J-aggregate structure in a chloroform solvate of a 2,3-dicyanopyrazine dye - Separation of two-dimensional stacking dye layers by solvate formation

Article abstract of DOI:10.1016/j.dyepig.2012.04.014

Here, we report the J-aggregate structure of a chloroform solvate of 5-t-butyl-2,3-dicyano-6-[4-(dimethylamino)styryl]-pyrazine with a strong intramolecular charge transfer system. The dye was found to form a two-dimensional brick-wall structure separated

Full text of DOI:10.1016/j.dyepig.2012.04.014

Dyes and Pigments 95 (2012) 431e435
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Short communication
J-aggregate structure in a chloroform solvate of a 2,3-dicyanopyrazine dye e
Separation of two-dimensional stacking dye layers by solvate formation
,
b
a
a
c
Shinya Matsumoto a , Emi Horiguchi-Babamoto , Ryohei Eto , Saori Sato , Takashi Kobayashi ,
*
Hiroyoshi Naito c, Motoo Shiro d, Hiromi Takahashi e
a Department of Environmental Sciences, Graduate School of Environment and Information Sciences, Yokohama National University, 79-2 Tokiwadai, Hodogaya-ku,
Yokohama 240-8501, Japan
b Department of Pharmaceutical Sciences, Faculty of Pharmaceutics, Musashino University, 1-1-20 Shinmachi, Nishitokyo-shi, Tokyo 202-8585, Japan
c Department of Physics and Electronics, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Nakaku, Sakai, Osaka 599-8531, Japan
d X-ray Research Laboratory, Rigaku Corporation, 3-9-12, Matsubaracho, Akishima-shi, Tokyo 196-8666, Japan
e System Instruments Co., Ltd., 776-2 Komiyamachi, Hachioji-shi, Tokyo 192-0031, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Here, we report the J-aggregate structure of a chloroform solvate of 5-t-butyl-2,3-dicyano-6-[4-(dime-
thylamino)styryl]-pyrazine with a strong intramolecular charge transfer system. The dye was found to
form a two-dimensional brick-wall structure separated by chloroform molecules in the solvated crystals,
which exhibited intense red fluorescence. The absorption maximum of the solvate was also found to
show a bathochromic spectral shift. These observed optical characteristics were interpreted in terms of
excitonic intermolecular interaction based on two-dimensional brick-wall structure which is one of the
proposed structures for J-aggregates.
Received 21 March 2012
Received in revised form
24 April 2012
Accepted 25 April 2012
Available online 3 May 2012
Keywords:
J-aggregates
Ó 2012 Elsevier Ltd. All rights reserved.
Pyrazine dye
Chloroform solvate
Red fluorescence
Exciton interaction
Crystal engineering
1. Introduction
considered on the basis of theoretical treatments [2]. There are also
several reports on photographic images of J-aggregates using
J-aggregates are one of the well-known supramolecular systems
of organic dyes [1]. They exhibit an intense, sharp absorption band
with resonance fluorescence in the longer wavelength region
compared with those of a component dye monomer. These
particular spectral features may be ascribed to a resonance
quantum effect known as exciton interaction within the aggregates
[2]. A representative example of their practical applications is as
a photosensitizer for colour photographic films [1]. This industrial
opportunity stimulated research into J-aggregates. One of the main
research interests in J-aggregates is the structure. The structure of J-
aggregates was first proposed with respect to the surface adsorp-
tion of J-aggregates on ionic substrates [3]. With much experi-
mental evidence of the formation of J-aggregates on the surface of
ionic substrates [1], the structure of J-aggregates was further
microscopic techniques [4]. Comprehensive analyses of the crystal
structure of cyanine dyes showed that many cyanine dyes crystal-
lize in a
pep stacking structure and the proposed one- or two-
dimensional molecular arrangement of J-aggregates is recognized
as a part of their three-dimensional crystal structures [5]. However,
the detailed structure and dimensionality of J-aggregates are still
unknown except for the X-ray structure of a J-aggregate LB-film of
a merocyanine dye reported by Kato et al. [6] The size of J-aggre-
gates is also controversial: a range of some molecules to some
thousands molecules have been reported as the number of
component molecules within one J-aggregate [1e,7]. An alternate
opinion is that J-aggregates are a macroscopic molecular assembly
composed of meso-scale molecular assemblies [8]. The reported
analysis of the J-aggregated LB-film [6] demonstrated that a two-
dimensional infinite dye network is also a form of J-aggregates,
whilst that film may possess structural defects like mosaicity in
single crystals.
* Corresponding author. Tel.: þ81 45 339 3366; fax: þ81 45 339 3345.
E-mail address: smatsu@edhs.ynu.ac.jp (S. Matsumoto).
0143-7208/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2012.04.014

432
S. Matsumoto et al. / Dyes and Pigments 95 (2012) 431e435
In spite of the lack of clarity of the structure of J-aggregates, their
2.2. Synthesis of 5-t-butyl-2,3-dicyano-6-[4-(dialkylamino)styryl]
pyrazine
spectral features have attracted much attention in state-of-the-art
applications of organic dyes such as optical materials [7c,7d,9],
organic solar cells [10], and organic light-emitting devices [4e,11].
Stable J-aggregates in solid films [12] or in powdered form are
required for these applications. It is also very important that these
bulk J-aggregates can be prepared through a conventional method.
To achieve these demands, J-aggregate formation in a stable crys-
talline state [13] is a promising solution even though there is
currently no design strategy for obtaining J-aggregate crystals.
Here, we report the crystal structure and solid-state optical prop-
erties of a chloroform solvate of a 2,3-dicyanopyrazine dye as an
example of a potential crystal structure towards the design of
J-aggregate crystals.
To a solution of 5-t-butyl-2,3-dicyano-6-methylpyrazine (2.00 g,
10 mmol) in benzene (50 mL) was added piperidine (0.3 mL,
3 mmol) and 4-(dimethylamino)benzaldehyde (1.49 g, 10 mmol).
The mixture was refluxed for 37 h. The reaction mixture was cooled
to room temperature and then poured into a 2% HCl solution. The
precipitate was filtered off and the filtrate was extracted with EtOAc
three times. The combined organic layers were washed with brine
and dried over Na2SO4. After removing the solvent, the residue was
isolated by silica gel column chromatography (benzene) to give
a solid product. This solid and the precipitate obtained by filtration
were mixed and further purified by recrystallization from chloro-
form to obtain the target compound (1.56 g, 31%) as reddish brown
crystals. Yield 31%; mp 263e264 ꢀC; Anal. Calcd. for C20H21N5: C,
72.48; H, 6.39; N, 21.13%. Found: C, 71.92; H, 6.28; N, 20.97%; IR
2. Experimental section
(KBr)/cmꢁ1 2225 (CN); 1H NMR (CDCl3)
d 1.54 (s, 9H), 3.10 (s, 6H),
6.74 (d, J ¼ 8.0 Hz, 2H), 7.29 (d, J ¼ 16.0 Hz, 1H), 7.55 (d, J ¼ 8.0 Hz,
2.1. Materials and equipments
2H), 8.03 (d, J ¼ 16.0 Hz, 1H); 13C NMR (CDCl3)
d 29.4, 39.3, 40.2,
112.1, 113.8, 114.3, 116.3, 123.4, 126.0, 130.1, 130.3, 143.0, 152.1, 152.9,
4-(Dimethylamino)benzaldehyde was purchased from Hayashi
Pure Chemical Industries Ltd. 5-t-Butyl-2,3-dicyano-6-methyl-
pyrazine [14] was prepared as described in the literature.
162.8; MS (EI) m/z 331 (Mþ).
Melting points were measured using
a
Yanaco MP-500P
2.3. X-ray structural analysis
micro-melting-point apparatus. NMR spectra were obtained by
a Jeol JNM-ECX-400 spectrometer. EIMS spectra were recorded
on a Jeol JMS-GC mateII spectrometer. IR spectra were recorded
on a Jasco FT-IR-4100 system for samples in a KBr pellet form.
The elemental analysis was performed with an Elementar Vario
EL III elemental analyzer. UVevis absorption and fluorescence
spectra in solution were taken on a Shimadzu UV3100PC UV/vis
spectrometer and a Shimadzu RF5301PC fluorescence spectrom-
eter, respectively. Solid-state absorption spectra were measured
using a System Instruments SIS-50 surface and interface spec-
trometer based on optical waveguide spectrometry. Fluorescence
quantum efficiency in the solid-state is determined using a blue
diode laser (CrystaLaser), an integrating sphere (Labsphere), and
photo-multichannel analyzer (Hamamatsu, PMA-11). After
measured a reflected laser beam from a quarts substrate, we
measured a reflected laser beam and fluorescence from the
crystals placed on the substrate. Then, we calculated photon
numbers of the fluorescence and the adsorbed laser beam.
Fluorescence quantum efficiency was obtained as a ratio between
the two-photon numbers [15]. Electronic states of this dye in the
solvate were estimated by a semi-empirical molecular orbital
calculation. The ground state dipole moment and the transition
dipole moment were calculated by using the AM1 and the INDO/
S Hamiltonians, respectively, on the basis of the crystal structure.
These calculations were performed with a CAChe 5.2 program
package [16].
Data collection of the solvate crystal was performed by a Rigaku
RAXIS RAPID imaging plate diffractometer with graphite mono-
chromated CueKa radiation. The structure was solved by direct
method (SIR 2002 [17]) and refined by full-matrix least-squares
calculations. The non-hydrogen atoms were refined anisotropically.
All hydrogen atoms were simply located at the calculated positions
and their parameters were constrained. All calculations were per-
formed using the Crystal Structure 3.8 crystallographic software
package [18]. Crystal data: C20H21N5ꢂCHCl3, M ¼ 450.80, ortho-
rhombic, a ¼ 6.7574(5) Å, b ¼ 11.1520(6) Å, c ¼ 14.9279(9) Å,
V ¼ 1124.9 (1) Å3, T ¼ 223.1 K, space group Pmn21, Z ¼ 2, 11891
reflections measured, 2202 independent reflections (Rint ¼ 0.101).
The final R1 value (I > 2s
(I)), the final wR(F2) value (all data), and the
GOF was 0.075, 0.2163 and 0.900, respectively. The Flack parameter
was ꢁ0.05(4). The CCDC deposition number is CCDC834637.
3. Results and discussion
Dye 1, 5-t-butyl-2,3-dicyano-6-[4-(dimethylamino)styryl]-pyr-
azine, shown in Fig. 1 was synthesized as one of a series of
compounds to study the effect of the amino substituent on thin film
growth by vacuum deposition [19]. This dye system is also known
to exhibit good fluorescence properties in the crystalline state
[14,20]. We carried out single crystal growth of
1 using
liquideliquid diffusion from chloroform and c-hexane. Lustrous red
a
b
NMe
2
NC
NC
N
N
t-Bu
Fig. 1. (a) Chemical structure of 1 and (b) molecular structure of its chloroform solvate (30% probability level ellipsoids).

S. Matsumoto et al. / Dyes and Pigments 95 (2012) 431e435
433
Fig. 2. Photographs of dye 1 under (a) a room light and (b) a UV light.
platelets were observed in the glass tube within a few days, and,
after that, the red crystals transitioned into black block crystals and/
or reddish-purple flakes in the next several days. The red crystals
were found to emit brightly under a UV lamp (Fig. 2). They are
stable at about 263 K without any colour change.
We initially thought that 1 had three crystal polymorphs;
however, the red one was proved to be a chloroform solvate.
Thermogravimetric (TG) analysis revealed that the red crystals
include one chloroform molecule per dye (Fig. 3). The other two
crystals were identified as polymorphic forms without any solvent
molecules and their structures and optical properties will be
reported elsewhere.
X-ray structural analysis of the solvated crystals was carried out
at 223 K. The analytical data indicated that the structural unit is
composed of one dye molecule and one chloroform molecule,
which confirms the results of the TG analysis. The molecules are
oriented on a mirror plane parallel to the bc plane. This indicates
that the molecules have a planar conformation. Fig. 4 illustrates the
crystal structure of the solvate. The crystal structure is character-
ized by a two-dimensional stacking dye layer and a layer composed
of chloroform molecules. These two layers alternate along the b-
axis to form the three-dimensional crystal structure. Fig. 4 also
shows a portion of the two-dimensional dye layer viewed from
both the a-axis and the b-axis. The dye molecules are stacked along
the a-axis in a brick-wall fashion [2c] with a slip angle of ca. 24ꢀ.
This dye chromophore has an unsymmetrical intramolecular
charge transfer (CT) system. Therefore, 1 has a large dipole moment
in the ground state along the CT direction of the molecular plane. It
should be noted that the polar molecules are aligned in the same
direction in all the stacking layers. This is intriguing because
molecules with a large ground state dipole moment generally align
themselves to minimize electrostatic repulsion resulting in a non-
polar or less-polar crystal structure [21]. The transition dipole
moment of the visible absorption was estimated to be aligned in the
same direction as the ground state dipole moment. Therefore, this
two-dimensional layer can be regarded as a model structure for J-
aggregates, specifically the two-dimensional brick-wall structure.
The effect of the exciton interactions in this J-aggregate struc-
ture was estimated by using the extended dipole model [1d,2d,6]
on the basis of a nearest neighbour approximation. The interac-
tion from six neighbouring dye molecules in the same layer
induced a bathochromic energy shift of 991 cmꢁ1. The contribution
from two neighbouring dye molecules in the neighbouring layers
was a hypsochromic energy shift of 155 cmꢁ1. Therefore, the total
from eight neighbouring molecules resulted in a bathochromic
energy shift of 836 cmꢁ1. This suggests that this solvate has an
excitonic electronic nature resembling that of J-aggregates and this
was imparted by the two-dimensional brick-wall structure, even
though the transition dipole moments in the stacking layer are
aligned with a small declination, which may result in negligible
breaking of the symmetry limitation for light absorption based on
exciton interactions [2].
We then measured the optical properties of this solvate. The
powder samples were characterized by the crystal structure of the
solvate using powder X-ray diffraction. The absorption and fluo-
rescence spectra of 1 (in toluene) and the solvate are illustrated in
Fig. 5. Generally it is not easy to obtain an absorption spectrum of
dye crystals because dye solids have a strong extinction over the
visible region. The measurements were then performed using
optical waveguide spectroscopy [22]. Dye 1 exhibits an absorption
maximum around 480 nm in toluene with a molar extinction
coefficient of ca. 2.5 ꢃ 104. Its fluorescence in toluene was recog-
nized around 556 nm with 21% quantum efficiency, which was
measured using quinine sulphate as a reference. The Stoke’s shift is
2848 cmꢁ1. The absorption spectrum of the solvate was found to
2
1
30
20
10
0
shift drastically towards the bathochromic region. Its
l max was
around 559 nm with a broad shoulder around 460 nm. The bath-
ochromic shift of the absorption maximum from a toluene solution
to the solvate was estimated to be 2944 cmꢁ1. This spectral shift is
qualitatively in agreement with the calculated bathochromic shift
(836 cmꢁ1) based on exciton interactions. Fluorescence of the
solvate was measured by using a homemade set-up with excitation
at 375 nm. Its quantum efficiency was estimated in comparison
with purified anthracene crystals. The fluorescence spectrum
comprised a sharp fluorescence band around 619 nm and a broad
component in the longer wavelength region. Its quantum efficiency
is ca. 10%. This sharp peak was found to decrease gradually during
the measurements with a simultaneous increase of the longer
wavelength component. The longer fluorescence component was
0
-1
-2
-3
-4
-10
-20
-30
30
80
130
180
230
Temperature / oC
Fig. 3. The TG/DTA data for the solvate.

434
S. Matsumoto et al. / Dyes and Pigments 95 (2012) 431e435
Fig. 4. Crystal structure of the solvate.
We also thank Dr Shinji Ishihara at Yokohama National University
for the elemental analysis.
Toluene soln
Solvate
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