Toward modulation of the naphthopyran photochromism: A miniemulsion copolymerization strategy

Article abstract of DOI:10.1039/c3nj01313h

Novel naphthopyran derivative based photochromic polymer nanoparticles are prepared via miniemulsion copolymerization with methyl methacrylate (MMA) and n-butyl methacrylate (BMA) monomers. The systematic modulation of the photochromic rate can be achieved via tuning the ratio of MMA and BMA monomers to lead the naphthopyrans to swing between rigid and soft surroundings. Transmission electron micrographs (TEM) and dynamic light scattering (DLS) images prove that the nanoparticles are discrete and smooth. The typical spectral characteristics of naphthopyran in the polymer nanoparticles indicate that the photochrome is incorporated into the nanoparticles successfully. In addition, the polymer nanoparticles exhibit good photochromic reversibility and stability upon UV irradiation and thermal back reaction. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c3nj01313h

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Toward modulation of the naphthopyran
photochromism: a miniemulsion
copolymerization strategy
Cite this: New J. Chem., 2014,
38, 2348
Dehua Li, Meiduo Zhang, Guang Wang* and Shuangxi Xing*
Novel naphthopyran derivative based photochromic polymer nanoparticles are prepared via miniemulsion
copolymerization with methyl methacrylate (MMA) and n-butyl methacrylate (BMA) monomers. The systematic
modulation of the photochromic rate can be achieved via tuning the ratio of MMA and BMA monomers to
lead the naphthopyrans to swing between rigid and soft surroundings. Transmission electron micrographs
(TEM) and dynamic light scattering (DLS) images prove that the nanoparticles are discrete and smooth.
The typical spectral characteristics of naphthopyran in the polymer nanoparticles indicate that the
photochrome is incorporated into the nanoparticles successfully. In addition, the polymer nanoparticles
exhibit good photochromic reversibility and stability upon UV irradiation and thermal back reaction.
Received (in Montpellier, France)
24th October 2013,
Accepted 26th February 2014
DOI: 10.1039/c3nj01313h
www.rsc.org/njc
the original colorless state and the molecules undergo an
important intramolecular rotation in the process.5,6
Introduction
Photochromic compounds, such as diarylethenes, spiropyrans,
Photochromic naphthopyrans are well known for their great
spirooxazines, azobenzene, and naphthopyrans, have attracted potential in various applications such as optical switches4,7,8
considerable attention1–3 because they display a reversible and ophthalmic lenses,9,10 where fading speed plays a crucial
molecular structure transformation, which results in a change role. Therefore, it is of great importance to tune the fading
from colorless to colored states upon ultraviolet (UV) light speed simply and efficiently to meet the requirements of the
irradiation. Among them, naphthopyrans are one type of applications. Modification of naphthopyran molecules with
important photochromic compounds with special properties, different substitution groups is an adaptable strategy because
such as the absence of the background color and good thermal it can change the intrinsic electronic nature or structure to
reversibility.4
further influence the fading speed.11–14
When exposed to UV light, the photochromic reaction of the
Incorporating photochromic dyes into a polymer followed by
naphthopyrans occurs via the heterolytic cleavage of the C–O changing the local host environment and the media in which
bond in the pyran moiety (Scheme 1). Two main isomeric open the dyes are incorporated is another useful and efficient
forms (merocyanines) are produced in the process, which strategy to tune the kinetics of the switching. For example,
present intense colors due to their extended conjugation and Evans et al. incorporated the photochromic dyes into the
quasi-planar structures. Ceasing UV irradiation leads the polymeric backbone that significantly affected the speed of
colored forms thermally or photochemically to return back to coloring and decoloring performance of the photochromic
compounds.5,6,15–19 Perrier and coworkers reported the controlled
switching of the naphthopyran derivative via a reversible addition
fragmentation chain transfer process to change the polymer
frameworks. In addition, they further used the strategy of block
copolymers to change the properties of the photochromic dye in
nanostructures.4,20 In our previous studies, we found that the
fading speed of naphthopyran could be dramatically reduced in
layer-by-layer assembled films via hydrogen bonding.21 However,
Scheme 1 Generic photochromic process for naphthopyran derivatives.
tuning the switching speed of naphthopyrans in nanoscale is less
reported.
Miniemulsion polymerization has been widely used to prepare
organic–inorganic hybrid and hollow polymeric nanoparticles.22–24
This process directly transforms the monomer droplets into
Faculty of Chemistry, Northeast Normal University, Chang Chun 130024, Jilin,
P. R. China. E-mail: wangg923@nenu.edu.cn; Fax: +86-431-85098768;
Tel: +86-431-85099544
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polymeric nanoparticles that display a similar composition and 8-Methacryloxyethoxy-[3-biphenyl-3-phenyl]-3H-naphtho-
morphology relative to the liquid droplets. In this case, most of [2,1-b]pyran (M1)
1H NMR (500 MHz, CDCl3, d): 7.88 (d, J = 9 Hz, 1H, Ar–H), 7.54–
7.51 (m, 8H, Ar–H + –O–C–CQCH–C–), 7.41–7.38 (m, 2H, Ar–H),
7.34–7.29 (m, 3H, Ar–H), 7.27–7.24 (m, 2H, Ar–H), 7.20 (d, J = 9 Hz,
1H, Ar–H), 7.17–7.15 (m, 1H, Ar–H), 7.05 (d, J = 2.5 Hz, 1H, Ar–H),
the primal materials in the droplets are preserved in the
nanoparticles, hence the monomer droplet can be seen as a
nanoreactor.25,26
Herein, we utilized the miniemulsion copolymerization to
prepare a series of nanoparticles containing naphthopyran
derivative, poly(methyl methacrylate) (PMMA) and poly(n-butyl
5.58–5.57 (m, 1H, CQCH2), 4.52 (t, J = 4.5 Hz, 2H, CH2), 4.27 (t, J =
methacrylate) (PBMA). Changing the surroundings of naphtho-
pyrans from rigid to soft by varying the relative amounts of
18.3 (CH3), 63.1 (CH2), 66.0 (CH2), 82.3 [–O–(CH)C(CAr)2], 108.0
PMMA and PBMA allows tuning the naphthopyran’s photo-
chromic behavior.
6.31 (d, J = 10 Hz, 1H, –O–C–CHQC–C–), 6.14 (s, 1H, CQCH2),
5 Hz, 2H, CH2), 1.94 (s, 3H, CH3). 13C NMR (125 MHz, CDCl3, d):
(CAr), 114.3 (CAr), 118.8 (CAr), 119.3 (CAr), 119.7 (–O–C–CHQCH–C–),
123.0 (CAr), 125.3 (CAr), 126.1 (CQCH2), 126.8 (CAr), 127.0 (CAr),
127.1 (CAr), 127.3 (CAr), 127.4 (CAr), 127.5 (CAr), 128.0 (CAr), 128.1
(CAr), 128.5 (–O–C–CHQCH–C–), 128.7 (CAr), 130.2 (CAr), 136.0
Experimental
Materials
(CQCH2), 140.3 (CAr), 140.7 (CAr), 143.9 (CAr), 144.8 (CAr), 149.1
(CAr), 155.0 (CAr), 167.4 (CQO). MS (ESI+): m/z calcd for C37H30O4:
538.21; found: 539.2 [M]+.
Sodium dodecyl sulfate (SDS, 99%, Aladdin) and n-hexadecane
(HD, 99%, Aldrich) were used as received. Potassium persulfate
(KPS, 99.99%, Aladdin) was recrystallized twice from water and
dried under vacuum. Methyl methacrylate (MMA, 99.5%, Aladdin)
8-Methacryloxyethoxy-[3,3-diphenyl]-3H-naphtho[2,1-b]pyran (M2)
1H NMR (500 MHz, CDCl3, d): 7.80 (d, J = 9.5 Hz, 1H, Ar–H), 7.52
(d, J = 9 Hz, 1H, –O–C–CQCH–C–), 7.47–7.45 (m, 4H, Ar–H),
and n-butyl methacrylate (BMA, 99%, Aladdin) were purified
7.30–7.27 (m, 4H, Ar–H), 7.23–7.20 (m, 3H, Ar–H), 7.16–7.13
upon distillation under reduced pressure and kept refrigerated
(m, 2H, Ar–H), 7.04 (d, J = 2.5 Hz, 1H, Ar–H), 6.20 (d, J = 10 Hz,
until use. Ultrapure water (resistivity 4 18 M cmꢀ1) was used
1H, –O–C–CHQC–C–), 6.07 (s, 1H, CQCH2), 5.51–5.50 (m, 1H,
for all experiments.
CQCH2), 4.46 (t, J = 5 Hz, 2H, CH2), 4.21 (t, J = 5.5 Hz, 2H, CH2),
1.87 (s, 3H, CH3). 13C NMR (125 MHz, CDCl3, d): 18.3 (CH3),
Synthesis of naphthopyran derivative monomers
63.1 (CH2), 66.0 (CH2), 82.3 [–O–(CH)C(CAr)2], 108.0 (CAr), 114.3
(CAr), 118.8 (CAr), 119.2 (CAr), 119.5 (–O–C–CHQCH–C–), 123.0
(CAr), 125.2 (CAr), 126.1 (CQCH2), 127.0 (CAr), 127.5 (CAr), 128.0
(CAr), 128.1 (CAr), 128.4 (–O–C–CHQCH–C–), 130.1 (CAr), 136.0
All naphthopyran derivatives were prepared according to the
method described in the literature,27 and the synthetic route
was presented in Scheme 2. The procedures employed for the
preparation of monomers (M1 as an example) are summarized
as follows: 6-(2-hydroxyethoxy)naphthalen-2-ol was fabricated
by 2,6-naphthalenediol that reacted with 2-bromoethanol at
(CQCH2), 144.8 (CAr), 149.1 (CAr), 155.0 (CAr), 167.3 (CQO). MS
(ESI+): m/z calcd for C31H26O4: 462.18; found: 462.1 [M]+.
85 1C, then 6-(2-hydroxyethoxy)naphthalen-2-ol (1 equiv.) was treated
with 1-([1,10-diphenyl]-4-yl)-1-phenylprop-2-yn-1-ol (1.2 equiv.) in
the presence of para-toluenesulfonate (10 mol%) as the catalyst
for 24 h at ambient temperature. Finally the product reacted with
methacryloyl chloride (1.2 equiv.) to provide the desired monomer.
8-Methacryloxyethoxy-[3-(4-ethoxy-phenyl)-3-phenyl]-3H-
naphtho[2,1-b]pyran (M3)
1H NMR (500 MHz, CDCl3, d): 7.87 (d, J = 9.5 Hz, 1H, Ar–H), 7.53
(d, J = 9 Hz, 1H, –O–C–CQCH–C), 7.48–7.46 (m, 2H, Ar–H),
7.37–7.35 (m, 2H, Ar–H), 7.32–7.29 (m, 2H, Ar–H), 7.25–7.22
(m, 1H, Ar–H), 7.17–7.14 (m, 2H, Ar–H), 7.05 (d, J = 2.5 Hz, 1H,
Ar–H), 6.82–6.80 (m, 2H, Ar–H), 6.24 (d, J = 9.5 Hz, 1H, –O–C–
CHQC–C–), 6.15 (s, 1H, CQCH2), 5.59–5.58 (m, 1H, CQCH2),
4.53 (t, J = 5 Hz, 2H, CH2), 4.28 (t, J = 5 Hz, 2H, CH2), 4.00 (q, J = 7
Hz, 2H, –OCH2CH3), 1.95 (s, 3H, CH, CH3), 1.37 (t, J = 7 Hz, 3H,
–OCH2CH3). 13C NMR (125 MHz, CDCl3, d): 14.8 (CH2CH3), 18.3
(CH3), 63.1 (CH2), 63.3 (CH2), 66.0 (CH2), 82.2 [–O–(CH)C(CAr)2],
108.0 (CAr), 113.9 (CAr), 114.2 (CAr), 118.8 (CAr), 119.2 (CAr), 119.3
(–O–C–CHQCH–C–), 123.0 (CAr), 125.2 (CAr), 126.1 (CQCH2),
126.9 (CAr), 127.3 (CAr), 128.0 (CAr), 128.3 (–O–C–CHQCH–C–),
128.3 (CAr), 128.4 (CAr), 130.1 (CAr), 136.0 (CQCH2), 136.7 (CAr),
145.1 (CAr), 149.1 (CAr), 154.9 (CAr), 158.3 (CAr), 167.3 (CQO). MS
(ESI+): m/z calcd for C33H30O5: 506.21; found: 507.2 [M]+.
Synthesis of 1,4-phenylene bis(2-methylacrylate) (PBM)
Hydroquinone (0.55 g, 5 mmol) and triethylamine (1.53 mL, 11 mmol)
were dissolved in dry CH2Cl2 and stirred in an ice–water bath.
Scheme 2 Synthetic route of naphthopyran monomers.
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A solution of methacryloyl chloride (1.06 mL, 11 mmol) instrument with a heating rate of 10 1C minꢀ1 under an argon
in dry CH2Cl2 was added dropwise into the above mixture, atmosphere. Mass spectra were recorded on a Q-Trap mass
and the mixture was stirred for 10 h at room temperature. The spectrometer (Applied Biosystems/MDS Sciex, Concord, ON,
precipitated amine hydrochloride was filtered off; the filtrate Canada) equipped with an electro-spray ionization (ESI) source.
was washed with water (3 ꢁ 100 mL) and then dried with Dynamic light scattering measurements (DLS) were performed
anhydrous sodium sulfate. The purified product was obtained on the samples using a Zetasizer Nano ZS. Transmission
by column chromatography on silica gel (petroleum ether : ethyl electron micrographs (TEM) were obtained using a JEOL-2011F
acetate = 30 : 1) as a white solid, 1.11 g, 90% yield. 1H NMR microscope operating at an acceleration voltage of 200 kV. All of
(500 MHz, CDCl3, d, ppm): 7.14 (s, 4H, Ar), 6.34 (s, 2H, CQCH2), the experiments were carried out at room temperature.
5.75 (s, 2H, CQCH2), 2.06 (s, 6H, CH3).
Results and discussion
Preparation of polymer nanoparticles
The three naphthopyran monomers, denoted as M1, M2, and M3 Formation of nanoparticle dispersions
(Scheme 2), were used to prepare photochromic nanoparticles
via miniemulsion polymerization, denoted as NP-M1, NP-M2,
monomers according to the literature.27 The photochromic
and NP-M3 dispersions hereafter. Unless otherwise stated, the
preparation process is as follows.
To synthesize the photochromic tunable polymer nano-
particles, we firstly prepared the photochromic naphthopyran
nanoparticles were obtained via free radical polymerization in
In a typical experiment, the organic phase was prepared by
typical miniemulsion. The monomer droplets formed by ultra-
adding naphthopyran derivatives, MMA, BMA, PBM and HD
sonication include MMA, BMA and dyes as copolymerization
and stirring for 30 min to form a homogeneous solution (as
monomers, PBM as the cross linker and hexadecane as the
shown in Scheme 3). The organic phase was added to an
co-stabilizer. Then KPS as the initiator was added to the
aqueous emulsifier (SDS) solution and stirred for 30 min
monomer mixture to start the polymerization. After a period
(1000 rpm), and then the mixture was ultrasonicated in an
of time, the nanoparticle dispersion was produced. The inner
ice-bath for 15 min to obtain a miniemulsion. The miniemulsion
rigidity of nanoparticles was modulated by changing the ratio of
was transferred to a 50 mL flask equipped with a condenser and
MMA and BMA, for the objective of modulating the photochromic
was purged with nitrogen for 30 min before immersed in an oil
rate of naphthopyran in the nanoparticles. All amounts of the
bath with a thermostat. An aqueous solution of KPS was added
chemical components are presented in Table 1, the final
to the mixture to initiate the polymerization and the process was
molecular weight and polydispersities (PDI) of the copolymers
are also included. Polymerization in miniemulsion displays a
narrow distribution of PDI, as seen in Table 1.
maintained at 60 1C for 6 h. Finally, the as-prepared nanoparticle
dispersions were obtained after filtering. 40 ml of dispersion was
diluted with 4 mL of water for absorbance studies.
As seen in Fig. 1A, the TEM image of a sample (NP-M1d)
displays that most of the polymer nanoparticles are smooth
and spherical with a mean size diameter of 70 nm. The
Characterization
1H NMR spectra were recorded on a Varian Unity Inova Spectro-
diameter distribution of the particle sample was determined
meter (500 MHz) at room temperature using d-chloroform as
by DLS. The nanoparticles show a narrow size distribution with
solvent. Gel permeation chromatography (GPC) was performed
an average diameter of 80 nm (Fig. 1B). This is coincident with
using a Waters-410 system with THF as the eluent and standard
the results of TEM, considering both the degree of hydration
poly-styrene samples for calibration. A CHF-XM35 parallel light
system with a 500 W xenon lamp was used as the UV source of
irradiation. Thermal analysis by differential scanning calori-
Table 1 Polymerization conditions and characteristics of the nanoparticle
samples
metry (DSC) was carried out using a NETZSCH DSC 200F3
b
d
d
WMMA : WBMA
(g)
% Solid
contentc
Dm
Mn
Mw
Samplea
(nm)
(kDa)
(kDa)
PDId
NP-M1a
NP-M1b
NP-M1c
NP-M1d
NP-M2a
NP-M2b
NP-M2c
NP-M2d
NP-M3a
NP-M3b
NP-M3c
NP-M3d
1.2 : 0
0.6 : 0.6
0.24 : 0.96
0 : 1.2
25.6
24.5
22.1
23.1
24.3
25.4
19.8
20.1
27.8
18.5
31
63.2
1977
2052
1731
2052
1782
1999
1973
2040
1908
1726
2014
1883
2316
2376
2062
2376
2109
2354
2454
2479
2182
2221
2333
2210
1.17
1.15
1.19
1.15
1.18
1.17
1.24
1.21
1.14
1.28
1.15
1.17
82.66
69.93
71.16
61.06
72.3
65.63
82.74
71.95
70.20
86.82
80.06
1.2 : 0
0.6 : 0.6
0.24 : 0.96
0 : 1.2
1.2 : 0
0.6 : 0.6
0.24 : 0.96
0 : 1.2
19.3
a
The DYE/PBM/HD/SDS/KPS feed is 0.012/0.012/0.072/0.06/0.024 g,
respectively. b Hydrodynamic diameter estimated by DLS. Solid content
c
was determined gravimetrically. d Determined by GPC analysis of polymer
nanoparticles using polystyrene calibration and THF as the eluent.
Scheme 3 Preparation of polymeric nanoparticles via miniemulsion
polymerization.
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Table 2 Photokinetic analysis of the bleaching of naphthopyran in polymer
nanoparticlesa
b
d
lmax t1/2 k1
Sample (nm) (s)
(sꢀ1
k2
Tg
(1C)
)
(sꢀ1
)
A1
A2
Ath
NP-M1a 476 279 0.0283 0.0014 0.0150 0.0151 0.0123 112.8
NP-M1b 477
NP-M1c 476
NP-M1d 477
75 0.0383 0.0029 0.0271 0.0228 0.0120 57.3
27 0.0742 0.0136 0.0521 0.0582 0.0177 44.8
21 0.1026 0.0234 0.0221 0.0806 0.0163
12 0.5551 0.0316 0.1876 0.2349 0.0076
M1c
475
Fig. 1 (A) TEM image and (B) the corresponding size distribution of
photochromic nanoparticle NP-M1d determined by DLS in an aqueous
suspension at room temperature.
NP-M2a 469 136 0.0208 0.0012 0.0104 0.0107 0.0141 114.2
NP-M2b 468
NP-M2c 469
NP-M2d 468
64 0.0322 0.0025 0.0288 0.0221 0.0123 69.2
31 0.0507 0.0030 0.0499 0.0294 0.0184 42.0
20 0.1549 0.0030 0.0247 0.0923 0.0258 36.5
11 0.8448 0.0453 0.0977 0.3189 0.0020
M2c
467
NP-M3a 485 128 0.0256 0.0024 0.0091 0.0092 0.0097 114.0
and polydispersity effects.28 All the above results indicate that
miniemulsion polymerization is an adaptable method to produce
uniform polymer nanoparticles.
NP-M3b 482
NP-M3c 489
NP-M3d 488
56 0.0430 0.0031 0.0159 0.0139 0.0059 49.3
19 0.6351 0.0143 0.0263 0.0356 0.0050 47.3
10 0.8335 0.0610 0.0175 0.0219 0.0033 25.7
M3c
484
5
2.1621 0.1179 0.0718 0.2318 0.0042
a
Decoloration monitored at lmax of the colored form at ambient tempera-
Photochromic properties
b
ture (23 1C) in the dark. Time taken for decreasing to half of its original
value. M(1–3) used as reference compounds (5 ꢁ10ꢀ5 M, in CH2Cl2).
c
All the thermal decoloration of the nanoparticle dispersions was
investigated in the dark at room temperature after continuous UV
irradiation. As depicted in Scheme 1, exposure of the naphthopyran
closed form (FC) to continuous UV light irradiation results in
two major colored merocyanine forms, transoid-cis (TC) and
transoid-trans (TT). The TC isomer thermally returns back to
FC quickly, while the TT isomer reverts to FC slowly through a
two-step process, TT–TC–FC.29,30
d
Thermal analyses of polymer nanoparticles using a NETZSCH DSC
200F3 instrument.
is often used to represent and compare the decoloration
kinetics of the naphthopyrans within solid media: A(t) =
A1eꢀk t + A2eꢀk t + Ath, where A(t) is the optical density at the
1
1
lmax, A1 and A2 are contributions to the initial optical density
Fig. 2 shows the absorption profile of NP-M1d before and
after UV illumination. The apparent absorbance band between
400 and 600 nm upon UV irradiation indicates that the photo-
chromic naphthopyran derivatives are successfully incorporated
into the polymer nanoparticles. It is noteworthy that the absorp-
tion curves present a gradual downward trend from short to long
wavelength, which is due to the light scattering effect of particles
in water.31 The bleaching process of the nanoparticles was
investigated by recording absorbance intensities at the lmax of
the open form of naphthopyran with time.
A0, k1 and k2 are the rates of the fast and slow components,
respectively, and Ath is the residual coloration.17,21,29,32,33
The values of these constants for each series of nanoparticle
conjugates are given in Table 2. The value of t1/2 is another
important parameter, which is defined as the time taken for the
optical density reducing to half of its original optical density of
the colored form. The smaller numerical values of t1/2, the
faster fading speeds. Because there is the light scattering effect
on the particles in water, the t1/2 is calculated as the time
decreased to the half of the difference between the maximum
absorbance intensity after UV irradiation and the original
absorbance intensity before UV irradiation at the lmax of the
open form of naphthopyran.
The photochromic decoloration rates have also been inves-
tigated by a mathematical modeling. A biexponential equation
All naphthopyran derivatives give slow kinetics in the
polymer conjugates compared to their behavior in solution in
Table 2. In addition, photochromic dyes incorporated into the
PMMA matrices display larger t1/2 values and lower thermal
bleaching constant k1 relative to the PBMA matrices. As
expected, the weaker the rigidity of the local environment of
naphthopyran molecules, the greater free volume and freedom
to move for the photochromic compounds.34 Each set of t1/2
fading speeds in Table 2 shows an accelerated trend with an
increase in the ratio of the PBMA component, as the environ-
ment of naphthopyrans is changed from rigidity to softness.
Furthermore, a general reduction in Tg is found with an
increase in the ratio of the PBMA component; this indicates
that the micro-environment of naphthopyrans changes to a softer
one with an increase in the ratio of the PBMA component. These
results are depicted diagrammatically in Fig. 3 (NP-M(1a–1d) as
examples), which shows the time dependence of the fading rate of
Fig. 2 Absorption spectra of NP-M1d before and after UV irradiation.
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Fig. 3 Normalized absorbance versus time for the decoloration of nano-
Fig. 5 The t1/2 (seconds) fading speed of photochromic conjugates versus
particle samples NP-M(1a–1d).
the mass ratio of MMA and BMA, and including reference compounds.
decreasing normalized absorbance. The decoloration rate
increases significantly as the ratio of the PBMA component is
increased. The fading speed as measured by t1/2 ranges from 22
to 279 s. The addition of the PBMA component to the copolymer
NP-M1 conjugates gave a ca. 360% increase in the fast component
k1 of the decoloration of NP-M1d relative to NP-M1a (1a 0.02825, 1b
0.03829, 1c 0.07415, 1d 0.1026). A smaller increase in k2 is also
presented among the set of copolymer NP-M(1a–1d) conjugates.
The above result suggests that the method of copolymerization
in miniemulsion can effectively tune the fading speed of
naphthopyran over a wide range, and the monomer droplets
as nanoreactors in the process retain most of the raw materials
directly in the polymer particles.
in naphthopyran conjugates increases. Similar results are obtained
with copolymer NP-M(3a–3d) conjugates. These results demonstrate
that the fading rate of naphthopyran can be modulated by adjusting
the amount of BMA in nanoparticles.
Fig. 5 shows the relationship between bleaching speed and
the ratios of MMA and BMA components. As the amount of
BMA component increases, all curves display a downward
trend, while the method approaches the limit to change the
fading speed as in the complete PBMA conjugates that
approach the fading rate of naphthopyran in solution. A
comparison of the t1/2 of NP-M1a with respect to its corre-
sponding compounds (NP-M2a and NP-M3a) displays that
NP-M1a gives the lowest decay rate value, because the
naphthopyran substituted by bulky phenyl more likely experi-
enced the steric hindrance during rotating mobility in the
rigid host. In contrast, NP-M3a exhibits the fastest decolora-
tion rate due to the p-ethoxy moiety substitution on the aryl
portion that positively promotes the decoloration.6
The fading curves of the copolymer NP-M(2a–2d) in Fig. 4
also show a growing trend of the fading speed with a narrow
range from 20 to 136 s. The values of the fast component k1 of
the decoloration present an upward trend (2a 0.02083, 2b
0.03223, 2c 0.05072, 2d 0.1549) as the added BMA component
On the whole, the set of copolymer NP-M(3a–3d) conjugates
presents the fastest fading speed and the group of copolymer
NP-M(1a–1d) conjugates shows the slowest bleaching speed.
Apart from electronic substitution, the steric effect and the
rigidity imposed by the local environment dominate to influence
the photochromic performance of naphthopyran in polymeric
structures, the photochromic behavior of each set of polymer
nanoparticles can be successfully tuned based on different ratios
of MMA and BMA to change their micro-environments from
rigidity to softness.
The reversibility coloration and decoloration of polymer
nanoparticle dispersion at 477 nm (NP-M1d as an example)
was investigated, as shown in Fig. 6. After 5 cycles of UV
irradiation for coloration and thermal back reaction in the
dark for decoloration, the value of absorbance intensity appears
to decrease only by 4.4% based on that of the first irradiation owing
to a slight degradation. This indicates that the naphthopyran based
polymer nanoparticles have good resistance to photo-chemical
fatigue.
Fig. 4 Normalized absorbance versus time during thermal decoloration
of nanoparticle samples NP-M(2a–2d).
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NJC
6 F. Ercole, N. Malic, T. P. Davis and R. A. Evans, J. Mater.
Chem., 2009, 19, 5612.
7 S. Delbaere, G. Vermeersch, M. Frigoli and G. H. Mehl,
Org. Lett., 2006, 8, 4931.
8 S. P. Delbaere, G. Vermeersch, M. Frigoli and G. H. Mehl,
Org. Lett., 2010, 12, 4090.
9 W. Zhao and E. M. Carreira, J. Am. Chem. Soc., 2002,
124, 1582.
10 M.-J. R. P. Queiroz, A. S. Abreu, P. M. T. Ferreira, M. M.
Oliveira, R. Dubest, J. Aubard and A. Samat, Org. Lett., 2005,
7, 4811.
11 W. Zhao and E. M. Carreira, Chem.–Eur. J., 2007, 13, 2671.
12 R. M. Christie, J. D. Hepworth, C. D. Gabbutt and S. Rae,
Dyes Pigm., 1997, 35, 339.
13 S. Han and Y. Chen, J. Mater. Chem., 2011, 21, 4961.
14 K. Guo and Y. Chen, J. Mater. Chem., 2010, 20, 4193.
15 F. Ercole, N. Malic, T. P. Davis and R. A. Evans, J. Mater.
Chem., 2009, 19, 5612.
Fig. 6 Photochromic cycles of NP-M1d, monitored at 477 nm, before and
after sequential steps of irradiation of UV light and put in the dark.
16 N. Malic, J. A. Campbell and R. A. Evans, Macromolecules,
2008, 41, 1206.
Conclusions
In conclusion, a series of polymeric nanoparticles were fabricated
via a one-step miniemulsion polymerization based on three
classes of naphthopyran derivatives with different ratios of
MMA and BMA monomers. All nanoparticle dispersions displayed
a normal photochromic phenomenon, and the fading speed of
each set of naphthopyran nanoparticles improved as the ratio of
the PBMA component increased, because this could lead the
photochrome to change from a rigid to a relatively soft micro-
environment. Moreover, the nanoparticle dispersion exhibited
fairly good reversibility between the FC form and merocyanine
isomers triggered by UV irradiation and thermal back reaction.
This research work provides a new approach to modulate the
photochromic rate and may widen the potential application of
naphthopyran in optical switches.
17 R. A. Evans, T. L. Hanley, M. A. Skidmore, T. P. Davis,
G. K. Such, L. H. Yee, G. E. Ball and D. A. Lewis, Nat. Mater.,
2005, 4, 249.
18 N. Malic, J. A. Campbell, A. S. Ali, M. York, A. D’Souza and
R. A. Evans, Macromolecules, 2010, 43, 8488.
19 N. Malic, I. J. Dagley and R. A. Evans, Dyes Pigm., 2013,
97, 162.
20 W. Sriprom, C. Neto and S. Perrier, Soft Matter, 2010, 6, 909.
21 M.-D. Zhang, Y.-F. Zhang, D.-H. Li, G. Wang and J.-X. Li,
New J. Chem., 2013, 37, 1385.
22 K. Landfester, Angew. Chem., Int. Ed., 2009, 48, 4488.
23 K. Y. van Berkel and C. J. Hawker, J. Polym. Sci., Part A:
Polym. Chem., 2010, 48, 1594.
24 D. Crespy and K. Landfester, Polymer, 2009, 50, 1616.
25 K. Landfester, Macromol. Rapid Commun., 2001, 22, 896.
26 K. Landfester, U. Pawelzik and M. Antonietti, Polymer, 2005,
46, 9892.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (no. 50873019) and the Science and
Technology Department of Jilin Province (no. 201205004) and
Research Fund for the Doctoral Program of Higher Education
of China (20120043110007).
27 X. Zhao, G. Wang and K. Zhang, J. Appl. Polym. Sci., 2013,
127, 1794.
28 Q. Jin, G. Liu and J. Ji, J. Polym. Sci., Part A: Polym. Chem.,
2010, 48, 2855.
29 S. Delbaere, B. Luccioni-Houze, C. Bochu, Y. Teral,
M. Campredon and G. Vermeersch, J. Chem. Soc., Perkin
Trans. 2, 1998, 1153.
30 S. Delbaere and G. Vermeersch, J. Photochem. Photobiol., A,
2003, 159, 227.
31 J. Chen, F. Zeng, S. Wu, Q. Chen and Z. Tong, Chem.–Eur. J.,
2008, 14, 4851.
32 X. Zhao, G. Wang and K. Zhang, J. Appl. Polym. Sci., 2012,
124, 4157.
Notes and references
1 Photochromism, ed. G. H. Brown, Wiley, New York, 1971.
2 Organic Photochromic and Thermochromic Compounds,
ed. J. C. G. Crano and R. J. Guglielmetti, Plenum Press,
New York, 1999, vol. 1 and 2.
3 F. Ercole, T. P. Davis and R. A. Evans, Polym. Chem., 2010, 1, 37.
4 W. Sriprom, M. Neel, C. D. Gabbutt, B. M. Heron and 33 J. Biteau, F. Chaput and J.-P. Boilot, J. Phys. Chem., 1996,
S. Perrier, J. Mater. Chem., 2007, 17, 1885.
100, 9024.
5 F. Ercole, N. Malic, S. Harrisson, T. P. Davis and R. A. Evans, 34 N. Malic and R. A. Evans, J. Polym. Sci., Part A: Polym. Chem.,
Macromolecules, 2010, 43, 249. 2012, 50, 1434.
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