Gelation-induced enhanced fluorescence emission from organogels of salicylanilide-containing compounds exhibiting excited-state intramolecular proton transfer: Synthesis and self-assembly

Article abstract of DOI:10.1002/chem.200902615

Self-assembly structure, stability, hydrogen-bonding interaction, and optical properties of a new class of low molecular weight organogelators (LMOGs) formed by salicylanilides 3 and 4 have been investigated by fieldemission scanning electron microscopy (

Full text of DOI:10.1002/chem.200902615

FULL PAPER
DOI: 10.1002/chem.200902615
Gelation-Induced Enhanced Fluorescence Emission from Organogels of
Salicylanilide-Containing Compounds Exhibiting Excited-State
Intramolecular Proton Transfer: Synthesis and Self-Assembly
Manoj Kumar Nayak,[b] Byung-Hwa Kim,[a] Ji Eon Kwon,[a] Sanghyuk Park,[a]
Jangwon Seo,[a] Jong Won Chung,[a] and Soo Young Park*[a]
Abstract: Self-assembly structure, sta-
bility, hydrogen-bonding interaction,
and optical properties of a new class of
low molecular weight organogelators
(LMOGs) formed by salicylanilides 3
and 4 have been investigated by field-
emission scanning electron microscopy
(FESEM), X-ray diffraction (XRD),
UV/Vis absorption and photolumines-
cence, as well as theoretical studies by
DFT and semiempirical calculations
with CI (AM1/PECI=8) methods. It
was found that salicylanilides form gels
in nonpolar solvents due to p-stacking
interaction complemented by the pres-
ence of both inter- and intramolecular
hydrogen bonding. The supramolecular
arrangement in these organogels pre-
dicted by XRD shows lamellar and
hexagonal columnar structures for ge-
lators 3 and 4, respectively. Of particu-
lar interest is the observation of signifi-
cant fluorescence enhancement accom-
panying gelation, which was ascribed to
the formation of J-aggregates and in-
hibition of intramolecular rotation in
the gel state.
Keywords: fluorescence · gels · pro-
ton transfer · salicylanilides · self-
assembly
Introduction
bonding,[4] donor–acceptor interactions,[5] and metal coordi-
nation.[6] Subsequently, higher aggregates are formed
through intercomplex interactions (hydrogen bonding, p-
stacking interactions, or van der Waals forces) to form the
fully gelated network. Organogelators can self-assemble into
various types of aggregates such as fibrils,[7] strands, and
tapes.[8] These aggregates are highly entangled and cross-
linked through a junction zone[9] to form a three-dimension-
al (3D) network that immobilizes excess solvent molecules.
The organogels have many applications such as biocatalysts,
hydrometallurgy, and cosmetics. Hence, intense research has
focused on the synthesis and study of many types of organo-
gelators.[10] Recently, there has been increasing interest in
the development of functional gel systems with p-conjugated
molecular structures due to their potential application
in various optoelectronic fields, including enhanced
charge transport,[11] light harvesting,[12] fluorescence, and
sensing.[4a,13]
Over the past decade, low-molecular-weight organic gelators
have attracted considerable interest because of their diverse
applications as supramolecular soft materials.[1] Organogels
are nonflowing viscoelastic materials produced by adding a
small amount of organogelator (usually less than 1 wt%) to
an excess of organic solvent. The formation of organogels is
facilitated by highly directional self-assembly through non-
covalent interactions such as electrostatic interaction,[2] in-
termolecular hydrogen bonding,[3] intramolecular hydrogen
[a] B.-H. Kim, J. E. Kwon, Dr. S. Park, Dr. J. Seo, J. W. Chung,
Prof. S. Y. Park
Center for Supramolecular Optoelectronic Materials
and WCU Hybrid Materials Program
Department of Materials Science and Engineering
Seoul National University, ENG 445
San 56-1, Shillim-dong, Kwanak-ku, Seoul 151-744 (Korea)
Fax : (+82)2-886-8331
In an attempt to obtain a new functional organogelator
with potential photonic applications, we have designed and
synthesized photoactive organogelators containing peripher-
al alkyl amide for gelation and salicylanilide (2-hydroxy-N-
phenylbenzamide) for fluorescence emission (Scheme 1). To
realize a rather simple but very efficient organogelator
structure, long alkyl chains with an amide group, which is
E-mail: parksy@snu.ac.kr
[b] Dr. M. K. Nayak
Current address: Department of Chemistry
Texas A&M University at Qatar
PO Box 23874, Doha (Qatar)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200902615.
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photoluminescence with specific focus on the spectroscopic
difference between solution and gel states. Theoretical cal-
culations were carried out to better understand the driving
force for gelation as well as the mechanism of ESIPT emis-
sion. Structural effects of ESIPT molecules on twisted intra-
molecular charge transfer (TICT) induced fluorescence
quenching were analyzed by means of DFT calculations.
Results and Discussion
Design principle, synthesis, and self-assembly structure of
organogels: ortho-Substituted N-heptyl amide and O-heptyl
ester derivatives of salicylanilide-containing compounds 3, 4,
and 10 and benzanilide-type non-ESIPT analogues 7 and 8
were synthesized according to the procedure depicted in
Scheme 2. The cyclized intermediate 2-hydroxyphenyl be-
zoxazine-4-one derivatives 1 and 2 were prepared by dehy-
drocyclization between salicylic acid and the corresponding
anthranilic acid with triphenyl phosphite in pyridine in 20–
30% yield. The coupling reaction between benzoyl chloride
and the corresponding anthranilic acid in pyridine to afford
2-phenyl bezoxazine-4-one intermediates 5 and 6 proceeded
in 50% yield. Subsequently, amidolysis of 1, 2 and 5, 6 with
n-heptylamine in pyridine led to the final ESIPT gelator
products 3, 4 and non-ESIPT analogues 7, 8, in yields of 60–
80%. Intermediate compound 9 was obtained by hydrolysis
Scheme 1. Four-level photochemical and photophysical processes in an
ESIPT system, represented by 3.
widely known for its gelation power, were selected and at-
tached to a salicylanilide part with unusual fluorescence.
Fluorescence emission from salicylanilide derivatives has
been widely investigated and it was concluded that the emis-
sion originates from excited-state intramolecular proton
transfer (ESIPT).[14] Salicylani-
lides have also been the subject
of extensive investigations in
medicinal chemistry, due to
their ability to serve as inhibi-
tors of the protein tyrosine
kinase epidermal growth factor
receptor (EGFR PTK), related
to cancer, psoriasis, and reste-
nosis.[15] However, the self-as-
sembly behavior and related
properties of salicylanilide de-
rivatives remain unexplored in
the field of functional gels and
supramolecular organic soft
materials. Our target salicylani-
lide gelators (3 and
4
in
Scheme 2) were designed to un-
dergo both intermolecular and
intramolecular hydrogen bond-
ing. As a reference compounds,
we synthesized benzanilide ana-
logues 7 and 8 to investigate
the specific role of ESIPT-
active salicylic hydroxyl groups
in 3 and 4 in the gelation prop-
erties. We carried out SEM and
XRD studies to characterize
the structure of gels, and mea-
Scheme 2. Synthesis of salicylanilide-containing organogelators 3 and 4 and benzanilide-type non-ESIPT ana-
logues 7 and 8. a) Triphenyl phosphite, pyridine, 1008C, 2 h; b) pyridine, 1008C, 24 h; c) Pyridine, RT, 5 h;
sured UV/Vis absorption and d) pyridine, 1008C, 24 h; e) triphenylphosphine, diethylazodicarboxylate (DEAD), RT, 24 h.
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Enhanced Fluorescence Emission of Organogels
FULL PAPER
of 2 with aqueous potassium hydroxide and subsequently es-
terified with heptanol by using the Mitsunobu reagent from
triphenylphosphine and diethyl azodicarboxylate to give 10
in 61% yield. The molecular structures were fully character-
ized by FTIR and 1H NMR spectroscopy, MS-EI/HRMS,
and elemental analysis.
Gelation ability was estimated in various organic solvents
(Table 1). Organogelators 3 and 4 induce gelation in nonpo-
lar solvents such as dodecane and n-hexane with critical ge-
the amide I and nNH bands as well as the blueshift of the dNH
band indicate that intermolecular hydrogen-bond formation
occurs in the gel state. Moreover, the absence of any bands
above 3400 cmꢀ1 in the gel state indicates the presence of
strongly hydrogen bonded NH groups in the supramolecular
gel network. The NH stretching frequency of 3312 cmꢀ1 in
the solid state suggests the presence of strong intermolecular
hydrogen-bonding association in the dried gel. Also, shifting
of antisymmetric (nas) and symmetric (ns) CH2 stretching fre-
quency bands was observed
from 2929 and 2854 cmꢀ1 in the
Table 1. Basic properties of organogels.
solution phase to 2925 and
Compound
Form
gel
CGC
Solvent
Transparency
opaque
Tgel [8C]
2854 cmꢀ1 in the gel state, 2925
and 2853 cmꢀ1 in the solid state,
respectively. The decrease in
fluidity of the hydrophobic
chains due to the formation of
strong aggregates by van der
Waals interaction is evident
3
0.2 wt%
n-hexane,
dodecane
n-hexane,
dodecane
70
4
gel
0.2 wt%
translucent
80
7
8
10
needle-shaped crystals
diamond-shaped crystals
needle-shaped crystals
lation concentrations (CGC) as low as 0.2 wt%, but not in
polar solvents. Likely, the amide bond provides the main
force for gelation through intermolecular hydrogen bonding,
which can be indirectly evidenced by means of model com-
pound 10, which has an ester bond instead of an amide
bond. Indeed, model compound 10 did not immobilize sol-
vents but formed crystals, that is, intermolecular hydrogen
bonding of the alkyl amide group is an important factor for
gelation. Moreover, intramolecular hydrogen bonding of the
salicylanilide group in 3 and 4 is also essential to the gela-
tion behavior, since benzanilides 7 and 8, which lack that
structural element, cannot gelate solvents but form needle-
or diamond-shaped crystals. This implies that intramolecular
hydrogen bonding in the salicylanilide unit contributes to a
planarization effect that not only allows ESIPT but also im-
proves p stacking for the gelation.
Figure 1. FTIR spectra of gelator 4 in a) solid powder, b) 1 wt% gel in n-
Considering the structures of the LMOGs prepared in
this work, and the well-known fact that hydrogen bonding
interaction among amide groups is one of the main driving
forces for the self-assembly of organogelators in organic sol-
vents, FTIR spectroscopy is an important tool for investigat-
ing the different noncovalent interactions involved in gela-
tion.[16,17] To obtain structural information on self-assembled
materials, FTIR spectra were measured in three different
states: solution (1 wt%) in chloroform, gel (1 wt%) in n-
hexane, and solid powder. In solution, four main characteris-
tic peaks appeared at 3395, 2929 and 2854, 1607, and
1523 cmꢀ1 for OH or amide NH asymmetric stretching (nOH
or nNH), antisymmetric (nas) and symmetric (ns) CH2 stretch-
ing, amide C=O stretching (nCO, amide I), and NH bending
(dNH, amide II), respectively (see the spectra of 4 and 8 in
Figure 1 and Figure S5 (Supporting Information);Table S1 of
the Supporting Information lists band assignments). In the
gel (dried gel or powdered solid) state (Figure 1) the nOH or
nNH, amide I, and dNH peaks are shifted to 3390 (3312), 1604
(1600), and 1524 (1532) cmꢀ1, respectively. The redshifts of
hexane, and c) 1 wt% of chloroform solution.
from this particular shift in the CH2 stretching frequency.[17a]
Thus, it is concluded that hydrogen bonding and van der
Waals interaction are the major factors contributing to gela-
tion of 3 and 4. In contrast, in non-ESIPT analogue 8 in so-
lution (solid) the nNH and amide I bands appear at 3367
(3418) and 1602 (1663) cmꢀ1, that is, wavenumbers typical of
non-hydrogen-bonded systems[16d,17c,d] (see Figure S5 in the
Supporting Information), and evidence of their nonpartici-
pation in the aggregation processes. Further insight into the
hydrogen-bonding abilities of gelators was obtained by theo-
retical consideration of all possible pairing modes of dimers.
These calculations provide us with a more general picture of
the ability to form hydrogen-bonded complexes, and the
major driving force for gelation is discussed on the basis of
DFT calculations (vide infra).
To obtain visual insights into the aggregation mode, we
observed the xerogel morphologies of n-hexane/dodecane
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S. Y. Park et al.
gels 3 and 4 with a field emission scanning electron micro-
scope (FE-SEM). As shown in Figure 2, many stiff fibrillar
aggregates with diameters of several tens of micrometers
were observed for 3. The SEM images of xerogel 4 showed
flexible fibrillar aggregates with 100 nm diameter, which are
entangled with each other to give a 3D network.
Three plausible isomeric dimers were built up with head-to-
head intermolecular hydrogen bonding of the salicylanilide
X-ray diffraction (XRD)[18] has great potential for eluci-
dating the molecular structure of organogels and providing
information about long-range ordering in the molecular self-
assembly, from which a packing model of molecules in the
gel phase can be proposed. Figure 3 shows the XRD pattern
of the xerogel of 3 prepared in dodecane (1.0 wt%), which
has three reflections at 18.40, 9.16, and 6.16 ꢁ in the low-
angle region (reciprocal spacing ratio of 1:1:1) correspond-
ing to (100), (200), and (300) of a lamellar packing. A single
XRD peak observed at 7.63 ꢁ most likely originates from
the length of the parent salicylanilide moiety without the
alkyl chains. The interlamellar spacing of 18.40 ꢁ is some-
what larger than the molecular lengths of 15.55 and 16.07 ꢁ
for gelators 3 and 4, respectively, calculated by the AM1
method. Thus, an interdigitated arrangement of molecules in
the gel is proposed to be the most plausible structure. Inter-
molecular alkyl-chain interdigitation leading to lamellar p
stacking or hydrogen-bonding stacking of gelator 3 was as-
sumed on the basis of these XRD studies. The X-ray diffrac-
tion pattern of the dodecane xerogel of 4 showed four low-
angle reflections, at 25.00, 14.57, 12.35, and 9.42 ꢁ (recipro-
p
cal ratio of 1: 3:1= ), which correspond to columnar hexago-
2
nal packing.[19] An additional singular peak was found at
7.89 ꢁ. Even though the chemical structure of gelator 4 is
almost the same as that of 3, the arrangement of molecules
was totally different, possibly
Figure 2. FESEM images of xerogels of a) 3 and b) 4.
due to the two methoxyl
groups. Given the different
packing structures of xerogels 3
and 4, it is apparent that the
two methoxyl groups exert sig-
nificant steric control on their
packing modes. Because of the
steric requirement of the two
methoxyl groups, it seems likely
that molecules of 4 can not self-
assemble face-to-face. Instead,
a
somewhat tilted stacking
structure is favored due to the
two methoxyl substituents. This
specific arrangement is consid-
ered to produce a columnar
structure arranged in hexagonal
close packing.
To investigate the driving
force of self-assembly in 3 and
4, we calculated the energies of
model aggregate structures sta-
bilized by cooperative p -stack-
ing, hydrogen bonding, and van
Figure 3. XRD patterns of xerogels of 3 (top), 4 (bottom), and schematic representation of arrangements in
der Waals interactions by DFT. gels.
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Enhanced Fluorescence Emission of Organogels
FULL PAPER
stacked>T shape>chair form
also suggests the same order of
gelation capability. This result
implies that the gelation is pri-
marily driven by the p-stacking
interaction and is significantly
assisted by inter- and intramo-
lecular hydrogen bonding. It is
noteworthy that the calculated
dihedral angle of about 16–198
between two planes of p-
stacked dimers favors the for-
mation of slightly tilted geome-
tries, as revealed by XRD stud-
Figure 4. Fully optimized structures for all three dimer systems of gelator 3 at the B3LYP/6-31G** level of
theory in the gas phase. Dark gray balls represent carbon atoms, light gray balls hydrogen, blue balls nitrogen,
and red balls oxygen.
unit (chair form), tail-to-tail intermolecular hydrogen bond-
ing of the alkyl amide unit (T shape), and head-to-tail p-
stacked dimers (Figure 4). The structure and energetics of
these hydrogen-bonded and stacked dimers of gelator 3
were investigated by DFT B3LYP/6-31G** computations in
the gas phase (Table 2). The calculated interaction energies
for the chair form, T shape, and p-stacked dimers are 3.10,
7.33, and 9.81 kcalmolꢀ1. Hence the order of stability of p-
ies on dodecane xerogel, such as lamellar and hexagonal col-
umnar structures for gelators 3 and 4, respectively. This is
considered beneficial in attaining higher efficiency of fluo-
rescence emission by favorably reducing the intermolecular
vibronic interactions which would otherwise induce nonra-
diative deactivation processes.
Gelation-induced enhanced fluorescence emission: To study
the spectroscopic properties of self-assembled aggregates,
UV/Vis absorption and photoluminescence spectra were re-
corded in dodecane, as it was a good gel-forming solvent.
Propanol, which is miscible with dodecane, was used to dis-
turb the intermolecular hydrogen bonding. Two samples
were prepared to compare the gel to the solution state:
99.5 wt% dodecane and 0.5 wt% organogelator (gel state),
and 66.3 wt% dodecane, 33.2 wt% n-propanol, and 0.5 wt%
organogelator (solution state). Figure 5a shows the UV and
PL spectra for gelator 3 and Figure 5b shows the UV and
PL spectra for gelator 4. The gel state gave a markedly
broader band[10d] absorption spectrum in comparison to the
solution state. The apparent absorption tail in the longer
wavelength region is due to the scattering effect produced
by gelation. An emission with large Stokes shift observed at
470 nm in both gel and solution state is thought to originate
Table 2. Energetic [kcalmolꢀ1] and geometric parameters of dimers (gas
phase, 298.15 K).[a]
Conformer Bond lengths [ꢁ]
DEcec Dcec
DH
ꢀ
chair form
T shape
Oa···H Od 2.71 (inter-) 3.10
3.39
8.15
ꢀ1.9
ꢀ
Oa···H Od 1.66 (intra-)
ꢀ
Oa···H Nd 1.78 (intra-)
ꢀ
Oa···H Nd 1.98 (inter-)
7.33
9.81
ꢀ5.8
ꢀ
Oa···H Od 1.65 (intra-)
ꢀ
Oa···H Nd 1.81 (intra-)
ꢀ
p stacked
Oa···H Nd 2.18 (inter-)
10.67 ꢀ8.5
ꢀ
Oa···H Od 1.59 (intra-)
ꢀ
Oa···H Nd 1.75 (intra-)
[a] DEcec : binding energy, difference in electronic energies after including
the counterpoise correction; Dcec : equilibrium dissociation energy, energy
difference including zero-point energy and counterpoise correction; DH:
thermodynamic corrections for enthalpy in the gas phase.
Figure 5. Normalized UV/Vis absorption and photoluminescence spectra of a) 3 and b) 4 in solution (solid line) and gel (dashed line) state.
Chem. Eur. J. 2010, 16, 7437 – 7447
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S. Y. Park et al.
from the proton-transferred keto (K) form of 3 shown in
Scheme 1. Complete absence of enol emission suggests that
the excited enol form (3-E*) decays mainly to the excited
keto form (3-K*) by effective ESIPT rather than to the
ground enol form (3-E) with normal emission in both solu-
tion and gel state, which was further confirmed theoretically
(see Table 3). The intensity of photoluminescence in the gel
10 wt% of 3 was 0.182. Similarly, FPL of 4 are 0.093, 0.002,
and 0.002 in cyclohexane, chloroform, and n-propanol, re-
spectively, while the FPL of 0.135 for PMMA film doped
with 10 wt% of 4 indicates “rigidochromism” effected by re-
stricted intramolecular twisting motion of the salicylanilide
group in the gel or polymer film.
To further study the relationship between molecular struc-
tures and optical properties, the
geometrical
keto–enol tautomers and their
energy-minimized, preferred
parameters
of
Table 3. Energetic characterization of keto form (3-K) and enol form (3-E) in ground (S0) and excited (S1)
states in the gas phase.
B3LYP/6-31G**
3-E
3-K
B3LYP TD/6-31G**
excited state (S1)
3-E*
3-K*
conformations were calculated
by DFT B3LYP/6-31G** and
semiempirical calculations at
the AM1/PECI=8 level of
theory. The selected parameters
are summarized along with ex-
perimentally observed absorp-
tion and emission maxima in
ground state (S0)
relative energy [kcalmolꢀ1
]
0
13.14
5.06
2.39[a]
347[a]
–
0
barrier height [kcalmolꢀ1
excitation energy [nm]
]
23.65[a]
325.97
314;[a] 323[b]
411.45
405[a]
emission energy [nm]
461[a]
472[b]
[a] Semiempirical AM1/PECI=8 calculation. [b] Experimentally observed data in cyclohexane.
state was sevenfold enhanced over that in the solution state.
We speculate that in the solution of 3, nonradiative decay
process such as internal conversion (IC) and twisted intra-
molecular charge transfer (TICT) preferentially take place,
as reported previously for benzanilide[20] and salicylideneani-
line derivatives.[21] In contrast, it seems that these detrimen-
tal nonradiative decay processes are suppressed in the gel
state because of reduced motional relaxation due to gela-
tion. Significant fluorescence enhancement was observed to
accompany gelation, which was attributed to the intermolec-
ular stacking and inhibition of intramolecular rotation in the
gel. Analogous phenomena were previously observed by
others,[12b,22] whereby inhibation of intramolecular rotation
and formation of J-aggregates were also shown to enhance
fluorescence emission. Enhanced fluorescence emission was
observed not only from the dodecane gel, but also from
crystal and powdered solid states of gelators 3 and 4. In the
PL spectrum of 4, even more dramatic fluorescence en-
hancement was observed compared to 3. The PL intensity
from the gel state was almost fifty times larger than that
from the solution state. The fluorescence quantum efficien-
cies FPL of ESIPT for gelator 3 are 0.108, 0.032, and 0.024 in
cyclohexane, chloroform, and n-propanol, respectively, while
FPL of poly(methyl methacrylate) (PMMA) film doped with
Table 3. Selected bond lengths, dihedral angles, and charge
densities on the atoms involved in ESIPT, which were com-
puted by Mulliken population analysis from optimized
B3LYP geometries, are shown in Figure 6, which clearly dis-
plays a planar salicylanilide with twisted alkyl amide group.
Note that the geometry of the keto form in the ground state
ꢀ
(S0) was optimized while keeping the Oa H distance the
ꢀ
same as the Od H distance in the optimized geometry of
the enol form, where Od and Oa refer to the donor and ac-
ceptor oxygen atoms in the enol form, respectively. Other-
wise, the optimization will revert to the enol form. The cal-
culations demonstrate that the enol form is most stable, and
the keto form is 13.14 kcalmolꢀ1 higher in energy. This is un-
derstandable because proton transfer in the enol form re-
sults in two OH groups being bonded to an olefinic carbon
atom in the keto form. Moreover, there is partial loss of aro-
maticity of the benzene ring, as suggested by Sobolewski
and Domcke.[23] However, in the first excited singlet state
(S1) the keto form is 5.06 kcalmolꢀ1 more stable than the
enol form. The AM1 calculations also predict similar results,
except that the enol form is 19.47 kcalmolꢀ1 more stable
than the keto form in the S0 state, whereas the keto form is
9.08 kcalmolꢀ1 more stable than the corresponding enol
form in the S1 state under isolated conditions.
Figure 6. Optimized structure of keto–enol forms of gelator 3 at correlated DFT (B3LYP) level with 6-31G** basis set in the gas phase. Dark gray balls
represent carbon atoms, light gray balls hydrogen, blue balls nitrogen, and red balls oxygen.
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Enhanced Fluorescence Emission of Organogels
FULL PAPER
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. Al-
ternatively, one could view this as a hydrogen-atom transfer.
In either case, one needs to identify the “reaction coordi-
nate” and investigate the potential-energy change along it.
ꢀ
We chose to vary rACHTNUGTRNEUNG(Od H) and optimize the rest of the
structural parameters for value of this parameter using the
AM1/PECI=8 method (Figure 7). The resulting potential-
Figure 8. Potential-energy profiles for the keto form of 3 in the ground
(K) and excited (K*) singlet states with respect to the torsional angles
ꢀ
ꢀ
ꢀ
around the Ar CO, N CO, and Ar N bonds (AM1/PECI=8 calcula-
tion).
ꢀ
the aromatic amide bond (Ar CO) in the excited state, the
potential curve shows the maximum value at 08 and the min-
imum at 908. The appearance of the S1 potential-energy sur-
face indicates that there is no barrier for large-amplitude
ꢀ
twisting about the Ar CO bond. Furthermore, the maxi-
mum at 908 in the S0 state produces a smaller gap between
the ground and first excited states. As for the rotation
Figure 7. Potential-energy profile for intramolecular proton transfer in
ꢀ
around the amide bond (N CO) in the S1 state, the poten-
the ground and excited state of 3 obtained by varying r(OdꢀH) and opti-
G
tial-energy curve shows a maximum at 908 and thereby pro-
mizing the remaining structural parameters for each choice of r
by means of AM1/PECI=8 calculation.
N
duces an estimated activation barrier of 18.09 kcalmolꢀ1. In
ꢀ
the potential-energy profile for rotation around Ar N in the
excited state, we found only one maximum at 1058 with an
interconversion barrier of 17.54 kcalmolꢀ1. Gelation-induced
fluorescence emission in 3 and 4 could then be rationalized
and supported by combining these theoretical calculations
with the previously reported findings given below: in a rigid
energy 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 of
23.65 kcalmolꢀ1 for enol to keto transformation (3-E!3-K)
is large enough to make ground-state intramolecular proton
transfer (GSIPT) unviable under thermal conditions, where-
as upon photoexcitation a much smaller 3-E*!3-K*inter-
conversion of 2.39 kcalmolꢀ1 in the S1 state allows highly ef-
ficient ESIPT to give the strongly Stokes shifted tautomer
emission (10155 cmꢀ1), which is in good agreement with ex-
perimental observation (9773 cmꢀ1). After decaying to the
ground state, the keto form reverts to the original enol form
over the reverse proton-transfer barrier (3-K!3-E) of
5.67 kcalmolꢀ1. Moreover, the intrinsic four-level process (3-
E!3-E*!3-K*!3-K!3-E) provides an ideal scheme for
stimulated emission by easy population inversion of the
proton-transferred keto form.
ꢀ
ꢀ
ꢀ
matrix at 77 K, the flexible Ar CO, N CO, and Ar N bond
rotations are inhibited. The fluorescence intensity increases
45–50 times with respect to that observed at 294 K.[20a] Upon
light absorption, the Franck–Condon excited state maintains
the enol geometric structure of the fundamental state and
then the subsequent intramolecular excited-state proton
transfer enol*!cis-keto* takes place. In a fluid solution
(low viscosity) at 293 K after ESIPT, that is, when the
system achieves a cis-keto* character, the MO calculations
suggest that the most probable rotation involving the keto-
ꢀ
rotated species is the Ar CO bond. Under such conditions,
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*.[21] This species, ac-
cessed after ESIPT, involves a twisted intramolecular charge
transfer state (TICT), which brings about the fluorescence
quenching observed in solution. Accordingly, the fluores-
cence enhancement in the solid state is due to prevention of
TICT by kinetic constraint, which blocks large-amplitude
twisting motion. In essence, the enhanced fluorescent behav-
Figure 8 shows the geometry-dependent potential energies
in the S0 and S1 states for the keto form of ESIPT gelator 3.
We analyzed the rotations around the three flexible bonds
that can change their conformations in excited states: two
ꢀ
ꢀ
aromatic amide bonds (Ar CO and Ar N) and the amide
ꢀ
bond itself (N CO) by changing geometrical parameters of
interest from the optimized geometry of 3. For twisting of
Chem. Eur. J. 2010, 16, 7437 – 7447
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
7443

S. Y. Park et al.
(3.70 g, 26.8 mmol) were dissolved in pyridine (50 mL) and the solution
stirred for 30 min. Triphenyl phosphite (7.0 mL, 26.7 mmol) was added to
this solution. The mixture was heated to 1008C for 2 h. The reaction mix-
ture was poured into cold water and extracted with dichloromethane.
The solution was dried with anhydrous magnesium sulfate. After removal
of the solvent, the crude product was purified by column chromatography
on silica gel with ethyl acetate/n-hexane (1/4). The product was recrystal-
lized from ethanol to give 2.30 g of pure product (yield 30%). 1H NMR
(CDCl3, 300 MHz): d=12.40 (s, 1H), 8.08 (d, 1H), 7.55 (s, 1H), 7.46 (t,
1H), 7.00 (m, 3H), 4.05 (s, 3H), 4.01 ppm (s, 3H); m/z (MS-EI) calcd:
299.28, found 299; elemental analysis calcd (%) for C16H13NO5: C 64.21,
H 4.38, N 4.68; found: C 64.26, H 4.40, N 4.55.
ior of gelators is attributed to its rigid structure, which pre-
ꢀ
cludes extensive twisting about the Ar CO bond along the
reaction coordinate.
Conclusion
Low-molecular-weight organogelators 3 and 4 containing
salicylanilide-based ESIPT moieties were synthesized and
characterized. Highly fluorescent organogels are easily
formed in nonpolar solvents such as dodecane and n-hexane
by virtue of the self-assembled lamellar and hexagonal col-
umnar structures of 3 and 4, respectively. By combining the
results of UV/Vis absorption and photoluminescence studies
and DFT and semiempirical (AM1) calculations, the molec-
ular packing model in the gel phase was deduced. The gela-
tors are self-assembled into complex 3D networks, and their
aggregation into fibrous superstructures is driven by p-stack-
ing interactions between the central salicylanilide moieties,
hydrogen-bonding interactions among the amide groups,
and van der Waals interactions among the alkyl groups. In-
terestingly, the fluorescence emission intensity of the orga-
nogels is 10–50-fold higher than that of the solution phase.
Nonradiative relaxation via a TICT is reduced in the hydro-
gen-bonded supramolecular assembly and gel states, and
this leads to enhanced ESIPT fluorescence emission.
N-Heptyl-2-(2-hydroxybenzamido)benzamide (3): Compound
1 (1.0 g,
4.2 mmol) and n-heptylamine (0.60 g, 5.2 mmol) were dissolved in pyri-
dine (50 mL). The solution was heated at 1008C and refluxed under N2
atmosphere for 24 h. The reaction mixture was poured into cold water
and neutralized with 1n HCl solution, after which the precipitate was
collected by filtration. The crude product was purified by column chro-
matography on silica gel with ethyl acetate/n-hexane (1/10). The product
was recrystallized from ethanol to give 1.50 g of pure product (yield
59%). 1H NMR (CDCl3, 300 MHz), d=12.37 (s, 1H), 12.34 (s, 1H), 8.67
(d, 1H), 7.80 (d, 1H), 7.52 (m, 2H), 7.43 (t, 1H), 7.16 (t, 1H),7.00 (m,
2H), 6.18 (s, 1H), 3.50 (q, 2H), 1.62 (m, 2H), 1.30 (m, 8H), 0.88 ppm (t,
3H); IR (KBr pellet): n˜ =3300 (nOH or NH), 1655 (nCO) cmꢀ1; m/z (MS-EI)
calcd: 354.44, found: 354; elemental analysis calcd (%) for C21H26N2O3:
C 71.16, H 7.39, N 7.90; found: C 71.40, H 7.50, N 7.87.
N-Heptyl-2-(2-hydroxybenzamido)-4,5-dimethoxybenzamide (4): Com-
pound 2 (1.0 g, 3.3 mmol) and n-heptylamine (1.0 g, 8.6 mmol) were dis-
solved in pyridine (30 mL). The solution was heated to reflux at 1008C
for 24 h. The reaction mixture was poured into cold water and neutral-
ized with 1n HCl solution. The precipitate was collected by filtration.
The crude product was purified by column chromatography on silica gel
with ethyl acetate/n-hexane (1/10). The product was recrystallized from
ethanol to give 0.90 g of pure product (yield 65%). 1H NMR (CDCl3,
300 MHz): d=12.60 (s, 1H), 12.40 (s, 1H), 8.40 (s, 1H), 7.79 (d, 1H),
7.41 (t, 1H), 7.00–6.92 (m, 3H), 6.23 (s, 1H), 3.99 (s, 3H), 3.89 (s, 3H),
3.46 (q, 2H), 1.63 (m, 2H), 1.34 (m, 8H), 0.86 ppm (t, 3H); 13C NMR
(CDCl3, 75 MHz): d=168.85, 168.76, 162.01, 152.21, 144.52, 134.65,
134.31, 126.37, 119.23, 118.40, 115.06, 112.23, 109.24, 105.16, 56.43, 56.09,
40.16, 31.67, 29.48, 28.90, 26.92, 22.53, 14.01 ppm; IR (KBr pellet): n˜ =
3312 (OH or nNH), 1650 (nCO) cmꢀ1; m/z (MS-EI) calcd: 414.49, found:
414; elemental analysis calcd (%) for C23H30N2O5: C 66.65, H 7.30, N
6.76; found: C 66.51, H 7.35, N 6.74.
Experimental Section
General: 1H NMR spectra were recorded on
a Jeol JNM-LA300
(300 MHz) spectrometer in CDCl3 or [D6]acetone with TMS as internal
standard. Molar masses of compounds were measured on a Jeol JMS-
AX505WA in electron-impact (EI) mode. Elemental contents of com-
pounds were measured with an EA1110 (CE Instruments, Italy). FTIR
studies were performed on a Jasco 200 model spectrometer. Solid sam-
ples were recorded as intimate mixtures with powdered KBr. Liquid and
gel samples were recorded in a liquid cell equipped with CaF2 windows
and a 0.2 mm lead spacer. Data were registered at 2 cmꢀ1 resolution with
32 scans. UV/Vis absorption and fluorescence spectra were recorded on
Shimadzu UV-1650PC and Shimadzu RF-500 spectrofluorimeter, respec-
tively, with emission and excitation slit width of 3 nm each. Field-emis-
sion scanning electron microscopy (FESEM) images were obtained with
a JSM-6330F (Jeol). X-ray diffraction studies were performed in trans-
mission mode with Cu K Co radiation by NICEM (Bruker D5005 diffrac-
tometer).
2-Phenyl-4H-3,1-benzoxazin-4-one (5): Anthranilic acid (1.0 g, 7.3 mmol)
was dissolved in pyridine (20 mL) and the solution stirred for 10 min.
Benzoyl chloride (2.05 g, 14.6 mmol) was added to this solution. The so-
lution was stirred for 2 h at room temperature. The reaction mixture was
poured into cold water and the precipitate collected by filtration. The
crude product was recrystallized from ethanol to give 0.65 g of pure prod-
uct (yield 40%). 1H NMR ([D6]acetone, 300 MHz): d=8.32 (t, 1H), 8.29
(d, 1H), 8.21 (d, 1H), 7.97 (m, 1H), 7.73 (d, 1H), 7.68–7.61 ppm (m,
4H); m/z (MS-EI) calcd: 223.23, found: 223; elemental analysis calcd
(%) for C14H9NO2: C 75.33, H 4.06, N 6.27; found: C 75.26, H 4.01, N
6.22.
2-(2-Hydroxyphenyl)-4H-3,1-benzoxazin-4-one (1): Anthranilic acid
(10.0 g, 72.9 mmol) and salicylic acid (10.10 g, 73.1 mmol) were dissolved
in pyridine (100 mL) and the solution stirred for 30 min. Triphenyl phos-
phite (72.5 mol, 19.0 mL) was added and the solution was stirred at
1008C for about 2 h. The reaction mixture was poured into cold water
and extracted with dichloromethane. The solution was dried with anhy-
drous magnesium sulfate. After removal of the solvent, the crude product
was purified by column chromatography on silica gel with ethyl acetate/
n-hexane (1/3). The product was recrystallized from ethanol to give
3.10 g of pure product (yield 18%). 1H NMR (CDCl3, 300 MHz): d=
12.45 (s, 1H), 8.23 (d, 1H), 8.07 (d, 1H), 7.84 (t, 1H), 7.61 (d, 1H), 7.50
(m, 2H), 7.05 (d, 1H), 6.97 ppm (t, 1H); m/z (MS-EI) calcd: 239.23;
found: 239; elemental analysis calcd (%) for C14H9NO3: C 70.29, H 3.79,
N 5.86; found: C 70.39, H 3.79 N 5.80.
6,7-Dimethoxy-2-phenyl-4H-3,1-benzoxazin-4-one (6): 2-Amino-4,5-dime-
thoxybenzoic acid (1.0 g, 5.1 mmol) was dissolved in pyridine (20 mL)
and stirred for 10 min. Benzoyl chloride (1.4 g, 10.0 mmol) was added to
this solution and the mixture was stirred for 2 h at room temperature.
The reaction mixture was poured into cold water and the precipitate col-
lected by filtration. The crude product was recrystallized from ethanol to
give 0.78 g of pure product (yield 54%).1H NMR (CDCl3, 300 MHz): d=
8.29 (d, 2H), 7.58–7.51 (m, 4H), 7.13 (s, 1H), 4.05 (s, 3H), 4.01 ppm (s,
3H); m/z (MS-EI) calcd: 283.28, found: 283; elemental analysis calcd
(%) for C16H13NO4: C 67.84, H 4.63, N 4.94; found: C 67.89, H 4.65, N
4.92.
2-Benzamido-N-heptylbenzamide (7): Compound 5 (0.5 g, 2.2 mmol) and
n-heptylamine (0.80 g, 6.9 mmol) were dissolved in pyridine (20 mL). The
solution was heated to reflux at 1008C for 24 h. The reaction mixture was
2-(2-Hydroxyphenyl)-6,7-dimethoxy-4H-3,1-benzoxazin-4-one (2): 2-
Amino-4,5-dimethoxybenzoic acid (5.0 g, 25.3 mmol) and salicylic acid
7444
www.chemeurj.org
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 7437 – 7447

Enhanced Fluorescence Emission of Organogels
FULL PAPER
poured into cold water and neutralized with 1n HCl. The precipitate was
collected by filtration. The crude product was purified by column chro-
matography on silica gel with ethyl acetate/n-hexane (1/10). The product
was recrystallized from ethanol to give 0.60 g of pure product (yield
80%). 1H NMR (CDCl3, 300 MHz): d=12.10 (s, 1H), 8.79 (d, 1H), 8.04
(d, 2H), 7.55–7.48 (m, 5H), 7.09 (t, 1H), 6.33 (s, 1H), 3.45 (q, 2H), 1.62
(m, 2H), 1.35 (m, 8H), 0.88 ppm (t, 3H); IR (KBr pellet): n˜ =3340 (nNH),
1663 (nCO) cmꢀ1; m/z (MS-EI) calcd: 338.44, found 338; elemental analy-
sis calcd (%) for C21H26N2O2: C 74.52, H 7.74, N 8.28; found: C 74.47, H
7.86, N 8.45.
Series 56) and a PMT detector (Hamamatsu, PD471) attached to a mon-
ochromator (Acton Research, Spectrapro-300i). The detailed analytical
procedure to obtain solid-state FPL has been described elsewhere.[25]
Computational details: DFT calculations were used to obtain the geome-
tries and energetics of the internally hydrogen-bonded organogelator 3.
Beckeꢂs three-parameter exact exchange functional (B3)[26] combined
with the gradient-corrected correlation functional of Lee–Yang–Parr
(LYP)[27] was employed to optimize the conformations of molecules in
gaseous phase with the 6-31G** basis set. At the respective ground-state
optimized geometries, time-dependent DFT (TDDFT)[28] calculations
using the B3LYP functional were performed to obtain vertical excitation
energies. Considering the dimer structure of gelator 3 as the first step to-
wards understanding the structure of xerogel, geometries for the dimers
were computed with no symmetry restrictions by using the B3LYP func-
tional in combination with the 6-31G** basis set. Frequency calculations
were also carried out to ensure that an energy minimum was obtained in
each case. The energy of interaction between gelator pairs was calculated
as the difference between the energies of the dimer and the individual
moieties. All interaction energies calculated by the B3LYP/6-31G**
method were corrected for the basis set superposition error (BSSE) by
using a standard counterpoise method.[29] The ground-state B3LYP and
excited-state TDDFT calculations were carried out with the Gaussian 03
computational package.[30] Since organogelator 4 differs from 3 only in
the 4,5-dimethoxy groups at the phenyl ring, time-consuming higher level
theoretical calculations were not performed for 4.
2-Benzamido-N-heptyl-4,5-dimethoxybenzamide (8): Compound
6
(0.30 g, 1.05 mmol) and n-heptylamine (0.38 g, 3.2 mmol) were dissolved
in pyridine (20 mL). The solution was heated at 1008C and refluxed for
24 h. The reaction mixture was poured into cold water and neutralized
with 1n HCl solution. The precipitate was collected by filtration. The
crude product was purified by column chromatography on silica gel with
ethyl acetate/n-hexane (1/10). The product was recrystallized from etha-
nol to give 0.26 g of pure product (yield 62%). 1H NMR (CDCl3,
300 MHz): d=12.41 (s, 1H), 8.59 (s, 1H), 8.02 (d, 2H), 7.51 (m, 3H),
6.95 (s, 1H), 6.36 (s, 1H), 3.97 (s, 3H), 3.86 (s, 3H), 3.4 (q, 2H), 1.61 (m,
2H), 1.31 (m, 8H), 0.85 ppm (t, 3H); 13C NMR (CDCl3, 75 MHz): d=
168.90, 165.45, 152.23, 144.10, 135.68, 134.75, 131.73, 128.73, 127.22,
111.83, 109.40, 104.72, 56.41, 56.01, 40.09, 31.66, 29.50, 28.90, 26.92, 22.52,
14.00 ppm; IR (KBr pellet): n˜ =3418 (nNH), 1662 (nCO) cmꢀ1; m/z (MS-EI)
calcd: 398.50, found 398; elemental analysis calcd (%) for C23H30N2O4: C
69.32, H 7.59, N 7.03; found: C 69.25, H 7.68, N 6.93.
In addition to high-level DFT calculations, semiempirical methods offer
an attractive alternative for studying potential-energy surfaces in ground
and excited states. Procedures such as the AM1 method[31] allow exami-
nation of potential-energy surfaces without geometric assumptions. With
inclusion of limited configuration interaction, especially through the
single and pair double excitation (PECI) procedure,[32] spectral properties
of several conjugated organic systems have been reliably reproduced.[33]
Therefore, we simulated the spectral properties of ESIPT gelator 3 using
AM1/PECI=8 calculations in order to guide the synthetic efforts to-
wards materials with enhanced performance and to help interpret the ex-
perimental data. The geometry-dependent potential energies in the
ground (S0) and first excited (S1) states which were calculated by chang-
ing geometrical parameters of interest from the optimized geometry. In
these calculations, all the geometrical parameters were optimized at each
point.
2-(2-Hydroxybenzamido)-4,5-dimethoxybenzoic acid (9): Compound
2
(0.42 g, 1.4 mmol) and KOH (1.0 g) were dissolved in water (10 mL). The
solution was stirred for 30 min at 1008C and then poured into cold water
and neutralized with 1n HCl. The precipitate was collected by filtration.
The crude product was purified by column chromatography on silica gel
with ethyl acetate/n-hexane (1/3). The product was recrystallized from
ethanol to give 0.37 g of pure product (yield 83%). 1H NMR (CDCl3,
300 MHz): d=12.17 (s, 1H), 12.09 (s, 1H), 8.58 (s, 1H), 7.77–7.73 (d,
1H), 7.57 (s, 1H), 7.48–7.42 (t, 1H). 7.03 (d, 1H), 6.98–6.93 (m, 2H), 4.05
(s, 3H), 3.94 ppm (s, 3H); m/ (MS-EI) calcd: 317.29, found: 317.0; ele-
mental analysis calcd (%) for C16H15NO6: C 60.57, H 4.77, N 4.41; found:
C 60.68, H 4.62, N 4.29.
Heptyl 2-(2-hydroxybenzamido)-4,5-dimethoxybenzoate (10): Compound
9 (0.25 g, 0.79 mmol) and heptanol (0.28 g, 2.4 mmol) were dissolved in
tetrahydrofuran (8 mL). Triphenylphosphine (0.42 g, 1.6 mmol) and
DEAD (0.28 g, 1.6 mmol) were added subsequently. The solution was
stirred for 24 h at room temperature. The reaction mixture was poured
into cold water and extracted with ethyl acetate. The crude product was
purified by column chromatography on silica gel with ethyl acetate/n-
hexane (1/10). The product was recrystallized from ethanol to give 0.20 g
of pure product (yield 61%). 1H NMR (CDCl3, 300 MHz): d=12.39 (s,
1H), 12.29 (s, 1H), 8.55 (s, 1H), 7.83 (d, 1H), 7.60 (s, 1H), 7.47 (t, 1H),
7.00 (m, 2H), 4.37 (t, 2H), 4.16 (s, 3H), 3.87 (s, 3H), 1.82 (m, 2H), 1.43
(m, 4H), 1.32 (m, 4H), 0.89 ppm (t, 3H); m/z (MS-EI) calcd: 415.48,
found: 415; elemental analysis calcd (%) for C23H29NO6: C 66.49, H 7.04,
N 3.37; found: C 66.53, H 6.97, N 3.51.
Acknowledgements
This research was supported in parts by Basic Science Research Program
through the National Research Foundation of Korea (NRF) funded by
the Ministry of Education, Science and Technology (CRI; RIAMI-
AM0209(0417-20090011)).
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Enhanced Fluorescence Emission of Organogels
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publication in Chemistry–A European Journal Early View. The Editor.
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