Sensitivity and Resolution Development of Spiropyran-based Molecular Photoswitches

Article abstract of DOI:10.1002/jccs.201600159

The phenylazo moiety and its donor- and acceptor-substituted derivatives are studied as effective auxochromes to improve their sensitivity and resolution for distinguishing between the spiro (SP; OFF) and mero (ON) forms in molecular photoswitching applications. Thus, 13 azospiropyran derivatives were synthesized and their spectroscopic and photokinetic behaviors were studied. The quality of photochromic reactions of the synthesized photochromic compounds were compared using a dose–response model. Interestingly, by replacing the nitro group in 6-nitrospiropyran (ε = 0.42 × 10⁴ M^?1 cm^?1) with a simple phenylazo moiety, the SP form is still colorless and the color intensity of the merocyanine (MC) form is improved desirably by extending the conjugation length (1a, ε = 1.35 × 10⁴ M^?1 cm^?1). The presence of a hydrophilic OH group or a CH₃ group at the para position of phenylazo moiety revealed more or less the same photochromic properties as 1a. The OCH₃ group substituted at position 6 of the phenylazo moiety at the para position of the azobenzene moiety effectively increased the photochromic properties with the maximum k-value for SP to MC switching. Meanwhile, Cl, Br, COOH, and NO₂ groups at the para position of the azobenzene moiety revealed the reduction in photochromic properties compared to 1a.

Full text of DOI:10.1002/jccs.201600159

JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
Article
1Sensitivity and Resolution Development of Spiropyran-based Molecular Photoswitches
Maryam Heydaripour,a Farahnaz Nourmohammadianb,c,*and Naghi Saadatjooa
aDepartment of Chemistry, Semnan University, 19111-35131, Semnan, Iran
bDepartment of Organic Colorants, Institute for Color Science and Technology, 1668836471, Tehran, Iran
cCenter of Excellence for Color Science and Technology, 1668836471, Tehran, Iran
(Received: April 28, 2016; Accepted: August 3, 2016; DOI: 10.1002/jccs.201600159)
The phenylazo moiety and its donor- and acceptor-substituted derivatives are studied as effective
auxochromes to improve their sensitivity and resolution for distinguishing between the spiro (SP;
OFF) and mero (ON) forms in molecular photoswitching applications. Thus, 13 azospiropyran
derivatives were synthesized and their spectroscopic and photokinetic behaviors were studied. The
quality of photochromic reactions of the synthesized photochromic compounds were compared
using a dose–response model. Interestingly, by replacing the nitro group in 6-nitrospiropyran
(ε = 0.42 × 104 M−1 cm−1) with a simple phenylazo moiety, the SP form is still colorless and the
color intensity of the merocyanine (MC) form is improved desirably by extending the conjugation
length (1a, ε = 1.35 × 104 M−1 cm−1). The presence of a hydrophilic OH group or a CH3 group at
the para position of phenylazo moiety revealed more or less the same photochromic properties as
1a. The OCH3 group substituted at position 6 of the phenylazo moiety at the para position of the
azobenzene moiety effectively increased the photochromic properties with the maximum k-value
for SP to MC switching. Meanwhile, Cl, Br, COOH, and NO2 groups at the para position of the
azobenzene moiety revealed the reduction in photochromic properties compared to 1a.
Keywords: Azospiropyran; Photoresponsive; Photochromic dye; Spiropyran; Absorption.
INTRODUCTION
The design and synthesis of fine-tunable photo-
Spiropyrans reversibly switch between the colorless
spiro (SP) form which is nonpolar and uncharged, and
a colored merocyanine (MC) form with a polar, conju-
gated, zwitterionic structure.28,29
It has been known that the introduction of
electron-withdrawing substituents at positions 6 and
8 on the chromene part lowers the energy of MC form
and makes it long-lived at room temperature and so
enhances the photochromic properties.30,31 Conse-
quently, the majority of the investigations of molecular
switches based on spiropyran have focused on various
derivatives of spiropyrans containing a nitro group in
the chromene part.32
Recently, we synthesized and studied the photore-
sponse of bis-azospiropyrans connected with a conju-
gated aromatic bridge.33–35 These photoswitches
showed a high molar absorption coefficient in the MC
form.33 Such a high molar absorption coefficient
improves the sensitivity and contradistinction between
the two ON (MC) and OFF (SP) levels of finely photo-
responsive molecules. Previous to that study, there were
responsive molecules are of great scientific interest and
intense activity in different fields of chemistry. In fact,
photoresponsive systems with controllable light-induced
color have a variety of applications due to their poten-
tial use in state-of-the-art technologies.1–10
Photochromic molecules reversibly respond to
light by the photoisomerization process, in which some
physicochemical properties, such as absorption and
emission spectra, electrochemical property, magnetic
properties, dipole interaction, and refractive index, can
be tuned.11–14 These photoinduced tunable properties
give rise to different applications in photooptical
switching devices,15–19 sensors,20 biology,21–23 optical
storage,8,24 displays,25 and nonlinear optics.26
Among the molecular photoswitches, spiropyrans
as a family of photochromic compounds constitute an
important class mostly due to their molecular binding
abilities and the sharp distinction between the molecu-
lar properties of their photoinduced states.16,27
* Corresponding author: Email: nour@icrc.ac.ir
J. Chin. Chem. Soc. 2016
© 2016 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1

Article
Heydaripour et al.
only a few studies on using an azo chromophore as a
part of photochromic spiropyran molecule.36–39 Hence,
there is a lack of systematic study on azospiropyrans
aimed at taking advantage of the electron-withdrawing
character of phenylazo moiety to lower the energy state
of the MC form and deepening the color of the solution
at the photostationary state by extending the conjuga-
tion system.
expected functional groups as aldehyde, hydroxide,
and NO2.
The peaks at 3150–3465 cm−1 in the FT-IR spec-
tra of 4a–h were assigned to O–H stretch, and those of
the azo groups were observed at 1454–1481 cm−1. The
peaks at 2856–2927, 1616–1668, and 2675–2867 cm−1
are related to C–H aromatic (Ar), C O, and C H
stretch of the aldehyde functional groups, respectively.
The absorption at 1452–1488 cm−1 in the FT-IR
spectra of 1a–h revealed azo groups; the peak at
2960–2968 cm−1 was due to C H, Ar, and the peak at
1602–1606 cm−1 was related to C(3) C(4) stretch. The
stretching vibration of the C(2) N was observed at
1278–1317 cm−1, and the C(2) O stretching frequency
This achievement persuaded us to study azospiro-
pyrans’ photoresponses and figure out the substitution
effects on mono azospiropyrans-based molecular
switches. Accordingly, azospiropyran derivatives were
synthesized using several p-substituted anilines with
electron donor or acceptor groups for azo moieties, and
then coupling them with salicylaldehyde, 5-nitrosalicy-
laldehyde, or 3-methoxysalicylaldehyde. The photophy-
sical properties of the synthesized photochromic
molecules were also investigated. Thus, in this study we
present the spectroscopic and kinetic results of 13 azos-
piropyran derivatives. Eight derivatives (1a–h) contain
a group at position 6 of the phenylazo moiety; four
(2a–d) possess two groups, one in position 6 of the azo
moiety and a NO2 group on chromene moiety; and the
last derivative (3) possesses an NO2 group in the pheny-
lazo part and a methoxy group on the chromene moi-
ety. Spectroscopic studies of all synthesized
azospiropyran photochromic molecules were compared
with those of two spiropyrans terminated with NO2 or
H groups at position 6 of the chromene moiety (7a
and 7b).
1
occurred at 1016–1037 cm−1. The H-NMR spectra for
1a–h clearly showed singlet signals at 1.25–1.35 ppm
(3H) and 1.35–1.65 ppm (3H) for two CH3 at C(3)
(CH3)2 (Scheme 1). The proximity of two CH3 of indo-
line moieties (C(3) (CH3)2) to the olefinic proton or
oxygen moiety of the pyranyl group made them mag-
netically nonequivalent.40 A singlet signal appeared at
2.70–3.02 ppm (3H) for the N CH3 protons of 1a–h.
The olefinic protons of 1a–h revealed two doublets, one
at 4.20–5.81 ppm for H C(3) with large coupling con-
stants (J = 10.2–10.4 Hz), and the other for H C(4) at
the aromatic regions. The former doublet for olefinic
protons with large coupling is very characteristic of the
spiropyran system.
For 5a–d, the peaks at 3120–3130 cm−1 in the FT-
IR spectra refer to the O H stretch; the peaks at
2852–2933, 2692–2921, and 1612–1623 cm−1 corre-
spond to C H (Ar) , C H, and C O stretch of the
aldehyde functional groups, respectively, and the peaks
at 1510–1546 and 1332–1348 cm−1 were related to the
N O stretch. The peaks due to the azo groups were
RESULTS AND DISCUSSION
Diazotization of p-substituted anilines with subse-
quent coupling with any of the salicylaldehyde, 5-nitro-
salicylaldehyde, or 3-methoxysalicylaldehyde led to the
corresponding 4a–h, 5a–d, and 6 in high yields
(70–90%). Afterward, azospiropyrans 1a–h, 2a–d, and
3 were obtained in good yields (50–90%) by the cou-
pling reaction of the synthesized azo-salicylaldehyde
derivatives 4 to 6 with Fischer’s base, i.e., 1,3,3-tri-
methyl-2-methyleneindoline (Scheme 1).
The structures of the synthesized azospiropyrans
1–3 were characterized using FT-IR and 1H-NMR
spectroscopic data. Since the patterns of 4–6 are of
known azo structures in the literature, they were puri-
fied and characterized by FT-IR to confirm the
observed at 1440–1488 cm−1
.
For 2a–d structures, the absorption frequencies
in FT-IR spectra occurred at 2966 cm−1 for aro-
matic C H; the peak at 1604–1606 cm−1 was related
to C(3) C(4) stretch; the peak at 1487–1488 cm−1
was due to the azo groups; and the peaks at
1494–1510 and 1322–1338 cm−1 corresponded to the
N O stretch. The stretching vibration of C(2) N
was observed at 1259 cm−1, and the C(2) O stretch-
ing frequency occurred at 1012–1014 cm−1. The 1H-
NMR spectra for 2a–d clearly showed two singlet
signals at 1.25–1.35 ppm (3H) and 1.35–1.65 ppm
2
www.jccs.wiley-vch.de
© 2016 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
J. Chin. Chem. Soc. 2016

JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
Sensitivity and Resolution Developing
H
C O
H
C O
H
C O
H
O
H
O
H
O
OMe
N
O2
NH2
NaNO2,HCl
0-5°C
R
H
C O
H
C O
H
C O
H
O
H
O
H
O
N
N
N
OMe
N
N
O2
N
N
N
O2
R
4a-h
5a-d
6
R
Fischer's base, CHCl3, reflux
3
3
'
4
2
N
N
N
O
O
O
N
N
N
N
O2
N
N
OMe
N
R
N
O2
1a-h
2a-d
3
R
R
R
1a
H
2a
H
O
1b OH
2b
2c
H
1c
1d
1e
H
C
H
OC
3
H
3
N
N
O
2d N
O2
OC
Cl
O
3
1f Br
N
O2
1g
COOH
O2
7a
7b
1h N
Scheme 1. Synthesis of azospiropyran photochromic dyes 1a–h, 2a–d, and 3.
(3H) for two CH3 at C(3) (CH3)2 protons (Scheme
1). singlet signal appeared at 2.80–3.11 ppm
at 1488 cm−1was due to the azo groups, and the peaks
at 1465 and 1340 cm−1 corresponded to N O stretch.
The stretching vibration of the C(2) N was observed at
1280, and the C(2) O stretching frequencies occurred
at 1020 cm−1. The 1H-NMR spectra for 3 clearly
showed two singlet signals at 1.31 ppm (3H) and
1.57 ppm (3H) for magnetically nonequivalent CH3 at
C(3) (CH3)2 protons (Scheme 1). A singlet signal
appeared at 2.70 ppm (3H) for N CH3 protons, and a
singlet signal appeared at 3.01 ppm (3H) for OCH3
protons of 3. The olefinic protons of 3 revealed two
characteristics of the spiropyran system doublets: one at
4.25 ppm for H C(3) with large coupling constants
(J = 10.1 Hz), and the other olefinic protons H C
(4) at the aromatic regions.
A
(3H) for N CH3 protons of 2a–d. The olefinic pro-
tons of 2a–d revealed two very characteristic doub-
lets, one at 4.13–4.31 ppm for H C(3) with large
coupling constants (J = 10.1–10.2 Hz), and the other
for olefinic protons H C(4) at the aromatic regions.
The peaks at 3145 cm−1 in the FT-IR spectrum of
6 refers to O H stretch. The peaks at 2958, 2852, and
1654 cm−1 were due to the aromatic C H, C H, and
C O stretch of the aldehyde functional groups, respec-
tively. The azo groups were observed at 1456 cm−1, and
the peaks at 1514 and 1338 cm−1 were related to N O
stretch.
The absorption frequencies in FT-IR spectra of
3 revealed an aromatic C–H at 2964 cm−1. The peak at
1606 cm−1 was related to C(3) C(4) stretch, the peak
The synthesized photochromic molecules revealed
interesting
substitution-dependent
photophysical
J. Chin. Chem. Soc. 2016
© 2016 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.jccs.wiley-vch.de
3

Article
Heydaripour et al.
Fig. 1. UV/Vis spectra of 1 (a), and 2, 3 (b) in CH2Cl2 (C = 10−4 mol/L) after 3 min exposure to UV light at room
temperature.
responses to UV light as presented in the following
sections.
Azospiropyrans 1a–h were exposed to 254 nm UV
light at room temperature, and their photochromic
behaviors were studied using UV/Vis spectroscopy. The
difference between the wavelength of maximum absorb-
ance of azospiropyrans 1a–h in closed and open forms
was in the range 200–250 nm (Figure. 2). To investigate
the ability to get the irradiated azospiropyrans back to
their former SP state, thermal back reaction of 1a–h
was performed at 120–146ꢀC in DMF (Figure. 2). It is
worth noting that, due to the stability of the MC form
of azospiropyrans, the spontaneous thermal back reac-
tion in the dark and therefore photochemical bleaching
Photophysical studies of the photochromic dyes
Absorption spectra of the synthesized photochro-
mic compounds 1, 2, and 3 in CH2Cl2 (C = 10−4 mol/L)
were studied after being exposed to 254 nm UV light for
less than 3 min until reaching steady state (Figure. 1).
To study the substitution effect, the molar absorp-
tion coefficients of azospiropyrans 1a–h, 2a–d, and 3,
and also nonsubstituted spiropyran 7a and 6-nitro BIPS
7b (Scheme 1), which were synthesized according to the
literature,34,41 were measured and compared. The
results revealed that azo substitution increases the color
strength of the mero forms. Donor or acceptor groups
have significant effects on the photophysical properties.
The results are presented in Table 1. Two λmax values
are reported for 1c, 1e, and 1f because both affect their
related MC colors.
In this study, the apparent molar absorption coef-
ficient of the simple spiropyrans, when the color of
solutions reached the steady state, was 0.31 × 104 M−1
cm−1 for 7a and 0.42 × 104 M−1cm−1 for 7b. At the
same irradiation, depending on the substitutions, ε
increased to 0.84–2.2 × 104 M−1 cm−1 in the presence
of an azo moiety in the mero form of the synthesized
azospiropyrans. The high color strength of azospiropyr-
ans in the MC form arises from the extension of the
conjugation system of the spiropyran backbone with an
azo moiety (Scheme 2). It is well known that a higher
molar absorption coefficient leads to higher sensitivity
and resolution, which in turn provides better distinction
between the SP and MC forms in photoswitching
applications.
Table 1. Wavelength of maximum absorbance (λmax) and
molar absorption coefficients of azospiropyrans 1a–
h, 2a–d, and 3, and also 7a, b at the same
concentration 10−4 M in CH2Cl2
Compounds1
R
λmax1 (nm)
ε (M−1cm−1) × 104
1a
1b
1c
1d
1e
1f
1g
1h
2a
2b
2c
2d
3
H
OH
CH3
OCH3
Cl
570
570
494, 580
569
484, 580
472, 565
568
510
577
577
569
1.35
1.12
1.76,0.80
1.1
1.71,0.40
1.76,0.72
1.44
0.82
1.30
1.08
1.02
Br
COOH
NO2
H
OH
OMe
NO2
NO2
H
577
608
569
577
1.54
2.2
0.31
0.42
7a
7b
NO2
1 For some dyes, two wavelengths of maximum absorbance at
visible light are mentioned due to their effects on the final colors.
4
www.jccs.wiley-vch.de
© 2016 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
J. Chin. Chem. Soc. 2016

JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
Sensitivity and Resolution Developing
R
the SP form and easily switched to the highly colored
mero form, with λmax = 550–573 nm (Figure 3).
7
N
N
O
O
R
A diazotized 4-nitroaniline was also coupled with
3-methoxy salicylaldehyde for synthesizing azospiro-
pyran 3. The presence of the electron-donor group
(OMe) on the salicylaldehyde moiety and also the
acceptor group (NO2) on the azo substituent produced
yellowish azospiropyrans in the SP form, which turned
into green (Figure 4). The produced green color was
obtained from a mixture of the yellow SP form (λmax
416 nm) and the blue mero form (λmax 608 nm). Most
often, spiropyrans produce red shade in colored form
and produce blue color with the spiropyran-based struc-
ture 3, which can be very interesting (Figure 4).
O
N
R2
R2
N
N=N
N
1,3
2
R1
R1
NO2
N
N
O
O
NO2
O
N
N=N
N=N
R
R
The quality of photocoloration reactions of the
synthesized photochromic compounds is compared
empirically with a dose–response model fitting. Pre-
liminary evaluations showed that developing the visi-
ble peaks of all compounds cannot be explained by
a simple exponential model. This suggests that the
coloration mechanism of all compounds is not unimo-
lecular. Knowing that photocoloration of spiropyrans
is complex and is due to short- and medium-lived
transient species to reach the final stable merocyanin
isomer,42 for the case of azo-spiropyrans a sigmoidal
model can be fitted relatively well with the lack of ran-
dom residuals. Photokinetic examination of the
13 synthesized spiropyran derivatives were studied by
the fitting of the obtained absorbances at the λmax of
their MC form during irradiation with 254 nm UV
light to an exponential model (Equation 1). The data
points were fitted adequately in view of the adjusted r2
(Table 2) and as illustrated in Figure 5. While the
dose–response model (Eq. (1)) is not a mechanistic
model, as a mathematical framework that simulates
the trends of time vs. absorbance curves for all com-
pounds, the derived parameters (Table 2) are practical
for comparing the kinetics as well.
Scheme 2. Spiropyran (7) and azospiropyrans (1–3),
and their differences in mero form’s con-
jugation length
by common low-energy light sources is not possible.
However, the repetition of ON and OFF cycles of an
azospiropyran molecule by a laser source was shown
elsewhere,33 and this will be investigated later.
Within the synthesized azospiropyrans, 1a–h, 1b,
and 1d were produced in colorless SP forms due to
electron-donor groups such as OH and OMe at the
para position of the azo moiety. Similar to nonsubsti-
tuted azospiropyran 1a, these spiropyrans switched to
highly colored mero forms (λmax ≥ 550 nm) too. The
electron-acceptor groups such as Cl (1e) and Br (1f) in
this series gave yellow azospiropyrans and revealed a
blue shift in their mero form color (λmax ≤ 484 nm).
The SP form of compound 1 h showed more stability
than the other synthesized derivatives and was not eas-
ily switched to MC form in common polar solvents like
chloroform, dimethyl sulfoxide, DMF, DCM, acetone,
acetonitrile, and ethyl acetate. It also did not have
enough solubility in ethanol and methanol. Only the SP
form of this compound easily turned to the red MC
form in the presence of the highly polar silica gel
medium (Figure. 2, 1 h). However, in the absence of sil-
ica gel, a red solution in dichloromethane was obtained
ðAbs1 −Abs0Þ
Abs = Abs0 +
ð1Þ
m-tÞk
1 + 10ðt
where, k is the kinetic of SP to MC photo isomeriza-
tion, Abs0 and Abs1 are the absorbance at λmax of the
MC forms before irradiation (time = 0) and at photo-
stationary state,respectively, t is irradiation time, and tm
is the time of the middle of the curve.
only
after
22 min
of
UV
irradiation.
5-
Nitrosalicylaldehyde was coupled and diazotized with
p-substituted anilines possessing either highly electron
donor (OH, 2b, and OMe, 2c) or acceptor (NO2, 2d)
groups. These azospiropyrans are almost colorless in
J. Chin. Chem. Soc. 2016
© 2016 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.jccs.wiley-vch.de
5

Article
Heydaripour et al.
R
N
N
N
O
1
1a, R=H: Bleached at 122
1b, R=OH:Bleached at 132°C
1c, R=CH3: Bleached at 120°C
1d, R=OCH3: Bleached at 122°C
1e , R=C1 : Bleached at 138°Cl
1f, R=Br: Bleached at 136°C
1g, R=COOH: Bleached at 146°C
1h, R=NO2
Fig. 2. UV/Vis spectra of 1a–h (C = 10−4 mol/L in CH2Cl2) before (SP) and after (MC) 3 min exposure to 254 nm UV
light at room temperature. The mero forms were transformed to spiro forms at 120–146ꢀC in DMF.
According to Table 2, the maximum k-value for
SP to MC switching was obtained for 1d with OMe
substitution at position 6 of the phenylazo moiety. The
minimum k belongs to 1c possessing the Me group at
the same position.
cm−1). Interestingly, by replacing the nitro group with a
simple azobenzene (1a), the SP form is still colorless but
the color intensity of the MC form is improved consid-
erably by long conjugation (1a, maximum absorbance
1.25, ε = 1.35 × 104 M−1 cm−1).
According to Figure 5, as expected, nonsubsti-
tuted spiropyran 7b revealed low color intensity in the
It should be noted that the obtained data pre-
sented in Table 2 is at the same concentration
(5 × 10−5 M). However, for better kinetic examination,
the solutions of 7a and 7b were more concentrated
(2 × 10−4 M).
MC
form
(maximum
absorbance
0.6,
ε = 0.31 × 104 M−1 cm−1) compared to 6-nitro BIPS
(7a, maximum absorbance 1.4, ε = 0.42 × 104 M−1
6
www.jccs.wiley-vch.de
© 2016 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
J. Chin. Chem. Soc. 2016

JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
Sensitivity and Resolution Developing
Fig. 3. UV–Vis absorption of 2a–d (C = 10−4mol/L in CH2Cl2) after 3 min exposure to 254 nm UV light at room tem-
perature. The mero forms are reversible to spiro forms at 128–146ꢀC at the same concentration 10−4
M
in DMF.
Fig. 4. UV/Vis spectra of 3 (C = 5 × 10−5 mol/L in CH2Cl2), after 3 min exposure to 254 nm UV light at room temper-
ature. The mero form was transformed to spiro form at 146ꢀC in DMF.
The presence of a hydrophilic OH group or a
CH3 group at the para position of azobenzene moiety,
1b and 1c respectively, revealed more or less the same
photochromic properties as 1a. However, OCH3 group
at the para position of the azobenzene moiety (1d)
increased the photochromic properties more effectively
than 1a, while Cl, Br, COOH, and NO2 groups at the
para position of azobenzene moiety, 1e, 1f, 1g, 1h,
respectively, revealed reduced photochromic properties
compared to 1a.
J. Chin. Chem. Soc. 2016
© 2016 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.jccs.wiley-vch.de
7

Article
Heydaripour et al.
Table 2. Derived parameters from dose–response curve fitting
on azospiropyrans 1a–h, 2a–d, and 3, and also
nonsubstituted spiropyrans 7a, b using time
vs. absorbance data
(FT) NMR instrument, at 400 MHz in CDCl3; δ is
reported in ppm relative to SiMe4, J in Hz. A 254 nm
UV hand-held lamp (8 watt/cm2) was used as the exci-
tation light source for the photochromic ring-opening
reactions.
k
Span1
Adj. r2
1a
1b
1c
1d
1e
1f
1h
1g
2a
2b
2c
2d
3
0.015
0.013
0.009
0.055
0.013
0.018
0.016
0.029
0.024
0.011
0.036
0.010
0.018
0.067
0.024
2.22
1.10
3.65
1.89
3.27
5.68
0.51
1.97
1.00
1.07
1.54
1.39
0.99
0.053
0.176
0.9985
0.9945
0.9994
0.9968
0.9985
0.9991
0.9962
0.9948
0.998
0.9985
0.9971
0.9985
0.9954
0.9102
0.9852
Preparation of compounds 4a–h, 5a–d, and 6
General procedure
A solution of NaNO2 (1.6 g,
23.2 mmol) in H2O (5 mL) was added to salicylalde-
hyde (21.8 mmol) in H2O (10 mL) including NaOH
(1.8 g) at 5ꢀC. The resulting solution was added dropwise
to the solution of amine derivatives (23.2 mmol) in HCl
(10 mL, 1%) at 0ꢀC. The mixture was stirred for 30 min
at 0ꢀC and then for 30 min at room temperature. After-
ward, it was filtered and washed three times with distilled
water. The products were crystallized in EtOH several
times to afford pure 4a–h, 5a–d, and 6 (Scheme 1).
(E)-5-(2-Phenyldiazenyl)-2-hydroxybenzaldehyde (4a).
Yield (90%). m.p. 127–129ꢀC. IR (KBr): 3217 (OH),
2852 (C H aldehyde), 2971 (CH ), 1634 (C O), 1477
(N N), 1274 (C N).
(E)-2-Hydroxy-5-((4-hydroxyphenyl)diazenyl)benz-
aldehyde (4b). Yield (90%). m.p. 135–136ꢀC. IR (KBr):
3407 (OH), 2852 (C H aldehyde), 2927 (CH ), 1616
(C O), 1456 (N N), 1257 (C N).
(E)-5-(2-p-Tolyldiazenyl)-2-hydroxybenzaldehyde (4c).
Yield (90%). m.p. 151–152ꢀC. IR (KBr): 3191 (OH),
2860 (C H aldehyde), 2923 (CH ), 1650 (C O), 1454
(N N), 1271 (C N).
(E)-5-(2-(4-Methoxyphenyl)diazenyl)-2-hydroxyben-
zaldehyde (4d). Yield (90%). m.p. >300ꢀC. IR (KBr):
3404 (OH), 2840 (C H aldehyde), 2962 (CH ), 1620
(C O), 1458 (N N), 1280 (C N), 1029 (C O).
(E)-5-(2-(4-Chlorophenyl)diazenyl)-2-hydroxybenzal-
dehyde (4e). Yield (90%). m.p. 207–209ꢀC. IR (KBr):
3184 (OH), 2925 (CH ), 2867 (C H aldehyde), 1668
(C O), 1477 (N N), 1284 (C N).
(E)-5-(2-(4-Bromophenyl)diazenyl)-2-hydroxybenzal-
dehyde (4f ). Yield (90%). m.p. 214–216ꢀC. IR (KBr):
3188 (OH), 2871 (CH ), 2867 (C H aldehyde), 1668
(C O), 1475 (N N), 1286 (C N).
7a
7b
1 Span = Abs1 − Abs0.
The other possibility for the photochemical proc-
ess is the well-known cis–trans isomerization of the azo
bonds, which would lead to a decrease in color
intensity.24,26,33,34,43–46 However, such isomerization of
the azobenzene moieties should have a negligible
impact on the resulting immensely increased color
intensities resulting from the mero forms spiropyrans.
Therefore, the cis–trans isomerization of the azo bonds
is discounted.
To better understand the new aspects of the
photochromic properties of azospiropyrans, more inves-
tigations on solvatochromism and photokinetics,
e.g., photochromic quantum yield determination, are in
progress and the results will be published soon.
EXPERIMENTAL
General remarks
Chemicals were purchased from Merck and used
without further purification. Melting points were meas-
ured by a Büchi B-545 apparatus, and the values are
uncorrected. UV/Vis spectra were recorded on a
Multispec-1501-Shimadzu UV/Vis spectrophotometer.
IR spectra were recorded on a Perkin-Elmer Spectrum-
One-BX FT-IR spectrometer in KBr, and the ν
(E)-4-((3-Formyl-4-hydroxyphenyl)diazenyl)benzoic
acid (4g). Yield (70%). m.p. >300ꢀC. IR (KBr):
2817–3055 (COOH), 2677 (CH ), 2675 (C H alde-
hyde), 1679 (C O), 1454 (N N), 1286 (C N).
values are reported in cm−1
recorded on a Bruker-400-Avance Fourier transform
.
1H-NMR spectra were
8
www.jccs.wiley-vch.de
© 2016 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
J. Chin. Chem. Soc. 2016

JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
Sensitivity and Resolution Developing
Fig. 5. Kinetic study of UV/Vis spectra of a dose–response model for absorbance at the λmax of the dichloromethane
solutions of 1–3 against irradiation time (s) at 254 nm UV light.
(E)-5-(2-(4-Nitrophenyl)diazenyl)-2-hydroxybenzal-
dehyde (4h). Yield (80%). m.p. 192–194ꢀC. IR (KBr):
3404 (OH), 2856 (CH ), 2852 (C H aldehyde), 1664
(C O), 1456 (N N), 1286 (C N), 1521, 1348 (N O).
(E)-2-Hydroxy-5-nitro-3-(phenyldiazenyl)benzalde-
hyde (5a). Yield (80%). m.p. 112–115ꢀC. IR (KBr):
3217 (OH), 2920 (CH ), 2692 (C H aldehyde), 1623
(C O), 1515, and 1348 (N O), 1477 (N N),
1274 (C N).
(E)-2-Hydroxy-3-((4-hydroxyphenyl)diazenyl)-5-
nitrobenz-aldehyde (5b). Yield (80%). m.p. 277–278ꢀC.
IR (KBr): 3122 (OH), 2692 (C H aldehyde), 2852
(CH ), 1612 (C O), 1546 and 1332 (N O), 1440
(N N), 1286 (C N).
(E)-3-(2-(4-Nitrophenyl)diazenyl)-2-hydroxy-5-nitro-
benz-aldehyde (5d). Yield (70%). m.p. >300ꢀC. IR (KBr):
3118 (OH), 2971 (CH ), 2921 (C H aldehyde), 1622
(C O), 1515 and 1340 (N O), 1488 (N N),
1284 (C N).
(E)-3-(2-(4-Nitrophenyl)diazenyl)-6-hydroxy-2-meth-
oxybenz-aldehyde (6). Yield (80%). m.p. 225–226ꢀC. IR
(KBr): 3145 (OH), 2958 (CH ), 2852 (C H aldehyde),
1654 (C O), 1514 and 1338 (N O), 1456 (N N), 1265
(C N), 1130 (C O).
Preparation of compounds 1a–h, 2a–d, and 3
General procedure
A solution of 4a–h, 5a-–d, or
6 (1 mmol) in 50 mL CHCl3 was heated under reflux
conditions, and then 2- methylene-1,3,3-
(E)-3-(2-(4-Methoxyphenyl)diazenyl)-2-hydroxy-5-
nitrobenz-aldehyde (5c). Yield (70%). m.p. 169–170ꢀC.
IR (KBr): 3130 (OH), 2921 (CH ), 2844 (C H alde-
hyde), 1623 (C O), 1510 and 1342 (N O), 1485
(N N), 1299 (C N), 1020 (C O).
trimethylindoline (1 mmol) in 5 mL CHCl3 was added
dropwise. The mixture was refluxed for 2 h and then
cooled to room temperature. The precipitate was
J. Chin. Chem. Soc. 2016
© 2016 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.jccs.wiley-vch.de
9

Article
Heydaripour et al.
filtered and crystallized in EtOH to afford pure 1a–h,
2a–d, and 3, respectively (Scheme 1).
10.2, H C(3)); 6.53–7.26 (m, 11 H, Ar, and
H C(4)).
(E)-1’,3’,3’-Trimethyl-6-(phenyldiazenyl)spiro[chro-
mene-2,2’-indoline] (1a). Yield (90%). m.p. 206–207.5ꢀC.
IR (KBr): 2968 (CH ), 1606 (C(3) C(4)), 1485 (N N),
1311 (C(2) N), 1018 (C(2) O).
(E)-1’,3’,3’-Trimethyl-6-((4-nitrophenyl)diazenyl)
spiro-[chromene-2,2’-indoline]
(1h.Yield
(90%).
m.p. 202–203ꢀC. IR (KBr): 2962 (CH ), 1606 (C(3) C
(4)), 1521and 1346 (N O), 1456 (N N), 1278 (C(2)
1
(E)-4-((1’,3’,3’-Trimethylspiro[chromene-2,2’-indo-
N), 1022 (C(2) O). H-NMR (CDCl3) δ 1.20 (s, Me);
3
lin]-6-yl)diazenyl)phenol
(1b).
Yield
(90%).
1.33 (s, Me); 2.77 (s, NMe); 5.80 (d, JHH = 10.3, H C
m.p. 193–195ꢀC. IR (KBr): 2966 (CH ), 1606 (C(3)
C(4)), 1487 (N N), 1259 (C(2) –N), 1012 (C(2) –O).
1H-NMR (CDCl3) δ ppm: 1.33 (s, Me); 1.62 (s, Me);
(3)); 6.56–8.37 (m, 11 H, Ar, and H C(4)). 13C-NMR
(CDCl3) δ 20, 25, 28 (CH3); 52, 53 (C(3’), C(2));
121, 127 (C(4), C(3)); 106–158 (18 C, Ar)
3
2.84 (s, NMe); 4.20 (d, JHH = 10.2, H-C(3));
(E)-4-((1’,3’,3’-trimethyl-6-nitrospiro[chromene-2,2’-
6.53–7.25 (m, 11 H, Ar, and H C(4)).
indolin]-8-yl)diazenyl)phenol
(2b).
Yield
(70%).
(E)-1’,3’,3’-trimethyl-6-(p-tolyldiazenyl)spiro[chro-
mene-2,2’-indoline] (1c). Yield (90%). m.p. 181–182ꢀC.
IR (KBr): 1456 (N N), 2960 (CH ), 1604 (C(3) C
(4)), 1317 (C(2) N), 1016 (C(2) O). 1H-NMR
(CDCl3) δ ppm: 1.32 (s, Me); 1.70 (s, Me); 2.67 (s,
m.p. 168–168.5ꢀC. IR (KBr): 2970 (CH ), 1604 (C(3)
C(4)), 1508 and1338 (N O), 1487 (N N), 1259 (C(2)
1
N), 1012 (C(2) O) ). H-NMR (CDCl3) δ ppm: 1.29 (s,
3
Me); 1.54 (s, Me); 2.80 (s, NMe); 4.20 (d, JHH = 10.2,
H C(3)); 6.53–7.25 (m, 10 H, Ar, and H C(4)).
3
NMe); 2.34 (s, Ph-Me); 4.26 (d, JHH = 10.2, H C
(E)-8-((4-Methoxyphenyl)diazenyl)-1’,3’,3’-trimethyl-
6-nitrospiro[chromene-2,2’-indoline] (2c). Yield (90%).
m.p. 169–170ꢀC. IR (KBr): 2962 (CH ), 1606 (C(3) C
(4)), 1510 and 1322 (N O), 1488 (N N), 1311 (C(2) N),
(3)); 6.57–7.92 (m, 11 H, Ar, and H C(4)).
(E)-6-((4-Methoxyphenyl)diazenyl)-1’,3’,3’-trimethyl-
spiro- [chromene-2,2’-indoline] (1d). Yield (50%). m.
p. 193.5–195ꢀC. IR (KBr): 2966 (CH ), 1604 (C(3) C
1
1018 (C(2) O). H-NMR (CDCl3) δ ppm:1.28 (s, Me);
1
(4)), 1452 (N N), 1307 (C(2) N), 1018 (C(2) O). H-
1.63 (s, Me); 2.86 (s, NMe); 3.4 (s, OMe); 4.15 (d, 3JHH
10.1, H C(3)); 6.57–8.24 (m, 10 H, Ar, and H C(4)).
=
NMR (CDCl3) δ ppm: 1.32 (s, Me); 1.70 (s, Me); 2.67
3
(s, NMe); 3.4 (s, OMe); 4.26 (d, JHH = 10.2, H C(3));
(E)-1’,3’,3’-Trimethyl-6-nitro-8-((4-nitrophenyl)dia-
zenyl)spiro[chromene-2,2’-indoline] (2d). Yield (50%). m.
p. 169.5–170ꢀC . IR (KBr): 2968 (CH ), 1604 (C(3) C
(4)), 1494 and 1338 (N O), 1488 (N N), 1259 (C(2)
6.57–7.92 (m, 11 Arꢁ ꢁ ꢁH, and H C(4)).
(E)-6-((4-Chlorophenyl)diazenyl)-1’,3’,3’-trimethyl-
spiro-[chromene-2,2’-indoline] (1e). Yield (90%).
m.p. 176–178ꢀC. IR (KBr): 2962 (CH ), 1602 (C(3)
C(4)), 1454 (N N), 1317 (C(2) N), 1016 (C(2) O).
1H-NMR (CDCl3) δ ppm: 1.37 (s, Me); 1.67 (s, Me);
1
N), 1014 (C(2) O). H-NMR (CDCl3) δ ppm: 1.38 (s,
3
Me); 1.55 (s, Me); 2.86 (s, NMe); 4.15 (d, JHH = 10.1,
H C(3)); 6.53–7.25 (m, 10 H, Ar, and H C(4)).
3
2.90 (s, NMe); 4.20 (d, JHH = 10.2, 2H C(3));
(E)-5-Methoxy-1’,3’,3’-trimethyl-6-((4-nitrophenyl)
diazenyl)-spiro[chromene-2,2’-indoline] (3). Yield (80%).
m.p. 192–194ꢀC . IR (KBr): 2964 (CH ), 1606 (C(3)
C(4)), 1488 (N N), 1465 and 1340 (N O), 1280 (C
(2) N), 1020 (C(2) O). 1H-NMR (CDCl3) δ ppm:
1.33 (s, Me); 1.65 (s, Me); 2.90 (s, NMe); 3.7 (s, OMe);
6.57–7.95 (m, 11 H, Ar, and H C(4)).
(E)-6-((4-Bromophenyl)diazenyl)-1’,3’,3’-trimethyl-
spiro-[chromene-2,2’-indoline] (1f ). Yield (90%). m.
p. 173.5–175ꢀC. IR (KBr): 2964 (CH ), 1606 (C(3) C
1
(4)), 1454 (N N), 1317 (C(2) N), 1016 (C(2) O). H-
3
NMR (CDCl3) δ ppm: 1.31 (s, Me); 1.54 (s, Me); 2.84
4.25 (d, JHH = 10.1, H C(3)); 6.53–7.25 (m, 10 H,
3
(s, NMe); 4.20 (d, JHH = 10.4, H C(3)); 6.53–7.70
Ar, and H C(4)).
(m, 11 H, Ar, and H C(4)).
(E)-4-((1’,3’,3’-Trimethylspiro[chromene-2,2’-
CONCLUSIONS
indolin]-6-yl)dia-zenyl)benzoic
acid
(1g).
Yield
Electron-donor substitution on the azo moiety in
azospiropyrans made colorless SP and purple mero forms
with high color strength. However, electron-acceptor sub-
stitution on azo moiety were made yellowish SP and red
mero forms, with more stable SP forms. The presence of
(50%). m.p. 157–159ꢀC. IR (KBr): 2962 (CH ),
1606 (C(3) C(4)), 1452 (N N), 1313 (C(2) N),
1016 (C(2) O). 1H-NMR (CDCl3) δ ppm: 1.33 (s,
Me); 1.54 (s, Me); 2.84 (s, NMe); 4.20 (d, JHH
3
=
10
www.jccs.wiley-vch.de
© 2016 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
J. Chin. Chem. Soc. 2016

JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
Sensitivity and Resolution Developing
9. T. Tatsuma, Bull. Chem. Soc. Jpn. 2013, 86, 1.
electron-acceptor group on the azo moiety and the elec-
tron donor on the salicylaldehyde moiety effected a red
shift to longer absorption wavelengths in comparison
with the reverse condition, i.e., the electron-donor group
on the azo and the acceptor on the salicylaldehyde moi-
ety. Increasing the molar absorption coefficient of non-
substituted spiropyrans in mero forms in the presence of
an azo moiety was effective in improving the spiropyran
structures, which led to higher sensitivity and resolution,
and hence better distinction between the SP (OFF) and
mero (ON) forms in photochromic applications. In azos-
piropyrans, electron-donor groups at the para position of
the azo moiety produced colorless SP forms (OFF)
switching to highly colored MC forms (λmax ≥ 550 nm).
In contrast, electron-acceptor substitutions reduced the
possibility of SP–MC color differentiation.
10. T. Inoue, M. Inokuchi, Chem. Lett. 2015, 44, 911.
11. Y. S. Nam, I. Yoo, O. Yarimaga, I. S. Park, D. H. Park,
S. Song, J. M. Kim, C. W. Lee, Chem. Commun. 2014,
50, 4251.
12. R. Ranganathan, K. Sasikumar, P. Keblinski, J. Appl.
Phys. 2015, 117.
13. S. Ruetzel, M. Diekmann, P. Nuernberger, W. Walter,
B. Engels, T. Brixner, J. Chem. Phys. 2014, 140, 224310.
14. L. Zhang, L. W. Dai, Y. Rong, Z. Z. Liu, D. Y. Tong,
Y. J. Huang, T. Chen, Langmuir 2015, 31, 1164.
15. N. Tanaka, A. Okazawa, A. Sugahara, N. Kojima, Bull.
Chem. Soc. Jpn.. 2015, 88, 1150.
16. N. Darwish, A. C. Aragones, T. Darwish, S. Ciampi,
I. Diez-Perez, Nano Lett. 2014, 14, 7064.
17. A. Radu, R. Byrne, N. Alhashimy, M. Fusaro,
S. Scarmagnani, D. Diamond, J. Photochem. Photobiol.
A. Chem. 2009, 206, 109.
Colorless azospiropyrans obtained by coupling of
5-nitrosalicylaldehyde with diazotized p-substituted ani-
18. F. Nourmohammadian, T. Q. Wu, N. R. Branda, Chem.
Commun. 2011, 47, 10954.
19. E. Uchida, R. Azumi, Y. Norikane, Chem. Lett. 2014,
43, 1619.
lines switched to highly colored mero forms (λmax
:
550–573 nm).
20. P. K. Patel, V. K. Johns, D. M. Mills, J. E. Boone,
P. Calvo-Marzal, K. Y. Chumbimuni-Torres, Electroana-
lysis 2015, 27, 677.
Possessing an electron-donor group (OMe) on the
salicylaldehyde moiety and also an acceptor group
(NO2) on azo substitute produced yellowish azospiro-
pyrans in SP form, which switched to green color. Spir-
opyrans usually produce red shade in their colored
21. M. Hammarson, J. R. Nilsson, S. M. Li, P. Lincoln,
J. Andreasson, Chem. Eur. J. 2014, 20, 15855.
22. F. Jonsson, T. Beke-Somfai, J. Andreasson, B. Norden,
Langmuir 2013, 29, 2099.
form, so producing
interesting.
a green color can be very
23. T. Igarashi, S. Kuwahara, K. Katayama, Bull. Chem.
Soc. Jpn. 2013, 86, 1071.
24. E. Buncel, Can. J. Chem. 2000, 78, 1251.
25. J. W. Kim, Y. Jung, G. W. Coates, M. N. Silberstein,
Macromolecules 2015, 48, 1335.
ACKNOWLEDGMENT
The authors are grateful to the Semnan University
Research Council for the partial support to this work.
26. Z. Sekkat, H. Ishitobi, D. Yasumatsu, S. Kawata, In Lin-
ear, Nonlinear, and Power-Limiting Organics, M. Eich,
M. G. Kuzyk, C. M. Lawson, R. A. Norwood Eds.,
2000, 4106, p. 133.
27. J. Tao, Y. H. Li, P. Zhao, J. S. Li, Y. Duan, W. J. Zhao,
R. H. Yang, Biosens. Bioelectron. 2014, 62, 151.
28. B. Seefeldt, R. Kasper, M. Beining, J. Mattay,
J. Arden-Jacob, N. Kemnitzer, K. H. Drexhage,
M. Heilemann, M. Sauer, Photochem. Photobiol. Sci.
2010, 9, 213.
REFERENCES
1. G. Szaloki, O. Aleveque, J. L. Pozzo, R. Hadji,
E. Levillain, L. Sanguinet, Phys. Chem. B 2015, 119, 307.
2. Q. K. Qi, J. Y. Qian, S. Q. Ma, B. Xu, S. X. A. Zhang,
W. J. Tian, Chem. Eur. J. 2015, 21, 1149.
3. H. T. Nguyen, L. T. T. Nguyen, T. V. Le, Des. Mono-
mers Polym. 2015, 18, 271.
4. Y. N. Zhou, J. J. Li, Q. Zhang, Z. H. Luo, Langmuir
2014, 30, 12236.
29. M. Sakuragi, K. Aoki, T. Tamaki, K. Ichimura, Bull.
Chem. Soc. Jpn. 1990, 63, 74.
30. E. Berman, R. E. Fox, F. D. Thomson, J. Am. Chem.
Soc. 1959, 81, 5605.
31. H. Gorner, Phys. Chem. Chem. Phys. 2001, 3, 416.
32. H. Dürr, H. Bouas-Laurent, Photochromism: Molecules
and Systems: Molecules and Systems, Gulf Profes-
5. G. F. Zhang, T. Chen, C. Li, W. L. Gong, M. P. Aldred,
M. Q. Zhu, Chin. J. Org. Chem. 2013, 33, 927.
6. C. Brieke, A. Heckel, Chem. Eur. J. 2013, 19, 15726.
7. N. Shao, J. Jin, H. Wang, J. Zheng, R. Yang, W. Chan,
Z. Abliz, J. Am. Chem. Soc. 2009, 132, 725.
8. J. Y. Wang, C. G. Feng, W. X. Hu, Prog. Chem. 2006,
18, 298.
J. Chin. Chem. Soc. 2016
© 2016 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.jccs.wiley-vch.de
11

Article
Heydaripour et al.
sional Publishing Amsterdam; Boston: Elsevier, 2003,
ISBN 0080538835.
40. J.-F. Zhu, H. Yuan, W.-H. Chan, A. W. M. Lee, Org.
Biomol. Chem. 2010, 8, 3957.
41. É. R. Zakhs, V. M. Martynova, L. S. Éfros, Chem. Het-
erocycl. Compd 1979, 15, 351.
33. F. Nourmohammadian, A. A. Abdi, Spectrochim. Acta
Part A Mol. Biomol. Spectrosc. 2016, 153, 53.
34. F. Nourmohammadian, A. A. Abdi, Bull. Korean Chem.
Soc. 2013, 34, 1727.
42. G. Such, R. A. Evans, L. H. Yee, T. P. Davis,
J. Macromol. Sci. Polym. Rev. 2003, 43, 547.
43. M. Dumont, A. El Osman, Chem. Phys. 1999, 245, 437.
44. J. G. Zhi, B. Y. Zhang, B. L. Zang, G. H. Shi, J. Appl.
Polym. Sci. 2002, 85, 2155.
45. H. Ishitobi, Z. Sekkat, S. Kawata, Mol. Crys. Liq. Cryst.
2000, 344, 107.
46. M. L. Rahman, M. M. Yusoff, G. Hegde, M. N.
F. A. Malek, N. A. Samah, H. T. Srinivasa, S. Kumar,
J. Chin. Chem. Soc. 2014, 61, 571.
35. S. G. Kandi, F. Nourmohammadian, J. Mol. Struct.
2013, 1050, 222.
36. B. Hellrung, H. Balli, Helv. Chim. Acta 1989,
72, 1583.
37. É. Zakhs, L. Zvenigorodskaya, L. Éfros, Chem. Hetero-
cycl. Compd 1973, 9, 1463.
38. S.-R. Keum, K.-B. Lee, P. M. Kazmaier, E. Buncel, Tet-
rahedron Lett. 1994, 35, 1015.
39. S.-R. Keum, M.-J. Lee, S. Swansburg, E. Buncel,
R. P. Lemieux, Dyes Pigm. 1998, 39, 383.
12
www.jccs.wiley-vch.de
© 2016 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
J. Chin. Chem. Soc. 2016