Solution 1H NMR confirmation of folding in short o-phenylene ethynylene oligomers

Article abstract of DOI:10.1021/ja053530+

Oligomers based on an o-phenylene ethynylene (oPE) backbone with polar substituents have been synthesized using Sonogashira methods. Folding of these extremely short oligomers was confirmed via 1D and 2D (NOESY) NMR methods. Utilizing electron-rich and el

Full text of DOI:10.1021/ja053530+

Published on Web 11/18/2005
1
Solution H NMR Confirmation of Folding in Short
o-Phenylene Ethynylene Oligomers
Ticora V. Jones,† Morris M. Slutsky,† Roberto Laos,‡ Tom F. A. de Greef,§ and
Gregory N. Tew*,†
Contribution from Polymer Science & Engineering, UniVersity of Massachusetts,
Amherst, Massachusetts 01003, Chemistry Department, UniVersity of Florida,
GainesVille, Florida 32611, and Materials Technology, EindhoVen UniVersity of Technology,
5600 MB EindhoVen, The Netherlands
Received May 30, 2005; Revised Manuscript Received September 27, 2005; E-mail: tew@mail.pse.umass.edu
Abstract: Oligomers based on an o-phenylene ethynylene (oPE) backbone with polar substituents have
been synthesized using Sonogashira methods. Folding of these extremely short oligomers was confirmed
via 1D and 2D (NOESY) NMR methods. Utilizing electron-rich and electron-poor phenylene building blocks,
variations of these oPE oligomers have been synthesized to determine the folded stability of π-rich vs
π-poor vs π-rich-π-poor systems. Slight variations in temperature offer a route, aside from solvent
denaturation, to probe the stability of the folded structure. This is the first report of an NMR solution
characterization of folding for a PE backbone without hydrogen bonds.
Introduction
Foldamers have attracted great interest because of their
implications and similarity to folded proteins.1 Solvent, tem-
perature, metal-ligand interactions,2 electrostatic differences,3
hydrogen bonding,4 and structurally rigid design elements have
all promoted folding. These various interactions have been
studied for a wide variety of structures from those with
biological origin such as â and other peptides,5 peptoids,6
saccharides,7 and nucleic acids8 to those that lack any biological
construct but similarly form helical, extended, and/or turned
structures.
Here we report the 1D and 2D solution NMR characterization
of new o-phenylene ethynylene (oPE) oligomers containing
polar triethylene glycol monomethyl ether (Teg) side chains.
NMR characterization shows that even the short tetrameric
oligomers of this backbone, shown in Figure 1, are capable of
folding. Previous work with m-phenylene ethynylenes (mPEs)
† University of Massachusetts.
‡ University of Florida.
§ Eindhoven University of Technology.
(1) (a) Cheng, R. P. Curr. Opin. Struct. Biol. 2004, 14, 512-520. (b) Gellman,
S. H. Acc. Chem. Res. 1998, 31, 173-180.; (c) Hill, D. J.; Mio, M. J.;
Prince, R. B.; Hughes, T. S.; Moore, J. S. Chem. ReV. 2001, 101, 3893-
4011.
(2) Stadler, A. M.; Kyritsakas, N.; Lehn, J. M. Chem. Commun. 2004, 2024-
2025.
(3) Nguyen, J. Q.; Iverson, B. L. J. Am. Chem. Soc. 1999, 121, 2639-2640.
(4) Gong, B.; Zeng, H. Q.; Zhu, J.; Yuan, L. H.; Han, Y. H.; Cheng, S. Z.;
Furukawa, M.; Parra, R. D.; Kovalevsky, A. Y.; Mills, J. L.; Skrzypczak-
Jankun, E.; Martinovic, S.; Smith, R. D.; Zheng, C.; Szyperski, T.; Zeng,
X. C. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 11583-11588.
(5) Cheng, R. P.; Gellman, S. H.; DeGrado, W. F. Chem. ReV. 2001, 101,
3219.
Figure 1. oPE tetramers 1-3. Electron-rich (red) and electron-poor (blue)
rings are shown. Relevant protons for each tetramer are labeled. Each ring
of each oligomer has three protons that are labeled with respect to their
J-coupling and splitting pattern, respectively: a (8.4 Hz, d), b (2.1 Hz, d),
and c (8.4 and 2.1 Hz, dd).
(6) Wu, C. W.; Sanborn, T. J.; Huang, K.; Zuckermann, R. N.; Barron, A. E.
J. Am. Chem. Soc. 2001, 123, 6778-6784.
(7) Chakraborty, T. K.; Ghosh, S.; Jayaprakash, S.; Sharma, J.; Ravikanth,
V.; Diwan, P. V.; Nagaraj, R.; Kunwar, A. C. J. Org. Chem. 2000, 65,
6441
created systems that fold into pucklike helices upon solvent
change.9
(8) Vilaivan, T.; Suparpprom, C.; Harnyuttanakorn, P.; Lowe, G. Tetrahedron
Lett. 2001, 42, 5533-5536.
(9) Nelson, J. C.; Saven, J. G.; Moore, J. S.; Wolynes, P. G. Science 1997,
277, 1793-1796.
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J. AM. CHEM. SOC. 2005, 127, 17235-17240
17235

A R T I C L E S
Jones et al.
This system has been characterized using a variety of methods
including electron paramagnetic resonance (EPR),10 circular
dichroism (CD),11 and fluorescence.12a,b Despite these extensive
investigations, 2D solution NMR characterization of folded PE
structures, without extensive hydrogen bonds,4 has not been
reported.
tetramers 1-3.17 At the same time, the protons expected to
experience no π-π stacking interactions (rings 2 and 3) were
observed to remain at ∆ppm ≈ 0 throughout the titration,
completely consistent with the expected folded structure.
Further, when all three oligomers are compared, the differences
in ∆ppm are also in agreement with expectations. Oligomer 2
is expected to fold better than 1 due to complimentary
electrostatics. Rings 1 and 4 of 2 are π-poor and π-rich,
respectively, while these two rings in 1 are both π-rich. The
electron-poor system of oligomer 3 has the largest ∆ppm for
rings 1 and 4, suggesting closer stacking interactions,13 although
π-poor systems may be more sensitive spectroscopically.18
The overall upfield shifting, or ∆ppm, of the signals corre-
sponding to the protons on rings 1 and 4 in tetramer 1 are
relatively small; however, it is clear that the signals from the
protons on rings 2 and 3 are not affected by a change in solvent
as evidenced in the top graph of Figure 2a. When the data for
tetramer 2 are examined (middle graph of Figure 2a), it is again
very clear that the signals from the protons on rings 2 and 3 do
not shift upfield upon solvent change, indicating no conforma-
tional change that moves them to within proximity of a
π-stacking event, which is perfectly consistent with the expected
helical structure. The data obtained by solvent titration for
tetramer 3 (bottom graph of Figure 2a) are even more dramatic.
It is clearly evident that the average upfield shifts (∆ppm) for
the signals corresponding to the protons on rings 1 and 4 are
greater than those for the signals corresponding to the protons
on rings 2 and 3, which strongly supports a helical folded
conformation.
Helical molecular models shown in Figure 2b19 predict and
confirm folded conformations for each tetramer; side chains have
been omitted for clarity. The variations in tetramer structures
are primarily seen in the slip-stacking angle and distance
between rings 1 and 4. A vertical line is shown for each tetramer
which is perpendicular to the plane of ring 4 along with a second
line extending from the center of ring 4 through the center of
ring 1. The angle between these lines helps approximate the
slip-stacking offset while the distances between rings are
measured through the center of each ring. Exact face to face
stacking of the rings would be indicated by an offset angle of
0° and a distance between rings similar to that of constrained
and π-stacked benzene rings, or 3.4 Å.20 Given the conforma-
tional constraints of this oPE system, the electronics and/or
dipoles of each ring should have an influence on the details of
the geometry of the stacked rings,13 which in turn impacts the
folded conformation.21
In comparison to mPE oligomers, few oPE sequences have
been synthesized,12c-e and no folding systems have been
reported thus far. Computational and simple torsional consid-
erations suggest that the oPE oligomers have a high probability
of folding even with very short sequences.13 Moreover, the
aspect ratios for folded mPE and oPE oligomers are considerably
different, leading to distinct geometric shapes. For example,
dodecamers of mPE and oPE would form pucks and tall
cylinders, respectively. Therefore, the ability to create oPE
sequences that fold is being explored, and this is our initial report
that confirms folding in these systems.
Results and Discussion
The oPE oligomers reported here were synthesized using
standard Sonogashira methods reported earlier12d in good yield
to afford three tetramers of different electronic compositions.
The oligomers shown in Figure 1 include an electron-rich
tetramer, 1 (Et4), where all of the Teg side chains are attached
to the oPE backbone through ethers, a mixed system, 2 (EsEt3),
with one electron-poor ester ring and three electron-rich ether
rings, and an electron-poor tetramer, 3 (Es4).
Previous work suggested halogenated solvents would promote
a predominantly random conformation14 while more polar
solvents drive a folded structure. If indeed these short tetramers
fold in solution, it is well documented that π-π stacking shifts
aryl protons upfield to smaller ppm values.16 A solvent titration
study shown in Figure 2a was conducted at constant temperature
and concentration to determine the effects of π-π stacking as
a function of solvent composition for the three tetramers. Each
graph represents a solvent titration series for each of the three
tetramers at increments of 10 vol % acetonitrile (ACN) in
chloroform (CDCl3) from 0% ACN to 100% ACN; all data were
referenced to the standard, tetramethylsilane. The ppm shifts
for protons a, b, and c of each ring were averaged to represent
a single data point per ring at each measured concentration.
The original 0% ACN (or 100% CDCl3) value was set to zero
to normalize all of the data, and then the change in ppm (∆ppm)
was plotted as a function of solvent composition.
∆ppm, when the solvent is changed from CDCl3 to ACN,
indicates a clear upfield shift for the aryl protons of rings 1 and
4 predicted to be involved in π-π stacking for all three
The repulsion that might be expected for the electron-rich
rings of tetramer 1 (Et4, top of Figure 2b) is observed by the
35° slip-stack angle between rings 1 and 4. For tetramers 2
(EsEt3) and 3 (Es4) the slip-stack angle is approximately 20°
and 11°, respectively. These predictions fall directly in line with
the data shown in Figure 2a. The angle of slip-stack or offset
should directly correlate with the degree of π-π stacking
(10) Matsuda, K.; Stone, M. T.; Moore, J. S. J. Am. Chem. Soc. 2002, 124,
11836.
(11) Brunsveld, L.; Prince, R. B.; Meijer, E. W.; Moore, J. S. Org. Lett. 2000,
2, 1525-1528.
(12) (a) Prince, R. B.; Saven, J. G.; Wolynes, P. G.; Moore, J. S. J. Am. Chem.
Soc. 1999, 121, 3114-3121. (b) Lahiri, S.; Thompson, J. L.; Moore, J. S.
J. Am. Chem. Soc. 2000, 122, 11315-11319. (c) Grubbs, R. H.; Kratz, D.
Chem. Ber. 1993, 126, 149-157. (d) Jones, T. V.; Blatchly, R. A.; Tew,
G. N. Org. Lett. 2003, 5, 3297-3299. (e) Shotwell, S.; Windscheif, P. M.;
Smith, M. D.; Bunz, U. H. F. Org. Lett. 2004, 6, 4151-4154.
(13) Blatchly, R. A.; Tew, G. N. J. Org. Chem. 2003, 68, 8780-8785.
(14) Hill, D. J.; Moore, J. S. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 5053-
5057.
(17) The average ring chemical shift going from CDCl3 to ACN is +0.05 ppm
on the basis of model compounds such as the monomers, dimers, and
trimers, which cannot fold.
(18) Ghosh, S.; Ramakrishnan, S. Macromolecules 2005, 38, 676-686.
(19) Wavefunction-Spartan (molecular mechanics, MMFF minimization).
(20) Lee, M.; Shephard, M. J.; Risser, S. M.; Priyadarshy, S.; Paddon-Row:
M. N.; Beratan, D. M. J. Phys. Chem. A 2000, 104, 7593-7599.
(21) The structures shown in Figure 2b are the minimized conformations;
however, it should be noted that the system is dynamic, so this is really
only one snapshot.
(15) Neuhaus, D.; Williamson, M. The Nuclear OVerhauser Effect in Structural
and Conformational Analysis, 2nd ed.; John Wiley & Sons: New York,
2000.
(16) Pickholz, M.; Stafstrom, S. Chem. Phys. 2001, 270, 245-251.
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Folding in Extremely Short oPE Oligomers
A R T I C L E S
Figure 2. (a) NMR titration curves of 1-3 from CDCl3 to ACN. Upfield shifting of rings 1 and 4 is evident, while rings 2 and 3 do not move. Key: (top)
1, Et4, (middle) 2, EsEt3, (bottom) 3, Es4. (b) Energy-minimized (MMFF) conformation of tetramers 1-3 folded into a helix. The Teg side chains are
omitted for clarity. Angles indicate the offset between rings 1 and 4 in the helical conformation. Distances given are between the centers of rings 1 and 4.
Table 1. Average ∆ppm (CDCl3-ACN) Shifts for Aryl Protons for
experienced by each ring, which would therefore be reflected
in the magnitude of upfield shifting experienced by the protons
1-3
ring no.
1 (Et4)
2 (EsEt3)
3 (Es4)
of rings 1 and 4. This is consistent with the observations here
in which the system with the smallest angle of offset (3, Es4) is
the system in which the largest degree of upfield shifting occurs.
In terms of the distances that can be measured from the center
of ring 1 to the center of ring 4, tetramers 1-3 are separated
by approximately 4.3 Å (Et4), 4.1 Å (EsEt3), and 3.8 Å (Es4),
respectively. This again provides evidence to support the data
obtained in Figure 2a, suggesting that protons of tetramer 3 (Es4)
are most affected in the helical conformation. In addition, the
tilt, or face-to-edge stacking, between rings 1 and 4 is very small.
The models show a trend in which this small tilt is observed to
increase from tetramer 3 to tetramer 1. Table 1 summarizes the
ring proton shifts (∆ppm) for the end points of the titrations of
tetramers 1-3 and shows that in all cases rings 1 and 4 shift
upfield while rings 2 and 3 move very little, which is perfectly
consistent with the expected helical structure.
1
2
3
4
-0.09
0.01
0.02
-0.13
0.00
0.00
-0.21
-0.04
-0.02
-0.19
-0.05
-0.10
assignments were made using a combination of J-coupled
correlation spectroscopy (COSY) and heteronuclear multiple-
bond correlation (HMBC) NMR experiments to identify which
protons are attached to each ring of the tetramers. For tetramer
1, the model shown at the top of Figure 3b suggests that protons
1a, 4a, and 4b would experience the largest change in chemical
environment. The NMR spectra shown alongside the model in
Figure 3a (top) support this observation as these three protons
experience the largest change from CDCl3 to ACN. Proton 4b
is influenced by the aromatic ring, while protons 1a and 4a are
influenced by the triazene and acetylene functionalities, respec-
tively. At the same time, very little upfield shift is observed for
proton 4c, while proton 1c actually shifts downfield. In general,
we observe this small downfield shift for the aromatic protons
By examining the 1D NMR traces of all the aromatic protons
(Figure 3a) and coupling these observations with the molecular
models of tetramers 1-3 (Figure 3b), more details regarding
the ppm shifting of each aryl proton can be found. Ring
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A R T I C L E S
Jones et al.
Figure 3. (a) NMR traces for each concentration of ACN in CDCl3 for tetramers 1-3 (1.25 mM, 400 MHz, 298 K). Individual signals from rings 1 and
4 have been labeled and were assigned by a combination of COSY and HMBC 2D NMR experiments. Tetramethylsilane was used as the reference: (top)
1, Et4, (middle) 2, EsEt3, (bottom) 3, Es4. (b) Top down views of molecular models.
of model compounds which is most likely due to the change in
dielectric constant of the solvent.12b,17 The standard tetrameth-
ylsilane corrects this shift. At the same time, the inherent
downfield shifting of the protons going from CDCl3 to ACN17
provides further support that π-π stacking of rings 1 and 4
cause the upfield shifts we observe in ACN. The overall upfield
shifts for 1 are the smallest of the three tetramers, again
indicating a very weak interaction between rings 1 and 4 overall.
For tetramer 2 (EsEt3) the model indicates that the protons
of rings 1 and 4 that should be most affected by the folded
conformation are 1b, 1c, 4a, and 4b. Proton 1b is located under
the triazene group, proton 1c is influenced by the ester function,
proton 4a is located near the acetylene bond, and proton 4b is
located above ring 1. Interestingly, protons 1b and 4c of tetramer
1 as well as protons 1a and 4c of tetramer 2 do not experience
significant upfield shift. These protons, although labeled dif-
ferently due to chemical connectivity, are found in the same
locations on the two foldamers (see Figure 3b, top and middle).
The molecular model of tetramer 3 (Es4) indicates that the rings
are nearly on top of one another, suggesting that all of the
protons on rings 1 and 4 should be affected and all their signals
should shift upfield accordingly. Indeed, this is what is observed
in the NMR spectra at the bottom of Figure 3a. In fact, all of
the proton signals associated with rings 1 and 4 shift upfield
considerably more than those of tetramer 1 or 2.
In addition to the 1D data, we speculated that the terminal
TMS (trimethylsilane) group would provide an excellent NMR
probe in NOESY experiments for two main reasons. The
chemical shift of the TMS falls at a unique region in the spectra
(∼0.2 ppm) compared to those of the rest of the protons in the
molecule, and the methyl groups extend beyond the aromatic
backbone plane by ∼2.0 Å. Due to the inherent distance
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Folding in Extremely Short oPE Oligomers
A R T I C L E S
Table 2. Calculated NOESY Distances
TMS to Xa
molecule
distance (Å)
2 (EsEt3)
3 (Es4)
2.29
3.36
a X ) proton 3c for 2 and a methyl of triazene for 3.
NOE observed for 2 in Figure 4b, and that rings 1 and 4 are
closer to face-to-face stacking than face-to-edge stacking since
face-to-edge stacking would most likely move the two end
groups (triazene and TMS) away from each other. Extracting
distance data based on the integrals of the NOESY/ROESY
cross-peaks (see Table 2) confirms the close proximity of the
TMS and ring 3 (2, EsEt3) or the TMS and triazene end groups
(3, Es4).22 At room temperature, the TMS is ca. 2.29 Å away
from proton 3c of 2 and the triazene is ca. 3.36 Å from the
TMS of 3.
When tetramer 1 was examined by NOESY, no NOE signal
was observed. The lack of NOE peaks for tetramer 1 is most
likely due to the presence of the two electron-rich rings which
are not expected to promote folding as strongly and/or force
the two rings (1 and 4) further apart due to the higher electron
density, which consequently pushes the TMS protons out of
the NOE distance correlation range (4-5 Å). The 1D proton
chemical shift values discussed in Figures 2 and 3 support
folding of this oligomer, but are also consistent with the lack
of NOE cross-peaks since the ∆ppm is the smallest of all three
oligomers and the distance (measured by modeling) is the
largest.
Figure 4. Partial NOESY spectra of 2, EsEt3 (1.25 mM, 400 MHz, 299
K, mixing time 0.1 s), for the TMS and aryl regions in CDCl3 (a) and
ACN (b). No cross-peak is observed in CDCl3, while a strong NOE is
present in ACN. Partial NOESY spectra of 2 (1.25 mM, 400 MHz, mixing
time 0.1 s) in ACN revealing the TMS to aryl interaction at 288 K (c) and
310 K (d).
Other NOESY studies on folding oligomers4 have
indicated that low temperature is required to obtain an NOE
cross-peak. All of the work presented thus far was collected at
room temperature. Taking into account this possible temperature
dependence, we performed a limited temperature study to
determine whether the NOE signals we observed for tetramer
2 were temperature dependent. Upon cooling of tetramer 2,
EsEt3, to 288 K, a new NOE signal appears between the
TMS protons and proton 3a so that two NOEs are present
(Figure 4c). These signals correspond to distances of 2.42 and
2.97 Å, respectively, for protons 3c and 3a. Conversely, when
the temperature is increased to 310 K (Figure 4d), the NOE
for proton 3a completely disappears and the original proton
3c NOE is diminished by 95%. This temperature variation in
NOE is consistent with a folding pathway in which rings 1 and
4 would interact more strongly with one another as the
temperature is reduced, leading to the additional NOE
signal. Conversely, elevating the temperature reverses this
effect, resulting in the observed decrease in the two NOE
signals and intensities. A 1D study of tetramers 2 and 3
(Figure 6) over the same temperature range reveals the
expected downfield shift of the aromatic protons on rings 1 and
4 with increasing temperature and tetramer 3 shifts more
than tetramer 2. The larger observed shifts for the protons
of tetramer 3 are likely due to the fact that 3 is stacked
Figure 5. Partial ROESY spectra of 3, Es4 (1.25 mM, 600 MHz, 298 K,
mixing time 0.3 s), for the TMS and triazene regions in CDCl3 (left) and
ACN (right). No cross-peak is observed in CDCl3, while a strong NOE is
present in ACN.
dependence of NOESY, which is typically 4-5 Å,15 and the
likelihood of similar stacking distances, this additional ∼2.0 Å
plays an important role.
Tetramer 2, EsEt3, was examined by 2D (NOESY) 1H NMR
studies at room temperature in CDCl3 and ACN (Figures 4a
and 4b). The TMS peak appears at 0.2 ppm, and the aryl peaks
corresponding to ring 3 appear at 7.51 ppm (3a), 7.07 ppm (3b),
and 6.96 ppm (3c) in ACN. A strong NOE between the TMS
protons and proton 3c was observed in ACN but is completely
absent in CDCl3 as shown by the partial NOESY spectra in
Figures 4a and 4b. The NOE interactions are consistent with a
folded structure in which the electron-poor ring 1 stacks with
the electron-rich ring 4 (see Figure 3b, middle).
1
When tetramer 3 was examined by 2D (ROESY) H NMR
studies (Figure 5) at room temperature in CDCl3 and ACN, a
strong NOE cross-peak was observed between the terminal
TMS of the molecule and a methyl from the triazene end
group of ring 4 only in ACN. This cross-peak is completely
absent in CDCl3, indicating tetramer 3, Es4, folds in ACN but
not in CDCl3. This NOE further indicates the TMS group
continues in the progression of the helix, consistent with the
(22) As the distance between protons a and c on one ring can be approximated
as 2.44 Å, a relation between the integration of the observed cross-peaks
and the distances can be determined (see the Supporting Information for
more details).
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A R T I C L E S
Jones et al.
Figure 6. NMR traces in ACN for tetramers 2, EsEt3 (top), and 3, Es4 (bottom), at 288, 299, and 310 K. Downfield shifting is evident as temperature is
increased. Tetramethylsilane was used to reference each spectrum. Individual signals from rings 1 and 4 have been labeled and were assigned by a combination
of COSY and HMBC 2D NMR experiments.
Conclusions
more closely than 2. This is consistent with the data discussed
Very short novel oPE oligomers are reported and shown to
fold at room temperature by 1D and 2D NMR. NOESY provides
the first high-resolution data on solvent-driven PE oligomers
and confirms oPE as a versatile and suitable scaffold to construct
foldamers. Tetramers 2 and 3 appear to fold best with the
potential to unfold at higher temperature, indicating an additional
route for denaturation. Additional studies to determine the
structure most suitable for stable helix formation with the oPE
systems is required. The 1D chemical shifts appear to be more
sensitive to aromatic stacking than NOE interactions. oPE
oligomers will generate high aspect ratio objects when folded
and are being studied for self-assembly into larger, tertiary-
like structures as well as for the display of chemical functions
in 3D space.
above. In addition, since 3 is more closely packed, a small
change in distance would influence the ∆ppm more. Another
intriguing possibility is that 2 is more stable due to compli-
mentary electrostatics. Iverson3 showed that complementary
π-systems (π-rich and π-poor) associated more than two π-poor
systems by 1 order of magnitude. His study formed a charge-
transfer complex, unlike ours; however, the trend is similar.
Currently, we cannot distinguish these two possibilities, although
the first one in which 3 packs more tightly is most consistent
with the data presented. What is clear is that the observations
in the 1D spectra (Figure 6) agree with the NOE variable-
temperature data.
When the TMS region for Figures 4 and 5 is examined
closely, it is evident that the signal for the TMS protons also
shifts upfield in ACN. The TMS end group of tetramer 3 (Es4)
is most affected, shifting upfield 0.06 ppm. The magnitude of
shifting, although small, is directly related to the tetramer studied
and shows a larger shift going from 1 to 3. This is additional
support that the TMS end group lies over the plane of ring 3.23
Acknowledgment. We thank L. Charles Dickinson for NMR
assistance. T.V.J. thanks the Ford Foundation for financial
support. G.N.T. thanks the NSF-CAREER Award for funding.
We thank Prof. Rich A. Blatchly for many helpful discussions.
Supporting Information Available: Complete synthetic pro-
cedures for preparation of oligomers 1-3. This material is
available free of charge via the Internet at http://pubs.acs.org.
(23) Table S1 in the Supporting Information provides specific data for this
calculation.
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