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. 2015 Sep 30:6:8508.
doi: 10.1038/ncomms9508.

A tunable azine covalent organic framework platform for visible light-induced hydrogen generation

Vijay S Vyas  1 , Frederik Haase  1   2 , Linus Stegbauer  1   2 , Gökcen Savasci  2 , Filip Podjaski  1 , Christian Ochsenfeld  2   3 , Bettina V Lotsch  1   2   4
Affiliations

A tunable azine covalent organic framework platform for visible light-induced hydrogen generation

Vijay S Vyas et al. Nat Commun. .

Abstract

Hydrogen evolution from photocatalytic reduction of water holds promise as a sustainable source of carbon-free energy. Covalent organic frameworks (COFs) present an interesting new class of photoactive materials, which combine three key features relevant to the photocatalytic process, namely crystallinity, porosity and tunability. Here we synthesize a series of water- and photostable 2D azine-linked COFs from hydrazine and triphenylarene aldehydes with varying number of nitrogen atoms. The electronic and steric variations in the precursors are transferred to the resulting frameworks, thus leading to a progressively enhanced light-induced hydrogen evolution with increasing nitrogen content in the frameworks. Our results demonstrate that by the rational design of COFs on a molecular level, it is possible to precisely adjust their structural and optoelectronic properties, thus resulting in enhanced photocatalytic activities. This is expected to spur further interest in these photofunctional frameworks where rational supramolecular engineering may lead to new material applications.

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Figures

Figure 1
Figure 1. Design and synthesis of the Nx–COFs.
(a) A tunable triphenylarene platform for photocatalytic hydrogen evolution. Replacement of ‘C–H' by ‘nitrogen atoms' at the green dots changes the angle between central aryl and peripheral phenyl rings, which leads to varied planarity in the platform. (b) Synthesis of Nx–COFs from Nx–aldehydes and hydrazine.
Figure 2
Figure 2. 13C cross-polarization magic angle spinning solid-state NMR of the Nx–COFs.
The azine C=N peak (marked a) appears at ≈160 p.p.m. while the phenyl peaks (marked b) and characteristic central aryl peaks (marked c,d,e) show minimal changes with respect to their precursor aldehydes.
Figure 3
Figure 3. Structure and stacking analysis of the Nx–COFs.
(a) PXRD patterns of the Nx–COFs compared with the simulated pattern calculated for the representative N3–COF. (b) View of extended stacks of Nx–COFs in space filling model along the stacking direction (nitrogen, blue; carbon, grey; hydrogen, white). Note that an eclipsed stacking arrangement was assumed for simplicity.
Figure 4
Figure 4. SEM and TEM images of Nx–COFs.
(a) SEM images of N0–COF, (b) N1–COF, (c) N2–COF and (d) N3–COF indicating morphological variation along the series. (e) TEM image of N2–COF showing hexagonal pores, with fast Fourier transform (FFT) of the marked area (red circle) in the inset. (f) TEM image of N3–COF with enlarged Fourier-filtered image (upper inset) of the marked area and representative selected area electron diffraction pattern (lower inset). Scale bars, 5 μm (a,b,c,d); 50 nm (e and f); 20 nm (f, upper inset).
Figure 5
Figure 5. Optical and photocatalytic properties of Nx–COFs.
(a) Diffuse reflectance spectra of Nx–Alds and Nx–COFs recorded in the solid state. (b) Absorption spectra of precursor aldehydes Nx–Alds in dichloromethane at 22 °C. (c) Hydrogen production monitored over 8 h using Nx–COFs as photocatalyst in the presence of triethanolamine as sacrificial electron donor. (d) Photonic efficiency (PE) measured with four different band-pass filters with central wavelengths (CWLs) at 400, 450, 500 and 550 nm.
Figure 6
Figure 6. General structure of the model systems used for theoretical calculations.
N0–: X=Y=Z=C–H. N1–: X=Y=C–H; Z=N. N2–: X=C–H; Y=Z=N. N3–: X=Y=Z=N.
Figure 7
Figure 7. Kohn-Sham HOMO and LUMO energies of different model systems with Nx central core.
(a) Nx–Ald and Nx–PhAz; (b) hexagons with hydrazone (Nx–HxHz) and aldehyde terminations (Nx–HxAl); and (c) stacked hexagon layers of N3–COF with hydrazone (N3–HxHz) and aldehyde terminations (N3–HxAl).
Figure 8
Figure 8. Schematic representation of two possible pathways after photoexcitation of Nx–COFs.
Quenching the hole on the COF by the sacrificial electron donor leads to a radical anionic state for the COF (radical anion pathway, red arrow). The opposite order leads to the radical cationic pathway (dotted black arrows). Energies in red depict calculated vertical electron affinities as differences in total energies between radical anionic and neutral states of Nx–HxHz model systems at PBE0–D3/Def2–SVP level. Asterisk (*) denotes the excited state.
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References

    1. Ding S. Y. & Wang W. Covalent organic frameworks (COFs): from design to applications. Chem. Soc. Rev. 42, 548–568 (2013). - PubMed
    1. Feng X., Ding X. S. & Jiang D. Covalent organic frameworks. Chem. Soc. Rev. 41, 6010–6022 (2012). - PubMed
    1. Fang Q. et al.. Designed synthesis of large-pore crystalline polyimide covalent organic frameworks. Nat. Commun. 5, 4503 (2014). - PubMed
    1. El-Kaderi H. M. et al.. Designed synthesis of 3D covalent organic frameworks. Science 316, 268–272 (2007). - PubMed
    1. Yu J.-T., Chen Z., Sun J., Huang Z.-T. & Zheng Q.-Y. Cyclotricatechylene based porous crystalline material: synthesis and applications in gas storage. J. Mater. Chem. 22, 5369–5373 (2012).

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