Synthesis and electrochemical characterization of new optoelectronic materials based on conjugated donor-acceptor system containing oligo-tri(heteroaryl)-1,3,5-triazines

Article abstract of DOI:10.1016/j.electacta.2010.03.076

A series of novel oligoarylenes based on donor-acceptor system, containing triazine moiety as an electron-transporting central core, have been prepared by electrochemical polymerization. The redox behaviour of poly(2,4,6-tri[p-(2-(3,4-ethylenedioxythienyl

Full text of DOI:10.1016/j.electacta.2010.03.076

Electrochimica Acta 55 (2010) 4858–4864
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Synthesis and electrochemical characterization of new optoelectronic materials
based on conjugated donor–acceptor system containing
oligo-tri(heteroaryl)-1,3,5-triazines
Krzysztof R. Idzika,b,∗, Peter Raptac,d, Piotr J. Cywinskie, Rainer Beckerta,1, Lothar Dunschc,∗∗
a Institute of Organic and Macromolecular Chemistry, Friedrich Schiller Universität Jena, Humboldstraße 10, D-07743 Jena, Germany
b Department of Chemistry, Faculty of Medicinal Chemistry and Microbiology, Wroclaw University of Technology, Wybrzeze Wyspianskiego 27, 50-370 Wroclaw, Poland
c Leibniz-Institute for Solid State and Materials Research, Group of Electrochemistry and Conducting Polymers, D-01171 Dresden, Germany
d Department of Physical Chemistry, Faculty of Chemical and Food Technology, Slovak University of Technology, Radlinskeho 9, SK-81237 Bratislava, Slovak Republic
e Department of Physical Chemistry, Institute of Chemistry, University of Potsdam, Karl-Liebknecht-Strasse 24-25, 14476 Potsdam-Golm, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Aseriesofnoveloligoarylenesbasedon donor–acceptorsystem, containingtriazine moietyas an electron-
transporting central core, have been prepared by electrochemical polymerization. The redox behaviour of
poly(2,4,6-tri[p-(2-(3,4-ethylenedioxythienyl))-phenyl]-1,3,5-triazine) was studied by cyclic voltamme-
try and triple in situ ESR/UV–vis–NIR spectroelectrochemistry to get more details on the type of charge
carriers within the film. To obtain desired oligoarylenes, triazine-core monomers possessing various
electrochromic side groups have been synthesized by the Stille cross-coupling methodology. The struc-
tures have been confirmed by 1H NMR, 13C NMR, and elemental analysis. Monomers show good chemical
stability in common organic solvents such as chloroform, dichloromethane or toluene and also exhibit
excellent thermal stability over wide range of temperatures. Furthermore, their photophysical properties
have been established with the use of fluorescence spectroscopy. Electrochemical results accompanied
with fluorescence spectroscopy suggest that these derivatives of triazine can be successfully used in the
fabrication of organic light-emitting diodes (OLEDs).
Received 9 December 2009
Received in revised form 1 March 2010
Accepted 25 March 2010
Available online 2 April 2010
Keywords:
Donor–acceptor system
Triazine derivatives
Stille coupling
Electrochemical polymerization
Fluorescence lifetime
© 2010 Elsevier Ltd. All rights reserved.
1. Introduction
benzene core due to improved electron withdrawing properties
and possessing of a larger nucleophilic susceptibility than the lat-
Donor–acceptor (D–A) systems, containing triazine unit as
acceptor and heteroaryl group such as thiophen, furan or EDOT as
donor, deserve much attention because of their high potential to
be applied in molecular electronics and optoelectronic devices.
Over the past decade, considerable interest in the synthesis
and characterization of compounds containing the 1,3,5-benzene
or 1,3,5-triazine core has been observed [1]. Nevertheless, the
application of the 1,3,5-triazine unit is advantageous over the 1,3,5-
ter. In the field of supramolecular chemistry, triazine derivatives are
commonly used, as they are well known to form hydrogen bonds
and to favour p–p contacts, as recently shown to be involved in
attractive anion–p interactions [2–4].
Furthermore, triazines have higher electron affinity (EA) val-
ues than 1,3,4-oxadiazoles (PBD) and 1,2,4-triazoles [5]; therefore
they have been widely explored as building blocks for electron-
transporting materials (ETMs) to be used for organic light-emitting
diodes (OLEDs) [6–9]. Dendrimers containing triazine core are
interesting because of their unique functional properties and
potential applications in such fields as light power limiting, elec-
troluminescence [10–14], medicine [15,16], and active materials
in optoelectronic such as light-emitting and photovoltaic devices
[6–9].
∗
Corresponding author at: Institute of Organic and Macromolecular Chemistry,
Friedrich Schiller Universität Jena, Humboldstraße 10, Room 101, D-07743 Jena,
Germany. Tel.: +48 508893793; fax: +49 3641948212.
∗∗
Corresponding author at: Leibniz-Institute for Solid State and Materials
Importantly, the combination of triazine with an aryl group as
for instance thiophene, furan or ethylenedioxythiophene improves
chemical stability of the monomer and provides the poly-
mer built from such monomers with enhanced mobility of the
charge carrier. Polythiophene derivatives with optimized prop-
erties have been already used as active electrodes in several
Research, Group of Electrochemistry and Conducting Polymers, D-01171 Dresden,
Germany. Tel.: +49 351 4659 660; Fax: +49 351 4659 811.
E-mail addresses: krzysztof.idzik@pwr.wroc.pl (K.R. Idzik), rainer.beckert@uni-
jena.de (R. Beckert), l.dunsch@ifw-dresden.de (L. Dunsch).
1
Institute of Organic and Macromolecular Chemistry, Friedrich Schiller Univer-
sität Jena, Humboldstraße 10, Room 209, D-07743 Jena, Germany.
Tel.: +49 3641948230; Fax: +49 3641948212.
0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2010.03.076

K.R. Idzik et al. / Electrochimica Acta 55 (2010) 4858–4864
4859
Scheme 1. Synthesis of monomers.
electrochemical/electronic devices [17,18]. It was also observed
that poly(3,4-ethylenedioxy thiophene), PEDOT, is superior to its
parent polythiophene in many categories crucial to organic elec-
trochromic materials such as rapid switching and lower oxidation
potential [19]. The low band gap of PEDOT allows the polymer to be
almost transparent in the doped state and blue in the neutral state
[20,21].
Germany) outside the box. ESR spectra were recorded by the
EMX X-band CW spectrometer (Bruker, Germany). UV–vis–NIR
spectra were taken by TIDAS (J&M, Germany) spectrometer. Both
the ESR spectrometer and the UV–vis–NIR spectrometer were
linked to a HEKA potentiostat PG 285. Triggering was performed
by the software package PotPulse 8.53 (HEKA Electronic, Ger-
many).
Increased understanding of conduction and redox-cycling pro-
cesses of conducting polymers and oligomers has been achieved
in the last decade by the application of in situ spectroelectro-
chemical techniques. Therefore the present work is focused on the
newly synthesized star-shaped compounds with the central 1,3,5-
trazine ring and their analysis by voltammetry as well as by in situ
UV–vis–NIR and EPR spectroelectrochemistry in order to follow
different products of their electrochemical oxidation.
2.2. Photophysical characterization
The electronic absorbance and fluorescence spectra were
recorded on a Lambda UV/Vis spectrophotometer (Perkin Elmer)
and a Fluoromax 3 spectrophotomether (Jobin Yvon), respectively.
For the purposes of time-resolved fluorescence spectroscopy, the
frequency doubled output (SHG) of a Titan Sapphire Laser (Tsunami
3960; Spectra Physics) was used to excite samples. The lumines-
cence was detected in a right angle configuration to the incoming
2. Experimental
beam.
A multichannel plate (ELD EM1-132/300; Europhoton
GmBh) coupled to a FL920 fluorescence lifetime spectrometer
(Edinburgh Instruments) was used for the signal detection. The
time-resolved emission was collected in the time-correlated sin-
gle photon counting mode. The fluorescence quantum yields were
measured on a PL Quantum Yield measurement system C9920-02
with an integration sphere (Hamamatsu, Japan).
2.1. Oxidative polymerization and spectroelectrochemistry
The cyclic voltammograms were obtained with a PAR 273
potentiostat (EG&G, USA) in a three-electrode system using plat-
inum wires as working and counter electrodes and a silver wire
as pseudo-reference electrode. Ferrocene (Fc) was added as the
internal standard. Dichloromethane (DCM, dried) and acetonitrile
(AN, puriss. absolute) both purchased from Fluka and ferrocene
(p.a., ≥98.0%) purchased from Merck were used as received.
Tetrabutylammonium hexafluorophosphate (TBAPF6, Fluka, dried
under reduced pressure at 340 K for 24 h prior to use) was used
as the supporting electrolyte. Polymer films were prepared by
potentiodynamic polymerization of corresponding monomers on
platinum-wire electrode in dichloromethane (DCM) solutions of
0.001 M monomer and 0.2 M supporting electrolyte by repet-
itive cyclic voltammetry. The so-prepared film was rigorously
washed in a monomer-free electrolyte. The polymerization of
3c was also done potentiostatically at 0.2 V vs. Fc/Fc+ in 0.2 M
TBAPF6/DCM using laminated conducting ITO-coated glass plate
for spectroelectrochemistry of so-prepared polymer p(3c). In situ
ESR(electron spin resonance)/UV–vis–NIR spectroelectrochemical
experiments were performed in the flat ESR cell filled with 0.2 M
TBAPF6/AN. The cell was filled and tightly closed in the glove
box (water and oxygen content below 1 ppm) while the experi-
ments were done in the optical ESR cavity (ER 4104OR, Bruker,
2.3. Synthesis
We designed the molecular structures of a group of monomers
containing triphenyltriazine moieties and elaborated multiple-step
reaction routes for their synthesis (Scheme 1). The cyclization
of 4-bromobenzonitrile was achieved by the addition of trifluo-
romethanesulfonic acid in chloroform, which gave arylbromide 2.
The palladium-catalyzed Stille cross-coupling reaction of deriva-
tive 2 with 2-(tributylstannyl)arylenes gave the desired products
3a–e in yield of 59–82%. All the monomers were characterized by
NMR spectroscopic methods and elemental analysis.
2.3.1. General
All chemicals, reagents, and solvents were used as received
from commercial sources without further purification, except
tetrahydrofuran (THF) and toluene, which were distilled over
sodium/benzophenone. 1H NMR and 13C NMR spectra were
recorded in CDCl3 on a Brucker 300 MHz and 250 MHz spectrome-

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ter. Chemical shifts are denoted in ı unit (ppm) and were referenced
to internal tetramethylsilane (0.0 ppm). The splitting patterns are
designated as follows: s (singlet), d (doublet), t (triplet) and m
(multiplet). Preparative column chromatography was carried out
on glass columns of different sizes packed with silica gel Merck 60
(0.035–0.070 mm).
3. Result and discussion
3.1. Electropolymerization of monomers
Different processes during oxidation of compounds 3a–e at
a scan rate of 0.1 V s−1 were recorded in their dependence on
the terminal arm moieties. Dominant dimerization reactions were
observed for 3b, 3d and 3e oxidation [22]. Only for monomers
3a and 3c the oxidation leads to the electrodeposition of an elec-
troactive material on the electrode surface. The half-peak oxidation
potential of the monomer 3a is Ep/2 = 0.88 V vs. Fc/Fc+ in DCM con-
taining 0.2 M TBAPF6. In the first cyclovoltammetric scan going to
the anodic potentials up to 0.7 V vs. Fc/Fc+ no redox processes were
observed within the potential region from −0.6 V to +0.7 V vs. Fc/Fc+
(black line in Fig. 1a). Going to the region of the first oxidation
peak for 3a new redox processes are observable in the consecu-
tive scans (2nd, 3rd and 4th cyclovoltammetric scan in Fig. 1a) and
a fast adsorption on the working electrode takes place.
2.3.2. Preparation of 2,4,6-tri(p-bromophenyl)-1,3,5-triazine (2)
Trifluoromethanesulfonic acid (6.00 g, 40 mmol) was quickly
added to a vigorously stirred solution of 4-bromobenzonitrile
(3.64 g, 20 mmol) in dry CHCl3 (150 mL) at 0 ◦C. After stirring for 1 h,
the solution was stirred for 24 h at room temperature. Then 150 mL
of water was added and the mixture was stirred for 2 h. The solu-
tion was filtrated and the filtrate was washed twice by water and
cold chloroform. The white microcrystals were dried in exsiccator
(2.9 g, 80%, Mp > 320 ◦C).
The further electropolymerization process made by potential
scanning between –0.8 V and +1.1 V vs. Fc/Fc+ is shown in Fig. 1b
(5th–9th cyclovoltammetric scan). The increase in current with
every potential cycle, resulting from the redox processes of the
2.3.3. General procedure for the preparation of
2,4,6-tri[p-(2-aryl)-phenyl]-1,3,5-triazine (3a–e)
To 2,4,6-tri(p-bromophenyl)-1,3,5-triazine (2) (1.09 g, 2.0 mmol)
dissolved in 250 mL of anhydrous toluene under a nitrogen atmo-
sphere was added a solution of 2-(tributylstannyl)aryl (6.6 mmol)
and Pd(PPh3)4 (0.460 g, 0.4 mmol) in 150 mL of toluene. The result-
ing mixture was stirred for 4 days at 120 ◦C. After this time, the
mixture was cooled down to room temperature. Water was added
and the resulting solution was extracted three times with 50 mL
portions of CHCl3. The combined organic layers were washed with
50 mL of brine, dried over MgSO4 and evaporated to dark brown oil.
The crude product was purified by column chromatography (silica
gel, hexane/AcOEt, 10:1).
2,4,6-Tri[p-(2-thienyl)-phenyl]-1,3,5-triazine (3a). Light yellow
solid (Mp = 309–310 ◦C, 80%). 1H NMR (300 MHz, CDCl3) ı = 8.71 (d,
J = 8.4 Hz, 6H); 7.77 (d, J = 8.4 Hz, 6H); 7.46 (dd, J = 3.6, 0.9 Hz, 3H);
7.36 (dd, J = 5.0, 0.8 Hz, 3H); 7.14–7.11 (m, 3H). 13C NMR (300 MHz,
CDCl3) ı = 171.0; 143.6; 138.2; 135.1; 129.6; 128.3; 126.0; 125.8;
124.2. Elemental analysis for: C33H21N3S3 Calc.: C, 71.32; H, 3.81;
N, 7.56. Found: C, 71.47; H, 3.61; N, 7.68.
2,4,6-Tri[p-(2-furyl)-phenyl]-1,3,5-triazine (3b). Deep yellow
solid (Mp = 208–209 ◦C, 76%). 1H NMR (250 MHz, CDCl3) ı = 8.80 (d,
J = 8.4 Hz, 6H); 7.88 (d, J = 8.4 Hz, 6H); 7.58 (d, J = 3.3 Hz, 3H); 6.85
(d, J = 3.3 Hz, 3H); 6.56–6.54 (m, 3H). 13C NMR (250 MHz, CDCl3)
ı = 171.0; 153.5; 143.0; 135.0; 134.4; 129.4; 123.8; 112.0; 107.0.
Elemental analysis for: C33H21N3O3 Calc.: C, 78.09; H, 4.17; N, 8.28.
Found: C, 78.41; H, 4.33; N, 8.11.
2,4,6-Tri[p-(2-(3,4-ethylenedioxythienyl))-phenyl]-1,3,5-triazine
(3c). Braun solid (Mp = 156–157 ◦C, 82%). 1H NMR (250 MHz,
CDCl3) ı = 8.71 (d, J = 8.3 Hz, 6H); 7.90 (d, J = 8.4 Hz, 6H); 6.39 (s,
3H); 4.39–4.28 (m, 12H). 13C NMR (250 MHz, CDCl3) ı = 170.9;
142.4; 139.1; 137.1; 134.2; 129.2; 125.6; 117.0; 99.0; 64.8; 64.4.
Elemental analysis for: C39H27N3O6S3 Calc.: C, 64.18; H, 3.73; N,
5.76. Found: C, 64.31; H, 3.60; N, 5.98.
2,4,6-Tri[p-(2-thiazolyl)-phenyl]-1,3,5-triazine (3d). Yellow solid
(Mp = 282–284 ◦C, 65%). 1H NMR (250 MHz, CDCl3) ı = 8.80 (d,
J = 8.4 Hz, 6H); 8.16 (d, J = 8.5 Hz, 6H); 7.96 (d, J = 3.2 Hz, 3H); 7.42
(d, J = 3.2 Hz, 3H). 13C NMR (250 MHz, CDCl3) ı = 171.2; 167.5;
144.2; 137.4; 137.3; 129.6; 126.8; 119.5. Elemental analysis for:
C30H18N6S3 Calc.: C, 64.49; H, 3.25; N, 15.04. Found: C, 64.17; H,
3.62; N, 15.32.
Fig. 1. Potentiodynamic polymerization of 3a in 0.2 M TBAPF6/DCM, scan rate
0.1 V s−1. (a) Black line 1 – first cyclovoltammetric scan going to 0.7 V as reference,
red line 2 – 3a oxidation starting at −0.4 V, green line 3 – third CV scan in the same
potential region, blue line 4 – fourth CV scan starting at 0.1 V and going both to the
anodic and cathodic region. (b) Further potentiodynamic polymerization from 5th
to 9th CV scan at starting potential of 0.1 V (all potentials are referred vs. Fc/Fc+,
platinum-wire working electrode). (For interpretation of the references to colour in
this figure legend, the reader is referred to the web version of this article.)
2,4,6-Tri[p-(2-oxazolyl)-phenyl]-1,3,5-triazine (3e). Green solid
(Mp > 310 ◦C, 59%). 1H NMR (250 MHz, CDCl3) ı = 8.87 (d, J = 8.5 Hz,
3H); 8.68 (d, J = 8.3 Hz, 6H); 8.25 (d, J = 8.5 Hz, 3H); 7.39 (d, J = 8.2 Hz,
6H). 13C NMR (250 MHz, CDCl3) ı = 171.7; 161.6; 148.1; 139.1;
138.2; 133.7; 129.3; 126.5. Elemental analysis for: C30H18N6O3
Calc.: C, 70.58; H, 3.55; N, 16.46. Found: C, 70.37; H, 3.40; N, 16.72.

K.R. Idzik et al. / Electrochimica Acta 55 (2010) 4858–4864
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Fig. 3. Cyclic voltammograms obtained during potentiodynamic polymerization of
3c in 0.2 M TBAPF6/DCM at scan rate of 0.1 V s−1 in the potential region from −1.6 V
to 0.9 V vs. Fc/Fc+ using starting potential −0.6 V for each CV scan (line numbers
represent the corresponding CV scan; first two CV scans were performed in broader
potential region; platinum-wire working electrode).
in DCM containing 0.2 M TBAPF6 (Fig. 2a). The polymerization made
by potential scanning in this potential region leads to a fast adsorp-
tion on the working electrode resulting in a smooth film. In the next
step, after washing in free monomer electrolyte, the film was redox
cycled in 0.2 M TBAPF6 in acetonitrile. During this pre-treatment
process the rest monomers and short chains within the film are oxi-
dized, which leads to further cross-linking within the film (Fig. 2b).
The film prepared in such manner exhibits completely reproducible
redox behaviour in 0.2 M TBAPF6/acetonitrile electrolyte similar
to the recently electrochemically synthesized polymers contain-
ing star-shaped molecules based on a triazine core substituted
with short chain oligotiophene [29]. We observed two well-defined
voltammetric main peaks with E1/2 = −0.35 V and E1/2 = 0.10 V vs.
Fc/Fc+ with the peak separation of 0.02 V indicating well-defined
adsorbed film and a reversible oxidation. The phenomenon we
observed gives evidence for good stability and high conductivity
of the film. It was recently confirmed that electropolymerization of
star-shaped oligomers containing oligothiophene arms resulted in
the formation of highly redox active, cross-linked hyperbranched
Fig. 2. (a) Cyclic voltammograms obtained during potentiodynamic polymerization
of 3c in 0.2 M TBAPF6/DCM at scan rate of 0.1 V s−1 in the potential region from −1.6 V
to 0.25 V vs. Fc/Fc+ using starting potential −0.6 V for each CV scan (line numbers
represent the corresponding CV scan). (b) CV trace of p(3c) in monomer-free 0.2 M
TBAPF6/AN electrolyte at scan rate of 0.1 V s−1 (platinum-wire working electrode, 6
consecutive CV scans).
film, indicates a continual growth of conducting material on the
electrode. Generally, very complex redox behaviour was observed
both during electropolymerization of 3a and for prepared polymer
film on the electrode. This indicates a wide variety of oxidation
products. Different chain lengths, regions with various degrees of
crystallinity as well as the presence of dimers are expected. Better
and less ordered polymer structures of different crystallinity have
been already reported for other polythiophene derivatives [23–26].
The presence of the segments of different effective conjugations
also causes complexity of such electrochemically formed film [27].
Additionally, concerning the occurrence of new cathodic peak in the
back scan with reduction potential strongly shifted to the negative
potentials in comparison to the oxidation peak of 3a (peak marked
with asterisk in Fig. 1b) we propose the presence of relatively stable
short dimeric dicationic structures (sigma-dimers) within the film
(for a review on self-association of radical ions see [28]). Concern-
ing all observations and processes described above we can conclude
that polymer film p(3a) prepared electrochemically represents a
complex heterogeneous system.
In contrast to 3a the oxidation potential of the monomer 3c is
much lower (half-peak potential Ep/2 = 0.11 V vs. Fc/Fc+) and going
up to 0.25 V vs. Fc/Fc+ only two new well-defined reversible redox
couples arise at −0.35 V and 0.10 V vs. Fc/Fc+ upon repetitive cycling
Fig. 4. UV–vis–NIR spectra measured in situ during the cyclic voltammetry of
p(3c) deposited on ITO electrode in 0.2 M TBAPF6/AN (scan rate v= 0.002 V s−1) in
the potential region from −0.8 V to +0.2 V vs. Fc/Fc+. 21 UV–vis–NIR spectra were
recorded during cyclovoltammetric scan.

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K.R. Idzik et al. / Electrochimica Acta 55 (2010) 4858–4864
Fig. 5. (a) Spectroelectrochemical cell (1 – p(3c) deposited on ITO electrode, 2 – laminated ITO plate with a gold foil contact, 3 – Ag wire reference electrode, 4 – ESR flat
cell). (b) Chronoamperometric ESR/UV–vis–NIR spectroelectrochemistry of p(3c) film on ITO electrode recorded in 0.2 M TBAPF6/AN before (black lines) and after switching
the potential (coloured lines). Insets in (b)–(f): corresponding ESR spectra measured after potential switch (experimental curves are marked with the corresponding applied
potential in volts vs. Fc/Fc+). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
polymers, which led to systems with good conductivities, indicat-
ing the presence of multiple pathways for charge carriers provided
by the branching units [30].
characterization in solution similar to other triazine–thiophene
conjugated systems [29].
Similar to 3a complex redox behaviour was also indicated for 3c
by cycling in the large potential region from −1.6 V to 0.9 V vs. Fc/Fc+
(Fig. 3) with a variety of redox processes. This indicates the forma-
tion of complex reaction mixture with cross-linked hyperbranched
structures, dimeric forms and multistep redox processes as will
be discussed below. It should be noted that very low solubility
of such highly cross-linked networks hindered detailed structural
3.2. Spectroelectrochemistry of polymer p(3c)
The redox behaviour of polymer film p(3c) prepared by the elec-
trochemical polymerization of 3c in DCM solutions was studied
by triple in situ ESR/UV–vis–NIR spectroelectrochemistry to get
more details on the type of charge carriers within the film. Opti-
cally transparent indium tin oxide (ITO)-coated glass was used as

K.R. Idzik et al. / Electrochimica Acta 55 (2010) 4858–4864
4863
electrode material. The UV–vis–NIR spectra taken simultaneously
during cyclovoltammetry of p(3c) film in acetonitrile (AN) contain-
ing 0.2 M TBAPF6 on ITO working electrode (scan rate 0.002 V s−1
are shown in Fig. 4.
)
Absorbance maximum of neutral p(3c)0 film was found at
520 nm confirming a high pi-conjugation length. Two polaronic-
like transitions at 795 nm and 1300 nm are observed with a slight
shift to higher energies by increasing potential. During further oxi-
dation still at first oxidation peak several optical transitions are
observed and they shift to higher energy up to stable 685 nm and
1137 nm. Concerning the literature data and the shape of the first
oxidation peak measured for the investigated polymer film p(3c)
we propose that the first voltammetric peak consists of two con-
secutive one-electron transfers. It was generally observed with long
chain oligomers and other extended pi-systems that the simulta-
neous transfer of two electrons take place upon oxidation [31]. At
a very low oxidation state the di-ionic spinless structures of the
polymer chains are dominant. Simultaneously a small amount of
polaronic states p(3c)1+ (ESR active states) is formed due to a com-
proportionation reaction with the neighbouring neutral segments.
Formally this corresponds to a successive two-electron transfer
with the first and second formal oxidation potentials in inverted
order. Consequently at first oxidation peak the increase of ESR
intensity is observed at the beginning of oxidation which then
decreases upon further doping. By increasing the electrode poten-
tial to the value slightly behind the first peak a slight decrease of ESR
intensity was observed confirming ESR silent character of dications
p(3c)2+ [22].
At more anodic potentials in the region of the second voltam-
metric peak a new well-defined optical band at 940 nm appears
upon oxidation accompanied with further small peaks at 1400 nm
and new absorption above 1600 nm. This band is attributed to
the ESR active trication p(3c)3+ as rapid increase of ESR intensity
was observed at this stage of oxidation. By increasing the elec-
trode potential further to the higher potentials (0.7 V vs. Fc/Fc+)
new strong optical energy transition was observed at 815 nm (see
Fig. 5d). Simultaneously the ESR intensity decreases going from the
third to the fourth oxidation step confirming the formation of ESR
silent tetracation p(3c)4+. This assignment was further supported
by additional chronoamperometric ESR/UV–vis–NIR spectroelec-
trochemistry of p(3c) film on ITO electrode (Fig. 5) recorded in
0.2 M TBAPF6/AN before and after switching from the certain poten-
tial to another one. Potential step chronoamperometry, switching
between the various charged states, accompanied with ESR and
UV–vis–NIR spectroscopy gives unambiguous attribution of exper-
imental spectra to each redox state. ESR and UV–vis–NIR spectra
were measured immediately after potential change to the fixed
value. Fig. 5b shows experiment with potential step from −0.8 V
(black line in UV–vis–NIR spectra) to –0.3 V vs. Fc/Fc+ (see four
coloured UV–vis–NIR spectra immediately measured after poten-
tial change and corresponding ESR spectrum in Fig. 5b). Rapid
and reversible changes were observed in a sub-second range after
potential change confirming high redox stability and reversibility
of the polymer upon anodic charging. Similar behaviour was found
for switching between all redox states (Fig. 5b–f). A maximum ESR
intensity was found for the tricationic state (Fig. 5c) while mini-
mum ESR intensity was observed for neutral state and tetracation
(Fig. 5d,f). These experiments confirm reversible redox behaviour
of polymer p(3c) upon oxidation switching between all redox states
even going up to tetracation.
Fig. 6. Electronic absorbance, fluorescence excitation and emission spectra of
5 × 10−6 M solution of 3a (TTT) in dichloromethane.
excitation and emission accompanied with quantum yield and flu-
orescence lifetime have been separately determined in CH2Cl2 for
each compound. Electronic absorbance, fluorescence excitation and
emission spectra of 5 × 10−6 M solution of 3a in CH2Cl2 are pre-
sented in Fig. 6 and 3b in Fig. 7. The electronic absorbance spectrum
of compound 3a exhibits two well-separated bands responsible for
two electronically active transitions to the excited states. The first
sharp band peaks at 265 nm as being consequence of ␲–␲* transi-
tion and the second structureless responsible for n–␲* transitions
has maximum at 345 nm. Noticeable these compounds have high
molar extinction coefficients indicating that the light is strongly
absorbed at the excitation wavelength, which is desirable for opto-
electronic application. As typically observed for free molecules in
solution, the fluorescence excitation has the same shape and rel-
ative ratio between peaks as absorbance spectrum. Consequently,
excitation of the molecule at 340 nm leads to the emission of fluo-
rescence having the half width of 70 nm (determined by the fitting
to the Lorentz function) in the range from 395 nm to 465 nm peak-
ing at 425 nm (green light). High quantum yield being in the range
from 0.36 to 0.42 depending on a derivative is another advantage
supporting the idea of application these materials in optoelectronic.
3.3. Photophysical properties of triphenyltriazine derivatives
In order to establish the photophysical properties of the
newly synthesized triphenyltriazines, electronic absorbance, molar
extinction coefficient at the excitation wavelength, fluorescence
Fig. 7. Electronic absorbance, fluorescence excitation and emission spectra of
5 × 10−6 M solution of 3b (TFT) in dichloromethane.

4864
K.R. Idzik et al. / Electrochimica Acta 55 (2010) 4858–4864
Table 1
as an electron-transporting central core, thought separated from
the thiophene, furane, or EDOT groups by the p-phenylene spac-
ers, can display excellent redox stability. Anodic oxidation of
monomers 3a and 3c leads to the electrodeposition of an elec-
troactive material on the electrode surface. Oligomers containing
EDOT group provide the final polymer film with well-defined and
reversible redox behaviour in contrast to the oligomers based on
thiophene and furane. Spectroelectrochemical experiments con-
firm reversible redox behaviour of polymer p(3c) upon oxidation
switching between all redox states even going up to tetracation. The
electrochemical behavior of the oligothiophene–triphenyltrazines
(3a) showed greater deviations from those of the corresponding
linear oligothiophenes than expected at first instance.
Maxima of electronic absorbance, fluorescence excitation and emission spectra
accompanied with molar extinction coefficient at the excitation wavelength, quan-
tum yield and fluorescence lifetime of all triphenyltriazine derivatives studied by
steady-state and time-resolved fluorescence spectroscopy.
Compound
ꢀabs (nm)
ε (L mol−1 cm−1
)
ꢀem (nm)
ꢁf
ꢂf (ns)
3a
3b
3c
265, 340
265, 345
265, 365
26,780
32,930
38,460
415
425
450
0.36
0.38
0.42
1.65
2.29
2.33
Acknowledgements
P.R. thanks Alexander von Humboldt Foundation for financial
support (Project 3 Fokoop DEU/1063827 with funds from the Fed-
eral Ministry for Education and Research). The support of SGA VEGA
by the Slovak Scientific Grant Agency (Project No. 1/0018/09) to
P.R. is also acknowledged. K.R.I. thanks the European Union (SNIB,
MTKD-CT-2005-029554) for the financial support.
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derivatives have been obtained in good yields. Triazine moiety