Photo-reversible Pb2+-complexation of insoluble poly(spiropyran methacrylate-co-perfluorohydroxy methacrylate) in polar solvents

Article abstract of DOI:10.1039/b305966a

The first demonstration of photo-reversible Pb²⁺-complexation of an insoluble spiropyran-carrying copolymer in aqueous solutions is presented.

Full text of DOI:10.1039/b305966a

Photo-reversible Pb2+-complexation of insoluble poly(spiropyran
methacrylate-co-perfluorohydroxy methacrylate) in polar solvents
Takayuki Suzuki,* Yohei Kawata, Shinsuke Kahata and Tatsuya Kato
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: +81(476)
46-8735; Tel: +81(476) 46-8735
Received (in Cambridge, UK) 28th May 2003, Accepted 27th June 2003
First published as an Advance Article on the web 10th July 2003
The first demonstration of photo-reversible Pb2+-complexa-
tion of an insoluble spiropyran-carrying copolymer in
aqueous solutions is presented.
line) and methacryloyl chloride in toluene. Subsequently, the
radical polymerization of SPMA in toluene was performed
under a nitrogen atmosphere at 60 °C with 2,2A-azobis(isobutyr-
onitrile) (AIBN) as an initiator to afford poly(spiropyran
methacrylate), P(SPMA), (Mw = 1.5 3 105, Mw/Mn = 2.1).
Similarly, P(SPMA-FHMA) was synthesized from SPMA and
FHMA in acetonitrile under nitrogen atmosphere at 60 °C with
AIBN for 12 hours. The ratio of SPMA in P(SPMA-FHMA)
was 15 mol%, as determined by elemental analysis.
As shown in Fig. 1(A), the absorption spectrum of a solution
of SPMA in CH3OH–H2O (9 : 1 v/v) in the dark exhibited a
purplish-blue band (lmax = 560 nm) that is attributable to the
open form of SPMA. Solutions of SPMA in toluene or THF did
not exhibit any absorption bands in the visible regions. Our
results indicate that the open form of SPMA was stabilized in
polar environments. Addition of a tenfold molar excess of
Pb(ClO4)2 into the solution resulted in the appearance of a
yellow band (lmax = 435 nm) in the dark, as shown in Fig. 1(B),
along with the disappearance of the 560 nm band. These
changes in the absorption spectra suggest the generation of a
different open form of SPMA, which could be the Pb2+-
complex. Subsequently, as shown in Fig. 1(C), the yellow band
disappeared upon irradiation with visible light ( > 420 nm). The
435 nm band appeared again in the dark, and attained the same
absorbance as spectrum (B) in the equilibrium.
The photo-reversible isomerization of SPMA in a CD3OD–
D2O (9 : 1 v/v) solution containing Pb2+ ions was similarly
observed using 1H NMR spectroscopy; in this case, the photo-
responding reversible isomerization corresponded to the
changes in the gem-CH3 signals of SPMA (1.14 and 1.27 ppm
in the closed form, and 1.82 ppm in the open form). Using the
values of concentrations of the open form of SPMA, as
determined from 1H NMR measurements in the dark, the molar
absorption coefficient of the open form was calculated as 2.0 3
104 (dm3 mol21 cm21 at 435 nm). Stabilization of the open form
of SPMA in polar solvents did not prevent the photo-induced
ejection of the complexed Pb2+ ions.
The attractive features of photochromic spiropyrans have
prompted a number of studies to investigate the derivatives of
these compounds and their metal-binding characteristics.1,2
Upon irradiation with UV and visible lights, spiropyrans
isomerize between the closed and open forms,3 in which the
open form is comparatively more polar. Metal ions can
influence this isomerization process by associating with the
open form through the consequently electron-rich oxygen atom.
In contrast, visible light produces high concentrations of the
closed form, and thus hinders metal-binding. Spiropyrans,
therefore, show potential in providing photo-reversible metal-
complexation. However, strong polar solvents, such as water
and alcohols, can thermally stabilize the metal-complexed open
form significantly, resulting in an irreversible metal-complexa-
tion. In these polar solvents, visible light is not sufficient to
transform the metal-complexed open form to its closed form.
Reversible photo-induced switching of metal-complexation in
aqueous solutions was first demonstrated by Collins and co-
workers by utilizing quinolinospiropyran,4 in which the en-
hancement of thermodynamic stability of the closed form of
spiropyrans was crucial in promoting the photo-reversiblity of
metal-complexation in polar solvents. Recently, we have
reported on photo-reversible metal-complexations for polar
fluoroalcohols.5 To the best of our knowledge, there are no
published accounts of photo-reversible metal-complexations
that involve an insoluble spiropyran-carrying copolymer in
aqueous solutions. Insoluble materials are attractive for metal-
ion adsorption, such as adsorbents or sensors of metals, because
of their relative ease of use. Herein, we describe the photo-
reversible Pb2+-complexation for a synthetic spiropyran metha-
crylate (SPMA) in polar solvents (mixtures of methanol and
water). Furthermore, we present the first demonstration of
reversible photo-induced Pb2+-complexation of an insoluble
copolymer consisting of SPMA and perfluorooctylhydroxy
methacrylate (FHMA),6 P(SPMA-FHMA) (Scheme 1), in
aqueous solutions.
Although purplish-blue P(SPMA) was insoluble in methanol
and water, the polymer became soluble after addition of
Synthesis of SPMA was carried out by the reaction between
1A,3A,3A-trimethyl-6-hydroxyspiro(2H-1-benzopyran-2,2A-indo-
Fig. 1 Absorption spectra of a CH3OH–H2O (9:1 v/v) solution of SPMA.
(A): [SPMA] = 0.5 mM, (B): [SPMA] = 0.1 mM, [Pb(ClO4)2] = 1.0 mM,
(C): same solution as (B), measured under visible light irradiation.
Scheme 1 P(SPMA-FHMA).
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Pb(ClO4)2. The resulting solution showed an absorption band
(lmax = 435 nm), which was identical to spectrum (B) in Fig.
1. In contrast, P(SPMA-FHMA) was completely insoluble in
methanol and water, even after complexation with Pb2+ ions.
The color of the solid P(SPMA-FHMA) in H2O–CH3OH (8 : 2
v/v) changed as follows: 1) it existed as purplish-blue in the
dark, as shown in Fig. 2(A); 2) upon addition of Pb2+ ions,
started turning yellow, initially at the surfaces of the solid, as
shown in Fig. 2(B); 3) after diffusion of the Pb2+ ions into the
solid, turned completely yellow, as shown in Fig. 2(C); 4) upon
irradiation of visible light, although the interior was still yellow,
the surface of the solid turned white, as shown in Fig. 2(D). The
white color of the solid is attributable to the closed form of the
SPMA portion (15 mol%) and the FHMA portion (85 mol%) of
P(SPMA-FHMA). Moreover, the solid turned yellow again in
the dark. Removal of the ejected Pb2+ in the solution upon
irradiation with visible light resulted in the purplish-blue solid
of P(SPMA-FHMA) in the dark as observed in Fig. 2(A).
To determine the ejection of Pb2+ ions, square-wave
voltammetry of the Pb2+-complex of SPMA or P(SPMA-
FHMA) solutions in CH3OH–H2O (9 : 1 v/v), in the presence of
0.1 M LiClO4, were carried out using amalgamated Au in a
conventional three-electrode electrochemical cell. Voltammetry
of SPMA in the dark or under visible light did not yield any
waves within the 20.2 to 20.8 V region (vs. Ag/AgCl).
P(SPMA-FHMA) was insoluble in the solvent. Next, in the
absence of SPMA or P(SPMA-FHMA), our studies showed the
cathodic wave of free Pb2+ ions (20.35 V vs. Ag/AgCl), which
was consistent with the results of Weber and co-workers.7
Exposure of the solution to visible light had no effects on the
voltammogram prior to the addition of SPMA or P(SPMA-
FHMA). Increases in the concentration of SPMA in solution
caused the Pb2+(free)-cathodic wave (20.35 V) to decrease in
intensity, as shown in Fig. 3(a), and moreover, also caused the
appearance of another Pb2+-cathodic wave (20.60 V vs. Ag/
AgCl). Conversely, under irradiation of visible light, the
intensity of the cathodic wave of the free Pb2+ ions (20.35 V)
increased with simultaneous decrease of the other wave (20.60
V). The Pb2+ (free)-cathodic wave decreased again in the dark.
These results are indicative of Pb2+-complexation with SPMA,
which corresponds with the photoisomerization of SPMA (the
closed form Ù the open form). Incomplete ejection of Pb2+ ions
into the solution (41%) from the metal-complexed open form
Fig. 3 Cathodic reduction of Pb2+ ions in a CH3OH–H2O (9 : 1 v/v)
containing (a) SPMA and (b) P(SPMA-FHMA). [Pb2+] = 0.04 mM (2);
after addition of SPMA (2.0 mM) or P(SPMA-FHMA)8 (8); the same
solution measured after visible light irradiation (Ω).
SPMA should be the result of insufficient exposure of the
solution to visible light. The results of our photochemical
studies of Pb2+ ions in the presence of P(SPMA-FHMA) are
illustrated in Fig 3(b). The Pb2+(complexed)-cathodic wave
(20.60 V) was not observed during the process of the
complexation, nor during the photo-induced ejection of Pb2+
ions; however, the intensity of the Pb2+(free)-cathodic wave
(20.35 V) changed reversibly in response to visible light
irradiation. Our results suggest that P(SPMA-FHMA) remained
insoluble upon complexation with Pb2+ ions. Exposure of only
the surface of solid P(SPMA-FHMA) to visible light corre-
spondingly caused the ejection of only a portion of Pb2+ ions
(13%).
In conclusion, this report demonstrates the first photo-
reversible Pb2+-complexation from an insoluble copolymer,
P(SPMA-FHMA), in aqueous solutions.9 Modifications of
these systems designed to improve the photo-reversibility in
aqueous solution, for example, efficient exposure of solid
P(SPMA-FHMA) to visible light, and a degradation test with
repeated irradiation of light, are currently under investigation.
This work was partially supported by a Grant-in-Aid
Scientific Research (No. 13555262 and No. 13750837) from the
Ministry of Education, Culture, Sports, Science and Technol-
ogy, Japan, and Tokyo Ohka Foundation for the Promotion of
Science and Technology.
Notes and references
1 (a) R. C. Bertelson, in Photochromism, ed. G. H. Brown, Wiley, New
York, 1971; (b) R. J. Guglielmetti, in Photochromism, eds. 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, 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,
J. Chem. Soc., Chem. Commun., 1989, 1477.
3 R. C. Bertelson, in Organic Photochromic and Thermochromic Com-
pounds, eds. J. C. Crano and R. J. Guglielmetti, Plenum Press, New York,
1999.
4 G. E. Collins, L-S. Choi, K. J. Ewing, V. Michelet, C. M. Bowen and J.
D. Winkler, Chem. Commun., 1999, 321.
5 T. Suzuki, F.-T. Lin, S. Priyadashy and S. G. Weber, Chem. Commun.,
1998, 2685.
6 FHMA, as received from DAIKIN Chemicals Sales Corporation, is an
isomeric mixture of Rf–CH2CH(OH)CH2-mAcr (80–85%) and Rf–
CH2CH(CH2OH)-mAcr (20–15%).
7 M. T. Stauffer, D. B. Knowles, C. Brennan, L. Funderburk, F.-T. Lin and
S. G. Weber, Chem. Commun., 1997, 287.
8 The 50-fold molar excess of P(SPMA-FHMA) in the SPMA unit of
P(SPMA-FHMA) to Pb2+ ions was added in the solution.
Fig. 2 Color changes of solid P(SPMA-FHMA) in H2O–CH3OH (8 : 2 v/v):
(A) in the dark, (B) 15 s after addition of Pb(ClO4)2, (C) a minute after
addition of Pb(ClO4)2, (D) for a minute under visible light irradiation.
9 In addition to Pb2+
,
the photo-reversible metal-complexation by
P(SPMA-FHMA) was examined using UV–visible spectroscopy for
Zn2+, Cu2+, Ni2+, and Mn2+
.
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