Hydrogen-Bonded Crystalline Molecular Machines with Ultrafast Rotation and Displacive Phase Transitions

Article abstract of DOI:10.1002/chem.202001156

Two new crystalline rotors 1 and 2 assembled through N?H???N hydrogen bonds by using halogenated carbazole as stators and 1,4-diaza[2.2.2]bicyclooctane (DABCO) as the rotator, are described. The dynamic characterization through ¹H T₁ relaxometry experiments indicate very low rotational activation barriers (E_a) of 0.67 kcal mol^?1 for 1 and 0.26 kcal mol^?1 for 2, indicating that DABCO can reach a THz frequency at room temperature in the latter. These E_a values are supported by solid-state density functional theory computations. Interestingly, both supramolecular rotors show a phase transition between 298 and 250 K, revealed by differential scanning calorimetry and single-crystal X-ray diffraction. The subtle changes in the crystalline environment of these rotors that can alter the motion of an almost barrierless DABCO are discussed here.

Full text of DOI:10.1002/chem.202001156

Accepted Article
Accepted Article
Title: Hydrogen Bonded Crystalline Molecular Machines with Ultrafast
Rotation and Displacive Phase Transitions
Authors: Abraham Colin-Molina, Marcus J. Jellen, Joelis Rodríguez-
Hernández, Miguel Eduardo Cifuentes-Quintal, Jorge
Barroso, Rubén A. Toscano, Gabriel Merino, and Braulio
Rodríguez-Molina
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.202001156
Link to VoR: https://doi.org/10.1002/chem.202001156
01/2020

10.1002/chem.202001156
Chemistry - A European Journal
FULL PAPER
Hydrogen Bonded Crystalline Molecular Machines with Ultrafast
Rotation and Displacive Phase Transitions
Abraham Colin-Molina,1 Marcus J. Jellen,2 Joelis Rodríguez-Hernández,3 Miguel Eduardo Cifuentes-
Quintal,4 Jorge Barroso,4 Rubén A. Toscano,1 Gabriel Merino,4* Braulio Rodríguez-Molina.1*
1 Instituto de Química, Universidad Nacional Autónoma de México, Circuito Exterior, Ciudad Universitaria, 04510, Ciudad de México, México.
2 Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095, United States.
3 Centro de Investigación en Química Aplicada (CIQA), Blvd. Enrique Reyna Hermosillo, No. 140, Saltillo, Coahuila 25294, México.
4 Departamento de Física Aplicada, Centro de Investigación y de Estudios Avanzados, Unidad Mérida. Km 6 Antigua Carretera a Progreso. Apdo. Postal 73,
Cordemex, 97310, Mérida, Yuc., México.
Supporting information for this article is given via a link at the end of the document
Abstract: Two new crystalline rotors, 1 and 2, assembled through N-
H…N hydrogen bonds, using halogenated carbazole as stators and
diaza[2.2.2]bicyclooctane (DABCO) as the rotator, are described. The
dynamic characterization through 1H T1 relaxometry experiments
indicate very low rotational activation barriers of 0.67 kcal/mol for 1
and 0.26 kcal/mol for 2, indicating that DABCO can reach a THz
frequency at room temperature in the latter. These Ea values are
supported by solid-state density functional theory computations.
Interestingly, both supramolecular rotors show a phase transition
between 298 and 250 K, revealed by differential scanning calorimetry
and single-crystal X-ray diffraction. The subtle changes in the
crystalline environment of these rotors that can alter the motion of an
almost barrierless DABCO are discussed here.
volume-conserving motions. With this in mind, the pairing of the
appropriate molecular components should facilitate the
development of molecular rotors with fast rotation. Such a
combination of constituents shall not be restricted to covalent
bonds, and therefore the use of supramolecular interactions, like
hydrogen or halogen bonds, can be invoked to develop novel
rotors with very low rotational barriers.29
The directional nature of hydrogen bonds can be used to generate
novel structures with
a great deal of complexity. 30 The
directionality and cooperativity of this supramolecular interaction
have been successfully employed in the solid-state to create
cocrystals with emergent physicochemical properties not present
in the initial components.31 By using this strategy, a cocrystalline
rotor, with an unsubstituted carbazole stator and DABCO rotator,
has been recently reported by us, showing dynamics at the
molecular and macroscopic scales.32 These results prompted us
to design new molecular architectures, given the fact that
carbazole can be bestowed with numerous functional groups
across its skeleton, bearing derivatives for attractive interactions
during crystallization, and consequently improving the internal
dynamics of the rotator.
Introduction
Inspired by nature, several research groups around the world
have developed artificial molecular machines with internal
dynamics or functional properties.1,2,3,4,5,6 A considerable effort
has been invested in both the design and synthesis of compounds
with different types of motion in solution7,8 and the solid-state,9,10
aiming to understand better the type and frequency of the motion
at the nanoscale. Thus, several motors, rotors, and rotaxanes
have been reported recently.11,12,13,14 Among those, solid-state
systems with fast intramolecular motion are of particular interest
to developing responsive materials (muscles, actuators, sensors,
porous materials), akin to the functions of living organisms.15
Molecular rotors are crystalline entities composed of static groups
(stators) surrounding smaller dynamic components (rotators).
Previous reports have shown that stators should be voluminous
to generate pockets within the crystal lattice and provide enough
space for the rotator to undergo the intended motion. For this
purpose, molecular components such as trityl, triptycene,
pentiptycene, steroids, and more recently carbazole derivatives
have been used as stators.16,17 From these, carbazole-based
rotors are attractive to us given their good tendency to crystallize,
high thermal stability, and in some cases, interesting solid-state
emission.18,19,20
DABCO
1
i) THF solution or
ii) Mechanosynthesis
X = -Cl (3), -Br (4)
X = -Cl (1), -Br (2)
2
Figure 1. Left: synthesis of supramolecular of rotors 1 and 2. Right: Molecular
conformations observed in their corresponding crystals.
In this work, we describe the synthesis and dynamic
characterization of two symmetric rotors that feature either 3,6-
dichloro- (rotor 1) or 3,6-dibromo-9H-carbazole stators (rotor 2),
encasing a DABCO rotator (Figure 1). From the X-ray data
diffraction, it is determined that these carbazole derivatives are
excellent stators that tether the globular DABCO by means of
strong NH…N hydrogen bonds. According to the variable
temperature 1H spin-lattice experiments, both rotors display
rotational barriers of less than 1 kcal/mol, among the lowest Ea for
supramolecular rotors reported to date. Interestingly, both
compounds show a displacive phase transition that is unusual in
The choice of the rotator also plays an essential role in achieving
fast intramolecular motion. 21 Globular rotators like
bicyclo[2.2.2]octane (BCO), 22 4,4-diazabicyclo[2.2.2]octane
(DABCO), 23 , 24 p-carborane, 25 or fullerene, 26 might display
rotational frequencies faster than those reached with phenylene
groups,27,28 due to their larger size and symmetry that may enable
1
This article is protected by copyright. All rights reserved.

10.1002/chem.202001156
Chemistry - A European Journal
FULL PAPER
purely organic compounds, evidenced by variable temperature
single crystal X-ray diffraction, 1H relaxometry, and calorimetric
experiments. It is found that only the internal motion of compound
1 is affected by this transition. A series of density functional theory
(DFT) computations are carried out, using periodic boundary
conditions, to understand how the crystal structure and bonding
of these rotors enable such low Ea values. Our computed barriers
support the experimental values, and the noncovalent interaction
index (NCI) provide a way to understand the existent contacts in
the cocrystals. Both insights of these sophisticated internal
dynamics in ultrafast crystalline rotors can be useful to take
advantage of their functionality on future applications.
b)
a)
2.028(3) Å
176.4(3)
1.892(3) Å
176.9(2)
d)
c)
Results and Discussion
2.11(4) Å
Synthesis and X-ray studies of cocrystals 1 and 2
163.3(4)
Our purpose is to use halogenated carbazoles to produce bulky
stators and facilitate attractive interactions among the halogen
atoms on the periphery.33 This approach is not trivial given that
the substitutions reduce the ability of the carbazole derivatives to
form the desired 2:1 cocrystals with DABCO, affording the 1:1
species instead (not discussed here). Let us focus on the
supramolecular nature of these rotors because the lack of a
covalent bond may result in low electronic barriers to rotation and,
in conjunction with the globular shape of the DABCO, very high
rotational frequencies could be obtained (Figure 1).
2.11(4) Å
163.3(4)
Figure 2. a) Molecular structure of rotor 1, b) Cavity around the DABCO
(magenta) surrounded by four neighboring stators, c) Molecular structure of
rotor 2, d) Segment of the cavity around the DABCO (magenta) surrounded by
six stators. In both cases, the hydrogen-acceptor distances and D-H…A angles
(º) are indicated with standard deviations.
The desired crystalline rotors were obtained as small crystals
using a 2:1 ratio (stator/rotator) by slow evaporation of a THF
solution or by mechanochemical synthesis using Liquid-Assisted
Grinding. Out of three possible precursors, named 3,6-dichloro-
The X-ray diffraction data of both compounds show the intended
hydrogen-bonded triad, but with two completely different
supramolecular arrays, with two stator orientations (Figure 2).34 In
rotor 1, the two carbazole components are placed at a
perpendicular conformation surrounding a highly disordered
DABCO.35 In this rotor, there are two hydrogen-acceptor bond N-
H…N distances of 1.892(3) and 2.028(3) Å. In both interactions,
the angle NH…N is close to linearity 176.9(2)º and 176.4(3)º,
respectively, suggesting that these hydrogen bonds are strong.
The corresponding DFT computations provided excellent values
for the lattice constants of 1 with an error of 1.3-1.6%. In particular,
the computed hydrogen bond lengths of 1.88 Å and an NH…N
angle of 177.6º are in good agreement with the experimental
results. Although it is a difficult task to estimate the hydrogen bond
interaction energy within the crystal, TPSS-D3(BJ)/def2-TZVP
computations in the gas phase (including the ZPE correction)
indicate that this value is of 10.0 kcal/mol.
The molecular rotor 2 also show two hydrogen bonds that tie a
disordered DABCO, 36 with slightly longer hydrogen-acceptor
bond distances (2.11 Å) and lower linearity (163.3º) compared to
rotor 1. This deviation results from the twisted conformation in this
rotor, which resembles a ‘tilted ladder’ where the ‘steps’ of 3,6-
dibromo-9H-carbazole are antiparallel components that lie on
different planes while tethering the DABCO, which is inclined
approximately 22º. In 2, the crystal packing displays the expected
symmetric type-II halogen bond, with a Br…Br distance of 3.735 Å
and an angle of ca. 170º.37 The globular DABCO is surrounded
9H-carbazole
(compound
3),
3,6-dibromo-9H-carbazole
(compound 4), and iodocarbazole (compound 5, not included),
only two of them were successfully cocrystallized with DABCO.
Solution 1H NMR afforded the first indication that the rotors
displayed the desired hydrogen bond in solution. The
corresponding stators (3 or 4) and the DABCO rotator were
dissolved in CDCl3 in a 2:1 ratio and the -NH signal of the
halocarbazole indicated major changes in its chemical shifts with
respect to the starting materials ( = 0.67 and  = 0.45, for 1
and 2, respectively). Given that the solubility in chloroform of the
carbazole derivatives was slightly limited, we shifted to THF-d8 to
solubilize them completely, but the spectroscopic changes were
less noticeable (see Figure S1 and S2). Fortunately, the better
solubility in THF allowed us to grow crystals suitable for single
crystal X-ray studies from the evaporation of the corresponding
solutions (Figure S3, S4 and Table S1).
by planar neighboring halocarbazoles, residing within
a
rectangular cavity. The computed lattice constant values in 2 have
an error percentage of 0.4-2.6%. Note that the crystal has a
triclinic phase, so the lattice parameters α, β, and γ are no longer
2
This article is protected by copyright. All rights reserved.

10.1002/chem.202001156
Chemistry - A European Journal
FULL PAPER
90º, and the computations yield an error of less than 0.7%. The
computed hydrogen bond distances and NH...N angles in the
crystal are 1.87 Å and 178.1º, respectively, and the estimated
interaction energy in the gas phase for this hydrogen bond is
again 10.1 kcal/mol.
(Figure 3). This suggests that the rotational frequencies of the
DABCO in 1 and 2 are faster than 600 MHz even at 175 K. The
activation energies corresponding to rotation (Ea) are obtained
from the slope of Ln(1/T1) against the reciprocal of the
temperature (1000/T), as reported in other instances, which are
remarkably low for both rotors 1 (0.67 kcal/mol for 1 and 0.26
kcal/mol for 2, see Figures S20 and S21). These barriers
represent only a fraction of that found in the rotor using
unsubstituted carbazole and DABCO (2.6 kcal/mol).32 These
values indicate that the rotational frequencies at 295 K for rotors
1 and 2 should be ca. 760 GHz and 1.4 THz, respectively, and
both compounds should be able to reach faster frequencies above
room temperature, because they are thermally stable cocrystals.
Similar fast rotations have only been reported in covalent rotors
with bicyclo[2.2.2]octane as the rotational component, with an
energy barrier of 1.15 kcal/mol 42 or a distribution of energy
barriers ranging from 0.37 to 1.29 kcal/mol. 43 The computed
rotational barriers, using periodic boundary conditions, are 1.26
and 0.77 kcal/mol for 1 and 2, respectively. These values are
close to the experimental values, although within the limits of the
range of any computational method.
Bulk characterization, internal dynamics by 1H T1 spin-lattice
relaxation and DFT computations in the solid state
In order to study the internal dynamics of these supramolecular
rotors, larger amounts of each cocrystal were prepared
mechanochemically using the Liquid-Assisted Grinding (LAG)
approach.38 The phase purity of the samples was corroborated by
powder X-ray diffraction data using Le Bail fittings (see Figure S15
and S16), The thermal stability of molecular rotors was studied by
DSC and TGA analyses showing good stability under 120 °C (see
Figures S7-S11). Given that the hydrogen bond acts as the
supramolecular axis as evidenced by FTIR in the NH stretching
(see Figures S13 and S14), the globular DABCO is expected to
show a very fast reorientation process. This rapid motion can be
an efficient relaxation process for restoring the bulk magnetization
of the sample in the NMR spectrometer.39
Dynamic characterization by VT X-ray diffraction
b)
a)
The 1H T1 plot of rotor 1 indicates that some data deviate from the
projected linearity (Figure 3a). The discrepancies occur below
room temperature, and later the linearity is recovered as the
temperature approaches 175 K. In contrast, only a small deviation
is observed in the plot of rotor 2 (Figure 3b). This 1H T1 behavior
of compound 1 could come from variations in the vibrational or
rotational modes of the molecular components, giving rise to new
relaxation pathways within the crystals.44
y= 0.347x - 4.994
R2= 0.982
Ea= 0.689 kcal/mol
y= 0.124x - 3.595
R2= 0.989
Ea= 0.246 kcal/mol
y= 0.139x – 3.679
R2 = 0.999
Ea= 0.276 kcal/mol
y= 0.330x - 4.657
R2= 0.972
Ea= 0.655 kcal/mol
d)
c)
a)
b)
A2
D2
A1
D1
230 K
298 K
250 K
c)
d)
770
750
730
710
690
2815
2795
2775
2755
2735
Figure 3. a) 1H T1 relaxation experiments of rotor 1, b) 1H T1 relaxation
experiments of rotor 2. c) Computed rotational potential of the DABCO rotator
in 1, d) Computed rotational potential of the DABCO rotator in 2.
190
240
290
190
240
290
Temperature (K)
Temperature (K)
Figure 4. Changes in the relative orientation of the carbazole stators at different
temperatures. a) rotor 1 and b) rotor 2. Cell volume upon cooling from 298 K to
190 K: (c) rotor 1 displays an expansion of the cell at ca. 250 K, d) rotor 2 does
not show an evident cell expansion.
Considering that the rotation in these systems is a thermally-
activated process and that the reorientation is characterized by a
correlation time c, variable-temperature 1H T1 experiments were
carried out to obtain the corresponding activation parameters.40
At low temperatures, the reorientation of DABCO should be
reduced until they match the Larmor frequency of the observed
nucleus, in this case, 1H at 600 MHz. This would be identified
when the T1 values reach a minimum.41 For rotors 1 and 2, the 1H
T1 saturation-recovery experiments were carried out from 360 K
to 175 K (see Figures S22-S41), the minimum temperature
available in the spectrometer.
To explore in detail this intriguing behavior from a structural point
of view, we grew a fresh single crystal of 1 and performed low-
temperature X-ray diffraction studies from 298 K to 200 K (Figure
4). From the analyses of those experiments, it is noted that rotor
1 shows changes in the relative conformation of the molecular
components upon cooling, which affect the relative positions of
the carbazole and changed the alignment of DABCO, making the
N-H…N bond distance (D2) and angle (A2): 2.04 Å /170.9°, 2.05
In both rotors, the relaxation T1 times shortened as the
temperature of the samples reduced, although the T1 minima
were not observed due to instrumental temperature limitations
3
This article is protected by copyright. All rights reserved.

10.1002/chem.202001156
Chemistry - A European Journal
FULL PAPER
Å /170.0°, and 2.04 Å /169.5° at 298, 250, and 230 K, respectively
(Figure 4a). The same set of experiments for compound 2 reveal
only minor structural variations in the X-ray diffraction data (Figure
4b). However, the phase transition was detected by DSC (see
Figure S12).
transitions in 1 and 2, the structures at room temperature (before
transition) and 230 K (after transition) were superimposed,
revealing a displacement of 0.51 Å (rotor 1) or 0.31 Å (rotor 2)
occurring on the perpendicular axis to the plane of the stator. It is
important to emphasize that the displacive transition described
here was not studied in the previously reported parent rotor with
Despite the observed structural changes in rotor 1, the X-ray
diffraction data refinements at all temperatures show that the
crystal system and space-group have not changed (P21/c), and
therefore this process is an isostructural phase transition. Most
notably, an expansion of the cell axes and the volume is observed
at ca. 250 K (Figure 4).45
unsubstituted carbazole stator, but rather
a thermosalient
(disintegrative) phase transition at high temperatures.
To rationalize the distinctive behavior between the rotors, it is
mandatory to identify the number and type of intermolecular
contacts within the crystal. Each carbazole-DABCO-carbazole
triad is surrounded by 10 or 14 rotors in compounds 1 and 2,
respectively (Figure 6). Although the conformation differs and the
number of neighbors is different in each compound, the DFT
periodic computations indicated that the interaction energy
between a rotor and all its neighbors is ~60 kcal/mol in both cases
(58.0 and 60.0 kcal/mol for 1 and 2, respectively). These similar
interaction energies could indicate that: A) the type of interactions
in the crystal is the same or that B) the interactions are different
but compensate each other so that they result in equivalent
magnitudes. Hypothesis A was discarded since the number of
neighbors and the crystal arrangement for each rotor are different.
To explore if hypothesis B is valid, it was necessary to distinguish
the type and number of contacts. To this end, the NCI (Non-
Covalent Interactions) index was used to identify the contacts of
each rotor. It is impractical to determine how many and what type
of contacts surround a specific rotor from the NCI of the unit cell
(see Figure S42). Therefore, we considered the first neighbors of
a given rotor to form pairs (moieties).
a)
b)
c)
Distance (Å)= 0.21
Distance (Å)= 0.74
Distance (Å)= 0.15
298 K vs 250 K
250 K vs 230 K
230 K vs 200 K
Figure 5. Changes in the atomic positions in rotor 1 at different temperatures.
All atoms of the 3,6-dihalocarbazole portion that contain labelled atoms Cl(3)
and Cl(4) were overlapped. a) Comparison between 298-250 K, b) Comparison
between 250-230 K, and c) Comparison between 230-200 K. The comparison
was carried out calculating the centroids of each carbazole moiety.
According to the classification of phase transitions proposed by
Buerger,46 the observed structural changes should correspond to
a second-order phase transition called displacive. This kind of
transition has been mostly reported in hybrid compounds47 and
metallic salts, 48 and therefore its presence may be of great
relevance to fully understand the behavior of crystalline molecular
machines. It has been reported that these transformations require
small energy and are associated with changes in the atomic
positions of the molecular components due to molecular
translations or rotations.49 In the both rotors reported here, in
addition to expected changes in their atomic coordinates, the
calorimetric experiments clearly indicate a small amount of
absorbed heat.
A
comparison of the crystalline structures at different
temperatures reveals the change in the atomic positions of rotor
1. Figure 5a highlights the displacive rearrangement with a
centroid-centroid distance of 0.74 Å suffered by the stator above
250 K, but this change is less noticeable in the range of
temperatures between 250 and 200 K (Figures 5b and 5c). The
translational changes found below 250 K are considered enough
evidence of the displacive transition that occurred inside the
crystal with the largest change along the c-axis (Figure S5 and
Table S2). Similar structural reconfigurations have been reported
in molecular systems with ferroelectric response with a DABCO
component in their structures.50
Figure 6. Non-covalent interactions (NCIs) analyses. Plots in pairs or dimers
found in the crystals of rotors 1 and 2.
In the case of rotor 1, there are three different moieties of
neighboring units (Figure 6): a) four moieties labeled 1-A with
π…H-C, CH…CH, and C-Cl…H-C contacts, b) four moieties labeled
1-B with the similar interactions as seen in 1-A but with different
orientation and, c) two moieties 1-C with Cl…Cl interactions
In contrast, compound 2 only shows small changes in its
relaxation behavior (vide supra), calorimetric analysis as well as
minor variations in the single crystal X-ray data collections at
different temperatures (see Figure S6 and Table S3). In order to
emphasize the structural differences after the displacive phase
4
This article is protected by copyright. All rights reserved.

10.1002/chem.202001156
Chemistry - A European Journal
FULL PAPER
between rotors. In contrast, for the case of 2, there are five
different pairs of neighboring units: a) two moieties 2-A with π…H-
C, and C-Br…H-C contacts, b) four moieties 2-B with mostly with
π…H-C contacts, c) two moieties type 2-C, which exhibit C-
Br…hydrogen contacts of the rotator and also with the aromatic
carbazole, d) four moieties 2-D with C-Br…H-C interactions, and
e) two moieties 2-E, which show Br…Br contacts. So, the nature
of the interactions in each rotor could be similar, but the number
is different, so the conformational differences in the crystal
structure between 1 and 2 balance the reduced number of
contacts.
during cooling, specifically at 250 K, causing noticeable structural
distortions that were pinpointed by 1H spin-lattice and calorimetric
measurements. In stark contrast, rotor 2 does not display changes
in relaxation behavior. The lack of change is attributed to the
conformation of its components that uphold minor structural
changes. These insights reveal that is possible to attain very high
rotational frequencies in the solid-state and that the internal
motion is sensitive to the structural changes in the crystalline
environment in this type of molecular machines.
Acknowledgements
The fact that the intermolecular interactions in each rotor have
virtually the same energy may suggest that the relaxation
anomalies observed in 1 are due to the variation of the cavity
hosting the DABCO, considering the significant changes in the c-
crystallographic axis, which modifies the initial alignment (and
dynamics) of the rotator. In the case of rotor 2, the ladder
conformation of the components may act akin to a "shock
absorber" and, therefore, small structural changes do not
significantly affect the relaxation of the DABCO.
ACM and JB thank Conacyt for PhD fellowships. This project was
supported by PAPIIT (Grant IN209119) and CONACYT (Grant
A1-S-32820). We acknowledge the UCLA Department of
Chemistry and Biochemistry for solid-state 2H NMR experiments
(NSF DMR-1700471 and MRI-1532232) and the technical
assistance from F. Garcia-Gonzalez, F. J. Pérez Flores, (MS), M.
A. Peña González, E. Huerta Salazar (NMR), A. Nuñez-Pineda
(DSC and TGA), D. Martínez-Otero (XRD), and J. Arcudia (DFT
computations).
Based on the previous analyses, an asymmetric rotor where one
3,6-dichloro- and one 3,6-dibromo-9H-carbazole stator
simultaneously tether one DABCO rotator was attempted to
produce a rotor with modulated properties. The three components
(in a 1:1:1 ratio) were dissolved and followed by 1H NMR in
solution, and two proton signals at 9.40 and 9.32 ppm were
observed (Figure S1). These chemical shifts were taken as
indicative of the presence of the asymmetric rotor in solution.
When this three-component solution was left to evaporate (either
from CHCl3 or THF), the resulting crystalline precipitate showed a
mixture of supramolecular rotors 1 and 2, as evidenced by the Le
Bail fitting using powder X-ray diffraction data (see Figures S17,
S18 and S19). The self-assembly process of 1 and 2 was not
expected and it can be explained in terms of a lower energetic
cost that favors the symmetric rotors, although more detailed
subsequent studies are needed to gather deeper insights.
Keywords: Crystalline Molecular Machines • THz Frequency •
Displacive Phase Transition • Non-Covalent Interactions • 1H T1
relaxation .
Summary and Outlook
Two ultrafast supramolecular rotors
1
and
2
featuring
halocarbazoles and DABCO have been obtained by establishing
strong N-H…N hydrogen bonds. This interaction was already
observable from 1H NMR in solution and transferred to the solid-
state rendering symmetric structures and conformations, with 2
also displaying halogen bonds. Preliminary experiments indicate
that symmetric rotors are the preferred entities in the solid-state
over asymmetric ones.
The two rotors show very distinctive supramolecular arrays with
differences in the conformation of the halocarbazole stators.
However, both compounds display very low activation barriers to
rotation 1 (ca. Ea = 0.67 kcal/mol) and 2 (ca. 0.26 kcal/mol) for
organic crystals, as determined by 1H spin-lattice experiments.
These values are supported by DFT computations using periodic
boundary conditions. Taken these values into consideration, the
rotational frequencies for 1 and 2 at room temperature should be
ca. 760 GHz and 1.4 THz, respectively.
Notably, both crystalline rotors have different responses when
they are cooled down. Rotor 1 has an unexpected cell expansion
5
This article is protected by copyright. All rights reserved.

10.1002/chem.202001156
Chemistry - A European Journal
FULL PAPER
1 Lancia, F., Ryabchun, A., Katsonis, N., Nat. Rev. Chem., 2019, 3, 536-551.
33 Colin-Molina, A., Pérez-Estrada, S., Roa, A. E., Hernández-Ortega, S.,
Rodriguez,
M., Brown,
S.,
Rodríguez-Molina, B.,
Chem.
2 Groppi, J., Baroncini, M., Venturi, M., Silvi, S., Credi, A.,Chem. Commun.,
2019, 55, 12595-12602.
Commun. 2016, 52,12833-12836.
34 Rotor 1 was refined in a monoclinic centrosymmetric space group P21/c.
Although the diffraction patterns at low and room temperatures can be indexed
in the tetragonal space group P41 (P43) the structure could not be refined
successfully in this space group. In the case of rotor 2, the only space group
that gave good quality figures of merit was P-1 at low and room temperature.
3 Kassem, S., Van Leeuwen, T., Lubbe, A. S., Wilson, M. R., Feringa, B. L.,
Leigh, D. A., Chem. Soc. Rev., 2017, 46, 2592-2621.
4 Pezzato, C., Nguyen, M. T., Kim, D. J., Anamimoghadam, O., Mosca, L.,
Stoddart, J. F., Angew. Chem. Int. Ed., 2018, 57, 9325-9329.
5 Harris, J. D., Moran, M. J., Aprahamian, I., Proc. Natl. Acad. Sci. USA, 2018,
35 The DABCO rotator was solved with
a four-site disorder, showing
occupancies of 0.338, 0.256, 0.215, 0.191, at room temperature.
36 In rotor 2 the DABCO rotator was solved with a two-site disorder, with 0.60
and 0.40 occupancies at room temperature.
115, 9414–9422.
6 Braga, D., Grepioni, F., Maini, L., d’Agostino, S., Eur. J., Inorg. Chem., 2018,
3597-3605.
37 Cavallo, G., Metrangolo, P., Milani, R., Pilati, T., Priimagi, A., Resnati, G.,
Terraneo, G., Chem. Rev., 2016, 116, 2478-2601.
7 Kumbhar, H. S., Deshpande, S. S., Shankarling, G. S., ChemistrySelect, 2016,
1, 2058-2064.
38 Hasa, D., Rauber-Schneider, G., Voinovich, D., Jones, W., Angew. Chem.
Int. Ed., 2015, 54, 7371-7375.
8 Wu, Y., Wang, G., Li, Q., Xiang, J., Jiang, H., Wang, Y., Nat. Commun., 2018,
9, 1953.
39 Beckmann, P. A., Dougherty Jr. W. G., Kassel, W. S., Solid State Nucl. Magn.
Reson., 2009, 36, 86-91.
9 Kaleta, J., Bastien, G., Wen, J., Dracinsky, M., Tortorici, E., Cisarova, I., Beale,
P. D., Rogers, C. T., Michl, J., J. Org, Chem., 2019, 84, 8489-8467.
10 Inukai, M., Fukushima, T., Hijikata, Y., Ogiwara, N., Horike, S., Kitagawa, S.,
J. Am. Chem. Soc., 2015, 137, 12183-12186.
40 R. Kubo and K. Tomita, J. Phys. Soc. Jpn., 1954, 9, 888-919.
41 V.I. Bakhmutov, 2004, Practical NMR relaxation for chemists, Wiley.
42 Rodríguez-Molina, B., Pérez-Estrada, S., Garcia-Garibay, M.A., J. Am.
Chem. Soc., 2013, 135, 10388-10395.
43 Simonov, S., Zorina, L., Wzietek, P., Rodríguez-Fortea, A., Canadell, E.,
Meziere, C., Bastien, G., Lemouchi, C., Garcia-Garibay, M.A., Batail, P., Nano
Lett., 2018, 18, 3780-3784.
11 Harada, J., Yoneyama, N., Sato, S., Takahashi, Y., Inabe, T., Cryst. Growth
Des., 2019, 19, 291-299.
12 Zhang, Z.-X., Zhang, T., Shi, P.-P., Zhang, W.-Y., Ye, Q., Fu, D.-W., J. Phys.
Chem. Lett., 2019, 10, 4237-4244.
44 Harris, R.K., Wasylishen, R. E., Duer, M. J., 2009, NMR crystallography,
Wiley.
13 Balema, T. A., Ulumuddin, N., Murphy, C. J., Slough, D. P., Smith, Z. C.,
Hannagan, R. T., Wasio, N. A., Larson, A. M., Patel, D. A., Groden, K.,
McEwen, J.-S, Lin, Y.-S., Sykes, E. C. H., J. Phys. Chem. C, 2019, 123,
23738-23746.
45 DSC experiments at low temperatures displayed an endothermic event as
indicated in the SI.
46 Buerger, M. J., 1951, Phase transformation in Solids, John Wiley and Son.
47 Liu, Y., Zhu, C.-L., Zheng, X.-Y., Qin, L.-L., Yang, S.-X., Liu, Z.-Q., R. Soc.
Open Sci., 2018, 5, 180738.
14 Dong, J., Zhang, K., Li, X., Qian, Y., Zhu, H., Yuan, D., Xu, Q.-H., Jiang, J.,
Zhao, D., Nat. Commun., 2017, 8, 1142.
15 Urry, D.W., Angew. Chem. Int. Ed., 1993, 32, 819-841.
16 Meng, H., Zhao, C., Nie, M., Wang, C., Wang, T., Nanoscale, 2018, 10,
18119-18123.
17 Thibeault, D., Auger, M., Morin, J.-F., Eur. J. Org. Chem., 2010, 3049-36067.
18 Aguilar-Granda, A., Colin-Molina, A., Jellen, M.J., Núñez-Pineda, A.,
Cifuentes Quintal, M.E., Toscano, R.A., Merino G., Rodríguez-Molina, B.,
Chem. Commun., 2019, 55, 14054-14057.
48 Matvienko, A. A., Maslennikov, D. V., Zakharov, B. A., Sidelnikov, A. A.,
Chizhik, S. A., Boldyreva, E. V., IUCrJ, 2017, 4, 588-597.
49 Anwar, J., Zahn, D., Adv. Drug Deliv. Rev., 2017, 117, 47-70.
50 Ye, H.Y., Zhang, Y., Noro, S.-I, Kubo, K., Yoshitake, M., Liu, Z.-Q., Cai, H.-
L., Fu, D.-W., Yoshikawa, H., Agawa, K., Xiong, R.-G., Nakamura, T.,
Sci. Rep., 2013, 3, 2249.
19 Parrot, E. P. J., Tan, N. Y., Hu, R., Zeitler, J. A., Tang, B. Z., Pickwell-
MacPherson, E., Mater. Horiz., 2014, 1, 251-258.
20 Alam, P., Leung, N. L. C., Cheng, Y., Zhang, H., Liu, J., Wu, W., Know, R. T.
K., Lam, J. W. Y., Sung, H. H. Y., Williams, I. D., Tang, B. Z., Angew.
Chem. Int. Ed. 2019, 58, 4536-4540.
Entry for the Table of Contents
21 Karlen, S. D., Reyes, H., Taylor, R. E., Khan, S. I., Hawthorne, M. F., Garcia-
Garibay, M. A., Proc. Nat. Acad. Sci. USA, 2010, 107, 14973-14977.
22 Rodríguez-Molina, B., Pérez-Estrada, S., Garcia-Garibay, M. A., J. Am.
Chem. Soc., 2013, 135, 10388-10395.
96.8°
3.73 Å
Displacive
phase
transition
Peripheral
halogen
interactions
169.9°
23 Jiang, X., Duan, H.-B., Jellen, M. J., Chen, Y., Chung, T. S., Liang, Y.,
Garcia-Garibay, M. A., J. Am. Chem. Soc., 2019, 141, 16802-16809.
24 d’Agostino, S., Fornasari, L., Grepioni, F., Braga, D., Rossi, F., Chierotti, M.
R., Gobetto, R., Chem. Eur. J., 2018, 24,15059 –15066.
25 Prokop, A., Vacek, J., Michl, J., ACS Nano, 2012, 6, 1901-1914.
26 Lorbach, A., Maverick, E., Carreras, A., Alemany, P., Wu, G., Garcia-Garibay,
M. A., Bazan, G. C., Phys. Chem. Chem. Phys., 2014, 16, 12980-12986.
27 Setaka, W., Inoue, K., Higa, S., Yoshigai, S., Kono, H., Yamaguchi, K., J.
Org. Chem., 2014, 79, 8288-8295.
298 K
28 Bracco, S., Miyano, T., Negroni, M., Bassanetti, I., Marchio, L., Sozzani, P.,
Tohnai, N., Comotti, A., Chem. Commun., 2017, 53, 7776-7779.
29 Aakeroy, C. B., Panikkattu, S., Chopade, P. D., Desper, J., CrystEngComm,
2013, 15, 3125-3136.
250 K
30 Li, Z., Wu, L.-Z., 2015, Hydrogen Bonded Supramolecular Structures,
Springer.
1x1012
cycles by second at room temperature!
31 Liu, R., Wang, H., Jin, W. J., Cryst. Growth Des., 2017, 17, 3331-3337.
32 Colin-Molina, A., Prasad-Karothu, D., Jellen, M. J., Toscano, R. A., Garcia-
Garibay, M. A., Naumov, P., Rodríguez-Molina, B., Matter, 2019, 1,
1033-1046.
6
This article is protected by copyright. All rights reserved.