Hybrid functional mesostructured thin films with photo-oxidative properties in the visible range

Article abstract of DOI:10.1039/b911742c

Hybrid mesostructured thin films functionalised with organic photosensitiser molecules demonstrated high efficiency for the decontamination of polluted atmosphere via singlet oxygen production.

Full text of DOI:10.1039/b911742c

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Hybrid functional mesostructured thin films with photo-oxidative
properties in the visible rangew
a
a
b
b
´
Philippe Saint-Cricq, Thierry Pigot, Lionel Nicole, Clement Sanchez
and Sylvie Lacombe*a
Received (in Cambridge, UK) 16th June 2009, Accepted 7th July 2009
First published as an Advance Article on the web 22nd July 2009
DOI: 10.1039/b911742c
Hybrid mesostructured thin films functionalised with organic
photosensitiser molecules demonstrated high efficiency for the
decontamination of polluted atmosphere via singlet oxygen
production.
Two different sensitisers were tested: a laboratory made one,
9,14-dicyanobenzo[b]triphenylene-3-carboxylic acid (PS-1),
and a silylated derivative of the commercial anthraquinone-
2-carboxylic acid (PS-2) (Fig. 1). Both of them are known to
react efficiently with oxygen to give reactive oxygen species
(ROS) either by energy transfer with the formation of
Photoinduced generation of singlet oxygen in various materials
(silica, zeolites, polymers, textiles, . . .) is of great importance
for oxidation purposes, in particular in the field of fine
chemistry (mild oxidation of high added value molecules),1
photodynamic therapy (PDT),2 oxidation micro-reactors3 and
water sterilisation.4 These materials are based on photo-
sensitisers (PS) embedded or grafted on various matrices
(polymers, cellulose or inorganic oxides). Silica-based materials
processed via sol–gel chemistry are good candidates for photo-
sensitisation because of their high transparency (especially in
the UV-visible range), photostability,5 mechanical strength6
and chemical inertness. Silica-based monoliths containing
different PS have already demonstrated a high selectivity and
efficiency for the oxidation of nauseous sulfur compounds.7
Singlet oxygen was shown to be efficiently generated in micro-
porous silica monoliths,8 with a lifetime significantly enhanced
compared to those observed in water or alcoholic solvents.
Here, we propose to investigate the photo-oxidative properties
of meso-organised hybrid matrices (recently shown to enhance
optoelectronic or sensing properties of various matrices9)
shaped as thin films functionalised with organic photo-
sensitiser molecules and synthesised via the evaporation-
induced self-assembly (EISA) method.10 We demonstrate that
such matrices, presenting large opened porosity, high surface
area and important porous volume, improve molecular
diffusion and accessibility to the organic functions grafted
inside matrices. Moreover, the EISA approach which presents
an impressive versatility in terms of processability (materials
shaped as monoliths, particles, films, micro- and macro-
patterns obtained via a great variety of commercial deposition
techniques10,11) could favour fast industrial development.
1
singlet oxygen O2 or by electron transfer leading to radical
intermediates.12 Periodically organised mesoporous hybrid
thin films (POMTF) functionalised with organic photosensitisers
were synthesised via two routes depending on the nature of PS
molecules (presence or absence of alkoxysilane functions).13
With PS-2, a ‘‘delayed’’ one-pot procedure was used. This
approach, which notably limits the influence of functional
organosilanes on the mesostructuration process, is based on
a pre-hydrolysis-condensation step of inorganic precursors
(TEOS) followed by the addition of functional organosilanes
just prior the sol deposition. With PS-1, an approach
combining the ‘‘one-pot’’ and post-functionalisation pathways
was selected. It consists of ‘‘delayed’’ one-pot functionalisation
of an organosilane bearing a reactive group (here an amino one)
followed by post-modification (here a peptidic coupling
reaction), after surfactant removal, of this reactive group with
the desired organic molecule.
Briefly,
a pre-hydrolysed solution containing TEOS,
ethanol, deionised water and HCl was mixed with a solution
of CTAB, ethanol and dilute HCl. The final solution (molar
composition: 1TEOS : 22EtOH : 5H2O : 0.014HCl : 0.14CTAB)
was then aged at room temperature for 3 days prior to
the addition of functional organosilanes (APTS neutralised
with concentrated HCl or PS-2, molar ratio: 1TEOS : 0.01APTS
or 1TEOS : 0.02PS-2). Thin films were prepared by dip
coating silicon wafers and quartz substrates. In order to
obtain cubic Pm3n mesostructure, optical homogeneity and
transparency, the relative humidity (RH) was kept at 40% for
the first 30 s and at 70% for the next 5 minutes.14 Films were
heated at moderate temperature (130 1C for stiffening the silica
network without decomposed grafted molecules) for 2 days
and then washed with alcohol in order to remove surfactants.
a
´
UMR CNRS 5254, Universite de Pau et Pays de l’Adour, IPREM,
Helioparc Pau Pyrenees, 2 rue du president Angot, 64053 Pau cedex 9,
´
´
´
´
France. E-mail: sylvie.lacombe@univ-pau.fr;
Fax: +33 (0)559 407 422; Tel: +33 (0)559 407 579
b
`
´
Chimie de la Matiere Condensee de Paris, UMR CNRS 7574,
UPMC-Paris 6 – Colle`ge de France, 11 place Marcelin Berthelot,
75231 Paris, France
w Electronic supplementary information (ESI) available: Absorption
and emission–excitation spectra of POMTF-1, absorption spectra of
PS-2 and POMTF-2, ellipsoporosimetry of POMTF-1, emission
spectra of the lamps, scheme of the one-pass reactor, synthesis and
characteristics of Si-powder-1. See DOI: 10.1039/b911742c
Fig. 1 Molecular structure of PS-1 and PS-2.
Chem. Commun., 2009, 5281–5283 | 5281
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For films functionalised with PS-1 (namely POMTF-1), a
further coupling step between the amino group grafted in thin
films and the carboxylic function of PS-1 was achieved with
peptidic coupling reagents such as N-hydroxysuccinimide
(NHS) and dicyclohexylcarbodiimide (DCC).15 A solution of
DMF containing PS-1 (0.13 mmol), NHS (1.1 equivalent) and
DCC (1.1 equivalent) was ultrasonicated for 10 minutes.
Then films were soaked into the resulting solution and ultra-
sonicated for 15 more minutes. Films were rinsed in
various solvents such as dichloromethane and cyclohexane
to remove any adsorbed photosensitiser molecules. Absorption–
emission spectra of POMTF-1 confirm the presence of
well-grafted PS-1 inside thin films without a significant altera-
tion of its photoluminescence properties and their high trans-
parency and homogeneity (see ESIw). From the thickness of
the film (about 270 nm, determined by ellipsometric
experiments, see ESIw) and the molar absorption coefficient
of PS-1 and PS-2 in several solvents (13 000 molꢁ1 L cmꢁ1 at
418 nm and 6100 at 323 nm, respectively) the calculated
concentration of the PS in POMTF-1 is about 0.14 mol Lꢁ1
(corresponding to a coupling yield of around 80% of
the amino functions available in the APTES-film), and
0.45 mol Lꢁ1 in POMTF-2.
Fig. 3 Water isotherm (square: adsorption, triangle: desorption) of
POMTFs and pore size distribution.
under identical conditions. One sealed tube contained two
POMTF-1, the second one contained two POMTF-2, and the
third tube contained two films without any PS (blank
POMTF). Then 10 mL of DBS (4.4 ꢂ 10ꢁ5 mol) were added
in each sealed tube through a rubber septum. Each tube was
irradiated with 4 fluorescent lamps (l = 420 nm, 4 mW cmꢁ2
,
see ESIw) for 16 hours. This experiment was duplicated by
leaving a set of three identical sealed tubes in the dark. Finally
a standard experiment was carried out with Si-powder-1,
prepared from commercial silica gel functionalised with
3-aminopropyl moieties (Acros organics, particle size 40–63 mm,
SBET 330 m2 gꢁ1 determined by nitrogen adsorption–
desorption volumetry, pore size 6–7 nm, grafted with 1
according to the previous procedure, B4 ꢂ 10ꢁ7 mol mgꢁ1
or 0.8 mol Lꢁ1 assuming silica density around 2, see ESIw).
5 mg of Si-powder-1 deposited on a glass film were irradiated
at 420 nm in another sealed tube containing 35 mL of DBS.
The results are presented in Table 1.
The synthesis conditions (composition and RH) described
above were carefully chosen in order to obtain thin films
presenting a cubic Pm3n mesophase. In fact, it was reported
that cubic Pm3n structure is highly mechanically stable and
shows higher permeability compared to those of the other
CTAB mesostructures (2D-hexagonal p6m, 3D-hexagonal,
P63/mmc).16 A typical XRD pattern and TEM image17 of
hybrid POMTFs synthesised in this study are given in Fig. 2.
Water adsorption–desorption isotherms of thin films
performed via environmental ellipsometric porosimetry
technique are consistent with reported results for Pm3n
POMTFs (Fig. 3).18 Thin films present a mono-disperse pore
size distribution and a porous volume of 35%. Due to the
well-known unidirectional contraction of thin films perpendi-
cular to substrate, pores are ellipsoidal with a dimension of
2.8 by 2.0 nm, which is in a good agreement with XRD and
TEM results.
As expected, the blank film gave no oxidation product under
irradiation. In the other experiments at 420 nm, after 16 hours
of irradiation, the presence of sulfoxide (DBSO) and sulfone
(DBSO2) derivatives indicates the formation of singlet oxygen.
Both POMTF-1 and POMTF-2 are photoactive, but the yield
of oxidation product is much higher for the first one. The
lower efficiency of POMTF-2 may be explained by a lower molar
absorption coefficient of the dye at 420 nm (50 molꢁ1 L cmꢁ1),
as well as the weaker singlet oxygen quantum yield of
anthraquinone.7
The enhanced efficiency of POMTF-1 compared to
Si-powder-1 is obvious from the results of Table 1. The ratio
of DBS to sensitiser is much higher with the films (3000 vs. 100)
whereas the global yield of oxidation products are of the same
order of magnitude. This may be the result of the high
transparency of the films and less scattered light than in the
powder. Moreover, compared to solution selectivity,z the
noticeable disappearance of disulfide formation is observed
with both catalysts, while the ratio of sulfone to sulfoxide is
much higher with POMTF-1 than with Si-powder-1. This
increased selectivity for sulfone, already reported by
Clennan et al. inside zeolites relative to acetonitrile homo-
geneous solutions, was assigned to a different environment of
the key intermediate persulfoxide involved in the complex
singlet oxygenation of sulfides, and was also shown to be
sensitive to water content and sulfide loading inside zeolites.19
The different selectivity of POMTF-1 relative to Si-powder-1
may be the result of either different porosity, water content or
sulfide loading.
The photo-oxidative properties of the POMTFs were then
investigated with the oxidation of two organic sulfide
compounds: di-n-butylsulfide (DBS) and dimethyl sulfide
(DMS). Two different experiments were carried out: in static
closed reactors for gaseous DBS and in a ‘‘one-pass’’ gas flow
reactor for more volatile DMS.
In the first set of experiments, three couples of POMTFs
(2 ꢂ 1 cm) were placed in three different Pyrex tubes, prepared
Another point of interest is the efficiency of POMTF-1
under laboratory daylight (line 3, Table 1): oxidation products
Fig. 2 XRD pattern (left) and TEM picture-[210] plane (right) of
Pm3n hybrid POMTFs.17
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5282 | Chem. Commun., 2009, 5281–5283

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Table 1 Products obtained under irradiation at 420 nm for 16 hours
of gaseous DBS in the presence of POMTF or grafted silica powder
(DBSO: di-n-butyl sulfoxide, DBSO2: di-n-butyl sulfone
of the excited PS because of the presence of large amounts of
pre-adsorbed DMS. Thus only sulfide is activated, most
probably by electron transfer,11 yielding disulfide as the main
initial product.
[PS]/
mol Lꢁ1
DBS
l/nm (%)
DBSO DBSO2
(%)
(%)
rb
In summary, mesoporous organised thin films containing
two different sensitisers have been prepared. Hybrid POMTFs
are efficient in both static and dynamic mode under visible
light. The photo-oxidative properties of these highly trans-
parent films at the gas–solid interface over sulfide pollutants
were shown to be enhanced compared to more light-scattering
silica powders. Mesoporosity is not only responsible for the
high transparency of the films, but also for different selectivity,
probably arising from the different environments of sulfides
inside the pores.
Blank film
POMTF-1
POMTF-1
POMTF-2
0
420
420
—
420
420
100
5
75
74
11
0
0
51
0
Traces
22
0
0.14
0.14
0.45
44
25
26
67
3000
3000
850
a
Si-powder-1 0.8
100
a
b
Laboratory daylight. Molar ratio between DBS and sensitiser.
were also obtained, but in lower yield due to shorter
irradiation time under visible (natural/artificial) light and
lower light power.
Notes and references
z Irradiation at 420 nm of an acetonitrile solution of DBS (2.5 ꢂ 10ꢁ2 M)
The oxidation of another well-known volatile pollutant,
DMS, was carried out in a one-pass gas-phase reactor. Six
POMTF-2 (2 ꢂ 1 cm) samples were placed in a thermo-
regulated reactor (scheme in ESIw). Dry air containing DMS
at a defined concentration (about 110 ppmv) was flowed over
the films at a 20 mL minꢁ1 flow rate. The reactor was
irradiated with 16 lamps at 420 nm and the gas at the
outlet of the reactor was analysed by GC-FID. A typical
concentration curve with POMTF-2 is given in Fig. 4.
After an equilibrium adsorption step in the dark, lamps
were switched on. The DMS concentration sharply dropped,
while disulfide concentration first increased and then smoothly
dropped. After 5 hours irradiation, a photostationary state
was reached: dimethyl sulfone (DMSO2) and dimethyl
sulfoxide (DMSO) appeared in the gas flow, while DMDS
concentration decreased. 76 hours of irradiation were needed
to recover the initial DMS concentration. In this case, the
molar ratio between total introduced DMS and the sensitiser
was about 1 : 1800.
containing
distribution, sulfoxide : sulfone : disulfide 65 : 19 : 10.
1
(8.2
ꢂ
10ꢁ5 M) leads to the following product
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Fig. 4 DMS oxidation products detected during irradiation of
POMTF-2 at 420 nm (DMSO: dimethyl sulfoxide, DMSO2: dimethyl
sulfone, DMDS: dimethyl disulfide Me–S–S–Me).
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This journal is The Royal Society of Chemistry 2009
Chem. Commun., 2009, 5281–5283 | 5283