Solvophobically driven π-stacking of phenylene ethynylene macrocycles and oligomers

Article abstract of DOI:10.1021/ja002129e

Phenylene ethynylene macrocycles and oligomers with three different side-chain linking groups (ester, benzyl ether, and phenyl ether) were synthesized to investigate their tendency to undergo solvent induced π-stacked organization. ¹H NMR, UV, and fluorescence spectroscopies were used to probe two types of π-stacked supramolecular organizations: the intramolecular conformational ordering of the oligomers, and the intermolecular aggregation of the macrocycles. One important conclusion is that solvent can play a very dramatic role in modulating the strength of the interactions that drive the association of these π-stacked structures. The other important conclusion is that in a given solvent, the nature of the side chain linking group strongly influences the π-stacking propensities. It was found that macrocycles and oligomers with the ester side chain linking group were prone to adopt π-stacked structures in a range of solvents, whereas the corresponding macrocycles with benzyl ether and phenyl ether side chain linking groups showed only limited ability to π-stack, even in the most polar solvent examined (DMSO). In the interest of manipulating the helix-coil folding transition of phenylene ethynylene oligomers, a heterosequence consisting of monomers with ester and benzyl ether side chain linkages was synthesized. The folding transition of the heterooligomer was intermediate to that observed for the corresponding homooligomers, suggesting that the backbone sequence can be used to tune the stability of conformations that are based on π-stacked organizations.

Full text of DOI:10.1021/ja002129e

J. Am. Chem. Soc. 2000, 122, 11315-11319
11315
Solvophobically Driven π-Stacking of Phenylene Ethynylene
Macrocycles and Oligomers
Shreyasi Lahiri, Julie L. Thompson, and Jeffrey S. Moore*
Contribution from the School of Chemical Sciences, UniVersity of Illinois, Urbana, Illinois 61801
ReceiVed June 13, 2000
Abstract: Phenylene ethynylene macrocycles and oligomers with three different side-chain linking groups
(ester, benzyl ether, and phenyl ether) were synthesized to investigate their tendency to undergo solvent induced
1
π-stacked organization. H NMR, UV, and fluorescence spectroscopies were used to probe two types of
π-stacked supramolecular organizations: the intramolecular conformational ordering of the oligomers, and
the intermolecular aggregation of the macrocycles. One important conclusion is that solvent can play a very
dramatic role in modulating the strength of the interactions that drive the association of these π-stacked structures.
The other important conclusion is that in a given solvent, the nature of the side chain linking group strongly
influences the π-stacking propensities. It was found that macrocycles and oligomers with the ester side chain
linking group were prone to adopt π-stacked structures in a range of solvents, whereas the corresponding
macrocycles with benzyl ether and phenyl ether side chain linking groups showed only limited ability to π-stack,
even in the most polar solvent examined (DMSO). In the interest of manipulating the helix-coil folding transition
of phenylene ethynylene oligomers, a heterosequence consisting of monomers with ester and benzyl ether side
chain linkages was synthesized. The folding transition of the heterooligomer was intermediate to that observed
for the corresponding homooligomers, suggesting that the backbone sequence can be used to tune the stability
of conformations that are based on π-stacked organizations.
Introduction
Macromolecular self-organization is exemplified by the ability
of natural biopolymers to adopt regular conformations in
solution, such as the helical, double-stranded form of DNA.1
These ordered conformations are stabilized by noncovalent
interactions such as hydrogen bonding, hydrophobic, and van
der Waals forces.2 Utilizing these same interactions to control
the self-organization of nonbiological macromolecules presents
an interesting challenge, especially in polar solvents where
unscreened hydrogen bonding interactions are weak.3 Unlike
hydrogen bonds or metal-ligand coordination interactions for
which there generally exists a good understanding of the
chemical affinities and preferred bonding geometries, much
remains to be learned before nonspecific supramolecular forces
such as solvophobic interactions4 and π-stacking of aromatic
units can be used by design.5,6
Figure 1. Schematic diagram depicting (a) the intermolecular associa-
tion of macrocycles and (b) the helical conformation of oligo(phenylene
ethynylenes). Solvent-induced π-stacking interactions are presumed to
be the dominant force that stabilizes these supramolecular arrangements.
It has long been known that the strength of π-stacking inter-
actions is very sensitive to solvent.7-10 It is also known that
π-stacking tendencies are strongly influenced by the electro-
static properties of the interacting rings.6,11 Previously in our
laboratory, macrocycles consisting of phenylene ethynylene units
have been shown to associate in chloroform due to π-stacking
interactions (Figure 1).12 In chloroform (the only solvent
investigated prior to this study), the magnitude of the association
constant depended significantly on the group that linked the
phenylene ethynylene backbone to the alkyl side chain. Macro-
cycles with the ester linked side chains were observed to
dimerize to a significant degree, whereas macrocycles with the
benzyl ether and phenyl ether side chain linking groups did not
exhibit observable intermolecular aggregation.
(1) Saenger, W. Principles of Nucleic Acid Structure; Springer-Verlag:
New York, 1984.
(2) Friedman, R. A.; Honig, B. Biophys. J. 1995, 69, 1528-1535.
(3) Lawrence, D. S.; Jiang, T.; Levett, M. Chem. ReV. 1995, 95, 2229-
2260.
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(5) Davis, A. M.; Teague, S. J. Angew. Chem., Int. Ed. Engl. 1999, 38,
736-749.
(6) Hunter, C. A.; Sanders, J. K. M. J. Am. Chem. Soc. 1990, 112, 5525-
5534.
(7) Sheppard, S. E.; Geddes, A. L. J. Am. Chem. Soc. 1944, 66, 1995-
2009.
(8) Leonard, N. J. Acc. Chem. Res. 1979, 12, 423-429.
(9) Newcomb, L. F.; Gellman, S. H. J. Am. Chem. Soc. 1994, 116, 4993-
4994.
(10) Guckian, K. M.; Schweitzer, B. A.; Ren, X.-F.; Shiels, C. J.; Paris,
P. L.; Tahmassebi, D. C.; Kool, E. T. J. Am. Chem. Soc. 1996, 118, 8182-
8183.
(11) Burley, S. K.; Petsko, G. A. AdV. Protein Chem. 1988, 39, 125-
189.
(12) Shetty, A. S.; J., Z.; Moore, J. S. J. Am. Chem. Soc. 1996, 118,
1019-1027.
10.1021/ja002129e CCC: $19.00 © 2000 American Chemical Society
Published on Web 11/02/2000

11316 J. Am. Chem. Soc., Vol. 122, No. 46, 2000
Lahiri et al.
Phenylene ethynylene oligomers with an ester group linking
the aromatic backbone to a polar tri(ethylene glycol) side chain
have been demonstrated to collapse into a π-stacked helical
conformation in polar solvents due to solvophobic interactions
(Figure 1).13,14 If the trends observed for the macrocycles carry
over to phenylene ethynylene oligomers, then a strategy to
manipulate the conformational transitions of the oligomers is
apparent. To explore this issue, oligomeric sequences with
widely varying solvophobic and π-stacking propensities are
required. To this end, we selected three different groups (ester,
phenyl ether, benzyl ether) to link a polar tri(ethylene glycol)
side chain to the backbone. Macrocycles and oligomers were
synthesized from the corresponding monomers, and the stability
of their π-stacked arrangement was probed. We show that the
stability of these solvent-induced π-stacked arrangements is
dramatically altered by subtle changes in monomer structure.
These findings open the way to modulating the folding
transitions of the oligomers by altering monomer composition,
analogous to the way that R-helix stability depends on the
peptide’s amino acid sequence15 or the melting transition of
DNA strands depends on the stacking preferences of the
nucleotide bases.16
Figure 2. Plot of 1H NMR chemical shifts of the aromatic protons at
0.6 mM and room temperature for macrocycles 1-3 as a function of
solvent dielectric constant 1 (9), 2 ([), and 3 (b).
intermolecular association involving π-stacking.18 As can be
seen from Figure 2, macrocycles 2 and 3 did not exhibit
significant changes in chemical shifts over a range of solvents
tested.19 This implies that either 2 and 3 are not prone to stacking
in these solvents or that the difference in chemical shift between
the monomer and aggregate is negligible. In contrast to 2 and
3, the chemical shift of macrocycle 1 exhibited significant
upfield shifting as the solvent polarity increased (Figure 2). The
chemical shifts of 1 were plotted against several bulk solvent
parameters;20 solvent dielectric gave the best correlation for a
single bulk solvent parameter.
A more quantitative analysis was performed by determining
the association constants of the macrocycles in different solvents.
1H NMR chemical shifts were measured as a function of
concentration to determine the association constants of the
macrocycles (Table 1, Figure 3).21 These data were analyzed
using nonlinear least-squares regression and the equal K or
isodesmic (KE) model of indefinite self-association.22 For
macrocycle 1, the room-temperature KE values were extremely
Results and Discussion
Previous studies on phenylene ethynylene macrocycles were
conducted on macrocycles possessing simple n-alkyl side
chains.12 Due to poor solubility, their association behavior could
only be tested in a limited range of solvents. To address this
issue, a more polar side chain was sought. Tri(ethylene glycol)
side chains function as powerful solubilizing appendages making
it possible to study macrocycle association in a wide range of
solvents and thus explore π-stacking tendencies under stronger
solvophobic conditions. Following this line of reasoning,
macrocycles 1-3 were synthesized using procedures previously
developed in our laboratory.17
(18) Giessner-Prettre, C.; Pullman, B.; Borer, P. N.; Kan, L.-S.; Ts'o, P.
O. Biopolymers 1976, 15, 2277-2286.
(19) The corresponding nonassociating dimers i, ii, and iii of the
macrocycles were used as controls for these aggregation studies. The
chemical shifts of i, ii, and iii were found to be independent of solvent,
demonstrating that the chemical shifts of the macrocycles are a result of
association and not inherent to solvent-dependent chemical shifts.
All three macrocycles exhibited marked solubility in a wide
selection of solvents, ranging in polarity from toluene to
methanol. They were not soluble, however, in hydrocarbon
solvents such as hexanes, nor were they significantly soluble
in pure water. The aggregation behavior of 1-3 was qualita-
1
tively assessed by measuring the H NMR chemical shifts in
various solvents at a concentration of 0.6 mM. It is well
established that upfield chemical shifts are a signature of
(13) Nelson, J. C. SolVophobically DriVen Folding of Non-Biological
Oligomers and the Solid-Phase Synthesis of Phenylacetylene Oligomers;
University of Illinois at Urbana-Champaign: Urbana, IL, 1997.
(14) Prince, R. B.; Saven, J. G.; Moore, J. S.; Wolynes, P. G. J. Am.
Chem. Soc. 1999, 121, 3114-3121.
(15) Chakraborty, A.; Baldwin, R. L. AdV. Protein Chem. 1995, 46, 141-
176.
(16) Santa Lucia, J., Jr.; Allawi, H. T.; Seneviratne, P. A. Biochemistry
1996, 35, 3555-3562.
(17) Zhang, J.; Pesak, D. J.; Ludwick, J. J.; Moore, J. S. J. Am. Chem.
Soc. 1994, 116, 4227-4239.
(20) Gajewski, J. J. J. Org. Chem. 1992, 57, 5500-5506.
(21) Due to solubility and detection limits, it was only possible to study
a limited range of concentrations for the macrocycles.
(22) Martin, R. B. Chem. ReV. 1996, 96, 3043-3064.

π-Stacking of Phenylene Ethynylene Macrocycles
J. Am. Chem. Soc., Vol. 122, No. 46, 2000 11317
Figure 3. Plot of chemical shift vs log(concentration) (mg/mL) of 1
in different solvents. The curves represent the best fit of these data to
the equal K model of indefinate association (brown, acetone; blue, THF;
green, benzene; red, chloroform).
Table 1. Association Constants of Macrocycles 1 and 3 in
Different Solvents
macrocycle
solvent
Ke a (M-1
)
1
1
1
1
3
CHCl3
THF
benzene
acetone
acetone
50
350
1200
15000
140
a KE values were determined by nonlinear least-squares fitting of
chemical shift data to the equal K (KE) model of indefinite self-
association.22
solvent dependent, ranging from 50 M-1 in CDCl3 to 15 000
M-1 in acetone-d6. In general, KE increased with solvent polarity.
A surprising result, however, was that the association constant
of 1 in benzene was intermediate to that measured in THF and
acetone. The reason for this behavior is presently unknown.
In contrast to the strong association of 1, macrocycle 3
exhibited a KE value of only 140 M-1 in acetone-d6. We
conclude from this that the data for 3 in Figure 2 reflect its
weak self-association. The most likely interpretation of these
results is that the stacking propensity of the hexakis(phenylene
ethynylene) macrocycles in a given solvent is very sensitive to
the nature of the side-chain linking group and the macrocycle
with the ester side chain linkage displays the greatest tendency
to aggregate. Certain linking groups (e.g., phenyl ether, benzyl
ether) severely disfavor π-stacking, even under strongly solvo-
phobic conditions. Whether these linking groups disfavor
π-stacking because of electrostatic reasons (e.g., electron-rich
rings) or because of steric effects (e.g., conformational differ-
ences at the ring/side-chain juncture) cannot be determined at
this time.
Figure 4. Normalized fluorescence spectra (excitation wavelength )
290 nm) at room temperature of (a) 4, (b) 5, and (c) 6 in the indicated
solvents. Spectra were normalized to a constant optical density of 0.1
at 290 nm. The inset shows the corresponding absorption spectra. The
aqueous acetonitrile solvent mixtures are 1:1 by volume.
troscopy has previously been shown to be useful for monitoring
the conformational transition of this backbone in dilute solution
where unimolecular species predominate.14,23 In particular for
4, the absorbance ratio of the 303 to 288 nm π f π* bands
decreases abruptly upon helix formation.14,23 This behavior is
believed to be due to differences in oscillator strengths for the
cisoid and transoid conformations of diphenylacetylene. A
decrease in the ratio of these bands was observed in the UV
spectra of 5; however, compared to 4 solvents of greater polarity
were required in order to induce the conformational transition
(inset Figure 4a,b). The UV spectra of 6 were substantially
Since the putative helical conformation of phenylene ethyn-
ylene oligomers involves π-stacked structures, the intramolecular
folding transition is expected to track closely with intermolecular
aggregation tendencies of the macrocycles (Figure 1). To test
this idea, octadecamers 5 and 6 were synthesized. UV spec-
(23) Prest, P.-J.; Prince, R. B.; Moore, J. S. J. Am. Chem. Soc. 1999,
121, 5933-5939.

11318 J. Am. Chem. Soc., Vol. 122, No. 46, 2000
Lahiri et al.
Figure 5. Normalized fluorescence spectra (excitation wavelength )
290 nm) at room temperature of 7 in CHCl3 and CH3CN. Spectra were
normalized to a constant optical density of 0.1 at 290 nm. The inset
shows the corresponding absorption spectra.
Figure 6. Fluorescence titration data for oligomers 4 ([), 5 (9), and
7 (b). Fluorescence intensities at 350 nm are plotted against the volume
percent chloroform in acetonitrile. Intensities were normalized to a
constant optical density of 0.1 at 290 nm (the excitation wavelength).
different in that only one broad band was observed, and
therefore, these data did not provide any information about the
backbone conformation (inset Figure 4c).
Table 2. Solvent Denaturation of 4 and 7a
[CHCl3]1/2
(vol %)
m
∆G (CH3CN)
oligomer
(cal‚mol-1
)
(kcal‚mol-1
)
Fluorescence spectroscopy has also been used to assess the
solution conformation of the oligomers in different solvents at
low concentration (unassociated). Previously, it was shown that
the 350 nm emission of 4 in its folded form was quenched and
red-shifted (i.e., excimer-like emission) relative to the random
coil conformation.14 The fluorescence spectra of 5 and 6 in
chloroform indicate disordered conformations as expected
(Figure 4). In acetonitrile, partial quenching at 350 nm was
observed, but there was no substantial excimer-like emission
at longer wavelengths. Therefore, fluorescence spectra were
recorded for 5 and 6 in a 1:1 mixture of acetonitrile and water.
The spectra of the octadecamers in this more polar solvent were
both quenched and red-shifted suggesting the possibility of helix
formation. However, it is also possible that in this highly polar
aqueous solvent composition nonspecific aggregation occurs as
well.
From the UV and fluorescence data it is evident that the
nature of the side chain linking group plays an important role
in determining the stability of the ordered helical conformation
of the phenylene ethynylene oligomers. This behavior correlates
well with that observed for macrocycle association, supporting
the notion that solvophobically induced π-stacking interactions
are the dominant force stabilizing the folded conformation.
In the interest of manipulating the folding transition of these
oligomers, a heterooligomer (7, Chart 1) consisting of monomers
with ester and benzyl ether side chain linkages was synthesized.
In acetonitrile, the UV spectrum of 7 exhibited a significant
decrease in the absorbance of the 305 nm band relative to the
289 nm band. In chloroform these bands were of equal intensity.
These results suggest that 7 collapses into a helical conformation
in acetonitrile (Figure 5, inset). The fluorescence spectrum of
7 was consistent with the UV results in that an excimer-like
emission in acetonitrile was observed, implying helix formation.
4
7
72
57
99
42.1
-7.1
-2.5
a [CHCl3]1/2 is the solvent composition required to reach the midpoint
of transition. m describes how rapidly the free energy of transition
changes with solvent composition.14
Solvent denaturation studies previously have been used to
quantify the conformational transitions of these oligomers.14
Earlier results on octadecamer 4 showed that this oligomer
undergoes a cooperative conformational transition as the solvent
composition changes from chloroform to acetonitrile.14 To
determine the folding transition and thus the free energy of
folding for heterooligomer 7, a similar solvent denaturation
experiment was performed (Figure 6, Table 2). For pure
acetonitrile, solvent denaturation studies indicate that the helical
conformation of 7 is ca. 2.5 kcal/mol more stable than the
unfolded conformation. When compared to the 7.1 kcal/mol
value measured for 4,14 it is apparent that the incorporation of
the six benzyl ether units has significantly destabilized the
heterooligomer’s helical conformation. This example illustrates
how it is possible to tune the helix-coil transition of these
oligomers.
Conclusions
Solvophobicity can be an effective way to drive supramo-
lecular organizations involving π-stacked structures. However,
certain aromatic ring systems are much more prone to solvent-
induced stacking than others. This was demonstrated by studying
the intermolecular association of phenylene ethynylene macro-
cycles in different solvents as a function of side chain linking
group. For a given type of linking group, there is at least
qualitative correlation between the intermolecular association
Chart 1

π-Stacking of Phenylene Ethynylene Macrocycles
J. Am. Chem. Soc., Vol. 122, No. 46, 2000 11319
is the difference in chemical shift between monomer and dimer.22 The
chemical shifts vs concentration data were analyzed by nonlinear least-
squares fitting using Mathematica 3.0 (Wolfram Research).
of macrocycles, and the stability of the helical conformation of
the corresponding oligomers. The folding transitions of these
phenylene ethynylene oligomers can be tuned by combining
different monomers to make heterosequences which exhibit
folding behavior intermediate to that observed for their corre-
sponding homosequences.
P ) Pmonomer - ∆(1 + (1 - (4KEct + 1)1/2)/(2KEct))
(1)
Fluorescence and Absorption Measurements. Fluorescence spectra
were recorded on a Photon Technology International (PTI) QM-1
fluorimeter using a 1-cm quartz cuvette in the right angle geometry at
room temperature (23 °C). The absorption of the solutions for the
fluorescence measurements was approximately 0.1 at 290 nm (excitation
wavelength). The UV absorption spectra were recorded on a Shimadazu
(model UV-160A) spectrophotometer using 1-cm quartz cells. The
solvent denaturation experiment on compound 7 and calculations of
free energy were conducted based on previously reported methods.14
Experimental Section
Synthesis and Characterization. Compounds 2, 3, and 5-7 were
synthesized according to previously reported procedures using a
divergent/convergent growth strategy (see the Supporting Information).
The syntheses of 1 and 4 have already been reported.13,14 All compounds
were purified by silica gel column chromatography. Characterization
of intermediates leading to the monomers and oligomers included
methods of 1H NMR, 13C NMR, mass spectrometry (EI), and elemental
analysis. Oligomers 5-7 and macrocycles 2 and 3 were characterized
Acknowledgment. This work was supported by the National
Science Foundation (Grant No. CHE 97-27172). This material
is also based upon work supported in part by the U.S.
Department of Energy, Division of Materials, under Award No.
DEFG02-ER9645439, through the Frederick Seitz Materials
Research Laboratory at the University of Illinois at Urbana-
Champaign.
1
by H NMR and MALDI-TOF mass spectrometry and were assessed
for purity by high-performance liquid chromatography (HPLC). HPLC
was performed using a Rainin binary gradient system equipped with
two SD-200 pumps, a Si 80-125-C5 analytical column (4.6 × 250
mm), and a UV detector operating at 290 nm. All compounds were
greater than 99% pure based on peak areas.
1H NMR Experiments To Determine Association Constants of
Macrocycles. In a given solvent, NMR samples were prepared of each
macrocycle over a range of concentrations. The concentration range
was determined by the solubility of the macrocycles, detection limit
on the 600 MHz NMR, and sample availability. The association
constants were calculated based on an equal K model described by
Martin (eq 1), where P is the observed chemical shift, Pmonomer is the
chemical shift of monomer, KE ) association constant/(molecular weight
of macrocycle), ct is the concentration (mg/mL) of the sample, and ∆
Supporting Information Available: Synthesis and charac-
terization data of macrocycles 2 and 3 and oligomers 5-7,
calculations of free energy of folding of 7, and calculations for
association constants of macrocycles 1 and 3. This material is
available free of charge via the Internet at http://pubs.acs.org.
JA002129E