Cyclodextrin-/photoisomerization-modulated assembly and disassembly of an azobenzene-grafted polyoxometalate cluster

Article abstract of DOI:10.1039/c8dt05146a

Herein, a mono-lacunary Keggin-type polyoxometalate (POM), [SiW₁₁O₃₉]^8?, grafted with an azobenzene group through Sn ion bridging was prepared, and the formed organic-inorganic hybrid cluster was characterized via elemental analysis, NMR, TGA, and IR techniques. A vesicular structure of the hybrid cluster assembly in aqueous media was observed in the TEM image, and it dissociated in the presence of α-/β-, γ-cyclodextrins (α-/β-, γ-CDs); this dissociation was driven by the host-guest interactions. The monodispersed inclusion complex further reassembled into smaller micelles under irradiation with 365 nm light, and this transformation was reversibly controlled by alternating the irradiation with 450 nm light. Moreover, in the case of the POM-Azo/β-CD system, reassembly from the monodispersed state to the vesicular state was achieved by the addition of a competitive guest molecule. Thus, the reversible host-guest interactions combining reversible photoisomerization of the azobenzene group provided multiple ways to modulate the assembly and disassembly of the POM hybrid as well as the changes between different assemblies. The present study inspires the potential use of these kind of hybrid POMs in enhanced catalytic reactions and recycling.

Full text of DOI:10.1039/c8dt05146a

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DOI: 10.1039/C8DT05146A
Journal Name
ARTICLE
Cyclodextrin-/Photoisomerization- Modulated Assembly and
Disassembly of An Azobenzene-Grafted Polyoxometalate Cluster
Received 00th January 20xx,
Accepted 00th January 20xx
Hongbo Li, Fengrui Jiang, Guohua Zhang, Bao Li, and Lixin Wu*
A mono-lacunary Keggin-type polyoxometalate (POM) [SiW11O39]
8‒ grafting an azobenzene group through a Sn ion bridging
DOI: 10.1039/x0xx00000x
was prepared and the formed organic-inorganic hybrid cluster was characterized by means of elemental analysis, NMR, TGA,
and IR. A vesicular structure of the hybrid cluster assembly in aqueous media was observed in TEM image and it dissociated
in the presence of α-/β-, - cyclodextrins (α-/β-, - CDs) driven by host-guest interaction. The monodispersed inclusion
complex further re-assembled into smaller micells under the irradiation of 365 nm light, and this transformation was
reversibly controlled by alternating irradiation of 450 nm light. Moreover, in the case of POM-Azo/β-CD system, the re-
assembly from monodispersed state to vesicle was also achieved by the addition of a competitive guest molecule. Thus, the
reversible host–guest interaction combining reversible photoisomerization of azobenzene group provided multi ways to
modulate assembly and disassembly of POM hybrid as well as changes between different assemblies. The present study
inspired the potentials of this kind hybrid POMs using in enhanced catalytical reactions and recycles.
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like those of surfactant enwrapped POM systems.14-17 The assembled
structure of a POM that covalently grafted alkyl chains was also
reported to achieve a morphologic alteration between spheres and
tubes through electrochemistry controlling to the surface charges of
POMs.18 Though the POM component usually serves as a responsive
unit, the operation to the grafting organic group is more favourable
when further functionalization of POM clusters is required in their
assembled system.19-23
Host-guest interaction is widely used in the modulation of self-
assemblies of organic molecules by introducing external recognition
components because such an interaction can break the original
driving force occurred between guest groups or collect the separated
building units to aggregate together.24, 25 Thus, when a suitable guest
group is linked to a POM via covalent bond, an additive host molecule
can act as the stimuli agent to trigger the interaction change between
hybrid POM clusters while the POM’s structure property maintains.
Following the strategy, to drive the assembly and disassembly of a
hybrid POM cluster, we herein report a covalently hybridized Keggin
type POM by incorporating a tin ion onto a lacunary silicatungstate
as an active site for further modification. Following by grafting a
Introduction
Self-assembly of colloidal particles and particle like giant
molecules displays series of interesting structures and significant
functional potentials beyond general small molecules, such as
diverse liquid crystalline phases,1 photonic crystals,2 template
induced self-assemblies for synthesis of porous materials,3 and so
forth. Among those known particles with sizes from micro- to nano-,
even smaller scale, polyoxometalates (POMs) represent a type of
anionic nanosized inorganic clusters featuring a broad range of
precise compositions and uniform multi-topological architectures
with rich physicochemical properties,4 and receive special attentions
on their self-assembly studies over the past decades.5,6 Because the
assemblies of these molecular nanoclusters can be used in controlled
catalytic reactions,7 interfacial growth of hybrid catalysts and
connecting layers of solar cells,8,9 it is of importance to understand
the assembly and disassembly process in solution systems toward
stimulus-response modulation. Liu and his co-workers have firstly
demonstrated the formation of “blackberry” structure and assembly
process of several naked giant POM clusters versus the change of
solvent polarity.10-13 With the assistance of covalently grafted organic
groups and other inorganic clusters, the POMs’ assembly becomes
easy to be built and modulated via an enhanced hydrophobic force,
spacer group and utilizing
a click reaction, the photoactive
azobenzene (Azo) guest group was connected as the end group,
affording an organic-inorganic hybrid cluster with formula of
TBA0.7H4.3SiW11O39SnC21H23N6O (POM-Azo, where TBA is
tetrabutylammonium). The hybrid POM exhibited amphiphilic
behaviour and conducted dynamic assembly structure
transformations from vesicular state to a dispersed state via both
UV- light irradiation and the host-guest interaction with cyclodextrins
(CDs). The reversible change can be also realized by visible light
State Key Laboratory of Supramolecular Structure and Materials, College
of Chemistry, Jilin University, Changchun, 130012, China. E-mail:
wulx@jlu.edu.cn.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See DOI:
10.1039/x0xx00000
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irradiation, heating, and the addition of another guest molecular Infrared (FT-IR) spectra in pressed KBr pellets were recorded on a
component (Scheme 1). Importantly, the present study provides a
Bruker Vertex 80v spectrometer at range ofD4O0I0: 1−0.41003090/Ccm8D−T10w5i1t4h6Aa
resolution of 4 cm−1. The UV-Vis spectra were obtained on a Varian
CARY 50 Probe instrument. Dynamic light scattering (DLS) was
carried out on a Zetasizer Nano ZS instrument at 25 °C. Circular
dichroism spectrum (CDS) was performed on a PMS−450 instrument
with a step of 0.5 nm and a resolution of 4 nm s−1 in water at 25 °C.
Scanning electron microscopic (SEM) images were collected on a
JEM-2100F electron microscope with accelerating voltage of 200 KV.
Transmission electron microscopic (TEM) images were obtained on a
JEOL JEM 2010 without staining. The cryogenic transmission electron
microscopy experiment (cryo-TEM) was carried out on a film of
solution and moved to a grid under controlled humidity and
temperature condition. Excess solution was removed by filter paper
and the film was put into liquid ethane quickly in order to freeze with
liquid nitrogen. Then images were collected on a JEOL JEM 2010
instrument.
Synthesis of NH2(CH2)3N3. To the doubly distilled water (10 mL)
dissolving 3-aminopropanyl bromide (3.28 g, 15.00 mmol) was added
sodium azide (3.25 g, 50.00 mmol) dissolving doubly distilled water
(15 mL). After 16 h of refluxing, diethyl ether and then potassium
hydroxide (4.00 g) was added to the reaction solution under an ice-
water bath (lower than 10 oC). Organic phase was separated and then
dried over K2CO3. The evaporation of organic solvent gives yellowish
oily product in yield of 80%. 1H NMR (500 MHz, CD3CN, 25 oC): δ=1.23
(s, 2H), 1.62 (m, 2H) 2.70 (t, 2H), 3.27 (t, 2H).
Scheme 1. Schematic formation process of POM-Azo assembly and
its controlled transformations to disassembly under the multiple
stimulus conditions.
Synthesis of HC≡CCH2O(C12H9N2). The guest molecular component
bearing azobenzene group was synthesized according to procedures
described in literature.28 Briefly, to
a
solution of 4-
convenient route to modulate the assembly and disassembly of POM
cluster particles.
hydroxyazobenzene (0.50 g, 2.53 mmol) in anhydrous acetone (30
mL) was added K2CO3 (1.74 g, 12.61 mmol). After stirring at room
temperature for 0.5 h under nitrogen atmosphere, propargyl
bromide (1.50 g, 12.61 mmol) was added to the reaction. The mixture
was subsequently stirred at room temperature for 24 h. The obtained
residue was purified by column chromatography over silica gel (200–
Experimental Section
Materials. Mono-lacunary POM (K8SiW11O39) and initial organic
grafting agent Cl3Sn(CH2)2COOH were prepared following the
procedures as described elsewhere.26, 27 All organic agents including
α-, β-, and - CD, Adamantane-1-carboxylic acid that was used in its
sodium salt (Ad-COONa) in neutral condition, and all solvents were
obtained from commercial suppliers. Diethyl ether and acetone were
dried through distillation over CaH2. HCl gas was produced by slowly
adding concentrated hydrochloric acid to concentrated sulfuric acid
upon stirring, dried over concentrated sulfuric acid, and then lead in
reactor. All other compounds and solvents were used without
further purification. Distilled and deionized water was used in all
experiments.
300
mesh)
with
a
mixture eluent
of petroleum
ether/dichloromethane in 5:1 (v/v), giving an orange solid in 91%
yield. 1H NMR (500MHz, DMSO-d6, 25 °C): δ=3.66 (t, 1H), 4.94 (d, 2H),
7.21 (t, 2H), 7.54 (t, 1H), 7.59 (d, 2H), 7.85 (d, 2H), 7.92 (t, 1H). Anal.
Calcd for [C15H12ON2, M=236.27]: C 76.25%, H 5.12%, N 11.86%.
Found: C 76.55%, H 5.12%, N 11.77%.
Synthesis of TBA5[SiW11O39Sn(CH2)2COOH] (TBA-POM-COOH).
Following a similar procedure,27 to acetonitrile (30 mL) was added
Cl3Sn(CH2)2COOH (0.18 g, 0.59 mmol), tetrabutylammonium bromide
(1.31 g, 4.06 mmol), and then previously prepared mono-lacunary
POM K8SiW11O39. (1.22 g, 0.41 mmol) The mixture was stirred under
nitrogen atmosphere for 1.0 h at room temperature. After filtration
and evaporation of excess solvent, white solid was obtained. The
crude product was dissolved in acetone (5 mL) and ethanol (5 mL),
and then precipitated from diethyl ether, giving purified product
TBA-POM-COOH as that reported in yield of 68%. 1H NMR (500 MHz,
CD3CN, 25oC): δ=1.03 (t, 64H), 1.24 (t, 2H), 1.40 (m, 48H), 1.68 (m,
Instruments. Proton nuclear magnetic resonance (1H NMR) spectra
were obtained on a Bruker Avance 500 MHz instrument with
tetramethylsilane (TMS) as internal reference. Elemental analysis (C,
H, N) were conducted on a Vario micro cube, while inorganic
elemental analysis for Si, Sn and W was performed on an OPTOMA
3300DV instrument. Thermogravimetric analysis (TGA) was carried
out on a Q500 Thermal Analyzer (New Castle TA Instruments) under
a flowing air with a heating rate of 10 °C min−1. Fourier Transform
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48H), 2.69 (t, 2H), 3.17 (t, 48H). Anal. Calcd for [C83H185N5SiW11O41Sn, filtrate was evaporated to dryness. The crude product was re-
DOI: 10.1039/C8DT05146A
M=4078.4]: C 24.44%, H 4.57%, N 1.72%, Si 0.69%, W 49.58%. Found: dissolved in acetone (10 mL) upon sonication and then treated with
C 24.82%, H 4.48%, N 1.75%, Si 0.69%, W 49.68%.
dichloromethane (10 mL) and diethyl ether (120 mL), and the formed
precipitate was dried in a vacuum dessicator overnight to give orange
solid product in 60% yield. 1H NMR (500 MHz, CD3CN, 25 °C): δ=1.00
(t, 60H), 1.21 (t, 20H), 1.41 (m, 40H), 1.66 (m, 40H), 2.22 (m, 2H), 2.66
(t, 2H), 3.17 (t, 40H), 3.20 (m, 2H), 4.53 (t, 2H), 5.33 (s, 2H), 7.26 (d,
2H), 7.52 (t, 1H), 7.58 (t, 2H), 7.89 (d, 2H), 7.95 (d, 2H), 8.27 (s, 1H).
IR (KBr, cm−1): v=537, 799, 879, 913, 965, 1004, 1064, 1106, 1149,
1348, 1383, 1478, 1541, 1585, 1598, 1632, 1659, 1723, 2879, 2939,
2966. Anal. Calcd for [C101H203N11SiW11O41Sn, M=4396.78] C 27.60%,
H 4.65%, N 3.50%, Si 0.64%, W 45.99%, Sn 2.70%. Found: C 27.65%,
H 4.66%, N 3.42%, Si 0.64%, W 46.31%, Sn 2.70%.
Synthesis of TBA5[SiW11O39Sn(CH2)2CO] (TBA-POM-CO). To conduct
the amidation reaction, the above prepared TBA-POM-COOH was
firstly transferred into (TBA)5[SiW11O39Sn(CH2)2CO] (TBA-POM-CO).
To 10 mL acetonitrile solution of TBA-POM-COOH (500 mg, 0.13
mmol) was added triethylamine (40 μL, 0.14 mmol). After 5 min of
stirring to the mixture solution, isobutyl ester of chloroformate (20
μL, 0.16 mmol) was added, and then the solution was stirred under
nitrogen atmosphere. Cationic exchange resin and 10 ml of acetone
were added to the reaction solution with stirring for another 1.0 h.
Oily crude product was obtained after filtration and evaporation of
the excess solvent. The purification was performed by dissolving in Synthesis of H4.3TBA0.7[SiW11O39Sn(C6H11ON)(C2HN3)CH2O(C12H9N2)]
acetone (3 mL) and dichloromethane (5 mL), and precipitating from (POM-Azo). POM-Azo was obtained through running TBA-POM-Azo
diethyl ether (50 mL) with twice of washing with diethyl ether. over a silica gel (200–300 mesh) column with CH3CN as eluent
Dryness in vacuum gives purified intermediate product in yield of because TBA cations is hard to be exchanged via normal ionic
20%. 1H NMR (500 MHz, CD3CN, 25 oC): δ=1.01 (t, 64H), 1.36 (m, 2H), exchange columns. After evaporating excess solvent, the final
1.42 (m, 48H), 1.67 (m, 48H), 2.80 (m, 2H), 3.17 (t, 48H). IR (KBr, product exhibiting good solubility in water was obtained as an orange
cm−1): v=474, 506, 527, 542, 647, 737, 768, 801, 840, 878, 920, 966, solid in 75% yield. 1H NMR (500 MHz, CD3CN, 25 °C) : δ=0.92 (t, 2H),
1007, 1211, 1379, 1481, 1729, 2872, 2934, 2961. Anal. Calcd for 0.94 (t, 8.4H), 1.32 (m, 5.6H), 1.58 (m, 5.6H), 2.03 (m, 2H), 2.36 (t,
[C83H184N5SiW11O40Sn, M=4061.4]: C 24.55%, H 4.57%, N 1.72%, Si 2H), 3.07 (m, 2H), 3.17 (t, 5.6H), 4.43 (t, 2H), 5.27 (s, 2H), 7.28 (t, 2H),
0.69%, W 49.79%. Found: C 24.81%, H 4.35%, N 1.75%, Si 0.69%, W 7.53 (d, 1H), 7.58 (t, 2H), 7.87 (d, 2H), 7.92 (d, 2H), 7.94 (t, 1H), 8.37
49.78%.
(s, 1H). IR (KBr, cm−1): v=524, 538, 801, 912, 965, 1007, 1146, 1251,
1466, 1499, 1556, 1653, 2876, 2921, 2961. Anal. Calcd for
[C32.2H52.5N6.7SiW11O41Sn, M=3358.99]: C 11.52%, H 1.58%, N 2.79%,
Si 0.84%, W 60.21%, Sn 3.54%. Found: C 11.48%, H 1.61%, N 2.80%,
Si 0.84%, W 60.25%, Sn 3.55%.
Synthesis of TBA5[SiW11O39Sn(CH2)2CONH(CH2)3N3] (TBA-POM-N3).
The Azo grafting POM complex was synthesized in acetonitrile
following the similar procedures described in literature,27 To the
acetonitrile solution (30 mL) of above prepared TBA-POM-CO (1.33
g, 0.34 mmol) was added triethylamine (57 μL, 0.41 mmol) and Sample preparation. Typically, sample solutions (normally 8.4 μM)
NH2(CH2)3N3 (127 μL, 1.36 mmol). The mixture solution was stirred were prepared at room temperature (25 °C) by dissolving POM-Azo
overnight. After the addition of cationic exchange resin and acetone in distilled water and allowed to stand for 15 min to get self-assembly
(10 mL), the reaction solution was stirred for another 1 h, then equilibrium. The host-guest interaction was carried out by treating
filtered. The filtrate was concentrated by rotary evaporation to oily the sample solution with cyclodextrins at a molar ratio of 1:1 and
crude product. By adding acetone (7 mL), ethanol (7 mL) and diethyl then stirring for 0.5 h. Photo-isomerization experiments were carried
ether (30 mL) to the purified product was precipitated. Drying in out by exposing the sample solution in a quartz cell to a UV light
1
vacuum gives white solid in 84% yield. H NMR (500 MHz, CD3CN, (WHF 203, 8 W, 365 nm) at a distance of 5 cm from light source. The
25 °C): δ=1.01 (t, 60H), 1.19 (m, 2H), 1.43 (m, 40H), 1.67 (t, 40H), 1.81 visible light irradiation was performed in a similar way using a 200-W
(m, 2H), 2.65 (m, 2H), 3.19 (t, 40H), 3.30 (m, 2H), 3.42(t, 2H), 6.91 (s, incandescent light source. The samples for TEM were prepared by a
1H). IR (KBr, cm−1): v=537, 799, 879, 913, 965, 1004, 1064, 1106, 1149, drop-casting method on Cu grids, and diluted with a drop of water to
1348, 1383, 1478, 1541, 1629, 1658, 1726, 2095, 2879, 2939, 2966. get a good assembly dispersion, drying overnight. The sample for
Anal. Calcd for [C86H191N9SiW11O40Sn, M=4160.51]: C 24.83%, H SEM was prepared by casting the self-assembly dispersion onto a
3.63%, N 3.03%, Si 0.68%, W 48.61%, Sn 2.85%. Found: C 24.55%, H silicon wafer. After standing for 30 s, the excess solution was
3.65%, N 2.98%, Si 0.69 %, W 48.92%, Sn 2.85%.
removed using a filter paper. The sample preparation for Cryo-TEM
was performed with a thin water film on a lacey-supported grid
under certain temperature and humidity conditions. The filter paper
was used to remove excess solution. The prepared Cu grid was placed
in liquid ethane and then in liquid nitrogen. The thin water film on
the grid was preserved utilizing a cryotransfer device. Finally, the
prepared samples were used for cryo-TEM imaging measurement.
Synthesis of TBA5[SiW11O39Sn(C6H11ON)(C2HN3)CH2O(C12H9N2)]
(TBA-POM-Azo). TBA-POM-N3 (0.83 g, 0.20 mmol) in acetonitrile (5
ml) was mixed with HC≡CCH2O(C12H9N2) (0.095 g, 0.40 mmol).
CuSO4·5H2O (0.05 g, 0.20 mmol) in 5 mL of water and sodium L-
ascorbate (1.59 g, 8.00 mmol) in 5 mL of water were added to the
solution, and subsequently, the solution was stirred under nitrogen
atmosphere at room temperature for 24 h. Cation-exchange resin
and acetone (50 mL) were added to the reaction solution, and then
stirred for 1.0 h until the precipitate disappeared. After filtration, the
Results and discussion
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Characterization and self-assembly of POM-Azo. As shown in diameter of ca. 60 nm. The TEM image indicates the spherical
Scheme 2, TBA-POM-Azo was synthesized by an initial amidation of assembly and the formation of vesicular likDeOaIs:s1e0m.10b3li9e/sC8bDyTc0a5s1t4i6nAg
3-aminopropylazide, and then a clik reaction with propargyl
sample solution onto a copper grid and then sucking with filter paper.
Scheme 2. Detailed synthetic steps of POM-Azo.
azobenzene moiety to an organically grafted lacunary Keggin-type
POM, TBA-POM-COOH. To increase the solubility of the hybrid
cluster in aqueous media and exclude the influence deriving from
counterion, TBA cations were substituted with protons by an ionic
exchange reaction over a silica gel though the replacement was still
not complete even after three cycles due to the tight binding of TBA.
The obtained POM-Azo was characterized by elemental analysis, FT-
IR and NMR spectroscopy, and TGA. IR spectrum (Fig. S1) of POM-
Azo shows bands at 2876 and 2961 cm−1, which are attributed to
methyl and methylene C–H stretching vibrations of residual TBA and
organic linker. The peaks at the region of 1500–1700 cm−1 are
ascribed to amide I and II bands, which partially overlap with the
vibration of virgin POM. In addition, the POM-Azo framework is
confirmed by the existence of characteristic peaks in the region of
800−1100 cm−1, e.g. 1007 cm−1 (Si−O), 965 cm−1 (W−Oa), 912 cm−1
(W−Ob), and 801 cm−1 (W−Oc), where Oa: terminal O atom; Ob: bi-
bridging O atom; and Oc: tri-bridging O atom. The 1H NMR spectrum
demonstrates the successful synthesis and purification of POM-Azo
and existence of ca. 0.7 of TBA based on the integral of residual peaks
(Fig. S2). The TG measurement (Fig. S3) indicates 81.7 wt% remaining
of inorganic residual at ca. 470 °C, which is in perfect agreement with
calculated 82.2 wt% of inorganic component content as fully oxidized
products. The elemental analysis result (Table S1) is also in
accordance to the calculated values based on the molecular formula
of target chemical composition, further confirming the successful
preparation of the hybrid complex. Importantly, both TGA and
elemental analysis point to the ratio of inorganic and organic
component in POM-Azo.
Fig 1. DLS curves of (a) pure POM-Azo in aqueous solution versus the
light irradiation, and (b) POM-Azo in inclusion system of α-CD at
molar ratio of 1:1 before and after UV-/visible light irradiation for 0.5
h at 25 °C.
The observed layered substructure appears at the edge of self-
assembled spheres (Fig. 2a). To exclude the influence from solvent
evaporation, Cryo-TEM is carried out for an in-situ measurement to
demonstrate the self-assemblies forming in aqueous solution (Fig.
2b). Definitely, we observed spherical assemblies from sample
solution with size scale in agreement with those appeared in DLS and
dried sample TEM results. As azobenzene group serves as the
hydrophobic tail, the amphiphilic nature of the hybrid cluster drives
the formation of both aggregation morphology and packing structure
in aqueous solution. The ordered aggregation of azobenzene groups
can be identified through the change of UV-visible spectrum.29 In
contrast to the absorption band position of azobenzene group in its
monodispersed state, ordered aggregation leads to a band shifting in
the spectrum because of π-π interaction induced change of
electronic energy level. Due to the less effect of host-guest
interaction on the electronic state, the absorption band of included
azobenzene group is ascribed to the absorption of monodispersed
state. Thus, the observed blue shift (from 343 nm to 336 nm) of
absorption band (Fig. S4) demonstrates the ordered assembly for
POM-Azo alone in aqueous solution and it is rational for the hybrid
cluster to form a bilayer structure in the vesicular assemblies.
Even after covalent connection of hydrophobic azobenzene
moiety, the ionic substituted POM-Azo maintains soluble in water
and performs increased amphiphilic nature. DLS measurement (Fig.
1a) reveals the aggregation of the hybrid cluster with a hydration
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Modulation of POM-Azo assembly via CD inclusion. Since the displays a morphologic consistence, and the observed high contrast
DOI: 10.1039/C8DT05146A
azobenzene moiety is known as a typical guest group to have domain with size of ca. 1.0 nm supports the mono- dispersion of
inclusion interaction with some CD hosts, the addition of CDs can be POM-Azo in the presence of α-CD.
used for the modulation of the self-assemblies of POM-Azo. To
Modulation of POM-Azo assembly via photo-irradiation. While
Fig 2. (a) TEM and (b) Cryo-TEM images of POM-Azo in aqueous
solution with a concentration of 8.4 μM at 25 °C, and TEM images of
the solution after exposing to (c) UV light for 0.5 h and then to (d)
visible light for another 0.5 h.
detect the inclusion interaction, aqueous solution of POM-Azo is
treated with equimolar amount of α-CD, and the mixture is
incubated for 0.5 h at room temperature. It is seen that with the
addition of α-CD, the induced Cotton signal of azobenzene group
Fig 3. Circular dichroism spectra of POM-azo (black); POM-azo and β-
CD with the molar ratio of 1:1 at the initial (red), after UV light
irradiation for 0.5 h (blue), and after visible light irradiation for 0.5 h
(green); POM-azo, β-CD and Ad-COONa with the molar ratio of
appears and it can be well interpreted to derive from the transfer of
inner chirality in CD cavity due to the host-guest interaction (Fig.
31
3a).30,
In 1H NMR spectrum, chemical shifts sourcing from
azobenzene group move larger and broaden after α-CD is added, also
confirming the inclusion interaction (Fig. S5). In addition, H−H
correlations between POM-Azo (in trans- state) and the inner cavity
of α-CD are observed in 2D NOESY NMR spectrum of POM-Azo and
α-CD mixture solution, further supporting the formation of inclusion
complex (Fig. S6). The correlations of protons attributing to H(h) and
H(i) from Azo group with the protons H(3) and H(5) deriving from
inner cavity also indicate the primary face of α-CD getting into
azobenzene firstly. Just like that the assembly of POM-Azo derives
from the hydrophobic interaction between organic moiety, the
shielding to the hydrophobic interaction triggers a dissociation of the
formed assemblies. The inclusion of α-CD isolates azobenzene
groups and relieves the hydrophobic interaction among these
organic groups. After mixing with α-CD, the observed POM-Azo
assemblies dissociate quickly from aggregated state with diameter of
ca. 60 nm to a dispersed state with a hydrodynamic diameter of ca.
3.6 nm, as shown in DLS measurement (Fig. 1b). Since this size scale
is very close to the inclusion complex comprising of POM-Azo and α-
CD, we believed that the inclusion complex becomes its
monodispersed state. The characterization of TEM (Fig. 4a and 4b)
1:1:1.1 (purple) in water at 25 °C.
acting as a guest unit and hydrophobic tail, azobenzene moiety is also
a photoactive group exhibiting trans- and cis- isomerization change,
which can be used for the modulation of POM-Azo assemblies.32, 33
As shown in Fig. S7a, most of trans-azobenzene groups of POM-azo
transform into cis-form upon UV light irradiation and the
isomerization equilibrium can be reached in ca. 1.0 min. Based on the
1
calculation of proton integral value in H NMR spectra (Fig. S8), the
trans-to-cis conversion efficiency is estimated to be ca. 92% and the
efficiency for reverse cis-to-trans transition is ca. 94% with visible
light irradiation of 450 nm in 2 min (Fig. S7b). The reversible
transformation from trans- to cis- transition does not show obvious
weakening in five cycles (Fig. S7c and S7d). The conformation of
azobenzene group in cis- state becomes unfavorable for the tight
packing due to increased hydrophilicity and the distorted spatial
structure, and thus the light irradiation of 365 nm to the original
POM-Azo solution leads to the regular spherical assembly fusing into
irregular morphology (Fig. 2c and 2d). Apparently, it is the distorted
conformation of azobenzene drives the assembly change.
Interestingly, the reverse conformation change of azobenzene group
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to the trans- state with 450 nm light irradiation promotes the α-CD, the disappeared Cotton signal clearly demonstrates the
DOI: 10.1039/C8DT05146A
assembly back to the regular spherical morphology, due to linear discharging of cis- azobenzene group from the CD’s cavity.
structure recovering of POM-Azo.
With this property, the photo isomerization of azobenzene group
attached in POM is then used for interaction modulation of included
Multiple modulation of POM-Azo assemblies via competitive
inclusion. For photo irradiation controlling process, the
transformation occurs from the assembly of POM-Azo alone in trans-
state to the assembly of POM-Azo in cis- state. In the inclusion
system with α-CD, the assembly of POM-Azo is dissociated only in
one way while the photoisomerization leads to the transformation
from monodispersed POM-Azo in trans- state to the assembly of
POM-Azo in cis- state.
To extend the reversible cycle of the stimuli-response and realize
a multiple modulation for the self-assemblies of POM-Azo, we use β-
CD to repeat the modulation of assembly and disassembly of POM-
Azo like that in the presence of α-CD. With the addition of 1:1 molar
ratio of β-CD, the cyclodextrin with larger cavity than α-CD, the POM-
Azo assembly dissociates following the same behaviors as that
occurred in the system containing α-CD, because of the identical
inclusion property of the two CDs. Similarly, a well-pronounced
chirality signal belonging to azobenzene group is observed in circular
dichroism spectrum of mixture aqueous solution containing
Fig 4. TEM images of POM-Azo solution mixed with α-CD mixture (1:1
equimolar POM-Azo (trans-) and β-CD (Fig. 3b). The change of
molar ratio) at the state (a, and local amplified b) before and (c) after
chemical shifts also appears in corresponding 1H NMR spectrum (Fig.
UV light irradiation (365 nm) for 0.5 h, and then (d) visible light
S9). These results indicate the host-guest interaction between POM-
irradiation for 0.5 h at 25 °C.
Azo and β-CD, which is further confirmed by the appearance of two
cross-correlation peaks between POM-azo (trans) and β-CD in 2D
NOESY NMR spectrum (Fig. S10). Moreover, the NOESY result
demonstrates that the primary face of CD in the inclusion complex
locates inside facing to the POM. The TEM observation to the POM-
Azo (trans- state):β-CD (1:1 molar ratio) mixture solution after
encountering a 15 min of sonication and 0.5 h of aging, shows that
POM-Azo system, where the strong inclusion of POM-Azo with α-CD
mainly occurs at the trans- state. Accompanying with the
illumination of 365 nm light to the POM-Azo solution mixing with α-
CD, the transition from isolated inclusion complex to spherical
assembly (Fig. 4c and 4d) is observed, which is quite similar to that of
POM-Azo alone subjecting to a UV light irradiation. Though the
POM-Azo becomes monodispersed (Fig. 5a). The subsequent UV light
polarity in cis- state becomes high with respect to the trans- state,
(365 nm) irradiation triggers the formation of POM-Azo into micelle
the azobenzene group is still hydrophobic and tends to decrease
like small assemblies as that of POM-Azo and α-CD mixture system
interfacial energy by forming assembly. Since the cis-isomer of POM-
due to its transformation of azobenzene group into cis- state (Fig. 5b
Azo is no longer complexed by α-CD, as a result, the observed
and 5c). The following visible light (450 nm) irradiation to the mixture
assembly of POM-Azo in cis- form in the inclusion system is quite
solution makes the assembly dissociated back to monodispersed
similar to that of POM-Azo alone subjecting to a UV light irradiation.
inclusion complex (Fig. 5d). The DLS and CDS measurements (Fig. S11
In agreement with TEM observation, the DLS measurement affords a
and Fig. 3b) indicate similar mechanism for the transformation cycle
hydrodynamic diameter of the aggregate at ca. 25 nm (Fig. 1b), much
between assembly and disassembly. Due to the larger cavity of β-CD
smaller than the speculated vesicular assembly in trans- state. This
than that of α-CD, the formed monodispersed state and the
assembly can be dissociated reversibly to the isolated state by simply
using a visible light irradiation with wavelength of 450 nm and the
assemblies can be further modulated. When a competitive guest
component, Ad-COONa, is added as the modulation agent (1.1
assembly and disassembly cycles can be also repeated several times
equiv.), its higher binding capability makes it easier replace
through alternate UV and visible light irradiations.
azobenzene from the inclusion with β-CD quickly. After aging for a
In addition to the UV-visible spectral characterization, the photo
while, TEM image shows the reappearance of vesicle like assemblies
isomerization induced dissociation and re-formation of host-guest
of POM-Azo (Fig. 6). Because of unloading from β-CD driving by
inclusion are evaluated by induced CDS change (Fig. 3a). After the
365 nm light irradiation to the included POM-Azo system, the
competitive inclusion of Ad-COONa, the free POM-Azo in trans- state
re-forms the assembly like that of the hybrid cluster alone. The DLS
induced Cotton signal close to 350 nm disappears completely, which
measurement also shows the size increase of complex assembly from
implies the getting out of guest azobenzene group from CD cavity.
ca. 5 nm to ca. 60 nm when adding Ad-COONa into the sample
But the signal re-emerges again after undergoing a 450 nm light
solution, demonstrating the transferring of monodispersed state of
irradiation. As the chirality induction only occurs inside the cavity of
included POM-Azo with β-CD to the larger assembly of free POM-Azo
in non-included state. Such a competitive inclusion of Ad-COONa can
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also be used to the photo isomerization system of POM-Azo with β- controls, together with the competitive inclusion agent, provides rich
DOI: 10.1039/C8DT05146A
CD after encountering a UV light (365 nm) irradiation. When a visible transformations (Scheme 1) of assembly structure for one POM in
light (450 nm) is used, the assembly of POM-Azo in cis- state does not solution system. The present results demonstrate that the POM-Azo
get back to the included state, but instead, forms vesicular hybrid cluster could be a multifunctional building block for self-
assembled materials. On the other hand, the strategy used in this
study can also help in searching effective strategies for controlling
the existing state of POMs, which are important for the applicable
catalysis in both homogenous and heterogenous systems.
assemblies like that of isolated POM-Azo. In agreement with β-CD, -
CD display a very similar behaviour in modulation the assembly of
POM-Azo and competitive inclusion (data not shown) though its
inner cavity is a bit larger.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
The authors acknowledge the financial supports from the National
Natural Science Foundation of China (21773090, 21574057), the
Changbaishan Distinguished Professor Funding of Jilin Province, and
the Program for JLU Science and Technology Innovative Research
Team (2017TD-10).
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