Light-Emitting Covalent Organic Frameworks: Fluorescence Improving

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Light-Emitting Covalent Organic Frameworks: Fluorescence Improving via Pinpoint Surgery and Selective Switch-On Sensing of Anions Zhongping Li, Ning Huang, Ka Hung Lee, Yu Feng, Shanshan Tao, Qiuhong Jiang, Yuki Nagao, Stephan Irle, and Donglin Jiang J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 18 Sep 2018 Downloaded from http://pubs.acs.org on September 18, 2018

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Journal of the American Chemical Society

Light-Emitting Covalent Organic Frameworks: Fluorescence Improving via Pinpoint Surgery and Selective Switch-On Sensing of Anions Zhongping Li,§,¶ Ning Huang,§ Ka Hung Lee,, Yu Feng,§ Shanshan Tao,§ Qiuhong Jiang,§ Yuki Nagao,¶ Stephan Irle, and Donglin Jiang*§ §

Department of Chemistry, Faculty of Science, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore

¶

Area of Materials Chemistry, School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Nomi 923-1292, Japan

Computational Sciences & Engineering Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6493, USA

Bredesen Center for Interdisciplinary Research and Graduate Education, University of Tennessee, Knoxville, TN 37996, USA

Supporting Information Placeholder ABSTRACT: Covalent organic frameworks (COFs)

offer ordered π structures that are useful for developing light-emitting materials. However, most COFs are weak in luminescence. Here we report the conversion of less emissive COFs into light-emitting materials via a pinpoint surgery on the pore walls. Deprotonation of the N–H bond to form an anionic nitrogen species in the hydrazone linkage can eliminate the nitrogen-related fluorescence quenching pathway. The resulting COF enhances the fluorescence in a linear proportion to the progress of deprotonation, achieving a 3.8-fold improved emission. This pinpoint N–H cleavage on the pore walls can be driven only by the fluoride anion while other halogen anions, including chloride, bromide, and iodide, retain inactive, enabling the selective fluorescence switch-on sensing of the fluoride anion at a ppb level.

greatly expand the diversity and availability of skeletons and pores. 4-10 These progresses in the synthesis also have driven a growing trend of exploring functions and properties that originate from the extended 2D polymer structures. By contrast to the broad scope of π building units for the structural design and framework synthesis, the development of photoactive COFs, including light-emitting function, is still in its infancy.11 Scheme 1. (A) Synthesis of hydrazone-linked TFPPyDETHz-COF. (B) The pinpoint surgery on the N–H unit of the hydrazone linkage that undergoes acidbase reaction with F– and its regeneration by reaction with acid. (C) The fluorescence enhancement mechanism. Generation of N– by deprotonation of the N– H unit eliminates the electron transfer from N lone pair to COF and enhances the luminescence.

Covalent organic frameworks (COFs) are a class of crystalline porous polymer that develops latticed structures of topologically linked organic building blocks.1-3 Especially, two dimensional (2D) COFs are unique in that they constitute covalently linked yet layered 2D polymers whose topologies, building blocks, and linkages can be predesigned. In recent years, various geometry diagrams, monomers, and condensation reactions have been explored, which ACS Paragon Plus Environment

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Light-emitting activity of 2D COFs is highly dependent on the building blocks and linkages.11 Since the layers are formed by π stacking, the luminescence of 2D COFs is affected by the aggregationenhanced and/or quenched natures of the vertices and linkers, which can be managed by stacking or exfoliation.11 By contrast, the linkage is an indispensable site that presents in both single layer and stacked frameworks; layer exfoliation or stacking can hardly change its role in luminescence. In many cases, the linkage dominates the luminescence of COFs.11e,11i,12 Especially, due to the linkages that can cause the dissipation of excitation energy via photoinduced electron

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Figure 1. (A) PXRD patterns of experimentally observed (red), Pawley refined (green), and their difference (black), simulated from the eclipsed AA-stacking mode (blue), and staggered AB-stacking mode (purple). (B) Crystal structure of TFPPy-DETHz-COF. (C) Nitrogen sorption isotherms measured at 77 K (filled dots, adsorption; open dots, desorption). (D) Pore size (red dots) and pore size distribution (black dots) profiles. (E) The FT IR spectra, (F) PXRD patterns, (G) nitrogen sorption isotherms, and (H) BET surface areas of TFPPy-DETHz-COF upon treatment in different solvents (curves in different colors are the same as shown in E).

transfer, the photoluminescence of COFs is deactivated; this is the reason why most COFs are less emissive. As a typical example, the nitrogencontaining imine and hydrazone linked COFs are poorly luminescent as a result of fluorescenc quenching by the nitrogen atoms of the linkages, even if highly luminescent vertices and linkers are employed. In this context, preventing linkageoriginated fluorescence quenching pathway to relieve the limitation inherent to the linkage would provide a promising way to luminescent COFs. Among various linkages of COFs, we looked at hydrazone because it contains an N–H bond that is active to be deprotonated.12 Here, we report the pinpoint surgery on the nitrogen site of the hydrazone linkage by scissoring the N–H bond upon deprotonation into an N– anion so that the fluorescence quenching pathway can be eliminated, enabling a great improvement of light-emitting activity of COFs. We synthesized a hydrazone-linked TFPPy-DETHzCOF by condensation of 1,3,6,8-tetrakis(4formylphenyl)pyrene (TFPPy) and 2,5diethoxyterephthalohydrazide (DETHz) in a mixture of mesitylene and dioxane (1/1 v/v) in the presence of AcOH (6 M) at 120 °C for 72 h and obtained a yellow powder in a yield of 86% (Scheme 1). TFPPy-DETHzCOF was a new COF and fully characterized using elemental analysis, Fourier transform infrared spectroscopy (FT IR), electronic absorption spectroscopy, 13 C solid-state cross polarization magic angle spinning nuclear magnetic resonance spectroscopy

(CP/MAS NMR), field emission scanning electron microscopy (FE SEM), and powder X-ray diffraction measurements (PXRD). TFPPy-DETHz-COF displayed a vibration band of the C=N bond at 1672 cm–1, which was the same as the model compound and suggested the formation of hydrazone linkages (Figure S1). The 13C CP/MAS NMR peaks at 160 and 150 ppm were assigned to the carbons of the C=O and C=N bonds, respectively (Figure S2). The signals at 140, 132, 125, and 115 ppm were assigned to the carbons of the pyrene vertices and phenyl linkers. The peaks at 13-15 and 65 ppm were attributed to the carbons of the methyl and methylene groups, respectively. FE SEM revealed that the presence of micrometer-scale long rods (Figure S3). TFPPy-DETHz-COF exhibited PXRD peaks at 3.34°, 4.58°, 6.54°, 9.84°, and 24.52°, which were assigned to the (110), (200), (220), (330), and (001) Miller indices, respectively (Figure 1A, red curve). The Pawley refined pattern (green curve) confirmed the diffraction peak assignments with negligible difference (black curve). Among various possible stacking modes, the AA stacking mode is the most stable form in energy (Table S1). The AA stacking mode (Table S2) adopts a P1 space group with a = 28.19 Å, b = 28.92 Å, c = 4.31 Å, and α = 78.35°, β = 92.98°, and γ = 96.55° yielding a PXRD pattern that agrees with the experimentally observed profile (blue curve). By contrast, the staggered AB mode (purple curve) could not meet the experimental results. TFPPy-DETHz-COF exhibited



a type-IV sorption curve (Figure 1C). The BrunauerEmmett-Teller (BET) surface area was 1090 m2 g–1 and the pore volume was 0.54 cm3 g–1 (Figure 1D, black dots). From the pore size distribution profile, TFPPy-DETHz-COF has a pore width of 2.1 nm (Figure 1D, red dots). We investigated the chemical stability by dispersing TFPPy-DETHz-COF in tetrahydrofuran (THF), water, and aqueous HCl (1 M) and NaOH (1 M) solutions, respectively, at 25 °C for 24 h. FT IR spectra revealed that TFPPy-DETHz-COF retained the vibration bands that are the same as those of assynthesized COF (Figure 1E). All the samples exhibited strong diffraction peaks in PXRD patterns without change in the peak position (Figure 1F). Moreover, the COF samples retained the mesoporous characters (Figure 1G) while the BET surface areas changed slightly (Figure 1H).

Figure 2. (A) Fluorescence spectral change of TFPPy– DETHz-COF upon addition of F . (B) Fluorescence – – spectra of TFPPy-DETHz-COF upon addition of Cl , Br , – – I , and NO3 . (C) Relative fluorescence intensity of TFPPy-DETHz-COF upon addition of anions. (D) The – plot of fluorescence intensity versus [F ]. (E) Fluorescence decay curves of TFPPy-DETHz-COF (blue curve) – 1 and the COF upon addition of F (red curve). (F) H NMR spectral change of the model compound (blue – curve) in CDCl3 upon addition of F (red curve).

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374 and 457 nm respectively (Figure S5); these emission bands are blue shifted from that of the COF. Upon excitation at 460 nm, TFPPy-DETHz-COF dispersed in THF at 25 °C emitted a weak green-yellow luminescence at 540 nm (Figure 2A, black curve) with an absolute fluorescence quantum yield of 4.5%. Interestingly, adding tetrabutylammonium fluoride to the THF dispersion of TFPPy-DETHz-COF causes a prominent enhancement of luminescence (Figure 2A, red curve). By contrast, other anions, including Cl–, Br–, I–, and NO3–, did not change the luminescence under otherwise the same conditions (Figure 2B and 2c). The fluoride anion increases the fluorescence intensity to 3.8 fold that of the original one, leading to an absolute fluorescence quantum yield of 17%. The fluorescence intensity increased in a linear proportion to the concentration of the F– anion and saturated after 4 ppm (Figure 2D). Time-resolved fluorescence spectroscopy revealed that TFPPy-DETHz-COF has a lifetime of 1.4 ns (Figure 2E, blue curve). Upon addition of F–, the lifetime increased to 2.7 ns (red curve). This increment indicates that the deprotonation by F– suppresses the electron transfer from the linkage to the pyrene skeleton, enhancing the light-emitting activity. These results suggest that the F– anion reacts with the N–H unit while other halogen anions are inert because this process is based on an acid-base reaction mechanism. We used 1H NMR spectroscopy to confirm the reaction between F– and the N–H unit. Since TFPPy-DETHz-COF is insoluble and it is difficult to conduct a solution NMR spectroscopy, instead we used the model compound for the mechanistic study. The proton peak of the N–H unit appeared at 11.2 ppm (Figure 2F, blue curve). Upon addition of F–, the peak disappeared (red curve). This distinct change confirmed the deprotonation of the N–H unit by F–, which is consistent with the acid-base reaction mechanism as proposed for the reaction between hydrazone and tetrabutylammonium fluoride.13 The pKa value of TFPPy-DETHz-COF in water was determined to be 4.95 ± 0.05 according to the reported method;14 bases (sodium salts) such as OH–, S2–, NO2–, and HPO42– in a mixture of THF and water can enhance the emission (Figure S6). In contrast, an azine-linked analogue Py-Azine COF without N–H sites did not exhibit any response to F–(Figure S7).

The TFPPy-DETHz-COF powder exhibited an electronic absorption band at 475 nm (Figure S4, red curve). By contrast, the powders of TFPPy (green curve) and model compound (black curve) showed absorption bands at 462 and 455 nm, respectively. DETHz and TFPPy when dissolved in THF emitted at ACS Paragon Plus Environment

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*[email protected] ORCID Donglin Jiang: 0000-0002-3785-1330 Notes

The authors declare no competing financial interests.

ACKNOWLEDGMENT

Figure 3. Fluorescence spectral change of TFPPy– – DETHz-COF upon addition of Cl (A, blue curve), Br – – (B, violet curve), I (C, green curve), and NO3 (D, sky – blue curve), followed by addition of F (red curves). Original COFs are black curves.

TFPPy-DETHz-COF is a fluorescence switch-on sensor because its intensity increases as [F–] is increased. TFPPy-DETHz-COF is highly sensitive to F– with a detection limit down to 50.5 ppb, which is ranked the best among F– sensors.13 Notably, interference experiments (Figure 3) revealed that the presence of other anions did not affect the fluorescence enhancement by F–, enabling the sole response to F–. Trifluoroacetic acid (TFA) can regenerate the N–H unit via protonation of the N– anion.13 Indeed, adding TFA to the solution to regenerate the N–H unit decreases the emission to an original level (Figure S8), enabling the reset of COFs. In summary, we have demonstrated a pinpoint surgery on the N–H unit of the hydrazone-linked COFs and the first example of COFs for anion sensing. Scissoring the N–H unit upon deprotonation into N– anion eliminates the electron transfer from the linkage to COF so that the fluorescence quenching is suppressed, enabling the improvement of lightemitting activity in a proportional fashion. We found that the enhancement is triggered only by the F– anion, while other halogen anions and NO3– are inert. The fluorescence switch-on sensing renders the COF able to detect F– at a ppb level. These results suggest a fundamental approach to photoluminescent COFs via pinpoint surgery. ASSOCIATED CONTENT Supporting Information Supporting information is available free of charge via the Internet at http://pubs.acs.org. Materials and methods, supporting figures, supporting tables, and supporting references.

AUTHOR INFORMATION Corresponding Author

D.J. appreciates the start-up grant of NUS (R-143-000-A28133). K.H.L. was in part supported by an Energy Science and Engineering Fellowship at the University of Tennessee, Knoxville. K.H.L. acknowledges fruitful discussions with Q.V. Vuong. S.I. acknowledges support by the Laboratory Directed Research and Development (LDRD) Program of Oak Ridge National Laboratory. ORNL is managed by UT-Battelle, LLC, for DOE under contract DE-AC05-00OR22725. Z.L. appreciates the support by JSPS KAKENHI grant number JP18J13699.

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Light-Emitting Covalent Organic Frameworks: Fluorescence Improving

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