Thermochromism of a novel organic compound in the solid state via crystal-to-crystal transformation

Article abstract of DOI:10.1016/j.molstruc.2006.04.025

A novel thermochromic organic compound, 1,5-bis(hydroxyethylamino)-2,4-dinitrobenzene (BDB), was synthesized and characterized using X-ray diffraction, differential scanning calorimetry, visible and infrared spectroscopy. It was observed that BDB exists in two different crystal structures of A- and B-forms at room temperature. In each crystal structure, there exist intra- and intermolecular hydrogen bonds between hydroxy, amino, and nitro groups. As the temperature is increased, the color of BDB crystalline powder changes from yellow to orange. The yellow color was recovered for a specific period of time when it is cooled again, indicating that thermochromism of BDB in the solid state is reversible. Both crystals of A- and B-forms are transformed to the C-form crystal at the transition temperature. From the results of X-ray diffraction, thermal data, and infrared spectra, it is suggested that the thermochromism of BDB originates from the crystal-to-crystal transition accompanying with the configurational transformation between nitro-form and acid-form via intramolecular hydrogen transfer.

Full text of DOI:10.1016/j.molstruc.2006.04.025

Journal of Molecular Structure 825 (2006) 70–78
www.elsevier.com/locate/molstruc
Thermochromism of a novel organic compound in the solid state
via crystal-to-crystal transformation
a,
a
b
c
a
*
Sang Cheol Lee , Young Gyu Jeong , Won Ho Jo , Hee-Joon Kim , Jinho Jang ,
d
a
Ki-Min Park , Ihn Hee Chung
a
School of Advanced Materials and Systems Engineering, Kumoh National Institute of Technology, Gumi 730-701, Republic of Korea
b
School of Materials Science and Engineering, Seoul National University, Seoul 151-742, Republic of Korea
c
School of Natural Sciences, Kumoh National Institute of Technology, Gumi 730-701, Republic of Korea
Research Institute of Natural Sciences, Gyeongsang National University, Chinju 660-701, Republic of Korea
d
Received 22 November 2005; received in revised form 1 April 2006; accepted 4 April 2006
Available online 2 June 2006
Abstract
A novel thermochromic organic compound, 1,5-bis(hydroxyethylamino)-2,4-dinitrobenzene (BDB), was synthesized and character-
ized using X-ray diffraction, differential scanning calorimetry, visible and infrared spectroscopy. It was observed that BDB exists in
two different crystal structures of A- and B-forms at room temperature. In each crystal structure, there exist intra- and intermolecular
hydrogen bonds between hydroxy, amino, and nitro groups. As the temperature is increased, the color of BDB crystalline powder chang-
es from yellow to orange. The yellow color was recovered for a specific period of time when it is cooled again, indicating that thermo-
chromism of BDB in the solid state is reversible. Both crystals of A- and B-forms are transformed to the C-form crystal at the transition
temperature. From the results of X-ray diffraction, thermal data, and infrared spectra, it is suggested that the thermochromism of BDB
originates from the crystal-to-crystal transition accompanying with the configurational transformation between nitro-form and acid-
form via intramolecular hydrogen transfer.
ꢀ
2006 Elsevier B.V. All rights reserved.
Keywords: Crystal-to-crystal transition; Configurational transformation; Thermochromism; Visible and infrared spectroscopy; X-ray diffraction
1
. Introduction
cess between two molecular species. For example, the ther-
mochromic mechanism of spiroheterocyclic compounds
such as spiropyrans and spirooxazines has been identified
to involve a thermally sensitive equilibrium between the
spiroheterocyclic form and the quasi-planar open merocy-
anine-like structure formed by ring opening via the CAO
bond breaking [1,2]. It has also been reported that the ther-
mochromic mechanism of Schiff bases involves an intramo-
lecular hydrogen transfer, producing enol–keto tautomeric
species [15,17], where the presence of the ortho-hydroxy
group is essential for the observation of thermochromism
[14,16,20]. Another important factor for these compounds
to exhibit the thermochromism is the molecular packing of
these compounds, i.e., the thermochromism is restricted to
planar molecular packing which enables the tautomeric
equilibrium to shift toward the keto-form absorbing at
longer wavelengths as the temperature increases [20,25].
Many organic compounds exhibit the change of color
reversibly or irreversibly with temperature variation, so-
called thermochromism [1,2]. Those compounds are of
increasing importance in various practical applications
such as temperature-indicating devices, temperature sensi-
tive light filters, optical switching, imaging systems, etc.
[
1,3,4]. The most well-known thermochromic organic com-
pounds are spiroheterocyclic compounds [5–10], Schiff
bases [11–21], and overcrowded ethylenes [22–24].
Extensive studies on these compounds have revealed
that the thermochromic mechanism is an equilibrium pro-
*
Corresponding author. Tel.: +82 54 478 7685; fax: +82 54 478 7710.
E-mail address: leesc@kumoh.ac.kr (S.C. Lee).
0
022-2860/$ - see front matter ꢀ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2006.04.025

S.C. Lee et al. / Journal of Molecular Structure 825 (2006) 70–78
71
The thermochromism of overcrowded ethylenes in solution
also arises from a thermal equilibrium between two distinct
and interconvertible isomeric species of A and B [26], where
the isomeric species of A has a severe repulsion between the
two overcrowded moieties that can be avoided by taking a
folded conformation. This feature has been confirmed by
X-ray crystallographic investigation and dynamic NMR
studies [22–24]. Among various proposed structures for
the thermo-induced species of B, the twisted conformation
has been supported by a variety of experimental and theo-
retical works [22–24,27].
purely A-form crystal at room temperature and Sample II
does contain B-form crystal.
The chemical structure of BDB (C H N O , 286.25 g/
10
1
14
6
4
mol) synthesized was identified using H NMR spectrosco-
1
py and elemental analysis. H NMR (DMSO-d ): d 8.98 (s,
6
1H); 8.48 (t, 2H); 5.97 (s, 1H); 5.06 (t, 2H); 3.68 (q, 4H);
3.44 (q, 4H). Elemental analysis of BDB: calcd C, 41.96;
H, 4.93; N, 19.57. Found: C, 41.92; H, 5.04; N, 19.53. Both
NMR and elemental analysis confirmed that BDB was suc-
cessfully synthesized.
Until now, most of studies still focus on these organic
compounds, because very few new thermochromic molecu-
lar systems have been developed [28,29]. In this study, we
introduce a novel thermochromic organic compound of
2.2. X-ray crystal structure determination
Single crystals of A- and B-forms suitable for X-ray
crystallographic work were grown by slow evaporation of
THF and ethanol solutions of BDB, respectively. Each
crystal on a glass fiber was mounted on a Siemens SMART
diffractometer equipped with a graphite monochromated
1
,5-bis(hydroxyethylamino)-2,4-dinitrobenzene
(BDB),
whose chemical structure is shown in Fig. 1, and investigate
its crystal structure and thermochromic behavior in the sol-
id state using X-ray diffraction, differential scanning calo-
rimetry, visible and infrared spectroscopy. From the
experimental results, the most feasible thermochromic
mechanism of BDB is proposed in terms of both physical
and chemical structural changes.
˚
Mo Ka (k = 0.71073 A) radiation source and a CCD area
detector. Data collection were performed at 298(2) K.
Crystal data and structure refinements for A- and B-forms
of BDB are summarized in Table 1. The crystal structures
were solved by a direct method with SHELXL [30], in
2
which full-matrix least-squares on F was used for refine-
2
. Experimental
ment including hydrogen atoms and isotropic displacement
parameters. All non-hydrogen atoms were refined aniso-
tropically. All hydrogen atoms were obtained from differ-
ence electron density maps, and refined isotropically.
2
.1. Preparation and characterization of material
BDB was synthesized via two steps in our laboratory.
First, m-dichlorobenzene (50 g, 0.34 mol) was reacted with
KNO3 (70 g, 0.693 mol) in a solution of concentrated
H SO (250 ml) at 120–135 ꢁC for 1 h. The reaction prod-
Table 1
Crystallographic data of BDB A- and B-form crystals
2
4
A-form
B-form
uct of 2,6-dichloro-3,5-dinitrobenzene was filtered, dis-
solved in boiling 95% ethanol (1000 ml), and
recrystallized at 0 ꢁC. In the second step, 2,6-dichloro-
Formula
C
286.25
298(2)
Orthorhombic
Pbca
10
H
14
N
14
O
6
10 14 14 6
C H N O
286.25
298(2)
Formula weight
Temperature (K)
Crystal system
Space group
Monoclinic
P2 /n
3
,5-dinitrobenzene (20 g, 0.084 mol) in ethanol (250 ml)
1
was added dropwise into ethanolamine (23 g, 0.377 mol)
in ethanol solution (50 ml). The mixed solution was stirred
for 4 h to complete the reaction between 2,6-dichloro-3,5-
dinitrobenzene and ethanolamine. The crude product of
BDB was filtered and purified by recrystallization using
ethanol or THF solution. It was found that BDB crystalliz-
es with two crystallographically different crystal structures,
i.e., A- and B-forms, depending on the solvent used for
recrystallization. The single crystals for A- and B-forms
are prepared from THF and ethanol solution, respectively.
In this study, sample codes for single crystals prepared
from THF and ethanol solution are denoted as Samples I
and II, respectively. Undoubtedly, Sample I consists of
˚
a (A)
17.4534(13)
7.0324(5)
19.3742(14)
90
90
90
2378.0(3)
8
1.599
0.134
1200
10.4691(8)
9.4703(7)
12.5781(10)
90
102.525(2)
90
1217.38(16)
4
1.562
0.130
600
0.5 · 0.5 · 0.4
2.30–26.37
6886
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
Volume (A )
Z
˚
3
3
Calculated density (g/cm )
Absorption coefficient (mm
F(000)
Crystal size (mm)
h range for data collection (ꢁ) 2.10–26.37
Reflections collected
ꢀ1
)
0.5 · 0.4 · 0.4
12,814
Independent reflections
2428 (Rint = 0.0305) 2483
int = 0.0280)
(
R
Data/restraints/parameters
Goodness-of-fit on F
2428/0/238
1.027
2483/0/272
1.031
2
O N
NO2
2
Final R indices [I > 2 r(I)]
R1 = 0.0318
wR2 = 0.0830
R1 = 0.0411
wR2 = 0.0886
0.218 and ꢀ0.152
R1 = 0.0415
wR2 = 0.1052
R1 = 0.0576
wR2 = 0.1142
0.210 and ꢀ0.331
R indices (all data)
HOCH CH HN
NHCH CH OH
2
2
2
2
Largest differences peak and
Fig. 1. Chemical structure of 1,5-bis(hydroxyethylamino)-2,4-dinitroben-
zene (BDB).
ꢀ3
˚
hole (e A
)

72
S.C. Lee et al. / Journal of Molecular Structure 825 (2006) 70–78
2
.3. Characterization of thermochromic behavior
lengths and angles are listed in Table 2. Molecular struc-
tures and perspective packing views of A- and B-forms
are also shown in Figs. 2 and 3, respectively. Both molecu-
lar structures exhibit intramolecular hydrogen bonding
interactions between nitro-oxygen atoms and adjacent ami-
no-protons. The A-form shows another intramolecular
hydrogen bonding interaction arising from two hydroxyl
groups of amino-alcohol chains. To the contrary, similar
hydrogen bonding is not found in the B-form. The crystal
structure of A-form also reveals that flexible amino-alcohol
chains are well ordered due to the intermolecular non-clas-
sical hydrogen bonding interactions [31] between hydrogen
atoms attached C8 and C10 and the nitro-oxygen atoms in
neighbors, as well as intramolecular hydrogen bonding
interactions between two hydroxyl groups of amino-alco-
hol chains. On the other hand, one of flexible amino-alco-
hol chains with C9 and C10 in the B-form crystal is
disordered into the two positions, as shown in Fig. 3a.
The relative probability of atomic positions of N4–C9–
C10–O6 and N4–C9 –C10 –O6 in the B-form crystal was
estimated to be 0.64–0.36, respectively. In both A- and B-
form crystals, all the nitro and amino groups except
hydroxyl ethyl chains are nearly coplanar with the benzene
ring as can be seen in Figs. 2a and 3a, resulting in facile
intra-hydrogen bonding interactions. The hydroxy ethyl
groups in the A-form have different torsional angles from
those in the B-form, leading to the different molecular
Temperature-dependent X-ray powder diffractograms of
Samples I and II were obtained using a wide-angle X-ray
diffractometer (GADDS, Bruker) equipped with a general
area detector and heating accessary.
Thermal properties of BDB crystals were measured with
a Perkin-Elmer DSC Pyris-1 equipped with an intercooler
system. Temperature and heat flow were calibrated using
the high-purity indium standard (156.6 ꢁC and 28.45 J/g).
Heating and cooling rates were 2 or 10 ꢁC/min.
For BDB crystalline film cast on a glass plate, visible
spectra were obtained as a function of temperature or time
using a UV/vis reflectance spectrophotometer (ColorEye
3
100, Gretag Macbeth, USA). Infrared spectra of BDB
crystal powder in KBr pellet were obtained using a Per-
kin-Elmer 2000 FT-IR spectrometer equipped with a cus-
tom-designed temperature-controlling accessory. Each
infrared spectrum at different temperature was obtained
ꢀ
1
0
0
by averaging 128 scans with the resolution of 2 cm
.
3
. Results and discussion
3
.1. Molecular and crystal structures
Crystal structures of BDB A- and B-forms were identi-
fied using single-crystal X-ray crystallography. Their bond
Table 2
˚
Bond lengths (A) and angles (ꢁ) in A- and B-form crystals
˚
Bond length (A)
Bond angle (ꢁ)
Bond
A-form
B-form
Bond
A-form
B-form
O1–N1
O2–N1
O3–N2
O4–N2
O5–C8
O6–C10
O6–C10
N1–C1
N2–C5
N3–C2
N3–C7
N4–C4
N4–C9
1.2375(16)
1.2307(15)
1.2265(16)
1.2356(15)
1.416(2)
1.2364(18)
1.2220(19)
1.2318(18)
1.239(2)
1.424(2)
1.547(6)
1.457(6)
1.437(2)
1.437(2)
1.340(2)
1.452(2)
1.346(2)
1.461(4)
1.639(8)
1.377(2)
1.431(2)
1.396(2)
1.393(2)
1.434(2)
1.378(2)
1.505(2)
1.516(6)
1.505(10)
O2–N1–O1
O2–N1–C1
O1–N1–C1
O3–N2–O4
O3–N2–C5
O4–N2–C5
C2–N3–C7
C4–N4–C9
121.52(12)
119.30(12)
119.18(11)
121.17(11)
119.52(11)
119.30(11)
125.06(12)
125.64(12)
–
120.41(11)
116.74(12)
122.84(11)
121.09(12)
122.57(12)
116.34(11)
124.92(12)
121.17(12)
122.81(12)
116.02(11)
116.29(12)
120.44(12)
123.26(11)
121.83(12)
116.05(14)
115.50(14)
114.16(12)
113.96(12)
–
121.49(15)
119.03(14)
119.48(13)
121.74(14)
119.19(15)
119.06(14)
123.27(15)
123.54(19)
115.5(3)
120.51(14)
117.00(14)
122.45(14)
120.13(15)
123.52(14)
116.35(14)
124.70(16)
120.64(16)
123.14(16)
116.21(15)
116.80(14)
120.56(14)
122.63(15)
121.54(15)
110.47(15)
111.87(16)
104.7(3)
1.4268(18)
–
0
1.4335(17)
1.4312(17)
1.3491(17)
1.4542(19)
1.3465(17)
1.4581(17)
–
1.3757(18)
1.4397(19)
1.3936(18)
1.3985(18)
1.4416(18)
1.3798(18)
1.525(2)
0
C4–N4–C9
C6–C1–C2
C6–C1–N1
N1–C1–C2
N3–C2–C3
N3–C2–C1
C3–C2–C1
C2–C3–C4
N4–C4–C3
N4–C4–C5
C3–C4–C5
C6–C5–N2
C6–C5–C4
N2–C5–C4
C1–C6–C5
N3–C7–C8
O5–C8–C7
N4–C9–C10
O6–C10–C9
0
N4–C9
C1–C6
C1–C2
C2–C3
C3–C4
C4–C5
C5–C6
C7–C8
C9–C10
1.518(2)
–
0
0
C9 –C10
104.0(3)
97.4(5)
103.6(5)
0
0
O6–C10 –C9
0
0
C10 –C9 –N4
–

S.C. Lee et al. / Journal of Molecular Structure 825 (2006) 70–78
73
a
O2
O3
H6A
N1
N2
C6
O1
O4
C1
C2
C5
C4
H3
H4
C3
N3
C7
N4
C9
H3A
H7B
H7A
H9B
H10A
H9A
O6
H8B
O5
C10
H10B
C8
H6
H8A
H5
b
3.34(1)
3
.35(1)
Fig. 2. (a) Molecular structure of the A-form with displacement ellipsoid of the 50% probability level. (b) Perspective packing view of the A-form down the
crystallographic a axis. Hydrogen atoms are omitted for clarity in the packing diagram.
packing, when Fig. 2b is compared with 3b. Nonetheless,
the molecular units are almost parallel to one another
due to the aromatic p–p interaction between benzene rings,
regardless of crystalline forms. The inter-planar distances
between benzene rings in the A-form are somewhat shorter
than those in the B-form (Figs. 2b and 3b), resulting in the
dense molecular packing in the A-form (Table 1). The dot-
ted lines in Figs. 2 and 3 for asymmetric units and molec-
ular packing of A- and B-forms indicate the intra- and
intermolecular hydrogen bonding. Structural parameters
for the intra- and intermolecular hydrogen bonds in A-
and B-forms are summarized in Tables 3 and 4,
respectively.
when heated above 140 ꢁC, regardless of the crystalline
form of A or B. When the crystals are cooled down to room
temperature, yellow color is recovered for a specific period
of time, indicating that the thermochromism of BDB is
reversible. It is noteworthy that this thermochromic
behavior occurs in the solid state without melting. The vis-
ible spectrum of the thermochromic behavior was also
obtained by measuring spectral reflectance (R) and by cal-
culating color strength via the Kubelka–Munk equation
2
[K/S = (1 ꢀ R) /2R]. When the K/S value (K, absorption
coefficient; S, scattering coefficient) is plotted against wave-
length, as shown in Fig. 4, it is realized that the absorption
band at 480 nm is dominant below 140 ꢁC and that the
band at 500 nm becomes predominant above 140 ꢁC
(Fig. 4a). This spectral shift from 480 to 500 nm with
temperature (bathochromic or red shift) corresponds to
the change of color from yellow to orange in the solid
state. When the heated crystalline film is cooled to room
3
.2. Thermochromic behavior
The BDB crystals exhibit optically yellow color at room
temperature. This yellow color changes quickly to orange

74
S.C. Lee et al. / Journal of Molecular Structure 825 (2006) 70–78
a
O3
O2
N2
C6
N1
O4
O1
C5
C1
C4
H4
C2
N3
H3
N4
C9
C3
C9
C10
C7
C10
O5
C8
H5
H6
O6
b
3.447(4)
Fig. 3. (a) Molecular structure of the B-form with displacement ellipsoid of the 50% probability level. (b) Perspective packing view of the B-form down the
crystallographic b axis. Hydrogen atoms and disordered part are omitted for clarity in the packing diagram.
Table 4
Table 3
Intra- and intermolecular hydrogen bonding geometry in B-form crystal
Intra- and intermolecular hydrogen bonding geometry in A-form crystal
D-Hꢁ ꢁ ꢁA
D-H
Hꢁ ꢁ ꢁA
Dꢁ ꢁ ꢁA
D-Hꢁ ꢁ ꢁA
D-Hꢁ ꢁ ꢁA
D-H
Hꢁ ꢁ ꢁA
Dꢁ ꢁ ꢁA
D-Hꢁ ꢁ ꢁA
N3–H3ꢁ ꢁ ꢁO1
0.85(2)
0.82(2)
0.87(3)
0.85(3)
0.97(2)
0.97(2)
1.95(2)
1.98(2)
2.05(3)
1.90(3)
2.52(2)
2.57(2)
2.644(2)
2.637(2)
2.893(2)
2.746(2)
3.414(2)
3.142(2)
137(2)
136(2)
162(3)
170(2)
154(2)
118(1)
O6–H6ꢁ ꢁ ꢁO5
N3–H3ꢁ ꢁ ꢁO1
N4–H4ꢁ ꢁ ꢁO4
C8–H8Aꢁ ꢁ ꢁO2
0.84(2)
0.85(2)
0.86(2)
0.97(2)
0.98(2)
0.82(2)
1.98(2)
2.06(2)
2.02(2)
2.55(2)
2.59(2)
1.93(3)
2.812(2)
2.691(2)
2.665(2)
3.384(2)
3.391(2)
2.753(2)
175(2)
131(1)
131(2)
145(2)
138(1)
175(2)
N4–H4ꢁ ꢁ ꢁO4
(
1)
2)
O5–H5ꢁ ꢁ ꢁO6
O6–H6ꢁ ꢁ ꢁO5
(3)
(
(
1)
(3)
C8 –H8A ꢁ ꢁ ꢁO4
(1)
(4)
(4)
C10–H10Bꢁ ꢁ ꢁO3
C8 –H8A ꢁ ꢁ ꢁO2
(2)
O5–H5ꢁ ꢁ ꢁO6
Symmetry codes: (1) 0.5 ꢀ x, 0.5 + y ꢀ 1, 0.5 ꢀ z + 1; (2) x, y + 1, z; (3)
Symmetry codes: (1) x, 0.5 ꢀ y, 0.5 + z ꢀ 1; (2) 1 ꢀ x, 0.5 + y, 0.5 ꢀ z.
0.5 + x, 0.5 ꢀ y + 1, 0.5 + z ꢀ 1; (4) 0.5 + x, 0.5 ꢀ y, 0.5 + z ꢀ 1.

S.C. Lee et al. / Journal of Molecular Structure 825 (2006) 70–78
75
a
6
5
4
3
2
1
0
Sample I
o
25 C
o
1
60 C
o
1
20 C
o
25 C
o
1
50 C
5
10
15
20
25
30
35
40
4
00
450
500
550
600
650
700
2
θ (degree)
Wavelength (nm)
b
6
5
4
3
2
1
0
Sample II
1
6
8
1
2
hr
o
2
5 C
hrs
hrs
2 hrs
4 hrs
o
1
50 C
5
10
15
20
25
30
35
40
400
450
500
550
600
650
700
2
θ (degree)
Wavelength (nm)
Fig. 5. X-ray powder diffractograms of Samples I and II at 25 and 150 ꢁC.
Fig. 4. Visible spectra of BDB crystalline films: (a) temperature-dependent
spectra; (b) time-dependent spectra. The optical images in (a) were taken
at 25 and 160 ꢁC. The spectra in (b) were obtained at 25 ꢁC after cooling
from 160 ꢁC.
the molecular modelling and the quantum mechanical cal-
culation based on the density functional theory, and will be
reported in future.
temperature, the original visible spectrum at room temper-
ature was recovered for a period of time, as shown in
Fig. 4b.
The thermochromic transition behavior of BDB crystals
is also characterized by differential scanning calorimetry
(DSC). Fig. 6 shows the DSC heating and cooling thermo-
grams of Samples I and II. The heating thermograms show
two endothermic peaks at 127 and 211 ꢁC for Sample I and
at 140 and 211 ꢁC for Sample II. Integrating all the data of
visible spectra, DSC and X-ray diffraction leads us to con-
clude that the endothermic peaks at 127 ꢁC for Sample I
and at 140 ꢁC for Sample II are attributed to the crystal-
to-crystal transition (from A- or B-form to C-form) for
the thermochromism and the peak at 211 ꢁC corresponds
to the melting point of the C-form crystal transformed
from A- or B-form crystal. It is believed that the different
In order to investigate the change of crystal structure at
the thermochromic transition, X-ray diffractograms of
Samples I and II were obtained as a function of tempera-
ture, as shown in Fig. 5. X-ray diffractograms of both sam-
ples exhibit sharp crystalline diffraction peaks before and
after the thermochromic transition. It is noticeable that
the X-ray diffractogram of Sample I is identical with that
of Sample II above the thermochromic transition tempera-
ture. These results lead us to conclude that the thermochro-
mism of BDB occurs via the crystal-to-crystal transition
and that the crystals of A- and B-forms are transformed
to an identical crystal designated as the C-form hereafter.
Here it should be noted that it is not possible to identify
the crystal structure of C-form developed above the ther-
mochromic transition temperature by the X-ray diffraction
method used in this study, because it is very difficult to
obtain a single crystal of the C-form. The molecular and
crystalline structure of C-form is under investigation using
crystal-to-crystal transition temperatures (T ) and enthal-
cc
o
cc
pies ðDH Þ of Samples I and II originate from different
molecular packing in A- and B-forms. On the other hand,
the almost same melting temperature (T ꢂ 211 ꢁC) and
m
o
m
enthalpy ðDH ꢂ 5:8 kcal=molÞ of the crystals transformed
from Samples I and II indicate that the C-form crystals
transformed from the A- or B-form are identical, which is
exactly consistent with the X-ray diffraction results. Since

76
S.C. Lee et al. / Journal of Molecular Structure 825 (2006) 70–78
Sample I
o
1
98 C
a
5.2 kcal/mol
b
c
d
e
f
o
1
27 C
o
2
11 C
5
.6 kcal/mol
5
.8 kcal/mol
0
50
100
150
200
250
o
Temperature ( C)
g
Sample II
o
2
00 C
5.1 kcal/mol
5
0
100
150
Temperature ( C)
200
250
o
Fig. 7. DSC heating curves at the heating rate of 10 ꢁC/min. The sample
was first kept at 160 ꢁC for 3 min, cooled to ꢀ15 ꢁC, and maintained for a
specified period of time: (a) 1 h; (b) 3 h; (c) 16 h; (d) 24 h; (e) 48 h; (f) 73 h;
(g) 168 h.
o
1
40 C
4
.2 kcal/mol
o
2
11 C
5
.9 kcal/mol
the crystal structure of C-form could not be determined by
the X-ray diffraction method as mentioned above, the tem-
perature-dependent infrared spectroscopy is inevitably
employed to investigate the configurational change during
the thermochromic transition. Fig. 8 shows infrared spectra
of Samples I and II at 25 and 150 ꢁC. Close examination of
Fig. 8 reveals that the infrared spectra of Samples I and II
at 25 ꢁC are slightly different from each other, since the
molecular conformation and the crystal packing of A-form
are different from those of B-form. On the other hand, the
spectra of Samples I and II at 150 ꢁC are nearly identical,
indicating that the crystal structures of Samples I and II
above Tcc are identical, which is in agreement with the
results from X-ray diffraction and thermal analysis. Table
5 summarizes the assignment of infrared bands of BDB
0
50
100
150
200
250
o
Temperature ( C)
Fig. 6. DSC heating and cooling thermograms for Samples I and II at the
heating and cooling rates of 2 ꢁC/min.
the cooling thermograms in Fig. 6 show only one exother-
mic peak at around 200 ꢁC, it is assumed that the exother-
mic peak corresponds to the crystallization from the melt
to the C-form crystal. Indeed, the second heating thermo-
gram of the just-cooled sample shows an endothermic melt-
ing peak at 211 ꢁC, which corresponds to the melting of the
C-form crystal, as shown in Fig. 7a. As the holding time at
ꢀ
15 ꢁC increases, two endothermic peaks at 127 and
crystals below and above T , which may provide some
cc
1
40 ꢁC appear and their intensities increase, as can be seen
information on the configurational change at the thermo-
chromic transition. First, two NH stretching bands at
in Figs. 7b–g. It reveals that the C-form crystal transforms
to the A- and/or B-form for the period of time at the hold-
ing temperature. Therefore, it is concluded that the two
endothermic peaks at 127 and 140 ꢁC in Fig. 7 arise from
the transformation of A- and B-form crystals regenerated
at the holding temperature to the C-form. In addition, it
is realized from the relative area of endothermic peaks at
ꢀ
1
3388 and 3346 cm
for Samples I and at 3388 and
ꢀ1
3367 cm for Sample II below T are merged into a single
broader band of 3365 cm above T . Here two sharp NH
cc
ꢀ1
cc
stretching bands of respective A- and B-forms indicate that
two hydroxyl ethyl groups have different conformations.
Second, the intensity of C@C stretching band of benzene
ꢀ
1
1
27 and 140 ꢁC that the transformation from the C-form
ring at 1620 cm decreases significantly above T , while
cc
ꢀ
1
to the A-form crystal is larger than the transformation
from the C-form to the B-form crystal.
a new strong C@N stretching band at 1642 cm appears.
Third, the intensity of the NO stretching bands at 1545,
2
ꢀ
1
Now one may have a question whether or not any con-
figurational change of BDB occurs during the thermochro-
mic transition. The answer might not be clearly unravelled
until the crystal structure of C-form is fully identified. Since
1480, and 1347 cm becomes much weaker above T .
cc
ꢀ
1
Finally, a new band at 3550 cm appears, as the tempera-
ture increases from 25 to 150 ꢁC. The new band at
ꢀ1
3550 cm is believed to arise from the OH stretching of

S.C. Lee et al. / Journal of Molecular Structure 825 (2006) 70–78
77
1620
newly formed NAOH groups. Based on the change of char-
acteristic infrared bands, it is speculated that the conforma-
Sample I
3
346
3388
1347
1
545
tion of BDB crystal changes from a nitro-form below T to
o
cc
2
5 C
3260
1480
an acid-form above T , as schematically shown in Fig. 9.
cc
As described above, there exist intra- and intermolecular
hydrogen bonds between hydroxyl, amino, and nitro
groups in each crystal structure of A- and B-forms below
1619
1642
1347
o
T . However, the intramolecular distance between amine
cc
3365
150 C
1545
3
550
proton and the oxygen atom of the nitro group is closer
than any other intermolecular distances for both A- and
B-form crystals, as listed in Tables 3 and 4. Therefore, it
is expected that the intramolecular hydrogen transfer from
the amine proton to the nitro-oxygen takes place domi-
nantly during the thermochromic transition, leading to
the configurational transformation from nitro-form to
acid-form. This thermochromic mechanism of BDB seems
to be quite similar to the one of Schiff-bases, which occurs
in the solid state through the configurational transforma-
tion between two equilibrium species via hydrogen transfer.
In short, it is valid to conclude from the results of X-ray
diffraction, DSC, and infrared spectra that the first order
crystal-to-crystal transition of BDB occurs simultaneously
with configurational transformation at the solid-state ther-
mochromic transition.
3
260
1480
3
600
3300
3000 1800
1500
1200
900
-
1
Wavenumber (cm )
Sample II
1620
o
1
347
25 C
3
367
1
545
3388
3
260
1480
1
619
1642
3
365
1347
o
1
50 C
3
550
1545
3260
1480
4. Conclusions
A novel organic crystal of 1,5-bis(hydroxyethylamino)
2,4-dinitrobenzene (BDB) was prepared, and its
3
600
3300
3000 1800
1500
1200
900
-
-
1
Wavenumber (cm )
thermochromic mechanism was investigated using X-ray
diffraction, DSC, visible and infrared spectroscopy.
BDB crystals were developed into two different crystal
structures (A- and B-forms) at room temperature,
depending on the solvent used for recrystallization. Both
A- and B-form crystalline powders exhibited reversible
change of color from yellow to orange in the solid state.
The thermochromic transition temperatures, which corre-
spond to the crystal-to-crystal transition temperatures,
were determined to be 127 ꢁC for A-form and 140 ꢁC
for B-form. It was found that both crystals of A- and
B-forms were transformed to the C-form crystal at the
thermochromic transition. Comparison of the infrared
spectra before and after the thermochromic transition
revealed that the configurational transformation between
nitro-form and acid-form via intramolecular hydrogen
Fig. 8. FT-IR spectra of Samples I and II at 25 and 150 ꢁC.
Table 5
Assignment of characteristic infrared bands of BDB crystals below and
above the thermochromic transition temperature
ꢀ
1
Wavenumber (cm
)
Assignment
Crystalline form
3
3
3
3
1
1
1
1
1
550
m(OH) from @ NAOH
m(NH)
m(NH)
C
388, 3367, 3346
A and/or B
C
365
260
642
620
545
480
347
m(OH)
m(CAN)
A, B, and C
C
m(CAC)
A, B, and C
A, B, and C
A, B, and C
A, B, and C
m(NO
m(CAC), m(NO
m(NO
2
)
2
)
2
)
O
N
O
O
O
N
O
N
N
O
H
O
H
O
H
H
N
N
N
N
HOCH CH2
CH CH OH
2 2
2
HOCH CH
CH CH OH
2 2
2
2
Nitro-form (below Tcc)
Acid-form (above Tcc)
Fig. 9. Schematic description for configurational transformation between nitro-form to acid-form at the thermochromic transition temperature (Tcc).

78
S.C. Lee et al. / Journal of Molecular Structure 825 (2006) 70–78
transfer occurs at the thermochromic transition. There-
fore, it is suggested that the thermochromic transition
of BDB in the solid state arises from the crystal-to-crys-
tal transition accompanying with the configurational
transformation.
[8] G. Castaldi, P. Allegrini, R. Fusco, L. Longo, V. Malatesta, J. Chem.
Soc. Chem. Commun. (1991) 1257.
[9] P.N. Day, Z.Q. Wang, R. Rachter, J. Phys. Chem. 99 (1995) 9730.
[
10] H.-J. Suh, W.-T. Lim, J.-Z. Cui, H.-S. Lee, G.-H. Kim, N.-H. Heo,
S.-H. Kim, Dyes Pigments 57 (2002) 149.
[11] M.D. Cohen, G.M.J. Schmidt, J. Phys. Chem. 56 (1962) 2442.
[
[
[
12] M.D. Cohen, B. Schmidt, S. Flavian, J. Chem. Soc. (1964) 2041.
13] J.W. Ledbetter Jr., J. Phys. Chem. 81 (1977) 54.
14] N. Hoshino, T. Inabe, T. Mitani, Y. Maruyama, Bull. Chem. Soc.
Jpn. 61 (1988) 4207.
15] S.M. Aldoshin, L.O. Atovmyan, O.S. Filipenko, in: Proceedings of
the 14th Conference on Applied Crystallography, vol. 2, 1990,
483.
5
. Supporting information available
CCDC 289537 and 289538 contain the supplementary
[
[
[
crystallographic data for BDB A- and B-forms in CIF for-
mat, respectively. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the
Cambridge Crystallographic Data Centre, 12, Union
Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033).
16] T. Inabe, I. Gautier-Luneau, N. Hoshino, K. Okaniwa, H. Okamoto,
T. Mitani, U. Nagashima, Y. Maruyama, Bull. Chem. Soc. Jpn. 64
(
1991) 801.
17] S.M. Aldoshin, I.I. Chuev, Crystal Chemical Design of Phototropic
Systems with Intra- and Inter-molecular Transfer of Protons: Results
and Prospects for Practical Application, vol. 6, International Union
of Crystallography/Oxford University Press, Oxford, 1992.
Acknowledgement
[
[
18] A. Takase, S. Sakagami, K. Nonaka, T. Koga, J. Raman Spectrosc.
This work was supported by Korea Research Founda-
tion Grant (KRF-2004-002-D00476).
2
4 (1993) 447.
19] E. Lambi, D. Gegiou, E. Hadjoudis, J. Photochem. Photobiol., A:
Chem. 86 (1995) 241.
Appendix A. Supplementary data
[20] E. Hadjoudis, A. Rontoyianni, K. Ambroziak, T. Dziembowska, I.M.
Mavridis, J. Photochem. Photobiol., A: Chem. 162 (2004) 521.
[
21] T. Fujiwara, J. Harada, K. Ogawa, J. Phys. Chem. B. 108 (2004)
035.
22] I. Agranat, Y. Tapuhi, J. Org. Chem. 44 (1979) 1941.
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.molstruc.
4
[
2
006.04.025.
[23] Y. Tapuhi, O. Kalisky, I. Agranat, J. Org. Chem. 44 (1979) 1949.
[
[
[
[
[
24] J.J. Stezowski, P.U. Biedermann, T. Hildenbrand, J.A. Dorsch, C.J.
Eckhardt, I. Agranat, J. Chem. Soc. Chem. Commun. (1993) 213.
25] E. Hadjoudis, Photochromism: Molecules and Systems, Elsevier,
Amsterdam, 1990.
26] K.A. Muszkat, Chemistry of Quinoid Compounds, vol. 2, John
Wiley, Chichester, 1988.
References
[
[
1] J.H. Day, Chem. Rev. 63 (1963) 65.
2] A. Samat, V. Lokshin, in: J.C. Crano, R.J. Guglielmetti (Eds.),
Organic Photochromic and Thermochromic Compounds, vol. 2,
Kluwer Academic/Plenum Publishers, New York, 1999.
3] L. Tansienhee, D. Lavabre, G. Levy, J.C. Micheau, New J. Chem. 13
27] R. Korenstein, K.A. Muszkat, S. Sharafy-Ozeri, J. Am. Chem. Soc.
9
5 (1973) 6177.
[
28] S. Mataka, K. Takahashi, M. Tashiro, W.H. Lin, S. Iwasaki, T.
Tsutsui, S. Saito, S. Akiyama, T. Yonemitsu, J. Heterocyclic Chem.
(1989) 227.
[
[
4] M.A. White, M. LeBlanc, J. Chem. Edu. 76 (1999) 1201.
5] D. Ollis, K.L. Ormand, I.O. Sutherland, J. Chem. Soc. Chem.
Commun. (1968) 1697.
2
6 (1989) 215.
[
29] H. Quast, T. Herkert, A. Witzel, E.M. Peters, K. Peters, H.G.
Vonschnering, Chem. Ber. 127 (1994) 921.
30] G.M. Sheldrick, University of Gottingen, Germany, 1997.
31] J.D. Dunitz, A. Gavezzotti, Angew. Chem. Int. Ed. 44 (2005)
[
[
6] F. Zerbetto, S. Monti, G. Orlandi, J. Chem. Soc. Faraday Trans. 80
[
[
(
1984) 1513.
7] S.P. Makarov, B.Y. Simkin, V.I. Minkin, Chem. Heterocycl. Compd.
1988) 140.
1
766.
(