Electron donor-free photoredox catalysis: Via an electron transfer cascade by cooperative organic photocatalysts

Article abstract of DOI:10.1039/c8cy01072b

Electron-donating sacrificial reagents are highly important for certain photo-redox reactions. However, the use of excessive amounts of sacrificial reagents, mostly amines, often leads to undesired side products and is especially troublesome for product purification. Herein, we take the light-induced electron transfer cascade process of natural photosystems as a role model and assemble organic photocatalysts into cooperative photocatalyst couples. The cooperative photocatalyst couples could undergo intermolecular electron transfer to facilitate the charge separation process and overcome the need for an extra electron donor. Time-resolved photoluminescence spectroscopy was conducted to precisely characterize the photo-excited dynamics within the cooperative photocatalyst couples. As a model photoredox reaction, the carbon-carbon formation reaction between heteroarenes and malonates, which usually requires electron-donating sacrificial reagents such as amines, was conducted to demonstrate the feasibility of the cooperative photocatalyst couples under visible light irradiation. A significant reaction conversion improvement from trace conversion for a single photocatalyst system to over 90% by cooperative photocatalyst couples was achieved.

Full text of DOI:10.1039/c8cy01072b

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ARTICLE
Electron Donor-free Photoredox Catalysis via Electron Transfer
Cascade by Cooperative Organic Photocatalysts
Received 00th January 20xx,
Accepted 00th January 20xx
Lei Wang, Irina Rörich, Charusheela Ramanan, Paul W. M. Blom, Wei Huang, Run Li, and Kai A. I.
Zhang*
DOI: 10.1039/x0xx00000x
Electron-donating sacrificial reagents are highly important for certain photo-redox reactions. However, the use of usually
excessive amount of sacrificial reagents, mostly amines, often leads to undesired side products and is especially
troublesome for product purification. Herein, we take the light-induced electron transfer cascade process of the nature
photosystems as roll model and assemble organic photocatalysts into cooperative photocatalyst couples. The cooperative
photocatalyst couples could undergo intermolecular electron transfer to ease the charge separation process and
overcome the need of an extra electron donor. Time-resolved photoluminescence spectroscopy was conducted to
precisely characterize the photo-excited dynamics within the cooperative photocatalyst couples. As model photoredox
reaction, the carbon-carbon formation reaction between heteroarenes and malonates, which usually requires electron-
donating sacrificial reagents such as amines, was conducted to demonstrate the feasibility of the cooperative
photocatalyst couples under visible light irradiation. A significant reaction conversion improvement from trace conversion
for single photocatalyst system to over 90% by cooperative photocatalyst couples was achieved.
www.rsc.org/
Nature photosystems offer a promising model to overcome
the charge recombination issue, where a series of stepwise
electron transfer processes is involved to enhance the
photogenerated charge separation and transfer process.12 So
Introduction
Electron-donating sacrificial reagents are of great importance
for photocatalytic redox reactions including chemical
transformation and water decomposition into hydrogen and
oxygen.1-3 Especially for semiconductor-catalyzed reactions,
the catalytic efficiency can be largely improved while the
photo-generated holes can be scavenged by the electron-
donating reagent molecules and the charge recombination can
be reduced.2 Examples of organic photoredox reactions
without the employment of sacrificial reagent were conducted
using reactive substrates such as aryl diazonium4
or trifluoromethanesulfonyl salts5 , as well as when the
solvent molecule could also act as electron donor.6 However,
for specific photoredox reactions involving less reactive
substrates, the use of electron donating reagents is necessary.
And the use of usually excessive amount of the donor reagents
during the catalytic cycle, mostly amines,7-10 often leads to
undesired oxidized side products and is especially troublesome
for product purification.11 To overcome the above-mentioned
challenge and maintain reduced charge recombination within
the semiconductor-based photocatalyst, an enhanced photo-
generated electron transfer pathway should be developed.
far, chemists have designed different photocatalytic systems
for enhancing the photocatalytic efficiency. For example,
bridged transition metal complexes13-15 have been reported to
greatly promote the photogenerated charge separation and
transfer.
Organic semiconductor (OS)-containing photocatalysts
have emerged as a promising alternative to the traditional
metal-containing photocatalysts due to their highly tunable
electronical properties via flexible structural design. Recent
research activities have demonstrated a vast number of
10, 16-20
structural design methods of small molecular8,
or
macromolecular21-30 OS systems as efficient photocatalysts for
visible light-promoted photoredox reactions. Nevertheless,
most developed OS photocatalysts, similar to traditional
transition metal complexes,31-38 are single photocatalytic
systems. As illustrated in Fig. 1a, the photocatalytic ability of
the OS system originates from the photo-generated
electron/hole pair, which can function as the reductive and
oxidative species. To maintain the efficient electron transfer
from the ground to the exited state of the OS, extra sacrificial
agents are usually needed due to direct recombination of the
electron/hole pair. To overcome the necessary use of electron-
donating sacrificial reagents, the single photocatalyst system is
not sufficient; a new concept of catalyst design is needed.
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In this study, three molecular organic semiconductors (OS), in
particular, 4,7-di(thiophen-2-yl) benzo[c][1,2,5]thiadiazole (Th-
BT-Th),
2-butyl-4,7-di(thiophen-2-yl)-2H-benzo[d][1,2,3]
triazole
(
Th-BTz-Th), and 4,7-bis(1-butyl-1H-1,2,3-triazol-4-
yl)benzo[c] [1,2,5]thiadiazole (TA-BT-TA) were designed. The
structures and energy band positions of the OSs are shown in
Scheme 1. The synthetic and characteristic details of the OSs
are described in the Experimental Section or Electronic
Supplementary Information (ESI).
Fig. 1. Difference between (a) conventional single organic photocatalyst system with
direct electron/hole recombination, sacrificial reagent needed; (b) cooperative
photocatalyst system with enhanced electron/hole separation via electron transfer
cascade, no sacrificial reagent needed. OS: organic semiconductor; SA: sacrificial
electron donor reagent; S: substrate.
Herein, we aim to take the mechanism of the light-induced
electron transfer cascade process of nature photosystems as
roll model and assemble organic photocatalysts into
cooperative photocatalyst couples. As illustrated in Fig. 1b, the
two molecular OS photocatalysts within the cooperative
couple consisted of different organic photocatalysts possessing
different highest occupied molecular orbital (HOMO) and
lowest unoccupied molecular orbital (LUMO) levels. By
coupling the OSs, an extra intermolecular electron transfer
could occur between the OSs, leading to enhanced photo-
generated electron/hole separation under light irradiation and
thereby more stable reductive and oxidative species. The
intermolecular electron transfer within the OS couple can
decay the direct recombination of the electron/hole pair and
overcome the mandatory requirement of extra electron
donors during the photoredox cycle. As model reaction, the
photocatalytic carbon-carbon formation reaction between
electron-rich heteroarenes and malonates, which usually
required electron-donating sacrificial reagents,9, 39 was chosen
to demonstrate the feasibility and photocatalytic activity of
the cooperative photocatalyst couple. No extra sacrificial
reagents were needed. The light-induced electron transfer
between the OSs within the cooperative photocatalyst couple
and the substrates could be revealed by mechanistic studies.
Fig. 2. (a) UV/Vis absorption and (b) fluorescence spectra of the three OS molecules.
(c), (d) PL decay times of the pristine materials and the blend of the cooperative
couples with exciton lifetimes collected at the fluorescence maximum. (e), (f) Jablonski
diagram illustrates energy and charge transfer in the photocatalyst couples,
respectively. Excitation wavelength: 400 nm. EET: excitation energy transfer; ET:
electron transfer.
The three OSs possessed different HOMO and LUMO band
positions, which could be determined via cyclic voltammetry
(Fig. S1). As shown in Scheme 1, Th-BTz-Th exhibited a
significantly high LUMO level at -1.68 V vs. saturated calomel
electrode (SCE), followed by TA-BT-TA with -1.20 V vs. SCE and
Th-BT-Th with -1.15 V vs. SCE. In comparison, TA-BT-TA
exhibited the lowest HOMO level at +1.54 V vs. SCE, followed
by Th-BT-Th with +1.12 V vs. SCE and Th-BTz-Th with +0.95 V
vs. SCE. In addition, the excited state redox potentials are
calculated to be +1.16 V and -1.01 V vs. SCE for Th-BT-Th,
+0.62 V and -1.35 V vs. SCE for Th-BTz-Th, and +1.14 V and -
0.80 V vs. SCE for TA-BT-TA, respectively. The date are listed in
Table S1 in the ESI using the calculation described in previous
report.40 Given the different energy band positions, we
assembled the three OSs in two photocatalyst couples for this
Results and discussion
Scheme 1. Structure and HOMO/LUMO levels (vs. SCE) of the OS molecules in both
designed photocatalyst couples.
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study, particularly, Th-BTz-Th/Th-BT-Th for cooperative the three OSs and thereby the largest overpotential for the
photocatalyst couple 1 and Th-BTz-Th/TA-BT-TA for couple 2.
Steady-state fluorescence quenching experiments showed
that by only exciting Th-BT-Th (at 500 nm) or TA-BT-TA (at 450
nm) in both couples, the fluorescence intensity of couple 2
exhibited a larger decrease in PL intensity than that of couple
1, indicating an enhanced electron transfer within couple 2,
which was likely caused by the larger distance between the
two HOMO levels (Fig. S2).
reduction of diethyl bromomalonate.
Table 1. Photocatalytic C−C bond formaꢀon between diethyl bromomalonate and
heteroarenes using different photocatalyst systems.a
Conv
Yield
[%]c
Entry
1
Catalyst
Substrate
Product
[%]b
To further study the photo-induced charge transfer within
the couples, time-resolved photoluminescence (TRPL)
spectroscopy was conducted to characterize the photo-excited
dynamics within the OS couples. The PL measurements of the
pristine materials and a 1:1 (mol-%) blend were all performed
in DMF solution with excitation at 400 nm (Fig. 2c and 2d). The
selected PL decays correspond to the emission maximum for
each component OS (Fig. 2c). The fluorescence features of the
different component materials overlap with each other.
Therefore, decay associated spectra (DAS) were derived by a
global fitting analysis41 of the TRPL data (Fig. S2). This allows us
to characterize the spectral distribution and lifetime of each
emitting species, particularly for the measurements of couple
1 and couple 2. In couple 1 (Fig. S2a), one can see that DAS 1,
centered at 460 nm, matches exactly with the fluorescence
spectrum of Th-BTz-Th and decays in 0.6 ns, which is quenched
with respect to the pristine donor (2.6 ns). DAS 2, which is
centered at 590 nm, matches the fluorescence spectrum of Th-
BT-Th and decays with the same ~12 ns lifetime as the pristine
Th-BT-Th sample, exhibiting no quenching. This indicates that
in couple 1 excitation energy transfer (EET) from Th-BTz-Th to
Th-BT-Th likely dominates the excited state dynamics, as
illustrated in Fig. 2e. In couple 2 (Fig. S2b), the DAS 1,
centered at 460 nm, is again originating from Th-BTz-Th and
decays in 0.97 ns, which is quenched with respect to the
pristine Th-BTz-Th (2.6 ns). DAS 2, centered at 530 nm, is
assigned to the emission of TA-BT-TA and has a decay time of
10.6 ns. Here, the fluorescence of both Th-BTz-Th and TA-BT-
TA are quenched with respect to the pristine materials. This
indicates an electron transfer from Th-BTz-Th to TA-BT-TA and
an enhanced charge transfer within couple 2 (Fig. 2f).
Th-BT-Th
trace
-
-
2
TA-BT-TA
0
3
4
Th-BTz-Th
Couple 1
9
-
43
39
5
Couple 2
Couple 2
93
98
87
85
6d
7e
Couple 2
Couple 2
78
92
75
85
8e
9e
Couple 2
85
72
80
60
Th-BTz-
Th/TA-BT-
TA (2:1)
Connected
cat.
10f
11f
12g
13h
31
0
22
-
-
We chose the photocatalytic carbon-carbon formation
reaction between electron-rich heteroarenes and malonates as
model reaction.9,
Couple 2
0
-
10
Previous studies demonstrated the
mandatory use of sacrificial reagents, mostly amines, which
aReaction conditions: 1 equiv. (0.38 mmol) heteroarene, 2 equiv. diethyl
bromomalonate, 0.1 equiv. photocatalyst (for the coupled system, the ratio
ought not to only act as electron donor, but also as electron
mediator between the radical intermediate and the photo- between the two OSs was 1:1) in 2.5 mL DMF, white LED lamp (0.07 Wcm-2), 24 h.
bDetermined by GC-MS. cIsolated yield. dblue LED (450 nm, 0.1 Wcm-2). e48h.
f0.05 equiv. photocatalyst, 24h. gno catalyst, under light. hno light.
generated hole of the photocatalyst (Fig. S4). We first tested
the three designed OSs as single photocatalyst system without
using sacrificial reagent. As expected, by using 3-
In contrast, by using both cooperative photocatalyst
methylbenzofuran and diethyl bromomalonate in the model
couples, the product could be obtained in significantly higher
reaction, no product was obtained using Th-BT-Th and TA-BT-
conversions, in particular, 43% for couple 1 and 93% for couple
TA as photocatalysts (entries 1-2 in Table 1). Interestingly,
2, respectively (entries 4-5). Both the apparent quantum yield
using Th-BTz-Th as photocatalyst led to a slight conversion of
(ΦAQY) and catalytic turnover number (TON) for the model
9%. This indicates that a minimal electron transfer could still
reaction (entry 5) using coupling 2 as photocatalyst were
occur between the photocatalyst and the substrate, which
calculated to be 0.04% and 8.7, respectively (ESI). A kinetic
corresponds with the highest LUMO level of Th-BTz-Th among
study confirmed the superior catalytic activity of couple 2
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compared to couple 1 via fluorescence quenching experiment further fluorescence quenching experiments. It was shown
by the substrates (Fig. S6). This is in agreement with the that the fluorescence of the OSs as single photocatalyst could
photophysical study (Fig. 2), indicating that the enhanced be only minimal quenched by adding diethyl bromomalonate
charge transfer between Th-BTz-Th and TA-BT-TA in couple 2 (Fig. S5). As a contrast, the fluorescence of both photocatalyst
led to a better catalytic efficiency than that of couple 1 couples could be gradually quenching by adding diethyl
containing Th-BTz-Th and Th-BT-Th. The reason could lie on bromomalonate (Fig. S6), confirming the possible electron
the larger HOMO energy gap (ΔEHOMO = 0.59 V) between Th- transfer between the photocatalyst couple and the substrate.
BTz-Th and TA-BT-TA than that (ΔEHOMO = 0.17 V) in couple 1
43
By using a single
between Th-BTz-Th and Th-BT-Th.42,
wavelength light source (blue LED, 450 nm), where only TA-BT-
TA of couple 2 could be excited, the model reaction exhibited a
high conversion of 98%. This indicates a hole transfer from TA-
BT-TA to Th-BTz-Th, leading a similar effect of an electron
transfer from Th-BTz-Th back to TA-BT-TA, as illustrated in Fig.
2f.
In comparison to the use of Th-BT-Th as single
photocatalyst with the employment of an extra sacrificial
electron donor, in particular, triphenylamine from our
previous report for the model reaction,44 where the apparent
quantum yield was calculated to be 0.96%, the apparent
quantum yield of the couple 2 was lower due to the longer
reaction time. However, the avoidance of the extra sacrificial
electron donor using the molecular photocatalyst couples is
still advantageous. It can efficiently prevent the post-
purification procedures caused by the addition of usually
excess amount of the amine donors.
Screening experiments using different substrates
confirmed the feasibility of the OS couple as photocatalyst
(entries 7-9). Other control experiments either without the
photocatalyst or without light led to no product formation
(entries 12 and 13), indicating the mandatory roles of both
components.
Fig. 4. Control experiments: (a) Study of the impact of the OS ratio on the reaction
conversion; (b) Comparison between cooperative photocatalyst couple 2 with the C6-
connected molecule Th-BTz-Th@TA-BT-TA; (c) Structure of Th-BTz-Th@TA-BT-TA.
Reaction conditions: 1 equiv. (0.38 mmol) 3-methylbenzofuran, 2 equiv. diethyl
bromomalonate, 0.05 equiv. photocatalyst in 2.5 mL DMF, white LED (0.07 Wcm-2, λ >
420 nm), 24 h, conversion determined by GC-MS.
To precisely study the concrete electron transfer process
within the catalytic cycle, we then employed ethyl
bromoacetate as substrate, which possessed a reduction
potential of -1.43 V vs. SCE (Fig. 3). By using couple 2 as
photocatalyst, it could be determined that only traces of the
final product could be obtained (Fig. S11). This reveals that the
photo-generated electron could indeed only transferred from
the LUMO of Th-BTz-Th (-1.68 V vs. SCE) to the LUMO of TA-
BT-TA (-1.20 V vs. SCE) in the first step, from which the further
electron transfer to the higher reduction level of ethyl
bromoacetate was not possible. Additionally, a direct electron
transfer from Th-BTz-Th to ethyl bromoacetate could not occur
either. In comparison, diethyl bromomalonate (Ered.=-1.00 V vs.
SCE) could be reduced by via electron transfer originated from
the LUMO of TA-BT-TA, leading to the formation of the
malonate radical, which further reacts with 3-
methylbenzofuran to form the final in the following step of
the catalytic cycle. An additional model reaction using both Th-
BTz-Th as the single photocatalyst system and triphenylamine
as sacrificial reagent led to the formation of the final product,
confirming that a direct electron transfer from Th-BTz-Th to
ethyl bromoacetate was only possible in the absence of TA-BT-
TA (Fig. S12). It is also worth to note that further scope
Fig. 3. Study of the electron transfer pathway using comparative substrates with
different reduction potentials. Light source: white LED lamp (0.07 Wcm-2, λ > 420 nm).
Interm.: intermediate.
To gain more mechanistic insight of the interaction
between the photocatalyst and substrates, we conducted
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reactions with other substrates led to high product yields using enhanced photo-induced intermolecular charge transfer
couple as phototocatalyst, demonstrating general between the organic photocatalysts within the couple. By the
2
a
feasibility of the cooperative photocatalyst design. It is worth cooperative photocatalyst design, no extra electron-donating
to note that a possible radical chain mechanism as introduced sacrificial reagents are needed and a new paradigm in the
by Yoon et al.45 could be clearly ruled out due to our previous photoredox reaction without undesired side reaction under
study on the influence of the oxidation potential of the the oxidation of the sacrificial reagents can be established. We
photocatalysts on the reaction rate.10
believe that the cooperative photocatalyst design ought not to
The reason for the lower efficiency using the cooperative be limited in chemical transformation reactions; it can also be
photocatalyts couple in comparison to the one catalyzed in the applied in the field of photocatalytic water splitting or CO2
presence of sacrificial reagents might be caused by different reduction.
factors. The multiple step electron transfer within the
photocatalyst couple and further to the substrate might not be
Experimental
efficient due to the low concentration of the catalysts. Another
important factor might be the electron transfer steps between
Materials and Methods
the intermediates and the photogenerated hole. For this point,
Chemicals were purchased from commercial sources and used
an additional study was conducted as described below.
as
received
without
further
purification.
4,7-
The photo-induced electron transfer within the
cooperative photocatalyst couple usually occurs based on the
rather random contact between the two OSs in the liquid
reaction medium. To create a non-random contact and
thereby a more defined electron transfer process, we then
connected Th-BTz-Th and TA-BT-TA via C6 alkyl chain as
molecular bridge as a control photocatalyst. As displayed in
Fig. 4c, a large molecule of Th-BTz-Th@TA-BT-TA containing
both OSs in couple 2 was synthesized. Remarkably, the
fluorescence intensity of Th-BTz-Th/TA-BT-TA strongly
decreased after connection compared to their unconnected
form as couple 2 (Fig. S8). This result suggests a strong
interaction between two OSs inside the large molecule. A
study on the impact of Th-BTz-Th/TA-BT-TA in couple 2 on
their photocatalytic efficiency showed that the highest
reaction rate was achieved by a ratio of 2:1 for of Th-BTz-
Th/TA-BT-TA (Fig. 4a), which in fact corresponds to the
connected molecule of Th-BTz-Th@TA-BT-TA. However, when
comparing the catalytic efficiency between the connected
catalyst of Th-BTz-Th@TA-BT-TA and unconnected couple 2
with the same ratio between both OSs, only a decreased
conversion was observed for the connected catalyst (Fig. 4b).
In spite of the better electron transfer occurred within
connected system, the reduced catalytic efficiency could be
likely caused by the sterical hindrance within the large
molecule or the competing charge recombination process,46
which limits the contact between the substrate and the
individual OSs in the large molecule and efficient electron
transfer between the catalyst and substrate. To overcome this
limitation, new molecular bridges other than the C6 alkyl chain
could be needed.
dibromobenzo[c][1,2,5]thiadiazole (97%) was bought from
Combi-Blocks. Sodium tert-butoxide (97%), tert-Butanol
(99.5%), Sodium borohydride (96%), 1-Bromobutane (99%), 2-
(Tributylstannyl)thiophene
(97%),
Ethynyltrimethylsilane
(98%), Sodium ascorbate (98%), Pd(PPh3)Cl2 (98%), diethyl
bromomalonate (92%), 3-Methylbenzofuran (97%), 1-
Methylindole (97%) and 3-Methylindole (98%) were bought
from Sigma-Aldrich. Other common used solvents were bought
from Fisher Scientific and VWR International. Glassware was
°
dried under heating by a heating gun at 600 C and cooled
under vacuum prior to use if necessary. All photocatalyst
reactions were conducted using common dry, inert
atmosphere techniques via a Schlenk tube. White LED (Silent
Air System White OL M-018) is a product of OSA Opto Light
Gmbh with an intensity of 0.07 Wcm-2 from a distance of 5 cm.
UV-Vis absorption and emission spectra were recorded on a
Perkin Elmer Lambda 100 spectrophotometer and J&M TIDAS
spectrofluorometer at ambient temperature, respectively.
Cyclic voltammetry measurement was performed with Autolab
PGSTAT204 potentiostat/galvanostat of Metrohm (a three
electrode cell system): glassy carbon electrode as the working
electrode, Hg/HgCl2 electrode as the reference electrode,
platinum wire as the counter electrode, and Bu4NPF6 (0.1 M
Acetonitrile) as supporting electrolyte with a scan rate of 100
mV/s in the range of -2 eV to 2.5 eV. H and 13C spectra were
1
recorded on a Bruker Avance 300 spectrometer, and d-CHCl3
was used as solvent unless otherwise noted. GC-MS spectra
were recorded on
a
SHIMADZU GCMS-QP2010 gas
spectrometer. Time-resolved
chromatograph mass
photoluminescence (TRPL) on a nanosecond timescale was
taken with a Streak Camera System (Hamamatsu C4742) in
slow sweep mode. The excitation wavelength of 400 nm was
provided using the frequency-doubled output of a commercial
titanium:sapphire amplifier (Coherent LIBRA-HE, 3.5 mJ, 1 kHz,
100 fs).
Conclusions
In conclusion, we have taken the light-induced electron
transfer cascade process in nature photosystems as role model
and developed a new sacrificial reagent-free photoredox
pathway by employing molecular organic semiconductors as
cooperative photocatalyst couples. The photocatalyst couples
consists of two molecular organic semiconductors with
complementary HOMO/LUMO band positions, leading to
Synthesis of the molecular organic semiconductor molecules
Synthesis of Th-BT-Th44
A 100 ml Schlenk tube equipped with a stirring bar and stopper
was heated under vacuum then cooling three times before 4,7-
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dibromo-2,1,3-benzothiadiazole (1.76 g, 6.00 mmol), 2- twice, dried by anhydrous MgSO4, solvent was removed under
(tributylstannyl)thiophene (4.76 ml, 15 mmol) and Pd(PPh3)2Cl2 vacuum. Purified with a column using CH2Cl2/Hexane = 1 : 1 as
(201.60 mg, 5 mol%) were added. 30 ml anhydrous THF was eluent give the final product as white solid (1.28 g, 45%). 1H
added via syringe and the reactor was degassed under vacuum NMR (300 MHz, CDCl3)
δ : 7.37 (s, 2H), 4.72 (t, J = 7.4 Hz, 2H),
and then back filled with nitrogen three times. It was heated 2.06 (m, 2H), 1.41-1.29 (m, 2H), 0.91 (t, J = 7.4 Hz, 3H). 13C
under 90 oC overnight, then poured into 200 ml water and NMR (300 MHz, CDCl3)
extracted with CH2Cl2 three times. The organic layer was 32.17, 19.82, 13.51.
washed with water twice, dried by anhydrous MgSO4, solvent
δ : 143.71, 129.51, 109.97, 57.22,
was removed under vacuum. Purified with a short column then Synthesis of Th
crystalized from methanol give a needle like reddish solid (1.14 stirring bar and stopper was heated under vacuum then
g, 63%). 1H NMR (300 MHz, CDCl3)
: 8.04 (dd, J = 3.7, 1.2 Hz, cooling three times before 4,7-dibromo-2-(n-butyl)-2H-
(1 g, 3.00 mmol) 1c), 2-
-BTz-Th: A 100ml Schlenk tube equipped with a
δ
2H), 7.88 (s, 2H), 7.39 (dd, J = 5.1, 1.2 Hz, 2H), 7.15 (dd, J = 5.2, benzo[d]-[1,2,3]triazole
(
3.7 Hz, 2H). 13C NMR (300 MHz, CDCl3)
128.02, 127.52, 126.81, 126.02, 125.79.
δ
: 152.66, 139.36, (tributylstannyl)thiophene (2.38 ml, 7.5 mmol) and
Pd(PPh3)2Cl2 (100.8 mg, 5 mol%) were added. 15 ml anhydrous
THF was added via syringe and the reactor was degassed
under vacuum and then back filled with nitrogen three times.
Synthesis of Th-BTz-Th
o
It was heated under 90 C overnight, then poured into 200 ml
water and extracted with CH2Cl2 three times. The organic layer
was washed with water twice, dried by anhydrous MgSO4,
solvent was removed under vacuum. Purified with a column
using CH2Cl2/Hexane = 1 : 1 as eluent give the final product as
light red solid (713 mg, 70%). 1H NMR (300 MHz, CDCl3)
δ : 8.03
(dd, J = 3.7, 1.2 Hz, 2H), 7.56 (s, 2H), 7.31 (dd, J = 5.1, 1.2 Hz,
2H), 7.12 (dd, J = 5.1, 3.7 Hz, 2H), 4.76 (t, J = 7.4 Hz, 2H), 2.16-
2.07 (m, 2H), 1.44-1.34 (m, 2H), 0.94 (t, J = 7.4 Hz, 3H). 13C
NMR (75 MHz, CDCl3)
δ : 142.10, 139.97, 128.12, 126.97,
125.55, 123.59, 122.78, 56.61, 32.08, 19.89, 13.57.
Synthesis of 1a: The suspension of 4,7-dibromobenzo[c]-1,2,5- Synthesis of TA-BT-TA
thiadiazole (2.94 g, 10 mmol) in 100 ml methanol was
S
S
previously cooled to -5 oC in water-ice bath. Then sodium
borohydride (8 g, 21.7 mmol) was added portion wise to the
suspension. The mixture was warmed up to room temperature
and stirred overnight. Then, 500 ml water was added, the
mixture was extracted with CH2Cl2, the organic layer was
washed with water twice, dried by anhydrous MgSO4, solvent
was removed under vacuum to give the final product as a pale
yellow solid in 80% yield. The H-NMR was consistent with the
literature and used without further purification.47
N
N
N
N
N
Si
NEt3, CuI
Si
Si
Br
Br
Pd(PPh3)2Cl2, THF, 60 o
C
2a
S
S
N
N3
N
N
C4H9
C4H9
N
N
N
N
KOH
N
N
MeOH
CuSO4, Na-ascorbate
t-BuOH, H2O, 50 o
2b
TA-BT-TA
C
Synthesis of 2a: A 100ml Schlenk tube equipped with a stirring
bar and stopper was heated under vacuum then cooling three
times before 4,7-dibromobenzothiadiazole (3 g, 10.2 mmol),
Pd(PPh3)2Cl2 (143.2 mg, 2%), CuI (77 mg, 4%) were added. 30
ml THF and 10ml NEt3 were added via syringe. The mixture was
cooled to freeze in liquid N2, then trimethylsilyl acetylene (4
ml, 30.6 mmol) was added. The reactor was degassed via the
freeze-pump-thaw method and reacted under 50 oC overnight.
Then it was poured into 100 ml water and extracted with
CH2Cl2 three times. The organic layer was washed with water
twice, dried by anhydrous MgSO4, solvent was removed under
vacuum. Purified with a column using CH2Cl2/Hexane = 1 : 1 as
eluent give the final product as light yellow solid (2.78 g, 83%).
The H-NMR was consistent with the literature reported.48
Synthesis of 1b: Dibromobenzene-1,2-diamine (2 g, 7.5 mmol)
(
1a) was added to a flask containing 15 ml glacial acetic acid.
Sodium nitrite (1.03 g, 15 mmol) was dissolved in 10 ml water
and added dropwise to the solution. The mixture was further
reacted for another 3 hours before it was cooled by ice to 0 oC.
The suspension was filtrated, washed with water until PH
reaches 7. The crude product was dried in oven at 70 oC under
vacuum overnight. The H-NMR was consistent with the
literature1 and used without further purification.
Synthesis of 1c: To a 100 ml schlenk tube was added 4,7-
dibromo-2H-benzo[d][1,2,3]triazole (2.4 g, 8.6 mmol)(1b) and
Nat-OBu (1.24 g, 12.9 mmol), then 20 ml methanol and 1-
bromobutyl(4.3 ml, 40 mmol) was added via syringe. The
mixture was degassed under vacuum slightly then back filled
with N2 three times. After stirred under 90 oC for 24h, the
mixture was poured into 200 ml water and extracted with
CH2Cl2 three times. The organic layer was washed with water
Synthesis of 2b: In a 100 ml two neck flask was added KOH
methanol solution (1 mol L-1, 50 ml) and 4,7-bis[2-
(trimethylsilyl)ethynyl]benzothiadiazole (1 g, 3 mmol)(2a). The
solution was stirred under room temperature for 2 h, and then
6 | J. Name., 2012, 00, 1-3
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COMMUNICATION
it was poured into 300 ml water and extracted with CH2Cl2 under vacuum. Purified with a column using CH2Cl2/Hexane = 1
three times. The organic layer was washed with water twice, : 1 as eluent give the final product as a white solid (1.82 g,
dried by anhydrous MgSO4, solvent was removed under 41%). 1H NMR (300 MHz, CDCl3)
δ : 7.38 (s, 2H), 4.73 (t, J = 7.4
vacuum. Purified with a column using CH2Cl2/Hexane = 1 : 1 as Hz, 2H), 3.33 (t, J = 7.4 Hz, 2H), 2.16-2.06 (m, 2H), 1.84-1.75
eluent give the final product as light brown solid (420 mg, (multi, 2H), 1.50-1.39 (multi, 2H), 1.34-1.29 (multi, 2H). 13C
876%). The H-NMR was consistent with the literature NMR (75 MHz, CDCl3)
reported.48
δ : 143.76, 129.64, 110.00, 57.24, 33.58,
32.39, 29.99, 27.54, 25.70.
TA-BT-TA
:
4,7-Diethynylbenzothiadiazole (400 mg, 2.17 Synthesis
of
3b
:
4,7-Dibromo-2-(bromohexane)-
mmol)(2b), 1-azidobutane (synthesis 1-azidobutane: a mixture benzo[d][1,2,3]triazole (1.8 g, 2.27 mmol)(3a), sodium azide
of 10 mmol 1-bromobutane, 20 mmol sodium azide in 100 ml (295 mg, 4.54 mmol) were dissolved in 20 ml DMSO and
DMSO was stirred over night at RT, then the reaction mixture stirred under room temperature overnight. Then the mixture
was poured into 500ml water and extracted with CH2Cl2 then was poured into 200 ml water and extracted with CH2Cl2 three
dried by anhydrous MgSO4, solvent was removed under times. The organic layer was washed with water twice, dried
vacuum, used as synthesized without column purification. by anhydrous MgSO4, solvent was removed under vacuum.
Caution: the excessive amount of sodium azide in water phase Product was used directly for the next step without any
muss be slowly added to
a solution of NaOCl before purification.
deposition.), CuSO4 (51.4 mg, 0.32 mmol), Na-ascorbate (87
mg, 0.44 mmol), 20 ml t-BuOH, 2 ml H2O were added into a Synthesis of 3c: In a 100 ml two neck flask was added 4,7-
100 ml two nech flask. The reactor was degassed under diethynylbenzothiadiazole (184 mg, 1 mmol)(3b), 4,7-dibromo-
vacuum and then back filled with nitrogen three times. The 2-(bromohexane)-benzo[d][1,2,3]triazole, CuSO4 (24 mg, 0.15
o
mixture was heated under 60 C overnight, then poured into mmol), Na-ascorbate (41 mg, 0.21 mmol), 10 ml t-BuOH, 1 ml
200 ml water and extracted with CH2Cl2 three times. The H2O. The reactor was degassed under vacuum and then back
organic layer was washed with water twice, dried by filled with nitrogen three times. The suspention was stirred
o
anhydrous MgSO4, solvent was removed under vacuum. under heating at 60 C for two days, then it was poured into
Purified with a column using CH2Cl2/Acetone = 10:1 as eluent 100 ml water and extracted with CH2Cl2 three times. The
then recrystallized from methanol give the final product as a organic layer was washed with water twice, dried by
sheet-like yellow solid (747 mg, 90%). 1H NMR (300 MHz, anhydrous MgSO4, solvent was removed under vacuum.
CDCl3)
1.90 (m, 4H), 1.43-1.32 (m, 4H), 0.94 (t, J = 7.4 Hz, 6H). 13C give the final product as a yellow solid (750 mg, 76%). H NMR
NMR (75 MHz, CDCl3) : 152.29, 143.10, 126.08, 123.74, (300 MHz, CDCl3) : 8.66 (s, 2H), 8.59 (s, 2H), 7.36 (s, 4H), 4.72
δ : 8.67 (s, 2H), 8.61 (s, 2H), 4.44 (t, J = 7.4 Hz, 4H), 2.00- Purified with a column using CH2Cl2/Aceton = 20:1 as eluent
1
δ
δ
122.66, 50.27,32.37, 19.79, 13.52.
(t, J = 7.4 Hz, 4H), 4.42 (t, J = 7.4 Hz, 4H), 2.16-2.07 (m, 4H),
2.00-1.94 (m, 4H), 1.42-1.39 (m, 8H). 13C NMR (75 MHz, CDCl3)
Synthesis of the connected catalyst Th-BT-Th@TA-BT-TA
δ : 152.26, 143.74, 143.14, 129.65, 126.15, 123.82, 122.58,
109.98, 57.12, 50.30, 30.11, 29.86, 25.94.
H
N
C6H12N3
N
N
C6H12Br
N
N
N
N
N
N
NaN3, DMSO
Nat-OBu, 1,6-DiBrhexane
Br
Br
Br
Br
Br
Br
MeOH, 90 o
C
Th-BT-Th@TA-BT-TA: In a capped vial, 3c (750 mg, 0.76 mmol),
2-(tributylstannyl)thiophene (1.64 ml, 4.56 mmol) and
Pd(PPh3)2Cl2 (53 mg, 10 mol%) were added. 10 ml anhydrous
THF was added via syringe and the reactor was degassed via
bubbling nitrogen for 5 min. The vial was put in a microwave
3b
3a
S
Br
N
N
Br
S
N
N
N
N
N
N
N
Br
N
Br
N
N
N
N
N
CuSO4, Na-ascorbate
t-BuOH, H2O, 60 o
C
N
o
reactor (120 C, 0.8 T) for 1 h, then poured into 100 ml water
3c
and extracted with CH2Cl2 three times. The organic layer was
washed with water twice, dried by anhydrous MgSO4, solvent
was removed under vacuum. Purified with a column using
S
S
S
SnBu3
S
N
N
N
N
N
N
N
Pd(PPh3)2Cl2, THF
microwave
N
N
N
N
N
S
S
CH2Cl2/Acetone = 20:1 as eluent give the final product as a
1
yellow-red solid (403 mg, 53%). H NMR (300 MHz, CDCl3)
N
N
δ :
Th-BT-Th@TA-BT-TA
8.59 (s, 2H), 8.55 (s, 2H), 7.98 (dd, J = 3.7, 1.2 Hz, 4H), 7.52 (s,
4H), 7.29-7.27 (m, 4H), 7.08 (dd, J = 5.1, 3.7 Hz, 4H), 4.74 (t, J =
7.4 Hz, 4H), 4.39 (t, J = 7.4 Hz, 4H), 2.16-2.12 (m, 4H), 1.98-1.93
Synthesis of 3a: To a 100 ml schlenk tube was added 4,7-
dibromo-2H-benzo[d][1,2,3]triazole (3 g, 10.8 mmol) and Nat-
OBu (1.56 g, 16.2 mmol), then 40 ml methanol and 1,6-
dibromohexane (8.3 ml, 54 mmol) was added via syringe. The
mixture was degassed under vacuum slightly then back filled
with N2 three times. After stirred under 90 oC for 24 h, the
mixture was poured poured into 200 ml water and extracted
with CH2Cl2 three times. The organic layer was washed with
water twice, dried by anhydrous MgSO4, solvent was removed
(m, 4H), 1.45-1.42 (m, 8H). 13C NMR (75 MHz, CDCl3)
δ :
152.21, 143.15, 142.09, 139.84, 128.10, 126.92, 126.04,
125.63, 123.72, 123.55, 122.82, 122.59, 56.52, 50.30, 30.15,
29.70, 26.03, 25.97.
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Journal Name
2.
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photocatalyst couple system (declared amount and ratio as
described in Table 2), diethyl bromomalonate (0.76 mmol, 2.0
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the freeze-pump-thaw method and irradiated under a white
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9.
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Comparison control experiment for the C-C bond formation
reactions
between
3-methylbenzofuran
and
ethyl
11.
12.
13.
14.
15.
16.
17.
18.
bromoacetate using Th-BTz-Th as catalyst and 4-
methoxyltriphenylamine as electron-donating sacrificial
reagent
A 25 ml Schlenk tube equipped with a stirring bar and stopper
was heated under vacuum then cooling for one time and then
back filled with argon before 3-mehtylbenzofuran (0.38 mmol,
1 equiv), 4-methoxyltriphenylamine (0.76 mmol, 2 equiv),
ethyl bromomalonate (0.76 mmol, 2.0 equiv), Th-BTz-Th (1
mol%), 2.5 ml DMF were added. The reactor was degassed via
the freeze-pump-thaw method and irradiated under a white
LED. The reaction time was determined by GC-MS when the
signal of starting material totally disappeared. After the
reaction was completed, the mixture was poured into 20 ml
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anhydrous MgSO4, and the solvent removed under vacuum.
The crude product was purified on silica gel using the indicated
solvent system to offer the desired product. 1H NMR (300
19.
20.
21.
22.
MHz, CDCl3)
7.4 Hz, 2H), 3.69 (s, 3H), 2.13 (s, 3H), 1.19 (t, J = 7.4 Hz, 3H). 13
NMR (75 MHz, CDCl3) : 169.14, 154.15, 146.12, 129.91,
δ: 7.40-7.32 (m, 2H), 7.20-7.11 (m, 2H), 4.11 (q, J =
C
δ
123.92, 122.23, 119.13, 112.81, 110.95, 61.35, 32.84, 14.19,
7.97. Molecular weight C13H14O3: calculated 218.09, found
218.10.
23.
24.
25.
26.
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Acknowledgements
The authors thank the Max Planck Society for financial
support. L.W. thanks the China Scholarship Council (CSC) for
scholarship. The work of I. R. is part of the research program of
the Dutch Polymer Institute (project #763).
R. S. Sprick, J.-X. Jiang, B. Bonillo, S. Ren, T. Ratvijitvech, P.
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27.
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29.
Conflict of interest
The authors declare no conflict of interest.
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