Aminoacidic units wired on poly(aryleneethynylene) platforms as highly selective mercury-responsive materials

Article abstract of DOI:10.1016/j.tetlet.2012.11.034

Novel highly ethynylated materials carrying aminoacidic side arms on poly(aryleneethynylene) (PAE) conjugated backbones have been prepared and fully characterized. The luminescent sensing properties of these materials toward metal ions were investigated.

Full text of DOI:10.1016/j.tetlet.2012.11.034

Tetrahedron Letters 54 (2013) 303–307
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Aminoacidic units wired on poly(aryleneethynylene) platforms as highly
selective mercury-responsive materials
Antonella Ricci a, Marco Chiarini a, Maria Teresa Apicella a, Dario Compagnone a, Michele Del Carlo a,
c
c
c,
Claudio Lo Sterzo a,b, , Luca Prodi , Sara Bonacchi , Diego Villamaina , Nelsi Zaccheroni c,
⇑
⇑
a Dipartimento di Scienze degli Alimenti, Facoltà di Agraria, Università degli Studi di Teramo, Via Carlo R. Lerici 1, 64023 Mosciano S. Angelo (Teramo), Italy
b Istituto CNR di Metodologie Chimiche (IMC-CNR), Sezione Meccanismi di Reazione, Dipartimento di Chimica, Università La Sapienza, 00185 Roma, Italy
c Dipartimento di Chimica ‘G. Ciamician’, Latemar Unit, Università degli Studi di Bologna, 40126 Bologna, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
Novel highly ethynylated materials carrying aminoacidic side arms on poly(aryleneethynylene) (PAE)
conjugated backbones have been prepared and fully characterized. The luminescent sensing properties
of these materials toward metal ions were investigated. All compounds showed high selectivity toward
Hg(II) ions, and a signal amplification in Hg(II) detection was observed for the polymeric compound in
comparison with other molecular ligands.
Received 2 August 2012
Revised 31 October 2012
Accepted 7 November 2012
Available online 14 November 2012
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Poly(aryleneethynylene)s
Aminoacids
Fluorescence
Mercury ion
Sensors
The widespread role of metal ions in human health ranges from
the requirement of daily intake of essential elements, to the toxic-
ity due to metal overload.1 Because of their large presence in the
environment, as a result of human activities such as farming,
industry or from contamination during food processing and stor-
age, sources of metal ingestion might be contaminated drinking
water, seafood, breast milk, smoking, together with plants and ani-
mals used in the diet. In this respect, frequent exposure, although
below current admitted levels, may result in accumulative effects
and, with the extension of lifespan in the western world, the total
accumulation in the body over a lifetime, with special attention to
heavy metals, should be taken into serious account.2
Therefore, the development of selective and tunable practical
sensors for the detection and quantification of metal ions is the
subject of considerable research efforts.3–5
In nature, metal binding is achieved with an unsurpassed de-
gree of selectivity using aminoacids and peptides.6–9 In principle,
mimicking nature, one could thus succeed in designing and tuning
the selectivity of peptides to achieve the desired sensing proper-
ties.10 Despite these premises, only recently sensing systems using
aminoacidic and peptide moieties as recognition units for metal
sensing have started to appear.11–17
In the past few years we entered this field proposing a new syn-
thetic access to highly conjugated small molecules and polymers
characterized by poly(aryleneethynylene)s backbones (PAEs), and
their use as sensing material in the construction of efficient
chemo- and bio-sensors.18–20 In particular20 we reported the prep-
aration of co-polymers made of alternating arylene and phenan-
throline or bithiophene moieties spaced by ethynylene units, and
their sensing properties toward metal ions.
More recently, we presented our preliminary results obtained
with the use of further elaborated structures made of aminoacidic
recognition sites covalently linked to conjugated PAEs backbones,
and their effectiveness toward metal ion sensing.21 The good selec-
tivity and sensitivity obtained for the Hg(II) ion, an environmental
heavy metal pollutant of major concern,22 encouraged us to foster
the development of the sensing potential of highly ethynylated
platforms decorated with aminoacids.
In this Letter we present the synthesis of different PAE struc-
tures carrying leucine moieties as side arms, and the investigation
of their photophysical behavior upon interaction with metal ions.
Starting from the commercially available terephthalic acid bro-
mides 1 and 2, formation of derivatives 5 and 6 bearing L-leucine
⇑
Corresponding authors. Tel.: +39 0861 266904; fax: +39 0861 266915 (C.L.S.).
E-mail addresses: closterzo@unite.it (C.L. Sterzo), nelsi.zaccheroni@unibo.it
pendants was achieved in two steps by adapting literature proce-
dures (Scheme 1).23 Subsequently, formation of the conjugated
structures DLM, TLM, and DLP was achieved by the use of the Stille
(N. Zaccheroni).
Present address: University of Geneva, Department of Physical Chemistry, CH
1211.
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.11.034

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A. Ricci et al. / Tetrahedron Letters 54 (2013) 303–307
O
O
O
O
O
C
C
C
OH
Br
C
Cl
Br
C
N
OCH3
H3N
Cl
OCH3
H
SOCl2
R
R
R
N
Br
H
HO
C
Cl
C
CH3O
O
C
O
O
C
O
1, R = H
2, R = Br
3, R = H
4, R = Br
5, R = H
6, R = Br
Scheme 1. Schematic representation of the synthesis of aromatic bromides carrying leucine side arms.
Pd-promoted cross coupling reaction19,24 between the tributyle-
thynyltin derivatives 7 and 8 and the aryl halides 5 and 6, in the
presence of tetrakis(triphenylphosphine)palladium(0) catalyst
(Scheme 2).
The
terephthaloyl](
[(2,20-ethyne-2,1-diyl)diterephthaloyl](
L,L,L,L
model
)di-leucine methyl ester (DLM) and N,N0,N00,N000-
)tetra-leucine methyl
compounds
N,N0-[2,5-bis(phenylethynyl)
L,L
ester (TLM), presenting, respectively, two and four leucine units
O
O
O
O
O
C
C
C
O
C
C
C
C
N
OCH3
C
N
C
OCH3
2
C
C
SnBu3
H
H
H
H
7
Br
Br
C
C
N
C
"Pd"
H
H
CH3O
O
N
C
C
C
CH3O
O
C
6
DLM
C
C
C
O
O
O
C
O
O
O
O
O
C
O
C
N
OCH3
C
N
OCH3
C
N
OCH3
Bu3Sn
C
C
SnBu3
H
H
8
2
Br
C
C
"Pd"
H
H
H
CH3O
O
N
CH3O
O
N
C
H3CO
O
N
C
C
O
C
O
TLM
5
O
O
O
C
N
OCH3
C
N
C
OCH3
Bu3Sn
C
C
SnBu3
H
8
Br
Br
C
"Pd"
H
H
CH3O
O
N
CH3O
O
N
C
C
O
DLP
6
Scheme 2. Synthetic scheme of the leucine functionalized conjugated materials DLM, TLM, and DLP.

A. Ricci et al. / Tetrahedron Letters 54 (2013) 303–307
305
‘wired’ on different ethynylated platforms were designed with the
purpose of investigating the influence of the different number and
reciprocal proximity of the aminoacidic units toward metal ion
binding efficiency. On the other side, the polymeric material
vibronic transitions.28 Moderate emission quantum yields have
been observed: 5 ꢀ 10ꢁ3 for TLM and DLP and 1 ꢀ 10ꢁ2 for DLM.
We studied the photophysical behavior of DLM, TLM, and DLP
in the presence of different metal ions in order to investigate (i)
the influence of different distributions of the leucine groups along
the PAE backbones on metal ion coordination, and (ii) the possible
occurrence of synergic effects able to enhance the sensing effi-
ciency in more complex structures.
poly[N,N0-(2-ethynylterephthaloyl)(
L,L)di-leucine methyl ester]
(DLP), characterized by the regular repetition of leucine units into
a long spanning conjugated backbone, represents an example of
the so-called molecular wire approach to sensing.25,26
The photophysical characterization of DLM, TLM, and DLP was
carried out in acetonitrile. The absorption spectra (Fig. 1a) show,
for all compounds, the broad non-structured band profile typical
Very interestingly, all compounds presented significant changes
in their photophysical properties after the addition of Hg(II) ions,
while a hundred-fold excess of Pb(II), Co(II), Cd(II), Ni(II), Cu(II),
Fe(II), Zn(II), and Ag(I) did not cause any appreciable modifications
either of the absorption or of the emission spectra. The only rele-
vant exception was the significant luminescence variation of the li-
gand TLM in the presence of Cu(II) or Fe(II) (see discussion below
of these species, that can be attributed to delocalized
tions characterized by lower energies as the -delocalization in-
P–P
⁄ transi-
P
creases.18,19,27 This is confirmed by the bathochromic shift of the
absorption maxima, that goes from 310 nm for TLM to 320 nm
for DLM, up to 370 nm for the polymer DLP. In addition, in the case
of TLM and DLM, also the different number of electron-
withdrawing imido side substituents enhances this effect.27 The
maxima of the fluorescence bands present the same trend
(Fig. 1b), but with even more pronounced differences, shifting from
363 nm for TLM, to 392 nm for DLM, up to 505 nm for DLP. The
luminescence of these systems has already been attributed to
localized states generated by the migration of excitons along the
conjugated path with more or less pronounced shoulders due to
29
and Fig. S3, 4, Supplementary data).
In the case of DLM the complexation of Hg2+ induced drastic
changes in the absorption and emission spectra. In the absorption
spectrum a new band arose above 400 nm, with a concomitant de-
crease of the intensity maximum at 320 nm (typical band of the
non complexed ligand) that shifted to about 350 nm (Fig. S1,
Supplementary data), an expected trend since the red shift of the
absorption maximum is the most common and well known effect
induced by metal complexation. In the emission spectrum
Figure 1. Normalized absorbance spectra: (a) normalized emission spectra; (b) in acetonitrile of TLM (dotted line), DLM (solid line), and DLP (dashed line).
Figure 2. (a) Emission spectra (kexc 300 nm) of DLM 3.0 ꢀ 10ꢁ5 M in acetonitrile upon addition of increasing amounts of Hg2+. Inset: fluorescence intensity profiles (kexc
300 nm; kem 390 nm) vs added equivalents of Hg2+; (b) emission spectra (kexc 300 nm) of TLM 5.5 ꢀ 10ꢁ5 M in acetonitrile upon addition of increasing amounts of Hg2+. Inset:
fluorescence intensity profiles (kexc 300 nm) vs added equivalents of Hg2+; kem 367 nm (filled circles), kem 455 nm (open circles); (c) emission spectra (kexc 375 nm) of DLP
8.810ꢁ5 M in acetonitrile upon addition of increasing amounts of Hg2+. Inset: fluorescence intensity profile (kexc 375 nm; kem 505 nm) vs added equivalents of Hg2+
.

306
A. Ricci et al. / Tetrahedron Letters 54 (2013) 303–307
(Fig. 2a), a complete quenching of the band centered at 392 nm
could be observed.
Further additions of metal ions yield complex structures displaying
different absorption spectra, but without affecting the efficiency of
the quenching process. Comparing the emission intensity changes
occurring for TLM and DLP in the presence of Hg(II), while for the
first one each single complexation event induces the quenching of
up to monomeric units, in DLP one binding event quenches up to 5
repeating units. This is a very interesting result that indicates the
occurrence of positive cooperation effects inducing an amplifica-
tion of the response variations in species presenting a higher num-
ber of monomeric repeating units.
The plots of both the absorption and emission (Fig. 2a, inset)
intensities vs [Hg2+] showed marked variations between 0 and
2 equiv of added metal, to reach then a plateau.
The interpolation of the obtained data with the global analysis
software (SPECFIT)30 showed evidence of the formation of the
two equilibria (1) and (2).
2þ
L þ Hg2þ¡½LHgꢂ
ð1Þ
For all compounds detection limits (LOD) were calculated by 3
r
2þ
4þ
½L þ Hgꢂ þ Hg2þ¡½LHg2ꢂ
ð2Þ
and literature32 methods. The obtained values, coincident by the
two methods, were 0.42 ppm, 0.52 ppm, and 0.34 ppm for DLM,
TLM, and DLP, respectively. Interestingly, the detection limit ob-
tained for DLP is significatively lower than that of TLM, an ampli-
fication effect in line with the molecular wire effect.25,26 In this
respect, although the LOD values we obtained are in line with
Hg2+ detection procedures based on the use of other conjugated
sensing materials,3–5 the unprecedented use of aminoacidic func-
tionalized PAE platform might disclose new opportunities in the
sensoristic field.
Absorption and emission data fitting provided consistent values
for the association constants relative to these equilibria:
K1 = (1.8 0.3) ꢀ 106 Mꢁ1 for Eq. (1), and K2 = (1.3 0.3) ꢀ 105 M
ꢁ1
for Eq. (2).
Adding increasing amounts of mercury ions to acetonitrile solu-
tions of TLM, we observed absorption variations similar to those of
DLM (Fig. S2, Supplementary data), while the emission properties
presented more drastic changes (Fig. 2b). In this case, the quench-
ing of the emission intensity that centered at 363 nm was accom-
panied by the formation of a new, non-structured, band with the
maximum at 460 nm.
This is a very interesting result, since the possibility of monitor-
ing two distinct signals, the emission quenching at 363 nm and the
emission enhancement at 460 nm, is very valuable from an appli-
cative point of view, allowing ratiometric sensing.5b,31 In the case
of TLM, this can be particularly useful for the selective detection
of mercury ions in samples containing also Cu(II) and/or Fe(II), that,
as we have already mentioned, have a lower but not negligible
affinity for this ligand. Despite the fact that Cu(II) and Fe(II) ions,
similarly to Hg(II), were both provoking the rising of the band at
460 nm, the intensity decrease of the band at 363 nm was exclu-
sively related to the presence of Hg2+ ions (Fig. S3–6,
Supplementary data).
Plotting the values of the TLM absorption and emission (Fig. 2b,
inset) intensities vs [Hg2+], the trends were very similar to that ob-
served for DLM with marked variations between 0 and 2 equiv of
added metal, to reach then a plateau. The treatment of both the
absorption and emission titration data provided consistent values
for the association constants of equilibria (1 and 2): K1 = (1.7
0.5) ꢀ 107 Mꢁ1 and K2 = (2.9 0.7) ꢀ 106 Mꢁ1, respectively.
It is interesting to note that these values are both ten times
higher with respect to those obtained for DLM, clearly evidencing
the effectiveness of the different ligand architecture toward metal
complexation.
Moreover it is worth noting that also the ‘small molecules’ DLM
and TLM present very interesting sensing properties toward Hg2+
ions, in contrast to what was previously observed for other mono-
meric units of similar aminoacidic-functionalized conjugated
systems.13
In conclusion, by the use of the Stille Pd-catalyzed synthetic
protocol, we have prepared oligomeric and polymeric systems,
characterized by poly(aryleneethynylene) (PAE) conjugated back-
bones carrying aminoacidic side arms. These systems resulted
highly selective toward Hg2+ ions, a species of particular interest
due to its dramatic toxicity and environmental impact.
To the best of our knowledge, the compounds DLM, TLM, and
DLP described in this study represent the first example of sensing
materials made of aminoacidic groups directly linked to PAE struc-
tures. Although more studies are necessary to understand the spe-
cific selectivity of these systems for mercury,33 and, at this stage
only studies in acetonitrile solvent are reported, while for practical
applications aqueous environments are preferable, the enormous
range of possibilities offered by the proper choice and assembly
of aminoacidic and/or peptidic recognition sites with the PAE back-
bone confers to these systems a great potential for the develop-
ment of innovative materials for applications in the analytical field.
The study of other PAE systems carrying different aminoacids
and oligopeptides as side arms is ongoing.
Acknowledgments
Markedly different absorption and emission outputs were
observed upon exposure of DLP to the mercury ions. For clarity,
in order to facilitate the direct comparison with DLM and TLM,
concentration of DLP is expressed in terms of concentration of
monomeric repeating units. In the absorption spectrum (Fig. S7,
Supplementary data), no appreciable variations were observed up
to the addition of 0.15 equiv of Hg2+ to an acetonitrile solution of
the ligand. Only for higher amounts of the metal ions a gradual de-
crease of the absorbance could be detected without, however,
reaching a plateau, even in the presence of a 20-fold excess of mer-
cury ions. On the contrary, the trend in the emission plot was dra-
matically different (Fig. 2c). A drastic but not complete quenching
of the emission intensity was observed up to 0.2 equiv of added
mercury, and then no further changes of the residual luminescence
could be evidenced even in the presence of a 20-fold excess of the
metal ion (Fig. 2c, inset).
This work has been supported by MIUR through PRIN 2009
2009Z9ASCA project, and by ‘Fondazione del Monte di Bologna e
Ravenna’.
Supplementary data
Supplementary data associated with this article can be found, in
the
online
version,
at
http://dx.doi.org/10.1016/
j.tetlet.2012.11.034.
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