Quantum Dots in Visible-Light Photoredox Catalysis: Reductive Dehalogenations and C-H Arylation Reactions Using Aryl Bromides

Article abstract of DOI:10.1021/acs.chemmater.7b01109

In the recent past, visible-light-mediated photoredox catalysis has made a huge impact on the development of new synthetic methods under very mild and ecologically benign conditions. Although semiconductor nanocrystals or quantum dots (QDs) possess suitable optoelectronic and redox properties for photoredox catalytic applications, surprisingly, their use for the activation of challenging chemical bonds in the synthesis of organic molecules is little explored. We report here the application of ZnSe/CdS core/shell QDs for the synthetically important photoredox catalytic activation of carbon-halogen bonds in dehalogenation and C-H arylation reactions using (hetero)aryl halides as bench-stable inexpensive bulk starting materials, under very mild reaction conditions. The outstanding catalytic activity of ZnSe/CdS core/shell QDs is a direct consequence of the high specific surface area and homogeneity of QDs in solution and their high photostability toward oxidation.

Full text of DOI:10.1021/acs.chemmater.7b01109

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Article
Quantum dots in visible light photoredox catalysis: Reductive
dehalogenations and C–H arylation reactions using aryl bromides
Anuushka Pal, Indrajit Ghosh, Sameer Sapra, and Burkhard König
Chem. Mater., Just Accepted Manuscript • Publication Date (Web): 06 Jun 2017
Downloaded from http://pubs.acs.org on June 7, 2017
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Graphical abstract
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Quantum dots in visible light photoredox catalysis: Reductive dehalogenations
and C–H arylation reactions using aryl bromides
Anuushka Pal†, Indrajit Ghosh‡, Sameer Sapra†, and Burkhard König‡*
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†Department of Chemistry, Indian Institute of Technology Delhi, Hauz Khas, New Delhi -
110016, India
‡ Institute of Organic Chemistry, University of Regensburg, D-93040 Regensburg, Germany
*Correspondence to: burkhard.koenig@ur.de
TABLE OF CONTENTS
ABSTRACT
In the recent past, visible light mediated photoredox catalysis has made a huge impact on the
development of new synthetic methods under very mild and ecologically benign conditions.
Although semiconductor nanocrystals or quantum dots (QDs) posses suitable optoelectronic and
redox properties for photoredox catalytic applications, surprisingly, their use for the activation of
challenging chemical bonds in the synthesis of organic molecules is little explored. We report
here the applications of ZnSe/CdS core/shell QDs for the synthetically important photoredox
catalytic activation of carbon–halogen bonds in dehalogenation and C−H arylation reactions
using (hetero)aryl halides, as bench stable inexpensive bulk starting materials, under very mild
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reaction conditions. The outstanding catalytic activity of ZnSe/CdS core/shell QDs is a direct
consequence of the high specific surface area and homogeneity of QDs in solution and their high
photostability towards oxidation.
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INTRODUCTION
Semiconductor nanocrystals or quantum dots (QDs) have received enormous attention in the
scientific community due to their fascinating optical and electronic properties dependent on size,
shape, composition, and heterostructuring.1–3 Their unique optoelectronic properties render them
suitable as promising candidates for applications in different areas, such as photovoltaics,4–7 light
emitting diodes,8–10 photodetectors,11,12 biological markers,13,14 and recently in photo-
catalysis.15,16
For photocatalysis, QDs are particularly suitable due to their efficient visible light
harvesting capabilities, the high extinction coefficients in the visible region, their tunable and
size dependent redox potentials. Their solubility can be tuned depending on the ligands exposed
at the surface, and the presence of a large fraction of surface atoms provides a highly reactive
surface area, leading to more efficient energy or electron transfer processes for catalytic
applications.17 Most importantly, their stability against photoirradiation makes them superior in
comparison to conventional organic dyes18 or commonly used transition metal complexes.
Previously, bulk semiconductors and metal nanoparticles have been explored for the
photocatalytic degradation of dyes,19 photo-oxidative degradation and removal of organic
pollutants for water purification,20 water splitting,21,22 and carbon–carbon bond forming
reactions.16,23,24 However, the heterogeneity imposed by bulk semiconductors retards their
catalytic activity. Therefore, considerable efforts have been made in exploring QDs as better
photocatalyst compared to their bulk counterparts. Recent reports on nanocrystal-based
photocatalysis describe water splitting using cadmium sulfide (CdS) and titanium dioxide,25–28
ferric oxide,29 quaternary metal oxides, mixed oxides,30,31 and the photodegradation of organic
molecules such as phenols,32 benzene, and aromatic ketones by tungsten oxide nanocrystals.33,34
CdS and cadmium selenide (CdSe) QDs have been used in the photocatalytic reduction of
aromatic nitro compounds and azides to amines.35,36 Niemeyer and Ipe employed QD/enzyme
nanohybrids for photocatalyzing the transformation of myristic acid to α- and β-hydroxymyristic
acid under UV irradiation.37 Bernt et al. have reported the photochemical carbon disulfide
formation from 1,1-dithiooxalate using CdSe QDs under aerobic conditions.38 Most recently, Li
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et al. have reported the visible light photocatalytic formation of disulfides via coupling of a
variety of thiols without the need of external oxidants.39 Jensen et al. demonstrated the
photocatalytic reduction of nitrobenzene to aniline using CdS QDs.40 Chauviré et al. have
investigated the redox behavior of CdSe/ZnS core shell QDs in aqueous medium and monitored
the oxidation reaction of 8-oxo-2′-deoxyguanosine, and the reduction of nitrophenylalanine
derivatives under visible light.41
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Although many reports are known on the semiconductor nanocrystal catalyzed
photodegradation, surprisingly none on the synthetically important carbon–carbon bond forming
reactions using aryl halides.42,43 Notably, the constructions of carbon–carbon bonds are of
particular importance for synthesizing new drugs or fine chemicals, or in simplistic of cases, for
the synthesis of arylated arenes or more importantly heteroarenes that are widely used in material
science for their interesting electronic and optical properties.
Here, we report our attempts towards the generation of highly reactive aryl or heteroaryl
radicals from their corresponding halides for synthetically important arylation reactions using
core/shell QDs and visible light. Upon single electron transfer, (hetero)aryl halides (Ar-X, where
X= Cl, Br) form their corresponding radical anions, which upon fragmentation and by releasing
halide anions generate the corresponding (hetero)aryl radicals. However, photoredox catalytic
activation of carbon–halogen bonds in aryl halides via single electron transfer in substituted aryl
halides possesses, among others, mainly two challenges: (i) their extremely high reduction
2+
potentials that are often beyond the reach of many conventional photocatalysts (e.g., Ru(bpy)3
or Eosin Y) and (ii) the two step fragmentation kinetics of Ar-X (where X= Cl, Br), which often
play a crucial role in determining the feasibility of such photoredox transformations. The
generation of aryl radicals from bench stable aryl halides typically involves a strong base, such
as potassium tert-butoxide, or nucleophiles under ultraviolet (UV) (Ex ≤ 350nm) irradiation (c.f.,
SRN1 reaction mechanism).44 Murphy and coworkers have shown that the electron transfer to aryl
halides could be possible from highly reactive neutral organic reducing agents, such as
N2,N2,N12,N12-tetramethyl-7,8-dihydro-6H-dipyrido[1,4]diazepine-2,12-diamine under UV-A
(365 nm) irradiation.45 Recently, König and co-workers have shown that the reduction of aryl
halides is possible using excited doublet states of organic radicals available via consecutive
photoinduced electron transfer (conPET) processes.46 We propose core/shell QDs that in the
presence of a sacrificial electron donor and upon visible light photoexcitation transfer an electron
to the aryl or heteroaryl halides generating aryl or heteroaryl radicals, which are trapped by either
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hydrogen atom donors or used for C–C bond forming reactions in the presence of suitable
reagents.
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RESULTS AND DISCUSSIONS
Our selection of ZnSe/CdS core/shell QDs as the photocatalyst was based on its suitable redox
potential for the envisaged conversions (see Figure 1a). We have selected a ZnSe core because of
its high reduction potential of –1.8 V vs NHE.31 However, as the excitonic absorption of ZnSe
QDs is not in the visible region, we have coated ZnSe with CdS in order to shift the absorption
towards the visible blue region and at the same time to form type II junctions for better charge
separation.3,47 Figure 1b displays the steady state absorption and photoluminescence spectra of
ZnSe-QDs. The first absorption of ZnSe is observed at 407 nm, and the respective
photoluminescence is centered around 420 nm. The sharp photoluminescence spectra with a
FWHM of 14 nm indicates the narrow size distribution of the QDs. Figure 1c displays the
absorption and the photoluminescence spectra of core/shell ZnSe/CdS QDs. The Stokes shift
after the shelling is ca. 110 – 120 nm. This high stokes shift indicates the formation of type II
junctions in the heterostructures. The powder XRD pattern of the core QDs and core shell QDs
match with their bulk counterparts from the JCPDS database (file number 892940) as shown in
Figure 1d. The diffraction pattern of core and core/shell QDs reflects the resemblance to the
wurtzite crystal of the bulk. In the core/shell structure, the diffraction peaks are shifted to lower
2θ values. The size distribution and size has been calculated by transmission electron microscopy
(TEM) and high-resolution transmission electron microscopy (HRTEM) images (Figure 1e and
1f). Here, the average size of core QDs is 3.4 nm and the average size of the core/shell QDs is 4
nm. The TEM images reveal spherical particles that retain their shape after shelling via the
successive ion layer adsorption reaction (SILAR). The energy dispersive X-ray spectroscopy
(EDS) confirms the elemental composition of Zn and Se in ZnSe in an atomic molar ratio of 45
and 55. However, in case of the core/shell QDs the molar composition of Zn, Se, Cd and S are
25, 29, 27 and 17%, respectively. The molar ratio between Cd and Zn indicate the formation of
one monolayer.
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Figure 1. (a) Bulk conduction band and valence band positions of ZnSe and CdS. Absorption
and photoluminescence spectra of (b) ZnSe, (c) ZnSe/CdS. (d) PXRD pattern of synthesized
ZnSe and ZnSe/CdS. TEM images of (e) ZnSe, (f) ZnSe/CdS. The insets of (e) and (f) represent
the size distribution histograms and HRTEM images of ZnSe and ZnSe/CdS.
Applications of QDs in visible light photoredox reduction of aryl halides. We sought
to use these QDs for photoredox reductive generation of aryl radicals from commercially
available, inexpensive, and most importantly, bench stable aryl halides. The use of aryl radicals
from aryl halides have recently gained enormous interest, because of their importance in C–H
arylation reactions with arenes and heteroarenes.46,48–51
When a mixture of 2-bromobenzonitrile (model substrate, see Table 1, and Figure 2),
core/shell QDs (0.6 mol%), and N,N-diisopropylethylamine (DIPEA, as a sacrificial electron
donor) was irradiated in hexane with blue LEDs (Ex = 455  15 nm) benzonitrile was formed as
confirmed by GC and GC–MS measurements. To increase the efficiency of this photocatalytic
method, the reaction conditions were optimized using various solvents and varying the amount of
the electron donor. Among other solvents, hexane and toluene were found to be the best
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performing solvents for the photoreduction reactions to take place in the presence of 8.0 equiv.
of DIPEA. After 24 h, the photoreduction product (i.e., benzonitrile) was obtained in 92% yield.
Control experiments confirmed that the presence of QDs, electron donor i.e. DIPEA, and blue
light photoirradiation are important for the photoredox catalytic reduction reactions to occur.
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Table 1: Control reactions for reductive dehalogenation of (hetero)aryl halides.
Entry Catalyst
(mol %)
DIPEA
(equiv.)
Reaction Condition
t / h Yield (%) [a]
1
2
0.6
0.6
8
–
455 nm, N2, 25 °C
455 nm, N2, 25 °C
24
92
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–
8
455 nm, N2, 25 °C
0 [b]
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–
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455 nm, N2, 25 °C
dark, N2, 25 °C
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24
0 [b]
0 [b]
0.6
Determined by GC using naphthalene as an internal standard. [b] The yield is too low to be detected in the
[a]
GC.
Using the optimized reaction conditions, we explored the scope of the reaction with other
aryl halides. This catalytic method is suitable for the reduction of aryl halides possessing electron
withdrawing groups (see Figure 2). Electron deficient heteroaromatic compounds can also be
dehalogenated in good yields. The photoreduction does not proceed for aryl halides with neutral
or electron donating groups that possess higher reduction potentials than the available reduction
potential of the QDs, investigated herein, showing the limitation of this method. However, the
scope comprises heteroaromatic chlorides, such as pyridines, quinoline, isoquinolines, which
were cleanly photoreduced using this catalytic method.
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Br
H
QDs (0.6 mol%)
DIPEA (8.0 equiv)
455 nm, 25 oC, Hexane
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R
R
Reduction of aryl bromides
H
H
H
H
H
CN
N
O
CF3
(46%, 48 hr)
CN
O
(92%, 24 hr)
H
(70%, 48hr)
(76%, 48 hr)
(72%, 24 hr)
H
H
H
N
H
Cl
N
Br
F3C
(51%, 48 hr)
(39%, 24 hr)
(69%, 48 hr)
(60%,48 hr)
(39%,48 hr)
Reduction of aryl chlorides
H
H
H
N
H
N
N
F3C
(23%, 48 hr)
CN
(0%, 48 hr)
(49%,24 hr)
(33%, 24 hr)
Figure 2. Photoreduction of aryl bromides and chlorides. Reaction conditions: Substrate
concentration 0.05 mmol; catalyst amount 0.32 μmol, 0.6 mol%; DIPEA 0.4 mmol, 8.0
equivalents. Photoreduction yields were determined by gas chromatography using naphthalene as
an internal standard. Notably, for aryl chlorides unreacted starting materials (when the yields are
relatively low) was observed in the GC chromatograms.
Next, we applied the photocatalytic method for C–H arylation reactions using different
trapping reagents to capture the generated aryl radicals. The abstraction of hydrogen atoms by
aryl radicals from the cation radical of the amine competes with the reductive photoredox
catalytic C–H arylation reactions. The amount of added trapping reagent affects the yields of C–
C bond formation reactions (more the better). Under this catalytic conditions pyrrole derivatives,
including unprotected pyrroles (entry 2 in Figure 3), were found to be effective in trapping the
aryl radical in reasonable yields. The yields of the C–H arylation reactions reaches 40% with the
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remaining amount of the starting material being converted into the corresponding reduction
product due to the competing hydrogen abstraction reactions. The mass balance in all photoredox
catalytic reactions are very good. Aryl and heteroaryl halide substrates could be used for C–H
arylation reactions using cheap pyrrole derivatives as trapping reagents, albeit their use in excess.
Alkaloids such as -nicotyrine (entry 3 in Figure 3) could be obtained by simple mixing of
commercially available 3-bromopyridine, trapping reagents, QDs in very low catalytic amount,
DIPEA, and photoirradiation with visible light under nitrogen.
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Figure 3. C–H arylation of aryl halides using substituted pyrroles as trapping reagents. Reaction
conditions: Substrate concentration 0.05 mmol; catalyst amount 0.32 μmol, 0.6 mol%; DIPEA
0.4 mmol, 8 equivalents; pyrroles 1.5 mmol, 30 equivalents. Notably, although not quantified in
all cases the reduction products were formed (via hydrogen atom abstraction of the aryl radicals
from the radical cation of DIPEA) as the byproducts.
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Spectroscopic investigations. Spectroscopic investigations were performed in order to
understand the mechanism of the catalytic cycle. 2-Bromobenzonitrile (model substrate) and
DIPEA did not show significant, if any, influence on the absorption spectra of QDs. 2-
Bromobenzonitrile also did not have any effect on the photoluminescence spectra of the QDs.
However, the photoluminescence spectra of QDs (in this case intensity, PL quenching) changed
dramatically upon successive addition of DIPEA (Figure 4). The observed quenching could be
attributed to the electron transfer from DIPEA to the excited QDs, as also suggested by previous
reports in the literature.52 However, no ground state spectral shift of QDs in the presence of
substrates and DIPEA is observed showing that neither etching nor aggregation of the QDs
occurs, indicating their stability.
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8
6
4
2
0
8
6
4
2
0
2.0
1.8
1.6
1.4
1.2
1.0
2.0
1.8
1.6
1.4
1.2
1.0
Addition of
2-bromobenzonitrile
Addition of
DIPEA
0
10 20 30 40
Conc. of 2-bromobenzonitrile(mM)
0.00 0.05 0.10 0.15 0.20 0.25
Conc. of DIPEA (M)
450 500 550 600 650 450 500 550 600 650
Wavelength (nm)
Wavelength (nm)
0.6
0.4
0.2
0.0
1.6
1.2
0.8
0.4
0.0
Addition of
Addition of
DIPEA
2-bromobenzonitrile
300 400 500 600 700
Wavelength (nm)
300 400 500 600 700
Wavelength (nm)
Figure 4. Changes in the photoluminescence (in this case intensity) and absorption of QDs upon
successive addition of DIPEA ((a) and (c)), and 2-bromobenzonitrile (model substrate, (b) and
(d)). In the insets (for Figure a and b) the Stern-Volmer quenching plots are shown. Note that in
Figure 3d the increase in the optical density below 300 nm is due to the absorption of 2-
bromobenzonitrile.
Proposed Mechanism of the catalytic cycle. The spectroscopic investigations, synthetic
results along with the literature reports52 support the proposed photoredox catalytic mechanism
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depicted in Figure 5. Upon visible light photoexcitation, QDs oxidize DIPEA to form the radical
anion of QDs (QD•–) and the radical cation of DIPEA (DIPEA•+). The QD radical anion transfers
an electron to the aryl halide giving the corresponding radical anion Ar–X•–.50,53,54 Upon release
of the halide ion (X–) an aryl radical is formed, which either abstracts a hydrogen atom, likely
from the radical cation of DIPEA, to form the dehalogenated reduction product or is trapped by
the investigated pyrrole derivatives, which upon successive oxidation and release of proton yield
the C–H arylated products under visible light.46,48–51
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Figure 5. Proposed mechanism of the photoredox catalytic cycle.
Photostability of QDs. Photoredox catalysts, especially organic dyes and coordination
compounds, are readily photodecomposed under visible light photoirradiation requiring high
catalyst loadings for photoredox transformations.49,55 In contrast, the QDs investigated herein are
exceptionally photostable when irradiated with blue LEDs (Ex = 455  15 nm). Indeed, no
photodecomposition, indicated by the spectral shift or changes in the optical density, was
observed upon prolonged photoirradiation (up to 48 h; see Figure 6). Within the UV–Vis
experimental concentration range, most organic dyes decompose within hours. Note that the
minimal enhancement that is seen in the absorbance value is due to the evaporation of solvents
over the period of time. The photostability of QDs in the presence of DIPEA is slightly reduced
as observed from the changes in the absorption spectra as the absorbance of QDs is blue shifted.
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The origin of the slightly reduced photostability of QDs in the presence of DIPEA is not clear at
present and requires further investigations.
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0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
(a) Only QDs
(b) QD with DIPEA
48 hr
48 hr
300 400 500 600 700 800 900
Wavelength (nm)
300 400 500 600 700 800 900
Wavelength (nm)
Figure 6. The absorbance of QDs on photoirradiation at 455 nm for 48 hours (a) QDs only, (b)
QDs with DIPEA. Notably, in Figure 6a the minimal enhancement of the absorbance value is
mainly due to the evaporation of solvents over the period of irradiation time. For the changes in
the optical density at a particular wavelength with irradiation time, see the supporting
information (Figure S2).
CONCLUSION
QDs have been for the first time employed as visible light photoredox catalyst for the reduction
as well as for the synthetically important C–H arylation reactions using aryl halides. The
required low catalyst loading, homogeneity in solution, tunable optoelectronic properties, and
photostability of the QDs render them as suitable photocatalysts with distinct advantages
compared to organic dyes and metal complexes. Upon single electron transfer (hetero)aryl
halides (Ar−X) generate the corresponding radical anions (Ar−X•−) that upon fragmentation
(and by releasing halide anions) generate the corresponding (hetero)aryl radicals. These radicals
either abstract a hydrogen atom from the radical cation of DIPEA to form the reduction products
or are trapped by suitable pyrrole derivatives present in the reaction media to afford the C−H
arylated products. The catalyst concentration, as low as 0.6 mol%, provides evidence for
extremely efficient photoredox catalytic generation of aryl or heteroaryl radicals from
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(hetero)aryl halides using visible light. The presented results suggest a broader application of
QDs in photoredox catalysis that include generation of aryl radicals for the functionalizations of
aryl or heteroaryls or formation of carbon–heteroatom bonds, such as carbon–phosphorus bonds
(using suitable trapping reagents; for example, trialkyl phosphites) using aryl halides under
visible light photoirradiation.
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EXPERIMENTAL SECTION
Materials Required
Tri-n-octyl phosphine (TOP, 97%), zinc stearate (Zn(st)2, technical grade), selenium powder
(99.9%), octadecene (ODE, 90%), sulfur (99.5%), cadmium oxide (CdO,99%), oleic acid (OA),
octadecylamine (ODA,90%), 1-butanol were procured from the Sigma Aldrich and were used
without further purification, Methanol and hexane (analytical grade) was purchased from VWR
and distilled prior to use.
Synthesis of ZnSe QDs
QDs were synthesized by slight modification of a previously reported hot injection method,56 in
which a mixture of zinc stearate (Zn(st)2, 4 mmol) and 1- octadecene (ODE, 40 ml) was heated at
300 °C in an argon atmosphere until a clear solution was formed. A 4.0 mmol portion of Se was
dissolved in 10 mL tri-n-octylphosphine (TOP) in a crimp-capped vial under inert conditions.
After attaining 300 °C, the prepared TOPSe solution was injected rapidly into the solution. The
growth of NCs was carried out at 290 °C until the desired size of the QDs was achieved as
monitored by the UV visible absorption spectrum. After attaining the desired size, the reaction
was cooled down to ~ 60 °C and phases were separated by a hexane and methanol mixture (1:1).
The solution was centrifuged for 30 minutes giving a solid precipitate and a yellow layer of QDs.
Hexane was added to the solid to extract more QDs. The yellow solution containing QDs was
purified by precipitating in 1-butanol and re-dispersing in hexane. UV-Vis/photoluminescence
spectroscopy and TEM measurements were performed to characterize the resulting nanocrystals
for estimation of their size and band gap. Figure 1b shows the absorption and photoluminescence
of synthesized quantum dots. The concentration of QDs has been determined by using the
method reported by Banin and coworkers (see also the supporting information).57
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Core/shell Quantum Dots
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Preparation of injection Stock solutions
A stock solution of 0.4 M cadmium, was obtained by heating of 8 mmol of CdO (1.024g), 32
mmol of oleic acid (10.1 ml) and 9.9 ml of ODE in a 50 ml flask under argon atmosphere at 280
°C under stirring until a clear solution was obtained. The mixture was cooled to 60 °C. A sulfur
stock solution of 0.4 M was prepared by heating a 0.4 mmol of sulfur (0.128 g) in ODE (10 ml)
to 150 °C under argon.
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Synthesis of core/shell QDs
Core/shell nanocrystals were synthesized by a slightly modified previously reported SILAR
method,47,58 in which the surface of the nanocrystals is cation rich to ensure higher luminescence
efficiencies. A 3.8  10–6 mole of ZnSe core QDs solution, 24 mL of ODE and 9 g of ODA were
heated to 100 °C under vacuum for half an hour to remove the traces of solvent. The temperature
was raised to 220 °C and the calculated amounts of the stock solutions of sulfur and cadmium
were added starting with sulfur. The time between the two successive injections were 10
minutes. After shelling, the reaction mixture was allowed to anneal at 235°C for 20 minutes to
improve the crystallinity of the QDs. The QDs solution was allowed to cool to 60 °C and phases
were separated by a hexane and methanol mixture and QD were precipitated with acetone. The
QDs were re-dispersed in hexane and stored in dark and inert conditions for further use.
Characterization
All photophysical experiments were performed in hexane at room temperature. UV-Vis
absorption spectra were recorded on a Varian Cary 50 spectrophotometer. Fluorescence emission
spectra were collected on a Horiba Jobin Yova spectrophotometer with the excitation wavelength
of 370 nm for core and 455 nm for core shell QDs. The emission and excitation slit widths were
fixed at 2 nm each.
Transmission electron microscope (TEM) images were recorded on a Tecnai G2 20
electron microscope operated at an accelerating voltage of 200 kV. Samples were prepared on
200-mesh carbon coated Cu grids by dropping very dilute QDs solution dissolved in hexane and
allowing the solvent to evaporate. The EDS was collected by Si detector. PXRD was recorded on
Agilent Technologies Gemini R Ultra with Cu Kα (λ= 1.54 Å).
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ACKNOWLEDGEMENTS
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We thank the GRK 1626 and DST-SERB Grant No. EMR/2015/000005 for financial support.
We are grateful to Prof. Josef Zweck (Physics department, University of Regensburg) for
allowing us to carry out the TEM and HRTEM measurements. We thank Prof. Arno Pfitzner
(Chemistry department, University of Regensburg) for extending the PXRD facility to us and Dr.
Marc Schlosser for carrying out the measurements. AP acknowledges UGC for SRF.
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SUPPORTING INFORMATION. Materials and methods, general procedures for the
photoreduction of arylhalides and C-H arylations, photo of the photochemical set-up,
spectroscopic characterization of the C-H arylated products.
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