Mechanistic study of the formation of multiblock π-conjugated metallopolymer

Article abstract of DOI:10.1016/j.poly.2016.06.021

Electropolymerization of a π-conjugated polymer from [N,N′-bis(salicylidene)-o-phenylenediamine]nickel(II) complex ([Ni(salophen)]) on the surface of indium-tin-oxide (ITO) was studied. Two structures of the polymer have been proposed based on the theorem

Full text of DOI:10.1016/j.poly.2016.06.021

Accepted Manuscript
Mechanistic Study of the Formation of Multiblock π-Conjugated Metallopoly-
mer
Camila R. Peverari, Diego N. David-Parra, Madalina M. Barsan, Marcos F.S.
Teixeira
PII:
S0277-5387(16)30255-8
DOI:
http://dx.doi.org/10.1016/j.poly.2016.06.021
POLY 12062
Reference:
To appear in:
Polyhedron
Received Date:
Accepted Date:
11 April 2016
13 June 2016
Please cite this article as: C.R. Peverari, D.N. David-Parra, M.M. Barsan, M.F.S. Teixeira, Mechanistic Study of
the Formation of Multiblock π-Conjugated Metallopolymer, Polyhedron (2016), doi: http://dx.doi.org/10.1016/
j.poly.2016.06.021
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Mechanistic Study of the Formation of Multiblock π-Conjugated
Metallopolymer
1
Camila R. Peverari , Diego N. David-Parra , Madalina M. Barsan and Marcos
2
3
1
F. S. Teixeira *
1
- Department of Chemistry and Biochemistry, Faculty of Science and Technology, São
Paulo State University (UNESP), Rua Roberto Simonsen 305, CEP 19060-900
Presidente Prudente, SP, Brazil.
2
3
- São Carlos Institute of Chemistry, University of São Paulo (USP), São Carlos – SP –
Brasil.
- Department of Chemistry, Faculty of Science and Technology, University of Coimbra,
3
004-535 Coimbra, Portugal.
*Corresponding author. Tel.: +55 18 3229 53 55; Fax: + 55 18 3221 56 82
E-mail address: funcao@fct.unesp.br (Teixeira, M. F. S.)
orcid.org/0000-0001-9355-2143
1

ABSTRACT
Electropolymerization of
a
π-conjugated polymer from [N,N'-bis(salicylidene)-o-
phenylenediamine]nickel(II) complex ([Ni(salophen)]) on the surface of indium-tin-oxide
ITO) was studied. Two structures of the polymer have been proposed based on the theorem
(
of molecular columns. The highlighted structures are the molecular column of the central
aromatic rings (MCCAR) and molecular column of the side aromatic rings (MCSAR). The
molecular structures were obtained by control of the potential scan rate in the
electropolymerization step. The MCCAR structures are stable and structurally compact,
whereas the MCSAR structures are less compact .Electrochemical measurements, atomic
force microscopy (AFM), UV-Vis spectroscopy, FTIR Raman spectroscopy and optical
microscopy support the formation of the multiblock π-conjugated polymer.
Keywords: electropolymerization; metallopolymer; π-conjugated polymer; molecular
columns; molecular conductivity.
2

1
. Introduction
Schiff bases are widely employed in the development of new materials for
nanotechnology. These molecules form typically stable ligands due the pi bonds of imine
units attached to aryl groups [1].
Goldsby and collaborators were among the first to report the formation of the
poly(Schiff base) on a platinum surface [2, 3]. Goldsby [4] and Audebert [5-7] described
reasonably the electropolymerization mechanism for the transition Schiff base complex as a
radical-radical coupling reaction. In this model, the polymers are considered as polaronic
conductors without the participation of the central metals in the charge-transfer process and
without variation of the oxidation state of the metal. However, Vilas-Boas et al. [8, 9]
described a theorem of the electropolymerization process occurring by interactions of the
electron-rich π system with the metal center of the adjacent complex, functioning as bridges
between the molecules. This model fits the data from previously reported research [10-12],
due to the "donor-acceptor" interactions between the π-conjugated unit and the transition
metal ion in the adjacent monomer. The conductivity of these polymers is related to the
electronic transfer (ET) from a metallic cation to the aromatic π-conjugated systems (inner-
sphere ET through sequential hopping) [13, 14]. In this way, the electropolymerization of the
[
N,N'-bis(salicylidene)-o-phenylenediamine]nickel(II) complex results in positively charged
structures that form molecular columns, the positive charges being stabilized by negative ions
inserted from the supporting electrolyte (counter ions) in the cavities of the polymers, which
results in the formation of highly ordered and regular structures [8]. However, since each
metal complex is formed by three aromatic moieties located on each side of the molecule
center and one central (Fig. 1), mainly two different metallopolymer structures may occur on
the conductor surface, both classified as molecular columns. This work was aimed to
investigate the electrochemical processes of the conjugated metallopolymer structures based
on [N,N'-bis(salicylidene)-o-phenylenediamine]nickel(II) on an ITO surface. The studies
3

comprise cyclic voltammetry (CV), electronic conductivity, atomic force microscopy (AFM),
UV-Vis, Raman spectroscopy and optical microscopy.
N
O
N
O
Ni
Fig. 1. Molecular structure of [N,N'-bis(salicylidene)-o-phenylenediamine]nickel(II) in
highlighted aromatic moieties.
2
. Experimental
2
.1 Synthesis of salophen ligand and [Ni(salophen)] complex
The ligand N,N'-bis(salicylidene)-o-phenylenediamine (salophen) [15, 16] was obtained
by reaction of salicylaldehyde (18 mmol) and 1,2-phenylenediamine (9 mmol) in absolute
ethanol and kept under reflux for 15 hours at 40 ºC to produce a yellow precipitate. It was
filtered, washed and recrystallized with absolute ethanol and finally dried in desiccator. The
metallic complex [Ni(salophen)] was prepared by addition of stoichiometric and equimolar
amounts of the Schiff base ligand (2.86 mmol) and nickel acetate in absolute ethanol. The
solution was kept under reflux for 3 h at 50 ºC with constant stirring to yield a red precipitate.
The precipitate was filtered in a Gooch crucible, washed with absolute ethanol and kept in a
-1
desiccator. The yield of the complex was about 85 %. FTIR (ν cm ) in KBr: 3339 –OHph
620 C=N; 1600 C=C, 1362 >C–Oph, 1023 breathing ring, 765 Ni–O. UV-vis (nm) in
CH
CN: 310 (π→π*), 330 (π→π*), 375 (n→π*), 473 (MLCT).
;
1
3
2
.2 Metallopolymer synthesis
Synthesis of poly[Ni(salophen)] on a conductive substrate was performed by cyclic
voltammetry. Measurements were carried out in a conventional electrochemical cell, with a
saturated calomel electrode (SCE) as reference electrode, platinum electrode as counter
2
electrode and indium tin oxide (ITO) coated glass (area = 1.0 cm ) as working electrode,
4

connected to a potentiostat/galvanostat µAutolab type III. Films of different thickness were
prepared by use of different potential cycles and scan rates. The ITO/glass conductors were
cleaned by sonication for 30 min., first in acetone then in isopropyl alcohol to remove any
residual organic contamination.
The metallopolymer was electrodeposited by continuous oxidative potential scans on
conductor substrate (working electrode) between 0.1 and 1.5 V vs. SCE, immersed in a
-1
-1
benzonitrile solution containing 5.0 mmol L of [Ni(salophen)] monomer and 0.1 mol L
tetrabutylammonium perchlorate (supporting electrolyte) under a nitrogen atmosphere.
CAUTION: Due to the high toxicity of benzonitrile, measurements were performed in a
fume hood)
(
2
.3 Characterization of the metallopolymer
Electrochemical characterization of the metallopolymer on the ITO was realized by
-1
cyclic voltammetry in aqueous solution of KCl 0.5 mol L . The electrochemical study was
carried out in aqueous solution to minimize the effect of ohmic resistance of the electrolytic
medium. Conductivity measurements were performed by linear sweep voltammetry at 25 mV
-
1
s . Current–potential profiles were generated by measuring the current as the voltage was
incremented from 0.35 to 0.85 V. Conductivity profiles were constructed by the difference
between faradaic current density and capacitive current density, and converted to electronic
conductivities by applying Ohm’s law:
ꢁ
(ꢂꢃꢄꢅꢆ)×ꢇ(ꢃꢄ)
(1)
ꢀ
=
∆
ꢈ(ꢉ)
where ∆ꢊ is applied potential, j is current density and d is layer thickness of the polymer.
UV-Vis spectroscopy measurements were carried out with a Perkin Elmer Lambda 25
spectrophotometer to determine the characteristic absorption band and to monitor the growth
of the metallopolymer deposited onto the ITO substrate. Solid-state absorption spectra of all
samples were obtained in a dry and neutral state (without potential application). Raman
5

spectroscopy measurements were obtained with a Renishaw Raman spectrometer with laser at
an excitation wavelehgth of 663 nm and a diffraction grating of 1800 lines per millimeter. The
exposure time was set at 10 s with three accumulations.
Morphologic features of the metallopolymer were analyzed by atomic force and optical
microscopy. Atomic force microscopy images of films were captured with a Nanosurf
(
Boston MA) EasyScan 2 AFM. The scan head imaged at an angle 45 degrees relative to the
axis of stretching. Collected images were analyzed with Nanosurf AFM software (v. 3.0.2.4)
to determine roughness and thickness. Optical microscopy was accomplished with an optical
microscope (Leica - DMLM series) coupled to a spectrograph, whose objective had a 50x
2
increase in providing a spatial resolution of around 1.0 µm , Peltier CCD detector (cooled to -
7
0ºC) and motorized XYZ platform (stepper motor – 0.1 mm), where the samples were
positioned.
3
. Results and Discussion
3
.1 Electropolymerization and electrochemical properties of the metallopolymer
In the electropolymerization step, different solvents were tested to find a suitable
medium for the film fabrication, i.e., carbon tetrachloride, hexane, chloroform,
dimethylformamide, acetone, dichloromethane, acetonitrile and benzonitrile. It was observed
that solvents with high dipole moments, such as acetonitrile (µ = 3.53 D) and benzonitrile (µ
=
4.01 D) [17] lead to better polymer formation, explained as follows: since the
electropolymerization reaction occurs due to formation of a short half-life cation radical,
solvents with high dielectric constant and low viscosity stabilize better the radical. Due to the
increasing concentration of cation radical on the ITO conductor substrate and consequently
increased polarity of the oligomeric molecules, benzonitrile was the best solvent for obtaining
the metallopolymer. The influence of supporting electrolyte tetrabutylammonium perchlorate
(
TBAP)
and
tetrabutylammonium
hexafluorophosphate
(TBAPF
6
)
in
the
electropolymerization were also investigated. Timonov et al. [18-20] have described the
6

influence of the counter-anion size of the supporting electrolyte during electropolymerization
on the voltammetric characteristics. The distribution of the molecular column over the
conductor surface may be controlled by use of charge-compensating ions from the supporting
electrolyte of different ionic radii. During the formation of the metallopolymer, the molecular
column acquires a positive charge during scanning oxidation, which is compensated by anions
from the supporting electrolyte. Between the two tested supporting electrolytes,
tetrabutylammonium perchlorate (TBAP) was more suitable for the metallopolymer synthesis.
6
In the TBAPF supporting electrolyte, the formation of the polymeric film was not observed
due to the small size of the hexafluorophosphate anion (0.198 nm) [21], hindering the
formation of the molecular columns which are too close, causing greater repulsion during
electropolymerization. The perchlorate anion (0.241 nm) [21] fits perfectly, providing an ideal
spacing in between the molecular columns, and improves the conductivity of the polymer
providing a more efficient electropolymerization.
(
A)
35th
(B)
1
0
0
.5
.8
.0
1st
1.0
0
.5
.0
0
-0.8
-
0.5
1.0
-1.5
-
0.5
1.0
1.5
0.5
1.0
1.5
Potential (V) versus SCE
Potential (V) versus SCE
Fig. 2. (A) Cyclic voltammogram for electropolymerization of [Ni(salophen)] in
-1
-1
benzonitrile/0.1 mol L TBAP at 200 mV s , showing 35 cycles, where the first one is red.
(
B) Three voltammogram cycles for electropolymerization of [Ni(salophen)] in
-
1
-1
st
nd
benzonitrile/0.1 mol L TBAP at 200 mV s . Black line = 1 cycle; Red line = 2 cycle; and
rd
Green line = 3 cycle.
Polymer film formation on the ITO surface was carried out by successive potential
-1
cycling at 200 mVs in benzonitrile medium. Fig. 2A shows that successive scans produce an
increase of both anodic and cathodic peak current. For subsequent studies it was established
7

that eight potential cycles are sufficient to obtain a metallopolymeric structure with good
ionic conductivity. Fig.2B shows one redox couple that occurs in the electropolymerization
step and can be assigned to the Ni(II)/Ni(III) process [23,32-35]. The anodic wave centered at
approximately 0.72 V vs. SCE, associated with the Ni(II)/Ni(III) couple (Eq. 1 and 2) in
organic solvent, is consistent with the behavior reported by Teixeira et al. [11, 12] for a model
"
molecular column" and where the metallopolymer containing nickel complexes are bound to
all available coordination sites on the polymer backbone.
poly[Ni(II)salophen](ꢋꢌꢍ) →ꢎꢎpoly[Ni(III)salophen]ꢏ
+ꢎeꢐ
(2)
(3)
(ꢋꢌꢍ)
poly[Ni(III)salophen]ꢏ
+ꢎe ꢎ→ poly[Ni(II)salophen](ꢋꢌꢍ)ꢎ
ꢋꢌꢍ)
ꢐ
(
Polymeric film formation on a conductive surface occurs during oxidation of the central
metal cation. Transition metals have the ability to share electron density with π-systems
through d-orbitals. On the basis of this fact, the formation mechanism of the polymeric film
on conductor surface can be ascribed to the interaction between d-orbitals of the metal centers
with π-orbitals of aromatic rings of adjacent monomers, similar to the sandwich structure
formation [22], which is best achieved when monomers of square-planar spatial structure are
used. It was reported in our previous studies [10, 11, 23] and by others [18, 19, 24], that the
structure of a salen ligand containing metallopolymer is formed between the face of an
electron-rich π system and an adjacent central cation of another complex. The existence of
three aromatic ring moieties in the salophen ligand structure (see Fig. 1) enables the formation
of mainly two different molecular structures on the conductor surface through interactions
between the metal cation and π system that can be classified as molecular columns structure.
This was evidenced by the cyclic voltammetric profile of the metallopolymer onto the ITO
surface in aqueous solution that exhibited two well-separated redox couples with midpoint
potentials at +0.31 V and +0.59 V versus SCE that are assigned to the redox process of
Ni(II)/Ni(III) (Supplementary material – MS1). The potential gap between the redox pairs
arises from changes in the electron density of the polymer backbone, which suggests the
8

existence of two different polymer structures with distinct electronic communication between
the nickel centers. Malev and collaborators [24] studied the possible causes responsible for
the fact that cyclic voltammograms of polymer nickel-Schiff base possess some shoulders or
are split into two peaks, although the total electrode process has a one-electron character. The
research conducted a quantitative analysis of electrochemical response and indicated a
mechanism based on the mobility of charge (polaron/bipolaron conductance and
disproportionation reaction of polarons). The coverage of the metallopolymer on the ITO
surface was confirmed by the linear dependence of the anodic and cathodic peak currents with
-
1
the potential scan rates (10 to 200 mV s ). This showed that the redox processes were
controlled by an adsorption mechanism on the conductor surface and considered as a
Nernstian reaction. In adsorption processes, the concentrations of the electroactive species (Γ)
on the surface can be calculated by a method reported by Sharp et al [25], based on which the
-2
surface coverages of the metallopolymer were about 0.7 and 0.2 nmol cm for redox couples
at 0.31 and 0.59 V vs. SCE, respectively.
To understand the formation of the two types of molecular columns, the influence of the
-
1
electropolymerization scan rate (5 - 200 mV s ) on the concentration of electroactive species
onto the surface was studied, and it was observed that an increase of the scan rate led to a
decrease in electroactive species concentration on the ITO surface (Supplementary material –
MS2). For a high scan rate, the polymer backbone is less stable, since there is not sufficient
time to generate enough cation radicals on the conductor interface; consequently, the
formation of metallopolymer structures is hindered. Lower scan rates were more efficient for
polymer coating. Surprisingly, the cyclic voltammogram in an aqueous solution of the
-1
metallopolymer formed at 5 mV s (Supplementary material – MS3), exhibited only a redox
couple at +0.45 V versus SCE, evidencing that at a low electropolymerization scan rate there
is a predominance of one of the molecular columns configurations, which enables electronic
9

communication between the nickel centers in the conjugated backbone. The surface coverage
-1
-2
of the metallopolymer formed at 5 mV s was 35.2 nmol cm .
The study of electronic conductivity can provide significant information about the
charge-transport mechanism of poly[Ni(salophen)] produced by using the two
-1
electropolymerization scan rates studied (5 and 200 mV s ). According to Abruña [26, 27],
the measurements of charge transport at chemically modified electrodes are related to
physical movement of species, electron self-exchange, morphology of the deposited layer, and
counterion transport. The conductivity profiles of poly[Ni(salophen)] shown in Fig.3 are also
indicative of redox behavior. The currents have been converted to electronic conductivities by
applying Ohm’s law and adjusted for the dimensions of the metallopolymer film. The
-1
metallopolymer produced at 200 mV s presented two peaks in the conductivity profile
corresponding to the two redox pairs observed in the cyclic voltammetry (Fig. 3A). The
difference in magnitude of the two conductivity peaks is consistent with changes in
coordination of the metallopolymer complexes and indicates a faster electron transport at 0.52
V, indicating molecular structures with good electronic communication between cation
centers. The smaller peak at 0.81 V shows that electronic conductivity via the redox sites on
the polymer backbone is not the preferential electron-transfer pathway. These observations
can be explained by the formation of the two different molecular column structures by
interactions between the d-orbitals of nickel centers with π-orbitals of the aromatic rings of
salophen adjacent, i.e., between the d-orbitals of the nickel with the π-orbital of the central
aromatic rings of the adjacent salophen, forming molecular columns of central aromatic rings
MCCAR (Scheme 1a) and d-orbitals of the nickel with π-orbital of phenolic rings of the
salophen adjacent, yielding molecular columns of side aromatic rings MCSAR (Scheme 1b).
Probably, the MCCAR structure are more compact than the MCSAR ones, since the π
electronic system is substantially compartmentalized in the molecular column, enabling better
electronic communication between redox centers.
10

(A)
(B)
120
8
0
0
0
4
0.4
0.6
0.8
0.4
0.6
0.8
Potential (V)
Potential (V)
Fig. 3. Conductivity measurement in the potential range from 0.35 V to 0.85 V. The film
-1
-
1
generated by electropolymerization at (A) 200 mV s and (B) 5 mV s . Films formed by 35
cycles. T = 298 K.
Scheme 1. Schematic representation of the spatial structures of (a) MCCAR (molecular
columns of central aromatic rings) and (b) MCSAR (molecular columns of side aromatic
rings). Red = nickel; Yellow = oxygen; Green = nitrogen; Cyan = aromatic ring.
-1
On the other hand, the conductivity profile of the metallopoymer formed at 5 mV s
-
7
-2
(
Fig. 3B) presented only one conductivity peak (σmax = 1.2 × 10 S cm ) at 0.65 V, indicating
-1
a higher relative conductivity in comparison with the film obtained at 200mV s . By
-1
conductivity investigations, we conclude that an electropolymerization scan rate of 5 mV s
produces a polymer film with a single spatial structure that enables rapid electron transfer
between localized redox states in the polymer backbone. Probably, the predominant structure
11

for this type of polymer is MCCAR as we have previously observed in the study of
voltammetry.
3
.2. UV–Visible absorption spectroscopy
The aim of the UV-Vis spectroscopy study was to investigate further the effect of scan
rate on the structural differences of the synthesized metallopolymer. The intense color
nd
obtained on the conductor surface with successive cycling (from the 2 to the 8 cycle) was
th
the first evidence of the film formation (Supplementary material – SM4). Substrates coated
-1
-1
with films at 200 mV s remained optically transparent, whereas the ones at 5 mV s turned
dark brown and more opaque. The absorption spectra measurements at ITO with two different
coatings were carried out at room temperature for visible wavelengths ranging from 330 to
7
60 nm in the neutral state of the metallopolymer and are shown in Fig.4.
Solid-state absorption spectra of all the samples obtained at 200 mV s gave essentially
-1
the same profile (Fig. 4 A) with an absorption maximum at 375 nm corresponding to the π–π*
transition of the metallopolymer due to aromatic rings, and the band gap was calculated to be
2
.7 eV from this transition. The most significant absorptions are a metal-to-ligand charge-
transfer band MLCT at 491 nm. Absorbance values do not increase linearly with the number
of electrodeposition cycles as expected; a more detailed analysis of the polymeric film
thickness investigated by atomic force microscopy is presented in Section 3.4. Moreover, no
displacement of peak bands with an increase in the number of ligand and metal centers during
the electropolymerization was observed [28]. This indicates the existence of film structures
with low interaction between polymer layers with a minimized overlapping of the orbitals
(
Supplementary material – SM5). Hence, the electronic transfer is not significant through the
polymer film and consequently the shift of wavelength is negligible for films with different
thicknesses.
12

nd
th
th
th
nd
th
th
th
(A)
2
4
6
8
cycle (B)
2
4
6
8
cycle
cycle
cycle
cycle
cycle
cycle
cycle
4
00
500
600
700
400
500
600
700
Wavelength (nm)
Wavelength (nm)
Fig. 4. UV-Vis spectra of an ITO substrate covered with the polymer film generated by
-
electropolymerization at (A) 200 mV s and (B) 5 mV s in the neutral state with different
1
-1
electrodeposition cycles.
-1
The substrate covered with the metallopolymer obtained at 5 mV s presented a
-
1
different spectrum than that obtained at 200 mV s , with two bands at 388 and 480 nm, plus a
new absorption band at 593 nm (Fig 4B). From the onset of the absorption maximum (λonset),
the optical band gaps were estimated to be of 2.15, 1.98 and 1.86 eV, respectively. The band
gaps increased for lower energies, this behavior reflects the electronic changes in the HOMO
and LUMO levels in backbone polymer.
The MLCT band showed simultaneously a bathochromic shift and broadening, with the
increase in polymerization cycle number, associated with a change in the polarity of the
environment of this MLCT charge-transfer band [29-32], and to the the extension of the π
conjugation bonds. The interaction between d-orbitals of nickel centers with π-orbitals of the
aromatic ring of salophen is initially compartmentalized in the solid state (388 nm), which
then, with increasing polymerization cycles, gradually merges with the other at 480 nm,
suggesting that π electrons are delocalized along the polymer backbone. This delocalization
implies stronger interactions between the central aromatic rings of the salophen adjacent to
the coordination site of nickel. Broadening of this MLCT is associated with the
electrogeneration of different oligomers with similar electronic properties, whereas the new
13

band (between 590 and 610 nm) is considered to originate from d-d transition [33, 34],
8
indicating the existence of axial coordination with the central metal cation (d square-planar
II
Ni ) and consequently the formation of three-dimensional networks.
3
.3. FTIR Resonance Raman spectra
Raman spectroscopy has proved to be important in the analysis of metal sandwich
species, in the elucidation of metal cation–π system interactions [35, 36]. The connection
between the electronic structure deduced from UV-Vis spectra and the geometrical structure
supported by the vibrational data might be established from of the Raman spectra. Fig.5
-1
presents Raman spectra obtained on the ITO with films obtained at 200 and 5 mV s
polymerization scan rates (Figs. 5B and 5C, respectively); for comparison the spectrum of the
non-polymerized material is presented in Fig. 5A.
(A)
(B)
(C)
500
1000
1500
2000
500
1000
1500
2000
500
1000
1500
2000
-
1
-1
Raman shift (cm )
-1
Raman shift (cm )
Raman shift (cm )
Fig. 5. Resonance Raman spectra of (A) non-polymerized material, (B) metallopolymer
-
1
-1
formed at 200 mV s and (C) 5 mV s . λex = 663 nm.
Analyzing the rotational and vibrational structures in the spectra, one can observe that
the common bands associated with the stretching of C-O and C=C bonds in the 1600–1100
-1
cm region are very little affected. The major difference is the appearance of peaks in the
-
00-300 cm region that can be attributed to the symmetrical metal-ring stretching in the
1
8
polymer. The values are compatible with the ones obtained for metal sandwich species [37-
14

4
0], providing evidence for the formation of the nickel–π system in the polymer. The fact that
-1
the spectrum of the metallopolymer obtained at 5 mV s has sharper peaks in this region
indicates the predominance of a nickel–π system in the polymer.
Another important aspect observed in the spectra of the metallopolymer is the strong
suppression of the fluorescence background in comparison with the spectrum of the non-
polymerized material. It is known that Schiff base complexes are fluorogenic materials and
that fluorescence emission occurs with laser activation. This suppression was more evident
-
1
for polymer obtained at 5 mV s due to the d-d transition of the metal as observed in the UV-
Vis absorption spectrum (Section 3.1). The emitted energy, due to resonance between the
emission bands of the complex induced by the laser, is reabsorbed by nickel to promote the
electronic transition in the d-d absorption band region, and consequently the decrease in
fluorescence background occurs.
3
.4. Surface morphologic characterization
Nanoscale morphology of the ITO covered with the metallopolymer was investigated by
means of atomic force microscopy (AFM); the images are shown in Fig. 6, with the roughness
and thickness values in Table 1. Images and thickness values were obtained only for the
-1
-1
metallopolymer obtained at 200 mV s , since the polymer film obtained at 5 mV s showed
roughness superior to 1 µm, preventing the AFM measurement. Fig. 6A shows that the
surface of the film is not smooth and contains cluster structures with diameters between 150 and
5
000 nm. These structures definitely suggest an amorphous morphology film, in agreement
with results obtained in the conductivity and UV-Vis studies (Sections 3.1 and 3.2).
15

A
B
C
D
rd
Fig. 6. AFM images of metallopolymer growth analyzed in (A) 2 (B) 4 (C) 6 and (D) 8
th
th
th
electrodeposition cycle on an ITO surface.
This thickness and roughness change with the increase in the electropolymerization
cycle was investigated. The flatter areas with smaller roughness were obtained for films after
2
and 6 electrodeposition cycles; the film formed with eight cycles had a more granular,
thicker and rougher morphology compared with films obtained during fewer polymerization
cycles.
Table 1: Values of thickness and roughness for the polymeric films formed at 200
-1
mV s in different electrodeposition cycles.
nd
th
4
th
th
Cycles
2
6
8
Thickness (nm)
Roughness (nm)
106
71
130
91
79.45
68
125.8
117
It is expected that the thickness of the polymeric film would increase linearly with
number of electropolymerization cycles. However, there is evidence in the UV-Vis spectra of
metallopolymer formation discontinuity on the conductor surface. A plausible explanation for
16

such up-and-down changes of the thickness of polymer can be related to the instability of the
stratification of molecular columns and consequently a "breakdown" structural.
Optical microscopy images shown in Fig. 7 reveal different textures of the two
-1
-1
polymers on the surface of ITO obtained at 200 mV s (Fig. 7A) and at 5 mV s (Fig. 7B).
The first image shows aggregates ranging from 1 to 2 µm in width, with a very irregular and
nonhomogeneous polymer surface, whereas the second image was more uniform and
homogeneous with clear compact globular structures of the polymer.
Fig. 7. Optical microscope images of the polymer surface deposited on ITO substrate
-1
-1
obtained at (A) 200 mV s and (B) 5 mV s for 8 cycles.
. Conclusions
In this work, we have demonstrated the formation of two structures based on the
4
theorem of molecular columns. Highlighted structures consist of molecular columns of central
aromatic rings (MCCAR) and molecular columns of the side aromatic rings (MCSAR).
Molecular structures were obtained by control of the potential scan rate in the
17

electropolymerization step. Electrochemical measurements, atomic force microscopy (AFM),
UV-Vis spectroscopy, FTIR Raman spectroscopy techniques and optical microscope images
support the formation of the multiblock π-conjugated polymer. When structured polymer
-
1
surfaces are required, the metallopolymer formed at 5 mV s is a good candidate due to its
compact structure and enhanced electrical conductivity. Taken together, the metallopolymer
films represent a dynamic and intriguing approach in their formation, and a better
understanding of the fundamental parameters of these assemblies will aid in their future
applications.
5
. Acknowledgments
The authors acknowledge FAPESP (2005/01296-4) and CNPq (137444/2010-9 and
34256/2014-1) for financial support. SJT ∴
2
6
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19

Mechanistic Study of the Formation of Multiblock π-Conjugated
Metallopolymer
1
Camila R. Peverari , Diego N. David-Parra , Madalina M. Barsan and Marcos F. S.
2
3
1
Teixeira *
1
- Department of Chemistry and Biochemistry, Faculty of Science and Technology, São Paulo
State University (UNESP), Rua Roberto Simonsen 305, CEP 19060-900 Presidente Prudente,
SP, Brazil.
2- São Carlos Institute of Chemistry, University of São Paulo (USP), São Carlos – SP –
Brasil.
3
3
- Department of Chemistry, Faculty of Science and Technology, University of Coimbra,
004-535 Coimbra, Portugal.
The highlighted structures are the molecular column of the central aromatic rings (MCCAR)
and molecular column of the side aromatic rings (MCSAR). The molecular structures were
obtained by the control of the potential scan rate in the electropolymerization step.
Electrochemical measurements, atomic force microscopy (AFM), UV-Vis spectroscopy,
FTIR Raman spectroscopy techniques and optical microscope images support the formation
of the multiblock π-conjugated polymer.
GRAPHICAL ABSTRACT
ELECTROCHEMI CAL
CONDUCTI VI TY
ELECTROCHEMI CAL
CONDUCTI VI TY
UV- VI SI BLE
RAMAN
UV- VI SI BLE
RAMAN
20