Synthesis, characterization, and UV–visible study of some new photochromic formyl-containing 1′,3′,3′-trimethylspiro[chromene-2,2′-indoline] derivatives

Article abstract of DOI:10.1007/s13738-021-02253-5

The spiropyran derivatives can exist in two forms, the closed-ring spiropyran form and the open-ring merocyanine (MC) form. The SP form could be converted into the MC form upon UV irradiation. In this work, some 1′,3′,3′-trimethylspiro[chromene-2,2′-indoline] derivatives containing the nitro and formyl groups are synthesized via the reaction of 1,3,3-trimethyl-2-methylene-5-nitroindoline with the corresponding salicylaldehyde derivatives. These compounds have different photochromic behaviors. The synthesized photochromic molecules are characterized by the FT-IR, ¹H-NMR, and ¹³C-NMR spectroscopic techniques. In order to investigate the photochromic properties of these compounds, the UV–visible spectroscopic analyses of their methanolic solutions before and after exposure to a UV lamp (in the spectral range of 360–400?nm) are studied. Graphic abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s13738-021-02253-5

Journal of the Iranian Chemical Society
https://doi.org/10.1007/s13738-021-02253-5
ORIGINAL PAPER
Synthesis, characterization, and UV–visible study of some new
photochromic formyl‑containing 1′,3′,3′‑trimethylspiro[chromene‑2,2′
‑indoline] derivatives
Zeinalabedin Sepehr1 · Hossein Nasr‑Isfahani1 · Ali Reza Mahdavian2 · Amir Hossein Amin1
Received: 14 January 2021 / Accepted: 16 April 2021
© Iranian Chemical Society 2021
Abstract
The spiropyran derivatives can exist in two forms, the closed-ring spiropyran form and the open-ring merocyanine (MC) form.
The SP form could be converted into the MC form upon UV irradiation. In this work, some 1′,3′,3′-trimethylspiro[chromene-
2,2′-indoline] derivatives containing the nitro and formyl groups are synthesized via the reaction of 1,3,3-trimethyl-2-meth-
ylene-5-nitroindoline with the corresponding salicylaldehyde derivatives. These compounds have diferent photochromic
behaviors. The synthesized photochromic molecules are characterized by the FT-IR, 1H-NMR, and 13C-NMR spectroscopic
techniques. In order to investigate the photochromic properties of these compounds, the UV–visible spectroscopic analyses
of their methanolic solutions before and after exposure to a UV lamp (in the spectral range of 360–400 nm) are studied.
Graphic abstract
Keywords Photochromism · Spiropyran · Spiro[chromene-2,2′-indoline] · Merocyanine
Introduction
their diferent absorption spectra, the two isomers of a pho-
tochromic switch may have diferent chemical and physi-
cal properties such as wettability, dielectric constant, redox
potential, polarity, and conductivity. In the recent decades,
the photochromic compounds have been increasingly stud-
ied by the researchers due to their ability to change the
parameters involved. The reversible photo-switches are
the mixtures of two photoisomeric species that can be
In general, photochromism can be introduced as a reversible
switching between two isomers of diferent colors under
light irradiation (generally, visible or UV). In addition to
* Hossein Nasr-Isfahani
Nasrisfahani@yahoo.com
Extended author information available on the last page of the article
Vol.:(0123456789)
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Journal of the Iranian Chemical Society
inter-transformed by lights of diferent wavelengths. In
some compounds, isomerization in one of the directions
that can take place thermally. In the procedures that are fast
enough, this photo-switching can even be benefcial where
the UV light applied in this isomerization direction leads
to photodegradation [1, 2].
1′,3′,3′-trimethylspiro[chromene-2,2′-indoline] derivatives.
The products obtained were characterized by the FT-IR, 1H-
NMR, and 13C-NMR spectroscopic techniques. In order to
investigating their photochromic properties, the UV–visible
spectroscopic analyses of the methanolic solutions of the
synthesized spiropyran derivatives are studied in the spec-
tral range of 250–750 nm before and after exposure to a UV
lamp (in the spectral range of 360–400 nm).
During the past years, the photo-responsive substances
have been studied numerously due to their various poten-
tial usages including artifcial muscles [3], optical memory
devices [4–6], displays and switches [7–9], photochromic
lenses [10, 11], drug delivery [12, 13], and soft-actuators
[14]. Such substances have the ability to absorb lights of
specifc wavelengths leading to an intra- or inter-molecular
alteration. These variations in the molecular confgura-
tion and structure are typically along with alterations in
the optical features, dipole moment, and molecular charge
[15]. Spiropyran (SP) is one of the most usual instances of a
photochromic molecule, which is colorless and hydrophobic
with a low polarity. This closed-ring isomer can transform
into the greatly polar open-ring isomer merocyanine (MC)
upon exposure to a UV light; however, the visible light can
induce an inverse reaction (MC–SP conversion) (Fig. 1).
Furthermore, various other external stimuli including the
temperature [16], acids and bases [17], and metal ions [18,
19] can promote this reversible isomerization. Temperature
and light are two key appealing environmentally friendly
elements in the daily applications.
Experimental
Materials
Salicylaldehyde, phenylhydrazine, 3-methylbutan-2-one,
nitric acid, and acetic acid (glacial) were supplied from the
Merck, and used as received. 3- and 5-nitrosalicylaldehyde
were prepared according to the literature [24, 25] via the
classical nitration of salicylaldehyde by nitric acid.
Synthesis
of 1,3,3‑trimethyl‑2‑methylene‑5‑nitroindoline (1)
1,3,3-trimethyl-2-methylene-5-nitroindoline (1) was pre-
pared in two steps. Step 1: 2,3,3-trimethyl-5-nitro-3H-in-
dole was obtained via the nitration of 2,3,3-trimethylindole
according to the literature [26, 27] with a few changes.
For this purpose, 2,3,3-trimethylindole (5 g, 31 mmol)
was added dropwise to concentrated H2SO4 (30 mL) and
cooled down to 0–5 °C. Sodium nitrate (2.67 g, 31 mmol)
was added portion by portion over 30 min. The reaction
mixture was stirred for an additional 30 min, and then was
poured into crushed ice (200 g) and neutralized by the
addition of solid sodium hydroxide. The solid precipitate
formed was collected by fltration, and was then recrys-
tallized in acetone/H2O, 50:50 (v/v), to yield an orange
crystal as the product (6 g, 94%). mp, 118–120 °C (ref,
Ozhogin and his co-workers have studied the efect of
carboxylic and aldehyde substituents on the photochromic
properties of spiropyran molecules. Their fndings show
that these substitutions increase their photocolorability
and enhance the lifetime of the merocyanine form [20].
Also, some other spiropyrans with electron-withdraw-
ing substituents have been synthesized and their various
properties such as their photochromic behavior have been
investigated [21]. In this work, the new photochromic
1′,3′,3′-trimethylspiro[chromene-2,2′-indoline] deriva-
tives were synthesized. 2,3,3-trimethyl-3H-indole was
prepared by the reaction of isopropyl methyl ketone with
phenylhydrazine via the Fischer synthesis according to the
literature [22, 23]. 1,3,3-trimethyl-2-methylene-5-nitroin-
doline was reacted with the corresponding salicylalde-
hyde derivatives in order to prepare the fnal photochromic
1
116–118 °C); H-NMR (300 MHz, CDCl3, 25 °C, TMS)
δ (ppm): 1.38 (s, 6H, 2CH3), 2.37 (s, 3H, CH3), 7.62 (d,
1H, J = 9 Hz), 8.17 (d, 1H, J = 3 Hz), 8.26 (dd, 1H, J = 9,
3 Hz). 13C-NMR (75 MHz, CDCl3, 25 °C, TMS) δ (ppm):
15.97, 22.73, 54.51, 117.16, 120.04, 124.53, 145.64,
146.73, 158.99, 194.16. FT-IR (KBr), ν (cm−1): 3091,
Fig. 1 Spiropyran-merocyanine
interconversion
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Journal of the Iranian Chemical Society
3025, 2973, 2935, 1600, 1521, 1513, 1452, 1423, 1330,
1243, 1207, 1112, 1052, 908, 846, 802, 740; Anal. Calcd.
for C11H12N2O2: C, 64.70; H, 5.88; N, 13.72; Found: C,
64.76; H, 5.94; N, 13.77. Step 2: 1,3,3-trimethyl-2-meth-
ylene-5-nitroindoline was prepared according to the litera-
ture [28]: 2,3,3-trimethyl-5-nitro-3H-indole (2 g, 1 mmol)
was dissolved in 10 mL of acetonitrile, and then CH3I
(2.8 g, 3 mmol) was added to the mixture in one portion.
The reaction mixture was stirred at room temperature for
48 h. The precipitate formed was fltered of and washed
with a little fresh acetonitrile and dried under vacuum.
The precipitate (2.5 g) was dissolved in distilled water
(20 mL) at room temperature, and a solution of K2CO3
(1.5 g in 10 mL of H2O) was added, and the mixture was
stirred for 60 min. The dark-red precipitate formed was
fltered of and washed with cold water and dried under,
vacuum. 1.5 g of 1,3,3-trimethyl-2-methylene-5-nitroindo-
line (70%) was obtained. mp, 91–93 °C (ref, 89–91 °C);
1H-NMR (300 MHz, CDCl3, 25 °C, TMS) δ (ppm): 1.39
(s, 6H, 2CH3), 3.16 (s, 3H, CH3), 4.13 (d, 1H, J = 3 Hz),
4.16 (d, 1H, J = 3 Hz), 6.55 (d, 1H, J = 9 Hz), 7.96 (d, 1H,
J = 3 Hz), 8.15 (dd, 1H, J = 9, 3 Hz). 13C-NMR (75 MHz,
CDCl3, 25 °C, TMS) δ (ppm): 29.08, 29.70, 43.47, 78.95,
103.93, 118.10, 126.27, 138.18, 139.98, 151.59, 161.23.
FT-IR (KBr), ν (cm−1): 3100, 2950, 2857, 1660, 1604,
1519, 1475, 1386, 1328, 1297, 1120, 1085, 935, 813;
Anal. Calcd. for C12H14N2O2: C, 66.05; H, 6.42; N, 12.84;
Found: C, 66.11; H, 6.48; N, 12.96.
Synthesis of 4‑hydroxy‑5‑nitroisophthalaldehyde
(2b)
Compound (2b) was prepared in two steps. Step 1: 2.52 g
(18 mmol, 1.2 eq) of hexamethylenetetramine was added
to a solution of 4-hydroxybenzaldehyde (1.83 g, 15 mmol)
in 20 mL of TFA. The reaction mixture was then stirred
at 85–90 °C for 24 h. After cooling to room temperature,
20 mL of concentrated hydrochloric acid was added, and the
mixture was stirred for 2 h. The solvent was distilled under
reduced pressure. The crude product was recrystallized in
acetone/water 50:50 (v/v). 1.25 g of 5-formyl-2-hydroxy-
benzaldehyde (yield, 55%) was obtained. mp, 109–111 °C;
1H-NMR (300 MHz, CDCl3, 25 °C, TMS) δ (ppm): 7.16
(d, 1H, J = 9 Hz), 8.10 (dd, 1H, J = 9, 3 Hz), 8.18 (d, 1H,
J=3 Hz), 9.97 (s, 1H, -CHO), 10.04 (s, 1H, –CHO), 11.58
(s, 1H, OH). 13C-NMR (75 MHz, CDCl3, 25 °C, TMS) δ
(ppm): 118.86, 120.35, 129.26, 136.54, 137.20, 166.28,
189.36, 196.23. FT-IR (KBr), ν (cm−1): 3174, 2834, 2751,
1695, 1664, 1614, 1563, 1482, 1369, 1270, 1159, 925,
846, 775, 715, 613; Anal. Calcd. for C8H6O3: C, 63.52; H,
3.97; Found: C, 63.60; H, 4.02. Step 2: 1 g (6.6 mmol) of
5-formyl-2-hydroxybenzaldehyde was added in small por-
tions to a mixture of nitric acid (1 mL, 68%) and concen-
trated sulfuric acid (2 mL) at − 5 °C, and the mixture was
stirred for 1 h. The reaction mixture was poured on 100 g
of crushed ice. The yellow solid precipitate formed was fl-
tered of and washed with cold distilled water. The crude
product was recrystallized in acetone/water 50:50 (v/v).
1.18 g of 4-hydroxy-5-nitroisophthalaldehyde (yield, 91%)
was obtained. mp, 125–127 °C; 1H-NMR (300 MHz, CDCl3,
25 °C, TMS) δ (ppm): 8.64 (s, 1H), 8.89 (s, 1H), 10.05 (s,
1H, -CHO), 10.47 (s, 1H, -CHO), 11.97 (s, 1H, OH). 13C-
NMR (75 MHz, CDCl3, 25 °C, TMS) δ (ppm): 125.68,
128.43, 131.92, 135.91, 137.27, 159.90, 187.71, 189.02.
FT-IR (KBr), ν (cm−1): 3062, 2861, 1695, 1664, 1608, 1531,
1436, 1355, 1292, 1247, 1174, 1159, 983, 935, 821, 767,
732, 696, 646; Anal. Calcd. for C8H5NO5: C, 48.94; H, 2.56;
N, 7.14; Found: C, 49.02; H, 2.61; N, 7.20.
Synthesis of 2‑hydroxy‑5‑nitroisophthalaldehyde
(2a)
5-nitrosalicylaldehyde (2 g, 12 mmol) was dissolved in
20 mL of anhydrous trifluoroacetic acid (TFA). Hexa-
methylenetetramine (2 g, 14.3 mmol, 1.2 eq) was then
added to the mixture. The reaction mixture was stirred
for 48 h at 110–120 °C. After cooling, HCl (1 M, 20 mL)
was added, and the mixture was stirred for another 1 h.
The precipitate formed was filtered off, washed with
cold water, and dried under vacuum. The crude product
was recrystallized in acetone/water 50:50 (v/v). 1.65 g
of 2-hydroxy-5-nitroisophthalaldehyde (yield, 72%) was
obtained. mp, 143–145 °C; 1H-NMR (300 MHz, CDCl3,
25 °C, TMS) δ (ppm): 8.89 (s, 2H, aromatic), 10.34 (s,
2H, –CHO), 12.29 (s, 1H, OH). 13C-NMR (75 MHz,
CDCl3, 25 °C, TMS) δ (ppm): 123.20, 132.14, 140.77,
166.95, 190.48. FT-IR (KBr), ν (cm−1): 3299, 3095,
3050, 2879, 1698, 1685, 1617, 1587, 1550, 1446, 1398,
1347, 1295, 1197, 1106, 971, 719, 611; Anal. Calcd. for
C8H5NO5: C, 49.23; H, 2.56; N, 7.18; Found: C, 49.30;
H, 2.63; N, 7.23.
General procedure for synthesis of photochromic
1′,3′,3′‑trimethylspiro[chromene‑2,2′‑indoline]
derivatives (3a‑d)
2.18 g (10 mmol) of 1,3,3-trimethyl-2-methylene-5-ni-
troindoline was added in small portions to a solution of the
salicylaldehyde derivatives (10 mmol) in 10 mL ethanol.
The reaction mixture was stirred at 40 °C for 24 h. The reac-
tion progress was monitored by TLC (1:3 ethyl acetate and
n-hexane). The yellow precipitate formed was fltered of
and washed with a little amount of fresh ethanol and dried
in vacuo (Scheme 1).
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Journal of the Iranian Chemical Society
Scheme 1 Synthesis of the photochromic 1′,3′,3′-trimethylspiro[chromene-2,2′-indoline] derivatives
1′,3′,3′‑trimethyl‑8‑formyl‑5′,6‑dinitrospiro[chromene‑2,
2′‑indoline] (3a)
1′,3′,3′‑trimethyl‑5′,6‑dinitrospiro[chromene‑2,2′‑indol
ine] (3c)
1
1
Yellow solid (55% yield): mp, 205–207 °C; H-NMR
Yellow solid (75% yield): mp, 201–203 °C; H-NMR
(300 MHz, CDCl3, 25 °C, TMS) δ (ppm): 1.31 (s, 3H, CH3),
1.44 (s, 3H, CH3), 2.96 (s, 3H, CH3), 6.02 (d, 1H, J=12 Hz),
6.63 (d, 1H, J=9 Hz), 7.12 (d, 1H, J=12 Hz), 8.02 (d, 1H,
J=3 Hz), 8.25 (m, 2H), 8.60 (d, 1H, J=3 Hz), 10.15 (s, 1H,
-CHO). 13C-NMR (75 MHz, CDCl3, 25 °C, TMS): δ (ppm):
20.14, 25.76, 28.99, 52.33, 106.09, 107.40, 118.31, 120.30,
121.16, 122.73, 124.43, 126.36, 126.87, 128.70, 136.47,
141.54, 152.29, 152.58, 160.16, 185.98. FT-IR (KBr), ν
(cm−1): 3072, 2925, 2869, 1691, 1650, 1606, 1529, 1496,
1454, 1382, 1320, 1265, 1186, 1110, 1101, 914, 796, 738,
684; Anal. Calcd. for C20H17N3O6: C, 60.76; H, 4.30; N,
10.63; Found: C, 60.81; H, 4.37; N, 10.69.
(300 MHz, CDCl3, 25 °C, TMS) δ (ppm): 1.26 (s, 3H, CH3),
1.38 (s, 3H, CH3), 2.90 (s, 3H, CH3), 5.89 (d, 1H, J=12 Hz),
6.58 (d, 1H, J=9 Hz), 6.83 (d, 1H, J=9 Hz), 7.04 (d, 1H,
J=12 Hz), 7.99 (d, 1H, J=3 Hz), 8.07–8.11 (m, 2H), 8.23
(dd, 1H, J=9, 3 Hz). 13C-NMR (75 MHz, CDCl3, 25 °C,
TMS): δ (ppm): 19.80, 25.72, 28.87, 51.87, 105.76, 105.92,
115.57, 118.11, 118.30, 120.14, 122.92, 126.18, 126.27,
129.10, 136.92, 141.01, 141.52, 152.87, 158.76. FT-IR (KBr),
ν (cm−1): 3068, 2969, 2925, 1648, 1608, 1577, 1504, 1326,
1263, 1182, 1124, 1085, 1018, 954, 817, 742; Anal. Calcd. for
C19H17N3O5: C, 62.12; H, 4.63; N, 11.44; Found: C, 62.19; H,
4.69; N, 11.49.
1′,3′,3′‑trimethyl‑5′,8‑dinitrospiro[chromene‑2,2′‑indol
ine] (3d)
1′,3′,3′‑trimethyl‑6‑formyl‑5′,8‑dinitrospiro[chromene‑2,
2′‑indoline] (3b)
1
Yellow solid (72% yield): mp, 196–198 °C; H-NMR
1
Yellow solid (53% yield): mp, 203–205 °C; H-NMR
(300 MHz, CDCl3, 25 °C, TMS) δ (ppm): 1.26 (s, 3H, CH3),
1.44 (s, 3H, CH3), 2.89 (s, 3H, CH3), 5.87 (d, 1H, J=10.5 Hz),
6.55 (d, 1H, J=9 Hz), 6.98 (t, 1H, J=7.5 Hz), 7.03 (d, 1H,
J=10.5 Hz), 7.35 (dd, 1H, J=7.5, 1.5 Hz), 7.74 (dd, 1H, J=9,
1.5 Hz), 7.98 (d, 1H, J=3 Hz), 8.20 (dd, 1H, J=9, 3 Hz). 13C-
NMR (75 MHz, CDCl3, 25 °C, TMS): δ (ppm): 19.81, 25.77,
28.81, 51.70, 105.79, 105.99, 118.22, 119.65, 119.97, 121.00,
125.47, 126.19, 129.30, 131.41, 136.95, 137.21, 141.07,
147.30, 152.76. FT-IR (KBr), ν (cm−1): 3068, 2971, 2925,
2873, 1652, 1606, 1521, 1498, 1459, 1361, 1322, 1268, 1180,
1105, 1018, 989, 919, 813, 738; Anal. Calcd. for C19H17N3O5:
C, 62.12; H, 4.63; N, 11.44; Found: C, 62.18; H, 4.71; N,
11.48.
(300 MHz, CDCl3, 25 °C, TMS) δ (ppm): 1.29 (s, 3H,
CH3), 1.45 (s, 3H, CH3), 2.92 (s, 3H, CH3), 6.00 (d, 1H,
J=12 Hz), 6.61 (d, 1H, J=9 Hz), 7.13 (d, 1H, J=12 Hz),
7.91 (d, 1H, J=3 Hz), 8.00 (d, 1H, J=3 Hz), 8.22 (dd, 1H,
J = 9, 3 Hz), 8.27 (d, 1H, J = 3 Hz), 9.93 (s, 1H, -CHO).
13C-NMR (75 MHz, CDCl3, 25 °C, TMS): δ (ppm): 19.66,
25.83, 28.92, 52.15, 106.17, 107.84, 118.26, 120.77,
121.64, 126.25, 128.20, 128.66, 128.96, 130.67, 136.57,
137.28, 141.47, 151.88, 152.40, 188.53. FT-IR (KBr), ν
(cm−1): 3072, 3064, 2931, 2827, 1695, 1643, 1606, 1509,
1322, 1270, 110, 1014, 917, 817, 754; Anal. Calcd. for
C20H17N3O6: C, 60.76; H, 4.30; N, 10.63; Found: C, 60.79;
H, 4.38; N, 10.66.
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Journal of the Iranian Chemical Society
The photochromic 1′,3′,3′-trimethylspiro[chromene-
2,2′-indoline] derivatives were prepared by the reaction of
1,3,3-trimethyl-2-methylene-5-nitroindoline with the cor-
responding salicylaldehyde derivatives. The FT-IR spec-
tra of the spiropyran compounds showed the absorption
bands at 1691 cm−1 (3a) and 1695 cm−1 (3b) related to the
stretching resonance of the C=O bonds of unreacted alde-
hyde groups. The stretching resonance of the C=C bond in
the pyran ring was observed at 1650 cm−1 (3a), 1643 cm−1
(3b), 1648 cm−1 (3c), and 1652 cm−1 (3d). Also, the proton
resonance of unreacted aldehydes at 10.15 ppm (3a) and
9.93 ppm (3b) and the appearance of carbon resonance
related to these groups at 185.98 ppm (3a) and 188.53 ppm
(3b) in the 13C-NMR spectra of compounds 3a and 3b
confrmed their structures.
Results and discussion
I n
t h i s
w o r k ,
s o m e
d e r i v a t i v e s
o f
1′,3′,3′-trimethylspiro[chromene-2,2′-indoline] bearing
the formyl and nitro groups were synthesized. The indole
section was the same for all of these spiro compounds.
1,3,3-trimethyl-2-methylene-5-nitroindoline (1) was pre-
pared in two steps. Step 1: 2,3,3-trimethyl-5-nitro-3H-in-
dole was obtained via the nitration of 2,3,3-trimethyl-
3H-indole. In the FT-IR spectrum of this compound, the
absorption bands observed at 1521 and 1330 cm−1 are typ-
ical for the asymmetric and symmetric stretching modes
of N–O bonds, respectively, and confrm the presence of
the nitro group in 2,3,3-trimethyl-5-nitro-3H-indole. Step
2: The reaction of 2,3,3-trimethyl-5-nitro-3H-indole with
methyl iodide resulted in the formation of 1,2,3,3-tetrame-
thyl-5-nitro-3H-indolium iodide. This salt was dissolved
in distilled water, and was deprotonated using an aqueous
solution of K2CO3 to produce 1,3,3-trimethyl-2-methylene-
5-nitroindoline as the fnal product. In the 1H-NMR spec-
trum of this compound, the proton resonance of the dias-
tereotopic methylenic protons at 4.13 ppm and 4.16 ppm,
the proton resonance of the 2CH3 protons at 1.39 ppm, and
the N–CH3 protons at 3.16 ppm with three proton reso-
nance aromatic protons confrmed the successful synthesis
of 1,3,3-trimethyl-2-methylene-5-nitroindoline.
Photochromism of 1′,3′,3′‑trimethylspiro[ch
romene‑2,2′‑indoline] derivatives
In the recent years, the spiropyran derivatives have been
among the most well-known photochromic compounds
that have attracted the attention of many chemists. Vari-
ous external stimuli including the UV irradiation, tem-
perature, acids and bases, and metal ions can promote
the isomerization of the closed-ring spiropyran (SP)
form to the open-ring merocyanine (MC) form. We were
interested in investigating the effect of UV irradiation
on the isomerization of SP to MC. Also, we were inter-
ested in studying the effect of the aldehyde group on
this isomerization. For these purposes, dilute solutions
(0.5 mM) of the compounds 3a, 3b, 3c, and 3d in metha-
nol were prepared. These solutions were exposed to UV
irradiation (360–400 nm) for 15 min, which changed
the color of the solutions. In the UV–visible spectra of
compounds 3a and 3b, the absorption bands shifted to
longer wavelengths due to the presence of the aldehyde
group on the benzopyran ring (Fig. 2). We believe that
the presence of the nitro group on the indole ring causes
an instability in the MC form. However, the concerted
presence of the nitro and aldehyde groups on the benzo-
pyran ring causes more stability in the MC form in the
compounds 3a and 3b compared to the compounds 3c
and 3d. The return delay times of the compounds 3a and
3b after UV irradiation were studied. The λmax values for
the compounds 3a and 3b were returned to their previous
values (before UV irradiation) after 3 min. The absorp-
tion graph for the compound 3a became quite similar to
that before UV irradiation in the UV–visible spectrum
after 3 min, while this was happened after 9 min for the
compound 3b.
2-hydroxy-5-nitroisophthalaldehyde (2a) was prepared
via the formylation of 5-nitrosalicylaldehyde with hexam-
ethylenetetramine in anhydrous TFA. In the FT-IR spectrum
of this compound, the absorption bands at 1698 cm−1 and
1685 cm−1 are related to the stretching resonance of the
two C=O groups along with the proton resonance of the
two aldehyde protons at 10.34 ppm, helping to confrm the
structure of this compound. The synthesis of 4-hydroxy-
5-nitroisophthalaldehyde was performed in two steps.
First, 4-hydroxyisophthalaldehyde was prepared via the
Duf reaction of 4-hydroxybenzaldehyde with hexameth-
ylenetetramine in TFA. The FT-IR spectrum of this com-
pound showed two absorption bands at 1695 and 1664 cm−1
related to the stretching resonance of the two C=O bonds of
the 1- and 3-crabaldehyde groups, respectively. Also, the
proton resonance of the two aldehyde protons at 9.97 and
10.04 ppm and the appearance of two carbaldehyde peaks at
189.36 and 196.23 ppm in the 13C-NMR spectrum of com-
pound (2b) confrmed the successful synthesis in step 1. In
the second step, 4-hydroxy-5-nitroisophthalaldehyde was
prepared via the nitration of 4-hydroxyisophthalaldehyde
in a HNO3/H2SO4 medium. The absorption bands observed
at 1531 cm−1 and 1355 cm−1 related to the N–O bond (asym-
metric and symmetric stretching modes, respectively) in the
FT-IR spectrum of 4-hydroxy-5-nitroisophthalaldehyde con-
frmed the presence of the –NO2 group in this compound.
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Journal of the Iranian Chemical Society
(Compound 3b)
1.2
1
(Compound 3a)
before UV irradiaꢀon
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
before UV irradiaꢀon
immediately a er UV irradiaꢀon
3 min a er UV irradiaꢀon
6 min a er UV irradiaꢀon
9 min a er UV irradiaꢀon
immediately a er UV irradiaꢀon
3 min a er UV irradiaꢀon
0.8
0.6
0.4
0.2
0
220
320
420
520
620
220
320
420
520
620
λ (nm)
λ (nm)
(Compound 3c)
(Compound 3d)
1.2
1
1.2
1
before UV irradiaꢀon
before UV irradiaꢀon
immediately a er UV irradiaꢀon
immediately a er UV irradiaꢀon
0.8
0.6
0.4
0.2
0
0.8
0.6
0.4
0.2
0
225
325
425
λ (nm)
525
225
325
425
λ (nm)
525
Fig. 2 UV–visible spectra of compounds 3a-d before and after exposure to UV irradiation
Acknowledgement The authors wish to express their thanks to the
Research Council of the Shahrood University of Technology for the
fnancial support of this research work.
Conclusions
In this work, we introduced the synthesis of two new
1′,3′,3′-trimethylspiro[chromene-2,2′-indoline] derivatives,
3a and 3b. In these photochromic compounds, the presence
of the electron-withdrawing nitro group in the para position
of the nitrogen atom of the indole section causes an instabil-
ity in the open-ring merocyanine (MC) form. This unstable
form prefers to isomerize to the closed-ring spiropyran (SP)
form, while the presence of the electron-withdrawing nitro
and aldehyde groups on the benzopyran ring stabilizes the
MC form. In these compounds, the absorption bands in the
UV–visible spectra shifted to the longer wavelengths (red
shift). The UV–visible spectra of compounds 3a and 3b
changed further after UV irradiation compared to the com-
pounds 3c and 3. It can be said that they have a more stable
MC form. The negative charge of phenolate can be delocal-
ized in the benzopyran section due to the presence of the
electron-withdrawing nitro and aldehyde groups on this part
of the molecule and causes a stability in the MC form. How-
ever, the presence of only one electron-withdrawing nitro
group on the benzopyran section of compounds 3c and 3d
could not stabilize the MC form as much as the compounds
3a and 3b. The UV–visible spectra of compounds 3c and
3d did not change after UV irradiation as much as the com-
pounds 3a and 3b.
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Authors and Afliations
Zeinalabedin Sepehr1 · Hossein Nasr‑Isfahani1 · Ali Reza Mahdavian2 · Amir Hossein Amin1
1
Zeinalabedin Sepehr
Faculty of Chemistry, Shahrood University of Technology,
Zsepehr53@gmail.com
Shahrood 3619995161, Iran
2
Ali Reza Mahdavian
Department of Polymer Science, Iran Polymer
a.mahdavian@ippi.ac.ir
and Petrochemical Institute, Tehran 14965/115, Iran
Amir Hossein Amin
amir_amin345@yahoo.com
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