Sugar-poly(para-phenylene ethynylene) conjugates as sensory materials: Efficient quenching by Hg2+ and Pb2+ ions

Article abstract of DOI:10.1002/chem.200400788

Three polar poly(para-phenylene ethynylene)s (PPE) were synthesized by utilizing the Heck-Sonogashira protocol. Two of the PPEs carry β-glucopyranose substituents. Depending upon the linker used between the glycol units and the backbone, the fluorescence

Full text of DOI:10.1002/chem.200400788

FULL PAPER
Sugar–Poly(para-phenylene ethynylene) Conjugates as Sensory Materials:
2+
Efficient Quenching by Hg2+ andPb
Ions
Ik-Bum Kim, Belma Erdogan, James N. Wilson, and Uwe H. F. Bunz*[a]
Abstract: Three polar poly(para-phen-
ylene ethynylene)s (PPE) were synthe-
sized by utilizing the Heck–Sonoga-
shira protocol. Two of the PPEs carry
b-glucopyranose substituents. Depend-
ing upon the linker used between the
glycol units and the backbone, the fluo-
rescence of these PPEs can be
quenched by Hg2+ and Pb2+ to a vary-
ing degree. Monomeric model com-
pounds that are substituted with only
one glucose unit are not efficiently
quenched. The presence of many glu-
cose substituents in one PPE assembly
led to a large increase in the binding
constant to Hg2+ and quenching of the
fluorescence was amplified.
Keywords: alkynes
polymers glucose
transition metals
·
conjugated
sensors ·
·
·
Introduction
Conjugated polymers are organic semiconductors. They
find widespread use in electro-optical devices as well as in
analytical detection schemes.[8–10] The highly fluorescent
PPEs[11,12] have been exploited as sensory materials for elec-
tron-poor aromatics,[13] fluoride ions,[14] streptavidin,[15] and
some metal cations.[16] As an important note, a subtle differ-
ence in analyte can greatly affect the sensing action of con-
jugated polymers, and in some cases, closely related analytes
can be discerned with surprising efficiency. The reason for
the often observed selectivity is not clear but can be exploit-
ed to detect a specific analyte, an example being Tourꢀs
iodide sensor.[17]
The interaction of saccharides with heavy metals has been
only sparingly investigated, despite the importance of cellu-
lose copper complexes that allow for the processing of cu-
prammonium rayon.[18] Few reports describe the interaction
of monomeric sugars with mercury or lead ions. It was
claimed that simple sugars form chelate complexes of un-
known structure that show stability constants of ꢀ105 at
high ionic strengths in water.[19a] A second study contended
that “there is no major interaction between d-glucose and
the zinc-group metals (Zn, Cd, Hg) in aqueous solution”.[19b]
A third study[19c] investigated the coordination of mercury
salts to sugar amino acids. The authors of this study con-
clude from their data that complexformation is dependent
upon the chosen counterion for the mercury salt. A cited ex-
ample was HgCl2, dissolving mostly undissociated into
water. The solid-state structure of Hg2+ and Pb2+ complexes
of sugars are largely unknown and only recently Klüfers
et al.[20] have obtained crystal structures of some sugar–tran-
sition-metal complexes. An instructive case is the reaction
of g-cyclodextrin with Pb(NO3)2 which led to a sixteen-fold
plumbated cyclodextrin torus (solid state). Not all sugars
Herein we report the surprisingly efficient fluorescence
quenching of sugar-substituted poly(para-phenylene ethyn-
ylene)s (PPE) 2 and 3 by Hg2+ and Pb2+ ions. Lead and
mercury are heavy metals that play a large role in environ-
mental politics due to their high general and specific neuro-
toxicity.[1] Adverse long-term health effects are significant
even upon exposure to trace concentrations of these ele-
ments. Mercury and lead are prevalent as environmental
pollutants generated by industrial processes including the
large-scale combustion of coal (mercury) and the long-
standing environmental pollution by lead paint (now
banned).[2] As a consequence, the detection and determina-
tion of lead and mercury is of great scientific, medical, and
economic interest. The classic approaches to lead and mer-
cury sensing are atomic absorption spectrometry, inductively
coupled plasma mass spectrometry, anodic stripping voltam-
metry, and X-ray fluorescence spectrometry, all of which re-
quire expensive hardware, sophisticated sample treatment,
and well-trained operators.[3] As a consequence, the develop-
ment of sensitive “dip-stick” tests for mercury and lead ions
is attractive. Progress has been made, however, with recently
reported Pb2+ or Hg2+ sensors that utilize either specific
DNA and peptides or small fluorescent dyes, such as hy-
droxyquinoline-substituted crown ethers.[4–7]
[a] Dr. I.-B. Kim, Dr. B. Erdogan, J. N. Wilson, Prof. Dr. U. H. F. Bunz
School of Chemistry and Biochemistry
Georgia Institute of Technology
770 State Street, Atlanta GA 30332 (USA)
Fax: ( +1)404-385-1795
E-mail: uwe.bunz@chemistry.gatech.edu
Chem. Eur. J. 2004, 10, 6247 – 6254
DOI: 10.1002/chem.200400788
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FULL PAPER
form crystalline complexes with lead ions. According to Klü-
fers et al.,[20] such complexes appear gelatinous, detracting
attention from biologically and medicinally interesting
sugar–transition-metal coordination. And while the unspe-
cific interaction of some polythiophene derivatives with lead
and mercury salts at high concentrations has been studied
by McCullough et al.,[21] the chromic response was attribut-
ed to the conformational change of the conjugated back-
bone. Herein, we report the synthesis of PPEs 1–3 and their
sensitive but differential fluorescence quenching by Hg2+
and Pb2+ ions.
of the TMS groups by tetrabutylammonium fluoride. To
access the model compound 7b, diiodide 6 was coupled to
4-methoxyphenylacetylene under standard Pd-catalyzed
coupling conditions.[11,23] The trimer 7b was obtained analyt-
ically pure in a 54% yield after precipitation and washing
with a mixture of ethyl acetate/hexanes (EtOAc/Hex, 1:3).
The glucose-substituted polymer 2 and its model com-
pound 11b were obtained in an analogous way. Starting
from 2,5-diiodo-4-methoxyphenol (8), attachment of the tri-
ethylene glycol unit using iodide 5b and K2CO3 produced 9.
A Pd-catalyzed reaction with (trimethylsilyl)acetylene led to
monomer 10, while glycosyla-
tion of 9 using glucose penta-
acetate in the presence of
BF3·OEt2 in dichloromethane[24]
gave rise to the clean formation
of 11a in a 76% yield. Only the
b-anomer was observed under
these conditions, analogous to
the literature.[24] If diiodide 11a
is coupled to 10 by using stan-
dard
Heck–Cassar–Sonoga-
shira[23] conditions, deacetylated
polymer 2 is formed in a 93%
yield with an Mn of 35”103 amu
Results
and an Mw/Mn of 1.52. Coupling of 11a to 4-methoxyphenyl-
acetylene under the same conditions gave the analytically
pure, deacetylated model compound 11b (63% yield) after
precipitation and washing with a mixture of EtOAc/Hex
(1:3). The polymers and the model compounds are sparingly
soluble in water, but dissolve well in aprotic polar solvents
such as DMSO or dimethylformamide (DMF) forming clear
blue/green fluorescent solutions. The concomitant deacetyla-
tion of glucose when preparing 2 and 11b is gratifying and
occurs as a result of the action of the nucleophilic piperi-
dine. Varying amounts of N-acetylpiperidine are formed as
byproduct. The in situ deprotection is a convenient and mild
way of removing the acetyl groups cleanly.
Synthesis: Polymers 1 and 2 and the model compounds 7b
and 11b were synthesized according to Schemes 1 and 2, re-
Optical properties of the polymers andtheir model com-
pounds: PPEs 1 and 2 and their model compounds exhibit
the expected optical behavior.[11] In solution, the optical
properties of 1 and 2 are only determined by the dialkoxy–
PPE backbone and are not influenced by the presence of
the glucose or oligo(ethylene glycol) side chains. In Figure 1,
absorption and emission spectra of polymer 2 and model
compound 11b are shown. The optical properties of 2 in
DMF are identical, within experimental error, to those re-
Scheme 1. Synthesis of the polymer 1 and the model compound 7b.
a) K2CO3, DMF; b) [PdCl2(PPh3)2], CuI/THF, piperidine, then Bu4NF;
c) [PdCl2(PPh3)2], CuI/THF, piperidine.
abs
em
ported by other groups (l =410–452 nm,
l
=455–
max
max
spectively, while polymer 3 was prepared according to
ref. [22a] (Ethex: 1,4-diethynyl-2,5-bis(ethylhexyl)benzene).
Starting from diiodohydroquinone (4), etherification with
476 nm) for alkoxy–PPEs.[11] The quantum yield (F) of 2 in
DMF is 0.63, while that of 11b in DMF is 0.76. The quantum
yields for 1 and 7b are similar, as are their spectral features.
the polar chloride 5a furnished diiodide
6
(34%)
(Scheme 1). Polymerization utilizing the acetylene gas meth-
od[22b] produced polymer 1 in excellent yield (85%) with a
number-average molecular weight (Mn) of 9.5”103 amu and
a polydispersity (Mw/Mn) of 1.85. Another monomer, 7a,
was obtained in a 31% yield by the Pd-catalyzed alkynyl-
ation of 6 with (trimethylsilyl)acetylene followed by removal
Sensing of leadandmercury—comparison of polymers to
model compounds
Polymers: Polymers 2 and 3 were originally designed to test
the binding of PPEs to lectins, that is, sugar binding pro-
teins. In binding studies with the lectin concanavalin A
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Sugar–Poly(para-phenylene ethynylene) Conjugates
6247 – 6254
The quenching of polymers 2
and
3 with other transition-
metal ions was investigated.
Most transition-metal ions did
not elicit any response, however
Pb2+ and Hg2+ did (Table 1).
In our first experiment we
exposed 2 to HgCl2 and found
that significant quenching of 2
occurred with a KSV of 11”103.
Polymers
1 and 3, however,
were not quenched by mercury
chloride. Mercury chloride is
largely undissociated in water,
and probably even less dissoci-
ated in DMF. In a second set of
experiments we exposed 1–3 to
mercury nitrate, which is more
ionic. Here the KSV value in-
creased to 38”103 for 2 and a
moderate KSV value of 2.9”103
for the non-glycosylated poly-
mer 1 was observed. Polymer 3
was not quenched at all by
Hg(NO3)2. Is the nature of the
counterion or the dissociation
constant of importance for the
fluorescence quenching of the
polymers? We chose mercury
Scheme 2. Synthesis of the model compound 11b and the polymer 2. a) K2CO3, acetone; b) [PdCl2(PPh3)2],
CuI/THF, piperidine, then Bu4NF; c) [PdCl2(PPh3)2], CuI/THF, piperidine; d) glucose(OAc)5, BF3·OEt2;
e)[PdCl2(PPh3)2], CuI/THF, piperidine, then Bu4NF.
acetate and mercury trifluoroacetate (tfa) as additional
quenchers, the acetate having a low dissociation constant in
DMF, while mercury trifluoroacetate is more dissociated.
Mercury chloride and mercury acetate are grouped as low-
dissociating salts, while mercury nitrate and mercury tri-
fluoroacetate show a considerably higher concentration of
free mercury ions. The quenching of 1–3 with mercury ace-
tate is similar to that observed for quenching with HgCl2,
and the results of the quenching experiments of 1–3 with
mercuric trifluoroacetate ([Hg(tfa)2]) resemble those ob-
tained for the nitrate, with [Hg(tfa)2] being a somewhat
better quencher than the nitrate. Glucose–PPE
2 is
quenched by mercury trifluoroacetate effectively (KSV =48”
103) while the oligo(ethylene glycol)-substituted PPE (1)
shows a quenching of 2.9”103 and polymer 3 is quenched
only weakly by mercury trifluoroacetate. To test the quench-
ing of these PPEs by an unspecific quencher we investigated
the effect of the addition of 4-nitrophenol to 1–3. The ob-
tained KSV values are considerably lower than those for the
mercury salts and all similar, in the range of 1–2”103. The
data suggest that the chemical nature of the counterion to
the Hg2+ ion does not play a significant role in the interac-
tion of the PPEs with the transition-metal ion, only the dis-
sociation seems to be important. Addition of sodium acetate
does not quench the fluorescence of polymers 1–3, further
reinforcing the trend of the chemical insignificance of the
chemical nature of the counterion.
&
Figure 1. Absorption ( ) and emission (c) spectra (a.u.=arbitrary
abs
em
units) of model compound 11b in DMF (top;
397 nm) and glucose–PPE
476 nm).
l
=369 nm,
=436 nm,
l
=
max
max
abs
em
2 in DMF (bottom;
l
l
=
max
max
(Con A), we found that upon addition of the lectin-cofactor
Mn2+ to the solution of glucose–PPE 2 and Con A, a small
but significant quenching of the fluorescence of 2 occurred.
The quenching effect of lead was investigated. PPEs 1–3
show significant KSV values. Polymer 3 showed the largest
value with KSV =72”103. Only lead acetate quenches the
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U. H. F. Bunz et al.
FULL PAPER
Table 1. The degree of quenching observed with Hg2+ and Pb2+ ions, in addition to some other quenchers, by
polymers 1–3 and model compound trimers 7b and 11b in DMF.
anomeric carbon. Mercury is a
heavy-metal ion and will
quench fluorescence of a dye
by enhanced spin–orbit cou-
pling when in close proximity.
As a quantitative measure of
quenching, the Stern–Volmer
equation [Eq. (1)] is a useful
tool. The quencher concentra-
tion is [Q], KSV is the Stern–
Volmer constant, F0 is the fluo-
rescence intensity measured
without additional quencher,
and F[Q] is the fluorescence in-
tensity with quencher at a given
[Q].[27]
1
KSV
7b
KSV
2
KSV
11b
KSV
3
KSV
7b
SV
11b
SV
K1SV/K
K2SV/K
HgCl2
NQ[a]
NQ
NQ
NQ
520
200
NQ
NQ
nd[c]
nd[c]
130
–
–
11”103
16”103
38”103
48”103
25”103
<10
NQ
NQ
>1000
>1000
35
53
NQ
<10
NQ
Hg(OAc)2
Hg(NO3)2
[Hg(tfa)2][b]
Pb(OAc)2
Pb(NO3)2
NaOAc
4-nitrophenol
paraquat
dichloride
2.9”103
2.9”103
15”103
<10
5.6
15
>1000
1.1”103
0.91”103
<10
<10
>250
72”103
<10
–
<10
–
–
–
1
NQ
–
–
4
<10
nd[c]
NQ
1.2”103
520
1.3”103
260
nd[c]
2.1”103
4.5”103
290
[a] NQ=no quenching observed. [b] tfa: trifluoroacetate. [c] nd=not determined.
ðF0=F Þ ¼ 1 þ KSV½QŠ or KSV ¼ fðF0=FÞÀ1g=½QŠ
ð1Þ
polymers; the nitrate has an almost negligible effect upon
the fluorescence of the polymers. The presence of the ace-
tate ion seems to be an auxiliary in the coordination of the
lead ions to the sugar-coated PPEs, but quenching is not a
function of the acetate anion as such.
½QŠ
The slope of the graph (F0/F[Q]) versus [Q] equals KSV.
The more sensitive a given system is to a specific quencher,
the steeper the Stern–Volmer plot and the higher the KSV
value. Quenching processes can be generalized as having
two mechanisms, namely, they can be static or dynamic in
nature.[27] In static quenching, the fluorophore and the
quencher form a complexin the ground state. Upon irradia-
tion, quenching of the excited fluorophore by the complexed
quencher occurs. In dynamic or collision quenching, the ex-
cited state of the fluorophore is quenched by collision with
the quencher. If either one of these mechanisms is predomi-
nant in a given system, a linear Stern–Volmer relationship
occurs. A linear relationship is observed in the herein inves-
tigated Stern–Volmer plots of the quenching of the PPEs 1–
3, as shown in Figure 2 for glucose–PPE 2. Un-aggregated
Model compounds: To investigate if the binding of the tran-
sition metals to the sugar-coated PPEs is cooperative[25] or if
the molecular-wire effect as investigated by Swager for a
PPE model system[26] plays a role, the model compoundsꢀ
(7b, 11b) quenching behavior towards mercury and lead
salts was investigated. Neither 7b nor 11b were quenched
when mercury chloride or mercury acetate were added to
them in DMF (Table 1). In the case of Hg(NO3)2 and
[Hg(tfa)2], both 7b and 11b were quenched modestly
(Table 1) with KSV values in the range of 0.2–1.1”103.
Quenching polymers and model compounds with 4-nitrophe-
nol and paraquat (1,1’-dimethyl-4,4’-bipyridinium) dichlo-
ride: To obtain a standard for the sensing abilities of the
model compounds and the polymers, a reliable nonbinding
quencher was investigated. Using 4-nitrophenol, KSV values
between 1–2”103 for polymers 1–3 were obtained. For the
model compounds, nitrophenol was not a viable quencher as
its absorption wavelength overlaps with the emission wave-
length of 7b and 11b. Instead, paraquat dichloride was a
chosen as the quencher. An alkoxy–PPE was quenched by
paraquat with a KSV value of 5–6”102.[26] We find similar
values for 1 (KSV =520) and 2 (KSV =260). The model com-
pounds 7b and 11b showed KSV values of 130 and 290, re-
spectively. However, polymer 3 displayed much more effi-
cient quenching (KSV =4.5”103) suggesting a ground-state
complexof 3 with paraquat dichloride.
PPEs in solutions show
a short fluorescence lifetime
(around 0.3–0.5 ns),[11] and their fluorescence lifetimes do
not change upon addition of quencher.[26] It can be assumed
that static quenching is the prevalent mechanism operating
in the cases investigated here.[26] In the case of static quench-
ing, the Stern–Volmer constant is equal to the binding con-
Discussion
The literature claims that sugars form complexes that bind
tightly to Hg2+ in solution. We have made monomeric (7b,
11) and polymeric (2, 3) glucose derivatives of arylene ethy-
nylenes. All of the phenylene ethynylenes are highly fluores-
cent and are attached to the glucose residue through the
Figure 2. Spectra showing the fluorescence emitted during the quenching
of glucose–PPE 2 by [Hg(tfa)2] in DMF: quencher concentrations (m)
from the bottom upwards are 1”10À4, 6”10À5, 4”10À5, 2”10À5, and 1”
10À5; the top spectrum is without quencher. The inset shows the Stern–
Volmer plot. The extracted Stern–Volmer constant is KSV =48”103.
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Sugar–Poly(para-phenylene ethynylene) Conjugates
6247 – 6254
stant of the quencher to the fluorophore;[28] KSV convenient-
ly delivers the binding constant of mercury and lead salts to
sugar derivatives.
hancement in a specifically designed model/PPE system.
The 60-fold enhancement was the largest reported and
might represent the upper limit for the molecular-wire effect
in solution.[26]
Mercury: We observed linear Stern–Volmer plots and in
some cases large KSV values using mercury salts as quench-
ers. Quenching is most efficient if a sugar unit is present (2)
but a triethylene glycol linker also seems necessary. Poly-
mers 1 and 3 are lacking either one of these features. If only
the ethylene glycol group is present (1) quenching is ob-
served for dissociated mercury salts, but no quenching is re-
corded for the less-dissociated acetate and chloride. Howev-
er, polymer 2 is quenched by all of the tested mercury salts.
The model compounds 7b and 11b do not show any quench-
ing with mercury chloride or acetate. However, in the case
of the trifluoroacetate and the nitrate, moderately efficient
quenching is observed (Figure 3). According to Table 1, the
In one group we see enhancements of quenching that
range from factors of 6 to 35. In a second group of experi-
ments we observed 2 to be quenched with Hg2+ salts by a
factor >1000 times better than model 11b. The enhanced
quenching can not be due only to the molecular-wire effect,
cooperative binding must also be present. In cooperative
binding (Figure 4),[25] two or more sugar substituents of one
Figure 3. Spectra showing the fluorescence emitted during the quenching
of model compound 11b by [Hg(tfa)2] in DMF: quencher concentrations
(m) from the bottom upwards are 1”10À3, 8”10À4, 6”10À4, and 4”10À4
;
the top spectrum is without quencher. The inset shows the Stern–Volmer
plot. The extracted Stern–Volmer constant is KSV =0.9”103.
Figure 4. Schematic representation of the cooperative quenching of glu-
cose–PPE 2 by mercury trifluoroacetate. Two or more sugar substituents
(crossed arrows) of one chain partake in the complexation of a single
mercury ion (sphere).
binding of the model compounds to Hg2+ range from KSV
=
200–1100; 7b and 11b bind better to Hg(NO3)2 than to
[Hg(tfa)2].
chain partake in the complexation of a single mercury ion
or HgX2-molecule to give a significantly higher KSV value
for 2 than for 11b thus explaining the differences in quench-
ing of 2 versus 11b with mercury acetate or mercury chlo-
ride.
The largest increases of quenching ability are found when
comparing the quenching of 11b with mercury acetate or
mercury chloride with the quenching of 2 to the same salts.
The increase of the binding constant values via KSV is larger
than 103. In the case of the nitrate and the trifluoroacetate,
the increase when going from model compound to polymer
is significantly smaller but still a factor of 35–53. When com-
The polymersꢀ (1–3) fluorescence is efficiently quenched
by lead acetate, with a KSV range of 15–72”103. Lead nitrate
does not quench the polymersꢀ fluorescence at all. The larg-
est KSV value resulted when 3 was quenched by lead acetate,
but model compounds did not show any quenching. We can
speculate that 1–3 form a complexwith lead ions in which
the acetate groups might act as (a) supporting and/or bridg-
ing ligand(s) and increase binding. The quenching enhance-
ment shown by lead acetate is large for 1–3 suggesting that
both the molecular-wire effect and cooperative binding are
important, that is, two or more sugar substituents are in-
volved in the binding event.[27] We suspect that the unusually
high acidities of Hg2+ and Pb2+[29] led to a higher than ex-
pected binding constant to alcohol functionalities. With the
paring 7b to 1, the increase in KSV is less pronounced (KS1V
/
K7SbV =6–15).
When comparing the model compound and polymer we
have two independent effects that enhance quenching. The
first one is the molecular-wire effect, as described by
Swager.[26] In this scenario, amplified quenching is achieved
by the connection of the sensory elements to the conjugated
backbone. Complexing one quencher will therefore shut
down the fluorescence of the whole chain. Reported en-
hancements in KSV range from 17 for a nonfunctionalized
PPE versus a nonfunctionalized PE-trimer to a 60-fold en-
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U. H. F. Bunz et al.
FULL PAPER
enhanced acidity of the OH groups in sugars (pKa =12.3–
12.4)[30] this binding should be somewhat more enhanced.
We can speculate that a hexacoordinated Hg2+ ion sur-
rounded by three glucose molecules, of which two are
mono-deprotonated, is involved in these complexes
(Figure 5). In this case, the trans-arranged 4- and 5- hydroxyl
Experimental Section
Synthesis of 2,5-diiodo-4-methoxyphenol (8): 1,4-Dimethoxy-2,5-diiodo-
benzene (9.28 g, 23.8 mmol) was dissolved in dichloromethane (300 mL).
The reaction flask was cooled to À788C in a dry ice/acetone bath. An 1m
solution of boron tribromide (23.8 mL, 23.8 mmol) in CH2Cl2 was slowly
added through the condenser. The reaction mixture was stirred under ni-
trogen at room temperature for 16 h. Then water (200 mL) was added to
the flask. The organic layers were dried over MgSO4 and evaporated to
dryness. The product was purified by column chromatography (ethyl ace-
tate/hexanes 1:2). The solid was obtained in a 68% yield (6.80 g). M.p.
1
À
115–1168C; H NMR (400 MHz, CDCl3): d=7.37 (s, 1H; Ar H), 7.00 (s,
1H; Ar H), 5.01 (s, 1H; Ar H), 3.78 ppm (s, 3H; C H); 13C NMR
(100 MHz, CDCl3): d=152.96, 149.88, 124.98, 119.67, 86.74, 84.50,
57.22 ppm; IR (KBr): n˜ =3258, 2043, 1487, 1393, 1199, 1056, 1024, 854,
770 cmÀ1; elemental analysis calcd (%) for C7H6I2O2: C 22.36, H 1.61;
found: C 22.24, H 1.66.
À
À
À
Synthesis of 9: Compound
8
(3.01 g, 8.00 mmol) and 2-[2-(2-
iodoethoxy)ethoxy]ethanol (2.50 g, 9.60 mmol) were dissolved in acetone
(30 mL). Potassium carbonate (11.0 g, 80.0 mmol) was added to the so-
lution. The reaction mixture was stirred under nitrogen at the reflux tem-
perature for 48 h. After the solution was cooled, the solid was filtered off
and washed with acetone. The solvent was evaporated and the reaction
mixture was extracted three times with chloroform. The organic layers
were dried over MgSO4 and evaporated to dryness. The product was pu-
rified by column chromatography (ethyl acetate/hexanes 1:1). A colorless
solid was obtained (3.10 g, 76%). M.p. 55–568C; 1H NMR (500 MHz,
À
À
CDCl3): d=7.23 (s, 1H; Ar H), 7.15 (s, 1H; Ar H), 4.08 (t, J(H,H)=
Figure 5. Speculative molecular model of a [Hg(glucose)3] complex. Note
that two of the three sugar moieties are deprotonated for charge neutrali-
zation.
À
À
À
4.6 Hz, 2H; C H), 3.86 (t, J(H,H)=4.6 Hz, 2H; C H, ), 3.79 (s, 3H; C
À
À
H), 3.77 (t, J(H,H)=4.4 Hz, 2H; C H), 3.71 (t, J(H,H)=4.6 Hz, 2H; C
À
H), 3.68 (t, J(H,H)=4.6 Hz, 2H; C H), 3.60 ppm (t, J(H,H)=4.55 Hz,
2H; C H); 13C NMR (75 MHz, CDCl3): d=153.41, 152.55, 123.63,
À
121.17, 86.46, 85.32, 72.46, 71.14, 70.48, 70.29, 69.59, 61.78, 57.11 ppm; IR
(KBr): n˜ =3421, 2887, 1486, 1435, 1352, 1216, 1124, 1062, 1022, 854,
777 cmÀ1; elemental analysis calcd (%) for C13H18I2O5: C 30.73, H 3.57;
found: C 30.86, H 3.59.
groups could operate in a chelating fashion. Alternatively
the 6-CH2OH groups and the 5-hydroxyl group of the glu-
cose substituent could be involved in a trimeric heavy-metal
chelate.
Synthesis of 10: Compound 9 (1.02 g, 2.00 mmol) and (trimethylsilyl)-
ethyne (0.7 mL, 5.0 mmol) were reacted in diisopropylamine/THF (1:4
v/v; 20 mL) in the presence of CuI (4 mg, 0.02 mmol) and [PdCl2(PPh3)2]
(14 mg, 0.02 mmol). The solution was stirred overnight at room tempera-
ture. The mixture was filtered to remove the ammonium salts and the sol-
vent mixture was removed in vacuo. The solid residue was dissolved in
THF and a solution of tetrabutylammonium fluoride (1m) in THF was
added. The reaction mixture was stirred for 10 min at room temperature
and the solvent was removed in vacuo. The product was isolated after
chromatography on a silica-gel column (ethyl acetate/hexanes/MeOH
Conclusion
In summary, novel and polar PPEs 1 and 2 were made by
using the Pd-catalyzed coupling reaction of the Heck–
Cassar–Sonogashira–Hagihara type.[11] Fluorescence quench-
ing of PPE 2 and its monomeric model compound 11b with
mercury and lead salts was investigated. For both metals we
found dramatic enhancements of the quenching when going
from 11b to the sugar-coated PPE 2. In some cases the en-
hancements are too large to be explained by the molecular-
wire effect alone. In these cases a cooperative binding effect
between two or more neighboring glucose units placed
along the conjugated backbone must play a significant role
in the quenching experiments. However, the large complexa-
tion constants for mercury ions with monomeric sugars re-
ported in the literature[18] are not borne out by our investi-
gation. In future experiments we will make PPEs that con-
tain additional auxiliary metal-sensing entities, such as thiol
and/or aza crown functionalities in addition to the sugar
substituents.
20:20:1) as
a
pale-yellow solid (275 mg, 45%). 1H NMR (500 MHz,
[D6]DMSO): d=7.10 (s, 1H; Ar H), 7.06 (s, 1H; Ar H), 4.57 (t,
J(H,H)=5.50 Hz, 1H; O H), 4.39 (d, J(H,H)=5.15 Hz, 2H; C H), 4.11
(t, J(H,H)=4.75 Hz, 2H; O C H), 3.78 (s, 3H; O C H), 3.73 (t,
À
À
À
À
À À
À À
À À
À À
J(H,H)=4.55 Hz, 2H; O C H), 3.62 (t, J(H,H)=4.3 Hz, 2H; O C H),
À À
3.54 (t, J(H,H)=4.3 Hz, 2H; O C H), 3.50 (t, J(H,H)=5.25 Hz, 2H;
O C H), 3.43 ppm (t, J(H,H)=5.15 Hz, 2H; O C H); 13C NMR
(75 MHz, CDCl3): d=154.33, 153.37, 118.24, 115.64, 113.32, 112.32, 82.85,
82.77, 79.49, 79.37, 72.38, 70.89, 70.27, 69.42, 69.32, 61.56, 56.21 ppm; IR
(KBr): n˜ =3433, 3276, 2869, 1630, 1499, 1456, 1393, 1276, 1221, 1055,
1014, 864 cmÀ1; elemental analysis calcd (%) for C17H20O5: C 67.09, H
6.62; found: C 66.00, H 7.14.
À À
À À
Synthesis of 11a: b-Glucosepentaacetate (1.52 g, 3.00 mmol) and diiodide
9 (1.17 g, 3.00 mmol) were dissolved in anhydrous CH2Cl2 (10 mL), then
BF3·OEt2 (0.3 mL, 2.4 mmol) was added slowly. The reaction mixture was
stirred at room temperature for 24 h and then poured into 5% aqueous
NaHCO3 (20 mL). The organic layer was separated, washed with 5%
aqueous NaHCO3 and water. The organic phase was dried over MgSO4
and evaporated to dryness. The product was isolated by column chroma-
tography (ethyl acetate/hexanes/methanol 20:20:1). The product was ob-
tained as oil (1.24 g, 76%). 1H NMR (500 MHz, CDCl3): d=7.15 (s, 1H;
It is of importance that sugar-substituted polymers are ca-
pable of detecting mercury and lead salts with quite high
sensitivity, particularly because hydroxyl groups are not the
classic ligands for soft metals.
À
À
À
Ar H), 7.07 (s, 1H; Ar H), 5.07 (t, J(H,H)=9.5 Hz, 1H; C H), 4.96 (t,
J(H,H)=9.7 Hz, 1H; C H), 4.87 (t, J(H,H)=9.6 Hz, 1H; C H), 4.51 (d,
J(H,H)=8.0 Hz, 1H; C H), 4.14 (q, 1H; C H), 4.00 (m, 3H; C H), 3.83
À
À
À
À
À
6252
ꢁ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.chemeurj.org Chem. Eur. J. 2004, 10, 6247 – 6254

Sugar–Poly(para-phenylene ethynylene) Conjugates
6247 – 6254
À
À
À
À
À
À
(m, 1H; C H), 3.76 (t, J(H,H)=4.95 Hz, 2H; C H, ), 3.71 (s, 3H; C H),
3.52 (4H; C H), 3.45 (4H; C H), 3.39 ppm (4H; C H); 13C NMR
(100 MHz, [D6]DMSO): d=153.00, 117.26, 113.64, 91.54, 72.36, 70.19,
69.86, 69.01, 60.18, 60.06 ppm; IR (KBr): n˜ =3426, 2928, 2871, 2204, 1514,
À
3.66–3.52 (m, 8H; C H), 1.97 (s, 3H), 1.93 (s, 3H), 1.91 (s, 3H),
1.89 ppm (s, 3H); 13C NMR (75 MHz, CDCl3): d=170.33, 169.92, 169.09,
169.05, 153.25, 152.49, 123.43, 121.05, 100.63, 86.34, 85.22, 77.66, 71.59,
71.10, 71.62, 70.70, 70.23, 70.20, 69.50, 69.01, 68.22, 61.81, 57.01, 20.76,
20.71, 20.62, 20.60 ppm; IR (KBr): n˜ =2939, 2876, 1754, 1483, 1454, 1437,
1365, 1348, 1217, 1174, 1122, 1060, 1038, 762 cmÀ1; elemental analysis
calcd (%) for C27H36I2O14: C 38.68, H 4.33; found: C 38.29, H 4.37.
1426, 1353, 1278, 1216, 1127, 1062, 947, 886, 860 cmÀ1
.
Synthesis of model compound 7b: Compound 7 (312 mg, 0.500 mmol)
and p-methoxyphenylacetylene (145 mg, 1.10 mmol) were dissolved in a
mixture of piperidine (2 mL), THF (2 mL), and methanol (2 mL) in a
Schlenk flask with
a flow of nitrogen and with magnetic stirring.
Synthesis of glucose–PPE 2: Diyne 10 (111 mg, 0.365 mmol) and diiodide
11a (276 mg, 0.329 mmol) were dissolved in a mixture of piperidine
(0.5 mL), THF (0.5 mL), and methanol (0.5 mL) in a Schlenk flask with a
flow of nitrogen and with magnetic stirring. [PdCl2(PPh3)2] (8 mg,
0.011 mmol) and CuI (2 mg, 0.011 mmol) were added to the flask. The re-
action mixture was stirred under the nitrogen at 408C for 16 h. The so-
lution was slowly added to acetone (300 mL). The precipitate was
washed with water. An orange solid was obtained (220 mg, 93%). The
weight average molecular weight (Mw) was estimated to be 53320 with a
polydispersity (Mw/Mn) of 1.52 (eluent: DMF). 1H NMR (500 MHz,
[D6]DMSO): d=7.17, 7.13, 4.95, 4.89, 4.86, 4.54, 4.46, 4.20, 4.12, 4.11,
3.86, 3.81, 3.66, 3.53, 3.44, 3.39 ppm; 13C NMR (100 MHz, [D6]DMSO):
d=153.75, 152.90, 117.44, 115.40, 113.63, 112.99, 103.02, 91.46, 76.88,
76.76, 73.40, 72.40, 70.24, 70.02, 69.92, 69.77, 69.12, 67.86, 61.06, 60.24,
56.35 ppm; IR (KBr): n˜ =3431, 2877, 2204, 1509, 1452, 1403, 1271, 1218,
1040, 856 cmÀ1; elemental analysis calcd (%) for C36H46O15: C 60.16, H
6.45; found: C 58.89, H 6.67.
[PdCl2(PPh3)2] (7 mg, 10 mmol) and CuI (2 mg, 10 mmol) were added to
the flask. The reaction mixture was stirred under nitrogen at 408C for
16 h and the solvent was removed in vacuo. Water (20 mL) was added to
the flask. The precipitate was collected and washed with water and ethyl
acetate/hexanes mixture (1:4). A pale-yellow solid was obtained (170 mg,
1
À
54%). H NMR (400 MHz, CDCl3): d=7.44 (d, 4H; Ar H), 7.00 (s, 2H;
À
À
Ar H), 6.85 (d, 4H; Ar H), 4.18 (t, 4H), 3.90 (t, 4H), 3.80 (m, 10H),
3.66–3.61 (m, 8H), 3.55 ppm (t, 4H); 13C NMR (100 MHz, CDCl3): d=
159.70, 153.39, 132.98, 117.36, 115.35, 114.20, 113.99, 95.02, 84.45, 72.42,
71.19, 70.50, 69.75, 69.63, 61.74, 55.29 ppm; IR (KBr): n˜ =3431, 2931,
2872, 1603, 1515, 1409, 1247, 1217, 1121, 1060, 1028, 833 cmÀ1
.
Synthesis of model compound 11b: Diiodide 11a (372 mg, 0.444 mmol)
and p-methoxyphenylacetylene (129 mg, 1.977 mmol) were placed in a
small Schlenk tube. Piperidine (2 mL), THF (2 mL), and methanol
(2 mL) were added under a flow of nitrogen. Magnetic stirring dissolved
the starting materials, upon which [PdCl2(PPh3)2] (7 mg, 10 mmol) and
CuI (2 mg, 10 mmol) were added to the flask. The reaction mixture was
stirred under nitrogen at 408C for 16 h and the solvent was removed in
vacuo. Water (20 mL) was added to the flask. The precipitate was collect-
ed and washed with water and ethyl acetate/hexanes mixture (1:3). A
pale-yellow solid was obtained (190 mg, 63%). 1H NMR (400 MHz,
Synthesis of 6: 2,5-Diiodohydroquinone (5.05 g, 14.0 mmol) and 2-[2-(2-
chloroethoxy)ethoxy]ethanol (5) (9.44 g, 56.1 mmol) were dissolved in
DMF (30 mL). Potassium carbonate (55 g, 0.4 mol) was added to the so-
lution. The reaction mixture was stirred under the nitrogen at the reflux
temperature for 3 d. After the solution was cooled, the solid was filtered
off. The solvent was evaporated and the reaction mixture was extracted
with chloroform and washed with water. The organic layers were dried
over MgSO4 and evaporated to dryness. The solid was crystallized from
acetonitrile and further purified by column chromatography (ethyl ace-
tate/methanol 97:3). A colorless solid was obtained (2.93 g, 34%). M.p.
77–788C; 1H NMR (500 MHz, CDCl3): d=7.22 (s, 2H; Ar-H), 4.09 (t,
J(H,H)=4.55 Hz, 4H), 3.86 (t, J(H,H)=4.55 Hz, 4H), 3.77 (t, J(H,H)=
4.55 Hz, 4H), 3.71 (t, J(H,H)=4.55 Hz, 4H), 3.68 (t, J(H,H)=4.55 Hz,
4H), 3.60 ppm (t, J(H,H)=4.55 Hz, 4H); IR (KBr): n˜ =3424, 2943, 2886,
1487, 1467, 1355, 1326, 1265, 1240, 1218, 1126, 1117, 1086, 1063, 1031,
À
À
À
CDCl3): d=7.44 (d, 4H; Ar H), 6.99 (s, 1H; Ar H), 6.93 (s, 1H; Ar
À
H), 6.83 (d, 4H; Ar H), 4.24 (d, 1H), 4.12 (m, 2H), 3.87–3.73, 3.62–3.42,
3.32 (t, 1H), 3.24 ppm (d, 1H); 13C NMR (100 MHz, CDCl3): d=159.68,
152.98, 133.14, 117.70, 114.09, 113.95, 113.31, 103.01, 95.02, 84.63, 76.33,
75.63, 73.28, 72.89, 70.89, 70.30, 70.14, 69.60, 69.49, 68.70, 61.51, 56.34,
55.30, 55.25 ppm; IR (KBr): n˜ =3418, 2933, 1603, 1515, 1404, 1248, 1218,
1171, 1030, 832 cmÀ1
.
Acknowledgement
884, 858, 835, 798 cmÀ1 13C NMR (75 MHz, CDCl3): d=152.76, 123.18,
;
86.28, 72.43, 71.07, 70.38, 70.11, 69.49, 61.64 ppm.
This work was supported by the National Institute of Health (NIH 1U01-
AI5-650-301). We thank Prof. Mohan Srinivasarao (Polymer, Textile, and
Fiber Engineering) and Prof. Christoph Fahrni (School of Chemistry and
Biochemistry) for helpful discussions and suggestions.
Synthesis of 7a: Diiodide 6 (6.26 g, 10.0 mmol) and (trimethylsilyl)ethyne
(2.45 g, 25.0 mmol) were reacted in diisopropylamine/THF (1:4 v/v;
100 mL) in the presence of CuI (24 mg, 0.13 mmol), Pd(OAc)2 (22 mg,
0.10 mmol) and PPh3(131 mg, 0.499 mmol). The solution was stirred over-
night at room temperature. The mixture was filtered to remove the am-
monium salts and the solvent was evaporated in vacuo. The solid residue
was dissolved in THF (50 mL) and 1m solution of tetrabutylammonium
fluoride (in THF, 16 mL, 16 mmol) was added. The reaction mixture was
stirred for 10 min at room temperature. The solvent was removed in
vacuo and the product was isolated on a silica-gel column (ethyl acetate/
[1] a) T. C. Hutchinson, K. M. Meema, Lead, Mercury, Cadmium and
Arsenic in the Environment, John Wiley, New York 1987; b) M. J.
Scoullos, G. H. Vonkeman, L. Thornton, Z. Makuch, Mercury, Cad-
mium, Lead: Handbook for Sustainable Heavy Metals Policy and
Regulation (Environment & Policy, V. 31), Kluwer Academic, 2001;
c) T. W. Clarkson, Crit. Rev. Clin. Lab. Sci. 1997, 34, 369–403.
[2] For informative websites dealing with the problem of mercury and
other heavy metals in the environment see: a) http://www.epa.gov/
opptintr/lead/; b) http://www.orcbs.msu.edu/AWARE/pamphlets/haz-
hexanes/methanol 20:20:1). A pale-yellow solid was obtained (1.30 g,
1
À
31%). H NMR (300 MHz, CDCl3): d=6.94 (s, 2H; Ar H), 4.09 (t,
J(H,H)=4.8 Hz, 4H), 3.81 (t, J(H,H)=4.8 Hz, 4H), 3.71 (t, J(H,H)=
4.8 Hz, 4H), 3.62 (t, J(H,H)=4.8 Hz, 4H), 3.54 (t, J(H,H)=4.8 Hz, 4H),
3.32 ppm (s, 2H); 13C NMR (75 MHz, CDCl3): d=153.66, 117.98, 113.28,
82.85, 79.38, 72.40, 70.92, 70.28, 69.43, 69.27, 61.56 ppm; IR (KBr): n˜ =
3405, 3241, 2944, 2861, 2103, 1716, 1501, 1494, 1455, 1401, 1350, 1273,
1223, 1197, 1134, 1059, 1032, 941, 862 cmÀ1; elemental analysis calcd (%)
for C22H30O8: C 62.55, H 7.16; found: C 62.15, H 6.85.
waste/mercuryfacts.html;
c) http://www.epa.gov/ttncaaa1/t3/fact_
sheets/hg17th.pdf; d) http://www.epa.gov/mercury/.
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Synthesis of (triethylene glycol)–PPE 1: Diiodide 6 was dissolved in a
mixture of piperidine (0.5 mL), THF (0.5 mL), and methanol (0.5 mL) in
a 25 mL Schlenk flask with a flow of nitrogen and magnetic stirring.
[PdCl2(PPh3)2] was added to the flask and the contents were treated with
acetylene gas according to reference [22b]. The reaction mixture was stir-
red at room temperature for 16 h. The solution was slowly added to ace-
tone (300 mL). The precipitate was washed with water. The number aver-
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1
À
ty (Mw/Mn) of 1.85. H NMR (400 MHz, [D6]DMSO): d=7.16 (2H; Ar
À
À
À
À
H), 4.54 (2H; O H), 4.20 (4H; C H), 3.81 (4H; C H), 3.66 (4H; C H),
Chem. Eur. J. 2004, 10, 6247 – 6254
www.chemeurj.org
ꢁ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
6253

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Received: July 30, 2004
Published online: November 3, 2004
6254
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Chem. Eur. J. 2004, 10, 6247 – 6254