Triggering the dynamics of a carbazole-: P -[phenylene-diethynyl]-xylene rotor through a mechanically induced phase transition

Article abstract of DOI:10.1039/c9cc05672f

A new rotor exhibits rich solvatomorphism behavior with eight X-ray structures obtained. A heterogeneous solid obtained by mechanical stress exhibited a dominant isotropic ²H line shape at high temperatures. The motion occurs only in the amorph

Full text of DOI:10.1039/c9cc05672f

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Triggering the dynamics of a carbazole-p-
phenylene-diethynyl]-xylene rotor through a
mechanically induced phase transition†
[
Cite this: Chem. Commun., 2019,
5, 14054
5
Received 22nd July 2019,
Accepted 30th October 2019
a
a
b
Andres Aguilar-Granda,
Abraham Colin-Molina,
Marcus J. Jellen,
´
ac
d
Alejandra Nunez-Pineda, M. Eduardo Cifuentes-Quintal,
´
˜
DOI: 10.1039/c9cc05672f
a
d
a
Ruben Alfredo Toscano,
Gabriel Merino
and Braulio Rodrıguez-Molina
*
´
´
rsc.li/chemcomm
A new rotor exhibits rich solvatomorphism behavior with eight
Our group has been interested in the synthesis of crystalline
X-ray structures obtained. A heterogeneous solid obtained by fluorescent rotors using carbazole as the rigid component with
2
9
mechanical stress exhibited a dominant isotropic H line shape at potential optoelectronic applications. Given its poor solubility,
high temperatures. The motion occurs only in the amorphous only one unstable solvate and one solvent-free structure of rotor 1
À1
10
component of this solid, with an E of 7.4 kcal mol and a low were obtained previously. Despite its flat architecture, carbazole
a
10
À1
pre-exponential factor A of 6.22 Â 10
s , which indicates that the allows for the rotation of the central ring in the solvent-free form
1
0a
motion requires the distortion of the molecular axis.
with moderate rotational frequencies of 6 MHz at 295 K. It is
worth mentioning that other components in 1 did not reveal
Molecular rotors are artificial molecular machines designed to motion at any explored temperature.
show intramolecular rotation. The study of their rotational
Based on these previous results, precise but small structural
dynamics has been the focus of numerous research groups changes in the rotator were performed to deliberately stop the
1
using a variety of spectroscopic techniques. Crystalline mole- 2-fold flip motion of the central phenylene. To this end, the
cular rotors may also display interesting properties for applica- molecular rotor 2 with a xylene derivative as the central consti-
tions as advanced materials. The rotational component should tuent was envisioned (Fig. 1). The methyl groups would be large
2
3
be able to reorient under the appropriate thermal, dielectric,
enough to hinder the rotation of the central fragment while
4
or optical influence. To achieve this reorientation freely, the preserving most of the electronic distribution (Fig. S24, ESI†).
rotary part (rotator) can be surrounded by bulky groups (stator)
5
6
or can be hosted within porous crystals.
The rotational dynamics depend not only on the size and
point symmetry of the rotator but also on the resulting inter-
7
molecular interactions. Finding the appropriate crystal array
that allows motion is a challenging task sometimes restricted
by the solubility of the molecules or the stability of the crystals
formed (i.e. unstable solvates or hydrates), and polymorphism
8
screening would help to uncover the desired solid form.
a
Instituto de Qu ´ı mica, Universidad Nacional Aut o´ noma de M ´e xico, Circuito
Exterior, Ciudad Universitaria, 04510, Ciudad de M ´e xico, Mexico.
E-mail: brodriguez@iquimica.unam.mx
b
Department of Chemistry and Biochemistry, University of California, Los Angeles,
California, 90095, USA
c
Centro Conjunto de Investigaci o´ n en Qu ´ı mica Sustentable UAEM-UNAM, Carretera
Toluca-Atlacomulco Km 14.5, Toluca, 50200, Estado de M ´e xico, Mexico
d
Departamento de F ´ı sica Aplicada, Centro de Investigaci o´ n y de Estudios
Avanzados, Unidad M ´e rida, Km 6 Antigua Carretera a Progreso, Apdo. Postal 73,
Cordemex, 97310, M ´e rida, Yuc., Mexico
†
Electronic supplementary information (ESI) available: Experimental procedures,
Fig. 1 Chemical structures of molecular rotors 1 (previously reported) and
2 (this work). In the case of 2, the attached methyl groups inhibit the
motion of the central aromatic ring and allowed the dynamics of the rings
(magenta) attached to the carbazole stator.
1
13
13
2
tables with crystallographic parameters, H and C NMR solution data, C and
H
solid state NMR data and DSC and TGA results. CCDC 1940315–1940321. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/c9cc05672f
1
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As described below, the substitution indeed restricted the
rotation of the central aromatic ring, according to variable
1
3
temperature (VT) solid-state C NMR CPMAS experiments
and VT X-ray studies. Gratifyingly, the methyl groups also
substantially improved the solubility of 2, which gave rise to
new solid-state forms, either as multiple solvates (forms I to V)
or solvent-free structures (VI–VIII).
Despite all this structural diversity and according to VT
H NMR experiments in the solid state, the rings directly attached
2
to the carbazole fragment in 2 are only able to show motion in an
amorphous solid that originates by applying mechanical stress to
form VII. We describe here how this motion is allowed by the
concomitant torsion of the whole structure. These findings shed
light on the subtle structural differences that govern the intra- Fig. 2 Packing arrays of representative examples of the solid forms reported
here: (a) toluene solvate I and (b) chloroform solvate V. (c) Solvent-free form
molecular motion in artificial molecular machines.
VI (obtained from slow evaporation of ethyl acetate).
Compound 2 and its deuterated derivative 2-d
by cross-coupling reactions (Scheme 1), using 4-iodophenyl-
carbazole 3 or its deuterium-derivative 3-d , and 1,4-diethynyl-[2,5- shown in Fig. 2. The supramolecular array in group A propa-
8
were synthesized
8
11
dimethyl-phenylene] 4. The compounds 2 and 2-d were obtained gates by means of CHÁ Á Áp interactions. The components of the
8
in good yields, 64 and 68%, respectively. The complete spectroscopic lattice are stoichiometrically distributed (1 : 1), with the aro-
assignment of 2 and 2-d was carried out by using 1D and 2D NMR matic guest located close to one carbazole fragment, showing a
8
(
HSQC and HMBC, see the ESI†), FTIR and mass spectrometry.
Compared with the previously reported compound 1, ponding guests are placed in channels (Fig. 2b).
the novel compound 2 showed a much-improved solubility. Being isostructural to other structures of group A, the
Slow evaporation in open vials afforded five solvates labelled I solvent-free form VI also showed parallel layers of molecules
CHÁ Á Áp interaction (Fig. 2a). Conversely, in group B the corres-
(
(
toluene), II (chlorobenzene), III ( p-xylene), IV (THF) and V propagating through the CHÁ Á Áp interactions, but between the
CHCl ). A solvate using dichloromethane was also obtained, central p-xylene ring and a neighbouring carbazole fragment
3
but due to its low thermal stability, the X-ray statistical para- (Fig. 2c). In this form, a significant interaction occurs between
1
2
meters were not ideal and will be not discussed in detail.
one methyl group and one adjacent carbazole component, with
Based on the crystallographic data, we classified the solvates a C–H–p distance of 3.34 Å and an angle of 1661, which would
into two isostructural solvatomorphic groups (labelled A ‘lock’ the potential motion of this component.
and B for better comparison). Group A contains the solvates
Differential scanning calorimetry (DSC) of solvent-free VI showed
with aromatic molecules inside the crystal lattice, as seen in a small but clear endothermic transition close to 490 K (217 1C), and
Table S1 (ESI†). Conversely, group B contains solvates THF and this was further investigated by using hot stage microscopy (HSM).
chloroform (and the unstable DCM).
By heating fresh crystals of form VI above 533 K (260 1C) for 1 hour,
A comparison of representative examples of each group they transform to a new form labelled VII. The resulting structure
forms I and IV) along with the solvent-free structure VI is showed that the molecule is disordered over two positions, with a
severe distortion of the molecular conformation, as it can be seen in
(
the comparison of the two structures (Fig. 3).
The solvates are stable at room temperature, showing an
endothermic peak between 348 K and 423 K, which was
associated with the desolvation. This statement was confirmed
by performing TG analyses and powder X-ray diffraction studies
of the resulting solids (see the ESI†). Based on these results, it
can be concluded that all solvates transform to form VII.
Subsequently, the internal rotation of the xylene portion, or
rather, the lack of it, was explored by variable temperature
1
3
solid-state C CPMAS NMR using the 2-d
8
analogue so the
corresponding C–D carbon atoms of phenylene would not
1
3
overlap the p-xylene signals on the C CPMAS spectrum.
Given the fact that form VII requires some preparation of the
sample (solvation and then desolvation or very high temperatures),
to probe the dynamics of the xylene portion we started with form VI.
13
The C CPMAS experiments from 325 K to 223 K showed only
minor changes in the chemical shifts of the xylene component,
which were attributed to small oscillations of this ring (Fig. 4).
8
Scheme 1 Synthesis of title compound 2 and its deuterated derivative 2-d .
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Fig. 3 Superposition of the two solvent-free structures of rotor 2 that are
obtained by desolvation of the solvates (form VI, blue) and its subsequent phase
transition (form VII, green, only one disordered position shown). The molecular
conformation becomes severely distorted upon the phase transition.
This was corroborated by collecting X-ray structures at different
temperatures, where no major changes were observed (Fig. S4, ESI†).
2
We also carried out VT H NMR studies by the spin echo
technique to document the behaviour of the ring adjacent to the
carbazole. Deuterium is a sensitive quadrupolar nucleus, so changes Fig. 5 (a) Experimental (solid line) and simulated (dotted-line) VT H NMR
2
8
spectra of 2-d , a solid with 83% amorphous (rotation in kHz) and 17%
crystalline (static) components. (b) Cone angles used to describe the
in the crystallographic environment caused by the reorientation of
the C–D bond with respect to the external magnetic field will
produce changes in the broad line shape, known as the Pake
rotation of the phenylene. (c) Arrhenius plot of ln Krot vs. 1/T.
13
pattern. Here, it is emphasized that this ring did not show motion
at any explored temperature in the previously reported rotor 1, so we
did not expect rotation in this component.
Unexpectedly, the mechanical treatment generated a hetero-
geneous solid with crystalline and amorphous components.
The crystalline part was different from previous solids and was
labelled VIII. To date, we have not been able to obtain single
crystals of this form, but DFT computations helped us postulate
a structure that reproduces very well the observed X-ray diffrac-
tion pattern (see the ESI† for details).
The new solid showed a promising narrow peak that
suggested motion. Heating up to 380 K made the isotropic line
shape dominant (Fig. 5a). It is worth noting that depending
on the time and strength of the milling of 2, we produced
We started with the selected structures of the two groups, solvates
I (toluene, group A) and V (chloroform, group B). The experiments
from room temperature up to 380 K (before desolvation) showed a
2
14
broad H line shape that is characteristic of a static segment.
The solvent within the crystals was removed by heating the
solvates at 423 K in a hot plate for 30 min, causing them to
transform to form VII. After this procedure, the deuterated
phenylenes were still static. Considering that traces of the
solvent could be obstructing the motion, we ground the solid
using an agate mortar for 10 minutes. The resulting solid was
evaluated by powder X-ray diffraction and compared with the
known solvent-free forms VI and VII.
mixtures with decreasing amounts of the crystalline component
batches A, B and C) and in each case the narrow signal was
15
(
less visible.
2
The isotropic H line shape in rotating 1,4-disubstituted
phenylenes requires the distortion of the molecular axis from
6
01 to 54.71 and the existence of multiple minima (43-fold) as
1
6
reported previously. DFT computations in the gas and the
solid-state corroborated the presence of a four-fold rotational
potential and the twist of the molecular axis (Fig. S25 and
Movies S1–S3, ESI†).
Considering the above, the deuterium experimental line
shapes in this work were successfully reproduced by using a
model based on 4-fold jumps over a symmetric rotational
potential (Fig. 5b). It is important to note that the amount of
the crystalline component in each batch was taken into account
as the static component for the line shape.
The batch with the largest amorphous content (83%)
À1
showed an activation energy of rotation E
and a pre-exponential factor A of 6.22 Â 10
a
of 7.38 kcal mol
s
1
0
À1
(Fig. 5c). As the
amount of the crystalline component in the heterogenous solid
increases, the isotropic line shape is less evident and is over-
lapped by the static Pake pattern. The fittings of the other batches
13
a
afforded slightly higher E and A values (see Fig. S20–S23 and
Fig. 4 Variable temperature C NMR CPMAS of rotor 2, highlighting the
Table S3, ESI†). The trend indicates that the rotation occurs only
in the amorphous component.
changes in the carbon signals from the central xylene at low temperatures,
denoted by an asterisk, see the C peak assignment in the ESI.†
13
1
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Conflicts of interest
There are no conflicts to declare.
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(
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Interestingly, the low pre-exponential factor in batch A reveals a
geometrically congested transition state, implying that the motion in
5
6
7
2
depends on the collective bending of the molecule during the
phenylene angular displacements. DFT computation (Fig. S25, ESI†)
À1
indicate that the rotor requires 6 kcal mol to rotate in the gas
phase, so the amorphous nature of the ground solid enables the
motion by providing a very fluid environment.
In summary, exchanging a central phenylene for a xylene
ring increases the solubility of the compound and gives rise to
several solid forms that were characterized by X-ray diffraction
8 Y. A. Abramov, M. Zell and J. F. Krzyzaniak, Toward a rational solvent
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(
Chart 1). The methyl groups also inhibited the rotation of the
9 W. Bernal, O. Barbosa-Garc ´ı a, A. Aguilar-Granda, E. P ´e rez-Guti ´e rrez,
J.-L. Maldonado, M. J. Percino and B. Rodr ´ı guez-Molina, Dyes Pigm.,
13
central ring, as corroborated by VT C NMR CPMAS. Interestingly,
the phenylene rings attached to carbazole showed motion after
grinding the crystals, a process that produces an heterogenous
solid (amorphous/crystalline) with an isotropic H NMR signal in
the solid-state. The motion in the solid occurs only in the amor-
phous component, and in a sample with 17% crystalline compo-
nent, a low activation barrier to rotation (E
observed, requiring a bent structure with a low pre-exponential
2
019, 163, 754–760.
1
0 (a) A. Aguilar-Granda, S. Perez-Estrada, A. E. Roa, J. Rodr ´ı guez-
Hern ´a ndez, S. Hern ´a ndez-Ortega, M. Rodriguez and B. Rodr ´ı guez-
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Granda, S. Perez-Estrada, E. Sanchez-Gonz ´a lez, J. R. A´ lvarez,
2
J. Rodr ´ı guez-Hern ´a ndez, M. Rodr ´ı guez, A. E. Roa, S. Hern ´a ndez-
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1 M. M. Haley, Modern Alkene Chemistry. Catalytic and Atom-
Economic Transformations, Angew. Chem., Int. Ed., 2015, 54, 8332.
À1
a
= 7.38 kcal mol ) was
1
10
À1
factor (A = 6.22 Â 10
s
).
12 Preliminary unit cell parameters: (a) 32.44(4) Å, (b) 5.75(3) Å,
c) 21.65(3) Å, (a) 901, (b) 110.741(3), (g) 901.
(
We are grateful for the financial support from PAPIIT IN209119
and CONACYT for the PhD scholarship 576483 (A. C.-M.). We
1
3 M. J. Duer, Introduction to Solid-State NMR Spectroscopy, Blackwell,
Oxford, UK, 2004.
acknowledge the UCLA Department of Chemistry and Biochemistry 14 T. R. Molugu, S. Lee and M. F. Brown, Chem. Rev., 2017, 117,
2
12087–12132.
for solid state H NMR experiments (NSF DMR-1700471 and MRI-
532232). We are grateful for the technical assistance from Dr Diego
Mart ´ı nez-Otero (X-ray), Dr Mar ´ı a del Carmen Garc ´ı a Gonzalez, Mar ´ı a
1
5 Three different batches of rotor 2 were ground and a mixture of
crystalline/amorphous components was always present: batch A
(17%/83%), batch B (24%/76%) and batch C (42%/58%), see also
1
2
´
˜
´
ESI† for their corresponding VT H spectra.
de los Angeles Pena Gonzalez, and MSc Elizabeth Huerta Salazar
MS and NMR). We thank Prof. Salvador P ´e rez-Estrada for initial
solid-state NMR measurements.
1
6 A. Colin-Molina, M. J. Jellen, E. Garc ´ı a-Quezada, M. E. Cifuentes-Quintal,
F. Murillo, J. Barroso, S. Perez-Estrada, R. A. Toscano, G. Merino and
B. Rodr ´ı guez-Molina, Chem. Sci., 2019, 10, 4422–4429.
(
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