Self-assembly of a new C60 compound with a L-glutamid-derived lipid unit: Formation of organogels and hierarchically structured spherical particles

Article abstract of DOI:10.1039/c0sm01109f

A new C60-based low-molecular weight gelator (LMWG 1) with a L-glutamid-derived lipid unit was synthesized and characterized. LMWG 1 can gel toluene, p-xylene and benzene. Moreover, its gelation ability can be further enhanced by addition of an exTTF deri

Full text of DOI:10.1039/c0sm01109f

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PAPER
Self-assembly of a new C60 compound with a L-glutamid-derived lipid unit:
formation of organogels and hierarchically structured spherical particles†
a
a
Xingyuan Yang,ab Guanxin Zhang, Deqing Zhang, Junfeng Xiang,a Ge Yangab and Daoben Zhua
*
Received 6th October 2010, Accepted 1st February 2011
DOI: 10.1039/c0sm01109f
A new C60-based low-molecular weight gelator (LMWG 1) with a L-glutamid-derived lipid unit was
synthesized and characterized. LMWG 1 can gel toluene, p-xylene and benzene. Moreover, its gelation
ability can be further enhanced by addition of an exTTF derivative (compound 2), for which the
intermolecular interaction between the C60 unit in 1 and the exTTF unit in 2 is responsible based on the
absorption and NOE difference 1H-NMR spectral studies. Additionally, gelation-induced CD signals
are observed for the gel with LMWG 1 and that of LMWG 1 doped with compound 2. Further studies
indicate that the self-assembly structures of LMWG 1 in the gel phases can be modulated by the
addition of compound 2. In addition, the self-assembly of LMWG 1 in DMSO leads to spherical
particles of several microns in size, and the surfaces of these spherical particles are hierarchically
structured with nano-fibers. Moreover, the thin-film of spherical particles exhibits water-repellent
hydrophobicity.
prepare nanostructures through gelation process.37 In fact,
Introduction
gelation can lead to fibrous self-assembly structures of LMWGs
C60 and its derivatives are some of the most fascinating
and tube-like nanostructures in some cases.38 Additionally, the
p-conjugated carbon nanomaterials.1–3 They exhibit attractive
structures and properties of gels from LMWGs can be further
conducting,4–7 optoelectronic8–12 and biological13–15 properties
modulated by incorporation of carbon nanotube, gold/silver
which enable them to be useful in a number of areas. In recent
nanoparticles and chemical species;39,40 In this way, new hybrid
years, nanostructures of C60 and its derivatives with controllable
materials can be prepared. Shinkai et al. described the hybrid
sizes and morphologies have received attention16–21 as these
self-assembly structures derived from porphyrin gelators and
nanostructures are interesting both from a fundamental and
found that the organogel structures could be reinforced through
practical point of view. Nanowires and nanotubes of C60 were
intermolecular porphyrin-C60 interactions.41 A similar result was
fabricated using template and electrochemical methods,22,23 and
reported recently by Xue et al.42 Nakamura et al. reported the
submicrometer C60 whiskers and tubes were prepared24 by using
a liquid–liquid interfacial precipitation method. Self-assemblies
of C60 derivatives into macroscopic globular objects25 and
flower-shaped structures were also reported.26
It is known that self-assembly of certain organic molecules,
referred to as low-molecular weight gelators (LMWGs),27–30
through weak intermolecular interactions such as H-bonding,
p–p stacking and van der Waals interactions, can lead to gela-
tion of organic solvents and formation of organogels.31–36 In the
gel phase, molecules of LMWGs are self-assembled into entan-
gled three-dimensional networks, which entrap and immobilize
solvent molecules by capillary forces. Therefore, it is possible to
aBeijing National Laboratory for Molecular Sciences, Organic Solids
Laboratory, Institute of Chemistry, Chinese Academy of Sciences,
Beijing, 100190, China. E-mail: dqzhang@iccas.ac.cn; Tel: +86 6263 9355
Scheme 1 The chemical structures of LMWG 1 and compound 2 and
bGraduate School of Chinese Academy of Sciences, Beijing, 100190, China
the corresponding synthetic approaches: a) N1,N5-didodecyl-L-gluta-
† Electronic supplementary information (ESI) available: Synthesis and
mide, EDC$HCl, DMAP, dry 1,2-dichlorobenzene, 80 ꢀC, 78%; b)
characterization; TEM images; XRD patterns; transient absorption
spectra; photographs of water contact angles at different surfaces and
details on the techniques used. See DOI: 10.1039/c0sm01109f
methyl 4-hydroxybenzoate, DEAD, triphenylphosphine, dry THF, room
temperature, 74%.
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Table 1 The gelation experimental results with LMWG 1 at 25 ꢀC in
fullerene nanowires of a C60-derived LMWG fabricated with
Langmuir–Blodgett method;43 but the gel structures of this C60-
derived LMWG were not detailed.
different solvents.a
Solvent
Gelation
In this report we want to describe a new C60 derived LMWG 1
(see Scheme 1) with a L-glutamid-derived lipid unit. It should be
noted that L-glutamid-derived lipid units have been incorporated
into organic molecules to construct LMWGs by taking the
advantage of intermolecular H-bonding and van der Waals
interactions.44,45 The results manifest that LMWG 1 can gel
several organic solvents; moreover, the gel structures can be
a
b
c
d
e
f
g
h
i
j
k
l
m
n
o
p
q
r
Toluene
p-Xylene
Benzene
DMF
THF
DMSO
1,2-Dichloroethane
Dioxane
Gt (2.3 mg mLꢁ1
Gt (2.3 mg mLꢁ1
)
)
)
Go (5.6 mg mLꢁ1
P
P
P
P
P
I
I
I
I
I
modulated and the sol–gel phase transition temperature (Tgel
can be enhanced by the addition of compound 2 (see Scheme 1),
an exTTF [9,10-bis(1,3-dithiol-2-ylidene)-9,10-dihydroan-
)
Dichloromethane
Ethyl acetate
CCl4
Ethanol
thracene] derivative which as a strong electron donor can
specifically bind C60 and its derivatives.46 In addition, the self-
assembly of LMWG 1 in other solvents such as DMSO leads to
hierarchically structured spherical particles.
Isopropanol
Acetonitrile
Methylcyclohexane
Cyclohexane
n-Hexane
I
I
I
I
n-Heptane
I
Results and discussion
a
Gt: transparent gel; Go: opaque gel; I: insoluble; P: precipitate. The
CGCs of the corresponding gels were measured at room temperature
(25 ꢀC) and listed in the parentheses. All gels were stable at room
temperature in closed tubes for at least 3 months unless indicated
otherwise. The Tgel for the corresponding organogels under CGC
condition were 25 ꢀC.
Synthesis
As outlined in Scheme 1, the synthesis of LMWG 1 started from
compound 3 which was condensed with N1,N5-didodecyl-L-glu-
tamide under a nitrogen atmosphere in the presence of
EDC$HCl and DMAP at 80 ꢀC for one day to give LMWG 1 in
78% yield. For the synthesis of compound 2, compound 4 was
reacted with methyl 4-hydroxybenzoate under a nitrogen atmo-
sphere in the presence of DEAD and triphenylphosphine at room
temperature for one night to give compound 2 in 74% yield. The
chemical structures of both LMWG 1 and compound 2 were
established with the corresponding H- and 13C-NMR data as
1
Fig. 1 Illustration of gel formation with LMWG 1 (2.5 mg mLꢁ1) in
toluene (left) and that with LMWG 1 (2.1 mg mLꢁ1) and compound 2 (0.9
mg mLꢁ1) in toluene (right).
well as mass spectra data (see Experimental section).
Gelation with LMWG 1 and tuning with compound 2
The gelation ability of LMWG 1 was examined in different
solvents, and the results are listed in Table 1. Among the solvents
tested, LMWG 1 cannot gel aliphatic hydrocarbons such as
cyclohexane, methylcyclohexane, n-hexane and n-heptane
because of its poor solubility in these solvents. However, LMWG
1 do gel aromatic solvents such as benzene, toluene, p-xylene, and
the corresponding critical gelation concentrations (CGC) at
25 ꢀC were also measured (see Table 1). As demonstrated in
Fig. 1, a transparent dark purple gel was formed by cooling a hot
toluene solution of LMWG 1 (2.5 mg mLꢁ1). The intermolecular
p–p (due to C60) H-bonding and van der Waals (due to L-glu-
tamid-derived lipid unit) interactions may be the driving-force
for the gelation process. Fig. 2 shows the partial 1H-NMR
spectra for the gel formed with LMWG 1 (3.6 mg mLꢁ1) in d8-
toluene at several temperatures and the corresponding solution
at 373 K. The signals around 7.40 ppm and 5.42 ppm due to the
amide units in LMWG 1 were gradually up-field shifted by
increasing the temperature. This result implies that the H-
bonding owing to the amide units in LMWG 1 exists in the gel
phase, which becomes weak after heating as anticipated.
interconnected. The high-magnification image (Fig. 3b) reveals
that the nano-fibers are ca. 50 nm wide and entangled; the nano-
fibers are composed of particles that stick together. TEM images
of the xerogel of LMWG 1 also show the interconnected nano-
fibers (see Fig. S1a and S1b†). Only one peak at 2.42ꢀ
ꢀ
(see Fig. S2†), corresponding to a d-spacing of 36.47 A, was
detected in the XRD pattern for the xerogel of LMWG 1 from
toluene. This d-spacing is close to the length of the dimer of
LMWG 1, which may be formed through the hydrophobic and
H-bonding interactions due to the L-glutamid-derived lipid unit
as depicted in Fig. S2.†
Notably, the gelation ability of LMWG 1 can be enhanced
after addition of compound 2. For example, gelation did not
occur by cooling the toluene solution of LMWG 1 with
a concentration of 2.1 mg mLꢁ1 which was lower than the cor-
responding CGC (see Table 1). However, as shown in Fig. 1
a dark-purple gel was formed after addition of compound 2
(0.9 mg mLꢁ1, 1.0 eq. vs. LMWG 1). Moreover, the addition of
compound 2 to the gel of LMWG 1 can also increase the gel–sol
transition temperature. As an example, Fig. 4 depicts the plot of
gel–sol transition temperature vs. the amount of compound 2
added to the solution of LMWG 1. The gel formed with a toluene
The xerogels of LMWG 1 were characterized with SEM, TEM
and XRD. Fig. 3 shows the SEM images of the xerogel formed
with LMWG 1 in toluene. The xerogel contains bundles of long
nano-fibers which are a few micrometres long and randomly
solution of LMWG 1 (3.4 mg mLꢁ1) melted at 46.2 C, and the
ꢀ
gel–sol transition temperature increased gradually by
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introducing compound 2. It increased to 59.5 ꢀC when 1.0 eq. of
compound 2 was added, and further addition of compound 2 led
to only slight enhancement of gel–sol transition temperature. The
1
absorption and H-NMR spectral investigations manifest that
such gelation ability enhancement after addition of compound 2
is likely to be due to the intermolecular interaction between the
exTTF unit in compound 2 and the fullerene unit in LMWG 1.
This is because exTTF and C60 are geometrically and electroni-
cally complementary and thus it may involve p–p stacking,
solvophobic and concave-convex interaction.47–49
The absorption spectra of compound 2 (1.0 ꢂ 10ꢁ5 M) and
LMWG 1 in different concentrations were recorded. In order to
show the spectral changes more clearly, the absorptions due to
LMWG 1 in each case were subtracted for each spectrum of the
mixture and the resulting spectra were displayed in Fig. 5.
Obviously, the absorptions around 433 nm due to the exTTF unit
gradually became weak and new absorptions in the range of
460–580 nm emerged by increasing the concentration of LMWG
1 in the solution. According to previous studies,46,48 such
absorption spectral variation can be ascribed to the charge-
transfer interaction between the exTTF unit in 2 and C60 unit in
LMWG 1.50
1
Fig. 2 Partial H-NMR spectra for the gel formed with LMWG 1 (3.6
mg mLꢁ1 in d8-toluene) at different temperatures: A, 313 K; B, 333 K; C,
353 K and that for the corresponding solution at 373 K.
Further evidence to prove the interaction between the exTTF
unit in 2 and the C60 unit in 1 was obtained by examining the
NOE 1H-NMR of the mixture solution of LMWG 1 and
compound 2. Fig. 6 shows the corresponding NOE spectrum
after irradiation specifically for the signal at 7.70 ppm due to the
four protons (a) of the anthracene unit in 2, and for comparison
1
the normal H-NMR spectrum of the mixture in the range of
6.90–8.10 ppm is also shown in Fig. 6. Not only protons b at
7.30 ppm due to three protons of the anthracene unit in 2 but also
protons c at 7.94 ppm and 7.55 ppm due to the protons of the
benzene unit in LMWG 1 exhibit NOE effect albeit small. This
result clearly indicates that the benzene unit in LMWG 1 and the
anthracene unit of exTTF in compound 2 are spatially neigh-
boring. This is indeed in agreement with the fact that there is
interaction among the molecules of LMWG 1 and compound 2
in solution.
Fig. 3 SEM image of the xerogel of LMWG 1 from toluene (a), and its
high-magnification image (b), and the resulting SEM image of the xerogel
of LMWG 1 and compound 2 (1.0 eq. vs. LMWG 1) from toluene (c), and
the corresponding high-magnification image (d).
Fig. 4 Variation of the gel–sol transition temperature (Tgel) of the
organogels formed with LMWG 1 (3.4 mg mLꢁ1) from toluene in the
presence of increasing concentration of compound 2 (from 0 to 4.0 eq.);
the measurements were conducted in a glass vial containing 0.5 mL
toluene, with a steel ball weighing 33 mg.
Fig. 5 The absorption spectra of a toluene solution of compound 2
(1.0 ꢂ 10ꢁ5 M) after the addition of different amounts of LMWG 1
(0–0.7 mM); the arrows indicate the progression of the titration; the
absorptions due to LMWG 1 were subtracted in each case.
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enhanced for the gel of LMWG 1 doped with compound 2 (see
Fig. 7). This is probably due to the contribution of exTTF unit in
compound 2 within the chiral assembly structure.
The xerogel of LMWG 1 doped with compound 2 was also
characterized with SEM, TEM and XRD. As shown in Fig. 3c,
the xerogel of 1 ‘‘doped’’ with 2 shows interconnected fibrillar
network. Close examination of the SEM image of the xerogel
shows that the average width of fibers is ca. 100 nm, two times of
that for the xerogel of LMWG 1 without compound 2; besides
nanofibers molecules of LMWG 1 and compound 2 are also
assembled into nanoflakes with length of ca. 900 nm and width of
ca. 250 nm. TEM images (see Fig. S1c and S1d†) also shows
interwoven nanofibers. It is obvious that the self-assembly
structure of LMWG 1 is modulated by the addition of compound
2. This is understandable, considering the intermolecular inter-
action between C60 unit in 1 and the exTTF unit in 2 as discussed
above. Only a peak at 2.24ꢀ (see Fig. S2†) was detected in the
XRD pattern for the xerogel of LMWG 1 and compound 2 from
Fig. 6 Partial 600 MHz 1H-NMR spectra of compound 2 (2.0 mM) and
LMWG 1 (2.0 eq. vs. compound 2) in DCCl3 solution: the normal
spectrum (up) and the NOE difference spectrum (bottom) from irradia-
tion of the H-a at 7.70 ppm.
ꢀ
toluene. This corresponds to a d-spacing of 39.40 A, being close
to the length of the dimer of 1. This may be understandable as the
exTTF unit in 2 may occupy the space formed by two neigh-
boring fullerene units in LMWG 1.
Additionally, the CD spectra of the gel of LMWG 1 and that
of LMWG 1 doped with compound 2 were measured and
depicted in Fig. 7. Both the solution of LMWG 1 and that of
LMWG 1 and compound 2 are CD silent as shown in Fig. 7. But,
the gel formed with LMWG 1 from toluene exhibits a positive
CD signal in the range of 320–390 nm at which the fullerene unit
in 1 absorbs. The appearance of CD signal indicates the forma-
tion of chiral self-assembly structures induced by the chiral
L-glutamid-derived lipid unit in LMWG 1. The gel of LMWG 1
doped with compound 2 exhibits new CD absorption in the range
of 390–520 nm, besides that in the range of 320–390 nm due to
the C60 unit in LMWG 1; this new CD absorption may result
from the interaction between the exTTF unit in 2 and the C60 unit
in LMWG 1. These CD spectral results may indicate that
molecules of LMWG 1 and compound 2 co-assemble into chiral
superstructures in the gel phase because of the intermolecular
interactions of C60 and exTTF units in LMWG 1 and compound
2, respectively. In addition, the CD signal intensity was slightly
Self-assembly of LMWG 1 into spherical particles
LMWG 1 cannot gel DMSO (see Table 1). Byꢀcooling the hot
ꢀ
DMSO solution of LMWG 1 slowly from 110 C to 25 C and
then keeping it undisturbed for more than 6.0 h, precipitates were
formed. The precipitates were dispersed and examined with
SEM, TEM and XRD. As shown in Fig. 8a–c, spherical particles
of several microns in size were formed. Moreover, the surfaces of
these spherical particles were hierarchically structured with
nano-fibers of ca. 60 nm in diameter (see the inset of Fig. 8b). The
corresponding TEM image gave similar result. These results
manifest that molecules of LMWG 1 can be also self-assembled
into hierarchically structured spherical particles. A peak at 2.09ꢀ
ꢀ
(see Fig. S2†), corresponding to a d-spacing of 42.22 A, was
detected in the XRD pattern for the thin-film composed of
spherical particles. This d-spacing is close to the length of the
dimer of LMWG 1 as depicted in Fig. S2.† Thus, it may be
concluded that the ‘‘building block’’ for the spherical particles
and that for nanofibers observed in the gel phase (from LMWG
1) are the same. Remarkably, the thin-film, which was formed by
depositing the suspension of spherical particles on silicon,
exhibited water-repellent hydrophobicity with a water contact
angle of 148.2ꢀ (see the inset of Fig. 8c). Note that this contact
angle is rather close to those of superhydrophobic surfaces
reported previously.26
It is interesting to note that the size and surface structure of the
spherical particles from LMWG 1 is also affected by the way the
corresponding DMSO solution is cooled. For instance, when
the DMSO solution at 110 ꢀC was cooled rapidly by putting the
solution directly in a 20 ꢀC water bath, smaller spherical particles
of 0.8–1.2 micrometre in diameter were generated as shown in
Fig. 8d and 8e where the SEM images are displayed. In addition,
the SEM and TEM examinations (see Fig. 8e and 8f) also
manifest that the surfaces of these spherical particles under this
condition became smoother. As shown in Fig. S4,† the film
formed by depositing the suspension of smaller spherical
Fig. 7 CD spectra of the solution and gel phases of LMWG 1 (1.2 mM)
in toluene and those of LMWG 1 containing compound 2 in toluene:
solution of LMWG 1 (black), gel of LMWG 1 (red), solution of LMWG
1 and 1.0 eq.of compound 2 (blue), gel of LMWG 1 and 1.0 eq. of
compound 2 (green). All the samples were measured in a 0.5 mm quartz
cell.
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Fig. 8 SEM image (a), high-magnification SEM image of the surfaces (b) and TEM image (c) of the spherical particles formed with LMWG 1
precipitated after cooling its hot solution in DMSO in an oil–water bath; the insets show the magnified SEM images and a photograph of a water droplet
on the surface (contact angle of 148.2ꢀ). SEM image (d), high-magnification SEM image of the surfaces (e) and TEM image (f) of the spherical particles
formed with LMWG 1 precipitated after cooling its hot solution in DMSO in a 20 ꢀC water bath.
particles on silicon also exhibits water-repellent hydrophobicity
with a water contact angle of 139.1ꢀ, while the xerogel film of
LMWG 1 from toluene shows a water contact angle of 142.0ꢀ.
Thus, it seems possible to assemble LMWG 1 into different
nano- and micro-structures by varying physical conditions such
as the rate of the cooling process.
(210 mg, 1.10 mmol) and DMAP (135 mg, 1.10 mmol) were
added to the mixture, and the resulting solution was stirred at
80 ꢀC for one day. After separation by column chromatography
on silica gel using CHCl3/THF (20 : 1, v/v) as eluent, LMWG 1
was obtained as a dark brown solid (533 mg, 78%). M.p. 262–264
1
ꢀ
ꢀC. HNMR (400 MHz, CDCl3:CS2 ¼ 5 : 1, 25 C, TMS): d ¼
7.90 (d, J ¼ 7.3 Hz, 2 H), 7.52 (t, J ¼ 7.4 Hz, 2 H), 7.45 (t, J ¼
6.0 Hz, 1 H), 7.20 (d, J ¼ 8.0 Hz, 1 H), 7.02 (s, 1 H), 6.13 (s, 1 H),
4.27–4.32 (m, 1 H), 3.19–3.24 (m, 4 H), 2.89 (t, J ¼ 7.8 Hz, 2 H),
2.39–2.51 (m, 3 H), 2.26–2.32 (m, 1 H), 2.15–2.21 (m, 2 H), 2.01–
2.06 (m, 1 H), 1.90–1.98 (m, 1 H), 1.49 (s, 4 H), 1.25 (s, 36 H),
0.88 (t, J /it> ¼ 6.5 Hz, 6 H). 13CNMR (75 MHz, DCCl3:CS2 ¼
Conclusion
In summary, a C60-based organogelator with a L-glutamid-
derived lipid unit (LMWG 1) was synthesized and investigated.
LMWG 1 can gel aromatic solvents such as toluene, p-xylene and
benzene. It is interesting to note that the gelation ability of
LMWG 1 can be further enhanced by addition of compound 2
with an ex-TTF unit. This is attributed to the intermolecular
interaction between the C60 unit in LMWG 1 and exTTF unit in
compound 2 on the basis of absorption and NOE difference 1H-
NMR spectral studies. Gelation-induced CD signals were
observed for the gel with LMWG 1 and that of LMWG 1 doped
with compound 2. In addition, molecules of LMWG 1 can self-
assemble in DMSO solution into spherical particles of several
microns in size, and the surfaces of these spherical particles are
hierarchically structured with nano-fibers. Moreover, the thin-
film of spherical particles on silicon exhibits water-repellent
hydrophobicity. These self-assembly structures of C60 may be
potentially useful in fabricating solar cells, and thus they deserve
further investigation.
ꢀ
5 : 1, 42 C, TMS): d ¼ 173.23, 172.77, 171.30, 149.25, 148.30,
146.39, 145.75, 145.69, 145.61, 145.35, 145.24, 145.20, 145.05,
145.00, 144.62, 144.31, 143.61, 143.56, 143.51, 142.69, 141.56,
138.20, 132.63, 129.03, 128.81, 80.39, 52.99, 52.46, 40.52, 40.32,
36.74, 34.41, 33.78, 32.72, 30.64, 30.47, 30.32, 30.27, 30.18, 27.81,
27.73, 23.59, 14.92. HRMS: m/z calcd for C100H69N3O3Na+:
1382.5231, found: 1382.5229.
Synthesis of compound 2
At room temperature methyl 4-hydroxybenzoate (152 mg, 1.0
mmol), compound 4 (205 mg, 0.5 mmol) and triphenylphosphine
(288 mg, 1.1 mmol) were dissolved in 10 mL of dry THF under
a nitrogen atmosphere. Diethyl azodicarboxylate (DEAD, 191
mg, 1.1 mmol, 173 mL) were added dropwise to the mixture via
a syringe, and the resulting solution was stirred at room
temperature over night. After separation by column chroma-
tography on silica gel using CH2Cl2 as eluent, compound 2 was
Experimental section
obtained as
a yellow solid (202 mg, 74%). M.p. >230
Synthesis of LMWG 1
ꢀC(decomp.). 1HNMR (400 MHz, DMSO, 25 ꢀC, TMS): d ¼7.92
(d, J ¼ 8.7 Hz, 2 H), 7.64–7.69 (m, 4 H), 7.34–7.37 (m, 3 H), 7.16
(d, J ¼ 8.8 Hz, 2 H), 6.74–6.76 (m, 4 H), 5.28 (s, 2 H), 3.81 (s,
Compound 3 (450 mg, 0.50 mmol) and N1,N5–didodecyl-L-glu-
tamide (265 mg, 0.55 mmol) were suspended in 20 mL of dry 1,2-
ꢀ
dichlorobenzene under
a
nitrogen atmosphere. EDC$HCl
3 H). 13C NMR (100 MHz, DMSO, 25 C): d ¼166.44, 162.71,
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137.51, 137.33, 135.45, 135.22, 135.19, 135.06, 134.93, 131.85,
126.86, 126.07, 125.44, 125.28, 124.37, 122.70, 120.95, 118.71,
118.65, 115.52, 69.77, 52.42. HRMS: m/z calcd for C29H20O3S4:
544.0295, found: 544.0300.
Hongbing Fu and Yishi Wu for the transient absorption
measurements.
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Acknowledgements
The present research was financially supported by NSFC, the
State Basic Program and Chinese Academy of Sciences. This
work was partially supported by the NSFC-DFG joint project
(TRR61). The authors thank professors Lei Jiang and Yong
Zhao for the measurement of contact angles, and professors
This journal is ª The Royal Society of Chemistry 2011
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Fig. S3† displays the differential absorption spectra after excitation
at 440 nm with femtosecond laser, and the decay of the absorption
at 683 nm with time. The absorption centered at 620 nm appears
instantaneously after photoexcitation, but it decays rather quickly.
At time delay of 16.4 ps, a new absorption around 683 nm emerges,
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