Synthesis and characterization of new spiropyran micrometer-sized photochromic fluorescent polymeric particles of narrow size distribution by a swelling process

Article abstract of DOI:10.1016/j.polymer.2015.01.068

Photochromic fluorescent uniform micrometer-sized particles were prepared in two ways: (1) Swelling of polystyrene (PS) and poly(styrene-butylmethacrylate) (P(S-BMA)) template particles with spiropyran dye (FSP) to obtain PS/FSP and P(S-BMA)/FSP uniform particles; (2) A similar swelling process, replacing FSP by AIBN, styrene and a spiropyran monomer (MSP), followed by co-polymerization of the monomers within the swollen microspheres to obtain P(S-BMA)/P(S-MSP) uniform particles. The spiropyran dyes are not fluorescent in water or in non-polar organic solvents. In contrast, the dyes encapsulated within the particles exhibit strong red fluorescence in the merocyanine conformation. Fluorescence measurements reveal that the P(S-BMA)/FSP microspheres exhibit higher fluorescence than the PS/FSP particles. The microspheres aqueous dispersion was exposed to alternate UV and visible light cycles and it was found that the fluorescence intensity of the PS/FSP and the P(S-BMA)/FSP particles decreased by 30% of its original value after 5 photoswitching cycles while that of the P(S-BMA)/P(S-MSP) particles remained the same.

Full text of DOI:10.1016/j.polymer.2015.01.068

Accepted Manuscript
Synthesis and Characterization of New Spiropyran Micrometer-Sized Photochromic
Fluorescent Polymeric Particles of Narrow Size Distribution by a Swelling Process
Sharon Bretler, Shlomo Margel
PII:
S0032-3861(15)00127-5
10.1016/j.polymer.2015.01.068
JPOL 17600
DOI:
Reference:
To appear in: Polymer
Received Date: 25 November 2014
Revised Date: 21 January 2015
Accepted Date: 27 January 2015
Please cite this article as: Bretler S, Margel S, Synthesis and Characterization of New Spiropyran
Micrometer-Sized Photochromic Fluorescent Polymeric Particles of Narrow Size Distribution by a
Swelling Process, Polymer (2015), doi: 10.1016/j.polymer.2015.01.068.
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ACCEPTED MANUSCRIPT

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Synthesis and Characterization of New Spiropyran
Micrometer-Sized Photochromic Fluorescent Polymeric
Particles of Narrow Size Distribution by a Swelling Process
Sharon Bretler and Shlomo Margel*
The Institute of Nanotechnology and Advanced Materials, Department of Chemistry,
Bar-Ilan University, Ramat-Gan 52900, Israel
*Corresponding author: E-mail: shlomo.margel@mail.biu.ac.il;
Telephone: (972)-35318861; Fax: (972)-37384053
E-mails: sherbo22@gmail.com

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Abstract:
Photochromic fluorescent uniform micrometer-sized particles were prepared in two
ways: (1) Swelling of polystyrene (PS) and poly(styrene-butylmethacrylate) (P(S-
BMA)) template particles with spiropyran dye (FSP) to obtain PS/FSP and P(S-
BMA)/FSP uniform particles; (2) A similar swelling process, replacing FSP by AIBN,
styrene and a spiropyran monomer (MSP), followed by co-polymerization of the
monomers within the swollen microspheres to obtain P(S-BMA)/P(S-MSP) uniform
particles.
The spiropyran dyes are not fluorescent in water or in non-polar organic solvents. In
contrast, the dyes encapsulated within the particles exhibit strong red fluorescence in
the merocyanine conformation. Fluorescence measurements reveal that the P(S-
BMA)/FSP microspheres exhibit higher fluorescence than the PS/FSP particles. The
microspheres aqueous dispersion was exposed to alternate UV and visible light cycles
and it was found that the fluorescence intensity of the PS/FSP and the P(S-BMA)/FSP
particles decreased by 30% of its original value after 5 photoswitching cycles while
that of the P(S-BMA)/P(S-MSP) particles remained the same.
Key words: Photochromism, Spiropyran, Photoswitching,
Fluorescence, Micrometer-sized particles, Encapsulation

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1. Introduction
Photochromism is defined as the reversible conversion of a chemical species induced
by electromagnetic radiation between two states having light absorptions in different
regions of the visible spectrum.[1] Color changes in photochromic compounds are
typically the result of the shift of optical absorption due to changes in molecular
structure or conformation.[2]
Photochromic particles have attracted much attention in the last several decades[3]
due to their high potential for various applications, such as optically rewritable data
storage, optical switching, chemical sensing, fluorescence microscopic techniques,
cell imaging, ink jet printing, tensile stress sensors, UV-sensor, security or
authentication purposes and photochromic fabrics and plastics.[4-10]
Spiropyran (SP) type compounds are one of the most widely used photochromic
species, and it is well-known that SP undergoes photo-reversible structural
transformations in response to external inputs such as light. The SP molecules can
adopt one of two stable states: the open-ring state, known as the merocyanine (MC)
form, and the closed-ring state, known as the spiro form.[2, 11, 12] For example,
Figure 1 shows the structural transformations of a typical SP compound, 1′,3′-
Dihydro-1′,3′,3′-trimethyl-6-nitrospiro[2H-1-benzopyran-2,2′-(2H)-indole]
(FSP),
upon irradiation with UV and visible light. Under UV light, the FSP molecules are
transformed into the open MC form (blue colored form), while when they are
irradiated with visible light or heat, they adopt the closed-ring Spiro form (colorless
form).

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Fig 1. The molecular mechanism responsible for the photochromic of FSP
Spiro and MC forms of the SP dye are not fluorescent in most solvents. However, it
was reported that the MC form of SP dyes incorporated within the hydrophobic core
of polymer particles exhibits strong red fluorescence (660 nm).[13, 14] Thus, the
spiro-MC reversible photoisomerization allows controlled switching of the
fluorescence by alternating UV and visible radiation.
Up to date, a few examples of micro-scale photochromic particles have been reported,
usually on the macro-scale and with a broad size distribution.[15-17] The reported
photochromic dyes were either covalently bound to the surface of the polymeric
particles or dispersed in polymer solids.[17-19]
The present study describes the synthesis and characterization of photochromic
fluorescent micrometer-sized particles of narrow size distribution. The uniformity of
the particles’ size leads to a more homogeneous system, resulting in a more exact,
repeatable and sharp measurements. This property is especially important for optical
measurements since optical properties are size dependent. The photochromic
fluorescent particles were formed by a swelling process[20-22] of template
microspheres with SP dyes. The template microspheres of narrow size distribution
were formed by dispersion polymerization of styrene (S) or S with butyl methacrylate
(BMA) to obtain polystyrene (PS) or polystyrene/polybutylmethacrylate (P(S-BMA))
template microspheres, respectively.

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In this study, we used two SP dyes; the free non-monomeric dye molecule 1′,3′-
Dihydro-1′,3′,3′-trimethyl-6-nitrospiro[2H-1-benzopyran-2,2′-(2H)-indole] (FSP), and
the monomeric dye 1′-(2-Methacryloxyethyl)-3′,3′-dimethyl- 6-nitro-spiro(2H-1-
benzo-pyran-2,2'-indoline) (MSP).
The SP dyes were entrapped within the template microspheres by two different
swelling processes: (1) Swelling of PS and the (P(S-BMA)) templates particles with
FSP to obtain PS/FSP and P(S-BMA)/FSP uniform particles, respectively; (2) A
similar swelling process, substituting the FSP by azobisisobutyronitrile (AIBN), S and
the SP vinylic monomer MSP, followed by co-polymerization of the monomers
within the swollen microspheres to obtain P(S-BMA)/P(S-MSP) uniform particles.
The present work investigates the fluorescence and photochromic properties of the SP
dye encapsulated in the two ways described above within two different uniform
template micrometer-sized particles.
2. Experimental part
2.1. Materials
The following analytical-grade chemicals were purchased from Aldrich and used
without further purification: 1′,3′-Dihydro-1′,3′,3′-trimethyl-6-nitrospiro[2H-1-
benzopyran-2,2′-(2H)-indole] (FSP, 98%), benzoyl peroxide (BP, 98%), sodium
dodecyl sulfate (SDS), poly(vinylpyrrolidone) (PVP, mw 360,000), methacryloyl
chloride, butyl methacrylate (BMA, 99%), ethanol (HPLC), 2-methoxy ethanol
(HPLC), triethylamine and anhydrous methylene chloride (HPLC). Styrene (S, 99%)
was purified by activated alumina to remove inhibitors before use. 2,2'-
azobisisobutyronitrile (AIBN) was recrystallized from ethanol. Water was purified by

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passing deionized water through an Elgastat Spectrum reverse osmosis system (Elga
Ltd., HighWycombe, UK)..
2.2 Synthesis of Uniform Micrometer-Sized PS/FSP Composite Particles
2.2.1 Synthesis of the PS Template Microspheres
PS template microspheres of 1.61±0.05 ꢀm hydrodynamic diameter were prepared by
dispersion polymerization of styrene in ethanol and 2-methoxy ethanol, according to
the literature.[23]
2.2.2 Synthesis of the PS/FSP Composite Microspheres
Uniform PS/FSP composite microspheres of 1.69±0.05 ꢀm diameter were prepared as
follows: 1.6 mL of the swelling solvent methylene chloride containing 2.5 mg
dissolved FSP [1% (w/w)] were first added to 10 mL SDS aqueous solution [0.75%
(w/v)]. Emulsion droplets of the swelling solvent were then formed by sonication of
the mixture at 4 °C for 1 min. 3.5 mL of an aqueous suspension of the PS template
microspheres (245 mg, 7% w/v) of 1.61±0.05 ꢀm hydrodynamic diameter were then
added to the stirred methylene chloride emulsion droplets. The obtained mixture was
then shaken for 4 h at room temperature. The completion of the swelling process of
the PS template particles (from 1.61±0.05 µm to 2.3±0.09 µm) was confirmed by
optical microscopy, ensuring that no small emulsion droplets of the swelling solvent
mixture remained. Evaporation of the entrapped methylene chloride was then
performed by shaking the open vial containing the aqueous mixture of the swollen
particles over night at room temperature. The obtained uniform PS/FSP photochromic
microspheres of 1.69±0.05 ꢀm hydrodynamic diameter were then washed by several
centrifugation cycles with ethanol and water, and then lyophilized. The swelling

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procedure was also accomplished in the presence of different concentrations of the
FSP dye dissolved in methylene chloride.
2.3 Synthesis of Uniform Micrometer-Sized P(S-BMA)/FSP Composite Particles
2.3.1 Synthesis of the P(S-BMA) Template Microspheres
P(S-BMA) template microspheres of 1.85±0.05 µm diameter were prepared by a
slightly modified version of the procedure described in the literature.[24] Briefly, in a
three-neck round-bottom flask equipped with a condenser and a mechanically stirred
solution (340 rpm), PVP (1.75 g) was dissolved in ethanol (100 mL) and water (13
mL) and heated to 73 °C. Nitrogen was bubbled through the solution for 15 min to
exclude air, after which a blanket of nitrogen was maintained over the solution during
the polymerization period. A solution containing the initiator BP (0.75 g, 3.1 mmol),
S (13.2 mL, 115.21 mmol) and BMA (6.6 mL, 41.5 mmol) was then added to the
reaction flask and stirred for 24h at 73 °C. The polymerization reaction was then
stopped by cooling the polymerization flask to room temperature. The microspheres
formed were washed by extensive centrifugation cycles with ethanol and water and
then dried by lyophilization.
2.3.2 Synthesis of P(S-BMA)/FSP Composite Microspheres
Uniform P(S-BMA)/FSP composite microspheres of 1.94±0.05 ꢀm diameter were
prepared by swelling the P(S-BMA) template microspheres of 1.85±0.05 µm with
FSP as described in paragraph 2.2.2.

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2.4. Synthesis of Uniform Micrometer-Sized P(S-BMA)/P(S-MSP) Composite
Particles
2.4.1. Synthesis of the monomer 1′-(2-Methacryloxyethyl)-3′,3′-dimethyl- 6-nitro-
spiro(2H-1-benzo-pyran-2,2'-indoline) (MSP)
Fig 2. Synthesis scheme for MSP
MSP was synthesized as described in the literature (Figure 2).[25, 26] Briefly, a
mixture protected against visible-light exposure containing 1′-(2-hydroxyethyl)-3′,3′-
dimethyl-6-nitrospiro(2H-1-benzo-pyran-2,2′-indoline)(SP-OH) (0.5 g, 1.42 mmol),
triethylamine (250 ꢀL, 1.8 mmol) and dry methylene chloride (7.5 mL) was cooled to
0
0 C under N2 atmosphere. Methacryloyl chloride (170 mmol) in dry methylene
chloride (2.5 mL) was then added dropwise over 0.5 h. After stirring for 24 h at room
temperature, the solvent was removed using a rotary evaporator. The crude product
was purified by silica gel column chromatography using methylene chloride
/petroleum ether (2:1, v/v) as eluent. The pure MSP was obtained as yellow crystals.
1 H NMR (300 MHz, CDCl3 ,TMS, δ ): 1.14 (s, 3H), 1.26 (s, 3H), 1.92 (s, 3H), 3.4–
3.59 (m, 2H), 4.3 (t, 2H), 5.56 (m, 1H), 5.87 (d, 1H), 6.07 (s, 1H), 6.72 (q, 2H), 6.89
(q, 2H), 7.09 (d, 1H), 7.17–7.23 (m, 1H), 7.99–8.04 (m, 2H).
2.4.2. Synthesis of P(S-BMA)/P(S-MSP) Composite Microspheres

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Uniform micrometer-sized P(S-BMA)/P(S-MSP) composite particles were formed by
swelling the P(S-BMA) template particles with methylene chloride containing
styrene, AIBN, and MSP, followed by co-polymerization of the monomers within the
swollen particles at elevated temperature. In a typical experiment, uniform P(S-BMA)
/P(S-MSP) composite particles of 2.24±0.08 ꢀm diameter were prepared by adding to
a 20-mL vial, 10 mL of an SDS aqueous solution [0.75% (w/v)] and 2 mL of
methylene chloride containing 3 mg of MSP (0.5 %(w/w), 400 ꢀL of S and 10 mg of
AIBN. Emulsion droplets of the swelling solvent were then formed by sonication of
the mixture for 1 min, as described in the previous paragraph. 3.5 mL of an aqueous
suspension of the P(S-BMA) template microspheres of 1.85±0.05 µm diameter (245
mg, 7% (w/v)) were then added to the stirred methylene chloride emulsion droplets.
After the swelling process was complete, evaporation of the entrapped methylene
chloride was performed. For the polymerization of the MSP and S within the swollen
P(S-BMA) template particles, the temperature of the particle mixture was then raised
to 73 °C for 18 h. The formed P(S-BMA)/P(S-MSP) composite particles were then
washed by several centrifugation cycles with water, ethanol, again water, and then
lyophilized. Various P(S-BMA)/P(S-MSP) composite particles were prepared by
changing the weight ratio [MSP]/[P(S-BMA)].
2.5. Determination of the Encapsulated FSP and MSP Dyes
A calibration curve of FSP and of MSP was obtained by measuring the absorbance of
series of standard FSP and MSP solutions (0.1553x10-3-0.25 mM) in THF at 333 nm.
The concentration of the encapsulated FSP and MSP was determined by dissolving
the particles containing the dyes (1 mg/mL) in THF and then measuring of the
absorbance at 333 nm.

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2.6. Characterization
Optical microscope pictures and fluorescent micrographs were obtained with an
Olympus microscope, model BX51. The particle average size and size distribution
were determined by measuring the diameter of more than 200 particles on optical
micrographs with the image analysis software AnalySIS Auto (Soft Imaging System
GmbH, Germany).
Surface morphology was characterized with a JEOL scanning electron microscope
(SEM) model JSM-840. For this purpose, a drop of dilute microsphere dispersion in
water was spread on a glass surface, and then dried at room temperature. The dried
sample was coated with gold in vacuum before viewing under SEM.
Hydrodynamic particle size and size distribution were determined by dynamic light
scattering (DLS) with photon cross-correlation spectroscopy (Nanophox particle
analyzer, Sympatec GmbH, Germany)
Fourier Transform Infrared (FTIR) analysis was performed with a Bruker Platinum-
FTIR QuickSnap TM sampling modules A220/01. The analysis was performed with
13 mm KBr pellets that contained 2 mg of the examined particles and 198 mg KBr.
The pellets were scanned over 200 scans at a 4 cm-1 resolution. Absorption spectra
were obtained using a Cary 100 UV Visible spectrophotometer (Agilent Technologies
Inc.).
Fluorescence spectra of the particles dispersed in aqueous solution were recorded by a
spectrofluorometer (Cary eclipse, Agilent Technologies.(Excitation and emission slits
were fixed at 5 nm, and λex was set at 600 nm.
Nuclear Magnetic Resonance (NMR) spectroscopy was performed on a Bruker AC
1
300 MHz spectrometer. H spectra were recorded in deuteron–chloroform. Chemical

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shifts are reported as parts per million (ppm) downfield from tetramethylsilane, as
internal standard in 1H NMR.
Differential Scanning Calorimeter (DSC) Model DSC Q200 (TA Instrument) was
used to measure the thermal properties of the samples under N2 atmosphere at a
warming scanning rate of 10 K. min-1.
3. Results and Discussion
Figure 3 exhibits the basic strategy for the synthesis of our photochromic fluorescent
microspheres, based on a single-step swelling process of the uniform template
micrometer-sized particles. This was accomplished in two ways: (1) Preparation of
the photochromic PS/FSP and P(S-BMA)/FSP composite particles by swelling of the
PS and P(S-BMA) templates microspheres with methylene chloride containing FSP,
followed by evaporation of the methylene chloride after completion of the swelling
process, as described in the experimental section; (2) Preparation of the photochromic
P(S-BMA)/P(S-MSP) composite particles by swelling of the P(S-BMA) template
microspheres with methylene chloride containing the MSP, S and AIBN, followed by
co-polymerization of the monomers within the swollen particles, as described in the
experimental section.

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Fig 3. Preparation of the photochromic PS/FSP, P(S-BMA)/FSP and P(S-BMA)/P(S-MSP) composite
particles via a single-step swelling process.
First, the simple single-step swelling procedure was accomplished as demonstrated by
light microscope pictures in Figure 4. Since the P(S-BMA) template particles are
filled with the swelling solvent (methylene chloride containing FSP), the diameter of
the swollen particles is significantly higher than that of the non-swollen particles,
2.94±0.07µm (Figure 4B) and 1.85 ± 0.05µm (Figure 4A), respectively. Evaporation
of the methylene chloride from the swollen particles at room temperature leads to a
decrease in the size of the formed P(S-BMA)/FSP composite particles to 1.94±0.05
µm (Figure 4C), slightly higher than that of the template P(S-BMA) particles.
Fig 4. Light microscope pictures of the template P(S-BMA) microspheres before (A) and after
swelling with methylene chloride containing FSP (B), followed by evaporation of methylene chloride
from the swollen particles (C).

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Similar behavior was observed for the PS/FSP microspheres. The PS template
microspheres are of size 1.61±0.05 ꢀm. After the swelling process was complete its
diameter increased to 2.3±0.09 µm, while evaporation of the methylene chloride led
to shrinking of the swollen particles to 1.69±0.05 µm. We should note that the size
distribution of the PS/FSP and P(S-BMA)/FSP particles remained, more or less, as
narrow as that of the template particles during both the swelling process and the
swelling solvent evaporation process.
Table 1 demonstrates the influence of the weight ratio [FSP]/[Template microspheres]
on the percentage of FSP entrapped in the PS/FSP and P(S-BMA)/FSP microspheres.
This Table indicates that increasing the concentration of the FSP dye leads to
increased content of the dye in both composite particles. In addition, Table 1 shows
that in both composite microspheres, at all the studied dye concentrations, most (more
than 90%) of the initial FSP content was entrapped within the templates microspheres.
Table 1. Influence of the weight ratio [FSP]/[Templates microspheres] on the weight % of FSP in the
P(S-BMA)/FSP and PS/FSP microspheres.
[FSP] [FSP]/[Template
Weight % of FSP in the template microspheres
(mg)
microspheres]
(w/w)
P(S-BMA)/FSP
Calculated Measured
PS/FSP
Calculated Measured
1.25
2.5
5
0.005
0.01
0.02
0.5
1
2
0.46
0.92
1.9
0.5
1
2
0.47
0.91
1.9
Spiropyran (SP) molecules, such as FSP, can present one of two stable states as
shown in Figure 1: Upon irradiation by UV light, the SP molecules adopt the open-
ring state, known as merocyanine (MC) form, while with visible light, they adopt the
closed-ring state, known as spiro form. The spiro form has no absorption band from
400 to 700 nm, while after UV irradiation a new absorption band at 590 nm appears

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due to the formation of the blue MC form. The absorption spectra of FSP in THF
before and after UV radiation are shown in Figure 5A. The absorption behavior of the
aqueous dispersion of photochromic particles could not unfortunately be measured
due to the light scattering effect of the microspheres.
Both, the spiro and MC forms of SP dyes are not fluorescent in water and non-polar
organic solvents. However, incorporation of the SP dye in the MC form into the
hydrophobic core of polymer particles leads to highly fluorescent particles.[14] This
result is strong evidence for the stabilization of the MC form within the hydrophopic
core of the particles. The SP dye incorporated within the hydrophobic cavities of
polymer particles has a protective shell that isolates it from non-radiative decay
pathways or electron-transfer pathways generated by collisions with solution
components. Thus SP embedded in hydrophobic core can exhibit stronger
emission.[27]
In our studies, the FSP dye was incorporated by a swelling process into the
hydrophobic core of the PS and the P(S-BMA) microspheres. Figure 5B shows that
both photochromic microspheres, P(S-BMA)/FSP (5B-a) and PS/FSP (5B-b),
dispersed in water are fluorescent when they are in the MC conformation (excitation
wavelength 600 nm and emission wavelength 640 nm). However, the fluorescence
intensity of the P(S-BMA)/FSP particles is significantly higher than that of the
PS/FSP particles although their FSP concentration is equal (as shown in Table 1).
Figure 5B also illustrates that when the PS/FSP and P(S-BMA)/FSP microspheres are
dissolved in toluene the obtained spiro (5B-f) and MC (5B-e) toluene solutions are not
fluorescent any more. This figure also shows, as expected, that the spiro form of the
PS/FSP and P(S-BMA)/FSP microspheres dispersion in water (5B-c & d) is also not
fluorescent. However, when these microspheres dispersion in water was irradiated by

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UV for 60 sec, the obtained microspheres aqueous dispersion (5B-a & b) became
strongly fluorescent, indicating that the FSP moieties have been successfully
incorporated within the hydrophobic core of the polymer microspheres.
Fig 5. A. UV–Vis absorption spectra of FSP in THF before and after UV irradiation for 60 sec.; B.
Fluorescence spectra of the P(S-BMA)/FSP and PS/FSP microspheres dispersed in water in which the
FSP dye is in the MC form (a and b, respectively) or in the spiro form (c and d); Similar fluorescence
spectra of the P(S-BMA)/FSP and PS/FSP microspheres dissolved in toluene in which the FSP is in the
MC form (e) or in the spiro form (f) . The various composite particles with FSP [1% (w/w)] were
prepared according to the experimental description.
It is well known from the literature that the addition of BMA or BA to polymeric
particles backbone lower the particles’ Tg, and thereby decrease the rigidity of the
system.[14] It is also well known from the literature that the rigidity of the host
compound has a major influence on the total environment of guest photochromic
molecules. More rigid environment are known to retard the photochromic
transition,[28, 29] or in our case the fluorescence intensity.
As expected, DSC measurements clearly point that the P(S-BMA) particles (Tg of 69
ºC) are less rigid than that of the PS particles (Tg of 102 ºC). These different
environments of the FSP dye within the particles should clearly have influence on the
fluorescence quantum yield of the entrapped dye.[13, 14] Indeed, as expected from

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the literature, the fluorescence intensity of the FSP dye entrapped within the less rigid
P(S-BMA) particles is higher than that of the dye entrapped within the PS particles, as
shown in Figure 5.
In order to test the photostability of the FSP molecules embedded in the composite
microspheres, the fluorescence intensities at 640 nm of the P(S-BMA)/FSP and the
PS/FSP particles were measured. The microspheres dispersion in water (1mg/mL) at
room temperature were alternately exposed to visible and UV light for 30 and 10 min,
respectively. The fluorescence intensities (640 nm) of the aqueous dispersion of the
particles under excitation wavelength of 600 nm were measured. Figure 6 shows that
in each photoswitching cycle of the P(S-BMA)/FSP and PS/FSP particle dispersion in
water the fluorescence intensity decreased and after five cycles, the fluorescence
intensity of the particles decreased to 30% of its original value.
Fig 6. Fluorescence switching. Fluorescence intensities (640 nm) of the photochromic P(S-BMA)/FSP and
PS/FSP particles dispersion in water (1 mg/mL) under 600 nm excitation wavelength were switched on
and off with alternating UV and visible light irradiation for 10 and 30 min, respectively.

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Instability of the SP type dye under UV irradiation, caused by degradation of the dye,
is generally observed.[14, 30] As we know, instability of the SP type dye is mainly
induced by bimolecular degradative reactions that involve the triplet-excited dye. The
FSP molecules entrapped in the PS/FSP and P(S-BMA)/FSP particles are not tightly
bound and are able to migrate within the polymer matrix, so the FSP molecules have
increased opportunity to be degraded by the UV irradiation.
In order to improve the stability of the entrapped FSP dye, a vinylic monomer (MSP)
composed of a side FSP dye was synthesized (Figure 2). The co-polymerization of the
MSP with S within the swollen P(S-BMA) template particles leads to restricted
mobility of the dye, which should minimize the bimolecular degradation reactions and
increase their photostabilty.
The behavior of the system required for the formation of the P(S-BMA)/P(S-MSP)
microspheres was similar to that described for the formation of the P(S-BMA)/FSP
particles. The P(S-BMA) template microspheres were of size 1.85±0.05 ꢀm. As a
result of swelling with methylene chloride containing S, MSP and AIBN (as described
in the experimental section) their diameter increased to 4.07±0.15 µm, followed by a
decrease to 2.95±0.11 µm after the methylene chloride evaporation process.
Polymerization of the S and MSP within the swollen particles at 73°C led to the
formation of P(S-BMA)/P(S-MSP) composite particles with diameter of 2.24±0.11
µm. The size decrease of the swollen particles due to the polymerization step is a
well-known phenomenon [31], and takes place probably as a consequence of the
removal of excess reagents (e.g., monomers and soluble oligomers) after the
completion of the polymerization.
Figure 7 exhibits SEM photographs of the P(S-BMA) template microspheres (A) and
of the produced P(S-BMA)/P(S-MSP) microspheres (B and C). These pictures clearly

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show the smooth surface of the P(S-BMA) template particles compared to the rough
surface of the P(S-BMA)/P(S-MSP) composite particles. This rough surface of the
composite particles is probably due to the formation of surface attached P(S-MSP)
nanoparticles as shown in Figure 7C. These surface bound nanoparticles were formed
due to surface polymerization initiated by the interaction of generated surface
oligoradicals with residual water soluble monomers. In addition, these pictures again
illustrate that the encapsulation and polymerization processes retained the narrow size
distribution of the P(S-BMA) template particles.
Fig 7. SEM images of the P(S-BMA) template microspheres (A) and the P(S-BMA)/P(S-MSP) microspheres (B
and C). Picture C represents a higher magnification of B.
Figure 8 illustrates the FTIR spectra of the P(S-BMA) template microspheres (A),
MSP monomer (B) and the photochromic P(S-BMA)/P(S-MSP) composite particles
(C). The P(S-BMA) spectrum (8A), as expected, is composed of peaks belonging both
to PS (700 cm-1 corresponds to vibrational band of C-C and 1492 cm−1 corresponds to
stretching band of aromatic CH) and PBMA (1150–1300 and 1730 cm−1, belonging to
the stretching bands of carboxylic groups). Figure 8B shows an IR spectrum of the
vinylic monomer MSP: the transition frequencies at 1730 cm−1 belonging to the
stretching bands of carboxylic groups, 1510 and 1600 cm-1 correspond to the NO2
stretching bands, 1333 cm-1 corresponds to N-C of aromatic amine , 1200-1300 cm-1
corresponds to the vibrational band of ether, and 952 cm-1 corresponds to the alkene C–

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H bend. Figure 8C shows the IR spectrum of the photochromic P(S-BMA)/P(S-MSP)
composite particles. This spectrum is composed of peaks belonging both to PS/PBMA
(700, 1492, 1150–1300, 1730 cm-1, etc.) and to the monomeric units of MSP (952,
1333, and 1510 cm-1), thereby demonstrating the presence of the photochromic dye in
the P(S-BMA)/P(S-MSP) composite microspheres.

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Fig 8. FTIR spectra of the P(S-BMA) template microspheres (A), MSP (B) and P(S-BMA)/P(S-MSP)
composite particles (C).

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The presence of MSP monomeric units within the P(S-BMA)/P(S-MSP) composite
microspheres, prepared according to the description in the experimental part, was
determined through a calibration curve of the absorbance at 330 nm of different
concentrations of MSP in THF as described in the experimental part. The results of
this study indicates that 80% of the initial 0.5 weight% MSP were polymerized within
the P(S-BMA) microspheres, so that the obtained weight% ratio [PMSP]/[ P(S-
BMA)] was 0.4 %.
The photochromic reaction is caused by the reversible spiroheterolytic cleavage of the
C-O sp3 bond under UV irradiation, yielding the MC colored form which has strong
visible absorption bands in the range 500-600 nm, resulting in its blue color. Back-
conversion from the MC form of the dye to the spiro form occurs under visible light
irradiation or thermally in the dark.
The fluorescent image of the photochromic P(S-BMA)/P(S-MSP) particles is
illustrated in Figure 9. This figure shows that the PMSP part of the particles is
distributed evenly throughout each of the microspheres. The insert in the figure shows
the photochromism and fluorescence of the P(S-BMA)/P(S-MSP) particles. It is
obvious to the naked eye that when the samples are irradiated with a hand-held UV-
light source, the colorless particles in the spiro form (Figure 9A) turn dark blue within
several seconds under 365 nm UV-irradiation (Figure 9B). The dark blue MC
particles also emit a strong red fluorescence under excitation at 600 nm wavelength
(Figure 9C). On standing in ambient light, the blue color of the polymer particles
faded after about 30 min. This process was reversible and was repeated many times.

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Fig 9. Fluorescent microscope picture of the photochromic P(S-BMA)/P(S-MSP) composite particles
prepared by a single-step swelling of the template microspheres with methylene chloride containing
MSP, S and AIBN according to the description in the experimental section. Inserts A, B and C
represent photomicrographs of the P(S-BMA)/P(S-MSP) composite particles dispersion in water
recorded before, after UV irradiation for 10 sec and under excitation at 600 nm, respectively.
Figure 10 exhibits the photo-stability of the PMSP dye belonging to the P(S-
BMA)/P(S-MSP) particles dispersion in water under alternate irradiation with UV and
visible light. For this purpose, the fluorescence intensities (640 nm) of the P(S-
BMA)/P(S-MSP) particles aqueous dispersion were measured under an excitation
wavelength of 600 nm after alternative exposure to visible and UV light for 30 and 10
min, respectively.

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Fig 10. Fluorescence switching. Fluorescence of the photochromic P(S-BMA)/P(S-MSP) particle
dispersion in water under 600 nm excitation wavelength was switched on and off with alternating UV
and visible light irradiation for 30 and 10 min, respectively.
The comparison between Figures 6 and 10 indeed indicates that the P(S-BMA)/P(S-
MSP) dispersion in water exhibits much better reversibility than FSP in P(S-
BMA)/FSP, as even after five cycles of photoswitching, the fluorescence intensity of
the P(S-BMA)/P(S-MSP) particles remained without significant decrease while that
of the P(S-BMA)/FSP particles lost 30% of its initial value. This result may be
explained by the fact that the MSP molecules in the P(S-BMA)/P(S-MSP) particles
are attached to the P(S-BMA) particles, restricting their mobility, thereby minimizing
the bimolecular degradation reactions and contributing to the increased photo-stability
of the particles.

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4. Conclusions and Future Plans
This work first describes the synthesis and characterization of PS/FSP, P(S-
BMA)/FSP and P(S-BMA)/P(S-MSP) photochromic uniform micrometer-sized
particles, prepared by different versions of the single-step swelling process of the PS
and P(S-BMA) template micrometer-sized particles of narrow size distribution.
These particles are colorless in visible light since the SP molecules entrapped within
the particles are in the spiro form. Upon UV irradiation the particles become
photochromic and fluorescent due to the transition of the SP entrapped molecules
from the spiro form to the MC form. The fluorescence intensity of the particles also
indicates that the MC form of the entrapped SP dye is stable. Fluorescence
measurements (excitation at 600 nm wavelength and emission at 640 nm) reveal that
at similar untrapped FSP concentration, the P(S-BMA)/FSP microspheres exhibit
greater fluorescence than the PS/FSP particles. The extent of reversibility between the
spiro form and the MC form of the entrapped SP dye was examined by exposing the
aqueous dispersion of particles to alternate UV and visible light cycles. The
fluorescence intensity of the particles containing the FSP dye (PS/FSP and the P(S-
BMA)/FSP) decreased by 30% of its original value after 5 photoswitching cycles
while that of the P(S-BMA)/P(S-MSP) particles remained the same.
Our future plans include further optimization of the photochromic P(S-BMA)/P(S-
MSP) particles, and efforts to prepare and characterize similar particles in the
nanometer scale range.

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Highlights
•
•
•
•
Photochromic uniform microparticles formed by a swelling process of template particles
Fluorescence/photochromic properties of the various composite particles were studied
Photostability of the SP molecules embedded in the composite particles were measured
P(S-BMA)/P(S-MSP) particles show better photochromic reversibility than P(S-BMA)/FSP