Supramolecular design for two-component hydrogels with intrinsic emission in the visible region

Article abstract of DOI:10.1039/c3cc38419e

We report, for the first time, an in situ formation of two-component hydrogels from pyridine derivatives of poly(aryl ether) dendrons and tartaric acid. The two-component system (dendron + acid) undergoes J-type aggregation, leading to fibrillar type self-assembly in THF-water mixture along with blue (470 nm) and green (500 nm) intrinsic emissions. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3cc38419e

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Supramolecular Design for TwoꢀComponent Hydrogels with Intrinsic
Emission in the Visible Region
P. Rajamalli, Supriya Atta, Sandeepan Maity and Edamana Prasad*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
We report, for the first time, an in-situ formation of twoꢀ
in spite of the absence of a conventional fluorophore unit. The
component hydrogels from pyridine derivatives of poly(aryl 50 present study indicates that two component hydrogels with
ether) dendrons and tartaric acid. The two component system
(dendron + acid) undergoes Jꢀtype aggregation, leading to
10 fibrillar type selfꢀassembly in THFꢀwater mixture along with
blue (470 nm) and green (500 nm) intrinsic emissions.
tunable light emitting properties can be synthesized even in the
absence of conventional fluorophore molecules attached to the
gel. Such gel systems are highly desirable for various applications
in organic electronics since real time monitoring of the
55 supramolecular assembly is feasible by fluorescence turn on.1c
Design and synthesis of two component gelators have been
emerged as an attractive area of reserach.1 Addition of a suitably
designed second component largely leads to enhanced
15 intermolecular interaction, leading to effective selfꢀassembly in
the system. Further hierarchical molecular level reꢀarrangements
in solution pave the formation of a gelꢀphase material. Among the
known examples of low molecular mass gelators, twoꢀcomponent
gelators have gained special attention because of the additional
20 level of control over the selfꢀassembly by changing the
composition of the gel components.2 Amidst the reported cases,
the two component gels have been usually formed through
complementary hydrogenꢀbonding interactions,3 donorꢀacceptor
interactions4 and metal–ligand bond formation.5 While the
25 physiochemical properties of twoꢀcomponent gelators have been
greatly beneficial for numerous gel based applications, reports on
such systems are relatively less compared to that of the singleꢀ
component gelators.6
N
N
N
HN
N
HN
CO
O
CO
O
O
O
60
O
O
O
O
O
O
O
O
O
O
O
N
I
II
65
N
N
HN
N
HN
CO
CO
O
O
O
O
O
O
O
O
III
70
IV
Dendrimers/dendrons are interesting candidates in this
30 context since they bear many repeating units within their
branched architectures, which are capable of generating multiple
nonꢀcovalent interactions.7 We have recently explored light
emitting single component gel systems based on poly(aryl ether)
dendrons, which contain polycyclic aromatic hydrocarbons such
35 as anthracene, pyrene or naphthalene.8 While two component gel
systems based on dendritic amino acids are reported9, poly(aryl
ether) dendrons or their derivatives have not been used to
generate two component gel systems.10 In the present study, we
have designed specific functional properties to poly(aryl ether)
40 dendrons, particularly to enhance the hydrogen (H)ꢀbonding
ability of the gelator. A pyridine group has been attached (see the
structure in Fig. 1) to one end of the dendron, which is likely to
selfꢀassemble with aliphatic acids through hydrogen bonding. The
present study describes the formation of two component gel
45 systems between the designed poly(aryl ether) dendrons and
tartaric acid in THFꢀwater mixture. The gelation leads to fine
fibrillar type selfꢀassembly, which interestingly exhibits novel
gelation induced emission (GIIE) property in the visible region,
Fig. 1 Structure of the compounds used in the present study
The dendrons shown in Fig. 1 were completely soluble in
common organic solvents such as tetrahydrofuran (THF)
dichloromethane (DCM), and chloroform. Aggregate formation
75 has not been observed in any of these solvents, which was
confirmed by dynamic light scattering (DLS) experiments. We,
initially, examined the gelation propensity of the dendrons in
THFꢀwater mixture (1:1 v/v), in the absence of other components.
Earlier results from our group indicated that presence of water
80 induces aggregation in poly(aryl ether) dendron derivatives.8 FEꢀ
SEM images of the dendron in THF/water are given in Figure 2a
and supporting information (Fig. S1, ESI). The iamges clearly
show the formation of 400ꢀ500 nm sized vesicles by first
generation compounds (I and III) and 1ꢀ2 ꢁm sized vesicles by
85 second generation compounds (II and IV). In order to verify
whether drying the sample results in significant changes in the
initial morphology of the aggregates, DLS experiments have been
carried out for the all compounds in THFꢀwater mixture. DLS
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studies indicated the presence of aggregates having distribution of 60 region, indicating that molecules are arranged in a lamellar
varying hydrodynamic diameters, with a mean diameter (Dh) of
400ꢀ500 nm and 1ꢀ2 ꢁm (Fig. 2b and Fig. S2, ESI). The results
are strikingly consistent with the SEM analysis described above
indicating that microscopic images reflect actual morphology of
pattern (Fig. S7, ESI). The lamellar phase arrangement with dꢀ
spacing values of 33.5 Å, 16.8 Å and 11.3 Å was consistent with
the results obtained from geometry optimization, performed using
space filling model of chemdraw 11. The complex formation
5
the aggregated systems in solution phase. The values of the 65 between the dendrons and tartaric acid has been analyzed through
hydrodynamic diameter also suggest that the microspheres
formed were giant vesicles and not micelles, since the diameter
for micellar structures would normally fall in the range of 3ꢀ100
Job plot and the results indicate a 2:1 complexation in the system
(Fig. S8, ESI). Based on the XRD pattern and Jobs analysis, a
schematic representation for the molecular arrangement in the gel
phase has been given in the supporting information (Fig. S9,
10 nm.11 The vesicels formation has also been confimed by AFM
analysis (Fig. S3, ESI).
70 ESI).
FEꢀSEM images (Fig. 3 and Fig. S10, ESI) of the dried
dendron/tartaric acid gels reveal that they have been composed of
long and bundled fibers that form highly dense entangled
networks. The average diameter of the fiber is in the range of
75 200–225 nm. Furthermore, a parallel analysis by AFM, and TEM
also revealed the formation of entangled fibrils by the poly(aryl
ether) dendron derivatives/TA mixture, which are richly
entangled to each other. Fig. S11 contains the AFM images of Iꢀ
IV. The images show fibers heights ranging between 60ꢀ80 nm
80 (Fig. S11, ESI). TEM image of the fibers from I and III are
shown in the supporting information (Fig. S12, ESI). The
formation of gel fiber is presumed to be the result of crossꢀlinking
between the Hꢀbonded and πꢀstacked dendron units by tartaric
acid units. Quite interestingly, upon mixing a THF solution of
85 poly(aryl ether) dendron derivatives and an aqueous solution of
tamarind immediately resulted in gel formation, demonstrating
the capability of the dendrons to form gels with tartaric acid
present in natural resources.
b
a
16
14
12
10
8
15
6
4
2ꢁm
2
0
0
2000
Diameter, nm
4000
20
Fig. 2 (a) SEM image and (b) DLS histogram plot for compound
IV in THFꢀwater (1:1 v/v).
Since addition of water did not result in gelation, 0.50
25 equimolar amount of tartaric acid (TA) in water has been added
to a 0.25 wt% solution of compound I in THF. The colorless
solution was suddenly turned to a yellow gel. It has been also
noted that sonication of the above mentioned microvesicles in
presence of TA also leads to the formation of identical gel. The
30 selfꢀassembly was most likely to be assisted by hydrogen bonding
between I and the acid additives.12 The presence of Hꢀbonding
has been confirmed from FTꢀIR spectral analysis of the gel where
intense peaks at 3219 cmꢀ1 indicate Hꢀbonded ꢀOH, ꢀNH
stretching and peaks at 1724cmꢀ1 and 1670 cmꢀ1 indicate ꢀCO
35 (acid) and ꢀCO (amide) stretching, respectively (Fig. S4, ESI).
Critical gel concentration (CGC) values for all the dendrons are
given in Table S1. The data in Table S1 shows that AB3 type
dendrons (I and II) form stable gel in the presence of additives
compared to that of AB2 type dendrons (III and IV). This
40 observation suggests that Hꢀbonding as well πꢀπ stacking
interactions play important roles in the gel formation. Moreover,
the propensity of gel formation is slightly reduced in compound
II, compared to that of compound I, due to the steric
encumbrance of benzyl units in compound II. Among the four
45 compounds, the propensity of gel formation follows the following
order: I > II > III > IV. This indicates that a delicate balance
between the Hꢀboding and πꢀπ stacking interactions is essential
for efficient gel formation in the two component system. The
intense πꢀπ stacking present in the system has been analyzed by
50 NMR experiments in solution phase as well as in partial gel phase
(Fig. S5, ESI). The results suggest that the aromatic protons were
shifted upfield, upon gelation, which further confirms the
presence of strong πꢀπ stacking inetractions.8a XRD studies (Fig.
S6, ESI) of the dried gel indicate that the gel fibers are highly
55 crystalline for compounds II, III and IV. A diffraction peak was
found at 2θ = 25.33°, which is also indicative of a wellꢀdefined
and compact πꢀπ stacking of dendron at a distance of 3.5 Å in the
gel fiber. However, the gel I/TA exhibits XRD pattern which
follows the scattering vector ratio of 1:2:3 in the lower angle
90
a
b
95
2ꢁm
2ꢁm
Fig. 3 FESEM images of xerogels in THFꢀwater (1:1) (a)
compound I/tartaric acid (2 : 1) and (b) compound III/tartaric
100 acid (2 : 1).
UVꢀvis investigations reveal that the compounds absorb at
300 nm in solution phase. In the presence of tartaric acid,
aggregated absorption of dendron has been red shifted to 350 nm
for compound I (Fig. 4). The generation of redꢀshifted absorption
105 peak at the longer wavelength region indicates
a Jꢀtype
aggregation of the dendron in the gel phase.13 Upon addition of
THF, the absorption bands is reversed to its original position.
Compound II also exhibited a red shifted absorption band at 335
nm, in presence of TA. The red shift is less compare to that of
110 compound I, consistent with the high CGC value for the
compound II.
Unexpectedly and fascinatingly, the novel two component gel
systems exhibit intense intrinsic luminescence, upon UV
exposure. The emission spectra of the gel phase of
115 dendron/tartaric acid (2 : 1 ratio) (dendron II =2.5 mg/mL) show
a broad emission peak around 440 nm with much enhanced
luminescence intensity compared to the dendrons in solution
2
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†
Electronic Supplementary Information (ESI) available: Synthetic
phase (Fig. 4b). It was intriguing to note that the emission
intensity was optimum at 2 : 1 ratio of dendron to tartaric acid
and emission intensity steeply decreased upon decreasing the
amount of tartaric acid. While compounds III and IV exhibit
emission around 470 nm and 440 nm, respectively, compound I
exhibits a red shifted green emission at 500 nm (Fig. S13, ESI).
Fig. 5 shows the photographs of I/TA and III/TA systems, upon
UV irradiation. Although the intrinsic emission color (green or
blue) depends on the type of the compound, no luminescence has
10 been observed for I/TAꢀ IV/TA systems in solution phase. The
time resolved luminescence study with time correlated single
photon counting (TCSPC) system indicate that the luminescence
lifetime in the gel state increases as the generation increases (Fig.
S14). For example, first generation compounds (I and III) shows
15 the lifetime of about 1ꢀ5 ns while the second generation
compound II shows an unusually high lifetime value of around
125 ns. Though the gel systems exhibit intense emission, further
experiments have to be performed to understand the origin of this
intrinsic emission.
60 procedure, characterization data, DLS, SEM, TEM and AFM images etc.
See DOI: 10.1039/b000000x/
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4
(a) UVꢀVis spectra of dendron (solution phase) and
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Fig. 5 Photographs of the gels: I/TA (a) and III/TA (b) in THFꢀwater
45 (1:1) under ambient light (left) and UV illumination (right).
In summary, intrinsically luminescent and novel two
component gel systems have been developed via Hꢀbonding and
πꢀπ stacking mediated supramolecular assembly between
poly(aryl ether) dendron derivatives and tartaric acid. The red
50 shifted absorption peaks of the gels compared with that of
solution is attributed to the Jꢀtype aggregation in the system. Two
different colors (green and blue) of intrinsic emission have been
achieved from the two component hydrogels through varying the
dendrons type and generation.
55 Notes and references
*Department of Chemistry Indian Institute of Technology Madras (IITM),
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Fax: (+91)44-2257-4202; E-mail: pre@iitm.ac.in
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