Self-organization of porphyrin units induced by magnetic field during sol-gel polymerization

Article abstract of DOI:10.1039/b616421h

The use of a magnetic field as a controlling factor during the hydrolysis-polycondensation of porphyrin precursors substituted by Si(OR) ₃ groups, induces a self-organization of porphyrin moieties due to the stacking of these units in the hybrid material and this study also confirms the effect of the magnetic field in the nano- and micrometric organization during the kinetically controlled polycondensation process. The Royal Society of Chemistry.

Full text of DOI:10.1039/b616421h

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Self-organization of porphyrin units induced by magnetic field during
sol–gel polymerization{
Fre´de´ric Lerouge,a Genevie`ve Cerveau,a Robert J. P. Corriu,*a Christine Sternb and Roger Guilard*b
Received (in Cambridge, UK) 9th November 2006, Accepted 10th January 2007
First published as an Advance Article on the web 5th February 2007
DOI: 10.1039/b616421h
reported that hybrid silica–porphyrin networks can be achieved
by sol–gel process,6 and that these ‘‘metalloporphyrinosilicas’’
containing iron(III) porphyrins for example, catalyze oxidation
reactions.6a Other applications in sensors,7 nonlinear optics8 and
hole-burning9 have also been studied for metalloporphyrins
immobilized in sol–gel matrices. More recently supramolecular
architectures have been reported in the case of porphyrin
precursors presenting cooperative effect of hydrogen bonding
and p–p stacking interactions leading to their immobilization in
new organic–inorganic hybrid materials.10
The use of a magnetic field as a controlling factor during the
hydrolysis–polycondensation of porphyrin precursors substi-
tuted by Si(OR)3 groups, induces a self-organization of
porphyrin moieties due to the stacking of these units in the
hybrid material and this study also confirms the effect of the
magnetic field in the nano- and micrometric organization
during the kinetically controlled polycondensation process.
The organization of the organic spacers inside of the matrices of
hybrid organic–inorganic materials obtained by sol–gel polymer-
ization1 has been evidenced recently in the case of precursors
containing two, three or even four Si(OR)3 groups (Scheme 1).2–4
It has been shown that these processes are under kinetic
control.5 Both van der Waals and hydrogen bonding interactions
between hydrophobic units involved during polycondensation at
silicon induce self organization of the organic units in the solid.5
These materials can be viewed as amorphous since X-ray
powder diffraction diagrams exhibit no Bragg peaks. In fact these
are non-crystalline organized materials because all the previous
organic units incorporated in the framework and studied until
now, whatever their geometry – linear rigid, linear with some
flexibility, twisted linear, planar, tetrahedral – and the experimental
conditions,5 exhibit X-ray diffraction diagrams showing broad
signals consistent with nanometric scale ordering. Unquestionable
existence of organization at the micrometric scale has been
revealed by birefringence in thin films of the materials.3c
In this paper we describe the effect of magnetic field as a
controlling factor for the self-organization of porphyrin moieties in
organic inorganic material prepared by hydrolytic polycondensa-
tion (sol–gel route). Two porphyrin precursors with completely
different structures were investigated. 5,10,15,20-Tetrakis[4-
(triethoxysilyl)phenyl]porphyrin zinc P1 presents a rigid zinc
porphyrin metallated core and the free base 5,10,15,20-tetrakis[4-
(3-triethoxysilyl)propylureido)phenyl]porphyrin P2 possesses flex-
ible arms containing ureido groups favoring H-bonds (Scheme 2).
The hydrolytic polycondensation was performed with (xerogels
X1M and X2M for P1 and P2, respectively) and without magnetic
field (xerogels X1 and X2 for P1 and P2, respectively).
The tetraethoxysilyl metalloporphyrin P1 was prepared as
described below. 5,10,15,20-Tetrakis(4-bromophenyl)porphyrin
was synthesized following the Adler–Longo procedure and the
metalation was done as reported in the literature.11 Silylation12 was
performed by using triethoxysilane in the presence of NEt3 and a
catalytic amount of [Rh(cod)(MeCN)2]BF4. P1 was obtained as a
purple solid in 65% yield. The tetraethoxy-ureido porphyrin P2
was synthesized by condensing four 3-(triethoxysilyl)propyl
isocyanate with the amino functions of the 5,10,15,20-tetrakis(4-
aminophenyl)porphyrin in the presence of triethylamine.13 The
free base amino porphyrin was obtained by reducing the tetranitro
derivative in the presence of SnCl2?2H2O.14
These two types of organization appear during two different
steps of the process: the nanometric one occurs during the
polymerization in solution, whereas the micrometric one takes
place in solid phase during the ageing step.5
In the present paper we describe the self-organization of bridged
organosilica precursors with porphyrin groups. It has been
The corresponding bridged porphyrylpolysilsesquioxanes X1,
X2, X1M and X2M were prepared by hydrolysis–polycondensa-
tion of the precursors in THF for P1 and in DMSO for P2 due to
Scheme 1 Sol–gel process of nanostructured hybrid organic–inorganic
materials.
aLaboratoire de Chimie Mole´culaire et Organisation du Solide, UMR
5637, Universite´ Montpellier II, cc 007, Place E. Bataillon, F-34095,
Montpellier Cedex 5, France. E-mail: corriu@univ-montp2.fr;
Fax: (+33) 4 67 14 38 52
bLaboratoire d’Inge´nierie Mole´culaire pour la Se´paration et les
Applications des Gaz, UMR 5633, Universite´ de Bourgogne, Faculte´ des
Sciences Mirande, 9 Avenue Alain Savary, BP 47870, 21078, Dijon
Cedex, France. E-mail: limsag@u-bourgogne.fr; Fax: (+33) 3 80 39 61 17
{ Electronic supplementary information (ESI) available: Syntheses of P1
and P2. See DOI: 10.1039/b616421h
Scheme 2 Tetraethoxysilyl porphyrins P1 and P2.
Chem. Commun., 2007, 1553–1555 | 1553
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Table 1 Properties of the xerogels
29Si CP MAS NMR (%)
Precursor
Xerogel
Conc./mol L21
tgel/min
SBET/m2 g21
T0
T1
T2
T3
L.C.a (%)
Birefringence (6103)
P1
P1
P2
P2
a
X1
X1Mb
X2
0.05
0.05
0.1
1920
1920
54
280
447
,10
,10
0
0
0
0
11
13
30
0
45
43
50
39
44
44
20
61
78
77
70
87
4.7
5.8
7.7
8.4
b
X2Mb
0.1
30
Degree of condensation calculated according to the general equation L.C. = [0.5 (T1 area) + 1.0 (T2 area) + 1.5 (T3 area)]/1.5. Under
magnetic field (4.7 Tesla)
the very low solubility of P2 in THF, at room temperature in the
presence of 2% mol of tetrabutylammonium fluoride as catalyst
(TBAF). The xerogels X1M and X2M were prepared in a
magnetic field of 4.7 Tesla{ at room temperature. The gels are
formed after various gelation times and the resulting solids were
treated as usual.5 The xerogels were isolated as dark purple solids
in almost quantitative yield (calculation based on a fully
polycondensed solid, 96–98%). The experimental conditions,
specific surface areas, spectroscopic data and birefringence values
for these solids are given in Table 1.
In the case of X1M and X1, the 29Si CP MAS NMR spectra
Scheme 3
displayed three resonances at y270, y280 and y289 ppm,
first signal has been attributed to the distance between two
assigned, respectively, to T1, T2 and T3 substructures. The level of
columns, the medium planes being defined by the zinc atoms
condensation, estimated in first approximation by deconvolution
presenting the higher electron density (Scheme 3). The weaker
of spectra,15 was observed in the same range (70–80%) (Table 1).
signal is attributable to the Si–O–Si units contribution.16
In the case of X2, three resonances at y249, y258 and
In the case of the solids obtained from P2, three broad signals
˚
were observed at 9.0, 5.9, and 4.3 A. This behavior is similar to
y265 ppm (T1, T2 and T3) were observed while for X2M the
spectra exhibited only two signals at y259 and y264 ppm. The
level of condensation was higher when the gel was performed
under magnetic field (87% versus 70%). The electronic spectra in
the diffuse reflectance mode were performed by dispersing the
xerogels in silica. The presence of a band in the Soret region
(lmax/nm: 422 for X1, 421 for X1M) and two Q bands (lmax/nm:
558 and 602 for X1, 560 and 603 for X1M) for each gel indicates
that the porphyrin rings were not modified during the hydrolysis-
polycondensation process.
that of other polysilsesquioxane materials prepared from pre-
cursors possessing H-bond interactions.17 It was difficult in this
˚
case to assign the distances of 9.0 and 5.9 A.
21
˚
More information has been obtained between 0.04 and 0.24 A
by SAXS experiments (Fig. 2). A diffraction signal was observed in
this area only in the case of the xerogels X1M and X2M prepared
21
under magnetic field, at 0.12 A (52.3 A) for X1M and at
˚
˚
21
0.077 A (81.9 A) for X2M, whereas without magnetic field (case
˚
˚
of X1 and X2), no signal was observed. Taking into account a
˚
As already described for all nanostructured hybrid materials
reported until now,5 the X-ray diffraction diagrams of X1, X2,
X1M and X2M did not exhibit sharp Bragg signals (Fig. 1), but
broad and intense signals were observed. It is well established that
these broad signals correspond without any doubt to the existence
of a nanometric scale level in the material.16 Assuming Bragg’s
law, the distances associated to the q values were calculated
(d = 2p/q).
distance of around 3.5 A between a zinc atom and the medium
plane of the next porphyrin unit,18 the distance of 52.3 A in the
˚
case of X1M can correspond to a stacking of 15 units. In the case
˚
of X2M, the intensity of the signal observed at 81.9 A is higher and
the signal is more narrow. This is indicative of a better
organization in the solid as expected because of hydrogen bonding.
This distance can correspond to the stacking of 23 units.
Such a behavior can be explained from the strong interaction
between the applied magnetic field and the diamagnetism induced
by the 18 p electrons of the macrocycle (Scheme 4). Thus all the
molecules should be forced to orient and organize one to each
other in the direction of the magnetic field lowering the entropy of
the polycondensation reaction and leading to the stacking of
porphyrin rings as shown in Scheme 4.
The shape of the X-ray diffraction diagrams (from 0.2 to
2.0 A21) was the same for all the solids prepared from P1 whatever
˚
the experimental conditions: two broad signals were observed at
21
21
˚
˚
˚
y0.61–0.63 A (10.0–10.3 A) and a weak one at y1.57–1.61 A
˚
(3.9–4 A). Considering a columnar stacking of the macrocycles, the
Fig. 1 X-Ray diffraction diagrams of X1, X1M and X2, X2M.
1554 | Chem. Commun., 2007, 1553–1555
Fig. 2 SAXS diagrams of X1, X1M, X2 and X2M.
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unambiguously that the nanometric and the micrometric scale
organizations take place during different steps of the polyconden-
sation process and are not governed by the same parameters.§
Notes and references
{ The intensity of the field was measured with a teslameter. The sample was
displayed under the magnet of a 250 MHz NMR Bru¨ker apparatus, near
the probe using a wood made device.
Scheme 4 Schematic comparison of the orientation of porphyrin units:
(a) in a magnetic field, (b) without magnetic field; and (c) interaction
between the magnetic field and the macrocycles.
§ Synthesis of the xerogels: The preparation of the xerogels was carried out
according to a general procedure. The preparation of X1 is given as an
example: 0.34 g (0.25 mmol) of P1 and 1.27 mL of dried THF were
introduced in a Schlenk tube. 1.27 mL of a solution containing 2.5 mL
(2.5 mmol) of TBAF (1 mol L21 in THF), 27 ml (1.52 mmol) of H2O and
1.24 mL of dried THF, were added. This mixture was introduced by
capillarity into the observation cell just before the gel point, and the
remaining solution was kept in the Schlenk tube. In both cases, after the
sol–gel transition, the dark gel was aged 6 days. Then, the gel obtained in
the Schlenk tube was crushed and washed twice with acetone, ethanol and
diethyl ether, and the resulting powder was dried at 120 uC under vacuum
for 3 h yielding a dark purple xerogel. In the case of X1M or X2M, the
process was performed in a magnetic field of 4.7 Tesla.
1 C. J. Brinker and G. W. Scherer, in Sol–Gel Science: The Physics and
Chemistry of Sol–Gel Processing, Academic Press, San Diego, CA, 1990.
2 D. A. Loy and K. J. Shea, Chem. Rev., 1995, 95, 1431.
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and R. J. P. Corriu, in Supplement Si: The Chemistry of Organic Silicon
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Chichester, 2001, ch. 10, p. 565; (c) B. Boury and R. J. P. Corriu,
Chem. Rec., 2003, 3, 120, and references therein.
Fig. 3 Birefringence pictures of (a) X1, (b) X1M, (c) X2, (d) X2M.
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Charreton, Sens. Actuators, B, 2000, 62, 136.
The sol, initially placed in a magnetic field, was taken up a few
minutes before the gel point and introduced by capillarity in a thin
Teflon-coated cell which was then sealed and placed in the
magnetic field. The sol–gel transition was observed in the cell after
15 min. The same experiment was performed to afford the xerogels
X1 and X2, but without applying a magnetic field. Both cells were
analyzed by microscopy in polarized light. An important shrinkage
and the formation of birefringent purple wires of gel without
cracks were observed in the case of P1. The same morphology was
observed for the gels X1 and X1M (Fig. 3).
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important shrinkage is a priory due to the low concentration of
the precursor. The birefringence intensity was slightly higher under
magnetic field (4.7 6 1023 for X1 and 5.8 6 1023 for X1M) and
the optical axis was oriented, as usual, perpendicularly to the axis
of the wires in both cases (Fig. 3). Furthermore, it is well known
that porphyrins, and especially rigid ones, tend to align according
one-dimensional aggregates,10 therefore the fiber-like morphology
observed for the gels X1 and X1M could depend on the
architecture of the precursor.
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Solids, 2000, 265, 41, and references therein.
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In the case of X2 and X2M, both gels exhibited the same
morphology, very different of that of the gels X1 and X1M
(Fig. 3(c) and (d)). Such morphology has been already observed in
the case of gels obtained from TTF or planar precursors having
flexible arms.18
The results presented here show clearly the drastic effect of the
magnetic field in the self-organization of the porphyrin units
during the polycondensation process. This effect is mainly
observed at the nanometric scale level, since the SAXS diagram
is totally different when a magnetic field is applied during the
gelation process. In contrast the micrometric scale organization is
the same in both cases (similar birefringence phenomena
and orientation of optical axis). Thus these results confirm
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Chem. Commun., 2007, 1553–1555 | 1555