Molecular Wire Effects in Phenyleneethynylene Oligomers: Surprising Insights

Article abstract of DOI:10.1002/chem.201705604

The synthesis and quenching behavior of a series of water-soluble, carboxylate-carrying phenyleneethynylene oligomers—monomer to tetramer—and their polymers are reported; their quenching behavior with different test analytes (paraquat, lead salts, mercury

Full text of DOI:10.1002/chem.201705604

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Accepted Article
Title: Molecular Wire Effects in Phenyleneethynylene Oligomers:
Surprising Insights
Authors: Uwe Heiko Bunz, Marcel Hergert, Markus Bender, and Kai
Seehafer
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Molecular Wire Effects in Phenyleneethynylene Oligomers:
Surprising Insights
Marcel Hergert,[a] Markus Bender,[a] Kai Seehafer,[a] and Uwe H. F. Bunz*[a, b]
Abstract: We report the synthesis and quenching behavior of a
series of water soluble, carboxylate-carrying phenyleneethynylene
oligomers
- monomer to tetramer – and their polymers and
investigate their quenching behavior with different test analytes
(paraquat, lead salts, mercury salts, picric acid, methylpyridinium
iodide) in water and compare the results to that of the conjugated
polymer. We find significant but analyte-dependent enhancement
effects. For monovalent quenchers only the molecular wire effect
applies but for divalent quenchers multivalency effects are also
important.
The molecular wire effect was first described by Swager et al.[1]
Receptors connected in series through a conjugated poly(para-
phenyleneethynylene) (PPE) backbone show linear signal
enhancement upon binding to a quencher molecule, paraquat,
as the fluorescence of the whole polymer chain is quenched, if
even only one of the receptors is occupied by the quencher
molecule. Enhancement factors of around 60-70 were observed;
that happened, even if much longer polymers were investigated,
where several hundred of the receptors were “wired in series”.
This observation is easily explained if one assumes that one
exciton samples around 60 receptors, attached to the PPE-
backbone. This was an important concept that has been
exceptionally successful.[2-6] While Swager had looked at the
monomeric units and the polymers, studies with defined
oligomers and their responses are rare perhaps because such
defined oligomers are a bit more difficult to synthesize[7] and it is
currently assumed that the increase in sensitivity is linear with
the number of receptor units.
Figure 2. GPC traces of esters 1-4 and ester protected polymers P1-P2.
Here we re-investigated these test analytes - but with a series of
oligomers 1-4, a short polymer P1 and a longer polymer P2
(Figure 1), prepared by Sonogashira coupling of suitable
arylalkynes and aromatic diiodides. GPC vs. polystyrene
standard suggests a PN (degree of polymerization) of 10 for
ester-P1 and a PN of 25 for ester-P2, but the structured GPC
traces (Figure 2) and the literature suggest that the PN is
lower.[9,10] Employing the defined oligomers 1-4 and the resolved
peaks of P2 (exact masses were determined by HR-MALDI) as
GPC calibration, the PN for ester-P1 is 6 and the PN for ester-P2
is 15, seeming fairly reasonable and therefore this PN is applied
in Figure 5.
In addition to the molecular wire effect one also observes
multivalency effects, when investigating the interaction of a
carboxylated PPE P with lead ions;[8] this we concluded from the
enormous observed amplification factors that exceeded those
observed by Swager et al. by a factor of 25, explained by
binding of two or more carboxylates from neighboring PE-units
to the lead cations.
Figure 1. Structures of oligomers 1-4 and polymers P1-P2.
[a]
Prof. U. Bunz, M. Hergert, Dr. M. Bender, Dr. K. Seehafer
Organisch-Chemisches Institut
Ruprecht-Karls-Universität Heidelberg
Im Neuenheimer Feld 270, 69120 Heidelberg.
E-mail: uwe.bunz@oci.uni-heidelberg.de
Centre for Advanced Materials (CAM)
Ruprecht-Karls-Universität Heidelberg
Im Neuenheimer Feld 225, 69120 Heidelberg.
Figure 3. Photographs of aqueous solutions under daylight (top) and at
365 nm (black light= of 1-4 and P1-P2 (from left to right).
[b]
Figure 3 shows photographs of aqueous solutions of 1-4 and
P1-P2; in Figure 4 the UV-Vis and emission spectra are
displayed. In Table 1 the optical data are collated. Some
interesting trends are notable. The absorption and emission
spectra of 4 and P1-P2 are already pretty similar to each other
Supporting information for this article is given via a link at the end of
the document.
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10.1002/chem.201705604
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and the emissive half-life decreases from monomer to P2 from
1.3 ns to 0.3 ns - typical for PPEs.[11-14] We note that the
fluorescence quantum yields drop from 62% for monomer and
dimer down to 43% in the case of the tetramer and to 11% and
9% for P1 and P2 (reported 8%).[15] The obtained numbers are in
good agreement with literature values.[16] The quantum yield of
the oligomers decreases with their length due to a number of
interlocking effects including vibrational deactivation by the
solvent water. The effects have been discussed in ref.[16] The
evolution of a second set of red-shifted emission features in the
larger oligomers and polymers is due to a changed Franck-
Condon envelope that allows for efficient 0-1 and 0-2 vibronic
transitions. This feature is not accessible for the smaller
oligomers, where only 0-0-transitions are observed.
acid as quencher. Here we find that the KSV-value is largely
independent of the oligomer size, and similar to the measured
pKa-value. We conclude, that picric acid just protonates the
oligomers and that we have therefore determined the pKa-value
by a somewhat different method. The discrepancy between the
apparent pKa-values obtained from picric acid and the one
employing a series of buffers (pH 1-7) is a consequence of picric
acid’s ability for Förster resonance energy transfer that
additionally quenches the phenyleneethynylenes’ fluorescence
and therefore gives an apparently higher acid strength. For lead
salts, mercury salts, paraquat, and for methyl pyridinium iodide
as quencher, we observe notable trends. For the monomeric unit,
divalent lead salts are the best quencher, followed by paraquat,
while mercury and methylpyridinium iodide salts display weak
quenching only (Table 2).
Table 2. Binding of oligomers 1-4 and polymers P1-P2 to different analytes
using Stern-Volmer quenching.
a)
Cpd. Pb(OAc)2
Ksv
HgCl2
Ksv
PQ2+
Ksv
MP+
Ksv
Picric acid pKa
Ksv
Rel Ksv
Rel Ksv
Rel Ksv
Rel Ksv
Rel Ksv
(log KSV
)
(log KSV
)
(log KSV
)
(log KSV
)
(log KSV
)
1
2
3
4
6.1 103
18
± 1
1.0 ± 0.1
(1.26)
2.4 102
± 14
1.0 ± 0.1
(2.11)
39
± 2
1 ± 0.1
(1.59)
2.9 104
3.4
3.6
3.7
4.4
4.5
± 6.0 102
1.0 ± 0.1
(3.79)
± 2.2 103
1 ± 0.1
(4.46)
3.7 104
± 7.3 102
6 ± 1.2
(4.50)
1.3 102
± 10
7 ± 0.1
(2.11)
1.2 103
± 38
5 ± 0.2
(3.08)
68
± 3
2 ± 0.1
(1.83)
1.9 104
± 1.4 103
0.7 ± 0.1
(4.28)
4.7 104
± 1.9 104
8 ± 3.2
(4.67)
5.3 102
± 15
29 ± 0.1
(2.72)
3.8 103
± 1.3 102
15 ± 0.5
(3.58)
1.0 102
± 5
3 ± 0.1
(2.02)
1.3 104
± 1.1 103
0.5 ± 0.1
(4.10)
Figure 4. UV-Vis and emission spectra of 1-4 and P1-P2 in water. The
concentration of the solutions were 10 M, and the excitation wavelength was
centred on the absorption maximum of the absorption features (see Table 1).
6.7 104
± 2.4 104
11 ± 4.0
(4.83)
1.4 103
± 84
77 ± 5.0
(3.14)
7.4 103
± 3.7 102
31 ± 1.5
(4.87)
1.4 102
± 9
4 ± 0.2
(2.15)
1.1 104
± 1.1 103
0.4 ± 0.1
(4.05)
Table 1. Optical properties of the water soluble oligomers 1-4 and the
polymers P1-P2.
Cpd.
Abs. Max
[nm]
Em. Max
[nm]
Stokes Shift
[cm-1]

[%]
2
[ns]
P1 4.4 105
± 3.0 105
72 ± 51
2.9 103
3.0 104
± 1.2 103
124 ± 5
(4.47)
2.0 102
± 12
5 ± 0.3
(2.30)
1.1 104
± 9.5 102
0.4 ± 0.1
(4.05)
± 1.9 102
163 ± 11
(3.47)
1
357
378
395
404
411
429
398
422
440
451
458
462
2886
2758
2589
2580
2497
1665
62
62
60
43
11
9
1.3
1.1
0.8
0.6
0.4
0.3
(5.65)
8.8 105 b)
8.0 102 b)
2.3 104 b)
2
P2 7.0 105
± 2.9 105
114 ± 47
(5.84)
1.4 104
± 2.0 103
785 ± 111 783 ± 141 7 ± 0.8
(4.15) (5.27) (2.48)
1.9 105
± 3.4 104
3.0 102
± 33
8.9 103
± 6.5 102
0.3 ± 0.1
(3.95)
4.5
3
4
P1
P2
a) the pKa values were obtained by dissolving the fluorophores in buffers
ranging from pH 1-7 and evaluating the apparent KSV. b) KSV from ref [8]
Table 2 collates the Stern-Volmer constants KSV from the
quenching of the fluorescence of lead and mercury salts, as well
as paraquat (PQ2+), methyl pyridinium iodide (MP+) and picric
acid with 1-4 and P1-2.
In none of the four analytes Förster resonance energy transfer is
expected. For the metal salts we expect heavy atom quenching,
while for paraquat and methylpyridinium iodide excited state
electron transfer is more probable. In all of the cases the
quenching is of static nature, as the emissive lifetimes bookend
between 0.3 and 1.3 ns – too short for dynamic quenching to
occur at the used fluorophore concentration of 0.7-20 M.
Upon going from monomer to polymer P2 via the oligomers one
would naively expect linear increase of the KSV-constants
measured for the monomer. We observe that indeed, in the case
of the simple pyridinium salts, where the KSV-constants increase
There are several noteworthy features in these data. First, the
measured quenching and binding constants of P1 and P2 are in
reasonable agreement with those obtained in our earlier work.[8]
We first determined the pKa-values of the oligomers and the
polymers and we find that the acidity decreases from pKa 3.4 for
the monomer to around 4.5 in the polymer; this change is in the
error range of our measurements. We then investigated picric
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10.1002/chem.201705604
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nearly linearly with the size of the oligomer/polymer. For the
doubly charged quenchers mercury and paraquat the KSV-values
increase 780-fold when going from monomer to the polymer but
not in a linear fashion (Figure 5). If we assume that P2 is roughly
a 15-mer, the overall enhancement factor above the molecular
wire effect is 52 for these two analytes. The end-increase in
amplification is - in P2 for those two quencher ions fairly similar,
even though the absolute KSV-values are different. However, in
the beginning of the oligomer series, the increase in the
amplification -a multivalency effect- is considerably distinct for
each analyte, suggesting that charge density might play a role,
while for the absolute values counter ion effects are in play.
Synthesis: See the Supporting Information for synthesis and
characterization of the oligomers 1-4, the polymers P1-P2 and the
respective precursors.
Determination of Stern-Volmer constants: Two 200 µM stock solutions
of each fluorophor were prepared in a) pH 7 PIPES and b) pH 7
KH2PO4/Na2HPO4 buffer. Analyte stock solutions with different
concentrations (10-1-10-4 M) were prepared in pH
(Pb(OAc)2, HgCl2) or KH2PO4/Na2HPO4 buffer
7
PIPES buffer
(paraquat,
methylpyridinium iodide, picric acid). Measurements were performed in
300 µL vials, filled up with the respective buffers. The fitting was done
using a modified Stern-Volmer equation by using the decreasing peak
height (see the Supporting Information).
Acknowledgements
The authors thank Sebastian Hahn for taking the pictures.
Conflict of interest
The authors declare no conflict of interest.
Keywords: molecular wire • sensor • quenching • poly(para-
phenyleneethynylene) • oligo(phenyleneethynylene)
[1]
[2]
[3]
Q. Zhou, T. M. Swager, J. Am. Chem. Soc. 1995, 117, 12593-12602.
T. M. Swager, Acc. Chem. Res. 1998, 31, 201-207.
Figure 5: Relative KSV of 1-4 and P1-P2 to lead (), mercury (), paraquat ()
and methylpyridinium (). The inlet shows the zoomed in region for the
relative KSV of 1-4.
C. Tan, E. Atas, J. G. Müller, M. R. Pinto, V. D. Kleiman, K. S.
Schanze, J. Am. Chem. Soc. 2004, 126, 13685-13694.
[4]
[5]
[6]
M. R. Pinto; K. S. Schanze, Proc. Natl. Acad. Sci. U.S.A. 2004, 101,
7505-7510.
Lead binds strongest to all of the fluorophores, showing by far
the highest KSV-constants (for further discussion, see the
Supporting information). Interestingly enough though, the
relative increase in the binding constants, when going from
monomer to polymer were only a factor of around 114, while for
mercury ions and paraquat the enhancement was a factor of
around 800.
K. Haskins-Glusac, M. R. Pinto, C. Y. Tan, K. S. Schanze, J. Am. Chem.
Soc. 2004, 126, 14964-14971.
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[7]
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38, 4560-4562.
In conclusion, we have prepared and investigated a series of
side-chain charged anionic oligo-PEs and PPEs; we find that
when going from monomer to polymer via the oligomers a
significant increase in Stern-Volmer quenching - beyond the
molecular wire effect, is observed for all examined quenchers,
with exception of methylpyridinium iodide. The added
enhancement effect points to multivalency type enhancement for
1-4 and P1-P2 if divalent or higher charged quenchers are used.
For monovalent cations, here the methylpyridinium salt, such
enhancement effects are absent and the molecular wire effect
alone is operative. In future we will investigate the binding of
libraries of different small PEs to different types of quenchers
and create low molecular weight sensor arrays for multiple
analytes.
[9]
a) U. H. F. Bunz, Macromol. Rapid Commun. 2009, 30, 772-805.
[10] a) J. S. Schumm, D. L. Pearson, J. M. Tour, Angew. Chem. 1994, 106,
1445-1448; Angew. Chem. Int. Ed. 1994, 33, 1360-1363. b) S. L.
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[11] U. H. F. Bunz, Chem. Rev. 2000, 100, 1605-1644.
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[14] C. Weder, Chem. Commun. 2005, 5378-5389.
[15] I.-B. Kim, R. Phillips, U. H. F. Bunz, Macromolecules 2007, 40, 5290-
5293.
[16] U. H. F. Bunz, K. Seehafer, M. Bender, M. Porz, Chem. Soc. Rev.,
2015, 44, 4322-4336.
Experimental Section
.
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COMMUNICATION
Layout 1:
COMMUNICATION
Size does matter, charge does
matter. We investigate
Marcel Hergert, Markus Bender, Kai
Seehafer, Uwe H. F. Bunz*
fluorescence quenching of a
series of phenyleneethynylene
oligomers using different
quenchers and show different
enhancement mechanisms
depending upon the charge and
the chemical nature of the
quencher.
Page No. – Page No.
Molecular Wire Effects in
Phenyleneethynylene Oligomers:
Surprising Insights
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