Electrochemical synthesis of thin films of polymers derived from [NiSalen] and [NiSalphen]

Article abstract of DOI:10.1023/B:RJAC.0000015726.55537.c1

Electrochemical synthesis of conducting polymeric Ni(II) complexes with N,N′-bis(salicylidene)-ethylenediamine and N,N′-bis(salicylidene)-o- phenylenediamine was studied. The kinetic of this process was examined. The possibility of preparing the polymeric films with a definite thickness ranging from 0.01 to 2 μm was assessed.

Full text of DOI:10.1023/B:RJAC.0000015726.55537.c1

Russian Journal of Applied Chemistry, Vol. 76, No. 10, 2003, pp. 1626 1630. Translated from Zhurnal Prikladnoi Khimii, Vol. 76, No. 10, 2003,
pp. 1669 1674.
Original Russian Text Copyright
2003 by Shagisultanova, Ardasheva.
MACROMOLECULAR CHEMISTRY
AND POLYMERIC MATERIALS
Electrochemical Synthesis of Thin Films of Polymers
Derived from [NiSalen] and [NiSalphen]
G. A. Shagisultanova and L. P. Ardasheva
Herzen State Educational University, St. Petersburg, Russia
Received April 8, 2003
Abstract Electrochemical synthesis of conducting polymeric Ni(II) complexes with N,N -bis(salicylidene)-
ethylenediamine and N,N -bis(salicylidene)-o-phenylenediamine was studied. The kinetic of this process was
examined. The possibility of preparing the polymeric films with a definite thickness ranging from 0.01 to
2
m was assessed.
It is known that electrochemical oxidative poly-
merization of Ni(II) complexes with N,N -bis(salicyl-
idene)ethylenediamine (Salen) and N,N -bis(salicyl-
idene)-o-phenylenediamine (Salphen) and their deriv-
atives in weakly or moderately electron-donor sol-
vents gives thin conducting films [1 8].
In this study, we analyzed in detail the electro-
chemical (potentiostatic conditions) synthesis of po-
lymers derived from [NiSalen] and [NiSalphen] and
determined the kinetic parameters of this process.
EXPERIMENTAL
Electrochemical synthesis of polymeric transition-
metal complexes with structurally different macro-
cyclic ligands, including those of the Salen type, has
been the subject of numerous recent studies [2, 9 15].
The resulting polymers with controllable thickness
have high redox conductivity; their reduced and (or)
partially oxidized forms exhibit photoelectric activity
[9 15].
The complexes [NiSalen] and [NiSalphen] were
synthesized by the procedure in [17]
Ni
Ni
.
.
.
.
.
.
.
.
.
.
.
.
HC N N CH
HC N N CH
H2C CH2
Thus, a wide variety of conducting polymeric
Cu(II), Pt(II), Co(II), Pd(II), and Ni(II) complexes in-
soluble in water and organic solvents have been grown
on solid supports and studied. The parameters of the
electrochemical synthesis and the kinetics of polymer
formation have been analysed. The charge diffusion
coefficient Dct in the polymer bulk, which character-
izes the rate of charge transfer in the polymer in a so-
lution of a supporting electrolyte, has been calculated
[2, 9 12]. The suggested mechanism of polymer for-
mation is based on existence of a single resonance
structure of the initial monomers and transformation
of this structure into another structure with redistribu-
tion of the electron density in the macrocycle [16].
[NiSalen]
[NiSalphen]
Electrochemical measurements were performed on
a PI-50-1 pulsed potentiostat in the potentiostatic and
potentiodynamic modes with linear potential sweep
1
at a rate of 0.01 to 0.5 V s , using a PR-8 program-
mer. We used an three-electrode electrochemical cell
with separated compartments. Platinum wire (99.99%
Pt) with surface area of 0.25 cm2, sealed in glass,
served as working electrode; a platinum gauze was
an auxiliary electrode. All potentials were measured
relative to a silver chloride electrode filled with
a saturated NaCl solution.
Although orangometallic polymers have been
studied extensively, optimization of their synthesis
and analysis of their structure, stability, and properties
remain a topical task.
The monomer concentration in the working solution
was 1 mM. The supporting electrolyte was 0.1 M so-
lution of tetraethylammonium perchlorate (Et4NClO4)
in acetonitrile (CH3CN) dried by the procedure de-
1070-4272/03/7610-1626 $25.00 2003 MAIK Nauka/Interperiodica

ELECTROCHEMICAL SYNTHESIS OF THIN FILMS OF POLYMERS
scribed in [18]. The solution was deaerated with high-
1627
purity argon.
The film thickness was calculated by the procedure
described in [19].
The cyclic voltammogram measured by single po-
tential sweep in an acetonitrile solution of [NiSalen]
is shown in Fig. 1. Comparison of the peak current
with those measured in propylene carbonate C3H6CO3
in a single potential sweep cycle [2, 9] confirmed that
the deposition rate of the polymer in CH3CN is higher
than that in C3H6CO3. Probably, diffusion of
the monomer to the near-electrode space in more
viscous C3H6CO3 is slower.
Fig. 1. Cyclic voltammogram of anodic polymerization
of [NiSaen] in 0.1 M Et NClO in CH CN (first cycle).
As a rule, the intensity of anodic and cathodic
peaks grows with increasing the number of potential
sweep cycles in solutions of nickel(II) complexes.
The cathodic peak current in acetonitrile solution of
[NiSalen] increases during five cycles of potential
4
4
3
3
1
c
= 1 10
M, V = 0.05 V s . (I) Current
complex
s
and (E) potential; the same for Fig. 2.
1
sweep from 0.0 to 1.6 V at a rate of 0.001 V s and
then decreases rapidly [7]. However, this phenomenon
cannot be interpreted unambiguously. Vilas-Boas et al.
[7] suggest that the decrease in the peak current in
the course of polymerization may be due both to
superoxidation caused by irreversible oxidation of
the ligand at high potentials and to lowering of
the complex concentration near the working electrode.
Stable multilayer poly[NiSalen] films with thick-
ness ranging from 0.01 to 2 m can be prepared
by potential sweep from 0.0 to +1.2 V at a rate of
1
0.05 V s .
As in our previous study [2], a dark yellow film
insoluble in water and organic solvents is deposited
on the electrode surface, which confirms the formation
of stable solid oxidation products of [NiSalen] in
the course of continuous potential sweep. A yellow-
brown conducting film of poly[NiSalphen] was pre-
pared similarly.
Fig. 2. Cyclic voltammograms illustrating solid-phase red-
ox processes in poly[NiSalen] films deposited in (1) 0.5,
(2) 1.0, (3) 2.0, (4) 5.0, (5) 7.0, (6) 10.0, (7) 17.0, and
(8) 43.0 min in the potentiostatic mode at E = 1.1 V.
a
1
CH CN, Et NClO , V = 0.01 V s ; the same for Figs. 3
and 4.
3
4
4
s
Polymeric [NiSalen] and [NiSalphen] were depos-
ited in the potentiostatic and potentiodynamic modes.
Potentiodynamic synthesis was performed by cyclic
sweeping of the potential of the working electrode in
a solution of the initial monomer in a certain range
for each complex: 0.0 1.2 V for [NiSalen] and 0.0
1.3 V for [NiSalphen]. The potentiostatic synthesis is
performed at a constant accumulation potential de-
termined experimentally. The accumulation potential,
at which the deposition rate of the electrochemically
active polymer is the highest was considered to be
the optimal. The potentionstatic accumulation mode
has an advantage over, the potentiodynamic procedure
in that it allows rapid deposition of thick films. In this
study, [NiSalen] and [NiSalphen] were polymerized
mainly in the potentiostatic mode at a potential of
1.1 0.05 V.
The electrodes coated with [NiSalen] and [NiSalphen]
were immersed in a solution of the supporting elec-
trolyte, and their potential was swept in the ranges
0.0 1.2 and 0.0 1.3 V, respectively. The cathodic and
anodic waves in the cyclic voltammograms character-
ize electron transfer in the bulk of the polymer. Cyclic
voltammograms recorded at the potential sweeping of
the electrode coated with poly [NiSalen] films of
1
varied thickness (sweep rate 0.01 V s ) in a solution
of pure supporting electrolyte are shown in Fig. 2.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 76 No. 10 2003

1628
SHAGISULTANOVA, ARDASHEVA
anodic peak at Ea 0.6 V in cyclic voltammograms
can be regarded as a quantitative characteristic of
the transition from thin to thick polymeric films,
which differ in structure and physicochemical prop-
erties.
There exist two forms of the polymeric substance:
yellow-brown oxidized, which turns dark brown with
increasing thickness of a polymeric film, and yellow
reduced. These forms determine the electrochromic
properties of a polymer. The oxidized form is de-
posited in the potentiostatic mode (0.5 60 min, E =
1.05 V) and can be rapidly converted to the reduced
form by potentiostatic reduction at E = 0.0 V in a so-
lution of pure supporting electrolyte. The reduction
time
is equal to the time of oxidative polymeriza-
red
tion. The oxidized form is spontaneously reduced in
a solution of the supporting electrolyte within 30
120 min (depending on the film thickness) after the
applied voltage is switched off. The thicker the film,
the slower its spontaneous reduction. It should be
noted that the cathodic potential never reaches 0.0 V
in the course of spontaneous reduction of the polymer.
Our experimental results show that the limiting, so-
called equilibrium potential of spontaneous reduction
is equal to the potential at which the anodic wave
in the cyclic voltammograms starts to grow, i.e., at
which the electrostimulated oxidation of the re-
duced form starts. Along with spontaneous reduction,
spontaneous oxidation of the polymer was observed.
The polymer reduced under potentiostatic conditions
at E = 0.0 V is oxidized in a pure supporting electro-
lyte after the applied voltage is switched off. The po-
tential of the electrode coated with the polymeric film
increases to a value equal to the potential recorded by
the end of the spontaneous reduction. Thus, the so-
called intermediate redox form is formed both by
spontaneous reduction and by spontaneous oxidation
of the polymer. The potential of this form depends on
the film thickens and the nature of a supporting elec-
trolyte.
Fig. 3. Thickness h of poly[NiSalen] films deposited
from solutions with monomer concentration of (1, 1 ) 0.5,
(2, 2 ) 0.75, (3, 3 ) 1.0, (4, 4 ) 1.5, and (5, 5 ) 2 mM vs.
the accumulation time
culated from the quantity of electricity spent for (1 5) ox-
idation and (1 5 ) reduction of the polymer.
. The film thickness was cal-
acc
Fig. 4. Thickness h of poly[NiSalen] films grown for
(1, 1 ) 3 and (2, 2 ) 7 min vs. the monomer concentration
in the solution c. The film thickness was calculated from
the quantity of electricity spent for (1, 2) oxidation and
(1 , 2 ) reduction of the polymer.
Our experimental results show that polymer thick-
ness h is one of the most important parameters de-
termining the rate of charge transfer in the polymer.
It can be seen that the anodic peak at Ea
0.6 V
appears and is shifted to positive potentials with an in-
creasing deposition time and, hence, the film thick-
ness. The shift of the first oxidation potential at Ea
To calculate the thickness h of a polymeric film,
we recorded cyclic voltammograms at slow potential
1
1
0.6 V (sweep rate 0.01 V s ) is the most pronounced
sweep (0.01 V s ). Under these conditions, the film
at the thickness of poly[NiSalen] film larger than
0.7- m. This peak disappears upon further growth of
the film. In this case, the cyclic voltammogram con-
tains a single wave at Ea 0.95 V (Fig. 2). We sug-
gest that the characteristics of redox processes in thick
polymer layers differ from those for thin films. Thus,
the shift and the subsequent disappearance of the first
is deposited by the adsorption mechanism ensuring
reduction or oxidation of all molecular fragments of
the polymeric chain. The dependence of h of poly-
[NiSalen] on the time of accumulation in the poten-
tiostatic mode at different concentrations of the com-
plex in solution is shown in Fig. 3. On the whole,
the polymer formation accelerates with increasing
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 76 No. 10 2003

ELECTROCHEMICAL SYNTHESIS OF THIN FILMS OF POLYMERS
monomer concentration in solution. In the initial de-
1629
position steps (up to 7 min), the dependence of h on
the monomer concentration at constant accumulation
time is linear (Fig. 4). Upon further electrode polar-
ization in a solution of the nickel complex (for more
than 15 20 min), polymerization decelerates and
the curve flattens out. When the films are deposited
from highly concentrated solutions, h passes through
a maximum and then decreases. At low monomer
concentration, the curve flattens out owing to a de-
crease in the monomer concentration near the elec-
trode. The influence of the concentration on the rate of
polymer growth is the most pronounced in going from
0.5 to 0.75 mM monomer solutions. At higher mono-
mer concentration (up to 2 mM), the dependence of
the thickness of polymeric film on the monomer con-
centration becomes less steep, being, on the whole,
nonlinear (Fig. 4).
Fig. 5. Thickness h of (1) poly[NiSalen] and (2) poly-
[NiSalphen] vs. the accumulation time . CH CN,
Et NClO , E = 1.1 V, continuous growth.
acc
3
4
4
a
It should be noted that the dependence of the thick-
ness of polymeric film grown from a solution with
monomer concentration of 2.0 mM on the accumula-
tion time passes through a maximum. Since the curves
measured in the first deposition steps are similar for
all monomer concentrations, we suggest that, in
20 min, the polymerization is decelerated and the curve
flattens out. However, it can be seen from Fig. 3 that
at the monomer concentration of 2 mM the curve has
ascending and descending portions, which reflects the
decrease in the electrical conductivity of the polymer.
At high monomer concentration in the working solu-
tion, the polymerization accelerates and a film with
a high content of structural defects grows [20], which
makes lower the redox conductivity of the polymer.
The dependences of the thickness of poly[NiSalen]
and poly[NiSalphen] films on the time of accumula-
tion on the electrode at the optimal potentials are
shown in Fig. 5 (curves 1, 2). It can be seen that the
poly[NiSalen] film is thicker than the poly[NiSalphen]
film grown under the same conditions. Probably, po-
lymerization of [NiSalphen] on the electrode sur-
face is sterically hindered owing to the presence of
the third phenyl ring in the ligand.
We examined the possibility of stepwise growth of
polymer films. An electrode coated with the polymer
for a definite time was immersed in a pure supporting
electrolyte and a cyclic voltammogram was recorded.
Then the electrode was placed in the monomer solu-
tion and the polymerization runs (up to 15 steps de-
pendening on the experimental conditions) were con-
tinued. We found that a polymeric film can be grown
stepwise. The thickness of the thus obtained poly-
meric film was close to that of the film grown in
a single step during the same time.
We calculated the polymer thickness from the area
under the cathodic (hc) and anodic (ha) portions
of the cyclic voltammograms (Fig. 3). As seen from
Fig. 3, the thicknesses of poly[NiSalen] films cal-
culated from the areas under the cathodic and anodic
portions become different when the thickness reaches
0.7 m. The thicknesses of these films, calculated
from the anodic portion are higher than those cal-
culated from the cathodic portion. Thus, the calculated
thickness of the oxidized conducting polymeric film
grown at any monomer concentration is slightly larger
than that of the reduced film. The higher the monomer
concentration in the working solution, the greater
the difference between the currents of anodic and
cathodic portions of the cyclic voltammograms and
hence, the greater is the difference between ha and hc.
The decreased activity of the polymer films in reduc-
tion is due to the presence of residual oxidized frag-
ments in the polymer, which are electron traps or have
CONCLUSIONS
(1) Electrochemical synthesis of polymeric nick-
el(II) complexes in the potnetiostatic mode was
studied. The best accumulation potential providing
fastest growth of the electroactive layer is 1.1 0.5 V.
(2) The redox conductivity of the polymer may de-
crease with increasing monomer concentration in
the working solution.
(3) The polymeric film can be grown by both con-
tinuous and stepwise deposition, which is of practical
importance.
a
frozen charge [21, 22].
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1630
SHAGISULTANOVA, ARDASHEVA
J. Photochem. Photobiol. A. Chem., 1997, vol. 103,
ACKNOWLEDGMENTS
no. 3, pp. 249 255.
This work was supported financially by the Rus-
sian Foundation for Basic Research (project no. 01-
03-33155), Ministry of Education of Russian Federa-
tion (grant E00.-5.0-210), and Committee for Science
and Higher School of St. Petersburg (grant of St. Pe-
tersburg in the field of science and engineering for
the year 2002, contract no. 207).
11. Orlova, I.A., Synthesis and Properies of New Photo-
and Electroactive Polymeric Transition Metal Com-
plexes with Schiff Bases, Cand. Sci. Dissertation,
Moscow, 1997.
12. Shagisultanova, G.A., Orlova, I.A., Ardasheva, L.P.,
and Popova, E.O., J. Macromol. Phys., 1998, vol. 136,
pp. 91 95.
13. Ardasheva, L.P., Luminescence Spectral and Electro-
chemical Properties of Platinum(II) and Palladium(II)
Complexes with Schiff Bases, Cand. Sci. Dissertation,
Moscow, 1999.
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