Photo/Thermal Dual Responses in Aqueous-Soluble Copolymers Containing 1-Naphthyl Methacrylate

Article abstract of DOI:10.1021/acs.macromol.1c00297

Photoresponsive polymers capable of luminescence switching are attracting significant interest due to their potential application in fluorescence patterning, bioimaging, optical data storage, and anti-counterfeiting. In this work, we have developed aqueous-soluble copolymers of 1-naphthyl methacrylate and oligo(ethylene glycol) methyl ether methacrylate [P(1-NMA-co-OEGMA)] that undergo a significant shift in fluorescence emission wavelength after UV irradiation. Irradiation of the 1-naphthyl methacrylate moieties results in the photo-Fries rearrangement to form hydroxy aryl ketones, which exhibit strong emission at 475 nm through excited-state intramolecular proton transfer (ESIPT) and excited-state proton transfer (ESPT). The resultant shift in fluorescence emission maximum from 338 to 475 nm after rearrangement can potentially be exploited for fluorescence patterning. Furthermore, the copolymers are thermally sensitive in aqueous solutions. The lower critical solution temperature (LCST) of the copolymers depends on the content of hydrophobic 1-naphthyl methacrylate units; the photo-Fries rearrangement results in a more polar structure, shifting the LCST to a higher temperature. Of note, the temperature-triggered volume phase transition of copolymer hydrogels selectively ″switches off″ fluorescence arising from the ESPT mechanism, while the ESIPT emission is unaffected. We also demonstrate that films formed by coating the copolymers onto various substrates can be selectively patterned to form gradients in fluorescence intensity. These versatile P(1-NMA-co-OEGMA) copolymers are simple to prepare at low cost, demonstrate effective photoswitching, and have excellent water solubility, thus ensuring potential applications in a number of important areas.

Full text of DOI:10.1021/acs.macromol.1c00297

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Article
Photo/Thermal Dual Responses in Aqueous-Soluble Copolymers
Containing 1‑Naphthyl Methacrylate
Jiacheng Zhao, Gerald Tze Kwang Er, Francis J. McCallum, Sisi Wang, Changkui Fu, Joshua A. Kaitz,
James F. Cameron, Peter Trefonas, III, Idriss Blakey, Hui Peng,* and Andrew K. Whittaker*
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ABSTRACT: Photoresponsive polymers capable of luminescence
switching are attracting significant interest due to their potential
application in fluorescence patterning, bioimaging, optical data
storage, and anti-counterfeiting. In this work, we have developed
aqueous-soluble copolymers of 1-naphthyl methacrylate and oligo-
(ethylene glycol) methyl ether methacrylate [P(1-NMA-co-
OEGMA)] that undergo a significant shift in fluorescence emission
wavelength after UV irradiation. Irradiation of the 1-naphthyl
methacrylate moieties results in the photo-Fries rearrangement to
form hydroxy aryl ketones, which exhibit strong emission at 475 nm
through excited-state intramolecular proton transfer (ESIPT) and
excited-state proton transfer (ESPT). The resultant shift in
fluorescence emission maximum from 338 to 475 nm after
rearrangement can potentially be exploited for fluorescence
patterning. Furthermore, the copolymers are thermally sensitive in aqueous solutions. The lower critical solution temperature
(LCST) of the copolymers depends on the content of hydrophobic 1-naphthyl methacrylate units; the photo-Fries rearrangement
results in a more polar structure, shifting the LCST to a higher temperature. Of note, the temperature-triggered volume phase
transition of copolymer hydrogels selectively ″switches off″ fluorescence arising from the ESPT mechanism, while the ESIPT
emission is unaffected. We also demonstrate that films formed by coating the copolymers onto various substrates can be selectively
patterned to form gradients in fluorescence intensity. These versatile P(1-NMA-co-OEGMA) copolymers are simple to prepare at
low cost, demonstrate effective photoswitching, and have excellent water solubility, thus ensuring potential applications in a number
of important areas.
Recently, a class of novel luminescence molecules called
aggregation-induced emission luminogens (AIEgens) was
introduced by the group of Tang.19,20 These molecules have
strong fluorescence in the aggregated or solid state but weak
emission in a dilute solution. By combining photoresponsive
molecules with AIEgens, a series of luminescent photo-
responsive materials in the solid state was reported.21−24
However, the synthesis of these constructs is synthetically
challenging. Other workers have exploited the reaction of
photoacid generators (PAG) and heterocyclic com-
pounds.25−29 These systems are based either on photoacid-
catalyzed deprotection of heterocyclic compounds to induce
tautomerism or on photoacid protonation of the nitrogen atom
INTRODUCTION
■
Photoresponsive materials have been widely investigated over
the past several decades due to their potential applications in
areas such as optical materials, photoswitches, memory storage,
and photovoltaic devices.1−6 A number of photoresponsive
systems such as those based on spiropyrans, diarylethenes,
azobenzene, and stilbene derivatives have been intensively
studied.7−10 Among these materials, solid-state photoswitching
molecules are particularly attractive for bioimaging and
biosensing, optical information storage, and fluorescence
patterning.11−16 However, certain systems based on azoben-
zene do not display luminescence, while the spiropyrans suffer
from quenching of fluorescence caused by aggregation in the
solid state.17 Steric hindrance of close-packed molecules
inhibits the photo-induced structural change and makes the
switching incomplete.18 Despite many advances, these
challenges highlight that the development of novel photo-
switching materials in the solid state is a significant challenge
but remains highly desirable.
Received: February 7, 2021
Revised: March 23, 2021
Published: May 7, 2021
https://doi.org/10.1021/acs.macromol.1c00297
© 2021 American Chemical Society
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Figure 1. (a) Schematic of photo-Fries rearrangement of 1-naphthyl acetate; (b) fluorescence and UV−vis absorbance spectra of 1-naphthyl acetate
in acetonitrile; (c) fluorescence and UV−vis absorbance spectra of 1-naphthol in acetonitrile; (d) UV−vis absorbance spectra (200−500 nm) of 1-
naphthyl acetate in acetonitrile ([1-NA] = 0.12 mM) after UV irradiation with varying times (0−540 s); (e) UV−vis absorbance spectra (300−800
nm) of 1-naphthyl acetate in acetonitrile ([1-NA] = 1.2 mM) after UV irradiation with varying times (0−540 s); (f) fluorescence spectra (λex =
260 nm) of 1-naphthyl acetate in acetonitrile ([1-NA] = 2.4 mM) after UV irradiation with varying times (0−540 s); (g) fluorescence spectra (λex
= 280 nm) of 1-naphthyl acetate in acetonitrile ([1-NA] = 2.4 mM) after UV irradiation with varying times (0−540 s); (h) fluorescence spectra
(λex = 370 nm) of 1-naphthyl acetate in acetonitrile ([1-NA] = 2.4 mM) after UV irradiation with varying times (0−540 s); (i) fluorescence
spectra (λex = 330 nm) of 1-naphthyl acetate in acetonitrile ([1-NA] = 2.4 mM) after UV irradiation with varying times (0−540 s); (j)
fluorescence spectra (λex = 330 nm) of 1-naphthyl acetate in acetonitrile ([1-NA] = 2.4 mM) after UV irradiation from 0 to 120 s; (k) fluorescence
spectra (λex = 330 nm) of 1-naphthyl acetate in acetonitrile ([1-NA] = 2.4 mM) after UV irradiation from 120 to 540 s; and (l) fluorescence
intensity (400 and 475 nm) of 1-naphthyl acetate in acetonitrile ([1-NA] = 2.4 mM) as a function of UV irradiation time.
of heterocyclic moieties to block excited-state intramolecular
proton transfer (ESIPT). The design of ESIPT systems with
UV-cleavable protecting groups has also been reported.30
However, in these systems, unreacted PAG as well as
photolytic/thermolytic byproducts can degrade patterning
resolution and impact upon long-term stability. To circumvent
the use of PAG and the need for postprocessing procedures
such as baking, other systems such as photodegradation-
triggered emission of poly(silylene-p-phenylene)31 and UV-
quenchable conjugated benzoxazole polymers with adjacent
hydroxyphenyl groups were developed.32 A remaining
challenge of the reported photoresponsive materials is the
hydrophobicity of the π-conjugated aromatic rings that renders
them insoluble in water and limits their application as
environment-friendly fluorescent films and coatings.
In the current study, we have developed a series of
copolymers of 1-naphthyl methacrylate and oligo(ethylene
glycol) methyl ether methacrylate [P(1-NMA-co-OEGMA)]
with both photoresponsive and thermal-responsive properties.
The photoactive 1-naphthyl methacrylate moieties can under-
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1
Figure 2. (a) Schematic of photo-Fries rearrangement of P(1-NMA83-co-OEGMA180) and H NMR spectra (DMSO-d6) of P(1-NMA83-co-
OEGMA180) after UV irradiation for varying times; (b) schematic of excited-state proton transfer (ESPT) and excited-state intramolecular proton
transfer (ESIPT) of photo-Fries rearrangement products of P(1-NMA83-co-OEGMA180); (c) UV−vis absorbance spectra (200−500 nm) of P(1-
NMA83-co-OEGMA180) in DI water (80 μg/mL) after UV irradiation for varying times (0−20 min); (d) UV−vis absorbance spectra (300−600
nm) of P(1-NMA83-co-OEGMA180) in DI water (1 mg/mL) after UV irradiation for varying times (0−20 min); (e) fluorescence intensity (λex
=
275 nm) of P(1-NMA83-co-OEGMA180) in DI water (2 mg/mL) after UV irradiation for varying times (0−20 min); (f) fluorescence intensity (λex
= 286 nm) of P(1-NMA83-co-OEGMA180) in DI water (2 mg/mL) after UV irradiation for varying times (0−20 min); (g) fluorescence intensity
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Figure 2. continued
(λex = 330 nm) of P(1-NMA83-co-OEGMA180) in DI water (2 mg/mL) after UV irradiation with varying times (0−20 min); (h) fluorescence
intensity (λex = 378 nm) of P(1-NMA83-co-OEGMA180) in DI water (2 mg/mL) after UV irradiation for varying times (0−20 min); (i) the
fluorescence intensity (338 and 475 nm, λex = 275 nm) of P(1-NMA83-co-OEGMA180) in DI water (2 mg/mL) as a function of UV irradiation time;
(j) the fluorescence intensity (338 and 475 nm, λex = 286 nm) of P(1-NMA83-co-OEGMA180) in DI water (2 mg/mL) as a function of UV
irradiation time; and (k) the fluorescence intensity (475 nm, λex = 330 nm and 378 nm) of P(1-NMA83-co-OEGMA180) in DI water (2 mg/mL) as
a function of UV irradiation time.
go the photo-Fries rearrangement on UV irradiation (254 nm)
to generate products with a new emission band at 475 nm.
Fluorescent patterns with gradients in intensity can be created
by tuning the UV exposure doses. The high photosensitivity of
the copolymers makes them compatible with conventional
photolithography techniques for fluorescence patterning. The
inclusion of the hydrophilic OEGMA monomer in the polymer
chains significantly enhances the aqueous solubility of the
copolymers as well as the flexibility and surface adhesion of
copolymer films. Solutions of the copolymers exhibit thermal
responsive properties, with an LSCT that can be tailored
through controlling the UV dose and 1-naphthyl methacrylate
content. Significantly, the fluorescence of the P(1-NMA-co-
OEGMA) hydrogels can be selectively ″switched off″ on
exclusion of water from the hydrogels above the volume phase
transition temperature. The emission from ESPT (excited-state
intramolecular proton transfer) decreases above the volume
phase transition, while the fluorescence excited by 365 nm UV
(ESIPT mechanism) remains unaffected. These versatile,
photo/thermal dual responsive copolymers with simple
photochemistry, effective photoswitching, low cost, and
excellent water solubility are extremely promising in the
applications of fluorescent patterning, data storage, and
counterfeit security.
1-naphthol is located at 295 nm (Figure 1c), the new peaks at
255 and 330 nm are assigned to the rearrangement product 4-
acetyl-1-naphthol (255 and 330 nm), consistent with the
results of Nakagaki et al.33 The fluorescence of the UV-
irradiated 1-naphthyl acetate solution at the corresponding UV
absorbance wavelength was also measured. Using excitation
wavelengths of 260 nm (Figure 1f) and 280 nm (Figure 1g),
the emission of 1-naphthyl acetate at 333 nm decreased with
UV irradiation time, suggesting a gradual decrease in the
concentration of 1-naphthyl acetate over time. Concomitantly,
a new fluorescence band maximum at 475 nm (λex = 370 nm)
increased in intensity with time, indicating the generation of 2-
acetyl-1-naphthol as a result of the photo-Fries rearrangement
(Figure 1h).40−43 The fluorescence intensity of 4-acetyl-1-
naphthol at 400 nm increased significantly and reached a
maximum at 120 s (Figure 1j) before decreasing gradually
(Figure 1k). This indicates that the 4-acetyl-1-naphthol
generated after rearrangement is not photostable, and its
photolysis may involve photocleavage of aromatic−aliphatic
ketones and hydrogen abstraction reactions.49 To quantita-
tively compare the change of fluorescence intensity with UV
irradiation time, peak deconvolution of the fluorescence
spectra was conducted. As shown in Figure 1l, both the
maximum fluorescence intensity and the area of the
fluorescence peak of 2-acetyl-1-naphthol increase with
irradiation time. However, the maximum intensity and the
area of 4-acetyl-1-naphthol against time show a different trend.
The peak intensity at 400 nm drops linearly after reaching the
maximum after UV irradiation for 120 s, while the area of the
peak remains constant. This observation indicates a red shift of
the fluorescence of 4-acetyl-1-naphthol after photolysis. It is
worth noting that the excitation and emission of the 1-
naphthol are not obvious due to overlap with the emission of
1-naphthyl acetate. The excitation and emission wavelengths of
1-naphthol (λex = 295 nm, λem = 340 nm) are close to those
of 1-naphthyl acetate (λex = 280 nm, λem = 333 nm). In
addition, it is reported that the yield of 1-naphthol after
photolysis of 1-naphthyl acetate in acetonitrile is significantly
lower than that of 2-acetyl-1-naphthol and 4-acetyl-1-naphthol
([2-acetyl-1-naphthol]/[4-acetyl-1-naphthol]/[1-naphthol] =
1:0.45:0.12).33 Similar results were also reported by Weiss et
al. who examined the distribution of photo-Fries rearrange-
ment products of 1-naphthyl esters in a range of different
solvents and reaction media.37,38,50
RESULTS AND DISCUSSION
■
Shift in Fluorescence of 1-Naphthyl Acetate after
Photo-Fries Rearrangement. The photo-Fries rearrange-
ment of 1-naphthyl acetate has been extensively explored,33−39
and it was found that the major rearrangement product, 2-
acetyl-1-naphthol, can undergo excited-state intramolecular
proton transfer (ESIPT) and emit strong fluorescence at 475
nm.40−43 In addition, the photocleavage product 1-naphthol
was reported to have a fluorescence band at 354 nm in a pure
alcohol solution.44−47 This fluorescence band decreased in
intensity upon the addition of water, and a new band appeared
at 480 nm that was attributed to the excited-state proton
transfer (ESPT) of 1-naphthol.44−47 To provide a more
complete understanding of the changes in the fluorescence of
the 1-naphthyl-containing polymers after UV irradiation,
especially in different solvents, we have reexamined the
photolysis of 1-naphthyl acetate.
It is well established that the photo-Fries rearrangement of
1-naphthyl acetate33−39 generates three photoproducts (2-
acetyl-1-naphthol, 4-acetyl-1-naphthol, and 1-naphthol) after
UV irradiation (Figure 1a). As shown in Figure 1b, 1-naphthyl
acetate has a strong UV absorbance band maximum at 280 nm
with a corresponding emitted fluorescence maximum at 333
nm. After UV irradiation, a new absorbance band that consists
of two peaks at 255 and 265 nm appeared, shown in Figure 1d,
while two weaker overlapping absorbance bands at 330 and
370 nm were also observed (Figure 1e). It was reported that 2-
acetyl-1-naphthol has UV−vis absorbance bands around 165
and 370 nm.40,48 Since the UV absorbance band maximum of
Shift in Fluorescence of P(1-NMA-co-OEGMA) follow-
ing Photo-Fries Rearrangement. The shift in the
fluorescence band of 1-naphthyl acetate after UV irradiation
suggests that polymers of 1-naphthyl methacrylate may be
suitable for fluorescence patterning. However, the poor water
solubility and high rigidity of poly(1-naphthyl methacrylate)
(Tg = 133 °C, Figure S2) limit its practical application as an
aqueous-compatible polymer. Copolymerization of 1-naphthyl
methacrylate with a polar, water-soluble monomer is expected
to significantly enhance the aqueous solubility of the polymers.
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Figure 3. (a) Schematic of fluorescent patterning of photoreactive polymer films on the surface of transparent substrates; (b) schematic of photo-
Fries rearrangement of P(1-NMA)83-co-OEGMA180) and fluorescent patterns (UV irradiation: 60 min) on the surface of glass and PET substrates
under different excitation wavelengths; and (c) schematic of photo-Fries rearrangement of P(1-NMA)186 and fluorescent patterns (UV irradiation:
60 min) on the surface of glass and PET substrates under different excitation wavelengths.
Therefore, copolymers with oligo(ethylene glycol) methyl
ether methacrylate (OEGMA; average Mn = 300) were
prepared with 65−85 wt % OEGMA (Figure S2), and it was
found that the copolymers were fully soluble in water. Aqueous
solutions of the copolymers were irradiated with UV light, and
their optical properties were examined. As shown in Figure 2c,
P(1-NMA83-co-OEGMA180) has a UV absorbance band
centered at 286 nm, which decreased in intensity after UV
irradiation. Meanwhile, two new peaks at 275 nm (Figure 2c)
and 378 nm (Figure 2d) from the ortho-site rearrangement
product became apparent. However, the UV absorbance bands
of the para-site product at ca. 255 and 330 nm were not
observed, suggesting that it was not formed after rearrange-
through excited-state proton transfer (ESPT) in an aqueous
solution.44−47 To understand the emission at 475 nm, 1-
naphthyl acetate and 1-naphthol were dissolved in different
cosolvent systems, and the fluorescence intensity of different
solutions was measured. As shown in Figure S1a, the
fluorescence intensity of 1-naphthyl acetate at 333 nm falls
when the water content in the cosolvent systems increases. A
decrease in fluorescence at 333 nm was also observed in the
UV-irradiated 1-naphthyl acetate solution, and a new emission
band at 475 nm appeared (Figure S1b). A similar trend is
shown in Figure S1c, in which a significant shift occurred in the
emission spectrum of 1-naphthol when the water content in
the solution was changed. The influence of water on the
fluorescence intensity of P(1-NMA83-co-OEGMA180) was
investigated as well. As shown in Figure S1d,f, the fluorescence
intensity of UV-irradiated P(1-NMA83-co-OEGMA180) at 475
nm increased with water content in the solution, suggesting the
important role of water in the excited-state proton transfer
(ESPT) of photo-Fries rearrangement products. The emission
at 475 nm can be excited via two different proton transfer
mechanisms (ESPT and ESIPT). In the ESPT process (Figure
2b), UV irradiation of 2-acetyl-1-naphthol moieties leads to a
1
ment. The H NMR (Figure 2a) and fluorescence spectra
(Figure 2g) further confirm that the ortho-site product is the
1
main rearrangement product. A broad peak in the H NMR
spectrum (DMSO-d6) at 14 ppm appeared after UV irradiation
and is characteristic of the hydroxyl proton of the ortho-site
product 2-acetyl 1-hydroxy naphthalene.51 In addition, an
emission peak at 400 nm was not observed under excitation at
330 nm. These findings are in good agreement with the results
reported by Iguchi et al. that the ortho-product is the main
rearrangement product when the photo-Fries reaction occurs
in a confined environment.52 Of note, the emission at 475 nm
increased in intensity, while the fluorescence at 338 nm
decreased, after UV irradiation, as shown in Figure 2e,f. These
results are in contrast to those of 1-naphthyl acetate, in which
the emission peak centered at 475 nm was not observed
(Figure 1f,g). It is noteworthy that the photolysis of 1-naphthyl
acetate was conducted in acetonitrile solution while that of
P(1-NMA83-co-OEGMA180) was performed in deionized water.
It was reported that 1-naphthol and its derivates are strong
photoacids and their fluorescence shifts from 333 to 475 nm
1
relatively nonpolar first excited-state Lb, which relaxes to the
second polar acidic excited-state 1La after solvation. The
removal of protons from 2-acetyl-1-naphthol moieties to
surrounding water molecules leads to ESPT.53,54 In contrast,
the fluorescence with a maximum at 475 nm (λex = 378 nm)
(Figure 2h) originates from the involvement of the excited-
state intramolecular proton transfer (ESIPT) process between
the enol and keto forms of 2-acetyl-1-naphthol moieties.40−43
Figure 2i,k shows that the yield of the rearrangement product
reached a maximum after exposure to UV irradiation for 10
min. Irradiation to longer times significantly increased the
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Figure 4. (a) Table of cloud point temperature (transmittance = 0.5) of different polymers; (b) LCST curves of different polymers at a polymer
concentration of 5 mg/mL; (c) shift of the LCST curve of the P(1-NMA47-co-OEGMA195) solution ([P(1-NMA47-co-OEGMA195)] = 5 mg/mL)
after exposure to UV irradiation for varying times; (d) shift of the LCST curve of the P(1-NMA83-co-OEGMA180) solution ([P(1-NMA83-co-
OEGMA180)] = 5 mg/mL) after exposure to UV irradiation for varying times; (e) shift of the LCST curve of the P(1-NMA83-co-OEGMA180
)
solution [P(1-NMA83-co-OEGMA180)] = 3.5 mg/mL) after exposure to UV irradiation for varying times; (f) cloud point temperature
(transmittance = 0.5) and solution transmittance above LCST of the P(1-NMA47-co-OEGMA195) solution ([P(1-NMA47-co-OEGMA195)] = 5 mg/
mL) as a function of UV irradiation time; (g) cloud point temperature (transmittance = 0.5) and solution transmittance above LCST of the P(1-
NMA83-co-OEGMA180) solution ([P(1-NMA83-co-OEGMA180)] = 5 mg/mL) as a function of UV irradiation time; and (h) cloud point
temperature (transmittance = 0.5) and solution transmittance above LCST of the P(1-NMA83-co-OEGMA180) solution ([P(1-NMA83-co-
OEGMA180)] = 3.5 mg/mL) as a function of UV irradiation time.
yield. Similar results were previously reported for the photo-
Fries rearrangement of poly(4-acetoxy styrene).55,56 However,
the rearrangement reaction became sluggish as the yield
increased, and a maximum yield of only 40−50% could be
achieved.
dry overnight at 40 °C before UV irradiation (Figure 3a). As a
control, films of P(1-NMA)186 homopolymer on glass and PET
were also prepared. As shown in Figure 3b, P(1-NMA83-co-
OEGMA180) formed excellent films on both substrates due to
the presence of the polar ethylene glycol segments and
nonpolar naphthyl methacrylate units. In contrast, the film of
P(1-NMA)186 showed poor adhesion to the glass surface,
reflecting the low affinity of the nonpolar naphthalene moieties
with the polar glass surface. Moreover, copolymerization of 1-
NMA with OEGMA not only improves the water solubility of
the copolymers but also promotes the flexibility of the cast
films. Compared to P(1-NMA83-co-OEGMA180), P(1-NMA)186
has higher rigidity due to the pendant naphthalene moieties
(Tg = 133 °C, Figure S2). As shown in Figure 3c, crazes were
observed on the surface of films of P(1-NMA)186 cast on the
PET sheet after bending. The mechanical properties of the
films can be simply tuned by controlling the content of
OEGMA in the copolymer chains; as shown in Figure S2, the
glass transition temperature (Tg) of the copolymers decreased
with increasing OEGMA content. After patterning through a
photomask, bright and clear fluorescent patterns were observed
Fluorescent Patterning Using P(1-NMA-co-OEGMA)
as Versatile Aqueous Patternable Copolymers. The
significant shift in the fluorescence maximum after photo-
Fries rearrangement makes the 1-naphthyl methacrylate
copolymers promising candidates for fluorescence patterning.
From the perspective of practical applications, a fluorescent
film or coating should have good photostability, high
patterning resolution, low toxicity, and high compatibility
with different surfaces and preferably be prepared by a green
and cost-effective process. To test the compatibility of P(1-
NMA83-co-OEGMA180) with different surfaces, glass slides
(hard and hydrophilic) and poly(ethylene terephthalate)
(PET) sheets (flexible and hydrophobic) were selected as
transparent substrates for fluorescence patterning. An aqueous
solution of P(1-NMA83-co-OEGMA180) was cast on the
substrate surface using a blade casting method and left to
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as shown in Figure 3b, consistent with the fluorescence spectra
of P(1-NMA83-co-OEGMA180). Films of a range of copolymers
with varying 1-NMA content were also illuminated to form
clear fluorescent patterns on the substrate, suggesting the
flexibility of the P(1-NMA-co-OEGMA) as an aqueous-soluble
fluorescent copolymer. Since the photo-Fries rearrangement
reaction is usually triggered by a UV light source, these P(1-
NMA-co-OEGMA) films can be used for fluorescence
patterning using UV photolithography. In addition, P(1-
NMA-co-OEGMA) has a UV absorbance band maximum at
230 nm (Figure 2c), suggesting that these materials are not
prone to photobleaching in practical applications.
dose (Figure 4,gf). A more pronounced change in trans-
mittance above the LCST after UV irradiation was observed
when the concentration of P(1-NMA83-co-OEGMA180) was
decreased from 5 to 3.5 mg/mL, suggesting the important role
of polymer concentration on the transmittance of the
aggregates formed above the LCST (Figure 4e,h). To
understand the effect of polymer concentration on the optical
properties of the polymer solution above the cloud point
temperature, the LCST curves of POEGMA460, P(1-NMA47-
co-OEGMA195), and P(1-NMA83-co-OEGMA180) at varying
concentrations were measured (Figure S4a−c). The relation-
ship between transmittance above the LCST and polymer
concentration in Figure S4d shows that POEGMA460 and P(1-
NMA47-co-OEGMA195) solutions turn opaque once the
polymer concentration is above 1 mg/mL, while the solution
of P(1-NMA83-co-OEGMA180) was translucent for concen-
trations up to 5 mg/mL. This behavior is suggested to be due
to the formation of copolymer aggregates during the heating
process. The formation of aggregates significantly increases the
solubility of the copolymers in the aqueous solution. These
results are in excellent agreement with the DLS results, which
also confirm the existence of large aggregates in the solution of
P(1-NMA83-co-OEGMA180).
Photo/Thermal Dual Responsive Behavior of P(1-
NMA-co-OEGMA) Hydrogels. The strong correlation
between fluorescence intensity and UV irradiation dose
(Figure 2) suggests that P(1-NMA-co-OEGMA) copolymers
are capable of forming fluorescent gradient patterns. As
demonstrated in Figure 5a, patterns with varying fluorescent
intensities can be created on the surface of films or hydrogels
by exposing in turn the individual letters within the photomask
to different UV irradiation doses. Hydrogels (wt % of 1-NMA
around 15%) were prepared by conducting the polymerization
LCST of Aqueous Solutions of P(1-NMA-co-OEGMA)
after UV Irradiation. The copolymerization of 1-NMA with
OEGMA not only increases the water solubility and flexibility
of the copolymers but also affects their solution properties. As
shown in Figure 4a,b, the homopolymer POEGMA460 has a
cloud point temperature in water (transmittance = 0.5) at 61
°C. The copolymerization with 1-naphthyl methacrylate
increases the hydrophobicity of copolymers and decreases
the corresponding LCST. As expected, the cloud point
temperature of the copolymers falls with an increase in the
1-NMA content in the copolymer chains. For example, the
cloud point temperature decreases to 36 from 61 °C when the
1-naphthyl methacrylate content increased from 0 to 15%. A
further decrease of LCST from 36 to 26 °C was observed when
the 1-naphthyl methacrylate content of copolymer was
increased from 15 to 25%. Note that the LCST curves in
Figure 4b were recorded during cooling due to the
condensation of moisture on the surface of the cuvette during
heating from low temperatures (Figure S3a,b). The solution of
P(1-NMA83-co-OEGMA107) remained partly cloudy at 5 °C
after the heating−cooling cycle, suggesting the formation of
aggregates during the LCST measurement. DLS results further
confirmed the presence of aggregates in the solution of P(1-
NMA83-co-OEGMA107). As shown in Figure S3d,e, the size of
single-chain particles (unimers) of POEGMA460 was around 13
nm. The copolymer with 15 wt % 1-NMA also formed unimers
in the solution (size = 13 nm). When the 1-NMA content was
increased to 25 wt %, aggregates with a size of around 38 nm
were observed in the solution of P(1-NMA83-co-OEGMA180),
while the size of aggregates increased to 80 nm when the 1-
NMA content was raised to 35 wt %. The increase in 1-NMA
content not only significantly increases the size of the polymer
aggregates but also prevents dissociation of the aggregates on
cooling below the LCST presumably due to enhanced π−π
stacking interactions between the pendant naphthalene
moieties.
The involvement of photoactive 1-naphthyl methacrylate
moieties in the polymer chains provides the possibility of
controlling the LCST of copolymers by irradiation with UV
light. As previously demonstrated by us for hydrogels of
OEGMA and the photosensitive monomer 4-acetoxystyrene,
the photo-Fries rearrangement leads to the formation of photo-
products significantly more polar than the original aromatic
ester unit and therefore to an increase in the volume phase
transition temperature.57 A similar increase in polarity and
LCST is expected for the 1-NMA copolymers on irradiation.
As shown in Figure 4,dc, the cloud point temperatures
(transmittance = 0.5) of both P(1-NMA47-co-OEGMA195) and
P(1-NMA83-co-OEGMA180) increased steadily with UV
irradiation time. In addition, the optical clarity (transmittance)
of the solution above the LCST increased with UV irradiation
Figure 5. (a) Schematic of fluorescent patterning with gradient
through varying the UV irradiation dose; (b) schematic of varying the
fluorescence intensity with temperature; and (c) change in fluorescent
pattern on the hydrogel surface at different temperatures and
excitation wavelengths.
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Sigmacote were purchased from Sigma-Aldrich and used as received.
1-Naphthol (99%) and pyridine (99%) were purchased from Merck
and Fluka, respectively. 2,2-Azobis(isobutyronitrile) (AIBN; 98%,
Fluka) was recrystallized from methanol for purification.
in dioxane in the presence of the cross-linking monomer
ethylene glycol dimethacrylate (EGDMA). Since UV irradi-
ation significantly raises the LCST of P(1-NMA-co-OEGMA)
in an aqueous solution, the volume phase transition temper-
ature (VPTT) of the UV-irradiated P(1-NMA-co-OEGMA)
hydrogel is assumed to also increase.57 Since water plays a
crucial role in the ESPT process, above the VPTT, significant
quenching of ESPT fluorescence is expected on dehydration of
the hydrogel (Figure 5b). In contrast, the fluorescence from
ESIPT is expected to remain less affected because the ESIPT
fluorescence of 2-acetyl-1-naphthol is reported to be
significantly stronger in hydrophobic microenvironments
than in water.58 Figure 5c demonstrates the change in
fluorescence at 475 nm across the volume phase transition of
the hydrogel. As expected, the fluorescence excited by 365 nm
light (ESIPT mechanism) did not change significantly during
the volume phase transition process of hydrogel, while the
emission from ESPT diminished progressively with an increase
in temperature (Figure S5). In addition, quenching of the
ESPT was observed to commence in regions of the hydrogel
with low exposure dose due to the increase of the VPTT.
These results demonstrate the potential for P(1-NMA-co-
OEGMA) to create programmed gradient fluorescent patterns
and for temperature to be used to tune the fluorescence
intensity of the patterns.
Synthesis of 1-Naphthyl Acetate (1-NA). 1-Naphthol (1.44 g,
10 mmol) and triethylamine (1.32 g, 13 mmol) were dissolved in 50
mL of THF and placed in an ice bath. Acetyl chloride (0.940 g, 12
mmol) was then added dropwise into the mixture solution. After
stirring at room temperature for 24 h, the generated salts were filtered,
and the filtrate was passed through a short basic aluminum oxide
column. The product was collected as colorless crystals after the
1
removal of solvents (yield ∼80%). H NMR (400 MHz, chloroform-
d) δ 7.84−7.76 (m, 2H), 7.68 (d, J = 8.3 Hz, 1H), 7.50−7.40 (m,
2H), 7.40 (t, J = 7.9 Hz, 1H), 7.21−7.15 (m, 1H), 2.40 (s, 3H).
Synthesis of 1-Naphthyl Methacrylate (1-NMA). Into an ice-
cold solution of 1-naphthol (7.21 g, 0.05 mol), 4-dimethylaminopyr-
idine (1.22 g, 0.01 mol), and pyridine (4.75 g, 0.06 mol) in anhydrous
DCM (300 mL), methacrylic anhydride (9.25 g, 0.06 mol) was added
in a dropwise manner. After stirring at room temperature for 48 h, the
solution was poured into 300 mL of ice-cold saturated NaHCO3 and
separated in a separation funnel. The aqueous layer was thrice
extracted with DCM before combining and drying using anhydrous
MgSO4. The extraction was further concentrated under reduced
pressure rotary evaporation. The product was further purified by flash
column chromatography with hexane/ethyl acetate (3/1, v/v),
yielding white crystals after the removal of solvents (yield ∼85%).
1H NMR (400 MHz, DMSO-d6) δ 8.04−7.97 (m, 1H), 7.91−7.79
(m, 2H), 7.61−7.51 (m, 3H), 7.36 (dd, J = 7.5, 1.1 Hz, 1H), 6.46 (p, J
= 1.0 Hz, 1H), 5.98 (t, J = 1.5 Hz, 1H), 2.13−2.07 (m, 3H).
Synthesis of Poly(1-NMA-co-OEGMA) via RAFT Polymer-
ization. The copolymerizations of 1-NMA and OEGMA monomers
with different monomer feed ratios were conducted. For example, in a
5 mL glass tube equipped with a rubber stopper, 1-naphthyl
methacrylate (1-NMA) (212 mg, 1 mmol), OEGMA (1.5 g, 5
mmol), CPDB (4.42 mg, 0.02 mmol), and AIBN (0.66 mg, 4 × 10−3
mmol) were dissolved in 2 mL of 1,4-dioxane before deoxygenating
with argon gas for 20 min. The polymerization was conducted at 70
°C for 24 h and terminated by inserting the tube into iced water for
10 min. The polymer solution was then poured into an excess of
diethyl ether for precipitation. The precipitate was redissolved in THF
and precipitated in diethyl ether twice. The precipitate was dried
under vacuum at 40 °C overnight, and a pink viscous oil-like liquid
was obtained.
Synthesis of Poly(1-NMA-co-OEGMA) Hydrogel. Inside a 20
mL glass vial that was pretreated with Sigmacote, 1-NMA (300 mg,
1.4 mmol), OEGMA (1.06 g, 3.5 mmol), EGDMA (0.032 mg, 1.6 ×
10−4 mmol), and AIBN (0.54 mg, 3.3 × 10−3 mmol) were dissolved in
2 mL of 1,4-dioxane before deoxygenating with argon gas for 20 min.
The polymerization was conducted at 70 °C for 24 h. The hydrogel
product was removed from the glass vial and dialyzed against DI water
for 1 week to remove dioxane and the unreacted monomer.
Preparation of Copolymer Films. Poly(1-NMA-co-OEGMA)
was dissolved in DI water at a concentration of 5 mg mL−1. The
copolymer solutions were cast onto the surface of substrates (PET or
glass) using a blade. The wet films were left at room atmosphere for
drying for 24 h.
Patterning with Photomask. The patterning of films was
performed using an unfiltered high-pressure Xenon UV lamp. The
light intensity (integrated power density) of 10 mW cm−2 at 254 nm
was measured by a Hamamatsu spectroradiometer. Patterned
structures were obtained by placing a photomask (Cr pattern on
quartz) directly onto the polymer film prior to UV exposure.
UV Irradiation Studies. UV irradiation studies were performed
using an unfiltered high-pressure UV lamp. The light intensity
(integrated power density) of 10 mW cm−2 at 254 nm was measured
by a spectroradiometer. 1-NA and poly(1-NMA-co-OEGMA) were
dissolved separately in acetonitrile and DI water and transferred to a
quartz cuvette (1 × 1 cm) for UV irradiation.
CONCLUSIONS
■
In this work, we prepared a series of P(1-NMA-co-OEGMA)
copolymers and studied in detail the changes in fluorescence
properties after illumination with UV light and consequent
photo-Fries rearrangement reactions. Photolysis of the model
compound 1-naphthyl acetate demonstrated that the main
photo-Fries rearrangement products exhibit new fluorescence
emissions centered at 475 nm (the ortho-site product) and 400
nm (the para-site product). However, the ortho-site product
was predominately formed during irradiation of P(1-NMA-co-
OEGMA) copolymers in an aqueous solution. It was found
that the fluorescence of the copolymers leading to emission at
475 nm was excited through two different mechanisms: ESPT
and ESIPT. Compared to its homopolymer, P(1-NMA-co-
OEGMA) exhibits good water solubility, excellent compati-
bility with different surfaces, as well as high flexibility, making it
a promising aqueous-soluble copolymer for spatial fluorescence
patterning. The photo-Fries rearrangement of P(1-NMA-co-
OEGMA) also significantly affects the behavior of the
copolymer in an aqueous solution. The change in polarity of
the products formed after rearrangement leads to an increase
in the LCST of the copolymers in the solution. Programmed
gradient fluorescent patterns were demonstrated in the P(1-
NMA-co-OEGMA) hydrogels. Changes in temperature could
be used to control the fluorescence intensity of the patterns.
These results indicate that P(1-NMA-co-OEGMA) is a
versatile aqueous-soluble copolymer with potential for
applications such as fluorescent patterning, data storage, and
counterfeit security.
EXPERIMENTAL SECTION
■
Materials. Acetyl chloride (98%), triethylamine (99%), tetrahy-
drofuran (anhydrous, 99%), dichloromethane (DCM; 99%), meth-
acrylic anhydride (94%), 4-dimethylaminopyridine (DMAP; 98%),
1,4-dioxane (99.8%), 2-cyano-2-propyl benzodithioate (CPDB; 97%),
oligo(ethylene glycol) methyl ether methacrylate (OEGMA; average
MW = 300), ethylene glycol dimethacrylate (EGDMA; 98%), and
Nuclear Magnetic Resonance (NMR). NMR spectra were
collected on a Bruker Avance 400 MHz spectrometer at 298 K
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using DMSO-d6 or CDCl3 as solvents. All chemical shifts are reported
in ppm (δ) relative to tetramethylsilane (TMS).
UV−Vis Spectra. The UV−vis absorbance of UV-irradiated 1-NA
and poly(1-NMA-co-OEGMA) was measured using a Shimadzu UV-
2450 UV−vis spectrophotometer (Shimadzu Scientific Instruments,
Columbia, MD). The absorbance spectra from 200 to 800 nm were
recorded.
Convergent Bio-Nano Science and Technology, The
University of Queensland, Brisbane, QLD 4072, Australia
Sisi Wang − Australian Institute for Bioengineering and
Nanotechnology and ARC Centre of Excellence in Convergent
Bio-Nano Science and Technology, The University of
Queensland, Brisbane, QLD 4072, Australia
Changkui Fu − Australian Institute for Bioengineering and
Nanotechnology and ARC Centre of Excellence in Convergent
Bio-Nano Science and Technology, The University of
Queensland, Brisbane, QLD 4072, Australia; orcid.org/
0000-0002-2444-607X
Joshua A. Kaitz − DuPont Electronics & Imaging,
Marlborough, Massachusetts 01752, United States
James F. Cameron − DuPont Electronics & Imaging,
Marlborough, Massachusetts 01752, United States
Peter Trefonas, III − DuPont Electronics & Imaging,
Marlborough, Massachusetts 01752, United States
Idriss Blakey − Australian Institute for Bioengineering and
Nanotechnology, The University of Queensland, Brisbane,
QLD 4072, Australia; orcid.org/0000-0003-2389-6156
Fluorescence Spectra. The fluorescence spectra of UV-irradiated
1-NA and poly(1-NMA-co-OEGMA) were recorded with a Perkin
Elmer LS55 luminescence spectrometer.
Differential Scanning Calorimetry (DSC). The glass transition
temperature (Tg) was obtained from DSC using a Mettler Toledo
DSC1 STARe system calorimeter. For the measurement, samples of
4−7 mg were heated from −100 to 150 °C in 10 mL min−1 of
nitrogen atmosphere, with heating and cooling rates of 10 °C min−1.
The thermal properties were determined from the second heating
cycle.
Cloud Point Temperature Measurement. The cloud point
temperatures of poly(1-NMA-co-OEGMA) aqueous solutions were
determined using a Varian UV−vis Cary 4000 spectrophotometer
fitted with a Cary temperature controller. A total of 4 mL of the
polymer solution was placed in a cuvette, and the visible absorption
was recorded at a wavelength of 500 nm over a temperature range
from 5 to 70 °C. The temperature was increased at a stable rate of 1
°C/min.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.macromol.1c00297
Notes
ASSOCIATED CONTENT
■
The authors declare no competing financial interest.
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.macromol.1c00297.
ACKNOWLEDGMENTS
■
This work was performed in part at the Queensland node of
the Australian National Fabrication Facility (ANFF) at the
University of Queensland node of the Australian Microscopy
and Microanalysis Research Facility (AMMRF) within the
Centre of Microscopy and Microanalysis (CMM). Financial
support from the Australian Research Council (LE110100028,
LE110100033, LP120100737, DP130103774, LE140100087,
CE140100036, and DP180101221) is also gratefully acknowl-
edged.
Additional fluorescence spectra of 1-naphthyl acetate
and P(1-NMA83-co-OEGMA180) in different solvents;
DSC, GPC, DLS, and LCST profiles of P(1-NMA-co-
OEGMA) copolymers; and quantitative analysis of the
change of hydrogel pattern contrast with temperature
(PDF)
AUTHOR INFORMATION
■
Corresponding Authors
REFERENCES
Hui Peng − Australian Institute for Bioengineering and
Nanotechnology and ARC Centre of Excellence in Convergent
Bio-Nano Science and Technology, The University of
Queensland, Brisbane, QLD 4072, Australia;
Email: h.peng@uq.edu.au
Andrew K. Whittaker − Australian Institute for
Bioengineering and Nanotechnology and ARC Centre of
Excellence in Convergent Bio-Nano Science and Technology,
The University of Queensland, Brisbane, QLD 4072,
Australia; orcid.org/0000-0002-1948-8355;
Email: a.whittaker@uq.edu.au
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https://doi.org/10.1021/acs.macromol.1c00297
Macromolecules 2021, 54, 4860−4870