Synthesis and characterization of amphiphilic silica nanoparticles covered by block copolymers branching photochromic diarylethene moieties on side chain

Article abstract of DOI:10.1016/j.dyepig.2014.11.010

Silica nanoparticles covered with polystyrene branching diarylethene moieties to the side chains are synthesized by reversible addition-fragmentation chain transfer polymerization on silica nanoparticles. The resulting nanoparticles are soluble in toluene

Full text of DOI:10.1016/j.dyepig.2014.11.010

Accepted Manuscript
Synthesis and characterization of amphiphilic silica nanoparticles covered by block
copolymers branching photochromic diarylethene moieties on side chain
Ryuta Seno , Seiya Kobatake
PII:
S0143-7208(14)00440-9
10.1016/j.dyepig.2014.11.010
DYPI 4588
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 15 September 2014
Revised Date: 6 November 2014
Accepted Date: 10 November 2014
Please cite this article as: Seno R, Kobatake S, Synthesis and characterization of amphiphilic silica
nanoparticles covered by block copolymers branching photochromic diarylethene moieties on side chain,
Dyes and Pigments (2014), doi: 10.1016/j.dyepig.2014.11.010.
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ACCEPTED MANUSCRIPT
Revised Manuscript for publication in Dyes and Pigments
Synthesis and characterization of amphiphilic silica nanoparticles covered by
block copolymers branching photochromic diarylethene moieties on side chain
Ryuta Seno and Seiya Kobatake*
Department of Applied Chemistry, Graduate School of Engineering, Osaka City
University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan
Corresponding author. Tel./fax: +81 6 6605 2797.
E-mail address: kobatake@a-chem.eng.osaka-cu.ac.jp (S. Kobatake).
ABSTRACT
Silica nanoparticles covered with polystyrene branching diarylethene moieties to the
side chains are synthesized by reversible addition-fragmentation chain transfer
polymerization on silica nanoparticles. The resulting nanoparticles are soluble in
toluene and tetrahydrofuran and show reversible photoisomerization upon alternating
irradiation with ultraviolet and visible light. The photoreversible isomerization follows
color change, which is called photochromism. During the photochromic reactions, the
nanoparticles are dispersed in solutions such as terahydrofuran. The modification by
further block copolymerization of N-isopropylacrylamide results in fabrication of
photochromic silica nanoparticles soluble in both tetrahydrofuran and water.
Keywords: Silica nanoparticle; RAFT polymerization; Photochromism; Diarylethene;
Sedimentation
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1. Introduction
Inorganic nanoparticles covered with organic polymers have drawn much attention
as organic/inorganic hybrid materials in recent years because of their applications for
optical materials, paints, makeup, column fillers, and solar cells [1-4]. Inorganic silica
nanoparticles have characteristics such as simple fabrications of controlled particular
size, low cost, weather resistance, chemical resistance, and safeness. Silica
nanoparticles covered with the functional dyes is useful as novel functional dyes and
pigments which have the potential for toughness and durability. The various types of
fluorescent dyes attached on silica nanoparticles have recently been used for reversible
fluorescence switch for biological imaging [5-10]. The fluorescence modulation is
based on a photochromic molecular switch incorporated into the silica nanoparticles.
The biocompatibility of the silica nanoparticles makes promising materials for in vivo
imaging applications. Such organic/inorganic hybrid materials can improve the
solubility in solvents by the modification of the covering agents.
Photochromic compounds exhibit reversible transformations between two isomers
upon irradiation at an appropriate wavelength of light [11,12]. The interconversion
between the two states is accompanied by changes in the physical properties such as
absorption spectra, fluorescence, refractive indices, dielectric constants,
oxidation–reduction potentials, and so on. Diarylethene is one of typical P-type
photochromic compounds and shows reversible photoisomerization upon alternating
irradiation with ultraviolet (UV) and visible light [13-16]. Silica nanoparticles with
diarylethene polymers are expected to be used for application as a photochromic
pigment having toughness and durability because of high dispersibility of the particle in
organic solvents in addition to thermal stability and fatigue-resistant properties.
Photochromic hybrid materials can be applied as photochromic decoloration,
photochromic glasses, UV sensors, photochromic optical waveguide, optical
memorydevices, holographic recording media, non-linear optics, and so on [17].
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The “grafting-from” and “grafting-to” techniques in polymer chemistry are
commonly used for connecting polymer chains on solid substrate [18-20]. Among them,
the former method can connect polymer with functional group on substrate, whereas the
latter method can initiate polymerization from substrate on which an initiator or a chain
transfer agent presents to form grafting polymer chains in high density. Particularly,
living radical polymerizations on the surface of materials are widely used for grafting
on substrate because of easy control of polymer chain length and end groups. There are
some reports on the surface-initiated living radical polymerizations such as atom
transfer radical polymerization and reversible addition-fragmentation chain transfer
(RAFT) polymerization [21,22]. Tsujii and coworkers found that high-density polymer
brushes can be obtained by living radical polymerization from solid substrate [21].
Moreover, they demonstrated that the dispersibility of silica nanoparticles covered with
polymer brushes in organic solvents is dominated by the nature of the polymers
fabricated onto the silica nanoparticles [23].
Here, we report on fabrication of silica nanoparticles covered with polystyrene
having photochromic diarylethene chromophores to the side chains, as shown in
Scheme 1. The silica surface-initiated polymerization was carried out by RAFT
copolymerization of styrene (St) and a diarylethene monomer (DE) using silica
nanoparticle covered with a RAFT agent. To modify the solubility of the nanoparticles
in both hydrophobic and hydrophilic solvents, block copolymerization with
N-isopropylacrylamide (NIPAm) was also performed. The photochromic behavior and
dispersibility of the resulting nanoparticles in THF and water are discussed.
[Scheme 1]
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2. Experimental section
2.1. Measurements
1H NMR spectra were measured using a Bruker AV-300N NMR spectrometer at
300 MHz. Deuterated chloroform (CDCl3) was used as the solvent and
tetramethylsilane (TMS) as internal standard, respectively. Transmission electron
microscopy (TEM; Hitachi H-7000) was performed at an accelerating voltage of 75 kV.
TEM samples were prepared by dropping an ethanol or tetrahydrofuran (THF) solution
of the particles on a carbon-coated copper grid and dried in air. The particle size was
determined from the TEM image. Absorption spectra were measured using a UV/visible
absorption spectrophotometer (Jasco V-560). Photoirradiation was conducted using a
200 W mercury-xenon lamp (Moritex MUV-202) or a 300 W xenon lamp (Asahi
Spectra MAX-301) as a light source. Monochromatic light was obtained by passing the
light through a monochromator (Jobin Yvon H10 UV or Jasco CT-10) and glass filters.
Gel-permeation chromatography (GPC) was performed using a Tosoh 8000 series GPC
system equipped with TSK-gel columns at 40 °C and using THF as the eluent.
2.2. Materials
All reagents were purchased from Wako Pure Chemical Industries. NIPAm and
2,2’-azobis(2,4,4-trimethylpentane) (ATMP) were recrystallized from n-hexane. DE was
synthesized according to the method described in our previous paper [24].
2.3. Synthesis of 1-[3-(methoxydimethylsilyl)propyloxycarbonyl]ethyl dithiobenzoate
(RAFT-Si)
RAFT-Si was synthesized according to the method described in literature as follows
[25]. Phenylmagnesium bromide was prepared from bromobenzene (12.4 mL, 118
mmol) and magnesium turnings (1.90 g, 78.2 mmol) in dry THF (30 mL). Carbon
disulfide (4.80 mL, 79.5 mmol) was slowly added to the THF solution of the Grignard
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reagent at 40 °C. To the mixture was slowly added 3-(methoxydimethylsilyl)propyl
2-bromopropionate (20.2 g, 71.2 mmol) at 40 °C. The mixture was stirred for 24 h at
room temperature. An adequate amount of ice water was added to the mixture to quench
the reaction. The reaction mixture was extracted with ether, washed with brine, dried
over MgSO4, filtrated, and concentrated in vacuo. The crude product was purified by
column chromatography on silica gel using n-hexane/ethyl acetate (8:2) as the eluent.
The pure product was obtained by a recycle HPLC (JAI LC-908) on gel columns
(JAIGEL-1H and 2H) using CHCl3 as the eluent in 72.2% yield (18.3 g). 1H NMR (300
MHz, CDCl3, TMS): δ (ppm) = 0.11 (s, 6H, Si(CH3)2OCH3), 0.56-0.65 (m, 2H,
COOCH2CH2CH2), 1.67-1.75 (m, 2H, COOCH2CH2), 1.68 (d, J = 7.4 Hz, 3H,
SCHCH3CO), 3.41 (s, 3H, Si(CH3)2OCH3), 4.09-4.17 (m, 2H, COOCH2), 4.75 (q, J =
7.4 Hz, 1H, SCHCH3CO), 7.26-7.57 (m, 3H, Aromatic H), 7.97-8.01 (m, 2H, Aromatic
H).
2.4. Synthesis of 1-(ethoxycarbonyl)ethyl dithiobenzoate (EEDB)
EEDB was synthesized according to the method described in literature as follows
[26]. Phenylmagnesium bromide was prepared from bromobenzene (10.5 mL, 100
mmol) in THF (50 mL) and magnesium turnings (2.43 g, 100 mmol) in dry THF (7 mL)
by heating for 20 min at 60 °C. Carbon disulfide (6.10 mL, 101 mmol) was slowly
added to the THF solution of the Grignard reagent at 40-50 °C. To the mixture was
slowly added ethyl 2-bromopropionate (18.1 g, 100 mmol) at 40 °C. The mixture was
stirred for 16 h at room temperature. An adequate amount of ice water was added to the
mixture to quench the reaction. The reaction mixture was extracted with ether, washed
with brine, dried over MgSO4, filtrated, and concentrated in vacuo. The crude product
was purified by column chromatography on silica gel using n-hexane as the eluent. The
pure product was obtained by a recycle HPLC (JAI LC-908) on gel columns
(JAIGEL-1H and 2H) using CHCl3 as the eluent in 37.8% yield (9.62 g). 1H NMR (300
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MHz, CDCl3, TMS): δ = 1.29 (t, J = 7.1 Hz, 3H, CH2CH3), 1.67 (d, J = 7.4 Hz, 3H,
CHCH3), 4.23 (q, J = 7.1 Hz, 2H, CH2), 4.75 (q, J = 7.4 Hz, 1H, CH), 7.34-7.43 (m, 2H,
Aromatic H), 7.51-7.58 (m, 1H, Aromatic H), 7.95-8.03 (m, 2H, Aromatic H).
2.5. Synthesis of silica nanoparticle by the Stöber method
A 28 wt% aqueous solution of ammonia and absolute ethanol (50 mL) were added
into a flask. To the solution was added a solution of tetraethoxysilane (TEOS) and
absolute ethanol (50 mL). The mixture was stirred for 24 h at room temperature. Silica
nanoparticles were obtained as solid by centrifuging for 10 min at 6000 rpm in ethanol.
The silica nanoparticles were purified by centrifuging 5 times in ethanol. The silica
nanoparticles obtained were dried in vacuo.
2.6. Synthesis of silica nanoparticle covered with RAFT agent
The silica nanoparticles obtained above were added to 2 N hydrochloric acid, and
the mixture was stirred for 24 h at room temperature. The silica nanoparticles were
washed by distilled water and purified by centrifuging 5 times in distilled water, and
dried. The silica nanoparticles were added to a dry THF solution (10 mL) of RAFT-Si
(500 mg, 1.4 mmol). The mixture was refluxed for 18 h. The resulting silica
nanoparticles were purified by centrifuging 5 times in n-hexane. The nanoparticles
(SNP-RAFT) were dried in vacuo.
2.7. Polymerization
RAFT polymerization was carried out in a glass tube sealed under vacuum.
Monomer, EEDB, SNP-RAFT, ATMP, and toluene were put in a glass tube. The tube
was degassed by several freeze-pump-thaw cycles and sealed off under vacuum. After
polymerization for a prescribed time at 100 °C, the resulting nanoparticles were
collected as solid by centrifugation for 10 min at 6000 rpm and washed 5 times with
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THF followed by drying in vacuo. The silica-free polymer was collected by
precipitation in methanol.
2.8. Block copolymerization
NIPAm, EEDB, AIBN, N,N-dimethylformamide (DMF), and SNP-poly(St-co-DE)
were put in a glass tube. The tube was degassed by several freeze-pump-thaw cycles
and sealed off under vacuum. After polymerization for a prescribed time at 60 °C, the
resulting nanoparticles were collected as solid by centrifugation for 10 min at 6000 rpm
and washed 5 times in THF followed by drying in vacuo. The silica-free poly(NIPAm)
was isolated by precipitation in n-hexane.
2.9. Sedimentaion behavior
The sedimentation behavior of the nanoparticles in solution was observed by the
light-transmission method of the nanoparticles in THF and in water. Before the
measurement, the solution of the nanoparticles was irradiated with 313 nm light to reach
the photostationary state. The number of the nanoparticles in solution was determined
from absorbance of the diarylethene closed-ring form at absorption maximum
wavelength in the visible region.
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3. Results and Discussion
3.1. Synthesis of RAFT-SNP
Silica nanoparticles were synthesized according to the technique established by
Stöber and coworkers [27]. The Stöber method has characteristic to produce a
monodispersed silica nanoparticle with small size distribution. The diameter of the
particle is controlled by adjusting the concentrations of TEOS, water, and ammonia.
Table 1 shows the preparation conditions and the properties of the resulting silica
nanoparticles. The reaction was performed by stirring the solution of TEOS, water, and
ammonia in ethanol for 24 h at room temperature under argon atmosphere, and the
resulting silica nanoparticles were isolated and purified by centrifuging the suspension
in ethanol. After drying the resulting silica nanoparticles, they were distributed in
ethanol, and casted on the copper grid to measure TEM. The average particle diameter
(d) and the standard deviation (σ) were obtained from the TEM image. Figure 1 shows
the TEM images of the resulting silica nanoparticles. The silica nanoparticles have
particle diameters of 60-350 nm and narrow particle size distributions (σ/d = 5-10%).
The relationship between the d value and the concentration of ammonia used for the
reaction was plotted in Figure 2. The d value can be controlled by the concentration of
ammonia because the particle size increased with increasing the concentration of
ammonia.
[Table 1]
[Figure 1]
[Figure 2]
The RAFT agent, RAFT-Si, is required to be immobilized onto the silica
nanoparticle surface. In general, the silane coupling agent with a methoxysilyl group to
attach the silica nanoparticle surface is used under the basic condition. Because the
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RAFT agent is unstable under the basic condition, it is necessary to use the
immobilization technique of the silane coupling agent without the basic condition. Thus,
the silica nanoparticles were treated with hydrochloric acid and washed with distilled
water, followed by refluxing the THF solution of RAFT-Si under argon atmosphere. The
siloxane bond was formed by the reaction of the ethoxysilyl group of RAFT-Si and the
silanol group on the silica nanoparticle surface. The resulting SNP-RAFT has a pale red
color originated from dithiobenzoate group. This means that the RAFT agent is
successfully immobilized onto the silica nanoparticle.
3.2. Synthesis and characterization of SNP-poly(St-co-DE)
RAFT copolymerization of DE and St using SNP-RAFT as the RAFT agent was
carried out at 100 °C. The RAFT polymerization using SNP-RAFT without a silica-free
RAFT agent cannot control Mn and Mw/Mn of the grafting polymers [22]. Therefore,
EEDB as the silica-free RAFT agent was added to the polymerization mixture. Table 2
shows the polymerization condition and the results of the polymerization. Two different
sizes of SNP were used for the polymerization. After the polymerization,
SNP-poly(St-co-DE) was collected as solid by centrifugation, and silica-free
poly(St-co-DE) was isolated by precipitation to methanol. Figure 3 shows TEM images
of SNP-poly(St-co-DE). The silica core maintains particle diameter and narrow particle
size distribution. The polymer shell around the silica core is observed in the image. The
thickness of the polymer shell in dry was found to be 3-6 nm from the image. This
means that RAFT polymerization was successfully carried out to form
SNP-poly(St-co-DE).
[Table 2]
[Figure 3]
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The composition of DE in silica-free poly(St-co-DE) was determined to be 34-39
1
mol% by H NMR spectroscopy, whereas the composition of DE in monomer is 25
mol%. The copolymer has a Mn of 17000-30000 and a low polydispersity (Mw/Mn =
1.1-1.2) determined by standard poly(St) calibration using GPC. The copolymer
attached to the silica nanoparticle surface is considered to have the similar
characteristics.
3.3. Photochromism of SNP-poly(St-co-DE)
Photochromic reaction behavior of SNP-poly(St-co-DE) was examined in THF and
in neat powder. Figure 4 shows color change in SNP60-poly(St-co-DE) with a diameter
of SNP in 60 nm upon alternating irradiation with UV and visible light. The THF
solution of the silica nanoparticle, which is well-dispersed in THF, turned blue by
irradiation with UV light. The color was thermally stable and was bleached by
irradiation with visible light. The powder of SNP-poly(St-co-DE) also shows blue color
upon irradiation with UV light. The color was bleached by irradiation with visible light.
The coloration and decoloration can be repeated several times. Figure 5 shows
absorption spectral changes of poly(St-co-DE) and SNP-poly(St-co-DE) in THF upon
alternating irradiation with UV and visible light. The open-ring form is colorless in THF.
Upon irradiation with 313 nm light, new absorption band appeared at 450-700 nm, and
the absorption band disappeared upon irradiation with visible light. The light scattering
of the silica nanoparticles was observed in the measurement as an increase in the base
line in Figure 5b. No change of the light scattering was observed during the
photochromic reactions. SNP-poly(St-co-DE) was found to exhibit photochromism as
well as poly(St-co-DE).
[Figure 4]
[Figure 5]
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Figure 6 shows the photocyclization and photocycloreversion reactivities of
SNP-poly(St-co-DE) and poly(St-co-DE) in THF upon irradiation with 313 nm light
and 600 nm light, respectively. There is no difference in the photocyclization and
photocycloreversion rates of DE for SNP-poly(St-co-DE) and poly(St-co-DE); thus, the
photoisomerization of DE takes place in high efficiency even in SNP-poly(St-co-DE). A
few retardation on photocyclization and photocycloreversion reaction rates of
SNP-poly(St-co-DE) is considered to be brought by light scattering of the silica
nanoparticles.
[Figure 6]
3.4. Synthesis and characterization of amphiphilic photochromic silica nanoparticles
Block copolymerization of NIPAm using SNP-poly(St-co-DE) was carried out in
DMF using AIBN as the initiator at 60 °C. Table 3 shows the polymerization conditions
and the results of the polymerization. The polymerization was successfully carried out
in a glass tube for 20 h or 40 h at 60 °C. Figure 7 shows TEM images of
SNP-poly(St-co-DE)-block-poly(NIPAm). The TEM images show that the particles
were well-dispersed in THF and spread on the copper grid without aggregation. The
polymer layer was also observed in the vicinity of the silica nanoparticles with the layer
thickness of 5-10 nm.
[Table 3]
[Figure 7]
The
degree
of
polymerization
11
for
poly(NIPAm)
segment
in

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1
poly(St-co-DE)-block-poly(NIPAm) can be determined from the H-NMR spectrum of
silica-free poly(NIPAm) obtained simultaneously by the polymerization. Figure 8 shows
1H-NMR spectrum of poly(NIPAm). The broad peak at 0.4-2.7 ppm is assigned to the
methylene and methine protons in the main chain of the polymer. The peaks at 1.14 and
3.8-4.4 ppm are assigned to methyl and methine protons of isopropyl group,
respectively. The broad peak at 5.5-7.7 ppm is due to the NH proton. The aromatic
protons of the dithiobenzoate group at the polymer end appeared at 7.39, 7.56, and 7.96
ppm. The degree of polymerization of poly(NIPAm) was determined base on the
comparison of the proton intensities of the aromatic protons at the polymer end and the
methine protons of the isopropyl group in the monomer unit. The results are shown in
Table 3.
[Figure 8]
3.5. Photochromism of SNP-poly(St-co-DE)-block-poly(NIPAm)
The resulting SNP-poly(St-co-DE)-block-poly(NIPAm) is soluble in THF and water.
The photochromic behavior of the nanoparticles was examined in THF and water upon
alternating irradiation with UV and visible light. Figure 9 shows absorption spectral
changes of the nanoparticles in THF and water. Upon irradiation with UV light, the
solution turned blue both in THF and water. The absorption maximum wavelength in
the visible region appeared at 622 and 613 nm in THF and water, respectively. The
maximum wavelength of silica-free poly(DE) and SNP-poly(St-co-DE) in THF is 592
nm. Therefore, the red shift of 30 nm is ascribed to the presence of the poly(NIPAm)
segment. Although the poly(NIPAm) segment is introduced to the polymer chain as a
block segment, it is considered to affect an absorption property of diarylethene in THF
and water. The pendant diarylethene chromophore in poly(St-co-DE) is considered to
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serve as surrounded by hydrophilic poly(NIPAm). Therefore, the shift is due to the local
structure around the diarylethene chromophore. The instability of diarylethene molecule
at the ground state brings about the red shift of the absorption spectrum of the
closed-ring form.
[Figure 9]
3.6. Dispersibility of SNP-poly(St-co-DE)-block-poly(NIPAm) in solution
The hydrophilicity of the silica nanoparticle covered with the diarylethene
chromophore can be expected to improve by introducing the poly(NIPAm) segment into
the outside of SNP-poly(St-co-DE) by block copolymerization. The dispersibility of
SNP60-poly(St-co-DE) and SNP60-poly(St-co-DE)-block-poly(NIPAm) in THF and
water was observed by the sedimentation behavior of the particles, as shown in Figure
10. In THF, both nanoparticles are well-dispersed. However, in water,
SNP60-poly(St-co-DE) was not dispersed because of the hydrophobic polymer chain. On
the other hand, SNP60-poly(St-co-DE)-block-poly(NIPAm) was well-distributed in
water as well as in THF.
[Figure 10]
The chain length dependence of poly(NIPAm) segment on dispersibility in water
was examined. Figure 11 shows the time dependence of the number of
SNP60-poly(St-co-DE)-block-poly(NIPAm)
(m
=
137)
and
SNP60-poly(St-co-DE)-block-poly(NIPAm) (m = 217). The polymer chain length of the
latter particles is 1.5 times longer than that of the former particle. Both of the
nanoparticles were dispersed highly in water and 88% of particles maintain a dispersion
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state even after 2 h. This means that the poly(NIPAm) segment is long enough to
disperse in water.
[Figure 11]
Poly(NIPAm) shows lower critical solution temperature (LCST) near room
temperature.
Therefore,
the
sedimentation
behavior
of
SNP60-poly(St-co-DE)-block-poly(NIPAm) was examined at 25 and 40 °C, as shown in
Figure 12. The sedimentation speed of the particles was estimated from the slope of the
linear relationship to be 1.1 × 1011 and 1.4 × 1011 particles L-1 min-1 at 25 and 40 °C,
respectively. The speed at 40 °C is 1.3 times faster than that at 25 °C. Although this is
due to the decrease in the hydrophilicity of the poly(NIPAm) segment at 40 °C, the large
difference of the sedimentation behavior was not observed. It is concluded that
SNP-poly(St-co-DE)-block-poly(NIPAm) is well-dispersed in water even at a
temperature above LCST.
[Figure 12]
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4. Conclusion
We succeeded in preparing a novel photochromic pigment that originates from silica
nanoparticles and photochromic polymers. The photochromic silica nanoparticles
showed photochromism in high efficiency in solution and neat powder. According to
introduction of the poly(NIPAm) segment by the block copolymerization, the silica
nanoparticles were well-dispersed in both THF and water. Such amphiphilic silica
nanoparticles covered with photochromic diarylethene chromophores can be useful for
applications such as organic/inorganic hybrid photochromic pigments.
Acknowledgments
This work was partly supported by a Grant-in-Aid for Scientific Research (C) (No.
24550161) from the Japan Society for the Promotion of Science (JSPS) and a
Grant-in-Aid for Scientific Research on Innovative Areas "Photosynergetics" (No.
26107013) from MEXT, Japan.
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[23] Ohno K, Morinaga T, Koh K, Tsujii Y, Fukuda T. Synthesis of monodisperse
silica particles coated with well-defined, high-density polymer brushes by
surface-initiated atom transfer radical polymerization. Macromolecules 2005; 38:
2137–42.
[24] Kuratani H, Kobatake S. Photochromism of diarylethene-functionalized
polystyrene with high conversion in a solid-state polymer film. Chem Lett 2006;
35: 628–9.
[25] Li C, Benicewicz BC. Synthesis of well-defined polymer brushes grafted onto
silica nanoparticles via surface reversible addition–fragmentation chain transfer
polymerization. Macromolecules 2005; 38: 5929–36.
[26] Farmer SC, Patten TE. (Thiocarbonyl-α-thio)carboxylic acid derivatives as
transfer
agents
in
reversible
addition–fragmentation
chain-transfer
polymerizations. J Polym Sci Part A: Polym Chem 2002; 40: 555–63.
[27] Stöber W, Fink A, Bohn E. Controlled growth of monodisperse silica spheres in
the micron size range. J Colloid Interf Sci 1968; 26: 62–9.
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Figure Caption
Scheme 1. Synthetic scheme of an amphiphilic photochromic silica nanoparticle.
Figure 1. TEM images (left) and histograms of diameter distribution (right) for silica
nanoparticles prepared by the Stöber method: [TEOS] = 0.11 mol L−1, [NH3] = 2.0 (a),
1.3 (b), 1.0 (c), 0.77 mol L−1 (d), [H2O] = 2.4×[NH3].
Figure 2. Relationship between the particle diameter of the silica nanoparticle and the
concentration of ammonia used for the reaction. An error bar expresses the standard
deviation of the particle diameter.
Figure 3. TEM images of SNP150-poly(St-co-DE) (d = 150 nm, Mn = 27500) (a) and
SNP60-poly(St-co-DE) (d = 60 nm, Mn = 29800) (b).
Figure 4. Photographs of SNP60-poly(St-co-DE) (Mn = 29800) before (left) and after
(right) irradiation with UV light in THF (a) and neat powder (b).
Figure 5. Absorption
spectral
changes
of
poly(St-co-DE)
(a)
and
SNP60-poly(St-co-DE) (Mn = 29800) (b) in THF: open-ring form ( - - - ) and the
photostationary state upon irradiation with 313 nm light ( ─── ).
Figure 6. Time-conversion plot of SNP60-poly(St-co-DE) (Mn = 29800) (●) and
poly(St-co-DE) (○) in photocyclization by irradiation with 313 nm light (a) and in
photocycloreversion by irradiation with 600 nm light (b).
19

ACCEPTED MANUSCRIPT
Figure 7. TEM images of SNP150-poly(St-co-DE)-block-poly(NIPAm) (m = 160) (a)
and SNP60-poly(St-co-DE)-block-poly(NIPAm) (m = 137) (b).
Figure 8. 1H NMR spectrum of silica-free poly(NIPAm) obtained by RAFT
polymerization of NIPAm with SNP150-poly(St-co-DE) in DMF for 20 h at 60 °C:
[NIPAm] = 3.5 mol L−1, [EEDB] = 16 mmol L−1, [SNP150-poly(St-co-DE) ] = 10 g L−1,
[AIBN] = 8.0 mmol L−1.
Figure 9. Absorption spectral change of SNP60-poly(St-co-DE)-block-poly(NIPAm)
(m = 137) in THF (a) and water (b) upon alternating irradiation with 313 nm light and
visible light: open-ring form (broken line) and the photostationary state upon irradiation
with 313 nm light (solid line).
Figure 10. Time-dependent change of the number of SNP60-poly(St-co-DE) (Mn =
17000) (a) and SNP60-poly(St-co-DE)-block-poly(NIPAm) (m = 137) (b) dispersed in
THF and water. The sedimentation behavior was observed in the photostationary state.
Figure 11. Time-dependent
SNP60-poly(St-co-DE)-block-poly(NIPAm)
SNP60-poly(St-co-DE)-block-poly(NIPAm) (m = 217) dispersed in water.
change
of
number
of
(a)
(b)
(m
=
137)
and
Figure 12. Time-dependent
change
of
the
number
of
SNP60-poly(St-co-DE)-block-poly(NIPAm) (m = 137) dispersed in water at 25 °C ( ● )
and 40 °C ( ○ ).
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ACCEPTED MANUSCRIPT
O
CH3
S
H3CO Si
CH3
O
CH3
S
O
CH3
O Si
CH3
RAFT-Si
dry THF
NH3, H2O
ethanol
S
Si(OEt)4
O
CH3
S
SNP
O
O
SNP-RAFT
O
F2
CH3
F2
F2
DE, styrene
RAFT copolymerization
S
S
S
O Si
CH3
O
H3C
n
CH3
CH3
=
S
SNP-poly(St-co-DE)
DE
O
CH3
O Si
CH3
NIPAm
RAFT block copolymerization
S
m
O
n
CH3
S
O
NH
SNP-poly(St-co-DE)-block-poly(NIPAm)
Scheme 1. Synthetic scheme of an amphiphilic photochromic silica nanoparticle.
21

ACCEPTED MANUSCRIPT
Figure 1. TEM images (left) and histograms of diameter distribution (right) for silica
nanoparticles prepared by the Stöber method: [TEOS] = 0.11 mol L−1, [NH3] = 2.0 (a),
1.3 (b), 1.0 (c), 0.77 mol L−1 (d), [H2O] = 2.4×[NH3].
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400
300
200
100
0
0.0
0.5
1.0
1.5
2.0
2.5
[NH3]/mol L−1
Figure 2. Relationship between the particle diameter of the silica nanoparticle and the
concentration of ammonia used for the reaction. An error bar expresses the standard
deviation of the particle diameter.
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ACCEPTED MANUSCRIPT
Figure 3. TEM images of SNP150-poly(St-co-DE) (d = 150 nm, Mn = 27500) (a) and
SNP60-poly(St-co-DE) (d = 60 nm, Mn = 29800) (b).
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Figure 4. Photographs of SNP60-poly(St-co-DE) (Mn = 29800) before (left) and after
(right) irradiation with UV light in THF (a) and neat powder (b).
25

ACCEPTED MANUSCRIPT
0.8
(a)
0.6
0.4
0.2
0.0
300
400
500
600
700
800
Wavelength/nm
0.8
0.6
0.4
0.2
0.0
(b)
300
400
500
600
700
800
Wavelength/nm
Figure 5. Absorption spectral changes of poly(St-co-DE) (a) and SNP60-poly(St-co-DE)
(Mn = 29800) (b) in THF: open-ring form ( - - - ) and the photostationary state upon
irradiation with 313 nm light ( ─── ).
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1.0
(a)
0.8
0.6
0.4
0.2
0.0
0
50 100 150 200 250 300 350
Time/s
1.0
0.8
0.6
0.4
0.2
0.0
(b)
0
2
4
6
8
10
Time/min
Figure 6. Time-conversion plot of SNP60-poly(St-co-DE) (Mn = 29800) (●) and
poly(St-co-DE) (○) in photocyclization by irradiation with 313 nm light (a) and in
photocycloreversion by irradiation with 600 nm light (b).
27

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Figure 7. TEM images of SNP150-poly(St-co-DE)-block-poly(NIPAm) (m = 160) (a)
and SNP60-poly(St-co-DE)-block-poly(NIPAm) (m = 137) (b).
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CHCl3
～
1.95
1.00
2.06
8.2
8.0
7.8
7.6
7.4
7.2
7.0
δ
/ppm
1425
X
160
163
8
7
6
5
4
3
2
1
0
δ
/ppm
Figure 8. 1H NMR spectrum of silica-free poly(NIPAm) obtained by RAFT
polymerization of NIPAm with SNP150-poly(St-co-DE) in DMF for 20 h at 60 °C:
[NIPAm] = 3.5 mol L−1, [EEDB] = 16 mmol L−1, [SNP150-poly(St-co-DE) ] = 10 g L−1,
[AIBN] = 8.0 mmol L−1.
29