Luminescent micro and nanogel formation from AB3 type poly(aryl ether) dendron derivatives without conventional multi-interactive gelation motifs

Article abstract of DOI:10.1039/c1nj20136k

We report the synthesis, gelation and photophysical properties of luminescent AB₃ type poly(aryl ether) dendron derivatives in the absence of conventional multi-interactive gelation motifs. The gelation process is controlled through employing partial polar solvent milieu, which significantly enhances the propensity of π-π interaction between the aryl units present in the system. The self-assembly leads to unprecedented gelation through entrapping solvent molecules in the fibrillar arrangement of poly(aryl ether) units. The strategy was further utilized to prepare an excimer based photoluminescent gel through incorporating anthracene units in the dendrons. The close proximity between the anthracene units in the gel renders the formation of anthracene excimers at room temperature, resulting in the emission of bright green light from the gel, upon UV excitation. The study suggests that the size and packing of the self-assembled fibre can be controlled by the generation and functional groups present in the dendron. Furthermore, the strategy envisages an easy approach to generate fluorescent Low Molecular-mass Organic Gelator (LMOG) through incorporating poly cyclic aromatic hydrocarbon units to the poly(aryl ether) dendrons, since the self-assembly is largely guided by π-π interactions. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique.

Full text of DOI:10.1039/c1nj20136k

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PAPER
Luminescent micro and nanogel formation from AB3 type
poly(aryl ether) dendron derivatives without conventional
multi-interactive gelation motifsw
P. Rajamalli and Edamana Prasad*
Received (in Montpellier, France) 18th February 2011, Accepted 20th April 2011
DOI: 10.1039/c1nj20136k
We report the synthesis, gelation and photophysical properties of luminescent AB3 type
poly(aryl ether) dendron derivatives in the absence of conventional multi-interactive gelation
motifs. The gelation process is controlled through employing partial polar solvent milieu, which
significantly enhances the propensity of p–p interaction between the aryl units present in the
system. The self-assembly leads to unprecedented gelation through entrapping solvent molecules
in the fibrillar arrangement of poly(aryl ether) units. The strategy was further utilized to prepare
an excimer based photoluminescent gel through incorporating anthracene units in the dendrons.
The close proximity between the anthracene units in the gel renders the formation of anthracene
excimers at room temperature, resulting in the emission of bright green light from the gel, upon
UV excitation. The study suggests that the size and packing of the self-assembled fibre can be
controlled by the generation and functional groups present in the dendron. Furthermore, the
strategy envisages an easy approach to generate fluorescent Low Molecular-mass Organic Gelator
(LMOG) through incorporating poly cyclic aromatic hydrocarbon units to the poly(aryl ether)
dendrons, since the self-assembly is largely guided by p–p interactions.
non-covalent interactions among the dendrons, which can
Introduction
‘immobilize’ the solvent milieu, thereby generate gel systems
with tunable properties.29–31
Study of self-assembly by organic molecules has gained
increasing attention due to the potential applications of such
molecular systems in developing functional soft materials.1–6
For example, the remarkable self-assembling property of
tetrathiafulvalene,7,8 oligothiophene,9,10 and diacetylene11–13
in a fibrillar fashion can potentially lead to the formation
of molecular wires and field-effect transistors. In general,
multiple non-covalent interactions such as hydrogen bonding,
p–p interactions, and/or ionic interactions have been utilized
for the self-assembly of molecules to generate functional soft
materials.14–19 In this context, dendrimers and dendrons
deserve special attention since the feasibility of multiple inter-
actions is very high in such systems due to their well-known
fractal geometry. Self-assembly in dendrimers has been inten-
sively investigated due to their potential candidacy in soft
matter chemistry and numerous structural patterns have
already been recognized in this regard.20–28 Recently, organo-
gelators based on dendrimers and/or dendrons have also
gained much interest because of the unique nature of multiple
In the reported cases of dendritic gelators, presence of large
number of amide functional groups, long alkyl chains and/or
peripheral functional groups for specific ‘head’-to-‘tail’ inter-
action were essential for effective gel formation.32–37 A recent
report from Zhang and Fan describes the self-assembly of
tetraphenylethene, perylenediimide, and spiropyran functiona-
lized poly(aryl ether) dendron derivative to a photoresponsive
organogel.38a,b However, the design of organogelators, parti-
cularly dendritic gelators, with no conventional gelation motifs
such as peptide, long alkyl chains, steroidal groups etc. remains
as a great challenge to date,38c,d even though such gelators have
better potential applications in optoelectronic systems.37
Herein, we report the unprecedented gel formation through
the self-assembly of AB3 type (also known as Percec type)
poly(aryl ether) dendrons containing no long alkyl chains or
bulky substituents. While Percec type dendrons have been
known for their self-assembly and gelation properties,39,40
most of the reported cases utilized multiple non-covalent
interactions for the gel stability. In the present study, a series
of poly(aryl ether) dendron derivatives were selected (Chart 1)
and the self-assembly is invoked through p–p interactions,
which was controlled by the polarity of the medium. The
results from the present study suggest that p–p interactions
Department of Chemistry, Indian Institute of Technology Madras,
Chennai 600 036, India. E-mail: pre@iitm.ac.in;
Fax: +91-44-2257-4202; Tel: +91-44-2257-4232
w Electronic supplementary information (ESI) available: Synthetic
procedure, SEM image and XRD pattern. See DOI: 10.1039/c1nj20136k
c
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Results and discussion
Chart 1 contains the structure of the poly(aryl ether) dendron
derivatives ((AB)3-G1, (AB)3-G2, (AB)2-G1, and (AB)2-G2
series, where G represents the generation of the dendrimer)
utilized in the present study.
All the sixteen poly(aryl ether) dendron derivatives were
synthesized according to a reported procedure42 with slight
1
modifications. The characterization was carried out using H
NMR, 13C NMR and mass spectrometry (the synthetic
procedure and spectral data are given in the Experimental
section as well as in the ESIw).
Gel formation by the dendrons
The gelation behavior of the molecules in Chart 1 was evaluated
in a series of organic solvents and solvent mixtures. The
experimental results suggest that for the compound in the
AB3 series (both G1 and G2), the compounds prefer to form
a gel in solvent mixtures, rather than in pure solvents. For
example, compound V is completely soluble in dichloromethane
and practically insoluble in hexane at identical concentration.
However, V readily forms a gel in a dichloromethane: hexane
mixture (1: 3% v/v), above the critical gel concentration (CGC),
Chart 1 Structure of AB3 and AB2 type poly(aryl ether) dendron
derivatives utilized in the present study.
which was 2 mg mLÀ1 43
Similar observations were made
.
predominate when partial polar medium is employed through
for other AB3 compounds in Chart 1, suggesting that polarity
of the medium plays a pivotal role in enhancing the gelation
process in poly(aryl ether) derivatives. The solubility data along
with the critical gel concentration (CGC) of the compounds
are given in Table 1. The lowest value of CGC obtained was
2 mg mLÀ1 for compound V in the dichloromethane: hexane
mixture (1 : 3% v/v).
proper solvent mixtures, which leads to
a remarkable
columnar type self-assembly in poly(aryl ether) dendron
derivatives without conventional gelation motifs. The close
packing of the poly(aryl ether) units enhances the feasibility
of the excimer formation upon photo-excitation of the gel.41
To the best of our knowledge, this is the first example of
excimer based fluorescent low-molecular-mass organogelators
(LMOG) by poly(aryl ether) dendrons, in the absence of usual
gelating motifs. Most importantly, the present study
demonstrates the importance of p–p interactions, which
A careful examination of the data in Table 1 indicates that
the propensity to form gel is greatly enhanced in the second
generation (G2) poly(aryl ether) dendrons, compared to that
in the first generation (G1). It is also clear from the table that
the presence of an alcoholic group does not favor the gelation
in the second generation (G2) AB3 dendron series (compounds
V, VI and VII). In order to explore the effect of functional
groups on the gelation process in G2 AB3 series, we have
compared the physical properties of V with VI and VII, where
the carboxylic acid functional group is replaced by methyl
ester and alcohol, respectively. Differential scanning calori-
metry (DSC) experiments indicate that the melting point
follows the order VII o VI o V (63 1C, 87 1C and 150 1C,
alone can be successfully utilized for
a columnar gel
formation in the case of poly(aryl ether) dendron derivatives.
Extensive investigation based on scanning electron micro-
scopy (SEM), atomic force microscopy (AFM) and X-ray
diffraction (XRD) studies have been carried out to
understand the morphology and molecular arrangement in
the gel structure. The fluorescence properties of the gels were
investigated using steady state fluorescence measurement
techniques.
Table 1 Gelation properties and critical gel concentrations (CGCs) of the poly(aryl ether) dendrons in various organic solvents and solvent
mixtures at 25 1C. The solvent ratio is specified in parentheses. S = solution, G = gel, PG = partial gel
Solvent
I
II
III
V
VI
VII
CH2Cl2 : hexane
G (8.5 mg mLÀ1
(2 : 1)
)
S
G(18 mg mLÀ1
(1 : 3)
)
)
G(2 mg mLÀ1
(1 : 3)
)
G(5 mg mLÀ1
)
PG
(1 : 5)
G(4 mg mLÀ1
CHCl3 : hexane
CHCl3 : CCl4
G(10 mg mLÀ1
(2 : 1)
)
S
S
G(17 mg mLÀ1
(1 : 3)
S
G(2.5 mg mLÀ1
(1 : 3)
)
)
PG
PG
PG
PG
PG
(1 : 5)
S
G(3 mg mLÀ1
(1 : 8)
)
G(4 mg mLÀ1
(1 : 8)
)
)
)
Ethyl acetate : hexane
THF : hexane
G(5.7 mg mLÀ1
(3 : 2)
)
G(50 mg mLÀ1
(1 : 3)
S
)
G(25 mg mLÀ1
(1 : 3)
)
)
)
G(4 mg mLÀ1
(1 : 1)
G(3.5 mg mLÀ1
(1 : 5)
)
G(10 mg mLÀ1
(3 : 1)
)
G(20 mg mLÀ1
(1 : 3)
G(4 mg mLÀ1
(1 : 1)
G(6 mg mLÀ1
(1 : 4)
)
Dioxane : hexane
G(10.5 mg mLÀ1
(3 : 1)
)
S
G(22 mg mLÀ1
(1 : 3)
G(4.5 mg mLÀ1
(1 : 1)
)
G(8 mg mLÀ1
(1 : 5)
)
c
1542 New J. Chem., 2011, 35, 1541–1548
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respectively). Parallel to this observation, we have noted that
VII forms partial gels in all the solvent mixtures examined and
VI forms gel only through sonication. This indicates that the
two point hydrogen bonding interaction enhances the propen-
sity of the self-assembly and gel formation in V, compared to
the dipole–dipole interaction in VI and relatively less strong
hydrogen bonding in VII. The importance of the hydrogen
bonding in the gel formation was evident in the first generation
(G1) AB3 series also. Quite contrary, the first and second
generation of AB2 type poly(aryl ether) dendrons ((AB)2-G1
and (AB)2-G2 series) bearing carboxylic acid functional
groups (IX and XIII, Chart 1) exhibited little gelation
properties in any of the solvent mixtures utilized in the
study, even at relatively higher concentrations. This suggests
that the cooperative effect originated from the hydrogen
bonding and p–p interactions leads to the gelation in the
present study. It has been recognized that relatively weak
p–p interactions can stabilize supramolecular aggregates due
to their presence in sheer large numbers.44–46
SEM and AFM imaging study
The unprecedented solvent induced self-assembly by tri-
substituted poly(aryl ether) dendron derivatives without conven-
tional gelation motifs was further examined through scanning
electron microscopy (SEM) and the images clearly show
intensely packed nano-fibrils of the self-assembled poly(aryl
ether) dendron derivatives. Fig. 2 shows the SEM images
of compounds I–III and V. As clear from the figure, all
compounds show amazingly well defined fibrillar type self-
assembly. The self-assembly confined to a definite shape has
direct impact on the gelation properties of the system. For
example, the SEM images of compound VI does not indicate
fibrillar type assembly, even though undefined self-assembly
was evident from the SEM images (Fig. S1, ESIw). Corollary
to this, no gelation was observed in the case of VI until the
system is sonicated. This observation was identical for anthra-
cene substituted second generation AB3 type poly(aryl ether)
dendrons as well (vide infra), suggesting that the self-assembly
of poly(aryl ether) dendrons in the fibrillar fashion enables
them to easily contain the solvent molecules inside the
entangled fiber-like segments.
NMR study to probe the p–p interactions
Furthermore, a parallel AFM analysis also revealed the
formation of well-defined fibrils by the poly(aryl ether)
dendron derivatives, which are entangled to each other, upon
gel formation. Fig. 3 contains the AFM images of all
compounds exhibiting gelation. The images show fibers with
two distinct heights: one set with 150–200 nm and another
Since the preliminary results indicate that p–p interactions
in the poly(aryl ether) dendron derivatives might be crucial,
the presence of p–p interaction in V was closely examined by
1H-NMR spectroscopy. The compound was dissolved in
CDCl3 in the presence of increasing concentrations of hexane
(from 10 to 80% v/v). The NMR spectra for compound V in
the presence of increasing amount of hexane is given in Fig. 1.
As anticipated, the aromatic as well as benzyl protons in V
were shifted up-field due to the enhanced self-assembly
through p–p interactions in the increased non-polar
environment (Fig. 1).36 The peaks at 7.2, which correspond
to the benzyl ring proton were shifted up field by 0.12d.
Also, peaks at 4.9 and 6.7 were also up field shifted in the
presence of hexane. The shifts for the entire peak were
monitored using TMS signal as the reference. Also, above
70% v/v of hexane, NMR experiments could not be performed
due to the sudden gel formation by the system in the
NMR tube.
with 50–80 nm. The AFM images clearly illustrate
a
correlation between the fiber thickness of the self-assembled
poly(aryl ether) dendron derivatives and the dendron
generation. For example, while the compound in the G1 series
forms fibers with approximately 1 mm width, the fiber width
from compounds in the G2 series is only a few nano-
metres (110 nm). This could be presumably due to the closely
packed molecular arrangement in G2 dendrons as a result of
the enhanced p–p interactions between the relatively large
numbers of aryl units present, compared to that in the G1
dendron.
Fig. 1 NMR spectra of V (concentration: 4 mg mLÀ1) in CDCl3 in
the presence of increasing amount of hexane. From a to e, the mol% of
hexane increases from 10%, 20%, 30%, 50% and 60%, respectively.
Fig. 2 SEM images of compounds (a) I, (b) II, (c) III, (d) V.
c
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Fig. 4 Powder XRD pattern of compound V taken at (a) lower angle
and (b) wide angle.
Table 2 Powder X-ray diffraction (XRD) data for compounds I to VI
Compound
Phase
Colh
Lattice parameter
dobs/A
hkl
I
a = b = 25.48
22.12
11.88
10.84
9.54
14.47
12.42
9.33
7.09
6.47
22.65
12.76
11.32
8.45
23.69
13.41
11.76
9.05
26.38
15.25
13.16
9.99
22.22
18.55
13.89
11.08
8.85
100
110
200
210
200
210
310
400
420
100
110
200
210
100
110
200
210
100
110
200
210
200
211
310
400
500
II
Cub
a = 28.74
III
IV
V
Colh
Colh
Colh
Cub
a = b = 26.18
a = b = 27.35
a = b = 30.46
a = 44.48
Fig. 3 AFM images of compounds (a) I, (b) II, (c) III, (d) IV, (e) V
and (f) VI.
The results from SEM and AFM taken together suggest that
poly(aryl ether) dendron derivatives in the AB3 series (both
first and second generations) self-assemble in suitable solvent
mixtures into well-defined fibrillar type arrangement. Since
solvent molecules can be contained well inside the entangled
fibrillar assembly, gelation was a natural outcome of the self-
assembly. Fig. S1 (ESIw) shows the photographs of compound
V in dichloromethane : hexane mixture (1 : 3% v/v), below and
above the critical gel concentration (a and b respectively).
VI
The wide angle region reflection peaks at 2y = 16.71
(d-spacing of 5.1 A) and 2y = 18.71 (d-spacing of 4.6 A) are
seemingly due to the crystal packing of rod like segments in the
aromatic units in the gel, consistent with the SEM and AFM
results (Table 2).47 The hexagonal columnar packing of the
poly(aryl ether) dendrons has been slightly altered to cubic
columnar when the functional group was changed from acid to
ester (Fig. S3, ESIw).
XRD analysis of the gel
In order to understand the molecular packing pattern of the
self-assembly, X-ray diffraction pattern of the compounds
shown in Chart 1 was analyzed. A peak in the wide angle
region characteristic of a typical p–p stacking distance of 3.6 A
was observed in all the X-ray diffraction patterns (Fig. 4,
Fig. S2 to S3, ESIw).36 Fig. 4 shows the XRD pattern of V,
which follows the scattering vector ratio of 1 : O3 : O4 : O7 in
the low angle region, indicating a columnar hexagonal lattice
with a = b = 30.46 A, where a and b are the lattice
parameters.47 It can be envisaged that the molecules self-
organize in a columnar hexagonal (Colh) fashion through
stacking the dimer of V, where the hydrogen bonded carboxyl
groups are located in the middle of the column. The hexagonal
columnar phase with a column diameter of 30.46 A was
consistent with the results obtained from geometry optimization,
which was performed using DFT at the B3LYP 3-21G
level of theory with the Gaussian 03 suite of programs.
Anthracene substituted poly(aryl ether) dendrons
In order to verify the hypothesis that only p–p interactions
play the leading role in governing the self-assembly and
gelation in AB3 type poly(aryl ether) dendron derivatives,
anthracene substituted poly(aryl ether) dendrons IV and VIII
were synthesized and the gelation properties were examined
in various solvent mixtures. The compounds IV and VIII
were designed to investigate whether p–p interactions alone
can drive the self-assembly in poly(aryl ether) derivatives at
appropriate polarity of the medium, as there are no potential
hydrogen bonding sites available in these compounds.
c
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Fig. 5 Powder XRD pattern (a) and SEM image (b) of compound IV.
Chart 2 Cartoon representation of the self-assembly of poly(aryl ether)
dendrons utilized in the present study.
Corroborating our hypothesis, compound IV readily self-
assembled in the chloroform–methanol mixture (1 : 3% v/v)
to hexagonal columnar arrangement, as evident from the
XRD of the sample (Fig. 5). The AFM and SEM images of
compound IV exhibited entangled nano-fibrils as shown in
Fig. 3 and 5, respectively. The SEM images of compound IV
are similar to other poly(aryl ether) dendron derivatives
utilized in the present study.
In the case of compounds I, II, III, V, VI and VII, the self-
assembly is also assisted by the hydrogen bonding or dipolar
interaction at the core of the dendron, which drives two such
units together, placing the whole assembly one on the top of
the other. However, in the case of compounds IV and VIII, the
self-assembly should be purely driven by p–p interactions. As
the steric repulsion is less in the case of compound IV, the
propensity of self-assembly as well as gel formation is high in
the case of compound IV. For compound IV, two such
molecules could come on top of one another and form the
fibrillar assembly. The XRD studies of compound IV provide
direct evidence for a hexagonal columnar arrangement in the
system, similar to that observed in other poly(aryl ether)
dendron derivatives utilized in this study.
Fig. 6 Steady state emission spectrum (left) (lexc. = 310 nm), and
UV-illuminated photo (right) of compound V.
emission from the aggregation of benzene units in V as a
result of the columnar arrangement of the system.48 More
interestingly, photoexcitation of IV by UV light readily provided
anthracene excimer emission around 505 and 530 nm.49 Fig. 7
contains the emission spectra of IV in the solution phase
The experimental data taken together indicate that the
pattern of self-assembly (i.e., columnar) remains unchanged
in all the poly(aryl ether) dendron derivatives examined in this
study. Experimental results indicate that p–p interactions
between the aromatic units can be invoked at appropriate
polarity of the medium, which can result in such unusual self-
assembly in the absence of any conventional, multi-interactive
gelation motifs. It is interesting to note that while the
poly(aryl ether) dendron is not planar, especially in the G2
case, the self-assembly is not affected by the non-planarity of
the benzene rings. A cartoon representation of the self-assembly
of AB3 type poly(aryl ether) dendrons in the presence of a
suitable solvent mixture is given below (Chart 2).
Fluorescence properties of the gel
Since unprecedented aggregation driven by the solvent might
result in the close packing of aromatic units in the poly(aryl
ether) dendrons, it is quite likely that they can form aggrega-
tion induced fluorescence emission.48 In order to verify this
hypothesis, we have carried out the fluorescence studies of the
compounds shown in Chart 1. Photoexcitation of V by UV
light readily provides an intense blue emission around 350 nm.
Fig. 6 contains the emission spectra of V in the gel form. The
blue luminescence with lmax 343 nm is attributed to the
Fig. 7 Steady state emission spectra from compound IV in solution
(dash) (solvent used is CHCl3) and in the gel phase (solid) {CHCl3 :
CH3OH (1 : 3% v/v)} (lexc. = 360 nm). Right: photographs of
compound IV, (a) in the presence and (b) in the absence of UV
illumination.
c
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(chloroform, 4 mg mLÀ1) and in gel form (chloroform : methanol
mixture CGC is 4 mg mLÀ1), along with the photographs of
the gel in the presence and absence of UV light. Compound IV
exhibited negligibly small emission intensity in chloroform.
On the other hand, clear anthracene excimer emission was
observed at 505 nm and 530 nm, upon photo-excitation of the
gel. The intensity of emission from the gel was 1118 folds
higher than that observed in the solution, confirming the
aggregation induced emission (AIE) in the system examined.
It has been reported in the literature that anthracene can
form mainly two types of excimers based on the geometry of
each individual anthracene units adopted in the excited state.50
The first one is relatively stable and two anthracene units
overlap at an angle of 551 to each other and has an emission
maximum close to 470 nm. The second one forms sandwich
type excimer where the two anthracene units are symmetrically
p-stacked with emission maxima at 560 nm. A recent report
suggests the emission from a third type of anthracene excimer
(T-shaped excimer) at 510 nm.51 The different emission
maxima observed in the emission spectrum of compound IV
in the gel form suggest the presence of a fraction of anthracene
monomer and more than one type of anthracene excimers in
the excited state.
Experimental section
Materials
The poly(aryl ether) dendron derivatives were synthesized
according to a reported procedure42 with slight modifications.
All the starting materials were obtained from Aldrich or SD
fine-chemicals Pvt. Ltd. India and used as received unless
otherwise mentioned. The organic solvents used were dried
according to standard procedures.
Instruments
1H and 13C NMR data were collected on a Bruker 400 MHz
spectrometer (1H: 400 MHz; 13C: 100 MHz). Mass spectra
were recorded using a Micromass Q-TOF mass spectrometer.
Luminescence experiments were carried out on a Horiba Jobin
Yvon Fluoromax-4 fluorescence spectrophotometer. The
scanning electron microscopic studies were carried out using
a FEI-Quanta Microscope. AFM samples were prepared by a
spin-coating method on a silicon wafer and images were
recorded using a Park-system XE-100 in the non-contact mode
regime. Powder-XRD patterns were recorded on a Bruker D8
Advance X-ray diffractometer using Cu-Ka radiation
(l = 1.54178 A). Melting points were recorded using Q200
MDSC from TA Instruments under a dry nitrogen atmosphere
The emission maxima at 505 and 530 nm in the present case
indicate that ‘partial’ and ‘sandwich’ type overlap between the
anthracene units occur in the self-assembled gel system, upon
photoexcitation.49 As evident from Fig. 7, the anthracene
based gel system exhibits bright green color upon shining
UV light, suggesting that excimer emission is quite intense
(lexc. = 505 nm and 530 nm) in the system. The gel systems
based on the simple poly(aryl ether) dendron derivatives were
found to be highly stable under normal laboratory conditions
for more than six months. It is clear from the data that solvent
induced aggregation in simple poly(aryl ether) dendron deri-
vatives can easily result in the excimer formation as a result of
the closely packed chromophoric groups in a columnar fashion.
This strategy can be useful to generate fluorescent gel systems in
the presence of poly aromatic hydrocarbons, without any
conventional multi-interactive gelation motifs in the system.
with a heating rate of 5 1C minÀ1
.
Synthetic procedures
1
Synthesis of (AB)3 G1-COOCH3(II)
Methyl-3,4,5-trihydroxybenzoate (9 g, 0.045 moles) and
potassium carbonate (24.84 g, 0.18 moles) in 130 mL of
1,4-dioxane were taken in a 250 mL round bottom flask.
Benzyl chloride (33 mL, 0.135 moles) was added followed by
the addition of a catalytic amount of tertiary butyl ammonium
iodide (1.4 g, 0.0045 moles). The solution was heated to reflux
with stirring for 24 hours. The solvent was removed under
reduced pressure using a rotary evaporator to afford an oil
that turned into a solid upon standing. The solid was
recrystallized from methanol. The yield of (AB)3 G1-COOCH3
1
was 18.5 g (90.5%). Spectral characterization: H NMR (400
Conclusion
MHz, CDCl3) d: 3.8 (s, COOCH3, 3H), 4.9 (s, ArCH2O, 2H),
5.01 (s, ArCH2O, 4H), 7.1–7.3 (m, ArH and PhH, 17H).
13C NMR (100 MHz, CDCl3) d: 52.25, 71.27, 75.15, 109.13,
125.26, 127.57, 127.96, 128.04, 128.21, 128.56, 136.69, 137.48,
142.46, 152.59, 166.66. MS: m/z calcd for C29H26O5: 454,
found: 455 [M + H]+.
In summary, we have shown the first example of the formation
of fluorescent gel utilizing low molecular weight simple AB3
type poly(aryl ether) dendron derivatives through selecting
solvent mixtures which render partial polar environment,
enabling self assembly. The results suggest that the gelation
properties of the dendrons, in the absence of conventional
gelation motifs, are largely guided by p–p interactions and
strongly dependent on the dendron generation. It might be
possible to utilize these LMOGs, especially those composed by
first generation AB3 type poly(aryl ether) dendrons, in various
optoelectronic applications since remarkable columnar type
self-assembly is achieved utilizing these compounds.52–55
Further studies to explore the possibility of incorporating
other polycyclic aromatic hydrocarbons to the poly(aryl ether)
dendrons to generate gel systems with tunable light emitting
properties are in progress in our laboratory and the results will
be published in due course.
2
Synthesis of (AB)3G1-CH2OH(III)
Lithium aluminium hydride (0.809 g, 0.0213 moles) was
suspended in 40 mL of freshly distilled THF in a dry three-
neck round-bottom flask under a nitrogen atmosphere. (AB)3
G1-COOCH3 (9 g, 0.0198 moles) was dissolved in 50 mL of
freshly distilled THF and added dropwise to the lithium
aluminium hydride solution. The reaction mixture was
refluxed with stirring for 2 h. The THF solution was cooled
to room temperature and transferred to a beaker. Water was
added very slowly dropwise to the vigorously stirred THF
solution until the gray color of the lithium aluminium hydride
c
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disappeared and a white solid was formed which was filtered
and washed with THF. Excess solvent was removed under
reduced pressure and the crude product was recrystallized
from 95% methanol/water mixture to get the alcohol ((AB)3
G1-CH2OH) with 90% yield (7.6 g). Spectral characterization:
1H NMR (400 MHz, CDCl3) d: 4.6 (s, CH2OH, 2H), 5.09
(s,ArCH2O, 2H), 5.15 (s,ArCH2O, 4H), 6.72 (s, ArH, 2H),
7.30–7.48 (m, PhH, 15H). 13C NMR (100 MHz, CDCl3) d:
65.42, 71.22, 75.26, 106.46, 127.45, 127.83, 127.90, 128.18,
128.52, 128.62, 136.66, 137.13, 137.81, 137.87, 153.02. MS: m/z
calcd for C28H26O4: 426, found: 449 [M + Na]+.
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Synthesis of (AB)3 G1-COOH(I)
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A solution of KOH (5 mL, 10 N) was added to compound 1
(5 g, 0.0110 moles) in 95 mL ethanol. The mixture was refluxed
for three hours and was acidified with concentrated HCl. After
refluxing the acidified reaction mixture for 15 minutes the
precipitate was filtered and washed with water several times to
get the product (AB)3G1-COOH with a yield of 4.4 g (90.9%).
1H NMR (400 MHz, CDCl3) d: 5.20 (s, ArCH2O, 6H),
7.30–7.49 (m, ArH and PhH, 17H). 13C NMR(100 MHz,
CDCl3) d: 71.29, 75.19, 109.72, 124.12, 127.58, 128.01, 128.23,
128.53, 128.59, 136.58, 137.40, 142.42, 152.62, 170.02. MS: m/z
calcd for C28H25O5: 440, found: 441 [M + H]+.
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4
Synthesis of An–G1(AB)3(IV)
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(AB)3 G1-CH2–OH (0.9 g, 0.002 moles) was dissolved in 10 ml
freshly distilled THF in a dry 100 mL two neck flask under
nitrogen and temperature was maintained around 0 1C. To
this solution NaH (100 mg, 0.004 moles) was added under a
nitrogen atmosphere and stirred for 5 minutes followed by the
addition of 9-chloromethyl-anthracene (0.5 g, 0.0022 moles).
The reaction mixture was kept at room temperature with
stirring for 2 hours. After completion of the reaction excess
NaH was quenched by methanol followed by the addition of
water to get a yellow precipitate which was filtered and washed
with methanol to give the product G1TAN in 89.4% yield
(1.1 g). Spectral characterization: 1H NMR (400 MHz, CDCl3)
d: 4.63 (s, ArCH2O, 2H), 5.07–5.10 (s, ArCH2O, 6H), 5.51
(s, AnCH2O, 2H), 6.71 (s, ArH, 2H), 7.31–8.36 (m, AnH and
PhH, 24H). 13C NMR (100 MHz, CDCl3) d: 63.85, 71.14, 72.23,
75.23, 107.50, 124.45, 125.01, 126.17, 127.48, 127.85, 128.17,
128.28, 128.63, 129.03, 131.12, 131.51, 134.15, 137.15, 137.95,
152.89. MS: m/z calcd for C43H36O4: 616, found: 634
[M + NH4]+.
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Chem., Int. Ed., 2001, 40, 2626.
Acknowledgements
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This work was supported by grants from Department of
Science and Technology (DST), under Grant number SR/S1/
PC-26/2007. We thank MME department, IITM, for the SEM
images.
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