Electrogenerated conductive polymers from triphenylamine end-capped dendrimers

Article abstract of DOI:10.1021/ma401085q

Electroactive end-capped dendritic macromolecules were designed and synthesized. Their structures contain triphenylamine moieties as part of the core or dendrons. The electrogenerated films produced with these monomers behaved as conductive dendritic poly

Full text of DOI:10.1021/ma401085q

Article
pubs.acs.org/Macromolecules
Electrogenerated Conductive Polymers from Triphenylamine End-
Capped Dendrimers
María I. Mangione and Rolando A. Spanevello*
́
Instituto de Química Rosario, Facultad de Ciencias Bioquímicas y Farmaceuticas, Universidad Nacional de Rosario−CONICET,
Suipacha 531, S2002RLK Rosario, Argentina
Angel Rumbero
́ ́
Departamento de Química Organica, Facultad de Ciencias, Universidad Autonoma de Madrid, Cantoblanco, 28049 Madrid, Spain
Daniel Heredia, Gabriela Marzari, Luciana Fernandez, Luis Otero, and Fernando Fungo*
Departamento de Química, Universidad Nacional de Río Cuarto, Agencia Postal 3 (X5804BYA), Río Cuarto, Argentina
S
* Supporting Information
ABSTRACT: Electroactive end-capped dendritic macromolecules
were designed and synthesized. Their structures contain triphenyl-
amine moieties as part of the core or dendrons. The electro-
generated films produced with these monomers behaved as
conductive dendritic polymers that can be reversible charged, both
in the core and in the peripheral units. The design of dendrimer
structures with the introduction of systematic changes allows to
establish relationships between their electro-optical properties with
molecular structural parameters. The films of polymeric material
hold good electrical conductivity, reversible electrochemical
processes and chemical stability. The results indicate that the use
of conjugated and rigid structures in dendritic macromolecules is
an important factor in order to obtain electropolymeric films. This work provides a model to design starburst dendrimers capable
to form electrochemically active polymers with potential applications in electronic and optoelectronic devices.
One of the materials used as hole transporting layer with
more recent development are dendrimers. These molecules,
with highly branched three-dimensional architecture, present
multiple chains emanating from a single core.5,6 Dendrimers are
monodispersed macromolecules possessing well-defined struc-
tures, that can be precisely tailored with discrete functionalities
to create multifunctional materials. Their unique structures and
properties make these macromolecules suitable subjects for a
wide range of biomedical and industrial applications, such as
drug delivery,7 multivalent bioconjugate sand multivalent
diagnostics for magnetic resonance imaging (MRI),8 extremely
efficient light-harvesting antenna,9 and homogeneous cataly-
sis.10 Recently, dendrimers have also found promising
applications as materials for organic electronic devices, such
as photoactive and electroactive materials in OLEDs,11−16 solar
cells,17−19 and transistors.20,21 Reported advantages of some
starburst molecules are the good charge transporting properties,
and low energy barrier for hole injection to the anode with
INTRODUCTION
■
The development of organic materials with application in
molecular organic electronics, nanotechnology, and optoelec-
tronic devices (like organic solar cells and organic light emitting
diodes, OLEDs) have shown a remarkable scientific activity due
to the relevance of these technologies in the generation and
rational use of energy.1−3 Optoelectronic devices are optical-to-
electrical transducers that can produce photoinduced charge
separated states, and/or light by charge recombination. Using
organic materials as optoelectronic devices is a challenging
target, because these materials must be designed in order to
include efficient light absorption-emission capability, correct
work function, appropriate electron and hole transporting
characteristics, malleability, adequate solid properties and
chemical stability. Two key factors in devices operation are
hole−electron transporting, and their injection through the
heterogeneous interfaces in the contact electrodes. In most
multiple layers optoelectronic devices, the hole transporting
layer (HTL) and the electron transporting layer (ETL) control
the charge balance and the injection-collection into/from
them.4
Received: May 24, 2013
Revised: May 27, 2013
© XXXX American Chemical Society
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Figure 1. Dendrimer structures.
electron block injection capability.14,20 In addition, these
macromolecules have a high glass-transition temperature (Tg),
which is a very important factor in order to avoid device
degradation by the morphological changes produced by Joule
effect during operation.22−26 Moreover, the branched three-
dimensional architecture nature of dendrimeric material is also
convenient to retain optoelectronic properties in solid state
devices, due to the fact that they can avoid aggregation effects.
This type of devices also demands materials with capability
for thin films formation using low cost technologies. The most
used methods to deposit organic material for optoelectronic
devices are thermal evaporation and solution-processing (drop
casting, spin coating, etc.). Although thermal evaporation with
the use of fine mask can produce well-patterned films, the
process is slow and demands expensive vacuum equipment. In
addition, thermal evaporation requires materials with high
sublimation capability and excellent thermal stability, which are
properties not easily obtained in polymers. Also, low-cost
solution processes, such as spin-coating and deep-coating,
widely used in the production of nonpatterned films, require
material with intrinsic high solubility, and usually produce large
amount of waste. An alternative important technique to
produce conducting films is the electropolymerization. The
deposition of organic material over a conducting substrate
through electrochemical polymerization is a versatile method
that allows synthesizing a conducting film in one step.27,28 The
technique permits to obtain films with different forms and
patterns, with easy control in the thickness.28−32 A number of
reports revealed that the polymeric films obtained electro-
chemically over conductive solid substrates are highly stable,
with excellent adherence to the electrode. These materials often
hold high charge transporting capability and considerable
fluorescence properties in some special cases.28−33 Additionally,
we have demonstrated that by controlling the electrodeposition
conditions, it is possible to optimize the morphology of the
surface, and consequently the film optoelectronicproperties.27
In this article, we report the synthesis and properties of
functional starburst monomers featuring the presence or
absence of electroactive central core connected by conjugated
branches to peripherals triphenylamine (TPA) moieties (Figure
1). The introduction of systematic changes in the design of
dendrimer structures allows to establish relationships between
their electro-optical properties with molecular structural
parameters. These new electropolymerizable macromolecules
exhibit interesting electronic properties that yield thin films of
polymeric material with good electrical conductivity, reversible
electrochemical processes and stability. Hence, this work
provides a model for designing starburst dendrimers capable
to form electrochemically active polymers with potential
applications in electronic and optoelectronic devices.
B
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128.25. 127.63, 126.75, 126.26, 124.31, 123.34, 122.00, 90.28, 89.64.
MALDI−TOF (m/e): calcd for C90H63N3, 1186.5 (MH+); obsd,
1186.4 (MH+).
EXPERIMENTAL SECTION
■
Instrumentation and Characterization Techniques. Melting
points were taken on a Leitz Wetzlar Microscope Heating Stage,
Synthesis of Dendrimer 3. To a solution of 15 (231.0 mg, 0.49
mmol) in dry toluene:triethylamine 1:1 (16 mL) were added
Pd(PPh3)4 (18.3 mg, 0.02 mmol) and CuI (4.3 mg, 0.02 mmol),
and the mixture was stirred for 15 min under argon atmosphere at
room temperature. 1,3,5-Triethynylbenzene (7) (18.4 mg, 0.12 mmol)
was added and the reaction was refluxed during 48 hs. The solvent was
evaporated and the crude product was purified by column
chromatography yielding 80.3 mg (0.07 mmol, 55%) of dendrimer
(3) as orange solid. Mp: 135−138 °C.
Model 350 apparatus and are uncorrected. H and 13C NMR spectra
1
were recorded on Bruker Avance-300 spectrometer with Me4Si as the
internal standard and chloroform-d as solvent. Abbreviations: s =
singlet, d = doublet, t = triplet, and m = multiplet expected but not
resolved. Mass spectra of dendrons were recorded in a Shimadzu
QP2010 Plus instrument, ion source temperature = 300 °C, and
detector voltage = 70 kV. Samples were analyzed by ultraviolet matrix
assisted laser desorption-ionization mass spectrometry (UV−MALDI
MS) and by ultraviolet laser desorption-ionization mass spectrometry
(UV−LDI MS) performed on the Bruker Ultraflex Daltonics TOF/
TOF mass spectrometer (Leipzig, Germany). Mass spectra were
acquired in linear positive and negative ion modes. Stock solutions of
samples were prepared in chloroform. External mass calibration was
made using β-cyclodextrin (MW 1134) with nHo as matrix in positive
and negative ion mode. Sample solutions were spotted on a MTP 384
target plate polished steel from Bruker Daltonics (Leipzig, Germany).
For UV−MALDI MS matrix solution was prepared by dissolving GA
(gentisic acid, 1 mg/mL) in water and dry droplet sample preparation
was used according to Nonami et al.34 loading successively 0.5 μL of
matrix solution, analyte solution and matrix solution after drying each
layer at normal atmosphere and room temperature. For UV-LDI MS
experiments two portions of analyte solution (0.5 μL × 2) were loaded
on the probe and dried successively (two dry layers). Desorption/
Ionization was obtained by using the frequency-tripled Nd:YAG laser
(355-nm). The laser power was adjusted to obtain high signal-to-noise
ratio (S/N) while ensuring minimal fragmentation of the parent ions
and each mass spectrum was generated by averaging 100 lasers pulses
per spot. Spectra were obtained and analyzed with the programs
FlexControl and FlexAnalysis, respectively. Reactions were monitored
by TLC on 0.25 mm E. Merck Silica Gel Plates (60F254), using UV
light (254 nm) and phosphomolybdic acid as developing agent. Flash
column chromatography using E. Merck silica gel 60H was performed
by gradient elution of mixture of n-hexane and increasing volumes of
dichloromethane or ethyl acetate. Reactions were run under an argon
atmosphere with freshly anhydrous distilled solvents, unless otherwise
noted. Yields refer to chromatographically and spectroscopically
homogeneous materials, unless otherwise stated.
Synthesis of Dendrimer 1 by Hydrogenation of Dendrimer
3. Dendrimer 3 29.9 mg (0.025 mmol) was dissolved in distillated
ethyl acetate (30.0 mL) and 19.5 mg of Pd/C 10% were added. The
fluorescent mixture was degassed by 5 cycles of hydrogen-vacuum and
then stirred under atmospheric pressure of hydrogen for 2 days. The
colorless mixture was filtered through a Celite pad, washed with
chloroform and the solvents were concentrated under reduce pressure.
Crude product (35.5 mg) was purified by column chromatography
yielding 24.5 mg (0.020 mmol, 81%) of dendrimer 13 as colorless oil.
1H NMR, δ (300 MHz, CDCl3, TMS): 7.23 (sa, 5H), 7.21 (s, 4H),
7.13 (s, 11H), 7.07 (d, J = 8.85 Hz, 20H), 7.04−6.95 (m, 14H), 6.85 (s,
3H), 2.88 (s, 10H), 2.86 (s, 14H). 13C NMR, δ (75 MHz, CDCl3,
TMS): 148.01, 145.70, 141.87, 139.55, 139.40, 136.67, 129.24, 129.14,
128.47, 128.34, 126.31, 124.67, 123.78, 122.35, 38.08, 37.74, 37.58,
37.47. MALDI−TOF (m/e): calcd for C90H81N3, 1204.6 (M+); obsd,
1204.5 (M+).
1H NMR, δ (300 MHz, CDCl3, TMS): 7.64 (s, 3H), 7.48 (dd, J =
12.1 Hz, J = 8.6 Hz, 12H), 7.38 (d, J = 8.6 Hz, 7H), 7.34−7.23 (m,
8H), 7.14−7.09 (m, 14H), 7.07−6.99 (m, 18H), 6.94 (s, 1H). 13C
NMR, δ (75 MHz, CDCl3, TMS): 147.69, 147.48, 137.99, 133.91,
132.05, 131.06, 129.34, 127.54, 126.26, 126.14, 124.65, 124.14, 123.36,
123.20, 121.34, 90.86, 88.73. MALDI−TOF (m/e): calcd for
C90H63N3, 1186.5 (MH+); obsd, 1186.4 (MH+).
Synthesis of Dendrimer 4. To a mixture of 13 (34.9 mg, 0.038
mmol) in dry toluene were added triethylamine 1:1 (14 mL)
Pd(PPh3)4 (7.0 mg, 0.006 mmol), CuI (2.0 mg, 0.011 mmol), and
triphenylphosphine (1.0 mg, 0.004 mmol), and the mixture was stirred
for 15 min under argon atmosphere at room temperature. Dendron 17
(63.0 mg, 0.167 mmol) was added and the reaction stirred and
refluxed overnight. After solvent evaporation, the crude product was
purified by column chromatography and crystallized by dichloro-
methane: hexane yielding 51.7 mg (0.024 mmol, 63%) of dendrimer 4.
The solid was recrystallized from acetone: hexanes yielding 39.8 mg of
4 as an orange solid. Mp: 200−203 °C. 1H NMR, δ (300 MHz,
CDCl3, TMS): 7.58−7.33 (m, 35H), 7.32−7.20 (m, 14H), 7.19−6.90
(m, 41H). 13C NMR, δ (75 MHz, CDCl3, TMS): 147.61, 147.48,
146.74, 137.58, 138.39, 132.05, 131.95, 131.12, 129.34, 129.08, 128.87,
127.67, 127.52, 126.77, 126.33, 126.26, 124.63, 124.31, 123.38, 123.18,
122.09, 121.89, 90.73, 90.59. MALDI−TOF (m/e): calcd for
C126H90N4, 1658.7 (M+); obsd, 1658.4 (M+).
Optical Characterization. Absorption and fluorescence spectra
were recorded on a Shimadzu UV-2401PC spectrometer and on a
Spex FluoroMax fluorometer, respectively. Spectra were recorded
using quartz cells (path length: 1 cm) using 1,2-dichloroethane (DCE)
as a solvent at room temperature.
Electrochemical Study and Polymer Films Electrodeposi-
tion. The redox properties of dendrimers were studied in 0.5−1.0 mM
range concentration in DCE solution containing 0.1 M tetra-n-
butylammonium hexafluorphosphate (TBAHFP) as support electro-
lyte using a potentiostat Autolab Electrochemical Instruments. The
working electrode consisted of a 2.16 × 10−3 cm2 inlaid platinum disk
that was polished on a felt pad with 0.3 μm alumina and sequentially
sonicated in water and absolute ethanol for 3 min each; and finally
dried with a hot air gun. The voltammetric experiments were carry out
using a silver wire as pseudoreference electrode and a platinum coil
was used as the counter electrode. Cyclic potential scans were made in
the electrochemical windows of the system solvent−electrolyte in
order to discard possible active electrochemically interferes. All
potential values in this study are expressed relative to a ferrocene/
ferrocenium redox couple (Fc/Fc+ = 0.40 V vs SCE) that was added to
the solution as an internal standard.35 The molecules studied were
electropolimerized on Pt and ITO (indium tin oxide, Delta
Technologies, nominal resistance 10 Ω/square) electrodes by cyclic
voltammetry technique under the experimental conditions described
above.
Spectroelectrochemical. The experiments were carried out in a
homemade cell built from a commercial UV−visible cuvette. The ITO-
coated glass with a polymer film was used as working electrode, the Pt
counter electrode was isolated from the monomer solution by a glass
frit, and a silver wire was used as pseudoreference electrode. The cell
was placed in the optical path of the sample light beam. The
background correction was obtained by taking an UV−visible
spectrum of a blank cell (an electrochemical cell with an ITO working
Synthesis of Dendrimer 2. To a mixture of 13 (64.8 mg, 0.07
mmol) in dry toluene were added triethylamine 1:1 (14 mL)
Pd(PPh3)4 (11.4 mg, 0.01 mmol), CuI (2.4 mg, 0.01 mmol), and
triphenylphosphine (2.5 mg, 0.01 mmol), and the mixture was stirred
for 15 min under argon atmosphere. Commercial phenylacetylene
(0.05 mL, 0.43 mmol) was added and the reaction was stirred
overnigthat 80 °C at this temperature. The solvent was evaporated and
the crude product was purified by column chromatography yielding
51.7 mg (0.06 mmol, 87%) of dendrimer 2 as yellow solid. Mp: 122−
1
125 °C. H NMR, δ (300 MHz, CDCl3, TMS): 7.56−7.52 (m, 7H),
7.50 (d, J = 4.6 Hz, 9H), 7.47−7.40 (m, 8H), 7.38−7.32 (m, 9H),
7.16−7.07 (m, 9H), 7.01 (d, J = 16.3 Hz, 3H). 13C NMR, δ (75 MHz,
CDCl3, TMS): 146.81, 137.48, 132.06, 131.95, 131.60, 128.90, 128.37,
C
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Scheme 1. Synthesis of Cores 7 and 13
Scheme 2. Synthesis of Dendrons 15 and 17
electrode without the polymer film) working under the similar
conditions and parameters for polymer experiments cited above.
Film Analysis. Topography of electrodeposited polymer films was
performed by scanning electron microscopy (SEM). The samples were
analyzed on a Carl Zeiss EVO MA 10 with electron beam energy of 18
KV.
12 prepared from the product of the Vislmeier−Haack
formylation of commercial triphenylamine 11.37 Then, Wads-
worth−Horner−Emmons reaction38 of this tris-formyl deriva-
tive 12 with phosphonate 1039 allowed us to obtain compound
13 in very good yield (88%, Scheme 1).
Synthesis of dendrons 15 and 17 were accomplished as
described in Scheme 2. Dendron 15 was obtained via
Wadsworth-Horner-Emmons reaction with phosphonate 1039
of the formyl derivative 1440 in 75% yield. Dendron 17 was
synthesized starting with the Sonogashira reaction of 15 with
the terminal protected alkyne 2-methylbut-3-yn-2-ol yielding
16 in 84% yield. Deprotection of 16 with potassium
hydroxide38 afforded 17 in 73% yields.
RESULTS AND DISCUSSION
■
Synthesis and Characterization of Dendrons and
Dendrimers. All dendrimers (Figure 1) were obtained by
Sonogashira coupling between cores 1,3,5-triethynylbenzene
(TEB) (7) or tris{4-[(E)-2-(4-iodophenyl)vinyl]phenyl}amine
(13) and the corresponding dendrons. The core 7 was
prepared according to a synthetic protocol36 starting from
commercial 1,3,5-tribromobenzene (5) as described in Scheme
1. The product 7 was obtained in two steps with 94% as overall
yield. Core 13 was synthesized from tris(4-formylphenyl)amine
Taking into account that phosphonate carbanions produce
predominately E stereochemistry of double bonds, a stabilized
phosphorus ylide was chosen instead of Wittig protocol because
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Scheme 3. Synthesis of Dendrimers 1, 2, 3, and 4
Figure 2. UV visible absorption and emission spectroscopy DCE in solution Normalized absorption (black line) and emission (red line) spectra of
studied dyes in DCE solution. The emission spectra were measured by excitation at the maximum of the lower energy bands.
better yields and regioselectivity of the alkene derivatives were
obtained.
Dendrimer 3 was prepared through a carbon−carbon
coupling reaction of the core 7 and dendron 15 using
Pd(PPh3)4 and CuI in toluene: triethylamine solvents mixture
(Scheme 3). The product was isolated after column
chromatography purification yielding 3 as orange solid in
55% yield. Dendrimers 2 and 4 were obtained under similar
Sonogashira coupling conditions between the nucleus 13 and
commercial phenylacetylene yielding 2 as a yellow solid in 87%
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yield (after column chromatography purification) and carbon−
carbon coupling reaction of 13 with dendron 17 afforded
dendrimer 4 as an orange solid in 63% yield after crystallization.
Catalytic hydrogenation of 3 with hydrogen under atmospheric
pressure using Pd/C as catalyst in ethyl acetate as solvent
afforded dendrimer 1 in 81% yield as a colorless oil (Scheme
3). The structures of the cores and dendrons were confirmed
TPA moieties. In dendrimer 2, which holds one TPA unit as
core, the high energy band moves to lower energy (321 nm, 19
nm red-shifted with respect 1 molecule), due to the presence of
three vinylene-modified phenyls groups in the TPA. Finally, the
spectrum of 4 showed a band with maximum at λ = 310 nm
which can be ascribed to the outer TPA moieties with one
conjugated vinylene group, similar to those observed in 3. Also,
an additional peak at λmax at 344 nm was presents in 4. This
transition can be assigned to the vinylene-modified the TPA
core. The molecular structure of 4 can be seen as combination
of 2 and 3, since its core is similar to 2 and the periphery or
shell resembles to 3. However, Figure 2 shows that light
absorption spectrum of 4 is not a lineal combination of 2 and 3
spectra (See Table 1), which indicates that the conjugated
linker allows interaction between the core and the periphery of
the dendrimer. On the other hand, the low energy bands
(λmax∼400 nm) that are present in all dendirmers except 1,
showed similar behaviors (see Table 1): a red shift is detected
in these π−π* transitions with the increment of conjugation
length. This tendency was observed in low and high polarity
solvents (toluene and DCE).
In addition, Figure 2 showed the photoluminescence spectra
of monomers in diluted DCE solutions measured by excitation
at the absorption maximum wavelength of the π−π*
transitions, except for 1, which was excited at maximum of
the light absorption by the TPA moieties. For 2, 3 and 4,
maximum bands for emission were detected at 493, 500, and
503 nm, respectively. Thus, the fluorescence spectra also
showed the effect of the difference in effective conjugation
length observed in absorption spectra. Furthermore, marked
red shifts was observed in the emission maxima when the
fluorescence spectra were measured in DCE with respect to the
same obtained in lower polarity medium (toluene, see Table 1),
which is in according with the properties of π−π* transitions.
Dendrimers Electrochemical Characterization and
Electrodeposition of Polymeric Films. Redox properties
and radical ions stability of the dendrimers were determined by
cyclic voltametry in DCE solution containing TBAHPP 0.1 M
as supporting electrolyte at room temperature, using a Pt disk
as a working electrode. Dendrimer 3 holds three TPA units as
electroactive group in their periphery; therefore, it is expected
that their oxidation behavior is ruled by these groups. Figure 3a
showed two consecutive cyclic voltammograms of 3. In the first
anodic scan, multiples oxidation peaks at 0.31, 0.43, and 0.64 V
are observed. When the potential scan is inverted three
reduction waves can be detected with peaks at 0.36, 0.11, and
−1.04 V. The second scan shows a similar profile, but with a
remarkable growing in the current. The electro-oxidation of
TPA was extensively studied,43,44 and it is know that the radical
cation formed by oxidation can be involved in a dimerization
reaction which produced tetraphenylbenzidine (TPB) deriva-
tives.27−47
1
by H NMR, 13C NMR spectroscopy and low resolution mass
spectrometry experiments and compared with the correspond-
ing bibliographic reference when necessary. Structures of
1
dendrimers were characterized by H NMR, 13C NMR, and
MALDI−TOF mass spectrometry experiments. In the con-
jugated macromolecules 2, 3, and 4 the observation of two sp
carbons corresponding to alkyne signals and the total number
of different carbon atoms signals in the 13C NMR spectrum
allowed us to confirmed the starburst structure of dendrimers.
Dendrimer 1 showed no alkyne signals in 13C NMR spectrum
and four new methylene peaks were observed. In the 1H NMR
spectrum broad signals at 2.86 and 2.88 ppm corresponding to
a total of 24 protons demostrated the completed hydrogenation
of all the alkenes and alkynes present in the starting material 3.
MALDI−TOF data confirmed the expected molecular mass
calculated for each dendrimer. Experimental details for nuclei
and dendrons synthesis and NMR data are described in the
Supporting Information.
Dendrimers Optical Properties. UV−visible absorption
and emission spectroscopy measurements were performed both
in solution and in solid films for the electrodeposited polymers.
The absorption and fluorescence spectra of this set of
molecules in DCE solution is shown in Figure 2, and the
main optical characteristics are summarized in Table 1. The
Table 1. Absorption and Emission Characteristics of the
Studied Dendrimers
b
λmax absorption (nm)
λmax emission (nm)
compound
toluene
DCE
toluene
DCE
a
1
302
302
357
463
450
469
362
500
493
503
2
3
4
319/413
308/399
321/416
310/399
310/346/423
310/344/423
a
b
Measured in cyclohexane. The emission spectra were measured by
excitation at the maximum of the lower energy bands.
dendrimers studied possess different structural characteristics.
Compound 2 hold sonly a TPA moiety in the core, while 1 and
3 have their TPA groups as outer substituent in the
dendrimeric structure. The main difference between den-
drimers 1 and 3 is the lack of conjugated branches in the first
one. Finally, dendrimer 4 shows fully conjugated TPA groups
both in the core and in the outer layer.
The analysis of the absorption spectra permits to observe
that 1 showed a single light absorption peak, while 2, 3, and 4
showed two optical transitions, one with maximum around of λ
= 300 nm and other approximately at 400 nm. Therefore, the
high energy bands in the dendrimers can be assigned to light
absorption by the triphenylamine moieties, and the lower
energy bands to π−π* transitions.41,42 The observed differences
between the wavelength maximum in Table 1 are related to the
effective conjugation lengths and group interaction effects. An
analysis in DCE, revealed that the band ascribed to TPA
absorption in 3 is 8 nm red-shifted respects to this band in 1
spectrum due the presence of one conjugated vinylene with the
Also, the generated TPB is an electroactive center that
undergoes two successive reversible oxidation processes, which
are responsible of the additional waves observed in cyclic
voltametry of 3.27−45 This mechanism allows the growing of a
electroactive polymeric structure when 3 is successively cycled
between −1.6 and 0.9 V. Typical multiple CV scans of this
compound on a Pt electrode are shown in Figure 3b. In
addition to the TPA dimerization mechanism, it is know that
TPA exhibits reactivity in the dicationic charged state, with
simultaneous parallel follow-up coupling reactions, which was
careful described in a recent paper by Yurchenko et al.48 These
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defined at 0.42 and 0.77 V and one is manifested as shoulders at
0.65 V. In the reverse scan three peaks are observed as
complementary reduction processes at +0.25, −0.43, and −0.86
V, showing an irreversible behavior typical of chemical reactions
coupled to the heterogeneous charge transfer. Therefore, these
results suggest that oxidation of the central core and the
periphery groups occurs at a similar applied potential in
compound 4, and also undergoes an analogous electro-
polymerization process. This assumption was confirmed by
the increasing current observed in the second CV scan (Figure
4a), and by the growing of an electroactive polymeric structure
when 4 was successively cycled between −1.6 and +1.0 V
(Figure 4b).
Additionally, the electrochemical study of dendrimer 2,
which also holds TPA core but lacks of peripheral electroactive
groups, showed (Figure 5) a quasireversible oxidation wave at
Figure 3. Cyclic voltammogram of the compound 3: (a) first (black)
and second (red) cycles; (b) repetitive cyclicvoltamograms. (c) Cyclic
voltammogram of the electro-deposited film derived from 3 on a Pt
electrode in DCE with 0.1 M of TBAHPP, v = 0.1 V s−1.
follow-up coupled reactions could contribute to the voltametric
waves associated with the main process. To confirm the
existence of an electroactive dendrimeric polymer, the working
electrode was transferred to a supporting electrolyte solution
free of monomer. The redox response of this modified
electrode is shown in Figure 3c where it can be observed one
oxidation peaks at 0.56 V and one catodic waves at 0.18 V.
These observations confirms that the oxidation process of TPA
outer layer results in the formation of TPB through a
dimerization reaction, which leads to the formation of a stable
and electroactive film on electrode surface.
Figure 5. Cyclic voltammogram of the molecules 1 and 2 in DCE with
Similarly to 3, dendrimer 4 has three TPA groups in the
outer layer of the dendrimer, but it holds an additional
electroactive triphenylamnine as central core. Consequently, 4
showed a similar electrochemical behavior, producing three
oxidation waves in the first cycle (Figure 4a), two peaks are
0.1 M of TBAHPP, on Pt electrode at a scan rate of 100 mV/s.
0.38 V. This behavior is expected because dendrimer 2 has an
amine group with blocked phenyl rings in the reactive para-
position which allows to form stable oxidized states.29−48 Thus,
in 4 dendrimer, the core oxidation should occur at a similar
applied potential. This is evidenced when the potential is cycled
until the first oxidation wave obtaining a reversible response
without film formation (see inset in Figure 4b). This conclusion
is also supported by the spectroelectrochemical studies carried
out on the electropolymerized films (see below)
As it was done with compound 3, with the aim of study the
electrochemical behavior of the electrogenerated polymer of 4,
the working electrode was removed and washed with DCE in
order to eliminate residual monomer content. Subsequently, it
was transferred to a three-electrode cell with supporting
electrolyte solution free of monomer to study the film redox
behavior. Figure 4c shows the film response, which confirms
that the oxidation processes of macromolecule 4 forms an
irreversibly adsorbed electroactive product on the electrode
surface. The film has a reversible oxidation wave with a
potential peak at 0.77 V, and shows a great stability when the
film is submitted to numerous oxidation−reduction cycles.
In the electrooxidation of the hydrogenated dendrimer 1,
which has a nonconjugated structure with TPA peripheral
groups, no stable film formation was obtained under the
present experimental conditions, in spite that the TPA
substituent oxidation drives to the radical cation coupling and
Figure 4. Cyclic voltammogram of the compound: (a) first (black)
and second (red) cycles; (b) repetitive cyclicvoltamograms. (c) Cyclic
voltammogram of the electro-deposited film derived from 4 on a Pt
electrode in DCE with 0.1 M of TBAHPP, v = 0.1 V s−1.
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Figure 6. SEM imagines of the molecules obtained at different magnifications of the films obtained from 4 (a, b) and 3 (c, d).
Figure 7. Absorption spectra of electrodeposited films on an ITO electrode at different applied potentials of the compounds 3 and 4.
TPB formation. This fact is corroborated by the typical two
reduction waves observed in the reverse scan43,45 (Figure 5b).
It is possible to suppose that the dimers and/or oligomers
formed in the electrode−solution interface are soluble, and the
metal surface−organic molecule interaction in this case is not
strong enough to allow film formation. The higher solubility of
dimers and/or oligomers of 1 could be related to the higher
rotational and conformational freedom degrees of this
dendrimer in comparison with related compounds 3 and 4.
Tridimensional idealized structures obtained by molecular
mechanic calculation (MM+) of the dendrimer 1 and 3 (see
Figure S20 in Supporting Information) show that in the case of
1 a globular shape is permitted because the rotational freedom
of the single bonds. Contrarily, in 3, a rigid star shape is
observed due to the multiple conjugated bonds present in the
dendrimer arms. It is possible that the globular fashion in 1
contributes to increase the solubility of the electrooxidation
products, affecting the film formation on the electrode surface.
These results indicate that, when designing a dendron, the use
H
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of conjugated and hence more rigid structures is an important
factor in order to obtain electropolymeric films.
holds absorption with λmax = 484 cation and the dication at 450
and 722 nm.48
Surface Analysis of Electrogenerated Films. It is well-
known that the morphology of organic polymeric films plays a
key role when they are used as components in optoelectronics
devices. The presence of cracks, pinholes or inhomogeneities in
the surface can produce electrical shortcuts or current paths
with different resistances, which affects the performance and
stability of the devices.28,27 The surface morphology of 3 and 4
polymeric films were analyzed by scanning electron microscopy
(SEM) and the obtained images are shown in Figure 6. In both
cases the polymeric structure full covered the ITO electrode
surface, without unfilled spaces. However, different morphol-
ogies can be easily perceived. In the film derived from
dendrimeric compound 3, the images in Figure 6, parts a and b,
showed the presence of an inhomogeneous and granulate
surface. The micrograph allowed to see irregularly shaped
grains. On the other hand, SEM images of polymer of 4 (Figure
6, parts c and d) shows a smooth surface, without grains, cracks,
or pinholes. It can be seen that the film fully covers the ITO
electrode, without leaving islands, and the surface morphology
is completely soft and free of agglomerations. These facts were
repetitive for all the formed films, although the growing
conditions (monomer concentration, solvent and electrolyte)
for both films were the same. Thus, the differences observed in
films morphology are related to the molecular structure of the
dendrimers. As it was previously mentioned, the main
difference is the presence of the TPA electroactive central
core in 4. As it is demonstrated by electrochemical and
spectroelectrochemical studies (see below), this core is oxidized
during the electrodeposition procedure. The positive charge
located in the central core could produce columbic repulsion
between different polymeric branches, with effect over both, the
nucleation and growing processes of the film, affecting their
surface morphology.
The spectroelectrochemical response of polymer derived
from compound 4 showed different features with respect to
those observed for dendrimer 3. In the neutral form a sharp
band with λmax = 319 nm is observed, which decreased upon
film oxidation. Concomitantly, a new band with λmax = 345 nm
is formed (inset in Figure 7). These results are in agreement
with an electrochemical process associated with TPA central
core of the dendrimer. The 319 nm band can be assigned to the
TPA in neutral forms; meanwhile the band at 345 nm is typical
of TPA cation radical. Furthermore, the absorption bands
detected at λmax = 568 and 688 nm also are related to the
formation of TPA cation radical. In parallel to the dendrimer
central core electrochemical process, the characteristic neutral
TPB absorption band, centered at λmax = 405 nm, decreased as
the applied potential becomes more anodic. The TPB cation
radical and dication absorption are superimposed with the long
wavelength TPA radical cation absorption bands. These facts
indicated that this conductive dendritic polymer can be
reversible charged, both in the core and in the peripheral
units. This polymer is fully conjugated with a starburst structure
that can prevent staking or aggregation between the redox
centers. These characteristics are of great importance in order
to obtain adequate charge transporting and optoelectronic
properties, which in turns are inherent to the development of
materials that could have applications in organic electronic and
optoelectronic devices.
CONCLUSION
■
A series of electroactive dendrimers was obtained by a versatile
convergent synthetic strategy which involved Sonogashira cross
coupling reaction to assemble the dendrons with the core.
Dendrons were easily prepared following standard synthetic
procedures. The studied compounds contain a branched
structure with multiple TPA groups, which allows independent
dimerization and electropolimerization reaction that modifies
the electrode surfaces with conductive electropolymers. The
dendrimer structures were systematically modified, and
relationships between optical, electrochemical and film surface
morphological characteristics with the molecular structural
parameters were established. This work provides a model to
design starburst dendrimers capable to form electrochemically
active polymers. These results showed that the rigidity reach by
the extended conjugation in dendrimer branches is a key factor
to obtain polymers films by electrodeposition technique. Also,
the presence of charged center in the dendrimer core can affect
the electropolymer surface quality film formation. A polymeric
material with smooth surface, good electrical conductivity and
stability was obtained. These material characteristics are of great
relevance for their potential application in the development of
organic optoelectronic devices.
Spectroelectrochemical Characterization of Electro-
generated Films. Spectroelectrochemical studies of the
electrodeposited films on ITO electrode were performed in
order to obtain information about the polymer molecular
structure and polymerization mechanism. Figure 7 shows the
absorption spectra at different applied potentials electro-
polymerized films obtained from 3 and 4.
The absorption spectra of the films were obtained as a
function of applied potentials between 0 and ∼1.3 V. The films
in their neutral state (0 V) are semitransparent (they show a
pale yellow color to the naked eye), with light absorption at λ
bellow ∼450 nm assigned to π−π* transition. As the potential
is increased to positive values, the intensity of these bands
decrease, while new bands appear, in the wavelength range
from 400 to 1000 nm, which produced a visible change in the
color of the films from yellow to red and finally to blue (Figure
7). The absorption spectra of the electrodeposited film of
dendrimer 3 revealed a decrease in intensity of the shoulder at
386 nm (that extends to the visible region) when the applied
potential became more anodic, while an increase of the two
absorption bands at λmax = 525 and 703 nm was observed. The
intensity of the second absorption peak rises and reaches a
maximum at 1.2 V. This absorption is ascribed to the fully
oxidized state of the electro-deposited film of 3. All these results
are in fully agreement with the presence of TPB in the film’s
molecular structures.29,45,48,47 Neutral TPB shows an absorp-
tion band with λmax = 352 nm; meanwhile, the TPB radical
ASSOCIATED CONTENT
■
S
* Supporting Information
Synthetic procedures and characterization data, H and 13C
1
NMR spectra, and MM+ calculation of the synthesized
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org
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(28) Li, M.; Tang, S.; Shen, F.; Liu, M.; Xie, W.; Xia, H.; Liu, L.;
Tian, L.; Xie, Z.; Lu, P.; Hanif, M.; Lu, D.; Cheng, G.; Ma, Y.
J.Phys.Chem.B 2006, 110, 17784−17789.
AUTHOR INFORMATION
Corresponding Author
*E-mail: (R.A.S.) spanevello@iquir-conicet.gov.ar; (F.F.)
ffungo@exa.unrc.edu.ar.
Notes
■
(29) Gervaldo, M.; Funes, M.; Durantini, J.; Fernandez, L.; Fungo, F.;
Otero, L. Electrochim. Acta 2010, 55, 1948−1957.
(30) Liddell, P A.; Gervaldo, M.; Bridgewater, J. W.; Keirstead, A E.;
Lin, S.; Moore, T. A.; Moore, A L.; Gust, D. Chem. Mater. 2008, 20,
135−142.
(31) Tang, S.; Liu, M.; Lu, P.; Xia, H.; Li, M.; Xie, Z.; Shen, F.; Gu,
C.; Wang, H.; Yang, B.; Ma, Y. Adv. Funct. Mater. 2007, 17, 2869−
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(32) Li, M.; Tang, S.; Shen, F.; Liu, M.; Li, F.; Lu, P.; Lu, D.; Hanif,
M.; Ma, Y. J. Electrochem. Soc. 2008, 155, H287−H291.
(33) Gu, C.; Tang, S.; Yang, B.; Liu, S.; Lu, Y.; Wang, H.; Yang, S.;
Hanif, M.; Lu, D.; Shen, F.; Ma, Y. Electrochim. Acta 2009, 54, 7006−
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank Consejo Nacional de Investigaiones Cientificas y
Tecnicas (CONICET-Argentina), Agencia Nacional de Pro-
■
mocion
́ ́
Cientıfica y Tecnologica (ANPCYT Argentina), and
Universidad Nacional de Rosario y Universidad Nacional de
́
Rıo Cuarto. M.I.M, R.A.S, L.F, L.O., and F.F. are scientific
members of CONICET.
(34) Nonami, H.; Fukui, S.; Erra-Balsells, R. J. Mass Spectrom. 1997,
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