Hydrogen-bonding directed assembly and gelation of donor-acceptor chromophores: Supramolecular reorganization from a charge-transfer state to a self-sorted state

Article abstract of DOI:10.1002/chem.201100606

Gelation[1] of various functional π systems has been extensively studied due to the possibility of modulating their photophysical properties in the gel state. Donoracceptor (D-A) charge-transfer (CT) interactions have been utilized to generate many elegan

Full text of DOI:10.1002/chem.201100606

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DOI: 10.1002/chem.201100606
Hydrogen-Bonding Directed Assembly and Gelation of Donor–Acceptor
Chromophores: Supramolecular Reorganization from a Charge-Transfer
State to a Self-Sorted State
Anindita Das,[a] Mijanur Rahaman Molla,[a] Ambar Banerjee,[b] Ankan Paul,[b] and
Suhrit Ghosh*[a]
Gelation[1] of various func-
tional p systems[2] has been ex-
tensively studied due to the
possibility of modulating their
photophysical properties in the
gel state. Donor–acceptor (D–
A) charge-transfer (CT) inter-
actions have been utilized to
generate many elegant supra-
molecular materials, such as ro-
taxanes and catenanes,[3] syn-
thetic ion channels,[4] liquid
crystals,[5] folded oligomers,[6]
Scheme 1. Structure of various donor and acceptor building blocks.
polymers,[7] and organogels.[8]
Recently we have demonstrated
self-sorting[9] in a mixed assem-
bly of
a
bis
(amide)-functionalized dialkoxynaphthalene
interaction-mediated gelation of this D–A pair in a moder-
ately nonpolar solvent tetrachloroethylene (TCE) and also
demonstrate the serendipitous discovery that when the sol-
vent was changed to a less polar methylcyclohexane (MCH),
the CT gel switched over to thermodynamically stable self-
sorted gel within few hours.
Self-assembly of NDI-2+DAN-4 (1:1, total concentra-
tion=25 mm) was performed in TCE. The compounds were
soluble only at elevated temperature to generate a colorless
solution, which when cooled down to room temperature
produced a deep red gel (Figure 1a) clearly suggesting alter-
nate co-stacking of the donor and acceptor chromophores.
NDI-2 and DAN-4 individually also showed spontaneous ge-
lation under identical conditions (Figure 1).
(DAN) donor and a naphthalenediimide (NDI) acceptor
due to synergistic effect of hydrogen bonding and p stack-
ACHTUNGTRENNUNG
ing.[10a] Control experiments indicated that if the number of
methylene groups (n) between the DAN/NDI chromophore
and the amide functionality can be adjusted so that n-NDI=
(n+2)-DAN, then alternate co-stacking of the D–A chro-
mophore could also be achieved. However, in the previously
reported D–A pair (DAN-2+NDI-0),[10a] although a CT
band was visible in solution, no gelation could be observed,
probably due to highly rigid assembly of NDI chromophor-
e.[10b] Thus to achieve CT gelation we have explored a new
D–A pair DAN-4+NDI-2 (Scheme 1) in which the rigidity
of the acceptor unit is reduced due to the inclusion of addi-
tional methylene units. In this communication we reveal CT-
[a] A. Das, M. R. Molla, Dr. S. Ghosh
Indian Association for the Cultivation of Science
Polymer Science Unit
2A & 2B Raja S. C. Mullick Road, Kolkata 700032 (India)
Fax : (+91)33-2473-2805
E-mail: psusg2@iacs.res.in
[b] A. Banerjee, Dr. A. Paul
Indian Association for the Cultivation of Science
Raman Center for Atomic, Molecular and Optical Sciences
2A & 2B Raja S. C. Mullick Road, Kolkata 700032 (India)
Figure 1. Left: SEM picture of the CT-gel (1:1 mixture of NDI-2
+
DAN-4) in TCE; Right: Photographs of gels in TCE derived from a)
NDI-2+DAN-4 (1:1), b) NDI-2, c) DAN-4, d) NDI-2+DAN-2. Total ge-
lator concentration=25 mm in each case.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201100606.
Chem. Eur. J. 2011, 17, 6061 – 6066
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To confirm that distance between the two amide groups in
D and A indeed is critical for the CT interaction, we tested
the gelation of the NDI-2+DAN-2 pair and observed gela-
tion without appearance of any red color (Figure 1d), sug-
gesting lack of alternate co-stacking. The morphology of the
CT gel was examined by scanning electron microscopic
(SEM) studies that revealed the presence of micrometer
long entangled bundles of fibers (Figure 1, width of smallest
fiber=62 nm), which is typical of a good gelator. To under-
stand the influence of hydrogen bonding on the self-assem-
bly and gelation we examined the effect of MeOH, a hydro-
gen-bonding competing solvent, on the UV/Vis spectra of
the NDI-2+DAN-4 mixture (Figure 2).
Figure 3. Photograph of the gel (1:1 NDI-2+DAN-4 in MCH, total con-
centration=10 mm) at various time intervals.
Supporting Information) and found them to be almost iden-
tical. This simple experiment reveals the self-sorted assem-
bly of the individual chromophores in the modified yellow-
ish gel.
Furthermore we examined the effect of concentration and
temperature on this unprecedented reorganization phenom-
enon in the gel state (Figure 4). The variation of the scatter-
ing intensity[11] at 700 nm was monitored as a function of
time at two different temperatures (Figure 4a) and gelator
concentrations (Figure 4b). It can be clearly seen that in
both cases, the scattering intensity increased and then finally
saturated. It is evident that the process is much faster at the
lower concentration and higher temperature. This is expect-
ed as the re-organization of the chromophores from the CT
state to the self-sorted one will be facilitated at lower con-
Figure 2. a) Effect of MeOH on the CT band of NDI-2+DAN-4 (1:1,
total concentration=5 mm) in TCE. b) Gel (in TCE, conc. 25 mm) to sol
transition in presence of 7% (v/v) MeOH.
In TCE there is a broad absorption band (lmax =550 nm)
that is characteristic of CT interactions between the NDI
and DAN chromophores.[6] With an increasing amount of
MeOH, the intensity of the CT band gradually decreased
and finally disappeared at only 7.0% (v/v) MeOH/TCE
(Figure 2a). Consequently, when 7.0% MeOH was added to
the CT gel, the red color disappeared with concomitant
transformation of the gel to homogeneous solution (Fig-
ure 2b). This clearly indicates that hydrogen bonding is the
most-influential factor for the self-assembly and in its ab-
sence, the CT interaction alone is not adequate for gelation.
We further examined the gelation in an aliphatic hydro-
carbon solvent, MCH, in which hydrogen-bonding interac-
tions are expected to be even stronger and observed sponta-
neous gelation with an intense red color (Figure 3). Howev-
er, much to our surprise, unlike in TCE, in this case the red
color gradually faded away and completely disappeared in
about 5 h to produce a yellowish gel (Figure 3). Consequent-
ly with increasing time the CT band in the UV/Vis spectrum
disappeared with concomitant increase in the base-line in-
tensity (Figure S1a in the Supporting Information), which is
probably due to the increased opaqueness of the switched
gel. To examine the mode of assembly in the modified
yellow gel, we compared the absorption bands appearing
due to n–p* and p–p* transitions in the D–A mixture with
that of the mathematical sum of the aggregated spectra for
the individual D and A chromophores (Figure S1b in the
Figure 4. Variation of scattering intensity @ 700 nm in the UV absorption
spectra of the dynamic gel (DAN-4+NDI-2 (1:1) in MCH) at two differ-
ent temperatures and concentrations. Concentration of gelator=5 mm;
path-length of cuvette=1 mm.
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Table 1. Gelation data in TCE and MCH.
NDI-2 DAN-4
0.75 11.0
centration due to less rigidity of the medium and at higher
temperature due to more available energy.
To rationalize this supramolecular transformation, we ex-
amined the FT-IR spectra of the gel in MCH (Figure 5a).
NDI-2+DAN-4 (1:1)
CGC [mM]/TCE
CGC [mM]/MCH
Tg [8C][a]/TCE
3.0
[d]
0.91
75
2.0
31
65
–
72
The freshly prepared red gel exhibited a broad band due to
Tg [8C][a]/MCH
>105[b]
>39[c]
À1
À
the N H stretching of the amide functionality at 3309 cm ,
[a] Concentration of the gelator=20 mm. [b] Solvent started boiling
beyond this temperature. [c] Tg varied significantly with time, please see
the Supporting Information (Table S1) for details. [d] The data could not
be retrieved due to dynamic nature of the gel.
To explain stability of the CT gel in TCE, we examined
gelation of the NDI-2 and DAN-4 individually in both TCE
and MCH (Table 1). The critical gelation concentration
(CGC) and gel-melting temperature (Tg) data (Table 1) indi-
cated much stronger gelation for NDI-2 relative to DAN-4.
Poor gelation for the donor is attributed to the relatively
weaker hydrogen-bonding interaction, owing to the en-
hanced flexibility due to the presence of more number of
methylene units[10b,c] between the chromophore and amide
functional groups. The D–D self-assembly is further destabi-
lized by electrostatic repulsion among the electron-rich
DAN chromophores.[6] The gelation data in TCE also re-
vealed almost identical thermal stability and even lower
CGC for NDI-2 gel relative to those of the CT gel. This is
rather surprising, because one would expect the CT interac-
tion to be stronger than A–A p-stacking. Thus to gain fur-
ther insights about the structural nuances of the A–A homo-
aggregates and the charge-transfer (D–A) dual aggregates, a
preliminary quantum chemical investigation was conducted.
The hybrid density functional method, BHLYP was used in
conjunction with 6-31G (d, p) basis sets to optimize the mo-
lecular geometries for NDI-2 and NDI-2+DAN-4
(Figure 6).[14,15] The optimized structure for CT-assembly ex-
Figure 5. a) Selected region of the IR spectra of the CT-gel (NDI-2+
DAN-4, concentration=10 mm) in MCH as a function of time. b) Rheo-
logical data in MCH for the gel (4.0 wt%) before (triangles) and after re-
organization (circles).
which gradually shifted towards lower values and finally sa-
turated at 3271 cmÀ1 after 6 h. Note that by this time the red
gel also has been switched over to the yellow one (Figure 3).
It is also interesting to note (Figure 5a) that the final
stretching frequency of the reorganized gel almost exactly
matches the mathematical sum[12] of the IR spectrum of the
individual donor and acceptor gels (indicated as “sum spec-
trum” in Figure 5a), which confirms that reorganization
eventually leads to self-sorting.
We also compared the rheological properties of the CT
and self-sorted gels in a stress-amplitude sweep measure-
ment (Figure 5b). For both samples, the storage modulus
(G’) is initially higher than the loss modulus (G’’), which is
typical of a gel phase.[13] With increasing stress, both G’ and
G’’ remained invariant up to a certain point and then deviat-
ed and crossed each other. This crossover point is consid-
ered as the yield stress (sy) ,which is a measure of the ro-
bustness of the gel. The value of sy for the self-sorted gel
was found to be 30.6 Pa, which is almost 30-fold higher than
that of the CT gel (1.17 Pa); this result supports the findings
of the IR experiment that stronger hydrogen bonding is ach-
ieved due to the reorganization.
Figure 6. Optimized molecular geometries of DAN-4+NDI-2 (top) and
NDI-2 homo-aggregate (bottom) at BHLYP/6-31G (d, p) level of theory.
Some of the alkyl and phenyl hydrogen atoms are not shown for clarity.
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S. Ghosh et al.
hibits intermolecular hydrogen bonding between the amide
functionalities of the donor and acceptor chromophores
(each 2.08 ꢁ), along with a CT interaction. For assembly of
NDI-2 alone, similar hydrogen-bonding interactions were
observed but with significantly shorter bond lengths(1.99
and 1.96 ꢁ). These results clearly suggest that the strength
of the hydrogen-bonding interaction is greater in case of A–
A homoaggregates compared to the D–A alternate stack-
ing.[16]
smaller number of methylene units.[10b] Of course the dis-
tance between the two amide groups in this D–A pair also
was found to be comparable. The CGC and Tg (at 20 mm
concentration) of DAN-3 in TCE were found to be 3.2 mm
and 598C, respectively, which indeed proved superior self-
assembly of DAN-3 compared to DAN-4 in TCE. As ex-
pected NDI-2+DAN-3 (1:1) produced red CT gel in TCE
(total concentration 10 mm), but unlike previous case (NDI-
2+DAN-4 in TCE) after 4 h the color completely disap-
peared suggesting reorganization from CT state to the self-
sorted state (Figure S5 in the Supporting Information).[19]
This observation provides further evidence that if the indi-
vidual self-assembly of both the donor and acceptor chro-
mophores are reasonably strong, eventually the self-sorted
state is achieved even though the kinetically controlled CT
state is initially formed.
We assume that it is just a coincidence that the strength
of (stronger hydrogen bonding+ A–A p-stacking) becomes
comparable to that of (weaker hydrogen bonding+D–A CT
interaction) and thus the thermal stability of the two types
of gels (pure NDI-2 and NDI-2+DAN-4) are found to be
almost identical (Table 1). However, it is the poor self-as-
sembly of the donor gel that favours the formation of pure
CT gel[17] instead of acceptor gel, because in that case maxi-
mum number of amide functionalities linked to both D and
A chromophores can be engaged in hydrogen bonding. As
we used 1:1 mixture of D and A chromophores, if a mixture
of CT and A gels is formed, then many of the D molecules
will not take part in aggregate formation, because they
cannot form a stable self-assembled structure on their own
in TCE, as evident from its poor gelation ability (Table 1).
Next we examined the gelation data in MCH, in which re-
organization was observed.[18] The CGC value for DAN-4
was found to be 2 mm (Table 1), which is almost four times
lower than that in TCE (Table 1), and the Tg (20 mm) was
found to be 658C, which is more than twice as much as that
in TCE (Table 1). Similarly for NDI-2 also the gelation data
indicates stronger self-assembly compared to that in TCE;
this fact can be attributed to stronger hydrogen-bonding in-
teraction in hydrocarbon solvent. However, in case of D–A
alternate co-stacking we do not anticipate strengthening of
hydrogen bonding to such extent, because solvent polarity
can only alter the electronic parameters but not the geomet-
rical constraint that was observed in their energy minimized
geometry (Figure 6).
This is evident from the observed Tg of the CT gel in
MCH (Table 1, and Table S1 in the Supporting Informa-
tion). It is interesting to note that soon after the formation
of the CT gel, the Tg was found to be 398C, which is even
lower than that in TCE (728C). This can be attributed to
the dynamic nature of the system, which inhibits precisely
defined self-assembly with long-range order at the early
stage of gelation. However, gradually as the gel switched
over to the stable self-sorted state, the Tg increased signifi-
cantly (Table S1 in the Supporting Information).
It is evident from the forgone discussion that self-sorted
state is prefered over the alternate co-assembly if the stabili-
ty of the individual self-assembled structure is higher, as is
the case in MCH. To examine whether this can also be es-
tablished by structural variation of the chromophores in-
stead of solvent variation we examined gelation of NDI-2
with a different donor DAN-3[10c] (Scheme 1). The rationale
behind choosing the new donor is that we anticipated homo-
aggregates of DAN-3 to be stronger than DAN-4, due to
In conclusion, we have demonstrated spontaneous self-as-
sembly and gelation in a bisACTHNURGTNE(NUG amide)-functionalized donor–
acceptor chromophore mixture by the synergistic effect of
hydrogen bonding and CT interactions/p-stacking. Two dis-
tinctly different modes of assembly (alternate D–A co-as-
sembly versus self-sorting) could be observed depending on
marginal variation in structure of the chromophores and/or
solvent polarity. When the structure of the chromophore
and nature of the solvent provided opportunity for stronger
hydrogen bonding, the system adopted the particular mode
of assembly in which the effect of hydrogen bonding could
be fully realized. Thus the initially formed, kinetically con-
trolled, CT-state (NDI-2+DAN-4 in MCH, NDI-2+DAN-3
in both MCH and TCE) re-organized to more stable self-
sorted state, as there was no geometrical constraint for hy-
drogen bonding. On the other hand, in a moderately non-
AHCTUNGTRENNUNG
polar chlorinated solvent like TCE,[20] as the strength of the
hydrogen-bonding interaction was inherently reduced com-
pared to that in MCH, the CT interaction plays a significant
role in deciding the mode of self-assembly and thus long-
lived CT state was achieved. Of course none of these possi-
bilities arose when the distance between two amide func-
tionalities were very different for the D and A chromophore
and in that case (NDI-2+DAN-2) self-sorting was always
observed.
Acknowledgements
We thank Department of Science and Technology (DST), New Delhi,
India, for financial support (Project No: SR/FT/CS-039/2008). A.D.
thanks CSIR (India) for a research fellowship.
Keywords: charge transfer · organogels · self-assembly ·
self-sorting · supramolecular chemistry
[1] For recent reviews on organogel see: a) S. Banerjee, R. K. Das, U.
Maitra, J. Mater. Chem. 2009, 19, 6649; b) M. Suzuki, K. Hanabusa,
Chem. Soc. Rev. 2010, 39, 455.
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[2] a) H. Engelkamp, S. Middelbeek, R. J. M. Nolte, Science 1999, 284,
785; b) F. S. Schoonbeek, J. H. van Esch, B. Wegewijs, D. B. A. Rep,
M. P. De Haas, T. M. Klapwijk, R. M. Kellogg, B. L. Feringa, Angew.
Chem. 1999, 111, 1486; Angew. Chem. Int. Ed. 1999, 38, 1393; c) S.-i.
Kawano, N. Fujita, S. Shinkai, Chem. Eur. J. 2005, 11, 4735; d) A.
Ajayaghosh, V. K. Praveen, Acc. Chem. Res. 2007, 40, 644 and refer-
ences therein; e) S. K. Samanta, A. Pal, S. Bhattacharya, Langmuir
2009, 25, 8567; f) J. Puigmartꢂ-Luis, V. Laukhin, ꢃ. P. del Pino, J. V.
Gancedo, C. Rovira, E. Laukhina, D. B. Amabilino, Angew. Chem.
2007, 119, 242; Angew. Chem. Int. Ed. 2007, 46, 238; g) H. Shao, T.
Nguyen, N. C. Romano, D. A. Modarelli, J. R. Parquette, J. Am.
Chem. Soc. 2009, 131, 16374; h) K. Sugiyasu, N. Fujita, S. Shinkai,
Angew. Chem. 2004, 116, 1249; Angew. Chem. Int. Ed. 2004, 43,
1229; i) S. Ghosh, X.-Q. Li, V. Stepanenko, F. Wꢄrthner, Chem. Eur.
J. 2008, 14, 11343; j) S. Yagai, M. Ishii, T. Karatsu, A. Kitamura,
Angew. Chem. 2007, 119, 8151; Angew. Chem. Int. Ed. 2007, 46,
8005.
[3] a) S. A. Vignon, T. Jarrosson, T. Iijima, H-R. Tseng, J. K. M. Sand-
ers, J. F. Stoddart, J. Am. Chem. Soc. 2004, 126, 9884; b) J. Y. Ortho-
land, A. M. Z. Slawin, N. Spencer, J. F. Stoddart, D. J. Williams,
Angew. Chem. 1989, 101, 1402; Angew. Chem. Int. Ed. Engl. 1989,
28, 1394; c) H. Y. Au-Yeung, G. D. Pantos, J. K. M. Sanders, Proc.
Natl. Acad. Sci. USA 2009, 106, 10466; d) H. Y. Au-Yeung, G. D.
Pantos, J. K. M. Sanders, J. Am. Chem. Soc. 2009, 131, 16030.
[4] P. Talukdar, G. Bollot, J. Mareda, N. Sakai, S. Matile, J. Am. Chem.
Soc. 2005, 127, 6528.
3867; e) M. K. Khan, P. R. Sundararajan, J. Phys. Chem. B 2008, 112,
4223; f) A. Wu, L. Isaacs, J. Am. Chem. Soc. 2003, 125, 4831. For
self-sorted assembly of donor-acceptor chromophores see: g) K. Su-
giyasu, S-i. Kawano, N. Fujita, S. Shinkai, Chem. Mater. 2008, 20,
2863; h) J. van Herrikhuyzen, A. Syamakumari, A. P. H. J. Schen-
ning, E. W. Meijer, J. Am. Chem. Soc. 2004, 126, 10021; i) K.
Mahata, M. L. Saha, M. Schmittel, J. Am. Chem. Soc. 2010, 132,
15933.
[10] a) M. R. Molla, A. Das, S. Ghosh, Chem. Eur. J. 2010, 16, 10084;
b) M. R. Molla, S. Ghosh, Chem. Mater. 2011, 23, 95; c) A. Das, S.
Ghosh, Chem. Eur. J. 2010, 16, 13622.
[11] The baseline intensity of the absorption spectra was found to in-
crease with supramolecular reorganization (Figure S3 in the Sup-
porting Information), probably due to opaque nature of the self-
sorted gel. This caused difficulty in monitoring the CT-band intensi-
ty as a function of time and thus we followed the increase in the
baseline intensity itself at 700 nm, which arises due to scattering to
approximately quantify the transformation rate under various condi-
tions with a assumption that the rate of switching is proportional to
the change in scattering intensity.
[12] Solvent dependent (CHCl3 and MCH) variation in IR spectrum of
the individual chromophores clearly indicated hydrogen bonding;
see Figure S4 in the Supporting Information for detail.
[13] S. R. Raghavan, B. H. Cipriano in Molecular Gels: Materials with
Self-Assembled Fibrillar Networks (Eds.: R. G. Weiss, P. Terech),
Springer, The Netherlands, 2006, Chapter 8.
[5] a) S. Mahlstedt, D. Janietz, A. Stracke, J. H. Wendorff, Chem.
Commun. 2000, 15; b) W. Pisula, M. Kastler, D. Wasserfallen,
J. W. F. Robertson, F. Nolde, C. Kohl, K. Mꢄllen, Angew. Chem.
2006, 118, 834; Angew. Chem. Int. Ed. 2006, 45, 819; c) L. Y. Park,
D. G. Hamilton, E. A. McGehee, K. A. McMenimen, J. Am. Chem.
Soc. 2003, 125, 10586; d) A. Okabe, T. Fukushima, K. Ariga, T.
Aida, Angew. Chem. 2002, 114, 3564; Angew. Chem. Int. Ed. 2002,
41, 3414; e) L. Schmidt-Mende, A. Fechtenkçtter, K. Mꢄllen, E.
Moons, R. H. Friend, J. D. MacKenzie, Science 2001, 293, 1119.
[6] a) R. S. Lokey, B. L. Iverson, Nature 1995, 375, 303; b) A. J. Zych,
B. L. Iverson, J. Am. Chem. Soc. 2000, 122, 8898; c) G. J. Gabriel, B.
L Iverson, J. Am. Chem. Soc. 2002, 124, 15174; d) G. J. Gabriel, S.
Sorey, B. L Iverson, J. Am. Chem. Soc. 2005, 127, 2637; e) V. J. Brad-
ford, B. L. Iverson, J. Am. Chem. Soc. 2008, 130, 1517.
[14] For a recent report on related computational studies see: S. De, D.
Koley, S. Ramakrishnan, Macromolecules 2010, 43, 3183.
[15] The present calculation is limited by the fact that we have not in-
cluded solvents Nevertheless, we believe this provides valuable sup-
porting information regarding the geometrical constraints for hydro-
gen bonding in case of CT gel formation due to miss-match in dis-
tance. As here we are only concerned about structural aspects and
not the energetic ones, we assume exclusion of solvents does not
have much influence on our conclusions. Moreover, all the gelating
solvents have very low dielectric constant, hence it is expected that
a gas-phase structure would be very similar. More detailed calcula-
tions with extended D–A stacks and including solvent molecules are
underway and will be published in a subsequent full paper.
[16] We were also curious to examine the energy minimized structures
for the D–A pair NDI-2+DAN-2, in which we always observed
self-sorting (Figure S7 in the Supporting Information). In this case
we observe a substantially longer hydrogen bond (2.19 ꢁ) between
one of the two amide functionalities, while the other one was of
very similar length to that observed in the NDI-2+DAN-4. An al-
ternative, almost iso-energetic, minimum was also found that lies
only 0.35 kcalmolÀ1 above the previously discussed structure and in
this structure only a single intermolecular hydrogen bond (2.09 ꢁ)
between the two interacting moieties could be noted. However,
under no circumstances could we arrive at a minimum in which both
the amide bonds could be involved in strong hydrogen bonding for
the alternate stack of NDI-2 and DAN-2. This corroborates well
with the experimental observation, that for this D–A pair we always
observed self-sorting and no alternate co-stacking. Alternatively, the
distance in D–D aggregates also display slightly longer intermolecu-
lar hydrogen bond lengths than their A–A counterparts, but general-
ly slightly shorter than the corresponding D–A species (Figure S8 in
the Supporting Information).
[7] a) S. Ghosh, S. Ramakrishnan, Angew. Chem. 2005, 117, 5577;
Angew. Chem. Int. Ed. 2005, 44, 5441; b) S. Ghosh, S. Ramakrishn-
an, Angew. Chem. 2004, 116, 3326; Angew. Chem. Int. Ed. 2004, 43,
3264; c) S. Ghosh, S. Ramakrishnan, Macromolecules 2005, 38, 676;
d) S. De, S. Ramakrishnan, Macromolecules 2009, 42, 8599.
[8] a) U. Maitra, P. V. Kumar, N. Chandra, L. J. D’Souza, M. D. Prasan-
na, A. R. Raju, Chem. Commun. 1999, 595; b) R. K. Das, S.
Banerjee, G. Raffy, A. D. Guerzo, J-P. Desvergne, U. Maitra, J.
Mater. Chem. 2010, 20, 7227; c) P. Babu, N. M. Sangeetha, P. Vijay-
kumar, U. Maitra, K. Rissanen, A. R. Raju, Chem. Eur. J. 2003, 9,
1922; d) A. Friggeri, O. Gronwald, K. J. C. van Bommel, S. Shinkai,
D. N. Reinhoudt, J. Am. Chem. Soc. 2002, 124, 10754; e) K. V. Rao,
K. Jayaramulu, T. K. Maji, S. J. George, Angew. Chem. 2010, 122,
4314; Angew. Chem. Int. Ed. 2010, 49, 4218; f) A. F. M. Kilbinger,
R. H. Grubbs, Angew. Chem. 2002, 114, 1633; Angew. Chem. Int.
Ed. 2002, 41, 1563; g) B. G. Bag, G. C. Maity, S. K. Dinda, Org. Lett.
2006, 8, 5457; h) J. R. Moffat, D. K. Smith, Chem. Commun. 2008,
2248; i) D. Rizkov, J. Gun, O. Lev, R. Sicsic, A. Melman, Langmuir
2005, 21, 12130; j) P. Mukhopadhyay, Y. Iwashita, M. Shirakawa, S-i.
Kawano, N. Fujita, S. Shinkai, Angew. Chem. 2006, 118, 1622;
Angew. Chem. Int. Ed. 2006, 45, 1592; k) T. Ishi-I, R. Iguchi, E.
Snip, M. Ikeda, S. Shinkai, Langmuir 2001, 17, 5825.
[9] For general references on self-sorted assemblies see: a) B. H. North-
rop, Y-R Zheng, Ki-Whan Chi, P. J. Stang, Acc. Chem. Res. 2009, 42,
1554; b) A. Pal, S. Karthikeyan, R. P. Sijbesma, J. Am. Chem. Soc.
2010, 132, 7842; c) A. Heeres, C. v. d. Pol, M. Stuart, A. Friggeri,
B. L. Feringa, J. v. Esch, J. Am. Chem. Soc. 2003, 125, 14252; d) Y.
Rudzevich, V. Rudzevich, F. Klautzsch, C. A. Schalley, V. Bçhmer,
Angew. Chem. 2009, 121, 3925; Angew. Chem. Int. Ed. 2009, 48,
[17] To confirm that the observed CT gel in TCE is not contaminated
with individual self-assembly of A and D, we examined the effect of
cooling rate on the CT band of the gel (Figure S2 in the Supporting
Information); no notable differences were observed. This indirectly
supports the hypothesis that only the CT state is formed during ge-
lation. If there were other types of the self-assembly, the relative
mole fractions of various types of structures are expected to vary as
a function of cooling speed. However, we realize that this is only in-
direct evidence and does not completely rule out the possibility of
co-existence of the CT and self-sorted states. More detail studies to
understand these aspects are underway in our laboratory.
Chem. Eur. J. 2011, 17, 6061 – 6066
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
6065

S. Ghosh et al.
[18] a) Molecular Switches (Ed.: B. L. Feringa), Wiley-VCH, Weinheim,
2001; b) G. P. Spada, S. Lena, S. Masiero, S. Pieraccini, M. Surin, P
Samorꢅ, Adv. Mater. 2008, 20, 2433; c) J. E. Betancourt, M. M-Hidal-
go, V. Gubala, J. M. Rivera, J. Am. Chem. Soc. 2009, 131, 3186;
d) A. Lohr, F. Wꢄrthner, Angew. Chem. 2008, 120, 1252; Angew.
Chem. Int. Ed. 2008, 47, 1232.
[19] In this case reorganization also happens in MCH and other hydro-
carbon solvents. Detailed kinetics studies are underway to gain fur-
ther insight.
[20] The stable CT state was also observed in other non-polar chlorinat-
ed solvents (Table S2 in the Supporting Information).
Received: February 24, 2011
Published online: May 3, 2011
6066
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ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 6061 – 6066