Synthesis and characterization of conjugated Dawson-type polyoxometalate-porphyrin copolymers

Article abstract of DOI:10.1039/c3dt50850a

Hybrid polyoxometalate-porphyrin copolymeric films were obtained by the electro-oxidation of zinc octaethylporphyrin (ZnOEP) in the presence of a Dawson type polyoxophosphovanadotungstate bearing two pyridyl groups (POM(py) ₂). The synthesis of a series of POM(py)₂ consisting of [P₂W₁₅V₃O₆₂]^9- functionalized with diol-amide or triol moieties, as well as the characterization of the copolymers are presented.

Full text of DOI:10.1039/c3dt50850a

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The electrochemical properties of several polyoxometalate-porphyrin films are related to the
geometry of the linkers installed on the organo-POM monomers.

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ARTICLE TYPE
Synthesis and Characterization of Conjugated Polyoxometalate-
Porphyrin Copolymers Obtained from a Dawson-Type
Polyoxophosphovanadotungstate
Iban Azcarate,a,b,c Iftikhar Ahmed,d Rana Farha,e,f Michel Goldmann, e,g Xiaoxia Wang, h Hualong Xu, h
5
Bernold Hasenknopf,*a Emmanuel Lacôte,*b,i Laurent Ruhlmann*c,d
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
Hybrid polyoxometalateꢀporphyrin copolymeric films were obtained by the electroꢀoxidation of zinc
octaethylporphyrin (ZnOEP) in the presence of a Dawson type polyoxometalate bearing two pyridyl
10 groups (POM(py)2). The synthesis of a series of POM(py)2 consisting of [P2W15V3O62]9– functionalized
with diolꢀamide or triol moieties, as well as the characterization of the copolymers are presented.
45 POM and the visible light photosensitizer. The recent progress in
organoꢀpolyoxometalate chemistry ensures that virtually any
organic function can be added to POM structures, whose design
can be synthetically controlled.16,17
Introduction
Polyoxometalates (POMs) are a structurally diverse family of
anionic metal oxide molecular analogues with applications in
15 analytics, medicine, catalysis and photocatalysis, electronics and
material science.1 One of the most interesting features of POMs is
their rich photoredox chemistry.2 Typically, upon light irradiation
electrons are promoted from an oxygenꢀcentered 2p orbital to an
empty metallic dꢀorbital, generating a highly reactive chargeꢀ
20 separated state. The photoexcited POM has an increased
oxidizing power, and organic substrates can be oxidized while the
POM is converted into its reduced form. The later can
subsequently be reꢀoxidized with molecular oxygen or by
reducing a second substrate.
25 Interestingly, owing to their robustness and large number of
metallic centers, POMs can undergo reversible and multiꢀelectron
photoredox processes without decomposition. However, in most
of the cases, the O → M LigandꢀtoꢀMetal Charge Transfer
(LMCT) absorption band lies in the UV region (λ= 200ꢀ400nm)
30 which strongly limits the use of POMs in solar visible light
conversion materials.
In order to overcome this problem, POMs can be linked to visible
light sensitive molecules via covalent, coordination or nonꢀ
covalent bonding.
35 The nonꢀcovalent approach is based on Van der Waals or
Coulombic interactions and does not need POM preꢀ
funcionalization. Since POMs are anionic clusters, they can form
electrostatic complexes with cationic chromophores such as Ru or
Re polypyridyl complexes3–5, organics dyes6–8, or porphyrins and
40 phthalocyanines.9–11
POMꢀporphyrin coordination complexes were also reported.12–15
However, stronger POMꢀchromophore bonds are required to
avoid dissociation and ensure better electronic communication.
The third approach is based on covalent linkage between the
50 Scheme 1 a) Schematic representation of the two step synthesis of POM-
porphyrin copolymer using molecular building blocks. b) Representation
of ZnOEP, TBA5H4[P2V3W15O62] and the tris- (tvb3,3) or diol-amide
(dvb3,3, dvb3,4 and dvb4,4) ligands.
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Preparation of the organic ligands
The use of a spacer adds versatility. Hence, functional molecular
devices with varied POM/chromophore ratios, spatial geometry,
etc. can be obtained.
Several covalent donorꢀacceptor dyads based on POMs were
prepared using ferrocenyl18,19, pyrene and perylene20–22, Ru and Ir
complexes21,23, and porphyrins.24–27
The introduction of POMꢀchromophore hybrids in polymeric and
surface supported materials is the next step toward functional
devices with catalytic, optic or photovoltaic properties.28
10 The alternate deposition of anionic POMs and cationic sensitizers
such as metal complexes29,30, organic dyes31,32 and porphyrins33,34
on surfaces using the layerꢀbyꢀlayer (LbL) technique generates
thinꢀfilms with controlled thickness. Some of us recently reported
the first POM cationic porphyrinꢀbased photovoltaic multilayered
15 film obtained by LbL showing photocurrent production under
visibleꢀlight irradiation.34
The preparation of the diolamine/triol bisꢀpyridine ligands started
with 1,3,5ꢀtribromobenzene (1, Scheme 2), which was
60 carbonylated to 3,5ꢀdibromobenzaldehyde (2) via its monoꢀ
lithiated anion.43 The aldehyde was olefinated to ester 3 via a
Wittig reaction.44 3 was a key intermediate to the four ligands of
this study, which were obtained via double Pdꢀcatalyzed
Sonogashira crossꢀcouplings. 3 was doubly coupled to 3ꢀ
65 ethynylpyridine in the presence of tetrakis(triphenylphosphine)
palladium, CuI and triethylamine,45 quantitatively leading to
evb3,3.
5
Peng reported the synthesis of a Fe(terpyridine)2ꢀLindqvist
molybdate coordination polymer35 as well as hybrid (co)polymers
with conjugated photoactive backbones and the POM inserted in
20 the mainꢀchain36,37 or as a sideꢀchain pendant.38,39 A photovoltaic
device constructed from the POM mainꢀchain containing polymer
showed a power conversion of 0.15%.37 More recently, we
reported the formation of a mixed POMꢀporphyrin copolymer
film
25 [MnMo6O18{(OCH2)3CNHCO(4ꢀC5H4N)}2]3– and ZnOEP via an
original electropolymerization/deposition process. The
with
an
Anderson
type
POM
photoactive film was used as heterogeneous photocatalyst for the
reduction of Ag+ under visible light.40
However, none of the polymers examined thus far were based on
30 POMs with reversible redox behavior, and conjugated spacers. In
the present paper, we introduce such covalently bonded
conjugated
POMꢀporphyrin
copolymers
using
our
[P2W15V3O62]9– diolamideꢀgrafting method41 toward devising
more efficient photoactive films. We report herein the bottomꢀup
35 synthesis and characterization of films obtained from bispyridineꢀ
substituted molecular organoꢀpolyoxometallic bricks with
different geometries, and examine the influence of the monomer
structure on the architecture of the polymers as a prerequisite to
understand their photocatalytic properties.
40 Results and Discussion
Our strategy for the synthesis of the films relies on the electroꢀ
copolymerization of porphyrins with several bisꢀpyridineꢀcapped
phosphovanadotungstate [P2W15V3O62]9– (Scheme 1). The
organoꢀPOM monomer was designed around 1,3,5ꢀtrisubstituted
45 benzene rings, which serve as conjugated turntables. Two of the
substituents were connected to the pyridines (two needed for
polymerization), while the last one was connected to the POM.
We used one porphyrin (Zinc octaethyl porphyrin, ZnOEP), and
selected four organic connectors. Three of them relied on the
50 diolamide coupling41 to establish full conjugation in the polymer
between the organic backbone and the pendant POM framework
through the incorporated carbonyl function. They differed by the
relative positions of the two nitrogen atoms of the pyridines (3,3;
3,4; 4,4), while the fourth was a triolꢀcapping ligand,42 chosen to
55 assess the exact role of the conjugation to the POM.
70
Scheme 2 Synthesis of the ligands dvb3,3, dvb3,4, dvb4,4 and tvb3,3
However, the same reaction from 4ꢀethynylpyridine
hydrochloride only gave the monoꢀcoupled product in poor yield.
We changed the solvent system to DMF/Et3N (2:1) and it was
75 then possible to isolate the bis ethynylpyridine derivative (evb4,4,
55%) together with some remaining monoꢀcoupled evb4,Br
(26%), which were separated by column chromatography.
Interestingly, the isolation of the latter allowed us to prepare the
dissymmetric cinnamic ester evb3,4 via a Sonogashira coupling
80 with 3ꢀethynylpyridine (65%). The diol and triol functions were
then installed via amide formation from 2ꢀaminoꢀ2ꢀethylꢀ1,3ꢀ
propanediol and 2ꢀaminoꢀ2ꢀ(hydroxymethyl)ꢀ1,3ꢀpropanediol, as
reported previously.41,42 The former delivered ligands dvb3,3 (for
2
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diolamide vinyl benzoate ethynyl 3ꢀpyridine, ethynyl 3ꢀpyridine),
dvb3,4 and dvb4,4 in 80%, 75% and 60% yields respectively.
The latter gave tvb3,3 (for triolamide vinyl benzoate ethynyl 3ꢀ
pyridine, ethynyl 3ꢀpyridine) in 60% yield.
of all cations. Peaks assignable to the clusters associated to one
TBA, one H+ or two TBA cations were also observed. Also, some
reduced forms of the hybrid most likely produced during the
ionization process were detected (minor peaks). The other POM
40 monomers behaved similarly (see SI).
5
Preparation of the organo-POM monomers
TBA5H4[P2V3W15O62] was reacted with the diolꢀamide ligands
dvb3,3, dvb3,4 and dvb4,4 in DMAc at 80°C under microwave
irradiation (Scheme 3, right).41 The desired organicꢀinorganic
10 POM hybrids POM-dvb3,3, POM-dvb3,4 and POM-dvb4,4
were isolated in 72%, 46% and 70% yield respectively after
several purifications by selective precipitation with ethanol or
ether from an acetonitrile solution. Triol tvb3,3 was converted to
POM-tvb3,3 upon heating it at 90°C in the presence of
15 TBA5H4[P2V3W15O62] for 2 weeks (50% yield).42 The four
1
organoꢀPOM monomers were fully characterized by H, 13C and
31P NMR, MS, IR, UVꢀVis spectroscopy, elemental analysis and
electrochemistry.
Fig. 1 Obtained ESI MS data for POM-dvb3,3 solution in CH3CN (50 µM)
NMR characterization
45 The 31P NMR of all hybrids reveals only two signals around –7
and –13 ppm. This confirms the presence of the two chemically
inequivalent phosphate templates of the Dawson structure, and
proves that the hybrid is free of unfunctionalized [P2W15V3O62]9–,
1
or any Pꢀcontaining degradation product. The H NMR spectra
50 present the signals from the grafted organic bipyridine ligands
(around 7 ppm), the diols (or triol) and the TBA counterions.
20
Scheme 3 Synthesis of POM-tvb3,3, POM-dvb3,3, POM-dvb3,4 and
POM-dvb4,4
Characterizations of the organo-POM monomers
25 IR characterization
The IR spectra of the hybrid monomers showed peaks at 1087
(ĠPꢀO), 950 (ĠW=O), 900 (ĠWꢀOcꢀW), 815 (ĠWꢀOeꢀW) cmꢀ1, which are
characteristic of the Dawson phosphovanadotungstate.46,47 The
signals corresponding to the organic part were found at the
30 expected wavenumbers, albeit weak due to the relatively low
content in the hybrids.
Fig. 2 a) 31P NMR spectrum of POM-dvb3,3 in CD3CN. b) 1H NMR spectra
55 of dvb3,3 in CDCl3 and POM-dvb3,3 in CD3CN, zoom of the aromatic
region.
Mass spectroscopy
Fig. 1 presents an ESIꢀMS spectrum obtained for the POM-
35 dvb3,3 hybrid. The main peak corresponds to the 5– ion stripped
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The chemical shift differences between the functionalized and the
free organic part (ꢀδ, in ppm) are reported in Table 1. The full
comparison of the chemical shifts of the free ligand and those of
VV were reported by Hill42 for [P2V3W15O62]9– and triolꢀcapped
[CH3C(CH2O)3P2V3W15O59]6–. Similarly, we have observed that
in
the
porphyrin/triolꢀcapped
[P2VV3W15O59{(OCH2)3C–
the grafted ones (Fig.2b for POM-dvb3,3) gave strong evidence 40 NHC(=O)(ZnTPP)}],27 the first reduction process of the
5
of the covalent grafting of the organic part. In particular, the
signal for the CH2OH is strongly deshielded (about 1.8 ppm)
upon attachment to the vanadate cap. This is likely caused by the
strong electron withdrawing effect of the POM. Interestingly, the
trivanadium cap involves one electron, while the second process
involves the two other VV atoms together.
In the present study, [P2VV3W15O62]9– shows two successive
reduction processes. However, its differential pulse
presence of the POM impacts all the signals of the organic part in 45 voltammogram presents a tiny splitting of the second process,
10 the diolꢀamide chemistry but only small chemical shift changes
were observed in the case of triol functionalized hybrids
(ꢀδmax = 0.07 ppm, except for CH2OV). We believe this is a
further indication that the carbonyl incorporation into the POM
suggesting that two close monoelectronic waves are present
(bottom of Fig 4). In the case of POM-tvb3,3, POM-dvb4,4,
POM-dvb3,4 and POM-dvb3,3, the splitting of the second
process is well observed. The reduction potential of the
structure induces a good electronic communication between the 50 trivanadate cap depends mostly of the type of grafting (compare
15 inorganic and the organic parts (effect over more than ten C–C
bonds), while in the case of triol functionalization, the electronic
effects of the POM are limited to the first C–C bond.
curves in Figure 4 with tvb and dvb grafting), as one would
expect for such structures (Table 2).
Table 1. 1H NMR chemical shift difference (ꢀδ in ppm) between the
20 POM(py)2 hybrid and the free ligand.
POM-
dvb3,3
POM-
dvb3,4
POM-
dvb4,4
POM-
tvb3,3
CH2OV
1.81
1.22
0.63
0.71
0.19
1.94
1.21
1.72
0.83
0.41
0.67
0.11
2.02
ꢀ0.02
0.04
0.07
0.04
HEt1)
HEt(2)
HAr(1)
HAr(2)
Ethylenic
0.54
0.92 / 0.67
0.20
Aromatic
3py
0.12
ꢀ
4py
H2
H3
0.06
ꢀ
0.23
0.18
ꢀ
0.24
0.14
ꢀ
0.04
ꢀ
Pyridine H4
0.06
0.24
0.33
0.10
0.23
0.31
0.07
0.07
0.07
Fig. 3 UV-vis absorption spectra of POM (black doted curve), dvb3,3
55 (light doted purple curve) and POM-dvb3,3 (plain purple curve) in CH3CN
solution (c = 6.25 10-6 M). POM = [P2V3W15O62]9-.
H5
H6
0.18
0.23
0.14
0.24
Finally, the difference between the oxidative and the reductive
peak potentials (ꢀEp) increases after each reduction of the VV.
For example, ꢀEp for the first, second and third reductions of VV
60 measured for POM-tvb3,3 are 104, 220, and 325 mV
respectively (Table 2). This indicates that the electron transfer
becomes increasingly slow, in agreement with the localization of
the added electrons in the vanadium cap, where consequently the
electron density would increase. The approach of the molecule
65 with the side of the partially reduced trivadanadate cap to
exchange further electrons at the negatively polarized electrode
thus becomes increasingly difficult because of the important
electrostatic repulsion.
UV-Visible charaterization
The UV spectra of the hybrids are the sum of the oxygenꢀtoꢀmetal
25 charge transfer (LMCT) bands and the organic ligand absorbance
around 280 nm (Fig. 3). The combined bands extend into the
visible, which explains the yellow color of the POM(py)2 hybrids.
Cyclic Voltammetry and Differential Pulse Voltammetry
30 The cyclic voltammograms and differential pulse voltammograms
of POM-tvb3,3, POM-dvb4,4, POM-dvb3,4, POM-dvb3,3 and
the parent POM [P2V3W15O62]9– show three monoelectronic
successive reductions, corresponding to the reversible reduction
of the three VV between +0.1 V and –0.9 V vs. SCE. It is the first
35 time that separate waves for the reduction of the three VV are
observed. Indeed, only two reversible redox processes concerning
70
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Fig. 4 a) Cyclic voltammograms of POM-tvb3,3 , POM-dvb4,4 , POM-dvb3,4 , POM-dvb3,3, and POM (TBA5H4[P2V3W15O62]) at c=5.10-4 molL-1 and v=10
mVs-1 . b) Differential pulse voltammograms at 25 mVs-1 in 1,2-C2H4Cl2/CH3CN (7/3), containing 0.1 molL-1 NBu4PF6. Working electrode: glassy carbon.
*Adsorption peak.
5
Synthesis and characterization of the Poly-ZnOEP-POM
Copolymers
The reactions generated H+ ions. According to previous work,
scanning between –1.30 or –1.50 V and +1.80 V reduced these
Copolymer Synthesis
40 protons to H2, and they were accumulated in solution when the
scan was limited from 0.00 V to +1.80 V. Even in the second
case, they did not perturb the coating of the electrodes. The
demetalation of the metalloporphyrins was controlled by UVꢀ
visible spectroscopy and did not occur.
We have shown that exhaustive electroꢀoxidation of zinc βꢀ
octaethylporphyrin (ZnOEP) in the presence of an excess of
10 nucleophile at a potential of the first oxidation allows the
formation of the monoꢀsubstituted zinc mesoꢀNu+ꢀ βꢀ
octaethylporphyrin (ZnOEP(Nu)+) in good yield.48–50 This
reactivity of oxidized porphyrins has been advantageously used to
develop an easy and original method of electropolymerization of
15 porphyrins when a difunctional nucleophile such as 4,4’ꢀ
bipyridine was used.51,52 When the iterative scans were performed
at a sufficiently high potential in the anodic part (i.e. allowing the
formation of the porphyrin dication), the formation of a
conducting polymer with viologen spacers between porphyrins
20 coating the working electrode was observed. Indeed, while the
formation of radical cations was sufficient to perform monoꢀ
substitutions on porphyrins, the electropolymerization process
required the formation of the dications, undoubtedly due to
kinetic problems (the dications react faster with nucleophiles than
25 the radical cations). Hence an E1(E2iCNmesoE2i+1CB)i=1→nE2(n+1)
mechanism can be proposed to explain the electropolymerization
process where CNmeso corresponds to the nucleophilic attack at the
meso position of the porphyrin which form an isoporphyrin.52,53
The isoporphyrin is then oxidized (electrochemical step E2i+1) and
30 the proton present initially on the mesoꢀcarbon is released
(chemical step CB). For a degree of polymerization n, the
corresponding global reaction can be written as:
45 Thus, it is possible to easily modulate the nature of the bridging
spacers between the porphyrin macrocycles. Indeed, instead of
using 4,4’ꢀbipyridine, this process of electropolymerization can
be extended to molecules with various spacers between two
pyridyl groups. The spacers can be selected for their specific
50 chemical or structural properties: rigid or not; long or short;
electron conducting or not; with conjugated πꢀbonds (alkene,
alkyne or aromatic chains) or successive σꢀbonds (alkyl chains),
etc., allowing one to modulate the communication between
porphyrin macrocycles within the polymeric chains.
55 We applied this concept to POMs and prepared POMꢀporphyrin
copolymer
films
with
the
Anderson
type
POM
[MnMo6O18{(OCH2)3CNHCO(4ꢀC5H4N)}2]3– and ZnOEP,40
which presented good photocatalytic54,55 and photovoltaic34
properties.
60 In the present work, all electropolymerizations have been carried
out under the same experimental conditions by iterative scans in
0.1
mol
Lꢀ1
solutions
of
tetrabutylammonium
hexafluorophosphate in 1,2ꢀC2H4Cl2/CH3CN (7:3) containing
ZnOEP (0.25 mmol Lꢀ1) and POM(py)2 (0.25 mmol
65 Lꢀ1) under an argon atmosphere.
Figure 5 illustrates the evolution of the CVs during the
electropolymerization of ZnOEP with POM-dvb4,4. Cyclic
scanning was applied at potentials between –1.45 and 1.90 V vs.
SCE or between ꢀ0.05 and +1.90 V vs. SCE. During the first scan
(n+1)ZnOEP + (n+1)PyꢀRꢀPy →
35 ZnOEPꢀ(Py+ꢀRꢀPy+ꢀZnOEP)nꢀPyꢀRꢀPy + (2n+1)H+ + (4n+2)eꢀ
(R = spacer)
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25 Table 2 Electrochemical data for POM-tvb3,3, POM-dvb3,3 , POM-
dvb3,4 and POM-dvb4,4 and copolymers poly-POM-tvb3,3-ZnOEP,
poly-POM-dvb3,3-ZnOEP, poly-POM-dvb3,4-ZnOEP and poly-
POM-dvb4,4-ZnOEP.a
in reduction, only signals assigned to the reduction of the POM-
dvb4,4 were detected between –0.05 and
–1.45 V vs. SCE.
In the second scan, one additional irreversible reduction peak
appeared between –0.75 to –0.95 V vs. SCE (peak a, Fig. 5),
which corresponds to the reduction of the pyridinium units of the
dipyridiniumꢀPOM spacers of the generated copolymer poly-
Oxidation
πꢀring
Reduction
5
VV/VIV
ꢀ0.11b ꢀ0.48b ꢀ0.85b
py+
πꢀring
POMꢀtvb3,3
POMꢀdvb3,3
POMꢀdvb3,4
POMꢀdvb4,4
ZnOEP
POM-dvb4,4-ZnOEP
(ZnOEPꢀPy+ꢀPOMꢀPy+)n]
(Scheme
(104)
(220)
(325)
3).51,52,56–61 The height of this peak grew with repetitive scans at
0.06b
(87)
ꢀ0.26b ꢀ0.43b
(117) (166)
10 the beginning of the deposition and then reached a plateau. The
irreversibility of the signal indicated that the generated pyridyl
radicals are not stable and react further62–64 as already observed in
the case of copolymer with Anderson type POM.40
0.06b
(84)
ꢀ0.29b ꢀ0.43b,*
(139)
0.06b
(113)
ꢀ0.31b
ꢀ0.41b,*
(222)
Similar behaviors were observed for the synthesis of copolymers
15 poly-POM-dvb3,4-ZnOEP, poly-POM-dvb4,4-ZnOEP and
poly-POM-tvb3,3-ZnOEP except that the reduction of the POM
VV/VIV couple) became more difficult to observe. Analogous
electrochemical behavior was observed in the coordination
complexes between [MnMo6O18{(OCH2)3CNHCO(4ꢀC5H4N)}2]3–
20 and [Ru(CO)TPP] (TPP = tetraphenylporphyrin) or ZnTPP where
it was noticed that the MnIII/MnII redox process was remarkably
slowed down with the quasiꢀcomplete disappearance of the
electrochemical signal, even at the glassy carbon electrode.13
0.94 /
0.68
ꢀ1.60b
ꢀ1.02irr,c ꢀ1.70c
ꢀ1.02irr,c ꢀ1.32irr,c
ꢀ0.87irr,c ꢀ1.47irr,c
ꢀ0.79irr,c ꢀ1.54irr,c
polyꢀPOMꢀ
tvb3,3ꢀZnOEP
e
e
e
e
e
e
e
e
e
polyꢀPOMꢀ
dvb3,3ꢀZnOEP
polyꢀPOMꢀ
dvb3,4ꢀZnOEP
polyꢀPOMꢀ
dvb4,4ꢀZnOEP
a
All potentials in V versus SCE were obtained from cyclic voltammetry
30 in 1,2ꢀC2H4Cl2/CH3CN (7:3) containing 0.1 mol Lꢀ1 (NBu4)PF6. Scan rate
= 0.1 V sꢀ1. b Working electrode: glassy carbon electrode. c ITO, S = 1 cm2
after 25 scans. The given halfꢀwave potentials in the case of the reversible
couple are equal to E1/2 = (Epa ꢀ Epc)/2. Under bracket: (ꢀEp, peak
splitting in mV). not observed. ꢀEp is the difference potential between
35 the oxidative and the reductive peak potentials. * Measured from DPV.
e
40
Scheme 4 Copolymer poly-POM-dvb3,4-ZnOEP, obtained with POM-dvb3,4 and ZnOEP
6
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conjugation with the central benzene group, leading to a
stabilisation of the reduced intermediate. In poly-POM-dvb3,3-
35 ZnOEP, no stabilisation can occurs since the delocalization of
the radical is only possible within the pyridyl ring. The more
stable reduced form was obtained at less negative potentials.
On the contrary, the porphyrin reductions were measured at
–1.32 V, –1.47 V and –1.57 V vs. SCE for poly-POM-dvb3,3-
40 ZnOEP, poly-POM-dvb3,4-ZnOEP and poly-POM-dvb4,4-
ZnOEP respectively. These results show that when the
delocalization of the radical of the reduced pyridinium is only
possible onto the pyridyl ring, the localization of the radical is
more on the nitrogen atom. It renders the reduction of the
45 porphyrin easier because of the possibility of delocalization onto
the macrocycle when the porphyrin is reduced. In the case of
poly-POM-dvb4,4-ZnOEP, the reduction potential of the
porphyrin is nearly the same as the potential measured for ZnOEP
alone. Since the reduced pyridinium radical is delocalized on the
50 entire dvb4,4 ligand, delocalization of the reduced porphyrin
radical is limited to the macrocyle. Thus, it explains a reduction
potential of the ZnOEP subunit close to the potential measured
for the starting ZnOEP.
55
Fig. 5 Cyclic voltammograms for the electropolymerization of ZnOEP
(0.25 mM) with POM-dvb4,4 (0.25 mM) in 1,2-C2H4Cl2/CH3CN (7:3)
TBAPF6 0.1mol L-1. Working electrode: ITO. S = 1 cm2. Scan rate: 0.1 V s-1.
(ꢀ) Start of the scan. Cyclic scanning (0.1 V s-1) was applied at potentials
between -0.05 and +1.90 V versus SCE (top) and between -1.45 and 1.90
V versus SCE (bottom).
5
This significant electronꢀtransfer rate constant attenuation is
believed to be the consequence of longꢀdistance electron transfer
10 from the working electrode to the electroactive site of the
molecule65 because of the hindered approach of the POM to the
electrode in the presence of coordinated porphyrin. A similar
explanation can also be proposed in the present work where the
coordination complex between ZnOEP and POM(py)2 can also be
15 formed.
Cyclic Voltammetry of the copolymeric films
20 The electrochemical behavior of these films has been studied by
cyclic voltammetry (Figure 6a). Typically, two irreversible
reduction peaks appeared, that correspond to the pyridium
reductions and the first reduction of the ZnOEP macrocycle
respectively (Table 2).
25 The pyridinium reductions were observed at –1.02 V, –0.87 V
and –0.79 V vs. SCE for poly-POM-dvb3,3-ZnOEP, poly-
Fig. 6 Top: Cyclic voltammograms of poly-POM-dvb3,3-ZnOEP, poly-
POM-dvb3,4-ZnOEP and poly-POM-dvb4,4-ZnOEP films (25 iterative
scans between 0 and 1.90 V versus SCE) on ITO 1,2-C2H4Cl2/CH3CN(7:3)
60 (NBu4)PF6 0.1mol L-1 at 100 mVs-1. Bottom: Cyclic voltammograms of 1
mM of K3Fe(CN)6, in 0.5 M Na2SO4 of modified ITO electrode with poly-
POM-dvb3,3-ZnOEP film (full violin line) and of noncoated ITO electrode
(dotted black line). v=100 mV s-1.
POM-dvb3,4-ZnOEP
and
poly-POM-dvb4,4-ZnOEP
respectively, showing the strong influence of the isomer on the
reduction potential of the pyridinium spacers. It can tentatively be
30 explained in terms of πꢀconjugation (see SI Fig S4). For poly-
POM-dvb4,4-ZnOEP, the delocalization of the pyridyl radical is
larger than for poly-POM-dvb3,4-ZnOEP because of the
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The redox behavior of poly-POM-tvb3,3-ZnOEP is close to
of the solutions became sharper in comparison to those recorded
that of poly-POM-dvb3,3-ZnOEP, suggesting that conjugation 45 on the solid films, but remained larger than for ZnOEP. This
to the POM is not crucial.
evolution suggests that for the most part intramolecular
interactions remained when the copolymers were in solution.
5
Permeability to Anionic Probes of the Films
50 X-ray Photoelectron Spectra (XPS)
The permeability of these films toward electrochemically active
probe molecules has also been studied. For bare ITO electrode
10 (unmodified), the redox probe Fe(CN)6 showed a quasiꢀ
reversible cyclic voltammetry (peakꢀtoꢀpeak separation 219 mV,
This type of characterization has been employed to determine the
elemental composition of the films. While XPS is not an accurate
quantitative measurement of the individual atoms, it helps to
3–
Fig. 6b dottedꢀline), indicating that the probe diffuses freely to 55 determine the elements present and their oxidation states within
the ITO electrode surface and undergoes electronꢀtransfer at the
electrode. In contrast, in the presence of poly-POM-dvb3,3-
15 ZnOEP film, the peak current decreased a lot and reached zero.
Similar behaviour were observed for poly-POM-dvb3,4-
ZnOEP, and poly-POM-dvb4,4-ZnOEP, but also for poly-
the film. For example, the XPS measurement for POM-dvb3,3-
ZnOEP (Fig. 8) confirms the presence of F, C, N, O, P, V and W
for the copolymer onto ITO electrode.
POM-tvb3,3-ZnOEP films, suggesting that all these films were
3–
impermeable to Fe(CN)6
.
20
Fig. 7 Normalized UV-vis spectra of the modified ITO electrodes with
poly-POM-dvb3,3-ZnOEP obtained after 25 iterative scans between 0
25 and 1.90 V versus SCE (dotted pink line), and of poly-POM-dvb3,3-ZnOEP
(purple line) and of ZnOEP (black line) in DMF solution.
UV-Visible Characterization of the copolymeric films
30
All the UVꢀvisible spectra recorded on ITO electrodes coated
with the copolymers presented identical characteristics. A typical
UVꢀvisible spectrum is represented in Figure 7. The large Soret
absorption band was red shifted compared to the ZnOEP
35 monomer. This evolution can be understood by the excitonꢀ
coupling theory concerning intraꢀ or intermolecular excitonic
interactions between the ZnOEP subunits.48,66–68 Similar spectral
effects in dimeric or trimeric porphyrins have been reported and
analyzed as dependent on the interporphyrin angles, the direction
40 of the transition dipole moments in the monomer subunits, and
also the distances between the porphyrins.69–71
60
Fig. 8 XPS spectra of the modified ITO electrodes with POM-dvb3,3-
ZnOEP obtained after 25 iterative scans between 0 and 1.90 V versus
SCE. a) Global XPS spectra, b) C1s, c) O1s , d) N1s, e) P 2p3, f) W 4f7, g) V
2p3.
65
The films were then removed from the ITO electrodes by
dissolution in DMF. The profile of the UVꢀvis absorption bands
8
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The film exhibited peaks corresponding to P2p (133.2 eV), O1s
copolymer with Anderson type polyanion. The morphology did
(530.6 eV), V2p (V2p1/2 = 522.0 eV and V2p3/2 = 516.2 eV) and 30 not depend on whether the coated electrodes were obtained by
W4f levels (W4f5/2 = 37.2 and W4f7/2 = 35.4 eV) coming from the
phosphorous, oxygen and tungsten atoms in the POM, while the
C1s and N1s signals (at 284.6, 402.2 and 686.6 eV, respectively)
result from the porphyrin ligand and the TBA counterions.
scanning over the potential range from –1.30 to +1.90 V (Figure
9b) or only from –0.05 to +1.90 V vs. SCE.
5
The rms surface roughness of the film obtained by 25 iteractive
scans from –0.05 to +1.90 V vs SCE was 6.32 nm for 1 ꢁm2 area.
F1s signals (686.6 eV) can also be observed since PF6ꢀ anions can 35 The AFM studies showed an increase in the rms surface
be trapped in the copolymer structure to balance pyridium charge.
Thus, both POM phosphates and PF6ꢀ contribute to the P2p peak.
10 The signal of the Zn element is not detectable.
roughness with the number of iterative scan.
Similar results have been obtained for poly-POM-dvb3,4-
ZnOEP (rms surface roughness: 5.53 nm) , poly-POM-dvb4,4-
ZnOEP (rms surface roughness: 4.21 nm) and poly-POM-
The XPS data confirms that we indeed incorporated the Dawsonꢀ
type phosphovanadotungstate and the zinc metalloporphyrin into 40 tvb3,3-ZnOEP (rms surface roughness: 3.20 nm).
the films, which is in agreement with the UVꢀvis absorption
results. Similar XPS spectra (see SI Fig S7) have been obtained
Control of the Thickness of copolymeric films
15 for poly-POM-dvb3,4-ZnOEP and poly-POM-dvb4,4-ZnOEP.
We have also examined the control of the thickness of the
45 deposited polymeric film. For that, two ways of preparation of the
polymer coated onto ITO electrode have been followed. In the
first experiment, coated electrodes were prepared just by
changing the number of iterative scans
n during the
electropolymerization. Plot of the absorbance at λ = 427 nm
50 (Soret band of the porphyrin) as a function of the number of scans
n (Fig. 10a and Fig.10b) shows effectively an increase of the
thickness during the first scans and then a constant value. In the
second experiment, the thicknesses of the films measured by
AFM confirmed this point (Fig 10c and Fig. 10d).
55 To measure the thickness, we removed the polymer by scratching
the film with a metallic tip, and clearly observed some puffiness
induced by the material removal. Analysis of the bottom
indicated that we reached the ITO substrate. The thickness was
estimated by comparing the height on each side, far from the
60 puffiness. Interestingly, the estimated thickness increased with
the scan number with nearly the same trend as the UVꢀVis
absorbance intensity.
Fig. 9 a) Picture of the ITO modified electrode with poly-POM-dvb3,3-
ZnOEP copolymer obtained after 25 iterative scans between 0 and 1.90 V
versus SCE. a) Tapping mode AFM topography of the film on ITO.
Such behavior fits with the decrease of the film conductivity
already observed during the electropolymerization (Figure 5).
65 Thus, the deposition which is initially very efficient becomes
gradually more difficult and eventually stops. In addition, in the
case of poly-POM-dvb4,4-ZnOEP, with the same number of
iterative scan, the thickness is nearly three times more important.
This result can be explained by the better conductivity of the
70 films when using the fully conjugated dvb4,4 ligand.
20 Bottom: Section analysis of the marked white dotted line on the image
Copolymer Morphology (Atomic Force Microscopy)
The coated electrodes were washed with CH2Cl2 to remove any
25 trace of the conducting salt present on the film and were
examined by scanning atomic force microscopy (AFM).
Copolymer poly-POM-dvb3,3-ZnOEP appeared in the form of
tightly packed coils as already observed in the case of the
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Fig. 10 a) UV-visible absorption spectra of poly-POM-dvb4,4-ZnOEP (onto ITO) with different numbers of iterative scans (between 0 and +1.90 V versus
SCE). Only one side is recovered by ITO. b) Plots of the absorbance, measured at λ = 427nm, versus the numbers of iterative scans for poly-POM-dvb3,3-
ZnOEP, poly-POM-dvb3,4-ZnOEP, poly-POM-dvb4,4-ZnOEP, and poly-POM-tvb3,3-ZnOEP. c) Atomic force micrograph (AFM, surface plot) image of the
modified ITO electrode with poly-POM-dvb4,4-ZnOEP obtained after 25 iterative scans. Bottom: section analysis. d) Thickness measured from AFM
versus different numbers of iterative scans (between -0.05 and 1.90 V vs. SCE). The red filled black square corresponds to the measurement obtained
from c).
5
(Shanghai, China) for funding of this work. We also thank the
Délégation Générale
fellowship.
à l’Armement (DGA) for a Ph. D.
Conclusion
In the present paper we have prepared four films from the anodic
10 electroꢀcopolymerization of organoꢀPOM bricks with designed
geometries and structures, and a porphyrin. The bricks as well as
the materials were unambiguously characterized by several
analytical techniques.
The conclusion of the aggregated characterizations show that the
15 conjugation to the POM may not be a crucial item, at least from
an electrochemical point of view, but the geometry of the
pyridines on the hybrid monomer is directly related to the
performances of the materials.
30 Notes and references
a UPMC Univ. Paris 6, Institut Parisien de Chimie Moléculaire, CNRS
UMR 7201, 4 Place Jussieu, 75005 Paris, France
b ICSN CNRS, Av. de la Terrasse, 91198 GifꢀsurꢀYvette Cedex, France.
c Université de Strasbourg, Laboratoire d'Electrochimie et de Chimie
35 Physique du Corps Solide, Institut de Chimie, 4 rue Blaise Pascal CS
90032 Fꢀ67081 Strasbourg Cedex, France.
d Université ParisꢀSud 11, Laboratoire de Chimie Physique, U.M.R. 8000
CNRS, Bâtiment 349, 91405 Orsay cedex, France.
Future work will focus on the assessment on the reactivity of the
20 materials under visible light irradiation.
e Institut des NanoSciences de Paris, UMR 7588 au CNRS, Université
40 Paris 6, 4 place Jussieu, boîte courrier 840, F ꢀ 75252 Paris, France.
f Laboratoire d’Analyse et Contrôle des Systèmes Complexes ꢀ LACSCꢀ
ECE Paris Ecole d’Ingénieurs, 37 Quai de Grenelle, F ꢀ 75015 Paris,
France,
Acknowledgments
We thank CNRS, Université ParisꢀSud (Orsay, France), the
University of Strasbourg (France), Université ParisꢀDescartes
(Paris, France), Université Pierre et Marie Curie (Paris, France),
25 ECE Paris Ecole d’Ingénieurs (France), and Fudan University
g Université Paris Descartes, 45 rue des Saints Pères, F ꢀ 75006 Paris,
45 France,
h Department of Chemistry, Shanghai Key Laboratory of Molecular
Catalysis and Innovative Materials and Laboratory of Advanced
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