Molecular recognition driven self-assembly and chiral induction in naphthalene diimide amphiphiles

Article abstract of DOI:10.1039/c2cc35438a

Naphthalene diimide amphiphiles functionalized with the dipicolylethylenediamine motif self-assemble with tunable chirality upon molecular recognition with various adenosine phosphates and competitive guest binding leads to the dynamic helix reversal of t

Full text of DOI:10.1039/c2cc35438a

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Chemical Communications
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Volume 48 | Number 89 | 18 November 2012 | Pages 10921–11036
ISSN 1359-7345
COMMUNICATION
Subi J. George et al.,
Molecular recognition driven self-assembly and chiral induction in
naphthalene diimide amphiphiles
1359-7345(2012)48:89;1-O

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COMMUNICATION
Molecular recognition driven self-assembly and chiral induction in
naphthalene diimide amphiphilesw
Mohit Kumar, Narendra Jonnalagadda and Subi J. George*
Received 27th July 2012, Accepted 31st August 2012
DOI: 10.1039/c2cc35438a
Naphthalene diimide amphiphiles functionalized with the dipicolyl-
ethylenediamine motif self-assemble with tunable chirality upon
molecular recognition with various adenosine phosphates and
competitive guest binding leads to the dynamic helix reversal of
these assemblies.
The molecular recognition driven 1-D helical self-assembly
of chromophores often employed non-directional electrostatic
interactions for guest binding.7 However, we envisioned that
chromophore functionalization with specific guest binding
groups would give a better control over the resulting self-
assembly.8 On the other hand, use of biologically benign guest
molecules, such as adenosine phosphates, would not only
facilitate an efficient self-assembly through additional hydro-
phobic and p–p interactions between the base units, but would also
act as a chiral handle for imparting chirality to the resulting
assemblies. Extensive studies on molecular phosphate sensors
suggest that the dipicolylethylenediamine–zinc complex (DPA–Zn)
motif can specifically bind to various adenosine phosphates
with high association constants.9 Hence we have designed NDI
amphiphiles substituted with DPA–Zn motifs (Scheme 1), in
order to promote guest induced self-assembly and chiral induction
through specific binding interactions. NDPA-Bola was synthe-
sized following the literature procedure9 whereas NDPA-
Amph was synthesized by a statistical reaction of 1,4,5,8-
naphthalenetetracarboxylic dianhydride with dodecylamine
and N,N-bis(2-pyridylmethyl)ethane-1,2-diamine followed by
zinc metallation.10
The absorption studies of NDPA-Bola (5 Â 10À5 M, 10 mM
aq. HEPES buffer) showed characteristic features of molecularly
dissolved NDI chromophores such as sharp absorption bands
(lmax = 381 and 361 nm). However, titration of NDPA-Bola
with increasing molar ratios of ADP (0–2 equivalents), resulted
in broadening of absorption spectra, along with reversal of
relative intensity of vibronic bands at 361 nm and 381 nm
and decrease in fluorescence intensity characteristic of NDI
chromophoric self-assembly (Fig. 1a).10 Corresponding Circular
Dynamic helical polymers and supramolecular one-dimensional
(1-D) assemblies with tunable handedness, by the specific molecular
recognition of chiral guests, have attracted immense attention as
model systems to understand the concepts of chiral amplification.1,2
Moreover, the molecular recognition properties along with the
dynamic nature of these helical systems would be very attractive for
the design of stimuli responsive and chirotechnological materials.
Although, supramolecular helical stacks constructed from the
assembly of chiral monomers have been well investigated,2 the
molecular recognition driven 1-D assembly of achiral monomers
and the resultant induction of chirality from the guests to the
achiral/racemic assemblies is seldom reported. In this respect, chiral
induction,3 (preferential) chiral solvation4 and chiral memory,5 well
known concepts in their macromolecular counterparts, have been
recently demonstrated in 1-D supramolecular systems using the
principles of host–guest chemistry. However, gaining detailed
mechanistic insights and the design of guest responsive, dynamic
helical 1-D assemblies remains challenging.
Herein we communicate the adenosine phosphates induced
one-dimensional (1-D) self-assembly and the resultant supra-
molecular chirality of naphthalenediimide (NDI) amphiphiles
i.e. NDPA-Amph and NDPA-Bola. Detailed spectroscopic
probing provided mechanistic insights into the dynamic molecular
recognition, chiral induction process and stability of the assemblies.
The binding of multivalent chiral phosphates resulted in high chiral
order, as evident from the unprecedented excitonic, bisignated
circular dichroism signals, in the resulting NDI assemblies.6
Although self-assembly of NDI derivatives has been extensively
studied,6 this is the first report of their guest induced chiral self-
assembly. We further present the dynamic reversal of the helical
handedness of NDI stacks through competitive guest binding.
Supramolecular Chemistry Laboratory, New Chemistry Unit,
Jawaharlal Nehru Centre for Advanced Scientific Research
(JNCASR), Bangalore, India 560064. E-mail: george@jncasr.ac.in;
Fax: +91 80 22082760; Tel: +91 80 22082964
w Electronic supplementary information (ESI) available: Supporting
figures and synthetic details. See DOI: 10.1039/c2cc35438a
Scheme 1 Molecular structures of NDPA-Bola and NDPA-Amph.
c
10948 Chem. Commun., 2012, 48, 10948–10950
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derivatives ensures that they can be self-assembled in a
THF–water mixture, even in the absence of guest binding.
Solvent dependent absorption studies (c = 7 Â 10À5 M)
showed that upto 70% aq. HEPES buffer in THF, it exists
in the monomeric state. However in 90% aq. HEPES buffer,
the molecule is self-assembled as evident from the broadening
of absorption bands with reversal of the relative intensity ratio
of the absorption maxima at 359 and 379 nm.10 NDPA-Amph
at these two states, i.e. monomeric and aggregated, showed
very different behaviour on binding to phosphates. Mono-
meric NDPA-Amph (70% aq. HEPES/THF), self-assembles in
a similar way as that of NDPA-Bola with various phos-
phates.10,11 For example co-assembly with ADP resulted in a
negative bisignated CD signal with positive and negative
maxima at 357 and 391 nm, respectively.10 Similarly, helicity
induction was observed on binding to AMP and ATP giving
negative and positive bisignated CD signals, respectively.10
Remarkably, when ADP was added to the pre-assembled NDI
amphiphiles (90% aq. HEPES/THF), chiral induction was not
observed, neither at room temperature nor upon cooling the
mixture from higher temperatures.10 As the amphiphilic
chromophoric assemblies are known to be less dynamic,12 it
is evident that at higher amounts of water, the pre-assembled
NDPA-Amph molecules are not dynamic enough to reorganize
into helical stacks upon guest binding.10 Hence it can be
concluded that, in the present system, guest induced molecular
organization is crucial for the induction of supramolecular
chirality. Remarkably, temperature dependent absorption and
CD measurements of the NDI/phosphate co-assemblies have
shown that, heating even upto 95 1C does not completely
destroy chiral organization indicating the high stability of the
stacks and guest binding.10
Fig. 1 Changes in (a) absorption, (b) CD and (c) Job plot of NDPA-Bola
upon titration with ADP. (d) Mirror image CD spectra of NDPA-Bola
with 1 equiv. of ADP and ATP (c = 5 Â 10À5 M, 10 mM aq. HEPES
buffer).
Dichroism (CD) spectra showed the gradual evolution of
strong Cotton effects, through an isodichroic point at the
zero-crossing (370 nm), indicating that ADP binding induces
a preferred helical handedness to the resulting assemblies of
achiral NDIs (Fig. 1b). Binding of ADP resulted in negative
bisignated CD spectrum, with negative and positive maxima at
395 and 360 nm respectively, characteristic of excitonically coupled
chromophores. The titration curve obtained by monitoring the
CD intensity at 395 nm showed saturation at 1 equivalent of ADP,
suggesting a 1 : 1 stoichiometry in the co-assembly.10 This is
further evident from the Job plot, where the CD intensity probed
at 395 nm showed maxima at 0.5 mole fraction of NDI (Fig. 1c).
This suggests that the divalent ADP molecules bind to two
DPA–Zn moieties of adjacent NDIs in the assembly, thereby
clipping the chromophores together, in line with the literature
(inset, Fig. 1c).9 This system presents one among the best
excitonically coupled NDI chromophoric assemblies known in
the literature,6 as they often showed weak bisignated CD signals
either due to their low self-association or lack of aromatic
interactions in the self-assembly.
Transmission Electron Microscopy (TEM) imaging of
NDPA-Amph (70% water in THF, 7 Â 10À5 M) assemblies
with 0.5 equiv. of ADP, stained with uranyl acetate showed
the formation of 1-D nanofibers with a uniform diameter of
8 nm (Fig. 2). Based on the energy minimized molecular
dimensions of ADP bound NDPA-Amph (B4 nm), this
indicates that the fibers are formed by the p-stacking and
solvophobic interactions of the NDI bilayers with non-
interdigitated alkyl chains as the hydrophobic interior (inset,
Fig. 2). TEM analysis of NDPA-Bola/ADP co-assemblies
revealed the formation of fibers with a 4 nm diameter, suggesting
that they are constructed from single NDI p-stacked columns
with bound adenosine phosphates on both DPA–Zn sites.10
While NDPA-Bola showed similar assembly behaviour on
titration with monovalent AMP,10 binding of ATP induces
opposite handedness to NDI assemblies as evident from the
positive bisignated CD signal, with positive and negative maxima
at 390 and 359 nm, respectively (Fig. 1d).10 The mirror image
Cotton effects of NDPA-Bola assemblies obtained with the ADP
and ATP clearly suggests the induction of chirality with opposite
handedness. Furthermore, slow cooling of the various NDI/
phosphate co-assemblies from 95 1C to 15 1C, did not show
any significant change in the intensity of the CD signal, indicating
that room temperature binding of the chiral guest to NDI indeed
leads to the most stable assemblies.10
In order to understand the role of guest-induced chiral
assembly, we further studied the effect of phosphate binding
on NDPA-Amph derivatives, in its monomeric as well as in the
assembled states. The amphiphilic nature of the NDPA-Amph
Fig. 2 (a) and (b) TEM images of a 7 Â 10À5 M solution of NDPA-
Amph/0.5 equiv. ADP (70% water in THF). Inset of b shows the
schematic of the corresponding helical stack.
c
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In summary, we showed a novel supramolecular design, based
on dipicolylethylenediamine functionality, for the adenosine
phosphate recognition driven one-dimensional helical assembly
of chromophores. Chiroptical probing has provided mechanistic
insights into the self-assembly process and showed that guest
binding driven molecular reorganization is indeed necessary for
helicity induction. We have also shown a novel strategy for the
dynamic helical reversal of supramolecular assemblies, based
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systems interacting with biologically benign chiral phosphates
will be further studied for chiral amplification and memory.
We thank Prof. C. N. R. Rao, FRS for his support throughout
this work, JNCASR and DST, Government of India for financial
support. Authors acknowledge Amrit (DLS), Usha (TEM) for
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11 Addition of higher equivalents of phosphates resulted in higher
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