Fluorescent coronene monoimide gels via H-bonding induced frustrated dipolar assembly

Article abstract of DOI:10.1039/c1cc14990c

Two stage self-assembly of novel coronene monoimide (CMI) based gels that results in resurfacing of monomer emission in the aggregated state is reported. This process is attributed to a frustrated head-head dipolar assembly forced by hydrogen bonding.

Full text of DOI:10.1039/c1cc14990c

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COMMUNICATION
Fluorescent coronene monoimide gels via H-bonding induced frustrated
dipolar assemblywz
Ankit Jain, K. Venkata Rao, Chidambar Kulkarni, Anjana George and Subi J. George*
Received 11th August 2011, Accepted 16th September 2011
DOI: 10.1039/c1cc14990c
Two stage self-assembly of novel coronene monoimide (CMI)
based gels that results in resurfacing of monomer emission in the
aggregated state is reported. This process is attributed to a
frustrated head–head dipolar assembly forced by hydrogen
bonding.
their anomalous organization behavior, we inferred that
monomer emission resurfaces as a supramolecular polymer
stage is achieved.12
The dipolar chromophores, CMI–ACH and CMI–CH
(Fig. 1a), have been designed with alkoxy substituted gallic
amide groups, which would promote co-operative and syn-
chronous functioning of H-bonding and p–p interactions.13
CMI–CH consists of (S)-3,7 dimethyloctyloxy chiral chains in
the gallic wedge and would provide a chiro-optical handle
to probe the molecular organization and self-assembly char-
acteristics. The CMI derivatives were synthesized according
to literature procedures (Fig. S1, ESIz) and characterized
by various spectroscopic techniques such as 1H-NMR and
MALDI-TOF MS.
Self-assembly of p-conjugated systems has resulted in immense
amount of research involving insights into the supramolecular
organization behaviors of molecules.1 Systematic and engi-
neered self-assembly of molecules with optical and electronic
functionality has led to the evolution of various assemblies,
such as p-conjugated organogels2 with potential applications
in organic electronics. However invariably, aggregation results
in hampering of fluorescence of these molecules, which in most
cases is an undesirable trait for their light-emitting properties.3
Various schemes of emission enhancement as a result of
aggregation have been probed, excimer emission,4 exciplex
emission,5 energy transfer,6 aggregation induced emission,7
aggregation induced enhanced emission8 to name a few.
However still search for a novel methodology for enhanced
emission during aggregation persists. On the other hand,
controlling the self-assembly of dipolar dyes to parallel head–
head organization has been a topic of great research interest
because of their potential application in non-linear optics.9
Herein we report a novel approach to achieve fluorescent
assemblies through hydrogen bonding mediated supra-
molecular polymerization of dipolar chromophores using
the concept of repulsion between dipoles as they are forced
into a head–head staggered organization. For this purpose we
studied the self-assembly of unsymmetrical coronene mono-
imide (CMI) derivatives and the critical effect of its molecular
dipole on the stacking interactions.10 High emission character-
istics of self-assemblies which could be translated in an
organogel phase make this first CMI based gelator unique in
its design and properties.11 Further probing spectroscopically,
In toluene, the compound self-assembles to the extent that
CMI–CH immobilizes toluene at a critical gelator concen-
tration of 2.3 mM. CMI–ACH however does not form an
organogel, but results in the formation of a viscous solution
(2.5 mM), suggesting self-assembly. Morphological characteristics
visualized through Field Emission Scanning Electron Micro-
scopy (FE-SEM) and Atomic Force Microscopy (AFM) of
both CMI derivatives showed the formation of a fibrous mesh
of high aspect ratio with minimum width of an individual fiber
being 5–6 nm (Fig. 1b and S2 (ESIz)). IR spectra of the
CMI–CH xerogel from toluene showed a lower energy amide
band (3283 cmꢀ1) compared to the dried film from chloroform
Supramolecular Chemistry Laboratory, New Chemistry Unit,
Jawaharlal Nehru Centre for Advanced Scientific Research
(JNCASR), Bangalore, India-560064. E-mail: george@jncasr.ac.in;
Fax: +91 80 22082627; Tel: +91 80 22082964
w This article is part of the ChemComm ‘Emerging Investigators 2012’
themed issue
z Electronic supplementary information (ESI) available: Experimental
procedures, additional morphological and spectroscopic data. See
DOI: 10.1039/c1cc14990c
Fig. 1 (a) Molecular structures of CMI–CH and CMI–ACH.
(b) AFM height image of CMI–CH fibers (2.5 mM, toluene) on a
glass substrate. (c) and (d) Confocal microscopy images of CMI–CH
in toluene and MCH respectively. Insets show the corresponding fiber
emission spectra.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 1467–1469 1467

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Fig. 3 (a) Temperature dependent CD spectra of 0.05 mM CMI–CH
in MCH. (b) Temperature dependent absorption (red) and CD (black)
cooling curves (dT/dt =ꢀ10 1C minꢀ1) for 0.05 mM CMI–CH in
MCH monitored at 476 nm (l = 1 mm, front face geometry).
Fig. 2 Temperature dependent (blue = 90 1C, black = 20 1C)
(a) UV-Vis spectra (2.5 mM) and (b) emission spectra (1.5 mM) of
CMI–CH in toluene (l = 1 mm, front face geometry, lexc = 400 nm).
Insets show the photographs of CMI–CH in solution and gel under
visible and UV light respectively.
concentrations as low as 2 ꢁ 10ꢀ5 M and hence the aggregation
studies could be performed at much lower concentrations
(0.05 mM) compared to that in toluene. In a temperature
drive from 60 1C to 20 1C (dT/dt = ꢀ10 1C minꢀ1) (Fig. 3), as
the molecule aggregates, at low temperatures we found that
only the gallic part is bisignated and not the CMI moiety
(beyond 350 nm), suggesting that though chirality from chiral
side chains has been transferred to chromophoric assembly,
the adjacent chromophores are not excitonically coupled. This
hints towards an anomalous disorientation among the adjacent
p-clouds of CMI moieties as the molecule aggregates.
(3402 cmꢀ1) (Fig. S3, ESIz) showing the presence of H-bonding
in the self-assembly. Visualization of CMI–CH gel fibers
through confocal microscopy showed that the self-assembled
fibers in toluene and methyl cyclohexane (MCH) exhibit green
and yellow fluorescence, respectively (Fig. 1c and d).14
Temperature dependent absorption spectra of toluene gel
(2.5 mM) showed that the solution state is more broadened
compared to the gel state (Fig. 2a) (see the ESIz). Simulta-
neously, comparison of fluorescence spectra of the gel state
with that of the solution state showed the high fluorescent nature
(less than 5% quenching) of self-assembly, with mere changes
in the vibronic features between the two (Fig. 2b). These
observations suggest that CMI chromophores experience a
monomer like environment as adjacent p clouds are interacting
minimally to each other in the gel phase,12 much contrary to
the conventional p–p induced aggregation features. To get an
insight into chromophore orientations during the self-assembly
we first carried out concentration and temperature dependent
NMR studies. Temperature dependent 1H-NMR of 2 mM
CMI–CH in toluene-d8 showed upfield shifts in H1 (coronene
ring protons adjacent to the imide ring, Fig. S4, ESIz) protons
of the coronene ring when cooled from 60 1C onwards, possibly
because of the shielding by the ring current of the adjacent
aromatic rings. However, interestingly as the temperature
decreased beyond 30 1C, as the molecule gelates toluene, H1
proton shows a downfield shift suggesting the change in the
environment around it, possibly because of a conformation
change which results in deshielding. Concentration dependent
1H-NMR studies of CMI–CH (0.01 mM to 5 mM) showed
upfield shifts of H1 proton as the concentration is increased up
to 1 mM and a downfield shift started as the concentration was
further increased to 5 mM, which is consistent with the
transition observed in temperature dependent studies (Fig. S4,
ESIz). Above data clearly hint towards a two stage assembly
process for CMI–CH.15
Moreover, the temperature dependent CD studies also
provided evidence of a two stage self-assembly process. Carefully,
analyzing the CD spectra we found that as the temperature
is decreased from 60 1C to 20 1C, the initial molecularly
dissolved state and the final aggregated state have a reverse
chirality transition state in between. Temperature dependent
absorption and CD traces followed at 476 nm unambiguously
show two states, first being in between 54 1C and 36 1C,
resulting in decrease of absorbance and the second in between
36 1C and 20 1C resulting in increased absorption (Fig. 3b).
These observations point towards a supramolecular confor-
mational change during the process of aggregation. Since the
design of our system is not symmetric across the coronene ring
there must be a component of the residual dipole moment of
the CMI moiety in the plane of the molecule. From the above
observations we believe that the first transition from the
molecularly dissolved state corresponds to dipolar head–tail
p–p stacking interactions to satisfy the dipolar interactions,
which is evident from the decrease in absorption, upfield shifts
in H1 proton signal and the negative Cotton effect which is not
bisignated at both gallic and CMI absorptions. However,
when H-bonding interactions set in at lower temperatures
(signified from the appearance of positive bisignated Cotton
effect in the gallic region, Fig. 3), maximum p–p overlap means
CMI dipoles would be in an energy intensive head-to-head
organization and thus would be forced to reduce the p cloud
overlap to cope up with the dipolar repulsion. This results in
a frustrated assembly of distorted chromophores, which is
a compromise between the attractive p–p interactions and
repulsive dipolar interactions. This second transition is evident
from the downfield shifts in the aromatic protons and the
reversal of the Cotton effect with a positively bisignated CD
signal for the gallic moiety and a non-excitonically coupled
positive Cotton effect for CMI. The spectroscopic changes
observed made us visualize CMI assemblies as a linear chain
However, the UV and PL studies were unable to provide
enough translation of the two stage aggregation process
of CMI–CH. In addition, the toluene absorption in the
250–300 nm, where the gallic part of the molecule absorbs
and the contribution of linear dichroism (LD) in the gel state,
prevented the use of chiro-optical properties to follow the self-
assembly. To probe the matter further we performed temperature
dependent circular dichroism studies of CMI–CH in MCH.
Although, CMI–CH is not soluble enough in MCH at higher
concentrations to form gels, it self-assembles to form fibers at
c
1468 Chem. Commun., 2012, 48, 1467–1469
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We thank JNCASR and Department of Science and Tech-
nology (DST), Government of India, for financial support.
K.V.R thanks CSIR for scholarship. The authors acknowledge
Prof. E. W. Meijer for stimulating discussions, Prof. S.
Balasubramanian for the computational facilities, Prof. G.
U. Kulkarni for Veeco-lab facilities, and Basavraj (AFM),
Selvi (FE-SEM), and Suma (confocal) for measurements.
Notes and references
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In conclusion, we have synthesized and elucidated the self-
assembly properties of novel coronene monoimide derivatives.
Detailed analytical probing revealed subtle molecular changes
that governed the spectroscopic properties of the aggregates.
Moreover it is one of the few examples where disjunction
between two usually complimenting forces, p overlap and H
bonds, has been shown. More importantly, the concept of
chromophore disorientation upon H-bonding enforcement, a
consequence of minimizing dipolar repulsion, if generalized to
other chromophores whose quantum yield decreases on aggre-
gation, can be of great significance. Moreover an arrangement
that forces the dipole to aggregate in a head–head fashion can
be advantageous for NLO activities. Though much work on
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base for molecular understanding for such future directions.
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14 The red-shifted yellow emission in MCH could be due to the better
p–p interaction because the dipoles will have a lesser interaction as
a result of less dielectric constant of MCH compared to toluene.
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c
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