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Liquid Phase Epitaxial Growth and Optical Properties of Photochromic Guest-Encapsulated MOF Thin Film Wen-Qiang Fu,† Min Liu,† Zhi-Gang Gu,*,‡ Shu-Mei Chen,*,† and Jian Zhang‡ †

College of Chemistry, Fuzhou University, Fuzhou, Fujian 350108, P. R. China State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, 350002 Fuzhou, P. R. China

‡

S Supporting Information *

ABSTRACT: The reversible photochromic molecule azobenzene was encapsulated into pores of metal organic frameworks (MOFs) using a modified liquid-phase epitaxial layer-by-layer method successfully. The obtained thin film not only has an oriented, homogeneous film with effective guest encapsulation but also has an isomerization between trans- and cisazobenzene in MOF pores under UV and visible light irradiation. Furthermore, the photoluminescent property was studied for azobenzene loaded HKUST-1 thin film with different temperature. This facile preparation strategy and optical property of photochromic guest encapsulation into porous MOF thin film will provide the development of new optical thin film materials possessing photoswitching and photoluminescent properties.

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species.19,20 Although there are some azobenzene groupcontaining MOFs for preparing new photoswitching MOFs,21−23 the synthesis of the molecules with azobenzene group for building MOFs is very difficult and timeconsuming.24−26 Therefore, the facile strategy is encapsulating the commercial azobenzene molecules with photochromism into porous MOF to develop composite materials with new or enhanced property. Although in the past decade some functional guests (e.g., quantum dot, dye molecules, lanthanide ions) have been encapsulated into the pores of MOFs by using one-pot hydrothermal method or immersing the guest solutions,27 the compositions of the obtained materials with these methods are not easy to control and limited to encapsulate guest with high efficiency. In our recent work, we have developed a modified liquid phase epitaxial (LPE) method for encapsulation of guest (Ti-oxo clusters and lanthanide coordination compounds) in the pores of HKUTST-1 (Cu3BTC2, H3BTC = 1,3,5-benzenetricarboxylic acid) with in situ layer by layer fashion.28,29 Thus, approach offers an efficient method to control the growth orientations, thickness, and homogeneity of thin films. More importantly, the well-defined layer-by-layer LPE assembly fashion encapsulates the functional guest into MOFs without disrupting the growth of the MOF structures. As an extension of our previous works, herein, we use modified LPE method to encapsulate azobenzene molecule directly into MOF thin film (Scheme 1 bottom and Scheme

INTRODUCTION The phenomenon of photochromism is well-known as the light induced reversible change of color, which has developed rapidly to improve the established materials and to discover new devices and sensors for applications.1−5 As photochromic molecules, azobenzene derivatives with two phenyl rings separated by an −NN− double bond represent the most widely studied photochromism systems.6−8 Upon irradiation with appropriate wavelength light, it is possible to switch between trans- and cis-isomerization. The large change in the geometry and polarity of the molecule associated with the reversible light induced isomerization has attracted considerable attention for a wide range of applications on data storage, optical switching, polarization holography, and liquid crystal devices due to its high light sensitivity.7,8 Incorporation of azobenzene in various self-assembled materials, such as supramolecules, polymers, and ionic liquids, has facilitated the modulation of their reversible functional properties.9−11 Particularly, so far many researchers have introduced the azocompounds on the porous solid materials to obtain photofunctional materials and study the photochemical behavior.12−15 As a class of hybrid inorganic−organic porous materials, metal−organic frameworks (MOFs) are crystalline materials formed by connecting metal ions via organic linkers, providing a candidate for many applications in gas storage and separation, catalysis, magnetism, and sensing.16−18 The high ordered micropores in MOFs provide as microcontainers for encapsulating functional guest to design new materials and study the interaction between host framework and guest © 2016 American Chemical Society

Received: June 19, 2016 Revised: July 17, 2016 Published: July 18, 2016 5487

DOI: 10.1021/acs.cgd.6b00935 Cryst. Growth Des. 2016, 16, 5487−5492

Crystal Growth & Design

Article

X-ray photoelectron spectroscopy (XPS) spectra for the azobenzene@ HKUST-1 thin film was measured by ESCALAB 250Xi. UV−vis spectra for the solid samples were performed on Lambda35. Fluorescence spectra for the sample in present work were performed on an Edinburgh Analytical instrument FLS920. Preparation of Functionalized Substrates. The COOHfunctionalized self-assembled monolayers (SAMs) on Au were prepared by immersing Au substrates into 1 mM/L ethanolic solutions of 16-mercaptohexadecanoic acid (MHDA) for 24 h and then rinsed with the pure ethanol and dried under nitrogen flux for the next preparation. The OH-terminated quartz glass substrates were treated with a mixture of concentrated sulfuric acid and hydrogen peroxide (30%) with a volume ratio 3:1 at 80 °C for 30 min and then cleaned with deionized water and dried under nitrogen flux for the next preparation. Fabrication of HKUST-1 Thin Film. HKUST-1 thin films used in the present work were grown on OH-terminated quartz glass substrate using the liquid-phase epitaxy (LPE) spray method, which yields thin film with [111] orientation. The HKUST-1 thin film was fabricated using the following diluted ethanolic solutions: copper acetate (1 mM) and BTC (1,3,5benzenetricarboxylic acid) (0.4 mM). The modified pump method is adopted in this work, which is described in an earlier work. The immersion times were 15 min for the copper acetate solution and 30 min for the BTC solution. Each step was followed by a spray step with pure ethanol to remove residual reactants. A total of 100 growth cycles were used for HKUST-1 grown on functionalized Au and quartz glass substrates in this work. Fabrication of trans-Azobenzene@HKUST-1 Thin Film. The trans-azobenzene@HKUST-1 thin film was fabricated using the following diluted ethanolic solutions: copper acetate (1 mM), BTC (1,3,5-benzenetricarboxylic acid) (0.4 mM), and trans-azobenzene solution (0.5 mM). The modified pump method is adopted in this work, which is described in an earlier work. The immersion times were 15 min for the copper acetate solution, 30 min for the BTC solution, and 10 min for trans-azobenzene solution. Each step was followed by a spray step with pure ethanol to remove residual reactants. A total of 100 growth cycles were used for trans-azobenzene@HKUST-1 grown on functionalized Au and quartz glass substrates in this work.

Scheme 1. Schematic Presentation of the Reversible Isomerization between trans- and cis-Azobenzene under Photoinduction (top); in Situ Layer-by-Layer Growth of trans-Azobenzene Encapsulated HKUST-1 Thin Film Using Modified LPE Method, Showing a cis- to trans-Azobenzene@ HKUST-1 Film (bottom)

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S1) since it can offer a high effective and homogeneous encapsulation. The commercial trans-azobenzene was selected as the photochromic guest species, which are well-known for preparing photoswitching materials and studying the photochemistry in dynamic behavior. The typical 3-D MOF HKUST130 containing large numbers of ordered micropores was chosen as the host framework for guest encapsulation. Here, the photochromic guests were encapsulated into MOF thin film using modified LPE method successfully and used for studying the photochromic behavior between trans- and cis-isomerization under photoinduction or temperature, which may open a facile strategy to develop new composite materials for the applications in optical sensors and devices.

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RESULTS AND DISCUSSION The azobenzene molecule is very important on the application of the light emission materials. Recently the azobenzene functionalized organic linkers used to assemble azo-MOF exhibits the expected isomerization when exposed to UV and visible light. However, the synthesis of azobenzene functionalized linkers is expensive and time-consuming. The facile way to synthesize the azo-containing MOF is encapsulating the azobenzene into MOF pores, which also can be shown with expected isomerization and applied for the photoswitching. The UV−vis spectra of azobenzene ethanolic solution is dominated by two absorbencies, the intense wavelength π−π* absorbance band at ∼317 nm and the weak n−π* absorbance band around 432 nm as shown in Scheme 1 (top) and Figure S1. After irradiation with 365 nm for 0, 1, 6, 11, 21, and 31 min, respectively, the n−π* absorbance around 432 nm increases, and the π−π* absorbance around 317 nm is decreased, indicating the occurrence of trans- to cis-photoisomerization. When the sample is exposed to the visible light irradiation for 1, 2, 3, 5, 10, and 20 min, respectively, the increased π−π* absorbance band at ∼317 nm and the decreased n−π* absorbance band at ∼432 nm showed that the spectrum changes continuously from the cis- to the trans-state, demonstrating a good reversible photochromic phenomenon under different wavelength light irradiation. While previous studies of functional guest were encapsulated into MOF pores with high orientation, homogeneity, and high

EXPERIMENTAL SECTION

Materials and Instruments. All the reagents and solvents employed were commercially available and were used as received without further purification. The quartz glass substrates were treated with a mixture of concentrated sulfuric acid and hydrogen peroxide (30%) with a volume ratio 3:1 at 80 °C for 30 min and then cleaned with deionized water and dried under nitrogen flux for the next preparation. The samples grown on functionalized Au substrate were characterized with infrared reflection absorption spectroscopy (IRRAS). IRRAS data were recorded using a Bruker Vertex 70 FTIR spectrometer with 2 cm−1 resolution at an angle of incidence of 80° relative to the surface normal. Powder X-ray diffraction (PXRD) analysis was performed on a MiniFlex2 X-ray diffractometer using Cu−Kα radiation (λ = 0.1542 nm) in the 2θ range of 5−20° with a scanning rate of 0.5° min−1. Scanning electron microscope (SEM) images for the morphology of thin films were measured by JSM6700. 5488



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Figure 1. Out of plane XRD (a), IRRAS (b), Raman spectra (d), and reflection spectra (f) of HKUST-1 thin film and azobenzene encapsulated HKUST-1 thin film prepared by in situ LPE method; optical image with rainbow color (c) and XPS spectra (e) of azobenzene encapsulated HKUST1 thin film prepared by in situ modified LPE method.

Figure 2. Optical morphology of HKUST-1 thin film (a) and azobenzene encapsulated HKUST-1 thin film (b). SEM image of azobenzene encapsulated HKUST-1 thin film.

with [100] orientation due to the flexible azobenzene molecules influence the perfect growth crystalline orientation. Using this method to grow trans-azobenzene@HKUST-1 film on MHDASAM functionalized Au substrate (MHDA = 16-mercaptohexadecanoic acid), the relative intensities of (200)/(400) were decreased compared to the pristine HKUST-1 thin film (Figure S2), which can demonstrate that the trans-azobenzene was loaded in the pores of HKUST-1.31,32 For studying the IRRAS (infrared reflection−absorption spectroscopy) spectra of transazobenzene@HKUST-1 films, the new IRRAS absorbance bands (Figure 1b) at 1535 cm−1 are ascribed to −NN− vibrations of trans-azobenzene and the change in the reflection adsorption spectra at ∼515 nm (Figure 1f), combining to the Raman shift (Figure 1d) at 1538, 1003, 826, and 742 cm−1 and color change in the optical images (Figure 2a,b) showing that the molecule trans-azobenzene was encapsulated into HKUST1 film successfully. Furthermore, the XPS spectra (Figure 1e) at 403 eV is identical to the N 1s in azobenzene. The optical image with rainbow color (Figure 1c) and SEM images (Figure 2c) showed a homogeneous composite film, and EDS data (Figure S3) displayed an effective encapsulation with 20% (calculated by the ratio of N/Cu).

effectiveness using LPE layer by layer method, the commercial azobenzene was encapsulated into MOFs using the same method. The setup of preparation procedure was shown in Figure S1, which was called modified LPE pump method. During the preparation, the Cu(OAc)2, H3BTC, transazobenzene ethanolic solutions, and pure ethanol solutions were injected into the reaction cell with hydroxyl-functionalized quartz glasses, and 16-mercaptohexadecanoic acid (MHDA) SAMs-functionalized Au substrate sequentially. The reaction solution was pumped out from cell after each reaction step. Then a thin film was obtained after repeating 100 cycles, which is named trans-azobenzene@HKUST-1 thin film. The obtained trans-azobenzene@HKUST-1 composite film was characterized by XRD, IRRAS, SEM, EDS, and XPS. The out of plane X-ray diffraction showed XRD peaks of all the samples at 6.8, 13.6, 11.6, and 17.5° are in accord with the simulated XRD peaks at (200), (400), (222), and (333) of HKUST-1, indicating that the framework of thin film maintained the crystallinity and grow along [100] and [111] orientations on the functionalized quartz glasses (Figure 1a). Compared to the simulated powder XRD of pristine HKUST-1 grown on quartz glass, there is a weak peak at (200) and (400) 5489



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In order to investigate photoisomerization of the obtained composite film, the OH-functionalized quartz glass was chosen as the substrate for preparation of trans-azobenene@HKUST-1 thin film. Compared to the UV−vis spectra of azobenzene solution, the UV−vis spectra of trans-azobenzene encapsulated HKUST-1 on quartz glass showed a different absorbance band using pristine HKUST-1 thin film with 100 cycles as background (Figure 3). The absorbance bands at ∼270 and

Figure 4. Photoluminescent property (a) of azobenzene@HKUST-1 thin film at the excitation wavelength 374 nm under 303, 323, and 353 K, respectively. Note: R- square is 0.892. (b) Linear relationship between relative intensity and temperature after fitting the relative intensity of the absorbance band at 560 nm.

intensity after increasing the temperature. Figure 4b showed there was a linear relationship between relative intensity and temperature after fitting the relative intensity of the absorbance band at 560 nm. The photoluminescent property of azobenzene loaded into HKUST-1 thin film under different temperature demonstrated the photoisomeric guest can be loaded into MOF thin film to form the dynamics optical film material with photoluminescent emission.

Figure 3. UV−vis absorption spectra of trans-azobenzene@HKUST-1 thin film and powder solid azobenzene. Time-dependent UV−vis absorption spectra of trans-azobenzene@HKUST-1 thin film grown on quartz glass with 365 nm irradiation for 0, 1, 6, 15, and 30 min, respectively; followed by visible light irradiation for 1, 6, 15, and 30 min, respectively.

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CONCLUSION In summary, we have applied a modified liquid phase epitaxial pump method for preparation of photochromic molecule azobenzene loaded MOF HKUST-1 with high oriented and homogeneous thin film. The obtained thin film not only had a photoisomerization behavior under ultraviolet (trans- to cis-) and visible light irradiation (cis- to trans-) when azobenzene was encapsulated into HKUST-1 pores but also had a temperaturedependent photoluminescent emission. This study of azobenzene encapsulation into porous MOF thin film can provide a facile strategy for preparing the photochromic thin film materials and open a new dynamic of optical properties for the development of multifunctional optical devices and sensors.

∼370 nm were in accord to that of solid powder azobenzene (Figure 3a). The absorbance band at ∼270 nm was decreased (trans- to cis-photoisomerization) after UV light irradiation for 1, 6, 15, and 30 min in the UV−vis spectra of azobenzene@ HKUST-1 film and was increased (cis- to trans-photoisomerization) after visible light irradiation for 1, 6, 15, and 30 min, showing that a photoswitching thin film was obtained. The photoluminescent property of azobenzene@HKUST-1 thin film has been investigated and shows the emission maxima at ∼560 nm (λex = 374 nm, Figure S4) as shown in Figure 4a, which is attributable to luminescent emission of powder azobenzene molecule due to the fact that Cu(II) in HKUST-1 can quench the luminescence of the host framework and is identical to the luminescent property of solid azobenzene (Figure S5). In addition, the temperature-dependent photoluminescent property of azobenzene@HKUST-1 thin film was carried under 303, 323, and 353 K showed a slightly decreased

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.6b00935. 5490



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Article

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Experimental and characterization details; additional figures and images; XRD patterns, EDS, and photoluminescent spectrum (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the NSFC (21425102 and 21521061). REFERENCES

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