Reversible light-driven polymerization of polyoxometalate tethered with coumarin molecules

Article abstract of DOI:10.1002/chem.201303933

A new photosensitive polyoxometalate (POM) organic-inorganic hybrid compound has been prepared by covalently tethering coumarin moieties onto a Mn-Anderson cluster. This compound has been fully characterized by ¹H NMR, ¹³C NMR, FTIR, and UV/Vis spectroscopy, and ESI-MS. This organic-inorganic hybrid compound can undergo reversible light-driven polymerization and this process has been characterized in detail. Polymerizing POMs: A new photosensitive polyoxometalate (POM) organic-inorganic hybrid compound has been prepared by covalently tethering coumarin moieties onto a Mn-Anderson cluster. This compound has been fully characterized by ¹H NMR, ¹³C NMR, FTIR, and UV/Vis spectroscopy, and ESI-MS. This compound can undergo reversible light-driven polymerization (see scheme) and this process has been characterized in detail. Copyright

Full text of DOI:10.1002/chem.201303933

DOI: 10.1002/chem.201303933
Communication
&
Polymerization
Reversible Light-Driven Polymerization of Polyoxometalate
Tethered with Coumarin Molecules
UnSong Tong,[a] Wei Chen,[a] Chris Ritchie,[b] Xiaoting Wang,[a] and Yu-Fei Song*[a]
applied as laser dyes, as efficient fluorescent labels and probes,
Abstract: A new photosensitive polyoxometalate (POM)
or as sensitizers for solar energy collectors. In addition, owing
organic–inorganic hybrid compound has been prepared
to their remarkable photophysical properties, coumarin-con-
by covalently tethering coumarin moieties onto a Mn–An-
taining compounds have been used in a number of areas rang-
derson cluster. This compound has been fully character-
ing from photonics to photochemistry and photobiology.[11–15]
ized by H NMR, 13C NMR, FTIR, and UV/Vis spectroscopy,
Previous studies have shown that a number of functional or-
and ESI-MS. This organic–inorganic hybrid compound can
ganic moieties can be covalently grafted onto Anderson-type
undergo reversible light-driven polymerization and this
compounds, by using tris(hydroxymethyl)aminomethane (de-
process has been characterized in detail.
1
noted as Tris) as a linker, by means of pre- and postmodifica-
tion methods.[16–18] Herein, a new photosensitive organic–inor-
ganic hybrid compound has been prepared by covalently teth-
Polyoxometalates (POMs) are a class of anionic metal oxides of
V, Mo, W, and Nb species, among others. POM chemistry con-
tinues to attract increasing interest, as evidenced by the large
number of new POM species reported in the literature.[1] POMs
have been widely applied in various fields, such as catalysis
and medicine, owing to their diverse and tunable chemical
properties.[2] The design of POM-containing organic–inorganic
hybrids, based on covalently grafting various organic function-
alities onto POMs, has been an important synthetic strategy to-
wards the development of POM-containing advanced function-
al materials.[3] The organic fragments could be used to modu-
late the chemo-physical properties of the resulting hybrid ma-
terials.[4–6] For example, hybrid materials based on POMs and
organic polymers display the following properties: electro-
chromism, photochromism, conductivity, electrochemistry, and
catalysis.[7] The design of functional organic moieties is current-
ly attracting broad interest and fluorescent chromophores with
a planar and rigid conjugation system are of particular inter-
est.[8]
ering coumarin moieties onto a Mn–Anderson cluster. The re-
sulting compound (3, Figure 1) has been fully characterized by
1H NMR, 13C NMR, and FTIR, and UV/Vis spectroscopy, and ESI-
MS. In addition, compound 3 can undergo reversible light-
driven polymerization and this process has been characterized
in detail.
Compound 3 was prepared by the reaction of Mn(CH3COO)3
and Coumarin-Tris (Compound 2, see the Supporting Informa-
tion) with TBA4[Mo8O26] (TBA=tetrabutylammonium) in MeCN.
Orange crystals were obtained by ether diffusion into a MeCN/
DMF solution of compound 3. Single-crystal X-ray diffraction
analysis of compound 3 clearly shows that the coumarin moi-
eties are tethered onto the Mn–Anderson cluster, on both
sides, by means of COꢀNH bonds with the Tris linker.
This B-type Mn–Anderson cluster consists of six edge-sharing
MoO6 octahedra arranged around a central MnO6 unit. Hydro-
gen bonding interactions ranging from 2.406 to 2.708 ꢀ can
Coumarin derivatives, known for their antioxidant, anticoa-
gulant, antibacterial, and anti-HIV properties, have been widely
applied in biology and medicine.[9,10] The coumarin unit dis-
plays strong absorption in the near-UV region and a high radi-
ative quantum yield, therefore, coumarin derivatives can be
[a] U. Tong,+ W. Chen,+ X. Wang, Prof. Y.-F. Song
State Key Laboratory of Chemical Resource Engineering
Institution Beijing University of Chemical Technology
100029 Beijing (P.R. China)
E-mail: songyf@mail.buct.edu.cn
songyufei@hotmail.com
[b] Dr. C. Ritchie
School of Chemistry
The University of Melbourne
Parkville, Victoria 3010 (Australia)
[+] These authors contributed equally to this work.
Figure 1. Single-crystal X-ray structure of compound 3, with the molecular
formula [(TBA)3(MnMo6O18{(OCH2)3CNHC12H9O4}2)] (TBA=N(C4H9)4, TBA+
omitted for clarity); ESI-MS spectrum of compound 3.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201303933.
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Communication
be observed between the organic ligands and the Anderson
cluster. ESI-MS measurements of compound found
3
m/z=2072.04 and 1829.74, corresponding to [(TBA)2-
(MnMo6O18{(OCH2)3CNHC12H9O4}2)]1ꢀ and [(TBA)H(MnMo6O18-
{(OCH2)3CNHC12H9O4}2)]1ꢀ, respectively. This result is in good
agreement with the molecular structure of compound 3.
Upon irradiation at 365 nm, the monomer of compound 3
forms a C-(A-C-C)n-A-C polymer through continuous intermo-
lecular polymerization, as shown in Figure 2a, in which C, A,
Figure 3. UV/Vis kinetic changes upon photoirradiation of compound 3 in
MeCN.
MeCN at 365 nm, with an increase of the irradiation times. The
absorbance decrease and (1/Ct-1/C0) are plotted against the ir-
radiation time t in Figure 3, in which C0 and Ct are initial con-
centration and the concentration at an irradiation time of t, re-
spectively. The linear fit of the kinetic data reveals pseudo-
second-order reaction kinetics with R2 =0.9915. The rate con-
stant, K, of the polymerization reaction is determined to be
0.0052 m3 molꢀ1 sꢀ1 on the basis of Equations (1) and (2). The
UV absorbance reaches stabilization in 90 min.
dCt
dt
¼ KCt2
ð1Þ
ð2Þ
1
1
ꢀ
¼ Kt
Ct C0
Figure 2. a) Schematic representation of the photopolymerization process,
b) UV/Vis absorption spectra of compound 2, Mn–Anderson cluster, and
compound 3 in MeCN (2ꢂ10ꢀ5 molLꢀ1), c) UV/Vis spectral change for com-
pound 3 in MeCN upon photoirradiation at 365 nm.
The photocontrolled polymerization of compound 3 has
been further characterized by H NMR and 2D COSY NMR spec-
1
troscopy before and after irradiation. As shown in Figure 4a,
the 1H NMR spectra clearly show that the H3 and H4 signals
appear at d=6.2 and 2.4 ppm, respectively, before irradiation,
then shift to d=3.6 and 1.7 ppm, respectively, after irradiation
for 90 min. These results can be explained by the fact that
photocrosslinking, through dimerization of the coumarin
groups, takes place upon irradiation of 3 at 365 nm, resulting
in the formation of the cyclobutane-type bridge. This result is
in accordance with the results from UV/Vis spectroscopy.
2D COSY NMR spectroscopy provides further support for the
photocontrolled polymerization process of compound 3, as
shown in Figure 4b. The experimental result shows that upon
irradiation at 365 nm, the signals at d=6.2 and 2.4 ppm de-
crease, whereas signals at d=3.6 and 1.7 ppm appear. This in-
dicates a correlation between the protons at the C3 and C4
positions. In other words, upon irradiation of compound 3, the
monomer and dimer species coexist in MeCN solution.
and n represent coumarin group, Mn–Anderson cluster, and
the number of cross-linking monomers of compound 3, re-
spectively. As shown in Figure 2b, compounds 2 and 3 show
characteristic UV absorption at 316 and 317 nm, respectively,
whereas the Mn–Anderson cluster does not. The photoirradia-
tion of compound 3 has been investigated, in MeCN with
a mercury lamp at 365 nm, and the resulting polymeric com-
pound was analyzed by UV/Vis spectroscopy. The UV absorb-
ance at 317 nm decreases dramatically with increasing irradia-
tion time, whereas the absorbance at 360 nm increases gradu-
ally (Figure 2c). This result indicates the formation of photoin-
duced cyclobutane-type dimers and that the polymerization
process takes place on both sides of the Mn–Anderson cluster.
In addition, upon irradiation with shorter wavelengths, for ex-
ample l=254 nm, these dimers can be converted back into
the starting material. As such, the photoinduced polymeri-
zation process of compound 3 is reversible.
The light-driven polymerization takes place upon irradiation
of compound 3, therefore, it can be expected that the particle
size in solution might increase accordingly. As such, dynamic
light scattering (DLS) analysis was carried out to provide fur-
ther proof for the formation of aggregates in solution. Figure 5
To obtain the kinetic parameters of the UV/Vis absorption
change for compound 3, experiments were performed, in
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Figure 5. Size distributions, with a concentration of 400 mm, of compound 3
before and after irradiation at 365 nm.
Figure 6. The color changes of compound 2, the Mn–Anderson cluster, a mix-
ture of compound 2 and the Mn–Anderson cluster, and compound 3 in
DMSO upon irradiation at 365 nm for 0 s and 90 min.
Figure 4. a) 1H NMR spectra of compound 3 before and after irradiation, and
b) 2D COSY NMR spectrum of compound 3 after irradiation.
shows that, compared with the particle size before irradiation
(165 nm), the size distribution after irradiation of 3 for 90 min
(400 mm, 2.5 mL) at 365 nm is centered at 955 nm. This result
indicates that polymerization through the continuous photodi-
merization of compound 3 is the dominant process.
As shown in Figure 6, upon irradiation for 90 min, both com-
pound 2, the Mn–Anderson cluster, and a mixture of com-
pound 2 and the Mn–Anderson cluster exhibit no obvious
color change. In contrast, compound 3 shows a significant
color change, from a light-yellow to deep-brown solution. This
can be attributed to the synergistic interactions between the
Mn–Anderson cluster and the coumarin moieties, leading to
photopolymerization.
To gain a better understanding of the resulting polymeric
material, average molecular weight and polydispersity analyses
were carried by gel-penetration chromatography (GPC, Agilent
2600 series). The column model was the Polymer Laboratory
(PL) gel 5 mm 104 ꢀ. Both the temperature of the column and
the detector was 258C. A sample of the C-(A-C-C)n-A-C polymer
was obtained by photoirradiation of compound 3 at 365 nm in
DMF for 90 min. 20 mL of sample solutions of compound 3 or
the C-(A-C-C)n-A-C polymer were then injected onto the
column at a DMF flow rate of 1.0 mLminꢀ1. The resulting GPC
traces were converted to GPC distributions by using a calibra-
tion curve. Figure 8 shows the GPC distribution curves for com-
pound 3 and the C-(A-C-C)n-A-C polymer of compound 3. The
average molecular weight (Mw) of the C-(A-C-C)n-A-C polymer
can be calculated to be 337.38 kDa with a polydispersity value
of 1.60.
To strengthen the potential applications of this reversible
light-modulated process, we have prepared film samples,
based on compound 3, in the presence of agarose. Upon irra-
diation at 365 nm, the film samples show dramatic color
changes (Figure 7a) and the relative intensity of photoemission
decreases dramatically (Figure 7b). Furthermore, this reversible
light-driven process can be realized by applying irradiation at
365 nm and 254 nm, as evidenced by the fluorescence spectra
of the film samples (Figure 7c).
To summarize, compound 3 has been successfully prepared
by covalently tethering coumarin moieties onto a Mn–Ander-
son cluster, resulting in the formation of a new organic–inor-
ganic hybrid, which has been fully characterized by single-crys-
tal X-ray crystal structure and ESI-MS. Photoirradiation of com-
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2 were recorded on a Xevo G2 QT mass spectrometer by using
methanol as solvent, whereas the ESI-MS spectrum of compound 3
was recorded on a Bruker APEX IV FTMS in negative mode by
using MeCN as solvent. Fourier transform infrared (FTIR) spectra
were recorded on a Bruker Vector 22 infrared spectrometer using
the KBr pellet method; intensities donated as vs=very strong, s=
strong, m=medium, w=weak, and vw=very weak. UV/Vis ab-
sorption analysis was recorded on a TU-1901 double-beam ultra-
violet–visible spectrophotometer. Elemental Analyses were com-
pleted by using a VarioEL cube from Elementar Analysensysteme
GmbH. The average molecular weight and the polydispersity analy-
ses were carried by gel-penetration chromatography (GPC, Agilent
2600 series).
Ethyl 2-((4-methyl-2-oxo-2H-chromen-7-yl)oxy) acetate[20] (1) and N-
(1,3-dihydroxy-2-(hydroxymethyl)propan-2-yl)-2-((4-methyl-2-oxo-
2H-chromen-7-yl)oxy)acetamide[5f] (2, denoted as Coumarin-Tris)
were prepared according to literature methods.
Figure 7. a) The color change of film samples; b) fluorescence spectra before
(black) and after (grey) irradiation for 90 min; c) reversible fluorescence re-
sponse over five consecutive cycles of 365 nm and 254 nm irradiation.
Synthesis of [(TBA)3(MnMo6O18-
{(OCH2)3CNHC12H9O4}2)]·2DMF·0.5H2O (3)
A
mixture of TBA4[a-Mo8O26] (2.00 g, 0.93 mmol), Mn-
(CH3COO)3·2H2O (0.37 g, 1.4 mmol), and (HOCH2)3CNHC12H9O4 (2)
(1.08 g, 3.2 mmol) in acetonitrile (50 mL) was heated at reflux for
24 h. The orange solution was cooled to room temperature and
then exposed to diethyl ether vapor for two weeks. The orange
precipitate was filtered and then dissolved in a mixture of MeCN/
DMF (1:1). Small orange crystals can be obtained after three weeks.
Yield 2.26 g (79%, based on TBA4[a-Mo8O26]). 1H NMR (400 MHz,
[D6]DMSO) d=7.96 (s, 2H), 7.74 (d, J=6.33 Hz, 2H), 6.98 (s, 2H),
6.20 (s, 2H), 4.95 (s, 4H), 3.25–3.07 (m, 24H), 2.40 (s, 6H), 1.57 (s,
24H), 1.32 (d, J=13.39, 6.58 Hz, 24H), 0.94 ppm (t, J=7.00 Hz,
36H). FTIR (KBr): n˜ =3341 (w), 2960 (s), 2936 (s), 2872 (s), 1716 (s),
1681 (s), 1617 (w), 1560 (vw), 1484 (s), 1387 (m), 1348 (w), 1256
(w), 1151 (w), 1129 (w), 1108 (m), 1040 (s), 941 (s), 921 (s), 904 (s),
666 cmꢀ1 (vs); elemental analysis calcd (%) for C86H155MnMo6N7O34.5
C 41.82, H 6.33, N 3.97; Found C 41.66, H 6.23, N 3.55.
:
Crystal data
2[(TBA)3(MnMo6O18{(OCH2)3CNHC12H9O4}2)]·4C3H7NO·H2O, M=4939.5;
¯
triclinic; space group P1 (2); a=18.4101(4), b=18.6082(3), c=
Figure 8. GPC curves of compound 3 and the C-(A-C-C)n-A-C polymer of
compound 3.
20.3260(4) ꢀ; a=64.329(2), b=65.195(2), g=60.647(2)8; V=
5276.57(22) ꢀ3. CCDC-963940 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
pound 3 at 365 nm resulted in polymerization through a light-
driven cyclobutane-type dimeric species. This process has been
characterized in solution by UV/Vis, 1H NMR, 2D NMR (COSY)
spectroscopy, and by DLS measurements. Film samples of com-
pound 3, in the presence of agarose, have been prepared. The
change in the fluorescence properties of these film samples
provides further evidence for a reversible light-modulated pro-
cess resulting from polymerization.
Preparation of film sample
The agarose films of compound 3 were synthesized by a facile gel-
casting method. Agarose powder (0.2 g) was added slowly to DMF
(10 mL) with vigorous stirring at 808C, to create a cloudy suspen-
sion, for 1 h. Compound 3 was then added to the clear casting so-
lution under vigorous stirring. After elimination of the micro-bub-
bles in a vacuum oven, the mixture was cast on indium-tin oxide
glass substrates, and dried at 808C overnight. Then, the self-sup-
porting film was peeled from the substrates.
Experimental Section
All chemicals were purchased from Alfa Aesar and were used with-
out further purification except for MeCN, which was dried by distil-
Acknowledgements
1
lation in the presence of CaH2 prior to use. H and 13C NMR spectra
This work was supported by the National Natural Science
Foundation of China (21076020, 21222104), the Fundamental
were recorded on a AV400 NMR spectrometer with [D6]DMSO sol-
vent. The electrospray mass spectra (ESI-MS) of compounds 1 and
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Research Funds for the Central Universities (ZZ1227), and the
Beijing Nova Program (2009B12).
Keywords: coumarins
·
covalent
functionalization
·
photopolymerization · polyoxometalates
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Received: October 8, 2013
Published online on January 16, 2014
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