Stable isoporphyrin copolymer: Electrochemical mechanism and behavior and photovoltaic properties

Article abstract of DOI:10.1016/j.electacta.2019.04.050

Two types of porphyrin copolymer thin films were obtained by electropolymerization of zinc-5,15-bis(p-tolyl)porphyrin. Electrogenerated porphyrin radical cation and dication are powerful electrophiles which can rapidly react with 1,1″-(1,3-propanediyl)bis

Full text of DOI:10.1016/j.electacta.2019.04.050

Accepted Manuscript
Stable isoporphyrin copolymer: Electrochemical mechanism and behavior and
photovoltaic properties
Moussa Boudiaf, Yiming Liang, Raphaël Lamare, Jean Weiss, Helen Ibrahim, Michel
Goldmann, Embarek Bentouhami, Vasilica Badets, Sylvie Choua, Nolwenn Le Breton,
Antoine Bonnefont, Laurent Ruhlmann
PII:
S0013-4686(19)30722-4
https://doi.org/10.1016/j.electacta.2019.04.050
EA 33998
DOI:
Reference:
To appear in: Electrochimica Acta
Received Date: 14 February 2019
Revised Date: 6 April 2019
Accepted Date: 7 April 2019
Please cite this article as: M. Boudiaf, Y. Liang, Raphaë. Lamare, J. Weiss, H. Ibrahim, M. Goldmann,
E. Bentouhami, V. Badets, S. Choua, N. Le Breton, A. Bonnefont, L. Ruhlmann, Stable isoporphyrin
copolymer: Electrochemical mechanism and behavior and photovoltaic properties, Electrochimica Acta
(2019), doi: https://doi.org/10.1016/j.electacta.2019.04.050.
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ACCEPTED MANUSCRIPT
Stable isoporphyrin copolymer: electrochemical mechanism and
behavior and photovoltaic properties
abc,‡
a,‡
a
a
c
Moussa Boudiaf,
Yiming Liang, Raphaël Lamare, Jean Weiss, Helen Ibrahim, Michel
cd
b
a
a
a
Goldmann, Embarek Bentouhami, Vasilica Badets, Sylvie Choua, Nolwenn Le Breton, Antoine
Bonnefont, and Laurent Ruhlmann,*
a
a
a
Université de Strasbourg, Institut de Chimie, UMR CNRS 7177, 4 rue Blaise Pascal, CS 90032,
7081 Strasbourg cedex, France. E-mail: lruhlmann@unistra.fr
Laboratoire de Chimie, Ingénierie Moléculaire et Nanostructures Université Sétif 1.
6
b
Département de génie des procédés faculté de technologie université EL OUED.
c
Institut des NanoSciences de Paris, UMR CNRS 7588, Sorbonne Université, 4 place Jussieu, boîte
courrier 840, F - 75252 Paris, France.
d
Université Paris Descartes, 45 rue des Saints Pères, F - 75006 Paris, France.
‡
†
Both authors contributed equally to this work.
Electronic supplementary information (ESI) available: experimental details, electrochemistry,
EQCM, UV-Vis-IR spectra, ESR, XPS, AFM, EIS, and photoelectrochemical responses. (Figs. S1–
S21). See DOI:
1

ACCEPTED MANUSCRIPT
Graphical Abstract
Electro-oxidation of zinc-5,15-bis(p-tolyl)porphyrin in the presence of dipyridyl ligand led to the
formation of copolymers containing stable isoporphyrin radicals or porphyrins depending on the
applied upper potential limit (1.0 V versus 1.6 V). The electrochemical properties as well as the
photovoltaic performances have been investigated. The photocurrent generation was significantly
improved in the case of the isoporphyrin radical copolymer.
2

ACCEPTED MANUSCRIPT
Abstract
Two types of porphyrin copolymer thin films were obtained by electropolymerization of zinc-5,15-
bis(p-tolyl)porphyrin. Electrogenerated porphyrin radical cation and dication are powerful
electrophiles which can rapidly react with 1,1''-(1,3-propanediyl)bis-4,4'-bipyridinium
+
+
-
hexafluorophosphate salt (bpy –(CH ) – bpy·2PF ) as nucleophiles to form copolymers containing
2
3
6
stable isoporphyrin radicals or porphyrins, depending of the applied upper potential limit (1.0 V
versus 1.6 V). Electrochemical routes leading to these electroactive copolymers are discussed and the
unusual redox properties of copolymers containing stable isoporphyrin radicals are studied. The
electropolymerization of the two copolymers was monitored by electrochemical quartz crystal
microbalance (EQCM). The copolymers were characterized by UV-Vis-NIR spectroscopy, X-ray
photoelectron spectroscopy (XPS), electrochemistry, electron spin resonance (ESR) and atomic force
microscopy (AFM). Their electrical properties have been studied by electrochemical impedance
spectroscopy and their photovoltaic performances have been investigated by photocurrent transient
measurements under visible-NIR light irradiation. The isoporphyrin radical copolymer exhibited
considerably better photocurrent generation when compared to the porphyrin copolymer.
Keyword: electropolymerization · porphyrinoids · isoporphyrin · pyridinium · viologen · thin
films · EPR · EQCM · Impedance · photocurrent generation
1
. Introduction
Isoporphyrins are a non-standard tautomeric form of porphyrin, in which one of the N-bound protons
is shifted to a peripheral meso-carbon atom, leading to the loss of aromaticity. Isoporphyrins have
been reported as the intermediates in heme oxidation or biosynthesis of chlorophyll [1-2]. In 1960,
Woodward [3] hypothesized the formation of isoporphyrin from phlorin by dehydrogenation in the
synthesis of chlorophyll[4]. Ten years later, Dolphin et al reported the synthesis of the first zinc-
isoporphyrin by electrochemical oxidation of zinc-meso-tetraphenylporphyrin (ZnTPP) in methanol
[5]. More generally, the formation of isoporphyrins results from a nucleophilic attack on an oxidized
porphyrin [6-15].
3

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On several occasions, solid state structures have confirmed the molecular structures of isoporphyrin
[16-18]. Isoporphyrins show remarkable redox behavior [19-26] and display a characteristic strong
absorption in the near-IR ranging from 750 nm to 950 nm suitable for possible applications in
photodynamic therapy [27]. A recent review on isoporphyrins details their potential use in medicine,
near-IR dyes or photosensitizers, and as model system in biology for natural porphyrin degradation
studies, [ 28 ] although isoporphyrins have been mostly considered as reaction intermediates,
including in previous work by our group.
Our group has shown that π-radical cation porphyrins as well as dications are powerful electrophiles
which can rapidly react with nucleophiles to form isoporphyrin intermediates [ 29 ]. Most
isoporphyrins reported until now show a high tendency to decompose either by ring opening or by
rearomatization to regenerate the initial porphyrin. In the case of
β-octaethyl porphyrins, the
corresponding isoporphyrins are not stable, due to the loss of a proton from the saturated meso-
carbon atom whereas in the case of tetraphenylporphyrin (TPP), the corresponding isoporphyrin is
reasonably more stable due to the substitution of the meso-carbon atoms by a phenyl group [30-31].
In some cases, stable isoporphyrins can also be obtained by classical synthetic procedures,
however, they always result from multistep chemical syntheses which hampers the overall yields of
the desired products [32-33]. In this context, electro-generated porphyrin π-radical cations and
porphyrin dications, offer a new, alternate and facile synthetic method for obtaining oligo- and poly-
porphyrin systems through two possible reaction pathways.
The first pathway consists in the direct coupling of porphyrinic π-radical cation which is mostly
effective in the case of 5,15-diarylporphyrins and 5,10,15-triaryl-porphyrins, or when
bearing protons are not sterically hindered (Scheme 1BD). In the absence of nucleophile, direct
meso–meso, meso– or even linked dimers of porphyrins can be obtained by electro-oxidation
β positions
β
β–β
of 5,10,15-triaryl-porphyrins and even longer oligomers were obtained by electrolysis at an applied
potential corresponding to the first oxidation of the porphyrin [34]. Furthermore, it was also shown
that according depending on the nature of the metal center, meso–β linked dimers could be
selectively obtained. Magnesium and zinc-5,15-disubstituted porphyrins give exclusively the
corresponding meso–meso directly linked dimers, whereas copper, palladium, nickel and also free
base porphyrins led exclusively to meso–
β linked dimers [35]. The original yields, lower than 50%,
were significantly increased to obtain yields as high as 95% for electrochemical couplings [36].
4

ACCEPTED MANUSCRIPT
In an extension of their pioneering studies, Osuka et al. also showed that meso–meso coupling of
zinc-5,15-diaryl-porphyrins leading to oligomers could be achieved selectively in the presence of 1.5
eq. of AgPF in CHCl containing 0.5% of N,N-dimethylacetamide [37].
6
3
In this context, Lucas, Vorotyntsev and coworkers reported electropolymerization of magnesium
porphin (all meso or carbon atoms bearing H), allowing the formation of long chains of porphin
connected through direct meso–meso links [38-46]
The second reactive pathway is the addition of a nucleophile on the electron deficient porphyrin
radical cations or dications which can occur at the “free carbons” (meso or carbon atoms bearing H)
β
.
β
of oxidized porphyrin [47-48]. This electro-synthetic method led to the formation of various
substituted porphyrins. In particular, bidentate nucleophiles such as the 4,4’-bipyridine [29],
dipyridyl ligand [49-52] or diphosphine [53-55] yielded porphyrin dimers or oligo-porphyrins with
significant yields.
Lucas and coworkers also investigated electrochemical nucleophilic substitutions on magnesium
porphin [56]. In this case, an electro-generation of the porphin radical cation in the presence of
pyridine led to the regioselective mono meso-substitution of the porphin in 73% yield.
We took advantage of the reactivity of oxidized porphyrins to develop an easy and original
method of porphyrin Electropolymerization. We will first describe the possibility to obtain a
copolymer of porphyrins with iterative scans by cyclic voltammetry from the mono-substituted 5-
+
ZnOEP(meso-bpy) porphyrin (ZnOEP = zinc-
β
-octaethylporphyrin) [57-58]. On the one hand, when
the positive potential limit was sufficiently high (i.e. allowing the formation of the porphyrin dication
2
+
+
5
-ZnOEP (meso-bpy) , the current increased progressively, showing the formation of a conducting
copolymer film on the working electrode. On the other hand, if the iterative sweeps were stopped at a
+
ꢀ
potential only allowing the formation of the π-radical cation of the porphyrin (5-ZnOEP (meso-
+
bpy) ) no change of the cyclic voltammograms was observed [ 59 ] indicating that the
electropolymerization did not occur. In this case, the oxidation wave of the porphyrin remained
reversible, showing that the electro-generated π-radical cation porphyrin did not react further.
Despite the fact that the formation of radical cation was sufficient to perform mono-substitutions
onto macrocycles, the electropolymerization failure can be attributed to the presence of the bulky
ethyl groups influencing the reaction kinetics. Without a doubt, the nucleophilic attack on the π-
radical cation is slower than the attack on the dication in the case of zinc-β-octaethylporphyrin
5

ACCEPTED MANUSCRIPT
(ZnOEP). Accordingly, if the characteristic time of the potential scan during the
electropolymerization is shorter than that of the nucleophilic attacks onto the π-radical cation
porphyrin, the substitution will not occur and the radical cation will be reduced during the cathodic
+
scan. An E (E C
E C ) mechanism has been proposed in the case of 5-ZnOEP(meso-bpy)
3 B n
1
2
Nmeso
porphyrin where the double-oxidation step at the beginning (steps E E ) allows the electro-
1
2n
generation of the dication porphyrin and then the copolymer formation [57-58].
We have also developed the easy electropolymerization of porphyrins with the direct use of
commercial and unsubstituted ZnOEP porphyrin, in the presence of 4,4’-bipyridine. The growth of
the copolymer was obtained only if the porphyrin dication was generated in the presence of 4,4’-
bipyridine.
In this work, a new strategy led to the formation of stable isoporphyrins integrated in the
copolymers. It is shown that stable isoporphyrins copolymers can be obtained during porphyrin
oxidation when sterically less hindered porphyrin is used, such as the zinc-meso-5,15-ditolyl-
porphyrin (ZnT P) which presents only two meso positions occupied by one substitutable proton at
2
positions C10 and C20 (Scheme 1). These new copolymers containing stable isoporphyrins will be
described and compared with the “classical” porphyrin copolymer prepared using the strategy
developed in our group [52,60].
The characteristic strong absorption of isoporphyrins in the near-IR region is also promising for the
development of develop near-IR dyes or photosensitizers for photovoltaic devices and the
photocurrent generation.
The formation of such type of stable isoporphyrin copolymer will be achieved by oxidation of
porphyrin such as zinc-5,15-ditolyl-porphyrin (ZnT P) in the presence of dipyridyl ligand.
2
•
Copolymers containing isoporphyrin radical (poly-ZnT isoP ) or porphyrin (poly-ZnT P)
2
2
depending of the upper potential limit (1.0 V versus 1.6 V) of the iterative potential cycles (Fig. 1)
+
will be obtained. The 1,1''-(1,3-propanediyl)bis-4,4'-bipyridinium hexafluorophosphate salt (bpy –
+
-
(
CH ) – bpy·2PF ) was used for this study.
2 3 6
The formation of these copolymers on ITO electrode has been monitored in-situ by Electrochemical
Quartz Microbalance (EQCM). The two copolymers were also characterized UV-Vis-NIR
spectroscopy, X-ray photoelectron spectroscopy, electrochemistry, ESR (Electron Spin Resonance),
AFM and electrochemical impedance spectroscopy (EIS). The photocurrent generation has been
6

ACCEPTED MANUSCRIPT
•
investigated under visible light illumination and the performances of poly-ZnT isoP and poly-
2
ZnT P copolymer thin films are compared.
2
2
. Experimental
2.1. Materials. All solvents were of reagent grade quality and used without further purification.
Zinc-β-octaethylporphyrin (ZnOEP) was purchased from Sigma-Aldrich and used without further
+
purification. The 1,1''-(1,3-propanediyl)bis-4,4'-bipyridinium hexafluorophosphate salt (bpy –
+
-
(
CH ) – bpy·2PF ) was synthesized according to procedures described in literature [61-62].
2 3 6
2
2
.2. Synthesis of ZnT2P and ZnAT2P.
.2.1. ZnT P.
2
The free base 5,15-bis(p-tolyl)porphyrin (H T P) [63-65] was synthetized according to published
2
2
procedures from 2,2’-dipyrromethane and p-tolualdehyde [66]. The free base was then metallated
with Zn(OAc) in THF to give zinc-5,15-bis(p-tolyl)porphyrin (ZnT P).
2
2
2
.2.2. ZnAT P.
2
Free-base porphyrin 5-(4-methoxyphenyl)-10,20-di-p-tolylporphyrin [67] was prepared by literature
procedures and subsequently metallated (288 mg, 0.48 mmol, 1 eq) in THF using zinc acetate
Zn(OAc) (1059 mg, 4.8 mmol, 10 eq). The solution was refluxed for 1h. The crude product was
2
purified by silica gel column chromatography with dichloromethane/cyclohexane (3/7) to afford 5-
(
4-methoxyphenyl)-10,20-di-p-tolylporphyrin zinc ZnAT P (294 mg, 0.45 mmol, 93%) as purple
2
solid.
1
H NMR (400 MHz, CHCl -d, 25°C): δ 10.20 (s, 1H, H meso), 9.37 (d, J = 4.5 Hz, 2H, H pyrr.), 9.10
3
(d, J = 4.5 Hz, 2H, H pyrr.), 9.01 (m, 4H, H pyrr.), 8.12 (d, J = 7.8 Hz, 6H, Ho-anis + Ho-tolyl), 7.58 (d, J
=
7.6 Hz, 4H, Hm-tolyl), 7.28 (d, J = 7.8 Hz, 2H, Hm-anis), 4.09 (s, 3H, OCH ), 2.73 (s, 6H, CH ) ppm
3
3
(Fig. S1).
1
3
C NMR (125 MHz, CHCl -d, 25°C): δ 159.37, 150.47, 150.43, 150.28, 150.01, 139.95, 137.29,
3
1
1
35.59 (CH), 134.68 (CH), 132.83 (CH), 132.14 (CH), 132.02 (CH), 131.77 (CH), 127.55 (CH),
20.82, 112.17 (CH), 105.85 (CH), 55.77 (CH ), 21.77 (CH ) ppm (Fig. S2).
3
3
7

ACCEPTED MANUSCRIPT
UV-Vis: λ= 419, 547, 587 nm.
+
ESI-TOF: m/z = 658.17. Calcd for C H N OZn ([M ]): 658.17.
4
1
30
4
TLC (silica): R : 0.33 (cyclohexane/dichloromethane, 50/50).
f
2.3. Electrochemistry and photoelectrochemistry. Voltammetric and electrochemical impedance
measurements have been performed with a standard three-electrode system using a PARSTAT 2273
-2
potentiostat. Glassy carbon or single-side coated indium-tin-oxide (ITO, SOLEMS, 25-35 Ω/cm )
2
plates, with a surface of about 1 cm have been used as working electrodes. A platinum wire has been
used as auxiliary electrode. The reference electrode was a saturated calomel electrode (SCE). It was
electrically connected to the solution by a junction bridge filled with the electrolyte.
Photoelectrochemical responses of the films were measured by illumination of the electrode using a
300 W Xe arc lamp (with λ > 385 nm long pass filter). A Pt wire is used as counter and reference
−
1
−1
electrode. The measurements were performed in water containing 5 mmol.L of I and 0.5 mol.L
2
-
-
−
−
of NaI. In solution I reacts with I to give I . I and I anions act as reversible redox mediator and
2
3
3
also as conducting electrolyte.
2.4. UV-visible-NIR Spectroscopy. UV–vis-NIR absorption spectra of the copolymers deposited on
ITO or in DMF solution, as well as the starting monomers, have been recorded using an Agilent
8453 spectrophotometer.
2.5. Electron Spin Resonance spectroscopy. ESR spectra were recorded with an ESP 300E
spectrometer (Bruker) operating at X-band and equipped with a high sensitivity resonator (4119HS-
W1). Spectra were recorded with 2 mW microwave power, a modulation frequency of 100 kHz and a
modulation amplitude of 0.1 mT. Computer simulations of the ESR spectra were performed with the
help of easyspin software [68].
2.6. X-ray Photoelectron Spectroscopy (XPS). XPS experiments were carried out using a Thermo
VG. Multilab ESCA 3000 spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) equipped
with an Mg KR radiation (h =1253.6 eV) or Al KR radiation (h = 1486.6 eV) source. The energy
2
shift due to electrostatic charging was subtracted using the sp carbon C 1s band at 284.8 eV coming
8

ACCEPTED MANUSCRIPT
from the isoporphyrin or porphyrin macrocycle. The data analysis was implemented by using the
XPSPEAK4.1 software.
2
.7. Synthesis of the poly-ZnT P. Electropolymerization has been carried out under an argon
2
−
1
−4
atmosphere in a 0.1 mol.L NBu PF CH CN/1,2-C H Cl (3/7) solution containing 2.5 × 10
4
6
3
2
4
2
−
1
−4
−1
+
+
−
mol.L of ZnT P or ZnOEP and 2.5 × 10 mol.L of bpy –(CH ) – bpy·2PF . ITO electrodes,
2
2 3
6
2
with a surface of 1 cm , were used as working electrode.
Electropolymerization has been tested by applying iterative potential scan between -1.00 V and
+
+
−
1
.00 V vs. SCE with ZnOEP in the presence of bpy –(CH ) – bpy·2PF but no copolymers and
2 3 6
film deposition could be obtained (Fig. 1) as indicated by the absence of modifications in the cyclic
voltammograms. The oxidation wave of the porphyrin remains reversible, showing that the
electrogenerated π-radical cation porphyrin does not react further.
In contrast, the formation of the copolymer is observed when zinc-5,15-tolyl-porphyrin (ZnT P)
2
is used instead of ZnOEP under the same conditions (concentrations, solvent and supporting
electrolyte). Electropolymerization occurs during iterative potential scans either between -1.00 V and
1.00 V (just after the first oxidation wave of the porphyrin) vs. SCE or between -1.00 V and 1.60 V
+
(
after the second oxidation wave of the porphyrin) vs. SCE in the presence of bpy –(CH ) –
2
3
+
−
bpy·2PF6 (Scheme 1 and Fig. 2). After electropolymerization, the modified working electrodes
were systematically washed with CH CN in order to remove traces of the monomers and of the
3
NBu PF eventuallyremaining on the electrodeposited films.
4
6
2.8. Electrochemical quartz crystal measurement (EQCM).
A QCA-922 (SEIKO EG&G instrument) system combined with Versa STAT 3 was used for carrying
out simultaneous electrochemical quartz crystal (EQCM) and cyclic voltammetric measurements.
The electrochemical cell was assembled in a glove box using an ITO AT-cut quartz crystal resonator
2
(
mirror finished; resonant frequency, 9.08 MHz ± 50 kHz; A=0.2 cm ; SEIKO EG&G., LTD) as
working electrode, a platinum wire as counter electrode, and an Ag/AgCl wire as a quasi-reference
electrode. The solution used here is the same as the one used for electropolymerization of the
copolymers on larger ITO electrodes. Iterative potential scans have been performed at a sweep rate
−
1
of 100 mV s at room temperature with simultaneous recording of the quartz resonance frequency.
9

ACCEPTED MANUSCRIPT
Changes in the quartz resonance frequency (ꢀf) have been converted into mass changes (ꢀm) on the
ITO-coated quartz during iterative cycling by applying Sauerbrey’s equation (Eq. 1):
2
1/2
ꢀ
f = −2f .ꢀm/A(ꢁ⋅ρ)
(Equation 1)
0
where f stand for the resonant frequency of the fundamental mode, ρ is the density of the crystal
0
−
3
2
(
2.684 g.cm ), A is the working electrode surface (0.2 cm ) of the ITO quartz crystal resonator, ꢁ is
1
1
−1 −2
the shear modulus of quartz (2.947×10 m .s ). Sauerbrey’s equation (Eq. 1) has been used to
calculate ꢀm considering a rigid and evenly distributed mass on the surface of the resonator.
2.9. Atomic force microscopy (AFM).
Atomic force microscopy (AFM) measurements have been conducted directly on the ITO surfaces
using a Dimension 3100 (Veeco) in the tapping mode under ambient conditions. Silicon cantilevers
(Veeco probes) with a spring constant of 40 N/m and a resonance frequency in the range of 300 kHz
have been used. The scanning rate was 1.0 Hz.
The thickness measurements have been carried out upon scratching the film with a metallic tip. The
thickness was then estimated by comparing the height on each side of the scratch.
3
. Results and Discussion
3
.1. Electropolymerization of zinc-5,15-ditolylporphyrin (ZnT P)
2
The molecular precursors porphyrins zinc-β-octaethylporphyrin (ZnOEP), zinc-5,15-bis(p-
+
+
−
tolyl)porphyrin (ZnT P), as well as dipyridyl ligands, bpy –(CH ) – bpy·2PF and close-related
2
2 3
6
compound zinc-5-(4-methoxyphenyl)-10,20-di-p-tolylporphyrin (ZnAT P) are depicted in Scheme 1.
2
1
0

ACCEPTED MANUSCRIPT
Scheme 1. Representation of A) zinc-β-octaethylporphyrin, ZnOEP, B) zinc-5,15-bis(p-
+
+
−
tolyl)porphyrin, ZnT P, C) bpy –(CH ) – bpy·2PF , D) zinc-5-p-anisyl-10,20-bis(p-tolyl)porphyrin
2
2 3
6
(
ZnAT P).
2
The syntheses of the copolymers were achieved via our previously reported
electropolymerization method [57-58-70]. It relies on the addition of dipyridyl ligands to an electro-
generated π-radical cation or dication porphyrin produced by iterative voltammetric scans (Fig. 2).
3.1.1. Reactivity of zinc-β-octaethylporphyrin ZnOEP
In our previous work, electropolymerization of zinc-
β
-octaethylporphyrin (ZnOEP) was carried
out at a sufficiently high positive potential value to produce doubly oxidized porphyrins (dication)
which are readily reacting with dipyridyl nucleophile to produce copolymers [57-58]. Furthermore,
mono-substituted ZnOEP porphyrin can also be obtained by electrosynthesis from the mono-
+
ꢀ
oxidized π-radical cations ZnOEP whereas electropolymerization requires the formation of
2
+
porphyrin dications ZnOEP , most probably due to kinetic issues (porphyrin dications react more
rapidly with pyridyl groups than the radical cation). An E(ECN mesoEC ) E mechanism has been
B n
previously proposed in the case of ZnOEP to account for the electropolymerization process where
CN meso relates to the nucleophilic attack at the meso position of the porphyrin to yield an
isoporphyrin intermediate [71]. This latter compound is then oxidized (electrochemical step) and the
1
1

ACCEPTED MANUSCRIPT
hydrogen atom initially located on the meso-carbon is released (chemical step C ). The
B
corresponding global reaction is written in Eq. 2 assuming that only the disubstitution on the
porphyrin occurs at meso positions (Schemes 1-2):
+
+
+
+
-
(
n+1) ZnP + (n+1) Py-R-Py → ZnP-(Py -R-Py -ZnP) -Py -R-Py + (2n+1)H + (4n+2)e (Equation 2)
n
+
+
(
where -R- = py -(CH ) -py and ZnP = zinc porphyrin)
2 3
In the case of ZnOEP, the disubstitution occurs at least at two of the four meso positions (cis or
trans di-substitution: substitution on the carbons C5 and C10 or C5 and C15 respectively) [71].
+
+
−
In this work, the nucleophile used is bpy –(CH ) – bpy·2PF and the copolymer is obtained in the
2
3
6
2
+
presence of ZnOEP only after generation of the porphyrin dications ZnOEP by iterative scans
between -1.3 V and 2.0 V) or by potentiostatic electrolysis at sufficiently high potential [52].
In the present work, we have found that when the anodic potential limit is only above the potential at
(
+
ꢀ
which the radical cation ZnOEP is generated (i.e. 0.8 V), no film is deposited on the ITO electrode
proving that the polymerization doesn’t occur in this case (Fig. 1). Moreover, the oxidation wave of
the macrocycle remains rather-irreversible showing that the electrogenerated radical cation does not
react further. It should be noticed that the reduction wave detected at -0.7 V corresponds to the redox
+
+
ꢀ
ꢀ
couple bpy –(CH ) – bpy/ bpy –(CH ) – bpy.
2
3
2 3
Fig. 1. Cyclic voltammograms recorded during 25 iterative scans conducted between −1.00 V and
−
1
+
(
0.80 V vs. SCE in a 1,2-C H Cl /CH CN (7/3) solution and NBu PF (0.1 mol.L ) of ZnOEP
2 4 2 3 4 6
-1
+
+
-
-1
2
0.25 mmol L ) in the presence of bpy –(CH ) – bpy 2PF (0.25 mmol L ). WE: ITO. S = 1 cm . v
2 3 6
1
2

ACCEPTED MANUSCRIPT
−
1
=
0.1 V s . Blue curve: first scan (n=1). Red curve: final scan (n = 25). (For interpretation of the
references to color in this figure legend, the reader is referred to the Web version of this article.)
In our former work, we concluded that, while the formation of radical cation is enough to obtain
monosubstituted porphyrins [59,72] via electrosynthesis (but necessitating much longer time than
cyclic voltammetry), the formation of dication is requested to perform the electropolymerization of
ZnOEP via iterative cyclic voltammetry. A kinetic restraint can explain this difference considering
that the nucleophilic attack is faster on the dication than on the radical cation. As a result, if the
iterative scans are stopped at a potential allowing only the formation of the radical cation, the
-1
characteristic time of the cyclic voltammetry at 100 mV.s might be too short for the nucleophilic
attack to occur. The presence in this case of the ethyl groups decreases drastically the kinetic of the
nucleophilic attack at the meso position C5, C10, C15 or C20 respectively. Also, the need to apply a
higher potential for electropolymerization is related to the degree of substitution of macrocycles and
the presence of two positively charged groups on each porphyrin once included in the copolymers.
Consequently, porphyrins are more and more difficult to oxidize resulting in an increase of potential
at which the polymerization occurs. This explanation is also supported by the fact that a higher
applied potential has always been required to perform electrosynthesis of multi-substituted
porphyrins through electrolysis [49,73].
It can also be noted that higher anodic potential limit should lead to longer copolymers chains.
Otherwise, if the upper potential is too low, but still higher than that of the first monomer oxidation,
only small oligomers should be obtained which may dissolve.
3.1.2. Reactivity of zinc-meso-5,15-ditolyl-porphyrin ZnT2P
In this study, we have decided to use the zinc-meso-5,15-ditolyl-porphyrin (ZnT P) which
2
presents only two meso positions occupied by one substitutable proton at positions C10 and C20.
Additionally, all the positions of ZnT P are occupied only by protons. Such porphyrin will provide
better accessibility for the nucleophilic attack of the pyridyl groups of bpy –(CH ) – bpy·2PF in
β
2
+
+
−
2
3
6
comparison to ZnOEP (Scheme 1). In the literature, the magnesium porphin showed only the
nucleophilic substitution with pyridine or triphenylphosphine at the meso-positions even when the
positions are only occupied by protons [ 74 ]. Electrosynthesis of Zn and Mg meso-
β
-
1
3

ACCEPTED MANUSCRIPT
triarylphosphonium porphyrin were also reported from Zn or Mg 5,10,15-triarylporphyrin showing a
nucleophilic substitution mainly at the meso and not the
β position [ 75 ]. Furthermore, the
electrochemical oxidation of pyridin-2-ylthio-meso substituted Ni(II) porphyrins afforded oxidative
C-N fusion of pyridinyl-substituted porphyrins again at the meso position giving cationic and
dicationic pyridinium-based products [76].
Lastly, in order to demonstrate that the copolymer can be obtained using only ZnT P which presents
2
two meso positions bearing H (i.e. position 10 and 20, methine bridges), we have synthesized the
zinc-5-(4-methoxyphenyl)-10,20-di-p-tolylporphyrin, ZnAT P (Scheme 1) in which one meso
2
position (C5) is occupied by a methoxyphenyl group. The experimental protocol for the synthesis of
+
+
−
ZnAT P is described in the ESI. In the presence of bpy –(CH ) – bpy·2PF and ZnAT P no
2
2 3
6
2
copolymer was obtained even after a large number of iterative scans between -1.0 V and 1.0 V (v =
-1
1
00 mVs , Fig. S3) showing that two meso positions bearing H are required to form the copolymer.
Additionally, for zinc 5,15-ditolyl porphyrin (ZnT P), the nucleophilic attack appears to be allowed
2
only at the 10 as well as the 20 meso positions of the porphyrin.
+
ꢀ
It must be noted that in the absence of nucleophile, the electrogenerated radical ZnT2P may
undergo a radical coupling reaction and the formation of one dimer with C-C bond formation
through meso–meso, meso–β or β–β link between two macrocycles. Indeed, previous works in the
literature using the 5,15-bis(3,5-di-tert-butylphenyl) porphyrin showed only the formation of a
mixture of meso–meso and meso–β linked dimers. These dimers presented a splitting of the Soret
band of the optical spectra [77-79].
However, when potential iterative scans are performed between -1.0 V and +1.0 V or +1.6 V
using ZnT P alone (in the absence of nucleophile) no film deposition onto the ITO electrode is
2
observed suggesting that the kinetic of radical coupling reaction is relatively slow. The formation of
dimer and eventually small oligomer which are soluble cannot be completely excluded.
+
+
−
Additionally, in the presence of bpy –(CH ) – bpy·2PF nucleophile, the radical coupling is still
2
3
6
not detected during the iterative scans, copolymers being obtained only with viologen or double
viologen. The UV-vis-NIR spectra of the copolymer deposited onto ITO electrode also confirm the
absence of dimer (ZnT P) or oligomer (ZnT P) through radical coupling in the solid state. The
2
2
2
n
control of the solution after the electropolymerization has been checked by UV-vis-NIR
spectroscopy. Only the starting ZnT P monomer could be detected in the solution, even after a large
2
1
4

ACCEPTED MANUSCRIPT
number of iterative scans, showing the absence of significant quantity of dimer obtained by radical
coupling under these conditions. UV-vis-NIR spectroscopy also confirms the absence of
demetallation of the starting ZnT P porphyrin in solution even after deposition of a large number of
2
films. It demonstrates that the Zn metal ion remains in the porphyrin ring during
electropolymerization and that there is no (ZnT P) dimer or oligomer (formed through radical
2
2
coupling) in detectable amount.
The general electropolymerization procedure used to deposit the porphyrin-based polymers onto
ITO electrode surfaces is described below. This process depends on the polarization of the working
+
ꢀ
electrode during the potential scan that is either at the first oxidation potential to produce ZnT2P
2
+
(iteration between -1.0 V and +1.0 V) or at the second oxidation potential to produce ZnT2P
(iteration between -1.0 V and +1.6 V). It involves a series of intermolecular cascade reactions
leading to formation of physisorbed oligomers and copolymers (see Scheme 2 for suggested
mechanism). The bridging bis-nucleophile used as a precursor is a the dicationic bispyridinium
+
+
-
bpy –(CH ) – bpy·2PF offering two terminal pyridine rings (Scheme 1).
2
3
6
All the information summarizing the conditions of the formation of the copolymers, the species
present in the starting solution, their concentrations and the anodic potential limit used in oxidation
are given in Table 1.
Table 1. Electropolymerization conditions.
Monomer
c = 0.25mM)
Copolymer
Iterative scan
(v = 0.1 Vs )
Solvent and
electrolyte
-1
(
between:
-1.00 V and +1.00 V
ꢀ
ZnT P +
poly-ZnT isoP
2
2
+
+
-
-
-
-
bpy –(CH ) – bpy·2PF
2
3
6
6
6
6
ZnT P +
poly-ZnT2P
No reaction
-1.00 V and +1.60 V
-1.00 V and +1.00 V
-1.00 V and +1.60 V
1,2-C H Cl
2
2
2
4
+
+
bpy –(CH ) – bpy·2PF
/CH CN (7/3)
2
3
3
ZnOEP +
solution +
0.1 M TBAPF6
+
+
bpy –(CH ) – bpy·2PF
2
3
a
ZnOEP +
poly-ZnOEP
+
+
bpy –(CH ) – bpy·2PF
2
3
a
copolymer already published, see reference 57.
1
5

ACCEPTED MANUSCRIPT
Scheme 2. Proposed plausible (E C
E C ) mechanism of the formation of the intermediate
2 B n
1
Nmeso
ꢀ
poly-ZnT isoP (green box) and the final poly-ZnT P (yellow box) copolymer explaining the
2
2
+
+
−
ꢀ
reactivity of a bpy –(CH ) – bpy·2PF during the oxidation of ZnT P. Poly-ZnT isoP is obtained
2
3
6
2
2
using iterative scan between -1.0 V and 1.0 V and poly-ZnT P is obtained in the case of iterative
2
scan between -1.0 V and 1.6 V.
+
+
-
During the electropolymerization of ZnT P in the presence of bpy –(CH ) – bpy·2PF , when
2
2 3
6
the anodic potential limit is 1.60 V, we observed that the oxidation peaks intensity increases (Fig. 2A)
which is correlated to the increase of the film thickness. The first scan toward negative potentials
reveals only one irreversible pyridinium-centered reduction. Starting with the second scan, in the
cathodic potential range, the reduction peaks observed at ca. -0.13 V (peak I) and -0.63 V (peak II) vs.
SCE for poly-ZnT P (Fig. 2A) have been attributed to electron transfers on the electrogenerated
2
viologen units introduced between two porphyrins [52,57-58,60]. Peak a is attributed to the reduction
of an oxidized isoporphyrin intermediate as previously described [5,29].
1
6

ACCEPTED MANUSCRIPT
The continuous increase of these reduction waves indicates a regular growth of the copolymer
film onto the ITO working electrode.
Fig. 2. Cyclic voltammograms recorded during 25 iterative scans conducted between A) -1.00 V and
+
1.00 V/SCE and B) -1.00 V and +1.60 V vs. SCE in a 1,2-C H Cl /CH CN (7/3) solution of ZnT P
2 4 2 3 2
-1
+
+
-
-1
(0.25 mmol L ) in the presence of bpy –(CH ) – bpy·2PF (0.25 mmol L ) and NBu PF (0.1 mol
2 3 6 4 6
-1
2
−1
L ). WE: ITO. S = 1 cm . v = 0.1 V s . (a) This peak is attributed to the reduction of an oxidized
isoporphyrin intermediate obtained if using 1.6 V as end potential during iterative scan. (b) This peak
is attributed to the one electron reduction of the pyridium directly connected with the isoporphyrin.
Peaks I’: first reduction of the viologen (one electron reduction). Peak II’: second reduction of the
viologen (two electron reductions). Peaks I and II: successive reduction of the double viologen
spacers. Blue curve: first scan (n=1). Red curve: final scan (n = 25). (For interpretation of the
references to color in this figure legend, the reader is referred to the Web version of this article.)
+
+
-
During the electropolymerization of ZnT P in the presence of bpy –(CH ) – bpy·2PF , when the
2
2 3
6
anodic potential limit is +1.00 V (Fig. 2B), below the first porphyrin-based oxidation potential, the
1
7

ACCEPTED MANUSCRIPT
+
ꢀ
π-radical cation ZnT P is produced. As a consequence, we observed significant changes during the
2
electropolymerization process (Fig. 2B). In the anodic part, the intensity of the oxidation peak near
+
0.79 V corresponding to the first oxidation of ZnT P, continuously increases with no shift during
2
ꢀ
the electropolymerization proving the formation of a new copolymer poly-ZnT isoP in this
2
potential range.
In the cathodic range, three successive reductions are observed at -0.18 V (peak I’), -0.44 V (peak b)
and -0.70 V (peak II’) (Fig. 2B). It must be noted that peak b is irreversible and peak I’ is
approximately half of the intensity than peak II’. These results point out to the formation of a
ꢀ
copolymer poly-ZnT isoP with different behavior than poly-ZnT P. We present below a detailed
2
2
ꢀ
characterization of the two copolymers, i.e. poly-ZnT P and poly-ZnT isoP . A comparison
2
2
between their different properties is made when appropriate.
3.2. Electrochemical Quartz Crystal Microbalance (EQCM) for copolymers deposition
ꢀ
The electrosynthesis of poly-ZnT isoP by electropolymerization with iterative scans between -1.00
2
and +1.00 V of (Fig. 3) has been monitored in-situ by EQCM. The variation of the quartz resonance
frequency (∆f) decreases when the number of potential cycle increases, which is related to the
increase of the amount of polymer deposited (∆m), calculated using Sauerbrey’s equation [80].
Besides, the trace of the first scan in Fig. 3B shows a significant decrease of the resonance
frequency and thus an increase of the deposited mass at the first oxidation of the porphyrin; i.e.
+
ꢀ
electropolymerization occurs upon the formation of the radical cation porphyrin ZnT2P in the
+
+
-
presence of bpy –(CH ) – bpy·2PF . The mass of the copolymer film increases with the number of
2
3
6
potential cycle n (Fig. 3B). Note that a change of slope can be observed for n > 10 which might be
related to a transition in the 3D architecture of the copolymer film and the incorporation of
supporting electrolyte as well as solvent molecules.
2
ꢀ
After 25 iterative scans, 28.01 µg/cm of poly-ZnT isoP are deposited. The calculated surface
2
2
+
2+
-
-9
coverage Γ in mole of repeat unit (ZnT P-bpy –(CH ) – bpy·4PF ) for the polymer is 18.8×10
2
2 3
6
-2
mol.cm . Similar behavior was observed for the electropolymerization of poly-ZnT P using
2
iterative scans between -1.00 and +1.60 V (Fig. S4). Coverages after 25 iterative scans were about
2
-9
-2
3
5.38 µg/cm . The calculated surface coverage Γ in mole of repeat unit was 23.76 × 10 mol.cm , a
ꢀ
similar value to the one obtained for poly-ZnT isoP .
2
1
8

ACCEPTED MANUSCRIPT
Fig. 3. A) Consecutive cyclic voltammograms (first 25 scans) between -1.0 V and +1.0 V and
electrochemical quartz crystal microbalance measurements (∆m) for the first 10 scans during the
ꢀ
-1
formation of poly-ZnT isoP obtained by the electropolymerization of 0.25 mmol L ZnT P with
2
2
-1
+
+
-
-1
0
.25 mmol L bpy –(CH ) – bpy·2PF (0.25 mmol L ) in 1,2-C H Cl –CH CN (7/3) in the
presence of 0.1 mol L NBu PF . Working electrode: ITO (A = 0.2 cm ) deposited on a 9.08 MHz
AT-cut quartz crystal. v = 100 mVs . B) Mass change (ꢀm) of the first 25 scans calculated from
Sauerbrey’s equation versus the number of scan n.
2 3 6 2 4 2 3
-1
2
4
6
-1
3.3. UV–Vis-NIR spectroscopy
ꢀ
UV-visible-NIR spectra on ITO electrodes coated with the copolymer poly-ZnT isoP and poly-
2
ZnT P have been measured at various thickness and compared (Fig. 4 and Fig. S5 respectively). The
2
absorption intensity of the chromophores increases linearly with iterative scan number (Fig. 4B).
ꢀ
These results have been confirmed by AFM experiments (Figs. 10-11 for poly-ZnT isoP ) where the
2
thickness was found to increase also linearly versus the number of iterative scans.
ꢀ
A typical UV-visible-NIR spectrum of poly-ZnT isoP obtained after electropolymerization
2
between -1.00 and +1.00 V exhibited a broad split Soret absorption band at λ = 432 nm and λ = 473
nm. The bands were red-shifted by 17 and 58 nm respectively compared to the ZnT P monomer
2
1
9

ACCEPTED MANUSCRIPT
(Table 2 and Fig. 4). The visible bands (Q bands), observed at 571 and 635 nm, are also red-shifted
by 25 and 51 nm compared to ZnT P and an additional band at 831 nm in the NIR region is also
2
detected (Fig. 4A).
These electropolymerized films have been found to be fully soluble in DMF. Analysis of poly-
ꢀ
ZnT isoP in solution by UV-Vis-NIR absorption spectroscopy revealed comparable spectra than
2
the ones recorded on the solid film (Fig. 5).
ꢀ
Fig. 4. A) UV-visible-NIR absorption spectra of poly-ZnT isoP onto ITO with different numbers of
2
-1
iterative scans between -1.00 and +1.00 V vs. SCE (v = 100 mVs ). Only one side is covered by ITO.
B) Plot of the absorbance measured at λ = 433 nm versus n the numbers of iterative scans. (For
interpretation of the references to color in this figure legend, the reader is referred to the Web version
of this article.)
2
0

ACCEPTED MANUSCRIPT
ꢀ
Fig. 5. Normalized UV-Vis-NIR spectra of the ITO electrode modified with poly-ZnT isoP ,
2
-1
obtained with 10 iterative scans between -1.00 and +1.00 V versus SCE at v = 100 mVs (blue line),
ꢀ
and poly-ZnT isoP (red line) and ZnT P (dark dotted line) in DMF solution. (For interpretation of
2
2
the references to color in this figure legend, the reader is referred to the Web version of this article.)
The superposition of the UV-visible-NIR absorption spectra of ZnT P in solution and of the
2
ꢀ
copolymer poly-ZnT isoP deposited on ITO are compared in Figure 5. It reveals that the Soret
2
band, attributed to the main porphyrin-based π-π* electronic transition, is much broader, split and
ꢀ
more red-shifted for poly-ZnT isoP on ITO electrodes than for ZnT P (Table 2).
2
2
The red shift of the Soret (B) and the Q bands might result from the presence of the electron-
withdrawing pyridinium groups as well as from the nonplanar conformation of the macrocycle.
Optical red shifts induced by the non-planarity of porphyrins are well documented [81-84] and have
been rationalized by a larger destabilization of the highest occupied molecular orbitals (HOMOs)
relative to the lowest unoccupied molecular orbitals (LUMOs) resulting in smaller HOMO to LUMO
gaps [85-88]. These changes can also be understood taking into account the presence of intra- and
intermolecular exciton-coupling between the porphyrin macrocycle within the copolymer [89-90].
The additional bands at 473 and 831 nm may be attributed to the isoporphyrin structure present in
the copolymer. Similar bands have been already observed in the case of stable isoporphyrin
monomer [16]. Interestingly, the broadening and the splitting of the Soret band as well as the
presence of one additional band in the NIR between 750 nm and 1000 nm are expected to be
advantageous to photovoltaic applications by extending the domain of solar light absorption.
2
1

ACCEPTED MANUSCRIPT
ꢀ
Poly-ZnT isoP isoporphyrin radical film was found to be highly stable in the presence of
2
ꢀ
oxygen (still stable even after one year). The isoporphyrin radical Poly-ZnT isoP is also stable in
2
DMF solution.
We have also investigated various copolymer poly-ZnT P (Fig. S5 and Scheme S1) coated films,
2
prepared by changing the number of iterative scans n used for electropolymerization (between -1.00
V and +1.60 V). They exhibited only one large Soret absorption band at λ = 429 nm with a red shift
of 14 nm compared to the ZnT P monomer (Table 2 and Fig. S5-S6). Again, a plot of the
2
absorbance recorded at λ = 429 nm (Soret band of the porphyrin) as a function of the number of
iterative scans n (Fig. S5) shows a quasi-linear increase of the intensity of the chromophore.
However, the additional band at 473 nm was nearly not detected. It suggested that at higher applied
potential, isoporphyrin copolymer intermediate could be consumed giving the final structure of the
copolymer with double viologen as spacers. But the consummation is not quantitative as seen in
Figures S5-S6 probably due to i) the important thickness of the film and ii) to the difficulty to fully
-1
oxidized the film if using scan rate of 100 mVs .
+
+
-
Table 2. UV–visible spectral data for ZnT P, and bpy –(CH ) – bpy·2PF in 1,2-EtCl , poly-
2
2 3
6
2
ꢀ
ZnT P and poly-ZnT P on optical transparent ITO electrodes. Under bracket: molar extinction
2
2
3
-1
coefficient (ε /10 L.M.cm ).
*
Compound
Soret band/nm
Q bands/nm π-π Band/nm
ZnT2P
415 (405.4)
546 (18.9),
5
84 (4.9)
561 (42.4),
11 (18.1)
+
-a
ZnT P-bpy SbCl
429 (584.8)
2
6
6
+
+
-
a
bpy –(CH ) –bpy 2PF
266 (40.8)
2
3
6
ꢀ
b
poly-ZnT isoP
432, 473 (shoulder)
571, 635, 831
2
ꢀ
c
f
poly-ZnT isoP
415, 423 (shoulder), 573, 631, 825
2
4
429
63 (shoulder)
d
poly-ZnT2P
573, 614, 811
573, 614 ,811
e
f
poly-ZnT2P
415, 429
a
b
c
d
e
f
In CH Cl.
3
Copolymers obtained by iterative scan between -1.00 V and +1.00 V vs. SCE onto ITO.
Copolymers obtained by iterative scan between -1.00 V and +1.00 V vs. SCE in DMF.
Copolymers obtained by iterative scan between -1.00 V and +1.60 V vs. SCE onto ITO.
Copolymers obtained by iterative scan between -1.00 V and +1.60 V vs. SCE in DMF.
Soret band of the ZnT P monomer which was encapsulated in the film during the electropolymerization and
2
ꢀ
liberated during dissolution of poly-ZnT isoP .
2
2
2

ACCEPTED MANUSCRIPT
3.4. Cyclic voltammetric investigations of the copolymeric films.
ꢀ
3
.4.1. Cyclic voltammetric investigations of the poly-ZnT isoP film.
2
Electroactive polymers deposited by cyclic voltammetry (n iterative scans) on ITO surfaces have
been characterized by electrochemical methods. The CV curves shown in Figure 6 have been
recorded with a copolymer grown on the electrode surface for various potential scans (n = 2, 3, 5, 10,
15, 20 cycles) between -1.0 V and +1.0 V. The electrode was then removed from the electrochemical
cell, washed with CH CN and used as working electrode in a clean electrolytic solution containing
3
only the solvent and the supporting electrolyte.
ꢀ
Three successive waves are observed for poly-ZnT isoP during the cathodic scan (Table 3 and Fig.
2
6AB), the first and the last being reversible. The first well defined, bell-shaped, reduction wave
observed at –0.18 V vs. SCE (peak I’) is attributed to the formation of viologen radical cations in the
ꢀ
+
+
+
copolymer (-ZnT isoP- py-py –(CH ) – py-py -ZnT isoP-). The second wave (peak a) detected at
2
2 3
2
-0.62 V is irreversible and very close to the last process (peak II’). This wave (peak b) corresponds
ꢀ
+
probably to the reduction of the pyridinium connected to the isoporphyrin (ZnT isoP- py-py –
2
+
ꢀ
(
CH ) – py-py -ZnT isoP-) while the last wave reversible well defined, bell-shaped, reduction wave
2 3 2
observed (peak II’) is attributed to the second viologen-centered electron transfer (two-electron
transfer per spacer giving ZnT isoP-py-py–(CH ) –py-py-ZnT isoP-). The reduction of the monomer
2
2 3
2
+
+
-
py-py –(CH ) – py-py2 PF is measured at -0.68 V.
2
3
6
ꢀ
Additionally, poly-ZnT isoP films electropolymerized between by -1.0 V and 1.0 V, can be
2
further oxidized up to 1.6 V (Fig. 7). Oxidation of the isoporphyrin radical units which give oxidized
isoporphyrin is then possible. At this stage, the hydrogen atom located on the meso-carbon can be
released giving copolymer with similar redox properties than poly-ZnT P. Thus, it is feasible to
2
ꢀ
oxidize the radical isoporphyrin poly-ZnT isoP copolymer and then remove the hydrogen atom
2
located on the meso-carbon giving the formation of poly-ZnT P with a double viologen spacer
2
(Scheme 3).
2
3

ACCEPTED MANUSCRIPT
+
+
-
Scheme 3. Electropolymerization scheme explaining the reactivity of bpy –(CH ) – bpy·2PF in the
2
3
6
presence of ZnT P in the case of iterative scan between -1.0 V and 1.0 V or between -1.0 V and 1.6
2
V.
2
4

ACCEPTED MANUSCRIPT
ꢀ
+
+
-
Fig. 6. Cyclic voltammograms of poly-ZnT isoP obtained with bpy –(CH ) – bpy·2PF and
2
2 3
6
ZnT P after A) n = 2, 3, 5, 10, 15 and 20 scans and B) after 10 scans between -1.0 V and +1.0 V in
2
2
−1
CH CN/1,2-C H Cl (3/7) with 0.1 M NBu PF . WE: ITO. S = 1 cm , v = 0.1 V s . Irreversible
3
2
4
2
4
6
peaks not labelled in anodic part correspond to the oxidation of the π-ring of the macrocycle.
2
5

ACCEPTED MANUSCRIPT
ꢀ
+
+
-
Fig. 7. Cyclic voltammograms of poly-ZnT isoP obtained with bpy –(CH ) – bpy·2PF and
2
2 3
6
ZnT P after n = 10 scans, using iterative scan between -1.0 V and +1.0 V in CH CN/1,2-C H Cl
2
2
3
2
4
2
−1
(
3/7) with 0.1 M TBAPF . WE: ITO. S = 1 cm . v = 0.1 V s . Blue curve: the direction of the
6
potential scan is reversed (in comparison to the black curve) using new electrode. Global reaction of
ꢀ
the conversion of poly-ZnT isoP to poly-ZnT P during oxidation at 1.60 V (blue curve) is
2
2
described in Scheme 3. Peaks I and II correspond to the formation of the bis-radical cation viologen
2
+
2+
+
ꢀ
+
ꢀ
(
couple V –(CH ) –V /V –(CH ) –V , peak I) and the second reduction of two bis-radical cation
2 3 2 3
0
0
+
ꢀ
+
ꢀ
0
0
viologen units of the spacers yielding V –(CH ) –V (couple V –(CH ) –V / V –(CH ) –V , peak
2
3
2 3
2 3
II). V = py-py. (For interpretation of the references to color in this figure legend, the reader is
referred to the Web version of this article.)
2
6

ACCEPTED MANUSCRIPT
+
+
-
Table 3. Electrochemical data for ZnOEP, ZnT P, bpy –(CH ) – bpy 2PF , poly-ZnT P, and
2
2 3
6
2
ꢀ
poly-ZnT isoP .
2
Ring oxidation
Reduction of spacer
Ring
reduction
+
ꢀ
Compounds
peak I or I’ bpy /bpy
peak II or II’
or peak b
a
ZnOEP
1.08
0.71
-1.66
(
130)
(128)
0.79
(90)
a
ZnT2P
1.08
-1.41
-1.84
(
150)
(160) (170)
+
irr
bpy –(CH ) –
-0.68
2
3
+
-
bpy·2PF6
ꢀ
irr
irr
irr
poly-ZnT isoP
+1.45
+0.89
-0.18
140)
-0.62
-0.70
(70)
2
(
irr
poly-ZnT2P
+1.35
+1.09
-0.13
(170)
-0.63
(120)
(
130)
a
Potentials in V vs. SCE were obtained from cyclic voltammetry in 1,2-C H Cl with 0.1
2
4
2
-1
-1
2
mol L TBAPF Scan rate = 100 mV s . Working electrode: ITO, S=1 cm .
6
.
b
Potentials in V vs. SCE were obtained from cyclic voltammetry in CH CN/1,2-C H Cl (3/7) with
3
2
4
2
-1
-1
0
.1 mol L TBAPF . Scan rate = 100 mV s .
6
c
-1
Potentials in V vs. SCE were obtained from cyclic voltammetry in H O with 0.5 mol L NaI.
2
-1
Working electrode: glassy carbon electrode Scan rate = 100 mV s (see Fig. S15).
.
The given half-wave potentials are equal to E = (E + E )/2. Under bracket: ꢀE = |E -E |.
1
/2
pa
pc
p
pa pc
3
.4.2. Cyclic voltammetric investigations of the poly-ZnT P film.
2
Electropolymerization using iterative scan between -1.00 V and 1.60 V vs. SCE in the presence of
+
+
−
ZnT P and bpy –(CH ) – bpy·2PF , was also performed to obtain poly-ZnT P film with double
2
2 3
6
2
viologen spacers (Scheme 2). Poly-ZnT P copolymers were studied in the electrolyte used for
2
electropolymerization that is CH CN/1,2-C H Cl (3/7) solution with 0.1 M NBu PF .
3
2
4
2
4
6
ꢀ
In contrast with poly-ZnT isoP , no irreversible wave was detected in the cathodic part and only two
2
successive reversible reductions (peaks I and II) were measured.
The two reduction peaks centered at ca. -0.13 V (peak I) and -0.63 V (peak II) correspond to the
2
+
2+
+
ꢀ
+
ꢀ
formation of the bis-radical cation viologen (couple V –(CH ) –V /V –(CH ) –V , V = py-py,
2
3
2 3
2
7

ACCEPTED MANUSCRIPT
0
peak I) and the second reduction of two bis-radical cation viologen units of the spacers yielding V -
0
+
ꢀ
+
ꢀ
0
0
(
CH ) -V (couple V –(CH ) –V / V –(CH ) –V , peak II) (Table 3, Fig. 8 and Fig. S6).
2 3 2 3 2 3
It must be noted that under the measurement conditions, the splitting of the second reduction wave of
the viologen, observed with the ZnOEP porphyrin [52] was not detected in the case of ZnT2P
porphyrin.
+
+
-
Fig. 8. A) Cyclic voltammograms of poly-ZnT P obtained with bpy –(CH ) – bpy·2PF and ZnT P
2
2 3
6
2
after n = 1, 2, 3, 5, 10, 15 and 20 scans, between -1.0 V and +1.6 V in CH CN/1,2-C H Cl (3/7)
3
2
4
2
2
−1
with 0.1 M NBu PF . WE: ITO. S = 1 cm . v = 0.1 V s . (For interpretation of the references to
4
6
color in this figure legend, the reader is referred to the Web version of this article.)
3.5. Electron spin resonance (ESR) spectroscopy
ꢀ
Copolymer poly-ZnT PisoP onto ITO glass prepared after 50 iterative scans was inserted directly
2
to the ESR cavity and the spectrum (Fig. S7 in ESI) recorded at room temperature shows spectral
contributions from two paramagnetic species. The dominant feature shows a broad signal of six lines
and comes from an impurity in the ITO glass. A less intense line is observed in the middle of the
spectrum and probably arises from an organic radical.
ꢀ
To confirm the presence of the organic radical, poly-ZnT isoP films onto ITO were prepared with
2
-1
the same conditions (25 iterative scans between -1.0 V and 1.0 V, v = 100 mV.s ).
ꢀ
Poly-ZnT isoP was then removed from ITO using DMF. The operation was repeated three times.
2
The obtained solutions were degassed and transferred to a capillary. The ESR spectrum shown in
2
8

ACCEPTED MANUSCRIPT
Figure 9 reveals probably the presence of the radical centered on the isoporphyrin (g=2.0026,
linewidth=6 G) [91]. Increasing the concentration of the deposited copolymer on the ITO glass leads
to an enhancement of the ESR intensity (Fig. S8 in ESI).
ꢀ
Fig. 9. X-Band EPR spectrum in DMF of poly-ZnT isoP at room temperature. The solution of
2
ꢀ
poly-ZnT isoP was prepared by washing with 160 µL of DMF the covered ITO obtained using 25
2
-1
scans between -1.0 V and 1.0 V, v = 100 mV.s . In order to have enough solution, the operation was
repeated three times.
Usually, isoporphyrin or radical cations porphyrins are reactive and undergo facile degradation.
ꢀ
In this study, poly-ZnT isoP isoporphyrin radical film is remarkably stable for one year at the solid
2
state (in air). This stability may be due to the i) electron-withdrawing groups which significantly
facilitate the reduction and stabilize the radical cation, ii) the delocalization of the radical through the
porphyrin as well as intermolecular π–π -stacking and π-dimerization between macrocycles.
3.6. Film Morphology (Atomic Force Microscopy)
ꢀ
3
.6.1. Film Morphology (Atomic Force Microscopy) of poly-ZnT isoP copolymer
2
.
The films obtained by iterative scan between -1.0 V and +1.0 V were studied by scanning atomic
ꢀ
force microscopy (AFM) (Figs. 10-11, Figs. S9-S10). In a characteristic picture for poly-ZnT isoP ,
2
copolymer appears on the surface as tightly packed coils with an average diameter of ca. 60-70 nm,
the height being around 8.0 nm for the film obtained after 3 iterative scans between -1.0 V and +1.0
V (Fig. 10). The rms surface roughnesses of the two films have been estimated at 2.0 nm for poly-
ꢀ
2
ꢀ
ZnT isoP , (calculated from an area of 1.0 µm in Figure 10). The poly-ZnT isoP copolymer
2
2
2
9