Synthesis and acid-responsiveness of an insulated π-conjugated polymer containing spiropyrans in its backbone

Article abstract of DOI:10.3390/molecules24071301

A π-conjugated polymer containing spiropyrans (SPs), which could be almost completely converted to protonated merocyanines (MCH⁺) and back to the SP form by adding an acid and a base, respectively, was developed. The insulation of the π-conjugated polymer, referred to as insulated spiropyran-containing poly(p-phenylene ethynylene) (ins-SP-PPE), using permethylated α-cyclodextrins (PM α-CD) suppressed the π-π interaction between the polymer chains containing MCH⁺, and the installation of PM α-CD improved the switching ability of SPs. The polymer exhibited repeatable acidochromism with almost complete conversion between the SP and MCH⁺ forms. Photoluminescence measurements were conducted and the acid-induced luminescence quenching of the polymer in the solution was observed, which stemmed from energy transfer from the PPE to MCH⁺ moieties. In the solid state, the quantum yield of ins-SP-PPE was more than twice that of the uninsulated polymer, which derived from the insulation effects. The acid-induced luminescence quenching was also observed in the solid state.

Full text of DOI:10.3390/molecules24071301

molecules
Communication
Synthesis and Acid-Responsiveness of an Insulated
π-Conjugated Polymer Containing Spiropyrans in
Its Backbone
Hiromichi V. Miyagishi, Takashi Tamaki, Hiroshi Masai and Jun Terao *
Department of Basic Science, Graduate School of Arts and Sciences, The University of Tokyo, 3-8-1 Komaba,
Meguro-ku, Tokyo 153-8902, Japan; hero-miyagishi28@g.ecc.u-tokyo.ac.jp (H.V.M.);
ctamaki@g.ecc.u-tokyo.ac.jp (T.T.); cmasai.h@mail.ecc.u-tokyo.ac.jp (H.M.)
*
Correspondence: cterao@mail.ecc.u-tokyo.ac.jp
ꢀ
ꢁꢂꢀꢃꢄꢅꢆꢇ
Academic Editor: Il Kim
Received: 6 March 2019; Accepted: 1 April 2019; Published: 3 April 2019
ꢀ
ꢁꢂꢃꢄꢅꢆ
Abstract:
converted to protonated merocyanines (MCH ) and back to the SP form by adding an acid and
a base, respectively, was developed. The insulation of the -conjugated polymer, referred to as
insulated spiropyran-containing poly(p-phenylene ethynylene) (ins-SP-PPE), using permethylated
A π-conjugated polymer containing spiropyrans (SPs), which could be almost completely
+
π
α
-cyclodextrins (PM
α
-CD) suppressed the
π
-
π
interaction between the polymer chains containing
+
MCH , and the installation of PM
α
-CD improved the switching ability of SPs. The polymer exhibited
+
repeatable acidochromism with almost complete conversion between the SP and MCH forms.
Photoluminescence measurements were conducted and the acid-induced luminescence quenching
of the polymer in the solution was observed, which stemmed from energy transfer from the PPE
+
to MCH moieties. In the solid state, the quantum yield of ins-SP-PPE was more than twice that of
the uninsulated polymer, which derived from the insulation effects. The acid-induced luminescence
quenching was also observed in the solid state.
Keywords: stimulus-responsive copolymer; sensors; spiropyran; acidochromism; supramolecular
polymer; poly(p-phenylene ethynylene); permethylated α-cyclodextrins
1
. Introduction
Spiropyrans (SPs), first reported by Fischer and Hirshberg in 1952 [
due to their ability to switch forms in response to various external stimuli, such as light, mechanical
force and acid [ ]. For example, irradiation of an SP with UV light causes isomerization of the SP to a
merocyanine (MC), involving changes in the
1], have been of great interest
2,3
π
-conjugated system, via heterolytic C-O bond cleavage.
+
Under acid stimuli, SPs are converted to protonated merocyanines (MCH ) (Scheme 1) [
of the color differences between SPs and MCs (MCH ), SPs have been widely applied in electronic
devices [5,6], bioimaging [7], and chemical sensors [8].
4
]. Because
+
Scheme 1. Acid- and base-induced conversion between spiropyrans (SP) and protonated merocyanines (MCH+).
Molecules 2019, 24, 1301; doi:10.3390/molecules24071301
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The incorporation of SPs into the backbones of
π
-conjugated polymers to allow the switching
of their optical and electronic properties through the conversion of the SP moieties to MC moieties
under external stimuli has already been reported. Ng et al. first described the incorporation of an
SP into a poly(p-phenylene ethynylene) (polyPPE) backbone in 2006 [
SP moieties in the backbone were incompletely isomerized to MC moieties under UV irradiation.
Incorporation of SPs into poly(p-phenylene) [10], poly(m,m,p-phenylene) [11] and polyfluorene [12
9] and demonstrated that the
]
has also been reported and the responses of the polymers to UV irradiation or mechanical force have
been discussed. However, the isomerization of SPs in conjugated polymers showed low conversion
and/or limited reversibility. As for UV light stimuli, one report indicated that the conversion of SPs in
poly(p-phenylene) was approximately 16% and the SPs exhibited irreversible isomerization [10]. As for
mechanical stimuli, one report indicated that the highest conversion of SPs was 44% and precipitation
was observed after the proportion of MC exceeded the amount, which might have been caused by
the π-π interactions between the MC moieties causing the polymer chains to have low solubility [12].
While several limitations of the isomerization have been demonstrated in conjugated SP polymers, the
isomerization behavior of SPs under acid stimuli has not been studied for conjugated polymers [13].
Our group has developed a method to suppress the
polymer chains using covalently linked permethylated
π
-
π
interactions between
-cyclodextrins (PM -CDs), which
15]. The insulation using PM
-conjugated polymers high solubility in organic solvents and high solid-state
emission quantum yields because the insulation prevented the interaction between the polymer
chains [16 17]. We have incorporated external-stimuli-responsive moieties into various -conjugated
polymers to prepare materials with changeable optical properties [14 18]. Herein, we report the
π-conjugated
α
α
three-dimensionally insulate the
-CDs gave the
π-conjugated main chains [14,
α
π
π-π
,
π
,
synthesis of an insulated polyPPE containing SP moieties in its backbone (ins-SP-PPE). The conversion
behavior of the SPs in ins-SP-PPE and the change in the optical properties of ins-SP-PPE were also
investigated under acid stimulus.
2
. Results and Discussion
2
.1. Synthesis of Ins-SP-PPE and Unins-SP-PPE
Copolymerization between SP monomer
4 and a derivative of PPE monomer 8 insulated with
PM
Indolenium salt
give SP , and subsequent deprotection of the trimethylsilyl group afforded SP monomer
Glaser coupling of the previously reported PPE and subsequent hydrolysis of the acetamide gave
uninsulated PPE monomer . Insulated monomer was synthesized by the self-inclusion of 15 20].
The quantitative insulation was confirmed by the characteristic downfield shift of the signals of the
aromatic protons on the insulated PPE (Figure S14). Monomer was kinetically isolated even in
nonpolar solvents because the activation energy of the dethreading of was high enough to prevent it
from occurring [20]. Monomer was copolymerized with after the conversion of the amine groups
on to azide groups (Scheme 3). As shown in Figure 1, the gel-permeation chromatography (GPC)
chart evidenced the formation of ins-SP-PPE with the consumption of the insulated PPE monomer
α
-CDs was conducted via Cu-catalyzed Huisgen 1,3-dipolar cycloaddition to give ins-SP-PPE [19].
and a derivative of salicylic aldehyde were heated in the presence of piperidine to
(Scheme 2).
1
2
3
4
5
7
8
7
[
,
8
8
8
4
8
8
.
In order to remove oligomers, the crude ins-SP-PPE was treated with preparative GPC to give purified
5
4
ins-SP-PPE (M
w
= 1.3
= 3.0
×
10 , M
n
= 5.1
×
10 ). In addition, uninsulated PPE polymer unins-SP-PPE (M
under the same conditions as those
probably stemmed from the low solubility
of uninsulated conjugated polymers caused by the π-π stacking between the polymer chains.
w
4
4
=
4.2
×
10 , M
n
×
10 ) was also synthesized from monomer
7
for the synthesis from
w n
8 (Scheme 4). The lower M and M

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Scheme 2. Synthesis of SP monomer 4 and insulated (p-phenylene ethynylene) (PPE) monomer 8.
Scheme 3. Polymerization of insulated spiropyran-containing poly(p-phenylene ethynylene)
(
ins-SP-PPE) using Huisgen 1,3-dipolar cycloaddition.
Scheme 4. Polymerization of uninsulated (unins)-SP-PPE using Huisgen 1,3-dipolar cycloaddition.
2
5
30
35
40
45
Retention time / min
Figure 1. Gel-permeation chromatography (GPC) analysis of monomer
ins-SP-PPE (red line) after polymerization.
8 (gray line) and crude

Molecules 2019, 24, 1301
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2
.2. UV-Vis Titration with Acid and Base Stimuli
Ultraviolet-visible (UV-Vis) spectroscopy titration was conducted to investigate the acidochromic
properties of ins-SP-PPE and unins-SP-PPE. The acid stimuli were used because one report suggested
+
that the acid stimuli were able to quantitatively convert SPs to MCH in the polymer backbone [13]. It
+
was expected that ins-SP-PPE would be converted to ins-MCH -PPE by an acid stimulus (Figure 2A).
−
5
The UV-Vis spectrum of ins-SP-PPE in CH Cl (2.0
3
×
10 M) contained an absorbance peak at
2
2
62 nm, which was attributed to PPE (Figure 2B), and the addition of trifluoroacetic acid (TFA) to the
solution led to the appearance of a broad absorption at greater than 420 nm, which was attributed
+
to MCH [
4], with an isosbestic point at 333 nm. Additionally, the almost complete conversion of
SP moieties in ins-SP-PPE was confirmed because the absorbance at 420 nm was saturated under
the addition of more than 0.1 M TFA (Figure 2C). The formation of precipitation could be denied
because any decrease of absorbance was not observed. Subsequently, to confirm the reversibility of
this acid-induced conversion, triethylamine (TEA) was added into the solution as a base (Figure 2D),
which resulted in the decrease of the absorption at greater than 420 nm, indicating that the conversion
+
+
from the MCH form to the SP form occurred. The almost complete conversion of MCH to SP was
also confirmed by the saturation of the absorbance. Similar acidochromic behaviors were observed
in unins-SP-PPE (Figure S27), which probably derived from the high solubility of bulky side chains
of PM
α
-CD in unins-SP-PPE or the low degree of polymerization of unins-SP-PPE. These results
+
confirmed the acidochromism between ins-SP-PPE and ins-MCH -PPE. This acidochromic behavior
was repeatedly observed upon the addition of acid and base (Figure 2E).
A
B
1
0.5
Acid (TFA)
0
300
350
400
450
500
550
600
Wavelength / nm
C
D
0.2
1
0.15
0.1
0.5
Base (TEA)
0
.05
0
0
0
0.05
0.1
0.15
L
0.2
300
350
400
450
500
550
600
-
1
Concentration of TFA / mol
Wavelength / nm
Figure 2. Cont.

Molecules 2019, 24, 1301
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E
1
0.5
0
0
1
2
3
4
5
Number of cycles
+
Figure 2.
) Ultraviolet-visible (UV-Vis) spectroscopy spectra of ins-SP-PPE (2.0
addition of different amounts of trifluoroacetic acid (TFA) (0–0.2 M). (
absorbance of ins-SP-PPE at 420 nm under the addition of TFA (0–0.2 M). (
(A) Acid- and base-induced structural conversion between ins-SP-PPE and ins-MCH -PPE.
−
10 M in CH Cl ) upon
2 2
5
(
B
×
C
) A change of normalized
) UV-Vis spectra of
10 M in CH Cl ) upon addition of different amounts of triethylamine (TEA)
D
−
5
ins-SP-PPE (2.0
×
2
2
(
0–0.2 M) following the TFA addition (0.2 M). (E) Repeatable switching of the absorbance of ins-SP-PPE
2.0 × 10− M in CH Cl ) at 420 nm by the addition of TFA and TEA.
5
(
2
2
1
2
.3. H-NMR Study
A proton nuclear magnetic resonance ( H NMR) study was conducted to investigate the
acidochromic behavior of ins-SP-PPE. As shown in Figure 3A, the SP signals were observed at 6.98,
.84, 6.62, and 5.79 ppm. The addition of TFA resulted in the downfield shift of these signals as clearly
1
6
shown by a vinyl proton of SP moieties (Figure 3B). This shift indicated the conversion of SP moieties
+
to MCH ones, not to any other species because the similar shift was observed in the unsubstituted SP,
0
0
0
0
1
,3 ,3 -trimethylspiro(2H-1-benzopyran-2,2 -indoline) (Figure S20). Similar phenomena were observed
in unins-SP-PPE (Figure S23), which is consistent with the results of UV-Vis titration. The shifted
signals of SP moieties in ins-SP-PPE was reset to the original position by adding an excess amount of
+
TEA (Figure 3C), which confirmed the reversible conversion between SP and MCH moieties.
A
B
C
1
Figure 3. A proton nuclear magnetic resonance ( H-NMR) spectra (aromatic region) of ins-SP-PPE
before (
A
, black line) and after adding TFA (
B
, 0.04 M, red line) and then TEA (
C
, 0.08 M, blue line).
+
Gray allows indicate the shift of signals deriving from vinyl protons of SP and MCH .
2
.4. Photoluminescence Study in the Solution and Solid State
Photoluminescence measurements were conducted to investigate the effects of the acid-induced
SP conversion. A peak around 520 nm was observed in the emission spectrum of ins-SP-PPE in CH Cl
2
2
−
5
solution (2.0
×
10 M) (Figure 4A), which stemmed from the PPE chains. After the addition of TFA

Molecules 2019, 24, 1301
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to the solution, significant quenching of the fluorescence was observed. A similar phenomenon was
previously reported by Jung et al. [ ], which was attributed to fluorescence quenching of polythiophene
8
via energy transfer between polythiophene and MC moieties. Accordingly, the quenching of ins-SP-PPE
+
was probably caused by energy transfer from the PPE moieties to the MCH moieties.
A
B
C
5000
4000
3000
2000
1000
0
ins-SP-PPE
unins-SP-PPE
Acid (TFA)
680
380
480
580
Wavelength / nm
Figure 4.
The solution of ins-SP-PPE was excited at 362 nm. Photographs under UV irradiation at 365 nm
of ( ) ins-SP-PPE and unins-SP-PPE films on a glass substrate and ( ) fluorescence quenching of
(A) Emission spectra of ins-SP-PPE upon adding different amounts of TFA (0–0.2 M).
B
C
ins-SP-PPE before (right side) and after (left side) dipping in aqueous 0.1 M HCl for 10 s. The
double-headed arrow marks the dipped region of the film.
The luminescence properties of the polymers and the acid responsiveness of ins-SP-PPE in
the solid state were also examined. Films of ins-SP-PPE and unins-SP-PPE were prepared on glass
substrates by drop-casting their solutions. Under irradiation with UV light (365 nm), the film of
ins-SP-PPE exhibited stronger emission than that of unins-SP-PPE (Figure 4B). The quantum yields
of ins-SP-PPE and unins-SP-PPE in the solid state were 5.4% and 2.4%, respectively. The fact that
the quantum yield of the insulated polymer was more than twice that of the uninsulated one was
attributed to the suppression of the π-π interaction between the conjugated polymers. Furthermore,
dipping the ins-SP-PPE film into 0.1 M HCl aqueous solution for 10 s resulted in the quenching of
luminescence only in the dipped region (Figure 4C). As mentioned above, this was because of energy
+
transfer between the π-conjugated backbone and MCH moieties.
3
. Materials and Methods
The general remarks, the synthetic procedures of monomer
4, 8 and unins-SP-PPE, and
the preparation methods of the films of ins-SP-PPE and unins-SP-PPE are available in the
Supplementary Materials.
Synthesis of Ins-SP-PPE
To a solution of PPE monomer
8
(45 mg, 16
µmol, 1.0 eq.) in dehydrated acetonitrile (1.6 mL),
tert-butyl nitrite (37
µ
L, 0.31 mmol, 20 eq.) and trimethylsilyl azide (62 µ
L, 0.47 mmol, 30 eq.) were
◦
added at 0 C and then stirred for 2 h at room temperature. After the reaction, the solvent was
evaporated in vacuum. The residue was diluted with chloroform and H O and extracted with
2
chloroform and diethyl ether. The combined organic layer was dried over MgSO , filtrated, and
4
evaporated in vacuum to afford the product as a yellow solid. Without further purification, the
mixture of the solid,
4
(5.3 mg, 16
µ
mol, 1.0 eq.), sodium ascorbate (19 mg, 94
µ
mol, 6.0 eq.), and
t
CuSO4 5H O (24 mg, 94
·
µmol, 6.0 eq.) in H O (391
µL) and BuOH (391
µL) was stirred overnight
2
2
at room temperature. After the reaction, the mixture was diluted with chloroform and saturated
aqueous NH Cl solution and extracted with chloroform. The combined layer was washed with
4
saturated aqueous NaHCO , dried over MgSO , filtrated, and evaporated in vacuum. To remove
3
4
the oligomers, the residue was purified by preparative GPC with chloroform as the eluent to afford

Molecules 2019, 24, 1301
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5
4
1
product ins-SP-PPE as a yellow solid (22 mg, 43%, M
(
1
w
= 1.3
×
10 , M
n
= 5.1
×
10 , PDI = 2.5); H-NMR
500 MHz; CDCl3):
H, –CH=CH–), 6.82 (d, J = 7.8 Hz, 1H, ArH), 6.63 (d, J = 8.3 Hz, 1H, ArH), 5.79 (d, J = 9.6 Hz, 1H,
CH=CH–), 5.13–2.97 (m, 186H, CD–H, OCH ), 2.82 (s, 3H, N–CH ), 1.40 (s, 3H, gem–CH ), 1.25 (s, 3H,
δ = 8.18–8.08 (m, 6H, triazole–H, ArH), 7.74–7.58 (m, 14H, ArH), 6.98 (d, J = 10.0 Hz,
–
3
3
3
gem–CH3).
4
. Conclusions
An insulated
π
-conjugated polymer, ins-SP-PPE, was synthesized by copolymerization of SP
via Cu-catalyzed Huisgen 1,3-dipolar cycloaddition. UV-Vis
titrations revealed that the polymer exhibited repeatable acidochromism and the SP moieties in the
monomer 4 and insulated PPE monomer 8
+
1
polymer were almost completely converted to MCH ones. H-NMR study supported the fact that the
+
SP moieties in the polymer converted to MCH moieties, not to any other species. In addition, the
acid-induced luminescence quenching of ins-SP-PPE was observed in the solution and solid state. We
will further investigate the chromic properties of ins-SP-PPE induced by other stimuli.
Supplementary Materials: Supplementary Materials, including general remarks, synthetic procedures, NMR
spectra, charts of GPC, UV-Vis spectra, emission spectra, and a photograph of a polymer film, are available online.
Author Contributions: J.T. designed and directed the project. H.V.M. performed the experiments, analyzed the
data, and wrote the manuscript. J.T., T.T. and H.M. supervised the experiments. All the authors have discussed
the results and approved the manuscript.
Funding: This research was supported by JST CREST, Grant Number JPMJCR1331, the Environment Research and
Technology Development Fund of the ERCA Japan, Grant Number 5RF-1802, JSPS KAKENHI, Grant Numbers
1
8H05158 and 17K14446, the Nagase Science and Technology Foundation, the Terumo Foundation for Life
Science and Arts, the Yazaki Memorial Foundation for Science and Technology, the Asahi Glass Foundation, The
Ogasawara Foundation for Promotion of Science & Engineering, The Amada Foundation, and the Izumi Science
and Technology Foundation.
Acknowledgments: The authors would thank to Shuichi Hiraoka and Tatsuo Kojima for their helpful access and
advices on mass measurements.
Conflicts of Interest: The authors declare no conflict of interest.
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