Substituent effect on the acid-induced isomerization of spiropyran compounds

Article abstract of DOI:10.1016/j.saa.2018.04.076

Spiropyran compounds are well known as an isomeric system, the closed spiropyran (SP) could be converted into the open merocyanine (MC) via acid-induced because stable protonated merocyanine (MCH) were formed by combination of MC and H⁺. In ord

Full text of DOI:10.1016/j.saa.2018.04.076

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 202 (2018) 13–17
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Substituent effect on the acid-induced isomerization of
spiropyran compounds
Lixia Cui a, Honghong Zhang a, Guomei Zhang a, Ying Zhou a, Li Fan b, Lihong Shi a, Caihong Zhang a,
⁎
,
Shaomin Shuang a, Chuan Dong b,
⁎
a
School of Chemistry and Chemical Engineering, Shanxi University, Taiyuan 030006, China
b
Institute of Environmental Science, Shanxi University, Taiyuan 030006, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Spiropyran compounds are well known as an isomeric system, the closed spiropyran (SP) could be converted into
the open merocyanine (MC) via acid-induced because stable protonated merocyanine (MCH) were formed by
combination of MC and H+. In order to understand how the substituent affect the isomerization of spiropyran.
A series of the chromene 7-subsituted spiropyran compounds were designed and synthesized. The photophysical
properties of them were studied comparatively by UV-absorption and fluorescence spectra in different pH. The
results demonstrated that the various substituents influenced not only the photophysical properties of SP, MC
and MCH forms, but also the pKa of the MC-MCH transformation. There was a good linearity relationship between
the pKa and the Hammett constant of substituent, the pKa was smaller when the Hammett constant of substituent
was larger.
Received 19 July 2017
Received in revised form 23 April 2018
Accepted 29 April 2018
Available online xxxx
Keywords:
Spiropyrane
Absorption spectrum
Fluorescence spectrum
pKa
© 2018 Elsevier B.V. All rights reserved.
Hammett constant
1. Introduction
acidification should be affected and the tunable pKa values in certain
pH range could be achieved. Raymo's group designed a switch based
There has been a continuing interest in the isomerization of
spiropyran in recent years. Spiropyran compounds are a class of impor-
tant photochromic compounds, which have a colorless ring-closed
“spiropyran” (SP) form that can be converted into the colored open
“merocyanine” (MC) form by UV irradiation [1–5]. Furthermore, it also
takes place ring opening in the presence of an acid to form protonated
merocyanine (MCH) and then converts back to the closed spiropyran
form (SP) with the addition of a base [6–10]. In many applications,
such as biological probes [11–13], liquid crystals [14], nanoporous
conducting particles [15–17], molecular switches [18–22] were de-
signed based on the acid-induced SP-MC isomerization. An advantage
of isomerization of spiropyran compounds is the dramatic difference
in the photophysical properties of the two isomers, only MC form can
emit fluorescence. Acid-induced is beneficial to stabilization of polar
MC form and can affect its photophysical properties [23–28]. Exploiting
the changes in the photophysical properties of the compounds accom-
panying their structures will be helpful for the development of the com-
pounds applicable in the fluorescence probes, molecular switches and
functional materials.
on the chromene 6-nitro substituted spiropyran, the results showed
that the electron-withdrawing group enhanced the interconversion be-
tween SP and MC states [29]. Chan's group incorporated an oxime moi-
ety [30] and a pH-responsive moiety [31] onto the chromene 8- of the
spriopyran, and designed ratiometric pH fluorescence probes based on
the excited state intramolecular proton transfer mechanism. Meizhen
Yin's group designed the new extreme pH sensors by the introduction
of the bulky substituent benzoic acid at the indole N-position and the
electron-withdrawing carboxyl group at the indole 6-position of
spiropyran [32]. It was clear that the chromene 6-, 8- and indole N-
subsituted spiropyran compounds significantly affect the molecular
charge distribution and the process of intramolecular charge transfer
of MC state. To the best of our knowledge, less attention has been paid
to the chromene 7-position of spiropyran, even less was known how
the various substituents with different electron withdrawing or donat-
ing power to affects the SP-MC isomerization and the pH range of
their control.
In the paper, we designed and synthesized a series of the chromene
7-subsituted spiropyran compounds, the photophysical properties of
them were studied comparatively by UV-absorption and fluorescence
spectra under pH stimulation. The results showed that pKa values of
the compounds changed gradually accompanying their different sub-
stituents (\\Cl,\\Br,\\H,\\CH3,\\OCH3 and\\N(CH2CH3)2). As we ex-
pected, the chromene 7-substituted spiropyran had a slight adjustment
on the pKa of the MC-MCH transformation. This study opened a new
When suitable substituents were introduced the chromene ring and
indole ring of SP (Scheme 1), the transformation from “SP” to “MC” by
⁎
Corresponding authors.
E-mail addresses: chzhang@sxu.edu.cn (C. Zhang), dc@sxu.edu.cn (C. Dong).
https://doi.org/10.1016/j.saa.2018.04.076
1386-1425/© 2018 Elsevier B.V. All rights reserved.

14
L. Cui et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 202 (2018) 13–17
ethyl iodide was refluxed in 60 mL of acetonitrile for 24 h. Upon cooling
to room temperature, the crude solid precipitated. It was filtered off and
washed with ether to give 5.60 g of pink crystals (89%).
1,2,3,3-tetramethyl-3H-indolium iodide (1.88 g, 10 mmol), 4-
substituted 2-hydroxy-benzaldehyde (10 mmol) and 2.0 mL piperidine
were dissolved in 25 mL of CH3CH2OH, and the reaction mixture was
refluxed with stirring for 12 h and then the solvents were evaporated
in reduced pressure. The residue was purified by column chromatogra-
phy on silica gel to give target compounds. Yield: 45–59%. The structure
of A–F fully characterized by 1H NMR, 13C NMR and HRMS-ESI. (see the
Fig. S4-Fig. S21).
Scheme 1. The molecular structure of spiropyran.
2.4. General UV–Vis and Fluorescence Spectra Measurements
way to design spiropyran-based fluorescence probes, molecular switches
and biological materials.
The stock solutions of the A–F (0.2 mM) were prepared in DMF. And
phosphate buffer saline (PBS) buffer was prepared in deionized water.
The solutions for spectroscopic determination were prepared by dilut-
ing the stock solution with 50 mM buffer (containing 100 mM NaCl
for constant ionic strength) at various pH values in a 1 cm quartz cell
(totally 3 mL). UV–vis and fluorescence spectra were obtained in
DMF: PBS (1: 9 v/v, PBS buffer, pH 7.4) solutions. After addition of
analytes for 10 min, spectra could be measured. All spectroscopic exper-
iments were carried out at room temperature. The desired solution pH
was obtained by addition a certain amount of NaOH (0.1 M) or HCl
(0.1 M) to adjust.
2. Experiment
2.1. Materials
All chemicals and solvents were analytical grade and were used with-
out further purification. 2,3,3-tetramethyl-3H-indole,ethyl iodide, 4-
(dimethylamino)benzaldehyde, 2-hydroxy-4-chlorobenzaldehyde, 2-
hydroxy-4-bromobenzaldehyde, 2-hydroxy-4-methoxybenzaldehyde,
2-hydroxy-4-methylbenzaldehyde and 2-hydroxybenzaldehyde were
purchased from the Sigma-Aldrich company. All other chemicals were
commercially available from the Beijing chemical reagent company.
3. Results and Discussion
2.2. Instruments
3.1. UV–Vis Absorbance Spectra of Compounds (A–F) in Different pH
All reactions were monitored by thin-layer chromatography on glass
plates coated with silica gel GF254 (0.25 mm). 1H NMR and 13C NMR
spectra were collected on a 400 MHz spectrometer. High-resolution
mass spectra were obtained on a Bruker Autoflex mass spectrometer
(MALDI-TOF).
UV-absorption spectra were recorded on a Shimadzu UV-265 spec-
trophotometer (Tokyo, Japan). Fluorescence excitation and emission
spectra were measured on a Hitachi F4500 spectrofluorometer (Tokyo,
Japan). A PO-120 quartz cuvette (10 mm) was purchased from Shanghai
Huamei Experiment Instrument Plants, China. Fluorescence measure-
ments were carried out with excitation and emission slit width of 5 nm.
In order to know how the different substituents to affect the SP-MC
isomerization, six chromene 7-substituted spiropyran compounds
(A~F) with different electron withdrawing or donating power (\\Cl,
\\Br,\\H,\\CH3,\\OCH3 and\\N(CH2CH3)2) were synthesized. We
first investigated their UV–vis absorbance spectroscopic properties in
a mixture of DMF: buffer (1: 9, 50 mM PBS buffer) at the concentration
of 50 μM and the detailed experimental data were summarized in
Table 1. Fig. 1 showed the UV–vis absorption spectra of compound A
under the different pH. From Fig. 1, it was found that compound A ex-
hibited two absorbance bands about 300 and 530 nm at pH = 8.0.
With decreasing pH of the solution, the intensity of absorption bands
at 300 and 530 nm decreased and a new band was increased at
422 nm, two clear isosbestic points at 320 and 480 nm were observed.
When the pH was about 2.0, the absorption bands at 530 and 300 nm
disappeared and only a sharp absorption band at 422 nm was observed.
The phenomenon was reversible when the pH value was changed from
8.0 to 2.0 and back to 8.0. Meanwhile, the color of the solution changed
from purple to shallower, then to yellow accompanying the pH change
2.3. Preparation and Characterization of Compound A–F
The synthesis routes of A–F were summarized in Scheme 2.
Compound 2(1-Ethyl-2,3,3-trimethyl-3H-indolinium iodide) was syn-
thesized following the literature methods [33]. A mixture of 3.20 g
(20.00 mmol) 2,3,3-trimethyl-3H-indinium and 3.58 g (23.00 mmol)
Scheme 2. The synthesis route of compound A–F.

L. Cui et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 202 (2018) 13–17
15
Table 1
Photophysical properties of the spiropyran compounds A–F.
Compound
A (\\Cl)
B (\\Br)
C (\\H)
D (\\CH3)
E (\\OCH3)
F (\\N(CH2CH3)2)
Yield (%)
50
48
52
59
54
45
λabs (nm)
422, 530
480
16,560 (422 nm)
4720 (530 nm)
3.5–5.6
4.46
480
540, 635
615
0.5–2.2
(pH = 2)
(pH = 8)
4.74
424, 538
486
23,212 (424 nm)
8788 (538 nm)
3.8–5.2
4.42
475
525, 635
605
420, 540
480
12,121 (420 nm)
4242 (540 nm)
4.1–6
5.10
495
532, 640
620
0.359–1.2
112
438, 550
504
21,727 (438 nm)
4030 (550 nm)
4.2–6.1
5.23
500
537, 640
617
446, 528
492
29,848 (446 nm)
17,848 (528 nm)
4.5–6.3
5.42
490
542, 600
607
0.18–11.2
96
410, 550
460
19,484 (410 nm)
73,454 (550 nm)
4.9–6.8
5.89
450
540, 597
570
1.5–13
130
Isosbestic point (nm)
ε (M−1 cm−1
)
pH range
pKa (abs)
λex (nm)
λem (nm)
Isoemission point (nm)
Intensities ratio
0.5–2.2
101
97
0.2–1.0
99
90
Stokes shift (nm)
118
105
100
5.15
72
5.52
50
5.97
pKa (em)
4.55
5.29
from 8.0 to 2.0 (insert in Fig.1). After increasing pH of the solution, the
yellow solution turned purple gradually. It was clear that compound A
exhibited a highly reversible response to pH.
absorption of the six compounds presented a similar response to pH,
with decreasing pH of the solution, the absorption intensity of MC at
520–550 decreased and the new band of MCH was observed at
410–450 nm. 2) The ε of MCH at 410–450 nm was higher than them
of MC at 520–550 nm, because the MCH form was stabler than the MC
form. 3) The pH response range and the pKa of six compounds were dif-
ferent, they may be caused by the different substituents.
The Fig. 2 was the plot of the absorption intensity of compound A at
422 nm against the pH of the solutions, affording a pKa value of 4.46. The
value between absorption intensity correlated nicely with the pH values
of the solutions (R2 = 0.997). Moreover, the molar absorption coeffi-
cient (ε) could be obtained by using Lambert-Beer's law according to
Fig. 1. The ε of compound A at 530 nm and 422 nm were
4720 M−1 cm−1 (pH = 8.0) and 16,560 M−1 cm−1 (pH = 2.0), respec-
tively. The change of UV–vis absorption spectra attributed to the isom-
erization of spiropyran compounds. In the UV–vis spectra of
compound A, the 300 nm absorption band was assigned to the SP
form, the absorption band at 530 nm belonged to the purple open MC
form. In the presence of acid, the MC form converted to yellow proton-
ated merocyanine (MCH) form and the corresponding band at 422 nm
was appeared. The interconversion of SP, MC and MCH forms were
outlined in Scheme 3. There was the equilibrium between MC and
MCH in different pH. The molar absorption coefficients of MC and
MCH were constant value. The absorption intensity was mainly deter-
mined by the concentration of MC and MCH. In fact, Fig.2 expressed
the relationship between MCH concentration and pH value.
3.2. Fluorescence Spectra of Compounds (A–F) in Different pH
The fluorescence emission spectra of the six compounds were inves-
tigated under the same condition and the detailed fluorescence data
were summarized in Table1. The fluorescent spectra of the compound
A excited at 480 nm in various pH from 2.0 to 8.0 were displayed in
Fig. 3. At pH = 2.0, compound A exhibited the emission peak at
540 nm. With an increase in pH, its fluorescence intensity at 540 nm
was reduced and a new emission band at 635 nm emerged concomi-
tantly. Apparently, the clear isoemission point was apparent at
615 nm, which indicated the fluorescence intensities ratio (F635/F540
)
increased from 0.5 to 2.2 associated with pH change from 2.0 to 8.0, giv-
ing a change of 4-fold enhancement between two plateaus of the pH ti-
tration curve. Non-linear curve fitting using Boltzmann equation, on the
basis of the plot of fluorescence intensities ratios (F635/F540) against pH
(inset Fig.3), the pKa of the probe was calculated to be 4.74. The result
was similar to the value by absorbance with pH. In essence, the emission
bands at 540 and 635 nm could be assigned to the MCH and MC forms,
respectively.
The UV–vis absorption properties of compounds B\\F were investi-
gated and the experimental data were shown in Table 1. The UV–vis ab-
sorption spectra of B\\F at different pH and non-linear curve fitting
were shown in Fig. S1. These results indicated that: 1) The UV–vis
The fluorescent spectra of compounds B–F and their non-linear
curves fitting at different pH were measured and shown in Table 1
and Fig. S2. For all these materials, It was found that: 1) The fluorescent
Fig. 1. UV–vis absorption of compound A (50 μM) at different pH in PBS buffer/DMF (9:1,
v/v). Inset: The color of the solution changed from yellow to purple with increasing pH.
(For interpretation of the references to color in this figure legend, the reader is referred
to the web version of this article.)
Fig. 2. Nonlinear fitting of the UV–vis absorption at 422 nm.

16
L. Cui et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 202 (2018) 13–17
Scheme 3. The interconversion of SP, MC and MCH forms of compound A–F.
emissions of MCH were at 525–545 nm, while the emissions of MC were
observed at 600–640 nm. Obviously, MC showed a remarkable long
emission wavelength near the NIR fluorescence region. 2) MC and
their protonated MCH of six compounds displayed the significantly
large Stokes shift about 99–130 and 50–105 nm, respectively.
3) Ratiometric fluorescence signals were exhibited in different pH solu-
tions, especially compound E showed highest sensitivity with a more
Fig. 4. Color change of compounds B, C and E solution with the increase of pH value from
2.0 to 10.0 under daylight. (For interpretation of the references to color in this figure
legend, the reader is referred to the web version of this article.)
3.4. Effects of the Substituents on the pKa of MC-MCH
The pKa of compounds A–F were determined from their absorption
and fluorescence spectra in solution of varying pH and shown in
Table 2. The data of Table 2 revealed that the pKa was affected by the
substituent type. The influence of substituent type on pKa values was
quantified using Hammett equation [34].
than 62-fold enhancement in the fluorescence intensity ratio (F600
/
F540). On the basis of these results, compounds A–F were suitable for
the development of new fluorophores.
pKa ¼ pKa0 þ ρσp
3.3. Effects of the Substituents on the Color of MC-MCH
where ρ is the proportionality constant reflecting the sensitivity of
pKa values to the substituent effects, σp is Hammett constant of substit-
uents, refers to unsubstituted spiropyran compound. pKa refers to
substituted spiropyran compounds.
The color change was obvious during the reversible conversion pro-
cess between MC and MCH in different pH. Unsubstituted spiropyran
(C) and its 7-substituted derivatives (B, E) with electron withdrawing
(\\Br) and donating substituent (\\OCH3) were chosen to compare
their color change. Fig. 4 showed the color of them (10−5 M) in different
pH. We found that the color sudden change point of B and E were about
at the pH ~5 and ~6, respectively. It was obvious that the corresponding
pH value of color sudden change point was B b C b E. The result was
agreement with the pKa value from spectra experimental.
The relation between determined pKa values and Hammett con-
stants was presented in Fig.5 and there was a good linearity relationship
between them. As a result, the chromene 7-substituted spiropyran has a
slight adjustment on the pKa of the MC-MCH transformation. The pKa
was smaller when the Hammett constant of substituents was larger.
Moreover, the Henderson-Hasselbalch equation was also used to
calculate the pKa of the MC-MCH transformation and the detailed data
were listed in Table S1 and Fig.S3. The calculation results showed that
the change regularity of the pKa of the spiropyran compounds with dif-
ferent substituent was consistent and good linearity relationships be-
tween pKa values and Hammett constants were obtained by two
different equations.
4. Conclusion
In summary, a series of the chromene 7-subsituted spiropyran com-
pounds with different electron withdrawing or donating substituent
(\\Cl,\\Br,\\H,\\CH3,\\OCH3 and\\N(CH2CH3)2) were prepared to
investigate the effects of substitution on isomerization of spiropyran.
The experimental results showed that: 1) For six compounds, acid-
Table 2
pKa and σp for various spiropyran compounds.
Compound
Substituent
σp
pKa (abs)
pKa (em)
A
B
C
D
E
F
\\Cl
\\Br
\\H
\\CH3
\\OCH3
\\N(CH2CH3)2
0.227
0.232
0
−0.17
−0.268
−0.72
4.46
4.42
5.10
5.23
5.42
5.89
4.74
4.55
5.15
5.29
5.52
5.97
Fig. 3. The fluorescence emission spectra of compound A (50 μM) at different pH values in
PBS buffer/DMF (9:1, v/v) with λex = 480 nm. Inset: Plot of the emission intensities ratios
(F635nm/F540nm) to pH variation in PBS buffer/DMF (9:1, v/v).

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The work described in this paper was supported by the National Nat-
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Natural Science Foundation of Shanxi Province (No. 201601D011017),
the Scientific Activities of Selected Returned Overseas Professionals in
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Appendix A. Supplementary data
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Supplementary data to this article can be found online at https://doi.
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