Structural Design Principle of Small-Molecule Organic Semiconductors for Metal-Free, Visible-Light-Promoted Photocatalysis

Article abstract of DOI:10.1002/anie.201603789

Herein, we report on the structural design principle of small-molecule organic semiconductors as metal-free, pure organic and visible light-active photocatalysts. Two series of electron-donor and acceptor-type organic semiconductor molecules were synthesized to meet crucial requirements, such as 1) absorption range in the visible region, 2) sufficient photoredox potential, and 3) long lifetime of photogenerated excitons. The photocatalytic activity was demonstrated in the intermolecular C?H functionalization of electron-rich heteroaromates with malonate derivatives. A mechanistic study of the light-induced electron transport between the organic photocatalyst, substrate, and the sacrificial agent are described. With their tunable absorption range and defined energy-band structure, the small-molecule organic semiconductors could offer a new class of metal-free and visible light-active photocatalysts for chemical reactions.

Full text of DOI:10.1002/anie.201603789

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201603789
German Edition: DOI: 10.1002/ange.201603789
Photoredox Catalysis
Structural Design Principle of Small-Molecule Organic Semiconductors
for Metal-Free, Visible-Light-Promoted Photocatalysis
Lei Wang, Wei Huang, Run Li, Dominik Gehrig, Paul W. M. Blom, Katharina Landfester, and
Kai A. I. Zhang*
Abstract: Herein, we report on the structural design principle
of small-molecule organic semiconductors as metal-free, pure
organic and visible light-active photocatalysts. Two series of
electron-donor and acceptor-type organic semiconductor mol-
ecules were synthesized to meet crucial requirements, such as
1) absorption range in the visible region, 2) sufficient photo-
redox potential, and 3) long lifetime of photogenerated exci-
tons. The photocatalytic activity was demonstrated in the
cells (OSCs), and organic thin-film transistors (OTFTs) owing
to their unique optical and electronic properties originating
from the visible-light-responsive p-conjugated backbone
structure.[5] The driving force of organic solar cells is electron
(hole) transfer at a donor–acceptor (D–A) interface followed
by charge separation. An interesting question is whether the
light-generated electron-transfer process, which is the oper-
ation principle of organic photovoltaic (OPV) devices can be
translated into a photocatalytic system for chemical reactions.
Recently, macromolecular semiconducting polymeric systems
have been introduced as metal-free and heterogeneous
photocatalysts for organic reactions[6] or light-induced hydro-
gen evolution[7] based on a similar principle. In comparison,
only a few examples of the SMOSs were reported as
homogeneous visible-light-active photocatalysts.[8] A deeper
understanding for the design of this new class of small-
molecule-based materials is desirable.
À
intermolecular C H functionalization of electron-rich hetero-
aromates with malonate derivatives. A mechanistic study of the
light-induced electron transport between the organic photo-
catalyst, substrate, and the sacrificial agent are described. With
their tunable absorption range and defined energy-band
structure, the small-molecule organic semiconductors could
offer a new class of metal-free and visible light-active photo-
catalysts for chemical reactions.
P
hotocatalysts, which can absorb in the visible region, have
As illustrated in Figure 1a, the light-induced electron
transfer in OPVs usually occurs from the electron donor to
the separated acceptor organic semiconductor (OC) material.
By transferring this process into the photocatalytic process,
the photo-generated electron/hole pair of the organic semi-
conductor could function as the reductive and oxidative
species, reacting with either electron-accepting or -donating
substrates, respectively, and completing the photoredox cycle
(Figure 1b). For both applications, the energetic band posi-
tions of the organic semiconductor materials could be
controlled by employing various building blocks.[9]
To reach high efficiency in the OPVs, the enhanced charge
separation and long exciton lifetime is of great importance.
This can be achieved by morphology control of the separated
D- and A materials in solid state.[10] For the photocatalytic
process, which is usually carried out in liquid phase, the same
attracted much attention in recent years as a sustainable tool
for chemical synthesis under mild and environmentally
benign conditions.[1] Beside the well-developed organometal-
lic complexes based on transition metals, such as [Ru-
(bpy)3]Cl2 and fac-[Ir(ppy)3],[2] or recently reported
common-metal-based systems, such as copper-containing
photocatalysts,[3] pure organic dyes have been also employed
as a metal-free variation of photocatalysts for various
reactions.[4] Nevertheless, there are some intrinsic drawbacks
associated with these molecular systems, for instance, high
cost and toxicity of rare metals, limited availability in nature,
and photobleaching, which can be troublesome for catalyst
stability and long-term usage. The lack of systematically
controllable band structures still remains a large challenge. It
is therefore important to develop a new class of metal-free,
pure organic photocatalysts with easily tunable absorption
ranges, and controllable oxidative and reductive potentials for
chemical transformations.
Small-molecule organic semiconductors (SMOSs) have
been used in a vast number of photoelectronic applications,
such as organic light emitting diodes (OLEDs), organic solar
[*] L. Wang, W. Huang, R. Li, Dr. D. Gehrig, Prof. P. W. M. Blom,
Prof. K. Landfester, Dr. K. A. I. Zhang
Max Planck Institute for Polymer Research
55128 Mainz (Germany)
E-mail: kai.zhang@mpip-mainz.mpg.de
Supporting information (experimental details and characterization,
scope of the photocatalytic dehalogenation reactions, fluorescence
quenching experiments, 1H and 13C NMR spectra) and the ORCID
identification number(s) for the author(s) of this article can be found
under http://dx.doi.org/10.1002/anie.201603789.
Figure 1. Functional principles of light-induced electron transfer in a) a
simplified organic semiconductor (OC)-based photovoltaic device and
b) an organic semiconductor-based photocatalytic system. S: sub-
strate.
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effect ought to be realized by combining the D and A moieties
in a single molecule.
in Figure 2.[11] The first half reaction is the generation of
malonate radical via the reduction of the malonate halide,
driven by the transfer of the photo-generated electron from
the photocatalyst onto the malonate halide, resulting in the
Herein, we report a structural design principle of electron-
donor and -acceptor small-molecule organic semiconductors
(SMOSs) as a new class of pure organic, metal-free and
visible-light-active photocatalytic systems with tunable
absorption areas and defined band structures. The oxidative
and reductive potentials of SMOSs could be easily controlled
by simple variation of the electron-donor and -acceptor
moieties in the molecule. The photocatalytic activity was
À
demonstrated in the intermolecular C H functionalization of
electron-rich heteroaromates with malonates under visible
light irradiation. It could be observed by mechanistic studies
that the light-induced electron transport between the SMOSs,
substrates, and the sacrificial electron-donating agent played
a significant role in enhancing the photocatalytic efficiency of
the SMOSs. Furthermore, the time-resolved photolumines-
cence spectroscopy demonstrated fluorescence lifetimes of
the designed SMOSs ranging from around 12 to 16 ns,
indicating long-living photogenerated excitons in the SMOSs.
Two series of electron donor–acceptor type small-mole-
cule organic semiconductors were designed to meet the
crucial requirements as organic molecular photocatalysts:
1) absorption range in the visible region, 2) sufficient light-
generated redox potential, and 3) long lifetime of the photo-
generated excitons. The structures of the SMOSs are dis-
played in Scheme 1. Containing the same benzothiadiazole
(BT) unit as the electron-acceptor moiety, Series 1 represents
À
Figure 2. Reaction mechanism of the intermolecular C H functionali-
zation of electron-rich heteroaromates with diethyl bromomalonates.
R1, R2, R3 =aryl or alkyl; interm=intermediate.
malonate radical and halide anion. The malonate radical
reacts with the heteroaromates to form the radical inter-
mediate 1. The second half reaction starts first with the
oxidation of the sacrificial agent, usually an amine, by the
photogenerated hole on the highest occupied molecular
orbital (HOMO) level, leading to the formation of cationic
radical of the amine and transfer of the extracted electron to
the lowest unoccupied molecular (LUMO) level of the
catalyst. The full reaction cycle is completed by the electron
transfer from the radical intermediate 1 to the oxidized amine
radical and the cationic intermediate 2, forming the final
product after losing one proton.
To investigate separately the electron transfer from the
photocatalyst onto the halide, we conducted first the light-
induced dehalogenation reaction of haloketone derivatives
using Series 1 as the photocatalysts and diisopropylethyl-
amine (DIPEA) as the electron-donating sacrificial agent
(Figure 3a and Table S1 in the Supporting Information).
Under the irradiation of a household energy saving light bulb
(23 W), Th-BT-Th exhibited the highest reaction rate (Fig-
ure 3b), reaching full conversion within 1 h with a low catalyst
load (1 mol%), followed by Ph-BT-Ph and BT-Ph. To confirm
the proposed electron-transfer pathway as displayed in Fig-
ure 3a, we then conducted fluorescence quenching experi-
ments using Series 1 as the photocatalyst (Figure S1–S3). It
was shown that the fluorescence of the SMOSs could be
gradually quenched by adding DIEPA, while the addition of
diethyl bromomalonate did not affect the fluorescence.
Another control experiment showed that without using
DIEPA, no product was obtained (entry 2, Table S1). This
result indicates that for the SMOS-based systems, the excited
electron could not be transferred directed to diethyl bromo-
malonate without employing an electron donor. This further
indicates that the electron originated from the sacrificial
Scheme 1. Structures of the two series of the SMOSs.
different electron-donor variations, such as single phenyl ring
(BT-Ph), two phenyl rings (Ph-BT-Ph), and two thiophene
units (Th-BT-Th). In comparison, Series 2 shows the opposite
structural design principle using thiophene as the electron-
donor unit in combination with different benzochalcogena-
diazoles, such as benzooxadiazole (Th-BO-Th), benzothia-
diazole (Th-BT-Th), and benzoselenadiazole (Th-BS-Th) as
the electron acceptor. The synthetic details and character-
izations of the SMOSs are described in the Supporting
Information.
À
We chose the intermolecular C H functionalization of
electron-rich heteroaromates with malonates to investigate
the photocatalytic activity of the SMOSs. The reaction
mechanism could be divided in two half cycles as displayed
2
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bromomalonate (Ered = À1.0 V vs. SCE) and oxidize
triphenylamine (Eoxi =+ 0.89 V vs. SCE). In com-
parison, despite the high oxidation potentials of Th-
BO-Th (Eoxi =+ 1.24 V vs. SCE) and Th-BS-Th
(Eoxi =+ 1.02 V vs. SCE), their reduction potentials
with À1.06 V and À1.08 V (vs. SCE) appeared
insufficient to reduce diethyl bromomalonate, indi-
cating that the reductive half cycle might be the
rate-determining step. The HOMO/LUMO band
positions of the SMOSs and the reduction and
oxidation potentials of the substrates and sacrificial
agents are displayed in Figure 4a. Additionally,
using BT-Ph and Ph-BT-Ph as catalyst lead either to
no or a very low yield of product (entry 7 and 8),
demonstrating the absorption in the visible range is
indispensable. Note that the catalytic efficiency of
Th-BT-Th is comparable with the state-of-art pho-
tocatalysts based on transition metals, such as
[Ru(bpy)3]Cl2, for the same type of photoredox
reactions.[13]
Figure 3. a) Reaction mechanism of light-induced dehalogenation reaction of
haloketones using SMOSs as photocatalysts. b) Debromination rates of a-
bromoacetophenone as model substrate using Series 1 as catalysts. c) HOMO
and LUMO band positions and d) UV/Vis absorption spectra of the SMOSs in
Series 1. R1, R2 =H, aryl, carboxylate.
To obtain more insight of the reaction mecha-
nism and to investigate the concrete electron-trans-
fer pathway in the catalytic cycle, we verified the
role of the sacrificial agent. It was reported that
sacrificial reagents could be replaced by bases to
agent via oxidative extraction by the photogenerated hole.
Similar light-induced electron mediation has been demon-
strated by employing polymeric systems as photo-
catalysts.[6a,b,e,12]
À
Table 1: Photocatalytic C H functionalization of electron-rich heteroar-
omates with diethyl bromomalonate using the SMOSs as photocata-
lysts.[a]
The redox potentials of the SMOSs were derived from the
HOMO and LUMO band positions as displayed in Figure 3c.
Significantly, the highest oxidative and reductive potentials of
Ph-BT (À1.35 V and + 1.83 V vs. saturated calomel electrode
(SCE)) did not lead to the highest reaction rate within the
series. This is because the redox potentials of all three
catalysts are sufficient enough for the oxidation of DIPEA
(+ 0.52 V vs. SCE) and for the reduction of a-bromoaceto-
phenone (À0.99 V vs. SCE). The superior catalytic activity of
Th-BT-Th should rather attributed to its broader absorption
range in the visible region up to 550 nm, while Ph-BTand Ph-
BT-Ph mainly absorb in the UV and blue region (Figure 3d).
The D–A structure of Th-BT-Th was more advantageous for
designing visible light-active photocatalysts.
Entry Catalyst Substrate
Product
Time Conv. Yield
[h]
[%][b] [%][c]
Th-BO-
Th
1
15
>99 82
Th-BT-
Th
2
0.75 >99 86
Th-BS-
Th
3
14
>99 85
>99 78
Th-BT-
Th
To complete the reaction cycle of the photocatalytic
4
2
2
À
intermolecular C H functionalization, we then tested the
functionalization reaction of 3-methylindole and diethyl
bromomalonate using Series 2 with similar absorption
ranges in the visible region (Figure S4). A blue LED lamp
(460 nm, 1.2 WcmÀ2) was used as light source to accelerate
the reaction rate. Unlike for the dehalogenation reaction,
triphenylamine was used instead of DIPEA to avoid the
hydrogen abstraction process by the diethyl malonatyl radical
and to increase the final yield.[11] The results are listed in
Table 1. It was shown that all three SMOSs could catalyze the
reaction in high conversion and yields, with Th-BT-Th again
being the most efficient photocatalyst within Series 2 using 3-
methylindole as model substrate (entry 2). The superior
catalytic efficiency of Th-BT-Th corresponded well with its
redox potential, which is sufficient to both reduce diethyl
Th-BT-
Th
5
>99 87
>99 90
Th-BT-
Th
6
1
6
7
8
Ph-BT
0
–
5
Ph-BT-
Ph
6
20
[a] Reaction conditions: 1 equiv (0.38 mmol) heteroarene, 2 equiv
diethyl bromomalonate, 2 equiv 4-methoxy-N,N-diphenylaniline,
1 mol% photocatalyst in 2.5 mL DMF, blue LED (l=460 nm).
[b] Determined by GC-MS. [c] Yield of isolated product after purification
over SiO2.
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À0.34 V vs. SCE (Figure S12), making the triphenyl amine
radical cation the most efficient oxidant with a theoretical
overpotential of 1.23 V.
Time-resolved photoluminescence spectroscopy revealed
long fluorescence lifetimes for all SMOSs ranging from
around 12 to 16 ns, indicating long-living photogenerated
excitons for both series (Figure 5a). Furthermore, two
tendencies could also be observed: 1) Series 2 with absorption
Figure 5. a) Fluorescence-delay spectra of the SMOSs and b) HOMO
(bottom) and LUMO (top) of Th-BT-Th calculated at the B3LYP/6-
31G* level.
Figure 4. a) HOMO and LUMO band positions of the SMOSs in
Series 2 and redox potentials of substrates and sacrificial agents.
b) Reaction times with different triphenylamine-based sacrificial agents
using Th-BT-Th as the catalyst.
range mainly in the visible region exhibited much higher
reaction yields than Series 1 which has absorption mainly in
the UV region; 2) Th-BT-Th with the longest fluorescence
lifetime of 15.8 ns showed the highest photocatalytic effi-
ciency (Figure S13). Theoretical calculations using the density
functional theory (DFT) at B3LYP/6-31G*level showed that
the electron densities are mainly focused on the electron-
donating thiophene and -withdrawing benzothiadiazole moi-
eties on the HOMO and LUMO for Th-BT-Th (Figure 5b).
This clearly demonstrates that the D–A-type molecules could
exhibit longer exciton lifetimes, than oligomers based on only
donor moieties, such as thiophene, which usually exhibit
fluorescence lifetimes shorter than 1 ns.[15] These observations
further confirmed that all three parameters, that is, absorption
range, sufficient energy band position, and long exciton
lifetime are required for highly efficient visible-light-driven
catalytic reactions using SMOSs.
In summary, we have developed donor–acceptor (D–A)
based small-molecule organic semiconductors (SMOSs) as
a new class of pure organic and metal-free photocatalytic
systems with tunable absorption range, defined and appro-
priate energy band positions, and long exciton lifetimes. The
photocatalytic activity could be well controlled via variation
of the electron-donor and -acceptor combination in the
neutralize hydrogen halides formed as side products during
the catalytic cycle.[14] However, using K2HPO4, CsCO3,
K2CO3, or KHCO3 as the base for our system did not lead
to any consumption of 3-methylindole, confirming the role of
the amine not only as a base, but also most likely for
mediating the electron transfer from the radical intermedi-
À
ate 1 formed after the C H functionalization of 3-methyl-
indole with the malonate radical back to the photo-generated
hole of the photocatalyst in the full catalytic cycle (Figure 2).
To support this idea, we tested 4-nitro-N,N-diphenylaniline
with an oxidative potential of + 1.15 V (vs. SCE) as sacrificial
agent. By using Th-BT-Th (Eoxi =+ 1.12 V vs. SCE) as photo-
catalyst, no product could be determined, confirming that the
oxidation potential of Th-BT-Th was not sufficient enough to
oxidize 4-nitro-N,N-diphenylaniline, making the electron
transfer from the amine to the HOMO level of the catalyst
impossible (Figure 4a).
Furthermore, two triphenyl amine derivatives, 4-methyl-
N,N-diphenylaniline (Eoxi =+ 0.82 V vs. SCE) and 4-methoxy-
N,N-diphenylaniline (Eoxi =+ 0.74 V vs. SCE) were employed
to manipulate the electron-transfer process from the radial
intermediate onto the oxidized amine. As illustrated in
Figure 4b, the reaction times of 90 min and 360 min of both
amines, respectively, were slower than that of triphenylamine
(Eoxi =+ 0.89 V vs. SCE) with 45 min. Similar observations
could also be made by employing either Th-BO-Th or Th-BS-
Th as the photocatalyst or using different substrates (Fig-
ure S5 and S6). This further confirmed the electron-transfer
pathway as illustrated in Figure 2 as the main electron-
transfer pathway. The other possibility of direct electron
transfer from the radical intermediate or eventually from the
indole should rather play a minor role within the catalytic
cycle. The oxidation potential of the radical intermediate 1 to
its oxidized cation intermediate 2 was calculated to be
À
molecular structure. Using the intermolecular C H function-
alization of electron-rich heteroaromates with malonate
derivatives as a model reaction, the structural design principle
of the SMOSs was demonstrated. The catalytic efficiency of
the SMOSs was comparable to that of state-of-art catalysts,
such as [Ru(bpy)3]Cl2. Furthermore, the mechanistic insight of
the light-induced electron transport between the organic
photocatalyst, substrate, and the sacrificial electron-donating
agent confirmed that not only the finely controlled redox
potential of the photocatalyst, but also the oxidation potential
of the amine-based sacrificial agent could enhance the
4
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Acknowledgements
We thank the Max Planck Society for financial support. L.W.,
W.H.,and R.L. thank the China Scholarship Council (CSC)
for scholarships.
À
Keywords: C H functionalization · metal-free catalysts ·
photocatalysis · small-molecule organic semiconductor ·
visible light
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Received: April 19, 2016
Published online: && &&, &&&&
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Photoredox Catalysis
Give and take: Small-molecule organic
semiconductor donor–acceptor systems
with a tunable absorption range and
defined energy-band structure are
designed as metal-free, pure organic,
visible-light-active photocatalysts to meet
crucial requirements, such as absorption
in the visible region, sufficient light-
generated redox potential, and long exci-
ton lifetime.
L. Wang, W. Huang, R. Li, D. Gehrig,
P. W. M. Blom, K. Landfester,
K. A. I. Zhang*
&&&& — &&&&
Structural Design Principle of Small-
Molecule Organic Semiconductors for
Metal-Free, Visible-Light-Promoted
Photocatalysis
6
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!