Tandem synthesis of photoactive benzodifuran moieties in the formation of microporous organic networks

Article abstract of DOI:10.1002/anie.201300655

Tiny pores: Benzodifuran moieties were introduced into microporous organic networks (MONs) through a tandem process consisting of Sonogashira coupling of 1,3,5-triethynylbenzene and 2,5-diiodo-1,4-hydroquinone and intramolecular cyclization. The resultant

Full text of DOI:10.1002/anie.201300655

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Chemie
DOI: 10.1002/anie.201300655
Porous Organic Materials
Tandem Synthesis of Photoactive Benzodifuran Moieties in the
Formation of Microporous Organic Networks**
Narae Kang, Ji Hoon Park, Kyoung Chul Ko, Jiseul Chun, Eunchul Kim, Hee-Won Shin,
Sang Moon Lee, Hae Jin Kim, Tae Kyu Ahn,* Jin Yong Lee,* and Seung Uk Son*
Over the last decade, microporous organic materials have
been extensively prepared through various coupling reac-
tions.[1] In the early stages, relatively simple aromatic building
blocks were used to prepare microporous organic networks
(MONs) and relevant studies have focused on their physi-
sorption behavior toward gas guests. Recently, more specific
functionalities were achieved by the introduction of designed
active sites into MONs.[2] Usually, the active sites could be
introduced by using the predesigned building blocks or by
postmodification[3] of the porous materials. If the active sites
could be concomitantly formed in the network formation
process for porous materials, this synthetic process would be
very efficient and ideal for functional materials.[4] For
example, we have demonstrated the successful incorporation
of active N-heterocyclic carbene metal species into metal–
organic frameworks (MOFs) during self-assembly
processes.[4a–c] However, this kind of synthetic approach is
relatively rare, especially in the synthesis of MONs.[4d]
Benzodifurans (BDFs) are very interesting materials
owing to their unique optical and electrical properties.[5]
Their electron-rich nature has enabled them to be applied
as redox-active hole transfer materials in organic light-
emitting devices.[5a] Moreover, very recently, anti-benzodi-
furan-based organic materials have attracted significant
attention as photo- and redox-active materials in solar cells
and organic field-effect transistors.[5b] anti-Benzodifurans can
be prepared in the intramolecular cyclization reaction of 1,4-
hydroquinone with two alkyne groups at the 2,5-positions.
Generally, tandem reactions in organic synthesis can be
defined as a consecutive series of intramolecular reactions.[6]
In well-designed tandem processes, the functional groups for
the following successive reactions can be generated in situ as
a result of the previous reaction. Through the introduction of
tandem processes to organic synthesis, the synthetic strategies
become more atom-economical, because the work-up and
isolation processes for intermediates can be reduced. Thus,
much effort has been made for the development of smart
tandem processes for complicated target organic materials.[6]
The Cooper research group and others have shown that
MONs can be prepared by Sonogashira coupling between
multialkyne connectors and multihalo arene building
blocks.[2e,h,7,8] We have continued to develop functional
MONs.[8] We speculated that the generation of benzodifuran
species can be induced in a tandem manner during the
formation of the MON through a Sonogashira coupling. As
far as we are aware, tandem synthetic strategies for the
preparation of functional MONs have not been reported.
Herein, we report the preparation of photoactive MONs with
benzodifuran moieties through tandem synthetic processes,
and their applications to photocatalytic coupling of primary
amines.
Figure 1 shows the synthetic strategy for the synthesis of
a MON containing benzodifuran moieties (BDF-MON).
[*] N. Kang, Dr. J. H. Park, K. C. Ko, J. Chun, E. Kim, Dr. H.-W. Shin,
Prof. T. K. Ahn, Prof. J. Y. Lee, Prof. S. U. Son
Department of Chemistry and Department of Energy Science
Sungkyunkwan University
Suwon 440-746 (Korea)
E-mail: taeahn@skku.edu
jinylee@skku.edu
sson@skku.edu
Dr. S. M. Lee, Dr. H. J. Kim
Korea Basic Science Institute
Daejeon 350-333 (Korea)
[**] This work was supported by grants NRF-2012-045064 (Midcareer
Researcher Program) and R31-2008-10029 (WCU program) through
NRF of Korea. J.H.P. thanks for grant NRF-2012-1040282 (Priority
Research Centers Program). J.Y.L. acknowledges grant NRF-2007-
0056343.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201300655.
Figure 1. Strategy for the synthesis of microporous organic networks
containing benzodifuran moieties.
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First, 1,3,5-triethynylbenzene was prepared by using
a reported method.[9] 2,5-Diiodo-1,4-hydroquinone was pre-
pared by iodization of 1,4-dimethoxybenzene and demethy-
lation.[10] Then, a 2:3 stoichiometric mixture of 1,3,5-triethy-
nylbenzene and 2,5-diiodo-1,4-hydroquinone was treated
under conventional Sonogashira coupling conditions using
diisopropylamine as a base (see the experimental section in
the Supporting Information).[11]
tion). A comparison with model compounds (Figure 2b,g)[5a]
confirmed that the changes in the solid-state 13C NMR spectra
resulted from the intramolecular cyclization of the 2,5-
dialkynyl-1,4-hydroquinone moieties formed in the Sonoga-
shira coupling (Figure 1). All peaks, including that at
104 ppm, matched well with the suggested structure of
benzodifuran moieties (Figure 2 f and g).
When the precipitates isolated after 30 min were further
treated with diisopropylamine at 908C for 47.5 h, the same
intramolecular cyclization was observed, which was verified
by 13C NMR spectroscopy (Figure 2e). This observation
implies that the intramolecular cyclization to benzodifurans
was triggered by the base-induced addition of phenoxide to
adjacent alkynes. Notably, a base-induced formation of anti-
benzodifurans from 2,5-dialkynyl-1,4-hydroquinones was
recently reported.[5]
The outer shape and inner porosity of the organic
materials were investigated by scanning electron microscopy
(SEM) and the Brunauer–Emmett–Teller (BET) method. In
the SEM images, the precipitates obtained after 30 min
showed a spherical shape (Figure 3a). There were nearly no
changes in the shape of the materials obtained after 2 h, 12 h,
and 48 h (Figure 3b–d). In contrast, BET analysis showed
dramatic time-dependent changes in the porosity of the
materials (Figure 3e). The precipitates obtained after 30 min
showed negligible microporosity with a surface area of
8 m2 gÀ1. After 2 h, 12 h, and 48 h, the BET surface area
values of materials increased to 14 m2 gÀ1, 348 m2 gÀ1, and
455 m2 gÀ1, respectively. Microporosity with mainly 1–2 nm
pore sizes was observed in precipitates obtained after 48 h.
Considering these observations and the solid-state 13C NMR
studies, it can be speculated that after the coarse connection
of building blocks to form spherical materials through the
Sonogashira coupling reaction, further network formation
During the first 30 min, a significant amount of yellow
precipitates gradually appeared. The reaction mixture was
further stirred for 48 h. The resultant dark-yellow precipitates
were isolated by centrifugation and washed with methanol,
acetone, methylene chloride, and ether. To characterize the
chemical reactions in the formation of MONs, the precipitates
obtained after 30 min, 2 h, 12 h, and 48 h were analyzed by
using solid-state 13C NMR spectroscopy. The 13C NMR spec-
trum of the precipitates obtained after 30 min showed peaks
of internal alkynes at 89 and 96 ppm (asterisks in Fig-
ure 2a,b). After 2 h, the intensities of the peaks gradually
decreased and a new peak appeared at 104 ppm (Figure 2c).
The intensity of the peak at 104 pm gradually increased
during further reaction and reached a maximum after 12 h
(Figure 2d). The peaks of the aromatic parts also changed
during the reaction. After 48 h, a completely different
13C NMR spectrum was obtained, when compared with that
of precipitates obtained after 30 min (Figure 2 f). Fourier-
transform infrared (FTIR) spectroscopy of the materials
obtained after 30 min, 2 h, 12 h, and 48 h showed the gradual
disappearance of the absorption peak at 3300–3600 cmÀ1 from
the hydroxy groups (Figure S1 in the Supporting Informa-
Figure 3. SEM images (a–d) and N2 adsorption/desorption isotherms at
77 K (e) of the precipitates obtained after 30 min, 2 h, 12 h, and 48 h by
reaction of 1,3,5-triethynylbenzene and 2,5-diiodo-1,4-hydroquinone under
Sonogashira coupling conditions. The inset in (e) displays the DFT pore-
size distribution curve (V=differential pore volume, d=pore size) of
precipitates obtained after 48 h (BDF-MON). Scale bar in SEM
images=1 mm. STP=standard temperature and pressure.
Figure 2. Solid-state 13C NMR spectra of the precipitates obtained after
30 min (a), 2 h (c), 12 h (d), and 48 h (f) by reaction of 1,3,5-triethynyl-
benzene and 2,5-diiodo-1,4-hydroquinone under Sonogashira coupling
conditions. 13C NMR spectra of model compounds (b,g). e) 13C NMR
spectrum of precipitates that were further treated for 47.5 h in
diisopropylamine solution using the materials isolated after 30 min.
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and chemical transformation to benzodifuran moieties oc-
curred concomitantly within the spheres. The network
formation in the ripening period induced the inner porosity
of the MONs.
According to thermogravimetric analysis, BDF-MON was
stable up to approximately 2868C (Figure S2 in the Support-
ing Information). Powder X-ray diffraction studies showed
that BDF-MON has amorphous characteristics (Figure S3 in
the Supporting Information), as generally reported for MONs
prepared by the coupling reaction of organic building
blocks.[7]
aromatic supramolecules can mediate electron transfer from
1,4-bis(dimethylamino)benzene to oxygen, thereby resulting
in the generation of a blue-colored cationic radical species.[12]
BDF-MON mediated the abstraction of an electron from 1,4-
bis(dimethylamino)benzene to form the blue-colored species
with strong absorption peaks at 565 nm and 614 nm under
irradiation with a blue LED (Figure 4b and see the Support-
ing Information for the detailed experimental procedure).
Thus, we investigated the photocatalytic transformation of
amines (electron donor) by using a catalytic amount of BDF-
MON under irradiation with a blue LED.
Recently, benzodifuran-containing organic polymers have
shown promising photoelectrical performances in polymer
solar cells.[5b] In these photovoltaic devices, benzodifuran
moieties acted as electron donors and photosensitizers.
Considering these photoredox behaviors, we examined the
photocatalytic properties of BDF-MON in the oxidative
conversion of primary amines. BDF-MON showed a signifi-
cant absorption in the visible-light region of 400–500 nm and
emission at 430–700 nm (Figure 4a). In the literature, it was
interpreted that the absorption originates from the p–p*
transition of benzodifuran moieties and pending side-con-
jugated arenes.[5c] It was reported that the photoactive
Visible-light-induced photocatalytic systems for the oxi-
dative conversion of primary amines into imines have been
reported very recently.[13] However, the reaction temperatures
were relatively high (ca. 808C).[13c–d] Catalytic systems work-
ing at room temperature are relatively rare.[13a] Herein, we
conducted photocatalytic studies of BDF-MON with benzyl-
amine under 1 atm oxygen at room temperature (Table 1).
After 20 h and 40 h under irradiation with a blue LED
a catalytic amount (8 mol% of BDF moieties to amines) of
BDF-MON resulted in 62% and 97% conversion of benzyl-
amine into the imine, respectively (entries 1,2 in Table 1).
After the reaction, BDF-MON was retrieved by centrifuga-
tion. The resultant solution was colorless and transparent,
which implies that the reaction proceeded in a heterogeneous
Table 1: Photocatalytic oxidative conversion of amines into imines by
BDF-MON.[a]
Entry
Amine
Product
Yield
[%][b]
1[c]
2
62(57)
97(89)
96(93)
8
3[d]
4[e]
5[f]
4
6
7
8
97(92)
96(91)
93(87)
9
95(91)
10
11
12
98(91)
35(29)
61(54)
Figure 4. a) UV/Vis absorption and emission spectra (exc. 410 nm) of
BDF-MON and emission spectrum of 2,6-diphenylbenzodifuran. The
emission from medium (CH3CN) was calibrated. b) UV/Vis absorption
spectra and photograph of the cationic radical species of 1,4-bis(di-
methylamino)benzene (100 mm in 3 mL CH3CN) generated by BDF-
MON (21 mg) and irradiation with a blue LED for 1 h in the presence
of oxygen. c) A suggested photocatalytic process of oxidative conver-
sion of benzylamine into imine by BDF-MON under irradiation with
a blue LED.[17]
[a] Reaction conditions: amine (1.0 mmol), O2 (1 atm), cat. BDF-MON
(21 mg, 8 mol% benzodifuran moieties to amines based on elemental
analysis), CH3CN (3.0 mL), 40 h, room temperature. [b] Determined by
1H NMR spectroscopic analysis (yields of isolated products in paren-
thesis). [c] Reaction time 20 h. [d] The recovered catalysts were used.
[e] Glassware was covered with Al foil under irradiation with a blue LED.
[f] No BDF-MON used under irradiation with a blue LED.
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manner. The recovered BDF-MON showed nearly no change
in the SEM image (Figure S4 in the Supporting Information),
and maintained catalytic activity with 96% conversion of
benzylamine after 40 h (entry 3 in Table 1). In a control
experiment, with the glassware completely covered by
aluminum foil, the catalytic systems with BDF-MON
showed 8% conversion of benzylamine for 40 h at room
temperature under irradiation with a blue LED, thus con-
firming that the chemical transformation was triggered by
photoirradiation (entry 4 in Table 1). Moreover, when the
reaction mixture was irradiated by a blue LED without BDF-
MON, only 4% conversion of benzylamine was observed
after 40 h, thus supporting that BDF-MON mediated the
oxidative conversion of benzylamine (entry 5 in Table 1). The
model compound, 2,6-diphenylbenzodifuran showed 29%
conversion of benzylamine after 40 h under irradiation with
a blue LED, possibly owing to its blue-shifted absorption
band.
photoredox pathway for oxidative conversion of benzylamine
by BDF-MON is quite reasonable and may behave according
to the oxidative quenching process.[15] Considering this and
the detection of benzaldehyde derivatives[13b] as intermediates
by 1H NMR spectroscopy (Figure S11 in the Supporting
Information), the reaction mechanism shown in Figure 4c
and Figure S8 in the Supporting Information is suggested.[17]
This work shows that a tandem synthetic strategy can be
successfully applied to introduce active species into MONs.
We believe that more diverse tandem synthetic approaches
for functional MONs can be designed by the delicate
combination of network-forming reactions for MONs, and
intramolecular transformation for the generation of func-
tional species within MONs.
Received: January 24, 2013
Revised: March 18, 2013
Published online: && &&, &&&&
Next, various substrates were screened in the optimized
catalytic system. The influence of the functional groups on the
phenyl ring of benzylamine was not significant (entries 6–9 in
Table 1). 2-Thienylmethylamine showed excellent conversion
(98%, entry 10 in Table 1). However, the methyl substituent
at the methylene group of benzylamine significantly retarded
the conversion (35%, entry 11 in Table 1). Interestingly, N,N-
dibenzylamine, known as a better substrate in conventional
oxidative imination,[13a,14] showed less conversion than benzyl-
amine, possibly owing to steric hindrance (entry 12 in
Table 1).
Recently, two main photocatalytic reaction mechanisms,
known as the photoredox process[13,15] or the singlet-oxygen
(1O2) pathway,[14,16] have been suggested for the photooxida-
tion of benzylamine. The former consists of 1) photoinduced
generation of an O2CÀ radical and a benzylamine cationic
radical through electron transfer, 2) the formation of benzal-
dehyde or phenylmethanimine (PhCHNH2), and 3) the
successive reaction of additional benzylamine with these
species (Figure 4c).[13] In comparison, the singlet-oxygen
pathway is based on energy transfer from the excited triplet
state (with long lifetimes, > 1 ms) of photosensitizers to triplet
oxygen molecules.[16,2h] The reaction pathway is dependent on
the properties of photosensitizers. In this study, the lifetime of
BDF-MON emission was relatively short (at least shorter
than 5 ns, Figure S5 in the Supporting Information). More-
over, the typical long-lived emission at 1268 nm from singlet
oxygen[15] was not detected in straightforward photophysical
studies on the irradiated BDF-MON and oxygen (Figure S6 in
the Supporting Information). The irradiation of 1,4-bis(di-
methylamino)benzene under oxygen in the presence of BDF-
MON resulted in a color change to blue (Figure 4b), implying
electron transfer from amine to oxygen mediated by BDF-
MON.[12]
Keywords: benzodifuran · cross-coupling ·
microporous materials · photocatalyst · tandem synthesis
.
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Communications
Porous Organic Materials
Tiny pores: Benzodifuran moieties were
introduced into microporous organic
networks (MONs) through a tandem
process consisting of Sonogashira cou-
pling of 1,3,5-triethynylbenzene and 2,5-
diiodo-1,4-hydroquinone and intramolec-
ular cyclization. The resultant benzodi-
furan-containing MON showed promis-
ing photocatalytic activities in the oxida-
tive conversion of primary amines into
imines.
N. Kang, J. H. Park, K. C. Ko, J. Chun,
E. Kim, H.-W. Shin, S. M. Lee, H. J. Kim,
T. K. Ahn,* J. Y. Lee,*
S. U. Son*
&&&& — &&&&
Tandem Synthesis of Photoactive
Benzodifuran Moieties in the Formation
of Microporous Organic Networks
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