Functional organogel based on a hydroxyl naphthanilide derivative and aggregation induced enhanced fluorescence emission

Article abstract of DOI:10.1016/j.jphotochem.2010.09.014

A new class of low molecular weight organogelator (LMOG) of hydroxyl naphthanilide moiety was suitably designed and synthesized and it forms gels through noncovalent interactions in hydrocarbon solvents. Self-assembly structure, hydrogen bonding interacti

Full text of DOI:10.1016/j.jphotochem.2010.09.014

Journal of Photochemistry and Photobiology A: Chemistry 217 (2011) 40–48
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Functional organogel based on a hydroxyl naphthanilide derivative and
aggregation induced enhanced fluorescence emission
Manoj Kumar Nayak∗
Department of Chemistry, Texas A&M University at Qatar, PO Box 23874, Doha, Qatar
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 17 June 2010
Received in revised form 29 July 2010
Accepted 21 September 2010
Available online 16 November 2010
A new class of low molecular weight organogelator (LMOG) of hydroxyl naphthanilide moiety was
suitably designed and synthesized and it forms gels through noncovalent interactions in hydrocar-
bon solvents. Self-assembly structure, hydrogen bonding interaction, and photophysical properties of
organogelator 3-hydroxy-naphthalene-2-carboxylic acid (2-heptylcarbamoyl-phenyl)-amide (2) have
been investigated by field emission scanning electron microscope (FE-SEM), FT-IR, UV–vis absorption
and photoluminescence combined with theoretical studies by hybrid density-functional theory (DFT)
B3LYP and semi-empirical calculations AM1 with CI methods. It was found that gelation is completely
thermoreversible, and it occurs due to the aggregation of the organogelator resulting in the formation
of a fibrous network due to the ␲–␲ stacking interaction complemented by the presence of both inter-
and intra-molecular hydrogen bonding. The self-assembled fibrillar networks in the gels were distinctly
evidenced by SEM observations. FT-IR studies confirm that the common driving force for aggregation
in the organogels and microsegregation in the mesophase is the occurrence of a tight intermolecular
hydrogen bonded network that does not persist in diluted solution. Gelator 2 is very weakly fluorescent
in solution, but its intensity is increased by almost 30–32 times in their respective gelled state depending
on the nature of the gelling solvents. The aggregation induced emission enhancement is ascribed to the
formation of J-aggregation and inhibition of intramolecular rotation in the gel state.
Keywords:
Naphthanilide self-assembly
J-aggregation
Gelation-induced enhanced fluorescence
emission (GIEFE)
Excited-state intramolecular proton
transfer (ESIPT)
Twisted intramolecular charge transfer
(TICT)
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
Moreover, some organogels displayed unique liquid crystalline
properties, which are potential candidates for developing optical
The search for efficient low molecular weight organic gelators
with possible structure–activity correlation is on rise because of
their diverse applications as supramolecular soft materials [1–6].
Organogels are usually assembled through self-aggregation of
the small gelator molecules to form entangled supramolecular
fibrillar networks via a combination of noncovalent interac-
tions such as hydrogen bonding, ␲–␲ stacking, electrostatic
forces, donor–acceptor interactions, metal coordination, solvopho-
bic forces, and van der Waals interactions. The physical organogel
is responsive to external stimuli such as thermo- [2], photo- [3],
chemo- [4], metal- [5], proton- [6], and mechano- [1], by which
the aggregate structure is stabilized or destabilized. In the sta-
bilization process, the organogel structure is changed to a more
ordered aggregation mode. On the other hand, in the destabiliza-
tion process, the aggregate is dissociated to reversibly form a fluid
liquid, indicating the gel-to-sol phase transition. The reversible
systems can be potentially applied to drug delivery systems [7]
and supramolecular switch system with memory function [8].
electronics [9]. Recently, there has been increasing interest in the
development of functional organogels with ␲-conjugated moieties
because of their potential applications involving nanomaterials
such as, sensors [10], molecular electronics [11], enhanced charge
transport [12], and light harvesting [13].
As an attempt to obtain a new functional organogelator with
potential photonic applications in this work, the photoactive
organogelator containing peripheral alkyl amide for gelation and
3-hydroxy-2-naphthanilide for fluorescence emission has been
suitably designed and synthesized as shown in Fig. 1. In order
to realize a rather simple but very efficient organogelator struc-
ture, long alkyl chains with an amide group, which are widely
known for its typical gelation power, were selected and attached
to the 3-hydroxy-2-naphthanilide part with unusual fluores-
cence. Fluorescence emission from 3-hydroxy-2-naphthanilide
[14], and its resemblance 3-hydroxy-2-naphthoic acid [15], and
N-(3-hydroxy-2-naphthamido)-Nꢀ-phenylthiourea [16] has been
widely investigated and it was concluded that the emission origi-
nates from the excited-state intramolecular proton transfer (ESIPT)
phenomenon. Hydroxyl naphthanilide derivatives have also been
the subject of extensive investigations in medicinal chemistry,
due to their ability to serve as potential cestodicidal agents [17]
∗
Tel.: +974 4423 0380; fax: +974 4423 0060.
E-mail addresses: manoj.nayak@qatar.tamu.edu, manoj@ipc.iisc.ernet.in
1010-6030/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2010.09.014

M.K. Nayak / Journal of Photochemistry and Photobiology A: Chemistry 217 (2011) 40–48
41
Fig. 1. The photochemical process leading to the formation of the trans-keto form from the enol form of naphthanilide gelator 2. The solid arrows indicate the absorption
phenomena or emission relaxation and the dotted arrows represent non-radiative relaxation.
and histochemical substrate for phosphatase [18]. However, the
self-assembly behavior and related properties of naphthanilide
derivatives remain unexplored in the field of functional gels and
supramolecular organic soft materials. Target molecule of naph-
thanilide gelator 2 (Scheme 1) was designed to be capable of
intermolecular as well as intramolecular hydrogen bonding. As
a reference compound, 3-methoxy-2-naphthanilide derivative 3
was synthesized to investigate the specific role of ESIPT active 3-
hydroxy-2-naphthanilide in 2 on the gelation properties. FE-SEM
and FT-IR analyses were performed to probe the structure of gels,
and also measured UV–vis absorption and photoluminescence (PL)
with specific focus on the spectroscopic difference between solu-
tion and gel states. Whereas, the theoretical calculations were
carried out to better understand the driving force for the aggre-
gation mechanism and the ESIPT emission (Fig. 2).
2. Experimental
2.1. General
Proton and carbon magnetic resonance spectra (1H NMR and 13
C
NMR) were recorded on a Bruker Avance 500 (500 MHz) instru-
ment using tetramethylsilane (0.03 (v/v)% TMS) in CDCl3 as the
internal standard (1H NMR: TMS at 0.00 ppm, CDCl3 at 7.24 ppm,
and 13C NMR: CDCl3 at 77.23 ppm). Proton coupling patterns were
Scheme 1. Synthesis of naphthanilide-containing organogelator 2 and its non-ESIPT analogue 3.

42
M.K. Nayak / Journal of Photochemistry and Photobiology A: Chemistry 217 (2011) 40–48
heated at 100 ◦C and refluxed under N2 atmosphere for 24 h. The
reaction mixture was poured into cold water and neutralized with
1 N HCl solution, after which the precipitate was collected by filtra-
tion. The crude product was purified by column chromatography
on silica gel with ethyl acetate/n-hexane (vol. ratio 1/10). The prod-
uct was recrystallized from ethanol to give 0.55 g of pure product
(yield 40%). m.p. 147.7 ◦C; 1H NMR (CDCl3, 500 MHz) ı = 12.66 (s,
1H), 11.81 (s, 1H), 8.66 (d, J = 8.30 Hz, 1H), 8.36 (s, 1H), 7.92 (d,
J = 8.26 Hz, 1H), 7.67 (d, J = 8.35 Hz, 1H), 7.52 (q, J = 7.17 Hz, 2H), 7.47
(t, J = 7.46 Hz, 1H), 7.31 (t, J = 7.47 Hz, 2H), 7.14 (t, J = 7.65 Hz, 1H),
6.33 (s, 1H), 3.48 (q, J = 7.0 Hz, 2H), 1.65 (q, J = 7.30 Hz, 2H), 1.42–1.30
(m, 8H), 0.85 ppm (t, J = 6.39 Hz, 3H); 13C NMR (CDCl3, 125 MHz)
ı = 169.29, 169.11, 157.50, 139.22, 137.48, 132.85, 129.59, 128.88,
128.56, 127.36, 126.66, 126.29, 123.93, 123.82, 122.39, 121.49,
117.86, 112.56, 40.44, 31.93, 29.73, 29.15, 27.19, 22.78, 14.21 ppm;
MS (EI) m/z 404.1 (M+, 14.5), 289 (M+, 100.0), 261 (M+, 5.4), 233
(M+, 1.5), 171 (M+, 10.4), 142 (M+, 9.2), 115 (M+, 9.8), 92 (M+, 1.8);
HRMS calcd for C25H28N2O3: 404.50, found: 404.21; IR (KBr pellet,
cm−1) 3414.5 (␯OH), 3309.5 (␯NH), 1595.2 (␯CO), 1530.8 (ıNH); Ele-
ment analysis calcd (%) for C25H28N2O3: C 74.23, H 6.98, N 6.93 and
O 11.87; found: C 74.0, H 7.01, N 6.95 and O 11.75.
Fig. 2. UV–vis absorption at left and photoluminescence spectra at right of gelator
2 in solution (25 wt% of dioxane in cyclohexane), black; cyclohexane gel, green; and
dodecane gel, red. (For interpretation of the references to color in this figure legend,
the reader is referred to the web version of this article.)
2.2.3. 3-Methoxy-naphthalene-2-carboxylic acid
(2-heptylcarbamoyl-phenyl)-amide (3)
described as singlet (s), doublet (d), triplet (t), quartet/quintet (q),
multiplet (m) and broad (br). MS were recorded on a Kratos Con-
cept H spectrometer (EI). Element contents of compounds were
measured with EA1110 (CE Instrument, Italy). Differential scan-
ning calorimetry (DSC) was performed on a PerkinElmer DSC-7 at
heating rate of 20 ◦C min−1. Infrared spectra were recorded using
a PerkinElmer precisely Spectrum One FT-IR spectrometer in the
range of 4000–500 cm−1. UV–visible absorption and fluorescence
spectra were recorded using Shimadzu UV-1650PC and Shimadzu
RF-500 spectrofluorimeter, respectively, with emission and exci-
tation slit width of 3 nm each. Field emission scanning electron
microscopy (FE-SEM) images were obtained with a JSM-6330F
(JEOL).
Compound 2 (0.3 g, 0.74 mmol) was dissolved in DMF (8 mL)
at room temperature. K2CO3 (0.12 g, 0.86 mmol) and MeI (0.12 g,
0.82 mmol) were added to this solution. The reaction mixture was
stirred under dark conditions for 24 h. This solution was poured
to cold water and precipitate was collected by filtration. The crude
product was purified by column chromatography on silica gel using
eluent 1–2 (v/v)% of ethyl acetate in n-hexane to give 0.25 g of pure
product (yield 80%). m.p. 156 3 ◦C; 1H NMR (CDCl3, 500 MHz)
ı = 11.75 (s, 1H), 8.73 (s, 1H), 8.69 (d, J = 8.36 Hz, 1H), 7.88 (d,
J = 8.15 Hz, 1H), 7.73 (d, J = 8.25 Hz, 1H), 7.48 (q, J = 7.74 Hz, 2H),
7.42 (d, J = 7.65 Hz, 1H), 7.36 (t, J = 7.50 Hz, 1H), 7.23 (d, J = 5.67 Hz,
1H), 7.07 (t, J = 7.47 Hz, 1H), 6.14 (s, 1H), 4.16 (s, 3H), 3.42 (q,
J = 6.70 Hz, 2H), 1.58 (q, J = 7.35 Hz, 2H), 1.36–1.25 (m, 8H), 0.82 ppm
(t, J = 7.05 Hz, 3H); 13C NMR (CDCl3, 125 MHz) ı = 168.99, 164.36,
155.21, 138.50, 136.25, 134.10, 131.81, 129.40, 128.47, 126.73,
126.43, 125.12, 124.56, 124.04, 123.44, 123.43, 107.60, 55.93, 40.18,
31.93, 29.89, 29.19, 27.19, 22.77, 14.19 ppm; MS (EI) m/z 418.1 (M+,
28.1), 304 (M+, 32.5), 276 (M+, 11.0), 234 (M+, 20.5), 185 (M+, 100.0),
170 (M+, 3.0), 152 (M+, 7.0), 127 (M+, 14.5), 120 (M+, 5.6), 114 (M+,
3.4); HRMS calculated for C26H30N2O3: 418.53, found: 418.22; Ele-
mental analysis calcd (%) for C26H30N2O3: C 74.61, H 7.22, N 6.69
and O 11.47; found: C 74.80, H 7.52, N 6.41 and O 10.85.
2.2. Synthesis of the organogelator [19]
2.2.1.
2-(3-Hydroxy-naphthalen-2-yl)-benzo[d][1,3]oxazin-4-one (1)
Anthranilic acid (3.0 g, 21.89 mmol) and 3-hydroxy 2-naphthoic
acid (4.12 g, 21.89 mmol) were dissolved in pyridine (35 mL) and
stirred for 30 min. Triphenyl phosphite (6.8 g, 21.89 mmol) was
added to this solution and stirred at 100 ◦C for 4 h. The reac-
tion mixture was poured into cold water and extracted with
dichloromethane. The solution was dried with anhydrous mag-
nesium sulfate. After removal of the solvent, the crude product
was purified by column chromatography on silica gel with ethyl
acetate/n-hexane (vol. ratio 1/3). The product was recrystallized
from ethanol to give 2.53 g of pure product (yield 40%). m.p. 136 ◦C;
1H NMR (CDCl3, 500 MHz) ı = 12.05 (s, 1H), 8.74 (s, 1H), 7.86 (d,
J = 8.20 Hz, 1H), 7.71 (d, J = 8.45 Hz, 1H), 7.53 (t, J = 7.50 Hz, 1H), 7.
2.3. Preparation of gels and determination of gelation
temperatures
Weighed amounts of organogelators were added to the sol-
vent and heated until all organogelators were fully dissolved. The
solution was then left as such, until it cooled down to room tem-
perature. The inverted test tube method was used to examine the
gel formation in different solvents and to determine critical gela-
tion concentration (CGC). The ball dropping method was used to
determine the sol–gel phase-transition temperature (Tgel).
48 (t, J = 7.85 Hz, 2H), 7.35 (m, 3H), 7.26 ppm (t, J = 6.92 Hz, 2H); 13
C
NMR (CDCl3, 125 MHz) ı = 168.89, 156.79, 150.47, 138.59, 133.38,
129.93, 129.80, 129.62, 127.42, 126.73, 126.65, 124.42, 121.90,
113.96, 112.27 ppm; MS (EI) m/z 289.10 (M+, 100.0), 261 (M+, 18.9),
233 (M+, 4.5), 217 (M+, 2.6), 171 (M+, 10.8), 142 (M+, 13.4), 114 (M+,
8.0), 92 (M+, 1.6); Element analysis calcd (%) for C18H11NO3: C 74.73,
H 3.83, N 4.84 and O 16.59; found: C 74.76, H 3.87, N 4.80 and O
16.61.
2.4. UV–vis absorption and photoluminescence (PL)
To get proper absorbance, a UV-cell for opaque gel was made
to give 1 mm path length, photoluminescence of opaque liquids
and solids was carried in the same cuvette by front-face detection
technique at an angle that minimizes reflected and scattered light.
Photoluminescence quantum efficiencies (˚PL) for solutions were
obtained using 9,10-diphenylanthracene as a reference [20]. On the
2.2.2. 3-Hydroxy-naphthalene-2-carboxylic acid
(2-heptylcarbamoyl-phenyl)-amide (2)
Compound 1 (1.0 g, 3.46 mmol) and n-heptylamine (1.98 g,
16.5 mmol) were dissolved in pyridine (32 mL). The solution was

M.K. Nayak / Journal of Photochemistry and Photobiology A: Chemistry 217 (2011) 40–48
43
Table 1
Basic properties of organogels.
Compound
Shape
CGC
Solvent
Transparency
Tgel (◦C)
2
3
Gel
0.25 wt%
Cyclohexane, dodecane
Translucent
Opaque
74
81
Precipitate
Cyclohexane, dodecane
other hand, ˚PL of PMMA film doped with 10 wt% of organogelator
2 and 3 were measured using a 6-in. integrating sphere (Lab-
sphere, 3P-GPS-060-SF) equipped with a 325-nm CW He–Cd laser
(Omnichrome, Series 56) and a PMT detector (Hamamatsu, PD471)
attached to a monochromator (Acton Research, Spectrapro-300i).
The detailed analytical procedure to obtain solid-state ˚PL has been
described elsewhere [21].
of 2 with equimolaramountofMeI andpotassium carbonate inDMF
led to the non-ESIPT analogue 3 in good yield of 80%. The molecu-
lar structures were fully characterized through Fourier transform
infrared (FT-IR) spectroscopy, 1H and 13C NMR, MS-EI/HRMS spec-
tral evidence and elemental analysis.
The gelation ability of organogelator 2 was tested in various
organic solvents and it was able to induce gelation in non-polar
solvents such as dodecane and cyclohexane with critical gelation
concentrations (CGC) as low as 0.25 wt%, but not in polar solvents
as summarized in Table 1. It is most likely that the intramolec-
ular hydrogen bonding of 3-hydroxy-2-naphthanilide group in 2
is essential for the gelation behavior, since its methoxy derivative
3 without that structural element is not able to gel a solvent but
form needle shape crystals (Fig. 3). This result implies that the
intramolecular hydrogen bonding in 3-hydroxy-2-naphthanilide
unit contributes to the planarization effect not only allowing ESIPT
but also improving the favorable ␲-stacking for the gelation (vide
infra).
In order to discern the nature of the microstructures that
may be present in such gels with such varying gelation capac-
ity and mechanical strength, the morphology of the xerogels
obtained from 2 in cyclohexane and dodecane was examined by
field emission scanning electron microscope (FE-SEM). The FE-
SEM images (Fig. 4) show their nanoscale assembly in different
organic solvents, and this indicates that compound 2 possesses
the directional forces required for the unidirectional intermolecu-
lar interactions between the gelator molecules to form the fibrous
nanoscale architecture [31]. Careful analysis revealed the flexible
fibrillar aggregates obtained from cyclohexane gel are approxi-
mately 50–60 nm in width and ∼2–5 ␮m in length. However, the
dodecane gels obtained from 2 showed relatively thicker fibers
of diameter ∼1–2 ␮m and several micrometers (∼15–25 ␮m) in
length. In all the cases one can observe a network structure com-
posed of fibrous aggregates. This means that the gel fibers present
probably consist of hierarchical structures.
2.5. Computational details
All molecular structures were fully optimized using the hybrid
B3LYP functional method [22], in combination with 6-31G(d,p)
Gaussian basis set. For each optimized structure a frequency analy-
sis at the same level of theory was used to verify that it corresponds
to a stationary point in the potential energy surface. As the use
of diffuse functions is essential for accurate determination of the
energetics, particularly for ion radicals and excited states [23],
to perform a singlet-point calculation on the basis of B3LYP/6-
31G(d,p) optimized structures. The excited-state properties were
calculated with the time-dependent density functional theory (TD-
DFT) [24] formalism, using the optimized ground state geometries.
TD-DFT in combination with the B3LYP hybrid functional and the
6-31G(d,p) basis set has previously been shown to provide accu-
rate energies for excited states within 0.2 eV (5 kcal mol−1) [25].
All calculations were performed with the Gaussian 03 package of
programs [26].
In addition to high level DFT calculations, semi-empirical meth-
ods offer an attractive alternative for studying potential energy
surfaces in ground and excited states. Procedures such as the AM1
method [27] allow examination of potential energy surfaces with-
out geometric assumptions. With inclusion of limited configuration
interaction, especially through the single and pair double excita-
tion (PECI) procedure [28], spectral properties of several conjugated
organic systems have been shown to be reliably reproduced [29,30].
Therefore, the spectral properties of ESIPT gelator 2 have been
simulated using AM1/PECI = 8 calculations in order to guide the
synthetic efforts towards materials with enhanced performances
and to help with the interpretation of the experimental data. The
geometry dependent potential energies in the ground (S0) and
first excited (S1) states were calculated by changing geometrical
parameters of interest from the optimized geometry. In these cal-
culations, all the geometrical parameters were optimized at each
point.
Considering the structure of the LMOG prepared in this work,
and the well-known fact that hydrogen bonding interaction
among the amide groups is one of the main driving forces for
the self-assembly of organogelators in organic solvents. FT-IR is
an important tool for investigating the different non-covalent
interactions involved in gelation [32–36]. To obtain structural infor-
mation about self-assembled material FT-IR was measured at three
different states: solution (1 wt%) in chloroform, gel (1 wt%) in
cyclohexane and solid powder. In solution state, four main charac-
teristic peaks appeared at 3448.5, 3315.2, 2925.5, 2854.0, 1594.9,
3. Results and discussion
and 1529.0 cm−1 for O–H, amide N–H asymmetric stretching (␯
,
OH
␯NH), antisymmetric (␯as) and symmetric (␯s) CH2 stretching fre-
3.1. Synthesis and self-assembly structure of organogels
quency bands, amide C O ␯
(amide I), and N–H bending ıNH
CO
(amide II), respectively (representative spectra of 2, Figs. 5 and
S6, and Table S1). As far as gel (solid powder) material was con-
cerned the N–H stretching, the amide I band and N–H bending
peaks are shifted to 3310.2 (3309.5), 1594.0 (1595.2), and 1525.5
(1530.8) cm−1, respectively. The red shifts of N–H stretching as well
as the blue shift of N–H bending band indicate that intermolecu-
lar hydrogen bond formation occurs in the gel state and no band
was observed above 3400 cm−1 in the gel state indicating the pres-
ence of hydrogen bonded NH in the supramolecular gel network.
Ortho-substituted N-heptyl amide derivatives of 3-hydroxy-2-
naphthanilide (2) and its non-ESIPT analogue (3) were synthesized
according to our earlier reported literature procedure [19]
and are depicted in Scheme 1. The cyclized intermediate
3-hydroxynaphthyl-benzoxazin-4-one (1) was prepared by dehy-
drocyclization reaction between 3-hydroxy 2-naphthoic acid and
anthranilic acid using triphenyl phosphite in pyridine in 40% yield.
Amidolysis reaction of 1 with n-heptylamine in pyridine led to the
ESIPT gelator 2 in 40% yield. Subsequently, O-methylation reaction

44
M.K. Nayak / Journal of Photochemistry and Photobiology A: Chemistry 217 (2011) 40–48
Fig. 3. Images 1 wt% of gelator 2 in dodecane and cyclohexane (bottom); 2 wt% of non-ESIPT analogue 3 is prepared in dodecane and cyclohexane (top) at room light and
under UV light at 365 nm.
The O–H stretching frequency observed in solid state appeared at
3414.5 cm−1 (␯OH 3448.5 cm−1 in solution phase) is also suggesting
the presence of strong intramolecular hydrogen bonding associ-
3.2. Photo-physicochemical properties of naphthanilides and
gelation-induced enhanced fluorescence emission
ation in that dried gel. Also, the shifting of antisymmetric (␯
)
This class of gelator being chromophoric allows evaluation of
their optical and photophysical properties on self-assembly. To
study the spectroscopic properties of self-assembled aggregates,
UV–vis absorption and photoluminescence spectra of compounds
2 and 3 were investigated in details in different solvents (shown
as
and symmetric (␯s) CH2 stretching frequency bands was observed
from 2929.5 and 2856.4 cm−1 in the solution phase to 2925.0 and
2853.8 cm−1 in the gel state, respectively. The decrease in fluidity
of the hydrophobic chains due to the formation of aggregates via
van der Waals interaction is evident from this particular shift in
the CH2 stretching frequency [33]. The facts indicate that the pres-
ence of intermolecular hydrogen bonding among the amide groups
and intramolecular OH· · ·O C hydrogen bonding and van der Waals
interaction plays an essential role in the self-aggregation of this
gelator 2.
in Figs. 2 and 6 and summarized data in Table 2). Absorption band
ab
maxima (ꢀ
) for all the bands of compound 2 are blue shifted and
max
εmax decreases with increase in the solvent polarity and their protic
nature of the solvents. Long wavelength (LW) tautomer emission
observed in cyclohexane is independent of ꢀexc, but in less polar
aprotic solvents dual fluorescence is observed only at ꢀexc > 350 nm.
Fig. 4. Field emission scanning electron microscope images of xerogel 2 in (a) cyclohexane and (b) dodecane.

M.K. Nayak / Journal of Photochemistry and Photobiology A: Chemistry 217 (2011) 40–48
45
Table 2
ab
Absorption band maxima (ꢀ
, nm) and fluorescence band maximum (ꢀflmax, nm) and fluorescence quantum yield (˚fl) of 2 (concentration = 1 × 10−5 M, ꢀexc = 370 nm) and
max
3 (concentration = 2 × 10−5 M, ꢀexc = 300 nm) in different solvents.
Solvents
2
3
ab
ab
ꢀ
, nm (log εmax
)
ꢀflmax, nm (˚fl)
ꢀ
, nm (log εmax
)
ꢀflmax, nm (˚fl)
max
max
Cyclohexane
Chloroform
Acetonitrile
1-Propanol
315, 375 (4.14), (3.51)
314, 374 (4.14), (3.46)
312, 372 (4.13), (3.45)
307, 368 (4.13), (3.42)
303, 367 (4.14), (3.39)
306, 366
446, 574 (0.002)
432, 564 (0.0016)
415, 563 (0.002)
425, 501 (0.042)
427, 526 (0.023)
393, 558 (0.021)
296, 355
364, 380 (0.054)
389 (0.015)
390 (0.012)
396 (0.029)
406 (0.021)
386 (0.120)
300, 356 (4.30), (3.27)
299, 356 (4.29), (3.23)
301, 356 (4.33), (3.26)
299, 355 (4.31), (3.24)
299, 352
Methanol
PMMA film (10 wt% of sample)
Gelation study (0.5 wt% sample)
Dodecane gel
Cyclohexane gel
Solution (25 wt% of dioxane in cyclohexane)
313, 378
312, 375
299, 352
439, 450, 467, 558
438, 450, 467, 557
428, 466, 572
Underline is to distinguish two most intense peaks from the structured fluorescence bands obtained in the gel states.
under similar environments, LW ꢀflmax is blue shifted. ˚fl of SW
emission band increases and that of LW emission band decreases
with increase in polarity and protic nature of the solvents. Whereas
absorption band maxima for a long wavelength band centered on
∼356 nm of compound 3 remain invariant. Only one small Stokes’
shifted fluorescence band maxima (ꢀflmax) is red shifted under the
same environments and ˚fl first decreases from non-polar to polar
aprotic then increases in polar protic solvents. Emission band is
independent of excitation wavelength (ꢀexc) in all the solvents, sug-
gesting that emission is taking place from the most relaxed excited
state in these solvents.
To examine the spectroscopic properties of self-assembled
aggregates, UV–vis and photoluminescence spectra of gelator 2
were measured in solution (25 wt% of dioxane in cyclohexane),
as well as in gels (cyclohexane/dodecane gels). Dioxane, which is
miscible with cyclohexane, was used to disturb the intermolecular
hydrogen bond. Three samples were prepared to compare the gel to
the solution state, consisting of 0.5 wt% organogelator and 99.5 wt%
dodecane/cyclohexane, and 25 wt% dioxane and 74.5 wt% cyclohex-
ane and 0.5 wt% organogelator, respectively. In other words, the for-
mer is in gel state and the latter is in solution state. Fig. 2 shows the
UV–vis and PL spectra for gelator 2 and Fig. 6 shows the UV and PL
spectra for non-ESIPT analogue 3. In the gels, J-aggregate formation
resulted in a ∼23–26 nm redshiftofthe UV–visabsorption band and
the PL intensity from these gel states was ∼30–32 fold increased
relative to a solution of the same concentration. The fluorescence
Fig. 5. FT-IR spectra of gelator 2 in (a) solution 12.0 mg/mL of CDCl3, (b) 1 wt% of
wet gel in cyclohexane, (c) 1 wt% of dried gel/xerogel in cyclohexane, and (d) solid
powder.
A continuous red shift observed in short wavelength (SW) ꢀmfl ax
with increase in polarity and hydrogen bonding nature of solvents
indicates a greater charge transfer interaction from the substituents
to naphthanilide ring and increase in the delocalization of lone pair
quantum yield (˚PL) of ESIPT gelator is 2.0 × 10−3, 1.6 × 10−3
,
of the hydroxy group and ␲ cloud of the
C O group through-
2.3 × 10−2, 2.1 × 10−2 in cyclohexane, chloroform, methanol and
PMMA film doped with 10 wt% of 2, respectively. Whereas ˚PL of
non-ESIPT methoxy derivative is 5.4 × 10−2, 1.5 × 10−2, 2.1 × 10−2
and 1.2 × 10−1 in cyclohexane, chloroform, methanol and PMMA
film doped with 10 wt% of 3, respectively. The maximum emis-
sion peaks of both gels are located at 558 nm with a large Stokes’
shift (180–182 nm) are observed relative to the absorption of the
gels. This emission is thought to originate from the proton trans-
ferred keto (K*) form of molecule 2 (Figs. 1 and S10). Presence of
enol emission suggests that the locally excited state decays (E*)
mainly a competitive process rather effective ESIPT with only tau-
tomer emission in both solution and gel state, which was further
confirmed theoretically as summarized in Table 3. Enhanced flu-
orescence emission was observed not only from the gels, but also
from solid state crystal and powder state of compounds 2 and 3.
These results suggest that the enhanced fluorescence emission of
the gel is caused by the formation of molecular aggregations.
To further study the relationship between their molecular
structures and optical properties, the geometrical parameters of
keto–enol tautomers and their energy-minimized, preferred con-
formations were calculated by DFT using B3LYP/6-31G(d,p) and
semi-empirical using AM1 calculations. The selected parameters
out the aromatic ring in S1 state (Fig. S11). On the other hand
Fig. 6. UV–vis absorption at left and photoluminescence spectra at right of non-
ESIPT chromophore 3 in selected solvents and a polymethylmethacrylate (PMMA)
solid film.

46
M.K. Nayak / Journal of Photochemistry and Photobiology A: Chemistry 217 (2011) 40–48
Table 3
Energetic characterization of keto form (K) and enol form (E) in ground (S0) and excited (S1) states in the gas phase.
AM1 (TDDFT)
S0
S1
HNAHPA-E
HNAHPA-K
HNAHPA-E
HNAHPA-K
E (kcal mol−1
)
0 (0)
12.1 (16.39)
8.41 (9.56)
16.9 (0.6)
166.5 (179.6)
160.4 (166.4)
42.8 (25.9)
508 (542)
706
13.4 (37.81)
2.55
33.1
179.5
162.3
45.6
0 (0)
6.73
24.6
166.0
162.7
40.0
ꢁ (D)
3.92 (6.34)
36.7 (9.4)
178.6 (176.4)
161.5 (168.6)
46.1 (26.1)
311 (385)
336
ϕ1 (◦)
ϕ2 (◦)
ϕ3 (◦)
ϕ4 (◦)
Excitation energy (nm)
Emission energy (nm)
are summarized along with experimentally observed absorption
and emission maxima in Table 3. Dihedral angles (ϕ), and dipole
moments (ꢁ) in both S0 and S1 states of keto–enol tautomeric forms
of compound 2 clearly display a planar naphthanilide with twisted
alkyl amide side chain (Figs. S10 and S11).
465 nm and quantum efficiency 0.108 and 0.182, respectively.
However, the potential energy profile for enol to keto and reverse
proton transfer from keto to enol transformations upon photoex-
citation reveals that the barriers of 4.39 and 8.83 kcal mol−1 for
naphthanilide gelator 2 whereas, these barriers are reduced to
2.39 and 5.67 kcal mol−1 for salicylanilide gelator [36]. Smaller
interconversion barrier (E* → K*) of 2.39 kcal mol−1 in S1 state of
salicylanilide gelator allows highly efficient ESIPT to give only
the large Stokes’ shifted tautomer emission (9773 cm−1) whereas,
higher interconversion barrier (E* → K*) of 4.39 kcal mol−1 in S1
state of gelator 2 moreover allows competing processes originating
from both locally excited (normal emission) and tautomer emission
(9245 cm−1) as a result of dual emission. Interestingly, the fluores-
cence enhancement accompanying gelation 2 is ∼4 folds higher
than salicylanilide gelation induced enhancement.
Apart from the above mentioned chromophores, there are sev-
eral others that have been developed and reported recently samples
with enhanced emission induced by molecular aggregation [34–38]
and suggested that the enhanced fluorescence can be explained
by the planar conformation in the solid state, which can activate
the radiation process [39,40]. In the solution state, the molecule
remains twisted to some extent due to steric interactions, which
suppress the radiation process. On the other hand, in the gel state
aggregation induced planarization due to strong intermolecular
hydrogen bonding extends the effective ␲-conjugation which acti-
vates the radiation process. These intermolecular hydrogen bonds
probably reduce the bond rotation within gelator molecule that
prevents the nonradiative transition and as a result of that fluores-
cence enhancement also occurs to some extent [41]. The absorption
It must be noted that the geometry of the keto form in the ground
state (S0) was optimized, keeping the Oa–H distance, the same as
that of Od–H distance in the optimized geometry of enol form. Oth-
erwise, optimization will reverse back to the enol form here, Od
and Oa refer to the donor and acceptor oxygen atoms in the enol
form, respectively. The results of the calculations demonstrate that
the enol form is most stable and the keto form is 16.39 kcal mol−1
higher in energy. But, in the first excited singlet state (S1) the keto
form is 37.81 kcal mol−1 more stable than the enol form. Results
of AM1 calculations also predict similar results, except that the
enol form is 12.1 kcal mol−1 more stable than the keto form in
S0 state, whereas the keto form is 13.4 kcal mol−1 more stable
than the corresponding enol form in S1 state under isolated condi-
tions. The transformation from enol to keto in the S0 and S1 states
can be thought of as arising from proton transfer from Od to Oa,
with concomitant redistribution of electron density in and around
the six-membered hydrogen bonded ring. Alternatively, one could
view this as a hydrogen atom transfer. In either case, one needs
to identify the ‘reaction co-ordinate’ and investigate the potential
energy change along the reaction coordinate. As the proton translo-
cation distance of the mobile hydrogen atom is considered to be a
key parameter for the construction of the ESIPT potential, the Od–H
distance r(O –H) was varied between what is normal for the primary
and what isdknown to be the equilibrium tautomeric Od–H distance.
At each such point, all other geometrical parameters were fully
optimized (with 8-configuration CI in S0 and S1 states), and the total
energy (ꢂEt) was plotted against r(O –H) using the AM1 method
d
(Fig. 7). The resulting potential energy (PE) profile again reveals that
the enol form is most stable in the ground state whereas the keto
form is most stable in the first excited singlet (S1) state. The barrier
for the enol to keto transformation is substantial: 22.37 kcal mol−1
,
large enough to make ground state intramolecular proton transfer
(GSIPT) unviable (E → K) under thermal conditions, whereas upon
photoexcitation, much smaller interconversion barrier (E* → K*)
of 4.39 kcal mol−1 in S1 state preferably allows ESIPT to give the
large Stokes’ shifted tautomer emission (9245 cm−1). After decay-
ing to the ground state, the keto form reverts to the original enol
form via reverse proton transfer barrier (K → E) of 8.83 kcal mol−1
.
Moreover, the intrinsic four-level process (E → E* → K* → K → E)
provides an ideal scheme for stimulated emission by easy popu-
lation inversion of the proton transferred keto form.
It is worth to be mentioned here that naphthanilide gela-
tor 2 gives dual emission (orange color) in cyclohexane solution
and solid film state at 446, 574 and 393, 558 nm and quantum
efficiency 0.002 and 0.021, whereas under similar condition its
resemblance N-heptyl-2-(2-hydroxybenzamido)benzamide (sali-
cylanilide gelator) [Ref. 36] gives only tautomer emission (blue
color) in cyclohexane solution and solid film state at 472 and
Fig. 7. Potential energy profile for intramolecular proton transfer in the ground and
excited states of 2 obtained by varying r(O –H) and optimize the rest of structural
parameters for each choice of r(O –H) usingdAM1/PECI = 8 calculation.
d

M.K. Nayak / Journal of Photochemistry and Photobiology A: Chemistry 217 (2011) 40–48
47
observed in 2 could then be rationalized and supported by com-
bining these theoretical calculations with the previously suggested
molecular orbital (MO) calculations on deactivation processes of
the N-salicylideneanils [44], and ortho-substituted N-heptyl amide
of salicylanilides [36], the most probable rotation involving the
keto-rotated species is the phenyl–CO bond. Under such condi-
tions, the fluorescence of the cis-keto* isomer can be quenched
by dynamic internal torsion processes in the excited state lead-
ing to the formation of twisted-keto* (Fig. 1). This species, accessed
after ESIPT, involves a twisted intramolecular charge transfer state
(TICT), which brings about the fluorescence quenching observed in
solution. Accordingly, the fluorescence enhancement in the solid
state is due to the prevented TICT by kinetic constraint which blocks
large amplitude twisting motion. In essence, the enhanced fluores-
cent behavior of gelators was attributed to its rigid structure, which
precludes extensive twisting about the naphthyl–CO bond (torsion
angle, ϕ1) along the ESIPT reaction coordinate. This allowed the
tuning of emission by controlling the degree of aggregation, which
might be useful in the design of fluorescent labels [45] and optical
sensors [46].
Fig. 8. Potential energy profiles for the keto forms of compound 2 in the ground
(K) and excited (K*) singlet states with respect to the torsional angle around the
naphthyl–CO, HN–CO, and phenyl–NH bond rotations using AM1/PECI = 8 calcula-
tion.
4. Conclusion
band of the gel with a red-shift of ∼23–26 nm relative to that in
solution, also contributes towards the molecular planarization to
the enhanced emission. It is well known that an increase in the
rigidity of a molecule can decrease molecular vibrations, probably
suppress the internal conversion (IC) or twisted intra-molecular
charge transfer (TICT) of an excited molecule, and may increase the
fluorescence quantum yield [38,42]. In addition, the fluorescence
intensity of the ␲-conjugated chromophores is correlated with the
␲ aggregation, such as H and J aggregations [40]. H aggregations, in
which the molecules are aligned in parallel to each other in a head-
to-head form, tend to increase the internal conversion from a higher
electronic state into a lower one so that the fluorescence emission
is effectively quenched. In contrast, the molecules are arranged into
a head-to-tail stack as in J aggregation, in which the transition from
the lower couple excited state of the molecule to the ground state is
allowed; as a result, the absorption peak will red-shift and the fluo-
rescence emission will be stronger than that of the monomer [43].
In our case, the gelator molecules form J aggregations and cross-link
into a solid-like 3D network in the gel phase, so the enhanced emis-
sion of the gel is attributed to the synergetic effect of the restricted
molecular motion and the formation of J aggregations.
In Fig. 8, the geometry dependent potential energies in the S0
and S1 states for the keto forms of ESIPT gelator 2 are displayed.
The rotations around the three flexible bonds have been analyzed
that can change their conformations in excited states; two aromatic
amide bonds (naphthyl–CO, ϕ1 and phenyl–NH, ϕ3) and the amide
bond itself (HN–CO, ϕ2) by changing geometrical parameters of
interest from the optimized geometry of 2. For the aromatic amide
bond (naphthyl–CO) twist in the excited state, the potential curve
shows the maximum values at 0◦ and the minimum at 105◦. The
appearance of S1 potential energy surface indicates that there is no
barrier for large amplitude twisting about the naphthyl–CO bond.
Furthermore, the maximum at 90◦ in the S0 state produces a smaller
gap between the ground and first excited states. As a result, the non-
radiative transition from the minimum in S1 to the ground state is
likely to be important; consequently, this structure can be expected
to lead to the quenching of the keto fluorescence. As for the rotation
around the amide bond (HN–CO) in the S1 state, potential energy
curve shows a maximum at 60◦ thereby producing an estimated
activation barrier of 11.44 kcal mol−1. In the potential energy pro-
file for the rotation around phenyl–NH in the excited state, we
have found only one maximum at 75◦ with an interconversion bar-
rier of 16.12 kcal mol−1. Gelation-induced fluorescence emission
Highly emissive organogelator 2 containing ESIPT moiety bear-
ing a chelating platform was synthesized and characterized.
Through combining the results of FT-IR, UV–vis absorption, pho-
toluminescence and DFT, semi-empirical (AM1) calculations, the
molecular packing model in the gel phase was deduced. It has
been demonstrated that gelator 2 is self-assembled into complex
3-D networks and their aggregation into fibrous superstruc-
tures is driven by ␲–␲ stacking interactions between the central
naphthanilide moieties, hydrogen-bonding interactions among the
amide, OH· · ·O C groups, and van der Waals interactions among
the alkyl groups. In the gels, J-aggregate formation resulted in a
∼23–26 nm red shift of the UV–vis absorption band and the PL
intensity from these gel states was ∼30–32 folds higher than that
of the solution phase. It was found that the nonradiative relaxation
process via TICT is reduced in the hydrogen bonded supramolecular
assembly and gel states leading to the enhanced ESIPT emission.
Acknowledgments
The author is gratefully acknowledged to Professor Soo Young
Park, Department of Materials Science and Engineering, Seoul
National University, Seoul, South Korea, for useful discussions and
comments. I would also like to be grateful to Professor Hassan S.
Bazzi, Department of Chemistry, Texas A&M University at Qatar,
Doha, Qatar for giving me technical knowledge in several fields.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jphotochem.2010.09.014.
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