Nonlinear optical and ferroelectric materials based on 1-benzyl-2-phenyl- 1H-benzimidazole salts

Article abstract of DOI:10.1039/c1ce05860f

A series of benzimidazole-based salts, obtained through the reaction of 1-benzyl-2-phenyl-1H-benzimidazole and inorganic acid, crystallize in the chiral P1 space group, which exhibit moderate second harmonic generation (SHG) responses and ferroelectric properties.

Full text of DOI:10.1039/c1ce05860f

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Nonlinear optical and ferroelectric materials based on 1-benzyl-2-phenyl-1
H-benzimidazole salts†
a
Yong-Tao Wang, Gui-Mei Tang, Chao He, Shi-Chen Yan, Qi-Chao Hao, Long Chen, Xi-Fa Long,
a
c
a
a
a
b
*
*
a
Tian-Duo Li and Seik Weng Ng
c
*
Received 8th July 2011, Accepted 2nd September 2011
DOI: 10.1039/c1ce05860f
developing new ferroelectrics materials constructed by unknown
organic compounds and their salts still remain sparse and a great
challenge. To the best of our knowledge, there is no literature about
the development of ferroelectric based on organic benzimidazole
salts. During the investigation on NCS solids with interesting NLO
and ferroelectric properties, we have realized that unsymmetrical
azole compounds easily construct NCS solids with SHG responses
and potential ferroelectricity.9 As our continuous program on
designing and constructing NCS solids with NLO and ferroelectric
materials assembled through unsymmetrical organic ligands,9,10 three
new NCS compounds, HLX (L ¼ 1-benzyl-2-phenyl-1H-benzimid-
azole; X ¼ ClO4 (1), NO3 (2), and Cl (3)), were prepared by the
methanol hydrate solution reaction of L with a series of inorganic
acids, HClO4, HNO3 and HCl, at room temperature, respectively
(Scheme 1). Herein, we report the syntheses, crystal structures, NLO,
ferroelectric, and photoluminescence properties of three benzimid-
azole salts 1–3.
A series of benzimidazole-based salts, obtained through the reaction
of 1-benzyl-2-phenyl-1H-benzimidazole and inorganic acid, crys-
tallize in the chiral P1 space group, which exhibit moderate second
harmonic generation (SHG) responses and ferroelectric properties.
Noncentrosymmetric (NCS) crystal solids have currently attracted
increasing attention in the field of material chemistry owing to
exhibiting a variety of technologically important properties from
optics to electronics, such as second harmonic generation (SHG)
nonlinear optical (NLO), ferroelectricity, piezoelectricity, and pyro-
electricity.1–3 Among these properties, NLO-active and ferroelectricity
behavior have widely been applied in the high technological fields,
such as NLO devices, optical communication, signal processing,
information storage, and ferroelectric random access memories.4
Notably, lacking an inversion center is the essential and the prereq-
uisite for these materials. To date, crystal engineering has extensively
provided insight into the fabrication of the desired functional mate-
rials. Although some progressive advances based on crystal engi-
neering have been made in the syntheses of NCS solids,2a,2e,2g,5–7 the
arduous challenges still remain in the design and preparation of
noncensymmetric or polar packing arrangement.
L was prepared by a slightly modified method described in the
literature11 and its characterization was in accordance with that
reported in the literature.11b The structures of 1–3 were also identified
by satisfactory EA, IR and X-ray diffraction (XRD). The phase
purity of the bulk samples of 1–3 was established by comparison of
their observed and simulated powder XRD patterns (Fig. S1–S3, see
ESI†). Interestingly, salts 1–3 crystallize in same chiral space group
(P1) though only the difference anions exist in each salt (Table S1†).
It is predicted that the present study will provide useful information
for constructing and designing a novel NCS or chiral crystal.
Single crystal X-ray crystallographic analysis of 1 reveals that an
asymmetric unit of 1 contains one mono-protonated L cation and
Recently, some neutral organic ferroelectrics have received
considerable attention in the ferroelectric field.1c,2c,8 However, ionic
organic compounds with ferroelectricity remain rare. Owing to the
existence of alignment of cations, ionic organic compounds would be
more effective in enlarging the polarity and ferroelectricity than
neutral organic compounds. Additionally, studies towards
aDepartment of Chemical Engineering, Shandong Provincial Key
Laboratory of Fine Chemicals, Shandong Polytechnic University, Jinan,
250353, P. R. China. E-mail: ceswyt@sohu.com; Fax: (+86) 0531 8900
0551; Tel: (+86) 0531 8900 0551
bFujian Institute of Research on the Structure of Matter, Chinese Academy
of Science, Fuzhou, 350002, P. R. China. E-mail: lxf@fjirsm.ac.cn; Fax:
(+86) 0591 8371 4946; Tel: (+86) 0591 8371 4946
cDepartment of Chemistry, University of Malaya, Kuala Lumpur, 50603,
Malaysia. E-mail: seikweng@um.edu.my; Fax: (+60) 603 7967 4193;
Tel: (+60) 603 7967 6778
† Electronic supplementary information (ESI) available: Synthesis,
elemental analysis, m.p., IR, XRD, Tables, photoluminescent figures,
crystallographic data in CIF and additional figures of salts 1–3. CCDC
reference numbers 795443–795445. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c1ce05860f
Scheme 1 The benzimdazole salts.
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ꢀ
one chlorate ClO4 anion (Fig. 1a). Hydrogen bondings are formed
between the NH (protonated N atom) and O of perchlorate, and the
C atoms from the phenyl ring in the 2-positon of the imidazole and O
atoms of perchlorate, which results in the formation of a chiral 1D
chain, as shown in Fig. 1b. The supramolecular layers are constructed
through the interchain p/p packing interactions between imidazole
rings and phenyl ones, which further results in 3D supramolecular by
means of weak C–H/O hydrogen bonds (Table S2, Fig. S4†).
Notably, in one crystal, all the phenyl rings, benzyl ones and benz-
imidazole ones are oriented in the same direction. It means that there
exists only one-handedness of 1 in one crystal, which results in the
chiral crystallization.
X-Ray crystal structural determination of 2 reveals that it consists
of a mono-protonated L cation and nitrate anion in an asymmetric
unit, as shown in Fig. 2a. There are two kinds of hydrogen bonding,
which are found between NH (protonated N atom of imidazole) and
O of nitrate anions, and C atoms of phenyl rings and O atom of
nitrate, which results in the chiral 1D chain along the a axis (Fig. 2b).
There exist weak C–H/p interactions formed by C atoms of phenyl
and phenyl rings, and p/p packing interactions between imidazole
rings and phenyl ones. The 2D supramolecular layers are further
constructed through these weak interchain interactions (Fig. S5,
Table S2†). Compared with 1, the weak interactions hardly exist
among the 2D supramolecular layers in 2 (Fig. S5b†). Despite this, all
aromatic rings are also oriented in the same direction, and only one-
handedness of 2 is observed, which is similar to that found in 1.
X-Ray structural analysis of 3 reveals that in the asymmetric unit,
it contains a mono-protonated L cation, chloride anion, and one
water molecule, as shown in Fig. 3a. There exist many H-bonds
between NH (protonated N atom of imidazole) and O of the water
molecules, and O atoms of water and Cl atoms, which result in the
formation of the chiral 1D chain (Fig. 3b). There are C–H/p
interactions formed by C atoms of phenyl and phenyl rings; these
weak interactions further extend the 1D chains to the 2D supramo-
lecular layers (Fig. S6, Table S2†). Being similar to 2, in 3, the
Fig. 2 (a) Ball-and-stick drawing of 2 in the asymmetric unit; (b)
Perspective views of the chiral 1D chain in 2 along the a axis. The red
dashed-lines stand for C(N)–H/O intermolecular interactions.
Hydrogen bond lengths and angles are provided in Table S2.†
interlayer weak interactions are not obviously observed too
(Fig. S6b†). Interestingly, all the same orientation of aromatic rings,
the sole one-handedness, and homochiral crystallization of 3 in one
crystal are observed, which is similar to those found in both 1 and 2.
As mentioned above, the noncentrosymmetric structures of 1–3
encouraged us to examine their NLO property. Primary experimental
results indicate that 1–3 are SHG-active. The SHG efficiency of
compounds 1–3 are weaker than that of KH2PO4 (KDP),12 which
may be as a result of the absence of a significant donor–acceptor
system in the molecules.
Given that salts 1–3 crystallize in a chiral space group P1
associated with polar point group C1, which belongs to ten
Fig. 1 (a) Ball-and-stick drawing of 1 in the asymmetric unit; (b)
Perspective views of the chiral 1D chain of 1 along the a axis. The red
dashed-lines stand for C(N)–H/O interactions. Hydrogen bond lengths
and angles are provided in Table S2 (see ESI†).
Fig. 3 (a) Ball-and-stick drawing of 3 in the asymmetric unit; (b)
Perspective views of the chiral 1D chain in 3 along the b axis. The red
dashed-lines stand for N–H/O and O–H/Cl intermolecular interac-
tions. Hydrogen bond lengths and angles are listed in Table S2.†
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non-centrosymmetric point groups (C1, C2, Cs, C2v, C4, C4v, C3, C3v,
C6, C6v) a prerequisite for ferroelectric behavior, we investigated their
potential ferroelectricity. Preliminary experimental results of salts 1–3
reveal that they are ferroelectricity-active because the present P–E
loops are different from the rugby-shaped P–E loops pointed out by
Scott.13 Additionally, the leak current of single crystal samples is
lower than 10ꢀ8 A cmꢀ2 (Fig. S7†). As shown in Fig. S8a,† salt 1
displays a ferroelectric behavior with remnant polarization (Pr) of 0.4
mC cmꢀ2 and coercive field (Ec) of 0.6 kV cmꢀ1. Saturation sponta-
neous polarization (Ps) of 1 is about 0.5 mC cmꢀ2, which is obviously
stronger than that of the Rochelle salt (NaKC4H4O6$4H2O, Ps ¼
0.25 mC cmꢀ2).2c Similarly, the Pr, Ec and Ps of salt 2 can be estimated
to be 0.13 mC cmꢀ2, 0.9 kV cmꢀ1 and 0.15 mC cmꢀ2, respectively,
comparable to that of the mononuclear complex (Fig. S8b).14 Salt 3
shows a ferroelectric feature with Pr (ca. 0.2 mC cmꢀ2), Ec (0.5 kV
cmꢀ1) and Ps (ca. 0.25 mC cmꢀ2), which are significantly higher to
those found in ferroelectric polymers (Fig. S8c†).5c,15 The large Ps and
Pr values may result from identical orientation of the polar cations.
However, their ferroelectricity may need to be further characterized
through other supplementary experiments, for example, differential
scanning calorimetry (DSC), specific heat (Cp) and dielectric prop-
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salts 1–3 at room temperature have been examined (Fig. S9†). For
comparison, the photoluminescence of the free ligand L was also
investigated. It is found that, in the solid state, compound L exhibits
an emission upon excitation at 280 nm with maxima at 357 nm,
which might be assigned to the intraligand p–p* transition.17 Salts
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In summary, we have successfully prepared three benzimidazole
salts based on crystal engineering strategies, which display NLO-
active and potentially ferroelectric-active. To our knowledge, the
promising ferroelectric property based on benzimidazole salts is
investigated here for the first time, opening up new avenues for
exploring organic ferroelectric materials. Currently studies on devel-
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properties are under way in our laboratory.
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Acknowledgements
This work was financially supported by the Project of Shandong
Province Higher Educational Science and Technology Program
(J09LB03) and the Shandong Distinguished Middle-aged and Young
Scientist Encouragement and Reward Foundation (BS2011CL034).
We thank Mr. Ru-Ping Yan, Xiao-Ling Wang and Yong-Ping Luo
for experimental helps. We also thank the University of Malaya for
supporting this study.
€
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