Folding of a donor-containing ionene by intercalation with an acceptor

Article abstract of DOI:10.1002/asia.201000528

Cationic ionenes that bear electron-rich 1,5-dialkoxynaphthalene (DAN) units within the alkylene segment were allowed to interact with different types of electron-deficient, acceptor-containing molecules in an effort to realize intercalation-induced folding of the ionenes; the collapse of the chains was expected to occur in such a way that the donor and acceptor units become arranged in an alternating fashion. Several acceptor-bearing molecules were prepared by the derivatization of pyromellitic dianhydride and naphthalene tetracarboxylic dianhydride with two different oligoethylene glycol monomethyl ether monoamines. This yielded acceptor molecules with different water solubility and allowed the examination of solvophobic effects in the folding process. UV/Vis spectroscopic studies were carried out by using a 1:1 mixture of the DAN-ionenes and different acceptor molecules in water/DMSO solvent mixtures. The intensity of the charge-transfer (CT) band was seen to increase with the water content in the solvent mixture, thereby suggesting that the intercalation is indeed aided by solvophobic effects. The naphthalene diimide (NDI) bearing acceptor molecules consistently formed significantly stronger CT complexes when compared to the pyromellitic diimide (PDI) bearing acceptor molecules, which is a reflection of the stronger π-stacking tendency of the former. AFM studies of drop-cast films of different ionene-acceptor combinations revealed that compact folded structures are formed most effectively under conditions in which the strongest CT complex is formed. Know when to fold 'em: When coerced to collapse in a polar solvent in the presence of an electron-deficient amphiphilic folding agent, suitably modified ionenes with a symmetrically positioned electron-rich 1,5-dialkoxylnaphthalene (DAN) unit appear to form an accordion-type folded structure wherein the units are arranged in an alternating fashion (see picture). D=Donor; A=Acceptor. Copyright

Full text of DOI:10.1002/asia.201000528

DOI: 10.1002/asia.201000528
Folding of a Donor-Containing Ionene by Intercalation with an Acceptor
Swati De and S. Ramakrishnan*[a]
Abstract: Cationic ionenes that bear
electron-rich 1,5-dialkoxynaphthalene
(DAN) units within the alkylene seg-
ment were allowed to interact with dif-
ferent types of electron-deficient, ac-
ceptor-containing molecules in an
effort to realize intercalation-induced
folding of the ionenes; the collapse of
the chains was expected to occur in
such a way that the donor and acceptor
units become arranged in an alternat-
ing fashion. Several acceptor-bearing
molecules were prepared by the deriva-
tization of pyromellitic dianhydride
and naphthalene tetracarboxylic dia-
nhydride with two different oligoethy-
lene glycol monomethyl ether monoa-
mines. This yielded acceptor molecules
with different water solubility and al-
lowed the examination of solvophobic
effects in the folding process. UV/Vis
spectroscopic studies were carried out
by using a 1:1 mixture of the DAN–
ionenes and different acceptor mole-
cules in water/DMSO solvent mixtures.
The intensity of the charge-transfer
(CT) band was seen to increase with
the water content in the solvent mix-
ture, thereby suggesting that the inter-
calation is indeed aided by solvophobic
effects. The naphthalene diimide
(NDI) bearing acceptor molecules con-
sistently formed significantly stronger
CT complexes when compared to the
pyromellitic diimide (PDI) bearing ac-
ceptor molecules, which is a reflection
of the stronger p-stacking tendency of
the former. AFM studies of drop-cast
films of different ionene–acceptor com-
binations revealed that compact folded
structures are formed most effectively
under conditions in which the strongest
CT complex is formed.
Keywords: amphiphiles
·
atomic
force microscopy · charge transfer ·
foldamers · ionene
Introduction
spaced (symmetric) or unequally spaced (unsymmetric) de-
pending on the choice of the two monomers. Since the early
studies by Rembaum and co-workers,[3] ionenes have fasci-
nated researchers because they represent a class of polyelec-
trolytes wherein the charges reside directly on the polymer
backbone and, importantly, the periodicity of the charges
can be very easily controlled. Early studies by Kunitake
et al.[4] suggested that symmetric ionenes that bear long al-
kylene spacers (>12 carbon atoms) tend to adopt an accor-
dion-type zigzag conformation in aqueous solutions. This
idea was later exploited by Hinze et al.[5] by using long-
chain ionenes to develop micelle-mimetic systems that were
utilized for improved separation and chemical analysis.
The introduction of functional units within the alkylene
segment of ionenes is a useful way to modulate their pro-
pensity to fold and also to introduce other novel properties.
Many examples of ionenes wherein functional units such as
azobenzene,[6] 4,4’-bipyridine,[7] anthracene,[8] porphyrin,[9]
and so on are incorporated have been reported. Incorpora-
tion of a rigid azobenzene unit within the alkylene segment
was also shown to induce a similar transition to a folded ac-
cordion-type structure in a solvent of suitable polarity.[6] The
solution properties of ionenes can also be modified by ex-
change of the counterion, which could be different halide
Ionenes are a fascinating class of polymers that carry peri-
odic charges separated by long alkylene segments; these hy-
drophobic segments in turn induce them to adopt an inter-
esting accordion-like folded structure when dissolved in
water.[1] The simplicity of their preparation, the rich possibil-
ities for structural variation, and potential for applications
have made ionenes a very attractive target. The majority of
the ionenes studied so far are cationic and carry quaternary
ammonium groups. They are generally prepared by the reac-
tion of a di-tert-amine with a dihalide under standard Men-
schutkin reaction[2] conditions to generate a polymer that
bears periodic quaternary ammonium groups, either equally
[a] S. De, Prof. S. Ramakrishnan
Department of Inorganic and Physical Chemistry
Indian Institute of Science, Bangalore 560012 (India)
Fax : (+91)080-2360-1552
E-mail: raman@ipc.iisc.ernet.in
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201000528. It includes experimen-
tal details of monomer synthesis, spectral characterization, and UV
dilution experiments.
Chem. Asian J. 2011, 6, 149 – 156
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FULL PAPERS
ions or a variety of organic anions (like lauryl sulfate).[10]
Other studies have focused on incorporating functional
units, such as semifluoroalkyl groups or fluorophores, as
pendant groups to examine their tendency to form layered
domains or to study their conformational transition, respec-
tively.[11] Recently, we demonstrated that, by inclusion of an
electron-rich donor, like 1,5-dialkoxynaphthalene (DAN),
and an electron-deficient acceptor, like pyromellitic diimide
(PDI), in an alternating fashion within the alkylene seg-
ment, the chain collapse to form accordion-type structures
can be reinforced. In addition, the occurrence of a charge
transfer (CT) between the donor and acceptor units induces
a spectral transition in the visible region, thereby providing
a useful spectroscopic handle for the study of the chain col-
lapse; for instance, as a function of solvent composition or
temperature.[12]
In the present study, we wish to report the synthesis of a
cationic ionene that carries only DAN units, which are sym-
metrically located within the alkylene segment. The solvo-
phobically-driven collapse of these chains in polar solvents
to form an accordion-type zigzag structure will bring adja-
cent donor units in close proximity; this would in turn pro-
vide an ideal hydrophobic location for intercalation of suita-
bly designed electron-deficient acceptor molecules, the addi-
tional driving motivation for the intercalation being the for-
mation of a CT complex as envisaged in Scheme 1. In our
previous reports, we had demonstrated the use of an exter-
nal folding agent to induce folding of a high-molecular-
weight polymer by means of a two-point interaction.[13] Simi-
lar studies that utilize a two-point interaction for folding oli-
gomeric systems have also been reported by several other
groups.[14] For the present study, four different acceptor-
bearing molecules were designed to optimize the intercala-
tion efficiency; these were based on pyromellitic diimide
(PDI) and naphthalene diimide (NDI). The intercalation
studies were done by monitor-
Scheme 1. Schematic representation of folding aided by interactions with
a folding agent.
functional units within the alkylene segment of either or
both the monomers. For the present study, we prepared a
DAN-containing dibromide (2) and a DAN-containing di-
tert-amine (3) as shown in Scheme 2. The alkylation of 1,5-
dihydroxynaphthalene using an excess amount of 1,6-dibro-
mohexane yielded 1,5-bis(6-bromohexyloxy)naphthalene
(2), which was then treated with an excess amount of dime-
thylamine to afford the second monomer (3). Polymeri-
zation was carried out by taking equal molar amounts of the
two monomers in a tightly capped pressure tube along with
the chloroform/methanol (2:1 v/v) solvent mixture. The con-
tents were maintained at 908C for 36 h. Although it is well
known that the Menschutkin reaction occurs most rapidly in
polar solvents,[2] a certain volume fraction of chloroform was
required for the dissolution of the two monomers. The con-
ing the CT-band intensity in
solvent mixtures of varying po-
larity. The studies reveal that
the intercalation is most fa-
vored when NDI carrying ac-
ceptors are used, and in solvent
mixtures of highest polarity.
Results and Discussion
Synthesis and Structural
Characterization
Cationic ammonium ionenes
are typically synthesized by the
reaction of an a,w-dibromoal-
kane with an a,w-bis(N,N-dime-
thylamino)alkane.[3] This ap-
proach can be readily modified
to yield several interesting ion-
enes that bear various types of Scheme 2. Synthetic scheme for the preparation of donor-containing monomers and the polymer DAN–ionene.
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Donor-Containing Ionene
centration of the monomers
was kept high to minimize the
formation of cyclic products.
The polymer was isolated by
precipitation into diethyl ether
and further purified by two re-
precipitations by using chloro-
form/methanol (1:1 v/v) as the
solvent and diethyl ether as the
nonsolvent.
Four
acceptor-containing
folding agents that were carry-
ing either a PDI or a NDI unit
were prepared from pyromellit-
ic dianhydride and naphthalene
tetracarboxylic acid dianhy-
dride,
respectively
(see
Scheme 3. Synthetic scheme for the preparation of PDI- and NDI-containing folding agents.
Scheme 3). The required anhy-
dride was treated separately
with two different oligoethylene
glycol monomethyl ether monoamines, one of which carried
a chiral carbon atom. Triethylene glycol monomethyl ether
monoamine was prepared from triethylene glycol mono-
methyl ether by means of the tosylate-azide route. The de-
tails of the synthesis, along with the synthesis of the chiral
glycol amine derivative, are provided in the Supporting In-
formation. Triethylene glycol derivatives were chosen to
impart the requisite hydrophilicity to the folding agent, and
this in turn was expected to provide an effective balance be-
tween solubility and intercalation efficiency. The combina-
tion of a relatively hydrophobic aryl–diimide core and the
hydrophilic oligoethylene glycol segments was expected to
provide the folding agent with an amphiphilic character that
would assist intercalation into the hydrophobic spaces creat-
ed in the folded ionene structure, yet at the same time was
expected to provide greater stability to the folded form. The
structures of the various folding
OE3 and NDI-OE3-C stand for those that carry the naph-
thalene diimide (NDI) units.
The proton NMR spectra of the two DAN-containing
monomers and the corresponding polymer, DAN–ionene,
are shown in Figure 1. The monomers exhibit the expected
peaks, as assigned in the figure, and their intensities were in
accordance with the expected structure. The three sets of
peaks, which arise from the 1,5-dialkoxynaphthalene unit,
remain largely unaltered, except for a small change in the
chemical-shift value of peaks a and c, which could be due to
the polar-solvent-induced collapse of the ionene that leads
to co-location of the DAN units. An intrinsic effect due to
the use of different solvents—namely, DMSO for the poly-
mer and CDCl3 for the two monomers—could also contrib-
ute to the observed differences. The most dramatic shift is
seen in the case of peak g due to methyl protons linked to
agents were confirmed by their
1H NMR spectra (see the Sup-
porting Information). One of
the triethylene glycol deriva-
tives was designed to include a
chiral carbon, in a b position
with respect to the amine; this
was done to examine whether
the use of a chiral folding agent
could introduce a helical twist
along the stacking direction.
The folding agents are labeled
as follows: PDI-OE3 and PDI-
OE3-C are the pyromellitic dii-
mide (PDI)-based molecules, in
which OE3 indicates the pres-
ence of a trioxyethylene seg-
ment and the hyphenated -C
Figure 1. a) and b) 1H NMR spectra of the donor-containing monomers in CDCl3, and c) the corresponding
refers to the presence of
a
chiral carbon, whereas NDI- DAN–ionene polymer in [D6]DMSO. The peaks marked with an asterisk are due to solvent/water.
Chem. Asian J. 2011, 6, 149 – 156
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S. Ramakrishnan et al.
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the amine. This peak moves considerably downfield, where-
as peak f in the dibromide monomer moves slightly upfield
in the polymer. The relative intensities of the various peaks
in the polymer spectrum are in accordance with the expect-
ed structure.
the NDI-based folding agents are more effective in folding
the DAN-ionone than the PDI-based molecules, as reflected
by the significantly more intense CT band, although for pre-
cise quantification one needs to recognize that the extinc-
tion coefficients of the CT band with PDI and NDI are also
likely to be different, as will be seen later.
The association constants can be readily estimated by the
dilution method,[15] which involves diluting a 1:1 mixture of
the donor and acceptor while simultaneously monitoring the
variation of the absorbance of the CT band. Assuming that
the absorbance at the lmax (CT) has no contribution from
the free donor and acceptor species, a linear equation that
relates the ratio of C/A, in which C is the concentration of
the donor (or acceptor) and A is the absorbance at lmax
Acceptor-Induced Folding of DAN–Ionene
To explore the possibility of inducing the folding of the
DAN–ionene by one of the acceptor-bearing folding agents,
we had to design a suitable experiment. It is well known
that the conformation of ionenes are strongly dependent on
the solvent polarity; simple aliphatic ionenes, wherein the
alkylene segments are more than 12 carbon atoms long,
have been shown to form folded, accordion-type structures
in polar solvents such as water.[4] This type of conformation-
al transition occurs due to the interplay between two factors,
the electrostatic repulsion between the ammonium cations
that keep the chain expanded in less-polar solvents and the
hydrophobic effects that cause the chain to fold in polar sol-
vents to avoid the exposure of the long alkylene segments to
the aqueous medium. As the DAN-ionenes are insoluble in
water unlike the simple alkylene ionenes, the experiments
were done in DMSO/water mixtures. In the first set of ex-
periments, the UV/Vis spectra of the DAN-ionene along
with an equimolar amount of the different folding agents
was recorded in DMSO/water (60:40 v/v), and the spectra
are shown in Figure 2. From these plots it is apparent that
(CT), to inverse of the square root of absorbance can be
1
readily derived.[16] A plot of this ratio versus (1/A) = would
2
give a straight line from which both the association constant
and the molar-extinction coefficient of the CT band can be
estimated. A typical plot of the dilution experiment for the
DAN–ionene with NDI-OE3 is shown in Figure 3. The esti-
mated values of the association constants for the different
folding agents are given in Table 1. It is clear that the forma-
tion constant with the NDI acceptors are nearly twice that
of the PDI-based acceptor. This is along the expected lines
and reflects the stronger CT complexation of DAN with
NDI, both because of the larger p surface and also the
Figure 3. Concentration-dependent change in the UV/Vis spectra of an
equimolar (with respect to ionene repeat unit) mixture of the DAN–
ionene and NDI-OE3 in 83% water in DMSO. The inset shows the plot
of the UV/Vis dilution method from which the association constant and
molar extinction coefficients are estimated.
Figure 2. UV/Vis spectra of an equimolar (with respect to ionene repeat
unit) mixture of the DAN–ionene and various folding agents in DMSO/
water solvent mixture (60:40 v/v). The concentrations of the DAN–
ionene and the folding agent were 2 mm each.
Table 1. The values of association constant (Ka) in different solvent compositions, estimated by the UV/Vis dilution method by using the CT absorbance
values.
Sample
Ka (in 83% water) [mÀ1
]
Ka (in 60% water) [mÀ1
]
Ka (in 40% water) [mÀ1
]
DAN–ionene+NDI-OE3
DAN–ionene+NDI-OE3-C
DAN–ionene+PDI-OE3
DAN–ionene+PDI-OE3-C
4519 (503)[a]
3088 (421)
2117 (238)
1959 (173)
3384 (450)
2418 (287)
1770 (155)
1500 (136)
2776 (336)
2090 (205)
1459 (109)
1200 (96)
[a] The values within parentheses are the respective molar extinction coefficients.
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Donor-Containing Ionene
stronger solvophobic motivation to intercalate within the hy-
drophobic space between two DAN units.[17] It is also seen
that the interaction weakens significantly upon methyl
group substitution at the b position with respect to the
imide N atom in the chiral derivatives (NDI-OE3-C and
PDI-OE3-C) as reflected by the smaller values of the for-
mation constant. This suggests that the substituent might be
sterically hindering the CT interaction between the donor
and the acceptor units.
chain. Thus, the increase in the association constant may be
ascribed to the solvophobically driven co-location of the
PDI (or NDI) rings in the vicinity of the DAN units. A simi-
lar variation was seen in all the other folding agents as
well.[16] The association constants for the different folding
agents in the various solvent compositions were estimated
by the UV/Vis dilution method, as before, and they are
listed in Table 1. It is evident from the table that for a given
folding agent, the formation constant increases roughly
1.5 times on going from 40 to 83% water in DMSO; this ap-
pears to be true for all four of the folding agents. One might
have expected the NDI-based folding agents to experience a
much larger increase compared to the PDI-based ones
based on hydrophobic effects alone. Although the extent of
increase (in terms of the relative ratios in the different sol-
vent mixtures) is slightly larger for the NDI case, the differ-
ence is not very significant.
In an effort to probe the formation of helically twisted
stacks in the presence of the chiral folding agent, we exam-
ined the circular dichroism (CD) spectra of mixtures of
DAN–ionene and NDI-OE3-C in 83% water in DMSO,
which are the conditions under which absorption studies
reveal the strongest interaction. No clear signature of the
generation of twisted stacks was evident; specifically, we ex-
amined the CT absorption region of the spectrum to see if
the formation of a twisted alternating D--A stack might lead
to a specific CD signal in this region. These studies suggest
either that the CT region could be insensitive to the forma-
tion of twisted stacks, or that the stack length is too small to
generate a CD signal. Further studies would be required to
address this aspect.
Solvophobic Effects on the Folding Process
In an effort to understand the effect of solvent polarity on
the folding process, the UV/Vis studies were carried out in
three different DMSO/water solvent compositions: 60:40,
40:60, and 17:83 (v/v), the last being the maximum water
that is tolerated by the DAN-ionene before it begins to pre-
cipitate out of solution. A plot of the variation of the UV/
Vis spectra as a function of solvent composition in the case
of DAN–ionene and NDI-OE3 is shown in Figure 4.
AFM Studies
In previous studies,[12] we demonstrated that AFM micro-
graphs of solution-cast films of D--A ionenes, which contain
an alternating placement of DAN and PDI units, reveal flat
pancake-like structures with a fairly uniform thickness that
averages about 3.5 nm, which was very close to the estimat-
ed thickness of the folded accordion-type conformation. To
check if we can observe similar folded structures in the pres-
ent two-component systems, we examined cast films pre-
pared from a dilute mixture (10 mm) of DAN–ionene and
the folding agent by using 95% water in DMSO as the sol-
vent. Although the solubility of the mixture is not high in
this solvent composition, it was reasoned that compact
folded structures would be formed effectively only from
such water-rich solutions.
In Figure 5, the AFM micrographs of drop-cast films of
the pure DAN–ionene polymer, along with those cast from
a mixture of the DAN–ionene and NDI-OE3 in different
ratios, are shown. This polymer-folding agent pair was
chosen for the AFM studies because it was shown to form
the most stable CT complex in solution. The pure DAN–
ionene also shows some flat structures of different sizes and
shapes. However, in the presence of varying mole ratios of
NDI-OE3, the micrographs exhibit a range of structural fea-
Figure 4. Variation of the UV/Vis spectra of an equimolar (with respect
to ionene repeat unit) mixture of the DAN–ionene and NDI-OE3 (2 mm
concentration) in different DMSO/water solvent mixtures.
It is clear that the intensity of the CT band increases dra-
matically with an increase in water content in the solvent
mixture. This increase may be rationalized based on two ef-
fects: first is the effect of increasing solvent polarity on the
chain conformation of the DAN–ionene, which would be ex-
pected to go towards the accordion-type zigzag-folded form
as the solvent polarity increases, and the second is the driv-
ing motivation for the acceptor to intercalate between two
adjacent DAN units of the ionene. Although the intrinsic
stability of the CT complex may be lowered in more polar
solvent based on the electrostatic model as suggested by Si-
nanoglu et al.,[18] in the present case solvophobic effects
appear to dominate, thus causing the association constant to
increase with an increase in solvent polarity. In other words,
the discomfort experienced by the relatively hydrophobic ar-
omatic diimide ring in a polar solvent drives it to the hydro-
phobic regions of the solvent-induced collapsing ionene
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S. Ramakrishnan et al.
FULL PAPERS
Figure 5. AFM micrographs of a) polymer DAN–ionene, b) folding agent NDI-OE3, c) DAN–ionene/NDI-OE3 in 1:0.5 ratio, d) DAN–ionene/NDI-OE3
in 1:1 ratio, and e) DAN–ionene/NDI-OE3 in 1:2 ratio. The insets show line scans that depict the heights and diameters of the aggregates.
tures. At a ratio of 1:0.5, very large aggregates are seen on
an uneven background, whereas in a 1:1 mole ratio slightly
more uniformly sized aggregates are seen with an average
height of slightly over 4 nm. When the mole ratio of the
folding agent is further increased to 1:2, only a few of the
aggregates are seen in the field of view with a height slightly
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Donor-Containing Ionene
greater than 5 nm; these structures are seen on a very rough
background with an average roughness greater than 0.5 nm.
Unlike in the single-component D--A ionenes,[12] in the pres-
ent system there is an equilibrium between D--A intercalat-
ed folded polymer and the free individual components. It is
clear that even in the absence of the folding agent, the
DAN–ionene appears to form collapsed structures: a control
sample of a simple deposit of NDI-OE3 from identical con-
ditions revealed the presence of very small aggregates on a
fairly rough background. Based on these experiments, we
can conclude that the DAN–ionenes do exhibit a strong ten-
dency to form CT-reinforced folded accordion-type confor-
mations in the presence of NDI-OE3. The folded structures
appear to form most effectively when the mole ratio of the
DAN to NDI is 1:1. However, in all cases the formation of
the folded form is not complete and this leads to a rough
background that arises from direct deposition of aggregates
of the acceptor molecules and/or the DAN–ionene.
Experimental Section
Materials
1,5-Dihydroxynaphthalene, pyromellitic dianhydride, 1,6-hexane diol, and
10% Pd/C were purchased from Aldrich Chemical Co. Acetic anhydride
was dried over P2O5 and distilled. Pyromellitic dianhydride was heated at
reflux with dry acetic anhydride before use. All the solvents were dried
following the standard procedures. Structures of all intermediates, mono-
mers, and polymers were confirmed by 1H NMR spectroscopy. NMR
spectra were recorded with a Bruker AV400 MHz spectrometer by using
CDCl3 or [D6]DMSO as the solvent and TMS as reference. The UV/Vis
spectra were recorded with a Varian 300 UV/Vis spectrophotometer.
Synthesis of Compound 2
Dry ethanol (50 mL) was placed in a round-bottomed flask and it was
purged with N2 for 5 min. Then Na metal (2.15 g, 93.5 mmol) was added
in pieces. After all the Na had reacted, 1,5-dihydroxynaphthalene 1
(5.0 g, 31.2 mmol) was added and the contents were purged with N2 for
5 min. This mixture was added dropwise to the ethanolic solution
(50 mL) of 1,6 dibromohexane (38.0 g, 155.7 mmol) under reflux and N2
purge. After additional heating at reflux for 12 h at 1008C, the ethanol
was removed and the residue was poured into water (100 mL). The solid
product was filtered and dried. Residual dibromohexane was removed by
stirring with petroleum ether (100 mL) to give the product (11.0 g, 73%).
Conclusion
M.p.
1308C;
1H NMR
OCH2CH2CH2CH2CH2CH2Br),
(CDCl3):
1.9
d=1.5
(m,
(m,
8H;
8H;
Ar-
Ar-
In conclusion, we have demonstrated that suitably modified
ionenes that carry a symmetrically positioned electron-rich
1,5-dialkoxylnaphthalene (DAN) unit, when coerced to col-
lapse in a polar solvent in the presence of an electron-defi-
cient (pyromellitic diimide or naphthalene diimide) amphi-
philic folding agent, appear to form an accordion-type
folded structure wherein the DAN and PDI (or NDI) units
are arranged in an alternating fashion. UV/Vis spectroscopic
studies of the folding process of the DAN–ionenes with four
different folding agents clearly showed that the NDI-based
molecules are more effective in generating the hypothesized
structure than PDI-based systems, as revealed by their sig-
nificantly higher association constants. Studies done in
DMSO/water solvent mixtures demonstrated that with in-
creasing water content, the CT-band intensity increases as a
consequence of the higher association constants between the
DAN and NDI units. The variation in the association con-
stants as a function of solvent polarity suggests that solvo-
phobic effects drive the folding agent to intercalate between
DAN units in the collapsed DAN–ionene structures. AFM
studies reveal the presence of aggregated folded structures,
the heights of which are roughly in accordance with the ac-
cordion-type folded structures, although free DAN–ionene
molecules and aggregated NDI units also appear to be pres-
ent, as revealed by the rough background surface in the line
scans. The use of a chiral folding agent does not appear to
generate the hypothesized twisted D--A stacks by the inter-
calation of the folding agent between the zigzag-folded
structures of the DAN–ionene. Alternate designs need to be
envisaged to realize such potentially interesting structures.
À
OCH2CH2CH2CH2CH2CH2Br), 3.4 (t, 4H;
CH2Br), 4.1 (t, 4H;
À
ArOCH2 ), 6.7 (d, 2H; Ar-H, o-OCH2), 7.3 (t, 2H; Ar-H, m-OCH2),
7.8 ppm (d, 2H; Ar-H, p-OCH2).
Synthesis of Compound 3
Dry methanol (10 mL) was placed in a high-pressure tube and dimethyl
amine gas (a large excess amount of about 5 mL) was condensed into it.
A solution of compound 2 (2.0 g, 4.1 mmol) in dry chloroform (10 mL)
was added to it, and the reaction mixture was heated at 908C for 36 h
under a tight seal. The excess amounts of dimethyl amine, methanol, and
chloroform were removed, and the residue was dissolved in water and
neutralized with NaOH (1.0 g). The precipitate formed was filtered, dis-
solved in chloroform, and dried over Na2SO4. Removal of the solvent
yielded the product (1.5 g, 88%). M.p. 558C; 1H NMR (CDCl3): d=1.3–
1.6 (m, 12H; ArOCH2CH2CH2CH2CH2CH2NACTHUNGTRNEUNG(CH3)2), 1.9 (m, 4H;
ArOCH2CH2CH2CH2CH2CH2NACTHUNGTRENN(UG CH3)2), 2.25 (s, 12H; CH2NACHTUNGTRENNUNG(CH3)2),
(CH3)2), 4.1 (t, 4H; CH2OAr), 6.8 (d, 2H; Ar-H, o-
OCH2), 7.3 (t, 2H; Ar-H, m-OCH2), 7.8 ppm (d, 2H; Ar-H, p-OCH2).
À
À
À
2.29 (t, 4H; CH2N
U
Synthesis of the Polymer (DAN–Ionene)
Compound 2 (1.2 g, 2.4 mmol) and compound 3 (1.0 g, 2.4 mmol) were
dissolved in a mixture of dry chloroform (4 mL) and dry methanol
(2 mL). The mixture was stirred for 36 h at 908C. The contents were pre-
cipitated into diethyl ether to obtain the polymer. The polymer was puri-
fied twice by dissolving it in a 1:1 mixture of chloroform and methanol
and precipitating into diethyl ether. The final yield of the polymer was
2.0 g (91%).
Synthesis of Compound PDI-OE3
Pyromellitic dianhydride (PDI; 1.0 g, 4.6 mmol) and triethylene glycol
monomethyl ether monoamine (1.57 g, 9.6 mmol) were dissolved in dry
DMF (10 mL). The reaction mixture was stirred at 1008C for 4 h. After
the reaction was complete, it was poured into water (10 mL) and extract-
ed with CHCl3. The CHCl3 layer was washed with brine and passed over
Na2SO4. Then it was purified by column chromatography to yield the
product (1.7 g, 75%). M.p. 628C; 1H NMR (CDCl3): d=3.30 (s, 6H;
À
OCH3), 3.48–3.79 (m, 20H; glycol protons), 3.97 (t, 4H; NCH2),
8.26 ppm (s, 2H; Ar-H).
The other folding agents—namely, PDI-OE3-C, NDI-OE3, and NDI-
OE3-C—were synthesized by following a similar procedure as that for
PDI-OE3. Details are given in the Supporting Information.
Chem. Asian J. 2011, 6, 149 – 156
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
155

S. Ramakrishnan et al.
FULL PAPERS
UV/Vis Spectroscopic Experiments
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AFM Images with Height Profile
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We would like to thank the Department of Atomic Energy for the ORI
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Received: July 31, 2010
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156
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