Competition between π-π Or furan-perfluorophenyl stacking interactions in conjugated compounds prepared from azomethine connections

Article abstract of DOI:10.1039/c1ce05575e

Conjugated compounds associating phenyl or pentafluorophenyl units linked via azomethine bonds to a central furan or furylene-vinylene moiety have been synthesized. The crystal structures of compounds end capped with perfluorophenyl units are analyzed in

Full text of DOI:10.1039/c1ce05575e

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PAPER
Competition between p–p or furan–perfluorophenyl stacking interactions in
conjugated compounds prepared from azomethine connections†
*
ꢀ
Charlotte Mallet, Magali Allain, Philippe Leriche and Pierre Frere
Received 17th May 2011, Accepted 21st June 2011
DOI: 10.1039/c1ce05575e
Conjugated compounds associating phenyl or pentafluorophenyl units linked via azomethine bonds to
a central furan or furylene–vinylene moiety have been synthesized. The crystal structures of compounds
end capped with perfluorophenyl units are analyzed in terms of intermolecular interactions involving
the C–H---F contacts, p–p interactions and furan–perfluorobenzene interactions. The compound with
a furan moiety as spacer presents a stacking mode defined by C–H–----F contacts and p–p interactions
while the compound based on a furylene–vinylene unit shows a packing pattern exclusively due to
strong furan–perfluorobenzene interactions.
materials.21–27 X-Ray analyses of the described structures
Introduction
revealed the formation of mainly two kinds of supramolecular
arrangements. In planar perfluoroarene–thiophene oligomers,
the stacking of the molecules is promoted by close cofacial
interactions between the electron-rich thiophene and the elec-
tron-poor perfluoroarene moieties.28,29 In contrast, when the
perfluorophenyl and thiophene units are not coplanar, the
organization shows a discrimination of systems and thiophenes
stack together via p–p interactions and separately from per-
fluorophenyl units which also interact with each other.24,30
Although conjugated systems based on the furan moiety have
emerged as a new alternative for the development of semi-
conducting materials,31,32 conjugated systems associating per-
fluoroarene and furan have been scantly studied.33,34 Moreover,
furan–perfluorophenyl interactions have not clearly been evi-
denced yet. There exists a study of furan–fluoroaromatic inter-
actions concerning the intramolecular interactions in
(heterocyclo)paracyclophane derivatives35 and one example of an
In the field of molecular materials, the control of the crystal
structures which determines the physical properties of the
materials requires intensive investigations in order to elucidate
the relationships between molecular structures and supramolec-
ular architectures. Weak intermolecular interactions such as
hydrogen bonding, halogen bonding, p–p stacking and donor–
acceptor interactions represent the main tools of crystal engi-
neering.1–7 Thus arene–arene interactions between aromatic
residues play a key role in the performance of conducting or
semi-conducting materials.8,9 Since the discovery of the benzene–
hexafluorobenzene co-crystallization by Patrick and Prosser,10
phenyl–perfluorophenyl interactions have been observed for
many co-crystals or in crystalline structures of mixed derivatives
built with both phenyl and perfluorophenyl groups.11–19 Such
interactions, often resulting in eclipsed or staggered face to face
p–p stacking, are the main driving forces for the self-organiza-
tion of the molecules in those solids. Although the understanding
of the role of the fluorine atoms is not yet clear, many theoretical
and experimental works have shown that the arene–per-
fluoroarene interactions could be considered as an interesting
supramolecular tool for crystal engineering.20 By contrast with
the great number of studies devoted to the arene–perfluoroarene
interactions, the works to elucidate the interactions between
perfluorophenyl and heterocycle units are surprisingly limited. In
the field of organic semiconductor materials, a recent trend
consists of associating perfluoroarene and thiophene units in
order to enhance the electron affinity of corresponding
X-ray structure of
a
furan–perfluorophenyl conjugated
compound recently published;34 however, the stacking mode in
the structure was not described.36
In the continuation of our current interest in conjugated
systems based on furan units as electroactive materials from
renewable biomass resources,37–39 we present here three conju-
gated compounds 1, 2 and 3 associating furan moieties with
phenyl or perfluorophenyl units via azomethine bonds (Scheme 1).
The use of furan cycles and azomethine bonds are part of
a green chemistry approach. Thus azomethine bonds are formed
by reaction between aldehydes and amines and produce only
water as waste. Several examples of conjugated materials based
on azomethine have been described.40–47
LUNAM Universiteꢁ, University of Angers, MOLTECH—Anjou UMR
CNRS 6200, SCL group, 2 boulevard Lavoisier, 49045 Angers, France.
E-mail: pierre.frere@univ-angers.fr; Fax: +33 241735405; Tel: +33
241735063
Moreover, furaldehyde and 5-hydroxylmethyl-furfural (HMF)
used as starting materials are well known as renewable materials
obtained by dehydration of glucose or by transformation of
lignocellulosic biomass.48–50
† CCDC reference numbers 818758–818760. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/c1ce05575e
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P ¼ 4.4 bar, power ¼ 250 W) during 3 ꢁ 10 min led to target
molecules 2 or 3 in 45% and 38% yield respectively.
Electronic properties
The electronic properties of compounds 1–3 have been analyzed
by UV-vis spectroscopy and by cyclic voltammetry (CV). Optical
and electrochemical properties are gathered in Table 1. The
comparison of the UV-vis data of compounds 1 and 2 shows that
the replacement of phenyl terminations by perfluorophenyl units
leads to a slight blue shift of the absorption bands probably due
to higher torsion of the azomethine bonds. The lengthening of
the conjugated system in 3 leads to a bathochromic shift of the
absorption band corresponding to a decrease of the HOMO–
LUMO gap from 3.41 eV for 2 to 2.88 eV for 3.
Scheme 1 Structures of compounds 1–3.
In this contribution, we report on the electronic properties and
on the solid state assemblies in crystals obtained from 1–3 and we
show the key role of the furan–perfluorophenyl interactions on
the supramolecular organizations of these later.
The electrochemical properties of compounds 1–3 have been
analyzed both in oxidation and reduction. Compound 1 presents
two irreversible oxidation processes respectively at 1.26 and 1.54
V corresponding to the formation of radical cation 1+ and
dication 1++. The donating ability of compound 2 is strongly
decreased due to the presence of the lateral perfluorophenyl
groups. Thus this later only undergoes the non-reversible
formation of a radical cation at the more anodic potential of 1.49
V. By contrast, the insertion of the furylene–vinylene (FV) unit as
spacer in compound 3 enhances the electron donor character and
thus allows an easier access to the oxidized states 3+ and 3++ as
shown by the two oxidation peaks at 1.18 and 1.47 V.
Results and discussion
Synthesis
Compounds 1–3 have been synthesized by condensation of the
dialdehyde 4 or 5 with aniline or pentafluoroaniline (Scheme 2).
Dialdehyde 4 was synthesized by oxidation of HMF51 while 5
was prepared in two steps from furaldehyde.38 The easy protocol
used for obtaining 1, consisting of heating a mixture of 4 and
aniline without solvent,52 could not be used with penta-
fluoroaniline, firstly because of the low reactivity of its nitrogen
atom due to the electron withdrawing effect of the fluorine atoms
and secondly because pentafluoroaniline is too volatile to allow
long time of reaction or high temperature. The reactions of
condensation were efficiently carried out by using a sealed vessel
under microwave irradiation. Thus, a mixture of dialdehyde 4 or
5 and 2.2 equivalents of pentafluoroaniline in methylene chloride
in the presence of a small amount of P2O5 (T ¼ 80 ꢀC,
Table
compounds 1–3
1
Electronic absorptiona and cyclic voltammetryb data of
Compound
lmax/nm
DEopt/eV
Eox1/V
Eox2/V
Ered1/V
1
2
3
371
364
430
3.34
3.41
2.88
1.26
1.49
1.18
1.54
—
1.47
ꢂ1.42
ꢂ1.31
ꢂ1.59
a
10ꢂ5 M in CH2Cl2. b 10ꢂ3 M in 0.1 M Bu4NPF6/CH2Cl2, v ¼ 200 mV sꢂ1
,
reference AgCl/Ag.
Scheme 2 Synthesis of oligomers 1–3.
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Compounds 1–3 present a reduction peak corresponding to the
formation of radical anion. As expected, the replacement of the
phenyl by perfluorophenyl groups allows a rise in the reduction
potential from ꢂ1.42 V for 1 to ꢂ1.31 V for 2. The lengthening of
the spacer with the donor FV units for 3 enhances the revers-
ibility of the reduction process but at a more cathodic potential
of ꢂ1.59 V. These results show that the linkage of phenyl or
perfluorophenyl groups to furan units by azomethine bonds
leads to conjugated systems presenting both donor and acceptor
properties. As expected the acceptor character is enhanced by the
fluorine atoms, but their electron withdrawing effects are out-
weighed by the multiplication of the furan units.
The molecules stack along the c axis in a head to tail overlap of
the molecules which prevents p-interactions between the conju-
gated systems. The regular distances between the planes formed
ꢂ
by the furan rings are about 3.3 A. The molecular organization is
ꢂ
mainly defined by C–H---Ph (d ¼ 2.63 A shown by dotted line in
Fig. 1c)53 interactions between the phenyl rings which present an
edge to face arrangement as observed in crystalline structures of
benzene or naphthalene.54
The replacement of the phenyl terminations by per-
fluorophenyl groups leads both to a conformational change and
to a strong modification of the interactions that determine the
stacking of the molecules. Compound 2 crystallizes in the triclinic
P1 space group. The molecule is unsymmetrical with an azo-
methine bond in a d-cis conformation while the second adopts
a d-trans one (Fig. 2a). The d-trans conformation presents
a better planarity with a torsion angle of the perfluorophenyl of
11ꢀ whereas the torsion of the second perfluorinated cycle rea-
ches 39ꢀ. This torsional angle is very close to the value observed
in structure 1 for the two arms in the same d-cis conformation.
The view of the structure shows that the molecules stack along
the a axis. Between two adjacent columns, molecules are in
contact through C–H–---F contacts55 with dH---F distances
Crystal structures
Slow evaporation of chloroform–ethanol solutions of
compounds 1–3 gave single crystals suitable for X-ray diffraction
analysis. The details of the data collections are gathered in
Table 2.
Compound 1 crystallizes in the monoclinic space group C2/c.
Its structure is defined from a half molecule which lies on a two-
fold axis. As shown in Fig. 1a, the two azomethine bonds are
coplanar with the central furan ring and adopt an E configura-
tion and a d-cis conformation (the nitrogen atoms are oriented in
the direction pointing to the oxygen atom). The lateral phenyl
rings are not coplanar with the central conjugated system
assuming a torsional angle of 38ꢀ with respect to the furan ring.
ꢂ
ranging between 2.50(4) and 2.77(5) A as indicated in Fig. 2b by
red dotted lines. The stacking of the molecules presents an
overlapping of quasi planar conjugated systems involving the
d-trans arm and the furan ring with a slight shift, so that a per-
fluorophenyl moiety is placed opposite of the azomethine bond
of the d-trans arm (Fig. 2c). The average distance between the
ꢂ
conjugated systems is approximately of 3.4 A which corresponds
Table 2 Details of the data collection and structural refinement of 1–3
to classical distances observed for p–p interactions. The two
ꢂ
ꢂ
shortest distances d1 ¼ 3.382(4) A and d2 ¼ 3.385(6) A involve
the carbon atoms of the two azomethine links which are in
contact with a carbon of furan ring and a carbon of a fluorinated
aromatic ring.
Crystal
1
2
3
Formula
C18H14N2O C18H4F10N2O C24H8F10N2O2
274.31
293
Molecular weight
Temperature/K
Crystal system
Space group
454.23
293
546.32
293
At the other end of the molecule, the perfluorophenyl moieties
of the d-cis arms overlap with a shift leading to a superposition of
the aromatic ring with the azomethine bond. The distances
Monoclinic Triclinic
C2/c P1
31.930(8) 4.7787(2)
Monoclinic
P21/c
6.4323(9)
7.278(1)
22.578(4)
90
ꢂ
a/A
ꢂ
ꢂ
between the planes of the perfluorinated cycles are close to 3.3 A
b/A
6.041(2)
7.424(1)
90
6.5029(4)
13.9073(5)
78.371(3)
83.747(3)
87.099(4)
ꢂ
c/A
ꢂ
and the shortest interatomic distance d3 ¼ 3.286(5) A is observed
a/deg
b/deg
g/deg
between the nitrogen atom of an imine linkage and carbon atom
ꢂ
97.04(1)
90
93.06(1)
90
of an aromatic ring. A second short distance d4 ¼ 3.330(5) A
3
ꢂ
V/A
Z
1421.2(6) 420.60(3)
1055.5(3)
2
Orange
1.719
544
involves fluorine and carbon atoms of two superposed per-
fluorophenyl rings. Thus the crystalline arrangement seems to be
oriented by a subtle combination of p–p-interactions between
conjugated systems and by electrostatic interactions between the
electronegative fluorine and nitrogen atoms and the electropos-
itive centre of perfluorinated rings.
4
Colorless Yellow
1
Crystal colour
Dc/g cmꢂ3
F(000)
m/mmꢂ1
1.282
576
0.081
1.793
224
0.189
0.171
Transmission(min/max)
q (min/max)/deg
Data collected
Data unique
Data observed [I > 2s(I)]
R(int)
Number of parameters
R1 [I > 2s(I)]
wR2 [I > 2s(I)]
R1 [all data]
wR2 [all data]
Absolute structure
parameter
0.553/0.998 0.949/0.986
3.43/28.05 3.01/27.96
0.972/0.992
3.61/29.00
19 128
2630
1397
0.099
188
0.0554
0.1056
0.1308
0.1339
—
The multiplication of the number of furan rings by the inser-
tion of a FV unit as spacer between the two perfluorophenyl rings
for 3 strongly modifies the packing mode in the crystal.
Compound 3 crystallizes in the centrosymmetric monoclinic
space group P21/c. The structure is defined from a half molecule
and the molecule is located on a crystallographic inversion
center. As shown in Fig. 3a, the central ethylenic bond is in an
E configuration and the two azomethine bonds are in an
E configuration and adopt a d-cis conformation. The two azo-
methine links are co-planar with the FV unit while the two per-
fluorophenyl rings assume a torsional angle of 45ꢀ with the plane
defined by the central conjugated system. Here the torsional
7548
1644
971
13 816
3764
2735
0.077
124
0.084
297
0.0577
0.0943
0.1299
0.1146
—
0.0568
0.1470
0.0872
0.1655
0.1(2)
GOF
Largest peak in final:
1.108
0.151/
ꢂ0.211
1.049
1.061
0.401/ꢂ0.323 0.212/ꢂ0.275
ꢂ3
ꢂ
difference/eA
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Fig. 1 X-Ray structure of 1. (a) Molecular structure of 1 with anisotropic displacement ellipsoids drawn at the 50% probability level. (b) Crystal
packing of 1 viewed along the cell c axis. (c) Stacking mode of molecules along the c axis, C–H---Ph interactions are shown in dotted lines.
angles are slightly larger than those obtained in structure 1 (38ꢀ)
and 2 (39ꢀ) for the arm in the same d-cis conformation. The
structure shows ribbons of molecules oriented along the c axis
which stack along the b axis. The ribbons are linked themselves
by C–H–---F contacts with dH---F distances ranging between 2.38
conformations adopted in the structures (Fig. 4). For both
compounds, the more negative potential regions (in red) are
located on the nitrogen, oxygen atoms and on the fluorine atoms
which point to the imine bonds. By contrast the internal part of
the hexafluorobenzene and the external part of the central
conjugated systems present positive potentials (blue).
ꢂ
(3) and 2.73(2) A (Fig. 3b). The stacking along the b axis is done
so that the furan cycles of a central FV unit of a molecule overlap
two perfluorophenyl cycles of two other molecules. The distances
ꢂ
between the two planes are about 3.4 A with several inter-atomic
As shown in Fig. 4 (right) in the case of 3, the superposition of
the perfluorobenzene and furan units in the crystal allows the
optimization of the interactions between the positive and nega-
tive regions. Thus the furan–perfluorophenyl interactions are the
main driving forces responsible for the molecular stacking.
For crystal 2, the unsymmetrical conformation of the molecule
leads to a slightly different repartition of the charges in the two
perfluorinated cycles (Fig. 4 left). The fluorine atoms that are in
line with the d-trans azomethine bond show a less negative
potential than the ones of the second perfluorobenzene unit. The
slip-stacking of the two d-trans arms thus mainly implicates p–p
interactions strengthened by small electrostatic character.
In conclusion, we have synthesized new examples of conju-
gated systems associating furan and perfluorophenyl groups. The
comparison of the crystal structures of three compounds varying
by the lateral phenyl or perfluorophenyl rings or by the nature of
the conjugated spacer evidences the key role of the per-
fluorophenyl groups in the stacking mode of the molecules.
Although compound 1 built with phenyl groups does not present
any p-stacking, the replacement of phenyl by perfluorophenyl
ꢂ
distances ranging between 3.20 and 3.50 A as shown by dotted
lines in Fig. 3c. The centroid to centroid distance measured
ꢂ
between perflurophenyl and furan rings is of 3.748(2) A. This
typical face to face stacking, often observed in the structures of
mixed fluoroarene–thiophene oligomers or in benzene–per-
fluorobenzene systems, results from non-covalent interactions
between the electron-deficient perfluorophenyl rings and the
electron-rich furan cycles. By contrast with the structure of 2,
such a disposition for 3 does not allow any p–p contact. Thus the
lengthening of the conjugated system by insertion of a FV unit
strongly favors the multiplication of furan–perfluorobenzene
interactions to the detriment of the p–p interactions.
For a better understanding of the arrangements observed in
the X-ray structures of 2 and 3 and to unravel the role of the
fluorine atoms and azomethine bonds in the stacking of the
molecules, we investigated the electrostatic potential (ESP)
surface23 of the two compounds 2 and 3 by considering the
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Fig. 2 X-Ray structure of 2: (a) Molecular structure of 2 with anisotropic displacement ellipsoids drawn at the 50% probability level. (b) Crystal
packing of 2 showing the intermolecular C–-H---F contacts by red dotted lines. (c) Stacking mode in crystal 2, the shortest interatomic distances between
overlapping molecules are shown by blue dotted lines.
groups in 2 allows a p-stacking of the molecules due to subtle
combination of p–p interactions and donor–acceptor interac-
tions. The replacement of the furan by the FV unit as spacer in
the conjugated systems leads to the formation of strong furan–
perfluorophenyl interactions which can be considered as the
main driving forces of the packing mode of 3 to the detriment of
the p–p interactions.
2,5-Bis(pentafluorophenyliminomethyl)furan: 2. A 10 mL tube
equipped with a magnetic stirring bar was filled with 75 mg of
dialdehyde 4 (0.6 mmol), 320 mg of perfluoroaniline (1.7 mmol) in
2 mL of CH2Cl2 and 50 mg of P2O5. The tube was sealed with
a rubber cap and irradiated three times for 10 min at 80 ꢀCandunder
a pressure of 4.4 bar with a power reactor of 200 W. The mixture was
cooled to room temperature, poured on 20 mL of water, extracted
twice with CH2Cl2 (2 ꢁ 20 mL) and the organic phase was dried on
MgSO4. After evaporation of the solvent the residue was purified by
flash chromatography on silica gel in the presence of several drops of
triethylamine (petroleum ether (40–60 ꢀC)/CH2Cl2, 1/1) to give
125 mg (0.27 mmol) of compound 2 (45% yield).
Experimental
Syntheses
2,5-Bis(phenyliminomethyl)furan: 152. A slight excess of aniline
was added to a 25 mL Erlenmeyer flask containing 1 g of di-
aldehyde 4 (8 mmol) at rt. The temperature rapidly rose thus
allowing the liberation of water vapor. After 10 min, the brown
solid obtained was washed by adding 3 mL of methanol. The
solid was recovered by filtration, recrystallized from methanol to
give 2 g of compound 1 (91% yield).
Yellow solid, Mp ¼ 98 C.
ꢀ
1H NMR (CD3COCD3): 8.59 (s, 2H), 7.34 (s, 2H).
13C NMR (CD3COCD3): 156.6, 154.2, 120.5.
19F NMR (CD3COCD3): ꢂ154.5 (dd, 4F, J ¼ 21 Hz,
J ¼ 5 Hz), ꢂ162.4 (t, 2F, J ¼ 21 Hz), ꢂ165.6 (dt, 4F, J ¼ 21 Hz,
J ¼ 6 Hz).
MS MALDI-TOF: calcd. for C18H4ON2F10 454.02; found
454.94 (M + H).
ꢀ
Yellow pale solid, Mp ¼ 156 C.
1H NMR (CD3COCD3): 8.53 (s, 2H), 7.43 (t, 4H, 3J ¼ 4.8 Hz),
7.33 (d, 4H, 3J ¼ 4.2 Hz), 7.28–7.25 (m 4H).
13C NMR (CD3COCD3): 154.6, 151.5, 147.9, 129.3 (2C),
126.6, 121.2 (2C), 117.1.
Elemental analysis for C18H4ON2F10: calcd. C 47.60, H 0.89,
N 6.17; found C 47.28, H 1.01, N 5.99.
1,2-Bis(5-pentafluorophenyliminomethyl-furyl)-E-ethene: 3. A
10 mL tube equipped with a magnetic stirring bar was filled with
0.13 g of dialdehyde 5 (0.6 mmol), 320 mg of perfluoroaniline
MS MALDI-TOF: calcd. for C18H14ON2 274.1; found
274.6 (M).
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Fig. 3 X-Ray structure of 3: (a) Molecular structure of 3 with anisotropic displacement ellipsoids drawn at the 50% probability level. (b) Crystal
packing of 3 showing the intermolecular C–H---F contacts by red dotted lines. (c) Stacking mode in crystal 3, the shortest interatomic distances between
overlapping molecules are shown by blue dotted lines.
(1.7 mmol) in 2 mL of CH2Cl2 and 50 mg of P2O5. The tube was
sealed with a rubber cap and irradiated three times for 10 min at
the organic phase was dried on MgSO4. After evaporation of the
solvent the residue was purified by flash chromatography on
silica gel in the presence of several drops of triethylamine
(petroleum ether (40–60 ꢀC)/CH2Cl2, 1/1) to give 125 mg of
compound 3 (38% yield).
ꢀ
80 C and under a pressure of 4.4 bar with a power reactor of
200 W. The mixture was cooled to room temperature, poured on
20 mL of water, extracted twice with CH2Cl2 (2 ꢁ 20 mL) and
Fig. 4 Electrostatic potential surface of compounds 2 (left) and 3 (right), the values are in units of hartree. The overlapping of the molecules is rep-
resented in the inset.
5838 | CrystEngComm, 2011, 13, 5833–5840
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Orange solid, Mp ¼ 204 C.
1H NMR (CD3COCD3): 8.37 (s, 2H), 7.20 (s, 2H), 7.14 (d, 2H,
3J ¼ 3.5 Hz), 6.65 (d, 2H, J ¼ 3.5 Hz).
3
13C NMR (CD3COCD3): 156.7, 154.3, 150.9, 121.9, 117.5,
113.6.
19F NMR (CD3COCD3): ꢂ153.2 (dd, 4F, J ¼ 21 Hz, J ¼ 6
Hz), ꢂ160.3 (t, 2F, J ¼ 21 Hz), ꢂ163.4 (td, 4F, J ¼ 21 Hz,
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546.95 (M + H).
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N 5.13; found C 52.18, H 1.41, N 4.96.
Calculation of electrostatic potential surfaces
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Electrostatic potential surfaces of compounds 2 and 3 were
calculated with Gaussian 09 by using DFT method. From the
geometry adopted in the X-ray structures, single-point energy
calculations were performed by using the B3LYP/6-31g basis set.
Electrostatic potential surfaces were created by using Gausview
for a density value of 0.002 electrons per au3.
X-Ray structures
X-Ray single-crystal diffraction data for the three compounds
were collected at 293 K on a BRUKER KappaCCD diffrac-
tometer, equipped with a graphite monochromator utilizing
ꢂ
MoKa radiation (l ¼ 0.71073 A). The structures were solved by
direct methods using SIR92 (Altomare et al., 1993) and refined
on F2 by full matrix least-squares techniques using SHELXL97
(G.M. Sheldrick, 1998). All non-H atoms were refined aniso-
tropically and the H atoms were found by Fourier difference
synthesis. Absorption was corrected by the SADABS program
(Sheldrick, Bruker, 2000) for compound 1 and by Gaussian
technique for compounds 2 and 3.
36 The study of the stacking of the molecules in the structure drawn from
the data obtained in the Cambridge Database (CSD code WUSKEB)
shows an overlapping of the pentafluorophenyl and furan rings. The
ꢂ
centroid to centroid distance is 3.586(1) A, thus indicating the
existence of furan–perfluorophenyl interactions.
Acknowledgements
ꢀ
37 A. Benahmed-Gasmi, P. Frere and J. Roncali, J. Electroanal. Chem.,
C. Mallet thanks the Ministry of Education for a PhD
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