Non-amphiphilic pyrene cored poly(aryl ether) dendron based gels: Tunable morphology, unusual solvent effects on the emission and fluoride ion detection by the self-assembled superstructures

Article abstract of DOI:10.1039/c2sm26151k

The design, synthesis and study of morphology as well as photophysical properties of novel low molecular weight gels (LMWG) based on non-amphiphilic pyrene cored poly(aryl ether) dendron derivatives are described. Solvent controlled self-assembly in the system results in nano-sized vesicles, which further aggregate to micro-sized vesicles and finally turned to entangled fibrillar type arrangement in the gel phase. The morphology transformations were investigated by dynamic light scattering (DLS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM) and confocal laser scanning microscopy (LSCM) experiments. The nano-/micro-sized vesicles and fibre aggregates exhibit an intense light emitting property from the pyrene moiety, which is attached to the dendron through an acylhydrazone spacer group. More interestingly, the luminescence properties were found to be controlled by the solvent polarity, in a rather unusual manner, due to the selective formation of pyrene 'excimer' and 'exciplex' in solvent controlled aggregates. Furthermore, the system has been utilized to detect fluoride ions through a reversible gel-sol transition, which is associated with a color change from yellow to red.

Full text of DOI:10.1039/c2sm26151k

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PAPER
Non-amphiphilic pyrene cored poly(aryl ether) dendron based gels: tunable
morphology, unusual solvent effects on the emission and fluoride ion detection
by the self-assembled superstructures†
*
P. Rajamalli and Edamana Prasad
Received 18th May 2012, Accepted 27th June 2012
DOI: 10.1039/c2sm26151k
The design, synthesis and study of morphology as well as photophysical properties of novel low
molecular weight gels (LMWG) based on non-amphiphilic pyrene cored poly(aryl ether) dendron
derivatives are described. Solvent controlled self-assembly in the system results in nano-sized
vesicles, which further aggregate to micro-sized vesicles and finally turned to entangled fibrillar
type arrangement in the gel phase. The morphology transformations were investigated by dynamic
light scattering (DLS), scanning electron microscopy (SEM), transmission electron microscopy
(TEM), atomic force microscopy (AFM) and confocal laser scanning microscopy (LSCM)
experiments. The nano-/micro-sized vesicles and fibre aggregates exhibit an intense light emitting
property from the pyrene moiety, which is attached to the dendron through an acylhydrazone
spacer group. More interestingly, the luminescence properties were found to be controlled by the
solvent polarity, in a rather unusual manner, due to the selective formation of pyrene ‘excimer’
and ‘exciplex’ in solvent controlled aggregates. Furthermore, the system has been utilized to detect
fluoride ions through a reversible gel–sol transition, which is associated with a color change from
yellow to red.
elegant works by Ajayaghosh and co-workers have demonstrated
Introduction
the formation of fluorescent vesicles from non-amphiphilic
oligomer structures.29,30 In another interesting study, Li et al.
have described vesicle formation by oligomers and foldamers,
which are non-amphiphilic in nature.31,32
In the recent past, low-molecular-mass organic gelators
(LMOGs) containing polycyclic aromatic hydrocarbons have
gained considerable attention due to their potential role in
various optoelectronic devices for enhanced charge transport
and emission intensity.1–5 Designing LMOGs with attractive
fluorescence properties is highly desirable in the context of
developing functional materials for sensing and imaging appli-
cations.6 Study of the self-assembly of fluorescent LMOGs to
various nano-architectures is vital in nanoscience and tech-
nology, particularly in the emerging fields of supramolecular
electronics.7–9 Among the varied shapes generated by the
molecular assembly, micelles, fibres and vesicles have gathered
much interest since they are important three-dimensional
molecular assemblies for biomimetics study, functional nano-
materials, resonance energy transfer study and building drug and
gene delivery systems.10–22 While a large number of amphiphilic
polymers and surfactants are known to spontaneously form
vesicles,23–28 only limited examples are available in the literature
for non-amphiphilic structures forming vesicles. For example,
Dendrimers are macromolecules with well-defined architec-
tures and flexible molecular framework at the nanoscale.33–36 Our
continued interest in the design and utilization of dendrimer
based low-molecular-mass organogels37,38 prompted us to
develop organogels with ‘tunable’ morphology and light emitting
properties. While amphiphilic and zwitterionic dendrimers/den-
drons forming flexible morphology are known in the litera-
ture,39,40 non-amphiphilic type dendrimers/dendrons, which
spontaneously self-assemble to various morphologies, have not
been reported so far. Herein, we describe the controlled forma-
tion of nano- to micrometer-sized vesicles and fibres from non-
amphiphilic pyrene cored poly(aryl ether) dendrons. To the best
of our knowledge, this is the first report on developing a fluo-
rescent organogel based on a non-amphiphilic poly(aryl ether)
dendron derivative which exhibits ‘vesicle’ as well as ‘fibre’
morphology. We have also investigated the role of solvent
polarity on the morphology and photophysics of the system and
the results reveal an unusual solvent dependency on the emission
wavelength from the self-assembled system. Furthermore, the
ability of the system to detect anions such as fluoride ions has
been analysed.6d
Department of Chemistry Indian Institute of Technology Madras (IITM),
Chennai, 600036, India. E-mail: pre@iitm.ac.in; Fax: +91 44-2257-4202
† Electronic supplementary information (ESI) available: Synthetic
procedure, characterization methods, SEM, TEM, AFM images, Jobs
plot, CD spectrum, etc. See DOI: 10.1039/c2sm26151k
8896 | Soft Matter, 2012, 8, 8896–8903
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depicts the variation of CGC with respect to the ET values of
different solvents. The plot shows a weak correlation between
solvent polarity parameter and CGC values. The Reichardt’s
parameter for the mixture of chloroform (ET -39.1) and meth-
anol (ET -55.4) is expected to be higher than that of the mixture
of chloroform and hexane (ET -31.4). Since the CGC values of
the gelators are not varied between the two different solvent
mixtures mentioned above, we conclude that polarity has a poor
role in regulating specific solute–solvent interaction at the
molecular level.45a We then compared the effect of solvents on
gelation ability in terms of Kamlet–Taft parameters, where
solvents are described in terms of their ability to donate (a),
accept (b) hydrogen bonds and generalized polarity parameter
(p*). Similar to the above-mentioned cases, a weak correlation
between Kamlet–Taft parameters and CGC values was observed
as shown in Fig. S2a and c.† Kamlet–Taft parameters vary
widely from polar solvents such as methanol (a-0.98, b-0.66,
p*-0.60) to non-polar solvents such as hexane (a-0.00, b-0.00,
p*-0.04) and comparable CGC values in chloroform–methanol
and chloroform–hexane mixtures suggest that the parameters
appear to be less important in the present case. The trend remains
identical for all the gelators examined in the study.
Results and discussion
Gel formation
The first and second generations (G1 and G2, respectively) of
AB2 and AB3 type poly(aryl ether) dendrons containing pyrene
moiety, attached through an acylhydrazone spacer unit, were
synthesized based on a reported procedure (Fig. 1).38 Pyrene
fluorophore was selected since its derivatives have been well
recognized and extensively utilized in designing biological
probes, artificial light-harvesting systems, photonic devices and
organic semiconductors.41–44
Compounds I to IV are sparingly soluble in moderately polar
solvent environments (Table 1), under ambient conditions.
However, the compounds were dissolved completely by heating
followed by sonication for three to four minutes. The system
suddenly turned to gel upon cooling to room temperature. The
critical gel concentration (CGC) values of the compounds were
determined in each solvent and are given in Table 1. In order to
analyse the solvent effect on the gelation properties quantita-
tively, we correlate the CGC values of the gelator with the solvent
parameters such as dielectric constant (3), Reichardt’s parameter
(ET), and the Kamlet–Taft parameter.45 A plot of dielectric
constant vs. CGC values of the compounds (Fig. S1a†) does not
provide a definite correlation, indicating that the dielectric
constant of solvents does not play a significant role in controlling
the specific solute–solvent interactions at the molecular level. A
similar trend is observed for the mixture of solvents examined in
this study. For example, the CGC values for compound IV did
not vary drastically in a mixture of chloroform (3-4.8) and
methanol (3-32.7), compared to that of chloroform and hexane
(3-1.9), even though the dielectric constant of the solvent
mixtures is expected to be distinct. We have also analyzed the
role of solvent polarity parameter (Reichardt’s parameter, ET)
on the gelation properties of the compounds described. Fig. S1b†
The CGC value provides another pathway to understand the
correlation between gelator action and gelator solubility. From
Table 1, it is clear that gelators show relatively high CGC values
in single solvents like CHCl3, THF, dioxane, DMF, etc.,
compared to solvent mixtures due to the high solubility of the
gelators in these solvents. However, slight decreases in the CGC
values are observed in the presence of highly polar (methanol or
water) or highly non-polar (hexane) solvents, which is likely due
to the fact that the presence of poor solvents provides improved
p–p interactions in the system.45c AB2 type poly(aryl ether)
dendrons form gel in several organic solvents at very low CGC
values. The propensity of gel formation by compounds III and
IV (AB2 type) was high compared to that of compounds I and II
(AB3 type). This can be attributed to the steric hindrance in
compounds I and II due to the increased number of benzyl units
which disrupts the planarity of the molecules and their self-
assembly.
The FT-IR spectrum of gel IV (from CHCl3) shows a single
amide (C]O) stretching vibration band at 1641 cmꢀ1 and an
intense NH stretching vibration at 3189 cmꢀ1 (Fig. S3†). This is
an unambiguous signature of the presence of a network of
hydrogen-bonded amides. In addition to this evidence, powder
XRD patterns of the compounds I to IV exhibit a reflection peak
in the wide-angle region, which is characteristic of a typical p–p
ꢀ
stacking distance in the range of 3.42–3.59 A (Fig. S4a to d†),
indicating that hydrogen bonding and p–p stacking interactions
hold the self-assembled system in the gel phase. The gel transition
temperature increases as the concentration of the gel increases
and Fig. S5† shows the linear relation between the gel transition
temperature (53–63 ꢁC) and the corresponding gel concentration
(0.2–1.2 wt%) for the first generation AB2 type dendron deriva-
tives in a THF–water mixture (1 : 1).
Tunable morphology
The morphology of the xerogels of the dendron derivatives in
chloroform was examined by scanning electron microscopy
Fig. 1 Structure of the compounds used in the present study.
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Table 1 Gelation properties and critical gel concentrations (CGCs) of dendrons in various organic solvents and mixture of solvents at 25 ꢁCa
Medium
I
II
III
IV
CHCl3
DCM
THF
Dioxane
Acetone
DMF
Ethyl acetate
Pyridine
Ethanol
P.G
P.G
G(8)
G(9)
G(5.5)
G(6)
G(4)
G(7)
G(4)
G(12)
(3)
S
I
G(10)
G(10)
G(6)
G (6) (1 : 1)
G(3.5) (1 : 1)
G(7) (1 : 1)
G(4) (1 : 1)
G(3.5) (1 : 1)
G(6)
G(7)
G(5)
G(6.5)
G(3.5)
G(20)
G(5)
G(5)
I
P.G
G(5)
G(9)
G(6) (1 : 1)
G(3.5) (1 : 1)
G(6) (1 : 1)
P.G
G(4)
G(5)
G(4)
G(9)
G(5)
G(25)
T.G(4)
S
G(5)
G(15)
G(8)
G(4)
I
P.G
G(3.5)
G(11)
G(4) (1 : 1)
G(4) (1 : 1)
G(5) (1 : 1)
P.G
I
Toluene
T.G(6)
G(15)
G(4)
G(5) (1 : 1)
G(3) (1 : 1)
G(8) (1 : 1)
G(3.5) (1 : 1)
G(3) (1 : 1)
Acetonitrile
Acetophenone
DMF : water
THF : water
Dioxane : water
CHCl3 : methanol
CHCl3 : hexane
P.G
G(4) (1 : 1)
a
The value given in parentheses is the CGC in mg mlꢀ1. For solvent mixtures, the ratio is also given in parentheses. G ¼ Gel, T.G ¼ Transparent gel, S ¼
Solution, P.G ¼ Partial gel, I ¼ Insoluble.
Fig. 2 SEM images of xerogel formed from first generation compounds
I and III (a and b, respectively) and second generation compounds II and
Fig. 3 SEM images of xerogel formed from compound IV in CHCl3–
MeOH at (a) 1 ꢂ 10ꢀ5 M, (b) 1 ꢂ 10ꢀ4 M, and (c) 3.5 ꢂ 10ꢀ3; (d) SEM
IV (c and d, respectively).
image of compound IV (1 ꢂ 10ꢀ4 M) in CHCl3–hexane.
(SEM). The SEM images are shown in Fig. 2. The four gelators
form nano-fibrillar type arrangement, where the thickness of the
fibres was relatively higher in the first generation dendrons,
compared to that formed from the second generation.
magnitude (1 ꢂ 10ꢀ4 M), the nano-spheres fuse together result-
ing in the formation of microspheres. Interestingly, the size of the
microsphere exhibited a solvent dependency. While the average
diameter of 1 mm was obtained for the microspheres in a CHCl3–
MeOH mixture, an increased diameter of 3 mm was obtained in a
CHCl3–hexane mixture (Fig. 3b and c). The SEM images clearly
reveal that the initially formed nano-sphere assembly could
further combine to generate twins, triplets or even multiplets,
suggesting that larger structures were formed by the fusion of the
smaller ones (see Fig. S6a†). The microspheres can further self-
assemble to form extended structures (nanofibres) in solvent
mixtures at concentrations above CGC (>1 ꢂ 10ꢀ3 M, Fig. 3d
and S6c†). The tunable morphology was, however, not observed
in the first generation dendrons (compounds I and III) where
The morphology of the xerogels of the dendron derivatives
formed from varied solvent mixtures was also examined by SEM.
While all the compounds exhibit fibrillar type assembly in single
solvents, fine-tuning of the polarity of the medium resulted in the
formation of spherical morphology in the system. Initial exper-
iments were carried out in CHCl3–MeOH (relatively polar) and
CHCl3–hexane (relatively non-polar) mixtures. The results from
SEM studies suggest that compounds II and IV (second gener-
ation) form well defined nano-spheres with a diameter range of
200–300 nm at concentrations below CGC (1 ꢂ 10ꢀ5 M)
(Fig. 3a). Upon increasing the concentrations by an order of
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predominantly nano-fibres were formed in solvent mixtures over
a wide range of concentrations (Fig. S6d†). The generation
dependency on the aggregate morphology indicates that the
conformational flexibility adopted by the system is crucial in
deciding the final self-assembled structure.
mixtures without any CD signals (for example, see Fig. S11†).
While the elegant work by Percec et al. demonstrated the self-
assembly and formation of helical structures from achiral den-
drons, formation of helical gel systems from achiral poly(aryl
ether) dendrons has not been reported yet.47,48
In order to verify whether drying results in significant changes
in the initial morphology of the aggregates, dynamic light scat-
tering (DLS) experiments were carried out at lower (10ꢀ5 M) and
higher (1 ꢂ 10ꢀ4 M) concentrations of compounds II and IV in a
CHCl3–MeOH mixture. DLS studies carried out at the lower
concentration range indicate the presence of aggregates having
distributions of varying hydrodynamic diameters, with a mean
diameter (Dh) of 200–300 nm (Fig. S7†). At a relatively higher
concentration range, the hydrodynamic diameter exhibited a
mean diameter (Dh) of 1.0–1.3 mm. The results are strikingly
consistent with the SEM analysis described above indicating that
microscopic images reflect actual morphology of the aggregated
systems. The values of the hydrodynamic diameter also suggest
that the microspheres formed were vesicles and not micelles,
since the diameter for micellar structures would normally fall in
the range of 3–7 nm.46
The hollow feature of the vesicle was confirmed by fluores-
cence microscopy images (Fig. 5) where there is a large difference
in the luminescence of the outer ring relative to that of the inner
areas. This clearly shows that vesicles are centrally hollow and
filled with fluorescent-silent solvent molecules. To our surprise,
the vesicles exhibited two different fluorescent colors in CHCl3–
MeOH and CHCl3–hexane mixtures (blue and green, respec-
tively) (vide infra).
To further examine the morphology, TEM images of the
compounds I to IV were taken from single solvents. The images
show nano-fibre formation, consistent with the SEM analysis
(Fig. S12a to d†). The TEM images for compounds II and IV
were also taken from a mixture of solvents and the images
revealed the hollow feature of the vesicles (Fig. 6). The magnified
TEM images (Fig. S13†) suggest that the wall thickness of the
vesicles of compound IV was ca. 6 nm when the gel is formed
from a CHCl3–hexane mixture and 3.1 nm when the gel is formed
from a CHCl3–MeOH mixture (Fig. S13†). The vesicle to fibre
transformation by the dendron derivatives was further confirmed
through TEM images (see Fig. 6b), where the fusion of initially
formed vesicles is clearly observed.
Next, atomic force microscopic images were taken for
compounds I to IV in chloroform. The nano-sized fibres in the
gel systems were evident in the AFM images and average height
of the fibres was found to be 200 nm and 40 nm for generations I
and II, respectively (Fig. S8a and d†). AFM images of II and IV
were also investigated in the mixture of solvents to get more
insight into the morphology of the initially formed nano-/micro-
meter assemblies and the subsequent transformation to super-
structures (Fig. 4 and S9†). The ratio of the diameter to the
height of the assembly was estimated to be 4 nm for the case of
compound IV, indicating that the soft structures are flattened
after transferring from the solution phase to silicon wafer. This
can be attributed partially to the slow evaporation of the solvent
molecules from the assembly and/or to the high local force
applied by the AFM tip. Surprisingly, the AFM images of the
dendron derived gels (I–IV) from toluene indicate the presence of
a one dimensional helical network structure and spiral nano-fibre
formation (Fig. 4b and S10†). The AFM images indicate that the
widths of these helical fibres are within the range of 80–110 nm.
The morphology of the single helix is shown to be twisted with a
left- and right-handed pattern and the helical pitches vary from
100–170 nm. Furthermore, left- and right-handed helical fibres
were present in equal quantities, thus resulting in overall racemic
Fluorescence properties
Although the poly(aryl ether) dendron derivatives are non-fluo-
rescent in the solution phase (<10ꢀ4 M in single solvents and
Fig. 5 LSCM images of vesicles formed from compound IV (1 ꢂ 10ꢀ4
M) (a) in CHCl3–hexane and (b) in CHCl3–MeOH at room temperature.
Fig. 4 AFM images of compound IV: (a) vesicles formed in CHCl3–
MeOH (1 ꢂ 10ꢀ4 M) and (b) helical fibres formed in toluene at room
temperature.
Fig. 6 TEM images of compound IV in CHCl3–MeOH (1 ꢂ 10ꢀ5 M) at
room temperature: (a) vesicle formation and (b) fusion of vesicles.
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<10ꢀ6 M in solvent mixtures), enhanced emission was observed
from all the compounds under aggregated conditions. For
example, Fig. 7 shows the photographs of compound IV in
chloroform (1 ꢂ 10ꢀ4 M) and the gel phase, under visible and
UV illumination. It is evident that there is no emission from the
solution phase under UV and visible light excitation. Nonethe-
less, the gel formation resulted in bright green emission under
UV light (Fig. 7).
unconventional solvent dependent wavelength shift for the
emission. For example, upon exciting the gel formed in a CHCl3–
MeOH mixture at 410 nm, a broad structureless band is observed
around 445 nm, associated by a bright-blue emission. In contrast,
a broad structureless bright-green emission is observed from the
gel around 525 nm in a relatively more non-polar environment
(CHCl3–hexane mixture). The emission maximum from the gel in
toluene was, however, in between (470 nm). Fig. 9 depicts the
emission spectra of compound IV in the gel formed from varied
solvents. The corresponding spectra for compound II is given in
the ESI (Fig. S14d†).
The fluorescence intensity from the gel phase was enhanced by
ꢃ750 fold compared to that in the solution phase (Fig. 8a). The
remarkable fluorescence enhancement from the gel systems can
be explained by gel-induced enhanced emission (GIEE).49 The
enhanced emission from the remaining compounds (I–III) is
shown in the ESI (Fig. S14†). Laser scanning confocal micros-
copy (LSCM) was utilized to obtain the images of the fluorescing
gels through drop casting on a microscope coverslip. The
confocal microscopic image of compound IV is given in Fig. 8b
which reveals the presence of a dense 3D network of greenish
fluorescent fibres.
These results are quite unexpected since the shift in the emis-
sion wavelength is independent of solvent polarity, suggesting
that the role of the solvent in controlling the photophysics of the
self-assembly of the dendron systems is subtle (vide infra).
Mechanism of the self-assembly
Results taken together suggest that gelation in generation I
dendrons always leads to fibre type self-assembled structures
irrespective of the solvent milieu and concentrations. The self-
assembly might occur through a two-step hierarchical supra-
molecular mechanism, as we have observed in the corresponding
anthracene analogues.38 First, the molecules are assembled
through hydrogen bonding and p–p interactions into one-
dimensional extended assemblies, followed by intertwining of the
chains to form the elementary fibre. However, the morphological
features of the generation II dendrons in the present study are
remarkably different from the anthracene cored dendron
analogues.38
The structureless features of the luminescence emission from
the gel systems (lmax ¼ 520–530 nm) suggest that the emission is
from pyrene excimer. The excited state luminescence lifetime of
the gels was measured and the fluorescence decays were best fit
for a triexponential curve with 9.46 ns (65.85%), 2.41 ns (17.33%)
and 0.43 ns (16.82) (Fig. S15†) and the highest lifetime value is
attributed to the pyrene excimer emission.50
Role of solvent in ‘tuning’ the emission
Since solvent polarity can control the size and morphology of the
self-assembled systems described, we hypothesized that the
solvent milieu could affect the emission properties from the gel
systems. The experimental result suggests that there is an
In a polar environment, the acylhydrazone spacer group is
likely to be exposed to a solvent medium, and the appended
benzyl rings as well as pyrene groups could ‘fold back’ due to
hydrophobic interactions. The self-assembly of such folded
dendrons results in the formation of vesicles in which pyrene and
benzene units are placed at close proximity and this can poten-
tially lead to exciplex formation upon photoexcitation.51 This is
consistent with the blue emission obtained from the LSCM
images and the emission spectra of the second generation den-
dron derivatives. On the other hand, it is likely to have
predominantly p–p stacking interaction between pyrene
Fig. 7 Photographs of compound IV in chloroform (1 ꢂ 10ꢀ4 M) and in
the gel phase under ambient light (left) and under UV illumination
(right).
Fig. 8 (a) Emission spectra of compound IV in the gel phase (red) and in
chloroform solution (1 ꢂ 10ꢀ4 M) (black); (b) LSCM image of the gel
formed from compound IV in CHCl3 at room temperature.
Fig. 9 Emission spectra of compound IV (gel) formed from varied
solvent environments.
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moieties during the self-assembly of the dendron derivatives in a
relatively less polar solvent, resulting in the formation of pyrene
excimer, upon photoexcitation. This is consistent with the
observation of the green emission from the second generation
dendron derivatives in a CHCl3–hexane mixture. The third type
of emission was observed at 470 nm in toluene. This can be
attributed to the pyrene excimer emission, where the pyrene units
are partially overlapped in the excited state, due to the helical
type of self-assembly.52 Based on the photophysical results, a
schematic representation of the above-mentioned processes is
proposed in Fig. 10.
Fig. 11 Photographs of (i) organogel formed from compound IV in
THF, (ii) after addition of TBAF (1 equiv.), and (iii) after addition of
water (0.2 ml).
As a corollary of the solvent dependent self-assembly, we
expect a difference in the wall thickness of the vesicle formed in
different solvent environments. The space-filling model of
compound IV in the energy minimized structure showed 1.6 nm
as the distance between the spacer group and the end groups,
implying that the folded double layer self-assembly of compound
IV would have a total length of 3.1 nm (path b in Fig. 10). This is
strikingly consistent with the results obtained from the TEM
analysis where the wall thickness of the vesicles in a chloroform–
methanol mixture was found to be 3.1 nm (Fig. S13†). Similarly,
the end to end distance for an extended structure of compound
IV was 3.1 nm. This is also concurrent with the wall thickness
obtained from TEM analysis (6 nm), where two such units were
self-assembled in a linear fashion (path a in Fig. 10). The results
suggest that the vesicles have a double layer outer surface.
Fluoride ion detection
The use of this low molecular weight fluorescent gel as a sensor
for various anions (Fꢀ, Clꢀ, Brꢀ, Iꢀ, ClO4ꢀ, CH3COOꢀ,
H2PO4ꢀ and HSO4ꢀ as Bu4N+ salts) was examined in THF. The
gels from compounds I to IV show intense variation in their
electronic absorption spectra only upon the addition of tetra-
butylammoniumfluoride (TBAF) at room temperature
(Fig. S16a to c†). The presence of Fꢀ ions not only changes the
color of the systems, but also disrupts the preformed gel into a
Fig. 12 (a) UV-vis absorption spectra of compound IV (5 ꢂ 10ꢀ5 M) in
the presence of 1 equiv. of various anions in THF and (b) increasing the
concentration of the fluoride ion (0.01–1 equiv.).
Fig. 10 Scheme representing different types of self-assembly of the dendrons to form a gel.
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solution through slow diffusion of the anion. The photographs of
the fluoride induced gel-to-sol conversion are given in Fig. 11.
The drastic color change of the gels due to the addition of fluo-
ride ions reflects the significant changes in the electronic structure
of the system. The UV-vis absorption spectra of compounds I–IV
in THF (5 ꢂ 10ꢀ5 M) in the presence of 1 equiv. of various
anions were taken at room temperature (Fig. 12 and S16a
and c†). As is clear from the figures, the absorption band of
compounds I–IV is shifted to a longer wavelength only in the
presence of fluoride ions.
patterns were recorded on a Bruker D8 Advance X-ray diffrac-
ꢀ
tometer using Cu-Ka radiation (l ¼ 1.54178 A). Dynamic light
scattering (DLS) experiments were done on a Malvern Zetasizer
nano-series 25 ꢁC, with a path length of 1 cm. The wavelength of
the laser used was 632.8 nm and the scattering angle was kept
at 90.
Synthesis
A solution of 1-pyrene carboxaldehyde (0.506 g, 0.0022 mole) in
methanol was added drop wise to a CHCl3 solution of compound
a (1 g, 0.0022 mole).38 The mixture was stirred for 3 hours. The
resulting precipitate was filtered off by suction and dried under
vacuum to yield I (1.45 g, 96.2%). 1H NMR (400 MHz, DMSO-
d6) d: 5.03 (s, ArCH2O, 2H), 5.23 (s, ArCH2O, 4H), 7.26–7.51 (m,
ArH & PhH, 17H), 8.11 (t, PyH, J ¼ 7.6 Hz, 1H), 8.25 (m, PyH,
2H), 8.36 (d, PyH, J ¼ 7.6 Hz, 4H), 8.59 (d, PyH, J ¼ 8 Hz, 1H),
8.84 (d, Hz, J ¼ 9.2 Hz, PyH, 1H), 9.49 (s, CH]N, 1 H), 11.97 (s,
CONH, 1H); 13C NMR (100 MHz, DMSO-d6) d: 70.08, 74.94,
107.74, 124.31, 124.88, 125.10, 125.29, 125.45, 125.71, 126.89,
127.52, 128.09, 128.20, 128.30, 128.44, 128.72, 128.78, 129.04,
130.69, 131.15, 131.78, 137.78, 138.04, 138.56, 152.01, 152.31,
168.18; HRMS (ES+): m/z calcd for C45H34N2O4: 666.2519
found: 667.2582[M + H]+.
The evidence for the reaction mechanism was obtained from
1H NMR experiments in DMSO-d6 (Fig. S17†). Before addition
of Fꢀ ions, the 1H NMR chemical shift values of –CH]N– and
–NH protons in the compounds were at 9.65 and 12.05 ppm,
respectively. After addition of 1 equivalent of Fꢀ ions, the NH
signal disappeared which suggested that the NH groups under-
went a deprotonation reaction.38
Conclusions
In summary, we have initiated the synthesis, morphology study
and photophysics of non-amphiphilic poly(aryl ether) dendron
based tunable fluorescent gel systems. The gels exhibit controlled
size and morphology, induced by concentration and solvent
polarity. While the second generation dendrons form nano- and
micro-sized vesicles below CGC, giant fibrillar type assemblies
are formed above CGC. The self-assembly was associated with a
ꢃ750 fold enhancement in emission intensity compared to that of
solution. Importantly, the gel system exhibited unconventional
solvent effects on emission wavelength, which is attributed to the
controlled formation of ‘excimer’ and ‘exciplex’ from the pyrene
moiety in the self-assembly. The current gel system was also
employed to detect Fꢀ ions, resulting in the sol–gel transition
with an intense color change from yellow to red.
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