Dielectric response of novel one-dimensional hydrogen-bonded molecular crystal [4,6-dmpH][Hca]

Article abstract of DOI:10.1016/j.physb.2009.12.047

The frequency dependent dielectric response due to proton transfer is observed for the novel one-dimensional (1D) hydrogen-bonded co-crystal, [4,6-dmpH][Hca] (4,6-dmp=4,6-dimethylpyrimidine, ca=chloranilic acid). This response is only observed for an as-grown crystal in a metastable state with defects of hydrogen bonds. The critical hydrogen-bond length and one-dimensionality might be deeply related to the metstable state.

Full text of DOI:10.1016/j.physb.2009.12.047

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Physica B 405 (2010) S341–S343
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Physica B
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Dielectric response of novel one-dimensional hydrogen-bonded molecular
crystal [4,6-dmpH][Hca]
Ã
Hironori Ohchi a, Kazuyuki Takahashi a, Jyun-ichi Yamaura a, Shinya Takaishi b, Hatsumi Mori a,
a Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan
b Graduate School of Science, Tohoku University, Aoba-ku, Sendai 980-8578, Japan
a r t i c l e i n f o
a b s t r a c t
The frequency dependent dielectric response due to proton transfer is observed for the novel one-
dimensional (1D) hydrogen-bonded co-crystal, [4,6-dmpH][Hca] (4,6-dmp=4,6-dimethylpyrimidine,
ca=chloranilic acid). This response is only observed for an as-grown crystal in a metastable state with
defects of hydrogen bonds. The critical hydrogen-bond length and one-dimensionality might be deeply
related to the metstable state.
Keywords:
Proton transfer
One-dimensional molecular materials
Dielectric response
Metastable state
& 2009 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental detail
Recently, proton transfer phenomena in molecular crystals
have attracted much interest in aim for functional materials such
as ferroelectrics and anhydrous proton conductors [1]. So far, we
have reported the dielectric response by an inter-molecular –
NyHyO– proton transfer for [H2ca][1,2-da]2 (H2ca=chloranilic
acid and 1,2-da=1,2-diazine in Fig. 1) [2]. This response was
observed on the way of the migration from N–HyO at 300 K to
O–HyN below 90 K, which is proved by the infrared absorption
spectra and the temperature dependence of lattice constants. This
proton migration induces the small broad peak of dielectric
constant because protons transfer only in a molecular unit of
[H2ca][1,2-da]2.
The acetonitrile solutions of 4,6-dmp (3.33 ꢀ 10ꢁ1 M, 2 mL)
and H2ca (6.19 ꢀ 10ꢁ3 M, 48 mL) were slowly mixed and stored in
Pertier cooler at 3 1C. After one night, a high quality of black block
crystals of 1 (41.1 mg) were harvested, filtered off, washed with
cold acetonitrile and dried in air.
The X-ray single crystal data of 1 were collected at room
temperature on a Bruker CCD system (SMART APEX, Mo K
a,
21 kW, 2
y
o601). The structure was solved by a direct method and
refined by the full-matrix least squares technique using Crystal
Structure (ver. 3.6.0, Rigaku Co. and Rigaku/MSC). Anisotropic
thermal parameters were applied to all non-hydrogen atoms.
Crystallographic data of [4,6-dmpH][Hca]: C12H10N2O4Cl2, mono-
˚
˚
˚
In order to enhance the dielectric response, the molecular
crystal with the higher dimensional hydrogen-bonded network is
aimed. Then, the molecular pair of 4,6-dmp and H2ca in Fig. 1 was
chosen. The pKa of 4,6-dmp (=2.7) [3] is a little larger than that of
1,2-da (=2.3) [3], which affords the proton migration crystal.
Moreover, the configuration of nitrogen atoms for 4,6-dmp is
preferable to construct the 1-D hydrogen-bonded supramolecular
structure.
clinic, P21/c, a=13.9361(9) A, b=23.156(2) A, c=8.2051(5) A,
3
˚
b
=97.886(3)1, V=2622.8(5) A , Z=8, R=0.0468 , RW=0.1112, and
GOF=1.009 by using independent 3637 reflections [I42.0
The measurement of dielectric constant was carried out by
quasi-four-probe capacitance method in use of an impedance
analyzer (Agilent Technologies 4294A) between 2–100 kHz and
300–360 K. The contacts were made by silver paint.
s(I)].
In this report, the preparation, the crystal structure analysis,
and the dielectric measurement of the novel hydrogen-bonded
co-crystal [4,6-dmpH][Hca] (1) were carried out. As a result, the
frequency dependent dielectric responses owing to proton
transfer in the metastable state were successfully observed.
3. Results and discussions
The crystal structure of 1 was shown in Figs. 2 and 3. The
crystallographically independent molecules are two 4,6-dmpH+
and two Hcaꢁ. The positions of proton were determined in
consideration of the C–O bond length of chloranilic acid; the C–O
˚
bond lengths are reported to be 1.322 A for neutral chloranilic
˚
˚
acid [4], 1.253 and 1.324 A for monoanion [5], and 1.253 A
Ã
for dianion [6], respectively. Since C–O bond lengths of 1 are
Corresponding author. Tel./fax: +81 471363444.
E-mail address: hmori@issp.u-tokyo.ac.jp (H. Mori).
1.250(2) and 1.320(2) (a and b in Fig. 2) and 1.249(2) and 1.320(2)
0921-4526/$ - see front matter & 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.physb.2009.12.047

ARTICLE IN PRESS
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H. Ohchi et al. / Physica B 405 (2010) S341–S343
Fig. 1. Molecular structures of 1,2-da, 4,6-dmp, and H2ca.
N+-H...O-
2.715(2)
O-H...N
2.861(2)
β
α
α
N+-H...O-
2.704(2)
β
O-H...N
2.875(2)
Fig. 2. Intermolecular hydrogen bonds for [4,6-dmpH][Hca]. The C–O bonds are
denoted as and
a
b.
Fig. 4. Temperature dependence of the first run of (i) real and (iii) imaginary parts
of dielectric constants for [4,6-dmpH][Hca]. (ii) The upper inset is frequency
dependence of tand. (iv)Temperature dependence of real part of dielectric constant
for the annealed sample. (a)2, (b)3, (c)5, (d)10, (e)20, and (f)100 kHz.
Fig. 3. One-dimensional hydrogen-bonded chains indicated by the thick arrows
As the metastable state, we propose the existence of some
defects such as divalent molecules, 4,6-dmpH22+, along the 1-D
hydrogen-bonded chain. The divalent molecules have instability
due to coulomb repulsion in 4,6-dmpH22+, so that protons could
move between the oxygen and nitrogen atoms to stabilize the
for [4,6-dmpH][Hca].
(
a
and
b), respectively, two chloranilic acids are monoanions.
Therefore, one proton transfers from H2ca to 4,6-dmp, and two
kinds of hydrogen bonds coexist: 2.704(2) A for N –H?O and
+
ꢁ
stable state. The Fig. 4 (ii) elucidates that tand /
(=e00 e0) extends to
˚
˚
2.875(2) A for N?H–O, and 2.715(2) and 2.861(2) (Fig. 2). The
low frequencies. The Cole–Cole plot does not show a Debye-arc-
type, but linear-type. The similar processes have been reported in
1D hydrogen-bonded chains; the dielectric loss in low frequencies
was observed in the long alcohol chains such as (C19H40O)n due to
a continuous proton transfer under electric field in 1D hydrogen-
bonded chain [7]. As for another example of 1D hydrogen-bonded
BSQB?4H2O (BSQB=bisquaricacidbiphenyl), the linear Cole–Cole
plot down to low frequencies were also observed. The continuous
proton relay is explained as protonic soliton model [8]. Therefore,
the dielectric constant and loss of 1 might be also originated from
the continuous motion of proton along 1D hydrogen-bonded
chains. Once the defects disappear by heating and aging, any
dielectric response is no longer observed because hydrogen
cannot transfer in the stable state.
Hcaꢁ and 4,6-dmpH+ molecules are arranged alternately to
construct 1-D zigzag chains. These 1-D chains are anti-parallelly
arranged to each other to afford the 2-D layer (Fig. 3). There are,
however, no intermolecular interactions such as
hydrogen bonds between 1-D chains.
p-stacking and
Fig. 4 shows the temperature dependence of the real part e0
and imaginary part e00 of the complex dielectric constant of 1. The
frequency-dependent peaks of e0 and e00 were observed around
352 K only in the initial heating process. This suggests proton
transfer between the acid and base might occur. However, both
annealed and three-months-aged crystals show no dielectric
response [Fig. 4(iv)]. Since the decomposition starts from 364 K
with heating the sample by the thermogravimetry (TG), the
dielectric response around 352 K is not derived from degrading of
a sample. The dielectric responses of freshly prepared samples are
reproducible in the initial heating, indicating that the as-grown
crystals should be in the metastable state.
Proton transfer is deeply related to the length and the dimension-
ality of hydrogen bonds. The hydrogen bond length r(N+–H?Oꢁ)=
0
0
˚
2.710 A of 1 is slightly longer than that of 5,5 -DMBP-H2ca [5,5 -
+
0
0
˚
DMBP=5,5 -dimethyl-2,2 -bipyridine, r(N –H?Oꢁ)=2.683 A], which

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H. Ohchi et al. / Physica B 405 (2010) S341–S343
S343
shows an antiferroelectric phase transition due to the proton transfer
Acknowledgments
[9]. Since the hydrogen bond of 1 is 1-D with critical lengths of
hydrogen bonds, the metastable state could appear and demonstrate
the dielectric response. The pressure dependence of dielectric con-
stant for 1 is under way to obtain larger and stable dielectric response.
This work was supported by Grants-in Aid for Scientific
Research (B) (No. 20340087) and on Innovative Areas of
Molecular Degrees of Freedom (No. 20110007) from the Ministry
of Education, Culture, Sports, Science and Technology.
4. Conclusions
In conclusion,
a unique 1D hydrogen-bonded dielectric
References
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response due to proton transfer was observed around 352 K only
for as-grown crystals in the metastable state with defects of
hydrogen bonds. The metastable state is originated from the
critical length and one-dimensionality of hydrogen bonds.
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CCDC 752373 contains the supplementary crystallographic
data of 1 for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_re
quest@ccdc.cam.ac.uk, or by contacting The Cambridge Crystal-
lographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK;
fax: +44 1223 336033.