Photo-induced charge-transfer complex formation and organogelation by a tripeptide

Article abstract of DOI:10.1039/c2sm25062d

A terminally protected tripeptide Boc-Phe-Phe-Tyr-OMe 1 and picric acid form a photo induced charge-transfer complex and organogels. The interactions between stacked aromatic units play a key role in the assembly process. UV light (366 nm) has been used a

Full text of DOI:10.1039/c2sm25062d

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PAPER
Photo-induced charge-transfer complex formation and organogelation by
a tripeptide†
*
Poulami Jana, Sibaprasad Maity, Suman Kumar Maity, Pradip Kumar Ghorai and Debasish Haldar
Received 10th January 2012, Accepted 20th March 2012
DOI: 10.1039/c2sm25062d
A terminally protected tripeptide Boc–Phe–Phe–Tyr–OMe 1 and picric acid form a photo induced
charge-transfer complex and organogels. The interactions between stacked aromatic units play a key
role in the assembly process. UV light (366 nm) has been used as a source of energy to cleave and
homogenize p-stacking in the supramolecular arrangement of peptide 1 and picric acid. CD, FT-IR,
NMR and powder X-ray diffraction studies exhibit distinct structural changes before and after light
irradiation. Field emission scanning electron microscopy of the xerogels reveal a morphological change
caused by photo induced charge-transfer complex formation. The fluorescence spectroscopy as well as
confocal microscopy studies show that these charge-transfer complexes have a significant red emission
at 672 nm.
Phe–Tyr–OMe 1 and picric acid. The peptide itself forms a gel in
various aromatic solvents like toluene, xylene, and 1,2-dichloro
benzene. The doping of picric acid into the organogel is not
enough to form the charge transfer complex. However, an
external stimuli like exposure to UV light (366 nm) changes the
colour of the organogel from pale green to brown.
Introduction
Organogels are highly studied materials with wide range of
applications like drug delivery,1 thermo- and mechanoresponsive
sensor materials,2 ion-selective membranes,3 and hardening
liquid waste.4 The noncovalent interactions such as p–p stack-
ing, hydrogen bonding, metal-ion coordination, dipole–dipole
interactions, and other van der Waals interactions are mainly
responsible for the molecular self-assembly and the gelation
process.5 Different external stimuli like pH change,6 solvent
polarity,7 light,8 ultrasound,9 ions,10 enzymes,11 and so forth are
generally used to manipulate the structure and function of the
gel. The gel phase is different from the solid or the liquid
phase, and provides a novel platform for photochemical or
photophysical process.12 There is considerable interest in the
development of gels responsive to external stimuli with a special
focus on photo-responsive organogels. Recently, organogelators
with interesting optical and electronic properties have been
developed based on porphyrins,13 phthalocyanines,14 and
conjugate oligomers.15 Glycoluril has reported as a photo-
responsive supergelator that can be switched reversibly from the
gel to the sol state by light irradiation.16
Results and discussion
The terminally protected tripeptide Boc–Phe–Phe–Tyr–OMe 1
(Fig. 1) containing a C-terminal L-tyrosine residue has been
synthesized by a conventional solution-phase methodology,
purified, characterized and studied. The gelation propensities of
the terminally protected tripeptide 1 has been studied in a variety
of organic solvents by dissolving a small amount of the
compound in 0.5 mL of the solvent under investigation by
heating. Upon cooling the complete volume of solvent is
immobilized and forms a transparent gel. We found that the
tripeptide 1 forms thermoreversible transparent gels in various
aromatic solvents including benzene, toluene, xylene and
1,2-dichlorobenzene (ESI Table 1†). The gelation has been
confirmed by the inverted test tube method.18 We have
As a part of our program aiming the fabrication of supra-
molecular materials,17 herein we present the formation of photo
induced two component organogel between peptide Boc–Phe–
Department of Chemical Sciences, Indian Institute of Science Education
and Research Kolkata, Mohanpur, West Bengal 741252, India. E-mail:
deba_h76@yahoo.com; deba_h76@iiserkol.ac.in; Fax: +913325873020;
Tel: +913325873119
† Electronic supplementary information (ESI) available: Synthesis and
characterization of peptides, 1H NMR, 13C NMR, solid state FTIR
spectra, ESI Table 1 & 2, ESI Fig. S1–4, Fig. S1–S13. CCDC reference
number 831903. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c2sm25062d
Fig. 1 The schematic presentation of the peptide 1 and picric acid.
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investigated the Tgel (gel to sol transition temparature) of tri-
peptide 1 in 1,2-dichlorobenzene (lowest MGC) which shows
that the Tgel value increases as the concentration of the gelator
increases (ESI Fig. 1†).
of picric acid to the peptide 1 gel in 1,2-dichlorobenzene exhibits
an absorption at 337 nm, (Fig. 2a) which is characteristic for
picric acid. On exposure to UV light, a new absorption band
appeared at 422 nm along with the 337 nm peak (Fig. 2b).
To obtain an insight into the photochemical properties of the
light-induced charge-transfer organogel, fluorescence spectros-
copy as well as confocal microscopy of the organogel were per-
formed. As shown in Fig. 3, the photo-induced charge-transfer
complex of peptide 1 and picric acid have an emission at 672 nm
i.e. a red emission (excitation at 415 nm) along with a second
emission at 435 nm in 1,2-dichlorobenzene. However, without
light irradiation there is only an emission at 435 nm (Fig. 3,
excitation at 415 nm), which is characteristic for picric acid.
The emission quantum yield value (Fem) has been measured to
be 0.19 (ꢂ10%).
The reported peptide 1 is an electron rich compound and we
were curious to examine its behaviour with an electron deficient
system in gel state. One of the attractive properties of donor–
acceptor organogels is their ability to assemble organic moieties
in close proximity without forming a precipitate.19 This may lead
to potential applications in optics and electrochemistry because
close packing of monomers improves conductivity and may
provide improved and even nonlinear optical properties. We
have incorporated an electron deficient molecule 2,4,6-trini-
trophenol (picric acid) in the peptide 1 organogel. One interesting
fact is that after mixing electron deficient picric acid with electron
rich peptide 1 in 1,2-dichlorobenzene, the gel adopts the pale
green colour of picric acid (Fig. 2d). Perhaps the addition of
picric acid to the organogel is not enough to form the charge
transfer complex. But, an exposure to external stimuli like UV
light (366 nm) changes the colour of the organogel from pale
green to dark brown (Fig. 2e). Hence, light acts as a source of
activation energy and helps to form the charge transfer complex.
To determine the stoichiometric requirements of the two
components upon gelation, Tgel is measured with increasing
concentration of picric acid at a constant concentration of
peptide 1 (46 mM). A plot of Tgel vs. the molar ratio of picric acid
(ESI Fig. 2†) shows the maximum value Tgel at a 1 : 1 molar ratio
of peptide 1 to picric acid. Also, we have investigated the MGC
value of 1 : 1 mixture of 1 and picric acid in various aromatic
solvents (ESI Table 2†). In the case of 1,2-dichloro benzene, the
minimum gelation concentration (MGC) for the 1 : 1 mixture of
compound 1 and picric acid is comparatively low. It is probable
that the electron deficient 1,2-dichloro benzene is not forming
a charge transfer complex with picric acid and rather acts only as
a solvent.
Moreover, confocal microscopic images (Fig. 4) have
confirmed the simultenious green emission (excitation at 488 nm)
and red emission (excitation at 561 nm) from the photo-induced
charge-transfer complex based organogel. Hence, now it is clear
that the dark brown colour (Fig. 2e) of the light-stimulated
organogel of compound 1 and picric acid is obtained by simul-
taneous green and red emission.
To determine if any chemical reaction takes place (formation
of new compounds by light irradiation of the tyrosine phenolic
OH, picric acid and 1,2-dichlorobenzene) or if the result is due to
supramolecular arrangements of the components, the mass
spectrum of the UV irradiated peptide picric acid organogel was
recorded. The mass spectrum confirms the presence of the
supramolecular complex of peptide 1 and picric acid rather than
any new compounds (ESI Fig. 4†). A comparative study of the
emission spectra of the organogel containing variable concen-
trations of picric acid with a constant peptide concentration
(0.186 mM) in 1,2-dichlorobenzene have been performed. Fig. 5
shows that, with increasing concentration of picric acid in the
system, the intensity of the emission at 672 nm increases.
To investigate the morphological changes of the organogel
FE-SEM experiments were performed.20 From Fig. 6a, one can
consistently observe a network structure composed of fibrous
aggregates for the peptide 1 xerogel from 1,2-dichlorobenzene.
Fig. 6b shows that on addition of picric acid the fibrous structure
is retained with some distortion. FE-SEM experiment shows that
on exposure to UV light nanocrystal (length ca 100 nm) are
To characterize the charge transfer complex formation UV/
Visible absorption and emission experiments were carried out.
The UV/Visible absorption spectra of peptide 1 (c ¼ 1 ꢀ 10ꢁ5 M)
in methanol shows a peak at 275 nm (ESI Fig. 3†). The addition
Fig. 2 UV/Vis spectra of peptide 1 and picric acid (a) before UV-light
exposure and (b) after UV-light exposure in the gel state formed from 1,2
dichlorobenzene. (c) The transparent organogel of peptide 1 in 1,2-
dichlorobenzene, (d) pale green gel after addition of picric acid and (e)
dark brown gel after exposure to UV light.
Fig. 3 Normalised emission plot of peptide 1 and picric acid (a) before
light exposure (black line) and (b) after UV light exposure (red line), in gel
state in 1,2-dichlorobenzene.
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obtained (Fig. 6c and d). The nanocrystals are arranged in
a linear array. The growth of nano crystal may be a causative
factor for the transformation of transparent organogel to an
opaque one.21
To gather information about the change of supramolecular
arrangements of the components under UV irradiation, solution
1
state H NMR experiments were performed. Since the peptide
contains a phenyl and a tyrosine ring, CDCl3 is the appropriate
solvent system to investigate the process by NMR. 1H NMR was
performed with the xerogel obtained from the peptide 1 picric
acid organogel both before and after UV light irradiation. On
light exposure, the peak of the NH of Phe(1) (6.21) and Phe(2)
(6.37) has sifted to 6.15 and 6.30 respectively (Fig. 7). Before UV
irradiation, the NMR spectrum is that of the peptide 1 and picric
acid mixture. But after UV irradiation the shift of the charac-
teristic peaks indicate that there is some change of environment
caused by complex formation.
Fig. 4 Confocal micrograph of light irradiated organogel (from 1,2-
dichlorobenzene) of peptide 1 and picric acid (a) green emission (exci-
tation at 488 nm) and (b) red emission (excitation at 561 nm).
CD is an excellent method of determining the change of
supramolecular arrangements. CD spectra of peptide 1 picric
acid complex in acetonitrile before UV exposure (Fig. 8b) have
positive bands at 205 nm and 225 nm. Fig. 8c exhibits the
significant change after UV irradiation and a new negative band
at 215 nm has appeared. Moreover, after light irradiation, a two
fold increase of intensity of the positive band at 225 nm is
observed.
Further information on the change of supramolecular
arrangements of the peptide 1 and picric acid under different
conditions was obtained from the FT-IR studies.22 The FT-IR
spectra of peptide 1 and picric acid before UV irradiation
(Fig. 9a) have bands at 3288 cmꢁ1 (N–H stretching vibrations)
and 1648 cmꢁ1 (C]O stretching vibrations) indicating presence
of hydrogen bonded conformation.23 After exposure to UV light
those bands have shifted at 3293 cmꢁ1 and 1636 cmꢁ1 exhibiting
a structural change by the complex formation (Fig. 9b).24
The powder X-ray diffraction (PXRD) data of the xerogels
indicate a large difference of molecular arrangement in the 1 : 1
peptide 1–picric acid gel before and after UV irradiation
Fig. 5 Emission spectra of charge transfer complex (in gel state in 1,2-
dichlorobenzene) with increasing picric acid concentration at constant
peptide concentration (0.186 mM). lex ¼ 415nm.
ꢀ
(ESI Fig. 4†). A peak corresponding to a d-spacing of 4.85 A
Fig. 6 FE-SEM images of (a) peptide 1 xerogel from 1,2-dichloroben-
zene, (b) peptide 1 and picric acid xerogel from 1,2-dichlorobenzene
before exposure to UV light, (c) and (d) peptide 1 and picric acid xerogel
from 1,2-dichlorobenzene after UV light irradiation showing a linear
array of nano crystals.
Fig. 7 Part of the 1H NMR spectra of the peptide 1 and picric acid 1 : 1
complex in CDCl3 (a) before UV light exposure and (b) after UV light
exposure, showing the up field shifting of peaks.
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To study the electrochemical properties of the charge-transfer
complex system, cyclic voltammetry was performed. The elec-
trochemical oxidation of Tyr is well-known and the oxidation
peak is found to be kinetically irreversible, connected with
radical formation.26 From the cyclic voltametric study, the
oxidation potential of tripeptide 1 (1.57 V, Fig. 10a) decreases to
1.47 V (Fig. 10b) in the 1 : 1 complex with picric acid. On UV
light exposure, the value further decreases to 1.36 V (Fig. 10c).
The results clearly indicates the effect of charge-transfer complex
formation by photo irradiation.
Geometry optimizations and vibrational frequency analyses
were carried out without any symmetry constraints at the level of
density functional theory (DFT) based methods as implemented
in the electronic structure program Gaussian 03.27 We have used
the Beck’s three parameter hybrid exchange functional28
combined with the Lee–Yang–Parr non-local correlation func-
tion abbreviated as B3LYP.29 The split-valence basis set with
diffuse functions, namely 6-311+G, have been employed for all
atoms. Vibrational frequencies were calculated for optimized
molecular structures to verify that no negative frequencies were
present for minimum energy structures. However the DFT
studies on the peptide 1–picric acid complex show that the
complex is highly stable due to charge transfer and p–p inter-
Fig. 8 Solution state CD spectra of (a) pure peptide 1 in acetonitrile, (b)
peptide 1 and picric acid 1 : 1 complex before light exposure, (c) peptide 1
and picric acid 1 : 1 complex after UV exposure showing the conforma-
tional change in acetonitrile.
ꢀ
ꢀ
actions (3.44 A between picric acid and Tyr and 3.66 A between
picric acid and Phe(2)). The HOMO to LUMO energy differ-
ences of peptide 1 and picric acid are 29.08 kcal molꢁ1 and
22.12 kcal molꢁ1 respectively. Whereas the energy difference
between the HOMO of peptide 1 and the LUMO of picric acid is
7.75 kcal molꢁ1, which is significant for this complex formation.
Fig. 11 exhibits the molecular orbital diagram of the complex.
We have synthesized another tripeptide Boc–Phe–Phe–Paba–
OMe 2 (Paba ¼ p-aminobenzoic acid) containing both electron
rich (donor) and electron deficient (acceptor) aromatic moieties.
This peptide also forms thermoreversible transparent gels in
various aromatic solvents including benzene, toluene and 1,2-
dichlorobenzene. But this tripeptide failed to form any photo
induced charge-transfer complex with picric acid. From X-ray
crystallography, it is evident that the asymmetric unit contains
two molecules of peptide 2 (namely A and B) and there is no
intramolecular hydrogen bond or intermolecular hydrogen
bond between molecules A and B.30 The backbone torsion angles
Fig. 9 Solid state FT-IR spectra of xerogel of (a) peptide 1 and picric
acid 1 : 1 complex before light exposure and (b) peptide 1 and picric acid
1 : 1 complex after UV exposure showing the conformational change.
ꢃ
ꢀ
(2q ¼ 18.27 ) accompanied by another peak at 10.06 A (2q ¼
8.8ꢃ) indicates the cross b-sheet structure of peptide 1.25 The
disappearance of this peak and increase of intensity of another
ꢃ
ꢀ
peak at 4.04–3.56 A (2q ¼ 23.5 ) is characteristic of a p–p
stacking interaction, clearly indicating the formation of the
charge–transfer complex after light irradiation.
Fig. 10 Voltammograms of (a) peptide 1, (b) peptide 1 with picric acid
before UV irradiation and (c) peptide 1 with picric acid after light
exposure.
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mediated by dicyclohexylcarbodiimide/1-hydroxybenzotriazole
(DCC/HOBt). Deprotection of the methyl ester was performed
using the saponification method. The intermediates were char-
acterized by 500 MHz and 400 MHz 1H NMR spectroscopy, 13
NMR spectroscopy and mass spectrometry. The final compound
was fully characterized by 500 MHz 1H NMR spectroscopy, 13
C
C
NMR spectroscopy (125 MHz, 100MHz), mass spectrometry,
and FT-IR spectroscopy. The peptides were purified by column
chromatography using silica (100–200 mesh size) gel as
stationary phase and an n-hexane–ethyl acetate mixture as
eluent.
Fig. 11 Molecular orbital diagram of the peptide 1–picric acid complex
showing (a) HOMO, (b) LUMO and (c) LUMO + 1.
(a) Boc–Phe(1)–OH (3). A solution of L-phenylalanine (3.30 g,
20 mmol) in a mixture of dioxane (40 mL), water (20 mL) and
1 M NaOH (20 mL) was stirred and cooled in an ice-water bath.
Di-tert-butylpyrocarbonate (4.8 g, 22 mmol) was added and
stirring was continued at room temperature for 6 h. Then the
solution was concentrated in vacuum to about 20–30 mL, cooled
in an ice-water bath, covered with a layer of ethyl acetate (about
50 mL) and acidified with a dilute solution of KHSO4 to pH 2–3
(Congo red). The aqueous phase was extracted with ethyl acetate
and this operation was done repeatedly. The ethyl acetate
extracts were pooled, washed with water and drier over anhy-
drous Na2SO4 and evaporated in a vacuum. The pure material
was obtained as a waxy solid. Yield 4.87 g, (18.35 mmol,
91.78%).
(f1, j1, f2 and j2) of peptide 2 are in the parallel b-sheet region
of the Ramachandran diagram. But the overall the peptide
backbone adopts a kink like shape including the p-aminobenzoic
acid moiety. From Fig. 12, it can be seen that there are strong
ꢀ
intramolecular p–p interaction (shortest C–C distance is 3.464 A
ꢀ
for molecule A and 3.545 A for molecule B) between the Phe(1)
and Paba(3) residues. The Phe(1) ring is acting as the donor and
the electron deficient Paba(3) ring is the acceptor. The light
irradiation has been failed to cleave this strong intramolecular
p-stacking. As a result there is no photo induced charge transfer
complex formation between peptide 2 and picric acid.
1H NMR (DMSO-d6, 500 MHz, d in ppm); 12.75 (br, 1H,
COOH); 7.28–7.09 (m, 5H, aromatic ring protons); 7.11–7.09 (d,
1H, J ¼ 10 Hz, Phe NH); 4.09–4.01 (m, 1H, CaH Phe); 3.02–
2.87(m, 2H, CbH Phe),1.36 (s, 9H, Boc). 13C NMR (DMSO-d6,
125 MHz, d in ppm): 173.57, 155.41, 138.00, 129.05, 128.09,
126.27, 80.24, 55.10, 36.39, 20.73. FT-IR: (cmꢁ1) 3336.62,
2980.24, 2928.43, 1718.09, 1508.24, 1396.37, 1368.93, 1252.56,
1165.90, 1053.15, 1028.98.
Experimental
General
All L-amino acids were purchased from Sigma chemicals. HOBt
(1-hydroxybenzotriazole) and DCC (dicyclohexylcarbodiimide)
were purchased from SRL.
Peptide synthesis
(b) Boc–Phe(1)–Phe(2)–OMe (4). 4.5 g (16.96 mmol) of Boc–
Phe–OH was dissolved in 25 mL dry DCM in an ice-water bath.
H–Phe–OMe was isolated from 7.31 g (33.92 mmol) of the cor-
responding methyl ester hydrochloride by neutralisation, subse-
quent extraction with ethyl acetate and ethyl acetate extract was
concentrated to 10 mL. It was then added to the reaction
mixture, followed immediately by 3.49 g (16.96 mmol) dicyclo-
hexylcarbodiimide (DCC) and 2.59 g (16.96 mmol) of HOBt. The
reaction mixture was allowed to come to room temperature
and stirred for 48 h. DCM was evaporated and the residue was
dissolved in ethyl acetate (60 mL) and dicyclohexylurea
(DCU) was filtered off. The organic layer was washed with 2 M
HCl (3 ꢀ 50 mL), brine (2 ꢀ 50 mL), 1 M sodium carbonate
(3 ꢀ 50 mL) and brine (2 ꢀ 50 mL) and dried over anhydrous
sodium sulphate and evaporated in a vacuum to yield Boc–
Phe(1)–Phe(2)–OMe as a white solid.
The peptides were synthesized by conventional solution-phase
methods using a racemization free fragment condensation
strategy. The Boc group was used for N-terminal protection and
the C-terminus was protected as a methyl ester. Couplings were
Yield 5.56 g (13.03 mmol, 76.82%). M.P. 121–122 ꢃC. 1H
NMR (CDCl3, 500 MHz, d in ppm): 7.27–6.89 (m, 10H, aromatic
ring protons). 6.19–6.18 (d, 1H, J ¼ 5 Hz, NH1); 4.85 (m, 1H,
CaH Phe1); 4.71–4.70 (d, 1H, J ¼ 5 Hz, NH2); 4.25 (m,
1H, CaHPhe2); 3.59 (s, 3H, OMe); 3.02–2.93 (m, 4H, CbH Phe1,
CbH Phe2); 1.35(s, 9H, Boc); 13C NMR (125MHz, CDCl3, d in
ppm): 171.36, 170.76, 155.30, 136.50, 129.23, 127.13, 80.24,
55.65, 53.28, 38.27, 31.94, 29.72. FT-IR: (cmꢁ1) 3330.85, 3063.74,
Fig. 12 The crystal structure of tripeptide 2. Intramolecular p–p
interactions are shown as a dotted line.
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3033.66, 2987.73, 2973.66, 2926.79, 2855.47, 1745.20, 1698.16,
1666.37, 1604.89, 1522.35, 1497.14, 1445.66.
at 298 K. Compound concentrations were in the range
1–10 mmol in CDCl3 and (CD3)2SO.
(C) Boc–Phe(1)–Phe(2)–OH (5). To 4.4 g (10.31 mmol) of
Boc–Phe(1)–Phe(2)–OMe, 25 mL MeOH and 2 M 15 mL NaOH
were added and the progress of saponification was monitored by
thin layer chromatography (TLC). The reaction mixture was
stirred. After 10 h, methanol was removed under vacuum; the
residue was dissolve in 50 mL of water, and washed with diethyl
ether (2 ꢀ 50 mL). Then the pH of the aqueous layer was
adjusted to 2 using 1 M HCl and it was extracted with ethyl
acetate (3 ꢀ 50 mL). The extracts were pooled, dried over
anhydrous sodium sulphate, and evaporated under vacuum to
obtained compound as a white solid. Yield 3.5 g (8.49 mmol,
82.30%).
1H NMR (DMSO-d6, 500 MHz, d in ppm); 12.75 (broad peak,
1H, COOH); 8.14–8.12 (d, 1H, J ¼ 10 Hz, NH2); 7.34–7.21(m,
10H, aromatic ring proton); 6.95–6.93 (d, 1H, J ¼ 10 Hz, NH1);
4.52–4.49 (m, 1H, CaH Phe1); 4.23–4.19 (m, 1H, CaH Phe2);
3.16–3.12 (m, 2H, CbH Phe1); 3.02–2.94 (m, 2H, CbH Phe2);
1.33 (s, 9H, Boc); 13C NMR (125MHz, DMSO-d6, d in ppm):
172.76, 171.84, 155.15, 137.67, 129.15, 128.05, 126.51, 78.51,
56.45, 54.55, 53.78, 39.85, 27.59. FT-IR: (cmꢁ1) 3339.51, 3030.84,
2978.61, 2928.46, 1691.18, 1664.23, 1524.12, 1454.93.
FTIR spectroscopy
All reported solid-state FTIR spectra were obtained with a Per-
kin Elmer Spectrum RX1 spectrophotometer with the KBr disk
technique.
Mass spectrometry
Mass spectra were recorded on a Q-Tof Micro YA263 high-
resolution (Waters Corporation) mass spectrometer by positive-
mode electrospray ionization.
Circular dichroism (CD) spectroscopy
Solution state CD study of peptide 1 in acetonitrile has been
carried out before and after UV irradiatꢃion on a JASCO J-815-
150S instrument at a temperature of 25 C.
Fluorescent spectroscopy
Fluorescent spectrum of the reported compounds were recorded
at different concentration in 1,2-dichlorobenzene on a fluores-
cent spectrometer (Perkin Elmer) and at excitation wavelength
415nm.
(d) Boc–Phe(1)–Phe(2)–Tyr(3)–OMe (1). 1 g (2.42 mmol) Boc–
Phe–Phe–OH was dissolved in 4 mL DCM in an ice-water bath.
H–Tyr–OMe 1.12 g (4.84 mmol) was isolated from the corre-
sponding methyl ester hydrochloride by neutralization, subse-
quent extraction with ethyl acetate and concentrated to 7 mL.
Then it was added to the reaction mixture, followed immediately
by 0.499 g (2.42 mmol) dicyclohexylcarbodiimide (DCC) and
0.370 g (2.42 mmol) HOBt. The reaction mixture was allowed to
come to room temperature was stirred for 72 h. The residue was
taken in 30 mL ethyl acetate and dicyclohexylurea (DCU)
was filtered off. The organic layer was washed with 2 M HCL
(3 ꢀ 50 mL), brine (2 ꢀ 50 mL), then 1 M sodium carbonate
(3 ꢀ 50 mL) and brine (2 ꢀ 50 mL) and dried over anhydrous
sodium sulfate and evaporated under vacuum to yield the tri-
peptide 1 as a white solid. Purification was done by silica gel
column (100–200 mesh size) and ethyl acetate and hexane 1 :ꢃ2 as
the eluent. Yield 1.2 g (2.03 mmol, 84.09%). M.P. 143–145 C.
1H NMR (CDCl3, 500 MHz, d in ppm): 7.29–6.69 (m, 14H,
aromatic ring proton); 7.05–7.04 (d, 1H, J ¼ 5 Hz, NH1); 6.43–
6.41 (d, 1H, J ¼ 10 Hz, NH2); 6.38–6.36 (d, 1H, J ¼ 10 Hz,
NH3); 4.77–4.75 (m, 1H, CaH Phe1); 4.70–4.68 (m, 1H, CaH
Phe2); 4.57–4.56 (m, 1H, CaH Tyr); 3.67 (s, 3H, OMe); 3.03–2.84
Confocal microscopy
The organogels from peptide 1 and picric acid in 1,2-dichloro-
benzene, before and after UV light exposure were drop cast on
glass slide and then dried under vacuum, and images were taken.
Field emission scanning electron microscopy
Morphologies of all reported gel materials were investigated
using field emission scanning electron microscopy (FE-SEM).
The organogels were dried and platinum coated and the micro-
graphs were taken in an FE-SEM apparatus (Jeol Scanning
Microscope-JSM-6700F).
UV/Vis spectroscopy
UV/Vis absorption spectra were recorded on a Perkin Elmer
UV/Vis spectrophotometer.
Gelation study
The gel forming ability of synthetic tripeptide 1 with or without
picric acid were studied in different organic solvents. 5–20 mg of
the corresponding compound was added in 0.5 ml of solvent
under investigation and made a clear solution by heating. The gel
appeared on cooling. The gel melting temperature of the resul-
tant organogel were determined by the inverted test tube method.
(m, 6H, CbH Phe1,CbH Phe2, CbH Tyr3); 1.39 (s, 9H, Boc); 13
C
NMR (125 MHz, CDCl3, d in ppm): 171.42, 170.04, 161.21,
155.48, 136.34, 130.32, 129.42, 128.73, 127.02, 115.79, 80.68,
70.10, 55.69, 53.54, 49.29, 37.97, 33.88, 29.71. FT-IR (cmꢁ1):
3305.85, 3063.24, 3029.20, 2929.15, 2851.88, 1738.06, 1687.78,
1648.47, 1517.25. ESIMS: m/z 612.45, [M+Na]+; Mcalcd 589.27.
Cyclic voltametry
The cyclic voltametry was carried out using Princeton Applied
Research Potentiostat/Galvanostat Model/236A. All experi-
ments were performed in the three-electrode mode using an
Ag/AgCl as a reference electrode and a platinum wire as counter
NMR experiments
€
All solution state NMR studies were carried out on a Bruker
AVANCE 500 MHz and Jeol JNM-ECS 400 MHz spectrometer
5626 | Soft Matter, 2012, 8, 5621–5628
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minimum energy structures.
Conclusions
In conclusion, we have demonstrated the fabrication of a novel
light induced organogel by a simple noncovalent supramolecular
approach. UV light has been used as a source of energy to cleave
and homogenize p-stacking and formation of charge-transfer
complex in an organogel medium. The photo irradiation has
changed the morphology of the organogel from fiber network to
nanocrystals array with a significant red emission. This photo-
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Acknowledgements
We acknowledge the DST, New Delhi, India, for financial
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