Unexpected photochemical debenzylation of 2,5-bis(dibenzylamino)-3,6-dichloro-p-benzoquinone

Article abstract of DOI:10.1016/j.dyepig.2015.05.036

Studies on the conformational polymorphism of organic dyes with bulky and flexible substituents showed that the benzyl moiety of 2,5-bis(dibenzylamino)-3,6-dichloro-p-benzoquinone can be easily eliminated under ambient light conditions to afford two analo

Full text of DOI:10.1016/j.dyepig.2015.05.036

Dyes and Pigments 121 (2015) 336e341
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Unexpected photochemical debenzylation of 2,5-bis(dibenzylamino)-
3,6-dichloro-p-benzoquinone
,
, **
Yuta Shimada a, Emi Horiguchi-Babamoto b *, Shinya Matsumoto a
a Department of Environmental Sciences, Graduate School of Environment and Information Sciences, Yokohama National University, 79-7 Tokiwadai,
Hodogaya-ku, Yokohama 240-8501, Japan
b Department of Pharmaceutical Sciences, Faculty of Pharmacy, Musashino University, 1-1-20 Shinmachi, Nishitokyo-shi, Tokyo 202-8585, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Studies on the conformational polymorphism of organic dyes with bulky and flexible substituents
showed that the benzyl moiety of 2,5-bis(dibenzylamino)-3,6-dichloro-p-benzoquinone can be easily
eliminated under ambient light conditions to afford two analogous derivatives, 2,5-bis(benzylamino)-
3,6-dichloro-p-benzoquinone and 2-amino-5-benzylamino-3,6-dichloro-p-benzoquinone. Herein, we
report the optical properties as well as the molecular and crystal structures of these three diaminodi-
chlorobenzoquinone dyes with the details of their crystallization. The responses of 2,5-
bis(dibenzylamino)-3,6-dichloro-p-benzoquinone under irradiation in solution and the crystalline
states are reported.
Received 1 April 2015
Received in revised form
25 May 2015
Accepted 28 May 2015
Available online 6 June 2015
Keywords:
Diaminodichlorobenzoquinone
Benzyl group
© 2015 Elsevier Ltd. All rights reserved.
Light-induced elimination
Crystal structure
Absorption spectra
Crystallization
1. Introduction
The light fastness of dyes and pigments is one of the important
subjects in dye chemistry from an industrial as well as scientific
Organic dyes often exhibit polymorphism in the crystalline state
[1e3]. This is an important phenomenon in pigment industries, and
recently, polymorphism in dye chemistry has attracted much in-
terest for functional dye applications, particularly in optoelectronic
applications [3e5]. We have been studying the polymorphism of
organic dyes accompanied by a color change. A series of pyrazine
dyes having a small chromophoric system with bulky and flexible
substituents was found to exhibit polymorphism with a drastic
color change [6]. Recently, we made an attempt to synthesize a
series of dyes having a small chromophoric system with bulky and
flexible substituents; 2,5-bis(dibenzylamino)-3,6-dichloro-p-ben-
zoquinone 1 was selected as one of the target dyes for polymorph
hunting (Fig. 1). Compound 1 could be easily transformed to anal-
ogous derivatives, 2,5-bis(benzylamino)-3,6-dichloro-p-benzoqui-
perspective. The light fading phenomena of dyes and dyed sub-
strates have been studied focusing on the fading mechanisms of
dyes [8e10]. There are several reports on photochemical deal-
kylation of amino groups of dyes [11,12], although an example with
a benzyl group is still unknown. A benzyl group is also well-known
as a protecting group for amino moieties in addition to that for
alcoholic hydroxy groups [13]. Catalytic hydrogenosis is a general
method for deprotecting a benzyl group from the protected moiety.
Photochemical elimination of the benzyl group of some benzyl
esters was reported by Ralim et al. using 2,3-dichloro-5,6-dicyano-
p-benzoquinone under a long wavelength UV light [14]. To our
knowledge, this paper is the first report on photoelimination of a
benzyl group from a benzylamino group.
This report describes the response of 1 to several light condi-
tions along with the crystal structure and absorption properties of
1, 2, and 3 as our first report on this finding.
none
2
[7] and
2-amino-5-benzylamino-3,6-dichloro-p-
benzoquinone 3, under ambient conditions by studying the crys-
tallization conditions of 1. This chemical transformation during
crystallization was found to be caused by light irradiation.
2. Results and discussion
2.1. Synthesis and crystallization
* Corresponding author. Tel./fax: þ81 42 468 8633.
** Corresponding author. Tel.: þ81 45 339 3366; fax: þ81 45 339 3345.
E-mail addresses: emibabam@musashino-u.ac.jp (E. Horiguchi-Babamoto),
smatsu@ynu.ac.jp (S. Matsumoto).
Compound 1 was synthesized from chloranil and dibenzylamine
in benzene in a moderate yield. The crystallization of 1 was carried
http://dx.doi.org/10.1016/j.dyepig.2015.05.036
0143-7208/© 2015 Elsevier Ltd. All rights reserved.

Y. Shimada et al. / Dyes and Pigments 121 (2015) 336e341
337
O
O
H
O
H
N
N
Cl
N
N
Cl
Cl
Cl
Cl
N
H
Cl
NH
2
O
O
O
1
2
3
Fig. 1. Chemical structures of three diaminodichlorobenzoquinone derivatives.
out to obtain a good single crystal for X-ray structure analysis and to
check the existence of any polymorphic forms. The crystallization
was carried out by the liquideliquid diffusion method in a thin
glass tube using a combination of chloroform and some selected
organic solvents as a good (soluble) and poor (insoluble or spar-
ingly soluble) solvents, respectively. The crystallization samples
were kept at 288 K in an incubator.
Three different colored crystals with different morphologies
were obtained as shown in Fig. 2: (i) form (I) as black blocks, (ii)
form (II) as thin red needles, and (iii) form (III) as thin green flakes.
Seven poor solvents were used: acetonitrile, diethyl ether, petro-
leum ether, n-hexane, c-hexane, methanol, and ethanol. Form (I)
appeared immediately in a glass tube after injecting a poor solvent
irrespective of the type of poor solvent. Form (II) appeared in a glass
tube usually approximately 12 h after the sample preparation. Form
(III) was obtained while studying different crystallization condi-
tions: solvent combinations, storage temperature, and the con-
centration of the chloroform solution. In general, three to four days
were required for obtaining form (III) when petroleum ether, n-
hexane, or c-hexane was used as the poor solvent.
Table 1
Crystallographic data of the three crystal forms.
I
II
III
Formula
Crystal system
Space group
Z
C34 H28 Cl2 N2 O2
C20 H16 Cl2 N2 O2
tetragonal
P42/n
4
C13 H10 Cl2 N2 O2
monoclinic
Cc
4
triclinic
P- 1
1
Cell constants
a/Å
b/Å
9.0230(4)
9.3144(4)
9.5441(4)
63.615(2)
78.370(2)
89.179(2)
701.21(5)
0.0684
16.65(1)
16.65(1)
6.296(5)
90
90
90
1744(3)
0.0862
223
8.44(2)
25.12(6)
7.28(2)
90
122.73(2)
90
1298(6)
0.0747
296
c/Å
ꢀ
ꢀ
a
/
b
/
ꢀ
g
/
V/Å3
R1
Temperature/K
296
structure was observed between the benzene rings of the benzyl
groups.
Fig. 4 shows the molecular and crystal structure of form (II). This
molecule also has an inversion center. The benzoquinone moiety of
2 was found to be almost planar. The benzyl groups are located on
the same plane as the benzoquinone ring, even though the phenyl
rings of the benzyl groups are perpendicularly twisted against the
benzoquinone ring. The molecules are stacked along the c-axis with
a 42 operation, forming a one-dimensional (1D) molecular column
as shown in Fig. 4(b). This molecular arrangement may be reflecting
the thin needle morphology of this form.
The molecular and crystal structures of form (III) are shown in
Fig. 5. The central benzoquinone ring of 3 in this form is almost
planar. The phenyl ring of the benzyl substituent is projected away
from the benzoquinone ring. The molecules are stacked along the
[10e1] direction by overlapping the central benzoquinone rings to
form a 1D column. As shown in Fig. 5(b), mutual H bonding be-
tween the amino and carbonyl groups was observed in a molecular
pair along the [101] direction. This H bonds contribute to the
alignment of the columns along this direction.
2.2. X-ray structure analysis
The X-ray structure analysis of these three forms revealed an
unexpected result. Form (I) is the crystal of 1, whereas the other two
forms were surprisingly not the expected polymorphs of 1, but the
crystals of different compounds, derivatives of 1 in which some of
the benzyl groups are eliminated. The crystallographic data are
shown in Table 1. Form (II), thin red needles, is the crystal of 2,5-
bis(benzylamino)-3,6-dichloro-p-benzoquinone 2. Form (III), thin
green flakes, is the crystal of 2-amino-5-benzylamino-3,6-dichloro-
p-benzoquinone 3.
The molecular and crystal structures of form (I) are shown in
Fig. 3. The molecule of 1 in this form has an inversion center. The
benzyl groups of one amino moiety are located in opposite di-
rections. The benzoquinone ring is slightly distorted, and the
carbonyl groups are slightly deviated from the central C6 ring. This
form has a triclinic system, and the molecules are only arranged in
accordance with
a translation operation. One pep stacked
2.3. Optical properties
The UVevisible absorption spectra of 1, 2, and 3 both in the
solution and crystalline states are shown in Fig. 6; their charac-
teristics are summarized in Table 2. The absorption spectra of 2 and
3 in chloroform show a typical benzoquinone spectrum, with broad
and weak n/
p
* band in a longer wavelength region and a strong
* bands of 2 and 3 were
p/p* band in a near-UV region. The n/p
Fig. 2. Obtained crystals with different color hues and crystal habits: black blocks (I),
thin red needles (II), and thin green flakes (III).(For interpretation of the references to
colour in this figure legend, the reader is referred to the web version of this article.)
observed at 539 and 545 nm, respectively, with a very small
extinction coefficient, thus showing a pale reddish-violet color in

338
Y. Shimada et al. / Dyes and Pigments 121 (2015) 336e341
Fig. 3. Molecular structure and arrangement of 1: (a) molecular structure (30% probability level), the top view (upper) and side view (lower), where all the H atoms are omitted for
clarity, (b) molecular arrangement viewed from the c-axis (upper) and b-axis (lower).
Fig. 4. Molecular structure and arrangement of 2: (a) molecular structure (30% probability level), the top view (upper) and side view (lower), where all the H atoms are omitted for
clarity, (b) molecular arrangement viewed from the c-axis (upper) and b-axis (lower).

Y. Shimada et al. / Dyes and Pigments 121 (2015) 336e341
339
Fig. 5. Molecular structure and arrangement of 3: (a) molecular structure (30% probability level), the top view (upper) and side view (lower), where all the H atoms are omitted for
clarity, (b) molecular arrangement viewed from the c-axis (upper) and the [101] direction (lower), where the mutual H bonds are shown in the dotted line.
the solution. In contrast, the major absorption band of 1 was found
at 452 nm with a relatively large extinction coefficient. Thus, this
solution shows an orange color. This band can be characterized as a
p/p
* band.
A small bathochromic shift in the major band of these com-
pounds was observed when the spectra were recorded in the
crystalline state. The color hues of the crystals were almost similar
to those of the solutions of 1 and 2, whereas the colors of the so-
lution and crystalline states of 3 differed significantly. The corre-
sponding n/p* peaks of 2 and 3 could not be clearly distinguished
in this measurement because of a spectral noise in the visible
region.
Remarkable color degradation was observed when a chloroform
solution of 1 was left under room light for approximately 2 h. The
crystals of 1 changed under room light with respect to their surface
appearance and color. In contrast, no color change occurred when
the chloroform solution of 1 was shielded by an aluminum foil.
These phenomena may be correlated to the crystallization condi-
tions of 1. Therefore, the optical response of 1 was investigated as
described in the next section.
2.4. Irradiation experiments
To investigate the effect of light in the single-crystal growth of 1,
its crystallization was carried out by exposing the solution of 1 to a
fluorescent lamp at 288 K in an incubator. Sample tubes covered by
aluminum foil were also placed under the same conditions as the
control samples. The results for the combination of seven poor
solvents are shown in Table 3. Single crystals of 1 were obtained in
all the solvent combinations under the light-shielding condition,
whereas two or three forms of crystals were obtained under fluo-
rescent light conditions. Under the irradiation conditions, trans-
parent pale colored crystals were also obtained in several solvent
combinations [15].
Fig. 6. UVevisible absorption spectra of 1, 2, and 3 (a) in chloroform and (b) in the
crystalline state. The inset of (a) 2 and 3 in the range 450e700 nm.
Under the same light conditions, an irradiation experiment was
also performed on
a
chloroform solution of 1. In
a
orange to pale yellow to pale pink. The thin-layer chromatography
(TLC) results indicate that 1 gradually disappeared, and then 2 and 3
were formed along with other uncharacterized products. Similar
color change phenomenon was also observed in benzene and
toluene under the same experimental conditions, even though the
solubility of 1 in these solvents is lower than that in chloroform.
1.0 ꢁ 10ꢂ3 mol dmꢂ3 solution of 1, the color of the solution gradually
changed from orange to colorless in 20e30 min. It took approxi-
mately 20 h to complete the color degradation of a concentrated
solution of 1 of approximately 1.0 ꢁ 10ꢂ2 mol dmꢂ3. In this con-
centration, the color of the solution gradually changed from dark

340
Y. Shimada et al. / Dyes and Pigments 121 (2015) 336e341
Table 2
3. Conclusions
Absorption characteristics of 1, 2, and 3.
The benzyl groups of 2,5-bis(dibenzylamino)-3,6-dichloro-p-
Chloroform
Solid
benzoquinone were easily eliminated under ambient light condi-
tions to afford two analogous diaminodichlorobenzoquinone dyes,
2,5-bis(benzylamino)-3,6-dichloro-p-benzoquinone and 2-amino-
5-benzylamino-3,6-dichloro-p-benzoquinone. The experiments
using a Xe lamp indicate that this finding can be applied for the
synthesis of a benzyl-substituted diaminodichlorobenzoquinone
derivative. The reaction mechanism and the electronic effect of the
benzyl substituent will be investigated in the future.
lmax [nm]
ε [mol-1dm3cmꢂ1
]
lmax [nm]
1
2
452
361
539
347
545
1.48 ꢁ 104
3.65 ꢁ 104
3.85 ꢁ 102
2.06 ꢁ 104
3.65 ꢁ 102
479
394
3
372
Table 3
Result of the crystallization under two different light conditions.
4. Experimental
Poor solvents
Crystal form
4.1. Materials and equipment
Shielding condition
Exposure condition
acetonitrile
diethyl ether
petroleum ether
n-hexane
c-hexane
ethanol
I
I
I
I
I
I
I
I, II
I, II
I, II, III
I, II, III
I, II, III
I, II
All the starting materials were used as received without further
purification. Chloranil was purchased from Tokyo Kasei Co., Ltd.
Dibenzylamine was purchased from Wako Pure Chemical In-
dustries, Ltd.
Melting points were measured using a MEL-TEMP (Barnstead
International. Inc.) apparatus. 1H NMR and 13C NMR spectra were
recorded using a Bruker DRX300 spectrometer in CDCl3 or DMSO-
d6. ESI-HRMS spectra were recorded using a Jeol AccuTOF JMS-
100LC (European Virtual Institute for Speciation Analysis) spec-
trometer. UVevisible absorption spectra in solution were recorded
using a PERKIN ELMER Lambda 750 spectrometer. Solid-state ab-
sorption spectra were recorded using a System Instruments SIS-50
surface and interface spectrometer based on optical waveguide
spectroscopy [16].
methanol
I, II
Then, the color change of 1 in chloroform was confirmed using a
Xe lamp; 20 mL of a 1.0 ꢁ 10ꢂ2 mol dmꢂ3 chloroform solution of 1
was irradiated using a Xe lamp under stirring and ambient condi-
tions. The same color change as the former experiment was
observed in these experimental conditions as shown in Fig. 7. The
yields of 2 and 3 were 26% and 29%, respectively.
The single crystals of 1 were also irradiated using a fluorescent
lamp. The block morphology of the sample crystals (Fig. 2) gradu-
ally changed under irradiation accompanied by surface melting as
shown in Fig. 8. The color change of the crystal could be clearly
recognized in the thin-crystal samples. The color change of the
crystals was the same as that of the solution by light irradiation. X-
ray diffraction measurements were also carried out along with the
color change observation. The results show that after irradiation,
the crystals lost their crystallinity, showing a halo pattern along
with the color change.
4.2. Synthesis
4.2.1. Synthesis of 2,5-bis(dibenzylamino)-3,6-dichloro-p-
benzoquinone (1)
To a benzene solution (25 mL) of chloranil (620 mg, 2.4 mmol),
dibenzylamine (2.0 mL, 10.1 mmol) was added dropwise at room
temperature. The mixture was stirred at room temperature for
16 h and then refluxed for 2 h. After the reaction was completed,
the mixture was cooled to room temperature. The reaction
Fig. 7. Observed color change of a chloroform solution (1.0 ꢁ 10ꢂ2 mol dmꢂ3) of 1 by Xe-lamp irradiation.
(a)
(b)
(c)
large crystal sample
thin crystal sample
Fig. 8. Surface change of a crystal of 1 by fluorescent lamp irradiation: (a) before irradiation, (b) irradiated for 24 h, and (c) irradiated for 4 d.

Y. Shimada et al. / Dyes and Pigments 121 (2015) 336e341
341
mixture was filtered to remove the resulting salt, and the filtrate
was evaporated under reduced pressure. The resulting residue
was purified by column chromatography on silica gel using ben-
zene as the eluent and then recrystallized from a mixture of
benzene/hexane.
CCDC1025777. Crystal data of form (II):
C
20H16Cl2N2O2,
M ¼ 387.26, tetragonal, a ¼ 16.65 (1) Å, b ¼ 16.65 (1) Å, c ¼ 6.296
(5) Å,
a
¼ 90ꢀ,
b
¼ 90ꢀ,
g
¼ 90ꢀ, Dcalcd ¼ 1.475 g cmꢂ3, V ¼ 1744
(3) Å3, T ¼ 223 K, space group P42/n, Z ¼ 4, 13995 reflections
measured, and 2403 unique reflections (Rint ¼ 0.0759). The final
Yield: 33%; mp 147e149 ꢀC; 1H NMR (CDCl3, 300 MHz)
R1 value (I > 2s
(I)), final wR(F2) value (all data), and GOF were
d
7.36e7.28 (m, 12H), 7.20e7.17 (m, 8H), 4.51 (s, 8H); 13C NMR
0.0862, 0.2537, and 1.204, respectively. The CCDC deposition
number is CCDC1025778. Crystal data of form (III): C13H10Cl2N2O2,
M ¼ 297.14, monoclinic, a ¼ 8.44 (2) Å, b ¼ 25.12 (6) Å, c ¼ 7.28
(CDCl3, 75 MHz) d 176.9, 150.0, 137.3, 128.6, 128.2, 127.6, 121.3, 56.4;
HRMS: Calcd for C34H28Cl2N2O2Na ([M þ Na]þ) 589.1426, found
589.1417.
(2) Å,
a
¼ 90ꢀ,
b
¼ 122.73 (2)ꢀ,
g
¼ 90ꢀ, Dcalcd ¼ 1.520 g cmꢂ3
,
V ¼ 1298(6) Å3, T ¼ 296 K, space group Cc, Z ¼ 4, 5494 reflections
measured, and 2540 unique reflections (Rint ¼ 0.0691). The final
4.2.2. Synthesis of two N-benzyl-substituted 2,5-diamino-3,6-
dichloro-p-benzoquinone derivatives by Xe-lamp irradiation
A chloroform solution of 1 (20 mL, 0.2 mmol) in a glass bottle
was irradiated by a Xe lamp (150 W, HAMAMATSU L9588-03-03,
0.6 W/cm2 at 452 nm) under stirring at room temperature for
4 h. After 4 h, the solvent was removed from the reaction mixture,
and the residue was separated by column chromatography on
silica gel. This operation was carried out in two steps. The first
column chromatography was performed using benzene as the
eluent to roughly separate 2 and 3. Next, the second column
chromatography was carried out using benzene for 2 and a
mixture of ethyl acetate/n-hexane (2:1) for 3 to afford 2 (19.9 mg,
26% yield) and 3 (17.1 mg, 29% yield). Because of the low solubility
of 2 and 3, 13C NMR data for these two compounds were not
obtained.
R1 value (I > 2s(I)), final wR(F) value (all data), and GOF were
0.0747, 0.1508, and 1.822, respectively. The CCDC deposition
number is CCDC1025779.
Acknowledgment
The authors are grateful to Prof. Takashi Amemiya (Yokohama
National University), Prof. Hajime Abe (Toyama University), and Dr.
Motoo Shiro (Rigaku) for their support in the Xe-lamp experiments,
ESI-HRMS measurements, and X-ray structure analyses,
respectively.
References
2,5-Bis(benzylamino)-3,6-dichloro-p-benzoquinone (2): mp
222e223 ꢀC; 1H NMR (DMSO-d6, 300 MHz)
d 8.56 (br s, 2H),
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C
20H16Cl2N2O2Na ([M þ Na]þ) 409.0487, found 409.0475.
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13H10Cl2N2O2Na ([M þ Na]þ) 319.0017, found 319.0015.
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anisotropically. All the H atoms were located at the calculated
positions. The parameters of the H atoms in form (I) were refined
by a riding model, and those in forms (II) and (III) were con-
strained. All the calculations for form (I) were performed using
the Crystal Structure 3.8 [18] crystallographic software package.
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a chromato-
[15] Unpublished result. This will be reported elsewhere.
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C
34H28Cl2N2O2, M ¼ 567.51, triclinic, a ¼ 9.0230 (4) Å, b ¼ 9.3144
(4) Å, c ¼ 9.5441 (4) Å,
a
¼ 63.615 (2)ꢀ,
b
¼ 78.370 (2)ꢀ,
g
¼ 89.179 (2)ꢀ, Dcalcd ¼ 1.344 g cmꢂ3, V ¼ 701.21 (5) Å3,
T ¼ 296 K, space group P-1, Z ¼ 1, 6379 reflections measured, and
2404 unique reflections (Rint
(I > 2 (I)), final wR(F) value (all data), and GOF were 0.0684,
0.1152, and 1.455, respectively. The CCDC deposition number is
¼
0.062). The final R1 value
[20] Macrae CF, Bruno IJ, Chisholm JA, Edgington PR, McCabe P, Pidcock E, et al.
Mercury CSD 2.0-new features for the visualization and investigation of
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s