Ferroelectric homochiral organic molecular crystals

Full text of DOI:10.1021/cg901032c

DOI: 10.1021/cg901032c
Ferroelectric Homochiral Organic Molecular Crystals
2010, Vol. 10
1025–1027
Tong Zhang,* Li-Zhuang Chen, Min Gou, Yong-Hua Li, Da-Wei Fu, and
Ren-Gen Xiong*
Ordered Matter Science Research Center, Key Laboratory of Micro-Inertial Instrument and Advanced
Navigation Technology, MOE, Southeast University, Nanjing 211189, P. R. China
Received August 26, 2009; Revised Manuscript Received January 2, 2010
Ferroelectrics based on organic molecular crystals such as
thiourea, cyclohexan-1,10-diacetic acid (CDA), trichloroaceta-
mide (TCAA) and tricyclohexylmethanol (TCHM) or some
organic base-inorganic acid salts such as HpyX (Hpy=pyridi-
nium and X=halogen, ClO4-, IO4-, etc.) and HdabcoX (dabco=
diazabicyclo[2.2.2]octane; X=ClO4-, BF4-, ReO4-, etc.) have
been the subject of research for a long time.1-3 However, to
explore or develop new ferroelectrics based on unknown pure
organic molecular crystals still remains largely unexplored and a
great challenge in which the ferroelectric research situation based
on homochiral organic compounds and their salts is especially
serious and unknown. To our knowledge, there are no examples
of homochiral (or optical active) organic compounds reported to
be potential ferroelectrics. During our systematical investigation
of inorganic-organic hybrid ferroelectrics, we found that pure
homochiral compounds (R)-2-methylpiperazine-1,4-diamine
(MPADA, 1) and (R)-2-methylpiperazinium bis(dichloroiodate)
((H2-MPA)(ICl2)2, 2) probably display ferroelectric properties.4
Herein we report their crystal structures and possible ferroelectric
properties because organic materials are emerging as an alter-
native to the inorganic counterparts which display efficient
molecular nonlinearity, inherent synthetic flexibility, ultrafast
response with better processability, higher laser damage thresh-
old, and ease of fabrication and integration into devices.
MPADA was prepared by the reaction of (R)-2-methylpiper-
azine with NaNO2 and HCl following H2 reduction, while
(H2-MPA)(ICl2)2 can be obtained by the reaction of (R)-
2-methylpiperazine with HIO4 and concentrated HCl at room
temperature, as shown in Scheme 1.
Figure 1. (left) An asymmetric unit representation of 1 showing
that the piperazine ring adopts to the chair conformation. (right) 2D
H-bonds net formed through N O and O O.
3 3 3
3 3 3
Scheme 1
1 recrystallizes in the solution of water to get 1 6H2O with a
3
chiral space group of P21 belonging to one of 10 ferroelectric
polar point groups, while 2 also directly crystallizes in the chiral
space group P21.5 Their powder X-ray diffraction patterns are
exactly matched with those fitting by single crystal diffraction
patterns. Figure 1 shows that the six-membered ring of piperazine
in an asymmetricunitof1 6H2O has a chair conformation. There
3
are manyH-bonds found between NH2 (amino group) andwater,
water and the N atom of piperazine ring, and water and water to
result in the formation of a three-dimensional (3D) framework
through H-bonds (see Supporting Information).
Figure2 clearly shows thatthe six-membered ringofpiperazine
in diprotonated (R)-2-methylpiperazine also adopts a chair con-
formation, while two dichloroiodate anions are almost parallel
with a small angle crossing each other. The iodine atom is two-
coordinated and not linear (177.3-178.8°). Also, there are many
H-bonds found between Cl and NH2 (protonated N atom).
Figure 2. (left) An asymmetric unit representation of 2 showing
that the piperazine ring adopts to a chair conformation while two
dichloroiodate anions are not parallel. (right) 2D H-bonds net
formed through N O and O O.
3 3 3
3 3 3
20.1 pC/N, respectively, still smaller than that of TGS (∼47 pC/
N,TGS=triglycine sulfate). The space group P21 is associated
with the point group C2, one of the 10 polar point groups (C1, C2,
Cs, C2ν, C4, C4ν, C3, C3ν, C6, C6ν) required for ferroelectric
Given that the product 1 6H2O and 2 both crystallize in a
chiral space group P21 while it also adopts a polar point group, its
3
behavior. Experimental results indicate that 1 6H2O and 2 dis-
piezolelectric properties were investigated. Preliminary studies of
single crystal samples indicate that 1 6H2O and 2 are piezo-
electricity-active with an approximate d33 value of 12.0 and
3
play ferroelectric behaviors, as shown in Figure 3, panels a and b
with a good dielectric hysteresis loop, respectively. In Figure 3a,
the coercive electric field (Ec) reaches 6 kV/cm, much higher than
that of TGS (∼0.3 kV/cm), while the remanent polarization (Pr)
andspontaneouspolarization(Ps) areabout 0.75and1.0 μC/cm2,
respectively, much smaller than that of TGS (3.5 μC/cm2) and
3
*To whom correspondence should be addressed. E-mail: tzhang@seu.
edu.cn (T.Z.); xiongrg@seu.edu.cn (R.-G.X.).
r
2010 American Chemical Society
Published on Web 01/25/2010
pubs.acs.org/crystal

1026 Crystal Growth & Design, Vol. 10, No. 3, 2010
Zhang et al.
Figure 5. The temperature-dependence permittivity of 2. (a) The
low temperature range, (b) high temperature range.
Figure 3. (a) Polarization versus applied electric field approxi-
mately along the b-axis in 1 6H2O. (b) Polarization versus applied
Scheme 2
3
electric field approximately along the b-axis in 2, while the inset is
the ferroelectric loop of TGS measured at the same conditions as
those of 1 6H2O and 2.
3
About the ferroelectric origin in the two systems, besides H-
bonds, we consider that the orientational disorder of two chair
conformations of the six-membered ring of piperazine may be
in part responsible for the ferroelectric property because this
will cause a reversible change of dipolar moment to result in
ferroelectricity in the electric field, as shown in Scheme 2. For
compound 2, we still consider that the diprotonated piperazien ring
can be treated as a chair-conformation neutral piperazine ring.
Thus, according to theoretical calculations, the dipolar moments of
state (I) and state (II) are 3.02 D and 3.15 D, respectively. Thus two
state energies are 417.88 and 417.87 au, respectively, and the energy
difference between two states is approximately 0.012a.u. (∼0.3 eV).
Similarly, the dipolar moments, state energy, and energy difference
in compound 2are 1.90 D and 1.92 D, 307.25 au and 307.24 au, and
0.0042 au (0.1 eV). Thus, a small energy difference can be easily
overcome by a high electric field.
Figure 4. (a) The temperature-dependence permittivity of 1 6H2O.
(b) Crystal morphology of compound 2 showing a good rhomb.
3
larger than that of Rochelle salt (0.25 μC/cm2). In Figure 3b, the
Ec of 2 reaches about 2 kV/cm, much smaller than that of
1 6H2O, while the Pr and Ps are about 0.75 and 1.6 μC/cm2,
respectively, much smaller than that of TGS and larger than that
3
of Rochelle salt and 1 6H2O. The crystal morphology of com-
3
pound 2 is depicted in Figure 4b showing a pale-yellow good
rhomb while 1 6H2O is a colorless block.
3
The preliminary dielectric constant measurements of 1 6H2O
and 2 show that they display a relatively high dielectric constant
3
reaching about more than 10 at room temperature. The tempera-
ture-dependence permittivity of 1 6H2O (due to the very low
3
In conclusion, the present work has demonstrated that the
homochiral organic small molecule with a six-membered ring will
open up an avenue for new organic ferroelectrics exploration.
melting point (<30 °C)) shows that the permittivity changes with
temperature remain basically unchanged. However, there is a
very small anomaly (or gradual increase) found in the measured
temperature ranges (about -20 °C), suggesting that probably
there is a phase transition occurring in this ferroelectric system.
However, differential scanning calorimetry (DSC) measurements
exclude the possibility of phase transition because there is no heat
anomaly in the DSC diagram (see Supporting Information) at the
same temperature (Figure 4).
Acknowledgment. This work was supported by Project 973
(Grant No. (2009CB623200)), the National Natural Science
Foundation of China (60977038, 60910187 and 90922005), and
Jiangsu province NSF (BK2008029).
Supporting Information Available: X-ray crystallographic cif file
and additional measurements. This material is available free of
charge via the Internet at http://pubs.acs.org.
Similarly, the temperature-dependence permittivity of 2
shows that there is no anomaly found in low and high tempe-
rature ranges (Figure 5). DSC measurements confirm this
aspect (see Supporting Information). Most of the organic
molecular ferroelectric crystals show that there are phase
transitions occurring in their solid-state changes. For example,
TCAA (its space group P21) displays two phase transition steps
associated with one small dielectric anomaly along the b-axis.
However, it only probably displays ferroelectric properties
approaching the phase transition temperature (at 353 K), Ps
reaching ∼0.25 μC/cm6 much smaller than those found in
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˚
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