Isomerization of spirobenzopyrans bearing electron-donating and electron-withdrawing groups in acidic aqueous solutions

Article abstract of DOI:10.1039/c0cp01989e

Spirobenzopyrans, which are well known as photochromic compounds, exist as thermodynamically stable protonated ring-opened isomers (protonated merocyanine form, McH) in an acidic aqueous solution in the dark. In the present study, we investigated effects

Full text of DOI:10.1039/c0cp01989e

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PAPER
Isomerization of spirobenzopyrans bearing electron-donating and
electron-withdrawing groups in acidic aqueous solutions
Taku Satoh, Kimio Sumaru,* Toshiyuki Takagi, Katsuki Takai and
Toshiyuki Kanamori
Received 1st October 2010, Accepted 10th February 2011
DOI: 10.1039/c0cp01989e
Spirobenzopyrans, which are well known as photochromic compounds, exist as
thermodynamically stable protonated ring-opened isomers (protonated merocyanine form, McH)
in an acidic aqueous solution in the dark. In the present study, we investigated effects of
substitution of the spirobenzopyrans on a ring-opening behavior in an aqueous system. We
prepared five polymerizable spirobenzopyrans that are substituted with a methoxy group or a
nitro group at the 60- or 80-positions and without a substituent. These monomers were
copolymerized with N,N-dimethylacrylamide to evaluate the spirobenzopyrans in aqueous
solution. Correlation between ring-opening rates and the kind and position of the substitution can
be summarized as follows: the substitution of an electron-donating methoxy group and the
substitution at the 80-position increased the ring-opening rate, whereas the substitution of an
electron-withdrawing nitro group decreased the rate. The effects of the substitution can be
explained by changes in the electron density of the oxygen atom of the spirobenzopyrans.
1. Introduction
Photochromic compounds undergo a reversible isomerization
by light-irradiation.1 This unique nature has attracted
considerable attention both for their fundamental characteristics
and potential applications in photonic devices. Spirobenzopyrans
that are typical of the photochromic compounds have been
intensively studied over the past few decades.2 Almost all
studies on characteristics of the spirobenzopyrans have been
performed in organic solvents or their binary mixtures with
water because of insolubility of those in water. A dominant
isomer of the spirobenzopyrans in organic solvents is a
colorless ring-closed form (spiro form; Sp) in general. An
isomerization of the Sp to a colored ring-opened form
Fig. 1 Chemical structures of spiropyran (Sp), merocyanine (Mc),
(merocyanine form; Mc) is induced by UV-light irradiation
and the reverse isomerization undergoes by visible-light irradiation
(Fig. 1). The isomerization also occurs thermally in the dark.
Moreover, a substitution,3–6 a solvent polarity,6,7 and the
presence of proton8–10 affect the isomerization behavior.
Recently, photoresponse of the spirobenzopyrans in
aqueous solutions has been investigated by conjugation to water-
soluble polymers11,12 or peptides.13–15 We have demonstrated a
reversible volume phase transition of spirobenzopyran-bearing
hydrogels in response to light-irradiation11 and have examined
protonated Mc (McH), and N-protonated Sp (SpH) and their equilibria.
A bridge region of the Mc and the McH allows conformational
changes in them.
their applications to a light-controllable valve and actuator to
control microfluidic systems.16 The volume changes in the
hydrogels include stages of visible-light-induced shrinking
and spontaneous re-swelling. The isomerization of a spiroben-
zopyran is essential in the volume changes. The present
problem is a slow swelling of the hydrogels, which is
likely due to a slow ring opening of an unsubstituted
spirobenzopyran.
Isomerization behaviors of the spirobenzopyrans in water,15
as well as that in organic solvents, are significantly affected by
substitutions. Therefore, a suitable choice of substituents is
Research Center for Stem Cell Engineering, National Institute of
Advanced Industrial Science and Technology (AIST), AIST Tsukuba
Central 5, 1-1-1, Higashi, Tsukuba, Ibaraki 305-8568, Japan.
E-mail: k.sumaru@aist.go.jp; Fax: +81 29-861-6278;
Tel: +81 29-861-6373
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CDCl3 at 25 1C. Tetramethylsilane or residual solvent was
used as internal reference. Multiplicities on the 1H NMR
spectra are represented as s, singlet; d, doublet; dd, double
doublet; ddd, double double doublet; and m, multiplet.
High-resolution mass spectra (HRMS) were recorded with a
JEOL JMS-700T Tandem MStation mass spectrometer
(JEOL) in the positive ion electron impact mode (EI+). Gel
permeation chromatography (GPC) was performed by using a
HLC-8121GPC/HT GPC system (Tosoh Co., Tokyo, Japan)
equipped with two serial-coupled Shodex LF-804 columns
(Showa Denko K.K., Tokyo, Japan) and a differential refracto-
meter under the following conditions: mobile phase, THF;
temperature, 50 1C; and flow rate, 0.5 mL minꢀ1. Polystyrene
standards (Tosoh Co.) were used as calibration standards.
UV–visible spectroscopy was performed on a V-560 UV/VIS
spectrophotometer (Jasco Co., Tokyo, Japan). Light-irradiation
was carried out by using a Lightningcure LC6 light generator
(Hamamatsu Photonics K.K., Hamamatsu, Japan) with blue-
light peaked at 436 nm with a total intensity of 80 mW.
2.2 Syntheses of poly(N,N-dimethylacrylamide) with pendant
spirobenzopyrans
2.2.1 1-(2-Carboxyethyl)-3,3-dimethylspiro(20H-10-benzo-
pyran-2,20-indoline) derivatives (3a–e). A mixture solution of
Scheme 1 Synthetic route to spirobenzopyran-bearing polymers
6a–e. (a) Piperidine, 2-butanone, reflux, 3 h. (b) 4-Nitrophenol,
DCC, DMAP, THF, rt, 4 h. (c) N-(3-Aminopropyl)methacrylamide
hydrochloride, triethylamine, DMF, rt, 4 h. (d) DMAAm, AIBN, dry
THF, 65 1C, 20 h.
1-(2-carboxyethyl)-2,3,3-trimethyl-3H-indolenium iodide
1
(2.00 g, 5.57 mmol), salicylaldehyde 2a (0.59 mL, 5.65 mmol),
and piperidine (0.56 mL, 5.66 mmol) in 2-butanone (16.7 mL)
was refluxed for 3 h. The reaction products were extracted
with CH2Cl2 and then the desired product was isolated by
silica gel column chromatography with a step gradient from
ethyl acetate/hexane in 3 : 1 (v/v) to methanol/ethyl acetate in
1 : 1 (v/v). After drying under reduced pressure, the purified
product 3a was obtained as a reddish brown powder in a yield
of 74% (1.37 g, 4.10 mmol). 1H NMR (600 MHz, CDCl3):
d 1.12 (3H, s, C3–CH3), 1.27 (3H, s, C3–CH3), 2.60 (1H, m,
CO–CH2–), 2.67 (1H, m, CO–CH2–), 3.52 (1H, m, N1–CH2–),
3.64 (1H, m, N1–CH2–), 5.64 (1H, d, J = 10 Hz, H30), 6.56
(1H, d, J = 8 Hz, H7), 6.65 (1H, d, J = 8 Hz, H80), 6.79 (1H,
ddd, J = 1 Hz, 7 Hz, and 8 Hz, H60), 6.82 (1H, d, J = 10 Hz,
H40), 6.84 (1H, ddd, J = 1 Hz, 7 Hz, and 7 Hz, H5), 7.01 (1H,
dd, J = 1 Hz and 8 Hz, H50), 7.05 (1H, ddd, J = 1 Hz, 7 Hz
and 8 Hz, H70), 7.07 (1H, dd, J = 1 Hz and 7 Hz, H4), 7.16
(1H, ddd, J = 1 Hz, 8 Hz and 8 Hz, H6).
necessary to achieve desired characteristics of spirobenzopyrans
in aqueous solutions. Effects of substitutions on the iso-
merization have been extensively investigated in organic
solvents,3–6 whereas little is known about those in water.15
Thus, it is important to obtain a clear relationship between the
kind and position of a substituent and the isomerization
behavior of a substituted spirobenzopyran in water.
In this study, we investigated effects of substitution on the
spontaneous ring opening of spirobenzopyrans in water. We
prepared water-soluble polymers with pendant spirobenzopyran
derivatives in which an electron-donating group and an electron-
withdrawing group were substituted at the 60- or 80-position
(Scheme 1). The spontaneous isomerization behavior of the
substituted spirobenzopyrans from the Sp to the McH in water
was evaluated by using the polymers with pendant
spirobenzopyrans.
Other derivatives 3b–e were prepared by a similar method to
that of 3a. In synthesis of 3e, a crystalline crude product was
formed at room temperature (rt) after the reflux. The purified
product was isolated by filtration and washing with acetone.
2. Materials and methods
2.1 General
1
Compound 3b. Yield of 69%, a reddish brown powder. H
N,N-Dimethylacrylamide (DMAAm) (Tokyo Kasei Co.,
Tokyo, Japan) was distilled before use. All other reagents
and solvents were used as received from commercial sources.
1H NMR, 13C NMR, 1H–1H COSY, and 1H–13C HMQC
spectra were measured on a JEOL JNM-LA 600 FT-NMR
system (JEOL, Tokyo, Japan) operating at 600 MHz and
150 MHz for 1H NMR and 13C NMR, respectively, except
for using a JEOL JNM-ECX 400 system (JEOL) operating at
NMR (600 MHz, CDCl3): d 1.11 (3H, s, C3–CH3), 1.27
(3H, s, C3–CH3), 2.61 (1H, m, CO–CH2–), 2.68 (1H, m,
CO–CH2–), 3.51 (1H, m, N1–CH2–), 3.64 (1H, m,
N1–CH2–), 3.74 (3H, s, OCH3), 5.67 (1H, d, J = 10 Hz,
H30), 6.56 (1H, d, J = 8 Hz, H7), 6.59 (1H, d, J = 3 Hz, H50),
6.59 (1H, d, J = 8 Hz, H80), 6.65 (1H, dd, J = 3 Hz and 8 Hz,
H70), 6.78 (1H, d, J = 10 Hz, H40), 6.84 (1H, dd, J = 7 Hz
and 7 Hz, H5), 7.06 (1H, dd, J = 1 Hz and 7 Hz, H4), 7.16
(1H, ddd, J = 1 Hz, 8 Hz and 8 Hz, H6).
1
400 MHz for H NMR. All NMR spectra were recorded in
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Compound 3c. Yield of 91%, a yellow powder. 1H NMR
(600 MHz, CDCl3): d 1.12 (3H, s, C3–CH3), 1.28 (3H, s,
C3–CH3), 2.60 (1H, m, CO–CH2–), 2.71 (1H, m, CO–CH2–),
3.54 (1H, m, N1–CH2–), 3.66 (3H, s, OCH3), 3.69 (1H, m,
N1–CH2–), 5.64 (1H, d, J = 10 Hz, H30), 6.55 (1H, d, J = 8
Hz, H7), 6.67 (1H, dd, J = 2 Hz and 7 Hz, H70), 6.72–6.75
(2H, m, H50 and H60), 6.80 (1H, d, J = 10 Hz, H40), 6.82 (1H,
dd, J = 7 Hz and 7 Hz, H5), 7.06 (1H, d, J = 7 Hz, H4), 7.14
(1H, ddd, J = 1 Hz, 7 Hz and 7 Hz, H7).
pale green powder in a yield of 92% (1.11 g, 2.43 mmol).
1H NMR (600 MHz, CDCl3): d 1.12 (3H, s, C3–CH3), 1.29
(3H, s, C3–CH3), 1.53 (2H, m, CH2–CH2–CH2), 1.98 (3H, s,
vinyl-CH3), 2.36 (2H, m, CO–CH2–), 2.57 (2H, m, CO–CH2–),
3.13 (2H, m, NH–CH2–), 3.20 (2H, m, NH–CH2–), 3.47
(1H, m, N1–CH2–), 3.73 (1H, m, N1–CH2–), 5.33 (1H, m,
vinyl CH2), 5.65 (1H, d, J = 10 Hz, H30), 5.75 (1H, m, vinyl
CH2), 6.62 (1H, d, J = 8 Hz, H7), 6.67 (1H, d, J = 8 Hz, H80),
6.83 (1H, dd, J = 7 Hz and 7 Hz, H60), 6.83 (1H, dd, J = 7 Hz
and 7 Hz, H5), 6.84 (1H, d, J = 10 Hz, H40), 7.06 (1H, d,
J = 7 Hz, H50), 7.06 (1H, d, J = 7 Hz, H4), 7.10 (1H, dd,
J = 7 Hz and 8 Hz, H70), 7.14 (1H, dd, J = 7 Hz and 8 Hz,
H6). HRMS (EI+): m/z calcd for C28H33N3O3 459.2522,
found 459.2519.
Compound 3d. Yield of 61%, a brown powder. 1H NMR
(600 MHz, CDCl3): d 1.14 (3H, s, C3–CH3), 1.26 (3H, s,
C3–CH3), 2.61 (1H, m, CO–CH2–), 2.67 (1H, m, CO–CH2–),
3.54 (1H, m, N1–CH2–), 3.63 (1H, m, N1–CH2–), 5.84 (1H, d,
J = 11 Hz, H30), 6.60 (1H, d, J = 8 Hz, H7), 6.72 (1H, d, J =
8 Hz, H80), 6.90 (1H, ddd, J = 1 Hz, 7 Hz, and 8 Hz, H5), 6.91
(1H, d, J = 11 Hz, H40), 7.09 (1H, dd, J = 1 Hz and 8 Hz,
H4), 7.20 (1H, ddd, J = 1 Hz, 8 Hz and 8 Hz, H6), 7.99–8.01
(2H, m, H50 and H70).
Other derivatives 5b–e were prepared by similar methods to
that of 5a.
1
Compound 5b. Yield of 90%, a pale grey powder. H NMR
(600 MHz, CDCl3): d 1.12 (3H, s, C3–CH3), 1.28 (3H, s,
C3–CH3), 1.53 (2H, m, CH2–CH2–CH2), 1.98 (3H, s,
vinyl-CH3), 2.36 (2H, m, CO–CH2–), 2.56 (2H, m, CO–CH2–),
3.17 (4H, m, NH–CH2–), 3.45 (1H, m, N1–CH2–), 3.72 (1H,
m, N1–CH2–), 3.75 (3H, s, OCH3), 5.34 (1H, m, vinyl CH2),
5.68 (1H, d, J = 10 Hz, H30), 5.75 (1H, m, vinyl CH2), 6.61
(1H, d, J = 8 Hz, H80), 6.61 (1H, s, H50), 6.61 (1H, d,
J = 8 Hz, H7), 6.67 (1H, d, J = 8 Hz, H70), 6.80 (1H, d,
J = 10 Hz, H40), 6.81 (1H, dd, J = 7 Hz and 8 Hz, H5), 7.14
(1H, dd, J = 8 Hz and 8 Hz, H6), 7.14 (1H, d, J = 7 Hz, H4).
HRMS (EI+): m/z calcd for C29H35N3O4 489.2628, found
489.2631.
Compound 3e. Yield of 99%, a dark grey powder. 1H NMR
(600 MHz, CDCl3): d 1.14 (3H, s, C3–CH3), 1.31 (3H, s,
C3–CH3), 2.64 (1H, m, CO–CH2–), 2.75 (1H, m, CO–CH2–),
3.55 (1H, m, N1–CH2–), 3.65 (1H, m, N1–CH2–), 5.81 (1H, d,
J = 10 Hz, H30), 6.56 (1H, d, J = 8 Hz, H7), 6.85–6.88 (2H,
m, H-5 and H-60), 6.89 (1H, d, J = 10 Hz, H40), 7.06 (1H, d,
J = 7 Hz, H4), 7.16 (1H, ddd, J = 1 Hz, 8 Hz and 8 Hz, H6),
7.35 (1H, dd, J = 2 Hz and 8 Hz, H50), 7.65 (1H, dd, J = 2 Hz
and 8 Hz, H70).
2.2.2 1-[2-(4-Nitrophenyl)oxycarbonylethyl]-3,3-dimethyl-
spiro(20H-10-benzopyran-2,20-indoline) derivatives (4a–e).
Compound 5c. Yield of 88%, a blue grey powder. 1H NMR
(600 MHz, CDCl3): d 1.09 (3H, s, C3–CH3), 1.29 (3H, s,
C3–CH3), 1.34 (2H, m, CH2–CH2–CH2), 2.00 (3H, s, vinyl-
CH3), 2.21 (2H, m, CO–CH2–), 2.49 (2H, m, CO–CH2–),
2.96 (2H, m, NH–CH2–), 3.05 (2H, m, NH–CH2–), 3.55
(1H, m, N1–CH2–), 3.75 (3H, s, OCH3), 3.98 (1H, m,
N1–CH2–), 5.34 (1H, m, vinyl CH2), 5.61 (1H, d, J = 10
Hz, H30), 5.75 (1H, m, vinyl CH2), 6.65 (1H, d, J = 7 Hz, H7),
6.73 (1H, d, J = 7 Hz, H70), 6.75 (1H, dd, J = 7 Hz and 8 Hz,
H5), 6.79 (1H, d, J = 8 Hz, H50), 6.81 (1H, dd, J = 7 Hz and
8 Hz, H60), 6.83 (1H, d, J = 10 Hz, H40), 7.01 (1H, d, J = 8
Hz, H4), 7.10 (1H, dd, J = 7 Hz and 7 Hz, H6). HRMS (EI+):
m/z calcd for C29H35N3O4 489.2628, found 489.2638.
A
mixture solution of 4-nitrophenol (0.608 g, 4.37 mmol),
N,N0-dicyclohexylcarbodiimide (DCC) (0.902 g, 4.37 mmol), and
4-dimethylaminopyridine (DMAP) (0.050 g, 0.41 mmol)
in THF (16 mL) was added dropwise to a solution of 3a
(1.33 g, 3.97 mmol) in THF (24 mL) with stirring. After the
stirring of the reaction mixture for 4 h at rt, the formed
precipitates were removed by filtration. The reaction products
were extracted with ethyl acetate and then the desired product
was isolated by silica gel column chromatography with a step
gradient from ethyl acetate/hexane in 1 : 4 to 1 : 1 (v/v). After
drying under reduced pressure, the purified product 4a was
obtained in a yield of 66% (1.20 g, 2.63 mmol).
Other derivatives 4b–e were prepared by a similar method to
that of 4a. The yields of compounds 4b, 4c, 4d, and 4e were
44%, 60%, 59%, and 57%, respectively.
Compound 5d. Yield of 78%, a pale yellow powder.
1H NMR (600 MHz, CDCl3): d 1.14 (3H, s, C3–CH3), 1.26
(3H, s, C3–CH3), 1.57 (2H, m, CH2–CH2–CH2), 1.97 (3H, s,
vinyl-CH3), 2.43 (2H, m, CO–CH2–), 2.56 (2H, m, CO–CH2–),
3.20 (4H, m, NH–CH2–), 3.50 (1H, m, N1–CH2–), 3.68 (1H,
m, N1–CH2–), 5.35 (1H, m, vinyl CH2), 5.73 (1H, m, vinyl
CH2), 5.86 (1H, d, J = 11 Hz, H30), 6.67 (1H, d, J = 8 Hz,
H7), 6.75 (1H, d, J = 8 Hz, H80), 6.87 (1H, dd, J = 7 Hz and
7 Hz, H5), 6.91 (1H, d, J = 11 Hz, H40), 7.07 (1H, d, J = 7
Hz, H4), 7.18 (1H, dd, J = 7 Hz and 8 Hz, H6), 7.98
(1H, s, H50), 8.00 (1H, d, J = 8 Hz, H70). HRMS (EI+):
m/z calcd for C28H32N4O5 504.2373, found 504.2370.
2.2.3 N-[3-[2-[3,3-Dimethylspiro(20H-10-benzopyran-2,20-
indolin)-1-yl]ethylcarbonylamino]propyl]methacrylamide derivatives
(5a–e). A solution of 4a (0.518 g, 2.90 mmol) in DMF
(6.0 mL) was added dropwise to a mixture solution of
N-(3-aminopropyl)methacrylamide hydrochloride (1.20 g,
2.63 mmol) and triethylamine (0.41 mL, 2.94 mmol) in
DMF (4.5 mL) with stirring. After the stirring for 4 h at rt,
the reaction products were extracted with ethyl acetate and
then the desired product was isolated by silica gel column
chromatography with a step gradient from ethyl acetate/
hexane in 3 : 1 (v/v) to ethyl acetate. After drying under
reduced pressure, the purified product 5a was obtained as a
Compound 5e. Yield of 76%, a dark blue grey powder.
1H NMR (600 MHz, CDCl3): d 1.11 (3H, s, C3–CH3), 1.28
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Table 1 The 13C NMR chemical shift assignments of the monomeric
spirobenzopyrans 5a–ea
obtained as a pale grey powder in a yield of 88% (w/w). The
content of the spirobenzopyran in the obtained polymers
was estimated to be 11 mol% by 1H NMR spectroscopy.
Weight-average molecular weight (Mw) and molecular weight
distribution (Mw/Mn) of the polymers were determined to be
2400 and 1.7, respectively, by GPC. 1H NMR (400 MHz,
CDCl3): 0.8–2.0 (54H, br, C3–CH3, CH2–CH2–CH2, vinyl-
CH3, DMAAm CH2, and DMAAm CH), 2.3–3.4 (64H, br,
CO–CH2–, DMAAm CH3, and NH–CH2–), 3.4–3.5 (1H, br,
N1–CH2–), 3.6–3.8 (1H, br, N1–CH2–), 5.6–5.7 (1H, br, H30),
6.5–6.7 (2H, br, H7 and H80), 6.7–6.9 (3H, br, H5, H60, and
H40), 7.0–7.2 (4H, br, H50, H4, H70, and H6).
Position
5a
5b
5c
5d
5e
Indoline ring
C2,20
C3
104.6
52.2
25.8
104.3
52.2
25.8
105.0
53.0
26.0
106.8
52.8
25.8
107.9
53.5
25.9
C3–CH3
20.1
20.2
20.0
19.8
20.2
C3a
C4
C5
C6
C7
C7a
136.4
121.8
119.1
127.6
106.6
146.6
136.4
121.8
119.0
127.6
106.6
146.6
135.3
121.7
118.4
127.6
106.5
146.7
135.9
121.8
119.7
127.8
106.9
146.4
135.0
121.7
119.6
127.8
106.8
145.8
Other polymers 6b–e were prepared by a similar method to
that of 6a.
Benzopyran ring
C30
119.5
129.5
118.7b
126.9
120.4
129.8
114.9
153.8b
—
120.5
129.4
119.0b
111.7
153.4b
115.3
115.5
147.9b
55.8
120.2
129.5
119.5b
119.3b
120.2
112.7b
146.8b
142.3b
56.0
121.9
128.3
118.7b
122.8
141.1
125.9
115.5
159.4b
—
122.4
128.6
121.9b
131.7b
119.7
125.4b
137.2b
148.1b
—
C40
C40a
C50
Polymer 6b. The molar ratio of 5b/DMAAm/AIBN was
C60
10 : 90 : 1, a yield of 87%, the spirobenzopyran content of 11
1
mol%, a pale green powder, Mw 1700, Mw/Mn 1.1. H NMR
C70
C80
(600 MHz, CDCl3): 0.8–2.0 (71H, br, C3–CH3,
CH2–CH2–CH2, vinyl-CH3, and DMAAm CH2), 2.3–3.3
(28H, br, DMAAm CH, CO–CH2–, DMAAm CH3, and
NH–CH2–), 3.4–3.5 (1H, br, N1–CH2–), 3.6–3.8 (4H, br,
N1–CH2– and OCH3), 5.6–5.7 (1H, br, H30), 6.5–6.7 (4H,
br, H80, H7, H50, and H70), 6.7–6.8 (2H, br, H40 and H5),
7.0–7.1 (1H, br, H4), 7.1–7.2 (1H, br, H6).
C80a
OCH3
Linker chain
CQO
172.5
168.6
39.9
36.2
35.9
172.6
168.6
40.0
36.3
35.9
173.1
168.3
39.6
37.5
36.5
171.6
169.0
40.0
36.0
35.7
172.7
168.4
39.7
36.7
36.1
N1–CH2
CO–CH2
NH–CH2
35.6
29.5
139.9
119.7
18.6
35.6
29.5
139.9
119.7
18.6
36.0
28.9
140.2
119.5
18.7
35.7
29.6
139.7
119.9
18.6
35.6
29.1
140.0
119.5
18.6
CH2–CH2–CH2
Vinyl-C
Vinyl-CH2
Vinyl-CH3
Polymer 6c. The molar ratio of 5c/DMAAm/AIBN was
10 : 90 : 1, a yield of 82%, the spirobenzopyran content of
11 mol%, a blue-grey powder, Mw 2200, Mw/Mn 1.2. 1H NMR
(600 MHz, CDCl3): 0.8–2.0 (29H, br, C3–CH3,
CH2–CH2–CH2, vinyl-CH3, and DMAAm CH2), 2.2–3.2
(72H, br, DMAAm CH, CO–CH2–, DMAAm CH3, and
NH–CH2–), 3.5–3.6 (1H, br, N1–CH2–), 3.6–3.9 (4H, br,
OCH3, N1–CH2–), 5.6–5.7 (1H, br, H30), 6.5–6.6 (1H, br,
H7), 6.7–6.9 (5H, br, H70, H5, H50, H40, and H60), 7.0–7.1
(1H, br, H4), 7.1–7.2 (1H, br, H6).
a
The NMR spectra were recorded in CDCl3 at 25 1C and the chemical
shifts are reported in ppm units. The denoted signals were assigned
by the calculation based on an additivity rule.
b
(3H, s, C3–CH3), 1.51 (2H, m, CH2–CH2–CH2), 1.98 (3H, s,
vinyl-CH3), 2.36 (2H, m, CO–CH2–), 2.59 (2H, m, CO–CH2–),
2.98 (2H, m, NH–CH2–), 3.18 (2H, m, NH–CH2–), 3.60
(1H, m, N1–CH2–), 3.88 (1H, m, N1–CH2–), 5.33 (1H, m, vinyl
CH2), 5.77 (1H, m, vinyl CH2), 5.80 (1H, d, J = 11 Hz, H30),
6.63 (1H, d, J = 8 Hz, H7), 6.81 (1H, dd, J = 7 Hz and
8 Hz, H5), 6.91 (1H, d, J = 11 Hz, H40), 6.92 (1H, dd, J = 7 Hz
and 8 Hz, H60), 7.01 (1H, d, J = 7 Hz, H4), 7.13 (1H, dd,
J = 8 Hz and 8 Hz, H6), 7.30 (1H, d, J = 7 Hz, H50), 7.70
(1H, d, J = 8 Hz, H70). HRMS (EI+): m/z calcd for
C28H32N4O5 504.2373, found 504.2382.
Polymer 6d. The molar ratio of 5d/DMAAm/AIBN was
5 : 95 : 1, a yield of 72%, the spirobenzopyran content of
5 mol%, a reddish purple powder, Mw 4100, Mw/Mn 1.4.
1H NMR (600 MHz, CDCl3): 0.8–1.9 (47H, br, C3–CH3,
CH2–CH2–CH2, vinyl-CH3, and DMAAm CH2), 2.3–3.4
(140H, br, DMAAm CH, CO–CH2–, DMAAm CH3, and
NH–CH2–), 3.4–3.5 (1H, br, N1–CH2–), 3.6–3.7 (1H, br,
N1–CH2–), 5.8–5.9 (1H, br, H30), 6.6–6.7 (1H, br, H7),
6.7–6.8 (1H, br, H80), 6.8–6.9 (1H, br, H5), 6.9–7.0 (1H, br,
H40), 7.0–7.1 (1H, br, H4), 7.1–7.2 (1H, br, H6), 7.9–8.0 (2H,
br, H50 and H70).
13C NMR assignments of compounds 5a–e are listed in
Table 1.
2.2.4 Poly(N,N-dimethylacrylamide) with pendant spiro-
benzopyran derivatives (6a–e). A mixture solution of the
monomeric spirobenzopyran 5a (0.501 mg, 1.09 mmol),
DMAAm (0.973 mg, 9.81 mmol), and 2,20-azoisobutyronitrile
(AIBN) (0.018 mg, 0.11 mmol) in dry THF (6.0 mL) (the
molar ratio of 5a/DMAAm/AIBN was 10 : 90 : 1) was
subjected to freeze–pump–thaw cycles to remove oxygen.
Nitrogen was filled to the degassed reaction equipments and
the mixture solution was stirred for 20 h at 65 1C. After
cooling to rt, the reaction mixture was poured into ether
(200 mL). The resulting precipitates were collected by filtration
and were dried under reduced pressure. The product 6a was
Polymer 6e. The molar ratio of 5e/DMAAm/AIBN was
5 : 95 : 5, a yield of 45%, the spirobenzopyran content of
3 mol%, a grey powder, Mw 2700, Mw/Mn 1.3. 1H NMR
(600 MHz, CDCl3): 0.8–1.9 (84H, br, C3–CH3, CH2–CH2–CH2,
vinyl-CH3, and DMAAm CH2), 2.3–3.4 (246H, br, DMAAm
CH, CO–CH2–, DMAAm CH3, and NH–CH2–), 3.5–3.6
(1H, br, N1–CH2–), 3.7–3.8 (1H, br, N1–CH2–), 5.8–5.9
(1H, br, H30), 6.5–6.6 (1H, br, H7), 6.7–6.8 (1H, br, H5),
6.8–7.0 (2H, br, H40 and H60), 7.0–7.1 (1H, br, H4), 7.1–7.2
(1H, br, H6), 7.2–7.3 (1H, br, H50), 7.6–7.7 (1H, br, H70).
c
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2.3 UV–visible spectroscopy of the spirobenzopyrans in
aqueous solutions
assignments were carried out by a conventional method. Since
the chemical shifts of the carbon atoms in the benzopyran
aromatic ring were significantly affected by the substitutions, a
part of the signal was difficult to directly assign by the NMR
measurements in this study. However, the chemical shifts of
the unassigned peaks were separated enough from each
other ( Z 4.5 ppm for the unassigned quaternary carbons).
Therefore, simple calculations based on an additivity rule were
valid to assign the signals. We used NMR data of benzene,
methoxybenzene (anisole), and nitrobenzene in CDCl321 in the
following calculation: at first, changes in chemical shifts of
benzene caused by methoxy and nitro substitutions were
calculated for each carbon at ortho-, meta-, and para-positions
of the substituents. Second, the chemical shifts of compounds
5b–e were estimated based on those of unsubstituted 5a and
the additivity of the substitution effects. The differences
between the calculated and the found values were within
ꢁ1.7 ppm, ꢁ2.9 ppm, ꢁ1.1 ppm, and ꢁ2.4 ppm in compounds
5b, 5c, 5d, and 5e, respectively, for the six carbons in the
benzopyran aromatic ring. The calculated values for signals of
the C40a and C80a agreed with the found ones when the
signal at around 120 ppm was assigned to the C40a. These
assignments for compound 5b were coincident with the
literatures.22,23
UV–visible spectra of polymers 6a–e were recorded both in
10 mM HCl and in neutral aqueous solution containing
10 mM triethylamine hydrochloride at 0.5 1C with stirring.
The neutral conditions were set up immediately before the
measurements by an addition of triethylamine to the solutions
of polymers 6a–e in 10 mM HCl. After the measurements in
the neutral conditions, the solution was acidified again with
1 M HCl to evaluate reversibility in the acid–base changes.
UV–visible spectra were also recorded before and after
light-irradiation in 2 mM HCl at 0.5 1C.
2.4 Spontaneous ring-opening rates of the spirobenzopyrans in
aqueous solutions
Solutions of polymers 6a–e in 2 mM HCl were irradiated
with blue-light at 25 1C. After stopping the irradiation, the
solutions were stored in the dark. Time-course of absorbance
of the McH was monitored during the light-irradiation and the
subsequent storage period.
3. Results and discussion
3.1 Syntheses
A series of spirobenzopyran-bearing polymers 6a–e were
synthesized according to the previous studies17–20 or with some
modifications of them (Scheme 1). First, a Fisher’s base 1 was
reacted with unsubstituted and appropriately substituted
salicylaldehydes 2a–e to give 1-(2-carboxyethyl)spirobenzo-
pyrans 3a–e.17 Second, compounds 3a–e and 4-nitrophenol
were condensed in the presence of DCC to yield the
corresponding 4-nitrophenyl ester derivatives 4a–e.18 Third,
monomeric spirobenzopyrans 5a–e were obtained by reaction
of compounds 4a–e with N-(3-aminopropyl)methacrylamide
hydrochloride in the presence of triethylamine.19 HRMS of
compounds 5a–e supported their formula weights. Detailed
chemical structures of compounds 5a–e were resolved by
1H NMR, 1H–1H COSY, 13C NMR, and 1H–13C HMQC
spectroscopies. Finally, polymers 6a–e were prepared by a free
radical random polymerization of the monomeric spiro-
benzopyrans 5a–e and N,N-dimethylacrylamide (DMAAm)
initiated by AIBN at 65 1C.20 The fed monomeric spiro-
benzopyrans 5a–e were 10 mol% for 6a–c and 5 mol% for
6d and 6e. The amounts of the spirobenzopyrans introduced in
the polymers were 11, 11, 11, 5, and 3 mol% in the order from
6a to 6e, which were determined by comparison of the
integrated intensities between the peak assigned to spiro-
benzopyran’s H30 proton and that assigned to DMAAm’s
methyl groups on the 1H NMR spectra. The Mw of the
polymers was determined to be in the range of 1700 to 4100
with a polydispersity between 1.1 and 1.7. Polymers 6a–e were
completely soluble in aqueous solutions at any pH.
3.2 UV–visible spectroscopy
3.2.1 Isomerization by acid–base changes. Fig. 2 shows
UV–visible spectra of the aqueous solutions of spirobenzopyran-
bearing polymers 6a–e acidified with 10 mM HCl and stored in
the dark. Each solution was yellowish due to the strong
absorbance band with peaks at around 400–470 nm. Since
the protonated Mc (McH) was a major and stable ring-opened
isomer under acidic and dark conditions, the spectra were
attributed to McH.24–26 Meanwhile, by neutralizing the acidic
solutions with the triethylamine, the color of solutions which
had been yellowish under acidic conditions turned reddish or
purple in no time, and returned to the former color in a
moment of the acidification by adding excess HCl. Considering
also that the both spectra exhibited strong absorbance bands
in the visible range suggesting the extendedly conjugated
systems, these spectral changes were strongly suggested to be
in the deprotonation and re-protonation of McH, and the
spectra observed just after the neutralization were attributed
to Mc (Fig. 2).
After neutralizing the acidic solutions with triethylamine,
the absorbance attributed to Mc gradually decreased in each
solution (Fig. 2, indicated by filled arrows). This decolorization
suggests the isomerization from the Mc to the Sp. UV–visible
spectroscopy recorded in organic solvents indicates that the
remaining absorption band at a wavelength shorter than
400 nm can be attributed to the Sp. In polymers 6d and 6e,
the band attributed to the Mc remained to some extent in
the neutral solutions. Furthermore, another absorption
band was observed at 400–500 nm, which seems similar to
that of 6-nitro-8-bromo-substituted spirobenzopyran in a
basic methanol solution.27
The 13C NMR chemical shift assignments of the monomeric
spirobenzopyrans 5a–e in CDCl3 are listed in Table 1. The
isomerization between Sp and Mc forms of the monomeric
spirobenzopyrans 5a–e highly favored the Sp form in
the CDCl3, in which the chemical structures of 5a–e seem
the same as those in a photoinduced ring-closed form. The
UV–visible spectra recorded after re-acidification (Fig. 2,
indicated by open arrows) were the same as those in the
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Fig. 3 UV–visible spectra of polymers 6a–e before and after blue-
light-irradiation (thick line and thin line, respectively) in 2 mM HCl at
0.5 1C. Each absorbance was normalized at 50 mM of the spiro-
benzopyran concentration.
Fig. 2 UV–visible spectra of polymers 6a–e in 10 mM HCl and
neutral solutions at 0.5 1C. The McH isomers in the HCl solutions
were converted into the Mc isomers by neutralization. Absorption
bands attributed to the Mc gradually decreased in the 5 neutral
solutions (indicated by filled arrows; storage for 0, 3, 10, and 30 min).
The McH was regenerated by re-acidification (open arrows, 3 h after the
re-acidification). Each absorbance was normalized at 50 mM of the
spirobenzopyran concentration. The table in the figure lists wavelengths
of absorption maxima.
The spectra of polymers 6a–e after the light-irradiation
showed an absorption band at a wavelength shorter than
400 nm. We obtained difference spectra by subtracting
contribution of the McH from the spectra recorded after the
irradiation (data not shown). The spectra at a wavelength
shorter than 400 nm were likely to be similar to those observed
in the neutral solutions and can be attributed to the Sp and
possibly N-protonated Sp (SpH).
original acidic solutions, indicating that the isomerization
caused by the acid–base changes was reversible.
3.2.2 Isomerization by light-irradiation. Previous studies
have demonstrated that the McH irradiated with visible-light
usually underwent the ring closure and proton dissociation to
give the corresponding Sp.28–30 Fig. 3 represents UV–visible
spectra of polymers 6a–e recorded before and after blue-
light-irradiation in 2 mM HCl at 0.5 1C. The light-irradiation
time needed to reach photostationary states (PSS) was
approximately 20, 210, 120, 60, and 60 s in the order from
6a to 6e. The absorbance of the McH was significantly
decreased by the light-irradiation, suggesting the light-induced
ring closure. The decreases in the absorbance associated with
the isomerization from the McH to the Sp were 98, 82, 77, 97,
and 86% in the order from 6a to 6e at the PSS. The values
were determined from changes in the absorbance in the range
of 400–500 nm, where the absorption is only attributed to the
McH in the acidic solutions. The incomplete ring closure
observed in polymers 6b, 6c, and 6e was due to concurrent
ring opening that occurred even under the light-irradiation in
the experiments.
3.3 Effects of the substitution on the ring-opening rates
The solutions of polymers 6a–e were irradiated with blue-light
until they reached PSS in 2 mM HCl at 25 1C. The absorbance
of the McH gradually increased after stopping the light-
irradiation, suggesting the spontaneous ring opening of the
Sp. We monitored time courses of absorbance of the McH in
polymers 6a–e under and after the light-irradiation (Fig. 4).
The ring-opening rates were estimated from the changes in the
absorbance.
Compared with the unsubstituted derivative (6a), the
methoxy-substitution (6b and 6c) significantly increased the
ring-opening rates, whereas a slight change and a decrease in
the rates were observed in the nitro-substitution (6d and 6e).
Table 2 lists the ring-opening rate constants (kSp-McH
)
calculated by fitting the experimental data to a single
exponential. The methoxy-substitution at the 80- and the
60-positions (6b and 6c) accelerated the kSp-McH up to 19.8
and 3.2 times, respectively, as compared to the unsubstituted
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The C–O cleavage of the spirobenzopyrans results in the
conversion of the oxygen atom of the ether linkage into the
phenolate anion. Thus, in this step, the Sp with higher electron
density of the 10O oxygen atom is likely to reduce the
activation free energy (DGz). Therefore, changes in the electron
density of the 10O will strongly affect the ring-opening rates.
We assume that these changes in the electron density are
substantial in the effects of the substitution.
We paid attention to a 13C NMR chemical shift of the C80a
carbon since a positive relationship was predicted between the
electron densities of the 10O and the adjacent C80a. An
increase in the electron density of the C80a, which reflects in
an upfield shift of its 13C NMR signal, is likely to cause an
increase in that of the 10O. In fact, the 13C NMR signal of the
C80a shifted to higher magnetic field with the increase in
the kSp-McH values (Table 2). Therefore, the chemical shift
of the C80a can be regarded as a potential indicator to predict
the kSp-McH values, although the relationship between the
13C NMR signal of the C80a and the kSp-McH value was
qualitative but not quantitative trend.
Fig. 4 Time-course of the absorbance of the McH in polymers 6a–e
after stopping the light-irradiation. The measurements were performed
in 2 mM HCl at 25 1C. The values of the absorbance before the light-
irradiation were normalized to be 1.0 for comparison.
Table 2 The ring-opening rate constants (kSp-McH) of the spiro-
benzopyrans in polymers 6a–e and the 13C NMR chemical shifts of the
C80a carbon of the monomeric spirobenzopyrans 5a–e
Furthermore, the assumption presented above can account
for the increase in the ring-opening rate by the 80-nitro-
substitution (6e). Although a nitro group is known as an
electron-withdrawing group, the 80-nitro-substitution led to
an upfield shift of the 13C NMR signal of the C80a, suggesting
the increase in the electron density of the C80a carbon. This
could contribute to the increase in the electron density of the
adjacent 10O oxygen which probably resulted in a decrease in
kSp-McH
/
Chemical shiftb/
Monomer ppm
Substituent Polymer s–1 ꢂ 103a
None
6a
6b
6c
6d
6e
0.49
1.57
9.70
0.03
0.61
5a
5b
5c
5d
5e
153.8
147.9
142.3
159.4
148.1
60-CH3O
80-CH3O
60-NO2
80-NO2
DGz for the ring opening of the Sp. The upfield shift of the 13
C
NMR signal by a nitro-substitution at an adjacent position
was observed not only in a spirobenzopyran, but also in
aromatic compounds such as nitrobenzene, o-nitrophenol,
and o-nitroanisole.21
a
b
In 2 mM HCl at 298 K. In CDCl3 at 298 K.
spirobenzopyran 6a. Furthermore, the 80-nitro-substitution
(6e) resulted in a slight increase in the kSp-McH. In contrast,
the 60-nitro-substitution (6d) reduced the kSp-McH to 0.06
times of that of the unsubstitution (6a). The substitution at the
80-position more accelerated the ring opening than that at the
60-position in both methoxy and nitro groups. In summary,
the effects of the substitution include the acceleration by a
methoxy group, the deceleration by a nitro group, and the
acceleration by the substitution at the 80-position. The result
of the 80-nitro-substitution (6d) is probably due to combina-
tion of the latter two effects.
3.4 Effects of N-protonation of the Sp on the ring-opening
rates
Relating to the assumption described above, we note effects of
the substitution on protonation at the N1 nitrogen of the Sp.
An apparent rate of the ring opening from the Sp to the McH
was reported to decrease with an increase in the proton
concentration because of formation of N-protonated Sp
(SpH) (Fig. 1).8 If the N-protonation behavior had been
affected by the substitution, the apparent ring-opening rate
would have been changed. In this case we cannot explain the
effects of the substitution on the order of the ring-opening
rates solely in terms of the electron density of the 10O oxygen.
In this study, however, the substitution is not considered to
affect the N-protonation behavior of the Sp, because the
indoline ring in the Sp is likely to have neither electronic
coupling with the benzopyran ring nor steric interaction with
the substituents. In fact, the 13C NMR spectra of the mono-
meric spirobenzopyrans 5a–e in the Sp form revealed that
there were no significant differences in the chemical shifts of
the indoline carbons in all of the derivatives (Table 1). This
means that the electron density of the atoms constituting the
indoline ring, including the N1 nitrogen, was not significantly
affected by the substitution. Therefore, the N-protonation
behavior of the Sp was likely to be similar in all of the
We suppose that the isomerization from the Sp to the McH
proceeds via C–O cleavage of the Sp and the subsequent
protonation of the resulting phenolate anion. It is reasonable
to assume that the C–O cleavage is the rate-determining step
because protonation occurs instantaneously in general in
acidic aqueous systems. In fact, the absorption band attributed
to the Mc was not observed in UV–visible spectra recorded
after stopping the light-irradiation, suggesting rapid conversion
of the Mc to the McH. The McH was expected to convert into
a thermodynamically stable conformation by rotation of the
C2,20–C30–C40–C40a bonds. UV–visible spectra of polymers
6a–e in acidic aqueous solutions right after the recovery from
light-induced ring closure coincided with those at 4 months
after the last irradiation. This suggests that ring-opened
isomers were converted into some stable conformation within
the time scale of 1/kSp-McH
.
c
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polymers 6a–e at a definite acid concentration. Consequently,
we can discuss the effects of the substitution on the kSp-McH
independently of those of the N-protonation.
11 (a) K. Sumaru, M. Kameda, T. Kanamori and T. Shinbo, Macro-
molecules, 2004, 37, 4949–4955; (b) K. Sumaru, M. Kameda,
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4. Conclusions
We investigated the effects of substitution on the thermal
ring-opening rate of the spirobenzopyrans in the acidic
aqueous solution. An electron-donating methoxy group and
an electron-withdrawing nitro group were introduced at the
60- or 80-position. The effects of the substitution on the
ring-opening rate were summarized as follows: the kSp-McH
values (1) were increased by the methoxy-substitution, (2) were
basically decreased by the nitro-substitution, and (3) were
increased by the substitution at the 80-position itself. We
consider that an essential effect of the substitution on the
ring-opening rate was the changes in the electrozn density of the
10O oxygen, because that is likely to affect DG values needed
for the C–O cleavage of the Sp. The 13C NMR of the Sp
showed that the signal of the C80a, which was bonded to the
10O oxygen, shifted to higher magnetic field with the increase
in the kSp-McH values. Thus, the chemical shift of C80a is
likely to be a potential indicator of the kSp-McH values.
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