Supramolecular alternate co-assembly through a non-covalent amphiphilic design: Conducting nanotubes with a mixed D-A structure

Article abstract of DOI:10.1002/chem.201202168

Mixing it up! The supramolecular alternate co-assembly of extended π-conjugated donor (D) and acceptor (A) molecules (i.e., oligo(phenylenevinylene) and perylenebisimide, respectively), to 1-D nanotubes with an unprecedented mixed stack D-A molecular structure is presented, through a non-covalent amphiphilic design strategy, which results in the formation of hydrogels with remarkable mechanical properties (see scheme). Copyright

Full text of DOI:10.1002/chem.201202168

DOI: 10.1002/chem.201202168
Supramolecular Alternate Co-Assembly through a Non-Covalent
Amphiphilic Design: Conducting Nanotubes with a Mixed D–A Structure
K. Venkata Rao and Subi J. George*[a]
Supramolecular polymerization of organic p-conjugated
monomers having optoelectronic functionality to form one-
dimensional (1-D) chains has been the topic of meticulous
research, as this provides an efficient bottom-up strategy for
the design of nanostructures for nano-sized electronics.[1] 1-
D assemblies of various p-conjugated molecules have thus
been constructed and the non-covalent synthetic design of
these systems often utilizes the characteristic p–p stacking
interactions of the extended aromatics, supported by direc-
tional H-bonding[2] or amphiphilic interactions.[3] However,
the use of these 1-D nanostructures as conducting compo-
nents in nanoelectronic devices definitely requires further
control over its function and structure. Thus the current re-
search is targeted to improve their conductivity as well as
the design of monodisperse[4] and multicomponent assem-
blies.[5] In recent years, the conductivity of these self-assem-
bled 1-D nanostructures has been improved by either the
modification of the molecular structure[6] or by external
doping.[7] However, the former strategy often requires tedi-
ous synthetic efforts and in the later approach, uniform
doping of the assemblies while preserving their morphology
is difficult. On the other hand, charge-transfer (CT) crystals
formed from donor (D) and acceptor (A) molecules, which
are organized either in a segregated (orthogonal; D–D–D…
and A–A–A…) or mixed (alternate; D–A–D…) fashion
have gathered immense attention because of their excellent
conducting properties.[8] In this context, we envisage that the
supramolecular one-dimensional analogues of these binary
CT complexes would provide new conducting nanostruc-
tures through inherent doping and hence can provide new
opportunities for nano-sized electronics.[9]
been recently predicted to have ambipolar CT proper-
ties[8b,12] and also shown to have interesting ferroelectric
properties.[13] Extended assemblies of D and A monomers
having mixed stack organization were attained by covalent
polymerization[14] and liquid-crystalline mesophase co-as-
sembly.[15] However, attempts to co-assemble these mole-
cules in solution to extended stacks, often resulted in phase-
separation[16] and hence a smart design principle is essential
for the supramolecular analogue of the mixed D–A struc-
ture.[17] Extended assemblies of aromatic donors and
methyl-viologen acceptors were constructed recently using a
“non-covalent amphiphilic design”, in which the co-facial
D–A pair formed due to CT interactions, resembles a surfac-
tant in their structure to facilitate the 1-D self-assembly in
water through amphiphilic interactions.[11a,b] Herein, we use
this non-covalent amphiphilic design principle to report the
supramolecular alternate co-assembly of extended p-conju-
gated D and A molecules, oligo(phenylenevinylene) (OPV)
and perylenebisimide (PBI) respectively, in water to form
hydrogels with excellent mechanical (elastic) and conducting
properties. In addition, we show that this self-assembly re-
sults in one-dimensional nanotubes with an unprecedented
mixed stack D–A molecular structure and promising con-
ductivity of the order of 10À2ScmÀ1.
Our molecular design consists of OPV (T-OPV) and PBI
(C-PBI) derivatives as donor and acceptor monomers (Sche-
me 1a), which were synthesized according to literature pro-
cedures.[18] Co-assemblies of OPV and PBI derivatives, with
segregated “p-n” architecture, have been achieved in litera-
ture by synergic hydrogen bonding and p–p interac-
tions.[10a–c] However, the mixed OPV-PBI architecture in ex-
tended assemblies is hitherto unknown, due to their self-as-
sociation through p–p interactions favoring an orthogonal
assembly.[16a] In the present case, the T-OPV is elegantly de-
signed with a T-shaped amphiphilic structure, which can
form a non-covalent amphiphilic pair with C-PBI, possibly
through synergistic p–p stacking, charge-transfer and elec-
trostatic interactions and hence would promote the co-as-
sembly in water as shown in Scheme 1b.[11] Furthermore, the
ionic nature of both molecules, would impart solubility to
the monomers and dynamic nature to the resulting co-as-
sembly in water.
Although segregated D–A organization has been achieved
in 1-D assemblies for photoconductivity studies,[10] the su-
pramolecular alternate co-assemblies with mixed D–A or-
ganization of the monomers is rarely studied.[11] Intriguingly,
extended assemblies having alternate D–A organization has
[a] K. V. Rao, Dr. S. J. George
Supramolecular Chemistry Laboratory
New Chemistry Unit
Jawaharlal Nehru Centre for
Advanced Scientific Research (JNCASR)
Jakkur P.O, Bangalore 560064 (India)
Fax : (+91)80-2208-2627
Interestingly, when the concentration of the equimolar
mixture of T-OPV and C-PBI monomers in water is in-
creased, the resulting solution gradually becomes viscous
(0.1 to 3.0 mm) and finally a dark wine-colored gel was
E-mail: george@jncasr.ac.in
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201202168.
14286
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COMMUNICATION
the charge-transfer interactions
and thus provides a proof for
the mixed-stack organization
of the monomers in the co-as-
sembly (Figure 1a).[10a] FE-
SEM image of the gels shows
typical highly interconnected
network of fibers, which was
further analyzed in detail using
atomic
force
microscopy
(AFM) and transmission elec-
tron microscopy (TEM) (see
below) (Figure 1b). Interest-
ingly this gel exhibits excellent
elastic
behavior,
which
prompted us to investigate
their mechanical properties
(Figure 1c and 1d). The stor-
age modulus (G’) of the T-
OPV/C-PBI
hydrogels
Scheme 1. a) Molecular structures of T-OPV and C-PBI. b) Schematic representation of the non-covalent T-
OPV/C-PBI CT amphiphile and its self-assembly into 1-D supramolecular structures.
(3.3 mm) measured as a func-
tion of angular frequency (w)
(1% strain) showed a signifi-
formed at a concentration of 3.3 mm, suggesting the forma-
tion of one-dimensional assembly (Figure 1a, inset). The
very intense absorption band at 700 nm and the non-fluores-
cent nature of the hydrogels, are characteristic features of
cant elastic response and was always greater than the corre-
sponding loss modulus (G’’). The value of G’ (ꢀ1000 Pa)
and the ratio of G’ to G’’ (ꢀ8) remain unaffected up to an
angular frequency of 70 radsÀ1 suggesting the good mechani-
cal stability and the elastic
nature of these gels. More im-
portantly, the gel did not col-
lapse when
a stress up to
100 Pa (13.5% strain, G’/G’’=
2.99) is applied at a constant
frequency of 1 Hz, which is
evident from the greater than
1 G’/G’’ values.[19] This is quite
remarkable, because most of
the organogels and hydrogels
made up of small molecules
have
a much lower critical
strain/stress region in which
they collapse.
Since both T-OPV and C-
PBI molecules have an extend-
ed p-conjugated backbone
with an amphiphilic structure,
they self-assemble in water to
J- and H- type assemblies, re-
spectively which were probed
in detail by spectroscopic and
microscopic
techniques.[18]
TEM of the T-OPV self-assem-
bly (0.1 mm) in water showed
the formation of 1-D nano-
Figure 1. Characterization of T-OPV/C-PBI CT hydrogels. a) Absorption spectrum of the CT gel (3.3 mm, l=
1 mm); inset shows the photographs of the gel exhibiting self-standing and viscous behaviors. b) FE-SEM
image of the hydrogel on a glass substrate. Storage (G’) and loss (G’’) modulus of the gel measured as func-
tions of c) angular frequency (w) at a constant strain of 1% and d) stress (s) or strain (g)% at a constant fre-
quency of 1 Hz.
structures
of
200–250 nm
length with widths in the range
of 10–15 nm (Figure 2a and b).
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S. J. George and K. V. Rao
Figure 2. a) and b) TEM images and c) schematic representation of T-
OPV (0.1 mm) cylindrical micelles in water.
Interestingly, a careful examination of these nanostructures
negatively stained with uranyl acetate (1 wt% in water)
showed that they consist of 3–4 laterally organized primary
fibers having a uniform diameter of 3.5 nm. The T-OPV am-
phiphile in its fully stretched T-shaped conformation has a
molecular length of 1.8 nm along the alky chain direction,
which suggests that the fibers might have a cylindrical micel-
lar organization of the OPV bilayers as already reported in
various rod-coil shaped amphiphiles (Figure 2c).[3d,e] FE-
SEM images of 0.1 mm of C-PBI in water on glass sub-
strates, showed the formation of microrods of 50–70 mm
length with widths of 2–3 mm.[18,20]
Figure 3. a) Absorption and b) emission spectra of T-OPV, C-PBI and
their 1:1 CT co-assembly in water (c=0.1 mm, l=5 mm, lexc =380 nm).
Insets of a and b show the photographs of the corresponding solutions
under normal and UV-light, respectively.
Since the absorption and fluorescence properties of p-
conjugated molecules are very sensitive to the intermolecu-
lar and donor–acceptor CT interactions, we have performed
detailed spectroscopic studies of the diluted solutions of the
T-OPV/C-PBI co-assembly in water, to obtain further in-
sight into their co-assembled microscopic structure. Titration
of C-PBI with T-OPV (1ꢁ10À4 m) resulted in a gradual red-
shift (up to 14 nm) in the absorption maximum of the latter,
with the concomitant appearance of the CT band at 700 nm,
which is very intense at 1:1 stoichiometry of the monomers,
suggesting a co-facial organization of the monomers (Fig-
ure 3a).[18,10a] PBI absorption spectral features, particularly
the intensity ratio of 0–0 and 0–1 absorption maxima that is
very sensitive to inter-chromophoric interactions,[21] also pro-
vided insight into the mixed organization of the D and A
monomers. At the initial stages of titration, the higher inten-
sity of 0–1 peak than the 0–0 peak (I0À1/I0À0 > 1) in PBI ab-
sorption is characteristic of intermolecular PBI interactions,
as observed in the homo-assembly of C-PBI (see above).[18]
However, upon co-assembly with T-OPV, I0À1/I0À0 ratio re-
verses, and the 0–0 peak gets intense compared to the 0–1
peak at 1:1 stoichiometry of the monomers, supporting the
mixed-stack D–A organization.[18] In addition, complete
quenching of the fluorescence is observed for both OPV and
PBI monomers, in the 1:1 co-assembly characteristic of the
non-fluorescent nature of the mixed CT D–A complexes
(Figure 3b). Interestingly, the excitation at 380 nm of the co-
assembly showed a non-linear quenching in the fluorescence
of T-OPV on titration with increasing equivalents of C-
PBI.[18] Detailed studies have indicated that this amplifica-
tion effect in the OPV donor quenching is due to the energy
transfer from the OPV molecules to the non-fluorescent
mixed CT-pairs that are incorporated into the self-assem-
bled chains and hence act as energy trap.[18] Similar ampli-
fied fluorescence quenching through Fçrster resonance
energy transfer (FRET) has been observed in molecular as-
semblies in which the energy acceptors are co-assembled
through p–p interactions.[22] The presence of energy transfer
in the co-assembly is further evident from the reverse titra-
tion of T-OPV with C-PBI, which showed only a linear
quenching of PBI fluorescence with increasing equivalents
of OPV, suggesting that OPV monomers are co-assembled
in an isolated way in the PBI stacks, forming ground-state
CT interaction with the neighbouring PBI molecules.[18] The
1:1 stoichiometry and alternate D–A sequence of the T-
OPV/C-PBI co-assembly was further confirmed by using
Jobꢂs plot and 1H NMR spectroscopy.[18] The formation of
mixed stacks from the pre-assembled donor and acceptor
molecules in water further indicates that the individual ag-
gregation of donor and acceptor molecules is dynamic
enough to reorganize to form more stable alternate D–A
co-assembly.
Detailed TEM and AFM studies were performed to get
insight into the microstructure of the one-dimensional nano-
structures formed by the self-assembly of wedge-shaped T-
OPV/C-PBI non-covalent amphiphile in water. AFM
images of a dilute, aqueous solution of these assemblies (1ꢁ
10À4 m) showed dense fiber networks, with extensive cross-
linking (Figure 4a). The dimensions of these nanofibers are
about 200–300 nm in width and with a length of more than
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Conducting Nanotubes with Mixed D–A Structure
COMMUNICATION
and AFM images of these substrates showed dense network
of CT nanotubes (Figure 5). Interestingly, I–V responses of
these nanotubes showed non-linear behaviour and the resist-
ance near V=0 obtained by analyzing the slope of the I–V
Figure 4. Microscopic studies of T-OPV/C-PBI co-assembly: a) and
b) AFM height images of the fibers and the inset of (b) shows the corre-
sponding cross-sectional analyses. c) and d) TEM images of the nano-
tubes. Electron density profile of the nanotube wall (marked as white bar
in the image) is also shown in Figure 4d. The dip in the profile could be
an indication of the existence of two bilayers in the nanotube wall of
7 nm (see the Supporting Information, Figure S11, for increased magnifi-
cation).
15 mm (Figure 4a and b). Interestingly, TEM images of these
1-D structures negatively stained with uranyl acetate
showed the presence of tubular morphologies with 35–
50 nm diameters, which is in agreement with the reported
nanotube formation of similar wedge-shaped amphiphiles
(Figure 4c and d).[23] AFM cross-sectional analysis of these
fibers showed a height of 30–40 nm, which is very close to
the diameter of the nanotubes, revealing the rigidity of
these tubular structures. This further suggests that the fi-
brous morphology visualized through AFM were bundles of
nanotubes, which could have formed by the lateral associa-
tion of these nanotubes through columbic interaction be-
tween their charged surfaces. Detailed TEM analysis of the
tubes further revealed a uniform wall thickness of 7 nm,
which is two times the calculated bilayer thickness (3.5 nm)
of the CT-amphiphile.[18] This indicates that the tube walls
are constructed by two coats of bilayers, as previously re-
ported for many amphiphilic self-assemblies.[18,23] Hence it
can be proposed that, the aromatic surfaces of the CT am-
phiphilic pair are organized perpendicular to the length of
the tubes promoting the 1-D self-assembly through synergic
p–p and hydrophobic interactions.
Figure 5. a) Tapping mode AFM image of the T-OPV/C-PBI nanotubes
(0.1 mm) drop-casted onto the gold-coated glass substrate and b) the cor-
responding C-AFM I–V curves. The black circle in the inset image of (b)
shows the fiber on which the I–V measurements were performed.
curve was 0.2 MW. The conductivity of the fibers having an
average height of ꢀ35 nm is found out to be 0.02ScmÀ1,
which is one of the highest value reported for a supramolec-
ular nanostructure without any external doping.[7c] These
measurements were further supported by two probe I–V
measurements on the nanotubes across the electrodes.[18]
The resistance found in this case is one order higher than C-
AFM measurements probably due to the more contact re-
sistance.
In summary, we have presented a new strategy for con-
ducting organic nanostructures with a supramolecular alter-
nate, mixed self-assembly of donor and acceptor molecules,
by using a non-covalent amphiphilic design. We demonstrate
that this amphiphilic design is a very powerful tool to pro-
mote efficient co-facial assembly of even large molecules
with structurally different extended p-conjugated backbones,
which are otherwise known to phase-segregate. The charge-
transfer nanotubes presented here, with an unprecedented
co-facial organization of PBI and OPV chromophores, ex-
hibits high conductivity even without external doping and
hence hold great promise in the emerging area of supramo-
lecular electronics.
To validate our mixed charge-transfer design for conduct-
ing nanostructures, conductive atomic force microscopy (C-
AFM) experiments were carried out on the T-OPV/C-PBI
nanotubes. The samples were prepared by drop-casting the
aqueous solution of the nanotubes onto a gold coated glass
substrate followed by vacuum-drying to remove the solvent
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S. J. George and K. V. Rao
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We thank JNCASR and Department of Science and Technology, Govern-
ment of India for financial support. We thank G. U. Kulkarni (Veeco
Lab), Basavaraj (AFM), NCUFP, Chennai (life-time), R. Ganapathy and
H. K. Nagamanasa (Rheology) for various measurements. K.V.R. thanks
CSIR for a research fellowship.
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Keywords: amphiphiles · charge-transfer · donor–acceptor
systems · hydrogels · nanotubes · self-assembly
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Received: June 19, 2012
Published online: October 2, 2012
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