Experimental confirmation of proton conductivity predicted from intermolecular hydrogen-bonding in spatially-confined novel histamine derivatives

Article abstract of DOI:10.1016/j.jssc.2021.122182

The new compounds with one (N-(2-(1H-imidazole-4-yl)ethyl)benzamide; A1), two (N,N′-bis(2-(1H-imidazole-5-yl)ethyl)terephthalamide; A2) and three (N,N′,N″-tris(2-(1H-imidazole-5-yl)ethyl)benzene-1,3,5-tricarboxamide; A3) histamine units were synthesized w

Full text of DOI:10.1016/j.jssc.2021.122182

Journal of Solid State Chemistry 299 (2021) 122182
Contents lists available at ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
Experimental confirmation of proton conductivity predicted from
intermolecular hydrogen-bonding in spatially-confined novel
histamine derivatives
,1,
Kanchanok Kodchakorn a, Piyarat Nimmanpipug a *, Suttinun Phongtamrug b,
,2,**
Kohji Tashiro c
a Department of Chemistry, Faculty of Science and Center of Excellence for Innovation in Analytical Science and Technology, Chiang Mai University, Chiang Mai, 50200,
Thailand
b Department of Industrial Chemistry, Faculty of Applied Science, King Mongkut’s University of Technology North Bangkok, Bangkok, 10800, Thailand
c Department of Future Industry-Oriented Basic Science and Materials, Graduate School of Engineering, Toyota Technological Institute, Tempaku, Nagoya, 468-8511,
Japan
A R T I C L E I N F O
A B S T R A C T
The new compounds with one (N-(2-(1H-imidazole-4-yl)ethyl)benzamide; A1), two (N,N0-bis(2-(1H-imidazole-5-
yl)ethyl)terephthalamide; A2) and three (N,N0,N00-tris(2-(1H-imidazole-5-yl)ethyl)benzene-1,3,5-tricarboxamide;
A3) histamine units were synthesized with an expectation of the realization of high proton conductivity in a high
temperature region in the presence of imidazole groups. The X-ray crystal structure analysis revealed the network
structures of the strong intra- and intermolecular hydrogen bonds. The thermal degradation temperatures were
much higher than those for the original histamine. The proton conductivities of the single crystals were measured
in a wide temperature range. The results of these three compounds were shown at different temperatures.
Referring to the crystal structure, the water molecules are confined in a local position and the protons cannot
spread freely over the whole crystal lattice. In this way, the information from X-ray analyzed crystal structure
allowed us to predict the proton transfer behaviors of these compounds, which were confirmed by performing the
proton conductivity experiment.
Keywords:
Histamine derivatives
Hydrogen bond network
Proton conductivity
1. Introduction
originated from the sensitive change of the conformation, tautomeriza-
tion, and intermolecular hydrogen-bonding network, may affect the
Histamine has an amino group and an imidazole ring in a molecule, as
shown in Scheme I. In addition, the internal rotations around the CH2‒
CH2 bond can induce the various orientations of these functional groups
[1–3]. These two functional groups may form intra- and inter-molecular
hydrogen bonds, but the network structure might be changed sensitively
by such an orientational change of the groups. These N-H⋅⋅⋅N type
hydrogen bonds are predicted to be useful for the generation of proton
hopping between neighboring units [4–8], allowing us to expect the
formation of an excellent proton carrier with high proton conductivity
over 10ꢀ3 S cmꢀ1 [9]. A complicated structural change of histamine,
proton conductivity of this molecule quite sensitively [10]. In such a
manner, it might be difficult to control the packing structure for
enhancing the proton conductivity. Another limitation to control of his-
tamine molecule is its hygroscopic character, which causes the disorder
of histamine structure when expose to air.
In order to utilize the above-mentioned functional histamine as a
proton transformer, we have fixed the histamine molecules to the ben-
zene rings as shown in Scheme I with the histamine’s functionality pre-
served (A1: N-(2-(1H-imidazole-4-yl)ethyl)benzamide (C12H13N3O), A2:
N,N0-bis(2-(1H-imidazole-5-yl)ethyl)terephthalamide
(C18H20N6O2),
* Corresponding author.
** Corresponding author.
E-mail addresses: piyarat.n@cmu.ac.th (P. Nimmanpipug), ktashiro@aichisr.jp (K. Tashiro).
Present address: Department of Chemistry, Faculty of Science and Center of Excellence for Innovation in Analytical Science and Technology, Chiang Mai Uni-
1
versity, Chiang Mai, 50200, Thailand.
2
Present address: Aichi Synchrotron Radiation Center, Knowledge Hub Aichi, Minami-Yamaguchi, Seto 489-0965, Japan.Frontier Research Center for Applied
Atomic Sciences, Ibaraki University in Ibaraki Quantum Beam Research Center, Shirakata 162-1, Tokai-mura, Naka, Ibaraki 319-1106, Japan.
https://doi.org/10.1016/j.jssc.2021.122182
Received 23 February 2021; Received in revised form 1 April 2021; Accepted 8 April 2021
Available online 16 April 2021
0022-4596/© 2021 Elsevier Inc. All rights reserved.

K. Kodchakorn et al.
Journal of Solid State Chemistry 299 (2021) 122182
Scheme 1. Chemical structure of histamine, A1, A2, and A3.
N,N0,N00-tris(2-(1H-imidazole-5-yl)ethyl)benzene-1,3,5-tri-
and
A3:
in 10 mL CH2Cl2 solution was slowly dropped into the histamine sus-
pension. After being stirred in the ice-water bath for 60 min and at room
temperature overnight, a precipitate was produced which was then fil-
trated and dried. The white precipitate was dispersed in 10 mL deionized
water and was brought to pH 11 by adding 2 N NaOH solution. Recrys-
tallization from methanol gave the colorless needle single crystals of A2.
N,N0,N00-tris(2-(1H-imidazole-5-yl)ethyl)benzene-1,3,5-tricarbox-
amide (A3): A suspension of histamine (300 mg, 2.7 mmol) in 10 mL
pyridine was stirred in an ice-water bath and the 1,3,5-benzenetricar-
bonyl chloride (240 mg, 0.9 mmol) in 10 mL CH2Cl2 solution was
slowly dropped into the suspension. After being stirred in the ice-water
bath for 60 min and at room temperature overnight, the obtained yel-
low precipitate was filtrated and dried. A yellow solid was dissolved in
10 mL deionized water and was adjusted to pH 11 by adding 2 N NaOH
solution. Recrystallization from methanol afforded the colorless needles
of A3.
carboxamide (C24H27N9O3)). These compounds are expected to build up
into 3D network structures [11,12] with the intermolecular hydrogen
bonds between the histamine functional groups, where the benzene rings
are stacked together as well as the positions and orientations of the
histamine parts are fixed in the crystal lattice. Recently, some order
structures were formed by local packing arrangement in a series of
side-chain liquid crystal with azobenzene and sulfonic acid groups, thus,
the proton conductivity was improved [13,14,26]. Moreover, a spatial
confinement of histamine molecules might result in the reduction of their
thermal mobility. In addition, the existence of ethylene bonds may
reserve the flexibility at the ends of the imidazole rings [15]. It might be
valuable to investigate the proton conductivity of these new compounds
from the viewpoints of their aggregation structures with the expectation
of the high proton conductivity.
In the present paper, we first investigated the crystal structures of
these compounds by the X-ray diffraction analysis. We predicted the
possibility of proton conductivity for these compounds by investigating
the detailed structural characteristics. Then, the temperature dependence
of the proton conductivity was measured for all of these 3 compounds to
confirm the structure-based prediction. This type of molecular design
may be quite useful for the development of a new compound with high
functionality like the high-temperature proton conductivity.
2.3. Measurements
The Fourier transform infrared spectroscopic (FTIR) measurements
were performed with the ATR accessory at a resolution of 2 cm-1 in the
frequency range 4000–400 cmꢀ1 using a PerkinElmer FrontierTM FTIR
spectrometer. Raman spectra were measured with a Japan Spectroscopic
Company NRS-2100 laser Raman spectrometer at the back-scattering
geometry with a laser beam of 532 nm wavelength as an excitation
2. Experimental
light source in the region 3400–50 cmꢀ1 at a resolution of 2 cmꢀ1 1H
.
NMR spectra of these three compounds were measured with a Varian
Inova 400 MHz spectrometer with a 5 mm multinuclear inverse probe at
296 K. Crystalline solid of each compound was dissolved with methanol-
2.1. Materials
Histamine (C5H9N3, >97% purity), benzoyl chloride (C6H5COCl, 99%
purity), terephthaloyl chloride (C6H4-1,4-(COCl)2, >99% purity), and
1,3,5-benzenetricarbonyl chloride (C6H3(COCl)3, 98% purity) were
purchased from Sigma-Aldrich® (Singapore). Hydrochloric acid (HCl),
sodium hydroxide (NaOH), dichloromethane (CH2Cl2), and methanol
(CH3OH) were bought from Merck, Germany. All chemicals used in this
study were of analytical grade.
d4 as a solvent for NMR spectroscopic analysis. Their molecular weights
were evaluated with an AXIMA Confidence MALDI-TOF mass spec-
trometer (MS) in positive ion mode. Each sample mixed in sodium
formate as an internal standard was ionized with a capillary exit voltage
of 4.5 kV. The MS data were recorded in the full scan mode in range of m/
z 50–1000.
The X-ray structure analysis was performed using a Rigaku VariMax
X-ray diffractometer equipped with a cylindrical imaging plate detector.
The diffraction data were collected in an oscillation mode at room tem-
2.2. Synthesis of histamine derivatives
N-(2-(1H-imidazole-4-yl)ethyl)benzamide (A1): A suspension of his-
tamine (100 mg, 0.89 mmol) in 10 mL pyridine was stirred in an ice-
water bath, and a benzoyl chloride (140 mg, 0.99 mmol) was added
dropwise to this suspension [16]. The suspension was stirred in an
ice-water bath at 0 ꢁC for 60 min, then, at room temperature for 4 h. The
deionized water (10 mL) was added and the pH was adjusted to 11 by
adding 2 N NaOH to obtain a clear solution. The A1 was recrystallized
from the solution.
perature using a Mo-Kα (λ ¼ 0.711 Å) radiation focused with a confocal
mirror. The crystal structure was solved by a direct method (SIR92) [17]
and refined by the full-matrix least-squares procedures for |F|2 (F:
structure factor). The structure analyses were carried out by the software
package of Crystal Structure [23].
The thermal analysis was performed with a TA Instrument Q2000
differential scanning calorimeter (DSC) at a scanning rate of 10 ꢁC/min
under nitrogen atmosphere in the temperature region of :50 ꢁC–200 ꢁC.
The degradation temperature was measured using a Mettler Toledo TGA/
DSC1 analyzer under a nitrogen atmosphere at a heating rate of 10 ꢁC/
min from 30 ꢁC to 700 ꢁC.
N,N0-bis(2-(1H-imidazole-5-yl)ethyl)terephthalamide (A2):
A sus-
pension of histamine (200 mg, 1.8 mmol) in 10 mL pyridine was stirred
in an ice-water bath and the terephthaloyl chloride (180 mg, 0.89 mmol)
2

K. Kodchakorn et al.
Journal of Solid State Chemistry 299 (2021) 122182
Temperature dependent FTIR spectroscopic measurements were car-
ried out by using an in-house temperature-controlled sample holder with
a Varian FTS 7000 Fourier transform infrared spectrometer measured in
isothermal steps from 30 ꢁC to 220 ꢁC. The sample was prepared in a KBr
pellet form and the transmission spectra were recorded in a range of
O stretching of amide at 1636 cmꢀ1. For A2 and A3, the characteristic
–
C
–
stretching mode of imidazole units and amide groups were also shown.
The 1H NMR spectrum indicated the chemical shifts of A1, i.e., at
7.82ꢀ7.74, 7.51 and 7.48ꢀ7.40 ppm (protons of aromatic ring), 7.59
and 6.87 ppm (protons of imidazole ring), and 3.62ꢀ2.90 ppm (protons
of CH2). The 1H NMR spectrum of A2 gave the chemical shifts at 8.67 and
6.87 ppm (protons of imidazole ring), 8.03 ppm (protons of aromatic
ring), and 3.50ꢀ2.78 ppm (protons of CH2). In the case of A3 the
chemical shifts, i.e., at 8.35 ppm (protons of aromatic ring), 7.60ꢀ6.90
ppm (protons of imidazole ring) and 3.65ꢀ2.92 ppm (protons of CH2)
were assigned. The MALDI-TOF MS spectrum also confirmed the A3
structure at m/z ¼ 512.60 corresponding to the molecular weight of the
A3 with Na ion (see the details of structural characterization in Fig. S1 –
S5).
4000–400 cmꢀ1 at a resolution of 2 cmꢀ1
.
Temperature dependent X-ray powder diffraction measurements
were performed with a high temperature reactor installed on a Rigaku
SmartLab X-ray diffractometer equipped with a D/teX detector. The
graphite-monochromatized Cu-K
α (λ ¼ 1.542 Å) beam was used as a
radiation source. The diffraction data was collected every 10 ꢁC stepwise
from 20 ꢁC to 200 ꢁC in the 2θ range of 5ꢁ–27ꢁ at a scanning rate of 10ꢁ/
min.
2.4. Proton conductivity measurement
3.2. Crystal structure and hydrogen bond network
The proton conductivity of the histamine derivatives was measured
by an impedance spectroscopic method with a
μꢀAUTOLAB Type-III
The crystal structures of the three histamine derivatives (A1, A2, and
A3) were determined by X-ray single crystal analysis. The crystallo-
graphic data are listed in Table 1. The fractional coordinates are listed in
the supplementary information.
In the case of A1 crystal structure, which belongs to a monoclinic
system of the space group P21 (Fig. 1), the molecules are bent by the
gauche conformation around the ethyl C-C bond. These molecules form
the intramolecular hydrogen bonds between the amide and histamine
end groups with the NH(amide)ꢀN(imidazole) distance of 2.14 Å. These
contracted molecules are connected with each other by the
connected to a PC running electrochemical impedance software in the
frequency range of 100 kHz–1 Hz with an AC signal 50 mV from 30 ꢁC to
200 ꢁC (Scheme 2). The sample was dissolved in methanol and dropped
on a glass filter paper and then dried at room temperature. This process
was repeated three times before drying under vacuum at room temper-
ature for 4 h. The thus-prepared samples were kept in desiccators until
the start of the measurement. A dried glass filter paper was assembled in
a sealed-off Teflon® cell using copper electrodes. At each temperature
(T), proton conductivity (
ance data based on Eq (1).
σ
ðTÞ, S cmꢀ1) was calculated from an imped-
Table 1
L
R ꢂ A
σ
ðTÞ ¼
(1)
Crystal structure of each histamine derivative.
A1
A2
A3
where L is the membrane thickness (cm), A is the cross-sectional area of
the electrode (0.4418 cm2), and R is the membrane resistance (ohm)
derived from the Nyquist plot.
C12H13N3O
C18H20O2N6Na⋅4H2O
C24H27N9O3⋅4H2O
Formula weight
Crystal system
Space group
a (Å)
215.15
monoclinic
P21
8.417(12)
5.551(7)
12.281(16)
90
103.97(4)
90
2
447.15
Monoclinic
P2/n
11.553(18)
4.489(7)
23.742(4)
90
92.43(3)
90
2
561.15
triclinic
P1
5.024(6)
14.595(16)
19.266(2)
78.97(2)
86.36(3)
83.85(3)
2
3. Results and discussion
b (Å)
c (Å)
3.1. Structural characterization
α
(ꢁ)
β (ꢁ)
Successful preparation of each derivative was confirmed by FTIR, 1H
NMR, and MALDI-TOF MS measurements. For A1, the characteristic
peaks were observed by FTIR, for example, hydrogen bonded NꢀH
stretching of amide and imidazole unit in range of 3200ꢀ3000 cmꢀ1, a
γ (ꢁ)
Z
Density (g cm-3
)
1.450
1.437
1.173
Scheme 2. Diagram of the apparatus for proton conductivity measurement.
3

K. Kodchakorn et al.
Journal of Solid State Chemistry 299 (2021) 122182
water and water-water molecules. In fact, the A2 crystal was found to
show the proton conductivity even in the solid state as will be discussed
later.
The A3 crystal belongs to a triclinic system of P1 space group and
contains four water molecules per one A3 molecule (Fig. 3). The large
triangular molecules form the 2-dimensional layer structure with many
intermolecular hydrogen bonds between the amide units and histamine
groups and also between the histamine groups. These layers are stacked
together by the benzene-benzene and inter-amide bonds. The hydrogen
bonding distances between the imidazole units and water molecules are
too long to give the effective proton exchange between them. But we may
have a possibility of the proton conductivity as we expected from the
trace of red dotted arrows in Fig. 3, though the proton hopping might
occur only locally.
3.3. Thermal properties of the imidazole-based compounds
In order to attain the realization of the proton conductivity in a wide
temperature region, the compounds must be stable enough at a high
temperature. The melting temperature (Tm) and the onset thermal
decomposition temperature (Td) of the compounds A1-A3 are demon-
strated in Fig. 4. The TGA thermograms of A2 and A3 (Fig. S6) showed
two-step decomposition, where the first started at low temperature and
the second started at 300 ꢁC. The first step occurred in the temperature
region from 50 ꢁC to 110 ꢁC and was attributed to a loss of solvent
molecules. The TGA thermogram of A2 showed the first weight loss at
about 4%, which related to the loss of one water molecule (~16 wt%)
from the unit cell. The similar weight loss of small molecules (9%),
corresponding to 12 wt% of three water molecules in the crystal unit cell,
was found in A3. The evaporation of water molecules was also detected
as a broad endothermic peak in the range of 50 ꢁC–140 ꢁC in the DSC
thermogram (Fig. S7). These confirmed the water molecules observed in
the crystal structure.
Fig. 1. Crystal structure of A1. The hydrogen bonds are indicated by red lines.
The pink, green and blue colors are imidazole, amide and benzene ring,
respectively. (For interpretation of the references to color in this figure legend,
the reader is referred to the Web version of this article.)
intermolecular amide-histamine hydrogen bonds to form a sheet struc-
ture [O(amide)ꢀNH(imidazole) distance of 1.87 Å]. These hydrogen-
bonded sheets are stacked together with benzene rings as separators.
The monoclinic crystal system of A2 in the P2/n space group contains
sodium ions (Naþ) and water molecules. The A2 molecules are almost
fully extended with the trans ethylene conformation. The hydrogen
bonds are formed among water molecules and between the A2 and water
molecules. As illustrated by red arrows in Fig. 2, the protons might be
transferred continuously between the imidazole groups and imidazole-
All synthesized compounds (A1-A3) were thermally stable up to 300
Fig. 2. Crystal structure of A2. The hydrogen bonds are indicated by blue lines. The pink, green and blue colors are imidazole, amide and benzene ring, respectively.
The protons may migrate following the red arrows. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of
this article.)
4

K. Kodchakorn et al.
Journal of Solid State Chemistry 299 (2021) 122182
Fig. 3. Crystal structure of A3. The hydrogen bonds are indicated by blue lines. The pink, green and blue colors are imidazole, amide and benzene ring, respectively.
The double arrows indicate the possible proton transfer with a low probability. (For interpretation of the references to color in this figure legend, the reader is referred
to the Web version of this article.)
3.4. Proton conductivity and its temperature dependence
In order to check the above-predicted proton conductive behaviors of
these three compounds, the experiments were performed.
<A1> Below the melting point (Tm, 147 ꢁC), no proton conductivity
was observed. The proton conductivity of 3.24ꢃ10ꢀ5 S cmꢀ1 was
detected above the melting point in the temperature range from 160 ꢁC to
200 ꢁC (Fig. 5a). In this case, the activation energy (Ea) can be estimated
on the basis of the Arrhenius-type equation originating from the hopping
mechanism similar to the previous studies [10,20,21]:
σ
¼ Ae‒Ea/RT
(2)
where R is a gas constant and T is an absolute temperature. The activation
energy (Ea) of proton transfer process of A1 was estimated to be 51.3 kJ
molꢀ1 (Fig. 5b, Table 2). The proton conductivity in the molten state at a
high temperature should correspond to the movement and transfer of
protons following the vehicle model similar to the study of Refs. [21,25]
As the H atoms in the amide-histamine hydrogen bonds are strongly
fixed, the jump of proton between the neighboring molecules is difficult
in the crystal lattice (Fig. 1). The packing stability of A1 comes from the
reservation of the hydrogen bonds in the high temperature region. As
Fig. 4. Melting (Tm) and degradation (Td) temperature of histamine (His), A1,
A2, and A3.
shown in Fig. 6, the intensities of IR bands characteristic of the hydrogen-
bonded amide groups (NH stretching mode at 3201 cmꢀ1 and C
O
–
–
stretching mode at 1636 and 1576 cmꢀ1) were kept constant up to the
melting point. After that, the free NH stretching (amide) band (at 3411
cmꢀ1) increased instead.
ꢁC, compared to histamine (Td ~200 ꢁC). This might have been affected
by the amide linkage, which degraded above 200 ꢁC [24]. Furthermore,
the reduced weight loss and the high resultant char yield at high tem-
perature indicated high thermal stability of these compounds [18,19].
The trifunctional compound (A3) exhibits the highest Td at 352 ꢁC and Tm
at 182 ꢁC, reflecting the strong hydrogen bonds between the amide units
and the histamine units. Considering the observed melting and degra-
dation temperature, the compounds A1-A3 may apply as
high-temperature proton conductors.
<A2> As predicted from the crystal structure (Fig. 2), the connection
of A2 molecules with water is important to exhibit the proton conduc-
tivity. However, since the A2 molecules are packed with the coexistent
water molecules, the proton conductivity may be lost when the water
molecules are evaporated at a high temperature. Different from the
above-mentioned in A1 case, the A2 compound showed the proton
conductivity in a solid state with the order of 10ꢀ4 S cmꢀ1. The con-
ductivity decreased in the temperature region higher than 70 ꢁC, which
5

K. Kodchakorn et al.
Journal of Solid State Chemistry 299 (2021) 122182
Fig. 5. (a) Nyquist plots at measuring temperature from 160 ꢁC to 180 ꢁC. (b) Arrhenius plot of conductivity of A1 under anhydrous condition. (c) Nominated
structural change and proton transfer mechanism of A1.
the proton hopping mechanism in the solid state. At such a point, the
Table 2
mechanism is similar to that of a histamine [10]. The conductivity due to
The proton conductivity and activation energy of the histamine derivatives (A1-
the Grotthuss-type mechanism may be expressed as
A3) in comparison with imidazole and histamine (H1).
0
(S cmꢀ1
)
Condition
Ea (kJ
molꢀ1
Reference
[22]
σ
¼ Aoeao þ a1Te‒(Eo þ E T)/[R(T‒To)]
(3)
Sample
σ
)
where ao and a1 are the coefficients describing frequency factor A ¼
[Aoexp(ao þ a1T)], which is affected by the thermal motion, and Eo and E0
are the parameters in temperature-dependent activation energy: Ea ¼ Eo
Imidazole
1.65ꢃ10ꢀ3
Solid,
23
Liquid
(30–120 ꢁC)
(at 348 K)
þ E0T. The natural logarithm of
σ is expressed as a function of temper-
Histamine
H1
9.40ꢃ10ꢀ6
2.70ꢃ10ꢀ4
[9]
[10]
Solid
18.90
(at 330 K)
ature as follows.
(30–80 ꢁC)
) ¼ InAo þ [ao þ a1T ‒ (Eo þ E0T)/R(T‒To)]
(4)
Histamine
derivatives
A1
In(
σ
3.24ꢃ10ꢀ5
2.64ꢃ10ꢀ5
1.00 ꢃ10ꢀ5
Liquid
51.30
This equation was applied and the thus-evaluated lnA and Ea values
(>160 ꢁC)
Solid
are shown in Fig. 7b.
A2/H2O
23.96
(at 310 K)
58.70
<A3> The proton conductivity was not detected in the temperature
region of 30–200 ꢁC. To investigate the structural change in more details,
the temperature dependence of the X-ray diffraction analysis was carried
out (Fig. 8). The crystal structure was found to change at around 90 ꢁC, as
known from the appearance of the new peaks at 2θ ¼ 9.5ꢁ, 11.5ꢁ, 20.0ꢁ
and so on, where the water-solvent molecules were evaporated as known
from the TGA and DSC curves. We found that, when a trace amount of
methanol was on the sample, a proton conductivity of about 10ꢀ5 S cmꢀ1
in the low-temperature range of 30–70 ꢁC was observed (Fig. S8). The
(30–60 ꢁC)
Solid
A3/CH3OH
(40–70 ꢁC)
(at 333 K)
corresponds to the start of water evaporation as seen in the TGA and DSC
data.
Once the water is lost, the proton conductivity disappears, being
understood reasonably by referring to the characteristic crystal structure.
In this way, the proton transfer in the A2 sample is supposed to occur by
6

K. Kodchakorn et al.
Journal of Solid State Chemistry 299 (2021) 122182
Fig. 6. Temperature dependent IR spectra of A1 (a)
ν(NH) and (b) Am
ν
(CO) modes and (c) the band intensity. The DSC thermogram is also shown here
for comparison.
activation energy of A3/CH3OH estimated from Arrhenius plot (Fig. S9)
was rather high (Table 2). This might be because of the strong interaction
among the A3 molecules in the crystal lattice. The additional labile
hydrogen-containing compound might induce the proton transfer in A3.
From our finding, the crystal packing modes of a series of novel
molecules with the plural histamine units confined have been investi-
gated. The crystal structure of A1 was obtained in anhydrous form, while
the water molecules was incorporated in the crystal lattices of A2 and A3.
In A1, the proton transfer interaction was promoted by a complex rear-
rangement of the local hydrogen-bond network at high temperature, as
depicted in Fig. 5c. For A2, the protons are expected to hop between the
imidazole groups and imidazole-water and water-water molecules. Pro-
ton transport along chains of hydrogen bonded polar molecules favors a
structural defect through the fluctuating hydrogen-bond network via a
series of elementary proton transfer steps as shown in Fig. 7c. As the
water molecules take part in the extended hydrogen-bond network, this
hydrogen-bonded pattern is speculated to be unstable in the high tem-
perature range. Comparing these derivatives to their origin, imidazole
was considered to be the most promising proton carrier among all cases
in Table 2 with a proton conductivity of 1.65ꢃ10ꢀ3 S cmꢀ1 but its uti-
lization is limited due to its volatility. High thermal properties of each
compound are remarkable improved corresponding to the strong
hydrogen bonding network among neighboring molecules. We found
that these histamine derivatives show a proton conductivity up to 10ꢀ5 S
cmꢀ1, lower that the smaller unit (imidazole and histamine) but the
values are comparable to the proton conductivity of imidazole-doped
organic crystalline covalent organic frameworks (10-4-10ꢀ5 S cmꢀ1at
130 ꢁC with Ea of 29.9-53.1 kJ mol-1) reported by Li et al. [14]. Among all
these novel derivatives and its parent structure i.e., imidazole and his-
tamine, A1 can be used at a high temperature above 160 ꢁC.
The expectation mentioned in the discussion about the crystal struc-
ture was unfortunately not realized. The water molecules are aggregated
together only locally and connected with the surrounding amide groups.
The proton transfer may be difficult judging from the localized and iso-
lated structure of water molecules. The result itself was not very satis-
factory, but the important point of this research is the fact that the proton
7

K. Kodchakorn et al.
Journal of Solid State Chemistry 299 (2021) 122182
Fig. 7. (a) Temperature dependence of proton conductivity of A2 in a natural logarithmic form. (b) Its frequency factor (lnA) and activation energy Ea plotted against
temperature. (c) Proposed proton hopping mechanism of A2.
conductivity was predicted on the basis of the X-ray analyzed crystal
structures and these predictions were checked by the actual experiments.
is expected to be essential in the hydrogen-bond network to shift the
structural defect across one hydrogen bond at high temperature. The A2
compound contains water molecules, which are connected with the
histamine units effectively over the whole crystal lattice. In the temper-
ature region of 30 ꢁC–60 ꢁC, the proton conductivity was detected, which
is due to the Grotthuss-type mechanism. Once the water was evaporated
above 70 ꢁC, the proton conductivity was lost because of fluctuated
hydrogen-bond network. In the case of A3, the proton conductivity was
not expected as hoped judging from the molecular packing mode with the
localized water clusters in the lattice. In fact, this compound did not show
any detectable proton conductivity in the experimental temperature re-
gion of 30 ꢁC–200 ꢁC. By referring to the crystal structure, the water
molecules are considered to be confined in a local position, and the
protons cannot spread freely over the whole crystal lattice. Extra meth-
anol molecules are speculated to serve as a proton transporter in this
case. The high activation energy in this situation may be regarding as to
difficulty of solvent reorganization in this complicated packing structure.
4. Conclusion
A series of the histamine derivatives (A1, A2, and A3) were synthe-
sized. The X-ray crystal structure analysis showed that these molecules
are aggregated with the intermolecular hydrogen bonds, in particular,
the hydrogen bonds with water molecules are unique in the case of A2
and A3. The strong hydrogen bonds and the rigid molecular shape
brought about the high degradation temperature (Td) of about 300 ꢁC,
much higher than that of histamine at about 200 ꢁC. The proton con-
ductivity was predicted from the analyzed crystal structure. The A1 show
the Grotthuss-type proton transfer mechanism because all the molecules
are connected strongly by the hydrogen bonds between the amide-
histamine units. In fact, A1 exhibited the proton conductivity only
above the melting point (147 ꢁC) up to 180 ꢁC. A complex rearrangement
8

K. Kodchakorn et al.
Journal of Solid State Chemistry 299 (2021) 122182
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://do
i.org/10.1016/j.jssc.2021.122182.
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