Carbon-Nanotube-Mediated Electrochemical Transition in a Redox-Active Supramolecular Hydrogel Derived from Viologen and an l -Alanine-Based Amphiphile

Article abstract of DOI:10.1002/chem.201600214

A two-component hydrogelator (16-A)₂-V²⁺, comprising an l-alanine-based amphiphile (16-A) and a redox-active viologen based partner (V²⁺), is reported. The formation the hydrogel depended, not only on the acid-to-amine stoichiometric ratio, but on the choice of the l-amino acid group and also on the hydrocarbon chain length of the amphiphilic component. The redox responsive property and the electrochemical behavior of this two-component system were further examined by step-wise chemical and electrochemical reduction of the viologen nucleus (V²⁺/V⁺ and V⁺/V⁰). The half-wave reduction potentials (E_1/2) associated with the viologen ring shifted to more negative values with increasing amine component. This indicates that higher extent of salt formation hinders reduction of the viologen moiety. Interestingly, the incorporation of single-walled carbon nanotubes in the electrochemically irreversible hydrogel (16-A)₂-V²⁺ transformed it into a quasi-reversible electrochemical system.

Full text of DOI:10.1002/chem.201600214

DOI: 10.1002/chem.201600214
Full Paper
&
Electrochemistry |Hot Paper|
Carbon-Nanotube-Mediated Electrochemical Transition in
a Redox-Active Supramolecular Hydrogel Derived from Viologen
and an l-Alanine-Based Amphiphile
Sougata Datta[a, b] and Santanu Bhattacharya*[a, b, c]
Abstract: A two-component hydrogelator (16-A)2-V2+, com-
prising an l-alanine-based amphiphile (16-A) and a redox-
active viologen based partner (V2+), is reported. The forma-
tion the hydrogel depended, not only on the acid-to-amine
stoichiometric ratio, but on the choice of the l-amino acid
group and also on the hydrocarbon chain length of the am-
phiphilic component. The redox responsive property and
the electrochemical behavior of this two-component system
were further examined by step-wise chemical and electro-
chemical reduction of the viologen nucleus (V2+/V+ and
V+/V0). The half-wave reduction potentials (E1/2) associated
with the viologen ring shifted to more negative values with
increasing amine component. This indicates that higher
extent of salt formation hinders reduction of the viologen
moiety. Interestingly, the incorporation of single-walled
carbon nanotubes in the electrochemically irreversible hy-
drogel (16-A)2-V2+ transformed it into a quasi-reversible
electrochemical system.
Introduction
growth and demolition of the effects of self-assembly in a re-
versible manner. Several attempts have been made to obtain
various LMWGs with stimuli-responsive properties, reacting to
stimuli, such as, heat,[16] ultrasound,[17] light,[18] pH,[19] redox ac-
tivity,[20] enzymatic manipulation,[21] host–guest interaction,[22]
and external additives.[23] In this context redox-active LMWGs,
which form an imperative part of the stimuli-responsive
LMWGs, have attracted recent attention. This is because of the
possibility to achieve control over physical and/or chemical
properties, that is, morphology (i.e., size and shape), surface
properties, conductivity, color, or solubility in a reversible
manner by shifting between the redox states.[24]
Realization of the importance of weak, noncovalent interac-
tions has spurred widespread efforts towards the evolution of
molecular systems endowed with new properties and func-
tions based on the principles of self-assembly.[1] A wide range
of materials that have potential applications in daily life, indus-
try, and various fields, including nanotechnology,[2] biomateri-
als,[3] stimuli-responsive systems,[4] and in catalysis[5] and so
forth have come out of these efforts. Among these, the self-as-
sembly of low-molecular-weight gelators (LMWGs) is a subject
of increasing interest, because this strategy allows the forma-
tion of various self-assembled architectures (i.e., helical and
bundles of fibers of high aspect ratio,[6] ribbons,[7] rings,[8] tu-
bules,[9] etc.). LMWGs include a wide variety of molecular enti-
ties, for example, long-chain hydro- and fluorocarbon or ste-
roid derivatives,[10] amino-acid analogs,[11] ureas,[12] carbohy-
drate-derived systems,[13] charge-transfer complexes,[14] and or-
ganic salts,[15] and so on.
Gel-to-sol transition processes associated with the chemical
and electrochemical reactions of various redox-active LMWGs,
for example, tetrathiafulvalene (TTF) derivatives[25] and organo-
metallic complexes,[26] especially ferrocene[27]-based com-
pounds, have been extensively investigated. Xu and co-work-
ers have reported a tripeptide-derived metallo-hydrogelator
that, not only self-assembles to form a hydrogel, but also ex-
hibits gel-to-sol transition upon oxidation of the metal center.
Interestingly, the nanofibers resulting from the self-assembly of
the hydrogelator have the width of a single molecule of the
hydrogelator.[28] Zhang and co-workers discovered a structurally
simple, stimuli-responsive hydrogelator based on ferrocene
and l-phenylalanine.[29] These authors showed the utility of the
hydrogel in the mobilization of fragile enzymes by cyclic vol-
tammetry experiment.[29b] Recently, Adams and Cameron have
explored a method for the electropolymerization of low-molec-
ular-weight hydrogelators derived from carbazole-protected
amino acids to form polymers with unique structures.[30] This
opens the possibility of using partially or completely polymer-
ized hydrogels for a range of sensing and bioelectronics appli-
cations. Thus, insertion of a redox-active moiety often offers
Attachment of a stimuli-inducible functional group at the
molecular level, may act as a “switch” for monitoring the
[a] Dr. S. Datta, Prof. Dr. S. Bhattacharya
Department of Organic Chemistry, Indian Institute of Science
Bangalore 560012, Karnataka (India)
E-mail: sb@orgchem.iisc.ernet.in
[b] Dr. S. Datta, Prof. Dr. S. Bhattacharya
Present address: Director’s Research Unit (DRU)
Indian Association for the Cultivation of Science
Jadavpur, Kolkata 700 032 (India)
[c] Prof. Dr. S. Bhattacharya
Jawaharlal Nehru Centre for Advanced Scientific Research
Bangalore 560 064, Jakkur (India)
Supporting information for this article can be found under http://
dx.doi.org/10.1002/chem.201600214.
Chem. Eur. J. 2016, 22, 7524 – 7532
7524
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
the corresponding LMWGs attractive electro-active and chemo-
responsive properties to tune the gel-to-sol transition by oxi-
dation/reduction processes, as well as though reactions with
electron donors/acceptors.
4,4’-Bipyridinium (viologen) derivatives are well-known redox
systems (V2+) capable of showing two successive reductive
processes, which are both chemically and electrochemically re-
versible.[31] The first reductive step results in the formation of
an intensely blue-colored radical cation species, which, upon
further reduction, forms a yellow-colored, neutral quinoid
moiety.[31] There are only a handful of reports of viologen-de-
rived gels.[32] George and co-workers reported co-assembly of
a noncovalent, donor–acceptor amphiphilic pair of coronene
and viologen derivative, which transformed into cylindrical mi-
celles to the generate hydrogel.[32c]
Single-walled carbon nanotubes (SWNT) have been modified
extensively in the last few years in either a covalent or nonco-
valent manner, because of a lack of solubility in water and in
various organic media. Covalent modification at the sidewalls
of SWNTs by attaching hydrophilic functional groups renders
them water dispersible. However, this results in serious altera-
tion of the electronic structure of the SWNTs by disruption of
the network of sp2-hybridized carbons.[33] An alternative strat-
egy to achieve solubilization of SWNTs in water is though non-
covalent wrapping of the SWNTs by using hydrophilic poly-
mers[34]/biomolecules[35]/small amphiphilic molecules[36] and so
forth. Fukushima et al. showed transparent gel formation by
mixing an imidazolium-type, room-temperature ionic liquid
with SWNTs.[37] We developed a gel–SWNT nanocomposite
from SWNTs and an organogelator based on a fatty acid amide
with limited dispersion ability.[38] The efficiency of incorporation
of SWNTs can be improved further by extending this idea to
a luminescent organogel system made of conjugated oligo-p-
phenylenevinylenes.[39]
Scheme 1. Molecular representation of redox-active and salt-type hydrogela-
tor, (16-A)2-V2+ and a schematic illustration of the associated two-compo-
nent hydrogelation process.
Results and Discussion
Synthesis
Compounds 1 and 2a–f (Figure 1a) were synthesized accord-
ing to Scheme S1 in the Supporting Information and were
characterized by 1H NMR, 13C NMR spectroscopy, FT-IR spectros-
copy, and mass spectrometry (ESI-MS; see the Experimental
Section in the Supporting Information).
Herein, we introduce an ion-pair, two-component system,
which is capable of immobilizing water, leading to hydrogela-
tion. The dicarboxylic-acid part of viologen imparts a redox-
active character (Scheme 1). Appendage of l-alanine though
a long-chain amide connector allows formation of an extensive
hydrogen-bonded network in the assemblies, where the hydro-
carbon chain introduces van der Waals interactions. Also in
place of l-alanine, l-phenylalanine is introduced to investigate
the influence of charge-transfer (CT) interactions, if any, be-
tween the phenyl and the viologen ring in the gelation pro-
cess. The mechanism of the systematic growth of the supra-
molecular assembly is investigated by small-angle X-ray diffrac-
tion (SAXD) analysis. The effect of variation of the acid-to-
amine stoichiometric ratio on the gelation process is examined
Figure 1. (a) Molecular structures of V2+ and 2a–f (n-A and n-F, where n=8,
12 and 16). Photographs of (b) an aqueous solution of V2+, (c,d) thermo-re-
versible hydrogels of 1:2 mixtures of V2+ and 16-A, (e) a viscoelastic sol of
a 1:2 mixture of V2+ and 16-F, and (f,g) precipitates of 16-A and 16-F, re-
spectively, in water (pH ca. 6.9).
1
by FT-IR, H NMR spectroscopy, electron microscopy, and rheo-
logical studies. Because of the amphiphilic nature of the mole-
cule, SWNTs can be dispersed effectively in the two-compo-
nent hydrogel. Electrochemical activity of the hydrogel and the
resulting SWNTs–hydrogel nanocomposite is analyzed by cyclic
voltammetry.
Gelation studies
Compound V2+ was checked for gelation in water in the pres-
ence of l-alanine or l-phenylalanine amides of aliphatic
Chem. Eur. J. 2016, 22, 7524 – 7532
www.chemeurj.org
7525
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
amines (2a–f) of various hydrocarbon chain lengths (Fig-
ure 1a). It was observed that long-chain C8 (8-A) and C12 (12-A)
amine derivatives of l-alanine formed turbid solutions in the
presence of V2+ in water, but never gels, not even upon pro-
longed aging or sonication. Only the C16 amine derivative of l-
alanine (16-A) was capable of forming stable hydrogels at ap-
propriate acid (V2+)-to-amine (16-A) ratio on standing for 15–
20 min at room temperature. These hydrogels transformed to
apparent solutions upon heating at 608C. Gelation characteris-
tics of various salt-type mixtures are listed in the Table S1 in
the Supporting Information. A 1:1 mixture of dicarboxylic acid
(V2+) and amine (16-A) could not induce gelation. However,
the 1:1.5 and 1:2 stoichiometric ratios of the acid (V2+)-to-
amine (16-A) mixtures readily formed opaque and thermo-re-
versible hydrogels (Figure 1c and d). Lowest minimum gelator
concentration (mgc) was observed with the (16-A)2-V2+ ([V2+
]=11.5 mm=mgc, [16-A]=23 mm). Addition of excess amine
(16-A) resulted in leaching of solvents and disruption of the
gelation process. Interestingly, aliphatic-chain (C8, C12 and C16)-
appended l-phenylalanine derivatives (n-F, where n=8, 12,
and 16) afforded clear viscoelastic fluids, which however, did
not gelate, under similar conditions even after standing for
a long time at room temperature (Figure 1e).
It is important to note that each of these l-alanine (n-A) and
l-phenylalanine (n-F) derivatives is insoluble in water on their
own (Figure 1 f and g), whereas V2+ is water soluble without
any additives (Figure 1b). However, the presence of V2+ in n-A
or n-F sols rendered the resulting mixture water-dispersible on
heating followed by brief sonication; presumably due to salt
formation. Presence of l-phenylalanine in the mixture was ex-
pected to induce precipitation instead of solubilization due to
greater hydrophobicity of the phenyl ring relative to that of
the methyl group of l-alanine. Solubilization of l-phenylalanine
derivatives (n-F) in the presence of V2+ might be a result of
charge-transfer (CT) interactions between the phenyl residue
and the viologen ring. This phenomenon is discussed in detail
below (1H NMR study). Addition of two equivalents of palmitic
acid or stearic acid instead of 16-A to the aqueous solution of
V2+ did not lead to gelation. This result clearly suggests the
importance of the amide linkage in the inter-molecular assem-
bly of gelator molecules, leading to gelation. The effect of vari-
ous salts on the gelation process was also investigated. Inter-
1
Figure 2. Stoichiometry-dependent partial H NMR spectra of (a,b) (16-A)n-
V2+ and (c–e) (16-F)n-V2+ in D2O at 258C, n=0.5, 1, 1.5 and 2. Concentration
of V2+ is 11.5 mm in all cases.
to-acid (V2+) stoichiometric ratio. The H NMR signal appeared
1
to be quite broad due to the gel formation as the acid
(V2+)-to-amine (16-A) stoichiometric ratio reached approxi-
mately 1:2. To understand the reason for formation of visco-
elastic fluids from l-phenylalanine derivatives in the presence
of V2+, H NMR spectra of (16-F)n-V2+ in D2O at 258C were re-
corded at various stoichiometries of 16-F (n=0.5, 1, 1.5, and
1
1
2). Similarly to above, the H NMR signal at 5.4 ppm shifted up-
field due to the salt formation. But, no significant line broaden-
ing occurred with the addition of 16-F (Figure 2c). This indi-
cates an isotropic distribution of molecules in the sol state,
thereby averaging out or minimizing the dipolar interactions
between the proton nuclei. Hence, salt formation is the
common characteristic of gel of (16-A)2-V2+ and sol of (16-F)2-
estingly, addition of various acetate salts of Na+, Cd2+, Mg2+
,
Ni2+, Co2+, Zn2+, Cu2+, Mn2+ did not disrupt the gelation pro-
cess. On the other hand, the integrity of the gel also remained
unaltered in the presence of various Na+ or Cu2+ salts with
counteranions, such as, ClÀ, BrÀ, NO3À, and citrate.
V2+
.
In addition to salt formation, the significantly different
1H NMR spectra of (16-F)n-V2+ in D2O compared with that of
(16-A)n-V2+ was probably also due to the CT from phenyl to vi-
ologen rings. Phenyl-ring protons appeared in the range of
6.9–7.2 ppm, whereas, the 1H NMR signals at 8.46 and
1H NMR study
1
Figure 2a shows H NMR spectra of (16-A)n-V2+ in D2O at 258C
1
at various stoichiometries of 16-A (n=0.5, 1, 1.5, and 2). The
1H NMR signal at 5.4 ppm of V2+ was assigned to the methyl-
ene protons attached to the ÀCOOH group; these showed sig-
nificant upfield shifts upon increasing the amine (16-A) con-
tent. This may be interpreted by an increase in the extent of
carboxylate anion formation with increasing the amine (16-A)-
8.93 ppm were assigned to the H nuclei of the viologen rings
in V2+. Each of these signals shifted upfield with increasing 16-
F content, suggesting a face-to-face stacking, involving the
donor and acceptor molecules (Figure 2d and e).[32c] Thus, the
aromatic p electrons of the phenyl ring induced shielding of
the viologen-ring protons. At the same time, phenyl-ring pro-
Chem. Eur. J. 2016, 22, 7524 – 7532
www.chemeurj.org
7526
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
1
tons exhibited a small upfield shift. Interestingly, the H NMR
with lengths >10–15 mm and uniform average diameters of
100–150 nm were observed (Figure 3b).
signals associated with the viologen-ring protons did not show
any significant shift with the increase of the 16-A/V2+ stoichio-
metric ratio (Figure 2b). This clearly suggests that CT is the
probable origin of the mechanism of sol formation of (16-F)2-
V2+. This is further supported by UV/Vis spectroscopy. A weak
absorption band was observed in the range of 450–500 nm
(Figure S1 in the Supporting Information) in case of (16-F)n-V2+
at various stoichiometries, indicating CT interaction between
the donor (16-A) and the acceptor molecules (V2+).[40]
The small-angle X-ray diffraction (SAXD) pattern of the xero-
gel of (16-A)2-V2+ showed six peaks with d-spacings of 4.61,
2.29, 1.54, 1.15, 0.923, and 0.771 nm. These were observed
with a d-spacing ratio of 1:1/2:1/3:1/4:1/5:1/6, indicating a la-
mellar pattern of the gel aggregates of (16-A)2-V2+ with an in-
terlayer spacing of 4.61 nm (Figure 3c).[6,16a]
Redox activity and cyclic voltammetry
To detect various redox states of the hydrogel, a layer of tolu-
ene was placed on the top of the hydrogel under an argon at-
mosphere (Figure 4a). This inhibits the quenching of the radi-
cal species by atmospheric oxygen, according to a published
protocol.[31b] Careful injection of a freshly prepared aqueous so-
lution of NaBH4 (3 equiv) into the hydrogel phase (11.5 mm) re-
sulted in the formation of a deep-blue-colored gel phase, pos-
sibly due to the formation of a viologen radical cation, accom-
panied by concomitant evolution of hydrogen gas.[31b] This
phase was further converted into a yellow-colored sol due to
the formation of fully reduced quinoid species after approxi-
mately 24 h and by this time a complete gel-to-sol transition
had occurred (Figure 4a).
FT-IR spectroscopy
FT-IR spectral analysis of V2+ and (16-A)n-V2+ at various stoi-
chiometries (n=0.5, 1 and 2) was performed to confirm that
salt formation was involved in the sol-to-gel transition process;
in other words, that electrostatic interactions are crucial for the
self-assembly. Figure S2 (see the Supporting Information)
shows FT-IR spectra of V2+ and (16-A)n-V2+ at various acid
(V2+)-to-amine (16-A) ratio. The IR band at 1742 cmÀ1 in the
FT-IR spectra of V2+ is assigned to the COOH group, which has
almost disappeared in the FT-IR spectra of (16-A)2-V2+. Addi-
tionally, a new shoulder appeared at 1669 cmÀ1, which indicat-
ed generation of a COOÀ anion, due to the salt formation.[41]
The above mentioned stepwise reduction processes were
followed by UV/Vis spectroscopy (Figure S4 in the Supporting
Information). Lee and co-workers have suggested that analo-
gous yellow-colored quinoid species were generated due to re-
duction of the viologen derivatives, which phase transferred
into the toluene layer because of the nonpolar character.[31b]
However, we found the quinoid species in the aqueous layer.
This indicates that the presence of dicarboxylic acid groups
offers sufficient polarity to partition into the aqueous layer. We
also used AFM to investigate the morphology of the aggre-
gates present in the yellow sol generated after reduction with
NaBH4. This showed that the fibrous structure of (16-A)2-V2+
transformed to a ‘collapsed’ aggregated structure due to a re-
duction-induced gel-to-sol transition (Figure S5 in the Support-
ing Information).
Morphological features and structural analysis
Scanning electron microscopy (SEM) images provide a possible
mechanistic insight to the growth of the supramolecular as-
sembly with an increased amine (16-A) proportion. Lower con-
centration of amine (16-A) in the salt-type mixture showed ex-
istence of aggregated organizations (Figure S3 in the Support-
ing Information). But tape-like fibrous network with diameters
ranging from 10–30 mm was clearly observed in the xerogel of
(16-A)2-V2+ (Figure 3a). The morphology of the freeze-dried
gel of (16-A)2-V2+ was further investigated by atomic force mi-
croscopy (AFM). Fibrous nanostructures of high aspect ratio
It may be instructive to compare the findings from other
redox-active systems. Kaifer and co-workers synthesized vari-
ous types of asymmetric redox-active dendrimers by integrat-
ing viologen or ferrocene and investigated rigorously by using
electrochemical studies to mimic activities of redox-active pro-
teins where the redox-active center was partially buried in the
polypeptide backbone and located “off center” in the protein
framework.[42] They also developed various synthetic water-
soluble hosts in which redox-active guests were incorporated
through host–guest interactions and investigated their proper-
ties to decipher intricate mechanistic details by considering
them as models for enzyme-bound substrates.[43] Thus the
presence of a redox-active moiety in a self-assembled matrix
may be useful to sense the environment and provide critical
information about the kinetics and thermodynamics associated
with the mechanism and growth of the self-assembly. They
first demonstrated that half-wave reduction potentials of the
viologen core shifted to more negative values with increased
Figure 3. (a) SEM, (b) AFM images, and (c) SAXD plot of the xerogel derived
from (16-A)2-V2+
.
Chem. Eur. J. 2016, 22, 7524 – 7532
www.chemeurj.org
7527
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
shows two distinguishable peaks associated with the two-step
reduction processes. But poor peak current was observed in
the anodic scan. This indicates that a very small amount of the
reduced species was available to undergo oxidation in the
anodic scan. Deviation from the reversibility is probably due to
the instability of the reduced species. The irreversible electro-
chemical behavior of these various acid (V2+)/amine (16-A)
stoichiometric mixtures do not originate from the alkyl chains
of (16-A); but rather from the intrinsic nature of the redox
core (V2+). The half-wave reduction potentials corresponding
to the first reduction step of various acid (V2+)/amine (16-A)
stoichiometric mixtures are reported in Table S2 in the Sup-
porting Information. Herein, we show a systematic growth of
a hydrogel by successive increase of the amine (16-A) propor-
tion. Figure 4b clearly shows that half-wave reduction poten-
tials shift to more negative values with the growth of the
supramolecular aggregate. Increased concentrations of the
amine (16-A) induces more salt formation and increases the
population of the amide group in the self-assembly. This re-
sults in increased polarity around the viologen moiety and hin-
ders the reduction (loss of positive charge) of the viologen
core. Hence, this behavior is quite similar to the behavior re-
ported with the Newkome-type of dendrimers observed by
Kaifer’s group. Similar trends of negative shifts of the half-
wave reduction potentials were also observed for samples that
had been drop-coated on the glassy carbon electrode followed
by careful drying.
Another interesting phenomenon was observed in the cyclic
voltammetry studies. Peak current (ip) associated with each of
the half-wave reduction potentials monotonically decreased
with the increase of amine (16-A) in (16-A)n-V2+. This finally
reached a minimum at a 1:2 acid (V2+)-to-amine (16-A) stoi-
chiometric ratio [ip(V2+)>ip(n=0.5)>ip(n=1)>ip(n=2)].
The peak current (ip) is proportional to the square root of
the diffusion coefficient (D) as described by the Randles–Sevcik
equation [Eq. (1)]:[42]
Figure 4. (a) Photographs showing the reduction of the viologen derivative,
(16-A)2-V2+ by NaBH4. (b) Cyclic voltammetry responses of (16-A)n-V2+ at
various acid-to-amine stoichiometric ratios (n=0.5, 1 and 2) in water on
glassy carbon electrode in presence of LiCl (0.1m) as the supporting electro-
lyte at a scan rate of 50 mVsecÀ1. The concentration of V2+ is 11.5 mm in all
cases (pH ca. 6.9). (c) Nyquist plots of V2+ and (16-A)n-V2+ at various acid-
to-amine stoichiometric ratios (n=0.5, 1 and 2); Concentration of V2+ is
11.5 mm in all cases.
1=2
ð1Þ
ip ¼ 0:4463 nFAC ðnFvD=RTÞ
In this equation, n is the number of electrons appearing in
half-reaction for the redox couple; v is the rate at which the
potential is swept (VsecÀ1); F is the Faraday’s constant
(96485 CmolÀ1); A is the electrode area (cm2); R is the universal
gas constant (8.314 JmolÀ1 K); T is the absolute temperature
(K); D is the diffusion coefficient of the analyte (cm2 secÀ1); C is
the concentration.
size of the Newkome-type dendrimers, thus indicating that
there is a thermodynamic hindrance for the reduction process
of dendrimers of higher molecular mass.[42] However, there is
no report that investigates the mechanism of growth of supra-
molecular gelation processes by systematic electrochemical
analysis.
If the temperature is at 258C (298.15 K), the Equation (1) can
be rewritten in a more concise form as:
Herein, we have developed a system with a redox-active vi-
ologen core, which is subjected to sense the supramolecular
environment by following the redox properties as the acid
(V2+)-to-amine (16-A) stoichiometric ratio is varied. We record-
ed cyclic voltammograms of (16-A)n-V2+ at various acid
(V2+)-to-amine (16-A) stoichiometries in presence of LiCl
(0.1m) as the supporting electrolyte (Figure 4b). (16-A)n-V2+
exhibits the two consecutive one-electron reductions, as antici-
pated for any viologen compound. The cathodic scan clearly
ip ¼ ð2:687 Â 105Þn3=2v1=2D1=2AC
ð2Þ
We calculated the diffusion coefficients of various salt-type
mixtures (n=0.5, 1, and 2) associated with the first half-wave
reduction potential by using Equation (2); these are listed in
Table S3 in the Supporting Information (n=1, A=0.09 cm2, C=
0.0115”10À3 molcmÀ3, v=50”10À3 VsÀ1). The calculated diffu-
sion coefficient in presence of 0.5 equiv of amine is 5.81”
Chem. Eur. J. 2016, 22, 7524 – 7532
www.chemeurj.org
7528
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
10À6 cm2sÀ1, which decreases to 2.87”10À7 cm2sÀ1 in case of
(16-A)2-V2+ at 11.5 mm.
Furthermore, the diffusion coefficient (D) is inversely propor-
tional to the viscosity (h) of the medium as described by the
Stokes–Einstein equation [Eq. (3)]:[42]
D ¼ kT=6phr
ð3Þ
In this equation, D is the diffusion coefficient of the
medium; k is the Boltzmann constant, T is the absolute tem-
perature (K); h is the viscosity of the medium; r is the radius of
the spherical particle.
On the basis of the Equations (2) and (3), the trend of de-
creased peak current can be rationalized as a consequence of
the increase in the viscosity of the medium (ip is inversely pro-
portional to h) with increasing amount of amine (16-A) in the
mixture. This explanation is further supported by the rheology
experiments (discussed below).
Figure 4c shows the complex impedance plots (Nyquist
plot) recorded with (16-A)n-V2+ at various acid (V2+)-to-amine
(16-A) ratios. The semicircle arc represents the CT resistance of
the sample and the working electrode. A shorter diameter arc
represents lower CT resistance and higher CT rate at the inter-
face. It is quite evident from the Nyquist plots[44] that the CT re-
sistance increases with an increased amount of amine (16-A) in
(16-A)n-V2+, which reflects the superior CT capacity of the
former over the other studied samples. The order of CT resist-
ance is: [n=2, (583.07 W)] > [n=1, (178.85 W)] > [n=0.5,
(112.31 W)] > [V2+ (74.01 W)]. Hence, this result clearly sup-
ports the findings obtained in the cyclic voltammetry experi-
ment.
In (16-A)2-V2+, the amide functionality links the redox-active
polar headgroup with the n-C16H33 chain. Interestingly, it was
found that (16-A)1.5-V2+ and (16-A)2-V2+ were able to disperse
pristine-SWNTs efficiently (Figure 5a and b). However, no dis-
persion could be achieved at the acid (V2+)-to-amine (16-A)
ratios of 1:0.5 and 1:1. Maximum dispersion was acquired for
the acid (V2+)/amine (16-A) stoichiometric ratio of 1:2. This
phenomenon ascribes to the importance of two equivalents of
16-A to satisfy the amphiphilic nature of the molecule.
The aqueous suspension of (16-A)2-V2–SWNTs was character-
ized by UV/Vis-NIR spectroscopy (Figure S6a in the Supporting
Information). Debundled SWNTs afforded distinctive peaks as-
sociated with the first interband transitions for the metallic
NTs, M11 (400–650 nm) and the first (S11, 900–1600 nm) and
the second (S22, 550–900 nm) interband transitions for the
semiconductor forms. These sharp van Hove peaks are a char-
acteristic feature of debundled, individually dispersed SWNTs
and indicate that (16-A)2-V2+ has a remarkable ability to de-
bundle SWNTs in water.[45] Moreover, a Raman spectrum (lex =
633 nm) of the sample evidenced a sharp peak, with a shoulder
in the high frequency region of 1500–1600 cmÀ1 (G+-band and
GÀ-band; Figure S6b in the Supporting Information). The low-
frequency range belongs to the radial breathing modes (RBM),
for which frequencies are dependent on the diameter of the
nanotubes.[45] The characteristic RBM bands at 190 and
210 cmÀ1 originated due to the presence of semiconducting
Figure 5. (a) Photographs of aqueous solutions of (16-A)n-V2+ (n=0.5, 1, 1.5
and 2), showing different extent of solubilization of SWNTs in water at differ-
ent acid (V2+)/amine (16-A) stoichiometric ratios. The concentrations of V2+
and SWNTs are 5 mm and 0.25 mgmLÀ1, respectively, in all cases. (b) Photo-
graph of a typical SWNTs hydrogel composite of (16-A)2-V2+. The concentra-
tions of V2+ and the SWNTs are 11.5 mm and 0.25 mgmLÀ1, respectively.
(c) SEM and (d) AFM images of the (16-A)2-V2+–SWNTs composite. The con-
centrations of (16-A)2-V2+ and the SWNTs are 5 mm and 0.1 mgmLÀ1, re-
spectively, in all cases. (e) Cyclic voltammetry responses of the SWNTs com-
posites of (16-A)1.5-V2+ and (16-A)2-V2+ in water on a glassy carbon elec-
trode in the presence of LiCl (0.1m) as the supporting electrolyte at a scan
rate of 50 mVsecÀ1. The concentrations of V2+ and the SWNTs are 11.5 mm
and 0.25 mgmLÀ1, respectively, in all cases (pH ca. 6.9).
and metallic SWNTs, respectively, where the intensities of the
two bands are almost equal. This result indicates that (16-A)2-
V2+ solubilizes both the metallic and the semiconducting form
of SWNTs.[46]
The superstructures created by an aqueous dispersion of the
nanocomposites of (16-A)2-V2+–SWNT were examined by SEM
and AFM. Interestingly, the insertion of SWNTs in the self-as-
sembly of (16-A)2-V2+ did not change the basic morphological
signatures of the self-assembled aggregates (Figure 5c). Frac-
tured fibers were observed, probably due to the embedding of
SWNTs inside the self-assembled (16-A)2-V2+. Results obtained
from the AFM experiment were in good accordance with the
SEM observations. (16-A)2-V2+–SWNTs composite in the aq.
suspension exhibited broken bundle type of nanostructures
(Figure 5d).
Figure 5e shows cyclic voltammograms of SWNTs compo-
sites of (16-A)1.5-V2+ and (16-A)2-V2+ in water. The concentra-
tion of the SWNTs is 0.25 mgmLÀ1 in each case. Interestingly,
prominent peaks in the anodic scan appeared along with the
Chem. Eur. J. 2016, 22, 7524 – 7532
www.chemeurj.org
7529
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
peaks in the cathodic scan (Table S4 in the Supporting Informa-
tion). However, the peak-to-peak splitting (DEp) is approximate-
ly 150 mV for both steps, which is larger than 59 mV (the re-
quired value for the reversible character of the system). The
ratio ipa/ipc associated with the first half-wave potential is 0.35
for the (16-A)1.5-V2+–SWNTs composite and became almost 0.8
at a 1:2 acid (V2+)/amine (16-A) stoichiometric ratio. Thus, in-
sertion of SWNTs in (16-A)2-V2+ transforms the electrochemi-
cally irreversible hydrogel to a quasi-reversible system.
decyl-chain-linked l-phenylalanine derivative (16-F) results in
sol formation due to CT from the phenyl group of the l-phe-
nylalanine to the viologen ring. The face-to-face stacking of
the phenyl ring and the viologen ring as a consequence of CT
interactions is manifested as distinct upfield shifts of the
1H NMR signals, associated with both the phenyl and the violo-
gen rings.
The redox activity of the hydrogel associated with the pres-
ence of the viologen moiety in (16-A)2-V2+ has been demon-
strated by stepwise NaBH4 reduction, which induced gel-to-sol
transition. Kaifer’s group first established the concept that half-
wave reduction potentials of the viologen core shift to more
negative values with increased of size of the Newkome-type
dendrimers, thus indicating an apparent thermodynamic hin-
drance for the reduction process for dendrimers with higher
molecular mass. Herein, we demonstrate the utility of this con-
cept for the first time in the supramolecular hydrogelation pro-
cess. Half-wave reduction potentials of the viologen core of
(16-A)n-V2+ shift to more negative values with an increased
proportion of the amine (16-A). Increment in the amine/acid
stoichiometric ratio enhances the polarity of the environment
around the viologen core; this in turn increases thermodynam-
ic hindrance in the reduction process. In addition, it was possi-
ble to efficiently solubilize SWNTs in the hydrogel phase. An
electrochemical analysis of the resultant nanocomposite un-
veiled that insertion of SWNTs converted the electrochemically
irreversible hydrogel to a quasi-reversible system. To the best
of our knowledge, this is the first report that shows such a re-
markable electrochemical transition of an electroactive gel
from irreversibility to quasi-reversibility by insertion of SWNTs.
These interesting findings should be useful in developing
novel electroactive nanocomposites for numerous applications
in the near future.
Rheological behavior
Cyclic voltammetry studies have already demonstrated that in-
crease in the amine (16-A) component resulted in a decrease
of the peak current, which reached a minimum in (16-A)2-V2+
due to the increase in the viscoelasticity of the resulting
media. To probe this finding, rheological studies were per-
formed to acquire valuable information pertaining to the corre-
lation between the structure and the viscoelastic properties
(Figure S7 in the Supporting Information). Figure S7a shows
a comparison of the oscillatory frequency sweep response of
(16-A)n-V2+ at various acid (V2+)/amine (16-A) stoichiometries.
All the samples were subjected to frequency sweeps with
a constant strain of 0.1%. G’ and G“ from (16-A)1.5-V2+ and
(16-A)2-V2+ remained independent of the frequency and the
elastic modulus (G’) was always greater than the associated
loss modulus (G”) in the frequency range of 0.1–50 radsÀ1,
thereby indicating the viscoelastic nature of these samples.
However, G’ and G“ from (16-A)1-V2+ showed a different be-
havior compared with other mixtures of these two-compo-
nents. At this ratio, G’ and G” showed an inclination to ap-
proach each other, and crossed over near 1 radsÀ1, eventually
G“ became greater than G’. This behavior indicates a rather
poor viscoelastic nature of the two-component self-assembly
of (16-A)1-V2+
.
Figure S7b shows the comparison of flow behavior of (16-
A)n-V2+ systems at different acid (V2+)/amine (16-A) stoichio-
metric ratios. At a fixed applied frequency of 1 Hz, (16-A)1.5-V2+
began to flow at approximately 29% critical strain, whereas
(16-A)2-V2+ at the identical concentration started to flow at
a critical strain of about 48%, thus ascribing significantly
higher viscoelastic, solidlike behavior of (16-A)2-V2+ compared
Acknowledgements
We thank Prof. Narayan Pradhan, Mr. Biplab K. Patra, and Mr.
Santimoy Khilari for assistance with the electrochemical experi-
ments. This work was supported by the Department of Science
and Technology, Government of India, New Delhi, India (J. C.
Bose Fellowship to S.B.). S.D. thanks CSIR for a senior research
fellowship.
with that of (16-A)1.5-V2+
.
Conclusions
Keywords: charge
nanocomposite · redox-active hydrogel · self-assembly
transfer
·
cyclic
voltammetry
·
In conclusion, we describe a novel supramolecular hydrogela-
tion by mixing an n-C16H33-chain-appended l-alanine amphi-
phile (16-A) and a bis(carboxymethyl)-4,4’-bipyridinium salt
(V2+) in a 2:1 ratio. The role of salt (ion pair) formation on the
self-assembly, leading to a lamellar-type of supramolecular or-
[1] S. Datta, S. Bhattacharya, Chem. Soc. Rev. 2015, 44, 5596–5637.
[2] S. Bhattacharya, A. Srivastava, A. Pal, Angew. Chem. Int. Ed. 2006, 45,
2934–2937; Angew. Chem. 2006, 118, 3000–3003.
[3] A. R. Hirst, B. Escuder, J. F. Miravet, D. K. Smith, Angew. Chem. Int. Ed.
2008, 47, 8002–8018; Angew. Chem. 2008, 120, 8122–8139.
1
ganization, is reflected in the stoichiometry-dependent H NMR
and FT-IR spectroscopic results. SEM and AFM experiments
reveal that these lamellar aggregates then lead to the forma-
tion of continuous 3D-fibrous networks, which act as hydrogel
matrices that enable entrapment of water. Interestingly, the
complexation of the dicarboxylic acid (V2+) with the n-hexa-
[4] a) X. Ma, H. Tian, Acc. Chem. Res. 2014, 47, 1971–1981; b) M. D. Segarra-
Maset, V. J. Nebot, J. F. Miravet, B. Escuder, Chem. Soc. Rev. 2013, 42,
7086–7098; c) M. Nakahata, Y. Takashima, H. Yamaguchi, A. Harada, Nat.
Commun. 2011, 2, 511; d) L. Hsu, G. L. Cvetanovich, S. I. Stupp, J. Am.
Chem. Soc. 2008, 130, 3892–3899; e) A. Kishimura, T. Yamashita, T. Aida,
J. Am. Chem. Soc. 2005, 127, 179–183.
Chem. Eur. J. 2016, 22, 7524 – 7532
www.chemeurj.org
7530
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
[5] a) J. Nanda, A. Biswas, B. Adhikari, A. Banerjee, Angew. Chem. Int. Ed.
2013, 52, 5041–5045; Angew. Chem. 2013, 125, 5145–5149; b) A. Di
Crescenzo, L. Bardini, B. Sinjari, T. Traini, L. Marinelli, M. Carraro, R. Ger-
mani, P. Di Profio, S. Caputi, A. Di Stefano, M. Bonchio, F. Paolucci, A.
Fontana, Chem. Eur. J. 2013, 19, 16415–16423.
[6] S. Datta, S. K. Samanta, S. Bhattacharya, Chem. Eur. J. 2013, 19, 11364–
11373.
[7] H. Shao, T. Nguyen, N. C. Romano, D. A. Modarelli, J. R. Parquette, J. Am.
Chem. Soc. 2009, 131, 16374–16376.
[8] H. Shao, J. Seifert, N. C. Romano, M. Gao, J. J. Helmus, C. P. Jaroniec,
D. A. Modarelli, J. R. Parquette, Angew. Chem. Int. Ed. 2010, 49, 7688–
7691; Angew. Chem. 2010, 122, 7854–7857.
[9] J. H. Jung, G. John, K. Yoshida, T. Shimizu, J. Am. Chem. Soc. 2002, 124,
10674–10675.
[10] a) S. Datta, S. Bhattacharya, Soft Matter 2015, 11, 1945–1953; b) M.
George, S. L. Snyder, P. Terech, C. J. Glinka, R. G. Weiss, J. Am. Chem. Soc.
2003, 125, 10275–10283.
[25] a) Y. Liu, W. Lei, T. Chen, L. Jin, G. Sun, B. Yin, Chem. Eur. J. 2015, 21,
15235–15245; b) C. Wang, F. Sun, D. Zhang, G. Zhang, D. Zhu, Chin. J.
Chem. 2010, 28, 622–626; c) J. Puigmartí-Luis, E. E. Laukhina, V. N. Lau-
khin, A. P. del Pino, N. Mestres, J. Vidal-Gancedo, C. Rovira, D. B. Amabili-
no, Adv. Funct. Mater. 2009, 19, 934–941; d) X. Yang, G. Zhang, L. Li, D.
Zhang, L. Chi, D. Zhu, Small 2012, 8, 578–584; e) J. Liu, J. Yan, X. Yuan,
K. Liu, J. Peng, Y. Fang, J. Colloid Interface Sci. 2008, 318, 397–404; f) J.
Puigmartí-Luis, V. Laukhin, A. P. del Pino, J. Vidal-Gancedo, C. Rovira, E.
Laukhina, D. B. Amabilino, Angew. Chem. Int. Ed. 2007, 46, 238–241;
Angew. Chem. 2007, 119, 242–245; g) C. Wang, D. Zhang, D. Zhu, J. Am.
Chem. Soc. 2005, 127, 16372–16373; h) T. Kitahara, M. Shirakawa, S.-i.
Kawano, U. Beginn, N. Fujita, S. Shinkai, J. Am. Chem. Soc. 2005, 127,
14980–14981; i) T. Akutagawa, K. Kakiuchi, T. Hasegawa, S.-i. Noro, T. Na-
kamura, H. Hasegawa, S. Mashiko, J. Becher, Angew. Chem. Int. Ed. 2005,
44, 7283–7287; Angew. Chem. 2005, 117, 7449–7453; j) T. Kitamura, S.
Nakaso, N. Mizoshita, Y. Tochigi, T. Shimomura, M. Moriyama, K. Ito, T.
Kato, J. Am. Chem. Soc. 2005, 127, 14769–14775.
[11] a) D. Mandal, S. K. Mandal, M. Ghosh, P. K. Das, Chem. Eur. J. 2015, 21,
12042–12052; b) R. Zou, Q. Wang, J. Wu, J. Wu, C. Schmuck, H. Tian,
Chem. Soc. Rev. 2015, 44, 5200–5219; c) B. Adhikari, J. Nanda, A. Bane-
rjee, Chem. Eur. J. 2011, 17, 11488–11496; d) Y. Gao, F. Zhao, Q. Wang, Y.
Zhang, B. Xu, Chem. Soc. Rev. 2010, 39, 3425–3433; e) G. Palui, J.
Nanda, S. Ray, A. Banerjee, Chem. Eur. J. 2009, 15, 6902–6909.
[12] a) D. K. Kumar, J. W. Steed, Chem. Soc. Rev. 2014, 43, 2080–2088; b) J. A.
Foster, R. M. Edkins, G. J. Cameron, N. Colgin, K. Fucke, S. Ridgeway,
A. G. Crawford, T. B. Marder, A. Beeby, S. L. Cobb, J. W. Steed, Chem. Eur.
J. 2014, 20, 279–291; c) J. W. Steed, Chem. Soc. Rev. 2010, 39, 3686–
3699.
[26] S. Kawano, N. Fujita, S. Shinkai, J. Am. Chem. Soc. 2004, 126, 8592–
8593.
[27] a) R. Afrasiabi, H.-B. Kraatz, Chem. Eur. J. 2015, 21, 7695–7700; b) B. Ad-
hikari, C. Singh, A. Shah, A. J. Lough, H.-B. Kraatz, Chem. Eur. J. 2015, 21,
11560–11572; c) B. Adhikari, H.-B. Kraatz, Chem. Commun. 2014, 50,
5551–5553; d) N. V. Lakshmi, D. Mandal, S. Ghosh, E. Prasad, Chem. Eur.
J. 2014, 20, 9002–9011; e) R. Afrasiabi, H.-B. Kraatz, Chem. Eur. J. 2013,
19, 17296–17300; f) J. Liu, P. L. He, J. L. Yan, X. H. Fang, J. X. Peng, K. Q.
Liu, Y. Fang, Adv. Mater. 2008, 20, 2508–2511.
[28] Y. Zhang, B. Zhang, Y. Kuang, Y. Gao, J. Shi, X. X. Zhang, B. Xu, J. Am.
Chem. Soc. 2013, 135, 5008–5011.
[13] a) N. Yan, Z. Xu, K. K. Diehn, S. R. Raghavan, Y. Fang, R. G. Weiss, J. Am.
Chem. Soc. 2013, 135, 8989–8999; b) S. R. Jadhav, P. K. Vemula, R.
Kumar, S. R. Raghavan, G. John, Angew. Chem. Int. Ed. 2010, 49, 7695–
7698; Angew. Chem. 2010, 122, 7861–7864; c) P. K. Vemula, J. Li, G.
John, J. Am. Chem. Soc. 2006, 128, 8932–8938.
[14] a) A. Das, S. Ghosh, Angew. Chem. Int. Ed. 2014, 53, 2038–2054; Angew.
Chem. 2014, 126, 2068–2084; b) M. Kumar, K. V. Rao, S. J. George, Phys.
Chem. Chem. Phys. 2014, 16, 1300–1313; c) A. Das, M. Rahaman Molla,
B. Maity, D. Koley, S. Ghosh, Chem. Eur. J. 2012, 18, 9849–9859.
[15] P. Dastidar, Chem. Soc. Rev. 2008, 37, 2699–2715.
[16] a) S. Datta, S. Bhattacharya, Chem. Commun. 2012, 48, 877–879; b) S.
Kiyonaka, K. Sugiyasu, S. Shinkai, I. Hamachi, J. Am. Chem. Soc. 2002,
124, 10954–10955.
[17] D. Ke, C. Zhan, A. D. Q. Li, J. Yao, Angew. Chem. Int. Ed. 2011, 50, 3715–
3719; Angew. Chem. 2011, 123, 3799–3803.
[18] a) W. Cao, X. Zhang, X. Miao, Z. Yang, H. Xu, Angew. Chem. Int. Ed. 2013,
52, 6233–6237; Angew. Chem. 2013, 125, 6353–6357; b) H.-Y. Lee, K. K.
Diehn, K. Sun, T. Chen, S. R. Raghavan, J. Am. Chem. Soc. 2011, 133,
8461–8463; c) C. Wang, Q. Chen, F. Sun, D. Zhang, G. Zhang, Y. Huang,
R. Zhao, D. Zhu, J. Am. Chem. Soc. 2010, 132, 3092–3096; d) K. Sun, R.
Kumar, D. E. Falvey, S. R. Raghavan, J. Am. Chem. Soc. 2009, 131, 7135–
7141.
[19] a) H. Komatsu, S. Matsumoto, S.-i. Tamaru, K. Kaneko, M. Ikeda, I. Hama-
chi, J. Am. Chem. Soc. 2009, 131, 5580–5585; b) S. Guha, M. G. B. Drew,
A. Banerjee, Chem. Mater. 2008, 20, 2282–2290.
[20] a) L. Chen, K. S. Mali, S. R. Puniredd, M. Baumgarten, K. Parvez, W. Pisula,
S. D. Feyter, K. Müllen, J. Am. Chem. Soc. 2013, 135, 13531–13537; b) X.
Miao, W. Cao, W. Zheng, J. Wang, X. Zhang, J. Gao, C. Yang, D. Kong, H.
Xu, L. Wang, Z. Yang, Angew. Chem. Int. Ed. 2013, 52, 7781–7785;
Angew. Chem. 2013, 125, 7935–7939; c) C. J. Bowerman, B. L. Nilsson, J.
Am. Chem. Soc. 2010, 132, 9526–9527; d) E. Krieg, E. Shirman, H. Weiss-
man, E. Shimoni, S. G. Wolf, I. Pinkas, B. Rybtchinski, J. Am. Chem. Soc.
2009, 131, 14365–14373.
[29] a) Z. Sun, Q. Huang, T. He, Z. Li, Y. Zhang, L. Yi, ChemPhysChem 2014,
15, 2421–2430; b) Z. Sun, Z. Li, Y. He, R. Shen, L. Deng, M. Yang, Y.
Liang, Y. Zhang, J. Am. Chem. Soc. 2013, 135, 13379–13386.
[30] P. S. Kubiak, S. Awhida, C. Hotchen, W. Deng, B. Alston, T. O. McDonald,
D. J. Adams, P. J. Cameron, Chem. Commun. 2015, 51, 10427–10430.
[31] a) M. Frasconi, I. R. Fernando, Y. Wu, Z. Liu, W.-G. Liu, S. M. Dyar, G. Barin,
M. R. Wasielewski, W. A. Goddard, III, J. F. Stoddart, J. Am. Chem. Soc.
2015, 137, 11057–11068; b) S. M. Kim, J. H. Jang, K. K. Kim, H. K. Park,
J. J. Bae, W. J. Yu, I. H. Lee, G. Kim, D. D. Loc, U. J. Kim, E. H. Lee, H. J.
Shin, J. Y. Choi, Y. H. Lee, J. Am. Chem. Soc. 2009, 131, 327–331.
[32] a) K.-P. Wang, Y. Chen, Y. Liu, Chem. Commun. 2015, 51, 1647–1649;
b) Q. Zhang, D.-H. Qu, X. Ma, H. Tian, Chem. Commun. 2013, 49, 9800–
9802; c) K. V. Rao, K. Jayaramulu, T. K. Maji, S. J. George, Angew. Chem.
Int. Ed. 2010, 49, 4218–4222; Angew. Chem. 2010, 122, 4314–4318; d) Y.
Liu, Y. Yu, J. Gao, Z. Wang, X. Zhang, Angew. Chem. Int. Ed. 2010, 49,
6576–6579; Angew. Chem. 2010, 122, 6726–6729.
[33] D. Tasis, N. Tagmatarchis, A. Bianco, M. Prato, Chem. Rev. 2006, 106,
1105–1136.
[34] T. Ogoshi, Y. Takashima, H. Yamaguchi, A. Harada, J. Am. Chem. Soc.
2007, 129, 4878–4879.
[35] M. Zheng, A. Jagota, E. D. Semke, B. A. Diner, R. S. McLean, S. R. Lustig,
R. E. Richardson, N. G. Tassi, Nat. Mater. 2003, 2, 338–342.
[36] a) D. Mandal, T. Kar, P. K. Das, Chem. Eur. J. 2014, 20, 1349–1358; b) S. K.
Mandal, T. Kar, P. K. Das, Chem. Eur. J. 2013, 19, 12486–12496; c) S. Brah-
machari, D. Das, A. Shome, P. K. Das, Angew. Chem. Int. Ed. 2011, 50,
11243–11247; Angew. Chem. 2011, 123, 11439–11443; d) S. Roy, A.
Baral, A. Banerjee, Chem. Eur. J. 2013, 19, 14950–14957.
[37] a) T. Fukushima, K. Asaka, A. Kosakaa, T. Aida, Angew. Chem. Int. Ed.
2005, 44, 2410–2413; Angew. Chem. 2005, 117, 2462–2465; b) T. Fu-
kushima, A. Kosaka, Y. Ishimura, T. Yamamoto, T. Takigawa, N. Ishii, T.
Aida, Science 2003, 300, 2072–2074.
[38] A. Pal, B. S. Chhikara, A. Govindaraj, S. Bhattacharya, C. N. R. Rao, J.
Mater. Chem. 2008, 18, 2593–2600.
[39] S. Samanta, A. Pal, S. Bhattacharya, C. N. R. Rao, J. Mater. Chem. 2010,
20, 6881–6890; S. Samanta, A. Pal, S. Bhattacharya, Langmuir 2009, 25,
8567–8578.
[40] a) M. E. Bush, N. D. Bouley, A. R. Urbach, J. Am. Chem. Soc. 2005, 127,
14511–14517; b) A. S. Jalilov, S. Patwardhan, A. Singh, T. Simeon, A. A.
Sarjeant, G. C. Schatz, F. D. Lewis, J. Phys. Chem. B 2014, 118, 125–133.
[41] a) A. Pal, H. Basit, S. Sen, V. K. Aswal, S. Bhattacharya, J. Mater. Chem.
2009, 19, 4325–4334; b) H. Basit, A. Pal, S. Sen, S. Bhattacharya, Chem.
Eur. J. 2008, 14, 6534–6545.
[21] Z. Yang, G. Liang, B. Xu, Acc. Chem. Res. 2008, 41, 315–326.
[22] Y.-L. Zhao, I. Aprahamian, A. Trabolsi, N. Erina, J. F. Stoddart, J. Am.
Chem. Soc. 2008, 130, 6348–6350.
[23] a) S. Datta, S. Bhattacharya, Chem. Commun. 2015, 51, 13929–13932;
b) S. Yamaguchi, I. Yoshimura, T. Kohira, S.-i. Tamaru, I. Hamachi, J. Am.
Chem. Soc. 2005, 127, 11835–11841.
[24] a) X. Yang, G. Zhang, D. Zhang, J. Mater. Chem. 2012, 22, 38–50;
b) W. R. Browne, B. L. Feringa, Chimia 2010, 64, 398–403; c) W. R.
Browne, M. M. Pollard, B. de Lange, A. Meetsma, B. L. Feringa, J. Am.
Chem. Soc. 2006, 128, 12412–12413.
Chem. Eur. J. 2016, 22, 7524 – 7532
www.chemeurj.org
7531
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
[42] a) W. Ong, A. Kaifer, J. Am. Chem. Soc. 2002, 124, 9358–9359; b) C. Car-
dona, A. Kaifer, J. Am. Chem. Soc. 1998, 120, 4023–4024.
[45] a) S. K. Mandal, T. Kar, D. Das, P. K. Das, Chem. Commun. 2012, 48, 1814–
1816; b) K. Nobusawa, A. Ikeda, J. Kikuchi, S. Kawano, N. Fujita, S. Shin-
kai, Angew. Chem. Int. Ed. 2008, 47, 4577–4580; Angew. Chem. 2008,
120, 4653–4656.
[46] a) B. Maji, S. K. Samanta, S. Bhattacharya, Nanoscale 2014, 6, 3721–
3730; b) H. Kataura, Y. Kumazawa, Y. Maniwa, I. Umezu, S. Suzuki, Y. Oht-
suka, Y. Achiba, Synth. Met. 1999, 103, 2555–2558.
[43] a) D. Podkoscielny, S. Gadde, A. Kaifer, J. Am. Chem. Soc. 2009, 131,
12876–12877; b) W. Wang, A. Kaifer, Angew. Chem. Int. Ed. 2006, 45,
7042–7046; Angew. Chem. 2006, 118, 7200–7204; c) W. Ong, J. Grind-
staff, D. Sobransingh, R. Toba, J. M. Quintela, C. Peinador, A. Kaifer, J.
Am. Chem. Soc. 2005, 127, 3353–3361; d) W. Ong, A. Kaifer, Angew.
Chem. Int. Ed. 2003, 42, 2164–2167; Angew. Chem. 2003, 115, 2214–
2217; e) I. Philip, A. Kaifer, J. Am. Chem. Soc. 2002, 124, 12678–12679.
[44] S. Khilari, S. Pandit, J. L. Varanasi, D. Das, D. Pradhan, ACS Appl. Mater. In-
terfaces 2015, 7, 20657–20666.
Received: January 16, 2016
Published online on April 5, 2016
Chem. Eur. J. 2016, 22, 7524 – 7532
www.chemeurj.org
7532
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim