; PD: Photodimerization.

However, proton NMR spectroscopy provides only indirect information about the molecular conformation and fails to show intermolecular interactions. So we tried to grow crystals of Z-MPPMNAN but failed to obtain high quality ones for analysis after long-term trials. On the other hand, yellow single crystals of Z-PPMNAN and its counterpart with I counteranion were obtained by slow vapor diffusion of their dichloro-

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methane/methanol mixtures and analyzed crystallographically. Their crystal structures were shown in Figure 1 and the associated data were provided in Table S1 and S2. Combing the crystal structures and the aforementioned 2D NMR results, it was reasonable to give a conclusion that Z-type MPPMNAN was obtained. To further support our conclusion, we carried theoretical calculations on the energy of MPPMNAN isomers. The difference in Gibbs free energy of the Z- and E-isomers (G = GZ  GE) was found to be 11.07 kJ mol1, indicating a higher thermodynamic stability of the former compound (Figure S19). After confirming the molecular conformation, we then investigated the photophysics of Z-MPPMNAN. Z-MPPMNAN was weakly emissive in CH3CN but with a gradual addition of water into the solution, enhanced photoluminescence (PL) was recorded (Figure S20). Since Z-MPPMNAN was insoluble in water, its aggregates should be readily formed in CH3CN/H2O mixtures with high water fractions. It means that aggregate formation has enhanced the PL of Z-MPPMNAN or in other words, Z-MPPMNAN is AIEactive.

Figure 1. Single crystal structures of (A) Z-PPMNAN and (B) ZMPPMNANI with I counteranion.

Z/E isomerization

Due to the preserve of cyanostilbene moiety, it is anticipated that Z-MPPMNAN may undergo Z/E isomerization under photo-irradiation. The UV/vis spectrum of ZMPPMNAN in CH3CN showed absorption at wavelength of up to 480 nm. This suggests that it may undergo isomerization under room light condition (Figure S21). 1H NMR spectroscopy was first used to follow this process. Quartz tubes filled with CD3CN solution of ZMPPMNAN was placed under room light irradiation and nitrogen gas protection or in dark room. Results showed that the untreated sample show no spectral change even after ten days (Figure S22). However, for the irradiated one, the resonances of its protons Hc, Hd, He, Hf and Hg were splitted into two sets: the original signals were assigned to the unreacted species, while the upfield-shifted ones originated from the E-MPPMNAN product (Figure 2A and B). Prolonging the irradiation time led a sharp decrease of the Z-isomeric signals accompanied with an increase of the peak intensity of the E-isomer (Figure 2BD). After 96 h, the resonance signals of both E- and Z- isomers showed almost un-

changed, suggestive of a stable equilibrium. By calculating the related integral peak, the ratio between the Zisomer and E-isomer was calculated to be 76:24 (Figure 2D). The non-100% conversion is possibly due to the overlapping of the absorption spectrum of EMPPMNAN with that of Z-MPPMNAN, which can also trigger the reverse E to Z isomerization (Figure S23). Meanwhile, the high-resolution mass spectrum of ZMPPMNAN did not show any new peaks after room light irradiation for 96 h, demonstrating that room light indeed promoted the Z to E isomerization of the cyanostilbene moiety rather than other photochemical reaction (Figure S24). Such photoisomerization is reversible and the E-isomer can be completely converted to the initial Z-state by heating the solution at 75C for 36 h (Figure 2DG).

Figure 2. 1H NMR spectra of as prepared Z-MPPMNAN (2.0 mM) (A) before and (BD) after room light irradiation for (B) 24 h, (C) 48 h and (D) 96 h in CD3CN. (EG) Spectrum after heating the solution of (D) for (E) 12 h, (F) 24 h and (G) 36 h at 75C.

Photocyclization

We further studied the response of Z-MPPMNAN to UV irradiation in CH3CN. To our surprise, upon UV irradiation at 365 nm, increased strong yellow fluorescence was observed, indicating that lightemission process of Z-MPPMNAN in CH3CN was photoactivatable. To obtain a more clear picture, we studied the photoactivation process using PL, UV-vis and NMR spectroscopies. As discussed before, ZMPPMNAN is an AIE-active molecule and shows weak orange emission when molecularly dissolved in solution. The emission, however, was significantly intensified and blue-shifted by 22 nm to 560 nm in the presence of UV light (Figure 3A and S25). The longer the irradiation time, the stronger was the emission (Figure 3B). The absorption of Z-MPPMNAN also shifted progressively to the shorter wavelength with decreased intensity by prolonging the exposure time (Figure 3C). Meanwhile, two new absorption peaks at 315 and 280 nm appeared, whose intensity increased with increasing the irradiation time. When such bright solution was treated with heating, neither emission change could be observed by na-



ked eye nor detected by a UV spectrophotometer (Figure 3D). Such result was quite different from the aforementioned reversible Z/E isomerization process, and suggested the formation of a stable compound.

Figure 3. (A) PL spectra of Z-MPPMNAN (10 M) in CH3CN solution irradiated with 365 nm UV light from a hand-held UV lamp for different time. ex = 385 nm. Inset: fluorescent images before and after UV irradiation. (B) Change of relative PL intensity (I/I0) at  = 560 nm with different irradiation time. (C) UV-vis

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spectra of Z-MPPMNAN (20 M) before and after irradiation with 365 nm UV light for 4 h followed by heating the solution at 75 C for 2 h. Inset: daylight images before and after UV irradiation. “” indicated heating. (D) Change of absorbance at  = 385 nm (red open circles) and  = 280 nm (red filled circles) at different irradiation time and at  = 280 nm (blue filled triangles) at different heating time.

The high-resolution mass and 1H NMR spectra obtained at different states of the photo-activation process were shown in Figures 4AC. The dynamic 1H NMR analysis of the irradiated solution showed that a new singlet peak at δ 9.56 ppm (Hα) and a doublet peak at δ 9.03 ppm (Hβ) gradually appeared, and their corresponding intensity was enhanced by lengthening the irradiation time (Figure 4A, spectrum i to v). Additionaly, if we gained more insight into the aromatic resonances at the up-field region, Z/E isomerization was dominated at the initial state of the photoreaction (Figure 4A, spectrum i to iii), while after 5 h, the photocyclodehydrogenation reaction governed the photo process (Figure 4A, spectrum iii-v). Combing the above phenomena, it was speculated that the sample first underwent Z/E isomerization, and subsequent photocyclization of E-MPPMNAN afforded unstable dihydrophenanthrene. This intermediate could be further oxidized to yield stable c-MPPMNAN (Figure 4D). The maximum photo-conversion from ZMPPMNAN to c-MPPMNAN was calculated from the 1H NMR spectra and reached as high as 99% after 70 h, indicative of the high efficiency of the photoactivation reaction (Figure 4B).

Figure 4. (A) Change of 1H NMR spectra of Z-MPPMNAN before and after UV irradiation at 365 nm of its solution (0.5 mM) for 0, 5, 15, 40, 70 h in CD3CN from a hand-held UV lamp. (B) Photo-conversion from Z-MPPMNAN to c-MPPMNAN at different time evaluated from 1H NMR spectra. (C) High-resolution mass spectrum of Z-MPPMNAN obtained after UV irradiation of its CD3CN solution for 70 h. (D) Proposed reaction routes for the formation of c-MPPMNAN from Z-MPPMNAN in CH3CN solution under UV light irradiation. (E) Single crystal structure of c-MPPMNAN-I with I counteranion.

After UV irradiation, a new peak at m/z 375.1492 emerged in the mass spectrum, which corresponded to molecular mass of Z-MPPMNAN minus the mass of hexafluorophosphate fragment and two hydrogen atoms

(Figure 4C). This definitely confirms the occurrence of the photocyclization. From both the PL and NMR spectra, we could see that the rate of photoconversion was fast at the beginning but became slow afterward. Finally,


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a platform was reached. This may be due to the gradual consumption of oxygen dissolved in the solvent. From the above experiments, the reaction mechanism should be concluded as follows: Z-MPPMNAN was efficiently converted to its E-form by UV irradiation, whose stilbene-type 6- electron ring-closure photocyclization reaction formed c-MPPMNAN. To better confirm our speculation, pure c-MPPMNAN was separated from the irradiated solution by high-performance liquid chromatography (HPLC) and was well characterized by 1H NMR, 13C NMR, mass, UV-vis and PL spectroscopies (Figures S2629). The obtained results were in great agreement with the data we discussed above. Single crystal X-ray analyses of c-MPPMNAN-I with I counteranion gave direct proof of the formation of photocyclized product (Figure 4E and Table S3). Unlike ZMPPMNAN, aggregate formation of c-MPPMNAN had its quenched fluorescence (Figure S30).

the suspensions.

Photodimerization

Molecules in the aggregated states will get close to each other under the driven of hydrophobic/hydrophilic interactions, - stacking, C-H interactions, etc, which may have a strong influence on their photoreaction. Combing the phenomenon that the fluorescence of ZMPPMNAN will be enhanced in the aggregated state, we wonder whether it can undergo different photoreactions in suspensions with high water fractions (fw) with obvious absorption or emission change. In the course of systematic exploration on the effect of fw on the photoreaction of Z-MPPMNAN under UV light irradiation at  = 365 nm, we observed quite different absorption and fluorescence behaviors from the aforementioned photocyclization process. After photo-irradiation, the colors of the suspensions with different fw all appeared colorless (Figure S31). Further detailed investigation by UV spectroscopy manifested a big difference. The UV spectral change of Z-MPPMNAN in CH3CN/H2O mixture with fw of 50% and 70% after 365 nm UV irradiation for different time (Figure 5A and B) resembled that of pure CH3CN (fw = 0%; Figure 3C). At fw = 90% or 99%, very conspicuous differences were found (Figure 5C and D). First, we noted that the higher fw, the lower was the peak intensity at  = 370 nm at the equilibrium. Especially at 99% water fraction, the absorption peak at  = 370 nm was almost vanished. Second, increase of fw in the solvent mixture sharply shortened the reaction time. The time required for the mixtures with fw = 0%, 50% and 70% to attain equilibrium obviously longer than that with fw = 90% and 99%. Third, for the suspensions with fw less than 70%, the sharp peak at 280 nm and the broad peak at around 315 nm increased continuously with time, while only the peak at 290 nm intensified in mixtures with 90% and 99% water fractions. In addition, the isosbestic points of mixtures with low and high water fractions were observed at 336 nm and 314 nm, respectively. This indicates that two absorbing species are present in

Figure 5. Changes of UV-vis spectra of Z-MPPMNAN (20 M) in CH3CN/H2O mixtures with fw of (A) 50%, (B) 70%, (C) 90% and (D) 99% upon 365 nm UV irradiation from a hand-held UV lamp for different time.

From the above characteristics, we could conclude that photocyclized compound, namely c-MPPMNAN, was the main product in pure CH3CN and 50% and 70% aqueous suspensions. However, different species with lower conjugation were generated in high fw. Then we checked the stability of the produced compound at 99% aqueous suspension by irradiating the solvent mixture with short wavelength light of 254 nm. The absorption at  = 290 nm gradually decreased, while the peak at  = 370 nm red-shift slightly with increased intensity. After 30 min, no further absorption change was observed, indicating that high-energy UV light could possibly trigger the partially reverse conversion (Figure S32). Similar phenomenon was observed in 90% aqueous mixture (Figure S33). The species generated at high fw was quite different from the stable photocyclized compound formed in low fw. To get more insight into the photo-process in aqueous suspensions with high water fractions upon UV irradiation, we carried the PL analysis. Results showed that weak blue emission and yellow fluorescence was observed at fw = 99 and 90%, respectively, while at fw = 50% and fw = 70%, comparatively stronger yellow emission was recorded (Figure 6A). This phenomenon suggested that the species formed at different fw possess different conjugation. We then further use PL spectroscopy to trace the photo-process. At 99% water fraction, the emission peak at  = 570 nm decreased very quickly upon photo-irradiation and a new peak with increased intensity appeared at 440 nm. If we investigated carefully the PL spectra, a relative weak emission peak actually existed at around 560 nm which was in accordance with the emission maximum of c-MPPMNAN in CH3CN



(Figure 6E). Such fluorescence change was exact opposite of that at fw = 0% (Figure S25). Then we further compared the PL spectra at different water fractions after photo-irradiation. At fw = 90%, the peak intensity at  = 440 nm was comparatively lower than that at fw = 99%. The trend was that the lower the water fraction, the lower was the emission intensity at  = 440 nm (Figure 6 and S25). From the UV analysis, photodimerization should be taken place at fw = 90% and 99% (Figure 5C and D). It is noteworthy that the UV spectra at fw = 90% and 99% change in a similar way upon irradiation to indicate the occurrence of photodimerization (Figure 5C and D). However, their PL spectra exhibited distinctly different patterns (Figure 6D and E), presumably due to the fact that some Z-MPPMNAN still underwent photocyclization at fw = 90%, which resulted in comparatively stronger emission at 560 nm.

Figure 6. (A) Fluorescent images of Z-MPPMNAN (20 M) in CH3CN/H2O mixtures with different fw (from left to right: 0%, 50%, 70%, 90% and 99%) before and after irradiation. PL spectra of Z-MPPMNAN (20 M) in CH3CN/H2O mixtures with fw of (B) 50%, (C) 70%, (D) 90% and (E) 99% upon 365 nm UV irradiation from a hand-held UV lamp for different time. ex = 300 nm.

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The low resolution electrospray ionization-mass spectrum of Z-MPPMNAN in 99% aqueous suspension after photo-irradiation for 1 h confirmed the formation of dimer: m/z 899.5 for [(Z-MPPMNAN)2PF6]+; m/z 753.5 for [(Z-MPPMNAN)2HPF6PF6]+; m/z 377.4 for [(Z-MPPMNAN)22PF6]2+ (Figure S34). Pure dimer was obtained by HPLC. Its enlarged electrospray ionization time of flight mass spectrum provided further evidence for the formation of dimer, named d-MPPMNAN (Figure S35). The topical peak at 377.1634 clearly showed its double positive charging characteristic as it corresponded to the molecular mass minus the mass of two PF6 counterions (Figure 7A). NMR measurement of Z-MPPMNAN and d-MPPMNAN in CD3CN was also carried out for comparison (Figure 7B, S3638). Obvious changes were observed for protons on naphthyl and phenyl rings. Notably, the appearance of a resonance signal at δ 5.82 for the cyclobutyl proton and the absence of signal for the characteristic double bond (He) resonance at δ 8.02 indicated the formation of dimer (Figure 7B). Similarly, two new signals appeared in the 13C NMR spectrum of Z-MPPMNAN after photodimerization at δ 47.58 and 54.21 assigned to the resonances of newly formed cyano-substituted cyclobutane ring (Figure S38). After confirming the formation of c-MPPMNAN and d-MPPMNAN at solvent mixture with low and high water fractions, respectively, we calculated the photoconversion as a function of irradiation time based on the aforementioned UV spectra (Figure 3C and 5). From Figure 8A, we observed that photodimerization proceeded in a much faster rate than photocyclization. This suggests that the formation of dimers at high fw is much easier than that of cyclized compound. Meanwhile, the ultimate conversion was high at all conditions (Figure 8B). For example, c-MPPMNAN could be obtained in almost quantitative yield (99%) by UV irradiation of pure CH3CN solution of Z-MPPMNAN. This was well correlated with the yield calculated from the 1H NMR spectrum in Figure 4B. d-MPPMNAN was also obtained in high yield (99%). This means that we can not only control the reaction route for either photocyclization or photodimeriztaion but also generate products in high efficiency.

Figure 7. (A) High-resolution mass spectrum of d-MPPMNAN. (B) 1H NMR spectra of (i) Z-MPPMNAN and (ii) d-MPPMNAN in CD3CN.


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Figure 8. (A) Photo-conversion from Z-MPPMNAN to cMPPMNAN/d-MPPMNAN in CH3CN/H2O mixtures with different fw at different irradiation time evaluated from the UV intensity change at  = 385 nm (open circles; photodimerization) or  = 370 nm (filled circles; photocyclization). (B) Photo-conversion from Z-MPPMNAN to c-MPPMNAN/d-MPPMNAN at different fw. a PD = photodimerization yield = (I0It)/I0 and b PC = photocyclization yield = (I0It)/(I0I1), where I0 = initial intensity of ZMPPMNAN, It = intensity of Z-MPPMNAN at different irradiation time, I1 = intensity of c-MPPMNAN at the same concentration (Figure S39). Concentration: 20 M.

der room light? To confirm this speculation, we tried to grow crystals by vapor diffusion of diisopropyl ether to a solution of Z-MPPMNAN in CH3CN under room light. Surprisingly but expectedly, after about two weeks, the amorphous and yellow-emissive Z-MPPMNAN changed to green-emissive crystals. Although the crystal quality was not good, the dimer structure obtained had firmly confirmed the occurrence of photodimerization (Figure 10 and Table S4). The crystallization process could be speculated as follows: tiny nanocrystals formed as nuclei first and during their slow size growth, Z-MPPMNAN gradually changed to its crystalline dimer after the absorption of short wavelength room light. The emission bathochromic-shift from 433 nm in CH3CN solution to 529 nm in the crystalline state of d-MPPMNAN reveals the presence of significant intermolecular interactions between its molecules (Figure S4244).81,82

Mechanism study

Why Z-MPPMNAN shows different photoreactions at different aggregated states? Considering that morphology difference may be the driven force for this phenomenon, we studied the morphology of the aggregates of ZMPPMNAN formed at different fw by scanning electron microscope (SEM). At fw = 0%, 50% and 70%, only spherical nanoaggregates were observed (Figure S40). When fw reached 90%, crystallization occurred, which was commonly observed in antisolvent systems with a large excess of one component over the other one (Figure 9A).66 If the fw was further raised to 99%, more microcrystals were observed (Figure 9BC and S41). This trend was in great accordance with the different photobehaviors observed at low and high fw. As discussed above, the photodimerization yield at fw = 99% was 99% and was slightly higher than that at fw = 90%. This shows that increasing the number of microcrystals will promote the photodimerization. The fluorescent image of Z-MPPMNAN at fw = 99% provided a more intuitive insight into the photodimerization process in the microcrystalline state. Besides the orange dots of ZMPPMNAN, some green dots circled by pink dashed circles were also found (Figure 9D). This was because the UV light from the fluorescence microscopy induced the photodimerization of Z-MPPMNAN microcrystals. Such observation further demonstrates the high efficiency of the photodimerization. Crystallization is a process where the molecules are organized regularly into a structure known as crystals. This usually occurs in two major steps: nucleation and crystal growth. As we discussed above, Z-MPPMNAN formed microcrystals in aqueous mixture with high fw, which could be further converted to d-MPPMNAN in the presence of UV irradiation. Since the cut-off absorption wavelength of Z-MPPMNAN reached the visible region of 480 nm (Figure S21). Thus, is it possible to get crystals of d-MPPMNAN during the crystallization un-

Figure 9. SEM photographs of Z-MPPMNAN aggregates formed in CH3CN/H2O mixtures with fw of (A) 90% and (B) 99%. (C) Partially enlarged picture of (B). (D) Fluorescent image of ZMPPMNAN at fw = 99%. The pink dashed circles in picture (D) distinguished the green emission dots from the orange ones.

Figure 10. (A) Synergistic room light and crystallization-induced photodimerization in aqueous solution. The pictures shown here are fluorescent photographs of Z-MPPMNAN and d-MPPMNAN in the solid state. (B) Single crystal structure of d-MPPMNAN.

It should be noted that four possible products may be generated by the photodimerization, namely, syn headto-head (syn-HH), anti head-to-head (anti-HH), syn head-to-tail (syn-HT) and anti head-to-tail (anti-HT) (Scheme S2). However, the formation of microcrystals favors the preorientation of the double bond of cyanostilbene of Z-MPPMNAN. This enhances its regioand stereo-selectivity for photodimerization to give iso-



mer with solely anti-HT conformation in an almost unity conversion yield. Now a question arises: which one (Z- or EMPPMNAN) serves as the precursor of the photodimer? Study on the crystal packing of the Z-isomer may help to understand more about the photodimerization but as we mentioned before, we failed to obtain crystals of Z-/EMPPMNAN. Luckily, we obtained crystals of their precursors E- and Z-BPMNAN (Figure 11 and Table S56). Pure E-BPMNAN was simply afforded by photoirradiation of a solution of Z-BPMNAN with hand-held UV lamp for 2 h in dichloromethane in 82% yield. One significant structural feature of E-BPMNAN was that the adjacent molecules were aligned in a well-ordered conformation for the topochemical [2 + 2] cycloaddition with a center-to-center distance of 3.85 Å. This value was shorter than the favourable distance (< 4.2 Å) for photodimerization in the solid state.8385 The key elements for the stable conformation of E-BPMNAN were derived from the CNHC hydrogen bonding with a distance of 2.83 Å (Figure S45). However, Z-BPMNAN showed antiparallel - stacking comprising the twisted stilbene CC bonds with a center-to-center distance of 4.61 Å. Besides the distance between the olefin units, appropriate angles (1, 2 and 3) were also considered to be geometric criteria for photodimerization. The ideal values were 0, 90°and 90°, respectively (Figure S46). From the summarized data in Table S7, we conjectured that E-BPMNAN was considered to undergo [2 + 2] cycloaddition reaction with more ease under photoirradiation. This also provides an indirect evidence that E-MPPMNAN crystal may also take a more favorable molecular packing for photodimerization than its Zisomer.

Page 8 of 12

tation and relaxation to afford triplet intermediate ZIM3_T1. The bond distance between the two double bonds was 4.91 Å in the ground state of dimer Z-IM1_S0 and 4.66 Å in the triplet state of dimer Z-IM3_T1. These values were in good agreement with those measured from the crystal structure of the model compound ZBPMNAN. In the next step, the two monomers approached each other and underwent a [2 + 2] cycloaddition reaction. Interestingly, the calculation results suggested that the dimerization took place in a stepwise manner considering the stability of the diradical triplet state species Z-IM4_T1. At last, the combination of the two carbon radicals gave the photodimerization product d-MPPMNAN. Scheme 2. Energy Profiles Calculated for the Dimerization of (A) Z-isomer and (B) E-isomer. All the Numbers Given were Relative Gibbs Free Energy and Electronic Energy (in Parenthesis) in kcal/mol with Respect to the Reference Point ZIM1_S0. (C) Proposed Reaction Routes for the Formation of dMPPMNAN from Z-MPPMNAN in Suspension with High fw under UV Light Irradiation.

Figure 11. (a) Crystal structures and packings of Z-BPMNAN and E-BPMNAN. The center-to-center distances between double bonds of adjacent molecules were 4.61 Å and 3.85 Å, respectively.

To obtain further mechanistic insight of the photodimerization, we performed DFT calculations on the proposed pathways. We first carried out calculation on the Z/E photoisomerization. The energy profiles given in Figure S47 verified the experimental observation that the isomerization process took place readily under irradiation. Afterwards, we calculated the pathways from both Z-isomer and E-isomer to d-MPPMNAN. From the energy profiles given in Scheme 2A, Z-isomer could form an antiparallel - stacking geometry followed by exci-

On the other hand, the pathway calculated for Eisomer generally followed the same procedure with some differences (Scheme 2B). The bond distances between the two double bonds of a - stacking dimer became 3.60 Å and 3.53 Å in the ground state and triplet


Page 9 of 12 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


excited state, respectively. This suggests the ready occurrence of the photodimerization. The following steps showed that dimerization between two E-isomers was more favorable as revealed by a lower energy barrier of 5.95 kcal/mol than that between two Z-isomers (18.6 kcal/mol). Thus, through the results from the SEM images, the crystal structure of model compounds and the DFT calculations, we proposed the reaction pathway as illustrated in Scheme 2C: Z-MPPMNAN first underwent isomerization to afford E-isomer upon photo-irradiation followed by the dimerization of two adjacent cyanostilbene units with the assistance of favorable orientation and distance in microcrystals. Table 1. Optical and electronic properties of Z-MPPMNAN, dMPPMNAN and c-MPPMNANa λabs (nm) CH3CN

CH3CN

Solid

Solid

CH3CN

Z-MPPMNAN

385

580

563

10

3.4 (6.5)

9.4

d-MPPMNAN

290

437

529

6.0

1.6 (1.3)

3.6

c-MPPMNAN

373

560

488

1.7

42.0

5.0

 (%)

λem (nm)

solid (ns)

Abbreviation: λabs = absorption maximum, λem = emission maximum,  = fluorescence quantum yield (data in the brackets indicated the quantum yields in CH3CN/H2O mixture with 99% water fraction), solid = fluorescence lifetime in the solid state (Figure S48). a

Z-MPPMNAN is a weak emitter, emitting faint orange emission at 580 nm with a fluorescence quantum yield (f) of around 3.4% measured by a calibrated integrating sphere (Table 1). Although it showed AIE feature, its 99% aqueous suspension showed only about two times higher PL intensity with f of 6.5% due to the TICT effect. Its photoactivatable characteristic enabled it to undergo facile photocyclization in acetonitrile to form yellow emissive c-MPPMNAN with a high f of 42.0%. Its microcrystals could undergo photodimerization to form faint blue emissive d-MPPMNAN (f = 1.3%). The f difference between solution (42.0%) and aggregated state (1.3%) after photoreaction was 32-fold. Considering the strong fluorescence contrast, this means that this molecule can work in a controlled “turn-on” or “turn-off” mode. Thus, a general strategy of applying ZMPPMNAN in information storage and reading is proposed as illustrated in Figure 12. A 96 well plate was used as a template to perform such study. Pure acetonitrile solution of Z-MPPMNAN was filled in the holes to make a pattern with letters “HK”, while the other parts were filled with 99% aqueous suspension. Before UV exposure, no obvious images were seen. When the plate was irradiated with UV light from a hand-held UV lamp, the letters emitted intense yellow light because of the activatable fluorescence from the photocyclization reaction, while the other parts underwent photodimeriztaion to lead to serious emission quenching. This generated a well-resolved 2D fluorescence pattern with enhanced signal-to-background ratio and sharp edges.

Figure 12. (A) Cartoon representation for the “turn-on” and “turn-off” process. (B) Two-dimensional fluorescent photopattern in CH3CN/H2O mixtures with fw of 0% (for letters “HK”) and 99% (for the other parts of the 96 well plate) of Z-MPPMNAN before and after irradiation with a hand held UV lamp at  = 365 nm. Concentration: 10 M.

CONCLUSIONS In this work, a simple cyanostilbene-based fluorophore with AIE characteristic was developed in high yield. By tuning the aggregate morphology or selecting different irradiation light, this molecule exhibited efficient, multiple and controllable photoreactions operated in the mechanism of 1) extraordinary Z/E isomerization under room light and thermal treatment in organic solvent, 2) UV-induced photocyclization with a concomitant dramatic fluorescence enhancement and 3) regio- and stereo-selective photodimerization in aqueous mixture with microcrystal formation. Experimental analyses gave direct evidences and visible insights into the photoreaction processes. DFT calculation provided more detailed mechanism for the photodimerization in the microcrystalline state. Fluorescent 2D photopattern was fabricated from Z-MPPMNAN, thanks to its enhanced signal-tobackground ratio before and after photo-irradiation with controllable turn-on and turn-off fluorescence behaviors in different states. Given that we not only realized multiple photoreactions in one single system, but also controlled the elusive photoresponsive reactions in a charming fashion for specific functionalities, we believe that the fundamental results present here will define a simple yet highly efficient way to construct multifunctional ondemand materials for unique applications. Meanwhile, considering the crystallization process in the aggregated state and the corresponding photophysical properties under light irradiation, doping this kind of fluorophore into polymer matrix to act as microactuators may introduce the materials with phototriggered macroscopic behaviors, which is of great interest and will be carried out in the near future. ASSOCIATED CONTENT Supporting Information Experimental procedures, methods and product characterization. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION



Corresponding Author [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We are grateful for financial support from the National Science Foundation of China (21788102), the National Basic Research Program of China (973 Program; 2013CB834701 and 2013CB834702), the Research Grants Council of Hong Kong (16308016, 16305015, C2014-15G, A-HKUST605/16 and N_HKUST604/14), and the Innovation and Technology Commission (ITC-CNERC14SC01). B. Z. T. is also grateful for the support from the Science and Technology plan of Shenzhen (JCYJ20160229205601482).

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Multiple yet Controllable Photoswitching in a Single AIEgen System

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