Role of complementary H-bonding interaction of a cyanurate in the self-assembly and gelation of melamine linked tri(p-phenyleneethynylene)s

Article abstract of DOI:10.1039/b912392j

Melamine-functionalized tri(p-phenyleneethynylene) 1 self-assembles to form opaque and weak gels in aliphatic solvents which turned transparent and stable upon addition of a cyanurate, affording supramolecular nanostructures with distinct physical propert

Full text of DOI:10.1039/b912392j

Chemical Communications
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Number 40 | 28 October 2009 | Pages 5925–6104
ISSN 1359-7345
COMMUNICATION
ShikiYagai, Ayyappanpillai Ajayaghosh et al.
Role of complementary H-bonding
interaction of a cyanurate in the
FEATURE ARTICLE
Nina Berova et al.
self-assembly and gelation of melamine Probing molecular chirality by CD-
linked tri(p-phenyleneethynylene)s
sensitive dimeric metalloporphyrin hosts

COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Role of complementary H-bonding interaction of a cyanurate in the
self-assembly and gelation of melamine linked tri(p-phenyleneethynylene)sw
Sankarapillai Mahesh,a Rajasekaran Thirumalai,a Shiki Yagai,*bc Akihide Kitamurab and
Ayyappanpillai Ajayaghosh*a
Received (in Cambridge, UK) 23rd June 2009, Accepted 28th August 2009
First published as an Advance Article on the web 14th September 2009
DOI: 10.1039/b912392j
Melamine-functionalized tri(p-phenyleneethynylene)
1
self-
electronic interaction between the p-conjugated molecules
(Fig. 1(a)). Quenching of the emission was saturated when
1 equiv. of dCA was added (Fig. 1(a), inset). The emission
intensity is decreased upon increasing the temperature to
45 1C, whereas above 45 1C, the emission intensity is enhanced
(Fig. 1(b) and (d)). Interestingly, in the case of the self-assembly
of 1 alone, the fluorescence of the molecule did not vary much
under similar experimental conditions. This is clear from the
plots of the fluorescence intensities of 1 in the absence and
presence of dCA at different temperatures (Fig. 1(c) and (d)).
Hierarchical self-assembly of compound 1 under higher
concentrations resulted in blue emitting gels in aliphatic
solvents such as hexane and methylcyclohexane. The critical
gelator concentration (CGC) in methylcyclohexane is 3 mM.
The gel is turbid which indicates the tendency of the aggregates
to precipitate or to form aggregates of large size which may
scatter light (Fig. 2(a)). In this case, the gel melting temperature
(Tgel) at the CGC is found to be 38 1C which gradually
increased with concentration. Remarkably, in the case of the
1 : 1 complex 1ÁdCA, a transparent gel is formed in aliphatic
solvents (Fig. 2(a)). The CGC of 1ÁdCA in methylcyclohexane
is 2 mM. In this case the Tgel is found to be 42 1C which is 4 1C
higher than that of 1 alone indicating that the 1ÁdCA gel is
stabilized by the complementary H-bonding interaction with
cyanurates. A comparison of the plots of Tgel against
assembles to form opaque and weak gels in aliphatic solvents
which turned transparent and stable upon addition of a cyanurate,
affording supramolecular nanostructures with distinct physical
properties.
Complex superstructures of organized biopolymers are created
by the interplay of complementary hydrogen-bonding inter-
actions. While nature uses amide hydrogen-bonding in protein
organization, complementary double and triple hydrogen-
bonding interactions between heteroaromatic components
are used in transferring genetic information of nucleic acids.1
Thus, multiple hydrogen-bonding interactions are vital in
determining the structure and properties of supramolecular
architectures.2 In recent years, complementary hydrogen-
bonding between heterocyclic compounds has been exploited
to regulate the structure and properties of functional
chromophores.3,4 Due to strong propensity for p-stacking,
linear p-conjugated systems when functionalized with H-bonding
moieties are known to form diverse supramolecular
architectures.5,6 Recently, we and others have been exploring
the use of p-phenyleneethynylene systems for the creation of a
variety of supramolecular architectures.7,8 In the present study
we demonstrate the self-assembly of a dual hydrogen-bonding
system 1 based on melamine linked tri(p-phenyleneethynylene)
(TPE) in the absence and presence of a complementary
cyanurate (dCA) (Scheme 1).
In methylcyclohexane, 1 (1 Â 10À5 M) showed an emission
maximum at 407 nm (see ESIw). The absence of significant
solvent dependence on the maximum suggests that this
compound exists as monomeric species under low concentration.
When aliquots of the ditopic dCA was added to the solution of
1 at a concentration of 1 Â 10À5 M, the emission was gradually
quenched and red-shifted, both characteristics diagnostic for
a Photosciences and Photonics, Group Chemical Science and
Technology Division, National Institute for Interdisciplinary Science
and Technology (NIIST), Trivandrum-695019, India.
E-mail: ajayaghosh62@gmail.com; Fax: (+) 91-471-249-0186
b Department of Applied Chemistry and Biotechnology, Graduate
School of Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku,
Chiba 263-8522, Japan. E-mail: yagai@faculty.chiba-u.jp;
Fax: +81-(0)43-290-3039; Tel: +81-(0)43-290-3368
c PRESTO, Japan Science and Technology Agency (JST),
4-1-8 Honcho Kawaguchi, Saitama, Japan
w Electronic supplementary information (ESI) available: 1: Experimental
section. 2: Synthesis. 3: Description of experimental techniques.
4: Absorption and emission spectroscopic studies. 5: IR studies. See
DOI: 10.1039/b912392j
Scheme 1 The self-assembly pathways of 1 and 1ÁdCA.
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This journal is The Royal Society of Chemistry 2009
5984 | Chem. Commun., 2009, 5984–5986

Fig. 1 (a) Change in the fluorescence intensity of 1 (c = 1 Â 10À5 M)
with different aliquots of dCA (c = 1 Â 10À5 M) and (b) variable-
temperature emission spectra of the 1 : 1 complex 1ÁdCA at 1 Â 10À5 M;
(c) and (d) are temperature-dependent plots of the fluorescence
intensity changes of 1 and for 1 : 1 complex 1ÁdCA, respectively.
Fig. 2 (a) Photographs of the gels in methylcyclohexane in the
absence and presence of dCA. (b) Gel melting temperature of 1 and
1ÁdCA at different concentrations; (c) and (d) TEM images of 1 in the
absence and presence of dCA, respectively; (e) and (f) AFM images of
1 in the absence and presence of dCA, respectively.
concentration is shown in Fig. 2(b). The enhanced stability
of the gels is rationalized by the synergistic effect of the
complimentary H-bonding with cyanurates as well as the
p-stacking of the TPE moieties.
corresponding to the carbonyl and the N–H stretchings of the
free amide groups at 1633 and 3288 cmÀ1, respectively
(see ESIw). In the case of the 1ÁdCA gel, the former band is
shifted to 1712 cmÀ1 and the latter band completely
disappeared from the original position (see ESIw). It can be
assumed that the latter band is embedded in a broad band at
around 3310 cmÀ1 involving the NH stretching of CA.
Transmission electron microscopy (TEM) images of a drop
casted dilute solution of 1 showed fibrous networks. The
average width of fibers is ca. 100 nm (Fig. 2(c)). These fibers
must have originated from the bundling of the one-dimensional
polymeric chains of 1 arising from amide–amide hydrogen-
bonding and p stacking. This observation is further confirmed
by atomic force microscopic (AFM) measurements of the
diluted gels in methylcyclohexane showing bundles of fibers
(Fig. 2(e)). The size of the smallest fiber is approximately
around 40 Æ 5 nm. On the other hand, TEM images of a dilute
solution of 1ÁdCA showed thinner fibers of 20 Æ 5 nm
(Fig. 2(d)) in width which are more compatible with the
solvent. In presence of solvent in the gel state, these thin fibers
may be uniformly dispersed. As a result, the gel is optically
transparent, which is in sharp contrast to the gel of 1 alone.
When drop-casted on a TEM grid, these thin and short fibers
may entangle and agglomerate as can be seen in the TEM
images. AFM images in this case convincingly showed the
formation of very short fibers with a length of ca. 200–250 nm
and width of ca. 50 Æ 5 nm (Fig. 2(f)).
The observed changes in the fluorescence emission, gelation
properties and morphological differences of 1 in the absence
and presence of dCA could be explained in analogy to a
previous study on the self-assembly of perylene bisimide.6g
In the case of 1ÁdCA, the formation of a 2 : 2 dimer is invoked
during the initial stage of the self-assembly (Fig. 3). In this
dimer, the molecule 1 is strongly bonded in an H-type fashion
through twelve H-bonds. These dimers may further self-
assemble to form hierarchical structures in a J-type fashion
through amide H-bonding. The variable-temperature
fluorescence changes thus could be explained by the selective
breakage of the inter-aggregate J-type amide–amide hydrogen
bonds upon increasing temperature up to 45 1C, thereby
forming the H-type dimer with more favourable p-overlap,
which reduces the fluorescence intensity. The H-dimers
subsequently dissociate into individual components upon a
further increase of temperature (45–90 1C), resulting in a
strong fluorescence from molecularly dissolved TPE chromo-
phores. In the case of 1 alone, initially, a linear supramolecular
polymeric assembly could be favored through amide–amide
FT-IR measurements of the gels confirm the contribution of
the amide groups to the formation of the fibrous structures.
A gel of 1 in methylcylohexane (3 mM) exhibited IR bands
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This journal is The Royal Society of Chemistry 2009
Chem. Commun., 2009, 5984–5986 | 5985

a Ramanna Fellow of DST. We thank D. Sanjai (Manager,
Technical Services) RGCB, Trivandrum for TEM measurements.
This is manuscript No. PPG-288 from NIIST.
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5986 | Chem. Commun., 2009, 5984–5986