A water soluble methanofullerene derivative: Synthesis, micellar aggregation in aqueous solutions, and incorporation in sol-gel glasses for optical limiting applications

Article abstract of DOI:10.1039/a908259j

Amphiphilic methanofullerene derivative 1 is water soluble and micellar aggregation has been evidenced by small-angle X-ray scattering measurements and UV/Vis spectroscopy. Thanks to the high solubility of 1 in polar solvents widely used in sol-gel proces

Full text of DOI:10.1039/a908259j

J O U R N A L O F
A water soluble methanofullerene derivative: synthesis, micellar
aggregation in aqueous solutions, and incorporation in sol±gel
glasses for optical limiting applications
Delphine Felder,a Daniel Guillon,a Roland LeÂvy,b Andre Mathis,c Jean-FrancËois Nicoud,a
Jean-FrancËois Nierengarten,*a Jean-Luc Rehspringerd and Jochen Schellb
C H E M I S T R Y
aGroupe des MateÂriaux Organiques, Institut de Physique et Chimie des MateÂriaux de
Strasbourg, Universite Louis Pasteur et CNRS, 23 rue du Loess, 67037 Strasbourg, France.
E-mail: niereng@ipcms.u-strasbg.fr
bGroupe d'Optique Non-lineÂaire et d'OptoeÂlectronique, Institut de Physique et Chimie des
MateÂriaux de Strasbourg, Universite Louis Pasteur et CNRS, 23 rue du Loess, 67037
Strasbourg, France
cInstitut Charles Sadron, Universite Louis Pasteur et CNRS, 6 rue Boussingault, 67000
Strasbourg, France
dGroupe des MateÂriaux Inorganiques, Institut de Physique et Chimie des MateÂriaux de
Strasbourg, Universite Louis Pasteur et CNRS, 23 rue du Loess, 67037 Strasbourg, France
Received 15th January 2000, Accepted 26th January 2000
Amphiphilic methanofullerene derivative 1 is water soluble and micellar aggregation has been evidenced by
small-angle X-ray scattering measurements and UV/Vis spectroscopy. Thanks to the high solubility of 1 in
polar solvents widely used in sol±gel processing, successful inclusion of 1 in the sol±gel could be easily achieved.
The optical limiting properties of the doped sol±gel samples have been evaluated and a fast S1±S0 relaxation
has been observed for these samples. This observation appears to be consistent with the presence of micellar
aggregates of 1 in the sol±gel. Indeed, the interactions between the fullerene spheres of neighbouring molecules
in the clusters may be at the origin of this fast S1±S0 relaxation as already shown in solid C60-®lms.
handling as compared to liquid solutions. It has been shown
that C60 keeps its limiting properties even after inclusion in
solid matrices such as sol±gel glasses,5 polymethyl methacrylate
(PMMA) matrices6 and glass±polymer composite samples.7 As
far as sol±gel glasses are considered, special procedures have to
be employed since good solvents for fullerenes are incompatible
with the sol±gel process.5a,8a The recent developments in the
functionalization of fullerenes9 allow the preparation of highly
soluble C60 derivatives, and the electronic properties such as
facile multiple reducibility,10 optical non-linearity8 or ef®cient
photosensitization11 that are characteristic of the parent
fullerene are maintained for monofunctionalized C60 deriva-
tives. As a part of this research, we now report the synthesis,
incorporation in sol±gel matrices, and optical limiting proper-
ties of methanofullerene 1.
Introduction
The development of optical power limiters with the aim of
generating devices for the protection of human eyes or sensors
against intense laser pulses of various wavelengths is a ®eld of
growing interest.1 In contrast to ordinary neutral density ®lters,
the transmission of an ideal optical limiting device drops
rapidly as the incoming light intensity exceeds a certain
threshold value.2 Therefore a sensor placed behind such a
device is protected against damage caused by strong laser
irradiation, but its sensibility at low light intensities is not
reduced as would be the case using an ordinary neutral density
®lter.2 To achieve this intensity dependent transmission of the
limiting device, different approaches are possible. Since much
of the potential threat is caused by nanosecond laser sources,
the transmission changes have to be very rapid. This excludes
commercial photochromous glasses as used in ordinary
eyeglasses, since their response time is too long.2 Non-linear
scattering and diffraction, two-photon absorption and reverse
saturable absorption (RSA) are rapid physical phenomena
more adequate for such a purpose.2 RSA is often considered as
one of the most interesting processes since it generally offers a
low threshold of the energy density for the onset of the limiting
action, very fast response times and the possibility of high
transmission changes.2 In reverse saturable absorbers, the
transmission decrease is the result of an increasing absorption
with increasing light intensity. The intensity dependent
absorption of these materials originates from larger absorption
cross sections of excited states compared to that of the ground
state.2 Such behaviour has been observed for buckminsterful-
lerene C60 over nearly the entire visible spectrum and several
studies on C60 solutions have shown its great potential as an
optical limiter.3,4 However, for technical purposes, the use of a
solid device is largely preferred due to its greater ease of
Results and discussion
Synthesis and solution studies
The synthesis of methanofullerene 1 is depicted in Scheme 1.
tert-Butyl ester 2 was obtained from 8-bromooctanoic acid (3)
following the esteri®cation conditions reported by S. W. Wright
and co-workers.12 This reaction is based on the treatment of
carboxylic acid 3 with isobutylene generated in situ from the
dehydration of tert-butyl alcohol in the presence of an acid
catalyst. Thus, the treatment of 3 with tert-butyl alcohol in the
presence of a dispersion of concentrated sulfuric acid on
powdered anhydrous magnesium sulfate in CH2Cl2 at room
temperature afforded tert-butyl ester 2 in 65% yield.
Treatment of 2 with 3,5-dihydroxybenzyl alcohol (4) and
K2CO3 in DMF at 70 ³C gave compound 5 in 52% yield.
Malonate 6 was prepared in 66% yield from malonyl dichloride
and 5 in CH2Cl2 in the presence of pyridine and 4-
DOI: 10.1039/a908259j
J. Mater. Chem., 2000, 10, 887±892
This journal is # The Royal Society of Chemistry 2000
887

butyl esters15 by treatment of 7 with tri¯uoroacetic acid in
CH2Cl2 gave tetracarboxylic acid 1 in 84% yield. All spectro-
scopic data for 1 are in full agreement with the proposed
formulation. Compound 1 is quite soluble in a wide range of
solvents: chlorinated organic solvents (CH2Cl2, CHCl3), THF,
alcohols and water at slightly alkaline pH. Whereas the
solubility has not been quanti®ed, compound 1 is soluble to at
least 5 mg ml21 in all the above mentioned solvents. The UV/
Vis spectrum of 1 in aqueous 0.1 M NaOH is broad when
compared to the absorption spectrum of parent compound 7
(Fig. 1).
According to previously reported studies,16 such a behaviour
may arise from the amphiphilic character of 1 which may cause
micellar aggregation in solution. The hydrophobic cores may,
dimethylaminopyridine (DMAP). The functionalization of C60
is based on the Bingel reaction.13 Nucleophilic addition of a
stabilized a-halocarbanion to the C60 core, followed by
intramolecular nucleophilic substitution, leads to clean cyclo-
propanation of C60. The a-halomalonate derivative is prepared
in situ from the reaction of the malonate with iodine.14
Treatment of C60 with 6, iodine and 1,8-diazabicyclo[5.4.0]un-
dec-7-ene (DBU) in toluene at room temperature afforded
methanofullerene 7 in 37% yield. Selective cleavage of the tert-
Fig. 1 UV/Vis spectrum of
1
in aqueous 0.1 M NaOH
(c~5.561026 M) and of 7 in CH2Cl2 (c~6.561026 M). The inset
shows the spectra obtained at higher concentrations (610).
Scheme 1 Preparation of compound 1. Reagents and conditions: i) t-BuOH, H2SO4, MgSO4, CH2Cl2, rt, 65%; ii) 3,5-dihydroxybenzyl alcohol (4),
K2CO3, DMF, 70 ³C, 52%; iii) malonyl dichloride, CH2Cl2, pyridine, DMAP, 0 ³C to rt, 66%; iv) C60, DBU, I2, toluene, rt, 37%; v) CF3CO2H,
CH2Cl2, rt, 84%.
888
J. Mater. Chem., 2000, 10, 887±892

in fact, form an actual cluster with the hydrophilic carboxy
groups sticking out into the polar solvent. Clustering seems to
prevail also in THF or alcohol solutions; the UV/Vis spectra
obtained for THF or EtOH solutions of 1 are still broad.
However, aggregation seems to be prevented upon further
decrease in solvent polarity. In CH2Cl2 the ground state
spectrum of 1 exhibits the same narrow and intense bands as
unclustered 7. The formation of aggregates of 1 in alkaline
water were con®rmed by small-angle X-ray scattering measure-
ments. This experiment was performed on a 5% solution of
methanofullerene 1 in aqueous 0.1 M NaOH. In the q range
explored, the intensity of the X-ray scattering I(q) is a
monotonously decreasing function of the scattering vector q.
The absence of any peak reveals that no interactions between
particles exist in the solution at this concentration. In the
Ê
Fig. 2 Transmission versus incident ¯uence at 532 nm of a sol±gel
sample containing compound 1.
Guinier domain (qRv1), a radius of gyration R of about 20 A
is obtained from the slope of ln[I(q)] vs. q2, but the too weak
intensity observed in the higher q range does not allow
description of the shape of the particles. However, in the
comparable or even slightly lower than that obtained with
inclusions of C60 in sol±gel matrices.5
spherical micelles assumption, a micellar radius RM of about
3
Ê 3
For the protection of the human eye, the unfocussed ¯uence
at the eye must not exceed 1 mJ cm22 per pulse. This means that
our doped glasses can not be used for protection in simple
eyeglasses, but they can only work ef®ciently if placed in an
intermediate focal plane of an optical instrument such as
binoculars. As the focalisation needs only to be mild, the
insertion of such a material showing a low threshold for
induced absorption should be easy.
Ê
26 A and a micellar volume VM of about 75610 A can be
calculated. From the calculated molecular volume
3
Ê 3
Vm~2.46610 A , an aggregation number NM of about
30 molecules is deduced. Given the amphiphilic character of
1, it is very likely that the micelles constitute clusters of C60
moieties surrounded by an aliphatic shell, with the solvated
ions forming an ionic monolayer at the interface with the
aqueous solvent.17 The corresponding area per polar head S is
The damage threshold of our samples is about 200 mJ cm22
.
Ê 2
about 70 A , in good agreement with values found in micellar
dispersions of sodium alkanoates.18
Up to this ¯uence, the effect is fully reversible. This damage
threshold is certainly determined by inhomogenities of the sol±
gel sample (internal and/or at the surface) and could probably
be considerably improved by polishing the surface and by
optimising the drying process.
Incorporation in sol±gel glasses and optical limiting properties
Incorporation of fullerene derivative 1 was ®rst attempted by
soaking mesoporous silica glasses with a solution of compound
1. The silica glasses were prepared by the sol±gel process as
outlined in the Experimental section. The porous silica samples
were soaked in a saturated THF solution of compound 1 or in a
saturated chlorobenzene solution of C60. After 30 minutes, the
samples were removed from the solutions and dried at 40 ³C for
one hour. In the case of C60, the resulting glass composites were
dark brownish. In contrast, the samples obtained from the
solution of 1 do not seem to be affected by the organic
substance. In fact, these composites were colourless and no
difference could be observed between the UV/Vis spectra of the
initial glass and the soaked glass whatever the average size of
porosity. Compound 1 seems to be unable to penetrate into the
pores of the glass. This effect could be ascribed to the micellar
aggregation of 1 in solution. The large size of the solvated
assembly of molecules (diameter of ca. 5.2 nm for the non-
solvated particles as determined by the small-angle X-ray
scattering measurements) may prevent the penetration into the
mesopores (maximum diameter: ca. 4.9 nm).
Time-resolved pump±probe experiments were performed in
order to obtain information about the involved molecular
states and their relaxation behaviour. Detailed knowledge of
these properties is necessary for successful modelling and
thereby optimisation of a limiting device.
In the pump±probe experiments, a powerful pump laser
pulse creates an important excited state population. The
transmission changes in the sample due to this excited state
population are monitored by a weak probe pulse. In time
delaying the probe pulse with respect to the pump pulse, the
temporal evolution of the transmission and hence of the excited
state population can be measured.
For isolated C60 molecules, the ®ve-level-model of Fig. 3 is
used to describe their reverse saturable absorption. C60 shows
singlet and triplet electronic states, each electronic state giving
rise to a manifold of associated vibronic states, shown by the
thin lines above each bold line indicating the zero-vibration
electronic state.
The dynamics following a photoexcitation of C60 in the
visible region is as follows: the molecules excited by the pump
However, successful inclusion of compound 1 in the sol±gel
could be achieved during the gelation process. A saturated
solution of compound 1 in a 1 : 1 mixture of water and THF
containing a small amount of ammonia was mixed quickly with
the sol±gel preparation in the casting boxes. A few seconds
after mixing, gelation occurred due to the pH change. The
resulting dark red glasses were perfectly homogeneous and
used as received for the optical measurements. The UV/Vis
spectrum of a sol±gel glass is similar to that recorded in
solution for compound 1. This broad spectrum suggests that
micellar aggregates of 1 are included in the glass.
The transmission of the sample at 532 nm and at room
temperature as a function of the incoming laser ¯uence is
shown in Fig. 2. With increasing pulse energy, the transmission
of the sample clearly decreases. The threshold for the onset of
the limiting action is located at about 3 mJ cm22, a value
Fig. 3 Five-level reverse saturable absorption model.
J. Mater. Chem., 2000, 10, 887±892
889

pulse will relax rapidly from one of the vibronic substates of S1
to the zero-vibration state of S1. Then part of the excited
molecules will relax directly to the S0 ground state, the other
part will undergo intersystem crossing to the metastable triplet
state T1. In C60, both the S1 and the T1 states have considerably
larger absorption cross sections than the ground state S0 in the
visible spectral region. Therefore, induced absorption can
originate from either excited state population or from the
molecules which have undergone intersytem crossing into the
metastable triplet state. The latter gives rise to a long-lasting
induced absorption.
The result of the time-resolved pump±probe experiment at
532 nm for a sol±gel sample doped with compound 1 is
presented in Fig. 4. It shows the difference in the transmission
of the sample either excited by the pump pulses or without
excitation. The steep decrease in transmission shortly after the
temporal coincidence of pump and probe pulses is due to the
directly pump pulse induced singlet excited state population.
This feature is followed by a slow re-increase in transmission,
indicating a relaxation of this initially created population. It
can be seen that this re-increase ends in a long-living plateau,
indicating a very long-living excited state population, different
from the initially created one but also capable of generating
reverse saturable absorption. By analogy with C60 and due to
the very long lifetime, the presence of the plateau can be
attributed to the existence of an excited triplet state population.
A model of the observed dynamics yields a time constant of
properties of C60. A fast S1±S0 relaxation has been however
observed for these samples. This observation appears to be
consistent with the presence of micellar aggregates of 1 in the
sol±gel. Indeed, the interactions between the fullerene spheres
of neighbouring molecules in the clusters may be at the origin
of this fast S1±S0 relaxation as already evidenced in solid C60-
®lms. Further studies are still under way in our laboratories in
order to have a better insight about this issue.
In conclusion, we have shown that the functionalisation of
the fullerene sphere with hydrophilic addends leads to highly
soluble derivatives that can be included in sol±gel matrices
during the gelation process. The optical limiting properties of
the resulting sol±gel glasses are similar to those of plain C60 in
sol±gel glasses. However, the micellar aggregation of com-
pound 1 is certainly at the origin of a fast S1±S0 relaxation and
affects the optical limiting behavior of the sol±gel samples
against nanosecond laser pulses. Further improvements could
be expected by preventing the micellar aggregation. Work in
this direction is currently under way in our laboratories.
Experimental
General methods
Reagents and solvents were purchased as reagent grade and
used without further puri®cation. All reactions were performed
in standard glassware under an inert Ar atmosphere.
Evaporation and concentration were done at water aspirator
pressure and drying in vacuo at 1022 Torr. Column chromato-
graphy: silica gel 60 (230±400 mesh, 0.040±0.063 mm) was
purchased from E. Merck. Thin layer chromatography (TLC)
was performed on glass sheets coated with silica gel 60 F254
purchased from E. Merck, visualization by UV light. Melting
points were measured on an Electrothermal Digital Melting
Point apparatus and are uncorrected. UV/Vis spectra (lmax in
nm (e)) were measured on a Hitachi U-3000 spectrophot-
ometer. IR spectra (cm21) were measured on an ATI Mattson
Genesis Series FTIR instrument. NMR spectra were recorded
on a Bruker AC 200 with solvent peaks as reference. FAB-mass
spectra (m/z; % relative intensity) were taken on a ZA HF
instrument with 4-nitrobenzyl alcohol as matrix. The scattering
experiments were performed using an experimental device
operating with linear collimation (in®nite height slit conditions)
490 ps for the depopulation of S1 for 1 in the sol±gel, as
5a,19
compared to 1000±1200 ps for C60
or 1500±1700 ps for
methanofullerenes20 in solutions. The weaker triplet state-
induced effect observed in our sample as compared to the
singlet state-induced effect can originate from a low triplet
quantum yield due to the interactions between the fullerene
spheres of neighbouring molecules in the micellar aggregates. It
has effectively been shown that in solid C60-®lms, the triplet
state is not noticeably populated due to fast S1±S0 relaxation
resulting from the fullerene±fullerene interactions.21
Conclusions
Amphiphilic methanofullerene derivative 1 is a water soluble
compound. Micellar aggregation has been evidenced by small-
angle X-ray scattering measurements and UV/Vis spectro-
scopy. As a result, due to their large size, the solvated assembly
of molecules is unable to penetrate mesoporous silica glasses.
However, thanks to the high solubility of 1 in polar solvents,
successful inclusion of 1 in the sol±gel could be easily achieved
during the gelation process. The absorption spectrum of the
resulting doped glasses suggests that micellar aggregates of 1
rather than isolated molecules are included in the glass. The
optical limiting properties of the doped sol±gel samples have
been evaluated and are comparable with the corresponding
Ê
of a monochromatic (l~1.5405 A; CuKa1) X-ray beam. The
scattered beams were recorded on a linear position sensitive
counter in the scattering q vector (4p/l6sinh range between
0.06 and 3.5 A . The solution scattering curves were corrected
Ê 21
by the subtraction of the background scattering from the
solvent.
tert-Butyl 8-bromooctanoate (2)
H2SO4 (2.31 mL, 22.41 mmol) was added to a stirred suspen-
sion of anhydrous MgSO4 (10.79 g, 89.63 mmol) in CH2Cl2
(150 mL) at room temperature. The resulting mixture was
stirred for 15 min and 3 (5 g, 22.41 mmol) was added. The
mixture was stoppered tightly and t-BuOH (10.51 mL,
112 mmol) was added with a syringe. After 18 h at room
temperature, aqueous Na2CO3 was added and the resulting
mixture was stirred until complete dissolution of MgSO4. The
aqueous layer was extracted twice with CH2Cl2; the combined
organic layers were washed with water, dried (MgSO4), ®ltered
and concentrated. Rapid ®ltration on a silica plug (CH2Cl2)
afforded 4.1 g of 2 (14.68 mmol, 65% yield). Pale yellow oil; 1H-
NMR (CDCl3, 200 MHz): 1.35±1.85 (m, 10 H), 1.44 (s, 9 H),
2.21 (t, J~7 Hz, 2 H), 3.40 (t, J~7 Hz, 2 H). 13C-NMR
(CDCl3, 50 MHz): 24.87, 27.91, 28.03, 28.35, 28.77, 32.64,
33.78, 35.41, 79.84, 173.07; IR (CH2Cl2): 1722 cm21 (CLO).
Fig. 4 Dynamics of induced decrease in transmission at 532 nm of a
sol±gel sample containing compound 1.
890
J. Mater. Chem., 2000, 10, 887±892

Compound 5
J~2 Hz, 4 H); FAB-MS: 1635.7 (10%, [MzH]z, calcd for
109H71O16: 1635.5), 720.2 (100%, [C60]z, calcd for C60: 720.0).
C
A solution of 2 (4 g, 14.32 mmol), 4 (942 mg, 6.82 mmol) and
K2CO3 (3.95 g, 28.65 mmol) in DMF (80 mL) was heated to
70 ³C for 20 h. The reaction mixture was then cooled to room
temperature, ®ltered, and evaporated to dryness. The resulting
dry residue was taken up in Et2O±H2O. After decantation, the
organic layer was washed with brine (36), dried (MgSO4) and
evaporated. Column chromatography (SiO2, Et2O±AcOEt
1 : 1) afforded 1.9 g (3.54 mmol, 52% yield) of 5. Colourless
glassy product; 1H-NMR (CDCl3, 200 MHz): 1.35±1.85 (m, 21
H), 1.44 (s, 18 H), 2.21 (t, J~7 Hz, 4 H), 3.96 (t, J~7 Hz, 4 H),
4.61 (d, J~6 Hz, 2 H), 6.36 (t, J~2 Hz, 1 H), 6.49 (d, J~2 Hz,
2 H); 13C-NMR (CDCl3, 50 MHz): 14.02, 24.84, 25.74, 27.94,
28.85, 29.02, 35.38, 64.99, 67.74, 79.81, 100.21, 104.82, 143.32,
160.24, 173.16; C31H52O7 (536.75): calcd C 69.4%, H 9.8%;
found C 69.1%, H 10.0%.
Mesoporous silica glasses
A solution of tetramethoxysilane (TMOS) in MeOH was added
to a mixture of water, MeOH, nitric acid and formamide, the
®nal composition of TMOS±MeOH±H2O±CHONH2±HNO3
was 1 : 10 : 5 : 1 : 0.063 (molar ratio).22 The reaction was
followed by FTIR which showed that full hydrolysis of
TMOS was reached in a few seconds. Then the condensation
reaction took place and gelation occurred within 2 to 3 h
depending on the temperature stated (normally at 40 ³C). The
gels were cast in small PMMA round boxes (diameter: 25 mm,
height: 8 mm) and dried at 40 ³C in a drying oven for 2 days. At
this step a large shrinkage occurred leading to pieces of
transparent xerogels of 10 mm in diameter and 0.4 mm in
height. These xerogel pieces were further heat treated at 500±
600 ³C. These samples are mesoporous as shown by physisorp-
tion measurement. The mean pore diameter is 3.46 nm and the
maximum pore diameter is 4.9 nm with a porous volume of
0.2133 cm3 g21. The soaking experiments with this mesoporous
glass substrates have been carried out as described in the text.
Compound 6
Malonyl dichloride (210 mg, 1.49 mmol) was added to a stirred
degassed solution of 5 (1.6 g, 2.98 mmol), pyridine (0.218 mL,
2.98 mmol) and DMAP (83 mg, 0.67 mmol) in CH2Cl2
(100 mL) at 0 ³C. The solution was warmed slowly to room
temperature (over 1 h) and stirred for 20 h. The resulting
CH2Cl2 solution was washed with water, dried (MgSO4),
®ltered and evaporated. Column chromatography (SiO2,
CH2Cl2±1% MeOH) yielded 1.13 g (0.99 mmol, 66% yield) of
6. Colourless glassy product; 1H-NMR (CDCl3, 200 MHz):
1.35±1.76 (m, 40 H), 1.44 (s, 36 H), 2.21 (t, J~7 Hz, 8 H), 3.49
(s, 2 H), 3.91 (t, J~7 Hz, 8 H), 5.10 (s, 4 H), 6.39 (t, J~2 Hz, 2
H), 6.46 (d, J~2 Hz, 4 H); 13C-NMR (CDCl3, 50 MHz): 24.82,
25.71, 27.92, 28.81, 28.86, 28.99, 35.34, 41.29, 66.98, 67.95,
79.68, 100.93, 106.12, 137.07, 160.24, 166.04, 173.00.
Inclusion of compound 1 in the sol±gel during the gelation process
A solution of 1 (12 mg mL21) in a 1 : 1 THF±H2O mixture
(2 mL) containing two drops of ammonia was added quickly to
a sol±gel preparation (same components in the same ratio as
described in the previous section) in the casting boxes. A few
seconds after mixing, gelation occurred due to the pH change.
At this step, compound 1 is included in the gel±glass. The
samples were dried at 40 ³C in a drying oven for 2 days. No
further heating has been carried out on these samples and they
were used as received for the optical measurements.
Compound 7
Optical measurements
DBU (0.107 mL, 0.71 mmol) was added under argon to a
stirred solution of C60 (207 mg, 0.286 mmol), I2 (91 mg,
0.35 mmol) and 6 (360 mg, 0.315 mmol) in dry toluene
(300 mL) at room temperature. After 6 h, the reaction mixture
was ®ltered over a short plug of silica (toluene) and the solvent
evaporated. Column chromatography (SiO2, CH2Cl2±0.5%
AcOEt) afforded 198 mg (0.11 mmol, 37% yield) of pure 7.
Dark red glassy product; UV/Vis (CH2Cl2): lmax (e)~258
(115 400), 326 (35 200), 425 (2330), 488 (1280), 685 nm (155);
1H-NMR (CDCl3, 200 MHz): 1.35±1.73 (m, 40 H), 1.43 (s, 36
H), 2.19 (t, J~6 Hz, 8 H), 3.87 (t, J~6 Hz, 8 H), 5.41 (s, 4 H),
6.38 (t, J~2 Hz, 2 H), 6.56 (d, J~2 Hz, 4 H); 13C-NMR
(CDCl3, 50 MHz): 24.94, 25.90, 28.06, 28.96, 29.05, 29.15,
35.47, 51.78, 67.93, 68.79, 71.35, 79.84, 101.52, 106.83, 136.49,
138.92, 140.74, 141.76, 142.08, 142.85, 143.70, 144.37, 144.56,
144.78, 144.88, 145.04, 145.11, 160.36, 163.26, 173.11; IR
(CH2Cl2): 1722 cm21 (CLO); C125H102O16 (1860.06): calcd C
80.7%, H 5.5%; found C 80.4%, H 5.7%.
To study the optical limiting properties, we measure the
transmission of our doped glass samples as a function of the
incoming ¯uence at room temperature. We use a frequency-
doubled (active/passive mode locked) Nd : YAG laser at
532 nm with a pulse duration of 30 ps and a repetition rate
of 5 Hz. At this wavelength, the linear transmission of the
sample is about 60%. This linear transmission is caused by the
inherent absorption of the active molecules by diffusion losses
due to inhomogeneities of the sample. To take into account the
intensity ¯uctuations of the laser source, a reference beam
created by a beam splitter from the same laser beam as the
excitation branch was used. The exciting pulses are focussed
onto the sample by a lens of f~16 cm into a spot of 6400 mm2.
The transmitted light of the measurement pulses as well as that
of the reference pulses is sent onto the same optical multi-
channel analyser. The incident intensity at the sample position
is changed by means of calibrated neutral density ®lters.
Compound 1
Dynamics of reverse saturable absorption
A solution of 7 (365 mg, 0.196 mmol) and TFA (25 mL) in
THF (100 mL) was stirred for 12 h at room temperature. The
reaction mixture was concentrated and CH2Cl2 was added. The
organic layer was washed with water until neutrality, dried
(MgSO4), ®ltered and evaporated to dryness. The resulting
product was dissolved in a minimum of THF. Precipitation
with hexane followed by ®ltration and drying under high
vacuum yielded 270 mg (0.165 mmol, 84% yield) of 1. Dark red
solid (mpw250 ³C); UV/Vis (in aqueous 0.1 M NaOH): see
Fig. 1; UV/Vis (CH2Cl2): lmax (e)~257 (95 000), 324 (27 500),
425 (3200), 488 (2200), 685 (130); 1H-NMR (CDCl3,
200 MHz): 1.35±1.73 (m, 40 H), 2.17 (t, J~6 Hz, 8 H), 3.87
(t, J~6 Hz, 8 H), 5.41 (s, 4 H), 6.39 (t, J~2 Hz, 2 H), 6.58 (d,
The laser source is the same ps Nd : YAG laser as outlined
above. Reference, pump and probe pulses are created from the
same laser beam by beam splitters. The transmitted test pulses
and the reference pulses are sent onto the same optical
multichannel analyser. We use a half-wavelength plate and
polarisers to achieve crossed linear polarisation between pump
and probe pulses. Coherent effects are therefore largely
suppressed and we observe exclusively the population
dynamics. Furthermore, the crossed polarisation is used
together with an analyser before the optical multichannel
analyser in order to suppress stray light from the strong pump
pulses. A discrimination system is used which ensures that only
laser pulses within a certain energy window are accumulated.
J. Mater. Chem., 2000, 10, 887±892
891

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Each measured point is the result of an accumulation of 200
laser pulses inside the energy window. By this means, energy
¯uctuations of the laser source can be largely suppressed.
Acknowledgements
We thank the CNRS for ®nancial support. Part of this work
has been supported by the INTAS project 97-11894 and the
European Comission (Contract ERBIMBICT 983556). We
also thank Dr A. Van Dorsselaer, H. Nierengarten and
R. Hueber for recording mass spectra, J.-D. Sauer for NMR
measurements, Dr B. Heinrich, M. Joucla and L. Oswald for
technical help, and Professor B. HoÈnerlage, Dr D. Ohlmann
and Dr A. Skoulios for helpful discussions.
13 C. Bingel, Chem. Ber., 1993, 126, 1957.
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17 The results obtained by small-angle X-ray scattering measure-
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