Electropolymerizable terthiophene-terminated poly(aryl ether) dendrimers with naphthalene and perylene cores

Article abstract of DOI:10.1021/ma201733a

We describe the synthesis and characterization of a new series of poly(aryl ether) dendrimers with both donor and acceptor moieties. By incorporating p-type terthiophene and n-type perylene and naphthalene diimides into one molecule, we have achieved a du

Full text of DOI:10.1021/ma201733a

ARTICLE
pubs.acs.org/Macromolecules
Electropolymerizable Terthiophene-Terminated Poly(aryl ether)
Dendrimers with Naphthalene and Perylene Cores
Ramakrishna Ponnapati, Mary Jane Felipe, and Rigoberto Advincula*
Department of Chemistry and Department of Chemical and Biomolecular Engineering, University of Houston, Houston,
Texas 77204-5003, United States
ABSTRACT: We describe the synthesis and characterization of a
new series of poly(aryl ether) dendrimers with both donor and
acceptor moieties. By incorporating p-type terthiophene and n-type
perylene and naphthalene diimides into one molecule, we have
achieved a dual electrochromic material. These dendrimers were
designed to act as electrochemically active precursor polymers,
which bear the electroactive terthiophene group that can be cross-
linked electrochemically to form conjugated polymer network
(CPN) films. Characterization of the optical and redox properties
shows that it is possible to incorporate donor and acceptor moieties
in the same molecule with retention of doping properties. The films made by electrochemical polymerization are highly active in
both the p- and n-doping processes and have been investigated by spectroelectrochemical methods.
’ INTRODUCTION
diimide-based dendrimers, into which the dendron peripheral
group terthiophene is introduced as a charge carrier transporting
tunable type emitter.12 These dendrimers are expected to show
specific light-antenna capacity and tunable charge carrier trans-
porting characteristics. The benzyl ether backbone increases the
solubility of these dendrimers. One advantage of the new
terthiophene dendrons is their electropolymerizability to yield
polythiophene, to yield Conjugated Polymer Network (CPN) films.
Extensive research is being carried out on electroactive chromo-
phores attached to an emissive core because of their luminescent
properties relevant to light-emitting diode applications.1 Recently,
dendrimers were investigated as an alternative source of electro-
luminescence.2 Dendrimers open up new opportunities for pre-
cisely placing charge-carrier transporting units at the periphery in a
three-dimensional nanoscale construction. One of the attractive
properties of electron-rich π-conjugated polymers like polycarba-
zole, polypyrrole, and polythiophene is their electrical conductivity,
which develops during doping of their π-conjugated systems.3
These conjugated polymers are proven to be generally good p-type
materials.4 Recently, conjugated perylene tetracarboxydiimide
(PDI) derivatives have attracted considerable attention owing to
their excellent chemical and thermal stability for various applica-
tions such as optical switches,5 photovoltaic devices,6 and dye
lasers.7 PDI derivatives are well-known n-type (electron acceptor)
charge generation sites.8 Incorporation of electron-rich polymer
and electron-poor dye molecules within a bulk material may lead to
efficient electronic and photonic materials.9
Electrochromism is another important application for these
materials. The conjugated polymers used in electrochromic
devices are normally of the p-doping type such as polythiophene.
The n-doping properties of aromatic diimides like naphthalene
tetracarboxydiimide and perylene tetracarboxydiimide have also
been reported.9 It is well-known that aromatic diimides generally
exhibit rather poor solubility in organic solvents. Solubility of
these diimides has been increased by attaching long alkyl chains
or soluble dendrons to these materials. By incorporating p-type
terthiophene and n-type perylene and naphthalene diimides into
one molecule, it may be possible to achieve a dual electrochromic
material. Herein we report a series of perylene and naphthalene
’ EXPERIMENTAL PART
Materials. Solvents were properly distilled and used immediately.
Other reagents and chemicals were commercially available. Analytical
grade chemicals were purchased from Aldrich Chemical Co. and were
used without further purification.
Characterization. NMR spectra were recorded using a General
Electric QE 300 spectrometer (1H 300 MHz). UVÀvis spectra were
recorded using an Agilent 8453 spectrometer. Fluorescence spectra were
obtained on a Perkin-Elmer LS-45 luminescence spectrometer. Ele-
mental analysis was done (samples sent) by Atlantic Analytical Micro-
labs. Cyclic voltammetry (CV) was performed on an Amel 2049
potentiostat and power lab/4SP system with a three-electrode cell. In
all the measurements, the counter electrode was platinum wire and
indium tin oxide (ITO) or gold coated glass as working electrode. The
ITO was pretreated with the RCA recipe (H2O/H2O2/NH3: 15.1 g/
26.6 g/8.57 g). The gold electrodes were cleaned with a plasma ion
cleaner (Plasmod, March). Ag/AgCl was used as a reference electrode.
Synthesis. Synthesis of Ethyl 2-(2,5-Di(thiophen-2-yl)thiophen-
3-yl)acetate (3T ET) (Scheme 1). The synthesis of 3T ET was carried by
Received: July 27, 2011
Revised:
September 4, 2011
Published: September 16, 2011
r
2011 American Chemical Society
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Scheme 1. Synthesis of Terthiophene Ethanol
Scheme 2. Synthesis of G1 OH Terthiophene Dendron
first synthesizing ethyl 2-(2,5-dibromothiophen-3-yl)acetate as reported
in the literature.10a The same literature procedure was modified to
synthesize 3T-ET. Ethyl 2-(2,5-dibromothiophen-3-yl)acetate (6.4 g,
10 mmol) and 2-(tributylstannyl)thiophene (15 g, 20 mmol) were added
to a 30 mL dry DMF solution of dichlorobis(triphenylphosphine)palladium
(1.3 g, 1.5 mmol). After three freezeÀthaw cycles, the mixture was heated at
100 °C for 48 h. The mixture was cooled to room temperature and poured
into a beaker containing 150 mL of water and subsequently extracted with
CH2Cl2. The extracted CH2Cl2 mixture was dried with Na2SO4. After
filtering and evaporation of the solvent, the crude product was purified by
chromatography on silica gel using toluene as an eluent. The final product
was obtained in 85% yield as pale yellow oil. The characterization of the
compound was found in accordance with the literature.10a
Synthesis of 2-(2,5-Di(thiophen-2-yl)thiophen-3-yl)ethanol (3T OH)
(Scheme 1). The compound 3T ET (2 g, 5.9 mmol) in 10 mL of THF
was added dropwise under nitrogen to an ice-cooled 100 mL THF
suspension of LiAlH4 (0.32 g, 8.4 mmol). Upon addition, the color of the
solution immediately turned red. After complete addition, the ice bath
was removed and the reaction was allowed to warm to room temperature
and constant stirring was maintained for 12 h. The reaction was
quenched by adding water and neutralized by 2 N HCl solution. The
red color of the solution immediately turned yellow upon neutralization.
The solvent was evaporated, and the resulting mixture was extracted
three times using CH2Cl2. The combined CH2Cl2 extracts were further
washed with water and brine. The organic layer was dried with Na2SO4.
After filtering and evaporating CH2Cl2, the reaction mixture was
chromatographed using 4:1 CH2Cl2:hexane as eluent mixture. The final
product was obtained in 90% yield as oil, which solidifies upon vacuum
or even at room temperature if kept for a longer time. 1H NMR
(CDCl3): (δ ppm) 7.31À7.04 (m, 7H), 3.88 (q, 2H, J = 6.4 Hz), 3.01
(t, 2H, J = 6.4 Hz). 13C NMR: 136.8, 135.8, 135.7, 135.2, 131.2, 127.8,
127.5, 126.4, 126.3, 125.7, 124.6, 123.8, 62.7, 32.4.
was added dropwise to a suspension of LAH in THF cooled to 0 °C with
an ice bath. The suspension was then allowed to warm to room
temperature and stirred until the reaction was completed as indicated
by TLC. The reaction was quenched by adding water, and the THF
removed under reduced pressure in a rotary evaporator. The resulting
layer was brought to a neutral pH with the addition of 2 N HCl solution
and extracted with CH2Cl2. The organic layers were combined, washed
with water, dried over sodium sulfate, filtered, and concentrated under
reduced pressure. The product was purified by silica gel flash column
chromatography using toluene as eluent.
General Method for the Conversion of Gn OH to Gn NH2 (Method C).
The conversion of Gn OH to Gn NH2 was carried by using a modified
Gabriel synthesis method. The mixture of alcohol, phthalimide, and
PPh3 in minimal THF was cooled to 5 °C with an ice bath and
sonicated for 5 min. Under sonication, a solution of DIAD was added
dropwise under nitrogen. The water temperature was allowed to warm
to room temperature, and precautions were taken to maintain the
temperature. Sonication was performed until the reaction was com-
pleted as indicated by TLC. The solvent was evaporated to get the
crude product as a white solid. Without further purification, the
protecting group was removed by refluxing the crude product in 1:1
THF:MeOH in the presence of hydrazine hydrate. The product was
purified by passing through a small pad of silica gel and methylene
chloride followed by methanol.
General Method for Dendrimer Synthesis (Method D). A mixture of
perylene 3,4,9,10-tetracarboxydianhydride or naphthalene tetracarbox-
ydianhydride, corresponding to Gn NH2, and zinc acetate dihydrate in
N-methylpyrrolidinone (NMP) was heated to reflux for 12 h. The warm
solution was diluted with water and extracted with methylene chloride.
The crude product was purified by column chromatography with
methylene chloride and silica gel.
General Method for Etherification Reactions (Method A). The
Mitsunobu etherification method was performed using sonication con-
ditions according to a literature procedure reported recently by our
group.12 The mixture of alcohol, phenol (or aromatic carboxylic acid),
and triphenylphosphine (PPh3) in minimal THF was cooled to 5 °C
with an ice bath and sonicated for 5 min. Under sonication, a solution of
diisopropylazodicarboxylate (DIAD) was added dropwise under nitro-
gen. The water temperature was allowed to warm to room temperature,
and precautions were taken to maintain the temperature. (Caution:
water in the sonicator turns very hot, if sonicated for a long time.)
Sonication was performed until the reaction was completed as indicated
by TLC. The product was purified using toluene as an eluent by silica gel
column chromatography.
Synthesis of Methyl 3-(2-(2,5-Di(thiophen-2-yl)thiophen-3-yl)ethoxy)-
5-(2-(5(thiophen-2-yl)-2-(thiophen-3-yl)thiophen-3-yl)ethoxy)benzoate
(G1 Ester) (Scheme 2). Following the general synthesis method A as
described above, a precooled solution of methyl 3,5-dihydroxybenzoate
(0.756 g, 4.6 mmol), 3T OH (3 g, 10.2 mmol), and PPh3 (2.67 g, 10.3
mmol) in THF under sonication was treated with a solution of DIAD
(2.06 g, 10.3 mmol) in 3 mL of THF under nitrogen. The solution was
sonicated for 3 h to afford a pale yellow solid product in 54% yield after
1
purification. H NMR (CDCl3): δ (ppm) 7.33À7.02 (m, 16H), 6.64
(t, 1H, J = 2 Hz), 4.21 (t, 4H, J = 7 Hz), 3.88 (s, 3H), 3.23 (t, 4H, J =
7 Hz). Calcd: C, 60.31; H, 3.94; O, 8.93; S, 26.83. Found: C, 60.68; H,
4.19; O, 8.72; S, 26.41.
Synthesis of (3-(2-(2,5-Di(thiophen-2-yl)thiophen-3-yl)ethoxy)-5-(2-
(5-(thiophen-2-yl)-2-(thiophen-3-yl)thiophen-3-yl)ethoxy)phenyl)-
methanol (G1 OH) (Scheme 2). Following the general synthesis method
General Method for Lithium Aluminum Hydride (LAH) Reduction of
Aromatic Esters to Benzylic Alcohols (Method B). The aromatic ester
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Scheme 3. Synthesis of G1 Per-terthiophene Dendrimer
Scheme 4. Synthesis of G2 OH Terthiophene Dendron
general synthesis method C was followed to get G2 NH2 in 95% yield.
1H NMR (CDCl3): δ (ppm) 7.33À7.00 (m, 28H), 6.56 (d, 4H, J = 2
Hz), 6.54 (d, 2H, J = 2 Hz), 6.47 (t, 1H, J = 2 Hz), 6.4 (t, 2H, J = 2 Hz),
4.91 (s, 4H), 4.16 (t, 8H, J = 7 Hz), 3.76 (s, 2H) 3.20 (t, 8H, J = 7 Hz).
Calcd: C, 62.44; H, 4.15; N, 0.95; O, 6.48; S, 25.98. Found: C, 631.27;
H, 4.80; N, 1.37; O, 7.28; S, 23.28.
Synthesis of G2 Per-Dendrimer (Figure 5). The general synthesis
method D was followed to get G2 Per-dendrimer in 42% yield. 1H NMR
(CDCl3): δ (ppm) 8.13 (d, 4H, J = 4 Hz), 7.87 (d, 4H, J = 4 Hz),
7.33À7.00 (m, 56H), 6.79 (s, 4H) 6.52 (s, 8H), 6.39 (s, 2H), 6.28 (s,
4H) 5.29 (s, 4H), 4.88 (s, 8H) 4.15 (t, 16H, J = 7 Hz), 3.19 (t, 16H, J =
7 Hz). Calcd: C, 64.42; H, 3.83; N, 0.84; O, 7.71; S, 23.19.
B as described above, the reaction of LAH (0.175 g, 4.7 mmol) in 300 mL
of THF with a solution of G1 ester (2 g, 2.7 mmol) in 20 mL of THF
afforded white solid in90% yield. 1H NMR (CDCl3): δ (ppm) 7.33À7.00
(m, 14H), 6.52 (d, 2H, J = 2 Hz), 6.38 (t, 1H, J = 2 Hz), 4.60 (s, 2H) 4.21
(t, 4H, J = 7 Hz),), 3.23(t, 4H, J = 7 Hz). Calcd: C, 61.01; H, 4.10; O, 6.97;
S, 27.92. Found: C, 61.26; H, 4.12; O, 6.78; S, 27.84.
Synthesis of (3,5-Bis(4-(9H-carbazol-9-yl)butoxy)phenyl)methanamine
(G1 NH2) (Scheme 3). The general synthesis method C was followed to
get G1 NH2 in 95% yield. 1H NMR (CDCl3): δ (ppm) 7.33À7.00 (m,
14H), 6.48 (d, 2H, J = 2 Hz), 6.38 (t, 1H, J = 2 Hz), 4.15 (t, 4H, J = 7 Hz),
3.75 (s, 2H) 3.19 (t, 4H, J = 7 Hz). Calcd: C, 61.10; H, 4.25; N, 2.04; O,
4.65; S, 27.96.
Synthesis of G1 Per (Scheme 3). The general synthesis method D was
followed to get G1 Per in 60% yield. 1H NMR (CDCl3): δ (ppm) 8.35
(d, 4H, J = 4 Hz), 8.13 (d, 4H, J = 4 Hz), 7.33À7.00 (m, 28H), 6.72 (s,
4H), 6.37 (s, 2H), 5.29 (s, 4H), 4.15 (t, 8H, J = 7 Hz), 3.19 (t, 8H, J = 7
Hz). Calcd: C, 65.17; H, 3.61; N, 1.62; O, 7.39; S, 22.21. Found: C,
64.73; H, 3.68; N, 2.13; O, 8.97; S, 20.49.
Synthesis of G1 NTCDI (Scheme 6). The general synthesis method D
1
was followed to get G1 NTCDI in 66% yield. H NMR (CDCl3): δ
(ppm) 8.70 (s, 4H), 7.33À7.00 (m, 28H), 6.65 (s, 4H), 6.34 (s, 2H),
5.27 (s, 4H), 4.13 (t, 8H, J = 7 Hz), 3.15 (t, 8H, J = 7 Hz). Calcd: C,
62.74; H, 3.64; N, 1.74; O, 7.96; S, 23.93.
Electrochemical Synthesis of Cross-Linked Dendrimers. The pre-
cursor dendrimers were electropolymerized using the cyclic voltamme-
try (CV) technique. In a three-electrode cell, 0.1 M tetrabutylam-
monium hexafluorophosphate (TBAH) was taken as a supporting
electrolyte along with 2.5 mM of each dendrimer dissolved in methylene
chloride in separate cells. The electropolymerization was performed by
sweeping the voltage at a scan rate of 50 mV/s from 0 to 0.8 V against
Ag/AgCl as a reference electrode and platinum as a counter electrode.
The ITO or were used as a working electrode and also as a substrate.
Synthesis of Methyl 3,5-Bis(3-(2-(2,5-di(thiophen-2-yl)thiophen-3-
yl)ethoxy)-5-(2-(5-(thiophen-2-yl)-2-(thiophen-3-yl)thiophen-3-yl)-
ethoxy)benzyloxy)benzoate (G2 Ester) (Scheme 4). Following the
general synthesis method A as described above, a precooled solution
of methyl 3,5-dihydroxybenzoate (0.221 g, 1.31 mmol), G1 OH (2 g, 2.9
mmol), and PPh3 (0.786 g, 3 mmol) in THF under sonication was
treated with a solution of DIAD (0.606 g, 3 mmol) in 2 mL of THF
under nitrogen. The solution was sonicated for 3 h to afford a yellow
’ RESULTS AND DISCUSSION
1
solid product in 45% yield after purification. H NMR (CDCl3): δ
Synthesis and Characterization. The synthesis of the den-
drons was performed using the classical Mitsunobu etherification
reaction. However, this reaction is very slow in the case of
sterically hindered molecules such as dendrons and dendrimers.
In order to achieve fast and synthetically useful yields, we have
utilized the sonication-mediated synthesis. The sonication-
mediated synthesis drastically reduces the reaction time from
weeks or days to only a couple of hours. We have achieved
comparable yields to other Frꢀechet type dendrons using simple
Mitsunobu etherification reaction performed under sonication.
Although in the past Leopre et al.11 have demonstrated a similar
approach using hindered phenols, surprisingly no group has
demonstrated the use of sonication chemistry to synthesize
dendrimers as in our first report.12 We have not only optimized
these conditions for etherification reactions, but have even
extended this method to synthesize imides. The dendrimers with
(ppm) 7.33À7.00 (m, 30H), 6.76 (t, 1H, J = 2 Hz), 6.55 (d, 4H, J = 2
Hz), 6.40 (t, 2H, J = 2 Hz), 4.94 (s, 4H), 4.16 (t, 8H, J = 7 Hz), 3.86 (s,
3H), 3.20 (t, 8H, J = 7 Hz). Calcd: C, 62.04; H, 4.00; O, 8.48; S, 25.48.
Found: C, 61.91; H, 4.12; O, 8.76; S, 25.21.
Synthesis of (3,5-Bis(3-(2-(2,5-di(thiophen-2-yl)thiophen-3-yl)ethoxy)-
5-(2-(5-(thiophen-2-yl)-2-(thiophen-3-yl)thiophen-3-yl)ethoxy)benzyloxy)-
phenyl)methanol (G2 OH) (Scheme 4). Following the general synthesis
method B as described above, the reaction of LAH (0.085 g, 2.2 mmol)
in 300 mL of THF with a solution of G2 ester (2 g, 1.3 mmol) in 15 mL
1
of THF afforded a yellow solid in 76% yield. H NMR (CDCl3): δ
(ppm) 7.33À7.00 (m, 28H), 6.58 (d, 2H, J = 2 Hz), 6.55 (d, 4H, J = 2
Hz), 6.51 (t, 1H, J = 2 Hz), 6.39 (t, 2H, J = 2 Hz), 4.92 (s, 4H), 4.60 (s,
2H), 4.16 (t, 8H, J = 7 Hz),), 3.20 (t, 8H, J = 7 Hz). Calcd: C, 62.40; H,
4.08; O, 7.56; S, 25.96. Found: C, 62.45; H, 4.07; O, 7.72; S, 25.76
Synthesis of (3,5-Bis(3,5-bis(2-(2,5-di(thiophen-2-yl)thiophen-3-yl)-
ethoxy)benzyloxy)phenyl)methanamine (G2 NH2) (Scheme 5). The
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Scheme 5. Synthesis of G2 Per-terthiophene Dendrimer
Scheme 6. Synthesis of G1 NTCDI Dendrimer
terthiophene groups at the periphery and perylene tetracarboxy-
diimide at the core were synthesized by a convergent synthetic
strategy. First, the Frꢀechet type dendrons with OH functionality at
the focal point were synthesized. The OH groups were converted
to amine groups by reacting the corresponding Gn OH with
pthalimide in the presence of triphenylphosphine and diisopropyl
azodicarboxylate. Pthalimide was deprotected in the presence of
hydrazine hydrate according to the literature13 and then grafted to
the PDI core. To compare the optical properties of these materials,
we have also synthesized a first generation terthiophene dendrimer
with naphthalene tetracarboxydiimide as a core. It is known that
perylene tetracarboxydianhydride (PDA) has very limited solubility
in common organic solvents, but when we functionalized with
terthiophene dendrons, the solubility of the resulting dendrimers
increased drastically. The synthesized dendrimers were character-
ized using NMR, elemental analysis, and MALDI-TOF mass
spectroscopy technique. With increasing dendron generation, the
1H and 13C NMR signals become more complicated and difficult to
discern the structures. To assess more clearly the identity of the
chemical structures, further characterization was performed using
MALDI-TOF mass spectrometry which confirmed exact mass
assignment with structure.
Figure 1. UVÀvis spectra of first and second generation dendrimers in
CH2Cl2 solution (optically matched at the perylene region).
dendrimers were investigated by UVÀvis absorption as well as
fluorescence spectroscopy. Figure 1 shows the absorption
spectra of the first and second generation dendrimers optically
matched at the perylene region in dichloromethane (ca. 5 Â
10À6 M). As can be seen, both the first and the second
generation dendrimers show very similar absorption character-
istics. The broad absorption peak at 345 nm is attributed to the
πÀπ* transitions of terthiophene14 while the peaks at 460, 490,
and 530 nm are attributed to the perylene diimide core.15 From
the first generation to second generation, the absorbance of the
Absorption and Luminescent Properties. The optical
properties of the terthiophene-functionalized perylene bisimide
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Figure 2. Emission spectra of first and second generation dendrimers in
CH2Cl2 solution excited at 345 nm (inset excited at 490 nm).
Figure 3. UVÀvis and emission (excited at 340 nm) spectra of G1
NTCDI in CH2Cl2.
terthiophene moiety increased proportionally to the dendrimer
generation in an exponential manner consistent with the
number of terthiophene groups, e.g., G1 = 4 and G2 = 8,
terthiophene units in first and second generation, respectively.
The presence of terthiophene units does not influence the
absorption behavior of perylene chromophore and vice versa,
indicatingnegligible interaction betweentheground states of these
two chromophores. Figure 2 shows the fluorescence spectra for
these samples excited at 345 nm. In these dendrimers, both the
peripheral chromophores and the focal dye are capable of con-
tributing to the absorption of light by the entire macromolecule. In
a dendritic antenna, the peripheral donor units collectphotons and
transfer the excitation energy through space to the core or focal
point acceptor. An interesting property to observe here is a
possible energy transfer from the peripheral terthiophene to the
central perylene unit through a singletÀsinglet F€orster resonance
energy transfer (FRET) mechanism as there is a slight overlap
between the emission spectrum of terthiophene and absorption
spectrum of perylene.16 To monitor this phenomenon as a
function of dendrimer generation, we have excited these samples
at 345 nm which is the terthiophene absorption region and
monitored from 360 to 620 nm. Both the first and the second
generation dendrimers exhibit perylene bisimide peak at around
540 nm along with an intense emission peak of the terthiophene at
around 430 nm. Upon irradiation at 345 nm, the emission peak
from perylene chromophore reveals an energy transfer from the
terthiophene units to the perylene core. The inset of Figure 2
shows the emission peaks of both dendrimers excited at 490 nm.
The emission of the perylene moiety in second generation was
drastically quenched more than in the case of first generation.
Because of the electron richness of the terthiophene groups, it is
not surprising that the emission of the perylene units is quenched
by photoinduced electron transfer (PET) process.17 To further
Figure 4. Cyclic voltammograms for (a) G1 Per and (b) G1 NTCDI.
support PET processes time-resolved emission spectroscopy
studies are underway.
Synthesis of G1 NTCDI was carried in a similar manner to the
perylene dendrimers. Figure 3 shows the UVÀvis absorption and
emission spectra of G1 NTCDI in dichloromethane solution.
Absorption peak with λmax at 345 nm is attributed to the πÀπ*
transitions of terthiophene14 while the peak shoulder at 380 nm
is attributed to the naphthalene diimide core. The fluorescence
spectrum of G1 NTCDI is very similar to terthiophene itself with
an emission peak at 440 nm. Both naphthalene core and
terthiophene emit in the same region.
Electrochemical Synthesis of Cross-Linked Dendrimers.
Further insight into the electronic properties of these materials
was investigated by cyclic voltammetry. The electrochemical
polymerization is an important method for forming cross-linked
films directly onto a conducting substrate. Our group has been
investigating the precursor polymer approach to form conjugated
polymer networks (CPN).18 This approach has recently been
applied toward the electrochemical cross-linking and deposition
of dendrimers.19 The resulting cross-linked film is highly robust,
mechanically stable, and insoluble in any organic solvents. The
electropolymerization of each dendrimer was performed in a
three-electrode cell, where the following electrodes were used:
platinum as a counter electrode, Ag/AgCl as a reference elec-
trode, and ITO-coated glass substrate as anode/working elec-
trode. The dendrimers were electrodeposited (cross-linked) as
films using 2.5 mM concentration in methylene chloride contain-
ing 0.1 M tetrabutylammonium perchlorate as a supporting
electrolyte and scanning between 0 and 0.8 V at a scan rate
of 50 mV/s.
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Figure 5. Cyclic voltammograms of the thin films in monomer free
acetonitrile solution (a) G1 Per and (b) G1 NTCDI.
Figure 4 shows the different CV traces for both first generation
perylene and naphthalene diimide dendrimers. Both the dendri-
mers show similar redox behavior with an onset potential of
0.62 V vs Ag/AgCl. From the second scan, the growth of the
conducting polymer is reflected by gradually increasing currents
in subsequent potential cycles and the simultaneous decrease in
the onset potential, which corresponds to the formation of the
polythiophene backbone.20 The slight increase in the peak
separation potential in the subsequent cycles reflected the
increase in the polymer film resistance as the thickness of the
film increased with increasing cycles. These films are very robust
and showed good stability toward repeated redox cycling. The
slight increase in the peak separation potential reflected the
increase in the polymer film resistance as the thickness of the film
increased with increasing cycles.21 Subsequently, the redox
properties of the dendrimers were investigated by cyclic voltam-
metry. The electrochemically deposited thin films of G1 Per and
G1 NTCDI were washed with dichloromethane and placed into
a monomer free electrolyte solution of dichloromethane. Figure 5
shows the cyclic voltammograms of G1 Per and G1 NTCDI
dendrimer films. They both exhibit good p- and n-doping
processes as these dendrimers were designed to exhibit electro-
chemical activity in both anodic and cathodic potential scans.
When an anodic potential is applied, the polyterthiophene
moiety gets oxidized and shows the p-doping properties, whereas
in the cathodic potential scan the perylene and naphthalenete-
tracarboxydiimide moieties get reduced and show n-dpoing
characteristics. Moreover, the electrochemical redox processes
of the films are very stable in the features of cyclic voltammetry
during subsequently repeated scanning. In the p-doping process,
the onset oxidation potential is around 0.62 V vs Ag/AgCl for
both dendrimers, which is typical for polyterthiophene moiety. In
the n-doping process, G1 Per shows a quasi-reversible reductions
at À1.6 V, which is typical for the reduction of perylene bisimides
Figure 6. Spectroelectrochemical changes for (a) G1 Per and (b) G1
NTCDI at different applied voltages.
to anionic species.22 Wedidnot observea secondreductiontoform
a dianionic species, which is expected for the perylene moiety.23 In
the case of G1 NTCDI, an irreversible reduction at À1.9 V was
observed. It is normal for aromatic diimides that the reduction
waves become less reversible at scan rates below 0.2 V/s, suggesting
that the radical anions undergo slow decomposition.24
To obtain further insight into the redox process of the
terthiophene-functionalized perylene and naphthalene bisi-
mides, spectroelectrochemistry of the electropolymerized films
was studied by recording their UVÀvis absorption at different
applied potentials. It is interesting to observe the electrochromic
properties of these materials as these materials contain both
donor and acceptor moieties. The changes in the absorption
spectra induced by p- and n-doping of G1 Per and G1 NTCDI
are presented in Figure 6 as a series of UVÀvis absorbance curves
correlated to different electrode potentials. In the neutral form at
0 V, G1 Per film exhibited strong absorption at a wavelength near
460 nm with two shoulder peaks at 500 and 540 nm. Upon
increasing the applied voltage from 0 to 0.6 V, no change was
observed in the UV spectrum. The intensities of the peaks at 500
and 540 nm start to decrease at potential around 0.8 V, where the
polymer starts to be doped. As the intensities of the peaks
decrease, there is a simultaneous appearance of a new band in
the longer wavelength region from 600 to 800 nm. This spectral
change was attributed to the formation of radical cations of
polyterthiophene showing the doped state of the polymer.25 On
the other hand, when the electrode potential was decreased from
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0 to À1.6 V, the absorption peaks at 460 and 500 nm decrease in
favor of new absorption peaks at 570, 610, and 650 nm. Further
decrease in the potential did not show any spectral changes. The
origin of new peakswas attributed to the formation of radical anions
of perylene moieties,26 which is in agreement with the electro-
chemical data. For G1 NTCDI, a stepwise increase of the electrode
potential to 0.8 V drastically quenches the absorption peak at
460 nm with the simultaneous formation of a new absorption band
in the longer wavelength region from 600 to 800 nm similar to that
of G1 Per which is attributed to the formation of radical cations of
polyterthiophene showingthe doped state of the polymer.27 On the
other hand, a stepwise decrease of the electrode potential from 0 to
À1.6 V allows a complete recovery of the absorption peak without
any other changes in the absorption spectrum. Further reduction in
the electrode potential to À2.5 V did not change the absorption
spectrum. The electrochemical reduction of naphthalene diimide to
the corresponding radical anion state leads to a change in the
absorption with the appearance of new peak at 470 nm. In this case,
this change was not visible as the polyterthiophene has an intense
absorption in this region. In case of G1 Per, the fabricated films
shows a clear dual electochromism by showing electrochromic
activity in both anodic and cathodic potential scans. These spectral
changes observed in the UVÀvis spectrum are difficult to see with
the naked eye, especially during cathodic scan as the shift in the
wavelength is very small. However, this study reveals protocols for
making libraries of materials that can be tailored to act as dual
electrochomic materials and leading to multifunctional electro-
chromic films. It will be interesting to investigate the electrolumi-
nescence behavior of these materials as well.
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’ CONCLUSION
In conclusion, we have synthesized terthiophene-terminated
dendrons and dendrimers with naphthalene and perylene di-
imide cores. Characterization of the optical and redox properties
of the novel materials shows that it is possible to design materials
with donor and acceptor moieties in the same molecule which
will retain their individual properties. Electrochemical polymer-
ization of these materials provided polymer films which have
been investigated by spectroelectrochemical methods. The films
obtained are highly active in both the p- and n-doping processes.
G1 Per with perylene core and terthiophene dendron periphery
showed dual electrochromism by showing the electrochromic
activity in both cathodic and anodic potential scans. Further
investigation of these materials is still underway.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: radvincula@uh.edu. Fax: 713-743-1755.
’ ACKNOWLEDGMENT
The authors acknowledge funding from NSF CBET-0854979,
DMR-10-06776, and Robert A. Welch Foundation, E-1551.
Technical support from INFICON Inc. and Metrohm is also
acknowledged.
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