Ultrafast Molecular Rotors and Their CO2 Tuning in MOFs with Rod-Like Ligands

Article abstract of DOI:10.1002/chem.201702930

A metal organic framework (MOF) engineered to contain in its scaffold rod-like struts featuring ultrafast molecular rotors showed extremely rapid 180 ° flip reorientation with rotational rates of 10¹¹ Hz at 150 K. Crystal-pore accessibility of

Full text of DOI:10.1002/chem.201702930

DOI: 10.1002/chem.201702930
Communication
&
Molecular Rotors
Ultrafast Molecular Rotors and Their CO2 Tuning in MOFs with
Rod-Like Ligands
Silvia Bracco,[a] Fabio Castiglioni,[a] Angiolina Comotti,*[a] Simona Galli,[b] Mattia Negroni,[a]
Angelo Maspero,*[b] and Piero Sozzani[a]
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to build solid state molecular rotors in a porous environment,
providing, in principle, the free volume needed for motion to
occur. However, in the MOFs presented in literature, the typi-
cally-adopted chemical bonds on which molecular rotors are
pivoted are rather inadequate to ensure a low barrier for rota-
tion.[6,10] Attempts to lower this energy barrier have been car-
ried out by destabilizing the ground state through the intro-
duction of lateral substituents on p-phenylene units in the
struts, although this causes the molecular mass of the rotor
and its inertia momentum to increase, leading to relatively
slow rotational frequencies.[11] To increase the rotational spin-
ning speed, an alternative strategy adopted in porous molecu-
lar crystals is to insert into the struts triple bonds as pivots for
the molecular rotor, thus for lowering the energy barrier for ro-
tation.[5,7]
Abstract: A metal organic framework (MOF) engineered
to contain in its scaffold rod-like struts featuring ultrafast
molecular rotors showed extremely rapid 1808 flip reor-
ientation with rotational rates of 1011 Hz at 150 K. Crystal-
pore accessibility of the MOF allowed the CO2 molecules
to enter the cavities and control the rotor spinning speed
down to 105 Hz at 150 K. Rotor dynamics, as modulated
by CO2 loading/unloading in the porous crystals, was de-
2
scribed by proton T1 and H NMR spectroscopy. This strat-
egy enabled the regulation of rotary motion by the diffu-
sion of the gas within the channels and the determination
of the energetics of rotary dynamics in the presence of
CO2.
In the present paper we show extremely fast rotor dynamics,
already 1011 Hz at 150 K, comparable to liquid matter, in a bis(-
pyrazolate)-based MOF architecture. Moreover, the CO2 enter-
ing the porous material could actively tune the spinning speed
and govern the mechanics of motion (Figure 1). This opens up
the perspective of actively manipulating rotor response in a
robust framework, simply by the adsorption of a gas.
Molecular rotors and motors have become an important issue
today, being a prominent challenge to present-day research
where the target is to reorganize molecular motion from a cha-
otic to a coherent state.[1] When opportunely governed, molec-
ular motion can be exploited for its response to electric-field
stimuli in optical devices, switches and sensors.[2] To date, light
energy has aided the promotion of unidirectional propulsion
for the fabrication of nanorobots and nanomachines.[3] Artificial
molecular pumps have been fabricated, mimicking rotary ma-
chines that, in nature, perform fundamental functions such as
proton pumps.[4] Thus solid state structures, periodically organ-
ized in 2D and 3D, have the potential to offer a reliable venue
for the controlled motion of molecular rotors, though they
often suffer from a tendency towards close-packing inducing
reciprocal interference among rotors.[5]
The porous and absorptive architecture was realized
through the robustness of metal-organic bonds and the use of
rigid molecular rods bearing the rotors. We selected a suitable
rod-like strut that contains a central p-phenylene unit (the
rotor) connected through ethynyl groups to two pyrazole moi-
eties 1,4-bis(1H-pyrazol-4-ylethynyl)benzene (BPEB) which in-
teracts with ZnII ions to fabricate the microporous MOF Zn-
BPEB (Figure S3).[12] The isostructural MOF d4-Zn-BPEB selective-
However, crystalline porous materials with their low density
and structural order can overcome this limitation, and are ex-
cellent candidates for the fabrication of molecular rotors in the
solid state.[6] Furthermore, porosity allows rotor exposure to
diffusing species through adsorption, this occurrence enlarging
the application perspectives.[7] Indeed, the easy rotor accessi-
bility results in an active manipulation of rotary motion
through interaction with guests,[6] revealing the feasibility of
molecular rotors as responsive elements sensitive to chemical
perturbation.
The architecture of porous materials can be realized by a va-
riety of chemical bonds and supramolecular interactions.[8] Spe-
cifically, in metal-organic frameworks (MOFs) metal organic
nodes and proper organic struts ensure both crystallinity and
robustness.[9] The large variety of MOFs offers a valid platform
[a] Dr. S. Bracco, Dr. F. Castiglioni, Prof. A. Comotti, Dr. M. Negroni,
Prof. P. Sozzani
Department of Materials Science and INSTM Consortium
University of Milano Bicocca, Via R. Cozzi 55, Milan (Italy)
E-mail: angiolina.comotti@mater.unimib.it
[b] Prof. S. Galli, Prof. A. Maspero
Dipartimento di Scienza e Alta Tecnologia and INSTM Consortium
Universitꢀ degli studi dell’ Insubria, Via Valleggio 11, 22100 Como (Italy)
E-mail: angelo.maspero@uninsubria.it
Supporting information and the ORCID identification number(s) for the au-
thor(s) of this article can be found under https://doi.org/10.1002/
chem.201702930.
Figure 1. 1,4-Bis(1H-pyrazol-4-ylethynyl)benzene linker (above) and Zn-BPEB
showing CO2 molecules diffusing in both small and large cavities (below).
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ly deuterated on the central p-phenylene moiety was prepared
to investigate the rotor dynamics. Straightforward evidence of
the dynamics in the presence and the absence of CO2 within
1
2
the pores was provided by H T1 relaxation times as well as H
solid-state NMR spectroscopy, which is a unique method to un-
derstand the motional trajectories and reveal the mechanism
of reorientation of the C-D vectors.[13]
The crystal structure of Zn-BPEB features 1D square channels
with 10 ꢁ ꢂ 10 ꢁ cross section and small pores with aperture
of 4.3 ꢁ diameter connecting the large channels (Figure S3), ac-
counting, respectively, for an empty volume of 1356 ꢁ3 and
696 ꢁ3 per unit cell, (calculated through an exploring sphere of
1.2 ꢁ radius and the contact surface model). Each p-phenylene
unit, which constitutes the mobile element capable of rotating,
is exposed to both channels. The tetrahedral metal node plays
a key role in the separation of the rod-like ligands, providing
the molecular rotor with enough room to rotate. The robust-
ness of the MOF structure is guaranteed by the coordination
of pyrazolate rings at the end of the rods with metal node
(Zn+2). The crystal and molecular structure was confirmed by
solid state NMR multiplicity of signals (Figure S6), compared
with PXRD structural refinement. The channels are empty and
no residual solvent is included. The porous materials exhibit a
BET surface area of 1214 m2 gÀ1 (Langmuir SA=1366 m2 gÀ1)
and pore volume of 0.64 cm3 gÀ1 (Figure S10).
2
Figure 2. a) Experimental (left) and simulated (right) H NMR spectra of d4-
Zn-BPEB. k corresponds to reorientational rates and q to libration amplitude.
b) 1H T1 relaxation times and k versus the reciprocal of temperature for Zn-
BPEB.
Structure analysis reveals that the minimum distance be-
tween adjacent axes of central p-phenylene rings is 7.3 ꢁ
along the c-axis (Figures S4). On the other hand, being an in-
terpenetrated network, in the ab plane each p-phenylene ring
faces both the ethynyl and the pyrazolate groups of the adja-
cent linker. The pyrazolate plane is at a distance of 5.2 ꢁ and
does not interfere with the rotor. Such a relatively large dis-
tance allows for the rotation of the p-phenylene ring and, con-
sidering the low rotation barrier about Csp2-Csp bonds (as the
p-phenylene-ethynyl bond), the p-phenylene moieties are ex-
pected to be efficient as fast molecular rotors.
Tomita relation. When spin-lattice relaxation times increase
with temperature, the system is exploring the extreme narrow-
ing limit and the dynamics are faster than the observation fre-
quencies (typically from 108 to 1012 Hz), complementing
1
2H NMR data. This was the case in Zn-BPEB in the H domain,
in which Ln(T1) proved to be inversely proportional to 1/T (Fig-
ure 2b and Supporting Information). At 150 K the reorienta-
tional frequencies k are as fast as 1011 Hz. The activation
energy was calculated to be 0.5 kcalmolÀ1. The p-phenylene
dynamics discovered in this MOF is orders of magnitude faster
than those observed in other MOFs, which are still in slow ex-
change regimes at room temperature (Table S1).[10,14] The ex-
tremely fast dynamics and the low energy barrier we observed
are a benchmark in the class of MOFs, and overcome the best
molecular rotation value recently published for non-porous
crystalline compounds.[15]
2
This was demonstrated by wide-line H NMR spectroscopy of
selectively deuterated d4-Zn-BPEP, on the central p-phenylene
ring of MOF. At room temperature the spectrum exhibits a re-
stricted spectral profile (with singularities separated by
27.7 kHz), which does not change appreciably down to 150 K,
except for a minor increase of the spectral shoulders. The si-
mulated line-shapes show that the mechanism of motion con-
sists of a rapid two-site 1808 flip reorentation of the p-phenyl-
ene moieties about their main axis, passing through the pivo-
tal ethynyl groups, and extremely fast reorientation (k>108 Hz)
of the aromatic C-D bonds (Figure 2a). Rapid librations of the
p-phenylene ring about the energy minimum, witnessed by
the weakening of the spectral shoulder, must also be taken
into account, together with the 1808 flip mechanism that is
preserved in the strut. Surprisingly, rates in the fast exchange
limit (>108 Hz) and libration amplitudes q of more than Æ208
occur even down to 150 K.
Moreover, the permanent porosity of the crystals enables
CO2 control of the molecular rotor dynamics while exploring
the MOF nanochannels. As a matter of fact, CO2 adsorption iso-
therms, run at various temperatures from 298 K down to 195 K
(Figures S12 and S13), enabled us to assess the loading of the
material at each temperature, and the adsorption energy at
low coverage. The interaction energy, as evaluated by the van’t
Hoff equation, is 25 kJmolÀ1, indicating a good interaction of
1
2
CO2 with the channel walls. H T1 and H NMR made it possible
to investigate the rotor dynamics in the presence of the gas
1
into the channels. At variance with the empty MOF, H relaxa-
1
In order to explore higher frequency regimes, H T1 spin-lat-
tion times of Zn-PBEB loaded with CO2 are close to the mini-
mum values and, below 260 K, increased with temperature,
with motional frequencies in the regime of motion of solids
(n<3 108 MHz at 7.04 T), with a frequency drop of a few
tice relaxation times were measured in Zn-BPEB, because they
provide information on the rotary frequencies at variable tem-
perature, following, the master curve generated by Kubo-
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orders of magnitude as compared to the empty sample. The
deuterated sample d4-Zn-BPEB was sealed under pressurized
CO2 at 9 bar and room temperature (denoted as d4-Zn-BPEB/
to Æ418 in addition to the two-site 1808 flip reorientation.
Characteristic tower-shaped profiles are produced by this
mechanism when large librations take place and, to date, they
have only been observed at much higher temperatures in
rotor-containing porous materials.[7b] Such large librations sug-
gest a higher rotor degree of freedom because a lower
number of CO2 molecules surround each rotor and CO2 mole-
cules undergo rapid diffusion along the channels.
2
CO2) and H NMR spectra were collected at variable tempera-
ture (Figure 3).
Grand Canonical Monte Carlo simulations established the
number and partition of CO2 molecules around a rotor, particu-
larly the distribution between the small and large pores as a
function of loading (Figure 4 and Figures S17–S20). At high
temperature and low loading, CO2 occupies only the small cav-
ities and 2 gas molecules (or less) surround a single rotor (Fig-
ure 4c, configuration I), while on decreasing the temperature
the system reaches its full loading and CO2 molecules also fill
the main channels (Figure 4d, configuration II).
2
Figure 3. Experimental H NMR spectra (black lines) of d4-Zn-BPEB/CO2 at
variable temperature. Blue and red profiles represent the fast and slow com-
ponents (configuration I and II, respectively). Rotational frequencies k and li-
bration amplitudes q for the two components are expressed in Hz and de-
grees, respectively.
Figure 4. a) The small pores and the large channel in Zn-BPEB described by
a sphere of 1.6 ꢁ radius. b) The molecular rotor surrounded by the small
pores in the channel walls. Exploration of the cavities by CO2: c) low and
d) high loadings (configuration I and II, respectively).
The dramatic change of the spectral series with respect to
that of the empty matrix in the same range of temperatures
from 150 K to 290 K, is apparent. The spectra were simulated
by considering two distinct components, represented in
Figure 3 as blue and red profiles. At 150 K the spin-echo
2H NMR spectrum exhibits a dominant profile associated with a
rotation rate k of 105 Hz and a two-site 1808-flip reorientation
mechanism (Figure 3, blue profile). Thus, the rotation rate is
slowed by more than 6 orders of magnitude compared to the
empty MOF (in the order of 1011 Hz at 150 K), owing to the
hampering effect of CO2 molecules filling the channels. At this
temperature virtually complete loading of the channels occurs
and CO2 molecules surround the rotor with the maximum effi-
ciency.
In the latter configuration, each rotor is surrounded by 3
CO2 molecules simultaneously: two in the small pores (one on
either side of the rotor) and one in the large channels (facing
the rotor at the short distance of 3.4 ꢁ). Thus, the CO2 mole-
cules wrap the rotor and slow down its motion, as observed in
the blue profile of Figure 3. In the temperature range from
150 K to 200 K, in the configuration II with the 3 CO2 molecules
about a single rotor progressively its rotational rates increases
on increasing the temperature (blue profile). The measured
energy barrier, by the Arrhenius plot, is 4.5 kcalmolÀ1, that is,
4 kcalmolÀ1 higher than that recorded in the empty sample
(Figure S8).[16] These results clearly show that CO2 controls the
rotor dynamics, yet without changing the main two-fold flip
mechanism. On increasing temperature and decreasing the
CO2 loading, the probability of 3 CO2 molecules surrounding si-
multaneously a single rotor decreases, and CO2 molecules flow
away from the larger channels, reducing the constriction on
the rotors, that gain mobility. Consequently, in the 2H NMR
spectra the red component with large librations (coupled with
fast 1808 flip reorentation) prevails. Anyway, the two configura-
On increasing the temperature, the CO2 loading of the
sample decreases (Tables S2) and consequently the fraction of
rotors fully surrounded by CO2 is reduced, as observed by the
decrease of blue area. At higher temperatures (>200 K) a nar-
rower profile, representing a motional dynamics faster than
108 Hz, clearly emerges and dominates progressively evolving
with temperature (Figure 3, red profile). This lineshape was si-
mulated with the occurrence of large librations from q= Æ328
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Acknowledgements
Cariplo Foundation, INSTM Consortium/Lombardy Region and
PRIN 2016 are acknowledged for financial support. G. Palmisa-
no (Universitꢃ dell’Insubria) is acknowledged for his constant
support in the synthesis of the ligands.
Conflict of interest
The authors declare no conflict of interest.
Keywords: dynamics · molecular rotors · porous materials ·
solid state NMR spectroscopy · supramolecular chemistry
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Manuscript received: June 26, 2017
Accepted manuscript online: July 4, 2017
Version of record online: && &&, 0000
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COMMUNICATION
MOF, I’m only dancing: CO2 gas flows
into the Zn-based MOF which sustains
ultrafast rotors and intervenes actively
in the motional mechanics, increasing
reversibly the energy barrier for rotation
and allowing active manipulation of
molecular rotor dynamics (see figure).
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Molecular Rotors
S. Bracco, F. Castiglioni, A. Comotti,*
S. Galli, M. Negroni, A. Maspero,*
P. Sozzani
&& – &&
Ultrafast Molecular Rotors and Their
CO2 Tuning in MOFs with Rod-Like
Ligands
Molecular rotors reorienting in times as short as 0.01 nanoseconds at 150K and
endowed with minimal constraints (Ea=0.5 kcalmol-1) were realized in MOFs. This
benchmark of motional flexibility, coupled with porosity, entailed unprecedented
sensitivity to gases, such as CO2, which could modulate extensively rotor dynamics.
Our results highlight how largely crystal dynamics are responsive to the gas
environment. For more information see the Full Paper by A. Comotti, A. Maspero et al.
on page && ff.
Chem. Eur. J. 2017, 23, 1 – 7
www.chemeurj.org
7
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers! ÞÞ