Evidence of aggregation induced emission enhancement and keto-enol-tautomerism in a gallic acid derived salicylideneaniline gel

Article abstract of DOI:10.1039/c1cc16232b

A novel salicylideneaniline type fluorescent organogelator based on a 3,4,5-(tri-dodecyloxy)benzoyl group immobilizes aromatic solvents. The resulting gels show enhancement in emission and thermochromic/non-photochromic behaviour during sol-to-gel transit

Full text of DOI:10.1039/c1cc16232b

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Evidence of aggregation induced emission enhancement and
keto-enol-tautomerism in a gallic acid derived salicylideneaniline gelw
Sougata Dattaa and Santanu Bhattacharya*ab
Received 7th October 2011, Accepted 4th November 2011
DOI: 10.1039/c1cc16232b
A novel salicylideneaniline type fluorescent organogelator based
on a 3,4,5-(tri-dodecyloxy)benzoyl group immobilizes aromatic
solvents. The resulting gels show enhancement in emission and
thermochromic/non-photochromic behaviour during sol-to-gel
transition.
Low-molecular-mass organic gelators (LMOGs) have attracted
much attention because of their unique properties and potential
uses in various fields of research and industry.1 Intermolecular
hydrogen-bonding and other weak non-covalent interactions
(e.g. van der Waals, p–p stacking and solvophobic effect,
charge-transfer, metal–ligand coordination etc.) are the driving
forces which bring about the supramolecular assemblies.
Non-covalent ‘cross-links’ and physical entanglements among
such assemblies create a three-dimensional network where the
solvent is immobilized.2
Fig. 1 (a) Molecular structure of 3,4,5-(tri-dodecyloxy)benzamide
based salicylideneaniline (1) with H-bonding donors/acceptors and
(b) a photograph of the organogel of 1 in toluene at 2 mM.
metal–salen complexes that exhibit excellent liquid-crystalline
properties.6 But, there is no report of supramolecular gelation
induced by such salicylideneanilines. Herein, we report the
first example of a new thermochromic salicylideneaniline type
organogelator consisting of a 3,4,5-(tri-dodecyloxy)benzoyl
group.
Salicylideneanilines (anils) have recently received particular
interest because of their excellent photochromic,3 thermochromic,4
solvatochromic5 and liquid-crystalline properties.6 Metal–salen
based complexes are also widespread in the literature because of
their extensive applications in catalysis and industry.7 The packing
of the anils confirmed the general trend: thermochromic =
‘‘close-packed-structure’’ and photochromic = ‘‘open-structure’’.8
In thermochromic anils, the salicylaldimine group is involved in
intramolecular H-bonding and the structural motif is arranged
in a planar conformation which introduces p–p stacking with
short inter-planar distances (3.5 A). In the photochromic anils,
only the latter moiety is planar and it forms an intramolecular
H-bond, whereas the aniline ring is rotated by B501 with
respect to this plane, thus limiting their close proximity.8
Recently, cholesterol based photochromic/thermochromic
salicylideneaniline type organogelators were introduced by Lu
and co-workers.9 There are a number of examples of 3,4,5-(tri-
dodecyloxy)benzoyl derivatives based on salicylideneanilines and
Firstly, the gelation ability of compound 1 (Fig. 1) was
confirmed in different aromatic hydrocarbons by the ‘‘stable-
to-inversion protocol of a test tube’’ method. It formed gel in
aromatic solvents such as benzene and toluene (minimum
gelator concentration [mgc] = 2 mM). Presence of four sets
of –NH–(CQO) linkages in 1 promotes facile intermolecular
H-bonding mediated association leading to gelation (see below).
However, it was soluble in CH2Cl2, CHCl3 and THF and was
insoluble either in ethanol or in n-butanol.
To discern the microstructures formed by the gel of 1 in
toluene, scanning electron microscopy (SEM) was performed.
It showed the presence of fibrous networks (Fig. 2a). The
morphology of the gel observed by atomic force microscopy
(AFM) (Fig. 2b) was in agreement with the SEM analysis.
Distinct fibrous structures (0.2–0.3 mm) along with bundles
were clearly visible.
a Department of Organic Chemistry, Indian Institute of Science,
Bangalore, India. E-mail: sb@orgchem.iisc.ernet.in;
Fax: +91-80-23600529; Tel: +91-80-22932664
To obtain further insights into the interactions among the
salicylideneaniline chromophores in the gel state, temperature-
dependent absorption spectra of the gelator 1 (2 mM) were
recorded in toluene. The intensity of the broad absorption in the
range 200–500 nm increased monotonically with the increase in
temperature. This was a consequence of gel-to-sol transition,
weakening the interactions between the chromophores and the
b Chemical Biology Unit, Jawaharlal Nehru Centre for Advanced
Scientific Research, Bangalore, India.
E-mail: sb@orgchem.iisc.ernet.in; Fax: +91-80-23600529;
Tel: +91-80-22932664
w Electronic supplementary information (ESI) available: Complete
experimental procedures, compound characterization data for the
products described in the text, and some of the spectroscopic and
X-ray diffraction analysis of the gelator 1. See DOI: 10.1039/c1cc16232b
c
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Chem. Commun., 2012, 48, 877–879 877

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Fig. 2 (a) SEM and (b) AFM images of toluene gel of 1 at a
concentration of 2 mM.
transition temperature (B50 1C) was calculated from the A350
vs. temperature plot (Fig. S1, ESIw). Relative to the sol, gelator
1 showed a red-shift of B8 nm in the gel phase, indicating a
J-type aggregation.10 This phenomenon was further supported
by the observation of a blue-shift in the absorption characteristics
of 1 in toluene during dilution (Fig. S2, ESIw). This observation
suggests that H-bonding among the amide linkages and probably
p–p stacking are responsible for the rigidity of supramolecular
structural motifs, which leads to the formation of J-aggregates.
Concentration-dependent IR-spectroscopy revealed significant
shifts of the –NH stretching frequency to higher wavenumber
on dilution, indicating the weakening of the H-bonding inter-
action (Fig. S3, ESIw). The N–H band appeared at 3275 cmꢀ1 in
‘strong’ gel (2 mM), 3305 cmꢀ1 in ‘weak’ gel (1 mM) and shifted
to 3310 cmꢀ1 in the sol state (0.2 mM).11
Fig. 3 (a) Photographs of the colourless sol at 353 K, yellow gel at
298 K and colourless solid gel at 77 K of 1 in toluene at 2 mM.
(b) Keto–enol tautomerism of thermochromic salicylideneanilines and
(c) temperature-dependent partial 1H-NMR spectra of 1 in [D6]benzene
at 2 mM.
spectra of 1 in toluene at 0.27 mM. Absorption intensity in the
range 450–500 nm decreased with increasing temperature,
elucidating a low population of the NH form at higher
temperature.
To explore the photochromic behaviour, the toluene gel of 1
was exposed to 365 nm UV light for a long time (41 h) at
room temperature. But, even after this UV-visible spectra
remained invariant and a gel-to-sol transition did not occur.
Interestingly, a greenish yellow gel of 1 in toluene was rendered
colourless at 77 K and was converted to nearly colourless sol
at 353 K. These may be attributed to the low population of the
keto form in sol. In order to investigate this equilibrium
process further, temperature-dependent UV-Vis spectroscopy
was performed at the mgc. Unfortunately, it became difficult
to detect the NH absorption band near 500 nm at such a high
gelator concentration because of the broad nature of the
absorption spectra. So, we investigated this phenomenon at
lower concentration (0.27 mM) of the gelator molecule. But, it
is notable that the intensity of absorption near 500 nm in the
UV-Vis spectra of 1 in toluene at mgc starts to decrease at
45 1C and is completely diminished at 75 1C (Fig. S1, ESIw).
Accordingly, we prepared a toluene solution of 1 (0.27 mM)
and froze it in liquid nitrogen. A greenish yellow solution at
298 K was converted to a colourless solid at 77 K. This was
then allowed to thaw naturally and the colourless solid started
to regain its greenish yellow colour (Fig. 3a). UV-Vis spectra
were recorded during warming at a time interval of 2 min (Fig. S4,
ESIw). The absorption in the range of 450–500 nm became
stronger with increasing temperature. This phenomenon
indicates that the tautomeric equilibrium markedly shifts to
the E–OH form at the liq. nitrogen temperature. The greenish
yellow solution of 1 in toluene also started to lose its colour on
heating and turned nearly into a colourless solution at 353 K,
illustrating a very low population of the E–NH form. Fig. S5
(ESIw) shows the temperature-dependent changes in absorption
Keto–enol-tautomerism of 1 (Fig. 3b) associated with the
sol–gel transition was further investigated by temperature-
dependent 1H-NMR spectroscopy, which is an efficient tool
for following the process of aggregation.12 Interestingly,
1H-NMR spectra of 1 in [D6]benzene at the mgc (2 mM) showed
significant differences from that of the 1H-NMR spectra in
CDCl3. Fig. 3c shows NMR signals near d = 13.7. These
originated from a combination of two closely spaced peaks,
which can be interpreted from the presence of an equilibrium
between the enol (–OH) and the keto (–NH) form. Both peaks
were of equal intensity in the gel phase at 25 1C and the peak
that appeared at more downfield was assigned to the OH
form because of higher electronegativity of O than that of N.
However, the relative intensity of the peak associated with the
enol form became stronger and sharper with increasing tem-
perature. This suggests that the gel phase of 1 contains an
approximately equal population of the enol and keto forms and
the population of the enol form increases with increasing
temperature during the gel-to-sol transition. The same inter-
pretation may be applied to explain a similar behaviour of the
NMR signals at d E 8 which were assigned to the H0 of the enol
form and H0 0 of the keto form. It indicates that the enol and
keto forms of 1 coexist in solution while in the gel the OH form
predominates in the sol phase. Ogawa and Arai13 proposed that
the energy differences between the ground states of the OH and
NH forms in solution are too large to allow enol to convert to
the keto form. However, here the NH form is stabilized in the
aggregates of the gel phase. This lowers the energy barrier
between the OH and NH forms than that in the solution, which
in turn favors the proton tautomerization leading to the formation
c
878 Chem. Commun., 2012, 48, 877–879
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In conclusion, we have demonstrated that 3,4,5-(tri-dode-
cyloxy)benzoyl appended salicylideneaniline, 1, acts as an
efficient gelator for aromatic solvents. Thermochromism of
its toluene gel could be explained by following the equilibrium
between the enol (OH) and the keto form (NH) using UV-Vis
absorption and temperature-dependent 1H-NMR spectro-
scopy. This is the first time it has been possible to investigate
keto–enol-tautomerism associated with the gel-to-sol transition
of salicylideneaniline by recognizing distinguishable NMR
signals of the tautomers. Furthermore, calculation of the
molecular length by energy minimization of a single molecule
and its comparison with the small-angle X-ray diffraction
pattern reveals that gelator molecules arrange themselves in
an interdigitated lamellar structure. Thus, the gelation process
is attributed to the J-aggregation of the self-assembly promoted
by H-bonding, p–p stacking and van der Waals interactions
among the individual salicylideneaniline units, which results in
an aggregation induced strong green emission in the gel phase.
This work was supported by J. C. Bose fellowship of DST to
S.B. S.D. thanks CSIR for a junior research fellowship.
Notes and references
Fig. 4 (a) Temperature-dependent changes in the fluorescence spec-
tra of gel 1 in toluene at 2 mM; the inset shows the plot of intensity vs.
temperature at 525 nm. (b) Photographs of the gel and hot sol of 1
in toluene under 365 nm UV light. (c) SAXD plot of the xerogel of 1.
(d) Molecular packing model in the xy plane through stacking with a
long period of 5.2 nm and (e) proposed molecular packing of
propagation of the aggregates of 1 along the direction (z-axis) of the
growth of supramolecular fibrous networks.
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Fig. 4a shows the temperature-dependent changes in the
fluorescence spectra of 1 (2 mM) in toluene when excited at
465 nm. The intensity of the peak at 515 nm associated with
the sol phase becomes stronger and sharper with decreasing
temperature and also red-shifts to 525 nm in the gel phase.14
Fig. 4b shows a strong green fluorescence emission of the
toluene gel of 1 under long UV-light (365 nm) and a weak
emission of the hot sol (B80 1C). This phenomenon is ascribed
to a combination of inhibition of the intramolecular rotation
and the formation of J-aggregates (head-to-tail arrangement
of the fluorophore).10 The sol-to-gel transition temperature
(B50 1C) is calculated from the emission intensity at 525 nm
vs. temperature plot and the result is in good agreement with
the results of the UV-Vis spectroscopy.
Small-angle X-ray diffraction (SAXD) was performed to unveil
the mechanism of packing of the supramolecular organization in
the xerogel (Fig. 4c). The diffraction pattern shows two peaks
at 5.2 nm and 2.6 nm, which are in the ratio of 1 : 1/2,
indicating a lamellar pattern15 of the aggregates of 1 with an
interlayer spacing of 5.2 nm which is less than the double of its
molecular length (3.91 nm) as calculated by optimization of
geometry of the single molecule using B3LYP/6-31G*. This
analysis indicates that the n-C12H25 chains interdigitate16 with
each other to introduce van der Waals interactions among the
long chains. It thereby renders the salicylideneaniline molecules
proximal to develop an extended supramolecular network
(Fig. 4d and e).
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Langmuir, 2009, 25, 8548.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 877–879 879