Assemblies of perylene diimide derivatives with melamine into luminescent hydrogels

Article abstract of DOI:10.1039/c1cc14189a

We report unique and spontaneous formation of hydrogels of perylene derivatives with melamine. The luminescent gel network is formed by H-type aggregation of the perylene core, supramolecularly cross-linked by melamine units. As a result of controlled aggregation in the extended nanofibers, strong exciton fluorescence emission is observed.

Full text of DOI:10.1039/c1cc14189a

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COMMUNICATION
Assemblies of perylene diimide derivatives with melamine into
luminescent hydrogelsw
Pradip K. Sukul,a Deepak Asthana,b Pritam Mukhopadhyay,b Domenico Summa,c
Luca Muccioli,c Claudio Zannoni,c David Beljonne,*d Alan E. Rowan*e and Sudip Malik*ae
Received 12th July 2011, Accepted 2nd September 2011
DOI: 10.1039/c1cc14189a
We report unique and spontaneous formation of hydrogels of
perylene derivatives with melamine. The luminescent gel network
is formed by H-type aggregation of the perylene core, supra-
molecularly cross-linked by melamine units. As a result of
controlled aggregation in the extended nanofibers, strong
exciton fluorescence emission is observed.
harvesting organogel of cholesterol based perylene derivatives,
whereas, Wurthner et al.7 developed a super organogelator
¨
from perylene derivatives with J-type aggregation. Even,
stimuli responsive supramolecular gels of perylene derivatives
have been reported by Rybtchinski et al.8 All these gelation
studies were carried out in organic medium. To the best of our
knowledge, there is no report on gelation of perylene deriva-
tives in water. Utilizing a rational design, herein we report a
two-component based spontaneous hydrogelation of perylene
derivatives with melamine (MM).9
The study of molecular gels1 has recently become an area of
great interest in the fields of supramolecular chemistry and
material science. Small molecules are held together by non-
covalent interactions such as hydrogen bonds, p–p inter-
actions, dipole–dipole interactions, van der Waals forces,
solvophobic interactions etc. These interactions are much
advantageous for the creation of various kinds of smart
materials due to their dynamic character. In particular, hydrogels
are of great importance due to their potential applications in
drug delivery,2 tissue engineering,3 and biosensing.4a
Two derivatives (PI and PBI) have been designed (Fig. 1a),
synthesized and unambiguously characterized. These are water
soluble due to the presence of four carboxylic acid groups.
Typically, hydrogels are prepared by mixing of the solution of
PI in water (2 Â 10À2 M) with the solution of MM in water
(4 Â 10À2 M) at equal volume ratio. Spontaneous gel is formed
for PI at comparatively low concentration (0.9% w/v) as it is
evidenced from the retardation of flow of mixture upon
inverting the vials.10 Spontaneous super-hydrogelation of a
two component based system is extremely rare in the
literature.7b,11
Perylenediimide dyes are an interesting class of chromo-
phores and fluorophores due to their enhanced stability, good
optical properties and pronounced capabilities of self-
assembly by means of p–p stacking. These are also the basic
materials of numerous organic electronics for light emitting
diodes, field effect transistors and photovoltaic cells.5 Owing
to pronounced hydrophobicity of perylene derivatives due to
the presence of an aromatic perylene core, their usage in an
aqueous environment has been restricted.6 There have been
tremendous efforts directed towards the design and synthesis
of perylene based gelators. Shinkai et al.4b reported light
FE-SEM images (Fig. 1b) of dried PI/MM gels reveal that
they are composed of long and bundled fibers that form highly
dense entangled networks. The average diameter of the fiber is
in the range of 20–200 nm. AFM studies of these loose gels
a Polymer Science Unit, Indian Association for the Cultivation of
Science, 2A & 2B Raja S. C. Mullick Rd., Jadavpur, Kolkata,
700032, India. E-mail: psusm2@iacs.res.in
b School of Physical Sciences, Jawaharlal Nehru University,
New Delhi-110067, India
c Dipartimento di Chimica Fisica e Inorganica and INSTM, University
of Bologna, viale Risorgimento 4, 40136 Bologna, Italy
d Laboratory for Chemistry of Novel Materials, University of Mons,
Place du Parc, 20, B-7000, Mons, Belgium.
E-mail: David.Beljonne@umons.ac.be
e Institute for Molecules and Materials, Radboud University of
Nijmegen, Heyendaalseweg 135, 6525 AJ Nijmegen,
The Netherlands. E-mail: A.Rowan@science.ru.nl
w Electronic supplementary information (ESI) available: Experimental
details for synthesis and characterization of PI and PBI, supplemen-
tary figures and molecular dynamics results. See DOI: 10.1039/
c1cc14189a
Fig. 1 (a) Molecular structure of PI, PBI and melamine (MM). FE-
SEM images of dried gels (b) [PI]/[MM] = 1 : 2 and (c) [PBI]/[MM] =
1 : 2 [PI] or [PBI] = 2 Â 10À2 M and [MM] = 4 Â 10À2 M [inset:
picture of an inverted quartz cuvette of 10 mm pathlength]. (d) AFM
topology of loose gel [PI]/[MM] = 1 : 2 directly cast on a freshly
cleaved mica surface.
c
11858 Chem. Commun., 2011, 47, 11858–11860
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show the presence of individual nanofibers having a diameter
of B12 nm (Fig. 1d). These short fibers are coiled to form
bundle of fibers having larger diameters and lengths (5–10 mm)
as shown in FE-SEM observations. The formation of gel fiber
is presumed to be the result of cross-linking by the MM
(H-bonding) between the p-stacked PI units, coated with a
water soluble carboxylic acid.
between 600–800 nm arises at high concentration. In contrast,
for PI/MM (1 : 2) a defined blue shifted emission is seen with
respect to PI solution.7,12 The luminescent network of gels can
be clearly observed under a confocal laser scanning micro-
scope upon excitation at 488 nm (Fig. 3c).10 Moreover, the
emission decay profile observed with nanosecond time-
resolved fluorescence shows the increase of life time in the
gel state (4.27 ns) with respect to PI (1.54 ns) or PI/MM
solution (2.00 ns) in water (Fig. 3d), indicative of exciton
emission rather than excimer emission.10
1H-NMR titration studies (Fig. S8, ESIw) of PI with gradual
addition of MM reveal an upfield shift of both types of
protons of PI (Ha and Hb stand for the protons on the perylene
core and the benzene moiety, respectively).13 The extent of
upfield shift of Hb (8.18 ppm to 8.01 ppm) is much higher than
that of Ha (7.15 ppm to 7.10 ppm), indicating the larger
shielding of the benzene ring caused by interaction between
carboxylic acid of PI and amino group of MM. The shift of
CQO stretching to B38 cmÀ1 lower frequency in FTIR
studies (Fig. S9, ESIw) indicates the participation of the
H-bond between PI and MM in the gel. XRD studies
(Fig. S10, ESIw) of the dried gel show that the gel fibers are
highly crystalline. A diffraction peak around 24.51 indicates a
well-defined and compact p–p stacking of perylene cores at the
distance of 3.6 A in gel fiber.
UV-Vis investigations reveal that PI in aqueous medium has
a different aggregation behavior in the presence of MM which
is optically silent over the region of 400–600 nm. PI having
concentration B2.5 Â 10À4 M shows two intense absorption
maxima at 502 nm (aggregated band) and 535 nm (monomeric
band). After mixing of PI with MM at a molar ratio of 1 : 2,
the position of these two maxima remains constant and a new
absorption maximum emerges at a lower wavelength, 452 nm
(Fig. 2a). The simultaneous formation of a loose gel is
confirmed by the naked eye. The generation of a blue-shifted
absorption peak at the lower wavelength region indicates a
H-type aggregation of the perylene core in the hydrogel. Upon
dilution of loose gel of PI/MM (2.5 Â 10À4 M to 3.75 Â 10À5 M),
the inversion of intensities of two bands at 502 nm and 535 nm
is observed and concomitantly the 452 nm peak is shifted to
468 nm (Fig. S3, ESIw), indicating monomer absorption. Time
dependent plots for PI/MM at [PI] = 2.5 Â 10À4 M indicate
the gradual increase of intensity of the 452 nm band with
decreasing intensities of 502 nm and 535 nm bands
(Fig. S4, ESIw). Temperature dependent aggregation of PI/
MM [2.5 Â 10À4 M] shows the presence of isosbestic points at
471 and 558 nm, revealing a clear transition between spectro-
scopically different states (Fig. S5, ESIw).
Fluorescence spectra (Fig. 2b) of the gel phase of PI/MM =
1 : 2 (CPI = 1 Â 10À2 M) show the broad emission peak at 677
nm with highly enhanced fluorescence intensity as compared
to PI only under identical conditions. It is intriguing to note
that the emission is optimum at 1 : 2 and decreases steeply
upon adding an additional amount of MM (Fig. S6, ESIw).
Upon irradiation at 365 nm, the PI/MM gel emits a bright
yellow fluorescence, whilst a solution of PI at the same
concentration in the absence of MM generates no visible
fluorescence. Concentration dependence of fluorescence
sspectra of PI and PI/MM (1 : 2) (Fig. 3a and b) reveals that
for the former solution a broad structureless emission band
To investigate the importance of the p–p stacking, we have
synthesized PBI having bromine atoms at the bay position of
the perylene core. Upon mixing at an appropriate molar ratio,
PBI also forms hydrogel with MM (Fig. 1c), however,
10 hours at room temperature are required to gelate water
and a highly entangled network of PBI/MM fibrils is observed.
For PBI gels, spectral studies (Fig. S11–S14, ESIw) also
confirm the defined aggregation in the presence of two molar
Fig. 3 Concentration-dependent fluorescence spectra of (a) PI only
and (b) [PI]/[MM] = 1 : 2 solution in water at room temperature
([PI] = 3.75 Â 10À5 (navy) to 3.75 Â 10À3 M (green) and lex = 475 nm).
Arrows indicate spectral change upon increasing the concentration
of PI [inset: PI only and fluorescent PI/MM hydrogels under UV
irradiation at 365 nm]. (c) CLSM image of hydrogel of 1 : 2.
(d) Time-resolved fluorescence decay of PI solution (red), PI/MM
solution (blue) and PI/MM hydrogel (green) at 25 1C (lex = 440 nm).
The sharp profile on the left is the lamp profile (black).
Fig. 2 (a) UV-Vis spectra of PI solution and loose gel of PI/MM at
room temperature (solid lines for gel and dashed lines for PI solution).
Typical molar ratio = 1 : 2, [PI] = 2.5 Â 10À4 M and [MM] =
5 Â 10À4 M [pathlength = 1.0 mm], (b) fluorescence spectra of
solution and gel at room temperature of the PI system. Typical molar
ratio = 1 : 2, [PI] = 1 Â 10À2 M and [MM] = 2 Â 10À2 M, at lex
=
475 nm [pathlength = 10 mm].
c
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finite angle between neighboring molecular transition dipoles
and energetic/positional disorder along the columns) is
favored over excimer emission (necessitating short intermolecular
distances) in the presence of MM (seen in Fig. 3b).
In conclusion, we report the first example of perylene based
hydrogels formed spontaneously upon mixing of a simple
perylene diimide derivative with melamine. These gels exhibit
highly intense fluorescence visible to the naked eye. The
resulting gel network consists of inner core of H-stacked
perylenes cross-linked by MMs and a water soluble carboxylic
acid at the outer surface. The observation that the perylene
derivative forms gel in aqueous medium is novel and will
encourage the design of perylene based gelators aiming for a
biological environment.
Fig. 4 (a and b) Charge-transfer character, electronic excitation
energy and radiative lifetime for PI dimers versus the intermolecular
spacing z, as calculated at the INDO/SCI level. (c) Simulated distribu-
tion of the intermolecular spacing for a couple of oppositely charged
PIs inside a stack, in the presence and absence of MM. (d and e) Two
MD snapshots showing the intercalation and the wrapping of MM
around a PI stack.
PKS is indebted to CSIR, India, for his fellowship. We
acknowledge Dr A. Bandyopadhyay of IICB for CLSM. SM
thanks Prof. A. K. Nandi of PSU, IACS, for his valuable
suggestion and EU for MC Fellowship (fp7-people-2007-4-2-iif).
The work was supported by the START-UP fund provided by
IACS. AER thanks NWO, VICI and EU ITN Superior.
equivalents of MM. Interestingly, the emission intensity of
PBI/MM gel is less than that of PI/MM gel. Assuming limited
heavy atom effects, it may be reasoned that the bay substituents
significantly reduce p–p interaction as evidenced from XRD
results (broadening of reflection at 24.51, Fig. S10b, ESIw).
To understand why these fibers remain fluorescent in water,
extensive molecular dynamics and simulations have been
performed and MM stabilizes the assemblies of PI molecules
into columns by gluing these together into a gel-like structure.
Here, the presence of the phenyl rings bearing the carboxylic
units at the N-terminal sites freezes the rotation angle (B301)
in both ground-state and excited-state potentials, so that the
intermolecular distance is the main geometric factor prompting
the formation of excimers as evidenced from the concentration
dependence studies of PI (Fig. 3a). INDO/SCI and TD-DFT
calculations of PI dimers extracted from MD simulations of
the columns where the distance between the molecular back-
bones has been gradually reduced from 6 A down to 3 A
(Fig. 4a and b) show an abrupt decrease in the lowest excitation
energy, increase in the associated radiative lifetime and in the
excited-state charge-transfer character at distances shorter
than B3.4 A, which are all signatures of excimer emission.14
The addition of MM affects the packing of the PI molecules
within the columns in multiple ways which have dramatic
impact on the luminescent properties of the gels. MD studies
reveal that MM molecules can be inserted into the columns
(Fig. 4d), thus not only preventing direct quantum-mechanical
coupling between PI molecules and hence excimer formation
but also slowing down energy migration to weakly emissive
excimer sites. Most importantly, MM molecules form a
continuous belt of H-bonds cross-linking the PI stacks and
preventing close contacts (below B3.3 A) between adjacent PI
molecules in the excited state (Fig. 4c–e) (the excited-state
potential is modeled here by adding partial charges on
adjacent PI cores to mimic possible excimer formation).10 As
a result, exciton emission (optically allowed as a result of the
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