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

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.