A generalized supramolecular strategy for self-sorted assembly between donor and acceptor gelators

Article abstract of DOI:10.1039/c1cc12915e

An effective supramolecular strategy for self-sorting between naphthalene-diimide (NDI) acceptor and dialkoxy-naphthalene (DAN) donor organogelators is reported. The concept is based on mismatch in the placement of the two amide functionalities in the don

Full text of DOI:10.1039/c1cc12915e

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COMMUNICATION
A generalized supramolecular strategy for self-sorted assembly between
donor and acceptor gelatorsw
Anindita Das and Suhrit Ghosh*
Received 18th May 2011, Accepted 6th June 2011
DOI: 10.1039/c1cc12915e
An effective supramolecular strategy for self-sorting between
naphthalene-diimide (NDI) acceptor and dialkoxy-naphthalene
(DAN) donor organogelators is reported. The concept is based
on mismatch in the placement of the two amide functionalities
in the donor and acceptor chromophores so that self-sorting
ensured maximum effect of H-bonding.
side of the chromophore. We envisaged that in such a case,
the number of intermolecular H-bonds among the amide
functionalities of the neighbouring chromophores can be
maximized in self-sorting, and thus the possibility of mixed
D–A alternate stacking can be eliminated. NDI-2, a structural
isomer of NDI-1 is used as a control molecule which is known
to form alternate D–A stacks with DAN-1, as H-bonding is
geometrically allowed in the CT-state for this D–A pair.9
Gelation of the individual chromophores were checked in
methylcyclohexane (MCH) where both NDI-1 and DAN-1
showed spontaneous gelation (Fig. 1) with a critical gelation
concentration (CGC) of 1.23 and 2.0 mM, respectively.10
Morphologies of self-assembled structures were examined by
transmission electron microscopy (TEM) where micrometre
long crosslinked-fibres were observed (Fig. S1w) which is
typical of an efficient gelator. We examined gelation of the
NDI-1 + DAN-1 (1 : 1) pair and observed formation of a light-
yellow gel (Fig. 1), and no red color (which is the characteristic
of CT-interaction between NDI and DAN chromophores)
appeared during gelation, suggesting that there is no molecular
mixing among the NDI and DAN chromophores. Contrary to
this, the DAN-1 + NDI-2 pair produced an intense red gel
(Fig. 1) indicating formation of CT-interaction among the
alternately stacked NDI and DAN chromophores.
In the recent past there has been considerable interest in
H-bonding mediated self-assembly of various organic p-systems1
and utilizing them as building blocks2 for gelation3 due to their
potential application in optoelectronic devices.4 In this context,
self-sorting5 among electron-rich (donor) and electron-deficit
(acceptor) aromatic compounds is desirable because this will
allow formation of distinct p- and n-type bundles of fibres
that further intermix and generate multiple junction points for
efficient electron transfer and transport of charge carriers.
However, the difficulty associated with such practice is the
possibility of an alternative mode of assembly, i.e. charge-transfer
(CT) interaction mediated alternate donor–acceptor stacking,6
and thus examples related to self-sorting among donor–acceptor
chromophores are still limited.7 Recently we have demonstrated
hydrogen (H)-bonding mediated orthogonal self-assembly,8
in a mixture of bis-amide functionalized NDI-acceptor and
DAN-donor, where mutual recognition between similar chromo-
phores took place to maximize the effect of H-bonding. Never-
theless, the strategy was found to be too sensitive on many
parameters, and even minor structural variation of the building
block or change in solvent polarity altered the mode of assembly
from self-sorting to alternate D–A co-stacking.9 Herein we reveal
a versatile supramolecular strategy for self-sorted assembly
between donor and acceptor chromophores and demonstrate
the concept using a classical DAN–NDI donor–acceptor pair.
In the present design, we appended the donor and acceptor
chromophores with two amide functionalities but deliberately
changed their positions in the individual chromophores
(Fig. 1). In DAN-1, the two amide groups were symmetrically
placed on both arms while for NDI-1 they were located on one
Clear indication of self-sorting from gelation experiments
prompted us to probe the self-assembly quantitatively by spectro-
scopic experiments. Fig. 2a shows solvent and temperature-
dependent variations in absorption spectra of NDI-1. The
spectrum in CHCl3 represents the monomeric form with well-
resolved absorption bands in the range of 300–400 nm due to
p–p* transition along the long axis of NDI-1 monomers. By
contrast, in highly non-polar medium such as MCH–CHCl3
(95 : 5) a strong hypochromic shift with significant bathochromic
Indian Association for the Cultivation of Science, Polymer Science
Unit, Kolkata, 700 032, India. E-mail: psusg2@iacs.res.in;
Fax: +91-33-2473-2805; Tel: +91-33-2473-4971
w Electronic supplementary information (ESI) available: Synthesis,
additional figures and experimental details. See DOI: 10.1039/
c1cc12915e
Fig. 1 Left: Structure of the amide-functionalized chromophores.
Right: Photos of gels in MCH (c = 10 mM) formed by DAN-1 (a),
NDI-1 (b), DAN-1 + NDI-1 (c), DAN-1 + NDI-2 (d) and NDI-2 (e).
c
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absence of a CT-band in the range of 450–500 nm (Fig. S3w)
also discounted the probability of CT-interaction mediated
alternate co-stacking. To test the ability of the design to
tolerate stoichiometric imbalance of the two chromophores,
absorption spectra of the D–A mixture were recorded with
varying molar ratios of NDI-1 and DAN-1, while keeping the
total chromophore concentration constant.8 From these data
absorbance at 388 and 326 nm, due to the NDI and DAN
chromophores, were plotted as a function of mole fraction
of DAN-1 and a perfect linear plot was observed (Fig. S4w)
which would not have been possible if molecular mixing had
occurred in presence of excess of any of the components.
As the self-sorting is based on H-bonding, we further
probed self-assembly by FT-IR. The NH-stretching frequency
of the amide group was recorded for individual gels and their
mixtures. Lower stretching frequency in MCH (Fig. 2d)
compared to CHCl3 (Fig. S5w) was observed in all three gels
as expected for H-bonding. The IR spectrum due to NH-
stretching frequency in the self-sorted (NDI-1 + DAN-1) yellow
gel was compared with mathematical summation of those
of the individual gels and was found to be nearly identical
(Fig. 2d), further confirming self-sorting of the individual
components in the mixed gel.
Fig. 2 Solvent and temperature dependent UV/vis spectra of
(a) NDI-1 (c = 0.1 mM) and (b) DAN-1 (c = 2.5 mM); (c) UV/Vis
spectrum of NDI-1 + DAN-1 (1 : 1) in MCH–CHCl3 (95 : 5) and
the mathematical sum of the individual spectra (c = 0.2 mM);
(d) IR-spectra of various gels in MCH (c = 2.5 mM for individual
chromophores).
shift (10 nm) was observed, suggesting offset stacking of NDI-1
11
chromophores on top of each other.
Similarly for DAN-1
distinct differences were noticed when its absorption spectra in
CHCl3 were compared with that in MCH (Fig. 2b). The major
absorption bands at 327 and 314 nm experienced a hyperchromic
shift with concomitant very small bathochromic shift and
the aggregated spectrum in MCH became more sharp than
the monomeric spectrum in CHCl3.12 Such opposite spectral
behavior compared to those observed for self-assembled NDI-1
(Fig. 2a) eliminates the possibility of face-to-face p-stacking
among DAN-1 chromophores.13 This can be attributed to the
electrostatic repulsion among the electron-rich DAN chromo-
phores when they are stacked on top of each other.14 It is
noteworthy that for DAN p-systems, generally edge-to-face15
or side-by-side (same plane)16 chromophoric arrangements have
been observed in their solid-state structures. In the present case,
as the geometry of the stacked-assembly is expected to be greatly
influenced by the restriction imposed due to H-bonding among
the amide groups (as evidenced by IR studies), side-by-side
chromophore arrangements is more likely. However, in the
absence of a crystal structure of a suitable model chromophore
we refrain from proposing an exact geometry for the self-
assembled DAN chromophores. Thermal stabilities of the
individual self-assembled structures of NDI-1 and DAN-1 were
compared by variable-temperature UV-vis studies. For NDI-I,
even at 65 1C there was no significant change in the aggregated
spectrum (Fig. 2a) indicating very strong self-assembly.
Conversely, for DAN-1 (Fig. 2b), thermal dissociation took
place at much lower temperature (45 1C), indicating weaker
self-assembly, which can be attributed to electrostatic repulsion
among the donor chromophores17 as well as increase in the
spacer length between the chromophore and the amide groups.18
We then examined spectral behavior of NDI-1 + DAN-1 (1 : 1)
pair in MCH–CHCl3 (95 : 5) (Fig. 2c) and noted the absorption
spectrum of the mixture was nearly identical with the mathe-
matical summation of the aggregated spectra of individual
chromophores confirming no molecular interaction between the
donor and acceptor chromophores in the mixture. Besides, the
Self-assembly was also probed by solvent and temperature-
1
dependent H NMR experiments. In Fig. 3, we show a part of
the NMR spectra where at least one signal from the DAN (Ha)
and NDI (Hb) ring proton could be assigned unambiguously. In
CDCl3, where the chromophores remain in monomeric form,
sharp signals for both Ha and Hb could be observed. However in
MCH where strong self-assembly was noticed, signals corres-
ponding to neither Ha nor Hb could be observed. Thus to
overcome this problem we chose TCE as the solvent for NMR
in which the self-assembly is inherently weak compared to
MCH.9 As anticipated at 30 1C in TCE the DAN proton Ha
appeared at 7.60 ppm which is significantly upfield shifted
compared to that in CHCl3 (7.82 ppm) suggesting self-assembly.
However, the Hb signal could not be seen at 30 1C even in TCE,
which further confirms very strong self-assembly for NDI-1.
We also examined the NMR spectrum of the mixed gel
(NDI-1 + DAN-1) at 30 1C where Ha protons are visible but
no signal for Hb was seen. Interestingly the chemical shift (d) for
Fig. 3 Solvent- and temperature-dependent proton NMR spectra
(selected region) of various gels and their mixtures. For TCE, 10%
C6D6 was used for locking the signal.
c
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Ha in the mixture (7.59 ppm) nearly matched with that of Ha
protons (7.60 ppm) when the experiment was carried out with
DAN-1 only. This clearly suggests that in the mixed system both
NDI-1 and DAN-1 maintain their identity in terms of self-
assembly. However as no signal due to Hb could be observed,
the experiments were performed at slightly higher temperature
(50 1C) hoping that slight destabilization of the aggregates will
cause appearance of Hb protons. In Fig. 3 it can be seen
indeed that the Hb signal is now visible at 8.36 ppm whereas
the Ha signal appeared at 7.69 ppm. Both these signals still
were significantly upfield shifted compared to those in CDCl3,
suggesting the existence of self-assembly, even at 50 1C. More
importantly, when the signals for Ha and Hb were compared
with the corresponding signals for the experiments with
individual gels, the respective chemical shift values were found
to be almost identical, clearly illustrating orthogonal self-
assembly of NDI-1 and DAN-1 in their mixture. Further for
DAN-1, going from CDCl3 to TCE, we observed a downfield
shift of the peak due to the amide proton, which at elevated
temperature experienced an upfield shift (Fig. S6w) as one
would expect for formation and disruption of H-bonding,
respectively. At no stage did we observe splitting of the amide
peak so indicating that both the amides were equally involved
in H-bonding.
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Having demonstrated self-sorting for the (NDI-1 + DAN-1)
pair we tested gelation of NDI-1 with various bis-amide
functionalized DAN-derivatives (Fig. S7w) with varying
distance between the two amide groups, and for all tested
D–A pairs, yellowish gels were observed in MCH (Fig. S7w)
without appearance of any red color, clearly suggesting
versatility of the current approach which was the major
drawback of our previous design.8,9 We further examined
the validity of this design in various structurally different
non-polar solvents (aromatic, chlorinated and hydrocarbon),
and in every occasion self-sorting was observed as indicated by
formation of a yellow gel (Fig. S8w).
V. Bc¸ hmer, Angew. Chem., Int. Ed., 2009, 48, 3867.
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In summary, we have illustrated a versatile molecular design
for H-bonding mediated orthogonal-assembly between aromatic
donor and acceptor chromophores. The molecular design relies
on placing two self-complementary H-bonding functionalities
(amide) symmetrically in the donor building block while
unsymmetrically in the acceptor one, so that for alternate
co-stacking, not all the amides can participate in H-bonding.
As H-bonding is stronger than CT-interaction, self-sorting
was observed for all the tested donor–acceptor pairs with
related structures. To best of our knowledge this is the most
effective design for self-sorting because, in principle, it is
applicable universally to any donor–acceptor pair without
compromising the stability of individual assembly.
7 For orthogonal self-assembly of donor and acceptor chromophores,
see: (a) T. L. Benanti, P. Saejueng and D. Venkataraman, Chem.
Commun., 2007, 692; (b) K. Sugiyasu, S.-i. Kawano, N. Fujita and
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9 A. Das, M. R. Molla, A. Banerjee, A. Paul and S. Ghosh,
Chem.–Eur. J., 2011, 17, 6061.
10 The CGC value of NDI-1 (1.23 mM) is comparable with that of
NDI-2 (0.91 mM) suggesting that unsymmetrical placement of the
amide functionalities does not affect the gelation efficiency.
11 H. Shao, T. Nguyen, N. C. Romano, D. A. Modarelli and
J. R. Parquette, J. Am. Chem. Soc., 2009, 131, 16374.
12 A. Das and S. Ghosh, Chem.–Eur. J., 2010, 16, 13622.
13 Enhanced emission in the aggregated state (Fig. S2w) also supports
absence of face-to-face p-stacking which is generally associated
with fluorescence quenching.
We thank DST, India, for financial support (Project
No: SR/FT/CS-039/2008) and Mr M. R. Molla for NDI-2
sample. A. D. thanks CSIR for a research fellowship.
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¨
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8924 Chem. Commun., 2011, 47, 8922–8924
This journal is The Royal Society of Chemistry 2011