Persistent Radicals of Self-assembled Benzophenone bis-Urea Macrocycles: Characterization and Application as a Polarizing Agent for Solid-state DNP MAS Spectroscopy

Article abstract of DOI:10.1002/chem.201701705

UV-irradiation of a self-assembled benzophenone bis-urea macrocycle generates μm amounts of radicals that persist for weeks under ambient conditions. High-field EPR and variable-temperature X-band EPR studies suggest a resonance stabilized radical pair th

Full text of DOI:10.1002/chem.201701705

A Journal of
Accepted Article
Title: Persistent Radicals of Self-assembled Benzophenone bis-urea
Macrocycles: Characterization and Application as a Polarizing
Agent for Solid-state DNP MAS Spectroscopy
Authors: Baillie A DeHaven, John T Tokarski, Arthur A Korous,
Frederic Mentink-Vigier, Thomas M Makris, Alexander M
Brugh, Malcolm D. E. Forbes, Johan van Tol, Clifford Russ
Bowers, and Linda S Shimizu
This manuscript has been accepted after peer review and appears as an
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content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.201701705
Link to VoR: http://dx.doi.org/10.1002/chem.201701705
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Persistent Radicals of Self-assembled Benzophenone bis-urea
Macrocycles: Characterization and Application as a Polarizing
Agent for Solid-state DNP MAS Spectroscopy
B. A. DeHaven,[a] J. T. Tokarski,[b] A. A. Korous,[a] F. Mentink-Vigier,[c] T. M. Makris,[a] A. M. Brugh,[d] M.
D. E. Forbes,[d] J. van Tol,[c] C. R. Bowers,[b] and L. S. Shimizu*[a]
Abstract: UV-irradiation of a self-assembled benzophenone bis-
urea macrocycle generates µM amounts of radicals that persist for
weeks under ambient conditions. High-Field EPR and variable
temperature X-band EPR studies suggest a resonance stabilized
radical pair through H-abstraction. These endogenous radicals were
applied as a polarizing agent for magic angle spinning (MAS)
dynamic nuclear polarization (DNP) NMR enhancement. The field-
stepped DNP enhancement profile exhibits a sharp peak with a
maximum enhancement of εon/off = 4 superimposed on a nearly
constant DNP enhancement of εon/off = 2 over a broad field range.
This maximum coincides with the high field EPR absorption
spectrum, consistent with an Overhauser effect mechanism. DNP
enhancement was observed for both the host and guests,
suggesting that even low levels of endogenous radicals can facilitate
the study of host-guest relationships in the solid-state.
Figure 1. A benzophenone bis-urea macrocycle self-assembles from
DMSO to form host
1 with encapsulated DMSO. A persistent
Introduction
paramagnetic species is generated when 1 is UV irradiated at 350 nm
with a Hanovia 450 W medium pressure mercury arc lamp. This long-
lived intermediate was used to create DNP enhancement over a broad
field range in MAS NMR experiments.
Dynamic Nuclear Polarization (DNP) has gained
widespread use as a means to improve the sensitivity of
nuclear magnetic resonance (NMR) signals.1-3 In material
science, solid-state DNP methods primarily rely on
exogenous radicals, such as TOTAPOL and AMUPol,
which are typically introduced by incipient wetness
impregnation in mM concentrations.4 Recent work suggests
that high field DNP enhancement may also be observed
with endogenous radicals.5 This manuscript probes the
structure of an unusually persistent endogenous radical in
bis-urea macrocycle 1 and its use as a polarizing agent
for
DNP
enhancement.
The photogenerated radical
was first noted when investigating the applications of
benzophenone bis-urea macrocycle 1 to facilitate selective
photooxidations.6,
7
The prolonged stability of these
endogenous radicals at room temperature appears to be a
consequence of the columnar assembly and crystal packing
of the porous organic crystals, where as no evidence of
radical formation is observed in solution.6 Herein, we probe
the structure of the radical, estimate its quantity, and
evaluate its lifetime by EPR spectroscopy. Finally, we
demonstrate that the low levels of endogenous radicals in 1
can be applied to hyperpolarize nuclei and enhance the
NMR signals of both the host and its encapsulated DMSO
guest (Figure 1).
[a]
Baillie A. DeHaven, Arthur A. Korous, Dr. Thomas M. Makris, and
Dr. Linda S. Shimizu*
Department of Chemistry and Biochemistry
University of South Carolina
Columbia, South Carolina 29208, USA
E-mail: shimizuls@mailbox.sc.edu
[b]
[c]
[d]
O
John T. Tokarski and Dr. Clifford R. Bowers
Department of Chemistry
University of Florida
Solutions of exogenous stable radicals such as AMUPol,
known as
“DNP juice”, are typically used in mM
Gainesville, Florida 32611, USA
concentration as polarizing agents for solid-state DNP MAS
NMR at ~100 K.1,
Dr. Frederic Mentink-Vigier and Dr. Johan van Tol
National High Magnetic Field Laboratory
Florida State University
1800 East Paul Dirac Drive, Tallahassee, Florida, 32310, USA
Alexander M. Brugh and Dr. Malcolm D. E. Forbes
Department of Chemistry and Center for Photochemical Sciences
Bowling Green State University
Bowling Green, Ohio 43403, USA
Supporting information for this article is given via a link at the end of
the document.
4
Under microwave irradiation, the
exogenous radical transfers its spin polarization to
neighbouring protons. The large spin polarization of the
protons generates a spin polarization gradient leading to
spin diffusion to nearby protons resulting in a uniform
proton hyperpolarization throughout the sample.8 The
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(Figure 3b). DMSO guests can be removed by heating and
other solvents and substrates can be readily loaded in the
channels.7 The assembled structure enforces close
contacts between the benzophenone groups to the
methylene H’s on neighbouring columns (2.44 – 2.81 Å,
Figure S4) and orients individual benzophenone units close
in space. 6 Molecular self-assembly and crystal packing in 1
dramatically quenches the phosphorescence lifetime from
µs to < 1 ns, likely through a non-radiative pathway.6
Mechanistic investigations suggest that columnar assembly
and packing stabilizes some type of photogenerated radical
that is stable for weeks in the dark at room temperature. No
evidence of radical formation is observed in solution where
the molecule only exists in monomer form. Our hypothesis
is that the solid-state structure may facilitate an H-
abstraction reaction to form the ground state triplet radical
pair (RP) shown in Figure 2A.
Figure 2. (A) Proposed photochemistry of 1, which suggests the known
photoreaction of benzophenone in the presence of an H donor. (B) X–
band EPR signal, pre- and post-UV exposure (simulation inset) centered
at an average g value of 2.006. (C) Dark-decay study of 1 (5 mg) after 1
h of UV irradiation. (D) Radical generation study shows that radical
signal reaches a maximum intensity after 5h.
Results and Discussion
We probed the structure of the photogenerated
radicals of 1 through solid–state X-band EPR studies on 1
and on
a
fully 15N-labeled derivative. Both samples
proton spin polarization can be transferred to other nuclei,
exhibited nearly identical broad peaks (g = 2.006, Figures
2B and S10). A simulation using parameters for a weakly
exchange–coupled RP (see inset Figure 2B and S13 for
more details and parameters) shows that the overall
spectral width and main features of the experimental
spectrum can be accounted for with such a model. This
also rules out the presence of a photochemically excited (or
thermally relaxed) molecular triplet state, which would be
expected to exhibit much broader line widths > 1000 G. The
similar broad peak observed for the 15N-labeled derivative
suggests that the triplet RP, drawn in Figure 2A, adopts a
conformation where the 15N hyperfine is less than half of the
natural line width (~14 G, Figure S13), which is reasonable
for the benzylic radical structure shown.
such as 13C, using
a Cross-Polarization (CP) pulse
sequence. Recently, Eichorn et al. demonstrated DNP in
pyruvic acid following low temperature UV-irradiation
without any exogenous radical.9,10 Instead, the quasi-stable,
short-lived UV-induced radicals were shown to afford
10
sizeable DNP enhancements at low temperatures.9,
Larger DNP enhancements are possible in solution and
have been observed in frozen media under constant
irradiation (photochemically induced DNP).11 With
exogenous radicals, optimum concentration is key, as high
radical concentration can result in excessive paramagnetic
4, 12
relaxation and line broadening.
Few studies have
examined DNP using endogenous radicals in the solid-state
or single crystals.5, 9, 10
The stability of the photoinduced RP was investigated
through dark decay studies, which were performed by UV
irradiating 1 (1 h, rt) and recording the EPR spectra over
time (0 h – 26 days) while storing the sample in the dark.
The double integration of the EPR signal is plotted vs. time
after UV-irradiation in Figure 2C. Little to no loss of signal
intensity is observed, suggesting that once generated, the
radicals created in 1 are stable for weeks. The small
fluctuations in the observed EPR signal intensity are likely
due to variations in the orientation of the crystalline sample
with respect to the magnetic field.15 The stability of the
photogenerated radical is likely a consequence of two
effects. First, it is probable that once generated, the radicals
are unable to terminate as benzophenone is known to do in
solution, due to the rigid structure of the macrocycles.
Second, the proposed photochemistry suggests that the
radicals are generated in positions that allow resonance
stabilization into the neighbouring benzene rings resulting in
enhanced stabilization of the radicals.
Recently, the Shimizu group found columnar
supramolecular assembly of benzophenone altered its
photophysics and lead to the formation of stable radicals as
an emergent property. Benzophenone is an extensively
studied chromophore with promising applications ranging
from photosensitization to genome sequencing and
materials chemistry.13 Its photochemistry affords a triplet
state as a result of fast intersystem crossing, which can
rapidly undergo H-abstraction to yield a ketyl radical. The
radical is only observed at low temperatures (77 K) as a
doublet with a g value of 2.0061 or as a radical anion via 1-
electron reductions.14
Compound 1 preorganizes two benzophenones within a
macrocycle. Upon recrystallization from hot DMSO, 1
assembles in needle-like crystals through predictable
bifurcated urea hydrogen-bonding interactions to afford
columns (Figure 1). 7 The crystals are robust and contain
accessible channels that are filled with DMSO guests
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Figure 3. Steady-state EPR studies of 1. (A) Variable temperature X-
band EPR study of 1 at 293 K (black line), 100 K (red line) and 10 K
(blue line) (B) CW high-field (240 GHz) EPR absorption curve of 1 at 6 K
vs. ppm (black line, see ESI for plot in mT). An EasySpin simulation of
the high field EPR (red line) for two S=1/2 electron spins, one weighted
x4. Inset: Plot of DNP enhancement vs. ppm observed in a field-step
study.
The concentration dependence of the radicals
Figure 4. Top: (A) Peak assignment of the benzophenone bis-urea
macrocycle, which self-assembles to form 1. (B) XRD structure of 1
generated by UV-irradiation of
1 was monitored for
depicting the encapsulated DMSO (space fill), the assembly of
1
exposures from 30 min to 7 h. Figure 2D plots the double
integration of the EPR signal vs. irradiation time and
indicates that the number of radicals reaches a maximum
after 5-7 h. After 7 h of UV-irradiation, 1 was slightly yellow
in colour but still suitable for single crystal X-ray diffraction.
No changes were observed in the X-ray structure (Figure
promotes the formation of a stable radical upon UV irradiation. Bottom:
CP MAS NMR spectra of thermally polarized 1 recorded at a spinning
speed of 11.3 kHz. (I) no UV, (II) 2 h. UV and (III) 4 h. UV.
electron at the site of a nucleus (i.e. Fermi contact
interaction). For a delocalized radical the probability is
small resulting in an averaged effect.17
1
S14) or in the H NMR, indicating that 1 is stable and the
absolute radical concentration is low. The maximum
concentration was approximated by calibration with
standard solutions of TEMPO in benzene under identical
conditions (Figure S15). 10, 16 The number of radicals
generated by 1 (4.5 mg) is similar to a 5.4 µM stock solution
(0.1 mL), which is equivalent to a radical forming in ~0.01%
of the macrocycles.
The high field EPR shown in Fig. 3B was acquired on
the NHMFL’s 240 GHz spectrometer and converted to a
ppm scale to assist in our interpretation of the field-stepped
DNP data. The solid-state high-field CW absorbance EPR
spectra of UV-irradiated 1 (25 mg) at 6K shows a broad
baseline component at g = 2.006 with a sharper transition at
g = 2.003. Spectral simulations carried out using the
EasySpin software are in agreement with two S=½ radical
species with a sharp isotropic signal at g = 2.003 as well as
Variable temperature EPR spectra were recorded to
resolve hyperfine couplings that may not be observed at rt.
First, spectra were recorded at high temperatures (293, 348
and 398K, Figure S16) for 1 and the 15N labelled 1. No
change in the g-factor or the coupling pattern was
observed, although the intensity of the signal decreased
with increasing temperature. Cooling the sample to 100K
did not markedly change the g-factor, although a slight
anisotropy was observed at g = 2.001 (Figure 3A, red
spectra). Further cooling to 10 K resulted in a change in
the EPR spectrum, leading to a powder pattern shape with
an overall shift in g-factor to 2.001 and a slight anisotropy.
This can be explained by the orientation-dependent dipolar
contribution of rigid radical pairs, which are often further
complicated by hyperfine interactions ultimately resulting in
line broadening due to motional averaging. By cooling the
sample, we were able to overcome the Boltzmann
distribution and rebuild the g-factor matrix leading to the
significant over population of the lower energy states. The
lack of hyperfine interactions at low temperatures is
consistent with delocalized radicals, since hyperfine
interactions are described by the probability of finding an
a second broad anisotropic signal.
An extrapolated
simulation to X-band indicates that the lines would overlap
at low field, consistent with the observations in Figure 3.
Variable temperature EPR and high-field experiments both
suggest the assignment as two radicals, possibly a
delocalized RP.
The thermally polarized CP MAS NMR spectra were
recorded at a spinning speed of 11.3 kHz after 0h, 2h and
4h of UV-irradiation (Figure 4). The 13C NMR peaks are
surprisingly sharp for both the host and included DMSO.
The spectra before and after UV-irradiation are nearly
identical and no paramagnetic broadening is observed,
consistent with the low estimated radical concentration and
a well-ordered structure.12 The THF loaded host crystals,
also show a sharp thermally polarized CP MAS NMR
spectra under similar conditions (Figure S24).
Solid-state MAS DNP experiments gave a maximum
enhancement factor εon/off = 4. The DNP enhancement
profile, shown in the inset of Figure 3B, demonstrates that
nearly constant DNP enhancement εon/off = 2 is obtained
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methods. Such DNP enhancement may be observable in
other structures.
Conclusions
We have demonstrated that UV-irradiation of
assembled benzophenone bis-urea macrocycles can
generate low ~µM concentrations of long-lived RPs that
persist for weeks at room temperature in the dark.
Labelling experiments ruled out nitrogen-centered radicals.
High field EPR data and variable temperature X-band EPR
Figure 5. Optimized DNP CP-MAS NMR enhancement observed at
14.085 T under CP DNP MAS NMR conditions demonstrating a total
enhancement of εon/off = 4. Recorded at 112 K and a spinning speed of
7.0 kHz, microwave on (red line) vs. microwave off (black line).
*Indicates spinning sidebands
studies suggest the formation of two radicals.
Our
hypothesis is that the columnar assembled structure of 1
facilitates an H-abstraction reaction and significantly
stabilizes the triplet RP. Stable and persistent organic
radicals are rare and typically belong to four structural
classes.20 These results suggest that additional organic
radicals may be stabilized by similar supramolecular
assembly. Thus, we are currently exploring building blocks
that contain other known sensitizers to investigate if their
crystalline solids also afford stable radicals upon photolysis.
In summary, we have demonstrated that the photo-
induced radical species generated by 1 can be utilized as a
polarizing agent to significantly enhance NMR signals for
both the host and its encapsulated guest DMSO, even
though the concentration of endogenous radicals in 1 is
orders of magnitudes below typical quantities of exogenous
agents used for DNP NMR. UV-irradiated 1 showed a
surprisingly broad DNP enhancement profile of over 120
mT, demonstrating that it is not necessary to tune the
microwave frequency in order to observe DNP
enhancement. The contribution of the cross-effect
mechanism appears to be insignificant, since the field-
stepped DNP profile exhibits only a single maximum with
εon/off > 0. These results suggest that the design and
incorporation of low levels of endogenous radicals into host
frameworks may be broadly applied for NMR signal
enhancement. Future DNP studies will focus on
investigating a variety of guest molecules to see if such
systems can be widely applied to study host: guest
interactions in the solid-state.
over a broad field range of approximately 120 mT with a
sharp peak in the enhancement factor of εon/off = 4. DNP
enhancement profiles are more typically limited to a much
smaller field range of 30-40 mT compared to our
18
experimentally observed 120 mT range for 1.2, Given the
broad EPR signal observed by 1, this profile is not
surprising. Figure 5 depicts the optimized DNP CP-MAS
NMR spectrum at the magnetic field that optimized the DNP
enhancement, where εon/off = 4 was recorded at 112 K and a
spinning speed of 7.0 kHz. Similar enhancement factors
were observed for all NMR peaks including the
encapsulated DMSO. The spinning side bands are a result
of the low spinning speed required to keep the sample
stabilized at the low temperatures employed. The non-
irradiated sample showed no DNP enhancement, which
indicates that the enhancement results from irradiation of
the sample and is not a microwave induced heating artifact
(Figure S25). The constant and positive sign of this field-
stepped study suggests an Overhauser mechanism.19
Much larger DNP enhancements can be observed
through traditional impregnation methods; therefore, the
UV-irradiated sample was impregnated with AMUPol (10
mM, 10-12 µL of a 6:3:1 glycerol-d8, D2O,H2O solution) and
the DNP CP-MAS NMR spectrum acquired using the
optimal field conditions for
1 (Figure S27). A DNP
enhancement of εon/off ~ 6 was observed for all peaks with
the exception of the glycerol peaks which were enhanced
Experimental
by a factor of εon/off
~ 20.
This higher observed
enhancement is likely due to the dual effect of the
endogenous and exogenous radicals in this sample. The
larger observed enhancement for glycerol is expected
because the exogenous radical is in more direct contact
with glycerol molecules present in the bulk DNP juice. It
should be noted that impregnating the sample did not yield
much higher polarization levels while introducing solvent
signals. These results suggest that in these porous organic
crystals, it may be more fruitful to use the endogenous
radicals formed by 1 to enhance the NMR signals of the
host:guest materials as opposed to traditional impregnation
EPR Sample Preparation
Neat crystals of 1 were added to a Norell quartz EPR tube,
purged under Argon gas, sealed under Parafilm, and
capped. The samples were UV-irradiated at 350 nm at rt
with a Hanovia 450 W medium pressure mercury arc lamp
cooled in a quartz immersion well. EPR signals were doubly
integrated three times and averaged from the 3305 G to
3370 G range using Xenon software (v 1.1b.66).
MAS DNP NMR Sample Preparation
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8
9
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UV irradiated sample: The sample was prepared by UV-
irradiating a crystalline sample of 1 (25 mg) for 7 hours
using a medium pressure Hanovia Hg lamp using Corning
glass filters to isolate the 366 nm Hg line. After UV
irradiation, the sample was packed neat into a 3.2 mm
sapphire rotor and DNP experiments were performed.
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into a 3.2 mm sapphire rotor and DNP experiments were
performed.
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irradiated host 1 sample (25 mg) was unpacked from the
sapphire rotor and impregnated with AMUPol (10 mM, 10-
12 µL of a 6:3:1 glycerol-d8, D2O, H2O solution) and the
DNP CP-MAS NMR spectrum acquired using the optimal
field conditions for 1.
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MAS DNP NMR Experimental
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MAS pulse sequence on
spectrometer (3.2 mm sapphire rotor) at the National High
Magnetic Field Laboratory. DNP experiments were
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Thanks to Zhehong Gan, Ivan Hung, Jose Amaya, Steven
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National Science Foundation (CHE-1608874, CHE-
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Field Laboratory is supported by the NSF (DMR-1157490)
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Keywords: benzophenone • DNP • EPR spectroscopy •
macrocycle • MAS
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Persistent organic radicals are
generated upon UV-irradiation as a
consequence of their self-assembled
structure. The endogenous host
framework radicals, present in ~µM
quantities, are characterized and
B. A. DeHaven, J. T. Tokarski, A. A.
Korous, F. Mentink-Vigier, T. M. Makris,
A. M. Brugh, M. D. E. Forbes, J. van Tol,
C. R. Bowers, and L. S. Shimizu*
Page No. – Page No.
applied as
a polarizing agent for
Persistent Radicals of Self-assembled
Benzophenone bis-urea Macrocycles:
Characterization and Application as a
Polarizing Agent for Solid-state DNP
MAS Spectroscopy
magic angle spinning (MAS) dynamic
nuclear polarization (DNP) NMR
spectroscopy where they displayed a
maximum enhancement of four for
both host and guest.
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