Temperature-controlled changeable oxygenation selectivity by singlet oxygen with a polymeric photosensitizer

Article abstract of DOI:10.1039/b617718b

A polymeric photosensitizer, poly(NIPAM-co-RB), consisting of N-isopropylacrylamide and rose bengal units, demonstrates a temperature- controlled changeable oxygenation selectivity by singlet oxygen in water. The Royal Society of Chemistry.

Full text of DOI:10.1039/b617718b

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Temperature-controlled changeable oxygenation selectivity by singlet
oxygen with a polymeric photosensitizer{
Hisao Koizumi, Yumi Kimata, Yasuhiro Shiraishi* and Takayuki Hirai
Received (in Cambridge, UK) 5th December 2006, Accepted 22nd January 2007
First published as an Advance Article on the web 12th February 2007
DOI: 10.1039/b617718b
A polymeric photosensitizer, poly(NIPAM-co-RB), consisting
of N-isopropylacrylamide and rose bengal units, demonstrates
a temperature-controlled changeable oxygenation selectivity by
singlet oxygen in water.
Development of photoreaction systems promoting selective
oxidation of molecules with molecular oxygen (O2) is one of the
most important subjects in photochemistry.1 Substrate- and
product-selective oxygenations have been achieved using several
organized and constrained media (zeolites, polymers, vesicles, and
microemulsions) as ‘‘microreactors’’.2 These host systems provide
cavities and/or surfaces to alter the photochemical behavior of the
molecules leading to selective oxygenation. Most of these systems,
however, have unique selectivity; in other words, the selectivity is
basically not changeable. Some systems can change the selectivity
by an addition of a third component;3 however, this results in an
Fig. 1 (A) Structure of poly(NIPAM-co-RB), where each unit is
1
randomly arranged along the chain. (B) Sensitized O2 oxygenation.
irreversible change. A photoreaction system, capable of changing
photoirradiation (l . 530 nm) to an O2-saturated aqueous
solution (pH 7) containing 1a and 2a (each 10 mmol) with
poly(NIPAM-co-RB) (0.58 mg containing 0.01 mmol RB and
5.01 mmol NIPAM units).{
the oxygenation selectivity reversibly by simple external stimuli
without contaminating the system, had not been proposed.
Our photoreaction system presented here promotes a substrate-
selective oxygenation in water, whose selectivity is reversibly
controlled by temperature as the external stimulus. We use a
polymeric photosensitizer, poly(NIPAM-co-RB), consisting of
N-isopropylacrylamide (NIPAM)4 and rose bengal (RB)5 units
as the thermosensitive and photosensitizing parts (Fig. 1A). In
that, the photoexcited RB produces a singlet oxygen (1O2),6 an
oxygenation agent, via an energy transfer to O2 (Fig. 1B). To the
best of our knowledge, this is the first reaction system capable of
changing the oxygenation selectivity by temperature. It is well-
known that polyNIPAM in water shows a reversible coil-to-
globule phase transition, associated with hydration/dehydration of
the polymer chain by temperature.4 Here we describe that this
changeable 1O2 oxygenation selectivity is driven by a hydrophobic
microenvironment formed inside the polymer behaving as an
intelligent microreactor, which promotes a temperature-controlled
selective encapsulation/elimination of substrates.
Fig. 2A shows temperature-dependent change in the quinone
yields obtained by photoirradiation of 1a and 2a together. With
RB§ (0.01 mmol) as the sensitizer (white symbols), both 1b and 2b
yields increase slightly with a rise in temperature, as is usually
1
observed for O2 oxygenation.7 This is because the temperature
increase accelerates the diffusion of RB, O2, and substrates in
solution, resulting in an enhancement of 1O2 formation and
substrate oxygenation. As summarized in Fig. 2B (white bar), 2b
selectivity [=[2b]/([1b] + [2b]) 6 100 (%)] obtained with RB is
Poly(NIPAMx-co-RBy) (x/y = 501/1) was easily synthesized by
copolymerization of NIPAM and vinylbenzyl chloride followed by
reaction with RB.{,{ Oxygenation selectivity was estimated with a
competitive transformation of phenol (1a) and 1-naphthol (2a) to
1
the corresponding quinones (1b and 2b; Fig. 1B), a typical O2
oxygenation reaction.6 The reaction was carried out by
Fig. 2 Temperature-dependent changes in (A) the yields of (circle) 1b
and (triangle) 2b and (B) the 2b selectivity [=[2b]/([1b] + [2b]) 6 100 (%)],
obtained by photoirradiation (0.5 h) of 1a and 2a together (each 10 mmol)
in O2-saturated aqueous solution (pH 7). The systems are: (white) with
0.01 mmol RB, (black) with 0.58 mg poly(NIPAM-co-RB), and (grey) with
0.01 mmol RB and 0.58 mg polyNIPAM (RB-free).
Research Center for Solar Energy Chemistry, and Division of Chemical
Engineering, Graduate School of Engineering Science, Osaka University,
Toyonaka, 560-8531, Japan. E-mail: shiraish@cheng.es.osaka-u.ac.jp;
Fax: +81-6-6850-6273; Tel: +81-6-6850-6271
{ Electronic supplementary information (ESI) available: Materials,
methods and figures S1–S7. See DOI: 10.1039/b617718b
1846 | Chem. Commun., 2007, 1846–1848
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almost constant (ca. 70%) at the entire temperature range,
indicating that the substrate selectivity is not changeable.
As shown in Fig. 2A (black triangle), with poly(NIPAM-co-
RB), 2b yield at 5–15 uC is similar to that obtained with RB (white
triangle). The 2b yield, however, increases drastically at 15–25 uC
and decreases at .25 uC. In contrast, 1b yield (black circle) at
5–25 uC is similar to that obtained with RB (white circle), but
decreases at .25 uC, resulting in zero yield at 35 uC.8 As shown in
Fig. 2B (black bar), 2b selectivity at 5–15 uC (ca. 70%) is similar to
that obtained with RB (white bar), but increases gradually at
.15 uC: the value reaches 97% at 35 uC. These indicate that the
Fig. 4 Change in turbidity (A650) and hydrodynamic radius (Rh) of
aqueous solution (pH 7) containing poly(NIPAM-co-RB) with tempera-
ture. (Inset) Change in absorption spectra of the polymer solution. For
detailed Rh data see Fig. S3.{
1
polymer changes the O2 oxygenation selectivity by temperature
and oxidizes 2a very selectively at higher temperature.
This changeable 1O2 oxygenation selectivity of poly(NIPAM-co-
RB) is driven by a heat-induced phase transition of the polymer
from coil to micelle, and then to globule state (Fig. 3). The micelle
contains a loosely-aggregated hydrophobic domain, while the
globule contains a strongly-aggregated hydrophobic core. Fig. 4
(white symbol) shows temperature-dependent change in turbidity
(A650) of aqueous solution containing poly(NIPAM-co-RB); an
obvious turbidity increase at .30 uC implies that almost all of the
polymer aggregates strongly at .30 uC (globule formation).9 As
shown in Fig. 4 (black symbol), dynamic light scattering (DLS)
analysis of the polymer solution detects a scattered light at .25 uC
(detection limit, 3 nm), indicating a formation of the hydrophobic
core.10 This implies that strong polymer aggregation occurs
Fig. 5 (A) 1H NMR spectra of 2a in D2O measured at (i) 35 uC without
polymer and measured at (ii) 35, (iii) 23, and (iv) 5 uC with poly(NIPAM-
co-RB). (B) Temperature-dependent change in integrated intensity of
aromatic CH resonance of (black circle) 1a measured with poly(NIPAM-
co-RB), (black triangle) 2a measured with poly(NIPAM-co-RB), and
(open square) RB measured with RB-free polyNIPAM. The intensities of
1a, 2a, and RB measured at 35 uC without polymer are set as 1. The
1
partially at .25 uC (Fig. 3). H NMR analysis of the polymer
dissolved in D2O (Fig. S2{) shows a gradual decrease in the
integrated intensity of CH resonance for the polymer chain and
the NIPAM unit at 5–25 uC. This indicates that, as described,11 the
hydrophobic domain forms within the polymer at 5–25 uC (micelle
formation) and the domain becomes more hydrophobic with a rise
in temperature. In contrast, at .25 uC, the signal intensity
decreases drastically (Fig. S2{). This is due to a removal of D2O
associated with the strong polymer aggregation (core formation).10
The temperature-dependent change in the structure of the polymer
can therefore be summarized in Fig. 3.
1
respective H NMR spectra for 1a and RB are shown in Fig. S4.{
integrated intensity of the aromatic CH resonance for 1a or 2a,
where the intensity of 1a or 2a measured at 35 uC without
the polymer is set as 1. The intensities of both 1a and 2a are ,1 at
5–15 uC, meaning that a part of 1a and 2a exists within the
hydrophobic domain of the polymer. At 15–23 uC, decrease in the
2a intensity is much larger than that of the 1a intensity, implying
that 2a is encapsulated more within the domain. This is because of
higher hydrophobicity of 2a due to its condensed aromatic rings.12
The selective accumulation of 2a within the domain accelerates
The 2b yield increase at 15–25 uC (Fig. 2A, black triangle) is
driven by the heat-induced growth of the hydrophobic domain,
which enhances the 2a uptake into the polymer. This is confirmed
by 1H NMR analysis of 1a or 2a in the presence of poly(NIPAM-
co-RB). Fig. 5A shows change in the spectra of 2a measured with
the polymer for instance. Fig. 5B summarizes change in the
1
oxygenation by O2 formed within the domain, thus resulting in
the 2b yield increase at 15–25 uC (Fig. 2A, black triangle).
At .25 uC (Fig. 2A, black symbols), both 1b and 2b yields
obtained with poly(NIPAM-co-RB) decrease as the temperature
rises. At 35 uC, the 1b yield is almost zero, although 2b yield is still
higher than that obtained with RB. The 2b selectivity at 35 uC is
97%, indicating that 2a oxygenation proceeds very selectively. This
high selectivity is explained by the heat-induced phase transition of
the polymer from micelle to globule containing a hydrophobic
core. As shown in Fig. 5B, the integrated aromatic proton intensity
of 1a and 2a measured with poly(NIPAM-co-RB) increases at
.25 uC, which is in accordance with the formation of the hydro-
phobic core (Fig. 4). This intensity increase is because the strong
polymer aggregation (core formation) expels 1a and 2a from the
polymer. The intensity of 1a at 35 uC becomes 1, indicating that
almost all of 1a exists in bulk water. At this temperature, 1a
oxygenation by 1O2 must occur in bulk water. Within the
Fig. 3 Schematic representation of temperature-dependent changes in
structure of poly(NIPAM-co-RB) and in behavior of substrates.
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Chem. Commun., 2007, 1846–1848 | 1847

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strongly-aggregated core, O2 can diffuse and form 1O2.13 However,
diffusion of 1O2 to bulk water is strictly limited by the rigid core.14
As shown in Fig. 4, the size of the core increases exponentially as
the temperature rises, indicating that 1O2 diffusion to bulk water is
restricted more. These suggest that 1b yield decreases at .25 uC
(Fig. 2A, black circle) because 1a is expelled from the polymer and
1
the diffusion of O2 to bulk water is limited.
In contrast, as shown in Fig. 2A (black triangle), the 2b yield
decreases at .25 uC, but the yield at 35 uC is still higher than that
obtained with RB. As shown in Fig. 5B, the integrated proton
intensity of 2a obtained with the polymer increases at .25 uC, as is
also the case for 1a, but is saturated at around 0.65. This means
that 2a still exists within the core even at 35 uC, although 1a is
expelled completely from the core. This is due to the higher
hydrophobicity of 2a.12 The selective 2a encapsulation to the core
therefore promotes selective oxygenation of 2a at 35 uC (Fig. 2B,
black bar). These indicate that the hydrophobic microenvironment
formed within the polymer determine the hydrophobicity differ-
ence between 1a and 2a, thus triggering the temperature-controlled
changeable oxygenation selectivity.
Fig. 6 Changes (A) in the yields of (circle) 1b and (triangle) 2b and (B) 2b
selectivity with time in poly(NIPAM-co-RB) system, where the reaction
temperature is changed after each 10 min (5 A 35 A 5 uC). The data
shown by white symbols and bars (run (a)) were obtained using the virgin
polymer. The data shown by black symbols and bars were obtained using
the polymer recovered after run (a). The recovery process is: heat the
sample to 40 uC, followed by centrifugation (5 min, 2 6 104 rpm).
This work was supported by the Grant-in-Aid for Scientific
Research (No. 15360430) and that on Priority Area (417; No.
17029037) from the Ministry of Education, Culture, Sports,
Science and Technology, Japan (MEXT).
It is notable that the copolymerization of the RB units with
NIPAM units is necessary for onset of the changeable oxygenation
selectivity. Fig. 2A (gray symbols) shows 1b and 2b yields obtained
by photoirradiation of 1a, 2a, and RB, together with RB-free
polyNIPAM." The 2b yield (gray triangle) increases at 15–25 uC,
as is also the case with poly(NIPAM-co-RB) (black triangle), but
the rate of increase is much smaller. As shown in Fig. 5B (white
square), integrated proton intensity of RB measured with
polyNIPAM decreases only slightly at 15–25 uC. This means that
RB is scarcely encapsulated within the hydrophobic domain of
polyNIPAM. The absence of RB within the domain cancels the 2a
accumulation, resulting in smaller 2b yield increase at 15–25 uC. As
shown in Fig. 2A (gray circle), 1b yield is similar to that obtained
with RB (white circle) at the entire temperature range. The 1a
oxygenation still occurs at 35 uC, although poly(NIPAM-co-RB)
suppresses completely (black circle). As shown in Fig. 5B, at 35 uC,
almost all of RB exists in bulk solution as well as 1a, thus allowing
Notes and references
{ Poly(NIPAM-co-RB): Mn = 55000; Mw/Mn = 2.6; ES (ET) = 211
(170) kJ mol21; Wfl (298 K) = 0.063; Wphos (77 K) = 0.0017; TLCST = 30 uC.
§ RB: ES (ET) = 211 (170) kJ mol21; Wfl (298 K) = 0.042; Wphos (77 K) =
0.0007.
" RB-free polyNIPAM: Mn = 35000; TLCST = 31 uC (Fig. S5 and S6{).
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with polyNIPAM system does not show temperature-controlled
changeable selectivity (Fig. 2B, gray bar).
Fig. 6 shows time-course variation in the 1b and 2b yields and
the 2b selectivity obtained with poly(NIPAM-co-RB), where the
reaction temperature is changed after each 10 min (5 A 35 A
5 uC). The data (white symbols and bars) clearly show that the
polymer can control the selectivity reversibly by temperature.
Another notable feature of the polymer is the high reusability with
a simple recovery process: heating the reaction mixture to 40 uC
followed by centrifugation (5 min, 2 6 104 rpm) affords .98%
polymer recovery, and the recovered polymer (black symbols and
bars) shows the same activity as does the virgin polymer.
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12 The log P values for 1a and 2a, calculated by ChemDraw Ultra
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In summary, we found that
a
polymeric sensitizer,
poly(NIPAM-co-RB), reversibly controls the 1O2 oxygenation
selectivity by temperature. This unprecedented photosensitizing
activity is driven by a heat-induced self-assembly of the polymer,
which promotes selective encapsulation/elimination of substrates.
The results presented here may contribute to the development of
selective photooxygenation processes and to the design of more
efficient photosensitizing materials.
1848 | Chem. Commun., 2007, 1846–1848
This journal is ß The Royal Society of Chemistry 2007