Assembly of naphthalenediimide conjugated peptides: Aggregation induced changes in fluorescence

Article abstract of DOI:10.1039/c3cc43538e

Naphthalenediimide appended peptide based self-assembly was studied. Interestingly, an aggregation induced drastic change in the fluorescence property and gel formation were observed depending on the solvent composition (chloroform:methylcyclohexane) at a fixed concentration of 1.6 mM at room temperature.

Full text of DOI:10.1039/c3cc43538e

ChemComm
View Article Online
View Journal | View Issue
COMMUNICATION
Assembly of naphthalenediimide conjugated peptides:
aggregation induced changes in fluorescence†
Cite this: Chem. Commun., 2013,
49, 6891
Shibaji Basak, Jayanta Nanda and Arindam Banerjee*
Received 11th May 2013,
Accepted 12th June 2013
DOI: 10.1039/c3cc43538e
www.rsc.org/chemcomm
Naphthalenediimide appended peptide based self-assembly was and NV (Fig. 1a) have a strong gelation ability to gelify various
studied. Interestingly, an aggregation induced drastic change in organic solvents (Table S1, ESI†). Moreover, these gelator peptides
the fluorescence property and gel formation were observed exhibit an aggregation induced drastic change in the fluorescence
depending on the solvent composition (chloroform : methylcyclo- behaviour from a very feeble violet emission to bright yellow
hexane) at a fixed concentration of 1.6 mM at room temperature. emission with the concomitant formation of a gel from the sol
state (Fig. 1b).
Naphthalenediimides (NDIs)1 are endowed with attractive proper-
The gelator peptide NF shows an interesting property called anti-
ties including molecular planarity, electron acceptability and the solvent gelation9 in a mixed solvent system (Table S2, ESI†) consist-
characteristic redox behavior and these make NDIs interesting ing of two solvents. Out of these two solvents, in one solvent the
candidates for electron and energy-transfer systems. NDI systems gelator NF does not dissolve, however, it forms a gel at higher
are gaining enormous interest as an organic n-type semiconductor concentration in the other solvent. At room temperature, into a
due to their potential applications in organic field effect transistors chloroform solution of NF, methylcyclohexane (MCH) is added and
(OFETs), photovoltaic devices, and flexible displays.2 However, gelation takes place instantly. In a 1 : 1 mixture of chloroform and
naphthalenediimides typically possess low quantum yields of MCH the minimum gelation concentration (MGC) is determined to
fluorescence unlike larger imide substituted dyes such as perylene- be 1.6 mM. The MGC value is much lower (10 times) than that of
bisimides.3 Recently, a few attempts have been made to increase the the pure chloroform solvent.
fluorescence efficiency of NDI based molecules by core substitu-
The morphology of these NDI based organogels in toluene
tion.4 Several research groups have reported that some NDI based was studied using field-emission scanning electron microscopy
molecules show aggregation induced green emission.5 Aggregation (FE-SEM) and results are presented in Fig. 1. Both gelators show
induced enhanced fluorescence emission (AIEE)6 is associated with the nano-fibrillar assembly in the gel state. FE-SEM reveals that the
several gel7 based materials. The enhanced fluorescence emission is gel nanofibers obtained from NF range from 50 nm to 80 nm in
quite exceptional because the fluorescence efficiency of organic
chromophores generally decreases in the solid state, although they
show a high fluorescence efficiency in solution. However, to the best
of our knowledge there is no report of NDI based peptide gels that
show aggregation induced fluorescence.
We are curious to investigate the self-assembly behaviour of NDI
containing peptides. The centrally located NDI moiety can be well
associated by using p–p interactions and the peptide moiety can be
assembled by utilising hydrogen bonding and other non-covalent
interactions. In the course of our study on the self-assembly and
gelation of short synthetic peptides,8 we found that compounds NF
Department of Biological Chemistry, Indian Association for the Cultivation of
Science, Jadavpur, Kolkata-700 032, India. E-mail: bcab@iacs.res.in;
Fax: +91-332473-2805; Tel: +91-332473-4971
† Electronic supplementary information (ESI) available: 1H NMR, 13C NMR and
Fig. 1 (a) Chemical structure of gelators NF and NV. (b) Photographs of the
peptide NF in pure chloroform and gel in the chloroform and MCH mixture (2 : 8)
in daylight and under UV light (365 nm). (c) and (d) are the FE-SEM and TEM
images of the NF gel. (e) and (f) are the FE-SEM and TEM images of the NV gel.
mass spectra of the reported compounds, experimental section, synthesis proce-
dure and synthetic scheme, XRD data, FT-IR data, cyclic voltammetry and I–V
plot. See DOI: 10.1039/c3cc43538e
c
This journal is The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 6891--6893 6891

View Article Online
Communication
ChemComm
width (Fig. 1c). Unlike gel nanofibers obtained from NF, the fibers
obtained from NV are relatively thicker and range from 200 nm to
425 nm in width and several micrometers in length as evident from
FE-SEM (Fig. 1e). TEM images (Fig. 1d and f) also indicate a network
of nano-fibrous structure for these two gels. Nano-fibers obtained
from the NF gel are very thin and the width ranges from 7 nm to
10 nm. These fibers are several micrometers in length. However,
fibers obtained from the NV gel are relatively wide with the widths
varying from 60 nm to 90 nm. These long fibers indicate that the
gelator molecules form a nano-architecture using unidirectional
interaction. The soft ‘solid-like’ gel phase formation has also been
supported by the rheological study (Fig. S7, ESI†). It is evident from
Fig. S7 (ESI†) that the storage modulus (G0) is higher than the loss
Fig. 3 A typical model of gelators self assembled in a J-aggregate fashion.
modulus (G00) for these two gels and no cross over point is noticed weak violet fluorescence under UV light exposure (illuminated at
within the experimental frequency regions (4–100 rad sꢀ1).
365 nm) (Fig. 1b). However, the addition of MCH into the chloro-
Solvent dependent UV-visible spectroscopic studies of these form solution of NF keeping the concentration fixed at 1.6 mM
gelators (NF and NV) were performed to explore the self-assembly results in a change in the aggregation behavior. At a composition of
pattern from monomeric chromophores to the self-aggregated state 7 : 3 (chloroform : MCH) aggregation of gelator NF has been noticed
by varying the composition of chloroform and MCH systematically and it is evident from the change in the fluorescence spectra
in mixtures keeping the gelator concentrations fixed at 0.05 mM. (Fig. 2b). Interestingly, at 1 : 1 (5 : 5) composition of the chloroform
One of the major driving force of self-assembly of these chromo- and MCH mixture, a gel was formed by the assembly of the gelator
phoric gelators is p–p stacking and this is evident from UV-visible NF and a strong yellow color was observed from the gel under UV
spectroscopy.1 Both gelators exhibit UV-visible absorption in the light exposure at 365 nm (Fig. 1b). Fig. 2b illustrates the aggregation
range of 300–400 nm and this corresponds to the p–p* transitions induced emission spectra of the gelator peptide NF at different
along the polarized long axis of the chromophore (NDI).1 In pure solvent compositions. In pure chloroform solution, a very small
chloroform, peaks appeared at 360 nm and 380 nm with a shoulder peak was observed at 410 nm corresponding to the non-aggregated
at 340 nm corresponding to the characteristic NDI moiety in the monomeric species of NDI at an excitation of 340 nm.5 At a
non-aggregated monomeric state of NF and NV. After the addition of composition of 7 : 3 (chloroform : MCH) an emission maximum at
MCH into the chloroform solution, intensities of all these peaks 550 nm was observed and the intensity of the peak was increased
decreased (Fig. 2a and Fig. S8, ESI†). An unexpected decrease in remarkably at the higher percentage of MCH in the chloroform and
intensity (hypochromic shift) of absorption up to 75% and 85%, and MCH mixture. With an increase in percentage of MCH (2:8) the
also a red shift (bathochromic shift) of 9 nm and 10 nm of the lowest emission peak was shifted to a longer wavelength 567 nm with a
energy absorption band (380 nm) were observed for aggregates of NF broad band around 480 nm. The PL spectrum obtained from NV
and NV respectively. The decrease in intensity and the red shift of (1.6 mM) is quite different (Fig. S9, ESI†) from that of NF in the
the absorption band indicate the involvement of J-type p–p aggrega- chloroform and MCH mixture. In only chloroform the gelator
tion1,3 (Fig. 3) in the assembled state of both gelator molecules.
molecule NV shows a small peak at 410 nm. However, addition of
Upon sol to gel transformation, these gelators show an unusual MCH into the chloroform solution triggers a drastic change in the
fluorescence output. The solvent composition dependent photo- fluorescence property. Emission maxima at 525 nm with two
luminescence property of the gelator peptides in the solvent mixture different shoulder peaks around 460 nm and 560 nm were observed
(chloroform and MCH) was studied to investigate the aggregation at a solvent composition of 1 : 1 (5 : 5) of chloroform and MCH
induced fluorescence behavior. The composition of chloroform and respectively. There was a slight variation in the peak position, but
MCH in mixtures can significantly alter the aggregation pattern of the intensity was increased drastically upon the variation of the
NF and NV. From UV-visible spectroscopy, it is evident that at higher solvent composition from 1 : 1 (5 : 5) to 2 : 8 of chloroform and MCH.
percentage of MCH, both gelators form J-type aggregates (Fig. 3). It is interesting to note that in the mixed solvent composition NV
The peptide NF at 1.6 mM concentration in chloroform shows a very fails to form any gel under similar conditions to that of NF. The
drastic change in the PL spectra and the observed emission color
can be explained on the basis of the aggregation induced enhanced
emission (AIEE) phenomenon. The weak violet color observed for
NF and NV is due to the monomeric state of the chromophore as
evident from the same PL emission (410 nm). The gelators NF and
NV contain phenylalanine and valine in the side chain, respectively,
and these amino acid residues interact differently in different
solvent compositions. The increase in percentage of MCH causes
the self-association of the gelator molecule using hydrogen bonding
and p–p stacking interactions. The enhanced fluorescence of the
Fig. 2 (a) Solvent dependent UV-visible study of NF (0.05 mM) in different
gelators at varying compositions of solvents in mixtures indicates
that the slipped NDI core forms a J-type aggregate.
chloroform and MCH mixtures. (b) Emission spectrum of NF (1.6 mM) sol to gel
state illustrating the remarkable change in the fluorescence output.
c
6892 Chem. Commun., 2013, 49, 6891--6893
This journal is The Royal Society of Chemistry 2013

View Article Online
ChemComm
Communication
X-ray diffraction studies of xerogels were carried out to from linearity and indicate the semiconducting nature of these
obtain the structural insight into the gelator molecules NF compounds. The magnitude of the current increases for gelator NV
and NV in the assembled gel state. Fig. S12 (ESI†) shows the compared to NF. It was observed from the FE-SEM studies that the
X-ray diffraction pattern of these xerogels. For the NF based gel fiber of NV is much thicker than NF. Thus, with a larger fiber
peaks at 2y = 18.91 and 2y = 9.41 are corresponding to the d diameter NV shows better electrical conductivity.
spacing value of 4.6 Å and 9.3 Å respectively (Fig. S12b, ESI†). It
In conclusion, we have demonstrated the self-assembly of two
clearly indicates the presence of a b-sheet type assembly.8 The naphthalenediimide based peptide gelators in various organic
p–p stacking peak of the NDI core appears at 2y = 24.91 with a solvents. These gels exhibit semiconducting and interesting aggre-
d spacing value of 3.5 Å for the NF based gel.1 Moreover, in the gation induced fluorescence properties. The gelator NF itself exhi-
small angle region (Fig. S12a, ESI†) a peak appeared at 2y = bits very feeble fluorescence in solution state, while it emits a bright
1.311 with a d spacing value 67.4 Å, which almost matches well yellow color in the aggregated gel state in the chloroform and MCH
with the calculated molecular length of gelator NF. For the NV mixture. These gels have been well characterized morphologically
based gel peaks at 2y = 18.61 and 2y = 9.11 correspond to the d and structurally. UV-visible and fluorescence spectroscopic data
spacing value of 4.78 Å and 9.6 Å, respectively, indicating the clearly suggest the presence of J-type aggregates in the self-
presence of a b-sheet structure in the gel state (Fig. S12d, ESI†). assembled state. Cyclic voltammetry and I–V measurements vividly
A peak at 2y = 24.01 with a d spacing of 3.68 Å indicates the indicate the semiconducting behavior of these gel phase materials.
presence of p–p stacking of the NDI core. In the small angle Moreover, the gelation of the peptide NF in different solvent
region (Fig. S12c, ESI†) the peak at 2y = 1.821 with a d spacing mixtures can be achieved at room temperature without heating.
value of 49.3 Å (D) is followed by several other peaks with d The aggregation induced bright fluorescence and semiconductive
values 23.9 Å (D/2), 16.1 Å (D/3), 11.95 Å (D/4), 9.6 Å (D/5), 8.0 Å nature of these gelators may find an important application in the
(D/6), 6.9 Å (D/7), 6.3 Å (D/8) and 5.75 Å (D/9). These data highly demanding optoelectronic devices.
suggest a lamellar type of structure in the self-assembled
gel state.
Notes and references
Hydrogen bonding interaction plays a crucial part in the aggre-
gation of NF and NV and was studied through FT-IR spectroscopy
(Fig. S13, ESI†). The solution (in chloroform) to gel (in toluene)
transition clearly reflects in the remarkable shift in FT-IR frequen-
cies. The N–H stretching frequencies, carbonyl stretching and N–H
bending peaks have been summarized in Table S3 (ESI†). Appear-
ance of the N–H stretching frequency at 3303 cmꢀ1, amide carbonyl
stretching frequency at 1642 cmꢀ1 and 1644 cmꢀ1 and N–H
bending at 1549 cmꢀ1 and 1545 cmꢀ1 for NF and NV gels,
respectively, is indicative of the b-sheet structure in the aggregate
state of molecules. From Table S3 (ESI†), it is evident that in
chloroform these gelators are not hydrogen bonded and molecules
remain in the monomeric state. The huge shift of FT-IR peaks in the
gel state indicates that gelator molecules remain in a highly
aggregated state. In addition to that, the presence of absorption
bands around 1660 cmꢀ1 and 1707 cmꢀ1 for both the gelators is
due to the symmetric and asymmetric stretching of the imide
carbonyl of the NDI group in the gel state.1 From the FT-IR study,
it is evident that the hydrogen-bonding interaction plays a crucial
role in the self-assembly and gelation of NF and NV.
The redox activity of gelators was studied by using cyclic
voltammetry.10 Cyclic voltammograms of these gelators in dichloro-
methane is presented in Fig. S14, ESI.† NF shows an average
reduction potential at ꢀ0.75 V vs. Fc/Fc+ (Fc: ferrocene) and the
corresponding LUMO energy is ꢀ3.66 eV. NV shows only one
reversible reduction at a potential of ꢀ0.51 V and the corresponding
LUMO energy is ꢀ3.90 eV. The room temperature bulk electrical
conductivity of this NDI based gelator was studied by drop casting
the gel material (in toluene) on the surface of an ITO glass. The
average thickness of the film on ITO glass was 240 nm and 320 nm
for NF and NV respectively. Fig. S15 (ESI†) shows the corresponding
I–V plot obtained from the NDI based gelators at room temperature.
The low voltage region of the curve follows Ohm’s law (V/I = R).
1 S. Tu, S. H. Kim, J. Joseph, D. A. Modarelli and J. R. Parquette, J. Am.
Chem. Soc., 2011, 133, 19125; H. Shao, T. Nguyen, N. C. Romano,
D. A. Modarelli and J. R. Parquette, J. Am. Chem. Soc., 2009, 131, 16374;
A. L. Sisson, N. Sakai, N. Banerji, A. Fu¨rstenberg, E. Vauthey and
S. Matile, Angew. Chem., Int. Ed., 2008, 47, 3727; R. S. K. Kishore,
O. Kel, N. Banerji, D. Emery, G. Bollot, J. Mareda, A. Gomez-Casado,
P. Jonkheijm, J. Huskens, P. Maroni, M. Borkovec, E. Vauthey, N. Sakai
and S. Matile, J. Am. Chem. Soc., 2009, 131, 11106; M. Kumar,
N. Jonnalagadda and S. J. George, Chem. Commun., 2012, 48, 10948.
2 H. E. Katz, A. J. Lovinger, J. Johnson, C. Kloc, T. Siegrist, W. Li,
Y.-Y. Lin and A. Dodabalapur, Nature, 2000, 404, 478; J. H. Oh,
¨
¨
¨
S.-L. Suraru, W.-Y. Lee, M. Konemann, H. W. Hoffken, C. Roger,
R. Schmidt, Y. Chung, W. Chen, F. Wu¨rthner and Z. Bao, Adv. Funct.
Mater., 2010, 20, 2148; H. E. Katz, J. Johnson, A. J. Lovinger and
W. Li, J. Am. Chem. Soc., 2000, 122, 7787.
3 S. V. Bhosale, C. H. Jani and S. J. Langford, Chem. Soc. Rev., 2008,
¨
37, 331; F. Wu¨rthner, T. E. Kaiser and C. R. S-Moller, Angew. Chem.,
Int. Ed., 2011, 50, 3376.
4 F. Wu¨rthner, S. Ahmed, C. Thalacker and T. Debaerdemaeker,
Chem.–Eur. J., 2002, 8, 4742.
5 M. Kumar and S. J. George, Nanoscale, 2011, 3, 2130; T. D. M. Bell,
S. V. Bhosale, C. M. Forsyth, D. Hayne, K. P. Ghiggino, J. A. Hutchison,
C. H. Jani, S. J. Langford, M. A.-P. Lee and C. P. Woodward, Chem.
Commun., 2010, 46, 4881.
6 S. S. Babu, K. K. Kartha and A. Ajayaghosh, J. Phys. Chem. Lett.,
2010, 1, 3413; B.-K. An, D.-S. Lee, J.-S. Lee, Y.-S. Park, H.-S.
Song and S. Y. Park, J. Am. Chem. Soc., 2004, 126, 10232; S. Datta
and S. Bhattacharya, Chem. Commun., 2012, 48, 877; S. K. Samanta
and S. Bhattacharya, J. Mater. Chem., 2012, 22, 25277.
7 J. R. Moffat and D. K. Smith, Chem. Commun., 2011, 47, 11864;
C. A. Lagadec and D. K. Smith, Chem. Commun., 2012, 48, 7817;
G. Cheng, V. Castelletto, R. R. Jones, C. J. Connon and I. W. Hamley,
Soft Matter, 2011, 7, 1326; G. Cheng, V. Castelletto, C. M. Moulton,
G. E. Newby and I. W. Hamley, Langmuir, 2010, 26, 4990; M.-O.
M. Piepenbrock, N. Clarke, J. A. Foster and J. W. Steed, Chem.
Commun., 2011, 47, 2095; M. D. S. Maset, V. J. Nebot, J. F. Miravet
and B. Escuder, Chem. Soc. Rev., 2013, DOI: 10.1039/c2cs35436e.
8 G. Palui and A. Banerjee, J. Phys. Chem. B, 2008, 112, 10107;
B. Adhikari, J. Nanda and A. Banerjee, Chem.–Eur. J., 2011, 17, 11488.
9 M. Suzuki, Y. Nakajima, M. Yumoto, M. Kimura, H. Hirai and
K. Hanabusa, Org. Biomol. Chem., 2004, 2, 1155; P. Sahoo, D. K.
Kumar, S. R. Raghavan and P. Dastidar, Chem.–Asian J., 2011, 6, 1038.
¨
However, in the higher voltage region, the I–V curves show deviation 10 C. Thalacker, C. Roger and F. Wu¨rthner, J. Org. Chem., 2006, 71, 8098.
c
This journal is The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 6891--6893 6893