Synthesis and characterization of porphyrin electrochromic and photovoltaic electropolymers

Article abstract of DOI:10.1016/j.orgel.2012.01.004

New asymmetric porphyrins, with electroactive functionalities were synthesized. 5,15-Bis[4(-N,N-diphenylamino)phenyl]-10,20-bis[3-(N- ethylcarbazoyl)]porphyrin and its Zn(II) derivative allow the electrochemical formation of conjugated, conducting and sta

Full text of DOI:10.1016/j.orgel.2012.01.004

Organic Electronics 13 (2012) 604–614
Contents lists available at SciVerse ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Synthesis and characterization of porphyrin electrochromic
and photovoltaic electropolymers
a
a
b
a
a
Javier Durantini , Gustavo M. Morales , Marisa Santo , Matías Funes , Edgardo N. Durantini ,
a
c
a,
⇑
a,
⇑
Fernando Fungo , Thomas Dittrich , Luis Otero , Miguel Gervaldo
a
Departamento de Química, Universidad Nacional de Río Cuarto, Agencia Postal Nro. 3, X5804BYA Río Cuarto, Córdoba, Argentina
Departamento de Física, Universidad Nacional de Río Cuarto, Agencia Postal Nro. 3, X5804BYA Río Cuarto, Córdoba, Argentina
Helmholtz Center Berlin for Materials and Energy, Institute of Heterogeneous Materials, Hahn-Meitner-Platz 1, D-14109 Berlin, Germany
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
New asymmetric porphyrins, with electroactive functionalities were synthesized. 5,15-
Bis[4(-N,N-diphenylamino)phenyl]-10,20-bis[3-(N-ethylcarbazoyl)]porphyrin and its Zn(II)
derivative allow the electrochemical formation of conjugated, conducting and stable poly-
meric films over semitransparent indium–tin oxide surface. The spectral changes observed
upon oxidation of the films produced different colorations. The polymeric materials showed
the generation of photoinduced charge separation states and charge migration upon por-
phyrin excitation, making them promising candidates for application in optoelectronics
devices.
Received 20 September 2011
Received in revised form 2 December 2011
Accepted 5 January 2012
Available online 17 January 2012
Keywords:
Porphyrin
Carbazole
Triphenylamine
Electropolymerization
Electrochromic
Surface photovoltage
Ó 2012 Elsevier B.V. All rights reserved.
1
. Introduction
In the development of optoelectronic devices, organic
electronics faces the difficulty of polymer film deposition
over conducting or semiconducting contact. Up to now,
two major approaches are used to deposit organic material
thin films in optoelectronic devices: thermal evaporation
and solution-processing. Thermal evaporation requires
materials with high sublimation capability and excellent
thermal stability, which are properties not easily obtained
in polymers. On the other hand, low-cost solution processes,
such as spin-coating and deep-coating, widely used in the
production of non-patterned films, require materials with
intrinsic high solubility, and usually produce large amounts
of waste. An alternative technique to produce promising
conductingpolymer films is the electropolymerization, with
the adoption of electroactive monomers. The polymeriza-
tion through electrochemical deposition techniques allows
synthesizing the polymeric film in one step with fine control
over the thickness; moreover, a mold in the working elec-
trode allows an excellent and simple pattern of the film.
A number of reports revealed that the polymeric films ob-
tained electrochemically over conductive solid substrates
molecular compounds are receiving great attention as
promising constituents due to their advantages over con-
ventional semiconductor electronics. Molecular engineer-
ing (often assisted by computational methods [1,2]) and
the development of synthesis routes allow the generation
of organic compounds with the desired physical properties
for application as building blocks of optoelectronic devices.
In this context it deserves special consideration the use of
organic conducting polymers [3–6]. It is reasonable to ex-
pect that the development of organic polymers with ade-
quate optical and electronic properties will introduce a
significant advance in the construction and application of
devices for energy generation, image display, lighting sys-
tems and others. However, the use of organic polymers in
⇑
Corresponding authors.
E-mail address: mgervaldo@exa.unrc.edu.ar (M. Gervaldo).
1
566-1199/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.orgel.2012.01.004

J. Durantini et al. / Organic Electronics 13 (2012) 604–614
605
There are several examples of generation of porphyrin
polymers by electrochemical [20,22,23] or chemical
(a)
N
M
N
[
16,19,24–26] methods and their use as components in
electronic or optoelectronic devices. Even more, a p/n het-
erojunction device where the donor and acceptor layers
were electrochemically deposited has recently been re-
ported [27].
In this work we show the monomers synthesis, poly-
mers electrosynthesis, spectroelectrochemical character-
ization and surface photovoltage spectroscopy of novels
N
N
N
N
N
N
5
,15-bis[4(-N,N-diphenylamino)phenyl)]-10,20-bis[3-(N-
ethylcarbazoyl)]porphyrin (PCBZTPA) and Zn(II)
,15-bis[4(-N,N-diphenylamino)phenyl)]-10,20-bis[3-(N-
5
ethylcarbazoyl)]porphyrin (PCBZTPA-Zn, Fig. 1) conducting
polymer films. Upon oxidation, both carbazol (CBZ) and tri-
phenylamine (TPA) moieties undergo the well-known rad-
ical cation dimerizations [7,12,37,47] producing dicarbazol
M=H
PCBZTFA
2
M=Zn(II) PCBZTFA-Zn
(
DCBZ) and tetraphenylbenzidine (TPB) units, which con-
(b)
duct to porphyrin electropolymer formation [7,12]. How-
ever, in general, the CBZ radical cation has coupling rate
constants that are 4–5 orders of magnitude higher than
those of the TPA groups [28], and as a consequence we ex-
pected that CBZ dimerization moieties led to the formation
of a linear polymer due to the cross positions of the CBZ
residues in the monomer structure, meanwhile TPA dimer-
izations allow a polymer cross-linkage during the electrop-
olymerization. CBZ dimers and polymers usually exhibit
high electrical conductivity and good optical quality, giving
them great potential for use in optoelectronic applications
N
N
N
N
M
N
N
[
29–33]. The films formed over conducting indium tin
oxide semitransparent electrodes showed the generation
of photoinduced charge separation states. This fact indi-
cates that electropolymerized films of PCBZTPA and
PCBZTPA-Zn could be promising in a variety of applica-
tions, for example for the construction of polymeric organ-
ic solar cells, since they are strong visible light absorbers
and good electronic conductors.
N
N
M=H
2
M=Zn(II)
2
. Experimental
2.1. PCBZTPA and PCBZTPA-Zn synthetic procedure
Fig. 1. Chemical structure of the monomers (a) and idealized polymer
structure (b).
PCBZTPA was synthesized from meso-[4-(N,N-diphenyl-
aminophenyl)]dipyrromethane. Thus,
a solution of 4-
are highly stable, showing compact layers, with excellent
stuck to the electrode and high charge transport capability
(N,N-diphenylamino)benzaldehyde (1.96 g, 7.17 mmol)
and pyrrole (24 mL, 346 mmol) was purged in an argon
atmosphere for 15 min at room temperature. The reactions
were initiated by addition of trifluoroacetic acid (TFA,
[
7–15].
A considerable number of organic structures has been
designed, synthesized and studied with the purpose of
being used as precursors of organic electropolymers in
the development of optoelectronic devices. In this frame,
many researchers inspired by the natural photosynthetic
apparatus incorporated porphyrins, chlorophyll deriva-
tives, and several related metallized and unmetallized tet-
rapyrrolic compounds in the development of organic
photovoltaic devices [16–21]. Porphyrins accept or donate
1
38
conditions. The reaction was terminated by addition of tri-
ethylamine (TEA, 760 L, 5.46 mmol) and diluted with
0 mL of dichloromethane (DCM). Finally, volatile com-
lL, 1.79 mmol) and stirred for 30 min under the same
l
5
pounds were removed under reduced pressure, and the
unreacted pyrrole was removed by vacuum distillation at
6
chromatography (silica gel, cyclohexane/ethyl acetate/
TEA 80:20:1), yielding 1.96 g (70%) of pure dipirrometane.
0 °C. The product obtained was purified by flash column
electrons easily through their large
p electron frameworks,
they are suitable for electronic conduction, and together
with their optical properties, porphyrins become in one
of most useful structures in organic optoelectronic devices.
TLC (silica gel, cyclohexane/ethyl acetate/TEA 80: 20:1)
1
R
f
= 0.25. H NMR (CDCl
3
, TMS) d [ppm] 5.43 (s, 1H,
meso-H); 5.94 (m, 2H, pyrrole-H); 6.17(q, 2H, pyrrole-H);

6
06
J. Durantini et al. / Organic Electronics 13 (2012) 604–614
6
.72 (m, 2H, pyrrole-H); 6.94–7,12 (m, 10 H, –ArH); 7.22
was changed with calibrated neutral density filters over
several orders of magnitude [34].
(
m, 4 H, –ArH); 7.98 (s, brs, 2H, pyrrole NH). ESI-MS [m/
+
z] 389.1892 (M ). A solution of N-ethyl-3-carbazolecarbal-
dehyde (0.56 g, 2.50 mmol) and dipyrromethane (1.00 g,
Atomic force microscopy (AFM) experiments were per-
formed using an Agilent 5500 SPM microscope (Agilent
Technologies, Inc.) working in Acoustic AC Mode. Commer-
cial silicon cantilever probes, with aluminum backside
coating, and nominal tip radius of 10 nm (MikroMasch,
2
1
.50 mmol) in 310 mL of DCM was purged with argon for
5 min. After that, trifluoroacetic acid (425 L, 5.50 mmol)
l
was slowly added and the solution was stirred for 60 min
at room temperature. Then, 2,3-dichloro-5,6-dicyano-1,4-
benzoquinone (DDQ, 1.50 g, 6.61 mmol) was added and
the mixture was stirred for an additional 18 h, open to
the atmosphere. The solvent was removed under reduced
pressure. The obtained product was purified by flash col-
umn chromatography (silica gel, hexane/DCM/TEA
ꢀ1
NSC15/Al BS/15, spring constant ranging 20–75 N m ),
were employed just under their fundamental resonance
frequencies of about 325 kHz.
The thickness of the polymer films were also measured
with a surface profiler (Dektak 8 Advanced Development
Profiler system, Veeco).
6
:93.8:0.2) obtaining 622 mg (42%) of pure PCBZTPA. TLC
silica gel, hexane/DCM/TEA 6:93.8:0.2) R
= 0.68. 1H NMR
CDCl
, TMS) (ppm) d ꢀ2.60 (brs, 2H, pirrol N–H); 1.68 (t,
H, –CH , J = 7.1 Hz); 4.64 (q, 4H, –CH
ꢀ, J = 7.1 Hz); 7.15
d, 4H, J = 7.8 Hz); 7.34–7.50 (m, 22 H); 7.52–7.64 (m, 4
H); 7.75 (d, 2H, J = 8.3 Hz); 8.02 (d, 4H, J = 7.8 Hz); 8.20
d, 4 H, J = 7.7 Hz), 8.34 (d, 2H, J = 8.3 Hz), 8.82–9.06 (d,
(
(
6
f
3
. Results and discussion
3
3
2
3.1. Absorption and emission properties
(
The absorption spectra of the porphyrins in DCM solu-
(
+
tion and of the films on ITO electrodes are shown in
Fig. 2a. The PCBZTPA and PCBZTPA-Zn porphyrins show
the typical Soret and Q-bands, characteristic of a free-base
porphyrin and the corresponding Zn(II) metalloporphyrin
8
H). ESI-MS [m/z] 1183.5176 [M+H] (1182.5097 calcu-
lated for ). PCBZTPA-Zn was obtained from
PCBZTPA. A solution of PCBZTPA (40 mg, 0.034 mmol) in
ml of DCM was treated with 3 mL of a saturated solution
of zinc(II) acetate in methanol. The mixture was stirred for
h in argon atmosphere at room temperature. After that,
84 62 8
C H N
8
[
35]. Also, the porphyrins in solution show an absorption
band in the UV region (ꢁ305 nm) due to the CBZ and TPA
substituents. The steady-state fluorescence emission spec-
trum of the porphyrins in DCM are shown in Fig. 2b. The
two bands are characteristic of free-base and Zn-substit-
ued porphyrins in solution, being these assigned to Q(0–
0) and Q(0–1) transitions [36].
1
the solution was treated with water (30 mL) and the or-
ganic phase was extracted with three portions of DCM
(
20 mL each). The solvents were evaporated under reduced
pressure. The reaction afforded 41 mg (97%) of pure
+
PCBZTPA-Zn. ESI-MS [m/z] 1245.4311 [M+H] (1244.4232
calculated for C84
.2. Instrumentation and measurements
The spectroscopic (absorption–emission), electrochem-
ical and spectroelectrochemical characterization of the
new organic materials was performed using the already
described set-up [7–12] Electrochemical studies were car-
ried out over Pt and indium tin oxide (ITO) electrodes
60 8
H N Zn).
3
.2. Electrochemical characterization
2
The electrochemical properties of PCBZTPA and
PCBZTPA-Zn monomers were investigated by cyclic vol-
tammetry in DCE containing TBAPF using a Pt working
6
electrode. Fig. 3a and b show the first anodic and cathodic
scan of the monomers. In order to obtain more detailed
information about the electrochemical processes CVs were
recorded at different inversion potentials. PCBZTPA shows
two reduction peaks and three oxidation processes whereas
PCBZTPA-Zn shows one reduction peak and four oxidation
processes. In the anodic scan the first oxidation peak de-
tected for PCBZTPA and the first two for PCBZTPA-Zn are
attributed to porphyrin macrocycle oxidation. The differ-
ence observed in the number of peaks could be attributed
to substitution of the two H central atoms in PCBZTPA by
Zn(II), which lowers the first oxidation potential in
PCBZTPA-Zn and as a result two peaks are observed. The
two remaining oxidation peaks of PCBZTPA detected at
more anodic potentials may be attributed to oxidation of
the TPA and CBZ groups attached to the porphyrin macrocy-
cle being the CBZ groups oxidized at more anodic potentials
than TPA [28,37]. For PCBZTPA-Zn a similar assignment
could be made with the difference that these two last peaks
occur at less anodic applied potentials than PCBZTPA, due
to the presence of the Zn(II) central metal.
(
Delta Technologies nominal resistance of 8–12 X/square)
in 1,2-dichloroethane (DCE) deoxygenated solution (nitro-
gen bubbling), with 0.10 M tetra-n-butylammonium hexa-
fluorophosphate (TBAPF ) as the supporting electrolyte. A
6
silver wire quasi-reference electrode was used. The Pt
working electrode was cleaned between experiments by
polishing with 0.3 lm alumina paste followed by solvent
rinses. After each voltammetric experiment, ferrocene
was added as an internal standard, and the potential axis
was calibrated against the formal potential for the Satu-
rated Calomel Electrode (SCE).
The measurements of modulated Surface Photovoltage
(
SPV) were performed in the fixed capacitor arrangement
with chopped light (modulation frequency 6 Hz) from a
quartz prism monochromator (SPM2) and a halogen lamp
(
100 W). The measurements were carried out in vacuum
using an already described set-up [34]. SPV transients were
excited with laser pulses (wavelength 600 nm, time of
2
laser pulses: 5 ns, intensity: about 3 mJ/cm ) and recorded
Continuous cycling of the monomers through the fur-
thest oxidation peak produces an increase in the intensity
of the oxidation and reduction currents and clearly new
with
a sampling oscilloscope (GAGE compuscope CS
1
4200) at resolution of 10 ns. The intensity of exciting light

J. Durantini et al. / Organic Electronics 13 (2012) 604–614
607
2
2
1
1
5
0
5
0
5
0
5
(a)
PCBZTPA film
PCBZTPA-Zn film
PCBZTPA sol
(a)
PCBZTPA-Zn sol
-
-
10
15
-
3
00
350
400
450
500
550
600
650
700
-2.0 -1.6 -1.2 -0.8 -0.4 0.0 0.4 0.8 1.2 1.6 2.0
Potential (V vs SCE)
Wavelength (nm)
1
2
0
8
6
4
2
0
2
4
6
(b)
(b)
PCBZTPA
PCBZTPA-Zn
1
-
-
-
-
2.0 -1.6 -1.2 -0.8 -0.4 0.0 0.4 0.8 1.2 1.6 2.0
5
50
600
650
700
750
800
Potential (V vs SCE)
Wavelength (nm)
Fig. 3. Cyclic voltammetry (at different inversion potentials) of PCBZTPA
a) and, PCBZTPA-Zn (b) in DCE solution containing 0.1 M TBAPF
Platinum working electrode, scan rate: 100 mV/s.
Fig. 2. (a) Electronic absorption spectra of PCBZTPA (solid black) and
PCBZTPA-Zn (solid red) in DCM solution, and PCBZTPA film (dashed black)
and PCBZTPA-Zn film (dashed red) on ITO. (b) Fluorescence spectra of
PCBZTPA and PCBZTPA-Zn in DCM solution. (For interpretation of the
references to color in this figure legend, the reader is referred to the web
version of this article.)
(
6
.
only support electrolyte both films showed two redox cou-
ples. The oxidation potentials of the films were similar to
the ones observed when the films were grown cycling
the most anodic oxidation peak, but the magnitudes of
the oxidation/reduction currents were smaller. This indi-
cates that the polymerization efficiency is lower when
the film is grown cycling the peak assigned to oxidation
of the TPA groups. No increase in the oxidation/reduction
currents was detected when the peaks assigned to porphy-
rin oxidation were continuously cycled.
Previous electrochemical results in porphyrins have
shown that substitution of the two hydrogen central atoms
by metals alters the reduction and oxidation processes
[38,39]. For example, substitution by Pd(II) increases the
oxidation potential by 160 mV whereas substitution by
Zn(II) decreases the oxidation potential [40]. Substitution
at the meso position of the macrocycle also modifies the
redox properties. We observed in a related porphyrin
substituted with TPA groups two oxidations peaks, being
the first one attributed to oxidation of the macrocycle (2
electron processes) and the second one to oxidation of
the TPA units [7]. Contrarily, substitution by four carbazole
redox systems can be detected, indicating the formation of
electroactive films on the working electrode surfaces
(Fig. 4a and b). To confirm the absorption of the films on
the electrode their electrochemical response were re-
corded in monomer-free electrolytic solution (Fig. 5a and
b). Two reversible redox couples are detected for PCBZTPA
and they appear at around 0.83 and 1.17 V. PCBZTPA-Zn
also shows two redox couples which occur at oxidation po-
tential values similar to those observed for PCBZTPA. Also
the oxidation/reduction peak currents for both films in-
crease linearly with the scan rate which is indicative of
an electro-active product irreversibly adsorbed on the elec-
trode. On the other hand, if the monomers are repetitively
scanned through the peaks assigned to oxidation of the tri-
phenylamine (TPA) units, an increase in the oxidation/
reduction current peaks is also detected, but this is smaller
than that observed when the furthest oxidation peak was
cycled. When the electrodes were removed from the
monomer solutions and placed in a solution containing

6
08
J. Durantini et al. / Organic Electronics 13 (2012) 604–614
4
3
2
1
0
0
0
0
80
(a)
(a)
60
40
20
0
10
20
30
40
0
20
40
60
80
-
-
-
-
-
-
-
-
0
.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00
0
.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00
Potential (V vs SCE)
Potential (V vs SCE)
3
2
1
0
0
0
(
b)
(
b)
4
5
3
1
0
5
0
10
20
30
0
15
30
-
-
-
-
-
-45
0
.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00
0
.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00
Potential (V vs SCE)
Potential (V vs SCE)
Fig. 4. Cyclic voltammograms of PCBZTPA (a) and PCBZTPA-Zn (b),
showing repetitive anodic sweeps at a platinum electrode immersed in
a solution of monomer as described in Fig. 2. A total of 10 scans are
shown.
Fig. 5. Cyclic voltammograms of electropolymerized films of PCBZTPA (a)
and PCBZTPA-Zn (b) resulting from Fig. 3. Scan rates: 200, 150, 100, 75,
50, 25 mV/s.
interacting. This ‘‘connection’’ between the different units
could affect the electrochemical properties. Oxidation of
the porphyrin may lead to charge delocalized not only over
the macroclycle, also over the CBZ and TPA groups. In the
same way the third electron could be removed from the
TPA, but the resulting positive charge may also delocalize
over the whole molecule.
(
CBZ) groups showed three oxidations, which were as-
signed to formation of porphyrin radical cation and dica-
tion, and to oxidation of the CBZ groups, respectively
[
12]. On the other hand, in general CBZ and TPA present
a similar electrochemistry [41–44]. Oxidation of any of
both monomers, CBZ or TPA, leads to formation of highly
reactive radical cations, which then couple with other rad-
ical cations to form dimers, being these dimers more easily
oxidized than the monomers. Moreover, related CBZ and
TPA based polymers have shown two oxidation processes,
which have been assigned to radical cation and dication of
dicarbazole (DCBZ) and tetraphenylbenzidine (TPB),
respectively [45–48]. Comparison of coupling rate con-
stants has shown that CBZ has a 4–5 order of magnitude
higher constant than those of TPA groups [28]. It must be
remarked that in PCBZTPA and PCBZTPA-Zn the CBZ and
TPA units are conjugated with the porphyrin macroclycle.
Therefore, it is highly possible that the monomers do not
behave as a porphyrin formed by three independent units
i.e. porphyrin macrocycle, two CBZ and two TPA units, but
like a whole molecule with all these groups connected and
3.3. Spectroelectrochemical characterization
Spectroelectrochemistry of the films was done in order
to gain more information about the oxidation processes
and to clarify the polymerization mechanism. When the
films are in the neutral state the spectra show the charac-
teristic porphyrin Soret bands and also the four and two Q
typical bands of free base and Zn(II) derivative, respec-
tively (Fig. 2a). These bands are quite similar to those ob-
served for the monomer in solution, indicating that the
macrocycles have not been altered during the polymeriza-
tion processes, although the bands of PCBZTPA film and
PCBZTPA-Zn film are broader and shifted in comparison

J. Durantini et al. / Organic Electronics 13 (2012) 604–614
609
with those of monomeric porphyrins in solution. These
facts indicate the presence of interaction between porphy-
rins in the branched film structure. Fig. 6a and b show
ilar for both films, which could indicate that the same
groups are oxidized. On the other hand, in case (2) similar
changes in the absorption spectra were obtained during
the oxidation processes (Fig. 6c and d). For the free base
film, growing of two bands can be seen at 490 and
710 nm. For the Zn derivative the same band at 490 nm ap-
pears, and the broad band observed at 720 nm (Fig. 6b) is
now shifted to 690 nm (Fig. 6d), and it is narrower com-
pared with the one seen in case (1). Also upon oxidation
PCBZTPA-Zn film presents the development of an absorp-
tion band in the near IR region (950 nm), and a bigger
bleaching in the UV zone is observed for both films in the
case (2).
It was mentioned above that both TPA and CBZ produce
dimers when they are oxidized. Oxidation of TPB solutions
have shown the apparition of a band at around 480 nm and
a second broad band at about 680 nm at more anodic po-
tential [49]. These two bands have been assigned to light
absorption by radical cation and dication of TPB. Also re-
lated polymers containing TPB in their structures have
shown similar absorption bands under oxidation, and a
band at around 350 nm attributed to TPB in the reduced
state [47,48]. On the other hand bands at 390 and
absorption spectra of the films (plotted as
ent applied potentials when the films were grown cycling:
1) the peak assigned to oxidation of TPA units and (2) the
DAbs) at differ-
(
peak assigned to oxidation of CBZ units. During oxidation
two bands develop for both films in (1). For the free base
the first one is located at about 485 nm and the second
band appears as broad band between 500 and 1000 nm
with a maxima at around 710 nm. For the Zn derivative
the first band appears at 510 nm, and the broad band has
a maximum at 720 nm. The bands at 485–510 nm start
growing at the onset potential of the first oxidation peaks
for both films, and then decrease at more anodic potentials.
At the beginning of the second oxidation peaks the broad
bands (maximum 710–720 nm) increase, and they reach
the highest absorption when the films are completely oxi-
dized. During the entire oxidation process the bleaching of
the Soret bands increases in intensity together with a small
change in the bands detected in UV region, indicating that
the porphyrin macrocycle electronic structures have been
modified. The changes in the UV–Vis spectra are very sim-
(
a)
(c)
500 mV
700 mV
900 mV
1100 mV
1300 mV
500 mV
700 mV
900 mV
1
100 mV
300 mV
1
3
00
400
500
600
700
800
900
1000
300
400
500
600
700
800
900
1000
Wavelength (nm)
Wavelength (nm)
(
b)
(d)
5
7
9
1
1
00 mV
00 mV
00 mV
100 mV
300 mV
500 mV
700 mV
900 mV
1100 mV
1300 mV
3
00
400
500
600
700
800
900
1000
3
00
400
500
600
700
800
900
1000
Wavelength (nm)
Wavelength (nm)
Fig. 6. Difference spectra of electropolymerized films of PCBZTPA (a) and PCBZTPA-Zn (b) in case (1), and of PCBZTPA (c) and PCBZTPA-Zn (d) in case (2)
deposited on ITO, at different applied potentials in DCE solution with 0.1 M TBAPF6.

6
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J. Durantini et al. / Organic Electronics 13 (2012) 604–614
7
80 nm have been observed during electrolysis of dica-
are yellow pale, during the p-doping this color starts to
change to greenish and finally, when the films are totally
oxidized, a uniform green-bluish coloration is observed.
As can be seen in the photographs, the electrogenerated
colors are homogeneously distributed across the electrode
surface, and the color changes are easily detected by the
naked eye. This implies that the electro-deposited films
have potential for electrochromic devices generation.
rbazol, which were attributed to the absorption spectra
of cation radical and dication of dicarbazol, respectively
[
41]. Oxidation of CBZ based polymers also depicted simi-
lar bands upon oxidation, and a band in the UV assigned to
the DCBZ units before oxidation of the film [14,50,51].
Moreover, in a recent paper, we reported the electropoly-
merization of a meso substituted porphyrin by four N-ethyl
carbazole groups. In the neutral state the film showed the
typical porphyrin bands and a shoulder in the UV zone.
Oxidation of the polymer presented a band that starts at
3.4. AFM images
4
80–500 nm and it extends to IR. A second broad band
The surface morphologies of the obtained polymer films
were examined with AFM. Fig. 8a shows a typical Acoustic
AC Mode AFM height mode image from the electrodepos-
ited film of PCBZTPA on ITO. No difference in topological
features was found for both PCBZTPA and PCBZTPA-Zn
electropolymerized films. As it was previously reported
for electrochemically generated porphyrin films, globular-
like microstructures are observed in the analyzed films
[7,20]. The observed microstructure must be a conse-
quence of the nucleation and further growing process itself
and not just a homogeneous decoration of the ITO surface
(inset in Fig. 8a). This conclusion is confirmed by compar-
ing the average surface RMS roughness measured over an
was observed at more anodic potentials, with a maximum
in the 600–750 nm range [12].
Based on the electrochemical and spectroelectrochemi-
cal studies, polymer structures in which the porphyrin units
are linked by TPB and DCBZ groups are proposed (Fig. 1b).
Oxidative polymerization does not happen when the peaks
assigned to oxidation of porphyrins are cycled. Continuous
cycling of any of the peaks corresponding to oxidation of
TPA or CBZ produces an electroactive film on the electrode
surface. Oxidation of the TPA or CBZ units in the monomers
conducts to the formation of radical cations which react
forming dimers, which are detected in the second cycle as
a new redox couple. Posterior cycling of the monomers leads
to the apparition of more radical cations which can produce
more dimers or react with other dimers adsorbed in the
electrode resulting in a longer chain. The difference in the
polymerization efficiency in cases (1) and (2) may be due
to a lower dimerization constant of TPA compared to the
CBZ. The bigger bleaching in the UV zone in (2) is also in con-
cordance with a lower dimerization constant. When the
electrode is cycled in (1) just the TPA groups are dimerized,
and the absorption in the UV zone is smaller than in (2). The
bands at 485 nm and about 710 nm for free base film and
area of 5
lm ꢂ 5
lm for the electrodeposited films, being
7.66 nm a much rougher surface than the ITO (1.32 nm).
In order to study the film thickness distribution on the
electrode, we used AFM scratching, a technique which
has been shown to be a suitable method for determination
of the absolute thickness of the electrochemically depos-
ited films [52]. Fig. 8b shows the profiles on different re-
gions of the electrode covered by the PCBZTPA
electrodeposited film. They clearly show the same thick-
ness in the studied areas, the PCBZTPA-Zn electropolymer-
ized films present the same behavior, concluding that the
films grow with a homogeneous thickness distribution
and without pinholes on the entire surface of the electrode.
Also, the film thicknesses were determinated using a pro-
filer, and the obtained values were correlated to the absor-
bance of the films. For both polymers it was observed that
the thickness increases linearly with the absorbance.
PCBZTPA showed that an increase in absorbance of 0.10
units at the Soret band corresponds to a ꢁ19 nm change
in the thickness, meanwhile PCBZTPA-Zn presented a
change of ꢁ10 nm for an increase of 0.1 units in the absor-
bance. The difference observed for the thicknesses of both
polymers could be related to the higher molar extinction
coefficient that present the Zn porphyrin derivatives com-
pared to the free bases. The obtained thicknesses values by
both techniques were very similar within the experimental
error. It must be remarked that the films used for thickness
determination were grown cycling the peak assigned to
oxidation of CBZ units due to the cycling of the peak as-
signed to TPA conducted to the formation of very thin
films.
5
10 and 720 nm for Zn derivative observed during oxidation
in case (1) could be assigned the radical cation and dication
of TPB units. On the other hand the bands at 490 and 710 nm
for PCBZTPA and 490 and 680 nm for PCBZTPA-Zn detected
under oxidation in case (2) could be attributed to the radical
cation and dication of DCBZ parts. The only difference be-
tween all the bands is for the PCBZTPA-Zn film grown cy-
cling at applied potentials where oxidation of the CBZ
units are produced. This film showed an absorption band
in the IR, and the broad band was shifted to shorter wave-
lengths. These spectralchanges are similar to those obtained
during electropolymerization of a porphyrin substituted by
four N-ethylcarbazol units [12].
Taking into account the above described results, it can
be concluded that when the films are formed by cycling
in the applied potential window where both functionalities
are oxidized, linear polymers with some degree of cross-
linkage are formed. This is due to the cross positions of
the CBZ residues in the monomer structure and their big-
ger (4–5 orders of magnitude) radical cation coupling rate
constants compared to those of the TPA groups.
The spectral changes observed upon redox switching
between oxidized and reduced forms of the films produced
different colorations. During these experiments we found
strong and reversible electrochromic effects (Fig. 7) in
ambient conditions. In the neutral state (0 V) the films
3.5. Steady state and time resolved surface photovoltage
spectroscopy
Fig. 9 shows the SPV (surface photovoltage) spectra of
PCBZTPA and PCBZTPA-Zn electropolymerized on ITO.

J. Durantini et al. / Organic Electronics 13 (2012) 604–614
611
Fig. 7. Photograph images of PCBZTPA film deposited on ITO at various applied potentials.
Fig. 8. (a) AFM images of ITO electrode surface (inset, scan size 1 ꢂ 1
l
m) registered after electrodeposition of PCBZTPA films (scan size, 5 ꢂ 5
l
m). (b) AFM
section analysis used to evaluate the film thicknesses distributions on the electrode. Films were grown cycling the peak assigned to oxidation of the CBZ
units.
The sign of the in-phase SPV signal is positive (negative) if
the photo-generated electrons are preferentially separated
towards the internal (external) surface. The absolute value
of the in-phase signal is in maximum and the phase-shifted
by 90° signal is zero, i.e. the phase angle is 0 or ±180°, if the
time constants of the increasing and decreasing signals are
much shorter than the modulation period. In contrast, the
in-phase signal is zero and the absolute value of the phase-
shifted by 90° signal is in maximum, i.e. the phase angle is
90° signals. As an example, the intensity dependence of
the modulated PV amplitude measured at 600 nm is plot-
ted in Fig. 10 for PCBZTPA-Zn in a double logarithmic scale.
The SPV amplitude was proportional to the intensity of the
illuminating light. This was a general behavior for both
types of samples. It manifested additionally in the spectral
dependence of the phase angle (cotan of the phase angle is
defined as the ratio between the phase-shifted by 90° and
in-phase signals) which was practically constant over the
whole spectral range where SPV signals could be detected
and which was independent of the intensity (see the insert
of Fig. 10 for PCBZTPA-Zn). A constant phase angle means
that the mechanisms of charge separation, transport and
recombination are independent of the photon energy, i.e.
excitations of the Q and Soret bands were equivalent with
respect to charge separation and relaxation processes.
The SPV spectra could be normalized to the photon flux
due to the facts that the PV amplitudes were proportional
to the intensity and the separation and relaxation pro-
cesses were independent of photon energy. The SPV spec-
tra normalized to the photon flux are shown in Fig. 11
for PCBZTPA and PCBZTPA-Zn. For the free base polymer
the SPV spectrum normalized to the photon flux presents
bands at 433, 517, 568 and 654 nm and the SPV signal lev-
eled out between 680 and 850 nm. The Zn derivative poly-
mer shows bands at 441, 554, 597 nm and a broad
shoulder at about 900 nm and leveling out at about
±90° or 270°, if the time constants of the increasing and
decreasing signals are much longer than the modulation
period. In this case, charge separation, transport processes
and recombination are very slow, for example, limited by
trapping with long trapping lifetimes. For PCBZTPA the
sign of the in-phase SPV signal is positive, which denotes
that the photogenerated electrons are separated towards
the internal surface. Contrarily PCBZTPA-Zn shows a nega-
tive in-phase SPV signal indicating that the photogenerat-
ed electrons are separated towards the external surface.
Various peaks appeared in the SPV spectra. As remark,
2
the electrode (quartz with SnO :F) strongly absorbed the
light at photon energies above 3.6 eV. It will be interesting
to compare SPV spectra directly with the absorption spec-
tra. For this purpose information about the intensity
dependence of the modulated SPV signals is needed. The
amplitude of a modulated SPV signal is the square root of
the sum of the squared in-phase and phase-shifted by

6
12
J. Durantini et al. / Organic Electronics 13 (2012) 604–614
4
3
2
1
00
00
00
00
0
PCBZTPA-Zn
in phase
phase-shifted by 90
(a)
0
1
00
1
50
00
50
0
slope 1
1
1
0
1
max intensity
max intensity * 0.025
1234
Photon energy (eV)
0
.01
0.1
1
-
-
-
-
-
100
200
300
400
500
Intensity at 600 nm (arb. un.)
Fig. 10. Intensity dependence of the modulated SPV amplitude measured
at 600 nm (PCBZTPA-Zn). The insert shows the spectra of the phase angle
measured at maximum light intensity and decreased by 40 times using
neutral filters.
1
0
0
.0
.5
.0
PCBZTPA-Zn
PCBZTPA
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
Energy (eV)
8
6
4
2
00
00
00
00
0
in phase
phase-shifted 90
(b)
0
4
00
600
800
1000
1200
Wavelength (nm)
Fig. 11. Spectra of the SPV amplitude normalized to the photon flux for
PCBZTPA (black line) and PCBZTPA-Zn (bold red line). (For interpretation
of the references to color in this figure legend, the reader is referred to the
web version of this article.)
-
-
-
-
200
400
600
800
resolved SPV measurements (Fig. 12), the electropolymers
showed different signs in the SPV signals in concordance
with the data obtained under chopped light illumination,
i.e. positive for the free base and negative for the Zn(II)
derivative. These experimental facts were observed for
several studied samples of different absorbances and thus
different thicknesses. In order to speculate about the type
of charge separation and charge transport, more detailed
knowledge about excitonic formation and transport, con-
ductivity type, and band bending is necessary. These char-
acterizations are under study, and the results will be
reported in an incoming paper. It should be remarked that
the transient SPV signals increased to about +40 mV and
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
Energy (eV)
Fig. 9. Spectra of the in-phase and phase-shifted by 90° surface photo-
voltage signals for PCBZTPA (a) and PCBZTPA-Zn (b) electropolymerized
on ITO.
ꢀ
40 mV within the laser pulse for PCBZTPA and
PCBZTPA-Zn, respectively, and continued to increase with-
ꢀ5
in about 10 s by several mV for PCBZTPA (Fig. 12a) and
ꢀ35 mV for PCBZTPA-Zn (Fig. 12b) at maximum light
intensity while the period during which the SPV signals in-
creased shifted to longer times with decreasing light inten-
sity. The increasing SPV signals in time are an indication of
1
200 nm. In both films the bands in SPV spectra matched
very well the absorption peaks of the polymers.
The directions of charge separation for both electropo-
lymerized films on ITO were also confirmed by time

J. Durantini et al. / Organic Electronics 13 (2012) 604–614
613
Innovación Productiva (MinCyT) join project (DA/10/06).
M.G., F.F., E.N.D., G.M., and L.O., are Scientific Members of
CONICET. J.D. and M.D.F. thank to CONICET, for research
fellowships.
(a)
5
0
0
PCBZTPA
Imax
Imax/100
Imax/10
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(b)
Imax/100
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