Perylene-Grafted Silicas: Mechanistic Study and Applications in Heterogeneous Photoredox Catalysis

Article abstract of DOI:10.1002/chem.201903539

A mechanistic study is herein presented for the use of heterogeneous photocatalysts based on perylene moieties. First, the successful immobilization of perylene diimides (PDI) on silica matrices is demonstrated, including their full characterization by means of electronic microscopy, surface area measurements, powder XRD, thermogravimetric analysis, and FTIR, ²⁹Si and ¹³C solid-state NMR, fluorescence, and diffuse reflectance spectroscopies. Then, the photoredox activity of the material was tested by using two model reactions, alkene oxidation and 4-nitrobenzylbromide reduction, and mechanistic studies were performed. The mechanistic insights into their photoredox activity show they have promising dual photocatalytic activity for both organic oxidations and reductions.

Full text of DOI:10.1002/chem.201903539

A Journal of
Accepted Article
Title: Perylene-grafted Silicas: Mechanistic Study and Applications in
Heterogeneous Photoredox Catalysis
Authors: Adela I. Carrillo, Ayda Elhage, Maria Luisa Marin, and Anabel
E. Lanterna
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To be cited as: Chem. Eur. J. 10.1002/chem.201903539
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Perylene-grafted Silicas: Mechanistic Study and Applications in
Heterogeneous Photoredox Catalysis
Adela I. Carrillo,[a],† Ayda Elhage,[a] M. Luisa Marin[a,],[b],* and Anabel E. Lanterna[a],*
Abstract: We present here a mechanistic study for the use of
heterogeneous photocatalysts based on perylene moieties. First, we
show the successful immobilization of perylene diimides (PDI) onto
silica matrices, including their full characterization by means of
electronic microscopy, surface area measurements, powder XRD,
TGA, and FT-IR, 29Si and 13C solid-state NMR, fluorescence and
diffuse reflectance spectroscopies. Then, we test the photoredox
activity of the material using two model reactions, alkene oxidation
problem, the use of perylene moieties grafted to mesoporous
silicas provides a way to fix the sensitizer in place while
performing heterogeneous photocatalysis. The use of
mesoporous silicas as catalyst supports is an interesting
approach due to their interesting structural properties – tuneable
pore diameter, large surface areas, high pore volumes, and
narrow pore-size distributions.[7] Silica has no inherent catalytic
property but can be used as a support functionalized either by
and 4-nitrobenzylbromide reduction, and perform mechanistic studies. framework
substitution or
by post-synthetic surface
The mechanistic insights of their photoredox activity show they have
promising dual-photocatalytic activity for both organic oxidations and
reductions.
modifications;[8] for instance, grafting perylene diimides.[9] These
silica-supported sensitizers (PDI-silica) allow easy catalyst
separation and excellent catalyst recovery.[3d,
10]
Additionally,
studies suggest the perylene-grafted silicas retain some emission
properties, and they can be quenched in the presence of amines[9]
– making them suitable for photoredox catalysis. Despite all these
advantages, the use of PDI-silica as photoredox catalysts is
virtually unexplored. Therefore, we show here mechanistic
studies for the use of these materials in both oxidations and
reductions of organic molecules. We start our analysis by studying
the photochemical properties of a heterogeneous catalyst
prepared by grafting PDI moiety into mesoporous silicas. We
show the generation of singlet oxygen (1O2) by photosensitization
of the material under visible light excitation, and prove the
versatility of the material to activate photoredox processes. For
that, we have selected two model reactions, i.e., oxidations of
alkenes into aldehydes and reduction of 4-nitrobenzyl bromide.[11]
Finally, we propose a rational mechanism for both oxidation and
reduction pathways.
Introduction
The future development of chemistry entails environmentally-
friendly and energy-sustainable alternatives for organic trans-
formations. Visible light photocatalysis can address these
challenges, as reflected by recent numerous reports.[1]
Additionally, heterogeneous photocatalysis provides both easy
separation of catalyst and light-induced selective organic
transformations.[1e, 2]
Perylenes are a family of fluorescent molecules that have been
used as pigments, colorants, photoreceptors, and, more recently,
as electronic materials because of their characteristic
combination of thermal- and photo-stability with optical and redox
properties.[3] However, owning the intrinsic insolubility of
perylenes, their use as photocatalysts in homogeneous phase
usually requires novel designs of perylene derivatives that are
soluble in the desired solvent.[4] For instance, Icli et al.[5] have
shown that engineered modifications of perylene diimides (PDI)
can improve solubility in organic solvents; nevertheless, different
substituents are necessary to work in higher polar media (e.g.,
CH3CN, CH3OH). Additionally, heavy aggregation is also
undesirable due to PDI self-quenching.[6] To overcome this
Results and Discussion
Material characterization
We use two different mesoporous silicas, MCM-41 and SBA-15,
as supports to immobilize the PDI (see experimental section). The
hybrid materials, PDI-MCM and PDI-SBA, are characterized by
means of TEM, N2 adsorption/desorption isotherms, powder XRD
patterns, TGA, and FT-IR, 29Si and 13C solid-state NMR,
fluorescence and UV-Vis spectroscopies, confirming the grafting
of the PDI on the silica materials (see SI).
[a]
Dr. Adela I. Carrillo, Dr. Ayda Elhage, Prof. Dr. M. Luisa Marin, Dr.
Anabel E. Lanterna
Department of Chemistry and Biomolecular Sciences
Centre for Catalysis Research and Innovation
University of Ottawa
10 Marie Curie, Ottawa K1N 6N5, Canada.
E-mail: marmarin@qim.upv.es; anabel.lanterna@icloud.com
Prof. Dr. M. Luisa Marin
Structural features of the PDI-MCM and PDI-SBA materials are
analysed using microscopic techniques. TEM pictures (Figure 1,
top) reveal that after their functionalization with PDI the
hexagonal porous arrangement typical of the silica (SBA-15) is
maintained (see also Figure S1 for PDI-MCM).
The textural properties of the synthesized materials are evaluated
by N2 adsorption/desorption isotherms (Table 1). As expected, in
all cases both the surface area and the mesopore volume
decrease after the incorporation of the anchored PDI into the
mesopores.[12] TGA analyses reveal that the amount of PDI
grafted to the silica is around 0.7 and 0.6 mmol/g of catalyst for
PDI-SBA and PDI-MCM, respectively. Powder XRD patterns are
[b]
Instituto de Tecnología Química UPV-CSIC
Universitat Politècnica de València
Consejo Superior de Investigaciones Científicas
Avenida de los Naranjos s/n, 46022 Valencia, Spain
As for the author Adela I. Carrillo, the views expressed are purely
those of the author and may not be regarded as stating an official
position of the European Commission]
[†]
Supporting information for this article is given via a link at the end of
the document.
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also in agreement with maintenance of the hexagonal order of the
mesopores after incorporating the PDI moiety (Figure S2).
The presence of the PDI in the hybrid materials is also verified by
using spectroscopic techniques. Figure S3 shows the FT-IR
spectra of PDI, MCM-41, SBA-15; together with PDI-MCM and
PDI-SBA hybrid materials. In all samples, bands observed at 1200,
1040 and 800 cm-1 are attributed to Si-O-Si vibrations and the
shoulder that appears at 960 cm-1 corresponds to the Si-OH
vibration.[13] Hybrid materials and the PDI also show the
characteristic peaks of the perylene moiety, i.e., peaks at 1695
and 1655 cm-1 due to the vibrations of the C=O groups and the
peak around 1595 cm-1 resulting from various vibrations of the
perylene aromatic core.[14]
Table 1. Textural properties of the PDI-silicas.
Sample
ABET[a] (m2/g)
VpBJH, [b](cm3/g)
MCM-41
PDI-MCM
SBA-15
628.0
233.2
670.1
229.0
0.22
0.03
0.69
0.24
PDI-SBA
[a]The Brunauer-Emmett-Teller (BET) surface area was estimated by
multipoint BET method using the adsorption data in the relative pressure
(P/P0) range of 0.05-0.3. [b]Mesopore volume from the isotherms at the
relative pressure of 0.8.
The effective incorporation of the PDI onto the mesoporous silica
materials is also supported by solid-state NMR spectroscopy.
Figures S4-S6 show the 29Si and 13C NMR spectra of PDI and
PDI-SBA (same as PDI-MCM). 29Si NMR shows two groups of
signals, the first one, between -92 and -110 ppm, confirms the
presence of the silica framework,[8a] thus the only signals present
in the silica matrix (SBA-15 or MCM-41),[15] and the second group,
between -45 and -70 ppm, acts as a confirmation of the covalent
linkage of the PDI [Tm = perylene-Si(OSi)m(OH)3-m, m = 2-3].[16]
Furthermore, 13C signals found in the PDI-SBA sample are in
agreement with the PDI 13C NMR spectrum.
Finally, the UV-Vis diffuse reflectance (DR) and fluorescence
spectra of the PDI-silicas (solid state measurements) show the
same profile regardless the nature of the silica (Figure 1, bottom).
Likewise, excitation wavelengths ranging from 400 to 600 nm
were employed to obtain emission spectra centred at ca. 660 nm,
regardless of the excitation wavelength employed.
Sensitized singlet oxygen formation
1
PDI-silicas are expected to generate O2 upon excitation, as the
PDI does in solution (see Fig. S7). Fluorescence quantum yields
of perylenes are reported to be higher than 0.97.[17] In addition,
singlet and triplet energies are ca. 54 and 27 kcalmol-1,
respectively.[18] This singlet-triplet energy gap allows formation of
singlet oxygen from the singlet excited state.[19] In fact, the
1
efficiency of the O2 production from perylene singlet states is
reported as 0.27 in acetonitrile.[20] To investigate the formation of
1O2 in the case of the heterogeneous materials, we select a
chemical method, specifically, the oxidation of 9,10-
dimethylanthracene (DMA) to its corresponding endoperoxide
(see SI).[13, 21] DMA exhibits distinct absorption bands between
310 and 410 nm, which are not obscured by PDI-silicas (Figure
S8), and thus can be easily monitored by UV-Vis spectroscopy.
Thus, selective excitation of PDI-silicas under visible light
excitation, in the presence of DMA under oxygen atmosphere was
performed. The consumption of DMA was monitored at 377 or 398
nm and the kinetics of the reaction was fitted to a pseudo-first
order model (See SI).
Figure 1. Top: TEM images of the hybrid PDI-SBA. PDI structure is shown in
cyan. Bottom: DR (black) and fluorescence (red) spectra of the PDI-SBA hybrid
material.
The dependence of kobs obtained for PDI-silicas and also for PDI
in homogeneous solutions at different PDI concentrations, is
shown in Figure S9. As it is usual for both homogeneous and
heterogeneous catalytic systems, the rate increases linearly with
the photocatalyst concentration, because of the increase in the
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absorbed photon flux. It is expected that the rate of the reaction
reaches a region where it stays constant, thus a region
representing concentrations of constant and optimal light
absorption. Note that due to the low solubility of PDI in the current
solvent, such region can only be reached when PDI is grafted to
the silicas. Thus, the amount of sensitizer that can be used is
higher in the cases of PDI-silicas. Indeed, when PDI-SBA is used,
we can observe the rate reaches a plateau at ca. 7x10–3 s–1. Also,
we note that the PDI reactivity remains the same in homogeneous
phase and in SBA when comparing the same PDI concentration
(~ 10 µM region). This shows that grafting PDI into silicas can
retain the PDI intrinsic catalytic activity, adding the advantages of
ease of separation and reusability (vide infra). Finally, it is worth
mentioning that the higher catalytic activity of PDI-SBA compared
to PDI-MCM can be rationalized in terms of the lower particle size
of PDI-SBA, that allows the photocatalyst to remain in suspension
more easily. SEM imaging (Figure S10) of the two different silicas
clearly shows different particle sizes, which agrees with the lower
stability of the PDI-MCM suspensions.
Table 3. kobs for the oxidation of DMA by 1O2, using PDI in homogeneous
and in heterogeneous conditions upon different excitation wavelengths.
Entry
Photosensitizer
max
kobs (10-4 s-1)
22.9
i
PDI-SBA
PDI-SBA
PDI-SBA
PDI
465[a]
530[b]
590[b]
465[a]
530[b]
590[b]
ii
17.8
6.2
iii
iv
v
6.5
PDI
18.6
<<1
vi
PDI
Irradiation power: ~0.36 W cm–2 [a]Cut on filter: 460 nm. [b]Cut on filter: 500
.
nm. Notice all samples have same PDI concentration (~10 µM).
In order to compare the activity of the two PDI-silicas, the same
experiment is carried out preparing suspensions with similar
absorption spectra (Figure S11). The values of kobs obtained for
PDI-silicas and also for PDI in homogeneous solutions –or their
physical mixtures– are shown in Table 2. As it is expected, the
same reaction rate is found for cases were PDI is in solution or
physically mixed to silicas. Noticeably, the PDI-SBA shows the
greatest rate constant, therefore it was selected for further studies.
Table 2. kobs for the oxidation of DMA by 1O2, using PDI in homogeneous
and in heterogeneous conditions.
Entry
Photosensitizer
kobs (10-4 s-1)
i
PDI
18.3
6.5
60
ii
PDI-MCM
PDI-SBA
MCM + PDI
SBA + PDI
iii
iv
v
22
16
LED current: 1A. 휆max: 530 nm. Irradiance: 0.27 W cm–2. Cut on filter: 500
nm. [DMA] = 23 휇M. All suspensions present similar absorption spectra
(See Figure S11).
Moreover, since PDI-SBA and PDI-MCM show broaden
absorption spectra than the parent PDI, we also test the
generation of 1O2 using different excitation wavelengths as shown
in Figure 2 top. For this, we use the same amount of PDI (~10
µM) in solution and in the PDI-SBA suspension. The results in
table 3 suggest that PDI-SBA can take advantage of a broaden
region of the visible spectrum, and are better illustrated in Figure
2 bottom.
Figure 2. Top: Normalized absorption spectra of PDI (black) and PDI-SBA
(red); and normalized emission spectra of 465 nm LED (blue), 530 nm LED
(green) and 590 nm LED (orange). Bottom: Normalized absorption spectra of
PDI (black) and PDI-SBA (red); and kobs at different excitation wavelengths (see
Table 3).
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Additionally, we test the reusability of PDI-SBA using the same
conditions described in table 2. Once the DMA was completely
consumed, the same amount of DMA was added to the
suspension and the evolution of the reaction was followed again.
After 5 cycles we observed the same reaction rate, suggesting the
photocatalyst capacity to generate 1O2 is maintained. Further, no
leaching PDI was found after reaction, thus the silica-supported
PDI act as real heterogeneous photocatalysts.
Electron transfer photocatalytic oxidation
PDI compounds are also believed to undergo reductive
photoredox catalytic cycles. In particular our PDI shows a redox
potential value (E1/2 (PDI•–/PDI) vs Fc/Fc+ = -0.95 V ( -0.57 V vs
SCE)) in agreement with those reported for related perylenes.[22]
Upon excitation, both reductive and oxidative properties of PDI
are enhanced[1d] and therefore, PDI excited state (PDI*) can
experience an electron transfer process with common electron
donors, such as amines, forming a PDI radical anion (PDI•–, E1/2
(PDI•–/PDI*) vs SCE = +1.77 V). The presence of this species is
easily detected by UV-vis spectroscopy due to the characteristic
absorption band of PDI•– around 700 nm. To prove this, we have
irradiated the PDI precursor in solution in the presence of an
amine (e.g., triethylamine –TEA). Figure 3A shows the formation
of the PDI•– upon 530 nm LED excitation and its quenching in the
presence of O2. Accordingly, the formation of the PDI•– species is
also demonstrated by using the PDI-SBA material (Figure 3B).
Photocatalytic applications
Based on the versatile properties of PDI, specifically on its
capability of generating 1O2, and also on its potential as a reducing
agent from its first singlet excited state, we envision that two
opposite yet complementary redox reactions can be tested in the
presence of heterogeneous PDI-SBA. Therefore, we select two
test reactions as “proof of concept”: the oxidation of stilbenes and
the reduction of 4-nitrobenzylbromide (4-NBB), summarized in
Scheme 1. Notice that our examples use single-photon excitation,
i.e., using any of the light sources shown in Figure 2 top, only PDI
species is excited as opposed to previous studies where
excitation of PDI•– (abs between ~650-850 nm, see Figure 3) was
suggested.[11a]
Figure 3. A: UV-vis absorption spectra of PDI in DMF in the presence of TEA
(100 mM) under Ar atmosphere in the dark (black) and upon 2 min irradiation
(red) with a 530 nm LED. Notice the formation of the characteristic absorption
band for PDI•– around 700 nm. PDI•– disappears quickly upon addition of O2
(blue). B: Absorption spectra of PDI-SBA suspension in DMF before (black) and
after addition of 25 mM TEA (red) under Ar atmosphere. The mixture was
irradiated for 5 min with a 465 nm LED (blue) –showing a new band around 700
nm corresponding to PDI•– formation– and then exposed to air (green).
The oxidative scission of stilbenes to form aldehydes has been
performed using expensive homogeneous Ru complexes and co-
oxidants such as NaIO4.[23] In our preliminary screening of
conditions, we find that PDI-SBA can catalyse this reaction for
different p-substituted t-stilbenes (Table S1) in
a more
environmentally-friendly process, using green light excitation,
molecular oxygen as oxidant, and heterogeneous conditions.[1c]
On the other hand, the reduction of 4-NBB is easily achieved by
green LED light irradiation of a catalyst suspension in CH2Cl2
under Ar atmosphere, using an amine as a sacrificial electron
donor (Table S2).
Scheme 1. Model reactions used to test the catalytic activity of PDI-SBA. Top:
Photocatalytic oxidation of t-stilbene. Bottom: Photocatalytic reduction of 4-
nitrobenzylbromide.
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These preliminary results suggest that PDI-SBA material has
great potential to catalyse oxidation or reduction reactions,
showing versatility to operate under different conditions. We
therefore perform different tests in order to understand the
mechanism operating under those conditions. Starting from the
oxidation process, we propose the mechanism depicted in
Scheme 2A, where the singlet excited state (PDI*) can be readily
quenched by t-stilbene (Table S2) to form the coloured radical
anion PDI•– as shown in Figure 4A. Subsequent quenching of the
radical cation derived from t-stilbene by O2, O2 or O2 leads to
oxidation products (Table S1). Additionally, we also prove that
PDI•– can be quenched by O2 (Figure 3A), more likely forming
reactive oxygen species.
In the case of the reduction of 4-NBB, a plausible mechanism
(Scheme 2B) involves the excitation of PDI to its singlet-excited
state (PDI*) followed by the injection of an electron from a
sacrificial electron donor (SED), e.g., an amine (Table S2). This
was confirmed by the formation of the PDI•– which can be restored
to the original PDI ground state by interaction with 4-NBB, leading
to the formation of benzyl radical. This is supported by the quick
disappearance of the PDI•– visible band upon addition of 4-NBB
(Figure 4B). The reaction would compete with quenching from O2
and therefore, it needs to be performed under deaerated
conditions.
1
•–
Scheme 2. A: Proposed mechanism for the photocatalytic oxidation of stilbenes
(Ar = phenyl). Note that [O] could be O2, 1O2 and/or O2•–. B: Proposed
mechanism for the photocatalytic reduction of benzyl bromides (Ar = 4-
nitrophenyl). SED: sacrificial electron donor.
Conclusions
In summary, we show the mechanism of action of perylene
diimide moiety grafted onto silica supports as versatile
heterogeneous photoredox catalysts. These PDI-silicas can
readily act as sensitizers for single oxygen formation, favouring
Figure 4. A: UV absorption spectrum of PDI in DMF in the presence of stilbene
(20 mM) under Ar atmosphere in the dark (black) and upon 5 min irradiation
(red) with a 465 nm LED. Notice the formation of the characteristic absorption
band for PDI•– around 700 nm. B: UV-vis absorption spectra of PDI in DMF in
the presence of TEA (100 mM) under Ar atmosphere in the dark (black) and
upon 2 min irradiation (red) with a 530 nm LED. Notice the formation of the
characteristic absorption band for PDI•– around 700 nm. PDI•– disappears
quickly upon addition of 25 mM of 4-NBB (blue).
the dispersion of
a greater amount of sensitizer in the
suspensions compared to the homogeneous PDI solutions. The
appeal of the material lies on its photocatalytic activity for both
oxidation and reduction transformations under mild reaction
conditions, along with the ease of separation and potential
reusability. In particular, the oxidative scission of stilbenes can be
performed under visible light and using only O2 (at atmospheric
pressure) as oxidant. In the case of the reduction of 4-nitro-benzyl
bromide, we find 81% total yield, which accounts for both dimer
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radiation (k = 1.541836 A), operating at 40 kV and 44 mA, at a scanning
velocity of 0.25 deg/min in the 0.7 < 2θ < 10 range. FT-IR was collected
on a Thermo Nicolet NEXUS 670 FT-IR instrument. Solid-state NMR
spectra were recorded at room temperature under magic angle spinning
(MAS) in a Bruker AV-400 spectrometer using a BL-4 probe with 4 mm
diameter zirconia rotors spinning at 10 kHz. 1H to 13C CP/MAS NMR
spectra were measured at 100.61 MHz and recorded with proton
and reductive products, under argon atmosphere in the presence
of amines as sacrificial electron donors. Finally, he mechanistic
studies for both oxidation and reduction processes can pave the
way to further study the versatility of this material with other
organic photoredox transformations.
decoupling. 29Si MAS NMR spectra were measured at 79.46 MHz. The 13
C
and 29Si spectra were referred to adamantane (CH2 signal at 38.3 ppm)
and TMS (0 ppm), respectively.
Experimental Section
Materials
UV-visible spectra were recorded using a Cary 100 spectrometer. For
diffuse reflectance (DR) spectra the same spectrometer equipped with a
Labsphere Inc. DRA-30I diffuse reflectance accessory was used.
Fluorescence spectra of PDI solutions were obtained with a Photon
Technology International (PTI) spectrofluorimeter at room temperature.
Fluorescence lifetimes were measured in an Easy-Life (from PTI) system
using a 530 nm LED pulse excitation and the lifetimes calculated in the
Easy-Life integrated software. Fluorescence spectra of solid samples were
acquired with a LS50 Perkin Elmer spectrofluorimeter. Singlet Oxygen
detection was carried out in a Laser flash photolysis system (LFP 111
system, Luzchem Inc., Ottawa, Canada) using a 532 nm excitation pulse
generated with a Nd-YAG laser operating at 10 mJ/pulse.
Tetraethylorthosilicate (TEOS, 98%) was used as silica precursor.
Cetyltrimethylammonium bromide (CTAB, 96%), and pluronic (P123) were
used as structure-directing agents. Aqueous ammonia solution (NH4OH,
30%) and hydrochloric acid were also used in the synthetic protocol to
obtain the final mesoporous materials. Perylene-3,4,9,10-tetracarboxylic
dianhydride (PTCD) and 3-aminopropyltriethoxysilane (APTES) were used
to synthesize the perylene derivative (PDI) to be anchored to the
mesoporous silica materials. 9,10-dimethylanthracene (DMA), t-stilbene,
t-stilbene oxide, 4-nitrobenzyl bromide (4-NBB), triethylamine (TEA),
diisopropylethylamine (DIPEA) were purchased from Sigma-Aldrich and
used as received.
Electrochemical measurements were carried out using a computer-
controlled potentiostat Autolab/PGSTAT 101 (Eco-Chimie) with NOVA
software. A three-electrode electrochemical cell was employed for the
electrochemical measurements. The working, counter, and pseudo-
reference electrodes were glassy carbon electrodes with diameters of 2
mm, and Pt-wire electrodes, respectively. The potentials were measured
with respect to ferrocene couple (Fc/Fc+) at room temperature in an
Eppendorf cup degassed with Ar using 0.5 mL of 1-Butyl-3-
methylimidazoliumtriflate. Scan rate: 0.2 V/s.
Synthesis of perylene diimide (PDI)
PDI was synthesized by modifying a reported protocol.[7b] Briefly, 2 g of
PTCD were purged with N2 in a round bottom flask. Then, 4 mL of APTES
were added and the reaction mixture was stirred for 20 h under N2
atmosphere at 150 ºC in the dark. After cooling to room temperature, the
dark red solid was placed into a Soxhlet system and washed for a week
with petroleum ether to remove the excess of APTES. Then, acetone was
used to extract the residue. The solvent was then slowly evaporated by
using a rotary evaporator to afford the desired PDI product.
Chemical detection of singlet oxygen formation
Synthesis of perylene-grafted silicas (PDI-silicas)
The photooxygenation of 9,10-dimethylanthracene (DMA) was carried out
in the presence of PDI, PDI-MCM or PDI-SBA in a mixture ethanol/water
90:10 at 293 K using a single LED working at 1000 mA with a maximum
emission at 530 nm. The incident light was filtered in order to cut out light
below 500 nm and eliminate any direct photochemical reaction of DMA.
The initial concentration of DMA was 23.3 µM, and its concentration was
measured by UV spectroscopy, following the decrease of the 377 nm or
398 nm band.
Mesoporous silica (0.2 g), MCM-41 or SBA-15, prepared by published
methods,[6a] were dehydrated in an oven at 200 ºC for 2 h. Then, 50 mL of
anhydrous toluene was added to the activated materials and this mixture
was stirred for 1 h in order to obtain a well dispersed suspension. Finally,
0.32 g of the PDI were added to the reaction mixture and refluxed overnight.
The obtained red PDI-silica solid was filtered, washed with fresh toluene
and acetone and air-dried. In a final step the solid was placed into a
Soxhlet system and was washed with acetone for 2 days to remove the
un-reacted PDI.
Photocatalytic activity
Photoredox reactions were performed using a light-emitting diode system
(LED) consisted of four, 10 W 530 nm LedEngin LZ4-40G110 emitters
attached to aluminum heat sinks in a custom design irradiator using a
thermal bath with water/ethanol circulation for temperature control (set T =
5°C). The intensity of each individual emitter at the distance the sample is
placed was measured (0.48 W cm–2).
Characterization
Transmission electron microscopy (TEM) images were collected with a
Philips CM-10 microscope operating at 100 kV. Scanning electron
microscopy (SEM) images were acquired on JSM-7500F FESEM (JEOL)
microscope, working voltage: 20 kV. The textural properties of the solids
(degassed for 4 h at 373 K at 5 x 10-5 bar) were determined from N2
adsorption at 77 K in an AUTOSORB-6 apparatus. The pore size
distribution was determined from the Barret-Joyner-Helender (BJH)
method applied to the adsorption branch of the obtained isotherms. The
surface area was determined using the multipoint BET method in the 0.05-
0.30 relative pressure ranges. Mesopore volume was measured at the
plateau of the adsorption branch of the nitrogen isotherm, P/Po = 0.8.
Thermogravimetric analysis (TGA) was collected on a TA Instruments
Q5000 IR Thermogravimetric Analyzer. Small-angle powder XRD analysis
was carried out with a Rigaku Ultima IV diffractometer using a CuKa
Identification and quantification of the reaction products was performed
using a Waters Integrity HPLC in tandem with a reverse phase C18 Zorbax
column or a Perkin Elmer GC coupled with a FID or Mass detector, and
the NMR spectrometer Bruker Avance II 400.
The oxidation of stilbene was performed in aerated CH3CN by ca. 3 mg of
the PDI-SBA, 15 mg of stilbene with green LED irradiation for around 20
h. Reaction progress was followed by GC-MS while quantification of
products was analyzed by CG-FID using naphthalene as external standard.
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The reduction of 4-NBB was carried out using ca. 4.5 mg of the PDI-SBA,
20 mg of 4-NBB (0.09 mmol) and 2 eq. of the DIPEA in 5 mL of strictly
deaerated dichloromethane. The reaction was irradiated with green LEDs
for 5 or 24 h. The reaction was analyzed by 1H NMR and quantified using
caffeine as external standard.
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Entry for the Table of Contents
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Unique photostability and
Adela I. Carrillo,[a],† Ayda Elhage,[a] M.
Luisa Marin[a,],[b],* and Anabel E.
photoredox properties of the
perylene diimide anchored on silica
supports, can be used for reductions
and oxidations photocatalytic
reactions, by simply changing
reaction conditions. This dual activity
is rationalized by mechanistic
studies, which can pave the road to
further exploration of these versatile
photocatalytic heterogeneous
materials.
Lanterna[a],
*
Page No. – Page No.
Perylene-grafted Silicas:
Mechanistic Study and Applications
in Heterogeneous Photoredox
Catalysis
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