Synthesis and photoresponsive properties of a molecularly imprinted polymer

Article abstract of DOI:10.1021/ol701618n

A photoresponsive molecularly imprinted polymer was prepared from a di(ureidoethylenemethacrylate)azobenzene monomer, using a methotrexate analogue as template. Photoisomerization of the 3D crosslinked polymer matrix allowed switching the substrate affinity by altering the geometry and spatial arrangement of the receptor binding sites. As a result, controlled release and uptake of the template (or analogous ligands) were obtained.

Full text of DOI:10.1021/ol701618n

ORGANIC
LETTERS
2007
Vol. 9, No. 20
3865-3868
Synthesis and Photoresponsive
Properties of a Molecularly Imprinted
Polymer
Christophe Gomy and Andreea R. Schmitzer*
Department of Chemistry, UniVersite´ de Montre´al, C.P. 6128 Succursale Centre-Ville,
Montre´al, Que´bec, H3C 3J7 Canada
ar.schmitzer@umontreal.ca
Received June 6, 2007
ABSTRACT
A photoresponsive molecularly imprinted polymer was prepared from a di(ureidoethylenemethacrylate)azobenzene monomer, using a methotrexate
analogue as template. Photoisomerization of the 3D crosslinked polymer matrix allowed switching the substrate affinity by altering the geometry
and spatial arrangement of the receptor binding sites. As a result, controlled release and uptake of the template (or analogous ligands) were
obtained.
Light offers many advantages as a means for manipulating
systems of either microscopic or macroscopic size: photons
can be applied with extreme spatial and temporal resolution
using modern laser techniques. Photons are perfectly clean,
leaving no remaining contamination and, in contrast to
matter, the photons do not interact with each other (at
moderate intensities) allowing for multiplexing. For these
reasons molecular assemblies with optical triggers are ideal
systems for studying molecular processes as well as for the
construction of molecular machines. Azobenzene has been
the most widely used optical trigger over the last decades
for the design and synthesis of a large variety of photo-
responsive systems.1-8 Incorporation of azobenzene moieties
into polymers creates materials with photocontrolled mechan-
(1) Willner, I.; Rubin, S. Angew. Chem., Int. Ed. Engl. 1996, 35, 367-
385.
(2) Willner, I. Acc. Chem. Res. 1997, 30, 347-356.
(3) Kumar, G. S.; Neckers, D. C. Chem. ReV. 1989, 89, 1915-1925.
(4) Shibaev, V.; Bobrovsky, A.; Boiko, N. Prog. Polym. Sci. 2003, 729-
836.
(5) Natansohn, A.; Rochon, P. Chem. ReV. 2002, 102, 4139-4175.
(6) Pieroni, O.; Fissi, A.; Angelini, N.; Lenci, F. Acc. Chem. Res. 2001,
34, 9-17.
(7) Harada, A. Acc. Chem. Res. 2001, 34, 456-464.
(8) Rau, H. In Studies in Organic Chemistry: Photochromism, Molecules
and Systems; Du¨rr, H. B.-L., H., Ed.; Elsevier: Amsterdam, 1990; Vol.
40, p 165-192.
* To whom correspondence should be addressed. Phone: (+1) 514-343-
6744. Fax: (+1) 514-343-7586.
10.1021/ol701618n CCC: $37.00
© 2007 American Chemical Society
Published on Web 08/30/2007

ical1-3 and optical4 properties and has also found extensive
applications in the search for innovative photoresponsive
materials as well as for photomodulation of biological
properties of peptides, proteins, and lipids.2,5-7 This is
because of the pronounced changes in geometry and polarity
that result upon its light-induced cis f trans isomerization,
leading to the high (photo)stability, the high isomerization,
and quantum yields8 as well as the high rate and full
reversibility of the isomerization. This process is complete
within 10 ps for the trans f cis and within 1 ps for the cis
f trans.9-13 At the photostationary states the trans and cis
isomers are formed, depending on the irradiation wavelength.
Conversely thermal cisftrans relaxation is a slow process,
but leads to 100% trans isomer.1 Less explored is the
possibility of incorporating such stimuli-responsive receptors
into rigid polymer matrices for controllable chemical and
drug delivery, analyte separation, and extraction. Receptor
sites that are capable of recognizing specific molecular
species can be conveniently imprinted into rigid polymer
matrices via a template-directed polymerization technique
known as molecular imprinting.14 With a suitable choice of
functional monomers, molecularly imprinted polymers (MIPs)
with a substrate affinity that can be switched by externally
applied stimuli should be possible. While azobenzene has
been frequently utilized as the backbone and in the side
chains of polymers and hydrogels for the fabrication of
photoresponsive materials,15-18 its application in photo-
responsive molecular recognition and host-guest binding in
MIP materials has seldom been reported.19-21 To our
knowledge, all previously decribed MIPs contained photo-
responsive azobenzene side chains. Incorporation of the
azobenzene group in the MIP’s backbone may lead to higher
differences in host-guest recognition properties of the two
isomers. In this work, new reported azobenzene-derivatived
functionalized monomers, (Scheme 1) were used to fabricate
a bulk MIP material containing bis(TBA)-N-Z-L-glutamate,
a methotrexate analogue, as the molecular template. The
Scheme 1. Di(ureidoethylenemethacrylate)azobenzene
Monomer Synthesis
thermodynamically less stable cis-form of the monomer was
used for the MIP preparation. Photoisomerization of azoben-
zene located in the backbone allowed the obtained material
to release the template most efficiently after irradiation at
440 nm and rebind the molecular template from solution after
irradiation at 365 nm.
The design of the novel functional monomer 1 was based
on the previously reported features that are relevant to the
creation of selective MIPs for oxyanions.22 First are the
bisurea binding functionalities, which exhibit strong affinity
for dicarboxylate moieties.23 The second feature is the
selection of polymerizable end groups. We chose methacry-
late groups, commonly used in chemical cross-linking and
known for providing materials with good thermal stability.
The methacrylate group is placed two carbons away from
the H-bond donating ureas to prevent destabilization of the
template/monomer complex during the polymerization pro-
cess. The monomer was synthesized from the p-phenylene-
diamine with KO2 by heating under reflux for 6 h.24 The
resulting azobenzene-4,4′-diamine was treated in a one-pot
reaction by N-hydro-C-alkylamino addition on 2-ethyliso-
1
cyanatemethacrylate (Scheme 1). H NMR titrations were
(9) Lednev, I. K.; Ye, T.-Q.; Hester, R. E.; Moore, J. N. J. Phys. Chem.
1996, 100, 13338-13341.
performed with both cis and trans isomers of monomer 1,
using bis(TBA)-N-Z-L-glutamate as guest compound. The
complexation-induced chemical shift (∆(δ)) of the urea
protons of both isomers of 1 was monitored. Addition of
increasing amounts of bis(TBA)-N-Z-L-glutamate (0-10
equiv) to DMSO-d6 solutions of 1 allowed extraction of
monomer/guest interaction information for each isomer
(Table 1). Titration data fit well to 1:1 binding isotherms
and association constants (Kass) were obtained by nonlinear
least-squares fitting.22 The competitive solvent DMSO-d6
prevented self-association of monomer and/or guest to occur
in the system. Although both inner and outer protons from
trans-1 are more acidic than those from cis-1, the cis
monomer gives rise to higher ∆(δ) than the trans isomer.
This is compatible with the hypothesis that the cis isomer is
topologically complementary to the oxyanion substrate. A
(10) Lednev, I. K.; Ye, T.-Q.; Matousek, P.; Towrie, M.; Foggi, P.;
Neuwahl, F. V. R.; Umapathy, S.; Hester, R. E.; Moore, J. N. Chem. Phys.
Lett. 1998, 290, 68-74. Natansohn, A.; Rochon, P. Chem. ReV. 2002, 102,
4139-4175.
(11) Na¨gele, R.; Hoche, R.; Zinth, W.; Wachtveitl, J. Chem. Phys. Lett.
1997, 272, 489-495.
(12) Fujino, T.; Arzhantsev, Y. S.; Tahara, T. J. Phys. Chem. A 2001,
105, 8123-8129.
(13) Satzger, H.; Spo¨rlein, S.; Root, C.; Wachtveitl, J.; Zinth, W.; Gilch,
P. Chem. Phys. Lett. 2003, 372, 216-223.
(14) Komiyama, M.; Takeuchi, T.; Mukawa, T.; Asanuma H. Molecular
Imprinting: From Fundamentals to Applications; Wiley-VCH: Weinheim,
Germany, 2003.
(15) Haitjema, H. J.; Buruma, R.; Alberda van Ekenstein, G. O. R.; Tan,
Y. Y.; Challa, G. Eur. Polym. J. 1996, 32, 1437-1448.
(16) Howe, L. A.; Jaycox, G. D. J. Polym. Sci., Part A: Polym. Chem.
1998, 36, 2827-2833.
(17) Izumi, A.; Nomura, R.; Masda, T. Macromolecules 2001, 34, 4342-
4348.
(18) Jaycox, G. D. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 566-
573.
(19) Gong, C. B.; Hon-Wah Lam, M.; Yu, H. AdV. Funct. Mat. 2006,
16, 1759-1767.
(20) Minoura, N.; Idei, K.; Rachkov, A.; Uzawa, H.; Matsuda, K. Chem.
Mater. 2003, 15, 4703-4704.
(22) Gomy, C.; Schmitzer, A. R. J. Org. Chem. 2006, 71, 3121-3125.
(23) Hall, A. J.; Manesiotis, P.; Emgenbroich, M.; Quaglia, M.; De
Lorenzi, E.; Sellergren, B. J. Org. Chem. 2005, 70, 1732-1736.
(24) Crank, G.; Makin, M. I. H. Aust. J. Chem. 1984, 37, 845-855.
(21) Minoura, N.; Idei, K.; Rachkov, A.; Choi, Y.-W.; Ogiso, M.;
Matsuda, K. Macromolecules 2004, 37, 9571-9576.
3866
Org. Lett., Vol. 9, No. 20, 2007

1
Table 1. Extrapolated H NMR Chemical Induced Shifts
b
(∆(δ))a and Association Constant (Kass
∆(δ) (ppm)
)
inner 1H
outer 1H
Kass (M-1
)
cis-1
trans-1
3.09
2.83
3.35
3.23
2800
2200
a The monomer concentration was 1 mM and the solvent was DMSO-
d6. b The association constants were determined from the experimental urea
proton chemical shifts by the nonlinear refinement method25.
600 M-1 increase in binding constant is observed for cis-1,
relative to trans-1 (Table 1). Though the difference is not
overhelmingly impressive, it has an important meaning
considering the nature of the measurement which has been
done in solution. It has been reported previously that similar
shorter receptors could form 2:1 assemblies, explaining the
possible association for trans-1/guest in solution. Neverthe-
less, once contained in a preorganized cis conformer
polymeric network, this effect should be attenuated, leaving
place to a higher difference of affinity of the substrate for
the cis relative to the trans isomer.
A molar EDMA crosslinker /monomer/ bis(TBA)-N-Z-L-
glutamate ratio of 8/4/1 was used for the 2,2′-azobis-(2,4-
dimethylvaleronitrile) (ABDV) initiated crosslinking polym-
erization in DMSO as solvent. Higher and lower crosslinker/
monomer ratios were tested, but the resulting polymeric
materials displayed no photoisomerization properties. It is
possible that at low crosslinker/monomer ratio, azobenzene
monomers tend to aggregate, leading to the reduction of free
volume necessary for the reorientation of the azobenzene
chromophores during isomerization. The monomer is first
irradiated at 365 nm and the polymerization mixture is
maintained under UV irradiation to maintain the thermody-
namically less stable cis configuration throughout the po-
lymerization process. For the MIP preparation, the monomer
and the bis(TBA)-N-Z-L-glutamate solution were kept at 40
°C for 30 min, to allow the monomer to bind the bis(TBA)-
N-Z-L-glutamate prior to addition of the crosslinker and the
polymerization initiator. The resultant bulk polymers were
ground using a combination of coffee grinder and ball mill,
then dry-sieved using a sieve shaker. Template removal was
achieved by Soxhlet extraction and repeated methanol
washing. The control polymer was prepared and treated in
exactly the same way as the MIP, except that no template
was used in the polymerization procedure.
Figure 1 shows the spectroscopic behavior of the resultant
MIP, imprinted with bis(TBA)-N-Z-L-glutamate, toward
irradiation at 365 and then 440 nm. Responses of the MIP
material to photoexcitation are similar to that of the
monomer, indicating that the azobenzene chromophore of
the monomer still possesses its photoswitching properties
after incorporation into a 3D crosslinked polymer network.
Photoisomerization was also observed in the control polymer
material (non-imprinted). Compared to the control material,
the relatively faster cis f trans isomerization rate of the MIP
material suggests that the imprinted receptor sites in the bulk
Figure 1. UV-vis spectra of the isomerization of monomer 1 and
MIP: (a) trans f cis isomerization of monomer 1 after 20 min
irradiation at 365 nm (50 µM in DMSO); (b) MIP toward irradiation
at 365 nm (1 mg in 2 mL DMSO, 20 min of irradiation).
polymer may be able to provide more free volume for the
photoswitching of azobenzene chromophores (data not
shown).
The ability of the empty MIP material to rebind bis(TBA)-
N-Z-L-glutamate was studied using batch-type rebinding
assays in DMSO and is shown in Figure 2. The density of
the imprinted receptor sites in the MIP was found to be 501
µmol/g cis-MIP and 270 µmol/g trans-MIP.
Figure 2 shows the change in free bis(TBA)-N-Z-L-
glutamate in DMSO in the presence of bulk MIP and control
material under alternating irradiation at 365 and 440 nm.
The addition of the empty MIP (10 mg) allowed binding of
3.5 µmoles of bis(TBA)-N-Z-L-glutamate from solution into
the MIP material. Taking into account the 0.5 µmoles of
bis(TBA)-N-Z-L-glutamate that was bound by nonspecific
sites in the polymer revealed from the control material and
from the amount that remain bound to MIP following release
at 440 nm a total of 3 µmoles of bis(TBA)-N-Z-L-glutamate
was bound by specific sites in 10 mg MIP, which is
equivalent to the uptake of 300 µmol/g cis-MIP. This also
implies that 60% of the receptor sites in the cis-MIP material
are occupied by the imprint molecules. Irradiation at 440
nm caused a steady increase in the amount of bis(TBA)-N-
Z-L-glutamate in the DMSO solution. A total of 2.8 µmoles
of bis(TBA)-N-Z-L-glutamate was released from the MIP
material sites. This photoregulated release from the MIP
material by 440 nm irradiation can be attributed to the
photoinduced cis trans isomerization of the azobenzene
Org. Lett., Vol. 9, No. 20, 2007
3867

amterene. Irradiation of the MIP material at 365 and 440
nm also allowed the partial release (40%) and uptake of
TBA-N-Z-L-methylesterglutamate. In the case of triamterene
(Figure 2), no significative release/uptake was observed.
These results indicate that the receptor sites in the MIP
material possess a specific affinity for bis(TBA)-N-Z-L-
glutamate, and TBA-N-Z-L-methylesterglutamate is less
sensitive than its analogue to the change in receptor
geometry.
We have demonstrated the utility of a new di(ureidoeth-
ylememethacrylate)azobenzene monomer for the preparation
of photoregulated MIP. The photoisomerization properties
of the azobenzene chromophore are retained in the rigid
polymer. Photoinduced cis f trans isomerization of the
azobenzene-containing polymer backbone is able to regulate
the receptor sites’ geometry and can regulate the release and
uptake of a substrate. An important factor to consider is that
this new cross-linking monomer combines interactive mono-
mer functionality with a cross-linking format. This new cross-
linking agent is readily copolymerizable under mild condi-
tions and has been used for noncovalent MIPs preparation
with potential improved performance, that more functionality
can be introduced without suffering performance losses due
to reduced cross-linking. Development of stimuli-responsive
MIP is advantageous for the improvement of the imprint
molecule’s release and may find applications in smart drug
delivery systems.
Figure 2. Photoregulated release and uptake of bis(TBA)-N-Z-L-
glutamate by the MIP and the control polymer. The initial amount
of ligands was 10 µmol for 10 mg MIP in 2 mL of DMSO.
chromophores in the MIP receptor sites, resulting in changes
in the receptor geometry. The host-guest interaction is
therefore weakened and the imprint is released into the
solution, by the switch of substrate affinity of the MIP
receptor sites. The near-quantitative uptake of the imprint
molecule after the trans f cis isomerization is an evidence
of the reversibility of the receptor-site configuration and
imprint affinity, without loss of specificity after three
irradiation cycles.
To examine the substrate specificity of the MIP material,
we studied the photoregulated release and uptake of TBA-
N-Z-L-methylesterglutamate and triamterene, two substruc-
tural models of the methotrexate drug. At the same substrate
concentration, the MIP material bound only 1.8 µmoles of
TBA-N-Z-L-methylesterglutamate and 0.4 µmoles of tri-
Acknowledgment. We are grateful to NSERC, FQRNT,
the Canadian Foundation for Innovation (CFI) and the
Universite´ de Montre´al for financial support. We thank
colleagues for careful reading and discussion of this manu-
script.
Supporting Information Available: Experimental pro-
cedure for the synthesis of monomer, polymers, and photo-
regulated release and uptake of the substrates. This material
is available free of charge via the Internet at http://pubs.acs.org.
OL701618N
(25) Connors, K. A. In Binding Constants: The Measurement of
Molecular Complex Stability; John Wiley and Sons Ltd: New York, 1987.
3868
Org. Lett., Vol. 9, No. 20, 2007