Electropolymerization of triphenylamine-dithiafulvene hybrid extended pi-conjugated systems

Article abstract of DOI:10.1039/b819675c

Extended hybrid conjugated systems based on a triphenylamine core with 1, 2 and 3 peripheral dithiafulvenyl units have been studied by cyclic voltammetry and UV-Vis absorption spectroscopy. The substitution pattern of the triphenylamine core determines tw

Full text of DOI:10.1039/b819675c

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PAPER
www.rsc.org/njc | New Journal of Chemistry
Electropolymerization of triphenylamine–dithiafulvene hybrid extended
pi-conjugated systemsw
Nicolas Cocherel, Philippe Leriche,* Emilie Ripaud, Nuria Gallego-Planas,
Pierre Frere and Jean Roncali
Received (in Montpellier, France) 4th November 2008, Accepted 17th December 2008
First published as an Advance Article on the web 22nd January 2009
DOI: 10.1039/b819675c
Extended hybrid conjugated systems based on a triphenylamine core with 1, 2 and 3 peripheral
dithiafulvenyl units have been studied by cyclic voltammetry and UV-Vis absorption spectroscopy.
The substitution pattern of the triphenylamine core determines two different electrochemical
coupling mechanisms leading to two types of electrogenerated extended conjugated electroactive
systems. The structure and properties of polymeric species are also presented.
Electrochemical and spectroscopic experiments on compounds
and corresponding polymers as well as theoretical calculations
provide a coherent picture of the behavior of these derivatives
Introduction
Triphenylamine (TPA)-based compounds have been widely
used as hole injection layers or as active materials in
upon oxidation.
light-emitting diodes.1 Recently, hybrid star-shaped systems
involving a TPA core derivatized with linear short-chain
conjugated systems have been reported, as well as their use
as active materials in organic field-effect transistors (OFET)
and organic solar cells.1–4 Thus, hole mobilities of up to
1 Â 10À2 cm2 VÀ1 sÀ1 have been obtained on OFET based
on TPA derivatized with oligothiophenes.2 Furthermore, it
has been shown that the creation of an internal charge transfer
by fixation of electron-withdrawing groups on TPA-based
conjugated systems leads to organic solar cells presenting
extended photoresponse and improved open-circuit voltage
efficiency and stability.3,4
Results and discussion
The synthesis of compounds 3 and 4 is described elsewhere.6
Compounds 1 and 2 have been synthesized by condensation of
corresponding 2-thioxo-1,3-dithioles onto the mono- and
dialdehyde of the TPA in the presence of triethylphosphite
(Scheme 1).7
The electrochemical and optical data of compounds 1–4 are
gathered in Table 1. All compounds present a large p–p*
absorbance band with a lmax that depends both on the number
of dithiafulvene groups and on the nature of the substituents
grafted on the dithiole cycle. The introduction of one, two
and three dithiafulvenyl moieties in the structure induces a
bathochromic shift of lmax from 378 to 408 nm. On the
other hand, the data for the fully substituted compounds
3 and 4 show that the position of lmax also depends on the
electron-donating ability of the dithiafulvenyl group, the
stronger donor alkylsulfanyl substituent in 3 leading to a slight
red shift compared to the benzo-substituted compound 4.
The cyclic voltammogram of all compounds exhibits two
On the other hand, recent years have witnessed a strong
renewal of interest for tetrathiafulvalene (TTF) derivatives as
active materials for OFET, and high hole mobility values have
been reported.5 In this context, we have recently undertaken
the development of a new series of extended conjugated
systems associating the TPA and TTF building blocks in
view of reaching new materials combining the isotropic
charge transport of TPA materials and the well known
propensity of TTF derivatives to develop strong intermolecular
interactions.6
oxidation processes with anodic peak potentials Epa1 and Epa
2
As such compounds may undergo polymerization upon
oxidation, their use as precursors for electrode material or
for electrochromics is also envisaged. Therefore, in the
continuation of our current interest in hybrid systems
associating TPA and TTF moieties, we report here on the
synthesis and properties of four TPA derivatives 1–4 grafted
with one to three dithiolylidene groups (Scheme 1). Upon
oxidation, these derivatives undergo a polymerization that
may imply two different electrochemical coupling mechanisms.
around 0.5–0.6 and 1.1–1.2 V (Table 1). In contrast with
optical data, comparison of the data for compounds 1a, 2
and 3 shows that Epa1 depends only slightly on the number of
dithiafulvenyl groups. Thus derivatives 1a, 2 and 3 present
very close oxidation potentials. On the other hand, comparison
of the data for compounds 1a and 1b or 3 and 4 shows that the
nature of the substituents grafted on the dithiafulvenyl group
significantly affects Epa1; the electron-withdrawing cyanoethyl
or benzo groups leading to positive shifts. These results
suggest that the first oxidation process essentially involves
the dithiafulvenyl units.
´
Department of Chemistry, Universite d’Angers, CNRS, CIMA,
2 Boulevard Lavoisier, 49045 Angers, France.
E-mail: Philippe.leriche@univ-angers.fr
w Electronic supplementary information (ESI) available: UV-vis
spectra of 1, 2 and 3; CV of derivative 4 and CV trace of poly(4).
See DOI: 10.1039/b819675c
This assumption is corroborated by ab initio quantum
calculations performed on compound 1a with the Gaussian
03 package8 of programs at a hybrid density functional theory
(DFT) level. Calculations have been carried out using the
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Scheme 1
Table 1 Optical and electrochemical properties of compounds 1–4
and of corresponding polymers
a
1b
Epa
2b
Epa
c
c
1c
2c
lmax
lmax
Poly
lmax
Eox
Poly
Eox
Poly
Polyox
1a
1b
2
3
4
378
377
399
408
398
0.53
0.59
0.53
0.51
0.63
1.12
1.09
1.18
1.18
1.01
—
—
376
386
401
—
—
353–644
345–640
360–489
—
—
0.44
0.44
0.52
—
—
1.00
1.17
1.20
a
b
10À5 M in CH2Cl2. Oxidation peak vs. SCE measured from cyclic
voltammetry, o10À3 M of compounds with 0.1 M Bu4NPF6 as
supporting electrolyte, v = 100 mV sÀ1
with 0.1 M Bu4NPF6 as supporting electrolyte.
.
Deposited on Pt electrode
c
U-B3LYP/6-31G* level for the cation radical states. The
SOMO of the radical cation is at À7.80 eV. In the radical
cation, the phenyl and dithiol rings are coplanar and the
SOMO is essentially located on the arm bearing the dithiafulvene
moiety. Coefficients are negligible on the terminal position of
phenyl rings while high coefficients are observed on the
carbons of the vinyl linkage (Fig. 1).
Fig. 2 CV of compound 1a in 0.10 M Bu4NPF6–CH2Cl2, scan rate
100 mV sÀ1
.
complex adsorption phenomena during its oxidation the
interpretation of its CV curve appears difficult.
This observation, in addition to calculation results, brings
The CV of compound 1a presents two successive
one-electron oxidation processes (Fig. 2). The first process is
irreversible and a blue coloration appears in solution around
the electrode during oxidation. Such a behavior may be
attributed to an electrochemical coupling. As 1b suffers from
further support to the hypothesis of an intermolecular
coupling of 1+ at the vinyl linkage between phenyl and
ꢁ
dithiole rings position.9–11
This supposition is confirmed by chemical experiments:
compound 1b has been chemically oxidized using DDQ and
HBF4.12,5b After reduction with zinc, compound 5, which
results from a coupling at the vinylic linkage was isolated in
60% yield together with a large amount of starting material
(Scheme 2). Note that derivatives 1b and 5, which both possess
cyanoethylsulfanyl groups, could be functionalized for further
applications.13 We can then assume that, upon oxidation of
derivatives 1, the vinyl linkage is favored compared to the
phenyl linkage.9–11
The CV of compound 5 exhibits two two-electron oxidation
steps with Epa1 and Epa2 at 0.61 and 1.14 V (Fig. 3), very close
to that of 1b. Indeed, dimerization on encombered dithiafulvene
moiety is known to lead to sterically constrained extended
TTF analogs with oxidation potentials rather similar to that of
Fig. 1 Calculated SOMO of 1+
.
ꢁ
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Fig. 4 Potentiodynamic electropolymerization of 2 (left) and 3 (right)
in 0.10 M Bu4NPF6–CH2Cl2, scan rate 100 mV sÀ1
.
followed by a non-reversible one-electron oxidation process.
For these two compounds 3 and 4, a quite different behavior is
observed when compared to that of 2. Indeed, cycling up to
the first oxidation step already triggers the electrodeposition
process (Fig. 4).
These results suggest that electrooxidation of compound 2
leads to a stable dicationic species and that it is therefore
necessary to form the trication radical at the second oxidation
to produce a reactive species able to electropolymerize. In
contrast, in the case of compounds 3 and 4, the reactive
tris-cation radical is directly generated at the first three-
electron oxidation step.
Scheme 2
Electrooxidation of compound 2 can lead to two different
reactions pathways yielding two different coupling products
linked either at the para-position of the phenyl ring or at the
dithiafulvenyl linkage similarly to compound 1b and to
related compounds previously investigated.10,14 In contrast,
for compounds 3 and 4, only the second reaction pathway is
possible.
Fig. 3 CV of compound 5 in 0.1 M Bu4NPF6–CH2Cl2 (dotted line)
Fig. 5 shows the CVs of the materials electrodeposited by
electrooxidation of compounds 2–4. These three CVs are very
and CV of the electrodeposited material on Pt in 0.1
Bu4NPF6–CH3CN, scan rate 100 mV sÀ1
M
.
1
similar and exhibit a first intense redox system with Epa at
the starting material.10,11 As the first oxidation process is
reversible, application of repetitive scans with a positive limit
set at Epa2 results in the deposition of an electroactive material
onto the electrode surface.
ca. 0.45 V and a less intense wave with Epa2 in the 1.00–1.15 V
region. The two redox processes present relative intensities of
2 : 1 for poly(2) and 3 : 1 for poly(3) and poly(4), respectively,
which corresponds to the relative quantities of dithiafulvenyl
and TPA moieties in the compounds. For the three polymers,
Epa2 is observed at a higher potential than for poly(5), which is
consistent with a shorter effective conjugation due to the
absence of phenyl–phenyl coupling. The less positive values
The CV of the resulting electrodeposited material exhibits
1
2
two reversible oxidation processes with Epa and Epa at 0.52
and 0.94 V, respectively (Fig. 3). These anodic peaks are
shifted toward negative potentials by 90 and 200 mV
compared to the starting compound 5, which is consistent
with an extension of the conjugated system resulting from
coupling on the terminal position of the phenyl rings.9 These
results can thus doubtless be interpreted by the electrodeposition
of a polymeric system resulting from the coupling on terminal
positions of phenyl rings.14
1
2
of Epa and Epa for poly(3) compared to poly(4) can be
attributed to the more electron-releasing effect of the
methylsulfanyl groups compared to the fused phenyl rings.
The high similarity of the three CVs, their difference with
that of poly(5) and the impossibility for compounds 3 and 4 to
undergo phenyl–phenyl coupling strongly support the conclu-
sion that electrooxidation of the three compound 2, 3 and 4
leads to a unique type of electrochemical coupling involving
the vinyl linkage of the dithiafulvenyl unit, as already
demonstrated for compound 1b and extended TTF derivatives.10
Films of compounds 2, 3 and 4 have been electropolymerized
on ITO-coated glass electrodes. The difficult polymerization
of 5 did not allow its electrodeposition on ITO. All electro-
deposited materials exhibit a stable CV with no evidence of
The CV of compound
2 exhibits a first reversible
two-electron oxidation wave peaking at 0.50 V, leading to a
stable dication followed by an irreversible wave at 1.20 V.
Repetitive cycling up to the first oxidation wave leaves the CV
unchanged, whereas cycling up and over the second wave leads
to the development of growing redox system typical of an
electrodeposition process (Fig. 4). For compounds 3 and 4, the
CV shows a first non-reversible three-electron oxidation wave
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Fig. 6 Spectroelectrochemistry of a film of poly(3) on ITO electrode
in 0.10 M Bu4NPF6–CH3CN at various applied potentials. Solid line
(neutral state at 0 V); short dash (fully oxidized state at 0.8 V); dotted
lines (intermediate states).
in their oxidized form and appear blue (Fig. 6). The oxidized
form of poly(4) presents a maximum at 489 nm, this blue shift
compared to the two other polymers can be attributed to its
lower donor ability.
Conclusions
Four new hybrid conjugated systems combining a triphenyl-
amine core and one to three dithiafulvenyl branches have been
synthesized. Electrooxidation of these compounds leads to
dimerization or polymerization products. Electrochemical
results together with theoretical calculations provide consistent
results indicating that electrochemical coupling occurs at the
vinylic linkage. Polymeric films of 2–4 present well defined
redox processes and electrochromic properties.
Experimental
General
Solvents were purified and dried using standard protocols.
1H NMR and 13C NMR spectra were recorded on a Bruker
AVANCE DRX 500 spectrometer operating at 500.13 and
125.7 MHz; d are given in ppm (relative to TMS) and coupling
constants (J) in Hz. Matrix-assisted laser desorption
ionization time-of-flight (MALDI-TOF) mass spectra were
recorded using a Bruker Biflex-III, equipped with a N2 laser
(337 nm). For the matrix, dithranol in CH2Cl2 was used. High
resolution mass spectra were recorded under FAB mode on a
Jeol JMS 700 spectrometer. UV-visible optical data were
recorded with a Perkin-Elmer lambda 19 spectrophotometer.
Thermal analyses were performed using a DSC 2010 CE
(TA Instruments). For cyclic voltammetry (scan rate
100 mV cmÀ1), the electrochemical apparatus consisted of a
potentiostat EG&G PAR 273A and a standard three-electrode
cell. As the working and counter electrodes, a platinum foil
and a platinum wire were used, respectively, while as a
reference a Ag/AgCl electrode served as reference. Electro-
chemical experiments were carried out in methylene chloride
or acetonitrile (spectroscopic grade) containing 0.10 M
tetrabutylammonium hexafluorophosphate (Fluka puriss,
Fig. 5 CV of poly(2) (top), poly(3) (middle) and poly(4) (bottom) in
0.10 M Bu4NPF6–CH3CN, scan rate 100 mV sÀ1
.
degradation after repetitive cycling on the first redox process
for several hundred cycles or after prolonged storage in air in
neutral or oxidized state. However, electrooxidation up to the
second oxidation process leads to a steady decrease of the film
electroactivity.
As shown in Table 1, the neutral polymer films show
absorption maxima at slightly shorter wavelengths than their
precursor molecules, suggesting that the effective conjugation
in the polymers is limited by distortions caused by steric
interactions.
The three compounds lead to homogeneous polymer films
that are yellow in their neutral state and present electrochromic
properties.15 Poly(2) and poly(3), in which the terminal
dithiole rings are substituted with electron-donating methyl-
sulfanyl groups, present two maxima around 350 and 640 nm
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used as received). Spectroelectrochemical experiments were
carried out with a home-made cell on polymers grown on ITO.
Note that experimental procedures for 3 and 4 have been
published elsewhere.6 Due to low solubility in organic solvents
and to low stability in acidic media, 13C NMR of compounds
appeared difficult and was not undertaken.
contains a large amount of unreacted 1b. The precipitate is
dissolved in acetonitrile, leading to a blue solution, 1 mL of
acetic acid, an excess of zinc is added and the mixture is stirred
for 45 minutes at room temperature. After filtration of zinc
and evaporation of the solvent, pure derivative 5 (30 mg, 60%
1
yield) is isolated as a yellow glassy solid. NMR H (CDCl3):
7.25 (t, 8H, J = 7.5Hz), 7.2 (d, 4H, 3J = 8.5Hz), 7.10 (d, 8H,
J = 8Hz), 7.04 (t, 4H, J = 7.5Hz), 6.97 (d, 4H, 3J = 8.5Hz),
Syntheses
3
3
3.06 (8H, q, J = 7 Hz), 2.76 (4H + 4H, 2 Â q, J = 7Hz).
HRMS: M+ (calcd) for C56H44N6NaS8: (1079.1291),
1079.1885.
4-[((4,5-Bis(methylthio)-1,3-dithiol-2-ylidene)methyl) phenyl]-
diphenylamine 1a. 178 mg (0.64 mmol) of 4-(diphenylamino)-
benzaldehyde and 248 mg (2 eq.) of 4,5-(dimethylthio)-2-thioxo-1,
3-dithiole are dissolved in 2 mL of boiling toluene, then 2 mL
of triethylphosphite are added. The mixture is refluxed for 5 h
under a nitrogen atmosphere. After cooling and addition of
methylenechloride, the mixture is washed with brine and dried
on magnesium sulfate. The residue is then chromatographed
twice (EP–CH2Cl2 5 : 1 and then EP–CH2Cl2 1 : 1) leading to a
yellow oily solid (80 mg, 28%). Decomp. E 45 1C. Rf = 0.81
(EP–CH2Cl2 1 : 1). NMR 1H (CDCl3): 7.27 (m, 4H), 7.10
(m, 6H), 7.04 (m, 4H), 6.42 (s, 1H), 2.44 (s, 3H, SCH3),
2.41(s, 3H, SCH3). MS (Malditof) C24H21NS4: ms M+:
451.1, theor.: 451.1. HRMS M+ (calcd) (451.0557) 451.0538.
Acknowledgements
Authors thank the SCAS of Angers for analytical experiments,
in particular, Dr J. Delaunay and Dr S. Fournier for MS.
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This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2009
806 | New J. Chem., 2009, 33, 801–806