An all-organic steroid-D-π-A modular design drives ferroelectricity in supramolecular solids and nano-architectures at RT

Article abstract of DOI:10.1039/c1cc12398j

Confluence of a modular design approach and self-assembly with a 'steroid-D-π-A' module generates spontaneous polarization in solids and for the first time in nano-architectures constituted from organogels, at room temperature (RT).

Full text of DOI:10.1039/c1cc12398j

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An all-organic steroid–D–p-A modular design drives ferroelectricity in
supramolecular solids and nano-architectures at RTw
Deepak Asthana,a Anil Kumar,b Abhishek Pathak,c Pradip Kumar Sukul,d Sudip Malik,d
Ratnamala Chatterjee,c Satyabrata Patnaik,b Kari Rissanene and Pritam Mukhopadhyay*a
Received 25th April 2011, Accepted 24th May 2011
DOI: 10.1039/c1cc12398j
Confluence of a modular design approach and self-assembly with
a ‘steroid–D–p-A’ module generates spontaneous polarization in
solids and for the first time in nano-architectures constituted
from organogels, at room temperature (RT).
(Ps B 20–60 mC cmꢀ2). Recently an H-bonding driven
all-organic two-component FE system4 and a metal–ligand
based design5 have been highly successful. Interestingly, a
module-based design strategy has also been extensively applied
with other all-organic materials e.g., semiconductors,6a,b FE
liquid crystals,6c,d NLO materials,6e gels,6f–h and receptors.6i
Such modular approach has an immense potential to achieve
the elusive RT ferroelectricity, high spontaneous polarization
and low Ec.
The search for novel ferroelectric (FE) materials has attracted
tremendous interest due to the exciting applications in FE random-
access memories (FERAM), FE field effect transistors (FEFET),
FE diodes, 1-D FE organic quantum magnet and multiferroic
materials.1 By definition, FEs are polar materials in which the
spontaneous electric polarization can be reversed by inverting the
applied external electric field. FE materials have been dominated
by inorganic compounds, which suffer from inability towards
solution processing, high temperature for sample preparation,
high electric coercive field (Ec) and hazards related to metal ion
toxicity.2 On the other hand, all-organic FE materials should
provide multi-fold advantages such as synthetic tailorability,
tunability of molecular interactions and solution processing.
Since the discovery of Rochelle Salt, the first FE material
with an organic component way back in 1921,3a surprisingly a
very few all-organic single-component (AOSC) FE solid
materials have been reported up to now.3 The discovery of
AOSC FE materials have been accidental and clear-cut design
strategies are yet to evolve. In general, existing AOSC FE
materials suffer from very low saturation and remnant electric
polarization (Ps and Pr of the order of 10ꢀ9 C cmꢀ2), high Ec
(V cmꢀ1) and low paraelectric to ferroelectric transition
temperature (Tc in range of 10–120 K). However, in two
recent reports, croconic acid3b and dehydroabietylamine-
based Schiff bases3c have shown exceptional FE properties
Herein, we report the first modular design strategy coupled
with supramolecular7 self-assembly approach which leads to
ferroelectricity in AOSC crystalline solids as well as for the
first time in organic nano-architectures. We could achieve
spontaneous polarization at RT which is three orders of
magnitude higher than the well-known AOSC FE materials.
As a proof of principle, we chose four modules to perform
the following explicit functions and integrated them to a
single-component FE material. Module A: a nitroaniline-based
donor–p-acceptor (D–p-A) unit to attain spontaneous polar-
ization due to its high dipole moment and ability to undergo
p–p interactions.8 Module B: an enantiomerically pure steroid
unit to aid in polar ordering and facilitate self-assembly via
van der Waals interaction.9 Module C: H-bonding units for
driving self-assembly process and Module D: a spacer unit
capable of adjusting and error-proofing the packing between
successive layers (Scheme 1a and b).
Compounds 1, 2 and 3 were synthesized and thoroughly
characterized by several analytical techniques (see ESIw).
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 School of Physical Sciences, JNU, New Delhi, 110067, India
c Department of Physics, IIT Delhi, 110016, India
d Polymer Science Unit, IACS, Kolkata, 700032, India
e X-Ray Crystallography: Department of Chemistry,
Nanoscience Center, University of Jyvaskyla, 40014,
JYU. E-mail: kari.t.rissanen@jyu.fi
¨
¨
w Electronic supplementary information (ESI) available: Experimental
details, synthesis and characterization. CCDC 822459. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c1cc12398j
Scheme 1 (a) M-A, M-B (*C, chiral unit), M-C and M-D, are the
various modules; (b) Structure of molecules.
c
8928 Chem. Commun., 2011, 47, 8928–8930
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Fig. 1 X-Ray crystal structure of compound 2, showing the H-bonded
rings, p–p stacking and steroid–steroid supramolecular interactions.
Fig. 2 Electric hysteresis loops at RT of 1, 2 and 3 in the form of
pellets recorded on a ferroelectric tester (model Precision Premier II)
made by Radiant Technologies, Inc.
The strong tendency of 1, 2 and 3 to form self-assembled
nano-architectures in a variety of solvents had posed a major
challenge in growing single crystals. However, after more than
a year of rigorous endeavour we have been able to grow
B1 cm long single crystals consistently as well as accomplish
nano-architectures of 2 (page S9w).
of the P vs. E curves; Pr of 0.265 mC cmꢀ2 and Ec of 3.67 kV cmꢀ1
was determined with the field E sweeping back to zero.
Compounds 2 and 3 also showed similar P–E loops at RT e.g.,
2 showed Ps, Pr and Ec of 0.511 mC cmꢀ2, 0.128 mC cmꢀ2 and
2.44 kV cmꢀ1 (SI 5–7, Table 2w).
Compound 2 crystallizesz in the non-centrosymmetric and
polar space group P1 which is one of the ten polar point
groups (C1, Cs, C2, C2v, C3, C3v, C4, C4v, C6 and C6v) necessary
for ferroelectricity to occur. The crystal lattice is constructed
from two individual pairs of 2, which are held together via four
NHꢁ ꢁ ꢁOQC hydrogen bonds (Nꢁ ꢁ ꢁO distances of 2.89/2.94
and 2.92/2.95 A for the dimer A/B and C/D, respectively)
(Fig. 1). These four N–Hꢁ ꢁ ꢁO H-bonds result in the formation
of two 14-membered supramolecular rings R1 and R2. These
supramolecular interactions further propagate utilizing the
four n-propanol solvent molecules which form H-bonds with
the three amide H-atoms or the nitro group of the adjacent
molecules (SI 1w). These pairs (A/B and C/D) form a pseudo-
symmetric tetramer, through p–p interaction of the nitro-phenyl
moieties with centroid-to-centroid distance of 3.82 A and
steroid–steroid van der Waals interactions ranging from 3.5 to
4.5 A. Interestingly, the nitro-phenyl rings make an angle of 81
and the phenyl planes are tilted by 0.023 A (SI 2w).
The highly attractive low Ec values (3-4 kV cmꢀ1) of 1–3
indicate a low energy barrier in reorienting the dipoles. This
is because of the favourable supramolecular H-bonding and
stacking interactions as is evident from the crystal structure of
2. In comparison, vinylidene fluoride11 a well-known FE organic
copolymer shows extremely large Ec of 500–1200 kV cmꢀ1 as it
faces large steric barriers in reorienting dipoles. In addition a
sizeable Pr value (remnant polarization at E = 0) in 1–3 reflects
the fact that a definite angle between the molecular dipoles exists
as can be clearly seen in the crystal structure of 2. Similarly, the
remnant dipoles reorient and undergo saturation polarization
even at 0.31 kV (discussed later) demonstrating the efficient
tunable nature of these supramolecular interactions.
Further, we evaluated the FE hysteresis properties of 1
(Fig. 3) and 2, 3 as a function of applied voltages (SI 5–7w).
Electric hysteresis loops are formed at voltages as low as
0.31 kV with Ps, Pr and Ec values of 0.124 mC cmꢀ2
,
0.041 mC cmꢀ2 and 0.70 kV cmꢀ1. As the voltage is increased
up to 3.1 kV, Ps, Pr and Ec values increase uniformly to
1.24 mC cmꢀ2, 0.57 mC cmꢀ2 and 7.56 kV cmꢀ1, respectively.
The Ps value obtained for 1 at 1.56 kV is more than three
orders of magnitude higher than for well-known AOSC FE
solid materials like benzil3d and tricyclohexylmethanol,3e
three-fold higher than 1,6-bis(2,4-dinitrophenoxy)-2,4-hexadiyne
(DNPH)3f and trichloroacetamide (TCAA)3g and two-fold higher
than Rochelle salt.
Compounds 1 and 3 produced stable crystalline materials. It
becomes clearly evident from the X-ray crystal structure of 2 that
each of the four modules (the p-ring, steroid moiety, H-bonding
units and the spacer) execute their predicted roles. Similarity of the
key modules in 1–3 and the molecular lengths (2.86–2.90 A) from
the optimized geometry points to a similar packing arrangements
in the bulk. Circular dichroism (CD) spectroscopy showed well-
defined positive CD signals for 1–3 which also confirms a similar
polar ordering in all of these compounds (SI 3–4w).
We further ascertained the polar ordering in these
compounds by performing second harmonic generation
(SHG) measurements applying the Kurtz–Perry method using
the fundamental 1064 nm light of a Q-switched Nd:YAG
laser.10 SHG efficiencies of 1, 2 and 3 were found to be
B0.10, 0.11 and 0.05 times, respectively, that of KH2PO4
(KDP) (Table 1, see ESIw). Therefore, the SHG activity
unequivocally confirms polar ordering in 1–3.
To verify the ferroelectricity at RT, the polarization
(P)–electric field (E) hysteresis curves of crystalline solid
samples were determined (in the form of pellets) using a FE
tester. Fig. 2 shows the P–E hysteresis loop, a typical FE
feature for compound 1. At an electric field of 10 kV cmꢀ1, Ps
of 0.614 mC cmꢀ2 was obtained as estimated by extrapolation
Fig. 3 Electric hysteresis loops at RT of 1 in the form of pellets at
different applied voltages, recorded on a ferroelectric tester (model
Precision Premier II) made by Radiant Technologies, Inc.
c
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Notes and references
z Crystal data for 2: 4C36H55N3O4ꢁ4C3H8O, M = 2615.70, triclinic,
space group P1, a = 12.9772(3), b = 14.3714(2), c = 22.6800(6) A,
a = 73.075(1), b = 79.981(1), g = 69.560(1)1, V = 3779.5(1) A3, Z = 1,
MoKa radiation (l = 0.71073 A), T = 123.0(1) K, m = 0.075 mmꢀ1
.
Brucker-Nonius Kappa-APEXII diffractometer, 27 543 reflections,
13227 independent (Rint = 0.0401), and 10668 observed reflections
[I Z 2s(I)], 1726 refined parameters, R = 0.06, wR2 = 0.13.
Fig. 4 (a) Organogels of 1, 2 and 3 formed in various organic
solvents; (b) and (c) nano-architectures formed by xerogels of 2 and
3, respectively. Scale bar represents 100 nm.
1 (a) S. J. Kang, I. Bae, Y. J. Park, T. H. Park, J. Sung, S. C. Yoon,
K. H. Kim, D. H. Choi and C. Park, Adv. Funct. Mater.,
2009, 19, 1609; (b) R. C. G. Naber, K. Asadi, P. W. M. Blom,
D. M. De Leeuw and B. de Boer, Adv. Mater., 2009, 21, 1;
(c) R. Schroeder, L. A. Majewski and M. Grell, Adv. Mater.,
2004, 16, 633; (d) F. Kagawa, S. Horiuchi, M. Tokunaga,
J. Fujioka and Y. Tokura, Nat. Phys., 2010, 6, 169;
(e) G. Rogez, N. Viart and M. Drillon, Angew. Chem., Int. Ed.,
2010, 49, 1921.
Dielectric measurements of 1, gave a value of 7.4 at RT,
which is 85% and 64% higher than DNPH and TCAA,
respectively. These measurements further revealed a sharp
transition at 105 1C, due to the crystalline to cholesteric phase
transition of the cholesteryl moiety (SI 9w).
It is worthwhile to consider that P–E loops can have some
contribution from the leakage current.12a However, large values of
Ps implies that leakage current, even if present, can be neglected as
in the case with improper ferroelectricity in spin frustrated
systems.12b We therefore measured the magnitude of leakage
current and ascertained that the origin of the ferroelectricity in
this material is due to the spontaneous polarization switching and
is not due to the leakage of charges arising from the conduction.
The leakage current measurement at RT of 1–3 show current
densities of 1.06–1.83 ꢂ 10ꢀ6 A cmꢀ2 for an applied field of
10 kV cmꢀ1 (SI 10w). These leakage current density values are
comparable or lower than for known FE materials.12c
2 (a) S. E. Park, S. Wada, L. E. Cross and T. Shrout, J. Appl. Phys.,
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4 For two-component organic ferroelectric solids, see: (a) S. Horiuchi
and Y. Tokura, Nat. Mater., 2008, 7, 357; (b) S. Horiuchi, R. Kumai
and Y. Tokura, Chem. Commun., 2007, 2321.
Furthermore, successful assimilation of FE properties in the
nano-domain would result in a wide range of innovative
materials.13 Although there has been much research toward
inorganic FE nano-materials, to the best of our knowledge,
there are as yet no reports for the organic counterparts. To
realize ferroelectricity in the nano-architectures the self-assembly
properties of 1–3 were studied in a range of organic solvents.
Compounds 1–3 formed thermo-reversible supramolecular
networks in a broad range of common organic solvents
(Fig. 4a, Table 4w).
5 For organic–inorganic hybrid ferroelectric solids: (a) T. Okubo,
R. Kawajiri, T. Mitani and T. Shimoda, J. Am. Chem. Soc., 2005,
127, 17598; (b) Q. Ye, Y.-M. Song, G.-Xi. Wang, K. Chen, D.-W. Fu,
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J.-S. Huang, Chem. Commun., 2008, 2762; (d) W. Zhang, R.-G. Xiong
and S. D. Huang, J. Am. Chem. Soc., 2008, 130, 10468.
6 For organic-based semiconductors, FE LCs, NLO materials, gels
and receptors, see: (a) D. Braga and G. Horowitz, Adv. Mater.,
2009, 21, 1473; (b) H. E. Katz, A. J. Lovinger, C. Kloc, T. Siegrist,
W. Li, Y.-Y. Lin and A. Dodabalapur, Nature, 2000, 404, 478;
(c) S. T. Lagerwall, Ferroelectric and Antiferroelectric Liquid
Crystals, Wiley-VCH, Weinheim, 1999; (d) W. Weissflog,
G. Pelzl, H. Kresse, U. Baumeister, K. Brand, M. W. Schroder,
M. G. Tamba, S. Findeisen-Tandel, U. Kornek, S. Stern,
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J. R. Nitschke, Science, 2009, 324, 1697.
SEM images (Fig. 4) revealed well-defined nano-architectures
such as nanorods, porous sheets, etc. in the xerogels (SI 11w).
As a representative example, we examined the FE properties of
the nano-architectures formed by 3, which clearly exhibits a
P–E hysteresis loop at room temperature having Ps, Pr and Ec
values of 0.498 mC cmꢀ2, 0.234 mC cmꢀ2 and 3.95 kV cmꢀ1
,
respectively at 1.25 kV (SI 8w). This new approach therefore
readily allows us to integrate FE property for the first time in
all-organic nano-architectures.
7 J.-M. Lehn, Supramolecular Chemistry: Concepts and Perspectives,
Wiley-VCH, Weinheim, 1995.
8 P. Mukhopadhyay, P. K. Bharadwaj, G. Savitha, A. Krishnan and
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To conclude, we have demonstrated the first applications of
a modular design approach combined with supramolecular
self-assembly to realize ferroelectricity in AOSC solids and for
the first time in all-organic nano-architectures constituted
from stable organogels. This approach has led to B1 cm long
FE crystals as well as nano-materials with the same molecule.
We therefore believe that this new and general strategy should
lead to many more innovative all-organic FE materials.
We thank DST for financial support and the Academy of
Finland (KR: proj. no. 212588 and 218325). We thank Prof.
P. K. Das and Mr Ravindra Pandey, IISC Bangalore, India
for the SHG measurements and AIRF, JNU, New Delhi,
India for the 500 MHz NMR, IR and CD facilities
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c
8930 Chem. Commun., 2011, 47, 8928–8930
This journal is The Royal Society of Chemistry 2011