Structural fine-tuning of (-donor-spacer-acceptor-spacer-)n type foldamers. Effect of spacer segment length, temperature, and metal-ion complexation on the folding process

Article abstract of DOI:10.1021/ma0478759

The synthesis of a series of polymers (PDA-nOE) containing an alternating arrangement of electron rich (donor) and electron deficient (acceptor) aromatic units, which are linked by flexible oligo(oxyethylene) (nOE) spacers, is reported. The length of the

Full text of DOI:10.1021/ma0478759

676
Macromolecules 2005, 38, 676-686
Structural Fine-Tuning of (-Donor-spacer-acceptor-spacer-)n Type
Foldamers. Effect of Spacer Segment Length, Temperature, and
Metal-Ion Complexation on the Folding Process
Suhrit Ghosh and S. Ramakrishnan*
Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India
Received October 14, 2004; Revised Manuscript Received November 12, 2004
ABSTRACT: The synthesis of a series of polymers (PDA)nOE) containing an alternating arrangement
of electron rich (donor) and electron deficient (acceptor) aromatic units, which are linked by flexible oligo-
(oxyethylene) (nOE) spacers, is reported. The length of the oligo(oxyethylene) spacer was varied from
tetra(oxyethylene) (n ) 4) to hexa(oxyethylene) (n ) 6), to examine the relative propensities of these
polymers to form folded structures by virtue of intrachain charge-transfer interactions between the
adjacent donor and acceptor units. Comparison of the proton NMR and UV-visible spectra of the three
different polymers clearly reveals that PDA)4OE, which has the shortest tetra(oxyethylene) spacer,
exhibits the greatest propensity to fold in the nascent form. A significant upfield shift of the aromatic
protons upon lowering of temperature confirmed this greater tendency of the PDA)4OE to fold; the
spectra of polymers with longer spacers, on the other hand, exhibited a very weak temperature dependence
indicating their sluggishness to fold in nascent form. However, in the presence of suitable alkali-metal
ions (or a polar solvent), the spectra of the polymers with longer oligo(oxyethylene) spacers also exhibit
a much stronger temperature dependence (similar to the PDA)4OE case), implying the occurrence of a
metal-ion (and solvophobic effect) assisted formation of folded structures. These NMR studies corroborate
well with similar variable temperature UV-visible spectral studies, wherein the intensities of the D-A
charge-transfer band is monitored as a signature of the folded structure; there were, however, some distinct
differences in the nature of the variation in the case of nascent polymer samples that appear to reflect
the differences in sensitivities of the two spectral techniques to the folding process. In an effort to better
understand the folding process, model compounds of the type donor-spacer-acceptor and analogous DAD
and ADA were synthesized. Comparison of the spectral changes seen in model compounds, both as a
function of temperature and during alkali-metal ion titrations, with those seen in the analogous polymers
helped confirm that the folded structures in the polymer clearly must involve intrachain charge-transfer
interactions that exceeds simple pairing of adjacent donor and acceptor units; i.e., the acceptors in the
folded polymer are sandwiched between donors and vice versa, thereby forming extended stacks.
Introduction
teractions. Gellman3 first used the term “foldamer” to
describe “any polymer with a strong tendency to adopt
a specific compact conformation”, which was more
recently elaborated upon by Moore and co-workers in a
very comprehensive review.4 During the past decade,
several classes of abiotic oligomers that adopt folded
conformations in solution have been developed. Ex-
amples of such systems are oligocarbamates,5 oligopy-
rrolidones,6 oligoureas,7 carbohydrate oligomers,8 oli-
goanthrilamides,9 crescent oligoamides,10 oligo(m-phen-
ylene ethynylene)s,11 pyrimidine-pyridine oligomers,12
etc., apart from peptidomimetic foldamers.13 Meijer and
co-workers showed that even supramolecular polymers,
formed by multiple H-bonds, could adopt a helical
superstructure by specific noncovalent interactions
between the monomeric units.14 In most of these sys-
tems, the chain adopts a specific conformation primarily
due to hydrogen bonding, steric and/or bond angle
constraints and solvophobic effects. Metal-ion coordina-
tion of backbone heteroatoms in specifically designed
oligomers has also been shown to generate folded
structures in solution.15
The stupendous growth of supramolecular science into
a major discipline that has helped bridge many tradi-
tional areas of science, such as chemistry, physics,
biology, and material science, stands testimony to the
growing importance of weak noncovalent interactions
across these various disciplines.1 Chemistry has been
greatly enriched by the recognition that several of these
weak interactions can be designed to work cooperatively
both to achieve extraordinarily elegant structures as
well to perform highly specific functions. While the
principles that govern organization of small molecules
have been reasonably mastered, designing synthetic
polymers that can adopt a specifically targeted second-
ary structure has been far more difficult.2 The motiva-
tion to generate synthetic polymers with a precisely
controlled conformation stems from the desire to emu-
late the elegance of nature on one hand, while at the
same time from the expectation that this will enable
one to bridge the gap between the molecular scale and
the macroscopic one, in terms of the evolution of
structural order.
Most of the above examples deal primarily with well-
defined oligomers. Although several examples of high
molecular weight polymers that adopt helical conforma-
tions in solutions are known, such as poly(trityl meth-
acrylate)s,16 polyisocyanates,17 cis-poly(acetylene)s,18
polyisocyanides,19 and poly(1,3-phenylene ethynylene)s,20
most of these deal with relatively stiff chains that
There has been a great deal of effort during the past
decade to understand the essential design elements that
would enable secondary structure formation in synthetic
macromolecules through intrachain, intersegment in-
* Author for correspondence. E-mail: raman@ipc.iisc.ernet.in.
10.1021/ma0478759 CCC: $30.25 © 2005 American Chemical Society
Published on Web 01/05/2005

Macromolecules, Vol. 38, No. 3, 2005
Structural Fine-Tuning of Foldamers 677
Scheme 1. Schematic Representation of the
Principles for Folding, along with Our Design for a
Foldamer
and co-workers recently reported well-defined oligomers
containing perylene units linked by oligo(oxyethylene)
units and studied their folding properties (aided by
directional π-stacking interactions).23 Most of these
studies on flexible foldamers have again dealt only with
precisely defined oligomers, except for a recent report
by Janssen and co-workers,24 wherein the authors have
postulated the formation of folded structures in poly-
mers having perylene units separated by oligo(oxybu-
tylene) units. In an effort to generate high molecular
weight polymers that can fold via intramolecular self-
assembly, we designed a polymer that contains 3-folding
elements (see Scheme 1): (i) an alternate placement of
aromatic donor and acceptor units that could form an
intrachain donor-acceptor charge-transfer complex,
analogous to the Iverson systems, (ii) a oligo(oxyethyl-
ene) (nOE) linking segment that could impart a solvo-
phobic motivation for folding,25 and (iii) the alkali-metal
ion complexation ability of the flexible OE segment that
could assist in the formation of folded structures by
restricting the conformational freedom of the loop, which
in turn could further aid the formation of the D-A
charge transfer complex. Early comprehensive studies
by Voglte and co-workers on podands clearly demon-
strated the possibility of open chain analogues of crown
ether to complex with metal ions and generate folded
structures.26 Several workers have exploited such metal-
ion induced folding to modulate the photophysical
properties in small molecules,27 and very recently in
polymers.28 Similarly, D-A charge-transfer interactions
have also been used to generate supramolecular systems
as well as to modulate the conformations in small
molecular systems.29
On the basis of the above design elements, in a
preliminary study, we reported the synthesis and fold-
ing properties of a D-A polyimide, PDA)6OE (see
Scheme 2), having a hexa(oxyethylene) linker segment.30
The UV-visible and NMR spectroscopic studies as a
function of solvent composition, and in the presence of
varying amounts of alkali-metal ions, provided evidence
in support of the postulate that the three aforemen-
tioned weak interactions could indeed work in tandem
to generate a folded structure. In this paper, we examine
the folding process in such (-donor-spacer-acceptor-
spacer-)n type polymers, as a function of the length of
the OE spacer in the nascent form as well as in the
presence of different alkali-metal ions. Furthermore, we
also present variable temperature spectroscopic studies
that reveal the synergistic effects of the solvophobic
interactions and metal-ion complexation on the folding
process. Studies on specifically designed model com-
pounds of the type donor-spacer-acceptor (DA), donor-
spacer-acceptor-spacer-donor (DAD), and acceptor-
spacer-donor-spacer-acceptor (ADA) are also presented,
which further endorse the hypothesis that extended
D-A stacking is present in the folded structures of the
polymers.
possess very limited conformational degrees of freedom
due to the imposition of steric and/or bond-angle con-
straints. There are only very few examples wherein
relatively flexible systems have been made to adopt a
well-defined conformation in solution. One simple con-
ceptual design for folding of flexible synthetic macro-
molecules can be schematically represented as shown
in Scheme 1, wherein the polymer backbone is consti-
tuted of equi-spaced segments linked by flexible spacers.
If such a polymer, for instance, is designed so that the
flexible spacers are hydrophilic and the equi-spaced
segments are hydrophobic, then in a polar medium the
hydrophobic segments might be expected to cluster
together (case A) in a process that could be termed as
intramolecular self-association; a situation similar to
micelle formation above the cmc.21 If, on the other hand,
an ordered intramolecular self-assembly has to occur,
it is essential to ensure that the interaction between
the equi-spaced segments is highly directional (case B).
Thus, incorporation of highly directional interactions
between equi-spaced segments along a flexible polymer
chain could form a general basis of intrachain self-
assembly or a directed folding process.
Experimental Section
General Data. All the reagents and solvents were purified
according to the prescribed procedure.31 1,5-Dihydroxynaph-
thalene and pyromellitic dianhydride (Aldrich Chemical Co.)
were purified by recrystallization from nitromethane and
acetic anhydride, respectively, and the latter was further
vacuum sublimed. Tetraethylene glycol and p-toluene sulfonyl
chloride were purchased from Aldrich Chemical Co. and used
as such. Pentaethylene glycol (1b) and hexaethylene glycol (1c)
were prepared following a literature procedure.32 All the NMR
In line with these general principles, Iverson and co-
workers described the folding of flexible oligomers in a
series of elegant papers,22 wherein they demonstrate the
possibility of forming folded structures in suitably
designed oligomers by virtue of specific intrachain
aromatic donor-acceptor charge-transfer interactions
(directional) aided by solvophobic effects. Similarly, Li

678 Ghosh and Ramakrishnan
Macromolecules, Vol. 38, No. 3, 2005
Scheme 2. Synthetic Scheme for the Preparation of the PDA-nOE Polymers
spectra were recorded in a Bruker 400 MHz NMR spectrom-
eter, and the absorption spectra were recorder using a Hitachi
U3400 spectrometer. The molecular weights were determined
by GPC using chloroform as the eluent at 30 °C, two PLgel
Mixed bed columns to effect separation and a Viscotek TDA,
model 300, as the detector. The universal calibration method
based on PS standards was used to estimate the molecular
weights. The variable temperature UV-visible experiments
were carried out using a Ocean Optics, SD2000 fiber optic-
based spectrometer that utilized a homebuilt dry N2 purged
chamber, along with a refrigerated circulatory setup.
Monomer Synthesis. Compound 2a. Tetraethylene glycol
(4 g, 0.021 mol) was dissolved in 100 mL of dry dichlo-
romethane, and the solution was cooled to 0 °C. To this were
added 7.2 g (0.031 mol) of freshly prepared Ag2O, 4.33 g (0.022
mol) of p-toluene sulfonyl chloride, and 0.66 g of KI, and the
resulting heterogeneous reaction mixture was stirred at 0 °C
for another 15 min. The reaction mixture was filtered through
a bed of silica gel, washed with ethyl acetate (3 × 10 mL),
and the combined filtrate was concentrated to get a light yellow
colored oil. The crude product was purified by column chro-
matography on silica gel column using chloroform/ethyl acetate
as the eluent. The pure product was obtained as a colorless
oil in 65% yield. 2b and 2c were prepared using the same
procedure. Yields: 2b, 70%; 2c, 67%.
were added compound 2a (5.78 g, 16.6 mmol), anhydrous K2-
CO3 (4.53 g, 32.8 mmol), and a catalytic amount of LiBr, and
the resulting reaction mixture was stirred under reflux condi-
tion for 36 h under a nitrogen balloon. After completion of the
reaction, the contents were filtered, the residue was washed
with acetonitrile several times, and the combined filtrate was
concentrated. It was redissolved in 50-mL of dichloromethane
and washed with a (3 × 50 mL) 3:1 mixture of brine and 10%
NaOH solution. The organic layer was dried over Na2SO4 and
concentrated to get a brown color oil as the crude product,
which was used for the next step without further purification.
Yield ) 90%. 4b and 4c were prepared using a similar
procedure from compound 2b and 2c in 80 and 92% yields,
respectively.
Compound 5a. Compound 4a (3.7 g, 7.24 mmol) was
dissolved in 10 mL of THF and to this was added an aqueous
NaOH solution (0.864 g in 6.5 mL of distilled water). The
resulting mixture was cooled to 0 °C, and a solution of TsCl
(3.11 g in 25 mL of dry THF) was added dropwise over a period
of 30 min, keeping the temperature of the reaction mixture
beween 0 and 5 °C. After the addition was completed, the
reaction mixture was stirred at room temperature for 3 h, and
the contents were poured into 60 mL of ice-cold water and
extracted with dichloromethane (3 × 30 mL). The combined
organic layer was washed with water and brine, dried with
Na2SO4, and concentrated to get a brown colored oil as the
crude product. It was purified over silica gel column using a
mixture of chloroform/ethyl acetate as the eluent. Yield ) 50%.
A similar procedure was followed to make the compounds 5b
and 5c starting from 4b and 4c, respectively. Yields: 5b )
45%; 5c ) 54%.
1H NMR (400 MHz, CDCl3): compound 2a, δ (ppm) ) 7.80
(d, 2H, Ar-H, o-SO3-), 7.33 (d, 2H, Ar-H, m-SO3-), 4.15 (t,
2H, -SO2OCH2-), 3.71-3.58 (m, 14H, oligoethylene H’s), 2.44
(s, 3H, Ar-CH3); compound 2b, δ (ppm) ) 7.81 (d, 2H, Ar-H,
o-SO3-), 7.45 (d, 2H, Ar-H, m-SO3-), 4.16 (t, 2H, -SO2OCH2),
3.73-3.59 (m, 18H, oligoethylene H’s), 2.45 (s, 3H, Ar-CH3);
compound 2c, δ (ppm) ) 7.72 (d, 2H, Ar-H, o-SO3-), 7.27 (d,
2H, Ar-H, m-SO3-), 4.09 (t, 2H, -SO2OCH2), 3.66-3.51 (m,
22H, oligoethylene H’s), 2.37 (s, 3H, Ar-CH3).
1H NMR (400 MHz, CDCl3): compound 5a. δ (ppm) ) 7.831
(d, 2H, Ar-H, p-OCH2), 7.770 (d, 4H, Ar-H, o-SO3), 7.352-
7.293 (m, 6H, Ar-H, m-OCH2 and Ar-H, m-SO3), 6.827 (d,
2H, Ar-H, o-OCH2) 4.280 (t, 4H, Ar-O-CH2), 4.118 (t, 4H,
SO2O-CH2), 3.978 (t, 4H, Ar-O-CH2CH2O-), 3.772 (t, 4H,
Compound 4a. To a degassed solution of 1,5-dihydroxy-
naphthalene (1.33 g, 8.31 mmol) in dry acetonitrile (50 mL)

Macromolecules, Vol. 38, No. 3, 2005
Structural Fine-Tuning of Foldamers 679
Ar-O-CH2CH2OCH2), 3.667-3.544 (m, 16H, rest of the
oligooxyethylene H’s), 2.41 (s, 6H, Ar-CH3); compound 5b, δ
(ppm) ) 7.850 (d, 2H, Ar-H, p-OCH2), 7.789 (d, 4H, Ar-H,
o-SO3,), 7.358-7.312 (m, 6H, Ar-H, m-OCH2 and Ar-H,
m-SO3), 6.840 (d, 2H, Ar-H, o-OCH2) 4.292 (t, 4H, Ar-O-
CH2), 4.137 (t, 4H, SO2O-CH2), 3.995 (t, 4H, Ar-O-CH2CH2O-
), 3.800 (t, 4H, Ar-O-CH2CH2OCH2), 3.706-3.560 (m, 24H,
rest of the oligoethylene H’s), 2.429 (s, 6H, Ar-CH3); compound
5c, δ (ppm) ) 7.84 (d, 2H, Ar-H, p-OCH2), 7.78 (d, 4H, Ar-
H, o-SO3,), 7.32-7.25 (m, 6H, Ar-H, m-OCH2 and Ar-H,
m-SO3), 6.825 (d, 2H, Ar-H, o-OCH2) 4.279(t, 4H, Ar-O-CH2),
4.13 (t, 4H, SO2O-CH2), 3.984 (t, 4H, Ar-O-CH2CH2O-),
3.790(t, 4H, Ar-O-CH2CH2OCH2), 3.698-3.549(m, 32H, rest
of the oligooxyethylene H’s), 2.42 (s, 6H, Ar-CH3).
Compound 6a. Compound 5a (2.42 g, 2.9 mmol) was
dissolved in 20 mL of dry DMF, 2 g of NaN3 (30 mmol) was
added to it, and the contents were stirred at 100 °C for 7 h.
The contents were cooled to room temperature and poured into
25 mL of ice-cold water. The product was extracted with
diethyl ether (3 × 25 mL), and the combined ether layer was
concentrated to obtain the crude product as a light yellow color
oil. The product was obtained as a light yellow oil after
purification on a silica gel column using a mixture of chloro-
form and ethyl acetate as the eluent. Yield: 75%. Compounds
6b and 6c were prepared following a similar procedure starting
from 5b and 5c, respectively. Yields: 6b ) 70%; 6c ) 72%.
1H NMR (400 MHz, CDCl3): compound 6a, δ (ppm) ) 7.852
(d, 2H, Ar-H, p-OCH2), 7.337 (t, 2H, Ar-H, m-OCH2), 6.834
(d, 2H, Ar-H, o-OCH2), 4.290 (t, 4H, Ar-OCH2), 3.981 (t, 4H,
Ar-OCH2CH2), 3.798 (t, 4H, Ar-OCH2CH2OCH2-), 3.343 (t,
4H, N3-CH2), 3.717-3.627 (m, 16 H, rest of the oligooxyeth-
ylene H’s); compound 6b, δ (ppm) ) 7.860 (d, 2H, Ar-H,
p-OCH2), 7.347 (t, 2H, Ar-H, m-OCH2), 6.841 (d, 2H, Ar-H,
o-OCH2), 4.297 (t, 4H, Ar-OCH2), 4.002 (t, 4H, Ar-OCH2CH2),
3.811 (t, 4H, Ar-OCH2CH2OCH2-), 3.365 (t, 4H, N3-CH2),
3.720-3.633 (m, 24 H, rest of the oligooxyethylene H’s);
compound 6c, δ (ppm) ) 7.846(d, 2H, Ar-H, p-OCH2), 7.33 (t,
2H, Ar-H, m-OCH2), 6.83 (d, 2H, Ar-H, o-OCH2), 4.284 (t,
4H, Ar-OCH2), 3.989(t, 4H, Ar-OCH2CH2), 3.797 (t, 4H, Ar-
OCH2CH2OCH2-), 3.361(t, 4H, N3-CH2), 3.705-3.634 (m, 32
H, rest of the oligooxyethylene H’s).
containing 7 drops of isoquinoline, was added to it, and the
temperature was gradually raised to 185 °C. The reaction
mixture was stirred at 185 °C for 7 h with continuous removal
of the water generated as a water/toluene azeotrope. The deep
red-colored viscous solution was poured into methanol to
obtain the polymer as orange colored fibrous precipitate. The
polymer was purified by fractionation using chloroform/
methanol. Yield ) 69%. PDA)5OE and PDA)6OE were
prepared by the same procedure. Yields: PDA)5OE ) 60%
and PDA)6OE ) 63%. 1H NMR (400 MHz, CDCl3) for
PDA)4OE, δ (ppm) ) 7.830 (s, 2H, imide ring H’s) 7.574 (d,
2H, Ar-H, p-OCH2), 7.154 (t, 2H, Ar-H, m-OCH2), 6.693 (d,
2H, Ar-H, o-OCH2), 4.190 (t, 4H, Ar-OCH2), 3.957 (t, 4H, Ar-
OCH2CH2), 3.875 (t, 4H, N-CH2) 3.764 (t, 4H, Ar-OCH2CH2-
OCH2-), 3.661 (broad peak, 16H, rest of the oligooxyethylene
H’s).
13C NMR (400 MHz, CDCl3): δ (ppm) ) 166.10, 154.15,
136.42, 126.32, 125.13, 117.25, 114.31, 105.54, 71.14, 70.78,
70.65, 70.29, 69.77, 67.81, and 37.93.
1
PDA)5OE. H NMR (400 MHz, CDCl3) δ (ppm) ) 8.059
(s, 2H, imide ring H’s) 7.712 (d, 2H, Ar-H, p-OCH2), 7.250
(merged with the chloroform peak in CDCl3. 2H, Ar-H,
m-OCH2), 6.765 (d, 2H, Ar-H, o-OCH2), 4.245 (t, 4H, Ar-
OCH2), 3.973 (t, 4H, Ar-OCH2CH2), 3.895 (t, 4H, N-CH2)
3.793-3.605 (m, 28H, Ar-OCH2CH2OCH2- and the rest of
the oligoethylene H’s).
13C NMR (400 MHz, CDCl3): δ (ppm) ) 166.15, 154.21,
136.76, 126.48, 125.12, 117.19, 114.49, 105.59, 71.00, 70.63,
70.05, 69.80, 67.79, and 37.84.
1
PDA)6OE. H NMR (400 MHz, CDCl3) δ (ppm) ) 8.073
(s, 2H, imide ring H’s) 7.71 (d, 2H, Ar-H, p-OCH2), 7.24 (t,
2H, Ar-H, m-OCH2), 6.75 (d, 2H, Ar-H, o-OCH2), 4.247 (t,
4H, Ar-OCH2), 3.976 (t, 4H, Ar-OCH2CH2), 3.903 (t, 4H,
N-CH2) 3.794 (t, 4H, Ar-OCH2CH2OCH2-), 3.756-3.529 (m,
32H, rest of the oligoethylene H’s).
13C NMR (400 MHz, CDCl3):166.16, 154.20, 136.7, 126.50,
125.08, 125.08, 117.55, 114.50, 105.60, 70.99-69.80, 67.83,
67.73, 37.85.
Fractionation of the PDA)nOE polymers. In a typical
fractionation procedure, 100 mg of the polymer was dissolved
in 7-8 mL of chloroform and then methanol was added
dropwise to this orange color solution until it became cloudy.
It was then stirred for 10-15 min and centrifuged. The
supernatant was decanted and the residue was dissolved in
minimum quantity of chloroform and reprecipitated from
methanol to get the high molecular weight fraction. The
polymers were dried under vacuum at 100 °C for 5-6 h. The
fractionated polymer was typically obtained in 20-25% with
respect to the nascent polymer taken.
Synthesis of the Model Compounds. Compounds 7a and
7b have been used for making ADA)4OE and ADA)5OE,
respectively. However, unlike during polymerization, here the
pyromellitic dianhydride was taken in excess, and the terminal
anhydride units were capped by reacting with excess n-
propylamine. The required compound was obtained by column
chromatography using silica gel as stationary phase and
mixture of chloroform/ethyl acetate as the eluent. For DA and
DAD type model compounds, the amine-containing donor units
were first prepared (as for the polymers) from 5-methoxynaph-
thol instead of 1,5-hydroxynaphthalene. The monoamine was
coupled with pyromellitic dianhydride, along with n-propy-
lamine (in a 1:1:1 mole ratio) leading to a mixture of DAD,
DA, and simple A-type compounds. The required DAD- and
DA-type models were separated by column chromatography
using silica gel as the stationary phase and chloroform/ethyl
acetate mixture as eluent. Further details of the experimental
procedure for the various model compound synthesis, along
with their spectral data, can be found in the Supporting
Information.
Compound 7a. 6a (1.2 g, 2.13 mmol) was dissolved in 10
mL of ethanol, and to this solution was added 80 mg of 10%
Pd-C. The reaction mixture was mechanically stirred under
50 psi of H2 pressure for 5 h, with intermittent recharging with
fresh hydrogen gas. After 5 h, the reaction was stopped and
the contents were filtered under suction. The filtrate was
concentrated to give a green color oil, which was purified on a
neutral alumina column (under a N2 atmosphere), using 2%
methanol in chloroform as the eluent, to get the pure diamine
monomer in 90% yield. Similarly, compounds 7b and 7c were
obtained starting from 6b and 6c, respectively. Yields: 7b )
88% and 7c ) 85%.
1H NMR (400 MHz, CDCl3): compound 7a, δ (ppm) ) 7.860
(d, 2H, Ar-H, p-OCH2), 7.346 (t, 2H, Ar-H, m-OCH2), 6.843-
(d, 2H, Ar-H, o-OCH2), 4.302 (t, 4H, Ar-OCH2), 4.001 (t, 4H,
Ar-OCH2CH2), 3.710 (t, 4H, Ar-OCH2CH2OCH2-), 3.483 (t,
4H, NH2CH2), 2.835 (t, 4H, NH2), 3.726-3.465 (m, 16 H, rest
of the oligooxyethylene H’s); compound 7b, δ (ppm) ) 7.859
(d, 2H, Ar-H, p-OCH2), 7.347 (t, 2H, Ar-H, m-OCH2), 6.845
(d, 2H, Ar-H, o-OCH2), 4.298 (t, 4H, Ar-OCH2), 4.000 (t, 4H,
Ar-OCH2CH2), 3.820 (t, 4H, Ar-OCH2CH2OCH2-), 3.483 (t,
4H, NH2CH2), 2.832 (t, 4H, NH2), 3.717-3.457 (m, 24 H, rest
of the oligooxyethylene H’s); compound 7c, δ (ppm) ) 7.84 (d,
2H, Ar-H, p-OCH2), 7.33 (t, 2H, Ar-H, m-OCH2), 6.83 (d, 2H,
Ar-H, o-OCH2), 4.285 (t, 4H, Ar-OCH2), 3.988 (t, 4H, Ar-
OCH2CH2), 3.797 (t, 4H, Ar-OCH2CH2OCH2-), 3.473 (t, 4H,
NH2CH2), 2.835 (t, 4H, NH2), 3.7-3.6 (m, 32 H, rest of the
oligooxyethylene H’s).
PDA)4OE. Diamine 7a (0.496 g, 0.971 mmol) was dis-
solved in 3 mL of distilled m-cresol, and to this stirred solution
was added pyromellitic dianhydride (0.211 g, 0.971 mmol). The
flask was then immersed into a preheated (80 °C) oil bath,
and the contents were stirred under N2 atmosphere for 2 h.
After cooling to room temperature, 4.2 mL of dry toluene,
Folding Studies. The alkali-metal salts were purified by
recrystallization, and the solvents CDCl3 and CH3CN were
dried before use. For NMR complexation studies, 1.5 mL of
the polymer/model compound solution was prepared in 1:1
CDCl3/CH3CN (concentration range: 1-3 mM) (solution A).
To 0.9 mL of solution A, a calculated amount of the appropriate

680 Ghosh and Ramakrishnan
Macromolecules, Vol. 38, No. 3, 2005
Table 1. Yield and Molecular Weights of the DA Polymers
polymer
yield (%)
Mn (PDI)a
PDA)4OE
PDA)5OE
PDA)6OE
69
60
63
43300 (2.1)
30000 (2.2)
49600 (2.0)
a Molecular weights of the fractionated samples determined by
GPC.
alkali-metal salt (∼10-15 molar equivalents with respect to
one oligooxyethylene segment) was added (solution B). 0.6 mL
of solution A was then taken in an NMR tube, and to it was
added solution B in 20-100 µL steps. This resulted in a series
of solutions having a fixed polymer/model compound concen-
tration but varying amounts of the alkali-metal salts. The δ
values in all the proton NMR studies are referenced with
respect to TMS. The UV-visible studies were also carried out
in a similar manner maintaining a fixed polymer/model
compound concentration of 1 mM, in the same solvent mixture.
1
Figure 1. Comparison of H NMR spectra (aromatic region)
of the various polymers with the mixture of (D + A) model
compounds (in CDCl3). The peak marked by an asterisk is due
to solvent.
Results and Discussion
Synthesis and Characterization. The synthesis of
the DA polyimides, PDA)nOE, was carried out as per
Scheme 2. In essence, 1,5-dihydroxynaphthalene was
first alkylated using the appropriate oligoethylene glycol
monotosylate,33 followed by tosylation of the terminal
hydroxyl groups in the coupled product, and subsequent
transformation of the tosylates to amines in two steps
via an azide intermediate. The diamine containing the
donor naphthalene unit (7) is then condensed with
pyromellitic dianydride under standard polyimide syn-
thesis conditions to generate the DA polymers.34
Scheme 3. Structure of Various Model Compounds
Wholly aromatic polyimides typically have very poor
solubility, however, the present samples with flexible
spacers are readily soluble in chloroform.35 Three dif-
ferent polymers with varying lengths of the OE seg-
ments, PDA)4OE, PDA)5OE, and PDA)6OE were
1
synthesized and characterized by H and 13C NMR to
confirm their structure.36 The molecular weights (Mn)
of the nascent polymers were in the range 15000-
20000, which after fractionation increased to 30000-
50000 (see Table 1).37 All the folding studies were
carried out with the high molecular weight fractionated
samples to ensure that the observed effects represent
the behavior of true polymers.38
magnitude of the upfield shift, one can conclude that
the extent of such C-T intrachain interactions among
the nascent polymers (in a good solvent, like chloroform)
is highest in the case of PDA)4OE and is lesser in the
other two cases.41
Penta- and hexa(ethylene glycols) were synthesized
from the smaller glycols according to reported proce-
dures.32 All the polymers formed intensely red-colored
elastic films upon solvent casting due to the presence
of strong charge-transfer interactions in the solid state.
Analogous OE spacers containing polyimides,39 which
do not contain the donor naphthalene units, on the other
hand, are white, confirming that the color is indeed due
to the CT complex formation.
Similarly, the relative absorbances of the charge-
transfer band in the UV-visible spectra of the three
polymers (see Figure 2) reveal the greater propensity
for PDA)4OE to form folded structures. Here again,
it is clear that linking of the donor and acceptor units
periodically along the polymer chain causes a significant
increase in the extent of CT interaction (compare the
absorbances in the polymers with that in the D + A
mixture). To ensure that the interactions in the poly-
mers are indeed intrachain, dilution studies (in the
range 0.75-0.06 mM) were done, and it was shown that
the CT band intensity follows the expected Beer-
Lambert behavior (see inset in Figure 2).
Variable Temperature Studies. Folding of poly-
mers to generate well-defined secondary structures
being an entropically disfavored process is expected to
exhibit fairly strong temperature dependence. To ex-
amine this, we first carried out variable temperature
1H NMR studies of all the three DA polyimides in a good
solvent, like chloroform. The NMR stack plot of these
1
The aromatic region of the H NMR spectra of the
three PDA)nOE samples is shown in Figure 1, along
with the peak assignments.40 One striking observation
when comparing these spectra is the relative position
of the aromatic signals, in particular the single peak
(Ha) corresponding to the pyromellitic-diimide unit,
which in the case of PDA)4OE is considerable upfield
shifted when compared to the other two polymers. In
all cases, note that all the aromatic peaks are upfield
shifted compared to those in a 1:1 mixture of the model
donor (D) and acceptor (A) molecules (see Scheme 3 for
their structures). As discussed in our earlier paper,30
such an upfield shift can be ascribed to the formation
of a D-A charge-transfer complex. On the basis of the

Macromolecules, Vol. 38, No. 3, 2005
Structural Fine-Tuning of Foldamers 681
acceptor (DA) and analogous DAD and ADA (Scheme
3), were synthesized and characterized by standard
techniques.40 In Figure 5, we compare the proton NMR
spectra of the two model compounds having a tetra-
(oxyethylene) spacer (4OE), along with that of the
corresponding polymer PDA)4OE and a 1:1 mixture
of D and A model compounds.
A few important observations regarding the relative
positions of the aromatic protons in these systems can
be made: (a) the chemical shift of acceptor proton of
the polymer lies close to that of model DAD, while that
of the model ADA is considerably downfield shifted; (b)
the chemical shift of donor protons (Hb shown in Figure
1) in ADA lies very close to that in the polymer, while
that of the model DAD appears downfield. These
observations confirm that the chemical shift of the
aromatic protons are sensitive at one level to the
formation of a simple DA pair (as evident from the
upfield shift of both D and A protons in all model
compounds when compared to those in the D + A
mixture), while the formation of a sandwiched structure,
wherein either the acceptor lies between two donors (or
vice versa), causes a further significant upfield shift. It
is also observed that, in all cases, the acceptor protons
appear to be intrinsically more sensitive to stacking
than the donor protons for reasons that are still unclear.
The greater tendency for the polymer with the shorter
spacer (PDA)4OE) to form folded structures is also
clearly reflected in the temperature dependence of the
NMR spectra of the model DAD model systems contain-
ing different OE spacers (see Figure 6); the acceptor
proton in DAD)4OE exhibits a very large upfield shift
causing it to move past the donor protons (Hb), while
the corresponding peak in DAD)5OE shifts very little.
Figure 2. Comparison of UV-visible spectra (CT region
alone) of the various polymers in chloroform, along with that
of a mixture of model compounds. Inset: Variation of ACT with
concentration for PDA)5OE in CHCl3.
studies for PDA)4OE and PDA)6OE is shown in
Figure 3.
It is seen that there is considerable upfield shift of
the aromatic proton signals with decreasing tempera-
ture in the case of PDA)4OE, while the shift is
minimal in the case of PDA)6OE. A plot of the
variation in the chemical shift of the acceptor aromatic
protons in the three different polymers as a function of
temperature is shown in Figure 4b. It is clear from this
plot that the change in δ value is significant only in the
case of PDA)4OE, while it is marginal in the other
two polymers. In contrast, however, all the three
polymers exhibit a continuous increase in the CT-band
intensity with decreasing temperature (Figure 4a). This
difference in the temperature dependences of the NMR
and UV-visible spectra appears to reflect the different
sensitivities of the two techniques to the folding process;
the NMR chemical shifts appear to be very sensitive to
the formation of extended stacks, which causes the
acceptor units to be sandwiched between two adjacent
donors and vice versa, while the absorbance of the CT-
band appears to primarily sense only the number of
D-A contacts induced upon folding.42
If we consider the fully folded and unfolded states to
be in equilibrium via an intermediate state wherein only
D-A contacts are established (as depicted in Scheme
4), it is apparent from the foregone discussions that the
folded form is relatively more stable in the case of
PDA)4OE than in the other two cases.43 While cooling
appears to cause an increase in the extent of CT complex
formation in all cases (based on UV-visible spectral
changes), in the case of PDA)4OE, it also results in
the formation of an extended array of alternately
To gain further understanding of the folding process,
three types of model compounds, namely, donor-spacer-
Figure 3. Variation of 1H NMR spectra of PDA)4OE (left) and PDA)6OE (right) as a function of temperature (in CDCl3). (/)
due to solvent.

682 Ghosh and Ramakrishnan
Macromolecules, Vol. 38, No. 3, 2005
chemical shifts of nascent PDA)6OE with those in the
presence of a polar solvent (40% (v/v) CD3OD in CDCl3)
and in the presence of excess KSCN (9 equiv per repeat
unit). The range over which the temperature depen-
dence could be studied was slightly limited by either
the precipitation of the polymer in the former case or
due to the precipitation of the salt in the latter.
However, it is apparent that the anticipated enhanced
temperature dependence is realized when the folded
state is stabilized either due to solvophobic interactions
and/or due to complexation of the OE loop around the
metal ion; the latter exhibits a stronger effect due to a
combined effect of both the factors (studies carried out
in 1:1 CDCl3/CH3CN) causing what appears to be
extended stacking.44
Alkali-Metal Ion Complexation Studies. It is well-
known that podands, as with their cyclic crown ethers
analogues, exhibit an ionic size-dependent complexation
with alkali-metals ions.26 We studied this selectivity by
examining the ability of various alkali-metal ions to
enhance the formation of the folded structure in the
polymers containing different OE spacers. The variation
of CT band intensity as a function of varying alkali-
metal ion concentration for all the polymers was exam-
ined; in the case of PDA)4OE no significant change
in the absorption spectra was noticed even in the
presence of the smallest Li ion, and therefore variation
in the presence of the larger ions was not examined.
The variation of the CT band intensity is plotted as a
function of different metal-ion concentrations, for both
PDA)5OE and PDA)6OE (see Figure 8). The OE
spacer length-based selectivity is evident when one
compares the changes that occur in the two cases; in
PDA)6OE, K ion causes the maximum change fol-
lowed by Na and Li, but in PDA)5OE, the effect of Li
ion is largest followed by Na and K.45
Figure 4. Variation of ACT (a, top) (in chloroform at 1 mM)
and δ value of acceptor proton (b, bottom) of the various
polymers as a function of temperature (in CDCl3).
The variation of the NMR spectra of both these
polymers in the presence of various metal ions was
similarly examined.46 A plot of the variation of the
chemical shift of the acceptor aromatic proton Ha
(shown in Figure 1) as a function of metal-ion concen-
tration (see Figure 8b) roughly parallels the changes
observed in the CT band intensity,47 thereby confirming
the size-selective nature of the binding of the loop.
Unlike in the variable temperature studies of nascent
samples, here both the spectroscopic techniques exhibit
significant and roughly similar changes, suggesting that
the complexation of the loop by a metal ion aids
extended stacking of the D and A units, thereby equally
affecting their NMR spectra. In other words, the metal
ion complexation of the OE spacers helps the polymer
chain go from a partially folded state toward a fully
folded one, as depicted in Scheme 4.
1
Figure 5. Comparison of H NMR spectra (aromatic region)
of polymer PDA)4OE with the analogous model compounds,
along with that of the mixture of (D +A) model compounds
(in CDCl3). (/) due to solvent.
Scheme 4. Schematic Depiction of the Evolution of
the Folded Structure in the Polymer
The metal-ion titration studies of the model com-
pounds reveal similar selectivity pattern, while at the
same time they also provide additional information that
reflects on the nature of the CT complex formed and
the extent of folding. To make a quantitative comparison
of the variations, in Figure 9, we plot the changes in
observed in the chemical shifts of the aromatic acceptor
proton (Ha) and donor proton (Hb), in PDA)5OE and
analogous models, as a function of metal-ion concentra-
tion. First, we observe that the chemical shift of the
acceptor proton of the nascent polymer PDA)5OE lies
very close to that of the corresponding DAD)5OE
model. Furthermore, the variation of the δ values of the
acceptor protons in the polymer, as a function of metal-
stacked D and A units (extended folded state), as
evident from the significantly larger shift of the acceptor
proton peaks in its NMR spectra upon cooling (see
Figure 4). These observations, in turn, suggest that
increasing the stability of the folded state (externally)
in either PDA)5OE or PDA)6OE should also result
in a similarly enhanced temperature dependence of
their folding process. It was shown in our earlier paper
that the formation of the folded state in PDA)6OE
could be significantly enhanced either by increasing the
solvent polarity and/or by complexing the OE loop with
a suitable alkali-metal ion.30 In Figure 7, we compare
the temperature dependence of the acceptor proton

Macromolecules, Vol. 38, No. 3, 2005
Structural Fine-Tuning of Foldamers 683
Figure 6. Variation of 1H NMR spectra (aromatic region) of DAD)4OE (left) and DAD)5OE (right) as a function of temperature
(in CDCl3). (/) due to solvent.
simple DA and ADA models is smaller and remain
clustered together in the same chemical shift region. On
the other hand, the variation of the δ values of the donor
aromatic proton (Hb) in the ADA model and the polymer
follows a similar pattern and fall in the same region,
while those of the DA and DAD models vary to a smaller
extent and in a parallel manner. These observations can
be rationalized as follows: in DAD and DA models the
donor can never be sandwiched, and likewise in ADA
and DA the acceptor cannot be sandwiched. Therefore,
the observation that the acceptor proton of polymer
Figure 7. Variation of δ value of the acceptor proton of
PDA)5OE behaves similar to that in the DAD model,
PDA)6OE as a function of temperature, under different
while the donor proton mimics the changes seen in the
conditions.
ADA model, provides further evidence in support of the
hypothesis that in the presence of a suitable alkali-metal
ion concentration, parallels that of the DAD model,
while the corresponding variation in the case of the
ion (Li+ for 5OE segment in the present example), the
Figure 8. Variation of normalized ACT (left) (in 1:1 chloroform-acetonitrile) of different polymers as a function of concentration
of various alkali-metal ions, and the change of δ values of the acceptor proton (right) with concentration of alkali-metal ions (in
1:1 v/v CDCl3/CH3CN). Concentration of metal ions is taken with respect to polymer repeat unit; each repeat unit has two EO
segments.

684 Ghosh and Ramakrishnan
Macromolecules, Vol. 38, No. 3, 2005
However, in the presence of increasing Na ion concen-
tration this coincidental overlap is lifted causing one set
of peaks to shift considerably upfield compared to the
other (the most affected donor peaks are Ha′ and Hc′).
Based on HMQC experiments, the peaks that shift more
are assigned to the ring that has the methoxy substi-
tutent, as shown in Figure 10.
We speculate that the origin for this lies in the exact
geometry of the charge-transfer complex in DAD)5OE,
which is significantly different in the case of the Na
complex when compared to the Li one.49 It appears that
complexation of the flexible OE spacer with the larger
size Na involves all the heteroatoms in the loop, while
in the case of Li, the heteroatoms close to the D and A
units are not directly involved. Because of this differ-
ence, the D and A units in the case of Li have greater
freedom to adopt the most optimum geometry for
effective CT complex formation. Two experimental
observations support of this hypothesis: one is the
significantly higher intensity of the CT band in the case
of Li complex (which is accompanied by a red-shift of
the λmax that not seen in the case of Na complex), despite
a lower value for the association constant (compared to
Na) obtained from the NMR spectral variation (see
Figure S28 in Supporting Information). The second
relates to the changes that occur in the aliphatic region
of the NMR spectra as a function of metal-ion concen-
tration; it is seen that the methylene protons adjacent
to the oxygen atom linked to the naphthalene unit shifts
downfield in the case of the Na complex, while in the
case of Li they shift very little (see Figure S29 in
Supporting Information). This kind of splitting of the
naphthalene protons in the presence of Na is seen only
when the donor units cannot be sandwiched between
two acceptors, as in the DA and DAD models, but no
such splitting is seen in the case of the polymers and
the ADA model compound.50 Furthermore, inducing CT
complexation by lowering the temperature also does not
Figure 9. Variation of δ value of donor protons (top) and
acceptor protons (bottom) of PDA)5OE and the analogous
model compounds as a function of LiClO4 concentration in 1:1
CDCl3/CH3CN. The donor peak of DAD merges with the
solvent/acceptor peak and changes very little beyond 2 equiv
of Li, and hence its δ value is not plotted beyond this point.
(see Figure S16 in Supporting Information).
polymer folds to form extended stacks built up of
alternating donor and acceptor units (approaching the
fully folded state depicted in Scheme 4).48
One other intriguing observation that was made while
studying the metal-ion titration of the DAD)5OE
model compound is the large splitting of the peaks
belonging to the naphthalene donor unit in the presence
of Na, which is less apparent in the presence of Li (see
Figure 10). To begin, each of the three pairs of aromatic
protons belonging to the two rings in the naphthalene
unit of DAD)5OE appear at near identical chemical
shifts, although they are chemically nonequivalent.
Figure 10. Variation of of 1H NMR spectra (aromatic region) of DAD)5OE in 1:1 CDCl3/CH3CN as a function of varying
concentrations of LiClO4 (left) and NaSCN (right). The numbers represent the mole ratio of the alkali-metal ion; (/) due to solvent.

Macromolecules, Vol. 38, No. 3, 2005
Structural Fine-Tuning of Foldamers 685
constant of the polymers with various alkali-metal ions. This
material is available free of charge via the Internet at
http://pubs.acs.org.
induce any such splitting in the DAD)4OE (see Figure
6), suggesting that in the optimum geometry for effec-
tive charge-transfer, the two aromatic rings of naph-
thalene remain undifferentiated. Thus, in the case of
Na complexation with DAD)5OE, the complexation of
the OE loop dictates the relative positioning of the donor
and acceptor, while Li complexation does not impose any
such constraint.
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In conclusion, a novel design for flexible high molec-
ular weight polymers that are capable of intrachain self-
assembly has been demonstrated. The design relies on
periodic placement of aromatic units of opposite electron
demand (donor and acceptor) along a polymer chain,
wherein an oligo(oxyethylene) (nOE) linker serves both
as a solvophobic motivator for folding and at the same
time is also capable of complexation with alkali-metal
ions thereby assisting the formation of a stacked array
of D and A units. By varying the length of the OE
spacer, it is shown that in the nascent form the polymer
PDA)4OE, having a tetra(oxyethylene) unit, exhibits
the highest propensity to form such folded structures.
However, upon complexation of the OE spacer with
alkali-metal ions of appropriate size the other polymers,
PDA)5OE and PDA)6OE, also formed folded struc-
tures containing a similar alternately stacked array of
D and A units. The selectivity of the OE spacers to ions
of varying size is also clearly revealed by both the NMR
titrations studies and analogous UV-visible studies.
Comparison of the spectral changes seen in the poly-
mers, as a function of temperature and upon metal ion
complexation, with those in analogous model com-
pounds, provide clear evidence for the formation of the
postulated intrachain stacked D-A arrangement. Al-
though the exact length scale over which such a stacking
occurs remains a challenging problem to address,51 it
is evident that in the folded structure the acceptors are
sandwiched between two donors and the donors between
two acceptors; this is most pronounced in a polar
medium containing an excess of the appropriate alkali-
metal ion. It was also concluded that while the com-
plexation of the OE spacer clearly enhances the charge-
transfer interaction, it does not necessarily allow the
optimum geometry for most effective CT interaction
between the D and A units; larger ions, which indulge
all the heteroatoms of the linker segment in the com-
plexation, restricts the mobility of the D and A units
leading to less effective interaction, while smaller ions
allow greater conformational freedom for more effective
CT interaction. In summary, we have demonstrated that
by the use of several weak but directional intrachain
intersegment interactions, suitably designed synthetic
polymers can be coerced into adopting a specific folded
conformation. Alternate designs for generating folded
structures as well as newer methods to examine folding
are currently being explored.
Acknowledgment. We would like to thank DST,
MHRD, and 3M-India for their generous support.
Supporting Information Available: Text and schemes
giving synthetic methods and characterization of all model
compounds and figures showing complete spectral data (1H and
13C NMR) of all polymers, GPC chromatograms, and all
spectral variations (not included in main text) of polymers and
model compounds as a function of temperature and metal-ion
concentration, and a table giving the apparent association
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Macromolecules, Vol. 38, No. 3, 2005
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further evidence in support of the hypothesis that while the
NMR chemical shifts are sensitive to the presence of sand-
wiched structures and reflects the formation of extended
stacked structures in the polymer, the CT absorbance fails
to detect this stacking. Since in the DA model only simple
pairing can occur but not extended stacking, one sees a
smaller change in the δ values compared to that seen in the
polymer, while the variations in the CT absorbance in both
systems are almost parallel.
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J. Am. Chem. Soc. 2003, 125, 1120. (b) Li, A. D. Q.; Wang,
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(24) Neuteboom, E. E.; Maskers, S. C. J.; Meijer, E. W.; Janssen,
R. A. J. Macromol. Chem. Phys. 2004, 205, 217.
(25) Oligo(oxyethylene) units have been extensively utilized to
provide solvophobic motivation for folding in oligomeric
systems. For examples, see ref 4.
(26) (a) Vogtle, F.; Weber, E. Angew. Chem., Int. Ed. Engl. 1979,
18, 753 (b) Lohr, H. G.; Vogtle, F. Acc. Chem. Res. 1985, 18,
65.
(43) The higher the stability of the fully folded state when
compared to the unfolded one, the greater would be the
temperature-dependence of the equilibrium constant; i.e., one
would expect the equilibrium to shift more toward the fully
folded form as one reduces the temperature.
(27) For examples see: (a) Ajayaghosh, A.; Arunkumar, E.; Daub,
J. Angew. Chem., Int. Ed. 2002, 41, 1766 (b) Suzuku, Y.;
Morozumi, T.; Nakamura, H.; Shimomura, M.; Hayashita,
T.; Bartsh, R. A J. Phys. Chem. B 1998, 102, 7910. (f)
Tummler, B.; Maass, G.; Vogtle, F.; Sieger, H.; Heimann, U.;
Weber, E. J. Am. Chem. Soc. 1979, 101, 2588. (g) Monti, D.;
Venanzi, M.; Manchini, G.; Marotti, F.; Monica, L. L.; Boschi,
T. Eur. J. Org. Chem. 1999, 1901.
(28) Balbo Block, M. A.; Hecht, S. Macromolecules 2004, 37, 4761.
(29) For examples see - (a) Amabilino, D. B. J. Am. Chem. Soc.
1995, 117, 1271. (b) Hunter. C. A.; Sanders, J. R. M. J. Am.
Chem. Soc. 1990, 112, 5525. (c) Maitra, U.; Vijay Kumar, P.;
Chandra, N.; D’Souza, L. Z.; Prasanna, M. D.; Raju, A. R.
Chem. Commun. 1999, 7, 595. (d) Percec, V.; Glodde, M.;
Bera, T. K.; Miura, Y.; Shiyanovskaya, I.; Singer, K. D.;
Balagurusamy, V. S. K.; Heiney, P. A.; Schnell, I.; Rapp. A.;
Spiess, H. W.; Hudson, D. D.; Duan. H. Nature (London)
2002, 417, 384. (e) Kost, D.; Peor, N.; Sod-Moriah, G.;
Sharabi, Y.; Durocher, D. T.; Raban, M. J. Org. Chem. 2002,
67, 6938. (f) Zhao, X.; Jia, M.-X.; Jiang, X.-K.; Wu, L.-Z.; Li,
Z.-T.; Chen, G.-J. J. Org. Chem. 2004, 69, 270.
(44) The solvent effect was examined using methanol/chloroform
mixtures as it exhibited a larger effect on folding, while an
aprotic solvent mixture, such as acetonitrile/chloroform mix-
ture, is more suitable for metal-ion titration studies.
(45) The effect of counterion was examined in a 1H NMR titration
study of PDA)5OE, using NaClO4 and NaSCN, and no
significant difference was noticed.
(46) The spectral changes of PDA)5OE and PDA)6OE with
increasing concentrations of various alkali-metal ions are
provided in the Supporting Information. The variation of the
spectral features in the aliphatic region in these spectra
provides further direct evidence for the fact that the com-
plexation indeed occurs with the OE spacer. The nonuniform
changes in the aliphatic region (some of the peaks move
upfield while others move downfield) of the spectra confirms
that the variation in the chemical shift is not induced by
changes in the dielectric (or ionic strength) of the medium
but is a direct reflection of complexation. Formation of the
CT complex between the donor and acceptor units could bring
some of the methylene protons, particularly those close to
the aromatic rings, under the influence of the aromatic ring
current, and therefore further influencing their chemical
shifts. Similar changes have been seen in the case of podands
by Vogtle et al. See ref 26a.
(30) Ghosh, S.; Ramakrishnan, S. Angew. Chem., Int. Ed. Engl.
2004, 43, 3264.
(31) Perrin, D. D.; Armarego, W. L. F.; Perrin, D. R. Purification
of Laboratory Chemicals, 2nd ed.; Pergamon Press: Oxford,
U.K., 1980.
(32) (a) Brunsveld, L. Ph.D. Thesis, Eindhoven University of
Technology, 2001. (b) Loiseau, F. A.; Hii, K. K. M.; Hill, A.
M. J. Org. Chem. 2004, 69, 639.
(47) In PDA)5OE, the changes seen in the δ values of the
acceptor protons in the presence of Li and Na are almost the
same while changes in their UV-visible spectra are distinctly
different. This is possibly because of differences in the
geometries of the CT complexes formed upon complexation
with Na and Li. The effect of differences in geometry appears
to be more significant on the ACT, but not on acceptor proton
chemical shifts. This is confirmed from the model compound
(DAD)5OE) studies, wherein the changes seen in the
chemical shifts of the acceptor proton under similar condi-
tions are roughly similar for both Na and Li (see Figure 10),
although they exhibit distinct differences in their ACT.
(48) We assume, of course, that the DAD and ADA model
compounds readily form sandwiched structures in the pres-
ence of alkali-metal ions. This assumption is supported by
the large difference in the NMR spectral variations between
these models and the corresponding DA analogue.
(49) Similar changes in the proton NMR spectra of naphthalene
donor containing adeamers were reported by Iverson and co-
workers, and were ascribed to differences in the geometries
of the D-A complexes. See ref 22c.
(50) When the donor is sandwiched both the aromatic rings of the
naphthalene unit it experiences similar changes and remains
undifferentiated.
(33) Bouzide. A.; Sauve, G. Org. Lett. 2002, 4, 2329.
(34) Liu, J. G.; He, M. H.; Li, Z. X.; Qian, Z. G.; Wang, F. S.; Yang,
S. Y. J. Polym Sci PartA: Polym. Chem. 2002, 40, 1572.
(35) The polymer PDA)3OE, with a tri(oxyethylene) spacer, was
also prepared but was found to have very poor solubility, and
hence its solution properties could not be examined.
(36) All the spectral data are available in the Supporting Informa-
tion.
(37) The fractionated samples exhibit unexpectedly high values
of polydispersity and this is because of the presence of
aggregates at the concentrations used for GPC (∼2 mg/cm3),
as evidenced from the light scattering detector signal. Ul-
trasonication of the solutions prior to injection reduces the
high molecular tail but does not completely remove it.
(38) In our earlier study,30 we had shown that the spectral changes
seen in the nascent polymer PDA)6OE were very similar
to those seen in the high molecular weight fractionated
samples. Similarly, representative studies using fractionated
samples of the other two polymers were also done, which
helped reaffirm the earlier observations.
(39) Polyimides from pyromellitic dianhydride and simple amine
terminated oligo(oxyethylene)s were prepared as controls and
these were white in color.
(40) For all the complete spectra refer to the Supporting Informa-
tion.
(41) For an earlier report on the effect of spacer length on CT
interactions in small molecule D-A systems see: Verhoeven,
J. W.; Dirkx, I. P.; Boer, T. J. Tetrahedron Lett. 1966, 4399.
(42) Comparison of the temperature dependences of the UV-
visible and NMR spectra of the polymer PDA)5OE with the
corresponding model DA-5OE in the presence of excess Li+
ion (see Figure S13 in Supporting Information) provides
(51) Dynamic light scattering studies on a representative polymer,
PDA)6OE, revealed that a dramatic narrowing of the size
distribution occurred in the presence of an excess of K ion,
possibly reflecting the breaking of aggregated structures,
which was also seen in the GPC chromatograms. However,
a quantitative estimate of extent of stacking could not be
obtained.
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