Apolar ortho-phenylene ethynylene oligomers: Conformational ordering without intermolecular aggregation

Article abstract of DOI:10.1039/b9nj00200f

This paper describes the characterization of solvent induced folding behavior for non-polar (NP) alkoxy substituted ortho-phenylene ethynylene (o-PE) oligomers. Oligomers of lengths up to nine units have been shown to adopt helical conformations in heptane by NMR and CD spectroscopy, while chloroform promotes extended conformations. Surprisingly, the molar ellipticity values found in heptane for these oligomers are very small compared to other literature values of meta-phenylene ethynylene (m-PE) folded systems; however, comparable molar ellipticity values were found for a closed macrocyclic o-PE suggesting the weak ellipticity is a molecular-feature rather than a quality of folding indicator.

Full text of DOI:10.1039/b9nj00200f

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www.rsc.org/njc | New Journal of Chemistry
Apolar ortho-phenylene ethynylene oligomers: conformational ordering
without intermolecular aggregationw
Jing Jiang, Morris M. Slutsky, Ticora V. Jones and Gregory N. Tew*
Received (in Gainesville, FL, USA) 19th May 2009, Accepted 27th September 2009
First published as an Advance Article on the web 24th November 2009
DOI: 10.1039/b9nj00200f
This paper describes the characterization of solvent induced folding behavior for non-polar (NP)
alkoxy substituted ortho-phenylene ethynylene (o-PE) oligomers. Oligomers of lengths up to nine
units have been shown to adopt helical conformations in heptane by NMR and CD spectroscopy,
while chloroform promotes extended conformations. Surprisingly, the molar ellipticity values
found in heptane for these oligomers are very small compared to other literature values of
meta-phenylene ethynylene (m-PE) folded systems; however, comparable molar ellipticity values
were found for a closed macrocyclic o-PE suggesting the weak ellipticity is a molecular-feature
rather than a quality of folding indicator.
Introduction
Results and discussion
Foldamers1 continue to attract interest in various fields such as
biomimetics, catalysis, supramolecular assembly, etc.2–8
Among the various types of foldamers, one class is based
on aromatic molecules including those with the phenylene
ethynylene backbone.9 Solvent has been routinely used in
the study of aromatic systems to induce or disrupt folding.
Taking advantage of the contrast between the side chain and
the backbone, it has been shown that a good solvent for the
side chain which is also a bad solvent for the backbone drives
folding. This has been referred to as solvophobic induced
folding. When a molecule with a polar side chain and an
apolar backbone is introduced to a polar medium, the
molecular heterogeneity of the structure is segregated such
that a secondary structure, a helix of ortho- or meta-phenylene
ethynylene oligomers, is formed.10,11 This helix allows the
non-polar (NP) backbone to collapse, reducing its interaction
with the solvent, while the polar side chains are favorably
solvated.
The NP o-PE oligomers were synthesized following previously
reported procedures (ESIw).11 Fig. 1 shows the chemical
structure of nonamer 4 to highlight the side chains and
molecular backbone. The introduction of NP side chains
allows us to explore whether the contrast between the alkyl
side chains and the highly conjugated apolar backbone
produces enough heterogeneity in the molecule to allow
solvophobically induced helix formation in alkane based
solvents. As shown in Fig. 1, our NP side chain is based on
(À)-S-methylbutane while many of the other molecules in the
literature have used longer side chains (typically greater than
9 carbons) to induce contrast.13–16
It has been shown previously through extensive NMR
studies that Teg substituted versions of these o-PE oligomers
take on compact helical structures. Detailed 1D and 2D NMR
analysis provided clear evidence of upfield shifts due to the p–p
stacking for only the signals corresponding to those aromatic
protons involved in folding. NOESY/ROESY 2D NMR data
also exhibited cross peaks between the terminal TMS and
aromatic or triazene protons on the oligomers that are only
possible if the o-PE is in a compact helical conformation.
In accordance with these previous studies, 1H NMR spectro-
scopy proves to be an indispensable tool for the oligomers
Previously, we reported the synthesis of short ortho-
phenylene ethynylene (o-PE) oligomers containing polar
triethylene glycol monomethyl ether (Teg) side groups. 1D
and 2D solution NMR spectroscopy clearly showed evidence
of helical folding in these oligomers.11 However, the folding
behavior of NP substituted o-PE oligomers has not yet been
reported. In this paper, a series of o-PE oligomers (1–4) with
lengths of up to nine units substituted with chiral alkoxy side
chains are studied by NMR, UV and circular dichroism (CD)
spectroscopy. UV and CD have been commonly used by
Moore et al. in their studies of the meta-phenylene ethynylene
(m-PE) system.10,12
Department of Polymer Science and Engineering,
University of Massachusetts, Amherst, MA 01003, USA.
E-mail: tew@mail.pse.umass.edu; Fax: +1 413 545 0082;
Tel: +1 413 577 1612
w Electronic supplementary information (ESI) available: Detailed
procedures and additional characterization of monomers and dimers.
See DOI: 10.1039/b9nj00200f
Fig. 1 Structure of alkoxy substituted nonamer 4.
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Fig. 2 Expansions of the aromatic regions of the 1H NMR spectra
for trimer 1 (n = 3), tetramer 2 (n = 4), and hexamer 3 (n = 6) in
CDCl3. Each ring has three protons that are labeled respective to their
J-coupling and splitting pattern: Ha (8.4 Hz, d); Hb (2.1 Hz, d); and
Hc (8.4 Hz and 2.1 Hz, dd).
1
Fig. 3 (a) The aromatic region in H NMR spectra of tetramer 2 in
16-heptane (top) and CDCl3 (bottom). (b) Molecular model of
d
described here. Fig. 2 shows the abbreviated structure for the
o-PE oligomers and the aromatic regions of trimer 1, tetramer
2, and hexamer 3 in CDCl3 at 400 MHz. Each unique aromatic
peak lies within the areas labeled as Ha, Hb and Hc.
Dispersion is exhibited in the aromatic regions as all 9, 12,
and 18 protons can be counted and assigned to specific ring
systems by J-coupled correlation spectroscopy (COSY).11c
To determine whether an alkane based solvent would have
an impact on the upfield shifting and therefore the p–p
stacking in the aromatic region of these NP o-PEs, deuterated
heptane (d16-hept) was used as the folding solvent. Fig. 3(a)
shows the aromatic region of tetramer 2 in d16-hept (top) and
in CDCl3 (bottom). Protons on rings 1 and 4 of this oligomeric
system are labeled. There are clear upfield shifts for protons
Ha1 and Ha4 as well as Hc1 and Hc4, when the solvent
changes from chloroform to heptane. This is exactly consistent
with the upfield shifts observed in our previous report, which
were caused by the p–p stacking of rings 1 and 4 (see Fig. 3(b)),
and confirmed by 2D NOESY studies.11 Fig. 3(a) shows that
proton Hb1 shifts upfield while proton Hb4 shifts downfield
slightly. Although the exact origins of these shifts are not
completely understood, based on the molecular model shown
in Fig. 3(b), Hb1 should shift upfield as it resides over the
triple bond while Hb4 might shift downfield since it is located
under the non-aromatic TMS group.
tetramer 2, top and side view.
Unfortunately, no NOE cross peaks are observed in these
systems due to the strong d16-hept solvent peak. Nevertheless,
we have previously documented the relationship between
through-space NOE signals and 1D upfield NMR shifts in
these o-PE folded structures. Therefore, the 1D NMR spectra
shown in Fig. 3(a) and 4 are completely consistent with folded
helical structures.
UV proved to be a useful tool for distinguishing folded and
unfolded states in m-PE oligomers.10,12 Six-unit macrocycles
were used to confirm the absorption bands assigned to the
cisoid conformations required in the folded structure. The
difference between transoid and cisoid conformations was
observed with changing solvent for the m-PE systems.
Unfortunately the same comparison cannot be made for
o-PE systems. Fig. 5(a–c) shows UV spectra for oligomers
1–4 in chloroform (a), heptane (b), and a comparison of
nonamer 4 with macrocycle 5 (c). There is little to no relationship
between the macrocycle and the o-PE oligomers as peaks at
1
The H NMR data in Fig. 4 for nonamer 4 were taken at
600 MHz in d16-hept (top) and CDCl3 (bottom). Though
particular assignments are difficult to make, a clear difference
is seen in the aromatic regions between d-heptane and CDCl3
and it appears that all of the aromatic peaks shift upfield in the
folding solvent (d16-hept), consistent with the expectation of a
fully folded structure. There is still excellent dispersion thus
large and uncontrolled aggregation can be ruled out even at
longer lengths of this o-PE system. While some of the Ha
peaks in the CDCl3 spectra are obscured by the solvent, it is
clear that all 9 Hb and Hc peaks are present and shift
upfield accordingly, which provides strong support for helix
formation in these NP o-PE oligomers.17
Fig. 4 1H NMR traces of the aromatic region of nonamer 4 in
d
16-heptane (top) and CDCl3 (bottom).
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285 and 315 nm in the macrocycle do not correlate well with
the peaks at 275 and 350 nm in the o-PE oligomers. This
difference is likely explained by the ortho-connection in this
PE macrocycle. The changes in UV profile in solvent for oligo-
mers 1–4 are extremely subtle and do not provide robust data for
clear interpretation, in sharp contrast to the m-PE systems.
Unlike the UV results, CD provides an additional mecha-
nism for probing helicity of the NP o-PE oligomers, given the
chiral nature of the alkoxy side chain. CD has been used
as a method to examine secondary structures of biological
macromolecules and foldameric systems.18–21 Fig. 6(a) shows
the CD spectra of nonamer 4 in chloroform (0.268 mM) at
0 1C as well as in n-heptane (0.167 mM) at 0, 10, and 20 1C.
The purpose of these experiments was to determine whether a
helical conformation is obtained in n-heptane versus an
extended conformation in chloroform. In addition, a decrease
in temperature should favor a folded structure. There is little
to no ellipticity for the nonamer in chloroform, even at 0 1C
and higher concentration while there is a comparatively
substantial signal for the nonamer in n-heptane even at room
temperature although the ellipticity is strongest at 0 1C.
Additional CD data for oligomers 2–4 indicate that there is
a slight length dependence for ellipticity in n-heptane while
Fig. 6 (top) CD spectra of nonamer 4 in CHCl3 (0.268 mM) and
n-heptane at 0, 10 and 20 1C at 0.167 mM. (bottom) CD spectra traces
of macrocycle 5 in solution and in the solid state on quartz.
each oligomer exhibits a lack of coherent signal in chloroform
at room temperature. This length dependence is reasonable as
tetramer 2 would represent only one and one third turn of a
helix while hexamer 3 and nonamer 4 represent two and three
turns of the helix, respectively. A temperature study of
hexamer 3 in heptane shows a stronger ellipticity with decreas-
ing temperature much like the data shown in Fig. 6(a) for the
nonamer. To determine whether concentration has an impact
on the obtained spectra, the concentration and path length
were altered by an order of magnitude each (concentration
increased, path length decreased) such that absorbance should
remain constant. The lack of dramatic change in the resulting
spectra leads to the conclusion that the data shown in Fig. 6(a)
are the result of non-aggregated NP o-PE entities in the helical
state, indicating a lack of large scale aggregation that could
create an undefined but CD-active structure.
While these data are encouraging, the molar ellipticity (De)
values are significantly less than what has been reported
for other synthetic macromolecular and biological systems in
the literature.10,22,23 In particular, the system most closely
related to these, the apolar m-PE foldamer exhibits ellipticity
values between À300 and 300 (MÀ1 cmÀ1) for an oligomer
with 3 helical turns.22,23 Additionally, the CD signals derived
from those NP m-PE systems appear to be time and tempera-
ture dependent, showing signs of intermolecular aggregation.
To further understand the CD behavior of these o-PE
oligomers, the CD spectrum of a model macrocycle was
studied in the solid state (see Fig. 6(b)) where this molecule
is known to form a highly ordered liquid crystal.24 The
observed CD signals of this ordered system show similarly
small De values compared to the solutions of folded nonamer 4
in Fig. 6(a).
Fig. 5 UV-Vis spectra of oligomers 1–4 in chloroform (a), n-heptane
(b) and a comparison of nonamer 4 with macrocycle 5 (c) (structure
shown in inset) at 12.5 mM.
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The CD signal intensity can be affected by various
parameters.25 Meijer et al. reported that the location of the
chiral center with relation to the chromophore greatly impacts
the resulting CD spectra.20 The angle of exciton coupled
transition moments has a large impact on the observed CD
intensity with a maximum observed at 701 which decays to null
at zero degrees.25e Naphthalene–diimide systems studied by
Matile et al. illustrate this nicely.25a,b Additionally, the
intensity decreases with increasing distance between the chromo-
phores and is proportional to the extinction coefficient.25e The
pitch of the helical twist can also impact the CD intensity. As a
result of these variables, the strict assignment of CD intensity
to helical content can be misleading. The small molar ellipticity
values of the ordered macrocycle lead to the conclusion that
these o-PE systems have inherently small CD signals.
compound were dissolved in separate flasks in triethylamine
(TEA) and transferred via syringe to the Schlenk flask
under N2. The Schlenk flask was gently degassed for 30 s
then backfilled with N2. The flask was sealed and placed in an
oil bath at 45 1C for 6–18 h and checked by TLC for
completeness. Once done, the reaction solution was diluted
with ether, filtered through a pad of Celite and concentrated.
The residue was then purified by flash chromatography.
Trimer 1
Trimer 1 was synthesized from TMS deprotected dimer (D-H)
and M5 (ESIw) following the general Sonogashira coupling
1
procedure with yield of 60%. H NMR (CDCl3, ppm) d: 7.45
(1H, d, J = 8.53 Hz), 7.44 (1H, d, J = 8.62 Hz), 7.35 (1H, d,
J = 8.63 Hz), 7.01 (1H, d, J = 2.57 Hz), 6.96 (1H, d, J = 2.60 Hz),
6.94 (1H, d, J = 2.51 Hz), 6.80 (1H, dd, J = 8.62, 2.60 Hz),
6.74 (1H, dd, J = 8.62, 2.60 Hz), 6.61 (1H, dd, J = 8.54,
2.56 Hz), 3.87–3.68 (8H, m), 3.63–3.50 (2H, m), 1.93–1.68
(4H, m), 1.63–1.37 (4H, m), 1.34–1.10 (12H, m), 1.04–0.84
(21H, m), 0.24 (9H, s). MS: m/z 732.
Experimental
General
All solvents were purified through a standard protocol. All
commercial chemicals were used without further purification.
1H NMR spectra were obtained from a Bruker DPX 400 MHz
spectrometer or 600 MHz spectrometer by means of a TXI
probe with Z-gradient capabilities. The temperature was
maintained at 305 K for all acquisitions. UV-visible spectra
were recorded on a Hewlett-Packard 8453. CD spectra were
taken on a JASCO J720 spectrometer in rectangular cuvettes.
The procedure for all monomers and dimer is shown in
the ESIw.
TMS deprotected trimer (1-H)
TMS deprotected trimer (1-H) was obtained from trimer 1 via
the general TMS deprotection procedure in quantitative yield.
1H NMR (CDCl3, ppm) d: 7.50 (1H, d, J = 8.69 Hz), 7.47
(1H, d, J = 8.85 Hz), 7.37 (1H, d, J = 8.61 Hz), 7.02 (1H, d,
J = 2.59 Hz), 6.99 (1H, d, J = 2.62 Hz), 6.95 (1H, d, J = 2.55 Hz),
6.80 (1H, dd, J = 8.61 Hz, 2.62 Hz), 6.77 (1H, dd, J = 8.61 Hz,
2.64 Hz), 6.63 (1H, dd, J = 8.55 Hz, 2.56 Hz), 3.9–3.5
(10H, m), 3.16 (1H, s), 1.95–1.7 (3H, m), 1.65–1.4 (3H, m),
1.36–1.08 (9H, m), 1.01 (3H, d, J = 6.74 Hz), 1.00 (3H, d, J =
6.73 Hz), 0.94 (3H, d, J = 6.74 Hz), 0.94 (6H, t, J = 7.62 Hz),
0.89 (3H, t, J = 7.6 Hz).
General TMS deprotection procedure
One equivalent of the TMS protected compound and 2.5
molar equivalents of K2CO3 with 5–10 mL of methanol
(and 5–10 mL THF if necessary for solubility) were stirred
in a nitrogen-flushed vial for 0.5 to 3 h. The reaction was
monitored by TLC. Upon completion, the solution was
diluted with ethyl acetate and water and washed twice with
water. After drying the ethyl acetate layer over MgSO4 and
evaporation of solvent, the residue was purified by flash
chromatography.
Triazene activated trimer (1-I)
Triazene activated trimer (1-I) was obtained from trimer 1 via
the general triazene activation procedure with a yield of 88%.
1H NMR (CDCl3, ppm) d: 7.49 (2H, d, J = 8.62 Hz), 7.38
(1H, d, J = 8.62 Hz), 7.37 (1H, d, J = 2.47 Hz), 7.10 (1H, d,
J = 2.46 Hz), 7.03 (1H, d, J = 2.51 Hz), 6.89–6.70 (3H, m),
3.90–3.59 (6H, m), 1.95–1.72 (3H, m), 1.66–1.42 (3H, m),
1.36–1.12 (3H, m), 1.07–0.84 (18H, m), 0.21 (9H, s).
General triazene activation procedure
A Schlenk flask with a stir bar was flame dried under vacuum
and backfilled with N2 three times. The triazene compound
was dissolved in enough distilled methyl iodide to make a
0.1 M solution and transferred to the Schlenk flask. The
Schlenk flask was then gently degassed for 30 s then backfilled
with N2 and closed. The reaction vessel was placed in a 110 1C
oil bath for 6–18 h, and monitored by TLC. Upon completion,
the reaction mixture was diluted with hexanes, filtered over
Celite, concentrated and purified by flash chromatography.
Tetramer 2
Tetramer 2 was synthesized from TMS deprotected trimer
(1-H) and M5 (see ESIw) following the general Sonogashira
coupling procedure with a yield of 45%. H NMR (CDCl3,
1
ppm) d: 7.51 (d, 1H, J = 8.72 Hz), 7.50 (d, 1H, J = 8.72 Hz),
7.45 (d, 1H, J = 8.44 Hz), 7.37 (d, 1H, J = 8.72 Hz), 7.06
(d, 1H, J = 2.73 Hz), 7.02 (d, 1H, J = 1.84 Hz), 7.01 (d, 1H,
J = 2.73 Hz), 6.94 (d, 1H, J = 2.49 Hz), 6.82 (dd, 1H, J =
2.76 Hz, 8.56 Hz), 6.79–6.73 (m, 2H,), 6.61 (dd, 1H, J = 2.55 Hz,
8.56 Hz), 3.96–3.54 (m, 12H), 1.90–1.68 (m, 4H), 1.63–1.46
(m, 4H), 1.36–1.18 (m, 10H), 1.03–0.83 (m, 24H), 0.24 (s, 9H).
MS: m/z 918.
General Sonogashira coupling procedure
A Schlenk flask with a stir bar was flame dried under vacuum
and backfilled with N2 three times. To this flask were added
0.05–0.1 equivalents (based on the acetylene compound) of
Pd(PPh3)2Cl2 and 0.1–0.2 equivalents of CuI. The 1–1.1
equivalents of the acetylene compound to 1 equivalent iodide
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Hexamer 3
smaller than other reported literature values, similarly small
values for the ordered macrocycle in the solid state were
obtained, supporting helix formation.
Hexamer 3 was synthesized from TMS deprotected trimer
(1-H) and triazene activated trimer (1-I) following the general
Sonogashira coupling procedure with a yield of 52%. 1H
NMR (CDCl3, ppm) d: 7.53 (2H, d, J = 8.59 Hz), 7.52 (1H,
d, J = 8.63 Hz), 7.50 (1H, d, J = 8.57 Hz), 7.46 (1H, d, J =
8.63 Hz), 7.36 (1H, d, J = 8.65 Hz), 7.12 (1H, d, J = 2.77 Hz),
7.11 (1H, d, J = 2.84 Hz), 7.06 (2H, d, J = 2.56 Hz), 6.99
(1H, d, J = 2.56 Hz), 6.95 (1H, d, J = 2.51 Hz), 6.84–6.72
(5 H, m), 6.63 (1H, dd, J = 8.65 Hz, 2.69 Hz), 3.89–3.52
(m, 16H), 1.97–1.68 (6H, m), 1.663–1.41 (8H, m), 1.39–1.08
(16H, m), 1.08–0.75 (40H, m), 0.26 (9H, s). MS m/z = 1290.
Acknowledgements
We thank the NSF for financial support (NSF CAREER
CHE-0449663). T.V.J. thanks the Ford Foundation for
financial support. G.N.T thanks the ARO and ONR Young
Investigator programs in addition to the PECASE program,
3M Nontenured faculty grant, and Dupont Young Faculty
Award for generous support. Tatyana Shalapyonok is
specially acknowledged for her contributions in the monomer
synthesis.
TMS deprotected hexamer (3-H)
TMS deprotected hexamer (3-H) was obtained from hexamer
3 through the general TMS deprotection procedure in quantitative
Notes and references
1
yield. H NMR (CDCl3, ppm) d: 7.50 (1H, d, J = 8.69 Hz),
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7.47 (1H, d, J = 8.85 Hz), 7.37 (1H, d, J = 8.61 Hz), 7.02
(1H, d, J = 2.59 Hz), 6.99 (1H, d, J = 2.62 Hz), 6.95 (1H, d,
J = 2.55 Hz), 6.80 (1H, dd, J = 8.61 Hz, 2.62 Hz), 6.77
(1H, dd, J = 8.61 Hz, 2.64 Hz), 6.63 (1H, dd, J = 8.55 Hz,
2.56 Hz), 3.9–3.5 (10H, m), 3.16 (1H, s), 1.95–1.70 (3H, m),
1.65–1.40 (3H, m), 1.36–1.08 (9H, m), 1.01 (3H, d, J =
6.74 Hz), 1.00 (3H, d, J = 6.73 Hz), 0.94 (3H, d, J =
6.74 Hz), 0.94 (6H, t, J = 7.60 Hz), 0.89 (3H, t, J = 7.60 Hz).
Nonamer 4
6 R. M. Meudtner and S. Hecht, Angew. Chem., Int. Ed., 2008, 47,
4926.
Nonamer 4 was synthesized from TMS deprotected hexamer
(3-H) and triazene activated trimer (1-I) following the general
Sonogashira coupling procedure with a yield of 49%. 1H
NMR (CDCl3, ppm) d: 7.53 (2H, d, J = 8.59 Hz), 7.52
(1H, d, J = 8.63 Hz), 7.50 (1H, d, J = 8.57 Hz), 7.46 (1H,
d, J = 8.63 Hz), 7.36 (1H, d, J = 8.65 Hz), 7.12 (1H, d, J =
2.77 Hz), 7.11 (1H, d, J = 2.84 Hz), 7.06 (2H, d, J = 2.56 Hz),
6.99 (1H, d, J = 2.56 Hz), 6.95 (1H, d, J = 2.51 Hz), 6.84–6.72
(5H, m), 6.63 (1H, dd, J = 8.6, 2.6 Hz), 3.89–3.52 (m, 16H),
1.97–1.68 (6H, m), 1.66–1.41 (8H, m), 1.39–1.08 (16H, m),
1.07–0.74 (40H, m), 0.26 (9H, s). MS m/z = 1850.
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A number of studies were performed to induce and observe a
helical secondary structure for these alkoxy substituted NP
o-PE oligomers. 1H NMR experiments exhibited upfield shifts
in the aromatic region indicating p–p stacking in d16-heptane.
All protons for nonamer 4 moved upfield while only protons
associated with rings 1 and 4 in tetramer 2 moved upfield,
consistent with helix formation. UV data proved too
ambiguous to identify distinctive changes with increasing
16 C. Dolain, C. L. Zhang, M. J. Leger, L. Daniels and I. Huc, J. Am.
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17 Individual proton resolution is lost here, so it could be possible
that one or more protons shifts downfield although the major
changes between the two spectra involve upfield shifts.
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o-PE length in heptane as opposed to chloroform.
A
comparison of the three-unit macrocycle to the o-PE oligomer
proved to be of little utility, as a correlation between the cisoid
structure and a folding oligomer could not be found due to the
ortho-connectivity of the macrocycle. CD spectra did exhibit
elements of length, solvent, and temperature dependence for
the resulting ellipticity values of these o-PE oligomers. These
signals appeared to be strongest for nonamer 4 in n-heptane at
0 1C. Though these values were 2–3 orders of magnitude
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This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2010
312 | New J. Chem., 2010, 34, 307–312