Photoregulated optical switching of poly(N-isopropylacrylamide) hydrogel in aqueous solution with covalently attached spironaphthoxazine and D-π-A type pyran-based fluorescent dye

Article abstract of DOI:10.1016/j.dyepig.2010.03.014

A thermoresponsive poly(NIPAM-co-SPO-co-fluorophore) hydrogel comprising poly(N-isopropylacrylamide) that was partially modified using an electron donor-π-conjugated-electron acceptor (D-π-A) pyran-based fluorescent dye using radical copolymerization as well as a photochromic spironaphthoxazine unit displayed a lower critical solution temperature transition from 28 to 50?°C in aqueous solution. The poly(NIPAM-co-SPO-co-fluorophore) copolymer exhibited excellent photochromic behavior in both aqueous solution and hydrogel state. The fluorescence intensity of the poly(NIPAM-co-SPO-co-fluorophore) was temperature-dependent. The relationship between fluorescence intensity change and size of the polymer aggregates was determined using dynamic light scattering. Reversible modulation of fluorescence intensity was achieved using alternating irradiation with UV and visible light. The internal microstructure of the poly(NIPAM-co-SPO-co-fluorophore) hydrogel was investigated using SEM.

Full text of DOI:10.1016/j.dyepig.2010.03.014

Dyes and Pigments 87 (2010) 158e163
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Photoregulated optical switching of poly(N-isopropylacrylamide) hydrogel
in aqueous solution with covalently attached spironaphthoxazine
and D-p-A type pyran-based fluorescent dye
,
b
a
c
Sung-Hoon Kim a , In-Jeong Hwang , Seon-Yeong Gwon , Young-A Son
*
a Department of Textile System Engineering, Kyungpook National University, Daegu, 702-701, Republic of Korea
b Department of Advanced Organic Materials Science and Engineering, Kyungpook National University, Daegu, 702-701, Republic of Korea
c Department of Advanced Organic Materials and Textile System Engineering, Chungnam National University, Daejeon 305-764, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
A
thermoresponsive
propylacrylamide) that was partially modified using an electron donor-
(D- -A) pyran-based fluorescent dye using radical copolymerization as well as a photochromic spi-
poly(NIPAM-co-SPO-co-fluorophore)
hydrogel
comprising
poly(N-iso-
Received 25 January 2010
Received in revised form
15 March 2010
Accepted 16 March 2010
Available online 21 March 2010
p
-conjugated-electron acceptor
p
ronaphthoxazine unit displayed a lower critical solution temperature transition from 28 to 50 ꢀC in
aqueous solution. The poly(NIPAM-co-SPO-co-fluorophore) copolymer exhibited excellent photochromic
behavior in both aqueous solution and hydrogel state. The fluorescence intensity of the poly(NIPAM-co-
SPO-co-fluorophore) was temperature-dependent. The relationship between fluorescence intensity
change and size of the polymer aggregates was determined using dynamic light scattering. Reversible
modulation of fluorescence intensity was achieved using alternating irradiation with UV and visible light.
The internal microstructure of the poly(NIPAM-co-SPO-co-fluorophore) hydrogel was investigated using
SEM.
Keywords:
Photochromicswitching
Thermoresponsive
Poly(N-isopropylacrylamide)
Low critical solution temperature (LCST)
Hydrodynamic radius
Spironaphthoxazine
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
a release of water bound to the polymer chain. In previous papers
[9,10], the present research groups has reported bistable photo-
Of various types of photochromic compound, spironaphthox-
azines (SPO) [1e3], have attracted much interest from the viewpoint
of fundamental elucidation of photochemical reactions and potential
application as optical memory. Photochromism in spironaphthox-
azine compounds generally involves the UV-induced dissociation of
the spiro CeO bond, from the oxazine ring to form a planar structure.
Several poly(N-isopropylacrylamide) (PNIPAM) copolymers con-
taining photochromic units such as azobenzene or spiropyran
derivatives have been reported [4e6], in which the PNIPAM copoly-
mers were controlled by temperature, light or chemical stimuli. AIn
the context of thermoresponsive polymers, PNIPAM and its copoly-
mers exhibit a lower critical solution temperature (LCST) in aqueous
solution by means of which, their hydration-dehydration states
change reversibly and rapidly in response to a small temperature
change [7,8]. This endothermic phase transition is accompanied by
switching in poly(N-isopropylacrylamide) with photochromic
spironaphthoxazine hydrogel for optical data storage applications.
D-p-Apyran-based, fluorescent dyes such as 4-(dicyanomethylene)-
2-methyl-6-(4-(dimethylamino)styryl)-4H-pyran (DCM) have been
developed as both photo- and electro-luminescent materials for
applications in dye lasers [11], sensors [12] and dye-sensitized solar
cells [13]. Their fluorescence properties are highly sensitive to
changes in external environment such as polarity and pH, owing to
their intrinsic intramolecular charge transfer character [14,15]. In
previous research [16], we reported the synthesis, photophysical
properties and unusual, aggregation-induced fluorescence changes
within copolymers comprising a novel pyran-based fluorophore. This
paper concerns the synthesis and characterization of a poly(N-iso-
propylacrylamide) hydrogel consisting of N-isopropylacrylamide,
spironaphthoxazine and D-p-A type pyran-based fluorescent dye
units as the thermosensitive, photochromic and fluorophore
components, respectively. Importantly, modulated fluorescence sig-
nalling was achieved by photochromic switching the spironaph-
thoxazine component of the polymer.
* Corresponding author. Tel.: þ82 53 950 5641; fax: þ82 53 950 6617.
E-mail address: shokim@knu.ac.kr (S.-H. Kim).
0143-7208/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2010.03.014

S.-H. Kim et al. / Dyes and Pigments 87 (2010) 158e163
159
2. Experimental
Melting points were determined using Electrothermal IA 900
apparatus and were uncorrected. Mass spectra were recorded on
a Shimadzu QP-1000 spectrometer using an electron energy of
70 eV employing a direct probe EI method. 1H NMR spectra was
recorded in CDCl3 using a Varian Inova 400 MHz FT NMR Spec-
trometer using TMS as internal standard. The Mn and polydispersity
(Mr/Mn) of the polymer was measured using a PL-GPC model 210
chromatograph at 25 ꢀC using THF as eluent and standard poly-
styrene as reference. Fluorescence spectra were measured on
a Shimadzu RF-5301PC fluorescence spectrophotometer; UVevis
spectra and transmittance were obtained on a Agilent 8457 UVevis
spectrophotometer. LCST values were determined by spectropho-
tometric detection of the changes in transmittance (lmax ¼ 465 nm)
of aqueous polymer solutions heated at a constant rate (0.2 ꢀC
minꢁ1) using a thermostatically controlled cuvette. Dynamic light
scattering (DLS) measurements were carried out on a Wyatt
Technology Dynapro Titan instrument using the 828.6 nm line of
a laser as excitation source. A high-pressure mercury lamop (Ushio,
SP3-250D) was used as UV radiation source and was calibrated with
a monochromator at 366 nm. Scanning electron microscopy (SEM)
images of the hydrogel were obtained using a Hitachi S-530 scan-
ning electron microscope.
Fig. 1. UVevis absorption spectral changes of poly(NIPAM-co-SPO-co-fluorophore) in
water (0.2 g/mL) upon irradiation with UV light. The inset figure shows the decolou-
ration at 605 nm.
methacryloyl chloride according to the method described previ-
ously [17,18] (Scheme 1).
2.1. Materials
2.2. Synthesis of the flurophore monomer 5
2,6-Dimethylpyran-4-pyrone, allyl cyanoacetate and 4-(diphe-
nylamino)benzaldehyde, Fisher’s base, 2,7-dihydroxy naphthalene
and methacryloyl chloride were purchased from Aldrich. N-Iso-
propylacrylamide (Aldrich) was purified by recrystallization from
a mixture of toluene/hexane (1/4) and dried under vacuum. 2,20-
Azobis(isobutyronitrile) (AIBN) was recrystallized from methanol.
All other chemicals were of the highest grade available and were
used without further purification. Solvents were analytically pure
and were employed without any further drying or purification.
The spironaphthoxazine 6 was prepared from 90-hydroxy-1,3,3-
trimethylspiro[indoline-2,3-[3H]naphtha[2,2-b][1,4-oxazine and
2.2.1. Allyl-cyano-2-(2,6-dimethyl-4H-pyran-4-ylidene)acetate 3
A mixture of 2,6-dimethylpyran-4-pyrone (3 g, 24 mmol) 1, allyl
cyanoacetate 2, (3 g, 24 mmol) and acetic acid (12.5 mL) was
refluxed for 24 h. The crude solid obtained was filtered, washed
with methanol and recrystallized from methanol several times.
Yield: 32%; mass (m/z) 231 (Mþ). 1H NMR (400 MHz, CDCl3):
d 2.27
(s, 6H), 4.65 (dt, Jab ¼ 1.6 Hz, Jac ¼ 1.6 Hz, Jad ¼ 5.2 Hz, 2H), 5.20 (dd,
Jac ¼ 1.6 Hz, Jbc ¼ 1.6 Hz, Jbd ¼ 10.4 Hz, 1H), 5.36 (dd, Jac ¼ 1.6 Hz,
Jbc ¼ 1.6 Hz, Jcd ¼ 17.2 Hz, 1H), 5.94e6.03 (m, 1H), 6.59 (s, 1H), 7.87
(s, 1H). Anal. calcd for C13H13NO3: C, 67.52; H, 5.67; N, 6.06. Found:
C, 67.17; H, 5.72; N, 6.03%.
(c)
(a)
H
O
H2
N
C
CN
(c)
(a)
H
O
(b)
O
H
H
O
H2
O
(d)
CN
C
O
CN
O
H
O
CHO
(b)
H
2
(d)
4
O
O
N
N
1
3
5
O
CN
O
CH3
C
H2
H2
C
H2
C
H
C
C
O
z
CN
x
y
O
O
HN
O
O
O
O
HN
O
N
N
O
O
N
N
5
7
N
O
N
O
N
N
6
Poly(NIPAM-co-SPO-co-fluorophore)
p-A charge transfer dye, 5, and poly(NIPAM-co-SPO-co-fluorophore).
Scheme 1. Synthesis of a D-

160
S.-H. Kim et al. / Dyes and Pigments 87 (2010) 158e163
0.7
0.6
0.5
0.4
0.3
0.2
0.1
100
Vis
Vis
Vis
Vis
80
60
40
20
0
UV
0
UV
UV
UV
20
30
40
50
60
20
40
60
80
100
120
Temperature(°C)
Time(Sec)
Fig. 4. Transmittance of a solution of poly(NIPAM-co-SPO-co-fluorophore) in water
(0.2 g/mL).C, heating; B, cooling.
Fig. 2. Cyclic absorbance changea at 605 nm of poly(NIPAM-co-SPO-co-fluorophore) in
water (0.2 g/mL) following UV and visible light irrafiation.
2.2.2. Allyl-2-(2,6-bis((E)-4-(diphenylamino)styryl)-4H-
pyran-4-ylidene)-2-cyanoacetate 5
600
A
solution of allyl-2-cyano-2-(2,6-dimethyl-4H-pyran-4-yli-
dene)acetate 3 (1 g, 4.3 mmol), 4-(diphenylamino)benzaldehyde 4
(1.4 g, 5.2 mmol), and piperidine (0.3 mL) in 1-propane (50 mL) was
refluxed for 24 h using a DeaneStark trap. After cooling the reaction
mixture, the resulting precipitate was filtered, washed well with 1-
propanol and dried. The reaction mixture was evaporated to
remove solvent and separated by column chromatography þusing
chloroform. Yield: 30%; mp 229e231 ꢀC; mass (m/z) 741 (M ). 1H
T /°C
45
400
20
60
Temperature(°C)
NMR (400 MHz, CDCl3):
d
4.69 (dt, Jab ¼ 1.6 Hz, Jac ¼ 1.6 Hz,
200
Jad ¼ 5.6 Hz, 2H), 5.23 (dd, Jac ¼ 1.6 Hz, Jbc ¼ 1.6 Hz, Jbd ¼ 10.8 Hz,
1H), 5.37 (dd, Jac ¼ 1.6 Hz, Jbc ¼ 1.6 Hz, Jcd ¼ 17.2 Hz, 1H), 5.87e5.94
(m, 1H), 6.60 (d, J ¼ 6.8Hz, 1H), 6.64 (d, J ¼ 6.8Hz, 1H), 6.73 (s, 1H),
7.02e7.35 (m, 30H), 8.04 (s, 1H). Anal. calcd for C51H39N3O3: C,
82.57; H, 5.66; N, 5.30. Found: C, 81.84; H, 5.78; N, 5.44%.
0
500
600
700
Wavelength(nm)
2.3. Synthesis of Poly(NIPAM-co-SPO-co-fluorophore)
Fig. 5. Fluorescence spectral changes of poly(NIPAM-co-SPO-co-fluorophore) in water
(0.2 g/mL) at various temperatures. The insect shows the fluorescence intensity at
565 nm.
The thermosensitive and photochromic hydrogel was prepared
by free radical copolymerization. N-Isopropylacrylamide 7 (3 g,
0.27 mmol), allyl-2-(2,6-bis((E)-4-(diphenylamino)styryl)-4H-pyran-
4-ylidene)-2-cyanoacetate 5 (0.1 g, 0.13 mmol),spironaphthoxazine
16000
14000
12000
10000
8000
6000
4000
2000
0
4.0
3.5
3.0
2.5
60ºC
60°C
2.0
1.5
20°C
1.0
0.5
20ºC
0.0
20
30
40
50
60
70
400
600
800
1000
Wavelength(nm)
Temperature(oC)
Fig. 3. Absorption spectra of poly(NIPAM-co-SPO-co-fluorophore) in water (0.2 g/mL)
Fig. 6. Hydrodynamic radius(Rh) of poly(NIPAM-co-SPO-co-fluorophore) in water
at various temperatures.
(0.2 g/mL) at various temperatures.

S.-H. Kim et al. / Dyes and Pigments 87 (2010) 158e163
161
3. Results and discussion
a
500
400
300
200
100
0
The synthetic route of the poly(NIPAM-co-SPO-co-fluorophore)
is depicted in Scheme 1. In our design and synthetic strategy, we
took the following points into consideration: at first, poly(N-iso-
propylacrylamide) is thermosensitive polymer hydrogel and
shows a lower critical solution temperature (LCST) due to the
presence of both hydrophilic amide groups and hydrophobic iso-
propyl groups in its side chains. Second, incorporation of the
photochromic spironaphthoxazine unit in the polymer system
Vis
UV
could lead to a photoswitching unit. We designed a new D-p-A
charge transfer dye, 5, containing a polymerizable functional allyl
group. As shown in Scheme 1, the synthetic method was very
simple. The 1H spectrum of 5 verified its well-defined chemical
structure. Characteristic NMR peaks due to the allyl group appeared
at 5.23(dd), 5.37(dd) and 5.94(m). Reaction of an fluorophore
monomer 5, spironaphthoxazine 6 and N-isopropylacrylamide 7 in
THF affords a orange-colored powder of poly(NIPAM-co-SPO-co-
fluorophore). The change in electronic absorption spectra of poly
(NIPAM-co-SPO-co-fluorophore) upon UV irradiation in aqueous
solution is depicted in Fig. 1.
450
500
550
600
650
700
Wavelength(nm)
b
500
UV
UV
UV
The visible range absorption increased gradually by UV irradia-
tion which is ascribed to the generation of the open merocyanine
form from the closed spiro form. When the sample of poly(NIPAM-
co-SPO-co-fluorophore) was left in the dark at room temperature
after irradiation, the absorbance at 605 nm decreased. The original
spectral pattern is reversibly recoverd within 20 s. Irradiation of the
merocyanine with visible light at wavelength greater than 500 nm
resulted in the rapid decolouration and the regeneration of the
closed spiro form. This photochromic behavior also exhibited a fairly
good reproducibility, as can be seen in Fig. 2, where consecutive
colourationedecolouration cycles are shown. UVevis spectrometry
is an easy and widely used method for the determination of the
LCST. In Fig. 3 we show the absorption spectra of aqueous poly
(NIPAM-co-SPO-co-fluorophore) solution at different temperature.
As solution is heated, it becomes turbid; these temperature-
responsive changes are entirely reversible. This kind of gelation
behavior induced by temperature is well-known as an LCST,
resulting from a balance between hydrogen-bond formation with
water and intramolecular hydrophobic forces [5].
400
300
200
Vis
Vis
Vis
0
1
2
3
Cycles
Fig. 7. (a) Fluorescence spectra of poly(NIPAM-co-SPO-co-fluorophore) on each pho-
tostationary state by alternating irradiation with UV and visible light. (b) Fluorescence
switching. Fluorescence of poly(NIPAM-co-SPO-co-fluorophore) under 445 nm exci-
tation is switched on and off with alternating UV and visible irradiation.
(SPO)(0.005g, 0.13mmol)6and2,20-azobis-(isobutyronitrile)(0.06g,
0.4 mmol) were dissolved in anhydrous THF (20 mL) under dry
nitrogen. After heating for 3 days at 65 ꢀC, the resultant mixture was
precipitated from ether and the resulting polymer was dried under
vacuum to give an orange-colored powder. Yield: 30%; Gel perme-
ation chromatography (GPC): Mn: 19800, Mw: 22000, MP: 16300,
Mw/Mn: 1.11.
The values of transmittance (A465) of aqueous solution con-
taining poly(NIPAM-co-SPO-co-fluorophore) are plotted against the
temperature of the solution in Fig. 4; an obvious transmittance
decrease at < 28 ꢀC:LCST ¼ 28e50 ꢀC. The results of thermo-
response of poly(NIPAM-co-SPO-co-fluorophore) solution charac-
terized by UVevis spectrometry are comparable with those from
fluorescence measurement.
strong fluorescent
weak fluorescent
CH3
CH3
H2
H2
C
H2
H2
H2
C
H2
H
C
H
C
H
C
H
C
C
C
C
C
C
O
C
z
x
y
x
z
y
O
O
O
O
UV
Vis
HN
O
HN
O
O
O
CN
O
CN
O
O
N
N
N
N
N+
-O
O
N
N
N
energy transfer
merocyanine
spiro
Scheme 2. Schematic representation of fluorescence modulation by photochromic switching. When spiro form is converted to the merocyanine bt UV light, the emission intensity
of fluorophore is reduced through energy transfer quenching. After exposure to visible light and photoisomerization from merocyanine to spiro form, the emission intensity is
increased.

162
S.-H. Kim et al. / Dyes and Pigments 87 (2010) 158e163
The temperature-dependent emission spectra of the poly(NIPAM-
4. Conclusion
co-SPO-co-fluorophore) in temperature range from 20 to 60 ꢀC is
shown in Fig. 5. Fig. 5 inset indicates the fluorescence intensity
temperature diagram of poly(NIPAM-co-SPO-co-fluorophore).
This shows the fluorescence intensity of the poly(NIPAM-co-
SPO-co-fluorophore) is significaltly increased with heating from 20
to 45 ꢀC when the excitation wavelength was 445 nm. The emission
enhancement at higher temperature is due to the formation of
hydrophobic domain near the isopropylacrylamide units. In
contrast, at gt; 45 ꢀC, the fluorescence intensity decreased. The
temperature dependence of the size of polymer particles as
obtained using dynamic light scattering method is shown Fig. 6.
At 45 ꢀC, the size of the polymer hydrogel is estimated to be
9000 nm, due to a formation of the large hydrophobic polymer
aggregates. Within the large polymer particles, the fluorophore
units scarcely absorb the excitation beam, thus resulting in
emission quenching which is accordance with the formation of
the huge hydrophobic domain [19]. Fig. 7(a) shows the fluores-
cence spectral change of poly(NIPAM-co-SPO-co-fluorophore)
with the excitation wavelength at 445 nm. The fluorescence
intensity change was regulated by the photochromic reaction.
Before irradiation with UV light, it showed an emission in 565 nm
In conclusion, photoswitchable spironaphthoxazine can be
integrated with D-p-A type pyran-based fluorophore into the
thermoresponsive poly(N-isopropylacrylamide). The phase tran-
sition temperature for poly(NIPAM-co-SPO-co-fluorophore) was
determined by UVevis spectroscopic method. The fluorescence
intensity of poly(NIPAM-co-SPO-co-fluorophore) increased in the
range at 20e45 ꢀC. This suggests that the hydrophobic domain
near the flurophore unit was increased with increasing temper-
ature. In contrast, at gt; 45 ꢀC, the fluorescence intensity
decreased, suggesting that the intensity of excitation beam could
be reduced by large hydrophobic polymer aggregates. The
particle size of the aggregates was estimated to be 9000 nm at
45 ꢀC by dynamic light scattering. We also revealed that the
fluorescence intensity of the D-p-A type fluorophore is efficiently
regulated by photochromic switching between spiro and mer-
ocyanine using alternate irradiation with UV and visible light
which can be attributed to energy transfer from the excited flu-
orophore to the merocyanine.
Acknowledgements
with lmax,
¼ 445 nm. Upon irradiation, with UV light, the
exi
fluorescent intensity of poly(NIPAM-co-SPO-co-fluorophore)
decreased. After irradiation with visible light, the original emis-
sion spectra is regenerated. The fluorescence of this polymer
obviously depends on the state of the spironaphthoxazine
photochromic switching between spiro and merocyanine using
alternate irradiation with UV and visible light. From what can be
seen in Fig. 7(b), the fluorescence intensity reversibly changed at
565 nm (excitation: 445 nm). The fluorescence quenching in
merocyanine is attributed to the energy transfer from the excited
fluorophore unit to the opened merocyanine units, because the
spectral overlapping in the range of 550e600 nm between the
absorption band of the merocyanine and the emission band of
the fluorophore. A feasible mechanism for energy transfer and
fluorescence switching is shown in Scheme 2.
Fig. 8 shows a few representative SEM image of the internal
matrix structure of poly(NIPAM-co-SPO-co-fluorophore) hydrogel
which has irregular fibrous aggregation and homogeneous coral-
like microstructure with a three dimensional net work structure.
This morphology indicated that poly(NIPAM-co-SPO-co-fluo-
rophore) could form stable hydrogel when the temperature was
above the LCST of poly(NIPAM-co-SPO-co-fluorophore).
This work was supported by Basic Science Research Program
through the National Research Foundation (NRF) grant funded by
the Korea government (MEST) (No. 2009-0063408).
References
[1] Lokshin V, Samat A, Metelitsa AV. Spirooxazines: synthesis, structure,
spectral and photochromic properties. Russian Chemical Reviews
2002;71:893e916.
[2] Durr H, Bouas-Laurent H, editors. Photochromism: molecules and systems.
Amsterdam: Elsevier; 1990. p. 493.
[3] Yuan W, Sun L, Tang H, Wen Y, Jiang G, Huang W, et al. A novel thermally
stable spironaphthoxazine and its application in rewritable high density
optical data storage. Advanced Materials 2005;17:156e60.
[4] Kretschmann O, Choi SW, Miyauchi M, Tomatsu I, Harada A, Ritter H.
Switchable hydrogels obtained by supramolecular cross-linking of adamantly-
containing LCST copolymers with cyclodextrin dimmers. Angewandte Chemie
International Edition 2006;45:4361e5.
[5] Garcia A, Marquez M, Cai T, Rosario R, Gust ZHD, Hayes M, et al. Photo-,
thermally, and pH-responsive microgels. Langmuir 2007;23:224e9.
[6] Matsubara K, Watanabe M, Takeoka Y. A thermally adjustable multicolor
photochromic hydrogel. Angewandte Chemie International Edition
2007;46:1688e92.
[7] Kumashiro Y, Ooya T, Yui N. Dextran hydrogels containing poly(N-iso-
propylacrylamide) as grafts and cross-linkers exhibiting enzymatic regulation
in
a specific temperature range. Macromolecular Rapid Communications
2004;25:867e72.
[8] Otake K, Inomata H, Konno M, Saito S. Thermal analysis of the volume phase
transition with N-isopropylacrylamide gels. Macromolecules 1990;
23:283e9.
[9] Wang S, Choi MS, Kim SH. Bistable photoswitching in poly(N-iso-
propylacrylamide) with spironaphthoxazine hydrogel for optical data
storage. Journal of Photochemistry and Photobiology A: Chemistry 2008;
198:150e5.
[10] Wang S, Choi MS, Kim SH. Multiple switching photochromic poly(N-iso-
propylacrylamide) with spironaphthoxazine hydrogel. Dyes and Pigments
2008;78:8e14.
[11] Antonov VS, Hohla KL. Dye stability under excimer-laser pumping. Applied
Physics B 1983;B32:9e14.
[12] de Silva AP, Gunaratne HQN, Gunnlaugsson T, Huxley AJM, McCoy CP,
Rademacher JT, et al. Signaling recognition events with fluorescent sensors
and switches. Chemical Reviews 1997;97:1515e66.
[13] Sansregret J, Drake JM, Thomas WRL, Lesiecki ML. Light transport in planar
luminescent solar concentrators: the role of DCM self-absorption. Applied
Optics 1983;22:573e7.
[14] Burland DM, Miller RD, Walsh CA. Second-order nonlinearity in poled-poly-
mer system. Chemical Reviews 1994;94:31e75.
[15] Zucchero AJ, Wilson NJ, Bunz UHF. Cruciforms as functional fluorophores:
response to protons and selected metal ions. Journal of the American
Chemical Society 2006;128:11872e81.
[16] Kwak G, Kim H, Kang IK, Kim SH. Charge transfer dye in various polymers
with different polarity: synthesis, photophysical properties, and unusual
Fig. 8. SEM micrographs of poly(NIPAM-co-SPO-co-fluorophore).

S.-H. Kim et al. / Dyes and Pigments 87 (2010) 158e163
163
aggregation-induced fluorescence changes. Macromolecules 2009;42:
1733e8.
[17] Suh HJ, Jin SH, Gal YS, Koh K, Kim SH. Synthesis and photochromism of pol-
yacethylene derivatives containing a spiroxazine moiety. Dyes and Pigments
2003;58:127e33.
[18] Lim WT, Cui JZ, Suh HJ, Lee HS, Heo NH, Kim SH. Synthesis and X-ray struc-
tural characterization of new spiroxazine. Dyes and Pigments 2003;56:7e25.
[19] Koizumi H, Shiraishi Y, Tojo S, Fujitsuka M, Majima T, Hirai T. Temperature-
driven oxygenation rate control by polymeric photosensitizer. Journal of the
American Chemical Society 2006;128:8751e3.