Self-assembly patterns of steroid-based all-organic ferroelectrics: Valuable insights from the single-crystals derived from an organogel and solution

Article abstract of DOI:10.1039/c4ce00013g

We report herein the first example of a single-crystal grown from a steroid-based organogel matrix. We have delineated the self-assembly process and compared the various non-covalent interactions with that of the single-crystals grown from solution, e.g.1·Ch showed 22 and 14-membered N-H-O H-bonded rings, while 1·Pr exhibited 24 and 14-membered rings. In addition, 1·Ch showed stronger cholesteryl-cholesteryl interactions than 1·Pr and 1·Ch exhibited 55% higher polarization.

Full text of DOI:10.1039/c4ce00013g

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Self-assembly patterns of steroid-based all-organic
ferroelectrics: valuable insights from the single-crystals
derived from an organogel and solution†‡
Cite this: CrystEngComm, 2014, 16,
4861
Deepak Asthana,a Sudhir K. Keshri,a Geeta Hundal,b Gyaneswar Sharmac
a
*
and Pritam Mukhopadhyay
Received 4th January 2014,
Accepted 28th March 2014
We report herein the first example of a single-crystal grown from a steroid-based organogel matrix. We
have delineated the self-assembly process and compared the various non-covalent interactions with that
of the single-crystals grown from solution, e.g. 1·Ch showed 22 and 14-membered N–H–O H-bonded
rings, while 1·Pr exhibited 24 and 14-membered rings. In addition, 1·Ch showed stronger cholesteryl–
cholesteryl interactions than 1·Pr and 1·Ch exhibited 55% higher polarization.
DOI: 10.1039/c4ce00013g
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well as in organic nanoarchitectures at RT.8 The design for
molecule 1 incorporates four modules: a donor–π-acceptor
Introduction
Ferroelectric (FE) materials have shown attractive applica-
tions in FE random-access memory (FERAM), FE diodes, etc.1
FEs are polar materials in which the spontaneous electric
polarization can be reversed by inverting the external electric
field. It has been realized that all-organic FE materials could
provide multi-fold advantages such as synthetic tailorability,
tunability of molecular interactions and solution processing
compared to the inorganic FEs. However, there are only a
very few reported all-organic single-component (AOSC) FE
materials.2 The existing AOSC FE materials suffer from low
saturation and remnant electric polarization (Ps and Pr in
10−9 C cm−2) and a high Ec value in V cm−1. Two recent
reports, based on croconic acid2b and a Schiff base,2c have
shown that exceptional FE properties (Ps ~20–60 μC cm−2)
can also be found in AOSC FEs. On the other hand, organic
two-component FE systems3 and metal–ligand based design4
have been quite successful. In this regard, application of
crystal engineering principles would further benefit design of
new classes of all-organic FEs.5,6
module, MA; a cholesteryl group, MB; H-bonding groups, MC;
and a spacer, MD.
1 exhibited remarkable self-assembly properties. It could
form stable organogels9 and could spontaneously gelate
acetic acid, carbon tetrachloride and a mixture of various
solvents, e.g. chloroform : methanol, chloroform : hexane, etc.
(see the ESI,‡ Table 1). Electron microscopy revealed that
these gels consisted of well-defined nanoarchitectures and
are dependent on the gelated solvent (Scheme 1a–b). The
high propensity to gelate organic solvents created a major
challenge to generate good quality single-crystals. After sev-
eral attempts with different solvents and their combinations
we obtained single-crystals by slow evaporation of n-propanol
(Scheme 1c). These crystals represented as 1·Pr crystallized in
the P1 polar space group, were found to be second harmonic
generation (SHG) active and exhibited spontaneous ferroelec-
tric properties at room temperature (RT).
With these results in hand, we wanted to modulate the
self-assembly characteristics by altering the supramolecular
interactions. Our goal was to grow single-crystals of 1 in non-
polar solvents, albeit the fact that 1 showed strong propensity
to gelate non-polar organic solvents. In this work, we demon-
strate the successful growth of single-crystals, 1·Ch, from an
organogel matrix constituted of chloroform and petroleum
ether (Scheme 1d–e). To our knowledge this forms the first
steroid-based organo-gelator10 that could be in situ crystal-
lized from an organogel matrix.
We recently reported the first modular design strategy
coupled with
a
supramolecular7 self-assembly approach,
which leads to ferroelectricity in AOSC crystalline solids as
a Supramolecular & Material Chemistry Lab, School of Physical Sciences, JNU,
New Delhi, 110067, India. E-mail: m_pritam@mail.jnu.ac.in; Fax: +91 11 26717537;
Tel: +91 11 2673 8772
b Guru Nanak Dev University, Amritsar, India. E-mail: geetahundal@yahoo.com
c School of Physical Sciences, JNU, New Delhi 110067, India
† This work is dedicated to Prof. S. Shinkai's 70th birthday.
‡ Electronic supplementary information (ESI) available: CCDC 974686 and
822459. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c4ce00013g
The single-crystal data for 1·Ch revealed an altered self-
assembly pattern, H-bonding interactions, and cholesteryl–
cholesteryl van der Waals interactions. The modulation of
these supramolecular interactions is anticipated to have a
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Table 1 Crystal data for 1
a) 1·Ch
Results and discussion
1·Ch crystallizes in the triclinic crystal system with the polar
space group P1, similar to 1·Pr, which comprises one of the
ten polar point groups necessary for ferroelectricity to occur
(Table 1). There were four crystallographically independent
molecules in the unit cell along with four independent
chloroform molecules (see the ESI‡ for the thermal ellipsoid
plot). The cell dimensions of 1·Ch were slightly elongated
and the cell volume was found to be marginally expanded
as compared to that of 1·Pr [a = 12.9772(3), b = 14.3714(2),
c = 22.6800(6), V = 3779.5(1) Å3].
Empirical formula
Formula weight
Temperature
C37H56Cl3N3O4
713.20
100(2) K
Wavelength
0.71073 Å
Crystal system
Space group
Triclinic
P1
Unit cell dimensions
a = 13.262(3) Å
b = 13.508(2) Å
c = 24.549(4) Å
3825.5(12) Å3
4
α = 76.154(8)°
β = 86.364(9)°
γ = 63.763(8)°
Volume
Z
Density (calculated)
Absorption coefficient
F(000)
Crystal size
Theta range for data
collection
1.238 Mg m−3
0.281 mm−1
1528
Self-assembly characteristics
0.21 × 0.16 × 0.10 mm3
1.71 to 25.40°
An infinite bi-layer like assembly of the polar nitrophenyl
rings (head) and the non-polar cholesteryl groups (tail) in
Reflections collected
50 819
a
head–tail–tail–head (H–T–T–H) arrangement could be
Independent reflections 24 785 [R(int) = 0.0503]
unravelled in 1·Ch (Fig. 1). In this H–T–T–H arrangement, the
two cholesteryl T's interact with the peripheral alkyl chains.
In the bilayer, the two nitrophenyl rings are stacked offset to
each other and the solvent chloroform molecules are at the
periphery of the bilayer and close to the nitrophenyl rings. In
the subsequent sections, we would discuss in detail the
H-bonding interactions, molecule–solvent interactions, π–π
stacking and cholesteryl–cholesteryl interactions.
In 1·Pr, a similar H–T–T–H bi-layer like assembly was
observed for the n-propanol molecules in close proximity to
the nitrophenyl rings (Fig. 2). It should be noted that a simi-
lar H–T–T–H arrangement of cholesteryl-based molecules has
been observed and characterized in detail in liquid crystals.
Refinement method
Full-matrix-block least-
squares on F2
Data/restraints/
parameters
24 785/20/1717
Goodness-of-fit on F2
Final R indices
[I > 2σ(I)]
0.975
R1 = 0.0628, wR2 = 0.1517
R indices (all data)
Absolute structure
parameter
Largest diff. peak
and hole
R1 = 0.0975, wR2 = 0.1753
−0.04(5)
0.909 and −0.581 e Å−3
CCDC no. (1·Ch)
b) CCDC no. (1·Pr)
974686
822459
significant role in tuning the nonlinear optical (NLO)11 and
FE properties of these materials. Furthermore, significant
control over the cholesteryl–cholesteryl interactions in the
solid state is important in the design of steroid-based drug
molecules.
H-bonding features
In 1·Ch an infinite H-bonded array was observed, formed by
the donor NH-groups of p-nitroaniline and the amide moiety
Scheme
1 A module-based design of all-organic ferroelectric 1;
a) organogel formation in acetic acid; b) a SEM image exhibiting the
nanorod like structures; c) single-crystals grown from n-propanol;
d) organogel formation in chloroform : petroleum ether; and e) in situ
formation of single-crystals from the organogel matrix.
Fig. 1 An infinite bi-layer like arrangement of the polar nitrophenyl
rings and the non-polar cholesteryl groups in 1·Ch. H atoms have been
removed for clarity. Colour code for atoms: C, grey; N, blue; O, red;
Cl, green.
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Fig. 2 An infinite bi-layer like arrangement of the polar nitrophenyl
rings and the non-polar cholesteryl groups in 1·Pr. H atoms have been
removed for clarity. Colour code for atoms: C, grey; N, blue; O, red.
and the acceptor O atoms of the nitro-group and the amide
moiety. A closer look at the array reveals two types of well-
defined rings (R1 and R2) and a chain that aids to achieve
the infinite propagation. The bigger H-bonded ring (R1) is
subtended by 22 atoms, while the smaller ring (R2) is
subtended by 14 atoms (Fig. 3, top panel). In R1, the charac-
teristic H-bonding parameters were found to be: N1–O11
2.959 Å, ∠N1–H1–O11 of 143° and N7–O3 2.954 Å, ∠N7–H7–O3
of 135°, while in R2 the H-bonding parameters were found to
be: N8–O2 2.87 Å, ∠N8–H8–O2 of 168° and N2–O10 2.858 Å,
∠N2–H2–O10 of 162°.
The solvent chloroform molecules did not participate in
the H-bonded ring and chain formation (Fig. 3, bottom panel).
However, a weak C–H–X type of bonding was unravelled
between the H atoms of the cholesteryl rings and the Cl
atoms, e.g. H12b–Cl8 2.775 Å, ∠C12–H12b–Cl8 of 153°;
H15a–Cl5 2.847 Å, ∠C15–H15a–Cl5 of 169°, etc. (see the ESI‡).
In contrast to 1·Ch, the solvent n-propanol molecules in
1·Pr participated in forming the infinite H-bonded array. In
this case, R1 is subtended by 24 atoms and the additional
two atoms (compared to 1·Ch) are from the H's of the two
propanol molecules. R2 is similar to 1·Ch and is subtended
by 14 atoms (Fig. 4, top panel). In ring R2, the notable
H-bond distances and angles are: N10c–O49 2.915 Å,
∠N10c–H10c–O49 of 175° and O49–O1d 2.915 Å, ∠O49–H49–O1
of 166°. In R2, the H-bonding parameters were found to be:
N7d–O12c 2.893 Å, ∠N7d–H7d–O12c of 158° and N7c–O12d
2.943 Å, ∠N7c–H7c–O12d of 168°.
Fig. 3 Top: an infinite array of N–H–O H-bonds in 1·Ch formed by two
types of rings, R1 (22-membered) and R2 (14-membered), and
H-bonded chains along the a-axis. Bottom: an infinite array of N–H–O
H-bonds along the a- and c-axes. Note that the solvent chloroform
molecules do not participate in forming the infinite rings and the
chains in the H-bonded structure.
tilted, which provides a net dipole to the bulk system as
verified by the SHG activity.8
Cholesteryl–cholesteryl van der Waals interaction
The cholesteryl moiety is known to play a significant role in
the design of soft materials like organogels, liquid crystals,12
well-defined nanoarchitectures, etc. In addition, understand-
ing and control over their interactions play a significant role
in modulating the efficacy of steroid-based drug molecules.
We are interested in delineating the cholesteryl–cholesteryl
interactions in 1·Ch and 1·Pr and arriving at a synthetic tool-
box for engineering soft materials.
In the case of 1·Ch several short C–C interactions less than
4.0 Å were observed between the cholesteryl moieties. Notable
short interactions were found between C61–C24 (3.515 Å),
C47–C21 (3.882 Å), and C99–C114 (3.997 Å). Such short chol-
esteryl–cholesteryl interactions have been observed in the sin-
gle crystals of metal cholesteryl–phosphine complexes.13 In
contrast, the cholesteryl–cholesteryl interactions in 1·Pr were
found to be much weaker, only one short C–C interaction
was observed between C19b–C34d (3.507 Å). Therefore, such
π–π stacking interactions
The donor–π-acceptor p-nitroaniline moiety is stacked in an
antiparallel fashion in 1·Ch and 1·Pr single-crystals (Fig. 5).
In the case of 1·Ch and 1·Pr, the centroid–centroid distances
between the two π-rings were found to be 4.316 Å and 3.851 Å.
These show a weak π–π stacking interaction in the organogel
matrix. In both of these cases the π-rings were found to be
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Fig. 6 Graphs showing polarization as a function of temperature (T)
obtained by pyroelectric current measurement of 1·Ch and 1·Pr.
The sample was cooled up to 150 K in the presence of an
electric field of 100 V (for details, see the ESI‡). It was then
heated uniformly up to 310 K at a constant rate and the
current was measured as a function of temperature using an
electrometer. The observed pyroelectric current during
the heating process was integrated with time to obtain the
change in spontaneous polarization (Fig. 6). As expected,
the polarization gradually decreased with the increase of the
temperature. Comparing the results from these two sets of
experiments, we found that the polarization value for 1·Ch
(0.045 μC cm−2) is 55% higher compared to that of 1·Pr
(0.029 μC cm−2).14b This can be attributed to higher asymme-
try in molecular packing and also to the resultant higher
dipoles emerging from 1·Ch. A closer look at Fig. 3 (top and
bottom panels) clearly shows that the donor–π-acceptor
p-nitroaniline moiety in 1·Ch has a greater tilt angle com-
pared to 1·Pr (Fig. 4) and hence, leading to the increase in
the net dipole in 1·Ch.
Fig.
4 Top: an infinite array of N–H–O and O–H–O H-bonds in
1·Pr formed by two types of rings, R1 (24-membered) and R2
(14-membered), and H-bonded chains along the b-axis. Bottom: an
infinite array of N–H–O and O–H–O H-bonds along the a- and b-axes.
Note the participation of the solvent propanol molecules (shown in
ball and stick representation) in forming the infinite rings and the
chains in the H-bonded structure.
Fig. 5 Top: π–π stacking interactions in 1·Ch; bottom: π–π stacking
interactions in 1·Pr.
Conclusions
In conclusion, we have provided the first crystallographic
snapshots of molecular interactions of a steroid-based all-
organic ferroelectric material. To the best of our knowledge,
this is the first example of a single-crystal (1·Ch) grown from
a steroid-based organogel matrix. We have delineated the
self-assembly process by delving into the H-bonding patterns,
π–π and cholesteryl–cholesteryl interactions and also the
solvent (chloroform) interaction mode with 1. Furthermore,
we compared these non-covalent interactions with that of
the single-crystals grown from n-propanol solution, e.g. 1·Ch
showed 22 and 14-membered N–H–O H-bonded rings, while
1·Pr exhibited 24 and 14-membered rings formed from the
combination of N–H–O and O–H–O H-bonds. In addition,
1·Ch demonstrated a stronger cholesteryl–cholesteryl interac-
tion compared to 1·Pr. These provide valuable insights into
modulating these interactions and tuning the optical and
solvent dependent modulation of cholesteryl–cholesteryl
interactions in a small molecule should be useful in tuning
the material properties associated with soft-materials.
Ferroelectric properties
Following the standard procedure,14 which has been recently
used to validate the FE properties of croconic acid,2b we stud-
ied the FE properties of the crystalline material of 1·Ch and
compared them with those of the previously reported 1·Pr.
We determined the temperature dependent changes in polar-
ization by measuring the pyroelectric current during a steady
temperature sweep. The pellets were coated with silver paste
on both sides and contacts were made using copper wires.
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electronic properties of soft materials. We determined the
temperature dependent changes in polarization by measuring
the pyroelectric current during a steady temperature sweep.
1·Ch exhibited higher polarization compared to 1·Pr as a
result of higher asymmetry in molecular packing and the
emergent net dipole.
an electric field of 100 V. After removing the applied field,
the sample was short-circuited to avoid the influence of the
charge stored during the process of poling for 10 minutes to
remove the possibility of any stray charge. The sample was
then heated uniformly up to 310 K at a constant rate and the
current was measured as a function of temperature using a
Keithle-6514 system electrometer.
Experimental
Materials and methods
Acknowledgements
All chemicals were obtained from Sigma Aldrich or
Spectrochem India and were used as received. Thin layer
chromatography (TLC) was carried out on aluminium plates
coated with silica gel mixed with a fluorescent indicator hav-
ing a particle size of 25 μm and was sourced from Sigma
Aldrich. 1H and 13C NMR spectra were recorded using a
Bruker 500 MHz spectrometer in DMSO-d6 with TMS as the
standard. Infrared spectra were recorded in a KBr pellet using
a Varian 3100 FT-IR instrument. The morphologies of the
xerogels were investigated using a field emission scanning
electron microscope (FE-SEM) apparatus (JEOL, JSM-6700F).
The samples were platinum coated with a thickness of 40 nm
by a sputtering technique in an argon atmosphere and were
observed at a voltage of 5 kV.
We thank the DBT-BUILDER and the DST-PURSE for funding
this research. We thank the MALDI-TOF MS, FT-IR, and 500 MHz
NMR facilities at AIRF, JNU. DA thanks the CSIR, India,
for the research fellowship. We acknowledge Dr. S. Patnaik,
SPS, JNU for providing the experimental facilities for the
ferroelectric studies.
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the top. A turbid layer was formed and the vial was closed
and kept at room temperature. The chloroform layer and
some part of the petroleum ether layer turned into a gel.
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organogel matrix.
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there were four crystallographically independent molecules
in the unit cell along with four independent chloroform mol-
ecules. All non-hydrogen atoms were refined anisotropically.
All calculations were performed using the WinGX package.15
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a fixed distance [0.88(2) Å] wherever needed.
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