To assemble or fold?

Article abstract of DOI:10.1039/c4cc05429f

This communication reports an elegant structure formation by an amide functionalized donor (D)-acceptor (A) dyad by stepwise folding and assembly. It adopts a folded conformation by intra-chain CT-interaction that subsequently dimerizes by inter-molecular

Full text of DOI:10.1039/c4cc05429f

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To assemble or fold?†
Anindita Das and Suhrit Ghosh*
Cite this: Chem. Commun., 2014,
50, 11657
Received 14th July 2014,
Accepted 6th August 2014
DOI: 10.1039/c4cc05429f
www.rsc.org/chemcomm
This communication reports an elegant structure formation by
an amide functionalized donor (D)–acceptor (A) dyad by stepwise
folding and assembly. It adopts a folded conformation by intra-chain
CT-interaction that subsequently dimerizes by inter-molecular
H-bonding to produce a folded dimer (FD) with a DAAD stacking
sequence. Incompatibility of the aromatic stacked face with
MCH triggers macroscopic assembly by solvophobically driven
edge-to-edge stacking of the FD with concomitant growth in the
orthogonal direction by D–D p-stacking leading to the formation of
a reverse-vesicle.
Scheme 1 Top: structure of NDI-PY and the control molecule; bottom:
possible modes of folding and assembly of the NDI-PY.
Charge-transfer (CT)-interaction mediated alternate stacking of
donor (D) and acceptor (A) p-systems,1 has successfully promoted
many ingenious supramolecular architectures2 with relevance to
To test this, in this communication we have studied the self-
optoelectronic applications. On the other hand, their segregated- assembly of a D–A dyad (NDI-PY) (Scheme 1) that contains an
assembly3 has also been studied with considerable interest electron deficient naphthalenediimide (NDI) chromophore
owing to its potential application in organic solar cells. connected to a pyrene (PY) moiety which is a well known D–A
However, hierarchical growth is limited in either case by the pair for CT-complex formation.2 The NDI-chromophore is
inherently weak nature of p-stacking or CT-interaction. Nature connected to a trialkoxybenzamide wedge that has a dual role
has cited innumerable examples where H-bonding governs the of solubilising the building block in a non-polar solvent as well
self-assembly and thereby the function of biotic systems. as promoting self-assembly through H-bonding among the
Inspired by nature, several synthetic systems have been amide groups. NDI-PY is so designed that in principle it can
reported on H-bonding-promoted supramolecular assembly of adopt different modes of self-assembly (Scheme 1) by interplay
isolated D and A chromophores by our group4 and others.5 of H-bonding and intra- or inter-molecular CT-interaction.
These examples show that the mode of co-assembly (alternate NDI-PY in MCH (C = 2.0 mM) produces an intense red solution
or segregated stack) is governed by relatively strong H-bonding (inset, Fig. 1a) suggesting the formation of a CT-complex,2
rather than D–A CT-interaction.2 This encouraged us to examine which is also confirmed from the appearance of a broad
whether H-bonding interaction can similarly dictate the mode CT-band at around 525 nm (Fig. 1a). This observation rules out
of self-assembly in covalently linked D–A systems which is the possibility of a segregated assembly (mode A, Scheme 1). Inter-
more challenging due to its possible conflict with intra-chain estingly, an equimolar mixture of PY and NDI-1 (Scheme 1) shows
CT-interaction which is much stronger compared to inter- neither red coloration nor a CT-band (Fig. 1a) substantiating the
molecular D–A complexation.4,5
formation of an intramolecular CT-complex (mode C, Scheme 1).
Furthermore the CT-band intensity (lmax = 525 nm) shows a
linear relationship with concentration (Fig. S1, ESI,† and
Fig. 1b) confirming intra-chain folding (mode C, Scheme 1).6
Variable temperature UV/vis studies (Fig. 1c) show a gradual
decrease in the CT-band intensity with increasing temperature
up to 65 1C followed by a sharp downfall and saturation at 85 1C
Indian Association for the Cultivation of Science, Polymer Science Unit,
Kolkata-700032, India. E-mail: psusg2@iacs.res.in
† Electronic supplementary information (ESI) available: Synthesis and additional
spectra data. See DOI: 10.1039/c4cc05429f
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Scheme 2 Proposed mode of folding followed by assembly by NDI-PY.
The other possibility (Fig. S6, ESI†) of stacking of the foldamer with more
usual . . .DADADA. . . sequence is eliminated as in that case H-bonding
among the amides was found to be not allowed by geometrical constraints
(Fig. S6, ESI†).
Fig. 1 (a) UV/Vis spectrum of the red solution (inset left) of NDI-PY and colorless
solution (inset right) of NDI-1 + PY (1:1) in MCH; C = 2.0 mM. (b) Variation
of absorbance (lmax = 525 nm) of NDI-PY as a function of concentration (10.0–
0.16 mM). (c) Effect of temperature on the CT-band intensity of NDI-PY
(C = 0.4 mM). (d) Variation of the CT-band intensity of NDI-PY (lmax = 525 nm)
with temperature in the absence and presence of MeOH.
that is reminiscent of reverse vesicles8 (Fig. 2a and Fig. S4, ESI†).
The average hydrodynamic diameter (Dh) of B200 nm obtained
from dynamic light scattering (DLS) (Fig. 2b) showed a larger
particle size compared to that obtained from TEM possibly due to
the shrinkage of the membrane in the dry state.9 Static light
scattering (SLS) measurements (Fig. S5, ESI†) revealed the radius
indicating (i) appreciable stability and (ii) possibly a two-step of gyration (Rg) to be 106 nm. The Rg/Rh(Dh/2) value (1.06) very
melting of the D–A foldamer. However in the presence of a close to unity further confirming a hollow spherical morphology.9
H-bond breaking protic solvent MeOH (Fig. S2, ESI,† and Fig. 1d)
Based on these experimental results it is proposed (Scheme 2)
shows a sharp fall in the CT-band intensity at a relatively much that initially NDI-PY adopts a folded structure by intramolecular
lower temperature suggesting significantly less stability of the CT-interaction that dimerizes by H-bonding to produce a folded
CT-complex or the folded state when the H-bonding effect is dimer (FD). Now a structural similarity can be drawn between
decoupled. The FT-IR spectrum of NDI-PY in MCH (Fig. S3, ESI†) the FD and a typical facial amphiphile.10 The stacked rigid
looked almost identical to that in the solid state and showed chromophores constitute a MCH incompatible face (similar to
sharp peaks at 1666 and 1580 cmÀ1 for amide-I and amide-II, the hydrophobic face of a facial amphiphile in water) while the
respectively, and a broad peak at 3410 cmÀ1 for –NH stretching presence of the alkyl chains make the other face of the FD
confirming H-bonding among the amide groups7 in the folded suitable for interaction with MCH. Thus the FD forms a bilayer
state. Combining all these spectroscopic pieces of evidence, it is structure by stacking of the edges that propagates orthogonally
now apparent that in solution, the flexible dyad (NDI-PY) adopts through PY–PY interaction to form an extended assembly which
a folded structure utilizing both intramolecular CT-interaction eventually bends to produce reverse vesicle (Fig. 2a) to avoid the
as well as intermolecular H-bonding.
unfavourable exposure of the edges to the medium.
To examine whether the NDI-PY foldamer remains as a
The MALDI experiment (Fig. 3a) showed a prominent peak
discrete structure or further propagates to form a higher ordered at m/z = 2919.54 corresponding to the dimer [dimer of NDI-PY +
assembly in MCH, we looked into the morphology. Transmission Na]+ of the NDI-PY and thus confirmed the formation of the FD.
electron microscopy (TEM) images revealed nearly spherical The zoomed image (inset, Fig. 3a) shows a typical isotope
hollow particles (diameter B100 to 120 nm) with a thin wall distribution with a 1 mass unit difference between the peaks
(thickness B1.6 nm as indicated by the white arrow in Fig. 2a) confirming a singly charged species. This model (Scheme 2) was
also supported by various other experimental pieces of evidence.
Firstly it accounts for the reduced stability of the CT-complex in
the presence of MeOH (Fig. 1d). Because H-bonding among the
amides restricts the conformational freedom of the acceptor
arm and thus reduces the number of possible conformations
of the unfolded D–A dyad. Thus the folding process happens
with
a relatively less entropy loss. Fluorescence studies
(Fig. 3b) show a strong emission band (lmax B 450 nm), which
can be attributed to pyrene excimer emission,11 as in that
region neither the monomeric NDI nor pyrene emits. This
supports the model (Scheme 2) for a higher order assembly
through PY–PY stacking. A relatively less intense peak at B650 nm
appears possibly due to emission from the CT-complex.12
Fig. 2 (a) TEM image of NDI-PY showing vesicular assembly. (b) DLS data
in MCH. C = 0.4 mM.
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DLS experiment (Fig. 3d and Fig. S8, ESI†). A very significant
drop in the average particle size from 190 nm to 15 nm with
increasing temperature from 25 1C to 85 1C illustrates rupture
of the reverse vesicular assembly. Notably, an initial increase in
the particle size around 30 1C may be due to the expansion of
the membrane (Scheme 2) by partial unfolding (Fig. 3c). The
powder X-ray diffraction (XRD) pattern (Fig. 3e) of a dried film
generated from the solution of NDI-PY in MCH reveals a sharp
peak in the low angle region (2y = 1.23) corresponding to an
interlayer spacing (d) of 71.2 Å that closely matches twice the
estimated length (Fig. S9, ESI†) of the foldamer (37 Â 2 = 74 Å)
and thus further supports the proposed model for bilayer
formation (Scheme 2). A relatively greater wall thickness
(1.6 nm) of the vesicles (Fig. 2a) compared to the bilayer length
indicates a multi-lamellar structure. A broad peak at around
2y = 23.2 (inset, Fig. 3e) corresponding to a spacing of 3.83 Å
confirms alternate stacking between pyrene and NDI.
A hierarchical assembly of an amide functionalized flexible
D–A dyad has been demonstrated by maneuvering multiple weak
interactions involving H-bonding, CT-interaction, p–p stacking
and solvophobic forces. While a H-bonding driven segregated-
assembly or intra-molecular CT-interaction promoted folding
were the two limiting possibilities, the system smartly adopted
an alternate path of self-organization to form a FD by satisfying
both H-bonding and CT-interaction. Even after significant
progress made in the field of foldamers in general13 and D–A
foldamers,6 understanding their macroscopic assembly14 is yet
to be explored which may have far-reaching consequences for a
more realistic bio-mimicking in terms of structure and function.
Amidst the numerous possibilities, the formation of only a
sequence specific (DAAD) structure that involves intra-chain
folding followed by macroscopic-assembly is reminiscent of
the protein structure by correlating the foldamer, FD and the
bilayer membrane to be synthetic mimics of secondary, tertiary
and quaternary structures of proteins, respectively.15
Fig. 3 (a) MALDI-TOF spectrum (selected region) of a film of NDI-PY
made from a solution in MCH. (b) Emission spectrum of NDI-PY in MCH.
Inset: plot of emission intensity at 448 nm (Fig. S7, ESI†) vs. temperature.
lex = 337 nm, concentration = 0.4 mM, solvent = MCH. (c) Transition
from folded state to partially unfolded state at elevated temperature.
(d) Variation in the particle size with temperature as obtained from the
DLS experiment (Fig. S8, ESI†). C = 0.4 mM. (e) Powder XRD data of dried
NDI-PY powder.
We thank Mr T. Mondal for the NDI-1 sample. AD thanks
CSIR for a research fellowship. SG thanks SERB for the funding
Intriguingly, variable-temperature fluorescence studies (inset, (SR/S1/OC-18/2012).
Fig. 3b and Fig. S7, ESI†) show a gradual increase in the
emission intensity up to B55 1C followed by a sharp decrease
Notes and references
and saturation at B85 1C. The initial increase in the excimer
emission up to 50 1C possibly indicates partial unfolding (not
complete melting) which still can allow PY excimer formation
outside the D–A stack (Fig. 3c) but reduces the possibility of
static quenching by the NDI acceptor. This also accounts for
the decrease in the CT-absorption intensity in the same tem-
perature window (Fig. 1d) at a slow rate possibly due to the
gradual decrease of the relative population of the D units in
the mixed stack. This also corroborates with the observed
(Fig. S7, ESI†) decrease in the CT-emission band (650 nm)
intensity with increasing temperature. However, above
B60 1C, thermal dissociation of the H-bonding leads to the
melting of the whole assembled structure causing a sharp
decrease in the intensity of the excimer emission (inset,
Fig. 3b) as well as CT-absorption (Fig. 1d). The thermorespon-
sive disassembly was also probed using a variable-temperature
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