A photo-switchable supramolecular hyperbranched polymer with aggregation-induced emission based on host-guest interaction

Accepted Manuscript A photo-switchable supramolecular hyperbranched polymer with aggregation-induced emission based on host-guest interaction Fei Qiao, Zhe Lian, Zhao Yuan, Meng-Ning Chen, Man Jiang, Rong-Zhou Wang, Ling-Bao Xing PII:

S0143-7208(18)32069-2

DOI:

https://doi.org/10.1016/j.dyepig.2018.12.055

Reference:

DYPI 7260

To appear in:

Dyes and Pigments

Received Date: 19 September 2018 Revised Date:

22 December 2018

Accepted Date: 23 December 2018

Please cite this article as: Qiao F, Lian Z, Yuan Z, Chen M-N, Jiang M, Wang R-Z, Xing L-B, A photoswitchable supramolecular hyperbranched polymer with aggregation-induced emission based on hostguest interaction, Dyes and Pigments (2019), doi: https://doi.org/10.1016/j.dyepig.2018.12.055. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT A photo-switchable noncovalently and covalently linked supramolecular hyperbranched polymers with aggregation-induced emission based on host-guest interaction between coumarin moieties and cyclodextrin

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Fei Qiao, Zhe Lian, Zhao Yuan, Meng-Ning Chen, and Ling-Bao Xing*

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We have prepared a dual-modality supramolecular polymer with aggregation-induced emission in aqueous solution by taking advantage of host−guest interactions between γ-CD and coumarin moieties.

ACCEPTED MANUSCRIPT A photo-switchable supramolecular hyperbranched polymer with aggregation-induced emission based on host-guest interaction

Xinga,b*

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School of Chemistry and Chemical Engineering, Shandong University of Technology, Zibo

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255000, P. R. China

CAS Key Laboratory of Molecular Recognition and Function, Institute of Chemistry, Chinese

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b

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Fei Qiao,a Zhe Lian,a Zhao Yuan,a Meng-Ning Chen,a Man Jiang,c Rong-Zhou Wang,a Ling-Bao

Academy of Sciences, Beijing 100190, P. R. China

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Resources and Environmental Engineering, Shandong University of Technology, Zibo 255000, P.

R. China

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*Corresponding author: Tel. /fax: +86 533 2781664. E-mail: [email protected].

ACCEPTED MANUSCRIPT Abstract

In this work, we report a simple strategy by mixing a coumarin appended tetraphenylethylene derivative (TPEC) and γ-cydodextrin(γ-CD) in aqueous solution, based on the host-guest

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interaction between TPEC and γ-CD to construct a dual-modality supramolecular hyperbranched polymer. The host-guest interaction between γ-CD and TPEC efficiently restricted the intramolecular rotation and the non-radiative relaxation channel, minimizing energy loss, thereby

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resulting in the strong emission of TPEC in dilute solution. Moreover, we achieved the

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conversion between non-covalent supramolecular polymers and its corresponding covalent supramolecular hyperbranched polymers under the irradiation of UV light, which can be reversibly controlled through the irradiation at the wavelengths of 254 nm and 365 nm. Furthermore, both non-covalent supramolecular hyperbranched polymers and covalent

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supramolecular hyperbranched polymers can self-assemble into spherical structures. This methodology provides a new idea for exploring a new type of supramolecular hyperbranched

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polymers with aggregation-induced emission.

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Keywords: dual-modality; reversible; host-guest interaction; photo-switchable; supramolecular polymer.

ACCEPTED MANUSCRIPT 1. Introduction

Our traditional molecular chemistry is based on covalent bonds between atoms or atomic groups, while study on molecules is based on intermolecular interactions, in which the chemistry

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of molecular aggregates is called supramolecular chemistry. Supramolecular chemistry is also called "chemistry beyond the concept of molecules", and it is a sublimation and surpassing of covalent bond molecular chemistry. Supramolecular chemistry involves not only inorganic

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chemistry, organic chemistry, physical chemistry, analytical chemistry and polymer chemistry, but

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also materials, information and life sciences. In recent years, supramolecular polymers formed self-assembly have attracted extensive research interest as an interdisciplinary subject of supramolecular chemistry and polymer chemistry [1-14]. A supramolecular polymer is defined as an array of repeating units connected by reversible and directional non-covalent bonds interactions

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such as multiple hydrogen-bonding [15-19], metal-coordination interactions [20-24], host-guest interactions [25-33], π-π stacking interaction [34-37], donor-acceptor interaction [38-42] as well

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as synergistic effect of multiple forces. As one of the non-covalent interactions, host-guest recognition system is a crucial bridge between supramolecular chemistry and macromolecular

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self-assembly. In the system of host-guest interactions, the host selectively identifies the object and coordinates with the object to form a complex. This working relationship is similar to the relationship between a lock and a key. People usually use macrocyclic molecules such as crown ethers, cyclodextrins, calixarenes, cucurbiturils, and pillararenes as frequently-used host in the process of supramolecular self-assembly [43-47]. The supramolecular assembly system constructed by non-covalent interactions has the adaptability to the environment and the responsiveness to the stimulus, including thermal response,

ACCEPTED MANUSCRIPT optical response, chemical response, redox response and other responsive supramolecular systems [48-50].

Moreover, light, as a kind of mild external stimulus without the introduction of external

matter, owing to facile controllability of space, intensity, wavelength and time, is often used to

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study the properties of supramolecular assembly systems. Especially, Tian and coworkers found γ-cyclodextrin can encapsulate two coumarin units in its cavity, in which the conversion from non-covalent bonds to covalent bonds between two coumarin units can be reversibly controlled

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[2,3]. Zhang and coworkers also developed a new method of fabricating covalently attached

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hyperbranched polymers by a combination of supramolecular polymerization and photochemistry between cucurbit[8]uril and an azastilbene derivative [51]. Covalent polymers or non-covalent polymers have their own advantages in the field of functional materials. However, little effort has been devoted to establish a bridge for the switch between covalent polymers and non-covalent

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polymers.

In this work, we designed and synthesized a 7-(4-bromohexoxy)-coumarin-substituted tetraphenylethylene derivative (TPEC) which could form supramolecular hyperbranched

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polymers self-assembly in aqueous solution by employing γ-CD-coumarin host-guest interactions.

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Tetraphenylethylene molecule is an aggregation-induced luminescence unit [52,53], the luminescence is obviously enhanced after the formation of the supramolecular polymers, which can be attributed that the intramolecular rotation was blocked by the combination of the TPEC molecule and γ-CD. Moreover, the supramolecular polymers can be constructed in a dual-modality state and photoswitchable with aggregation-induced emission [54-56] after assembly by coumarin group and γ-CD through host-guest interaction. The conversion of covalent and non-covalent bonds can be realized under the stimulation of different wavelengths of light.

ACCEPTED MANUSCRIPT Under the UV light irradiation at 365 nm, the non-covalent bonds of two coumarin groups are photodimerized to form covalent bonds. Moreover, this chemical process is reversible. Under the UV light irradiation at 254 nm, the covalent bonds photochemical cleavage and return to the state

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of non-covalent bonds. Furthermore, both non-covalent supramolecular polymers and covalent supramolecular polymers self-assemble into spherical aggregates in aqueous solution (Scheme 1). The construction of the photo-switchable non-covalently and covalently linked supramolecular

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idea for us to construct new functional materials.

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hyperbranched polymers with aggregation-induced emission in aqueous solution provides a new

Scheme 1

2. Experimental

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2.1 Materials

Tetra-(4-pyridylphenyl)ethylene [57-59] and 7-(4-bromohexoxy)-coumarin [3,4] were synthesized according to the literatures report. Acetonitrile and diethyl ether were purchased from

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Sinopharm Chemical Reagent Co., Ltd.

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2.2 Synthesis of TPEC

The synthetic method of TPEC is depicted in Scheme 2. Tetra-(4-pyridylphenyl)ethylene

(0.40 g, 0.63 mmol) and 7-(4-bromohexoxy)-coumarin (1.85g, 6.30 mmol) were dissolved in acetonitrile (40 mL). The mixture was stirred at 90 ℃ for 24 h. After the reaction is over, the solution was added dropwise to 400 mL of poor solvent, diethyl ether and filtered. We obtained yellow solid as product. 1H NMR (400 MHz, D2O) δ 8.74 – 8.69 (m, 8H), 8.05 – 7.99 (m, 9H), 7.67 – 7.61 (m, 8H), 7.44 – 7.38 (m, 10H), 7.23 – 7.16 (m, 7H), 6.67 – 6.58 (m, 8H), 6.43 – 6.40

ACCEPTED MANUSCRIPT (m, 4H), 4.60 – 4.56 (m, 8H), 3.94 – 3.89 (m, 8H), 2.22 – 2.16 (m, 8H), 1.80 – 1.74 (m, 8H). HRMS (ESI) (m/z): [M - 4Br]4+/4 calcd for C98H84N4O12, 377.44; found 377.40. 2.3. Characterizations

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Dynamic light scattering (DLS) measurement. DLS data were collected on a DynaPro NanoStar (Wyatt Technology) with a gallium-arsenide diode laser of 658 nm emission. The instrument has a temperature-controlled sample holder (precision of 0.1℃) for a quartz cuvette of 10 mL. Scattering

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data were collected at an angle of θ=90o and processed using the software program DYNAMICS

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V6, version 6.3.40. The autocorrelation functions were analyzed with the CONTIN method.

NMR spectra. 1H NMR spectra and 2D ROESY NMR spectra were recorded on a Bruker Avance 400 spectrometer (400 MHz). DOSY experiments were carried out with a Bruker Avance 600 NMR Spectrometer.

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ESI MS spectra. The electronic spray ionization (ESI) high-resolution mass spectra were obtained in the positive ion mode on a Bruker Daltonics Microflex spectrometer.

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UV-vis spectra. UV-vis spectra were obtained on a Shimadzu UV-1601PC spectrophotometer.

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Fluorescence spectroscopy. Steady-state fluorescence measurements were carried out using a Hitachi 4500 spectrophotometer. Fluorescence decay surfaces were determined by single photon counting technique using a FLS920 Edinburgh spectrometer. To determine the absolute quantum yield of the compound, the excitation wavelength was set at 320 nm. The scattering spectral range of blank and sample was from 320 nm to 380 nm, and the emission spectral range was from 320 nm to 660 nm.

ACCEPTED MANUSCRIPT Viscosity measurements. Viscosity measurements were carried out with a micro-Ubbelohde dilution viscometer at 25 ℃ in H2O.

Transmission electron microscopy (TEM) measurements. TEM images were obtained on a

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JEM 2100 and JEM 2100F operating at 200 kV. Samples for TEM measurement were prepared by

slow evaporation.

3. Results and discussion

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Scheme 2

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dropping the mixture aqueous solution on carbon-coated copper grid (300 mesh) and drying by

Tetra-pyridine-appended tetra-(4-pyridylphenyl)ethylene, as tetraphenylethylene derivative was synthesized according to our previous work. After carrying out one-step reaction as shown in Scheme 2, we gained supramolecular polymer monomer TPEC by recrystallization and filtration

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as a yellow solid in a yield of 75% which was confirmed by 1H NMR (Fig. S1) and ESI-MS (Fig. S2). It dissolves well both in organic solvents and water. Furthermore, this provides conditions for

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the construction of the dual-modality supramolecular polymers based on aggregation-induced

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emission in aqueous solution.

Fig. 1

Supramolecular hyperbranched polymers are formed through host-guest interactions, which

can be confirmed by 1H NMR spectroscopy of TPEC before and after the addition of γ-CD. When the host-guest interaction occurred, coumarin moiety would get into the cavity of γ-CD, the proton signals of the coumarin moiety (Hc, Hf and Hg) moved to high field because of the shielding effect of the cavity. We have observed the movement of the position in 1H NMR spectroscopy (Fig. S3).

ACCEPTED MANUSCRIPT This effectively proved that the coumarin moiety had entered to the cavity of γ-CD and formed supramolecular hyperbranched polymers. Furthermore, 2D ROESY NMR spectrum (Fig. S4) of the complex shows strong NOE signals between the protons of coumarin and the inner protons of

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γ-CD, which clearly demonstrates that the coumarin moiety of TPEC enters into the cavity of γ-CD to form the complex. In order to further demonstrate host-guest coordination between TPEC and γ-CD, a job's plot (Fig. 1) was used to confirm the binding stoichiometry and host-guest

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interaction between TPEC and γ-CD. We used the variable control method to keep the

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concentration of the mixture of TPEC and γ-CD unchanged (52.5 µM), and constantly changed the molar ratio of TPEC from 0 to 1. The intensity of the fluorescence emission at 560 nm was recorded each time when the molar ratio of TPEC was changed, so that the dependence of the intensity on the molar ratio of TPEC could be determined. The change of the emission reached a

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maximum at a ratio of 0.33 in job's plot. This result fully demonstrates that the optimum complexation ratio of TPEC and γ-CD is 1:2, indicating that the coumarin moieties appended in

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TPEC are efficiently encapsulated by γ-CD to form supramolecular hyperbranched polymers.

In order to further characterize the formation of supramolecular hyperbranched polymers,

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diffusion-ordered NMR spectroscopy (DOSY) was employed to determine the diffusion coefficients before and after supramolecular polymerization (Fig. S5). The average diffusion coefficients of TPEC and γ-CD were measured to be 3.21 × 10-10 m2 s-1 and 2.24 × 10-10 m2 s-1, respectively. After formation of supramolecular hyperbranched polymers, it gives an independent and relatively low diffusion coefficient with a value of 1.01 × 10−10 m2 s−1. The significant decrease of the diffusion coefficient implies that large polymeric species have formed. To gain additional evidence for the formation of supramolecular hyperbranched polymers, viscosity

ACCEPTED MANUSCRIPT measurements on solutions of TPEC+2γ-CD in H2O were performed at room temperature by using a micro-Ubbelohde viscometer. The supramolecular hyperbranched polymers assembled from the monomer exhibited a viscosity transition, and was characterized by a change in slope in

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the double logarithmic plot of specific viscosity versus concentration (Fig. S6). The slope of the curve was 0.38 in the low-concentration region, demonstrating a linear relationship between specific viscosity and concentration, which is characteristic of non-interacting assemblies of

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constant size. When the concentration increased above the critical polymerization concentration

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(CPC; approximately 3 mmol/L), a sharp increase in the viscosity was observed (slope = 0.86), indicating a transition from non-interacting assemblies to supramolecular hyperbranched polymers with increased size.

Fig. 2

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Furthermore, in order to demonstrate the formation of supramolecular hyperbranched polymer and the attractive aggregation-induced emission property, we employed UV-vis spectroscopy and fluorescence spectroscopy to illustrate this. As shown in Fig. 2a, TPEC displays a maximum

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absorption peak at around 280 nm and 320 nm due to the presence of coumarin moieties and

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tetraphenylethylene. The addition of γ-CD to the aqueous solution of TPEC causes a significant decrease in the absorbance of the coumarin units and tetraphenylethylene at around 280 nm and 320 nm, indicating that the coumarin groups are encapsulated in the cavity of γ-CD. There are two functional groups in TPEC, one is coumarin unit, and another is tetraphenylethylene skeleton. Moreover, they each have a maximum emission peaks at 425 nm and 560 nm, as shown in fluorescence spectroscopy (Fig. 2b). After adding γ-CD to the aqueous solution of TPEC, the emission peaks at 425 nm of coumarin unit decreased, and the emission peaks at 560 nm of

ACCEPTED MANUSCRIPT tetraphenylethylene skeleton increased, indicating that coumarin moieties are encapsulated in the cavity of γ-CD. At the same time, with the addition of γ-CD, the intramolecular rotation is blocked, the non-radiative decay channels are suppressed, and the excited molecules can only return to the

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ground state by radiation decay or non-radiative transition, therefore the intensity of tetraphenylethylene skeleton at 560 nm has an enhancement. On the other hand, there is not obvious increasement of emission intensity with continuing to increase the quantity of γ-CD on

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the basis of two equivalents, suggesting their strong binding affinity and the formation of a 1:2

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inclusion complex which was in good agreement with that confirmed by the job's plot (Fig. 1). In addition, such γ-CD-induced large fluorescence change of TPEC was clearly observed by naked eyes. When the mixed solution of TPEC and γ-CD was excited by using a UV lamp, we can see a strong yellow fluorescence appearing, further supporting the proposed mechanism above. And the

respectively.

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fluorescence quantum yields of TPEC before and after addition of γ-CD are 18.25％ and 21.44％,

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Fig. 3

In order to demonstrate reversibility of the switching process between non-covalent polymer

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and covalent polymer, UV-vis spectroscopy and fluorescence spectroscopy were employed. At first, we prepared a mixed aqueous solution of TPEC and γ-CD according to molar ratio at 1:2 under room temperature. With the increase of irradiation time at 365nm, the absorption at 320 nm and the fluorescence emission at 425 nm and 560 nm both decreased gradually (Fig. 3a and b), which exhibited the photodimerization of coumarin occured in the non-covalent bonds through encapsulating in the cavity of γ-CD. And the non-covalent bonds achieved the conversion to the covalent bonds. Meanwhile, with the increase of irradiation time at 254 nm, the absorption at 320

ACCEPTED MANUSCRIPT nm and the fluorescence emission at 425 nm and 560 nm both increased gradually (Fig. 3c and d), suggesting that photochemical cleavage was happened on the covalent bonds. Moreover, the form of the chemical bonds returned from covalent bonds to the initial state of non-covalent bonds. In

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addition, the absorption and fluorescence emission intensity returned back to the state before

different wavelength at 365 nm and 254 nm. Fig. 4 Fig. 5

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irradiation. This proved that the polymerization process was reversible under the irradiation of two

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After establishing the dual-modality supramolecular hyperbranched polymers in water, we further investigate the self-assembly behaviors of the complex between γ-CD and TPEC by using dynamic light scattering (DLS) and transmission electron microscopy (TEM). The aggregated species of the supramolecular hyperbranched polymers in aqueous solution was corroborated by

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DLS experiments (Fig. 4). CONTIN analysis of the DLS autocorrelation function of the supramolecular hyperbranched polymers shows different distribution of hydrodynamic diameter

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(Dh) of aggregates before and after irradiation at 365 nm. The main distribution of hydrodynamic diameter (Dh) of the aggregates centered at ~60 nm in aqueous solution before irradiation.

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Moreover, the hydrodynamic diameter (Dh) of supramolecular hyperbranched polymer is slightly smaller. The main distribution of hydrodynamic diameter (Dh) of the aggregates centered at ~40 nm in aqueous solution after irradiation for 12 hours at 365 nm UV light. This further demonstrated that the photodimerization was happened in the non-covalent bonds under the irradiation of 365 nm, and became covalent bonds. TEM was performed to gain a direct visualization of the size and morphology (Fig. 5). The polymers formed in both states are spherical aggregates. We can also find that the size of the supramolecular hyperbranched polymer

ACCEPTED MANUSCRIPT is obviously smaller by comparing Fig. 5c and d to Fig. 5a and b, which is consistent with the data of DLS.

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4. Conclusions

In summary, we have prepared a dual-modality supramolecular hyperbranched polymer with aggregation-induced emission in aqueous solution by taking advantage of host−guest interactions

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between γ-CD and coumarin moieties. Because of coumarin moieties were encapsulated in the cavity of γ-CD, the novel non-covalent polymers can be photochemically converted to the

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corresponding covalent polymers by UV light irradiation at 365 nm, and the covalent polymers can be depolymerized return to the state of non-covalent state by UV light irradiation at 254 nm. This chemical process is reversible. In addition, the intramolecular rotation of the phenyl rings of TPEC was hampered upon the addition of γ-CD, so the complex emits strong fluorescence in

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dilute solution. The simplicity and tunability highlights this methodology to construct novel photoswitchable supramolecular non-covalent/covalent polymers will provide a new effective

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method for construction of advanced functional materials.

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Acknowledgements

We are grateful for the financial support from the National Natural Science Foundation of China (21402108 and 51804188), Natural Science Foundation of Shandong Province (ZR2014BQ036 and ZR2018BEE015) and Key Laboratory of Photochemical Conversion and Optoelectronic Materials, TIPC, CAS (PCOM201814).

Appendix A. Supplementary data

Supplementary data associated with this article can be found in the online version.

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Scheme and Figure Captions

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Scheme 1. Schematic representation for the preparation of the supramolecular non-covalent linked polymer by host–guest interaction between TPEC and γ-CDs, and the photoswitching of its non-covalent linked polymer and corresponding covalent polymer by alternating light irradiation

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Scheme 2. Synthesis route of TPEC.

Fig. 1. Job's plot of the change in emission spectra at 560 nm; excitation wavelength was 320 nm; the total concentration of TPEC and γ-CD was fixed at 52.5 µM.

Fig. 2. (a) UV/vis spectra of TPEC (52.5 µM, black) and TPEC+2γ-CD (52.5 µM, red); (b)

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Fluorescence spectra of TPEC (52.5 µM, black) and TPEC+2γ-CD (52.5 µM, red). The inset photographs show the corresponding fluorescence changes of TPEC and TPEC+2γ-CD.

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Fig. 3. The absorption and fluorescence spectra changes of TPEC+2γ-CD in aqueous solution on

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irradiation at 365 nm (a, b), and then on irradiation at 254 nm (c, d), 52.5 µM, 298 K.

Fig. 4. DLS result of a) TPEC+2γ-CD aqueous solution and b) TPEC+2γ-CD irradiated 12 hours with 365 nm UV light.

Fig. 5. TEM result of a), b) TPEC+2γ-CD aqueous solution and c), d) TPEC+2γ-CD irradiated 12 hours with 365 nm UV light.

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ACCEPTED MANUSCRIPT Highlights Supramolecular polymers were constructed through host-guest interactions between γ-CD

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and coumarin.

Supramolecular polymers exhibited aggregation-induced emission.

Non-covalent and covalent supramolecular polymers can be reversibly controlled through the

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Supramolecular polymers self-assembled into spherical structures.