ISMSC 2007 - Università degli Studi di Pavia

A photocontrollable receptor for Cu II
Giacomo Dacarro, Paola Ricci, Angelo Taglietti
Dipartimento di Chimica Generale Università degli Studi di Pavia, Viale Taramelli 12, 27100
Pavia, Italy
Ligand LH2 shows the expected behaviour towards transition metal cations: it is able to bind
only Ni II and Cu II as a consequence of deprotonation of amido groups, to give neutral
complexes. Moreover, the different position in the Irving- Williams series accounts for the ability
of the ligand to discriminate between the two cations: at pH between 4.5 and 6.5 only Cu II is
bound by this receptor, even in presence of other cations. This behaviour is shared by a big
number of ligands based on the diamino-diamido donor set, [1] but in this case the receptor
features are photocontrollable. When linear LH2 is irradiated for a few minutes with a 366 nm
radiation in a degassed water/methanol (4:1) solution, the disappearance of the tipical 1 La
anthracene band is observed. 1 H-NMR
spectra confirms the formation of
macrocyclic ligand LCH2, which is
obtained via a [4s+4s
photocicloaddition. When the ligand is
in the cyclic form, no binding of Cu II and
Ni II is observed in the studied pH
range. Molecular modelling of the cyclic
ligand show a highly strained structure
and a cavity which is not preorganized
to bind cations. Thus, in this case the
high enthalpic demand for the
deprotonation of amidic groups is not
balanced by an appropriate energy gain
coming from complex formation.
Moreover, no cyclization is observed
when CuL is irradiated in the same
conditions. The macrocyclic compound
LCH2 reverts to the open form, at room
temperature and in the dark, following a
first order kinetic with an half time of 8
LH2
LCH2
NH HN
NH HN
O O
NH HN
NH HN
O O
NH HN
NH HN
hours. The rate of the back reaction can be controlled with temperature: for example it becomes
20 times faster if temperature is raised to 45 °C. This kind of photocicloaddition reaction, which
is well known in literature and applied to several photoswitchable systems, [2] was applied for
the first time to a poliamminic ligand and in aqueous solution, to obtain a photocontrollable
receptor selective for Cu II . In addition, variations of the acid-base properties are observed upon
irradiation: when the ligand is in the diprotonated form (pH0.5 pH units).
[1] L. Fabbrizzi, M. Licchelli, P. Pallavicini, A. Perotti, A. Taglietti, D. Sacchi, Chem. Eur. J.,
1996, 2, 1, 167
[2] a) H. Bouas-Laurent, A. Castellan, J. P. Desvergne, R. Lapouyade, Chem. Soc. Rev., 2000,
29, 43; b) Y. Molard, D. M. Bassani, J. P. Desvergne, N. Moran, J. H. R. Tucker, J. Org. Chem.,
2006, 71, 8523-8531
T
h
Cu 2+ , OH -
Cu 2+ , OH -
N
N
Cu +2
Cu +2
N
O O
h
N
O O
PSA 37
CuL
PSA 38
Synthesis of biomimicking receptors by the molecular imprinted polymers
(MIP) technique, for application in chemical sensors
Maria Pesavento, Girolamo D’Agostino, Antonella Profumo, Giancarla Alberti, Raffaela Biesuz,
Dip. Chimica Generale, Università di Pavia. Via Taramelli 12, I-27100 Pavia, Italy
The concepts of supramolecular assembling of molecules are applied to the preparation of
molecular imprinted polymers (MIP). These are solids containing sites exactly suited to
accommodate a particular molecule, the template.
Solids of this kind have a number of useful applications as selective receptors, for
example for the extraction of the template from complex matrices [1] and for sensors [2]. As a
matter of fact, the central part of a chemical or biochemical sensor is the recognition element,
which is responsible for specifically binding the target analyte, while the transducer translates
the chemical signal generated upon binding into a quantifiable output signal. The recognition
element is usually a biological molecule such as an antibody or enzyme. The biomolecules have
some drawbacks, so that biomimetic receptor systems capable of binding target molecules with
affinities and specificities similar to natural receptors have been synthesized. Whereas for small
target molecules, such as inorganic ions, artificial receptors can often be obtained through
rational design and chemical synthesis [3] this may prove difficult if the analyte is a large and
complex molecule. Molecular imprinting in synthetic polymers (MIP) is being increasingly
adopted in this case and have therefore been called ‘antibody mimics’ [4]
The scheme of the molecular imprinting procedure is here reported
The imprinting is obtained by associating the target molecule with polymerizable active
monomers (methacrylic acid in the case of the MIPs here synthesized) in a solvent, through
relatively weak bonds as hydrogen bonds, dipole-dipole and lipophilic interaction, and by bulk
polymerization in the presence of a crosslinker.
In this investigations the target molecules (templates) are some triazines (atrazine and cyanuric
acid), and a thioxantene, ITX (isopropyl-9H-thioxanthen-9-one).
A potentiometric sensor ISE for atrazine based on MIP membranes, was obtained [5]
The membranes were characterized electrochemically in aqueous solution.
It has been found that when a potential step of –850 mV vs Ag/AgCl(sat) is applied to a glassy
carbon electrode in contact with a MIP membrane a current flows, decreasing with time.
The faradic process is not ascribable to the target molecules, nevertheless the current
intensity depends on the concentration of the template in the aqueous solution phase. This is
due to a variation of the conductivity of the membrane resulting from the association of the
template to the specific site in MIP. The observed effect was very specific for the template. In
the case of analogous polymers, obtained in the absence of the template (NIP), the current was
independent of the concentration.
.
[1] A.Martin-Esteban, Fresenius J Anal Chem., 2001, 371, 370-795
[2] K.Haupt, K. Mosbach,) Chem Rev, 2000, 100,2495-2504
[3] J.-M Lehn, Supramolecular Chemistry; 1995, Wiley-VCH: Weinheim,.
[4] G. Vlatakis, L. I Andersson, R Muller, and K. Mosbach, Nature 1993, 361, 645-650.
[5] G D’Agostino, G. Alberti, R. Biesuz and M. Pesavento, Biosensors and Bioelectronics, 2006,
2, 145-152.