Folding a polymer via two-point interaction with an external folding agent: Use of H-bonding and charge-transfer interactions

Article abstract of DOI:10.1021/ma100189q

A polymer containing electron-rich aromatic donors (1,5-dialkoxynaphthalene (DAN)) was coerced into a folded state by an external folding agent that contained an electron-deficient aromatic acceptor (pyromellitic diimide (PDI)) unit. The donor-containing polymer was designed to carry a tertiary amine moiety in the linking segment, which served as an H-bonding site for reinforcing the interaction with the acceptor containing folding agent that also bore a carboxylic acid group. The H-bonding interaction of the carboxylic acid and the tertiary amine brings the PDI unit between two adjacent DAN units along the polymer backbone to induce charge-transfer (C-T) interactions, and this in turn causes the polymer chain to form a pleated structure. Evidence for the formation of such a pleated structure was obtained from NMR titration studies and also by monitoring the C-T band in their UV-visible spectra. By varying the length of the segment that links the PDI acceptor to the carboxylic acid group, we showed that the most effective folding agent was the one that had a single carbon spacer, as evident from the highest value of the association constant. Control experiments with propionic acid clearly demonstrated the importance of the additional C-T interactions for generating the folded structures. Further, solution viscosity measurements in the presence of varying amounts of the folding agent revealed a gradual stiffening of the chain in the case of the PDI carrying carboxylic acid, whereas no such affect was seen in the case of simple propionic acid. These observations were supported by DFT calculations of the interactions of a dimeric model of the polymer with the various folding agents; here too the stability of the complex was seen to be highest in the case of the single carbon spacer.

Full text of DOI:10.1021/ma100189q

Macromolecules 2010, 43, 3183–3192 3183
DOI: 10.1021/ma100189q
Folding a Polymer via Two-Point Interaction with an External Folding
Agent: Use of H-Bonding and Charge-Transfer Interactions
Swati De, Debasis Koley, and S. Ramakrishnan*
Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India
Received November 27, 2009; Revised Manuscript Received February 22, 2010
ABSTRACT: A polymer containing electron-rich aromatic donors (1,5-dialkoxynaphthalene (DAN)) was
coerced into a folded state by an external folding agent that contained an electron-deficient aromatic acceptor
(pyromellitic diimide (PDI)) unit. The donor-containing polymer was designed to carry a tertiary amine
moiety in the linking segment, which served as an H-bonding site for reinforcing the interaction with
the acceptor containing folding agent that also bore a carboxylic acid group. The H-bonding interaction of
the carboxylic acid and the tertiary amine brings the PDI unit between two adjacent DAN units along the
polymer backbone to induce charge-transfer (C-T) interactions, and this in turn causes the polymer chain to
form a pleated structure. Evidence for the formation of such a pleated structure was obtained from NMR
titration studies and also by monitoring the C-T band in their UV-visible spectra. By varying the length of
the segment that links the PDI acceptor to the carboxylic acid group, we showed that the most effective
folding agent was the one that had a single carbon spacer, as evident from the highest value of the association
constant. Control experiments with propionic acid clearly demonstrated the importance of the additional C-T
interactions for generating the folded structures. Further, solution viscosity measurements in the presence of
varying amounts of the folding agent revealed a gradual stiffening of the chain in the case of the PDI carrying
carboxylic acid, whereas no such affect was seen in the case of simple propionic acid. These observations were
supported by DFT calculations of the interactions of a dimeric model of the polymer with the various folding
agents; here too the stability of the complex was seen to be highest in the case of the single carbon spacer.
Introduction
between the DAN donors and PDI acceptor units, caused the
polymer chain to adopt a pleated structure leading to the genera-
Controlling the conformation of polymer chains in solution is
a challenge that is not only exciting from a purely academic
standpoint but it also holds the potential to spatially arrange
functional units around a polymer backbone and consequently
elicit unique collective properties that rely on such specific
arrangement. Whereas folding of well-defined oligomers1 has
been extensively studied using a variety of directional intrachain
interactions, such as H-bonding,2 π-stacking,3 metal-ion interac-
tions,4 charge-transfer complexation,5 etc., efforts to extend this
to higher molecular weight polymers have been far more limited.
Some years ago, we designed a few such polymeric systems
wherein charge-transfer interactions betweenalternatinglyplaced
electron-rich dialkoxynaphthalene (DAN) and electron-deficient
pyromellitic diimide (PDI) units, aided by metal-ion complexa-
tion and solvophobic interactions, caused the polymer chain to
adopt a specific folded conformation.6 Such charge-transfer (C-T)
induced folding was first studied by Iverson and co-workers5a-g
in well-defined oligomers and more recently elaborated by Zhao
et al.5h to generate alternate designs to fold oligomeric systems.
In all these studies, the C-T interactions served not only to
assist folding but it also served as a valuable spectroscopic
signature to study the folding process. Extending our approach
further, we designed a polymer carrying only PDI acceptor units
linked by oligo(oxyethylene) segments, which was then made
to fold upon interaction with a small molecule bearing a DAN
donor unit and an ammonium group.7 In this approach, a two-
point interaction: one between the ammonium group and the
oligo(oxyethylene) segment and the other a C-T interaction
tion of D-A stacks. The association constant between the folding
agent and the polymer was estimated to be around 850 M-1
,
which implied that the extent of folding was limited to only about
five to six D-A pairs. In an attempt to enhance the association
constant and extend the stacking further, we designed a new type
of donor containing polymer that carries a tertiary amine unit
in the spacer segment, which could interact strongly with a
suitably designed acceptor-bearing folding agent that also carries
a carboxylic acid group, as shown in Scheme 1. This acid-base
interaction, we reasoned, would bring the acceptor unit in a
suitable position so as to form a C-T complex with the adjacent
donors, resulting in the folding of the polymer chain. The
structure of the folding agent, specifically the length of the
segment that links the carboxylic acid to the DAN unit, was
varied to optimize the folding efficacy.
Experimental Section
1,5-Dihydroxynaphthalene, pyromellitic dianhydride and ac-
ryloyl chloride were purchased from Aldrich Chemical Co.
Pyromellitic dianhydride was refluxed with dry-distilled acetic
anhydride before use. All the solvents were dried following the
standard procedures.8 Structures of all intermediates, monomers
and polymers were confirmed by 1H NMR spectroscopy. NMR
spectra were recorded on a Bruker AV400 MHz spectrometer,
using CDCl3 as the solvent and TMS as reference. GPC measure-
ments were carried out using Viscotek triple detector system
(TDA model 300), coupled to a refractive index (RI), a right-
angle light scattering (RALS) and a differential viscometer (DV)
in series. The separation was achieved using a series of two PL gel
mixed bed columns (300 ꢀ 7.5 mm) operated at 30 °C using THF
*Corresponding author. E-mail: raman@ipc.iisc.ernet.in.
r 2010 American Chemical Society
Published on Web 03/05/2010
pubs.acs.org/Macromolecules

3184 Macromolecules, Vol. 43, No. 7, 2010
Scheme 1. Schematic Representation of Folding Aided by Two-Point Interactions with a Folding Agent
De et al.
as the eluent. Molecular weights were determined using the light
scattering calibration curve based on the data from the RI, DV
and RALS detectors. The UV-visible (UV-vis) spectra were
recorded using a Varian 300 UV spectrophotometer.
4H, -OCOCH2CH2), 4.2 (t, 4H, Ar-O-CH2CH2OCO), 4.5
(t, 4H, -CH2OCO), 6.7 (d, 2H, Ar-H, o-OCH2), 7.3 (t, 2H,
Ar-H, m-OCH2), 7.8 (2H, Ar-H, p-OCH2).
Synthesis of Model Compound (M). Monoacrylate 610 (0.30 g,
1.10 mmol) was dissolved in 2.5 mL of a mixture of dry THF and
dry DMF (4:1 v/v). n-Octyl amine (0.071 g, 0.55 mmol) was
added, and the reaction mixture was stirred in a sealed vessel at
50 °C for 15 days. The solvent was then removed, and the
contents were poured in water. It was then extracted with
EtOAc, and the organic layer was washed with brine. EtOAc
was removed, and the product was recrystallized from a mixture
of methanol and chloroform (2: 1 v/v) to get 0.27 g (yield =
73%) of model compound M.1H NMR (δ, CDCl3): 0.8 (t, 3H,
-N(CH2)7CH3), 1.27 (m, 12H, -NCH2(CH2)6CH3), 2.3 (t, 2H,
-NCH2(CH2)6CH3), 2.4 (t, 4H, -OCOCH2CH2), 2.7 (t, 4H,
-OCOCH2CH2), 3.93 (s, 3H, -OCH3), 4.2 (t, 4H, Ar-O-
CH2CH2OCO), 4.47 (t, 4H -CH2OCO), 6.76 (m, 2H, Ar-H,
o-OCH3 and o-OCH2), 7.3 (m, 2H, Ar-H, m-OCH3 and
m-OCH2), 7.8 (m, 2H, Ar-H, p-OCH3 and p-OCH2).
Synthesis of Compound 4. 1,5-Dihydroxynaphthalene 1 (1.5 g,
9.38 mmol) was taken along with K2CO3 (6.4 g, 46.4 mmol) and
a catalytic amount of KI in dry acetonitrile (25 mL) and stirred
for 1 h. Then, chloroethanol acetate (4.6 g, 37.5 mmol) was
added and the contents were refluxed for 72 h at 100 °C. After
the reaction was complete, CH3CN was removed, and the
contents were poured in alkaline water. The residue formed
was filtered and dried to get compound 3, which was then
directly used for the next step without further purification.
Compound 3 was taken with K2CO3 (3.0 g) in a mixture of
dry MeOH (15 mL) and dry THF (15 mL) and stirred for 12 h at
50 °C. Then, the solvents were removed, and the residue poured
in basic water. The precipitate formed was filtered and dried to
give 1.4 g (overall yield = 61%) of compound 4. mp 130 °C. 1H
NMR (δ, CDCl3): 2.0 (t, 2H, CH2OH), 4.0 (m, 4H, -CH2OH),
4.2 (t, 4H, Ar-O-CH2CH2OH), 6.8 (d, 2H, Ar-H, o-OCH2),
7.3 (t, 2H, Ar-H, m-OCH2), 7.8 (2H, Ar-H, p-OCH2).
Synthesis of Compound PDI-C1.5i,j Pyromellitic dianhydride
(1.0 g, 4.5 mmol) was taken in dry DMF (15 mL) along with glycine
(0.338 g, 4.5 mmol) and octyl amine (0.592 g, 4.5 mmol). The contents
were stirred for 12 h at 100 °C, after which the DMF was removed
under reduced pressure and the residue was purified by column
chromatography to get 0.540 g of compound PDI-C1 (yield=32%).
mp 240-245 °C. 1H NMR (δ, CDCl3): 0.90 (t, 3H, -NCH2CH2-
(CH2)5CH3), 1.35 (m, 10H, -NCH2CH2 (CH2)5CH3), 1.71 (m, 2H,
-NCH2CH2(CH2)5CH3), 3.77 (t, 2H, -NCH2CH2(CH2)5CH3),
4.57 (s, 2H, -NCH2COOH), 8.3 (s, 2H, Ar-H).
Synthesis of Compound PDI-C2. PDI-C2 was synthesized
following the same procedure as that of PDI-C1 using β-alanine
instead of glycine. Yield = 31%, m.p. = 230-232 °C. 1H NMR
(δ, CDCl3): 0.83 (t, 3H, -NCH2CH2(CH2)5CH3), 1.27 (m, 10H,
-NCH2CH2 (CH2)5CH3), 1.6 (m, 2H, -NCH2CH2(CH2)5CH3),
2.8 (t, 2H, NCH2CH2COOH),3.7 (t, 2H, -NCH2CH2(CH2)5-
CH3), 4.03 (t, 2H, NCH2CH2COOH), 8.22 (s, 2H, Ar-H).
Synthesis of Compound PDI-C3. PDI-C3 was synthesized
following the same procedure as that of PDI-C1 using
4-aminobutyric acid instead of glycine. Yield = 30%, mp
216 °C. 1H NMR (δ, CDCl3): 0.87 (t, 3H, -NCH2CH2(CH2)5-
CH3), 1.27 (m, 10H, -NCH2CH2 (CH2)5CH3), 1.69 (m,
2H, -NCH2CH2(CH2)5CH3), 2.07 (m, 2H, -NCH2CH2CH2-
COOH), 2.46 (t, 2H, -NCH2CH2CH2COOH), 3.75 (t, 2H,
-NCH2CH2(CH2)5CH3), 3.85 (t, 2H, -NCH2CH2CH2COOH),
8.27 (s, 2H, Ar-H).
Synthesis of Compound 5.9 Compound 4 (1.0 g, 4.0 mmol) was
taken in dry THF (5 mL) and stirred in ice-water bath for 5 min.
Then, Et3N (2.0 g, 19.8 mmol) was added dropwise. After 30 min,
acryloyl chloride (1.82 g, 20.0 mmol) in dry THF (5 mL) was added
dropwise. The reaction mixture was stirred at room temperature
for 3 h, followed by stirring at 50 °C for 7 h. After the reaction was
complete, THF was removed, and the contents were poured in
acidic ice-cold water. The residue was then filtered, dried, and
purified by column chromatography to get 0.44 g of compound 5
(yield = 30%). mp 80 °C. 1H NMR (δ, CDCl3): 4.3 (t, 4H,
Ar-O-CH2CH2OCO), 4.6 (t, 4H, -CH2OCO), 5.8 and 6.4 (d,
2H each, dCH2), 6.1 (m, 2H, -OCOCHd), 6.8 (d, 2H, Ar-H,
o-OCH2), 7.3 (t, 2H, Ar-H, m-OCH2), 7.8 (2H, Ar-H, p-OCH2).
Synthesis of Polymer (P). Diacrylate 5 (0.38 g, 1.06 mmol)
was dissolved in 3 mL of a mixture of dry THF and dry DMF
(5:1 v/v). Octyl amine (0.137 g, 1.06 mmol) was added, and the
reaction mixture was stirred in a sealed vessel at 50 °C for 15 days.
The reaction mixture was then poured in methanol, and the
precipitate formed was dissolved in a minimum amount of THF
and reprecipitated in MeOH once again to get 0.310 g (yield =
60%) of polymer P. Mw = 23 000 and Mn = 19 000 using
light scattering calibration. 1H NMR (δ, CDCl3): 0.8 (t, 3H,
-N(CH2)7CH3), 1.27 (m, 12H, -NCH2(CH2)6CH3), 2.3 (t,
2H, -NCH2(CH2)6CH3), 2.4 (t, 4H, -OCOCH2CH2), 2.8 (t,

Article
Folding Studies. For a typical NMR titration experiment,
Macromolecules, Vol. 43, No. 7, 2010 3185
Scheme 2. Synthetic Scheme for the Preparation of the Donor
Containing Polymer (P) and Model Compound (M)
2 mL of the polymer solution (2 mM) was prepared in a mixture
of CDCl3 and MeOH (20:1 v/v; solution A). An aliquot of this
solution (0.6 mL) was taken in the NMR tube, and a large excess
(∼10-15 equiv) of the acceptor folding agent was dissolved in
the remaining solution to form solution B. Solution A was then
titrated with solution B (in 20-100 μL steps), which resulted in a
series of solutions having fixed polymer concentration but with
varying amounts of the acceptor. The UV-vis titration experi-
ments were similarly performed using a mixture of CHCl3 and
MeOH (20:1) as the solvent. The spectra in Figure 5 were
obtained by subtraction of the pure polymer spectrum from
that of the spectrum of the mixture (polymer þ folding agent) to
permit clear visualization of the C-T band.
Viscosity Measurements. For viscosity measurements, 20 mL
of the polymer solution (10 mg/mL) was prepared in a mixture
of CHCl3 and MeOH (20:1 v/v; solution A). An aliquot of this
solution (4 mL) was taken with an excess (5 equiv) of the folding
agent PDI-C1 (or propionic acid) to make up solution B. Both
solutions were filtered using a 0.5 μm membrane filter prior to
the measurement. Viscosity measurements were carried out by
mixing different volumes of two solutions, which resulted in a
series of solutions having fixed polymer concentration but with
varying amounts of the folding agent (or carboxylic acid). The
measurements were done using an Ubelhode viscometer im-
mersed in a constant temperature bath maintained at 25 °C. The
flow times were measured using an automated Schott Gerate
instrument.
Computational Details. All DFT calculations were performed
using Gaussian03 program package.11 The density functional
calculations with the Becke’s half-and-half functional (BH&H)12
employed a basis set of double-ζ quality, which is denoted
LANL2DZ in Gaussian. The elements C, N, O, and H were
described by a Dunning/Huzinaga full double-ζ basis set.13 The
BH&H functional was employed because of its ability to
characterize adequately dispersion interactions.14 Geometries
were fully optimized, normally without symmetry constraints.
Harmonic force constants were computed at the optimized
geometries to characterize the stationary points as minima.
Zero-point vibrational corrections were determined from the
harmonic vibrational frequencies to convert the total energies,
Ee, to ground-state energies, E0. The rigid-rotor harmonic
oscillator approximation was applied to evaluate the thermal
and entropic contributions. Single-point solvent calculations
were performed on the optimized gas-phase geometries of
our model compounds and the corresponding complexes. We
employed the CPCM model,15 which is an implementation of
the conductor-like screening model (COSMO)16 in Gaussian03.
CHCl3 (ε = 4.9) was used as a solvent with UAKS (united atom
topological model) radii scheme for the respective atoms (C, H,
Scheme 3. Synthetic Scheme for the Preparation of the Acceptor
Containing Folding Agents (PDI-C1, PDI-C2, and PDI-C3)
N, and O). Enthalpy (ΔH298(sol)) and free energy (ΔG298(sol)
)
are relatively slow, polymers of moderate-to-high molecular
weights have been attained using this approach. Because Michael-
addition based polymerizations generate polymers bearing ter-
tiary amine groups along the polymer backbone, we chose to use
this appraoch for the present study. Therefore, to prepare a linear
polymer containing electron-rich aromatic donors linked by
a segment that carries a tertiary amine group, we synthesized a
donor containing diacrylate monomer (to serve as a Michael
acceptor) and reacted it with an aliphatic primary amine. The two
active hydrogens in the primary amine would be expected to add
to the acrylate units to yield the polymer (P) (Scheme 2).
Although the reactivity of a primary and secondary amine
(formed after the first addition) toward Michael addition is
different, it is of little consequence in the present context because
of the symmetry of the final polymer structure. In our first
attempt, we used a simple diacrylate (2) of 1,5-dihydroxy-
naphthalene and reacted it with n-octylamine in order to generate
the polymer. However, the aryl acrylate linkage was very suscep-
tible to cleavage; therefore, the polymerization process did not
change in CHCl3 were calculated from the solvent electronic
energy (ΔEsol) and respective thermal corrections from gas-
phase thermochemical values. (Refer to the Supporting Informa-
tion.) The effect of basis set superposition error (BSSE) on the
BH&H/LANL2DZ binding energies was determined through the
counterpoise method of Boys and Bernardi.17 The TDDFT (time-
dependent density functional theory)11 singlet excitation energies
of the complexes in chloroform were evaluated by a recent none-
quilibrium implementation of the CPCM model.16 To characterize
the C-T bands, only the five lowest spin-allowed singlet-singlet
transitions were taken into account. Transition energies and
oscillator strengths were interpolated by a Gaussian convolution
with a width of 0.2 eV.18
Results and Discussion
Michael addition has been extensively used for the preparation
of dendrimers and, more recently, for the preparation of linear
and hyperbranched polymers.19 Although these polymerizations

3186 Macromolecules, Vol. 43, No. 7, 2010
De et al.
Figure 1. 1H NMR spectra of the polymer (P) and model compound (M) in CDCl3. The peaks marked by asterisk are due to solvent or inadvertent
water.
Figure 2. 1H NMR spectra of the acceptor containing folding agents (PDI-C1, PDI-C2, and PDI-C3) in CDCl3. The peaks marked by asterisk are due
to solvent or inadvertent water.
occur effectively. Hence, we incorporated an ethylene spacer
segment by coupling the 1,5-dihydroxynaphthalene to chlor-
oethanol acetate, followed by hydrolysis and coupling to acryloyl
chloride. (See Scheme 2.) The reaction of the spacer containing
monomer (5) with n-octylamine yielded the polymer (P), having a
moderately high molecular weight of Mw = 23 000 and Mn =
19000 using the triple detector system. The n-octyl group on the
amine serves to enhance the polymer solubility; furthermore, the
relatively high boiling point of the amine ensured that the
stoichiometric balance between the two monomers was not
significantly altered during the prolonged polymerization at
50 °C. It is expected that the length of the spacer segment would
influence the stability of the folded state; the additional four
methylene units in this design, we argued, may provide greater
flexibility and facilitate adaptation to the preferred geometry of
the C-T complex formed with the external acceptor, although the
configurational entropy cost for folding is expected to increase.
Three acid-containing folding agents, bearing varying lengths of
the spacer linking the PDI and the acid group, were synthesized
(as per the Scheme 3) by reacting pyromellitic dianhydride with
an equimolar mixture of the appropriate amino acid and
n-octylamine. The required product was readily separated from the
mixture of the three possible products and typically was obtained in
∼30% yield. For a comparative study of the folding process, a
model compound (M) that represents the smallest foldable unit of
the polymer was also prepared as per the Scheme 2.10
The proton NMR spectra of the polymer (P) and the model
compound (M) are shown in Figure 1, along with the peak
assignments. The long chain octyl pendant group ensured ade-
quate solubility of the polymer in common organic solvents,
which is essential to carry out the folding studies. The relative
intensities of the various peaks were in accordance with the
expected structure. Three sets of peaks corresponding to the
aromatic protons of the naphthalene ring are seen in the spectrum
of the polymer, and these have been assigned as indicated in
the Figure; the most downfield peak (peak a) corresponds to the
proton para to the alkoxy substituent.20 The model compound
also exhibited a very similar spectrum, except for the additional
peak due to the terminal methoxyl protons at 3.9 ppm. The NMR
spectra of the PDI carrying acceptors, PDI-C1, PDI-C2,

Article
Macromolecules, Vol. 43, No. 7, 2010 3187
Figure 3. Variation in the aromatic region of the 1H NMR spectra of polymer P as a function of increasing amounts of an acceptor containing folding
agent, PDI-C1. The δ value of peak a, marked by an arrow was monitored to study the folding. A 2 mM solution of P in CDCl3/MeOH mixture (20:1,
v/v) was used for the titration.
and PDI-C3, are shown in Figure 2 along with their peak
assignments.
To study the folding of the polymer in the presence of the
folding agent, we carried out NMR titration experiments wherein
the relative mole ratio of the polymer and folding agent was
varied. As previously shown,6,7 the chemical shift of the donor
and acceptor protons is very sensitive to the formation of a C-T
complex; the chemical shift of the aromatic protons of both the
donor and acceptor units shifts upfield, the effect on the acceptor
proton typically being a little more pronounced. In Figure 3, we
show the variation in the aromatic region of the proton NMR
spectra of mixtures of the polymer P and folding agent PDI-C1 as
a function of increasing mole ratio of folding agent. It is evident
that all three sets of aromatic peaks belonging to the polymer
DAN unit experience an upfield shift with increasing concentra-
tion of folding agent, although the maximum shift is experienced
by the most downfield proton (peak a), which we shall henceforth
use to monitor the extent of folding. The titration of the model
compound M also shows a similar behavior, as evident from
Figure S3 of the Supporting Information; here again the max-
imum shift is seen for the most downfield proton. As shown by
Sanders et al.,20 this difference in the relative sensitivity of the
different protons of the DAN unit is a reflection of the precise
geometry of the C-T complex. The folding experiments were done
in a mixture of CDCl3 and methanol (20:1, v/v), keeping the
methanol content to the minimum required to ensure that both
the components are completely soluble. The strength of the
interaction between the folding agent and the polymer has been
previously shown to be greater in solvent mixtures of lower
Figure 4. Variation in the chemical shift (δ) of peak a of (a) the polymer
P and (b) the model compound M in the presence of various folding
agents PDI-C1, PDI-C2, PDI-C3, PDI-M, and PA. Solutions (2 mM)
polarity because of the solvophobic effect on the polymer con-
formation that facilitates complex formation.21
To optimizethe process of folding, three folding agents bearing
spacers of different length between the acceptor and the car-
boxylic acid group were prepared. Similar NMR titration experi-
ments were also carried out using the other two folding agents,
and the variation of the chemical shift of the most downfield
aromatic proton (peak a) of the donor as a function of mole ratio
of the folding agents was plotted (Figure 4a). As is evident, the
of theP and M inCDCl3/MeOH mixture (20:1, v/v) wereused for all the
studies.
maximum change in the chemical shift is observed when PDI-C1,
bearing a single carbon linker between the PDI and the carboxylic
acid, was used as the folding agent. When longer spacers, C2 and
C3, are present, the change observed in the chemical shifts is

3188 Macromolecules, Vol. 43, No. 7, 2010
De et al.
Table 1. Association Constants (Ka) for Complexes Were Calculated
by Using EQNMR Version 2.10 and from UV-Dilution Experiment
Ka from
EQNMR
Ka from
UV dilution
extinction
coefficient
(M-1 cm-1
)
no.
molecule
(M-1
)
experiment (M-1
)
1
2
3
4
5
6
P with PDI-C1
P with PDI-C2
P with PDI-C3
M with PDI-C1
M with PDI-C2
M with PDI-C3
1200
800
300
815
520
128
1325
593
40.5
37.8
a
a
1170
592
33.5
30.3
a
a
a C-T band was too weak to carry out the dilution experiements.
significantly smaller. A nonlinear least-squares fitting procedure
using EQNMR22 permitted the determination of the association
constants under the rapid exchange limit, and these values are
listed in Table 1. It is clear from the numbers that PDI-C1 is most
effective in inducing the polymer to fold; the association constant
was found to be ∼1200 M-1, whereas the other two folding
agents exhibited significantly lower values. A similar plot in
the case of the model compounds (Figure 4b) also revealed that
PDI-C1 exhibited the highest association constant, followed by
PDI-C2 and PDI-C3. A further interesting observation is that the
association constant was smaller in the model system when
compared with the polymer in all three cases, suggesting that
the solvophobic effect may be significantly higher in the poly-
meric systems than in the model compound. In other words, the
addition of methanol (a nonsolvent for the polymer) modulates
the polymer conformation toward one that spatially co-locates
adjacent donor units and consequently enhances C-T complex
formation with the folding agent, whereas such an effect is
expectedly much smaller in the case of the model compound.
To ascertain the importance of the C-T interaction in the folding
process, a control titration was carried out wherein the polymer
was titrated with increasing amounts of simple carboxylic acid,
namely, propionic acid (PA). The chemical shift of the most
downfield donor protons was unaffected by the presence of
propionic acid, suggesting that little, if any, interaction between
the adjacent DAN units occurs. A similar titration using bis(n-
propyl)-pyromellitic dimide (PDI-M) also showed very little
variation of the δ value, which confirmed the importance of the
additional H-bonding interaction. (See Figure 4.) These experi-
ments taken together reveal the importance of the two-point
interaction in enabling the formation of the pleated structure.23
Comparative examination of their solution viscosities, as will be
discussed later, further confirms the importance of both the
interactions in stiffening the polymer chain.
The C-T complex formation can also be readily studied by
monitoring the C-T band in the absorption spectra. Figure 5a
depicts the variation of the absorption spectra, in the region
where the C-T transition occurs, with increasing mole fraction of
the folding agent. A similar plot using the model compound with
the folding agent is shown in Figure 5b. The variation of the ACT
at λmax was plotted as a function of folding agent mole ratio for
the different folding agents, and these are shown as insets in
Figure 5. It is clear that the most instense C-T band is seen in the
case of PDI-C1, followed by PDI-C2 and PDI-C3, which is
generally in agreement with the trend seen in the NMR titrations.
UV dilution experiments24 were done to determine the associa-
tion constants between the polymer and different folding agents;
the values thus obtained are listed in Table 1. The UV dilution
experiments also provided an estimate of the molar extinction
coefficient associated with the C-T band, and these are also listed
in Table 1. The association constant values obtained from NMR
and UV dilution studies are in reasonable agreement, thus
reaffirming the conclusions that the efficacy of folding agents
follows the trend PDI-C1 > PDI-C2 > PDI-C3. In the case of
Figure 5. Variation of the UV-vis spectra of (a) polymer P and (b)
model compound M as a function of increasing amounts of the folding
agent, PDI-C1. Insets: variation of the absorbance of the C-T band of
the polymer P (inset a) and model compound M (inset b) as a function
of increasingamounts of the foldingagents: PDI-C1, PDI-C2, and PDI-
C3. Solutions of P and M (2 mM) in CHCl3/MeOH mixture (20:1, v/v)
were used for all studies.
PDI-C3, the UV dilution studies were difficult to perform
because of very low intensity of charge transfer band, and hence
the association constant value could not be determined.
Whereas the spectral changes seen in the NMR and UV-vis
studies suggest the formation of a pleated structure, it would be
useful to measure other chain conformational properties to
confirm this. To do so, we carried out solution viscosity measure-
ments of the polymer solution (1 wt %) in the presence of
increasing amounts of the folding agent, PDI-C1; the variation
of the inherent viscosity is plotted as a function of mole ratio of
the folding agent in Figure 6. It is clearly evident that a dramatic
increase in the solution viscosity is seen to occur beyond a
threshold concentration of the folding agent. This observation
suggests a stiffening of the chain on going from a flexible random
coil to a pleated structure. A control titration experiment was also
done wherein a similar experiment was performed in the presence
of a simple carboxylic acid, namely, propionic acid (PA). The
H-bonding of this carboxylic acid with the amine units along the
backbone alone is clearly inadequate to cause the conformational
transition, as evident from the very small increase in the inherent
viscosity (Figure 6); the additional C-T interactions between the
DAN units along the polymer backbone and the PDI unit of the
folding agent is essential to further stabilize the folded form.
In an effort to rationalize the observed trend in the folding
efficacy of the three folding agents, we carried out computational
studies using DFT methods. These studies were carried out for
the interaction of the model compound M0 with each of the
folding agents (C10, C20, C30); for ease of computation, the
N-alkyl group in both cases was taken as a methyl instead of

Article
Macromolecules, Vol. 43, No. 7, 2010 3189
the n-octyl, which is present in the actual systems. The BH&H/
LANL2DZ-optimized structures of the three donor-acceptor
complexes are shown in Figure 7. From the top view of the
optimized structures, it is evident that the most symmetric
placement of the donors on either side of the central acceptor
unit is seen in the case of M0-C10, whereas in both of the other
cases, the two donors are staggered and shifted off-center with
respect to the central acceptor unit. The BSSE-corrected ener-
getics for the formation of the complexes in chloroform (CHCl3)
are given in Table 2. In each of the complexes (M0-C10, M0-C20,
of formation for the complexes are in the order M0-C10 >
M0-C20 > M0-C30 (Table 2), which is in accordance with the
trend observed in the experimental values of formation constants.
(See Table 1.)
To separate the contributions of the N3 H1-O5 hydrogen
3 3 3
bonding interaction from those of π-π stacking (C-T), a series of
single-point BH&H/LANL2DZ calculations were performed.
Scheme 4 displays the approach utilized to separate out the
contribution from the two different interactions toward the
overall stabilization of the complexes. To retrieve the relative
contributions, the donor-acceptor complexes can be analyzed
under two different decomposition schemes (step A and step B;
see Scheme 4) that will furnish “incomplete complexes” I and IV.
In step A (Scheme 4), the two donor fragments are removed
without altering the relative orientations of the remaining con-
stituents. Now, the fragmented complex I is composed of two
independent subfragments II and III, which interact primarily
through H-bonding. Therefore, the interaction energies obtained
by subtracting the BSSE-corrected, single-point energies of
I from those of II and III for each of the complexes M0-C10,
and M0-C30), both hydrogen bonding (N3
H1-O5) as well as
3 3 3 3
C-T interactions are dominant. The free energy (ΔG298(sol)
)
Table 2. BH&H/LANL2DZ Calculated Binding Energies
(kilocalories per mole) for the Complexes M0-C10, M0-C20,
a,b
and M0-C30
c
complexes ΔEe ΔE0 ΔH298 ΔG298 ΔEsol ΔH298(sol) ΔG298(sol)
M0-C10 -57.2 -55.8 -57.4 -30.1 -46.1 -46.3
M0-C20 -51.7 -50.5 -51.8 -25.7 -42.9 -43.0
M0-C30 -45.6 -44.7 -45.9 -20.2 -37.6 -37.9
-19.0
-16.9
-12.2
a ΔEe is the electronic energy without the ZPE correction. ΔE0
includes the ZPE correction. ΔEsol is the solvent electronic energy.
ΔH298(sol) is the enthalpy change in CHCl3 including the thermal
enthalpic corrections from the gas phase. Similarly, ΔG298(sol) is the free
energy change in CHCl3 including the thermal free energy corrections
fom the gas phase. b All energies are counterpoise corrected. c In CHCl3.
Figure 6. Variation of the inherent viscosity of polymer P as a function
of increasing amounts of the folding agent, PDI-C1, or propionic acid
(PA). The polymer concentration was maintained at 1 wt % in CHCl3/
MeOH mixture (20:1, v/v), whereas the mole ratio of PDI-C1 or PA
(with respect to the polymer repeat unit) was varied.
Figure 7. (a) Optimized structures of the model complexes M0-C10, M0-C20, and M0-C30 with selected bond lengths (in angstroms) and angles (in
degrees). Hydrogen atoms of aromatic and spacer groups have been omitted for clarity. (b) Top view of the π-stacked aromatic framework for the three
different complexes. Color code: C, gray; O, red; N, blue; center-of-mass (c.o.m.) of the aromatic segments, brown.

3190 Macromolecules, Vol. 43, No. 7, 2010
Scheme 4. Energy Fragmentation Schemes for the Donor-Acceptor Complexes
De et al.
N3-H1 distances in the three complexes (1.263 (M0-C10), 1.397
Table 3. Energies (kilocalories per mole)for the Fragmentation of
the Complexes (M0-C10, M0-C20, and M0-C30; See the Text)
As Shown in Scheme 4a
0
0
0
0
˚
(M -C2 ), and 1.323 A (M -C3 ), respectively). This apparent
anomaly may be ascribed to the destabilization associated with
the spacer segments, which accounts for the higher overall
stability of M0-C20 when compared with M0-C30. One other point
in the present design is that the two-point interaction does not
directly impose a conformational constraint on the flexible
linking segment, unlike in our previous study where the interac-
tion of the oligo(oxyethylene) spacer with an ammonium unit
brings the adjacent PDI units in close proximity to the DAN unit
in the folding agent, thereby enabling C-T-induced stacking.7
Therefore, to assess the driving force for stacking in our system,
we examined the energetics associated with the interaction of a
preformed D-A pair with an additional donor unit to form a
DAD stack. Calculations suggested an additional stabilization to
the tune of about -13.9 kcal/mol,26 thereby supporting the fact
that the formation of the D-A stacks, as depicted in Scheme 1,
indeed enhances the stability of the system.27
complexes
H-bonding (step A)
π-stacking (step B)
b
b
ΔEe
ΔEsol
ΔEe
ΔEsol
M0-C10
M0-C20
M0-C30
-58.5
-43.3
-48.7
-50.1
-35.6
-40.9
M0-C10
M0-C20
M0-C30
-58.5
-43.3
-48.7
a All energies are counterpoise-corrected. b In CHCl3.
M0-C20, and M0-C30 (Table 3) yield the energies mainly associated
with the formation of N H-O hydrogen bonds.
3 3 3
Similarly, in step B, the spacer groups of the donor and
acceptor units were removed, and only the aromatic fragments
were retained, as in IV. To quantify the π-stacking (C-T)
interaction, complex IV was separated into the two donors (V)
and an acceptor (VI) (Scheme 4) moieties. The energies associated
with this process correspond to the donor-acceptor C-T inter-
action energies, and the trend seen for the three complexes reveals
that the strength varies in order M0-C10 > M0-C20 > M0-C30
(Table 3). From this simple decomposition process, it is evident
that the final stability of the complex appears to be more
dependent on the hydrogen bonding interaction between the acid
moiety of the folding agent and the amine unit of the model rather
than on the C-T interaction between the D and A units. A similar
observation was reported by Houk et al. in their exhaustive study
of the binding of bipyridinium-based guests by [3], [4] catenated
hosts.25 Interestingly, the H-bonding interaction in M0-C30 is
Time-dependent DFT calculations effectively capture the C-T
interactions in the complexes. The oscillator strength (f) for the
C-T excitation is highest for M0-C10 and lowest for the M0-C30
complex (M0-C10 (f = 0.0343), M0-C20 (f = 0.0327), M0-C30 (f =
0.0186)). Clearly, the intensity of the C-T excitation is influenced
by the relative orientation of the donor and acceptor frag-
ments.28,5e,g The geometry of the D-A-D complex can be
analyzed by comparing a few of the key parameters of the energy
minimized structures. First, the relative positions of the centers of
mass (d1, d2, and d3) can be examined: the angle d1-d2-d3,
which is a measure of their colinearity, was found to be 178.1,
162.6, and 155.3° for M0-C10, M0-C20, and M0-C30, respectively
(Figure 7a). Second, the relative orientations of the two DAN
units can be gauged from the angle between the lines O1-O2 and
O3-O4 (projected onto a single plane), which was determined
0
0
˚
stronger than M -C2 (N3-H1 = 1.397 A, ΔEsol = -35.6 kcal/
0
0
˚
mol for M -C2 versus N3-H1 = 1.323 A, ΔEsol = -40.9 kcal/
mol for M0-C30), although the overall stability of M0-C30 is lower
than M0-C20. (See Tables 2 and 3.) This is clearly reflected in the

Article
Macromolecules, Vol. 43, No. 7, 2010 3191
to be 3.2, 21.1, and 9.6° for for M0-C10, M0-C20, and M0-C30,
respectively. These values clearly indicate that the complex
M0-C10 is the most symmetric one, whereas the other two
complexes are significantly more asymmetric, although the
origins of the asymmetry in each case is different. The signifi-
cantly lower oscillator strength for the C-T transition in M0-C30,
when compared with the other two, could be a reflection of the
lower symmetry of the complex, although the relatively close
values of f in the other two cases suggest that other parameters
could also be important in governing the oscillator strength of the
C-T band. These values of f are qualitatively consistent with the
variation of the extinction coefficients of the C-T band seen in
these three cases (Table 1); the extinction coefficients in the cases
of PDI-C1 and PDI-C2 are roughly similar, whereas in the case of
PDI-C3, the C-T absorbance was too low to permit the dilution
experiments; this possibly reflects the significantly lower value of
the extinction coefficient, in addition to the lower Ka value in the
case of PDI-C3.
efforts should enable one to translate the conformational control
achieved in solution to the solid state. Work along these lines are
presently being pursued.
Acknowledgment. We would like to thank the Department of
Atomic Energy for the ORI-Award for the period 2006-2011.
D.K. is indebted to the Indian Institute of Science for providing
the Centenary Postdoctoral Fellowship and is grateful to the
Supercomputer Education and Research Centre (SERC) for
extending computational facilities.
Supporting Information Available: Experimental details of
model monomer synthesis, NMR titrations, EQNMR fitting,
UV-vis titrations, UV-vis dilution experiments, and computa-
tional details, such as absolute energies and optimized Cartesian
coordinates. This material is available free of charge via the
Internet at http://pubs.acs.org.
In conclusion, we have developed a simple methodology for
the preparation of a polymer carrying periodically placed elec-
tron-rich aromatic donors (DAN), wherein the spacer segment
linking the units carries a tertiary amine unit. The polymer was
made to fold by the interaction of a small-molecule folding agent
that carries an electron-deficient aromatic acceptor (PDI) and a
carboxylic acid. The formation of a pleated structure is facilitated
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