Ultra-Fast Molecular Rotors within Porous Organic Cages

Article abstract of DOI:10.1002/chem.201704964

Using variable temperature ²H static NMR spectra and ¹³C spin-lattice relaxation times (T₁), we show that two different porous organic cages with tubular architectures are ultra-fast molecular rotors. The central para-phen

Full text of DOI:10.1002/chem.201704964

A Journal of
Accepted Article
Title: Ultra-Fast Molecular Rotors within Porous Organic Cages
Authors: Ashlea R. Hughes, Nick J. Brownbill, Rachel C. Lalek,
Michael E. Briggs, Anna G. Slater, Andrew I. Cooper, and
Frederic Blanc
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To be cited as: Chem. Eur. J. 10.1002/chem.201704964
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Ultra-Fast Molecular Rotors within Porous Organic Cages
Ashlea R. Hughes, Nick J. Brownbill, Rachel C. Lalek, Michael E. Briggs, Anna G. Slater, Andrew I.
Cooper, and Frédéric Blanc*
Abstract: Using variable temperature 2H static NMR spectra and 13
C
spin-lattice relaxation times (T1), we show that two different porous
organic cages with tubular architectures are ultra-fast molecular rotors.
The central para-phenylene rings that frame the ‘windows’ to the cage
voids display very rapid rotational rates of the order of 1.2–8 x106 Hz
at 230 K with low activation energy barriers in the 12–18 kJ mol-1
range. These cages act as hosts to iodine guest molecules, which
dramatically slows down the rotational rates of the phenylene groups
(5–10 x104 Hz at 230 K), demonstrating potential use in applications
that require molecular capture and release.
Porous Organic Frameworks (POFs) are supramolecular
assemblies that have ordered architectures containing an
inherent void. Examples include Metal Organic Frameworks
(MOFs), Covalent Organic Frameworks (COFs), Zeolitic
Imidazolate Frameworks (ZIFs), and Porous Organic Cages
(POCs).[1–3] POCs differ from these other framework families
because they consist of discrete molecules containing both an
intrinsic void within the cages in addition to voids within the overall
lattice; because of these properties, POCs are also solution
Figure 1. Chemical structure and side view of the X-ray crystal structure of (a)
TCC2-R and (b) TCC3-R.[5] The cyclohexane groups are shown in red; other C,
grey; N, blue; H omitted for clarity in the crystal structure representation. The
green arrows indicate fast molecular rotation of the para-phenylene. The 13C
spin-lattice relaxation times (T1) obtained for selected carbons are given in the
figure, including T1 values after iodine loading (values in parentheses). The
deuterium-labelled positions on the para-phenylene are also shown for TCC2-
R-d12 and TCC3-R-d12
.
processable[4,5] and have been explored for
a range of
molecules controls guest loading into the molecular pore, and we
therefore set out to understand the response of these cages to
external stimuli, such as guest inclusion.
applications including catalysis, molecular separation and gas
storage.[6–8] For example, POCs have been shown to capture
guest molecules such as iodine, SF6, and hydrocarbons.[9] The
sequestration of iodine, which is an unwanted fission product,[10–
12] is of strong importance for the nuclear industry.[13] The selective
loading and retention of guests such as iodine often relies on
molecular flexibility and dynamics, and the understanding of these
processes plays a central role in the design of the next generation
of POFs.
Recently, a new family of POCs with a chiral, tubular covalent
cage (TCC) architecture has been discovered.[5] These TCCs
consist of three ‘walls’ bound by trans imine cyclohexane linkers.
Two of these structures (TCC2-R and TCC3-R) are shown in
Figure 1; they differ only by the additional acetylene moieties
between the phenylene rings in TCC3-R. The intrinsic pore within
these molecules permits guest sorption into the tubular
cavities.[3,14] The static imine cyclohexane linkers and rotating
phenylene groups allow these POCs to be classified as molecular
rotors.[15–17] It is likely that the window dynamics in these
2H solid echo NMR has become an extremely powerful tool
for the understanding of dynamics in the kHz timescale,[18–20]
2
which is enabled by the large change in the H NMR line shape
with temperature. This approach provides a qualitative description
of the motion as well as its rate and associated activation energies
in both pristine fast molecular rotors and those hampered by guest
loading (e.g. H2O, acetone, iodine, CH4 and hydrocarbons);[21–31]
these include mesoporous para-phenylenesilica,[21] poly aromatic
frameworks PAFs,[22] and MOFS.[26,27,29,31]
Additionally, 13C T1 values provide correlation times in the
MHz frequencies and complementary details of the molecular
2
reorientation. Here, we performed variable temperature H solid
echo NMR experiments and room temperature 13C T1
measurements on pristine and iodine loaded TCC2-R and TCC3-
R materials to understand the rotational dynamics of the cages
and their host-guest interactions.
TCC2-R and TCC3-R cages were synthesized using literature
procedures.[5] Cages deuterated on the para-phenylene rings
(i.e., TCC2-R-d12 and TCC3-R-d12, Figure 1) were prepared via
a similar approach[5] using 1,4-dibromobenzene-2,3,5,6-d4 and
these were used to record the 2H NMR data (see the Supporting
Information [SI] for details of all samples preparation and
characterisation). In these experiments, the natural abundance
2H signals (0.015 %) from the other hydrogens in the cyclohexane
linkers and trisubstituted benzene rings are not detected and the
2H NMR spectra only probe the dynamics of the para-phenylene
moieties of the cages.
[*]
A. R. Hughes, N. J. Brownbill, R. C. Lalek, Dr. M. E. Briggs, Dr. A.
G. Slater, Prof. A. I. Cooper, Dr. F. Blanc
Department of Chemistry, University of Liverpool
Crown Street, Liverpool, L69 7ZD
E-mail: Frederic.Blanc@liverpool.ac.uk
Dr. M. E. Briggs, Dr. A. G. Slater, Prof. A. I. Cooper
Materials Innovation Factory, University of Liverpool
51 Oxford Street, Liverpool, L7 3NY
Dr. F. Blanc
Stephenson Institute for Renewable Energy, University of Liverpool
Crown Street, Liverpool, L69 7ZD
2H static echo NMR spectra in the 105–298 K temperature
range were recorded on desolvated and iodine loaded TCC2-R-
Supporting information for this article is available.
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Figure 2. 2H static solid echo NMR spectra of (a) TCC2-R-d12 (blue), iodine loaded TCC2-R-d12 (dark blue), (c) TCC3-R-d12 (red), (d) iodine loaded TCC3-R-d12
(burgundy) and their corresponding simulated spectra (black dashed lines) obtained at various temperatures. The rotational rates, k, obtained from numerical
simulations of the NMR lineshapes are also given. Spectral artefacts are denoted with (#).
d12 and TCC3-R-d12 cages (Figure 2). With decreasing
temperature a gradual change in the 2H NMR line shape is
observed for these materials, however, the motion induced T2
anisotropy is dependent on the particular cage. Line shape
simulations of the 2H NMR spectra support a motion consisting of
a rapid two-site 180° flip reorientation of the para-phenylene ring
along its para axis, and this provides the rate of molecular
reorientation (k) at each temperature on the kHz timescale.[17,32,33]
Figure 2(a) and (c) show the variable temperature evolution of
by reducing the strong steric interactions of the ortho-hydrogens
and opening up the void space (Figure 1). Additionally, it is also
possible that small electronic factors play a role and for example
include the smaller degree of conjugation between the adjacent
phenylene rings in TCC3-R-d12 and TCC2-R-d12 likely contributes
to the smaller activation barrier seen for para-phenylene rotation
in the former.
Moreover, while the TCC2-R-d12 rate is comparable to other
organic frameworks,[25–27,34] the very fast reorientation rate value
obtained for the TCC3-R-d12 cage being larger than in any
exclusively organic systems reported previously below ~200 K
(Table S3).[16,17,22,27,30,35–37] In particular, below this temperature,
the dynamics of TCC3-R-d12 are faster than the ones observed
2
the H static echo NMR spectra of TCC2-R-d12 and TCC3-R-d12
cages. As the temperature decreases from 298 to 105 K, the
appearance of the outside horns around ± 60 kHz agrees with
slower rotation rates at low temperature and a static motional
regime at 105 K with the anticipated Pake doublet pattern. The
line shape evolution of both cages is analogous, as anticipated for
materials with similar tubular covalent architectures. However, the
2H NMR spectra of TCC3-R-d12 suggest a significantly larger jump
rate, as evidenced by the weakening of the spectral shoulders of
this cage at a lower temperature (230 K) compared to TCC2-R-
d12 (261 K), showing slower motion for the latter.
very recently for the para-phenylene reorientation in
a
bis(sulfophenylethynyl)-benzene frameworks based on an overall
similar architecture of a phenylene molecular rotor sandwiched
between two acetylene moieties (Figure 1(b)),[30] which previously
showed the largest reorientation rate for porous organic materials
to date.[30]
At temperatures higher than 298 K, the 2H NMR lineshape of
the TCC2-R-d12 and TCC3-R-d12 pristine cages is characteristic
of that of the fast motional regime with rates exceeding 108 Hz
(Figure S5(a) and (c)). No additional change in lineshape occurs
at higher temperature probably indicating an absence of C-D
Rotational rates of ca. 1.2 x106 and 8 x106 Hz were extracted
for TCC2-R-d12 and TCC3-R-d12 cages, respectively, at 230 K.
These data suggest that the presence of an acetylene group
enables easier rotation of the para-phenylene ring in TCC3-R-d12
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librational motion and is in sharp contrast to what is observed in
PAFs.[22] This difference likely arises from the more flexible nature
of the PAFs architecture compared to the relative rigidity of these
TCC2-R and TCC3-R cage structures (Figure 1).
A plot of ln k as a function of reciprocal temperature T-1 (Figure
3) shows a linear Arrhenius behaviour from which rotational
activation energies, Ea, of 18 and 12 kJ mol-1 were obtained (Table
1), along with extrapolated rotational rates at infinite temperature
(attempt frequencies), k0, of (9 ± 4) x1010 and (10 ± 7) x109 Hz for
TCC2-R-d12 and TCC3-R-d12, respectively. The larger Ea value
obtained for TCC2-R-d12 versus TCC3-R-d12 is again consistent
with stronger steric interactions in the terphenylene cage
structure. The k0 values obtained are on the low side of the ~1012
Hz[22] value often associated with para-phenylene rotation,
although these values vary significantly with the systems studied
and k0 in the 108-1041 Hz are known.[17,21,22,25–27,30,34,35,38,39] The
associated change in entropy (ΔS) is negative and is tentatively
assigned to correlated rotational motion (Table S2).[39,40]
Iodine was loaded into TCC2-R-d12 and TCC3-R-d12 using a
chemical vapour sublimation procedure (see SI), to determine if
guest addition hampers motional dynamics in these systems.[21,22]
Upon exposure to iodine at room temperature, the colour of the
cages changed from yellow to black, and the guest uptake was
monitored gravimetrically (see SI). This revealed a high loading of
10 and 12 iodine atoms per TCC2-R and TCC3-R cage molecule,
respectively, after 40 hours of iodine exposure. The variable
temperature 2H static echo NMR spectra of iodine loaded TCC2-
R-d12 and TCC3-R-d12 are given in Figures 2(b) and (d). There is
a clear difference in rotational rates of the molecular rotor
between the empty and guest-loaded materials. The less rapid
two-site ring flip is particularly evident at 230 K for the iodine
loaded materials, reducing the rotational rates to only 105 and 5
x104 Hz for TCC2-R-d12 and TCC3-R-d12, respectively (Table 1).
This is smaller than the change seen when iodine is loaded into
other porous materials,[22] which probably relates to the different
host – guest properties of the materials.
Figure 3. Arrhenius plot of the rotational rates, k, of the para-phenylene ring in
desolvated and iodine-loaded TCC2-R-d12 and TCC3-R-d12 molecular rotor
cages. The lines show the linear fit to the Arrhenius equation with the extracted
values being reported in Table 1. Error bars are estimated from comparison of
the 2H NMR line shape fit at various rotational rates. The errors associated with
the 154 K rates for iodine-loaded TCC2-R-d12 and TCC3-R-d12 are due to the
indistinguishable line shape between 1 and 9 kHz. The 2H NMR rotational rates
obtained at 230 K from the iodine released TCC2-R-d12 and TCC3-R-d12 cages
are also shown in empty circles.
data show that the presence of the iodine guest within the void of
the cages, and potentially in extrinsic voids between cages,
hampers but does not totally suppress molecular reorientation of
the para-phenylene rings. It has been reported that when the
motion of a guest molecule is restricted within a cavity, fast
librational motions are expected;[41] this is not apparent in these
TCC2-R and TCC3-R cages, since even when a guest is located
inside the cages, the lineshapes remain consistent with a 180°
site reorientation of the para-phenylene rings.
Finally, 13C T1 values were also monitored due to their strong
dependence on molecular motion on the MHz timescale.[41] The
considerably shorter room temperature 13C T1 values of 1.3–1.5 s
obtained in TCC2-R and TCC3-R (see Figures 1, S1 and Table
S1) for the CH carbons on the para-phenylene ring versus the
other carbons in the cages (appearing in the range of 4–7 s)
suggests an efficient relaxation mechanism and rapid molecular
reorientation of the cage windows, supporting the fast molecular
rotors of these cages. The 13C T1 relaxation times were found to
increase significantly upon loading (Figure 1, Table S1), being
consistent with the change in 2H solid echo NMR line shape noted
above, where guest addition into the central void slows
reorientation of the phenylene rings within the cage structures.
To conclude, we employed 2H solid echo NMR and
determined 13C T1 values to probe the rotational dynamics of the
para-phenylene rings that define access to the central void in two
porous chiral tubular covalent cages, TCC2-R and TCC3-R.
Table 1. Comparison of the activation energies barrier (Ea) and rotational
rates at 230 K (k230K) for all the TCC cages investigated.
Tubular covalent cages
TCC2-R-d12
Ea /kJ mol-1 [a]
18 (18-20)
21 (15-21)
12 (10-13)
21 (14-21)
k230 K/Hz
1.2 x106
1 x105
8 x106
5 x104
Iodine loaded TCC2-R-d12
TCC3-R-d12
Iodine loaded TCC3-R-d12
[a] Range of Ea values estimated from errors in the values of k are given in
brackets.
When iodine loaded TCC2-R and TCC3-R are heated above
room temperature, almost complete release of iodine over one
cycle is observed as detected by thermogravimetric analysis (see
SI) and visual inspection of the sample; as a result, large
reorientation rates are obtained in the corresponding 2H static
echo NMR spectra (Figures S6-S7) and highlight again that these
materials are responsive with the rotational dynamic being
modulated by the capture and release of a guest molecule.
Additionally, Arrhenius plots yield an increase of Ea with
respect to the guest-free cages: from 18 to 21 kJ mol-1 for TCC2-
R-d12 and from 12 to 21 kJ mol-1 for TCC3-R-d12 (Table 1). These
2
Using H NMR, we show that TCC2-R cages show reorientation
rates that are comparable with the fastest molecular rotors
reported for other porous frameworks; TCC3-R cages display
even faster dynamics (below 200 K), with a very small activation
energy barrier, which is ascribed to the facile rotation around the
acetylene bonds, due to a reduction in the steric hindrance
present. Iodine loading slows the phenylene rotation considerably,
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as further supported by the lengthening of the 13C T1s, while high
temperature treatment induces iodine release and reacceleration
of the phenylene rings reorientation rates. These data show that
the effect on cage dynamics is highly guest dependent, which
might have important implications for processes such as
competitive loading, molecular separation, and drug release.
These findings also emphasize that models of porosity derived
from static single crystal structures might be misleading, but that
‘time averaged’ models of the pore space could be equally
inappropriate because guest inclusion can switch the rotational
dynamics off. This suggests that computational models for
loading in such systems need to capture the interplay of guest
inclusion and rotational dynamics in the porous host.
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A. R. Hughes, N. J. Brownbill, R. C.
Lalek, Dr. M. E. Briggs, Dr. A. G. Slater,
Prof. A. I. Cooper, Dr. F. Blanc*
+ I2
Δ
Page No. – Page No.
- I2
Ultra-Fast Molecular Rotors within
Porous Organic Cages
TCC3-R-d12
8 MHz
0.05 MHz
3 MHz
Recently discovered chiral, tubular covalent cages are ultra-fast molecular rotors as
2
demonstrated by H and 13C solid-state NMR spectroscopies. The dynamics of the
cages para-phenylene walls are modulated by the capture and release of a guest
molecule (I2) on demand.
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