Anthracene-based indium metal-organic framework as a promising photosensitizer for visible-light-induced atom transfer radical polymerization

Article abstract of DOI:10.1039/c6ce00465b

Metal-organic frameworks (MOFs) provide an attractive platform for designing and synthesizing photoactive hybrid materials for photochemical reactions. We report here the utilization of a new visible-light responsive indium MOF for inducing the atom trans

Full text of DOI:10.1039/c6ce00465b

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Anthracene-based indium metal–organic
framework as a promising photosensitizer for
visible-light-induced atom transfer radical
polymerization†
Cite this: DOI: 10.1039/c6ce00465b
*
Xingyu Li,‡ Dashu Chen,‡ Yue Liu, Ziyang Yu, Qiansu Xia, Hongzhu Xing
*
and Wendong Sun
Metal–organic frameworks (MOFs) provide an attractive platform for designing and synthesizing photoactive
hybrid materials for photochemical reactions. We report here the utilization of a new visible-light respon-
sive indium MOF for inducing the atom transfer radical polymerization (ATRP) of methacrylate monomers,
where well-designed polymers with controlled molecular weights, narrow molecular weight distribution
and high retention of chain-end groups have been prepared. The kinetics study reveals that the MOF-
mediated ATRP shows characteristics of controlled radical polymerization (CRP). Besides, the polymeriza-
tion can be easily regulated by light. Furthermore, the heterogeneous MOF can be easily recovered from
the reaction and recycled for the photopolymerization. This study has involved photoactive MOFs materials
into a new photochemical reaction of polymer synthesis.
Received 27th February 2016,
Accepted 8th April 2016
DOI: 10.1039/c6ce00465b
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far fewer catalytic systems have been investigated for MOF
photocatalysts, such as the photopolymerization.
Introduction
Metal–organic frameworks (MOFs) are a class of crystalline or-
ganic–inorganic hybrid materials constructed by the coordina-
tion interaction between metal ions and organic linkers,
showing rich structures and multiple functionalities.1–11 Until
now, MOFs have shown versatile applications in gas storage,
separation, luminescence, chemical sensing, drug delivery,
heterogeneous catalysis, and so on.12–19 Very recently, visible
light responsive MOFs for artificial photocatalysis and photo-
synthesis have attracted much attention due to the increasing
global energy demand and environmental issues.20–24 MOFs
provide an interesting platform for designing and synthesiz-
ing visible light photocatalysts due to the easy incorporation
of the photoactive chromophores into their structures. Studies
of visible light responsive MOFs based on either metal–
organic or organic chromophores have been reported, where
these photoactive MOFs have made notable progress such as
degradation of organic pollutants,25,26 water splitting,27–29
and CO2 reduction.30,31 However, it is still a great challenge to
synthesize novel visible-light photoactive MOFs. Meanwhile,
Atom transfer radical polymerization (ATRP) mediated by
transition metal catalysts has emerged as one of the most
successful polymerization techniques for the easy access to
useful polymer materials.32–40 It provides the ability to pro-
duce polymers with controlled molecular weight, narrow mo-
lecular weight distribution, and high degrees of chain-end
functionalities. The mechanism of ATRP can be simplified to
a reversible redox system, where the transition metal catalyst
undergoes a reversible one electron oxidation and reduction
process. The application of photochemical redox reactions to
synthetic polymer chemistry brought significant advantages
compared to traditional techniques. Very recently, the photo-
induced ATRP has been rapidly developed owing to its easy
temporal and spatial control of the chain extension
process.41–44 Typically, the photoinduced ATRP relies on the
in situ reduction of the deactivator of a CuIJII) complex to the
activator of the CuIJI) complex for the polymerization. In view
of their photoinduced charge generation, MOFs are potential
heterogeneous photosensitizers that activate the copper com-
plex, inducing the ATRP.
Provincial Key Laboratory of Advanced Energy Materials, College of Chemistry,
Northeast Normal University, Changchun, Jilin 130024, China.
E-mail: xinghz223@nenu.edu.cn, sunwd843@nenu.edu.cn
† Electronic supplementary information (ESI) available. CCDC 1410541. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c6ce00465b
In the present work, we explore the possibility of utilizing
visible light photoactive MOFs material as a photoreducing
agent for the generation of a CuIJI) complex to induce the
ATRP. By the incorporation of the chromophore of an anthra-
cene group into a carboxylic ligand, a new three-dimensional
indium MOF showing broad-band absorption in the visible
‡ These authors contributed equally to this work.
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light region has been prepared. It has been demonstrated
that the as-prepared indium MOF is an effective photosensi-
tizer for inducing the photopolymerization of methacrylates,
showing characteristics of controlled radical polymerization.
product (2.5 g, 3.75 mmol), KOH was added (1.68 g, 30 mmol)
and the resulting solution was stirred under reflux. The
mixture was acidified with HCl, and then the precipitate
5,5′-(anthracene-9,10-diylbis(ethyne-2,1-diyl))diisophthalic acid
(H4L) was filtered and dried. 1H NMR (400 MHz, CDCl3) δ:
8.715 (d, 4H), 8.533 (s, 4H), 8.515 (s, 2H), 7.842 (d, 4H).
IR (KBr): ν = 2922, 1694, 1593, 1437, 1202 cm−1. The analytical
data are in accordance with that reported in the literature.45
Experimental section
Materials and physical studies
Methacrylate monomers of i-butyl methacrylate (i-BMA)
(98%, TCI), n-butyl methacrylate (n-BMA) (98%, TCI), methyl
methacrylate (MMA) (99.8%, TCI) and styrene (St) (99%, TCI)
were purified by passing through a basic alumina column to
remove the inhibitor. N,N,N′,N″,N′″-pentamethyldiethylene-
triamine (PMDETA) (99%, TCI), ethyl α-bromoisobutyrate
(EBiB) (98%, Accela), copperIJII) bromide (CuBr2) (99%, Acros),
acetonitrile (CH3CN) (99.9%, Acros), tetrahydrofuran (THF)
(99.99%, Acros) and other reagents were used as received
without further purification. The Fourier transform infrared
spectra (FTIR) were collected using a Bruker Vertex 70 in ATR
mode. Thermogravimetric analyses (TGA) were performed
with a heating rate of 10 °C min−1 using the Perkin-Elmer
TGA-7 thermogravimetric analyzer from room temperature to
800 °C under atmosphere. Powder X-ray diffraction (PXRD)
data were collected on a Rigaku D-MAX 2550 automated pow-
der diffractometer, using Cu-Kα radiation (λ = 0.15417 nm) at
room temperature with 2θ ranging from 3° to 40°. The UV–
vis spectra for solid state samples were taken on a HITACHI
U-4100 spectrophotometer while the spectra for liquid sam-
ples were taken on a SHIMADZU UV-2550 spectrophotometer.
The EPR spectra were recorded on a JES-FA 200 EPR spectro-
meter at 25 °C with a microwave power of 0.998 mW. The
sample was irradiated with a microwave frequency of 9.45
GHz and the measurement with a sweep width of 40 mT was
centred at 340 mT. The in situ experiments were carried out
using a 500 W xenon arc lamp where a 420 nm optical filter
was used to cut off the ultraviolet part of light. The stable
radical 2,2-diphenyl-1-picrylhydrazyl (DPPH) was used as a
standard for the calculation of g values. Gel permeation chro-
matography (GPC) measurements were conducted using an
Agilent Technologies PL-GPC-50 Integrated GPC system. THF
was used as an eluent at a flow rate of 1.0 mL min−1 at 30 °C.
The molecular weights were calibrated with PMMA standards
having a narrow molecular weight distribution.
Synthesis of NNU-32
A mixture of the organic ligand H4L (10 mg, 0.018 mmol) and
InIJNO3)3·4H2O (6.7 mg, 0.018 mmol) was dissolved in a mixed
solvent of DMF/H2O (1.6 mL, 8 : 1, v/v). Thereafter, 4 M HCl
(35 μL, aq.) was added, and the vial (20 mL) was capped and
placed in an oven at 65 °C for 5 days. The resulting yellow oc-
tahedron shaped crystals were collected and washed with
DMF several times. Yield: ca. 50% based on the ligand.
Single crystal X-ray diffraction
The single-crystal X-ray diffraction structure of NNU-32 was
collected on a Bruker Smart ApexIICCD diffractometer with
graphite-monochromated Mo Kα radiation (λ = 0.71073 Å)
under room temperature. Raw data of the structure was
processed using SAINT and absorption corrections data were
corrected using the multiscan program SADABS. The crystal
structure was solved by direct methods and refined by full-
matrix least-squares on F2 with anisotropic displacement
using SHELXTL (Table S1, ESI†).46 CCDC 1410541 contains
the supplementary crystallographic data for this paper.
Photoinduced ATRP
i-BMA (2 mL, 12.5 mmol), PMDETA (11.6 μL, 0.0558 mmol),
CuBr2 (4.2 mg, 0.0186 mmol), EBiB (14 μL, 0.093 mmol), ace-
tonitrile (0.5 mL, 9.57 mmol), and NNU-32 (20 mg) were
placed into a Schlenk tube, and the reaction mixture was
degassed by three freeze-pump-thaw cycles and left under
vacuum. The mixture was irradiated by the 300 W xenon arc
lamp equipped with a 520 nm band-pass optical mirror and
filter under magnetic stirring. The light intensity was 25 mW
cm−2 measured by using a power meter. After the reaction,
NNU-32 was first separated by centrifugation. The resulted
polymers were precipitated in methanol for several times and
then dried under vacuum. Monomer conversions were mea-
sured gravimetrically. The polymerization conditions for
n-BMA (2 mL) MMA (1.35 mL) and St (1.44 mL) are the same
as those for i-BMA.
Chain extension was performed using a macroinitiator
instead of EBiB under the following conditions: [i-BMA]0/
[Macroinitiator]0/[CuBr2]0/[PMDETA]0
macroinitiator PBMA-Br (Mn,GPC = 15160, Mw/Mn = 1.10) was
prepared by photoinduced ATRP for 8 h. PBMA-Br (141 mg,
9.3 × 10−3 mmol), i-BMA (2 mL, 12.5 mmol), CuBr2 (4.2 mg,
0.0186 mmol), PMDETA (11.6 μL, 0.0558 mmol), and NNU-32
(20 mg) were put into a Schlenk tube, and the reaction
Synthesis of ligand
The anthracene derived ligand H4L [5,5′-(anthracene-9,10-
diylbisIJethyne-2,1-diyl))diisophthalic acid] was synthesized from
9,10-dibromoanthracene and diethyl 5-ethynylisophthalate
via the Sonogashira coupling reaction. Typically, diethyl
5-ethynylisophthalate (0.5 g, 2 mmol), 9,10-dibromoanthracene
(0.3 g, 0.92 mmol), PdIJPPh3)2Cl2 and CuI were added into the
solution of DMF. The mixture was refluxed under an N2 atmo-
sphere. The reaction was quenched with water, extracted with
CHCl3, washed with brine and dried over MgSO4. Recrystalli-
zation from CHCl3 gave the product as a red solid. To this
= 135/0.1/0.2/0.6. The
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mixture was degassed by three freeze-pump-thaw cycles and
left under vacuum. The mixture was irradiated under the
same experimental conditions and the reaction was ceased
after 6 h. Finally, methanol was added to precipitate the poly-
mers, and the obtained polymer was dried under vacuum.
water molecules (calc. 6.7%). The weight loss (10.1%) from
120 to 420 °C is attributed to the gradual removal of DMF
and protonated dimethylamine (calc. 10.2%). The following
weight loss (66.6%) is attributed to the decomposition of the
organic ligand (calc. 65.8%). The remaining plateau (16.8%)
is assigned to the indium oxide In2O3 (calc. 17.3%). The
phase purity of the as-prepared sample was confirmed by
powder X-ray diffraction (PXRD), where the experimental and
simulated patterns matched well (Fig. S3, ESI†). NNU-32
shows no gas absorption after activation by the solvent ex-
change process, due to the loss of its structure during the ac-
tivating process.51
Results and discussion
Single crystal X-ray diffraction analysis reveals that the in-
dium MOF with a formula of (Me2NH2)[InL]·3(H2O)·0.5DMF
(denoted as NNU-32) crystallizes in the tetragonal space
group P42/mmc. In the structure, the mononuclear In3+ ion in
an 8-coordinated geometry is bonded to four chelating car-
boxylate groups, forming a tetrahedral 4-connected node. The
anthracene-derived carboxylate ligand binds to four In3+ ions
in a square-planar fashion (Fig. 1). A view along the crystallo-
graphic c-axis shows the primary square-shaped channels
running through the structure (Fig. 1). The approximate di-
ameter of the channel is 9.9 Å defined by the geometry of the
In-carboxylates units and the span of the tetratopic linker.
These channels are interconnected by square windows com-
posed of four indium centers and four ligands, which can be
viewed along the crystallographic a- or b-axes. Since each In3+
ion is bonded to four carboxylate groups, the framework is
negatively charged. The FTIR spectrum (Fig. S1, ESI†) shows
an intense peak at 1660 cm−1 (vCO), confirming the pres-
ence of DMF molecules. The peak due to the carboxylate
groups in the ligand (vsymIJCOO)) is around 1382 cm−1. The
peak at 2484 cm−1 is attributed to the vibration of the ammo-
nium group N−H+ belonging to protonated dimethylamine,47
where it is well known that DMF is easily decomposed, lead-
ing to the formation of dimethylamine.48,49 Several weak ab-
sorptions at 3068, 2926, 2855, 2809 cm−1 correspond to the
vibrations of the C−H bonds associated to the DMF mole-
cules and protonated dimethylamine. These vibrations
around 3000 cm−1 are characteristic of the stretching vst and
combination vibrations vcomb of the ammonium group.48,50
This broad band vibration reveals also the complex inter-
molecular hydrogen bonding between water, DMF molecules
and protonated dimethylamine.50 The thermogravimetric
curve of NNU-32 (Fig. S2, ESI†) shows a continuous weight
loss (6.5%) from room temperature up to 120 °C assigned to
The light absorption of NNU-32 was studied by UV-vis
spectrum in solid state at room temperature, where it dis-
plays a broad absorption band in the visible light region from
400 nm to 650 nm (Fig. 2a). The broad-band adsorption of
NNU-32 is attributed to the inhomogeneous broadening
resulting from the energy transfer and/or charge transfer pro-
cesses from ligand to metal ions (LMCT).52,53 The LMCT pro-
cess is also verified by the significantly lower energy absorp-
tion of NNU-32 as compared to the ligand (Fig. 2a). The
visible-light-induced charge generation was further studied
by electron paramagnetic resonance (EPR) spectroscopy. As
shown in Fig. 2b, the freshly prepared crystalline sample of
NNU-32 showed no EPR signal; whereas a characteristic EPR
signal attributed to free radical at g = 2.003 was detected
when NNU-32 was irradiated by visible light, suggesting the
ligand-based free radicals formation in the structure.54 The
ability of photoinduced radical formation from anthracene-
derived ligands was further verified by the observation of the
same EPR signal at g = 2.003 when the ligand in the solid
state was exposed to visible light. To further study the photo-
induced radical formation in NNU-32, as shown in Fig. 2c,
time evolution in situ EPR experiments were performed under
visible light irradiation. For NNU-32, the intensity of the EPR
signal at g = 2.003 increased rapidly under continuous visible
light illumination and saturated after 3 min. Thereafter, this
signal was sustained over ten minutes in the dark. The EPR
studies indicate the long-lived photoinduced charge genera-
tion of NNU-32, which resulted from the radical formation of
the ligand. It is notable that MOFs showing photoinduced
radical generation are rare.55–57
The above optical studies suggest that NNU-32 could be a
potential photoreducing agent for the in situ generation of
the CuIJI) activator in the ATRP reaction. Hence, the photo-
catalytic system of NNU-32/initiator/copperIJII) complex/mono-
mer was investigated, where NNU-32 is the photosensitizer
for the reduction of copper catalyst via the visible-light-
induced electron-transfer process. The reaction was carried
out using CuBr2/PMDETA as the catalyst and EBiB as the ini-
tiator in acetonitrile. The photopolymerization of the i-BMA
monomer was first investigated with
a molar ratio of
[i-BMA]0/[EBiB]0/[CuBr2]0/[PMDETA]0 = 135/1/0.2/0.6. To elimi-
nate the absorption interference from the Cu(II)/PMDETA
complex, the UV-vis spectroscopy of the reaction system in
the absence of NNU-32 was first studied where it showed
Fig. 1 (a) The structure of NNU-32 viewed along the c-axis. (b) A view
of the structure of NNU-32 showing the 4,4-net coordination between
the anthracene-based ligands and indium ions. The hydrogen atoms
and guest molecules are omitted for clarity.
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Fig. 2 (a) The UV–vis spectra of NNU-32 and the ligand in DMF. (b) Photoinduced EPR spectra of NNU-32 and the ligand. (c) Time-evolution varia-
tion of EPR intensities of NNU-32.
almost no absorption at 520 nm (Fig. S4, ESI†). And the con-
trol experiment showed that it was not reactive under this
wavelength (Table 1, entry 1), which is consistent with the
reported studies.58,59 Thereafter, the NNU-32 mediated ATRP
reaction was performed under the irradiation of 520 nm
monochromatic light.
Upon the addition of NNU-32, the photopolymerization
with ca. 71% monomer conversion was observed after 10 h ir-
radiation (entry 2). Gel permeation chromatography (GPC)
measurement showed that the molecular weight distribution
(Mw/Mn) of the resulting polymer was well controlled (Mw/Mn
= 1.11), indicating the feasibility of the NNU-32 mediated
ATRP reaction. A range of control experiments were carried
out to clarify the role of each component in the reaction. As
shown in Table 1 (entry 3), there was no product of polymer
when the polymerization was conducted in the absence of
EBiB, which states that the EBiB initiator is essential for the
reaction. When the photopolymerization was performed with-
and the resulting polymers were both in narrow molecular
weight distributions (1.09–1.12). When the amount of the
Cu(II)/PMDETA catalyst was decreased (entry 7 and 8), the
yields were relatively lower and the dispersibilities of the
resulting polymers were wider. The versatility of NNU-32 me-
diated ATRP was investigated by the polymerization of other
acrylate monomers such as n-BMA and MMA. For these
monomers (entries 9 and 10), narrow molecular weight distri-
butions were preserved with conversions of 31% (n-BMA) and
56% (MMA). Besides, the photopolymerization of styrene has
also been investigated; however, no corresponding polymer
has been obtained (entry 11). Because the photoactivity of
NNU-32 relies on the anthracene-based ligand, hence it is
necessary to investigate the photopolymerization by using the
ligand instead of the In-MOF. However, the ligand with car-
boxylate groups coordinates strongly with the Cu2+ ion,
resulting in the destruction of the Cu/PMDETA catalyst. In
this case, the 9,10-bis(phenylethynyl)anthracene (BPEA), a
molecule with the same backbone of ligand lacking the car-
boxylate groups, is used to induce the photopolymerization.
As shown in Table 1 (entry 12), no polymer was prepared un-
der the same photocatalytic conditions. This result reveals
that the as-prepared MOF is necessary and advanced com-
pared to the ligand for the photoinduced polymerization.
The evolution of the molecular weights and polydisper-
sities upon conversions was investigated based on the i-BMA
monomer. As shown in Fig. 3a, an induction period of ca. 3 h
out CuBr2 (entry 4), the reaction was out of control (Mn,GPC
=
31 3150 and Mw/Mn = 1.73) as evidenced by GPC. This phe-
nomenon can be reasoned by direct polymerization when the
EBiB initiator was activated by the radical cation of the ligand
via classical halogen abstraction.60 It is necessary to investi-
gate the influence of the catalysts for this photocatalytic sys-
tem. As shown in Table 1, the yield of polymerization was
slightly varied (66–74%) when the dosage of photosensitizer
NNU-32 was changed from 10 mg to 30 mg (entry 5 and 6),
Table 1 The results of NNU-32 mediated ATRP reactions under 520 nm
Entrya
[Monomer]0/[EBiB]0/[CuBr2]0/[PMDETA]0
Sensitizer
Monomer
Conversion [%]
Mn,thb [g mol−1
]
Mn,GPC [g mol−1
]
Mw/Mn
c
c
1
2
3
135/1/0.2/0.6
135/1/0.2/0.6
135/−/0.2/0.6
135/1/−/0.6
—
i-BMA
i-BMA
i-BMA
i-BMA
i-BMA
i-BMA
i-BMA
i-BMA
n-BMA
MMA
—
71
—
63
66
74
54
40
31
56
—
—
—
13 700
—
—
17 890
—
—
1.11
—
NNU-32
NNU-32
NNU-32
NNU-32
NNU-32
NNU-32
NNU-32
NNU-32
NNU-32
NNU-32
BPEA
4
12 080
12 650
14 190
10 350
7680
5 940
7 560
—
31 3150
16 500
18 230
21 430
32 640
13 370
10 910
—
1.73
1.09
1.12
1.19
1.34
1.14
1.12
—
5d
6e
7
135/1/0.2/0.6
135/1/0.2/0.6
135/1/0.1/0.3
135/1/0.02/0.06
135/1/0.2/0.6
135/1/0.2/0.6
135/1/0.2/0.6
135/1/0.2/0.6
8
9
10
11
12
Styrene
i-BMA
—
—
—
a
b
Polymerization was performed under 520 nm visible light irradiation, time = 10 h, NNU-32 = 20 mg. Mn,th = [i-BMA]0/[EBiB]0 × Mw,monomer
×
conversion. c Number average molecular weight (Mn,GPC) and molecular weight distribution (Mw/Mn) were determined by GPC. d NNU-32 = 10 mg.
e
NNU-32 = 30 mg. BPEA = 9,10-bis(phenylethynyl)anthracene.
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Fig. 3 NNU-32 mediated ATRP of i-BMA under 520 nm. (a) Kinetic plot. (b) Number average molecular weight (Mn,GPC) and dispersity (Mw/Mn) vs.
conversion. (c) GPC traces before (black) and after (blue) the chain extension process. Conditions for (a) and (b): [i-BMA]0/[EBiB]0/[CuBr2]0/
[PMDETA]0 = 135/1/0.2/0.6 in 2 mL of acetonitrile, NNU-32 = 20 mg. Conditions for (c): [i-BMA]0/[PBMA-Br]0/[CuBr2]0/[PMDETA]0 = 135/0.1/0.2/
0.6 in 2 mL of acetonitrile for 8 h, NNU-32 = 20 mg. All reactions were carried out at room temperature using 520 nm visible light.
was needed for the photoinduced polymerization, which is
likely a result of the slow photoreduction of CuIJII)/PMDETA
due to the heterogeneous nature of MOF photosensitizer. Af-
ter this induction period, the reaction exhibited first-order ki-
netics, indicating the constant concentration of radicals from
EBiB maintained through-out the reaction. Correspondingly,
a linear increase in molecular weight upon monomer conver-
sion was observed where all the polydispersity values at dif-
ferent conversions were relatively low (Mw/Mn = 1.09–1.12)
(Fig. 3b). The GPC measurements for the resulting polymers
revealed larger molecular weights compared to the calculated
ones. This result is likely due to the slow initiation where
EBiB was used as the initiator.61,62 The kinetic analysis
Fig. 4 NNU-32 mediated photopolymerization at 520 nm. Conditions:
proves that the photopolymerization after the induction pe-
riod was performed in a controllable way, exhibiting a living
polymerization feature.
[i-BMA]0/[EBiB]0/[CuBr2]0/[PMDETA]0
= 135/1/0.2/0.6 in 2 mL of
acetonitrile, NNU-32 = 20 mg.
The “living” nature of the system was also certified by a
chain extension experiment. A macroinitiator made by this
method was used instead of EBiB. As shown in Fig. 3c, the
chain-extended polymer shows a higher molecular weight
from MOFs to the copper complex. A proposed mechanism
for this photocatalytic reaction is illustrated in Scheme 1. In
the reaction, the CuIJII) complex is reduced by photosensitized
NNU-32 via single electron transfer. The resulting CuIJI) com-
plex reacts with the initiator of alkyl halide (R–X), forming
radicals (R˙) to initiate the polymerization. Meanwhile, the
resulting positive ligand cation in the MOF structure is re-
duced by the reductive amine of PMDETA to its pristine state,
leaving intermediate amine+X− which supports the chain-
end halogen for the dormant species and returns to amine.63
It is notable that there has been some research using
(Mn,GPC = 114 130) compared to the macroinitiator (Mn,GPC
=
15 160, Mw/Mn = 1.10) after 8 h of reaction, and the dispersity
of the resulting polymer is very uniform (Mw/Mn = 1.07). The
results of the chain extension experiment illustrated a well-
defined polymer with a halide group synthesized via the
NNU-32 mediated photoinduced ATRP reaction.
As shown in Fig. 4, the NNU-32 mediated ATRP can be
easily controlled by periodic light switching. When the reac-
tion was kept in the dark for 4 h, no polymer was obtained.
Then the reaction was exposed under 520 nm for 4 h, an
18% conversion of monomer was found. Afterward, the light
was turned off for another 4 h. The conversion of reaction is
nearly unchanged, indicating a negligible concentration of
the active radical present under dark conditions. Then the
light was turned on again, the polymerization was re-initiated
and reached 48% conversion, where the kinetic character is
almost the same as that observed in the first light-on process.
The light-switching experiment was further cycled for the
third time where a conversion of 80% was achieved.
The success of the NNU-32 mediated ATRP reaction im-
plies the potential of MOFs as photosensitizers for the poly-
mer synthesis, relying on the photoinduced electron transfer
Scheme 1 Proposed mechanism of the photoinduced ATRP.
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coordination polymers that control polymerization or
oligomerization.64–68 These studies had revealed that the
polymerization taking place in the nanochannel of coordina-
tion polymers supports unique advantage to control the
structure and orientation of the as-prepared polymer. The
NNU-32 mediated polymerization is different from these poly-
merizations as the reaction is driven by visible light and oc-
curs outside the channel of the MOF. Moreover, the photo-
polymerization could be more precisely controlled, leading to
a very narrow molecular weight distribution. The heteroge-
neous NNU-32 can be easily recovered from the reaction solu-
tion by centrifugation. As evidenced by the PXRD study, the
structure of NNU-32 was maintained after the ATRP reaction,
allowing its reuse in the subsequent reactions (Fig. S3, ESI†).
The recycle experiments revealed that NNU-32 can be reused
for at least three cycles without loss of activity (Fig. S5, ESI†).
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In summary, a new visible light responsive indium MOF,
NNU-32, has been prepared based on an anthracene deriva-
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MOF structure upon visible light irradiation is achieved by
the radical formation of the ligand. We further demonstrated
that NNU-32 could be utilized as a photosensitizer to conduct
a visible-light-induced ATRP reaction for polymer synthesis.
Results show that NNU-32 mediated ATRP for methacrylate
monomers took place in a controllable way, leading to a nar-
row molecular weight distribution and high retention of the
chain-end group. The kinetics study reveals that the reaction
shows characteristics of controlled radical polymerization.
Besides, it has been demonstrated that the MOF mediated
ATRP reaction could be easily controlled by light switching.
The success of MOF mediated photopolymerization exposes
that the design and synthesis of MOF photocatalysts by the
incorporation of chromophores is a feasible method. We fur-
ther anticipate that photoactive MOFs would find more
promising applications in polymer synthesis.
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Acknowledgements
This work is supported by the National Natural Science Foun-
dation of China (21473024 and 21101023) and the Natural Sci-
ence Foundation of Jilin Province (20140101228JC).
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