A phosphonic acid appended naphthalene diimide motif for self-assembly into tunable nanostructures through molecular recognition with arginine in water

Article abstract of DOI:10.1039/c3cc41259h

A naphthalene diimide motif bearing phosphonic acid functionalities has been found to be self-assembled with l- and d-arginine through chirality induced molecular recognitions and leads to the formation of micrometre long nanobelts and spherical aggregates at pH 9 in water, respectively.

Full text of DOI:10.1039/c3cc41259h

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A phosphonic acid appended naphthalene diimide
motif for self-assembly into tunable nanostructures
through molecular recognition with arginine in water†
Kamalakar P. Nandre,a Sheshanath V. Bhosale,*b K. V. S. Rama Krishna,a
Akhil Guptac and Sidhanath V. Bhosale*a
Cite this: DOI: 10.1039/c3cc41259h
Received 18th February 2013,
Accepted 4th April 2013
DOI: 10.1039/c3cc41259h
www.rsc.org/chemcomm
A naphthalene diimide motif bearing phosphonic acid functionalities acids and amongst them arginine is the most attractive, mainly due to
has been found to be self-assembled with L- and D-arginine through its involvement in selective protein binding with DNA and RNA.21 The
chirality induced molecular recognitions and leads to the formation phosphonic acid motif, which acts as a hydrogen bonding receptor,
of micrometre long nanobelts and spherical aggregates at pH 9 in for guanidium ion recognition, has been studied intensively.22
water, respectively.
Here we report the use of NDI bearing phosphonic acid
(Phos) self-assembled with L- and D-arginine (L- and D-arg)
Supramolecular self-assembly and self-organisation of small mole- through molecular recognition and leading to the formation
cules play an important role in nano- and biotechnology.1 Self- of well-defined long nanobelts and particular aggregates in
assembly in a controlled fashion within pseudo 1-D to form organized water at pH 9, respectively (Fig. 1; for detailed self-assembly,
nanostructures is the subject of current research.2 These paradigms see Scheme S1 in the ESI†).
have been widely studied with large aromatic macrocycles, especially
The absorption of Phos–NDI (1) ([1] = 0.5 Â 10À5 M), in water
in the design of conducting materials.3 Among them, 1,4,5,8-naphtha- (pH 9, pH adjusted by addition of 0.1 M NaOH) showed two well-
lene diimides (NDIs) have attracted much attention due to their resolved sharp absorption bands at 363 nm and 384 nm with a
tendency to form n-type rather than p-type semiconducting materi- shoulder at 347 nm, which is characteristic of the S0–S1 transi-
als.4 NDIs have allowed several research groups to probe the function tion.23 Fig. 2a shows the absorption spectra of 1 for various ratios of
of this dye molecule in a molecular and supramolecular sense.5 NDIs L- and D-arg in water at pH 9. It can be clearly seen that upon
appended with various motifs such as amino acids,6 pyridyl–metal gradual addition of L-arg (0–10 equiv.), a reduction in peak intensity
complexes,7 quaternion ammonium salts,8 phosphonates, carboxyl- along with a loss of fine structures at higher % v/v was observed.
ates9 and hydrophilic as well as hydrophobic character10 have been However after 2 equivalent additions, there were no further
used for decades for the formation of various supramolecular changes in the absorption spectrum thus indicating the
architectures such as nanotubes,11 nanobelts,12 organogels,13 hydro-
gels,14 synthetic ion channels,15 nanoparticles,16 fluorescent chemo-
sensors17 and artificial supramolecular photosystems.18 Amongst all,
the formation of nanobelts is an important task as the large area
interface of nanobelts, when deposited on electrodes, facilitates the
fabrication of optoelectronic devices with electrical contact.19
During the past few years, phosphorous atoms attached to
photoactive dyes have been identified as most versatile building
blocks in supramolecular chemistry.20 In particular, a phosphonic
acid group shows remarkable selectivity towards zwitterionic amino
a Polymers and Functional Material Division, CSIR-Indian Institute of Chemical
Technology, Hyderabad-50060, AP, India. E-mail: sidhanath.bhosale@gmail.com
b School of Applied Sciences, RMIT University, GPO Box 2476V, Melbourne, VIC-3001,
Australia. E-mail: sheshanath.bhosale@rmit.edu.au; Tel: +61 399252680
c CSIRO Material Science and Engineering and Department of Materials Engineering,
Monash University, Clayton, 3169 VIC, Australia
† Electronic supplementary information (ESI) available: Synthesis of 1, TGA/DSC/
FTIR spectra and details of all assemblies via TEM/SEM/AFM. See DOI: 10.1039/
Fig. 1 Schematic illustration of the proposed organized structure of Phos–NDI (1)
c3cc41259h
with L- and D-arginine assembled into nanobelts and spherical aggregates, respectively.
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Fig. 3 (a and b) SEM images and (c) TEM image of nanobelts in premixed
solution (1 : 2 molar ratio) of 1 (1 Â 10À4 M) with L-arginine (1 Â 10À2 M) in water
at pH 9. (d) XRD pattern of nanobelts (films prepared by evaporating premixed
solution of 1 and L-arginine (1 : 2 molar ratio) from water on a glass slide for 2 h
and plotted against the angle 2y at 298 K).
Fig. 2 UV/Vis absorption changes of Phos–NDI 1 (0.5 Â 10À5 M) upon adding
0–10 equiv. of (a) L-arg and (b) D-arg, (c = 1 Â 10À2 M), respectively. (c) Mirror
image circular dichroism spectra of premixed Phos–NDI 1 with 2 equiv. of D- and
L-arg as well as CD of competitive guest binding D-arg vs. L-arg. (d) Changes in CD
upon titration with L-arg (1 and 2 equiv.) in water at pH 9.
NDI self-assembly through p–p stacking of the central NDI core. A
similar effect was observed in the case of J- and H-aggregates.2,3
The mixture of 1 and L-arg (1 : 2 molar ratio) in water at pH 9
consists of nanobelts, as determined by means of scanning
electron microscope (SEM) images (Fig. 3a and b). A droplet of
saturation point (Fig. S1, ESI†). Practically, similar behaviour the water mixture was dropped onto a carbon tape and the tape
was observed in the presence of D-arg (Fig. 2b). was allowed to air-dry prior to imaging. One can clearly observe the
To gain an insight into the chiral transcription ability of presence of highly aligned and well defined nanobelts of Phos–NDI–
achiral 1 with two equivalents of L-arg and D-arg, we probed the L-arg. These nanobelts show quite uniform widths and thicknesses.
circular dichroism (CD) study (Fig. 2c),7,24 which exhibited The average width is B900 nm with a thickness of ca. 10–30 nm and
strong Cotton effects through an isodichroic point at the zero- the length is in the range of a few tens of micrometers (Fig. S4, ESI†),
crossing at 372 nm. CD was found to be inactive when only 1 leading to an aspect ratio (length over width).
(10À5 M) was employed in water at pH 9; however in the presence
Transmission electron microscopy (TEM) also supports the
of L-arg (2 equiv.) it showed strong negative and positive CD formation of nanobelt nanostructures. A droplet of premixed 1
bands in the region of 391 nm and 361 nm, respectively, and the and L-arg (1 : 2 molar ratio) in water was dropped onto a TEM grid
characteristic of excitonically coupled chromophores.
(a 400 mesh copper grid coated with a carbon film) followed by
On the other hand, Phos–NDI with D-arg showed an opposite staining with 2% uranyl acetate and the grid was allowed to dry in
bisignated CD signal with positive and negative maxima at air. TEM images clearly show that 1 and L-arg self-assembled into
390 nm and 359 nm, respectively. The mirror image Cotton a unique nanobelt nanostructure several micrometers in length
effects of 1 clearly suggest the chirality with opposite handed- with a uniform diameter of ca. 600–800 nm (Fig. 3c).
ness in the self-assembly. The CD signal induced by L- and D-arg
Further, to examine the mode of self-organization within the
at B360 nm was attributed to the p–p* transition of the NDI nanostructures, X-ray diffraction (XRD) measurements were carried
chromophore. Furthermore, upon varying molar ratios of L-arg out. Low angle XRD measurement displayed the strongest peak
to 1, the CD signal indicates gradual evolution with an increase at 2y = 27.51 corresponding to an interlayer d-spacing of 11.2 Å
in concentration of L-arg (Fig. 2d). To understand further in (the 012 diffraction of layered structure) and another diffraction
depth, we have studied competitive molecular recognition by peak at 2y = 26.11 with the other interlayer d-spacing of 7.5 Å
addition of L-arg to 1/D-arg using CD spectroscopy (Fig. 2c). A (the 101 diffraction of layered structure). The peaks at 39.91,
solution of Phos–NDI (c = 1 Â 10À5 M, water) with 2 equiv. D-arg 42.41, 47.21, 52.11 and 66.51 can be attributed to the second to
was titrated with the same amount of L-arg. Interestingly, addition the fifth order diffraction of the (012) plane respectively. The
of 2 equiv. of L-arg to 1/D-arg resulted in assemblies with a positive intense and sharp peak (012) indicated that the nanobelts grew
bisignated CD signal which exactly matched that of 1/L-arg stacks. preferentially along the axis.25
This clearly suggests the competitive replacement of D-arg with
L-arg from the assemblies, a result similar to the results obtained water at pH 9 (Fig. 4). TEM clearly showed that compound 1
for ATP over ADP.7
with D-arg (1 : 2 molar ratios) self-assembled into well-defined
Furthermore, the emission spectroscopy of 1 indicates spherical aggregates, with a mean diameter of 80–110 nm.
that upon gradual addition of L- and D-arg, enhancement of The formation of spherical aggregates in solution was confirmed
1 with D-arg was self-assembled into particular aggregates in
emission was observed for each addition (lext 384 nm; Fig. S2 by dynamic light scattering (DLS) study of the premixed 1 with
and S3 in ESI†). These results strongly suggest the formation of D-arginine (1 : 2 ratio, 1 Â 10À4 M). The average diameter of the
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Sid. V. Bhosale thanks the DAE-BRNS, Mumbai, India, for
financial support under the project no. 2009/37/39/BRNS.
Notes and references
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into a spherical form (Fig. S5 and S6 in ESI†). The average
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notably smaller than that observed in TEM analysis (see
Fig. 4 for the TEM micrograph). This may be due to the high
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Importantly and from a molecular recognition issue, Phos–NDI
itself or L-/D-arg alone or even a premixed solution of 1 and L-lysine
failed to give any well-defined nanostructures at any pH indicating
the subtleties of the self-assembly process, which may be due to lack
of H-hydrogen bonding (Fig. S7, ESI†). Importantly, nanostructures
of Phos–NDI with L or D-arg were only observed at pH 9, as lowering
the pH does not produce any nanostructures (Fig. S8, ESI†).
Although the self-assembly is predominantly driven by p–p stacking
interactions between NDI cores, electrostatic interactions and chir-
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in water (Fig. 1). We believe that self-assembly of 1 with L-arg leads
to nanobelt formation mainly due to two reasons: (i) molecular
conformation of two independent L-arg molecules arranged in a
head to tail manner and superimposed directly i.e. adjacent
molecules are linked by two N–HÁÁÁO hydrogen bonds between
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interactions may occur (Fig. S10 and Scheme S1, ESI†).
¨
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behaviour of a phosphonic acid appended naphthalene diimide
with L- and D-arg, which forms well-defined nanobelts and
spherical aggregates in water at pH 9, respectively, via chirality
induced molecular recognitions. The perfect morphological
features of the nanostructures of NDI–arginine complexes
may make them attractive for nano- and biomaterial research.
We also believe that the assembly concept reported in this
communication would be a valuable tool to identify protein
surface areas by distinctive sequence specific self-assembly.
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