Gelation and topochemical polymerization of peptide dendrimers

Article abstract of DOI:10.1039/c0nj00544d

A new class of peptide-based dendrons and dendrimers is presented that display unique vesicle-driven organogelation. We demonstrate the crosslinking of diacetylene-based dendrimers in the gel state, producing strongly fluorescent polydiacetylenes. The cro

Full text of DOI:10.1039/c0nj00544d

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PAPER
www.rsc.org/njc | New Journal of Chemistry
Gelation and topochemical polymerization of peptide dendrimersw
V. Haridas,*a Yogesh K. Sharma,a Rhiannon Creasey,b Srikanta Sahu,a
Christopher T. Gibsonb and Nicolas H. Voelcker*b
Received (in Montpellier, France) 11th July 2010, Accepted 27th September 2010
DOI: 10.1039/c0nj00544d
A new class of peptide-based dendrons and dendrimers is presented that display unique
vesicle-driven organogelation. We demonstrate the crosslinking of diacetylene-based dendrimers in
the gel state, producing strongly fluorescent polydiacetylenes. The crosslinked gels crafted from
peptide dendrimers may find practical use in various applications, including as scaffold materials
in tissue engineering and for drug delivery.
and their gel forming properties. BocLys(Boc)-OH was
reacted with propargyl amine in the presence of N-hydroxy-
Introduction
The relentless pursuit of chemists to design and synthesize
molecules and materials with functional properties has propelled
succinimide and dicyclohexylcarbodiimide (DCC), yielding 2a.
The use of the benzyloxycarbonyl group (Z) as a protecting
them to explore uncharted boundaries where the disciplines of
organic chemistry, materials science and biology amalgamate.
Further advances in this area require an understanding of
group produced 2b. The deprotection of Boc groups from 2a
using 50% TFA in CH2Cl2 and a further reaction with
BocLys(Boc)-OH using N-hydroxysuccinimide/DCC coupling
conditions produced dendron 3. Further deprotection using
50% TFA in CH2Cl2 and coupling with BocLys(Boc)-OH
resulted in the formation of dendron 4.9
We envisioned that the oxidative dimerization of these
molecular behavior and molecular carpentry.1 Self-assembly of
carefully designed molecular building blocks into complex
functional architectures as a paradigm has enabled the discovery
of new materials with programmable properties.2 The design of
new building blocks with defined and robust connectivities is
dendrons truncated with an acetylenic moiety would be an
a formidable challenge that relies heavily on an in-depth
understanding of structure–activity relationships.
Dendrimers have attracted the interest of materials scientists
attractive way to generate various rigid core dendrimers.
Indeed, the oxidative coupling of the dendrons with terminal
alkyne units in the presence of catalytic amounts of copper(I)
because they display a defined copy number of functional motifs
on their surface and possess a tunable yet monodisperse mass
chloride and N,N,N0,N0-tetramethylethylenediamine (TMEDA)
afforded symmetrical dendrimers 5a, 5b, 6 and 7 in good yields
and size, depending on the dendritic generation.3 The functional
properties of dendrimers can be tailored for applications such as
from 2a, 2b, 3 and 4, respectively (Scheme 1). The versatility of
this approach is further demonstrated by the creation of
drug delivery, organogels, catalysts and sensors.2,4,5 Dendrimers
asymmetrical dendrimers through the linkage of two different
offer enhanced flexibility in the design of materials over low
types of dendrons. In order to synthesize asymmetrical
molecular weight compounds due to their larger building
block size (steric impact), their large surface area and multiple
dendrimers, we coupled 2b and 3 in a Ni-catalyzed cross-
coupling reaction10 to the asymmetrical dendrimer 8 in 71%
yield (Scheme 2). Asymmetrical dendrimer 9 was synthesized
peripheral functional groups that give rise to multivalencies.
These advantages over low molecular weight organic compounds
prompted us to systematically investigate peptide dendrimers for
in a similar way by coupling dendrons 3 and 4.
With these dendrons and dendrimers in hand, we set out to
gelation properties. Chiral peptide dendrimers are gaining
screen their propensity to gel in different solvent systems.
Given that these dendrons contain multiple H-bond donors
importance as protein models and in biomaterial applications.6
Peptide dendrimers with large numbers of amide linkages can
and acceptors per molecule, we hypothesized that they should
associate in appropriate solvents to form networks of inter-
actions that generate controlled superstructures.2,7
show a propensity to aggregate, forming organogels in an
appropriate solvent system. The mechanism of supramolecular
gelation is a topic of immense interest, and recently it was
demonstrated that such gelation is very sensitive to solvent
polarity.11 Furthermore, recent work has shown that dendrimer
branching has a significant influence on gelation, since both
Results and discussion
Here, we report on the synthesis of peptide dendrons and
dendrimers8 (Scheme 1 and Scheme 2) based on lysine (Lys)
sterics and functionality change with dendrimer generation.2 It
is known that organogelation is the result of supramolecular
a Department of Chemistry, Indian Institute of Technology (IIT-D),
aggregation, which in turn is dependent upon the conformation
Hauz-Khas, New Delhi-110016, India.
of the gelator in the given solvent and the non-covalent
interactions (solvophobic forces) between the compound and
the solvent molecules. Predicting these parameters a priori is a
difficult task, so we screened 25 different solvent systems
ranging from single solvents to binary and tertiary mixtures
E-mail: haridasv@chemistry.iitd.ac.in
b Department of Chemical and Physical Sciences, Flinders University,
Adelaide, Australia. E-mail: nico.voelcker@flinders.edu.au
w Eleectronic supplementary information (ESI) available: Experimental
procedures and spectroscopic data of all new compounds. See DOI:
10.1039/c0nj00544d
c
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Scheme 1 Synthesis of symmetrical diacetylenic dendrimers 5a, 5b, 6 and 7.
of solvents of varying polarity (Table S1, ESIw). In a typical
procedure, the compound was dissolved in a polar solvent and
a non-polar solvent was added slowly until gel formation
occurred. Interestingly, each dendron required a mixture of
polar and non-polar solvent to induce gelation, and no gel
formation occurred in a single solvent, which reflects the
influence of both molecular structure and solvation factors.
It is noteworthy that a pre-made solvent mixture of appropriate
polarity is inadequate for gel formation because of the
insolubility of the compound in that solvent mixture.
In particular, we found that the addition of hexane to a
solution of 3 in ethyl acetate resulted in gelation (Fig. S1,
ESIw) at a hexane : ethyl acetate ratio of 3 : 2. Similarly, the
addition of ethyl acetate to a solution of 4 in methanol resulted
in gel formation at a ratio of 9 : 1 ethyl acetate : methanol
(Fig. 2A (inset)). The lower generation dendrons 2a and 2b did
c
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Fig. 1 Height mode AFM images in tapping mode of (A) a gel made
from 3 (hexane : ethyl acetate ratio 3 : 2) deposited on mica (z-scale =
10 nm) and (B) non-gelled 3 deposited on mica from an ethyl acetate
solution (z scale = 100 nm).
Fig. 2 (A) Height mode AFM image in tapping mode of a gel from 4
(ethyl acetate : methanol ratio 9 : 1) deposited on mica and (inset) a
photograph of the gel in an inverted tube. (B) TEM image of 4 in
methanol deposited on a Cu grid and stained with phosphotungstate.
The inset shows a zoomed-in view of two fused vesicles.
integral part of the gel formation process (Fig. 2B). The partly
formed gel was formed at room temperature by the addition of
0.15 mL of ethyl acetate to a solution of 4 (20 mg) in 0.04 mL
methanol in such a way that a translucent network of fibers
were formed. The further addition of ethyl acetate resulted in a
complete solid gel. We suggest that vesicle fusion induces a
morphological change that leads to fiber formation.
Scheme
2 Synthesis of unsymmetrical diacetylenic dendrimers
8 and 9.
not show any gelation, implying that a critical number of
hydrogen bonding moieties are needed for gelation. Also,
removal of protecting groups from 3 and 4 resulted in
complete loss of gelation in all of the tested solvent systems.
Atomic force microscopy (AFM) analysis of the gels from 3
(Fig. 1A) and 4 (Fig. 2A) revealed nanofibrous structures with
a bead-on-a-string architecture. During the investigation of
the underlying mechanism for gel formation, we observed that
both ethyl acetate solutions of 3 and methanol solutions of 4
contained structures with the appearance of vesicles of varying
size ranging from 100 to 500 nm, as judged by TEM, AFM
(Fig. 1B and Fig. S2 ESIw)12 and confirmed by dynamic light
scattering (Fig. S8 and S9 ESIw). The AFM images show
vesicle-like structures B100 nm high, albeit somewhat bigger
in lateral dimensions. This slightly distorted size is probably
due to a flattening of the vesicles upon adsorption on the
surface.13 Increasing the concentration of the dendrons led
to a concomitant increase in the concentration of discrete
vesicles. Furthermore, analysis of a solution of 4 with a partly
formed gel showed a beaded string-like topology reminiscent
of partially fused vesicles, implying that vesicle fusion is an
We then turned our attention to the diacetylene-core
dendrimers and investigated their ability to gelate in organic
solvents. Surprisingly, we found that only dendrimers 5a
(hexane : ethyl acetate ratio 3 : 2) and 5b (methanol :
chloroform : hexane ratio 1 : 2 : 2) showed any propensity to
gelate (Fig. 3A, Fig. S4 and 5 ESIw), whilst all attempts to
induce the gelation of higher generation symmetrical (6 and 7)
and asymmetrical (8 and 9) dendrimers failed in the
25 different solvent systems tested (ESIw). The bulky appended
Lys units in higher generation dendrimers may prevent
aggregation in solution as a consequence of increased steric
bulk and reduced flexibility. Gelation in the symmetrical
dimeric dendrimers 5a and 5b occurred independent of the
protecting group used. However, 5b afforded a denser gel than
5a by visual inspection. We attribute this to p–p interactions
from the benzyloxy-carbonyl groups, potentially facilitating
better packing in the gel state than Boc-protected 5b. The gel
transition temperature (Tg, 59 1C) of 5b was higher than that
of 5a (53 1C) (ESIw). Deprotection of 5a using TFA afforded
amine-terminated dimers that showed no gelation, consistent
with our observations for dendrons 3 and 4.
c
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Fig. 3 Height mode AFM images in tapping mode of a gel of 5a
(hexane : ethyl acetate ratio 3 : 2) (A) before and (B) after irradiation
with an Hg Lamp. The insets show photographs of the respective gels
in inverted tubes.
We further examined whether the rigid core dendrimers
with a central diacetylene unit could be photopolymerized,
generating polydiacetylenes (PDA), which would introduce
covalent crosslinks into the gels and render them very
interesting from a materials perspective. For this type of
topochemical polymerization to occur, molecules should
ideally be packed to a distance of approximately 4.9 A
between neighboring diacetylene moieties.14 We anticipated
that the diacetylene moieties in the gel state of 5a and 5b are
ideally juxtaposed for the 1,4-addition polymerization reaction
to proceed, with amide hydrogen bonds on both sides
of the central diacetylene units assisting molecular assembly
(Fig. S6 ESIw).15
Indeed, the UV irradiation of gels of 5a and 5b at 254 nm
immediately produced red-colored gels that were only
sparingly soluble in methanol, chloroform and ethyl acetate.
An AFM analysis of the photoirradiated gels gave evidence of
crosslinking in terms of straighter and tighter-packed fibers in
comparison to the morphology of the gels before irradiation
(Fig. 3B, Fig. S4C and D ESIw). The red color (lmaxCHCl3,
547 nm) is an indication of the formation of PDA with a
non-planar backbone.11c A non-planar backbone could arise
from the structural disturbance caused by the sterically
demanding Lys polymethylene chains. It is noteworthy that
PDA formation also occurred in the solid state at room
temperature without any irradiation, albeit at a very slow
rate (5 d).
Fig.
4
Fluorescence microscopy images (band pass excitation
(450–490 nm) and 515 nm long pass cutoff emission filters) of the
gel from 5b (A) before and (B) after UV exposure, and the gel from 5a
(C) before and (D) after UV exposure. The inset in (A) shows a
photograph of the gel from 5b irradiated through a mask with a
circular aperture.
indicating that the bulky dendritic structures sterically hinder
the approach of the diacetylenic moieties and thereby inhibit
polymerization. Photopolymerization brings about the potential
for the fabrication of complex cross-linked gel architectures,
such as arrays of gel spots. Indeed, when the gel from 5b was
exposed to UV light from an Hg lamp (B250 nm) through a
mask with a circular aperture, the exposed part was polymerized
(as indicated by the color change) whilst the masked area
remained a colorless gel (inset, Fig. 4). The patterning is an
interesting aspect of these materials, one which we will explore
in our future work. In addition to this, the gels prepared from
5a and 5b were fluorescent (Fig. 4), and the fluorescence
intensity significantly increased upon photopolymerization.
Cross-linked gels made from 5b showed stronger fluorescence
than those made from 5a (Fig. 4B and D). This autofluorescence
was observed only in the gel state. This observation is consistent
with an aggregation-induced fluorescence phenomenon,
although we cannot exclude other phenomena.16
Further evidence for photopolymerization came from our
confocal Raman microspectroscopy results. The Raman
spectra in Fig. S7 (ESIw) before UV irradiation showed a
band at 2256 cmꢀ1, corresponding to the stretching vibration
of carbon–carbon triple bonds of the diacetylene group. This
band was significantly reduced in intensity in the Raman
spectrum acquired after UV irradiation, and a new absorption
band at 2107 cmꢀ1, assigned to the stretching vibration of the
carbon–carbon triple bonds of the polydiacetylene chain,
appeared. Furthermore, a CQC vibration at 1501 cmꢀ1
became evident. These changes in the Raman spectrum are
consistent with our interpretation of photopolymerization.
The solutions of 5a and 5b did not polymerize under
identical conditions, demonstrating the importance of self-
assembly in this polymerization process. Interestingly dendrimers
6, 7, 8 and 9 did not show any tendency to gelate, but
dendrimer 8 showed polymerization in the solid state,
Whilst there are rare examples where the gel state is used to
facilitate topochemical polymerization,17 in those cases,
additional structure-directing groups like urea moieties are
required to enforce gelation via additional H-bonding.18 Here,
such moieties are not necessary for gelation to take place.
Furthermore, hydrogen bonding between the amide linkages
was sufficient to maintain the proper alignment of diacetylene
groups for polymerization.
Conclusions
In conclusion, we have reported a new class of peptide-based
dendrons and dendrimers with unique and tunable gelation
properties. We also demonstrated a unique vesicle-driven
c
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mechanism of nanofibrous gel formation. We have expanded
the structural repertoire by introducing covalent crosslinks
into diacetylene-core peptide dendrimers by using UV photo-
polymerization. Finally, we were able to spatially control the
dendrimer polymerization via UV lithography. We have
suggested potential uses for such peptide dendrimer gels and
their corresponding PDAs in diverse applications ranging
from drug delivery and biomaterials to organic electronics
and photonics.
Deprotection of dendron 4. To an ice-cooled solution of 4
(0.160 g, 0.091 mmol) in dry CH2Cl2 (1 mL) was added TFA
(3 mL) and the resulting solution stirred at RT for 2 h. The
reaction mixture was subjected to a vacuum to afford 0.080 g
1
of the compound. Yield: 92%; H NMR (D2O, 300 MHz) d
0.95–1.80 (m, 42H), 2.26 (s, 1H), 2.67 (br s, 8H), 2.87 (br s,
6H), 3.52–3.79 (m, 6H), 3.81–4.05 (m, 3H); IR (KBr) 3430,
2935, 1664, 1436, 1383, 1197, 1138 cmꢀ1; HRMS calc. for
C45H90N15O7 m/z 952.7148, found m/z 952.7155.
5a. To a solution of 2a (100 mg, 0.26 mmol) in 15 mL
CH2Cl2 was added Hay catalyst [prepared by stirring CuCl
(10 mg, 0.10 mmol) and TMEDA (0.033 mL, 0.22 mmol) in
CH2Cl2 (4 mL) for 5 min] and the mixture stirred under air.
After 3 h, more Hay catalyst (4 mL) was added and the
mixture left stirring for 24 h. The reaction mixture was diluted
with distilled CH2Cl2 (20 mL) and washed sequentially with
0.5 N aq. H2SO4, water, a saturated aqueous NaHCO3
solution, an aq. NH4CI + NH4OH (9 : 1) solution and water.
The organic layer was dried over anhydrous Na2SO4, filtered
and evaporated to yield 80 mg of 5a. Yield: 80%. [a]D ꢀ13.5
(c 0.2, MeOH); 1H NMR (300 MHz, CDCl3) d 1.13–1.90
(s + m, 44H), 1.58–1.81 (br m, 4H), 3.03 (br m, 4H), 3.82–4.25
(br m, 6H), 4.70 (br s, 2H), 5.45 (br s, 2H), 7.37 (br s, 2H);
13C NMR (75 MHz, CDCl3) d 22.70, 28.39, 28.47, 29.56,
29.90, 32.33, 40.16, 54.09, 67.88, 74.33, 79.16, 79.98, 155.91,
156.22, 172.51. IR (KBr): 3321, 3064, 2973, 2934, 2864,
1695(br), 1528, 1452, 1365, 1251, 1170 cmꢀ1. (HRMS):
calc. for C38H64N6O10Na m/z 787.4582, found m/z 787.4583.
Experimental
Methods and materials
All amino acids used were of L-configuration. Unless otherwise
stated, all reagents were used without further purification. All
solvents employed in the reactions were distilled or dried from
an appropriate drying agent prior to use. Reactions were
monitored wherever possible by thin layer chromatography
(TLC). Silica gel G (Merck) was used for TLC and column
chromatography was undertaken on silica gel (100–200 mesh)
in hexane, hexane–ethyl acetate or chloroform. The purity of
all the compounds was determined by TLC and HPLC
analysis. Melting points were recorded in a Fisher-Johns
melting point apparatus and are uncorrected. Optical rotations
were measured with a Rudolph Research Analytical Autopols
V Polarimeter; concentrations are given in g 100 mLꢀ1. IR
spectra were recorded on a Nicolet Protege 460 spectrometer
´ ´
as KBr pellets. 1H NMR spectra were recorded on a Brucker-
DPX-300 (1H, 300 MHz; 13C, 75 MHz) spectrometer using
tetramethylsilane (1H) as an internal standard. Coupling
5b. To a solution of 2b (100 mg, 0.21 mmol) in 15 mL of
CH2Cl2 was added Hay catalyst [prepared by stirring CuCl
(10 mg, 0.10 mmol) and TMEDA (0.033 mL, 0.22 mmol) in
CH2Cl2 (4 mL) for 5 min] and the mixture stirred under air at
room temperature. After stirring for 3 h, more Hay catalyst
(4 mL) was added and the mixture left stirring for 24 h. The
reaction mixture was diluted with 20 mL of distilled CH2Cl2
and washed sequentially with 2 N H2SO4, water, a saturated
aqueous NaHCO3 solution, an NH4CI + NH4OH (9 : 1)
solution and water. The organic layer was dried over
anhydrous Na2SO4, filtered and evaporated to yield 90 mg
1
constants are in Hz and the H NMR data are reported as s
(singlet), d (doublet), br (broad), br d (broad doublet),
t (triplet), q (quartet) and m (multiplet). HRMS were recorded
with an AB Sciex 1011273/A model instrument using the ESI
technique.
Chemical syntheses
4. To an ice-cooled and well stirred solution of Boc-Lys(Boc)-
OH (1) (1.447 g, 4.18 mmol) in 70 mL of dry CH2Cl2 were
added N-hydroxysuccinimide (481 mg, 4.18 mmol) and DCC
(862 mg, 4.18 mmol), and the mixture stirred for 10 min. To
this mixture was added a solution of tetra amine (408 mg,
0.929 mmol), obtained from the deprotection of 3 in 10 mL
CH2Cl2 and triethylamine (0.581 mL, 4.18 mmol). The reaction
mixture was stirred overnight, filtered, and the filtrate washed
with 0.2 N H2SO4, water and finally with a saturated aqueous
NaHCO3 solution. The organic layer was dried over anhydrous
Na2SO4, filtered and evaporated to yield 1.220 g of the
compound. Yield: 75%. [a]D + 28.66 (c 0.15, MeOH).
1H NMR (300 MHz, CDCl3) d 1.15–2.05 (br m, 114H), 2.20
(s, 1H), 2.95–3.70 (br m, 14H), 3.90–4.60 (br m, 9H), 4.72–5.25
(br m, 4H), 5.52–6.18 (br m, 4H), 6.68–7.20 (br m, 2H),
7.32–7.90 (br m, 5H); 13C NMR (75 MHz, CDCl3) d 14.1,
22.7, 24.9, 25.6, 28.5, 29.7, 31.7, 31.9, 32.7, 33.9, 38.8, 39.9,
40.3, 53.0, 54.3, 71.4, 79.0, 79.9, 156.3, 172.6, 172.9, 173.6,
174.2; IR (KBr): 3325, 3082, 2976, 2934, 2861, 2247, 1693,
1649, 1527, 1452, 1392, 1367, 1249, 1171 cmꢀ1. (HRMS): calc.
for C85H153N15O23Na m/z 1775.1161, found m/z 1775.1142.
1
of the 5b. Yield: 90%. [a]D ꢀ22.0 (c 0.1, MeOH). H NMR
(300 MHz, DMSO-d6) d 1.15–1.70 (br m, 12H), 2.99 (br m,
4H), 4.01 (br m, 6H), 5.03 (s, 8H), 7.37 (br m, 24H), 8.49 (br d,
2H). 13C NMR (300 MHz, DMSO-d6) d 23.18, 29.05, 29.36,
31.75, 55.00, 65.62, 65.95, 66.30, 76.51, 128.15, 128.26, 128.31,
128.84, 137.33, 137.61, 156.52, 156.65, 172.59. IR (KBr): 3300,
3067, 2932, 2861, 1691, 1652, 1539, 1449, 1383, 1331, 1259,
1150 cmꢀ1. (HRMS): calc. for C50H56N6O10Na m/z 923.3956,
found m/z 923.3963.
6. To a solution of 3 (400 mg, 0.476 mmol) in 15 mL of
CH2Cl2 was added Hay catalyst (4 mL) and the mixture stirred
under air at room temperature. After stirring for 3 h, more
Hay catalyst (4 mL) was added and reaction stirred for a
further 24 h. After completion of the reaction, as indicated by
TLC, the mixture was diluted with distilled CH2Cl2 (30 mL)
and washed successively with 0.5 N H2SO4, water, a saturated
aqueous NaHCO3 solution, an NH4CI + NH4OH (9 : 1)
solution and water. The organic layer was dried over
c
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anhydrous Na2SO4, filtered and evaporated to yield 320 mg
of 6. Yield: 80%. [a]D +43.5 (c 0.2, MeOH). 1H NMR
(300 MHz, CDCl3) d 1.20–2.10 (br m, 108H), 3.00–3.50
(br m, 12H), 4.00–4.40 (br m, 8H), 4.75–5.10 (br m, 4H),
5.55–6.00 (br m, 6H), 6.73 (br s, 2H), 7.03 (br m, 2H), 7.42
(br m, 2H). 13C NMR (75 MHz, CDCl3) d 22.5, 22.7, 25.4,
28.3. 29.0, 29.4, 31.0, 32.2, 38.5, 40.1, 53.0, 54.2, 71.5, 78.9,
79.4, 79.8, 79.9, 156.1, 171.6, 172.5, 173.2, 175.3. IR (KBr):
3328, 2975, 2935, 2868, 1692(br), 1654, 1525, 1452, 1369, 1251,
water. The organic layer was dried over anhydrous Na2SO4,
filtered, evaporated and purified by column chromatography
(ethyl acetate/hexane) to yield 70 mg of 9. Yield: 59%. [a]D
+4.26 (c 0.30, MeOH). 1H NMR (300 MHz, CDCl3) d
1.00–1.90 (br m, 168H), 2.80–3.55 (br m, 20H), 3.85–4.50
(br m, 12H), 4.60–5.45 (m, 8H), 5.50–6.20 (br m, 6H),
6.70–7.25 (br m, 4H), 7.30–8.10 (br m, 6H); IR (KBr): 3320,
3076, 2976, 2935, 2865, 1693, 1646, 1524, 1452, 1392, 1367,
1249, 1171 cmꢀ1. (HRMS): calc. for C126H224N22O34Na m/z
2612.6373, found m/z 2612.6288.
1171 cmꢀ1
. (HRMS): calc. for C82H144N14O22Na m/z
1700.0477, found m/z 1700.0497.
Gels
7. To a solution of 4 (150 mg, 0.086 mmol) in 15 mL of
CH2Cl2 was added Hay catalyst (4 mL) and the mixture stirred
under air at room temperature. After stirring for 3 h, more
Hay catalyst (4 mL) was added and reaction stirred for a
further 30 h. After completion of the reaction, as indicated by
TLC, the reaction mixture was diluted with distilled CH2Cl2
(30 mL) and washed with 0.5 N H2SO4, water, a saturated
aqueous NaHCO3 solution, an NH4CI + NH4OH (9 : 1)
solution and water. The organic layer was dried over
anhydrous Na2SO4 filtered and evaporated to yield 7. Yield:
67%. [a]D ꢀ30.0 (c 0.25, MeOH). 1H NMR (300 MHz,
CDCl3) d 1.15–2.05 (br m, 228H), 2.95–3.60 (br m, 28H),
4.05–4.60 (br m, 18H), 4.70–5.25 (br m, 8H), 5.52–6.15 (br m,
8H), 6.70–7.20 (br m, 4H), 7.32–7.90 (br m, 10H); IR (KBr):
3324, 3083, 2977, 2933, 2865, 1693, 1650, 1526, 1456, 1366,
1250, 1172 cmꢀ1. (HRMS): calc. for C170H304N30O46Na m/z
3525.2269, found m/z 3525.2422.
Preparation of gels. Compound 3 (10 mg) was dissolved in
0.2 mL of ethyl acetate and to this was added 0.3 mL of hexane
to obtain the gel with a Tg of 52 1C.
Compound 4 (20 mg) was dissolved in 0.04 mL of methanol
and to this was added 0.36 mL of ethyl acetate to obtain the
corresponding gel with a Tg of 53 1C.
Compound 5a (10 mg) was dissolved in 0.2 mL of ethyl
acetate and to this was added 0.3 mL of hexane to obtain the
corresponding gel with a Tg of 53 1C.
Compound 5b (10 mg) was dissolved in 0.1 mL of methanol
and to this was added 0.2 mL of chloroform to obtain a clear
solution. To this was added 0.2 mL of hexane to obtain the gel
with a Tg of 59 1C.
Measurement of gel transition temperature (Tg). The test
tube containing the gel was immersed in a thermostated water
bath and the temperature raised at a rate of 1 1C minꢀ1. Tg was
obtained as the temperature at which the gel disappeared.
8. To a solution of CuI (6 mg, 0.03 mmol) and NiCl2ꢁ6H2O
(8 mg, 0.03 mmol) in THF (4 mL) was added TMEDA
(0.02 mL, 0.13 mmol) and the solution stirred for 2 min at
room temperature. 2b (269 mg, 0.59 mmol) and 3 (100 mg,
0.12 mmol) were then added and the reaction mixture stirred
under air for 20 h at room temperature. The reaction mixture
was diluted with distilled CH2Cl2 (30 mL) and washed
sequentially with 0.5 N H2SO4, water, a saturated aqueous
NaHCO3 solution, an NH4CI + NH4OH (9 : 1) aq. solution
and water. The organic layer was dried over anhydrous
Na2SO4, filtered, evaporated and purified by silica gel column
chromatography to yield 110 mg of 8. Yield: 71%. [a]D ꢀ28
(c 0.25, MeOH). 1H NMR (300 MHz, CDCl3) d 1.20–1.70
(m, 52H), 1.70–1.90 (m, 8H), 3.00–3.28 (m, 8H), 3.90–4.50
(br m, 8H), 4.78 (br s, 2H), 5.07 (br m, 6H), 5.56 (br d, 2H),
6.40 (br m, 1H), 7.31 (m, 10H), 7.62 (br m, 3H); IR (KBr):
3319, 3066, 2935, 2864, 1769, 1691, 1646, 1530, 1455, 1367,
1257, 1168 cmꢀ1. (HRMS): calc. for C66H100N10O16Na m/z
1311.7216, found m/z 1311.7160.
Photopolymerization. The polymerization reactions were
performed on a freshly prepared gel in a glass vial by irradia-
tion under a high intensity mercury lamp (OmniCure Series
1000) for 3 min.
Microscopy methods
Scanning electron microscopy (SEM). A 10 mL aliquot of the
sample solution was dried at room temperature on a piece of
cleaved mica attached to a stub via carbon tape and coated
with B10 nm of platinum. SEM images were made using a
CAMScan MX2500 SEM equipped with a tungsten filament
gun operating at WD 13–20 mm and 10–15 kV. Some gels
were analyzed using a Zeiss EVO 50 SEM.
Atomic force microscopy (AFM). Sample images were
acquired using a Nanoscope IV Multimode AFM (Veeco)
operating in tapping mode in air. The cantilevers used
were NSC15 probes (MikroMasch). The spring constants were
between 30–60 N mꢀ1 while the resonant frequencies were
between 310–365 kHz. The images were taken in air at room
temperature with a scan speed of 1.5 lines sꢀ1 for 512 lines. Data
acquisition was undertaken using Nanoscope v5.30r3sr3 software,
while data analysis was performed using Nanoscope v6 software.
A 10 mL aliquot of the sample solution was transferred onto
freshly cleaved mica and allowed to dry in a laminar flowhood.
9. To a solution of CuI (6 mg, 0.03 mmol) and NiCl2ꢁ6H2O
(8 mg, 0.03 mmol) in THF (4 mL) was added TMEDA
(0.02 mL, 0.13 mmol) and the solution stirred for 2 min at
room temperature. 3 (192 mg, 0.228 mmol) and 4 (80 mg,
0.0456 mmol) were then added and subsequently the reaction
mixture was stirred under air for a further 20 h at room
temperature. After completion of the reaction, as indicated by
TLC, the mixture was diluted with distilled CH2Cl2 (30 mL)
and washed with 0.5 N H2SO4, water, a saturated aqueous
NaHCO3 solution, an NH4CI + NH4OH (9 : 1) solution and
Transmission electron microscopy (TEM). A 2 mL aliquot of
the sample solution was placed on a 400 mesh copper grid.
After 1 min, excess fluid was removed and the grid stained with
c
308 New J. Chem., 2011, 35, 303–309
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2% phosphotungstate in water. Excess staining solution was
removed from the grid after 2 min. Samples were viewed using
a JEOL 1200EX electron microscope.
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Confocal Raman microspectroscopy. Raman spectra were
collected with a WiTEC alpha300R microscope using a
40ꢂ (numerical aperture 0.6) and 532 nm (Elaser = 2.33 eV)
laser operating up to a possible maximum of approximately
60 mW. Raman data was collected by WiTEC Control software
and analyzed by WiTEC Project software with the surface
perpendicular to the excitation source. Stand-alone spectra
were collected using integration times of between 3 and 6 s
with 3 accumulations per spectrum. The Raman data presented
have cosmic rays removed but no other data processing
(e.g. background subtraction, averaging) has been performed.
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Fluorescence microscopy. Samples for fluorescence imaging
were prepared on clean coverslips, otherwise following the
sample preparation method for AFM. Fluorescence images
were collected by a Laborlux D fluorescence microscope
(Leitz) using filter no 1 (band pass excitation (450–490 nm)
and 515 nm long pass cutoff emission filters) with
4ꢂ objective.
a
Acknowledgements
We thank the Endeavour foundation for an Endeavour
Executive Award to V. H., and gratefully acknowledge
support from IIT Delhi, Flinders University, DST (New Delhi)
and the Australian Research Council.
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