Chiral induction by helical neighbour: Spectroscopic visualization of macroscopic-interaction among self-sorted donor and acceptor π-stacks

Article abstract of DOI:10.1039/c1cc11178g

Supramolecular induction of chirality to a π-stacked dialkoxynaphthalene (DAN)-fiber (made of achiral building blocks) from a neighbouring helical naphthalenediimide (NDI)-fiber is reported. CD-studies helped in understanding the nature of co-assembly in the donor-acceptor chromophore mixture from molecular to macroscopic scale.

Full text of DOI:10.1039/c1cc11178g

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
ChemComm
Cite this: Chem. Commun., 2011, 47, 8934–8936
www.rsc.org/chemcomm
COMMUNICATION
Chiral induction by helical neighbour: spectroscopic visualization of
macroscopic-interaction among self-sorted donor and acceptor p-stacksw
Mijanur Rahaman Molla, Anindita Das and Suhrit Ghosh*
Received 28th February 2011, Accepted 24th May 2011
DOI: 10.1039/c1cc11178g
Supramolecular induction of chirality to a p-stacked dialkoxy-
naphthalene (DAN)-fiber (made of achiral building blocks) from
a neighbouring helical naphthalenediimide (NDI)-fiber is reported.
CD-studies helped in understanding the nature of co-assembly in
the donor–acceptor chromophore mixture from molecular to
macroscopic scale.
of a structurally similar NDI-chromophore containing an
asymmetric centre in the peripheral alkyl chain (NDI-1,
Scheme 1) with achiral DAN-1 by circular dichroism (CD),
which revealed chirality transfer from the acceptor-stacks to the
donor-stacks. To the best of our knowledge, as yet there is no
report of such unique chirality induction by helical neighbours,
which in turn helped in spectroscopic visualization of the nature
of macroscopic level organization among the molecularly self-
sorted donor and acceptor supramolecular p-stacked fibers.
Self-assembly of NDI-1 was studied by absorption spectroscopy
(Fig. 1a) by varying the solvent from CHCl3 to methylcyclohexane
(MCH). In CHCl3 well resolved absorption bands are seen in the
range of 300–400 nm due to p–p* transition polarized along
the long axis of the chromophore, suggesting the presence of
monomers. With increasing amount of MCH, the spectra
remained almost invariant till 80 : 20 MCH–CHCl3 and beyond
this point with further increase in MCH composition, a strong
hypochromic shift with concomitant red shift (B3 nm) was
observed suggesting p-stacked assembly.8,9 The mole fraction of
the aggregate (aagg)8 is plotted as a function of solvent composition
in Fig. 1b and from this plot the a50 (critical solvent composition
where aagg = 0.5) was estimated to be 83% (v/v) MCH–CHCl3.
Helical assembly plays a key role in many elegant functions
performed by biopolymers in living systems with ultimate
perfection. Chemists have been fascinated to study helicity in
polymeric1 and supramolecular systems2 to mimic such biological
objects, if not by function, then at least from structural aspects. In
supramolecular systems, most often helicity is originated from
asymmetric building blocks by transfer of chirality from molecular
to macroscopic scale. It is well established that even a very small
fraction of a chiral building block is sufficient to induce chirality
in the entire supramolecular or polymeric assemblies by the
so-called ‘‘sergeant-and-soldiers’’ effect.3 Induction of chirality in
1-D supramolecular assemblies has also been observed by external
perturbations using various types of physical and chemical inter-
6
actions,4 chiral solvent,5 and even vortex effect generated by
unidirectional magnetic stirring.
We have recently demonstrated self-sorting7 among bis-amide
functionalized dialkoxynaphthalene (DAN-1, Scheme 1) donor
and naphthalenediimide (NDI-2, Scheme 1) acceptor due to
hydrogen-bonding and p-stacking.8 To gain further insight into
this interesting phenomenon herein we have studied self-assembly
Scheme 1 Structure of the various chromophores.
Indian Association for the Cultivation of Science,
Polymer Science Unit, 2A & 2B Raja S. C. Mullick Rd., Kolkata,
India. E-mail: psusg2@iacs.res.in; Fax: +91 33 2473 2805;
Tel: +91 33 2473 4971
w Electronic supplementary information (ESI) available: Synthesis, charac-
terization, detail experimental procedure and additional spectral data
related to probing self-assembly. See DOI: 10.1039/c1cc11178g
Fig. 1 Solvent dependent UV/vis spectra (a) and variation of aagg
with solvent composition (b) for NDI-1 at rt; Variation of CD spectra
of NDI-1 with solvent (c) and temperature (d) (solvent = 95 : 5
MCH–CHCl3). Concentration of NDI-1 = 0.5 mM. Arrows in
(a) indicate spectral variation from CHCl3 to MCH.
c
8934 Chem. Commun., 2011, 47, 8934–8936
This journal is The Royal Society of Chemistry 2011

View Article Online
Furthermore, it was observed that self-assembly could be
completely destroyed by addition of B5% (v/v) MeOH to the
chromophore solution in 95 : 5 MCH–CHCl3 (Fig. S1a, ESIw)
suggesting strong influence of H-bonding. The value of a50(T)
(temperature at which aagg = 0.5) for self-assembly in 95 : 5
MCH–CHCl3 was estimated to be 60 1C from variable-
temperature UV/vis experiments (Fig. S1, ESIw). Similar to
recently reported NDI-2,10 NDI-1 also showed very efficient
gelation ability11 in non-polar organic solvents with extremely
low critical gelation concentration (0.55 wt% in cyclohexane).
Further the self-assembly of NDI-1 was probed by CD. In
CHCl3 where the NDI-1 chromophores remain as monomers,
no CD band was observed, but in 95 : 5 MCH/CHCl3 an
intense positive Cotton effect was observed (Fig. 1c) in the
region of 300–400 nm which corresponds to the absorption
due to p–p* transition polarized along the long axis (band 1)
of NDI. The observed positive Cotton effect was attributed to
the formation of P-type helical assembly based on previously
reported systems.12 We further noticed that in presence of
5% (v/v) MeOH, the CD-band completely disappeared (Fig. 1c),
clearly suggesting that the CD-band originates due to self-
assembly. A variable-temperature CD-experiment (Fig. 1d)
revealed the Cotton effect even at 60 1C suggesting very good
thermal stability of the helical assembly. Atomic force
microscopy (AFM) images of NDI-1 (sample prepared from
solution in 95 : 5 MCH–CHCl3) revealed micrometre long
fibrils (Fig. 2) with average height and diameter of 5.9 Æ 0.5
and 70.6 Æ 1 nm, respectively. However, the resolution of the
images does not allow us to assign helicity conclusively.
Having established the self-assembly of NDI-1 alone we
now examined the co-assembly pattern for a mixture of
NDI-1 + DAN-1. NDI-1 is structurally almost identical to
NDI-2 except in one of the three peripheral alkyl chains which
contains the chiral centre, and thus it is expected to show similar
self-sorting behaviour8 with DAN-1. This was confirmed by
comparing the UV/vis spectrum of NDI-1 + DAN-1 (1 : 1) in
95 : 5 MCH–CHCl3 with that generated by mathematical
summation of the spectra of the two individual components
(Fig. S2, ESIw), and both were found to be almost identical,
suggesting self-sorting. Furthermore, absence of any charge-
transfer absorption band in the spectrum of the mixture
(Fig. S2, ESIw) confirmed lack of alternate co-assembly
between donor and acceptor chromophores. Now we examined
the co-assembly by CD (Fig. 3a). In CHCl3, NDI-1 + DAN-1
(1 : 1) mixture does not show any CD-signal due to lack of
Fig. 3 (a) CD spectra of NDI-1 and its mixture with DAN-1 under
various conditions. (b) Variation of CD intensity at 386 nm (solely due
to NDI-1) with temperature for NDI-1 alone and NDI-1 + DAN-1 (1 : 1)
in 95 : 5 MCH–CHCl3. Concentration of individual chromophore =
0. 5 mM.
aggregation. However an intense positive Cotton effect was
observed (green line) in 95 : 5 MCH–CHCl3 suggesting helical
self-assembly. Surprisingly in the CD-spectrum of the mixture,
in addition to the NDI-1 band at 386 nm, additional bands
were observed (orange line) in the region of 320–340 nm which
correspond to the DAN-1 absorption in the aggregated state8
although the chromophore does not contain any chiral center.
To confirm that the new band appears due to self-assembly of
DAN-1, the CD spectrum of the mixture was examined at an
intermediate temperature (55 1C) where self-assembly of NDI-1
should remain intact, but the relatively less-stable p-stacked
assembly DAN-1 should be converted to monomers.8,13 In
Fig. 3a, it can be clearly seen that at 55 1C the additional bands
in the CD-spectrum corresponding to the donor-absorption
disappear (blue line) and the spectrum is very similar to that
observed for NDI-1 alone (black line). This clearly demonstrates
that the new band in the 320–340 nm region indeed corresponded
to the self-assembly of DAN-1. Further we carried out variable-
temperature CD-experiments of the NDI-1 + DAN-1 mixture
(Fig. S3w) and compared the variation of intensity at 386 nm
(DAN molecule does not absorb in this region) with that for
NDI-1 alone (Fig. 3b). An almost identical nature of the curve
in both cases14 confirmed no significant interference of the
neighboring chromophoric-stack on the strength of self-
assembly for NDI chromophore, which is governed by inter-
chromophoric interaction among the NDI building blocks. To
explain this we considered the possibility of chiral induction by
the ‘‘sergeant-and-soldiers’’ effect due to insertion of small
fraction of chiral NDI-1 into the achiral DAN-1 stack. In such
a situation, one would expect appearance of a CT-band due to
donor–acceptor interaction. To check this we examined the
UV/vis spectra of DAN-1 mixed with 5% and 10% of NDI-1
but no CT-band was visible in either case (Fig. S6w). However,
one may argue that intrinsically weak and broad CT-bands
may not be detectable at very low concentration. However,
when we mixed the same amount of NDI-1 (5% and 10%) in
DAN-415 (see Fig. S5w for structure), we could trace a distinct
CT-band (Fig. S6w) clearly suggesting even at this concentration it
is detectable. Thus we eliminate the possibility of chiral induction
by ‘‘sergeant-and-soldiers’’ effect in the present study and propose
the following hypothesis (Scheme 2). Individual donor and
acceptor chromophores self-assemble in orthogonal fashion as
revealed from UV/vis studies. Such self-sorted NDI- and
DAN-homo-aggregates do not macroscopically differentiate
among them because both types of fibers are encased with
similar hydrocarbon chains. Thus clustering of individual
Fig. 2 (a) AFM height-images of NDI-1 in cyclohexane (conc. =
0.05 mM); inset: photo of the NDI-1 gel in cyclohexane at 0.55 wt%;
(b) zoomed region of the AFM-image shown in (a).
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 8934–8936 8935

View Article Online
3 L. J. Prins, P. Timmerman and D. N. Reinhoudt, J. Am. Chem. Soc.,
2001, 123, 10153; T. E. Kaiser, V. Stepanenko and F. Wurthner, J. Am.
¨
Chem. Soc., 2009, 131, 6719; A. Ajayaghosh, R. Varghese, S. Mahesh
and V. K. Praveen, Angew. Chem., Int. Ed., 2006, 45, 7729;
A. J. Wilson, J. V. Gestel, R. P. Sijbesma and E. W. Meijer, Chem.
Commun., 2006, 4404.
4 M. M. Green, S. Zanella, H. Gu, T. Sato, G. Gottorelli, F. K. Jha,
G. P. Spada, A. M. Schoevaars, B. Feringa and A. Teramoto, J. Am.
Chem. Soc., 1998, 120, 9810; A. P. H. J. Schenning, J. v. Herrikhuyzen,
P. Jonkheijm, Z. Chen, F. Wurthner and E. W. Meijer, J. Am. Chem.
¨
´
Soc., 2002, 124, 10252; S. J. George, Z. Tomovic, M. M. J. Smulders,
T. F. A. de Greef, P. E. L. G. Leclere, E. W. Meijer and A. P. H. J.
Schenning, Angew. Chem., Int. Ed., 2007, 46, 8206; M. P. Thompson
and R. P. Lemieux, J. Mater. Chem., 2007, 17, 5068; A. Ajayaghosh,
P. Chithra and R. Varghese, Angew. Chem., Int. Ed., 2007, 46, 230;
F. J. M. Hoeben, J. Zhang, C. C. Lee, M. J. Pouderoijen, M. Wolffs,
F. Wurthner, A. P. H. J. Schenning, E. W. Meijer and S. D. Feyter,
¨
Chem.–Eur. J., 2008, 14, 8579.
5 G. Celebre, G. D. Luca, M. Maiorino, F. Lemma, A. Ferrarini,
S. Pieraccini and G. P. Spada, J. Am. Chem. Soc., 2005, 127, 11736;
B. Isare, M. Linares, L. Zargarian, S. Fermandjian, M. Miura,
S. Motohashi, N. Vanthuyne, R. Lazzaroni and L. Bouteiller,
Scheme 2 Schematic of chiral-induction by helical neighbour.
Chem.–Eur. J., 2010, 16, 173; S. J. George, Z. Tomovic, A. P. H. J.
´
Schenning and E. W. Meijer, Chem. Commun., 2011, 47, 3451.
6 A. Tsuda, M. A. Alam, T. Harada, T. Yamaguchi, N. Ishii and
T. Aida, Angew. Chem., Int. Ed., 2007, 46, 8198; M. Wolffs,
S. J. George, Z. Tomovic, S. C. J. Meskers, A. P. H. J. Schenning
´
and E. W. Meijer, Angew. Chem., Int. Ed., 2007, 46, 8203;
fibers happens indiscriminately which results in chiral induction
from the helical NDI-1 fibers to the adjacent DAN-1 fibers
through hydrophobic interaction among the peripheral alkyl
chains. To support this hypothesis we studied self-assembly of
NDI-1 in the presence of another donor DAN-2 (Scheme 1)
which contains peripheral oligo-oxyethylene units instead of
hydrocarbon chains but in which the remaining part of the
molecule is structurally identical to DAN-1. Self-assembly of
DAN-2 alone16 was primarily investigated by solvent-dependent
UV/vis studies (Fig. S4w) which indicated J-type p-stacking.13
The CD-spectra of NDI-1 + DAN-2 in 95 : 5 MCH–CHCl3
was found to be fully matching to that for NDI-1 alone
(Fig. S7w). This clearly supports the hypothesis that indeed
the hydrophobic interaction among the peripheral alkyl chains
of the individual fibers is responsible for the observed chiral
induction for the (NDI-1 + DAN-1) pair which is not possible
for (NDI-1 + DAN-2) because of the immiscibility among
hydrocarbon and oligooxyethylene chains.17
S. Ghosh, X.-Q. Li, V. Stepanenko and F. Wurthner, Chem.–Eur. J.,
2008, 14, 11343.
¨
7 For general references on self-sorting see: A. Pal, S. Karthikeyan and
R. P. Sijbesma, J. Am. Chem. Soc., 2010, 132, 7842; J. v. Herrikhuyzen,
A. Syamakumari, A. P. H. J. Schenning and E. W. Meijer, J. Am. Chem.
Soc., 2004, 126, 10021; K. Mahata, M. L. Saha and M. Schmittel, J. Am.
Chem. Soc., 2010, 132, 15933; H. Northrop, Y.-R. Zheng, Ki-Whan Chi
and P. J. Stang, Acc. Chem. Res., 2009, 42, 1554; K. Sugiyasu,
S.-i. Kawano, N. Fujita and S. Shinkai, Chem. Mater., 2008, 20, 2863;
P. Mukhopadhyay, P. Y. Zavalij and L. Isaacs, J. Am. Chem. Soc., 2006,
128, 14093; A. Heeres, C. v. d. Pol, M. Stuart, A. Friggeri, B. L. Feringa
and J. v. Esch, J. Am. Chem. Soc., 2003, 125, 14252; J. R. Moffat and
D. K. Smith, Chem. Commun., 2009, 316.
8 M. R. Molla, A. Das and S. Ghosh, Chem.–Eur. J., 2010, 16, 10084.
9 S. V. Bhosale, C. H. Jani and S. J. Langford, Chem. Soc. Rev.,
2008, 37, 331; N. Sakai, J. Mareda, E. Vauthey and S. Matile,
Chem. Commun., 2010, 46, 4225; P. Mukhopadhyay, Y. Iwashita,
M. Shirakawa, S.-i. Kawano, N. Fujita and S. Shinkai, Angew.
Chem., Int. Ed., 2006, 45, 1592.
Herein we have probed H-bonding mediated assembly in
bis-amide functionalized chiral acceptor (NDI) and achiral
donor (DAN) molecules. Due to the mismatch in the distance
between the two amide groups, self-sorting was achieved
resulting in two types of homo-aggregated fibers. CD-experiments
revealed helical assembly for both the donor and acceptor
stacks although an asymmetric centre was present only in the
acceptor building block. This was attributed to induction of
helical-bias from the acceptor-stack to the donor-stack due to
mixing of the self-sorted fibers at macroscopic level by hydro-
phobic interaction among the peripheral alkyl chains. We are
exploring this as a general tool for spectroscopic visualization
of macroscopic-organization of chromophores and polymeric
mixtures which is highly relevant in bulk hetero-junction solar
cells.18
10 M. R. Molla and S. Ghosh, Chem. Mater., 2011, 23, 95.
11 For recent reviews or organogels, see: M.-O. M. Piepenbrock,
G. O. Lloyd, N. Clarke and J. W. Steed, Chem. Rev., 2010, 110, 1960;
A. Ajayaghosh and V. K. Praveen, Acc. Chem. Res., 2007, 40, 644;
P. Dastidar, Chem. Soc. Rev., 2008, 37, 2699; S. Banerjee, R. K. Das and
U. Maitra, J. Mater. Chem., 2009, 19, 6649; M. Suzuki and
K. Hanabusa, Chem. Soc. Rev., 2010, 39, 455.
12 H. Shao, T. Nguyen, N. C. Romano, D. A. Modarelli and
J. R. Parquette, J. Am. Chem. Soc., 2009, 131, 16374.
13 A. Das and S. Ghosh, Chem.–Eur. J., 2010, 16, 13622.
14 As observed in the melting curves, the absolute CD-intensity is slighly
different in two cases which probably arises due to difference in total
chromophore concentration in two separate experiments.
15 Recently we have demonstrated that the relative distances between the
two amide groups in this D–A pair allow effective CT-interaction. For
details, see: A. Das, M. R. Molla, A. Banerjee, A. Paul and S. Ghosh,
Chem.–Eur. J., 2011, 17, 6061.
16 Detail self-assembly studies with DAN-2 in organic and aqueous
medium are underway and will be published elsewhere.
17 For segregated chromophore assembly based on side-chain incompati-
bility, see: T. L. Benanti, P. Saejueng and D. Venkataraman,
Chem. Commun., 2007, 692; T. Sakurai, K. Shi, H. Sato,
K. Tashiro, A. Osuka, A. Saeki, S. Seki, S. Tagawa, S. Sasaki,
H. Masunaga, K. Osaka, M. Takata and T. Aida, J. Am. Chem.
Soc., 2008, 130, 13812.
Notes and references
1 E. Yishima, K. Maeda, H. Iida, Y. Furusho and K. Nagai, Chem.
Rev., 2009, 109, 6102; J. J. L. M. Cornelissen, A. E. Rowan, R. J.
M. Nolte and N. A. J. M. Sommerdijk, Chem. Rev., 2001, 101, 4039.
2 F. J. M. Hoeben, P. Jonkheijm, E. W. Meijer and A. P. H.
J. Schenning, Chem. Rev., 2005, 105, 1491; C. M. Drain,
A. Varotto and I. Radivojevic, Chem. Rev., 2009, 109, 1630.
18 J.-L. Bredas, J. E. Norton, J. Cornil and V. Coropceanu, Acc.
´
Chem. Res., 2009, 42, 1691.
c
8936 Chem. Commun., 2011, 47, 8934–8936
This journal is The Royal Society of Chemistry 2011