Unusual salt-induced color modulation through aggregation-induced emission switching of a bis-cationic phenylenedivinylene-based π hydrogelator

Article abstract of DOI:10.1002/chem.201201940

The synthesis, hydrogelation, and aggregation-induced emission switching of the phenylenedivinylene bis-N-octyl pyridinium salt is described. Hydrogelation occurs as a consequence of π-stacking, van der Waals, and electrostatic interactions that lead to a

Full text of DOI:10.1002/chem.201201940

DOI: 10.1002/chem.201201940
Unusual Salt-Induced Color Modulation through Aggregation-Induced
Emission Switching of a Bis-cationic Phenylenedivinylene-Based p
Hydrogelator
[
a, b]
[a]
Santanu Bhattacharya*
and Suman K. Samanta
16632
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 16632 – 16641

FULL PAPER
Abstract: The synthesis, hydrogelation,
and aggregation-induced emission
foundly influenced the order of aggre-
gation of the gelator molecules in
aqueous solution. Formation of a novel
chromophore assembly in this way
leads to an aggregation-induced switch
of the emission colors. The emission
color switches from sky blue to white
to orange depending upon the extent
of aggregation through mere addition
of external inorganic salts. Remarkably,
the salt effect on the assembly of such
cationic phenylenedivinylenes in water
follow the behavior predicted from the
well-known Hofmeister effects. Mecha-
nistic insights for these aggregation
processes were obtained through the
counterion exchange studies. The ag-
gregation-induced emission switching
switching of the phenylenedivinylene
bis-N-octyl pyridinium salt is described.
Hydrogelation occurs as a consequence
of p-stacking, van der Waals, and elec-
trostatic interactions that lead to a high
gel melting temperature and significant
mechanical properties at a very low
weight percentage of the gelator. A
morphology transition from fiber-to-
coil-to-tube was observed depending
on the concentration of the gelator.
Variation in the added salt type, salt
concentrations, or temperature pro-
that leads to
a room-temperature
white-light emission from a single chro-
mophore in a single solvent (water) is
highly promising for optoelectronic ap-
plications.
Keywords: aggregation · chromo-
phores · gels · salt effect · thermo-
AHCTUNGTRENNGNUr eversibility
Introduction
Herein, we report a novel p-hydrogel system based on the
conjugated bis-pyridinium phenylenevinylene (PPV) moiety
and its unique aggregation-induced emission switching prop-
erties. The emission color switches from sky blue to white to
orange upon simple addition of different inorganic salts in
varying concentrations in solution. Interestingly, the ob-
served effects follow the well-known Hofmeister series,
which offers a trend in the propensity of ions to induce the
folding/unfolding of proteins and other macromolecules in
aqueous media. To the best of our knowledge, such salt-in-
duced control of aggregation of small molecules (e.g.,
LMMGs) and the remarkable effect on emission switching
are hitherto unreported. Also, room-temperature white-light
emission from a single chromophore in a single solvent
(water) holds potential for optoelectronic applications and
makes the system especially interesting for a detailed inves-
tigation.
Supramolecular association of low-molecular-mass gelators
LMMGs) gives rise to viscoelastic gels with attractive ag-
(
[1]
gregation-induced properties. These aggregates in the form
[
2]
of rods, fibers, tubules, coils, and so forth provide a route
for the generation of nano- or microsized soft materials for
the fabrication of optoelectronic devices. Self-assembly of
photochromic gelators driven by noncovalent interactions
leads to a chromophore assembly, which is often responsible
for their impressive optoelectronic properties as well as
[
3]
[4]
their use in imaging and sensing applications. Indeed, in-
teresting photophysical properties have been recorded with
[5]
oligo(p-phenylenevinylene)-based systems. However, ach-
ieving control over the aggregation of the individual gelators
to produce emission switches depending upon the extent of
aggregation still remains a challenge. Also, most of the in-
stances that involve supramolecular association of these con-
jugated chromophoric molecules are restricted to gel forma-
tion in organic media. Aqueous solubility of these conjugat-
ed molecules is important because the charged conjugated
Results and Discussion
[6]
systems that involve oligo(p-phenylenevinylenes)
and
Synthesis: The chromophoric phenylenedivinylene bis-pyri-
dinium (PPV) gemini amphiphiles were synthesized by
using a fixed central PPV core in which two aliphatic chains
are connected through the pyridinium ends (1–4). In a two-
step procedure, different alkyl chains of desired lengths
were first covalently attached to 4-picoline by means of
quarternization. Then each pyridinium salt was subjected to
a base-induced aldol-type condensation with terephthalalde-
hyde to furnish the final product (see the Experimental Sec-
[7]
oligo(p-phenyleneethyneylenes) have received particular
attention due to their antibacterial and antiviral activities.
They are also potentially useful in imaging.
[
a] Prof. Dr. S. Bhattacharya, Dr. S. K. Samanta
Department of Organic Chemistry
Indian Institute of Science
Bangalore 560012, Karnataka (India)
Fax : (+91)80-23600529
E-mail: sb@orgchem.iisc.ernet.in
[
b] Prof. Dr. S. Bhattacharya
Chemical Biology Unit
Jawaharlal Nehru Centre for Advanced Scientific Research
Bangalore 560064, Jakkur (India)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201201940.
Chem. Eur. J. 2012, 18, 16632 – 16641
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
16633

S. Bhattacharya and S. K. Samanta
[
11]
tion). Each compound was characterized unambiguously by
tions. The increase in Tgel reached a plateau at a concen-
tration of approximately 7.5 mm. Beyond this, the addition
of further compounds into the mixture could not be solubi-
lized either by heating and/or sonicating.
1
13
using FTIR spectroscopy, H and C NMR spectroscopy,
mass spectrometry, and elemental analysis. The presence of
an all-trans configuration of the two vinyl moieties in 1–4
1
was confirmed from the H NMR spectra (Jꢀ16 Hz; Fig-
ure S1 in the Supporting Information).
Mechanical behavior of the gels: Rheological experiments
show an elastic response of the gels under applied external
mechanical stress. Rotational flow curve experiments show
that with increasing shear rate, the viscosity of the gel de-
creases linearly (slope approaching ꢁ1), thus indicating that
this gel is a shear thinning material (Figure S2a in the Sup-
porting Information). Under oscillatory stress amplitude,
the gels show an eightfold higher elastic modulus (G’) than
the viscous modulus (G“), thus indicating a dominant solid-
Gelation studies and gel-melting temperatures: Each com-
pound was dispersed in water and only compound 2 formed
an excellent hydrogel. Compound 1 precipitated and the
one with longer chains (3) showed a propensity for aggrega-
tion, whereas 4 was found to be sparingly soluble in water.
This indicates that a crucial hydrophobic/hydrophilic bal-
[12]
[8]
ance in 2 with two n-octyl chains is responsible for its gel-
ation in water with a minimum gelator concentration
MGC) of 1.76 mm (0.12 wt%). Thus, one molecule of 2
could gelate as many as approximately 30000 water mole-
cules, which deserves to belong to the category of ꢁsupergel-
atorsꢂ. The gel melting temperature (T ), obtained by the
[12b]
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
like behavior (Figure 1b).
The yield stress (s ), an ap-
y
(
plied stress above which the gel starts to flow, increased
with increasing concentration. The oscillatory frequency re-
sponse of the gel reveals that G’ is independent of frequency
AHCTUNGTRENNUNG
[9]
ꢁ1
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
over the entire angular frequency range (1–100 rads ) at
gel
[10]
dropping ball method, increased with increasing concen-
trations of 2 (Figure 1a). This indicates that with increasing
gelator concentration, the density of the gel assembly in-
creases to ensure participation of a larger number of gelator
molecules per unit volume in the thermally induced transi-
0.01% strain amplitude. The viscoelastic solidlike nature of
the gels increased with increasing concentration of 2 and it
became sufficiently strong (G’ꢀ2200 Pa at 7.5 mm), which
did not relax over the long timescale of the experiment (Fig-
[12c]
ure S2b in the Supporting Information).
1
1
H NMR spectroscopic studies of the gel: H NMR spectro-
scopic study of a gel of 2 in D O was performed at different
2
temperatures to understand the nature of interactions that
are responsible for the thermoreversible gelation. In the gel,
1
the H NMR spectroscopic signals broadened and almost
quenched owing to the restriction of the molecular tumbling
[13]
to produce zero-average dipolar coupling. Upon increas-
ing the temperature as the gel melted progressively, an iso-
tropic distribution of the molecules was achieved in solution
and their NMR spectroscopic signals started to appear grad-
ually (Figure S3 in the Supporting Information). Well-re-
solved peaks were observed above the Tgel (ꢀ468C for
2
.2 mm of 2). A concomitant increase in the d values with in-
1
creasing temperature indicates deshielding of the H signals
upon breakdown of the self-assembly. This indicates that the
self-assembly might be promoted by the p–p-stacking inter-
actions, and that participation of both aromatic and aliphatic
[13b–d]
protons are responsible for the gel formation.
Morphology under the microscope: Morphological studies
showed their topography in the gel matrix. Under a fluores-
cence microscope, a thin film of 2 showed yellow emission
and the presence of three-dimensional fiber bundles approx-
imately 1–2 mm in diameter was observed when excited at
3
40–400 nm (Figure 2a). SEM images of the xerogel ensured
the presence of a fibrillar network with a fiber diameter of
approximately 1–2 mm. Interestingly, the SEM images also
showed the presence of a coil-type morphology distributed
throughout the sample along with the fibers (Figure 2b,c). A
magnified image clearly shows the formation of the coils by
further assemblage of the individual fibers. Also, the individ-
Figure 1. a) Plot of Tgel versus concentration of 2 in water. b) Dynamic
stress amplitude sweep of the hydrogels with increasing concentration of
2
.
16634
www.chemeurj.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 16632 – 16641

Salt-Induced Color Modulation
FULL PAPER
Figure 2. a) Fluorescence microscopy image of a thin film of 2 (1 mm;
scale bar: 20 mm); b) SEM image of a xerogel of 2 (3 mm; scale bar:
1
0 mm) and c) its magnified image (scale bar: 5 mm). d,e) AFM amplitude
and f–j) topography images of the thin film of 2 obtained from d) the sol
0.75 mm) and e–i) the gel (3 mm); scale bars: 1 mm in each case).
(
ual coils combined together appear to form a tube with
shorter aspect ratio. AFM images confirmed the appearance
of the coil-type morphology above the MGC concentration
of 2 (Figure 2d–i). A thin film of 2 obtained from a ꢁsolꢂ
(
0.75 mm) in water showed only the presence of fibers of
high aspect ratio. This excludes the possibility of coil forma-
tion during solvent evaporation from the gels. The gel
(
3 mm) showed the presence of both the fiber and coil-type
morphologies throughout the sample. The coils were of
almost uniform diameter (ꢀ1 mm) and were polymorphic in
nature. Concentric circular coils were observed to have vari-
ous numbers of layers (2, 3, or multiple). Clearly, multiple
coils were assembled together to lead to the formation of a
Figure 3. a) XRD patterns of 2 (3 mm) with an inset showing the magni-
fied plot and b) probable self-organization and the molecular interactions
between molecules of 2.
[14]
[17]
tube.
of their optical and photophysical properties. The UV/Vis
absorption spectra (Figure 4a) of a dilute solution of 2
(20 mm) in water showed a maximum at 395 nm (S !S , e=
Packing pattern of the gelator molecules: X-ray diffraction
studies of the xerogel showed repetitive Bragg reflection
peaks, thus indicating a lamellar-type arrangement in the su-
0
1
ꢁ
1
ꢁ1
58500 cm m ) along with a small band at 247 nm (S !S ).
0
2
The same solution showed an emission maximum at 470 nm
when excited at 380 nm, which represents their ꢁmonomericꢂ
emission (Figure 4b). Interestingly, a drastic change in color
was observed from a sol to a gel of 2 (4 mm) either under
UV (365 nm) or normal daylight (Figure 5a). Thus, the pho-
tophysical properties in the gel state (aggregated species)
are vastly different from those in the solution (monomeric
species). This could be a consequence of aggregation in
water. Indeed, the aggregation of 2 in solution was accom-
plished upon the addition of a salt with the common ion
[15]
pramolecular self-assembly (Figure 3a).
The intense first
Bragg peak at 2.55 nm indicates a repetitive unit of 2 that is
nearly the double of the calculated aliphatic chain length of
the energy-minimized structure (Figure S4 in the Supporting
Information). The crystalline lamellar fibers showed bire-
fringent textures under a polarized optical microscope, thus
[16]
indicating an anisotropic packing of the PPV molecules
either in the thin film or in the hydrogel (Figure S5 in the
Supporting Information). A model might be proposed by
summing up all of these observations in a way that consists
of p stacking among the aromatic parts and van der Waals
interactions through the n-octyl chains in a repetitive
manner in its self-assembly (Figure 3b). The repulsion be-
tween charged pyridinium units might be mitigated by the
extensive hydration and partially slipped cation–p interac-
tions might further facilitate the self-organization by mini-
mizing the charge repulsion.
ꢁ
(Br ). Thus, when a solution of NaBr was added to the
aqueous solution of 2 (20 mm), the color of the solution
changed immediately and matched with that of the gel
under normal or UV light, thereby ensuring that an aggre-
gated state was achieved (Figure 5b). The added NaBr
might have caused a disruption in the hydration of 2 around
the pyridinium centers. The increase in the ionic strength of
the solution affects the delicate hydrophilic/hydrophobic
balance of 2 in solution, thus leading to the aggregation.
Photophysical properties of gels and solutions: This class of
molecules is chromophoric, which allows for the evaluation
Chem. Eur. J. 2012, 18, 16632 – 16641
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
16635

S. Bhattacharya and S. K. Samanta
Figure 5. a) Thermoreversible sol/gel phenomenon of 2 (4 mm) in water
shown under i,ii) daylight and iii,iv) 365 nm UV light. The sky-blue emis-
sion of the sol is a result of the monomeric species and the orange emis-
sion of the gel is a result of the aggregated gelator molecules. b) Aggre-
gation of 2 (20 mm) in the solution phase upon addition of NaBr (50 mm)
showing changes in the emission from sky-blue to orange at room tem-
perature. c) Aggregation-induced emission switches of 2 (20 mm) contain-
ing 30 mm of NaBr at different temperatures in the solution phase. Insets
show the corresponding emission colors under a 365 nm UV lamp.
d) CIE 1931 chromaticity diagram for the sky-blue, white, and orange
emission.
Figure 4. a) UV/Vis and b) fluorescence spectra showing titration of 2
20 mm) in water with NaBr (5 mm NaBr added each time), insets of (a)
and (b) show thermoreversible aggregation of 2 in the presence of 20 and
0 mm NaBr, respectively. Absorption and emission spectra show i,iii)
monomeric and ii,iv) aggregated species in the presence of 20 mm NaBr,
respectively.
(
at 432 nm indicates the existence of a thermoreversible equi-
librium between the monomeric and the aggregated states.
3
Aggregation-induced emission switches: When the fluori-
metric titration was carried out, the intensity of the mono-
meric emission at 470 nm decreased along with the appear-
ance of an aggregated emission band at 600 nm (Fig-
Effect of salt on the aggregation in solution: To probe this
salt-induced aggregation phenomenon, a solution of 2
[18]
(
(
20 mm) in water was titrated with a stock solution of NaBr
1m; Figure 4a). Under UV/Vis spectroscopy, the intensity
ure 4b). The 600 nm band reached saturation upon addi-
tion of 20 mm of NaBr and a further decrease in the 470 nm
band was observed upon gradual addition of NaBr. When
the temperature of the solution of 2 (20 mm) that contained
30 mm of NaBr was decreased, the aggregated emission
band at 600 nm emerged gradually with a concomitant de-
crease in the monomeric emission (470 nm), thereby ensur-
ing the thermoreversibility phenomenon (Figure 4b, inset).
Therefore, the addition of salt and variation in temperature
mimic the sol–gel-induced self-assembly process. When
these titrated solutions were viewed under a 365 nm UV
lamp, a transition from sky-blue emission (due to the mono-
meric species) to orange emission (due to the aggregated
species) was observed by means of white-light emission with
increasing salt concentrations (Figure S7 in the Supporting
Information). Also, the solution of 2 that contained 30 mm
NaBr showed a sky-blue emission at higher temperature
(>508C), an orange emission at low temperature (<158C),
of the 395 nm band decreased progressively along with the
appearance of a new band at 480 nm owing to the onset of
an aggregated state. An optimum aggregation was achieved
when the concentration of NaBr in the final solution
reached 20 mm, whereas the 480 nm band did not show any
significant changes. Beyond this concentration, further addi-
tion of NaBr only led to decreases in the 395 nm band, and
it finally reached a point of saturation at approximately
40 mm of NaBr.
The solution of 2 (20 mm) that contained 20 mm NaBr was
then subjected to temperature variations. At higher temper-
ature (608C), the spectra resembled that of a monomeric ab-
sorption (Figure 4a, inset). When the temperature was low-
ered, the intensity of the 395 nm band decreased and the
hump at 480 nm increased progressively until it saturated at
approximately 108C. The presence of a clear isosbestic point
16636
www.chemeurj.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 16632 – 16641

Salt-Induced Color Modulation
FULL PAPER
and a white-light emission at room temperature (25–308C;
Figure S8 in the Supporting Information). Thus, either by
varying the concentration of NaBr or the temperature of the
solution, different emission could be achieved depending on
the extent of aggregation of the monomers (Figure 6).
close to those of pure white-light emission (0.33, 0.33). The
corresponding excitation spectra also supports the aggrega-
tion-induced emission switches, and it resembled the varia-
ble-temperature absorption spectra (Figure S9 in the Sup-
porting Information). Thus, the temperature variation oper-
ated as an emission ꢁswitchꢂ, and the aggregation-induced
white-light emission from a single component in a single sol-
vent makes the system potentially useful for optoelectronic
applications. Other instances also bring about such effects
but notably require a mixture of two or more components
[19]
and/or solvents. Therefore, our findings demonstrate that
white-light emission can be obtained from a single chromo-
phore, which is always more desirable than that of a mixture
of chromophoric systems.
Fluorescence decay profiles of 2 were recorded in aque-
ous solution, as a hydrogel, and in the presence of NaBr at
different temperatures for the monomeric species, the aggre-
gated species, and the mixture (Figure S10 in the Supporting
Information). In solution, compound 2 (20 mm) showed a
monoexponential fluorescence decay with a time constant of
2
0
.1 ns (c =1.06) when the emission was recorded at 470 nm
(
(
Table S1 in the Supporting Information). The hydrogel of 2
3 mm) showed a multiexponential decay (lem =600 nm)
with the time constants of 4.7 (17%), 16.5 (70%), and 0.6 ns
2
(
13%), with an average lifetime of 15.5 ns (c =1.18).
The fluorescence decay curves for a solution of 2 (20 mm)
that contained 30 mm of NaBr at 60 (sky-blue emission), 25
white emission), and 108C (orange emission) were recorded
(
at two different wavelengths (470 and 600 nm). When re-
corded at 470 nm (at a monomeric emission band), a mono-
exponential decay was observed in each case (Table S1 in
the Supporting Information). However, multiexponential
decay curves were obtained when the emission was recorded
at 600 nm (aggregated emission band) with average lifetimes
Figure 6. A graphical representation depicting the salt-induced switch of
the emission colors, which resembles the disorder-to-order-induced sol–
gel transition. The sky-blue emission of the gelator alone in water trans-
formed into white light followed by orange emission upon progressive ad-
dition of NaBr at room temperature. The orange emissive solution exhib-
its further emission switches upon temperature variation under a 365 nm
UV lamp. The monomeric sky-blue emission and the aggregated orange
emission matched with that of the sol and gel, respectively, as a conse-
quence of the reversible order–disorder transitions.
2
2
of 5.9 ns (c =1.05) at 608C, 7.0 ns (c =1.09) at 258C, and
2
7
.3 ns (c =1.03) at 108C. The increase in lifetime values as
temperature decreased might be a result of the appearance
of relatively long-lived species that are formed owing to the
[20]
salt-induced aggregation of the gelator molecules.
White-light emission: We also recorded the excitation and
emission spectra for the same solution (20 mm of 2 that con-
tained 30 mm of NaBr) at 10, 25, and 658C after a visual
preview of their emission color under a 365 nm UV lamp.
The emission spectra were consistent with the previous ob-
servations and resembled that of a monomeric emission
Effect of other salts and Hofmeister series: To investigate
the role of addition of various inorganic salts on the aggre-
gation behavior of 2 in water, titration was carried out by
systematically varying the anions and then the cations as
well (Figure S11 in the Supporting Information). Among the
ꢁ
(
470 nm) at 608C, an aggregated emission (600 nm) at 108C,
anions, SCN showed the highest activity in terms of
and a wide-range emission (ꢀ425–700 nm) at 258C (Fig-
ure 5c). At 608C, only monomeric emission appeared and
the band corresponding to the aggregated emission was
absent. At 108C, the intensity of the aggregated emission
band was predominant over the monomeric band. However,
at room temperature, the intensity of both the bands was
comparable and it covered almost the whole visible spectral
region, thus leading to a white-light emission through a mix-
ture of both monomers and aggregates in the solution. The
CIE chromaticity diagram exhibited the coordinates for the
white-light emission (0.36, 0.35; Figure 5d), which are quite
quenching of the monomeric emission at 470 nm relative to
ꢁ
2ꢁ
ꢁ
ꢁ
ꢁ
the other anions examined (NO , SO , I , Br , Cl , and
3
4
ꢁ
F ). The addition of approximately 2 mm NaSCN into an
aqueous solution of 2 (20 mm) showed an effective quench-
ing of the monomeric emission, which led to the onset of an
ꢁ
aggregated emission at 600 nm. Among the halide series (F ,
ꢁ
ꢁ
ꢁ
Cl , Br , and I ), iodide appeared to be the strongest
quencher and an optimum quenching occurred with approxi-
mately 6 mm of NaI. Interestingly, although an efficient
quenching was observed with NaI, it did not produce the ag-
gregated emission band. This might be a result of the large
Chem. Eur. J. 2012, 18, 16632 – 16641
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
16637

S. Bhattacharya and S. K. Samanta
ꢁ
ionic radius of I , which did not allow the molecules of 2 to
lowed by hydration of the resulting system, which is primari-
ly responsible for the aggregation and not indirect influences
come in direct proximity to impart efficient p-stacking inter-
actions between the PPV aromatic units (see below). NaNO3
[23b]
that occur through changes in the water structure.
To ad-
(
(
ꢀ10 mm) showed a greater quenching efficiency than NaBr
dress this, anion displacement studies were also undertaken.
ꢀ40 mm). However, Na SO , NaCl, and NaF showed negli-
2
4
gible quenching. Under a 365 nm UV lamp, these titrated
solutions showed emission consistent with the above obser-
vations (Figure S12d in the Supporting Information). The
fluorescence emission of the aggregated band at 600 nm was
Anion-exchange studies: The anion displacement was stud-
ied by adding one anion followed by titration with the other
(Figure 7). This also explains the mechanism of the aggrega-
ꢁ
tion process at the molecular level. Although addition of I
more intense in the case of NaNO , which produced a stron-
(10 mm) into an aqueous solution of 2 (20 mm) shows fluores-
cence quenching, it resembles only monomeric emission
(470 nm). However, addition of NaSCN (10 mm) into this
solution led to the aggregated emission at 600 nm. This indi-
cates that SCN can replace I as the counterion, and the
resulting species gives rise to the aggregated state. When the
titration was performed in the reverse order, that is, when
the solution of 2 that contained NaSCN (10 mm) was titrated
with NaI, the aggregates could not be broken into the
3
ger yellow emission in the UV chamber than NaSCN. This
ꢁ
might be a result of a planar geometry of the NO3 ions.
The emission color was quenched in the case of NaI, since
[21]
ꢁ
ꢁ
iodide is known to be a strong quencher. However, the
emission color upon individual addition of Na SO , NaCl,
2
4
and NaF resembled that of the monomeric emission.
We then determined the Stern–Volmer quenching con-
stant (K ) on the basis of the titration of 2 with different
SV
anions. Here 2 acted as a fluorophore and the added salt as
a quencher. KSV indicates the sensitivity of a fluorophore to
a quencher and has been calculated by using Equa-
mono AHCTUNGTERUNNNmG eric species; only a fluorescence quenching was ob-
served. This also excludes the possibility of the simple
ꢁ
heavy-atom effect of I , since it could only quench the emis-
[22]
ꢁ
tion (1):
sion and not break the aggregation. Addition of NO3 or
ꢁ
Br to the solution of 2 that contained NaI did not show no-
F0
F
ticeable extent of aggregation except for a large excess
amount of Br . However, the addition of I to the solution
¼
1 þ K ½Qꢂ
ð1Þ
sv
ꢁ
ꢁ
of 2 in its aggregated state, which was achieved either by the
in which F and F are the emission intensities of the fluoro-
0
phore in the absence and presence of a quencher and [Q] is
the concentration of the quencher [mm]. A plot of F /F as a
0
function of [Q] yields a straight line, the slope of which af-
fords the KSV value (Figure S12 in the Supporting Informa-
ꢁ
ꢁ
ꢁ
tion). The K values follow the order SCN >I >NO
>
SV
3
ꢁ
2ꢁ
ꢁ
ꢁ
Br >SO4 >Cl >F (Table S2 in the Supporting Informa-
tion). These indicate that the relative ability of each anion
to quench the fluorescence emission of 2 is in agreement
with the known Hofmeister series, a trend of ions that gov-
erns the folding/unfolding of proteins and other macromole-
cules in aqueous media. Therefore, this observation sug-
gests that the same trend could also hold true for the small
molecules.
[23]
The effect of cations on the aggregation of 2 in water was
ꢁ
also checked with a fixed anion (Cl ), which, however, had
a less pronounced effect on the aggregation process. With
increasing cationic charge, aggregation increased and fol-
2
+
2+
+
+
+
lowed the order Ca >Mg >K >Na >Li , which is
also in agreement with the Hofmeister series.
Thus, addition of either anions or cations leads to the
changes in the assembly of 2 in water, and it is more pro-
nounced for the anions than the cations. For cations, the
extent of self-assembly depends on the number of cationic
charges that modify the extent of hydration of 2. The extent
of aggregation for the anions thus clearly depends primarily
on two factors: 1) the anion exchange with the preexisting
ꢁ
Br counterions followed by 2) the extent of hydration of
Figure 7. Effects on fluorescence emission upon exchange of anion:
a) NaI with NaSCN, b) NaSCN with NaI, c) NaI with NaNO , d) NaNO
3 3
with NaI, e) NaI with NaBr, and f) NaBr with NaI. Insets show the mag-
nification of the quenched spectra.
the resulting mixture that contains the replaced anions.
Therefore, the observed Hofmeister series is a result of the
direct interactions of the ions with the gelator molecules fol-
16638
www.chemeurj.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 16632 – 16641

Salt-Induced Color Modulation
FULL PAPER
ꢁ
ꢁ
addition of Br or NO , led to the breakdown of the assem-
therefore when 2 was titrated with NaBr, it led to a higher
3
bly in both instances. These observations indicate that the
anion exchange also follows the Hofmeister series.
state of aggregation.
The above phenomena might be explained by the model
depicted in the Figure 8. It is believed that in the presence
Conclusion
ꢁ
ꢁ
ꢁ
of anions such as SCN , NO , or Br , self-assembly is ach-
3
ieved through p stacking, and the anions are placed between
In summary, the collective analyses demonstrate the self-or-
ganization propensity of 2 in water, thereby leading to hy-
drogel formation through the optimization of p-stacking,
van der Waals, and electrostatic interactions. Variation in
the added salt type, concentration, or temperature pro-
foundly influenced the aggregation of 2 in aqueous solution.
Formation of a novel chromophore assembly in this way
leads to an aggregation-induced switch of emission colors.
Thus, a sky-blue emission at >508C, an orange emission at
the PPV units. However, as the size of the anion becomes
ꢁ
larger, as in the case of I , it does not facilitate the p stack-
ing between the PPV units, and hence aggregation-induced
emission at 600 nm was not observed.
<
158C, and a white-light emission at room temperature
(
25–308C) were observed. To the best of our knowledge,
this is the first observation of the control of aggregation and
emission switches of LMMGs by using different salts that
follow the well-known Hofmeister series. Mechanistic in-
sights for the aggregation process (through anion exchange
and alteration in hydration in this case) would be helpful to
explore these kinds of p gelators. The aggregation-induced
emission that shows room-temperature white-light emission
from a single chromophore in a single solvent (water)
makes the system useful for optoelectronic applications. Fur-
thermore, pharmacological usefulness of this type of charged
[6b,7]
(
cationic/anionic) conjugated molecule
adds to the po-
tential of these new bis-cationic phenylenevinylenes for cel-
lular imaging of events involved in killing bacteria and virus-
es. Investigative work is underway in this direction in our
laboratory.
Experimental Section
Materials and methods: All reagents and solvents were obtained from
the best-known commercial sources and were used without further purifi-
cation. FTIR studies were performed using a Perkin–Elmer FTIR Spec-
ꢁ
1
1
trum BX system and were reported in wavenumbers [cm ]. H and
1
3
C NMR spectra were recorded using a Bruker 400 Avance NMR spec-
trometer. Chemical shifts were reported downfield from the internal
standard, tetramethylsilane. Mass spectrometry of individual compounds
was performed using a MicroMass ESI-TOF MS instrument. Elemental
analysis was recorded using a Thermo Finnigan EA FLASH 1112
SERIES instrument.
ꢁ
Figure 8. Probable self-organization motifs of 2 dictated by a) Br (gray
spheres) and b) I (black spheres) ions.
ꢁ
Synthesis and characterization: Compound 2 was synthesized upon ap-
[
17a,b]
propriate modification of the reported literature procedure.
A mix-
ture of n-octyl bromide and 4-picoline (1:2 mol) was heated to reflux in
ethanol for 12 h to obtain the corresponding 4-picolinium salt. Then the
reaction mixture was cooled and an excess amount of 4-picoline was re-
moved under vacuum followed by repeated washing with hexane to give
ꢁ
ꢁ
2ꢁ
For anions such as F , Cl , or SO , the anion replace-
4
ments do take place. But due to the greater extent of hydra-
tion, the hydrophilic/hydrophobic balance could not be
maintained for their optimal self-organization in water.
rise to a white solid. The 4-picolinium salt thus obtained and ter
aldehyde were dissolved in a 2:1 molar ratio in ethanol, and an aqueous
solution (2 mL) of K CO (2 equiv) was added dropwise into the ethanol
ACHTUNGTRENNUNGe ph ACHTUNGTRENNUNGt hal-
A
H
U
G
R
N
U
G
A
T
N
T
E
N
N
ACHTUNGTRENNUNG
2
3
ꢁ
ꢁ
ꢁ
However, for SCN , NO , or Br after their anion replace-
solution under ice-cold conditions. The mixture was stirred under ice-
cold conditions for 2 h, and then the reaction was quenched by acidifica-
tion with 3n HCl. Ethanol was removed under vacuum, and the precipi-
tate was filtered and washed with ice-cold water. The crude product was
dissolved in methanol and precipitated in ethyl acetate, followed by fil-
tration to obtain a yellow solid. This cycle was repeated several times
3
ment, the hydrophobic effects dominate on account of a
lesser degree of hydration, thus leading them to the aggre-
gated state. Additional hydration of the added salts in aque-
ous media also helps the PPV to aggregate further, and
Chem. Eur. J. 2012, 18, 16632 – 16641
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
16639

S. Bhattacharya and S. K. Samanta
until the pure product was obtained as indicated from TLC. A similar
method was used to prepare compounds 1, 3, and 4.
A. Ajayaghosh, Angew. Chem. 2008, 120, 4769–4772; Angew. Chem.
Int. Ed. 2008, 47, 4691–4694.
[
3] a) T. Goodson, III, W. Li, A. Gharavi, L. Yu, Adv. Mater. 1997, 9,
639–643; b) F. Wꢃrthner, Chem. Commun. 2004, 14, 1564–1579;
c) S. S. Babu, S. Prasanthkumar, A. Ajayaghosh, Angew. Chem.
2012, 124, 1800–1810; Angew. Chem. Int. Ed. 2012, 51, 1766–1776.
4
,4’-[(1E,1’E)-1,4-Phenylenebis(ethene-2,1-diyl)]bis(1-hexyl pyridinium)
1
bromide (1): Yield 26%; H NMR (500 MHz, CD
6
4
3
OD): d=0.91–0.93 (t,
+
H; CH
H; N ꢁCH
3
), 1.27–1.38 (m, 12H; CH
2
), 2.01 (m, 4H; N -CH
2 2
-CH ), 4.54 (t,
+
2
), 7.53–7.56 (d, J=16.5 Hz, 2H; vinylic), 7.86 (s, 4H; aro-
matic), 7.96–7.99 (d, J=16.0 Hz, 2H; vinylic), 8.21- 8.22 (d, J=6.0 Hz,
[4] a) M. Ikeda, M. Takeuchi, S. Shinkai, Chem. Commun. 2003, 1354–
1355; b) J. F. Hulvat, M. Sofos, K. Tajima, S. I. Stupp, J. Am. Chem.
Soc. 2005, 127, 366–372; c) S.-J. Yoon, J. H. Kim, J. W. Chung, S. Y.
Park, J. Mater. Chem. 2011, 21, 18971–18973; d) S. Srinivasan, P. A.
Babu, S. Mahesh, A. Ajayaghosh, J. Am. Chem. Soc. 2009, 131,
15122–15123; e) K. K. Kartha, S. S. Babu, S. Srinivasan, A. Ajaya-
ghosh, J. Am. Chem. Soc. 2012, 134, 4834–4841.
1
3
4
(
1
H; aromatic), 8.82–8.83 ppm (d, J=6.0 Hz, 4H; aromatic); C NMR
125 MHz, CD OD): d=14.23, 23.46, 26.89, 32.29, 32.32, 62.04, 125.30,
25.59, 128.69, 130.11, 130.83, 138.64, 141.85, 145.35, 155.20 ppm; FTIR
3
(
neat): n˜ max =2926, 2851, 1615, 1522, 1479, 1359, 1209, 1172, 996, 874,
ꢁ
1
2+
7
26 cm ; TOF-MS: m/z calcd for C32
H
42
N
2
: 227.1674; found: 227.1676;
Br : C 62.55, H 6.89, N 4.56;
elemental analysis calcd (%) for C32
found: C 62.63, H 6.88, N, 4.84.
H
42
N
2
2
[
5] a) A. El-ghayoury, A. P. H. J. Schenning, P. A. van Hal, J. K. J. van
Duren, R. A. J. Janssen, E. W. Meijer, Angew. Chem. 2001, 113,
3772–3775; Angew. Chem. Int. Ed. 2001, 40, 3660–3663; b) A.
Ajayaghosh, S. J. George, J. Am. Chem. Soc. 2001, 123, 5148–5149;
c) A. P. H. J. Schenning, J. V. Herrikhuyzen, P. Jonkheijm, Z. Chen,
F. Wurthner, E. W. Meijer, J. Am. Chem. Soc. 2002, 124, 10252–
10253; d) F. J. M. Hoeben, I. O. Shklyarevskiy, M. J. Pouderoijen, H.
Engelkamp, A. P. H. J. Schenning, P. C. M. Christianen, J. C. Maan,
E. W. Meijer, Angew. Chem. 2006, 118, 1254–1258; Angew. Chem.
Int. Ed. 2006, 45, 1232–1236; e) M. Mba, A. Moretto, L. Armelao,
M. Crisma, C. Toniolo, M. Maggini, Chem. Eur. J. 2011, 17, 2044–
4
,4’-[(1E,1’E)-1,4-Phenylenebis(ethene-2,1-diyl)]bis(1-octyl pyridinium]
1
bromide (2): Yield 29%; H NMR (500 MHz, CD
6
4
3
OD): d=0.88–0.90 (t,
+
H; CH
H; N ꢁCH
3
), 1.30–1.39 (m, 20H; CH
2
), 2.01 (m, 4H; N -CH
2 2
-CH ), 4.54 (t,
+
2
), 7.53–7.56 (d, J=16.0 Hz, 2H; vinylic), 7.86 (s, 4H; aro-
matic), 7.96–8.00 (d, J=16.5 Hz, 2H; vinylic), 8.21- 8.22 (d, J=6.5 Hz,
1
3
4
(
6
H; aromatic), 8.82–8.83 ppm (d, J=6.5 Hz, 4H; aromatic); C NMR
125 MHz, CD OD): d=14.38, 23.65, 27.22, 30.09, 30.18, 32.38, 32.86,
2.03, 125.29, 125.57, 130.11, 138.63, 141.83, 145.35, 155.17 ppm; FTIR
3
(
neat): n˜ max =2920, 2852, 1615, 1519, 1468, 1353, 1328, 1210, 1173, 979,
ꢁ
1
2+
8
2
47, 723 cm ; TOF-MS: m/z calcd for C36
H
H
50
N
2
: 255.1987; found:
Br ·2H O: C 61.19,
55.1985; elemental analysis calcd (%) for C36
50
N
2
2
2
2
047; f) C. Lçwe, C. Weder, Adv. Mater. 2002, 14, 1625–1629;
H 7.70, N 3.96; found: C 61.35, H 8.02, N 4.18.
g) S. K. Samanta, A. Pal, S. Bhattacharya, Langmuir 2009, 25, 8567–
8578; h) S. K. Samanta, A. Pal, S. Bhattacharya, C. N. R. Rao, J.
Mater. Chem. 2010, 20, 6881–6890; i) S. J. George, T. F. A. de Greef,
R. Bovee, J. L. J. van Dongen, A. P. H. J. Schenning, E. W. Meijer,
Chem. Asian J. 2009, 4, 910–917; j) S. K. Samanta, S. Bhattacharya,
Chem. Eur. J., DOI: 10.1002/chem.201103855; k) S. K. Samanta, S.
Bhattacharya, J. Mater. Chem., DOI: 10.1039/c2jm35012b.
6] a) J. S. Treger, V. Y. Ma, Y. Gao, C.-C. Wang, H.-L. Wang, M. S.
Johal, J. Phys. Chem. B 2008, 112, 760–763; b) C. Zhu, Q. Yang, L.
Liu, F. Lv, S. Li, G. Yang, S. Wang, Adv. Mater. 2011, 23, 4805–4810.
7] a) Y. Tang, E. H. Hill, Z. Zhou, D. G. Evans, K. S. Schanze, D. G.
Whitten, Langmuir 2011, 27, 4945–4955; b) Y. Wang, T. S. Corbitt,
S. D. Jett, Y. Tang, K. S. Schanze, E. Y. Chi, D. G. Whitten, Lang-
muir 2012, 28, 65–70; c) Y. Wang, Y. Tang, Z. Zhou, E. Ji, G. P.
Lopez, E. Y. Chi, K. S. Schanze, D. G. Whitten, Langmuir 2010, 26,
4
,4’-[(1E,1’E)-1,4-Phenylenebis(ethene-2,1-diyl)]bis(1-decyl pyridinium)
1
bromide (3): Yield 30%; H NMR (400 MHz, CD
6
4
3
OD): d=0.86–0.89 (t,
+
H; CH
3
), 1.28–1.39 (m, 28H; CH
2
), 1.99–2.02 (m, 4H; N -CH
2 2
-CH ),
+
.52–4.56 (t, 4H; N ꢁCH
), 7.52–7.56 (d, J=16.4 Hz, 2H; vinylic), 7.86
2
(
s, 4H; aromatic), 7.96–8.00 (d, J=16.0 Hz, 2H; vinylic), 8.21–8.23 (d, J=
.8 Hz, 4H; aromatic), 8.82–8.84 ppm (d, J=7.2 Hz, 4H; aromatic);
6
1
3
3
C NMR (100 MHz, CD OD): d=14.40, 23.70, 27.21, 30.11, 30.37, 30.49,
0.56, 32.36, 33.02, 62.05, 125.30, 125.59, 130.11, 130.83, 138.64, 141.86,
[
[
3
1
1
2
45.35, 155.19 ppm; FTIR (neat): n˜ max =2919, 2850, 1615, 1469, 1353,
ꢁ
1
2+
329, 1211, 1176, 978, 847, 722 cm ; TOF-MS: m/z calcd for C40
83.2300; found: 283.2301; elemental analysis calcd (%) for C40
H
58
N
2
:
:
H
58
N
2
Br
2
C 66.11, H 8.04, N 3.85; found: C 66.10, H 8.21, N 3.77.
4
,4’-[(1E,1’E)-1,4-Phenylenebis(ethene-2,1-diyl)]bis(1-dodecyl pyridini-
1
um) bromide (4): Yield 35%; H NMR (400 MHz, CD
.89 (t, 6H; CH ), 1.22–1.39 (m, 36H; CH ), 1.99–2.02 (m, 4H; N -CH
CH ), 7.52–7.56 (d, J=16.4 Hz, 2H; vinylic),
), 4.52–4.55 (t, 4H; N ꢁCH
.86 (s, 4H; aromatic), 7.95–7.99 (d, J=16.0 Hz, 2H; vinylic), 8.20–8.22
d, J=6.8 Hz, 4H; aromatic), 8.81–8.83 ppm (d, J=6.8 Hz, 4H; aromat-
3
OD): d=0.86–
+
0
3
2
2
-
1
2509–12514; d) Y. Wang, T. D. Canady, Z. Zhou, Y. Tang, D. N.
+
2
2
Price, D. G. Bear, E. Y. Chi, K. S. Schanze, D. G. Whitten, ACS
Appl. Mater. Interfaces 2011, 3, 2209–2214.
8] a) M. George, S. L. Snyder, P. Terech, C. J. Glinka, R. G. Weiss, J.
Am. Chem. Soc. 2003, 125, 10275–10283; b) A. Pal, Y. K. Ghosh, S.
Bhattacharya, Tetrahedron 2007, 63, 7334–7348; c) J. Cui, Y. Zheng,
Z. Shen, X. Wan, Langmuir 2010, 26, 15508–15515.
9] a) H. Kobayashi, A. Friggeri, K. Koumoto, M. Amaike, S. Shinkai,
D. N. Reinhoudt, Org. Lett. 2002, 4, 1423–1426; b) A. Srivastava, S.
Ghorai, A. Bhattacharjya, S. Bhattacharya, J. Org. Chem. 2005, 70,
7
(
[
[
1
3
ic); C NMR (100 MHz, CD
3
1
2
3
OD): d=14.41, 23.72, 27.21, 30.11, 30.45,
0.48, 30.60, 30.72, 32.36, 33.06, 62.06, 125.03, 125.31, 125.59, 128.69,
30.12, 130.83, 138.65, 141.86, 145.36, 155.21 ppm; FTIR (neat): n˜ max =
ꢁ1
918, 2849, 1617, 1520, 1467, 1353, 1329, 1211, 1175, 979, 846, 722 cm ;
2
+
TOF-MS: m/z calcd for C44
H
66
N
2
: 311.5041; found: 311.3030; elemental
: C 67.51, H 8.50, N 3.58; found: C
analysis calcd (%) for C44
7.53, H 8.77, N 3.43.
H
66
N
2
Br
2
6
6
574–6582; c) Y. Qiao, Y. Lin, Z. Yang, H. Chen, S. Zhang, Y. Yan,
J. Huang, J. Phys. Chem. B 2010, 114, 11725–11730.
10] J. W. Chung, B.-K. An, S. Y. Park, Chem. Mater. 2008, 20, 6750–
755.
[
6
[
[
11] S. Bhattacharya, A. Pal, J. Phys. Chem. B 2008, 112, 4918–4927.
12] a) H. A. Barnes, A Handbook of Elementary Rheology, University
of Wales, Institute of Non-Newtonian Fluid Mechanics, Aberyst-
wyth, 2000, pp. 55–61; b) F. M. Menger, K. L. Caran, J. Am. Chem.
Soc. 2000, 122, 11679–11691; c) P. Terech, D. Pasquier, V. Bordas, C.
Rossat, Langmuir 2000, 16, 4485–4494;d) A. Pal, H. Basit, S. Sen,
V. K. Aswal, S. Bhattacharya, J. Mater. Chem. 2009, 19, 4325–4334.
13] a) D. C. Duncan, D. G. Whitten, Langmuir 2000, 16, 6445–6452;
b) J. H. Jung, S. Shinkai, T. Shimizu, Chem. Eur. J. 2002, 8, 2684–
Acknowledgements
We thank the DST for financial support, and INI and the Department of
Physics for various instrumental facilities. S.K.S is grateful to CSIR for a
Senior Research Fellowship. S.B. thanks the DST for a J.C. Bose Fellow-
ship.
[
[
[
1] a) P. Terech, R. G. Weiss, Chem. Rev. 1997, 97, 3133–3159; b) T.
Ishi-I, S. Shinkai, Top. Curr. Chem. 2005, 258, 119–160; c) S. Bhatta-
charya, S. K. Samanta, Langmuir 2009, 25, 8378–8381.
2] a) M. Llusar, C. Sanchez, Chem. Mater. 2008, 20, 782–820; b) S.
Yagai, S. Mahesh, Y. Kikkawa, K. Unoike, T. Karatsu, A. Kitamura,
2
690; c) D. K. Kumar, D. A. Jose, P. Dastidar, A. Das, Langmuir
2
004, 20, 10413–10418; d) Y.-T. Shen, C.-H. Li, K.-C. Chang, S.-Y.
Chin, H.-A. Lin, Y.-M. Liu, C.-Y. Hung, H.-F. Hsu, S.-S. Sun, Lang-
muir 2009, 25, 8714–8722.
16640
www.chemeurj.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 16632 – 16641

Salt-Induced Color Modulation
FULL PAPER
[
14] S. Yagai, S. Kubota, H. Saito, K. Unoike, T. Karatsu, A. Kitamura,
Praveen, C. Vijayakumar, S. J. George, Angew. Chem. 2007, 119,
6376–6381; Angew. Chem. Int. Ed. 2007, 46, 6260–6265; h) R.
Abbel, R. van der Weegen, E. W. Meijer, A. P. H. J. Schenning,
Chem. Commun. 2009, 1697–1699; i) R. Abbel, R. van der Weegen,
W. Pisula, M. Surin, P. Leclere, R. Lazzaroni, E. W. Meijer,
A. P. H. J. Schenning, Chem. Eur. J. 2009, 15, 9737–9746.
[20] R. Varghese, S. J. George, A. Ajayaghosh, Chem. Commun. 2005,
593–595.
A. Ajayaghosh, M. Kanesato, Y. Kikkawa, J. Am. Chem. Soc. 2009,
1
31, 5408–5410.
15] H. Basit, A. Pal, S. Sen, S. Bhattacharya, Chem. Eur. J. 2008, 14,
534–6545.
16] a) S. K. Samanta, A. Gomathi, S. Bhattacharya, C. N. R. Rao, Lang-
muir 2010, 26, 12230–12236; b) S. K. Samanta, K. S. Subrahmanyam,
S. Bhattacharya, C. N. R. Rao, Chem. Eur. J. 2012, 18, 2890–2901.
17] a) B. Juskowiak, M. Ohba, M. Sato, S. Takenaka, M. Takagi, H.
Kondo, Bull. Chem. Soc. Jpn. 1999, 72, 265–277; b) J. Tang, C.
Wang, Y. Wang, J. Sunb, B. Yang, J. Mater. Chem. 2001, 11, 1370–
[
[
6
[
[21] a) C. D. Geddes, Meas. Sci. Technol. 2001, 12, R53–R88; b) A.
Chmyrov, T. Sanden, J. Widengren, J. Phys. Chem. B 2010, 114,
11282–11291.
1
373; c) E. P. Petrov, F. Cichos, F. Wagner, St. Spange, C. von Borc-
[22] J. R. Lakowicz, in Principles of Fluorescence Spectroscopy, Springer,
New York, 2006, p.11, 3rd ed.
[23] a) F. Hofmeister, Arch. Exp. Pathol. Pharmakol. 1888, 24, 247–260;
b) Y. Zhang, P. S. Cremer, Curr. Opin. Chem. Biol. 2006, 10, 658–
663; c) L. Xu, X. Li, M. Zhai, L. Huang, J. Peng, J. Li, G. Wei, J.
zyskowski, Photochem. Photobiol. 2005, 81, 898–907; d) C.-X. Yuan,
X.-T. Tao, Y. Ren, Y. Li, J.-X. Yang, W.-T. Yu, L. Wang, M.-H.
Jiang, J. Phys. Chem. C 2007, 111, 12811–12816.
[
[
18] The excitation and emission maximum of the hydrogel of 2 ap-
peared at 470 and 590 nm, respectively, when recorded under the
front-face geometry. Therefore, the emission maximum at 600 nm in
the case of addition of NaBr ensured aggregated emission (see Fig-
ure S6 in the Supporting Information).
Phys. Chem.
B 2007, 111, 3391–3397; d) L. M. Pegram, M. T.
Record, J. Phys. Chem. B 2008, 112, 9428–9436; e) J. M. G. Swann,
W. Bras, P. D. Topham, J. R. Howse, A. J. Ryan, Langmuir 2010, 26,
10191–10197; f) H. Wang, Q. Feng, J. Wang, H. Zhang, J. Phys.
Chem. B 2010, 114, 1380–1387; g) C. P. Schneider, D. Shukla, B. L.
Trout, J. Phys. Chem. B 2011, 115, 7447–7458; h) Y. Zhang, P. S.
Cremer, Annu. Rev. Phys. Chem. 2010, 61, 63–83; i) A. Klaus,
G. J. T. Tiddy, R. Rachel, A. P. Trinh, E. Maurer, D. Touraud, W.
Kunz, Langmuir 2011, 27, 4403–4411.
19] a) Y.-L. Liu, Y.-H. Wu, C.-Y. Hsu, Nanotechnology 2009, 20, 235704;
b) X. Wang, J. Yan, Y. Zhou, J. Pei, J. Am. Chem. Soc. 2010, 132,
1
5872–15874; c) C. Vijayakumar, K. Sugiyasu, M. Takeuchi, Chem.
Sci. 2011, 2, 291–294; d) C. Giansante, G. Raffy, C. Schafer, H.
Rahma, M.-T. Kao, A. G. L. Olive, A. del Guerzo, J. Am. Chem.
Soc. 2011, 133, 316–325; e) Y. Hong, J. W. Y. Lam, B. Z. Tang,
Chem. Soc. Rev. 2011, 40, 5361–5388; f) M. R. Molla, S. Ghosh,
Chem. Eur. J. 2012, 18, 1290–1294; g) A. Ajayaghosh, V. K.
Received: June 1, 2012
Revised: August 31, 2012
Published online: November 23, 2012
Chem. Eur. J. 2012, 18, 16632 – 16641
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
16641