Angewandte
Chemie
monitoring the absorption changes at 399 nm during the
temperature-dependent studies of enantiomeric NDIs
(10À4 m) showed non-sigmoidal curves (Figure 1b). Fitting of
these cooling curves with a temperature-dependent nuclea-
tion–elongation model yielded an elongation temperature
(Te), the temperature at which polymerization begins, of
345.35 K.[18] As expected, probing the CD spectra also
revealed non-sigmoidal cooling curves for both NDIs.[17]
To study the self-recognizing/self-discrimination behavior
of RR-NDI and SS-NDI enantiomers during co-assembly,
temperature-dependent supramolecular polymerization of
enantiomeric mixtures (MCH/TCE, 99:1 (v/v), c = 2.5
10À5 m) with varying enantiomeric excess (ee) were carried
out under thermodynamic conditions and the spectral char-
acteristics of resulting assemblies were recorded at 298.15 K.
Absorption spectra of the self-assembled solutions hardly
showed any change, whereas the bisignated CD spectra
showed a monotonic increase in their intensity with an
increase in ee.[17] This is evident from the plot of g-value
against the ee values (%), which showed a linear change
(Figure 1c). This suggests that the assemblies constructed
from the racemic mixture of NDI monomers (zero ee) are
either self-recognition-driven homochiral stacks (supramolec-
ular conglomerates) or self-discrimination-driven heterochi-
ral stacks (supramolecular racemates). Although the similar-
ity between the absorption spectra of assemblies constructed
from various enantiomeric mixtures with that of enantiomer-
ically pure assemblies suggested the formation of homochiral
stacks, it was necessary to resort to more reliable probes to
confirm this observation. Interestingly, a plot of Te of various
assemblies versus corresponding ee values gives a V-shaped
curve, with a decrease in the elongation temperature with
decreasing enantiomeric excess. Minimum Te (313.73 K) was
attained when the ee is zero, hinting at the formation of
supramolecular conglomerates (Figure 1d).[17] Interestingly
the Te of a 1:1 mixture of RR-NDI and SS-NDI matched
exactly with that of the enantiomerically pure assemblies at
half the concentration, thus confirming the self-recognizing
stereoselective polymerization of the present system.[17]
A better understanding of the preference for this self-
recognition behavior was obtained through molecular mod-
elling studies. Homo- and heterochiral dimers of NDI
monomers (Figure 2) were considered as basic building
units of an assembly. The homochiral dimer is seen to be
favored over the heterochiral dimer by 15.1 kcalmolÀ1,
consistent with the spectroscopic studies. A difference in
chirality leads to significant differences in molecular packing,
resulting in an energy penalty for the heterodimer. Chiral
mismatch penalty offered by the optimized chiral core alone
(chopping off NDI) was accounted to be 5.1 kcalmolÀ1 at
B3LYP/6-31g(d).[17]
Next, we sought to construct orthogonal and mixed p-
n assemblies through the stereoselective supramolecular
polymerization of rationally designed D and A monomers.
In this regard, we have used similar chiral naphthol deriva-
tives (RR- or SS-DAN) as donor monomers.[9b] Although, the
DAN derivatives also self-assemble in MCH/TCE solvent
mixtures, the UV/Vis absorption spectral changes and CD
intensity are minimal upon stacking to follow the supra-
molecular polymerization process spectroscopically.[17] How-
ever, the bisignated CD signal in MCH/TCE solvent mixture
indeed showed the presence of excitonically coupled, helically
stacked DAN chromophores.[17] Transmission electron mi-
croscopy (TEM) studies further confirmed the self-assembly
of these DAN derivatives into one-dimensional nanostruc-
tures.[17] To realize the p-n assemblies, multi-component
supramolecular co-polymerization of enantiomeric mixtures
of D and A monomers were attempted in a similar fashion to
that of homo-polymerization described above (MCH/TCE,
99/1(v/v), c = 10À4 m, ÀdT/dt = 1 KminÀ1). Co-assembly of D
and A molecules to mixed p-n assemblies can be probed
spectroscopically by the appearance of a charge-transfer (CT)
absorption band.
Stereoselective
supramolecular
co-polymerization
between D and A monomer with similar chirality (RR-NDI
and RR-DAN) does indeed lead to alternately organized p-
n stacks, as evident from the appearance of strong CT band
with a maximum at 503 nm (Figure 3). Formation of mixed
CT stacks is further evident from the deep red color of the co-
assembled solutions.[17] Probing the intensity of the CT band
as a function of equivalents of DAN showed a saturation with
10 equivalents of DAN monomers. High-resolution mass
spectrometry of the co-assembled RR-NDI:RR-DAN
(10À4 m) solution showed a mass at 2055.2896 corresponding
to the 1:1 complex, reiterating the formation of co-facial CT
pairs.[17] Finally, TEM imaging of the CT solution showed the
formation of micrometer-long fibrous structures.[17] In con-
trast, supramolecularly polymerized solutions of D and A
monomers with opposite chirality (SS-NDI:RR-DAN) did
not show significant CT absorption, and the resulting sol-
Figure 2. a) and b) Side views of dimeric models of RR-NDI homodi-
mer and RR-NDI:SS-NDI heterodimer, respectively, calculated at the
DREIDING//B3LYP/6-311g(d,p) level of theory. Distances are specified
in . The swallow tail chains have been truncated to methyl groups to
decrease the computational cost.
Figure 3. Stereoselective supramolecular co-polymerization of D-A
monomers: a) Absorption spectra of co-assembled solutions of RR-
NDI or SS-NDI with RR-DAN, demonstrating the formation of alternate
and segregated D-A stacks, respectively. b) Plot of CT absorbance at
503 nm with increasing equivalents of RR-DAN.
Angew. Chem. Int. Ed. 2015, 54, 13053 –13057
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Narayan, Bhawani
