Photo-reversible Pb2+-complexation of thermosensitive poly(N-isopropyl acrylamide-co-spiropyran acrylate) in water

Article abstract of DOI:10.1039/b407342h

Photo-reversible Pb²⁺-complexation in a purely aqueous environment exhibiting a thermosensitive phase transition at a lower critical solution temperature was achieved using a synthetic copolymer composed of N-isopropyl acrylamide and spiropyran acrylate.

Full text of DOI:10.1039/b407342h

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21
Photo-reversible Pb -complexation of thermosensitive
poly(N-isopropyl acrylamide-co-spiropyran acrylate) in water
Takayuki Suzuki,* Tatsuya Kato and Hiraku Shinozaki
Department of Environmental Materials of Science, Faculty of Engineering, Tokyo Denki University,
2-1200 Muzai-gakuendai, Inzai, Chiba, 270-1382, Japan. E-mail: tsuzu@chiba.dendai.ac.jp;
Fax: 181(476)46-8735; Tel: 181(476)46-8735
Received (in Cambridge, UK) 14th May 2004, Accepted 16th July 2004
First published as an Advance Article on the web 13th August 2004
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1
Photo-reversible Pb -complexation in a purely aqueous
environment exhibiting a thermosensitive phase transition at a
lower critical solution temperature was achieved using a
synthetic copolymer composed of N-isopropyl acrylamide
and spiropyran acrylate.
60 uC with 2,2’-azobis(isobutyronitrile), affording I, (Mw ~ 7.4 6
10 , Mw/Mn ~ 2.7). The ratio of SPAA units in I was 2 mol%, as
4
1
determined by H NMR measurements.
An aqueous solution of I was prepared by adding I to
neutralized water (pH ~ 7.0) at 5 uC, below the LCST (29 uC,
mentioned below). The absorption spectrum of the purplish-red
solution shows a strong band at 533 nm (Fig. 1(A)), attributed to
the open form of the SPAA unit. This band is photobleached by
irradiating with visible light (w420 nm), but reappears on standing
in the dark. Solutions of I in non-polar THF or polar acetonitrile,
kept in the dark, do not exhibit absorption bands in the visible.
Clearly, the open form of the SPAA unit is stabilized in the more
polar water environment. In acidic solutions of I, an absorption
band at 422 nm (not shown), attributable to a protonated open
form of the SPAA unit, was observed along with an increase in
pH. The 422 nm band is absent in neutral or alkali solution, so the
concentration of this species is negligible.
The attractive features of photochromic spiropyrans have
prompted a number of studies of derivatives of these compounds
and their metal-binding characteristics. Upon irradiation with
UV or visible light, spiropyrans isomerize between the open and
closed forms, respectively. The open form is more polar. Metal
ions can influence this isomerization by association with the open
form through the electron-rich oxygen atom. However, visible light
generates the closed form, thus hindering metal binding. Strongly
polar solvents such as water and alcohols stabilize the metal-
complexed open form to such an extent that in aqueous solution,
visible light can no longer generate the closed form. However,
1,2
3
4
Collins and coworkers showed that the complexation of metal
Absorbtion measurements after addition in the dark of a tenfold
molar excess (relative to the SPAA unit) of Pb(ClO ) revealed a
ions by quinolinospiropyran could be made photo-reversible,
provided the closed form is stabilized by using acetonitrile–water
mixtures. Recently, other reports have described photo-reversible
metal-complexations of spiropyrans in fluoroalcohols and metha-
4
2
band at 433 nm (Fig. 1(B)) along with the almost complete
disappearance of the 533 nm band. These changes suggest the
generation of a different open form of the SPAA unit, which is
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-
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nol–water mixtures. Herein, we describe photo-reversible Pb
thought to be the Pb -complex. Fig. 1(C) shows the complete
complexation of a thermosensitive copolymer consisting of
N-isopropyl acrylamide (NIPAAm) and a synthetic spiropyran
acrylate (SPAA), I (Scheme 1), in water. Poly(N-isopropyl
acrylamide), P(NIPAAm), is thermosensitive in water showing a
disappearance of the 433 nm band upon subsequent irradiation
with visible light. This band reappeared after standing in the dark,
attaining the same absorbance as spectrum (B) under equilibrium
21
conditions. Thus, stabilization of the Pb -complexed open form
in an extremely polar solvent such as water did not prevent
isomerization to the closed form under visible light. The complexed
6
lower critical solution temperature (LCST). Above the LCST, the
aqueous solution generates a solid polymer and separates into two
phases. A thermosensitive copolymer that displays such phase
transition behavior at the LCST is attractive for use as a metal-ion
adsorbent. Above the LCST, the solid phase, containing bound
metal-ions, can be easily separated from solution. However,
because the solid phase forms a transparent solution below the
LCST, it is easy to subsequently recover these metal-ions from the
copolymer by irradiation with visible light.
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Pb ions were subsequently liberated by shifting the equilibrium
of the SPAA unit.
The transmittance of an aqueous solution of I suddenly
Synthesis of SPAA was carried out by reaction between 1’,3’,3’-
trimethyl-6-hydroxyspiro(2H-1-benzopyran-2,2’-indoline) and
acryloyl chloride in toluene. Copolymerization of NIPAAm and
SPAA was subsequently performed in ethanol under nitrogen at
Fig. 1 Absorption spectra of aqueous solution of I at 5 uC. (A) 0.04 wt% of
I ([SPAA unit] ~ 0.1 mM). (B) Solution after addition of Pb(ClO )
4 2
(1.0 mM) to solution (A). (C) The same solution as used in (B) measured
under visible light irradiation.
Scheme 1 I.
2
0 3 6
C h e m . C o m m u n . , 2 0 0 4 , 2 0 3 6 – 2 0 3 7
T h i s j o u r n a l i s ß T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

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Fig. 3 Cathodic reduction of Pb ions in aqueous solutions at 40 uC. (#)
Fig. 2 Temperature dependence of optical transmittance of aqueous
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1
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[
Pb ] ~ 0.04 mM in water, (%) after addition of I (0.23 wt%) to the Pb
solution of I, measured in the dark at a rate of about 11 uC min . (#)
2
1
solution, (') after irradiation with visible light to solution %.
0
.04 wt% of I in water, (%) addition of Pb(ClO
4 2
) (1.0 mM, [Pb ]:[SPAA
unit] ~ 10:1) to the solution of I (0.04 wt%). Inset: color changes of I in
water. 0.23 wt% of I at 10 uC (A) and at 40 uC (A’). 0.23 wt% of I and
7
of the system to improve photo-reversibility in water is currently
under investigation. One such study involves the investigation of
efficient exposure of a crosslinked hydrogel of I in the swollen state
(vLCST) to visible light.
4 2
Pb(ClO ) (5.0 mM) at 10 uC (B) and at 40 uC (B’).
decreased as temperature increased due to the thermosensitive
NIPAAm units, which exhibited an LCST at 29 uC, as shown in
Fig. 2. The increase in hydrophobicity of I would result in a lower
LCST than that of the P(NIPAAm) homopolymer (32 uC).
This work was supported in part by the Research Institute for
Technology (No. Q03M-04) at Tokyo Denki University.
4 2
Pb(ClO ) at a concentration of 1 mM did not influence the LCST
of the solution. At temperatures above the LCST, I precipitated in
water, as shown in Fig. 2(A’) and (B’). The dehydration of the
copolymer chain of I above the LCST may result in closing of the
open form of the SPAA unit, followed by the appearance of a white
precipitate of (A’). This supports the suggestion that water
thermodynamically stabilizes the open form of the SPAA units.
Notes and references
1 (a) R. C. Bertelson, in Photochromism, ed. G. H. Brown, Wiley,
New York, 1971; (b) R. J. Guglielmetti, in Photochromism, ed. H. Durr
and H. Bouas-Laurent, Elsevier, Amsterdam, 1990.
2
For recent examples, see: (a) J. Filley, M. A. Ibrahim, M. R. Nimlos,
A. S. Watt and D. M. Blake, J. Photochem. Photobiol. A: Chem., 1998,
21
In contrast, for the Pb solution a yellow precipitate of I was
obtained above the LCST (Fig. 2(B’)). This solid material is
expected to be useful for absorbing metal ions.
117, 193; (b) M. Inouye, K. Akamatsu and H. Nakazumi, J. Am. Chem.
Soc., 1997, 119, 9160; (c) K. Kimura, T. Yamashita and M. Yokoyama,
J. Chem. Soc., Perkin Trans. 2, 1992, 613; (d) T. Tamaki and
K. Ichimura, Chem. Commun., 1989, 1477.
3 R. C. Bertelson, in Organic Photochromic and Thermochromic
Compounds, ed. J. C. Crano and R. J. Guglielmetti, Plenum Press,
New York, 1999.
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To confirm the adsorption and desorption of Pb by I, square-
ions was
performed using an amalgamated Au working electrode, as
21
wave voltammetry of aqueous solutions of Pb
5
b
previously described. Addition of LiClO
4
(0.1 M) as a supporting
4
G. E. Collins, L-S. Choi, K. J. Ewing, V. Michelet, C. M. Bowen and
J. D. Winkler, Chem. Commun., 1999, 321.
electrolyte resulted in a shift of the LCST to lower temperature
8
(
4
27 uC) due to the salt effect of LiClO . Voltammetry was con-
5
(a) T. Suzuki, F.-T. Lin, S. Priyadashy and S. G. Weber, Chem.
Commun., 1998, 2685; (b) T. Suzuki, Y. Kawata, S. Kahata and T. Kato,
Chem. Commun., 2003, 2004.
ducted at 40 uC (wLCST) due to the difficulty in using viscous
polymer-soluble solutions. The cathodic wave of free Pb ions was
observed at 20.53 V vs. Ag/AgCl. After addition of I into the
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6
(a) Y. Maeda, T. Higuchi and I. Ikeda, Langmuir, 2001, 17, 7535;
(b) M. Hahn, E. G o¨ rnitz and H. Dautzenberg, Macromolecules, 1998, 31,
21
solution at 40 uC, the Pb -cathodic wave (20.53 V) decreased in
intensity, as shown in Fig. 3. The calculated Pb -adsorption
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5616; (c) H. Feil, Y. H. Bae, J. Feijen and S. W. Kim, Macromolecules,
1993, 26, 2496.
7 A letter ‘‘T’’ was behind the cuvette (thickness 1 cm) in order to confirm
the transparency of the solution.
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ability is 62% at [Pb ]:[SPAA unit] ~ 1:10 under initial
concentration conditions. After irradiation of the Pb -adsorbed
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precipitate of I with visible light, 48% of the Pb ions were
desorbed from the precipitate. A decrease in current intensity at
8
According to Maeda and co-workers, the LCST of P(NIPAAm)
homopolymer (32 uC) in water decreased in proportion to the added
2
0.53 V was again observed in the dark due to re-adsorption of
6a
NaCl (10 uC with 10 wt% of NaCl). Thus, the decrease in the LCST
21
Pb ions by I.
In conclusion, we demonstrated photo-reversible Pb -com-
from 29 to 27 uC of I in water after addition of 0.1 M (y1 wt%) of
LiClO was consistent.
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plexation with a thermosensitive copolymer, I, in water. For Zn
,
Cu ,Ni andMn ions,thephoto-reversiblemetal-complexation
of I was also examined by UV-visible spectroscopy. Modification
9
The P(NIPAAm) homopolymer containing the equivalent mole of the
NIPAAm unit in aqueous solution absorbed 15% of Pb ions in the
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same initial concentration of Pb ions at 40 uC.
C h e m . C o m m u n . , 2 0 0 4 , 2 0 3 6 – 2 0 3 7
2 0 3 7