Ultrasound stimulated nucleation and growth of a dye assembly into extended gel nanostructures

Article abstract of DOI:10.1002/chem.201301539

A squaraine dye functionalized with a bulky trialkoxy phenyl moiety through a flexible diamide linkage (GA-SQ) capable of undergoing self-assembly has been synthesized and fully characterized. Rapid cooling of a hot solution of GA-SQ to 0 °C results in se

Full text of DOI:10.1002/chem.201301539

FULL PAPER
Hybrid Materials
J. M. Malicka, A. Sandeep, F. Monti,
E. Bandini, M. Gazzano, C. Ranjith,
V. K. Praveen,* A. Ajayaghosh,*
N. Armaroli* .................. &&&& — &&&&
Ultrasound Stimulated Nucleation and
Growth of a Dye Assembly into
Extended Gel Nanostructures
Sound in action: A squaraine deriva-
tive gelates organic solvents upon
application of ultrasound and addition
of a miniscule amount of SWCNTs
considerably influences the self-assem-
bly process (see scheme).
Ultrasound-induced self-assembly
and gelation …
… is a hitherto unknown property
of a squarine derivative (GA-SQ).
In their Full Paper on page &ff.,
V. K. Praveen, A. Ajayaghosh,
N. Armaroli et al. report detailed
studies of the application of ultra-
sound to stimulate the nucleation
and growth of extended nanostruc-
tures by altering the conditions for
supersaturation. The influence of
nanoscale substrates (SWCNTs) on
the self-assembly of GA-SQ
through a heterogeneous nuclea-
tion process is also demonstrated.
This strategy could pave the way
to the rational design of nanostruc-
tured composites by cheap and
facile methods.
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DOI: 10.1002/chem.201301539
Ultrasound Stimulated Nucleation and Growth of a Dye Assembly into
Extended Gel Nanostructures
Joanna M. Malicka,[a] Anjamkudy Sandeep,[b] Filippo Monti,[a] Elisa Bandini,[a]
Massimo Gazzano,[a] Choorikkat Ranjith,[c] Vakayil K. Praveen,*[a]
Ayyappanpillai Ajayaghosh,*[b] and Nicola Armaroli*[a]
Abstract: A squaraine dye functional-
ized with a bulky trialkoxy phenyl
moiety through a flexible diamide link-
age (GA-SQ) capable of undergoing
self-assembly has been synthesized and
fully characterized. Rapid cooling of a
hot solution of GA-SQ to 08C results
in self-assembled precipitates consist-
ing of two types of nanostructures,
rings and ill-defined short fibers. The
application of ultrasound modifies the
conditions for the supersaturation-
mediated nucleation, generating only
one kind of nuclei and prompting the
formation of crystalline fibrous struc-
tures, inducing gelation of solvent mol-
ecules. The unique self-assembling be-
havior of GA-SQ under ultrasound
stimulus has been investigated in detail
by using absorption, emission, FT-IR,
XRD, SEM, AFM and TEM techni-
ques. These studies reveal a nucleation
growth mechanism of the self-assem-
bled material, an aspect rarely scruti-
nized in the area of sonication-induced
gelation. Furthermore, in order to
probe the effects of nanoscale sub-
strates on the sonication-induced self-
assembly, minuscule amount of
a
single-walled carbon nanotubes was
added, which leads to acceleration of
the self-assembly through a heteroge-
neous nucleation process that ultimate-
ly affords a supramolecular gel with
nanotape-like morphology. This study
demonstrates that self-assembly of
functional dyes can be judiciously ma-
nipulated by an external stimulus and
can be further controlled by the addi-
tion of carbon nanotubes.
Keywords: gels · hybrid materials ·
nucleation and growth · sonication ·
squaraine
Introduction
shape, function, and uses.[1–3] Typically, the methods em-
ployed in soft matter engineering are mild, in order to pre-
serve the integrity of the resulting self-assembled materials
in solution. In this regard, the discovery of sonication-in-
duced formation of self-assembled nanostructures and con-
sequent gelation of solvent molecules[4] was rather unexpect-
ed, as the ultrasonic waves were believed to disrupt the self-
assembly in solution.[5] However, the interaction of ultrason-
ic sound waves with soft matter is known in medical diagno-
sis and transdermal drug delivery.[6] Gels, akin to water, are
excellent media to transmit ultrasound energy, with the ad-
vantage of being more viscous than water and thus easier to
handle during the treatment. Another area of research in
which ultrasound has contributed to substantial progress is
the crystallization of molecules and materials.[7] Ultrasound
helps the primary nucleation process by modifying the con-
ditions for supersaturation, thereby reducing the induction
period between the attainment of supersaturation and the
commencement of nucleation and crystallization. In addi-
tion, the excellent mixing conditions maintained by sonica-
tion-induced shockwaves have been shown to reduce the ag-
gregation of crystals and allow the crystals to grow larger by
controlling local nucleus population. In spite of the lessons
from these totally different areas of research, supramolecu-
lar chemists overlooked these findings until Naota and
Koori first observed the sol-to-gel transition of a dinuclear
palladium complex that switches its conformation under the
Self-assembly of functional organic dyes to form extended
nanostructures is a topic of current interest due to their pos-
sible applications in electronic and optoelectronic devi-
ces.[1,2] Continuous research in this field has developed a
number of methods to construct nanostructures with desired
[a] Dr. J. M. Malicka,+ Dr. F. Monti, Dr. E. Bandini, Dr. M. Gazzano,
Dr. V. K. Praveen, Dr. N. Armaroli
Istituto per la Sintesi Organica e la Fotoreattivitꢁ
Consiglio Nazionale delle Ricerche (ISOF-CNR)
Via Gobetti 101, 40129 Bologna (Italy)
Fax : (+39)051–6399844
E-mail: nicola.armaroli@isof.cnr.it
praveen.karthikeyan@isof.cnr.it
[b] A. Sandeep,+ Prof. Dr. A. Ajayaghosh
Photosciences and Photonics Group
Chemical Sciences and Technology Division
National Institute for Interdisciplinary Science and Technology
(NIIST), CSIR, 695019 Trivandrum (India)
Fax: (+91)471–2491712, 249018
E-mail: ajayaghosh62@gmail.com
[c] Dr. C. Ranjith
Dipartimento di Chimica Organica
Universitꢁ degli Studi di Milano
Via Golgi 19, 20133 Milano (Italy)
[+] These authors contributed equally to this work.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201301539.
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influence of ultrasound to generate stabilizing intermolecu-
lar p-stacking interactions.[4]
The last few years have witnessed a surge of activities in
the field of sonication-induced gelation.[8] A variety of mole-
cules, such as metal complexes,[9] peptides[10,11] organic
dyes,[12] and hydrogen-bonded gelators[10–13] have been utiliz-
ed to accomplish sonogelation. Nevertheless, this approach
has rarely been exploited for the development of functional
gel-phase materials,[9g,10d,11d,e] especially those composed of
organic dyes.[12] For instance, Yi, Huang, and co-workers
have reported a family of cholesterol-appended naphthali-
mide molecules showing gelation[12a,b] and morphological
changes accompanied with gel-to-gel transition[12c] under the
influence of ultrasonic sound waves. Sonogelation of quina-
cridone derivatives functionalized with urea[12d] or cholester-
ol[12e,f] moieties have been demonstrated by Zhang, Wang,
and co-workers. Moreover, Jung et al., have observed that
sonication could modulate the aggregation as well as gelati-
on properties of a porphyrin–palladium(II) complex.[12g]
Squaraines are an interesting class of polymethine-type
dyes with resonance-stabilized zwitterionic structure. The
unique combination of photostability and optical properties
that span from the visible to the near-infrared region make
squaraine dyes attractive for various applications in the
areas of biology, chemistry, and material science.[14] Aggre-
gation properties of squaraines have been extensively stud-
ied in mixed solvents, organized media, and in the presence
of metal ions.[14b–e] However, there are only very few reports
on the formation of extended supramolecular nanostructures
of functional squaraine derivatives.[15] The first example has
come from the research group of Whitten, which reported
the self-assembly of a cholesterol-tethered squaraine deriva-
tive, forming gel fiber networks incorporating organic sol-
vents.[15a] Some of us have reported a cation-induced expres-
sion of supramolecular chirality and morphological transi-
tion from spheres to helices of a chiral tripodal squaraine
dye.[15c,d] In another study, it was demonstrated that metal
complexation promotes one-dimensional nanostructure for-
mation of a tailor-made squaraine dye, yielding an enhance-
ment of molar absorptivity due to the quantum confinement
effect.[15e] Evaporation-induced self-assembly of a simple
squaraine dye into micrometer-long aligned nanowires has
been reported by Zhang, Lee, and co-workers.[15f] Very re-
cently, Mayerhçffer and Wꢂrthner elucidated the thermody-
namic and mechanistic aspects of a hydrogen-bonded assem-
bly of a squaraine dye that forms extended fiber like aggre-
gates.[15h]
Scheme 1. Synthesis of GA-SQ gelator. Reagents and conditions: i) BOP
reagent, triethylamine, dry CH2Cl2, RT, 3 h, 80%; ii) dry 2-propanol,
tributyl orthoformate, 908C, 20 h, 37%.
of sonication-induced gelation of molecules.[12b,13f] Further-
more, the addition of a minuscule amount of single-walled
carbon nanotubes (SWCNTs) as a nanoscale substrate has
been found to considerably influence the morphological
properties of the GA-SQ sonogel by promoting a heteroge-
neous nucleation process.
Results and Discussion
The strategy adopted for the synthesis of squaraine gelator
GA-SQ from compounds 1–4 is shown in Scheme 1. Com-
pounds 1, 2, and 4 were prepared as per reported proce-
dures.[17] The preparation of compound 1 was achieved in
three steps (see Scheme S1 in the Supporting Information).
The first step was the reaction of methyl 3,4,5-trihydroxy-
benzoate with 1-bromododecane in the presence of
K2CO3.[17a] The ester group of the resulting compound was
hydrolyzed using KOH in ethanol to yield the corresponding
carboxylic acid derivative.[17a] The compound thus obtained
was converted to the aminoethylbenzamide derivative 1 by
reacting with excess ethylenediamine in the presence of ben-
zotriazol-1-yloxytris(dimethylamino)phosphonium
hexa-
fluorophosphate (BOP), an amide coupling reagent.[17b] Al-
kylation of N-methylaniline with ethyl 4-bromobutyrate in
the presence of NaOAc and I2, followed by hydrolysis using
5% KOH, afforded N-methyl-N-(carboxypropyl)aniline (2;
Scheme S2 in the Supporting Information).[17c,d] 3-[4-(N,N-
Dibutylamino)phenyl]-4-hydroxycyclobut-3-ene-1,2-dione
(4) was prepared by reacting squaryl chloride[17e] with N,N-
dibutyl aniline (Scheme S3 in the Supporting Informa-
tion).[17f]
Construction of self-assembled structures of squaraine
dyes continues to be important in the context of their re-
newed interest in light-energy harvesting and organic photo-
voltaics.[16] Herein we report the unique properties of the
squaraine derivative GA-SQ (Scheme 1), which forms ill-de-
fined assembly under normal conditions, but forms extended
nanostructures and gels upon application of ultrasound.
With the help of spectroscopic and microscopic techniques,
we demonstrate a nucleation and growth mechanism for the
observed self-assembly, an aspect rarely studied in the area
The amide-bond formation between aminoethylbenza-
mide derivative 1 and aniline 2 by using BOP in the pres-
ence of triethylamine gave 3 in 80% yield (Scheme 1). Re-
action of 3 with the semisquaraine derivative 4 in a 1:1 stoi-
chiometry, under reflux conditions in 2-propanol with tribu-
tyl orthoformate as the catalyst, resulted in the formation of
the squaraine gelator GA-SQ (Scheme 1). The crude prod-
uct first isolated by filtration was further purified by column
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V. K. Praveen, A. Ajayaghosh, N. Armaroli et al.
chromatography on neutral alumina and provided the pure
product as a greenish blue solid in 37% yield.
The FT-IR spectrum of GA-SQ in CDCl3 showed an in-
tense band at 1589 cmꢀ1 characteristic of pseudoaromatic
squarate moiety (Figure S1 in the Supporting Informa-
tion).[17f] The amide carbonyl group of GA-SQ clearly dis-
cerned as a sharp band at 1660 cmꢀ1 (Figure S1 in the Sup-
1
porting Information). In the H NMR spectrum of GA-SQ
in CDCl3 (Figure S2 in the Supporting Information), a sin-
glet at d=3.04 ppm corresponds to the resonance of -NCH3
protons. The signal at d=3.42 ppm is ascribed to the methyl-
ene protons of -CH2NCH3 and -CH2NCH2-, the resonance
of which overlaps to appear as a multiplet. The signals at
d=6.65, 6.70, 8.24, and 8.30 ppm are assigned to the protons
of the diphenyl squaraine unit. The peak at d=7.08 ppm is
attributed to the aromatic protons of the alkoxy-substituted
gallic acid moiety. 13C NMR (Figure S3 in the Supporting In-
formation) and gradient heteronuclear single quantum cor-
relation (gHSQC) (Figure S4 in the Supporting Information)
spectra in CDCl3 are in agreement with the structure of
GA-SQ. The MALDI-TOF mass spectrum exhibits the
1175.27 [M]+ ion peak along with 1197.92 [M+Na]+ and
1214.37 [M+K]+ peaks (Figure S5 in the Supporting Infor-
mation). The absorption and emission spectra of GA-SQ in
chloroform display a single band with the lmax at 635 and
650 nm respectively with a relatively small Stokes shift
Dnst =412 cmꢀ1 (Figure S6 in the Supporting Information).
These optical features are very similar to the standard squar-
aine dyes derived from the reaction of squaric acid with
N,N-dialkyl anilines.[17f]
Gel formation is normally obtained by heating a required
amount of gelator in a suitable solvent and subsequent cool-
ing of the resultant homogenous solution either to room
temperature or below.[18a,b] The driving force for the forma-
tion of gel nanostructures is considered to be a supersatura-
tion-mediated nucleation and growth.[18] The gelator GA-
SQ,[19] owing to the presence of hydrophobic dodecyl chains,
zwitterionic squaraine dye moiety, and amide functional
groups, exhibits a multifaceted solubility character. In rela-
tively polar solvents like CHCl3, CH2Cl2, and THF GA-SQ
is easily soluble and therefore, unsuitable for gelation stud-
ies. In aliphatic and aromatic solvents it is either insoluble
or becomes soluble by heating at high temperature, but pre-
cipitates quickly on cooling. Interestingly, in protic polar sol-
vents, such as aliphatic alcohols, GA-SQ becomes soluble at
high temperatures, while upon cooling it exhibits two types
of behavior. When a solution of GA-SQ in n-butanol is
heated to 708C and then let spontaneously cool down to
room temperature, no apparent changes were observed.
However, on rapid cooling to 08C by inserting the sample in
an ice bath, GA-SQ forms aggregates and the process is ac-
companied by a color change of the solution from cyan to
dark violet (Figure 1 inset and Figure S7 in the Supporting
Information). In order to visualize the morphology of these
aggregates we carried out detailed electron microscopic
studies. The aggregates of GA-SQ were cast on freshly
cleaved mica and carbon-coated copper grids, air-dried, and
Figure 1. a) SEM and b) TEM images of GA-SQ self-assembled aggre-
gates obtained by rapid cooling of a hot n-butanol solution (4 mm) to
08C. The inset shows the photograph of the corresponding self-assembled
precipitates formed in n-butanol, for a color picture see Figure S7 in the
Supporting Information.
imaged by scanning electron microscopy (SEM) and trans-
mission electron microscopy (TEM), respectively. SEM
images show the formation of nanorings and ill-defined fi-
brous structures (Figure 1a). TEM analysis confirms that
the aggregates entail nanorings surrounded by networks of
short fibers that are a few micrometers in length (Fig-
ure 1b).
Even though the aggregates of GA-SQ failed to entrap
solvent molecules and did not lead to gelation, the presence
of polymorphic nanostructures was quite encouraging. Previ-
ous studies have demonstrated that regulating the formation
of nuclei, its distribution and growth at the early stages of
the self-assembling process would modify the kinetic path-
ways of supramolecular gel formation and consequent mac-
roscopic properties.[18b–d] In this regard, application of ultra-
sound has gained a lot of attention. Ultrasound is known to
stimulate the primary nucleation process at a lower supersa-
turation level, which otherwise would not occur with other
methods.[7,8] Sonication also helps to break the seeds and
disperse it uniformly in solution. In addition, the cavitation
and streaming effect of the ultrasound waves ensure the
bulk-phase mass transfer of solute to the surface of growing
crystal, thereby promoting the secondary nucleation process.
Based on this knowledge, we speculated that the application
of ultrasound to a solution of GA-SQ might enhance the
formation and growth of nuclei responsible for the forma-
tion of nanorings and fibers, by altering the condition for su-
persaturation. If the secondary nucleation process in the
edges or sides of the developing nanostructure created per-
manent junction zones, an interconnected network structure
capable of trapping solvent molecules by capillary force and
van der Waals interactions might eventually arise.
To examine this possibility, detailed studies on the aggre-
gation and gelation were carried out. Homogeneous solu-
tions of various concentrations of GA-SQ in n-butanol were
prepared at 708C and then subjected to ultrasonication for
2 min (0.23 Wcmꢀ2, 37 kHz) in a bath maintained at room
temperature. As a control experiment, another batch of the
same solutions was cooled to room temperature without ap-
plying sonication. In the lower concentration region (0.01–
0.5 mm), the solutions cooled to room temperature in the
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Figure 2. Photographs of the GA-SQ solutions in n-butanol cooled to
room temperature a) in absence and b) presence of ultrasonic irradiation.
Numbers correspond to the following concentration: 1=0.01 mm, 2=
0.1 mm, 3=0.5 mm, 4=1 mm, 5=2 mm and 6=4 mm.
presence and absence of sonication show no difference
(Figure 2). Interestingly, at a higher concentration range
(1–4 mm), the sonication treatment gradually changed the
color of the solutions from cyan to dark violet, whereas con-
centrated solutions cooled in the absence of sonication do
not show any color variation (Figure 2). This finding under-
pins the key role of sonication, likely related to the modified
conditions of supersaturation that promote the aggregation
of molecules. Though at a concentration of 1 mm some ag-
gregate formation is observed, the solution turned into a
Figure 3. Absorption spectra of GA-SQ solutions after sonication a)
lower and b) higher concentration region (path length, l=0.05 mm).
Arrows indicates relative changes in absorption with increase in concen-
tration.
semi gel at
a
slightly higher concentration of 2 mm
sorption spectroscopy (Figure 3a and Figure S10 in the Sup-
porting Information). However, for concentrations 1–4 mm,
for which aggregation/gelation was observed (Figure 2b),
absorption spectra reveal the presence of a new blue-shifted
broad band with lmax at 540 nm and a tail in the region be-
tween 675 and 800 nm, along with a reduced absorption of
the monomeric squaraine at 640 nm. As the concentration
increases, the band corresponding to the monomeric squar-
aine decreases in intensity with a concomitant increase in
the absorption band at shorter wavelength with a tail in the
lower energy region (Figure 3b). The observed broad spec-
trum suggests strong association of squaraine units in the ag-
gregates of GA-SQ in a H-type fashion. The absorption fea-
tures of self-assembled GA-SQ closely matches the spectral
features of extended H-aggregates of squaraine amphiphiles
and gelators.[15a,20]
In light of the above results, it is important to gain insight
into the morphology of the self-assembled GA-SQ obtained
upon sonication. To this end, extensive microscopy studies
were performed on n-butanol aggregates and gel cast on
suitable substrates. SEM studies of a gel (4 mm) cast on
freshly cleaved mica surface reveal the formation of entan-
gled fibrous networks characteristic of self-assembled gels
(Figure 4a). The width of the fibers varies from 50 to
400 nm and the length is extended to several micrometers.
TEM micrographs of a diluted gel (2 mm) and aggregates
(1 mm) cast on carbon-coated copper grids are shown in Fig-
ure 4b and Figure S11 in the Supporting Information, re-
(Figure 2). A more stable gel is obtained at a concentration
4 mm, which prevented the flow of solvent upon tilting the
sample vial sidewise or upside down (Figure 2). Sonication-
induced gelation is also observed in other aliphatic alcoholic
solvents, such as ethanol and n-propanol, with slightly
higher critical gelator concentrations of 4.5 and 4.25 mm, re-
spectively.
Insight on aggregation and gelation process can be ob-
tained from the characteristic absorption properties of the
squaraine moiety that changes upon aggregation.[14,15]
However, the high extinction coefficient of squaraines
(~105 mꢀ1 cmꢀ1) together with the high concentrations used
for these studies (mm) make it challenging to apply this
technique. To circumvent these difficulties and enable a rea-
sonable analysis, we used a special type of demountable
flow cuvette with adjustable path length (for details see the
Experimental Section and Figure S8 in the Supporting Infor-
mation). UV/Vis absorption spectra of the investigated sam-
ples are presented in Figure 3 and Figures S9 and S10 in the
Supporting Information. In the case of samples not exposed
to sonication, the absorption spectra show a sharp band with
lmax at 640 nm, characteristic of the monomeric squaraine
dye; its intensity changes proportionally to the concentra-
tion (Figure S9a in the Supporting Information), as visible
by the naked-eye observation (Figure 2a). In the case of so-
nicated samples, solutions in the range 0.01–0.5 mm do not
undergo aggregation (Figure 2b), which is confirmed by ab-
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energy (frequency) when involved in hydrogen-bonding.[22]
The IR spectrum of GA-SQ in CDCl3 (4 mm, Figure S13a in
the Supporting Information) displays two bands at 3448 and
3342 cmꢀ1 that can be assigned as amide A bands. The ali-
phatic and aromatic amide carbonyl (amide I) appear as a
single band at 1660 cmꢀ1 and the amide II band at
1527 cmꢀ1. By contrast, the IR spectrum of GA-SQ in the
xerogel state exhibits a completely different behavior (Fig-
ure S13b in the Supporting Information). Thus, the amide A
bands appear at 3301 and 3265 cmꢀ1, with a shift of 147 and
77 cmꢀ1 respectively. The amide I band splits into two bands
displayed at 1650 and 1620 cmꢀ1, while the amide II shifts to
1539 cmꢀ1. The significant shifts of amide A and I bands to
the lower frequency and the amide II band to the higher fre-
quency region are clear indication of strong hydrogen bond-
ing in the gel state.[22] Another clear change is the shift of
symmetric and asymmetric CH2 vibrations of GA-SQ in gel
state to 2849 and 2918 cmꢀ1 in comparison to those in
CDCl3, where they are observed at 2856 and 2928 cmꢀ1 re-
spectively (Figure S14 in the Supporting Information). This
observation implies that the alkyl chains of GA-SQ in the
gel state are in all trans configuration and interdigitated.[23]
Similar observations were made in the case of GA-SQ self-
assembled precipitates formed by rapid cooling to 08C (Fig-
ure S15 in the Supporting Information).
Figure 4. a) SEM and b) TEM images of sonication induced self-assembly
of GA-SQ in n-butanol. The concentration used for SEM and TEM stud-
ies are 4 and 2 mm respectively.
spectively. The width and length of the thinnest fiber that
can be distinguished is approximately 30 nm and few micro-
meters, respectively. The branching of fibers and the tan-
gling of thin fibers to thicker fibrous structures can be also
visualized from the TEM micrographs. The atomic force mi-
croscopy (AFM) studies on a dilute n-butanol gel (2 mm)
cast on a freshly cleaved mica surface are also in agreement
with the fiber-like morphology of the gel assembly, as ob-
served with SEM and TEM. The AFM image of isolated
fibers and its corresponding section analysis are shown in
Figure S12 in the Supporting Information.
The morphological studies reveal that the formation of
well-developed fibrillar structures of micrometer length and
its entangling into network structures (Figure 4) are the key
factors responsible for the gelation of GA-SQ. This result is
in contrast with that obtained for the precipitated aggregates
(heat and rapid cool method), for which the formation of
thin fibers of few micrometers in length and ring-like struc-
ture are observed (Figure 1). The difference in morphology
suggests that sonication substantially favors the formation of
only one kind of nuclei by controlling the primary nuclea-
tion process and also helps the nuclei to grow further as
one-dimensional fibers by facilitating the secondary nuclea-
tion events. The increased rates of these nucleation process-
es are a reflection of sonication-mediated efficient mass
transfer from the bulk of the solution toward the growing
nuclei to form the fibrous structure. However, in the case of
rapid cooling, the slow rate of the primary and secondary
nucleation processes possibly hampers the complete growth
of the self-assembled nanostructures of GA-SQ, preventing
further development into a network like morphology.
To investigate the involvement of the squaraine moiety in
the observed aggregation properties of GA-SQ, tempera-
ture-dependent electronic absorption studies were per-
The above results prompted us to attempt a rationaliza-
tion of the noncovalent interactions responsible for the self-
assembly of GA-SQ. It is known that hydrogen bonding and
van der Waals interactions play a crucial role in the self-as-
sembly of molecules leading to gelation of solvents[2b,e,18,21]
and the best technique to probe this is FT-IR spectroscopy.
A secondary amide group is characterized by three distinct
ꢀ
IR absorptions corresponding to symmetric N H stretch
(amide A band), C=O stretch (amide I band) and a combi-
Figure 5. a) Temperature-dependent absorption spectra of GA-SQ n-bu-
tanol gel (2 mm) prepared by sonication (path length, l=0.05 mm). b)
The plot of fraction of aggregates (aagg) versus temperature fitted within
elongation (solid line) and nucleation (dashed line) regimes.
ꢀ
ꢀ
nation of the N H deformation and of the C N stretch
(amide II band).[22] These IR absorptions show change in
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formed, using the 2 mm n-butanol solution of GA-SQ that
responds to sonication (Figure 5a). As the temperature is in-
creased from 20 to 568C, the broad absorption in the wave-
length region from 470 to 570 nm and the tail band observed
between 675–800 nm exhibit a decrease in intensity with a
concomitant increase in the absorption at 640 nm. These
transitions are accompanied by two isosbestic points at 575
and 675 nm. The absorption spectra at higher temperatures
are found to be similar to those in CHCl3, in which GA-SQ
mainly exists as a monomeric species. The absorption blue
shift of the broad band with respect to the monomer sug-
gests that the squaraine units are involved in H-type aggre-
gation.[15c–e,h,20b] On the other hand, the presence of the red-
shifted tail extended to the near IR region suggests the con-
tribution of an intermolecular charge-transfer interaction in
the aggregate formation.[20a] The formation of H-type aggre-
gates was further confirmed by variable-temperature emis-
sion studies (Figure S16 in the Supporting Information). At
low temperatures, the fluorescence of GA-SQ self-assembly
is found to be completely quenched, which is a characteristic
feature of H-type aggregation.[15c–e,h] The emission band is
restored at higher temperatures due to conversion of self-as-
sembled GA-SQ into monomeric species.
The melting curve obtained from the temperature-de-
pendent absorption studies is shown in Figure 5b. In order
to demonstrate the involvement of nucleation and growth
process in the observed self-assembly of GA-SQ, we have
attempted to analyze the curve on the basis of the model
proposed by van der Schoot, Schenning and Meijer.[15h,24]
According to this model, the fraction of aggregated mole-
cules (aagg) in the elongation and nucleation regime can be
defined by the following Equations (1) and (2) respectively.
Figure 6. XRD patterns of GA-SQ a) xerogel film of n-butanol gel
(4 mm) prepared by sonication and b) film of self-assembled aggregates
obtained by rapid cooling of a hot n-butanol solution (4 mm) to 08C.
Inset shows schematic representation of possible lamellar packing dia-
gram of GA-SQ in the gel state.
two different sets based on the ratio of the d values. The
first group shows reflections with d spacing of 4.17 (100),
2.08 (200), 1.39 (300), 1.04 (400), 0.69 (600), 0.59 nm (700).
This implies that, in the gel state, molecules maintain a la-
mellar structure (Figure 6a inset) with the interlayer dis-
tance corresponding to 4.17 nm.[23b,25] This distance is found
to be in close match with optimized molecular length of
GA-SQ (Figure S18 in the Supporting Information). A
second group of peaks with d values of 1.74 (010), 0.86
(020), 0.43 (040), 0.29 (060) and 0.22 nm (080) indicate later-
al packing of the molecules in the lamellar arrangement.
The intense and broad reflection at 2q=20.68 (040) is found
to consist of two shoulder peaks with d spacing values of
0.41 and 0.39 nm and can be ascribed to stacking of aromat-
ic units assisted by secondary amide hydrogen bonding[26]
and crystalline packing of interdigitated alkyl chains respec-
tively.[23a,25d] Next, we have investigated the XRD of GA-SQ
self-assembled precipitates formed by heating and rapid
cooling to 08C. The diffractions peaks obtained from a thin
film of precipitates are found to be less sharp in comparison
to that of xerogel film (Figure 6b). Other noticeable differ-
ences are the absence of reflections corresponding to (100)
and (010) planes and the presence of a diffuse halo in the
wide-angle region due to distribution of reflections. The
result of XRD studies suggests unregulated arrangement of
molecules in the polymorphic nanostructures observed for
self-assembled precipitates.
ꢀhe
aagg ¼ aSATð1 ꢀ exp½
ðT ꢀ TeÞꢁ
ð1Þ
RTe2
he
RTe2
aagg ¼ Ka1=3½ð2=3Kaꢀ1=3 ꢀ 1Þ
ðT ꢀ TeÞꢁ
ð2Þ
The melting curve can be fitted with such elongation and
nucleation functions (Figure 5b and Figure S17 in the Sup-
porting Information). In the elongation regime the fit pro-
vides the molecular enthalpy released due to noncovalent
interactions, he (ꢀ194 kJmolꢀ1) and elongation temperature,
Te (320.5 K). The other parameters are the universal gas
constant (R), absolute temperature (T) and aSAT, a parame-
ter necessary to equate aagg/aSAT to unity. Fit of the nuclea-
tion regime gave the equilibrium constant Ka (5.3ꢃ10ꢀ3) for
the activation step at Te. The satisfactory fit of the melting
curve suggests the sonication-induced gelation involves a nu-
cleation and elongation process.[15h,19f,24]
To look closely into the packing of the molecules in the
self-assembled structures of GA-SQ that are formed under
different conditions, we have performed X-ray diffraction
(XRD) studies. Figure 6a shows the XRD pattern of the
GA-SQ xerogel film, which exhibits several well-resolved
peaks characteristics of long-range crystalline ordering of
the molecules. These peaks can be grouped and indexed in
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In principle, homogenous nucleation[27] from the interior
of a solution is difficult to occur.[18b] Generally, all nucleation
events are heterogeneous in nature and always involve some
kinds of substrates. Heterogeneous nucleation effectively de-
creases the energy barrier for nucleation.[18b] For example, in
the case of crystallization of molecules, seeding of crystal
enhances the crystallization process. Similarly, in the case of
polymers[28] and proteins,[29] the presence of substrates, such
as carbon nanotubes (CNTs) induce the crystallization proc-
ess. CNTs are good choice as a nanoscale substrate also in
supramolecular self-assembly, because of their propensity to
facilitate the epitaxial growth of interacting molecules.[30,31]
Furthermore, the involvement of CNTs in the nucleation
and growth process of a supramolecular soft material can
give rise to interesting properties. With this objective, we de-
cided to study the influence of SWCNTs on the sonication
induced self-assembling behavior of GA-SQ.
A very small amount of SWCNTs (ca. 0.1 mg per 0.5 mL)
was added to solutions of GA-SQ with concentration ranges
from 0.01 to 4 mm. These samples were first heated at 708C
and then subjected to sonication for 5 min. Control experi-
ments were also performed by sonicating another batch of
the solutions prepared without adding SWCNTs. After soni-
cation, the 0.01 mm solution of GA-SQ with SWCNTs
showed no color change, likewise the control sample. When
the concentration of GA-SQ was raised to 0.1 mm, the solu-
tion with SWCNTs changed its color indicating an aggrega-
tion process; at a concentration of 0.5 mm the formation of
dark violet aggregates was observed (Figure 7a). In contrast,
sonication was not found to affect the GA-SQ solution in
the reference samples without SWCNTs (Figure 2b). This
observation indicates that the synergic effect of sonication
and SWCNTs accelerates the aggregate formation of GA-
SQ even at low concentrations. This underpins SWCNT-
mediated heterogeneous nucleation and growth of GA-
SQ.[28,29,31] Interestingly, after sonication of 1 and 2 mm solu-
tions with SWCNTs, formation of a semigel and a stable gel,
respectively, were observed (Figure 7a). In the absence of
SWCNTs (see above) the semigel and gel of GA-SQ were
observed for concentrations of 2 and 4 mm, respectively
(Figure 2b). This means that the critical gelation concentra-
tion is reduced upon addition of a miniscule amount of
SWCNTs. Apart from this, addition of SWCNTs is found to
increase the stability of the composite gel as inferred from
its gel melting temperature (Tgel) of 538C, whereas the pure
GA-SQ gel showed a lower Tgel of 478C. Further evidence
to the role of ultrasonic waves in the SWCNT-mediated het-
erogeneous nucleation of GA-SQ solutions was obtained
from a control experiment in which the same solutions were
heated to 708C and left cooled spontaneously till room tem-
perature. Without application of ultrasound, none of them
were able to undergo aggregation (Figure S19 in the Sup-
porting Information).
The presence and involvement of SWCNTs in the ob-
served self-assembly of GA-SQ can be easily probed by op-
tical spectroscopy, through inspection of the peculiar absorp-
tion features of SWCNTs. Thus, the absorption spectrum of
an aggregated solution of GA-SQ (0.5 mm), prepared by
adding SWCNTs to a hot solution of GA-SQ followed by
sonication, was recorded. The absorption spectrum shows
the formation of the band corresponding to aggregated spe-
cies of GA-SQ (lmax =540 nm, Figure 7b). In addition, the
spectrum clearly exhibits well-defined absorption bands in
the near IR region, indicating van Hove singularities of dis-
persed SWCNTs (Figure 7c).[31,32] The absorptions corre-
sponding to the electronic transitions of semiconducting
nanotubes are observed between 750–900 nm (S22) and
1000–1500 nm (S11). The absorption features of metallic
nanotubes are overlapped with the strong absorption of
squaraines in the 400–700 nm region. The spectrum of
SWCNTs dispersed in GA-SQ aggregates was then com-
pared with that of SWCNTs dispersed in solution of sodium
dodecylbenzenesulfonate (SDBS) in D2O (Figure 7c). Even
though the effect of solvent cannot be completely ruled out,
a clear redshift and broadening of the first-order semicon-
ducting electronic transition S11 (1000–1500 nm) in the GA-
SQ aggregates is observable, most likely due to electronic
interactions of the aromatic units of GA-SQ with
SWCNTs.[32,33] Further evidence for the interaction between
GA-SQ and SWCNTs is obtained from FT-IR studies. The
ꢀ
aromatic C H bending mode of GA-SQ observed between
700–900 cmꢀ1 decreases its intensity in the presence of
SWCNTs, indicating the strong interaction of aromatic moi-
eties of GA-SQ with SWCNTs (Figure S20 in the Support-
ing Information).[34]
After having observed the impact of SWCNTs on the ag-
gregation and optical properties of the GA-SQ self-assem-
bly, we investigated their effect on the nanoscale morpholo-
gy. The TEM studies of GA-SQ aggregates formed at lower
Figure 7. a) Photograph showing the changes of GA-SQ n-butanol solu-
tions containing 0.1 mg of SWCNTs after ultrasound irradiation for
5 min. Numbers correspond to the following concentration: i=0.5, ii=1
and iii=2 mm. b) Absorption spectrum of sonication induced aggregates
of GA-SQ formed in the presence of 0.1 mg of SWCNTs (n-butanol, c=
0.5 mm, path length, l=0.5 mm). c) The absorption spectra of the same
sample in the range 800–1400 nm showing van Hove singularities of
SWCNTs. For a reference the absorption spectrum of SWCNTs dispersed
in SDBS is also shown.
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Conclusion
We have shown that a suitably designed squaraine dye
equipped with dodecyloxy galloyl diamide unit (GA-SQ),
undergoes ultrasound-triggered self-assembly and gelation, a
hitherto unknown property for this class of molecules. The
application of ultrasound is found to modify the conditions
for the supersaturation-mediated nucleation and growth of
GA-SQ self-assembly, which is apparent from the reluctance
of the squaraine derivative to undergo aggregation by heat-
ing and slow cooling without sonication. On the other hand,
self-assembly induced by just heating and rapid cooling to
08C results in the formation of precipitates consisting of pol-
ymorphic nanostructures. Extended morphological analysis
reveals that sonication prompts the formation of only one
kind of nuclei leading to extended crystalline fibrous struc-
tures. Interestingly, the sonication of GA-SQ solutions con-
taining a minuscule amount of SWCNTs promotes self-as-
sembling in a lower concentration regime, at which mere
sonication has no effect. The presence of the nanoscale sub-
strate yields a nanotape-like morphology, which highlights
the importance of nucleation events in shaping the final self-
assembled material.[18b–d,34] For the sonication induced gelati-
on of GA-SQ, nucleation and growth is mainly determined
by the crystalline packing of hydrophobic alkyl chains and
hydrogen bonding, which promote the p–p stacking of the
aromatic units. While, in the presence of SWCNTs, the ad-
sorption of molecules on the heterogeneous surface, leads to
a nucleation process assisted by p-stacking and hydrogen
bonding. In this scenario, the establishment of a crystalline
arrangement of the hydrophobic alkyl chains is more unlike-
ly.
Figure 8. Effect of SWCNTs on the morphology of GA-SQ assembly.
TEM images of GA-SQ a) aggregates (0. 5 mm) and b) gel (2 mm). c)
XRD patterns of the xerogel film (2 mm). The samples were prepared by
adding a miniscule amount of SWCNTs to a hot solution of GA-SQ in n-
butanol followed by sonication.
concentration (0.5 mm) showed the formation of micrometer
long supramolecular nanotapes along with nanoring-like
structures (Figure 8a). In some cases, the ring structures are
found to open as part of nanotapes and their bundles (Fig-
ure 8a inset and Figure S21 in the Supporting Information),
which indicate that they can be nanotape precursors. Inter-
estingly, an almost complete transformation of nanorings is
evident from the TEM micrograph of the GA-SQ gel
(2 mm) that displays mainly nanotape-like morphology with
bundles (Figure 8b). It should be noted that in the absence
of SWCNTs, GA-SQ gel showed a rather different fiber-like
morphology (Figure 4b). The substantial difference in mor-
phology observed in the presence of very small amounts of
SWCNTs indicates their influence in the self-assembling
process of the squaraine molecule.
The strategy presented in this study paves the way to the
rational design of nanostructured composites made of tech-
nologically relevant components such as functional dyes
(squaraines) and carbon allotropes (fullerenes, CNTs, and
graphene), with cheap and facile methods. The application
of these intimately mixed composites as active materials in
organic electronic devices for light-energy conversion and
related application is a subject worth of investigation.
Experimental Section
The observed difference in morphology is further con-
firmed by XRD studies. In contrast to the XRD profile of
sonication-induced gel (Figure 6a), the gel formed in the
presence of SWCNTs showed a XRD profile consisting of
only few peaks (Figure 8c). It displayed two diffraction
peaks at 2q=20.68 and 10.288 with d spacing of 0.43 and
0.86 nm respectively. The peak at 2q=20.688 is found to be
broad and contains a small shoulder peak presumably due
to the presence of reflection corresponding to stacking of ar-
omatic units assisted by hydrogen bonding.[26] The presence
of a diffuse halo in the wide-angle region indicates alkyl
chains are packed in a disordered manner.[26c]
Synthesis—general procedures: Unless otherwise stated, all starting ma-
terials and reagents were purchased from commercial suppliers and used
without further purification. CH2Cl2 was dried over CaH2 under N2 and
freshly distilled prior to use. Triethylamine was distilled in the presence
of KOH and stored over KOH for further use. The reactions were moni-
tored using thin layer chromatography on silica gel 60 F254 (0.2 mm;
Merck) or Al2O3 (0.2 mm; Merck). Visualization was accomplished using
UV light or phosphomolybdic acid solution. Preparative column chroma-
tography was performed on glass columns of different sizes hand packed
with silica gel 60 (particle size 0.040–0.063 mm, Merck) or activated neu-
tral Al2O3 (Sigma–Aldrich).
Synthesis—characterization techniques: NMR spectra were obtained on
a Varian Mercury-400 spectrometer equipped with a 5 mm probe. Chemi-
cal shifts are reported in ppm using tetramethylsilane (TMS) (dH =
0 ppm) or the solvent residual signal ([D6]DMSO: dH =2.49 ppm, dC =
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V. K. Praveen, A. Ajayaghosh, N. Armaroli et al.
39.50 ppm) (CDCl3: dC =77.00 ppm) as an internal reference. The reso-
nance multiplicity is described as s (singlet), d (doublet), t (triplet), m
(multiplet) and brs (broad singlet). IR spectra were recorded on a
Perkin–Elmer Spectrum BX spectrometer using KBr sealed cell and
plate for solution and film samples respectively. Matrix-assisted laser de-
sorption ionization time-off-flight (MALDI-TOF) mass spectra were ob-
tained on a Shimadzu AXIMA-CFR PLUS spectrometer using a-cyano-
4-hydroxycinnamic acid as a matrix.
119.65, 128.82, 132.76, 133.33, 140.88, 152.97, 153.75, 153.99, 168.06,
173.18, 183.42, 186.32, 187.46 ppm; MALDI-TOF-MS (matrix: a-cyano-4-
hydroxycinnamic acid): m/z calcd for C74H118N4O7 [M]+: 1174.90; found:
1175.27; m/z calcd for [M+Na]+: 1197.89; found: 1197.92;
m/z calcd [M+K]+: 1214.00; found: 1214.37.
Optical measurements: Electronic absorption spectra were recorded on a
Lambda 950 UV/VIS/NIR spectrophotometer (Perkin–Elmer). Photolu-
minescence spectra in chloroform were recorded on Edinburgh FLS920
spectrometer and temperature dependent photoluminescence was record-
ed on SPEX-Flourolog F112X spectrofluorimeter using front face geome-
try. Optical studies in chloroform were carried out in a 1 cm quartz cuv-
ette. Absorption studies of the aggregates/gel in n-butanol were carried
out in a demountable flow cuvette (Figure S8 in the Supporting Informa-
tion) with an option to vary path lengths using spacers of different thick-
ness (FLAB-50-UV-01, GL Sciences, Japan). Fluorescence studies in the
gel state were performed in a 1 mm quartz cuvette. All the solvents
(CHCl3, n-butanol) are spectroscopic grade (99.8%) and were used as re-
ceived.
Synthesis of 3: Compounds 1 (1.79 g, 2.5 mmol), 2 (0.48 mg, 2.5 mmol),
and BOP (1.1 g, 2.5 mmol) were dissolved in dry CH2Cl2 (50 mL). Trie-
thylamine (0.39 mL, 2.75 mmol) was added and the reaction mixture stir-
red at room temperature for 4 h under N2 atmosphere. The progress of
the reaction was monitored using TLC. After the completion of the reac-
tion, the organic layer was washed three times with brine, dried over an-
hydrous Na2SO4, and filtered, and the filtrate was dried under reduced
pressure. The residue was then subjected to column chromatography
over silica gel (chloroform/methanol=8:2 v/v) to afford compound 3 as a
white solid in 80% yield. FT-IR (CH2Cl2): nmax =3444, 2926, 2854, 1658,
1600, 1582, 1494, 1468, 1379, 1331, 1114 cmꢀ1 1H NMR (400 MHz,
;
Gelation studies: A weighed amount of the GA-SQ in an appropriate sol-
vent (0.5 mL) was placed in a sealed glass vial (1 cm diameter) and dis-
solved by heating at 708C. The solution was then subjected to sonication
(0.23 Wcmꢀ2, 37 kHz, bath temperature 258C, Elmasonic S10H) for
2 min. The sonication-induced gelation was considered successful if no
sample flow was observed upon tilting the sample vial upside down.
CDCl3, TMS, 258C): d=0.88 (m, 9H; -CH3), 1.26–1.42 (m, 48H; -CH2-),
1.45 (m, 6H; -OCH2CH2CH2-), 1.71–1.81 (m, 6H; -OCH2CH2-), 1.90 (m,
2H; -NCH2CH2CH2N-), 2.22 (t, J=7.2 Hz, 2H; -CH2C=O), 2.85 (s, 3H;
-NCH3), 3.13 (t, J=7.2 Hz, 2H; -CH2NCH3), 3.46–3.54 (m, 4H;
-NHCH2CH2NH-), 3.99 (m, 6H; -OCH2-), 6.12 (brs 1H; -NH), 6.68 (m,
3H; phenyl-H), 7.00 (s, 2H; phenyl-H), 7.04 (brs, 1H; -NH), 7.19 ppm
(m, 2H; phenyl-H); 13C NMR (100 MHz, CDCl3, 258C): d=14.09, 22.68,
22.80, 26.07, 26.10, 29.35, 29.37, 29.41, 29.58, 29.64, 29.68, 29.70, 29.72,
29.73, 30.31, 31.91, 33.54, 38.05, 39.85, 41.55, 51.77, 69.17, 73.45, 105.47,
112.27, 116.35, 128.69, 129.17, 141.00, 149.32, 153.03, 168.01, 174.29 ppm.
To study the effect of carbon nanotubes, SWCNTs purchased from
Unidym Inc (purified HiPco-SWCNTs, batch number: P2150) were used
as received. A miniscule amount of SWCNTs (ca. 0.1 mg per 0.5 mL) was
added quickly to a solution of GA-SQ in n-butanol prepared by heating
at 708C and the mixture was subjected to sonication for 5 min.
Synthesis of 3-(4-(N,N-Dibutylamino)phenyl)-4-hydroxy-cyclobut-3-ene-
1,2-dione (4): N,N-Dibutyl aniline (4.2 g, 13.2 mmol) in dry benzene
(50 mL) was added to a 100 mL round-bottomed flask containing squaryl
chloride[17e,f] (2.0 g, 13.2 mmol). After refluxing for 6 h, the reaction mix-
ture was cooled and poured into crushed ice under stirring. The organic
layer was separated, washed repeatedly with water (200 mL), dried over
anhydrous Na2SO4, and evaporated under vacuum. The residue dissolved
in a mixture of acetic acid (50 mL), water (50 mL), and 2n HCl (4 mL)
was refluxed for 2 h. After cooling to room temperature, the solid prod-
uct was isolated by filtration, washed with diethyl ether and dried. FT-IR
Scanning electron microscopy: SEM images were taken on a Zeiss EVO
18 cryo SEM Special Edn with variable pressure detector working at 20–
30 kV after sputtering with gold. Samples were prepared by drop casting
the aggregates (4 mm, rapid cooling method) and gel (4 mm, sonication)
of GA-SQ in n-butanol on freshly cleaved mica substrate. It was kept for
overnight to allow slow evaporation of the solvent and then further dried
in a vacuum desiccator for 12 h.
Transmission electron microscopy: TEM imaging was performed on a
JEOL-JEM0310 microscope with an accelerating voltage of 100 kV.
Specimens were prepared by drop casting self-assembled materials of
GA-SQ in n-butanol on carbon-coated copper grids (400 mesh) and
dried under air. The dried sample was then desiccated for 12 h. The ag-
gregates (rapid cooling method), aggregates (sonication) and gel (sonica-
tion) of GA-SQ were prepared using 4, 1 and 2 mm solutions respectively.
The doped aggregates (0.5 mm) and gel (2 mm) of GA-SQ were prepared
by adding a miniscule amount of SWCNTs to a hot solution of GA-SQ
followed by sonication for 5 min.
(KBR): nmax =3451, 1755, 1595, 1419, 1362, 1191, 1020, 746, 674 cmꢀ1
;
1H NMR (400 MHz, [D6]DMSO, 258C): d=0.91 (t, J=7.2 Hz, 6H;
-CH3), 1.29 (m, 4H; -CH2-), 1.48 (m, 4H; -CH2-), 3.36 (t, J=6.0 Hz, 4H;
-NCH2-), 5.06 (brs 1H, -OH), 6.73–6.75 (d, 2H; phenyl-H), 7.83–
7.86 ppm (d, 2H; phenyl-H); 13C NMR (100 MHz, [D6]DMSO, 258C):
d=13.75, 19.50, 28.61, 50.12, 111.98, 128.00, 150.00, 173.14, 194.55 ppm.
Synthesis of GA-SQ: Tributyl orthoformate (1 mL) was added to a
100 mL round-bottomed flask containing compounds
3
(0.45 g,
Atomic force microscopy: AFM images were recorded under ambient
0.50 mmol), 4 (0.15 g, 0.50 mmol), and dry 2-propanol (50 mL). The reac-
tion mixture was then stirred at 908C for 20 h under N2. The progress of
the reaction of was monitored using TLC. After the completion of the re-
action, the reaction mixture was cooled and filtered. The solid thus ob-
tained was washed with 2-propanol until the filtrate become almost color-
less. Column chromatography (chloroform/methanol=9:1 v/v) of the
crude product over neutral A12O3 gave the pure product in 37% yield.
FT-IR (CDCl3): nmax =3348, 3342, 2928, 2855, 1660, 1589, 1527, 1390,
conditions (298 K and 1 atm) by using a NTEGRA (NT-MDT) operating
with
a tapping mode regime. Micro-fabricated SiN cantilever tips
(NSG10) with a resonance frequency of 299 kHz, curvature radius 10 nm
and a force constant of 3.08–37.6 Nmꢀ1 was used. AFM imaging was car-
ried out using both height and magnitude profiles simultaneously. The
section analysis was done offline. Samples for the imaging were prepared
by drop casting diluted n-butanol sonogel of GA-SQ (2 mm) on freshly
cleaved mica surface. The samples were first air dried and further under
vacuum.
1
1361, 1177, 1110, 836, 790 cmꢀ1; H NMR (400 MHz, CDCl3, TMS, 258C):
d=0.87 (t, J=6.8 Hz, 9H; -CH3), 0.98 (t, J=7.2 Hz, 6H; -CH3), 1.25 (m,
48H; -CH2-), 1.41 (m, 10H; -OCH2CH2CH2- + -NCH2CH2CH2-), 1.59–
X-ray diffraction: For XRD, thin films of the samples were prepared by
transferring self-assembled materials of GA-SQ in n-butanol into the
cover slips and kept overnight for slow evaporation of the solvent. The
X-ray diffractogram of the samples were recorded on a PANalytical
X’Pert diffractometer equipped with a copper anode (lmean =0.15418 nm)
and a fast X’Celerator detector collecting signals for 350 s in each step of
0.058.
1.78 (m, 10H; -OCH2CH2-
-NCH2CH2CH2N-), 2.23 (t, J=6.8 Hz, 2H; -CH2C=O), 3.04 (s, 3H;
-NCH3), 3.42 (m, 6H; -CH2NCH3 -CH2NCH2-), 3.52 (m, 2H;
+
-NCH2CH2-), 1.93 (m, 2H;
+
-NHCH2-), 3.59 (m, 2H; -NHCH2-), 3.94 (m, 6H; -OCH2-), 6.65 (d, J=
9.2 Hz, 2H; phenyl-H), 6.70 (d, J=9.2 Hz, 2H; phenyl-H), 7.08 (s, 2H;
phenyl-H), 7.21 (brs 1H; -NH), 7.74 (brs 1H; -NH), 8.24 (d, J=9.2 Hz,
2H; phenyl-H), 8.30 ppm (d, J=9.2 Hz, 2H; phenyl-H); 13C NMR
(100 MHz, CDCl3, 258C): d=13.84, 14.10, 20.20, 22.67, 22.93, 26.08,
26.10, 29.36, 29.43, 29.59, 29.66, 29.71, 29.74, 30.33, 31.91, 32.86, 38.48,
40.12, 41.24, 51.24, 51.68, 69.12, 73.42, 105.58, 112.26, 112.48, 119.27,
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Acknowledgements
We acknowledge the European Commission for supporting V.K.P. (Marie
Curie International Incoming Fellowship, GELBRID; PIIF-GA-2010–
276574) and J.M.M. (Marie Curie Initial Training Network, FINELU-
MEN; PITN-GA-2008–215399) at CNR-ISOF. This work was supported
by the Italian Ministry of Research (MIUR) and Consiglio Nazionale
delle Ricerche (CNR) through PRIN 2010 INFOCHEM (contract no.
CX2TLM), FIRB Futuro in Ricerca SUPRACARBON (contract no.
RBFR10DAK6), MACOL PM. P04. 010, and Progetto Bandiera N-
CHEM. J.M.M. further thanks Istituto di Studi Avanzati, University of
Bologna, for the BRAINS-IN fellowship. A.S. is thankful to Council of
Scientific and Industrial Research (CSIR), Government of India for a re-
search fellowship. A.A. is grateful to the Department of Atomic Energy
(DAE), Government of India, for a DAE-SRC Outstanding Researcher
Award. The authors gratefully acknowledge Mr. C. K. Chandrakanth and
Mr. Kiran Mohan of NIIST for SEM and TEM measurements, respec-
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Received: April 22, 2013
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