Redox-active organic-inorganic hybrid polyoxometalate micelles

Article abstract of DOI:10.1039/c7ta00408g

A redox-active hybrid organic-inorganic polyoxometalate surfactant showed solvent-dependent self-assembly to form nano-scale architectures. The supramolecular assemblies exhibited contrasting electronic structure and redox activity to their molecular building units, and were found to be stable under electrochemical reduction and re-oxidation.

Full text of DOI:10.1039/c7ta00408g

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
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Redox-active organic-inorganic hybrid polyoxometalate micelles
Katharina Kastner,a Alexander J. Kibler,a Erno Karjalainen,b Jesum A. Fernandes,c Victor Sans,b, c
and Graham N. Newtona*
*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
A
redox-active hybrid organic-inorganic polyoxometalate long aliphatic alkyl chains, to form organic-inorganic hybrid
surfactant showed solvent-dependent self-assembly to form nano- molecules which can assemble to form micellar, vesicular and
scale architectures. The supramolecular assemblies exhibited liquid crystal-like states. Polarz, Song, Cronin, Liu and their co-
contrasting electronic structure and redox activity to their workers have pioneered the field of covalently functionalized
molecular building units, and were found to be stable under polyoxometalate-surfactants, and shed light on their complex
electrochemical reduction and re-oxidation.
architectures and self-assembly,13-24 however, relatively little is
known about the physicochemical properties of the nanoscale
hybrid assemblies, and in particular the translation of POM
functionality across the scales, from molecular to
supramolecular and bulk.
The development of new strategies to sustainably transform
chemicals, harness solar energy and a range of other catalytic
applications requires novel approaches to confine active
materials in a controlled fashion. In this regard, hybrid organic-
inorganic micellar and vesicular materials can exhibit enhanced
functionality, tunability and stability (pH, redox and thermal)
Covalently hybridised POM surfactants offer several
opportunities for development. Their composition,
morphology and physical properties can be tuned not only
through the chain length, conductivity and terminal functional
groups of the organic ligand, but also through control of the
shape, size, charge, photochemistry and electronic structure of
the POM head group. It is thus possible to design molecular
building blocks that will confer specific synergic properties to
2
compared to purely organic systems,1, leading to potential
applications in a wide range of catalytic processes.2, 3 Organic-
inorganic hybrid micellar assemblies have also been shown to
act as templates in precise metal nanoparticle synthesis for
energy storage and conversion applications.4
their organic-inorganic hybrid supramolecular assemblies.25
A
Polyoxometalates (POMs) are nanoscale molecular metal
oxides, typically constructed from oxo-bridged W, Mo or V ions
in their highest oxidation states.5 They exhibit rich, reversible
electrochemistry, excellent stability in acidic media and
function as photo-oxidation catalysts for a wide range of
further consideration in the development of these systems is
the nature (size, charge density and polarity) of the counter-
cations of the POM-surfactants, which also plays a crucial role
in the specific solubility of the compound.
7
organic transformations.6, Recently, they have also been
We recently reported that the incorporation of organic
shown to act as effective polar head groups in the formation of
supramolecular organic-inorganic assemblies. POM-based
micellar superstructures can be obtained via electrostatic self-
assembly of negatively charged POMs and positively charged
surfactant groups or organic polymers,3, 8-11 or through direct
POM organofunctionalisation.12 In this covalent approach, the
inorganic POMs are functionalised with organic groups, such as
Figure 1. A) The molecular structure of H61. Color code: blue polyhedra = {WO6}, purple
polyhedra = {PO4} or {PO3C}, red spheres = O, black = C, pink = H, yellow = S; B) The
cyclic voltammograms of {P2W18} and 16- collected vs. Fc/Fc+ in DMF and converted to
moieties into the core of inorganic POMs via phosphonate
linkers lowers the LUMO energy of the molecular metal oxide,

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and enhances its photosensitivity and photocatalytic activity.26 of the material characterisation. At concentrations up to the
To expand on this new concept, and probe the potential to mM regime, dynamic light scattering (DLS) measurements
translate the redox properties of molecular building units to identified no clear supramolecular aggregation in DMF, with
supramolecular assemblies, we developed
a redox-active the only particles falling into the hydrodynamic diameter (Dh)
hybrid POM with long aliphatic chains, and studied its solution range of 0.5 to 4.8 nm. TEM analysis confirmed that individual
state self-assembly behaviour and physicochemical properties POM-hybrid species were present (Figure S9).
(Figure 1).
A
Wells-Dawson-type-polyoxotungstate was
The electrochemical redox-activity of 16- was examined in
selected as the inorganic surfactant head group due to its rich DMF using tetrabutylammonium hexafluorophosphate
redox-chemistry and stability. The resultant material showed (TBA⋅PF6) as electrolyte. Figure 1B shows the cyclic
remarkable solvent-dependent self-assembly and the voltammogram of 16- in DMF in comparison to its purely
molecular and supramolecular phases exhibited contrasting inorganic parent-anion [P2W18O62]6- = {P2W18}. The first redox-
electrochemical behaviour.
process (corresponding to the reduction of 16- to 7-) is shifted
1
A POM surfactant was prepared by the condensation of (4- by ca. 400 mV towards more positive potentials compared to
((11-mercaptoundecyl)oxy)phenyl)phosphonic acid with the the equivalent process in {P2W18}. With regard to our previous
mono-lacunary Dawson-type anion, [P2W17O61]10- ({P2W17}) to findings,26 and those of others,27 this can be attributed to the
give the hybrid POM, H6[P2W17O57(H27C17O4PS)2]·3C4H9NO electron-withdrawing effects of the phosphonate groups.
(H6
chemical composition of H6
1
) (see Figure 1) following our standard approach.26 The Consequently, the LUMO is energetically lowered and the first
1
was confirmed by elemental redox process is shifted to a more positive potential.
1
analysis (EA and ICP), ESI-MS, H- and 31P-NMR, FTIR and TGA
After thorough drying under high vacuum at 70 °C
(see SI).
overnight, H6
1
was insoluble in deionized water at room
due
Due to the excellent solubility of H6
1 in polar aprotic temperature
solvents, N,N-dimethylformamide (DMF) was used for the bulk
Figure 2. A) TEM images of supramolecular assemblies formed by H61 in water after rapid heating to 70-80 °C; B) Particle size distribution curve determined from DLS for H61 in
water at 1 wt%; C) Cyclic voltammogram of 1.4 mM 16- in 0.1M H2SO4 showing two broad redox processes associated with the reduction of the supramolecular aggregates and
highlighting the potentials at which the reduction and oxidation processes were characterised spectroelectrochemically; D) Spectroelectrochemical analysis of the reduction and
re-oxidation of a solution of 0.5 mM 16- in 0.1M H2SO4.
2 | J. Name., 2012, 00, 1-3
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to its long hydrophobic alkyl substituents. Upon rapid heating time that the redox behaviour of micellar POM assemblies has
to temperatures of 70-80 °C, however, H6 fully dissolved in been probed by cyclic voltammetry studies.
water to form a clear, brown-orange solution, in stark contrast Potentiometric coulometry/bulk
to the yellow colour observed in DMF solution. TEM analysis of measurements conducted on the aqueous sample indicate
the deposited solution of H6 in water on a copper grid that the first broad redox-process of
6-, centred at 0.26 V vs.
showed uniform spherical structures with diameters between NHE, is a 2-electron process corresponding to a 16- 18- redox
10 and 25 nm (Figure 2A), a size range that was supported by couple. Spectroelectrochemical analysis shows two maxima
DLS measurements (Figure 2B). rising during bulk electrolysis at 0.05 V vs. NHE in the region
To probe the electronic structure of the supramolecular between 600 and 900 nm. These two bands are characteristic
hybrid-POM architectures, we investigated the POM-based intervalence charge transfer (IVCT) bands that
electrochemical redox behaviour of
6- (1.4 mM) in water with correspond to the two-electron-reduced species (Figure 2D).26
a 0.1 M H2SO4 electrolyte. DLS studies conducted on an To probe the electrochemical stability of the micellar
equivalent solution showed aggregates with mean structures, absorption spectra were collected for the fresh
1
electrolysis
1
1
/
1
a
hydrodynamic diameter Dh of 9 nm, indicative of small micellar (oxidised) sample in the absence of an applied potential, and
assemblies (Figure S16). In our three-electrode set-up a glassy spectroelectrochemical analysis (to 0.61 V vs. NHE) was
carbon electrode served as the working electrode, Ag/AgCl as performed to monitor the re-oxidation of the
reference electrode and Pt wire as the counter electrode. Two electrochemically reduced sample. The fresh sample showed
broad redox processes centred at E1/2 = 0.26 V and E1/2 = -0.11 no peaks in the range of 600 to 900nm, while those visible in
V vs. NHE were observed (Figure 2C). For comparison, under the reduced sample rapidly disappeared as the POMs were re-
identical conditions and at the same concentration as 1
6-, the oxidised. DLS measurements performed on the fresh sample,
cyclic voltammogram of {P2W18} showed redox processes the reduced sample, and the re-oxidised sample showed that
centred at 0.27 V and 0.10 V vs. NHE (Figure S14). Notably, the the particle size was retained across all samples, indicating
positive shift (compared to {P2W18}) of ca. 400mV seen for the that the electrochemical behaviour was non-destructive to the
first POM reduction potential of 16- in DMF is absent in the supramolecular system (Figure S16). Cyclic voltammograms
aqueous H2SO4 solution. This contrasting redox behaviour may collected before and after bulk electrolysis at 0.05 V and after
be due to intermolecular cooperativity or coulombic repulsion re-oxidation at 0.61 V vs. NHE showed that the E1/2 values for
between metal oxide head groups in the supramolecular the
aggregates of
6-. To the best of our knowledge, this is the first
redox
waves
remained
constant.
1
Figure 3. A) Schematic of the solvent-dependent supramolecular behaviour of H61; B) 31P-NMR of aqueous solution of H61 as DMF is added showing emergence of peaks as
aggregates are disrupted. The top spectrum was obtained for H61 in pure DMF; C) Voltammetric data for 16- in aqueous initial phase (grey dotted line), and upon DMF addition
(blue). The intensities are adjusted to compensate for the dilution.
To further characterise the solution behaviour of H6
and 1H-NMR analyses were conducted. 31P-NMR is an groups. It was notable, therefore, that in D2O the 31P-NMR
technique for the characterisation of organophosphonate- signals for H6 were broad and at very low intensities
1,
31P- equivalent organophosphonate (RPO32-; R = organic moiety)
1
hybridised POMs, as it typically gives three clear, sharp, well- compared to the signals that were observed in DMSO-d6 and
separated signals, two of which correspond to the two POM- DMF-d7 at similar concentrations. In contrast, corresponding
templating phosphate (PO43-) groups, and the other to the two measurements on {P2W18} in D2O give sharp signals (Figure S5).
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1
Furthermore, the alkyl chain signals were absent from the H- elucidate the self-assembly and solvent dependent dis-
NMR spectra (Figure S17). Stepwise addition of DMF-d7 to the assembly of the system, and develop these next-generation
D2O sample of H61 led to the emergence of the signals in both redox-tunable soft materials as stable, water-soluble
1
the H- and 31P-NMR spectra (Figure 3B and S17). The lack of supramolecular capsules for applications in a wide range of
signals in the D2O samples suggests that assembly of organic solvent-free catalytic systems.
supramolecular aggregates causes blocking or shielding of the
targeted nuclei. The growth of these signals upon DMF
Acknowledgements
addition indicates dynamic solvent dependence of the
supramolecular assembly process. Note that addition of
further D2O to the D2O NMR sample led to no change in the
spectra. Similar phenomena have been observed for
thermosensitive block-copolymers,28 where proton signals
disappeared upon thermal stimulation and subsequent coiling
of the polymer. Interestingly however, such behaviour has not
been reported for other POM-surfactant systems to date.
To develop the observations made upon addition of DMF
to the D2O NMR sample, small volumes (up to 2mL) of DMF
were added in a stepwise manner to a 2mL solution of 16- in
0.1 M aqueous H2SO4. Cyclic voltammograms showed the
gradual growth of two individually resolved redox processes at
E1/2 = 0.39 V and E1/2 = 0.23 V vs. NHE, replacing the one broad
redox process (centred at E1/2 = 0.26 V vs. NHE) measured
under purely aqueous conditions (Figure 3C). In addition, the
second redox wave became increasingly sharp and reversible
(∆Epc = 36 mV). The addition of less polar water-miscible
solvents to an aqueous solution is expected to result in a
negative shift in redox potentials.29 Addition of DMF to the
control sample, a 2mL solution of {P2W18} in 0.1 M aqueous
H2SO4, caused a negative shift in the POM redox waves, with
the first process shifting from E1/2 = 0.26 V to E1/2 = 0.12 V vs
NHE (∆E1/2 = -0.14 V) (Figure S15). The positive shift of +0.13 V
seen in the first redox process of 16- therefore cannot be
attributed to simple solvation effects. These observations
allow us to draw two conclusions: firstly, that the addition of
DMF to the aqueous system leads to the disruption of the
supramolecular aggregates and allows recovery of molecular
functionality, supporting our findings from the NMR
experiments; and secondly, that the supramolecular
aggregates have a remarkably different electronic structure
(orbital energies) to the molecular building blocks. Our further
studies will probe the relationship between the
supramolecular and electronic structures of these organic-
inorganic hybrid materials.
The authors gratefully acknowledge support from the EPSRC
Directed Assembly Network, The University of Nottingham, the
Nanoscale and Microscale Research Centre and The University
of Nottingham’s Advanced Molecular Material Research
Priority Area. VS thanks the Faculty of Engineering at the
University of Nottingham for the Dean Prize of Engineering. KK
gratefully acknowledges the DAAD for financial support. AJK
thanks the EPSRC Centre for Doctoral Training in Sustainable
Chemistry (EP/L015633/1) for his studentship. EK gratefully
acknowledges the Finnish Cultural Foundation (Suomen
Kulttuurirahasto) for funding. The authors also thank Prof.
Andrei Khlobystov for his help and advice on the TEM
measurements.
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