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. 2018 Jun 6;140(22):6969-6977.
doi: 10.1021/jacs.8b02839. Epub 2018 May 25.

Exploring Applications of Covalent Organic Frameworks: Homogeneous Reticulation of Radicals for Dynamic Nuclear Polarization

Wei Cao  1 , Wei David Wang  1 , Hai-Sen Xu  1 , Ivan V Sergeyev  2 , Jochem Struppe  2 , Xiaoling Wang  3 , Frederic Mentink-Vigier  3 , Zhehong Gan  3 , Ming-Xing Xiao  1 , Lu-Yao Wang  1 , Guo-Peng Chen  1 , San-Yuan Ding  1 , Shi Bai  4 , Wei Wang  1   5
Affiliations

Exploring Applications of Covalent Organic Frameworks: Homogeneous Reticulation of Radicals for Dynamic Nuclear Polarization

Wei Cao et al. J Am Chem Soc. .

Abstract

Rapid progress has been witnessed in the past decade in the fields of covalent organic frameworks (COFs) and dynamic nuclear polarization (DNP). In this contribution, we bridge these two fields by constructing radical-embedded COFs as promising DNP agents. Via polarization transfer from unpaired electrons to nuclei, DNP realizes significant enhancement of NMR signal intensities. One of the crucial issues in DNP is to screen for suitable radicals to act as efficient polarizing agents, the basic criteria for which are homogeneous distribution and fixed orientation of unpaired electrons. We therefore envisioned that the crystalline and porous structures of COFs, if evenly embedded with radicals, may work as a new "crystalline sponge" for DNP experiments. As a proof of concept, we constructed a series of proxyl-radical-embedded COFs (denoted as PR( x)-COFs) and successfully applied them to achieve substantial DNP enhancement. Benefiting from the bottom-up and multivariate synthetic strategies, proxyl radicals have been covalently reticulated, homogeneously distributed, and rigidly embedded into the crystalline and mesoporous frameworks with adjustable concentration ( x%). Excellent performance of PR( x)-COFs has been observed for DNP 1H, 13C, and 15N solid-state NMR enhancements. This contribution not only realizes the direct construction of radical COFs from radical monomers, but also explores the new application of COFs as DNP polarizing agents. Given that many radical COFs can therefore be rationally designed and facilely constructed with well-defined composition, distribution, and pore size, we expect that our effort will pave the way for utilizing radical COFs as standard polarizing agents in DNP NMR experiments.

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Conflict of interest statement

Notes

The authors declare no competing financial interest.

Figures

Figure 1
Figure 1
(a) Experimental PXRD pattern (black), Pawley refined pattern (red), difference plot (purple), and simulated pattern (blue) for the eclipsed model of PR(100)-COF. Inset: Extended structure of PR(100)-COF. C, gray; N, blue; O, red; H atoms omitted for clarity. (b) N2 adsorption (filled symbols) and desorption (empty symbols) isotherms of PR(100)-COF. Inset: pore-size-distribution of PR(100)-COF. (c) 129Xe NMR spectrum of PR(100)-COF; the pressure of xenon adsorbed in PR(100)-COF was controlled as 4.1 bar.
Figure 2
Figure 2
13C CP/MAS NMR spectra of PR(x)-COFs. Asterisks denote the spinning sidebands. Assignments of 13C chemical shifts of PR(x)-COFs are indicated in the chemical structure. Note that the 13C NMR signals in the radical region (146, 134, 76, 68, 31 ppm in Figure S41) are absent.
Figure 3
Figure 3
CW X band (9.5 GHz) EPR spectra of PR(x)-COFs recorded at 293 K in the solid state. Intensity was corrected for different receiver gain value and power attenuation. Upon the comparison of the double integral of the EPR signal to that of monomer 1, the derived radical concentration (Crad, from 0.02 to 1.80 mmol g−1) has been listed on the top right corner in each case. The Crad value showed a linear correlation with the ingredient proportion (x%, from 2 to 100%) of the radical monomer 1 (Figure S44).
Figure 4
Figure 4
DNP 1H NMR enhancement (εH, left) and 13C CP NMR enhancement (εC CP, right) as a function of the radical concentration (Crad) for PR(x)-COFs. DNP 1H MAS NMR spectra and 13C CP/MAS NMR spectra of PR(x)-COFs impregnated with H2O were recorded at 9.4 T and ~100 K with the microwave field on and off, respectively. The original spectra have been shown in Figures S65–S76.
Figure 5
Figure 5
(a) DNP 13C CP/MAS NMR spectrum of PR(10)-COF impregnated with H2O recorded at 9.4 T and ~100 K with the microwave (MW) field on and off, respectively. (b) DNP 15N CP/MAS NMR spectrum of PR(10)-COF impregnated with H2O recorded at 9.4 T and ~100 K with the microwave (MW) field on. Note that this spectrum could not be acquired without DNP. Asterisks denote the spinning sidebands (spinning rate of 8.0 kHz) in each spectrum.
Figure 6
Figure 6
DNP 13C CP/MAS NMR spectra of PR(15)-COF impregnated with 0.1 M H2O solution of [2,3-13C]-L-alanine recorded at different magnetic fields of 9.4 T (a) and 14.1 T (b), respectively. The spectra were acquired at ~100 K/~105 K with the microwave (MW) field on and off, respectively. Asterisks denote the spinning sidebands (spinning rates of 8.0 and 8.2 kHz, respectively).
Scheme 1
Scheme 1
Typical organic (bi)radicals for DNP NMR enhancement
Scheme 2
Scheme 2
Bottom-up construction of PR(x)-COFs for DNP NMR enhancement. The radical concentration in the COF framework can be explicitly controlled via the multivariate approach.
See this image and copyright information in PMC

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