Terthiophene-jacketed poly(benzyl ether) dendrimers: Sonication synthesis, electropolymerization, and polythiophene film formation

Article abstract of DOI:10.1021/ma1017023

We report the design and synthesis of a series of poly(benzyl ether)-type terthiophene-jacketed dendrimers. These dendrimers were designed as precursor macromolecules, which bear the electroactive terthiophene group that can be cross-linked by electrochem

Full text of DOI:10.1021/ma1017023

10414 Macromolecules 2010, 43, 10414–10421
DOI: 10.1021/ma1017023
Terthiophene-Jacketed Poly(benzyl ether) Dendrimers: Sonication
Synthesis, Electropolymerization, and Polythiophene Film Formation
Ramakrishna Ponnapati, Mary Jane Felipe, Jin Young Park, Jonathan Vargas, and
Rigoberto Advincula*
Department of Chemistry and Department of Chemical and Biomolecular Engineering, University of Houston,
Houston, Texas 77204-5003, United States
Received July 28, 2010; Revised Manuscript Received November 11, 2010
ABSTRACT: We report the design and synthesis of a series of poly(benzyl ether)-type terthiophene-jacketed
dendrimers. These dendrimers were designed as precursor macromolecules, which bear the electroactive terthi-
ophene group that can be cross-linked by electrochemical methods to form a conjugated polymer network
(CPN) polythiophene film. Characterization was done by NMR, elemental analysis, MALDI-TOF, and GPC
to confirm the chemical structure and macromolecular properties of the dendrimers. Spectroscopic studies of
thin films made by electropolymerization of these materials show the role that the dendrimer structure plays
in the UV-vis absorption and emission properties of a highly cross-linked conjugated polymer film compared
to linear polymers. The distinction is more evident in their emission behavior. Copolymerization studies in
particular were made to investigate the formation of more linear π-conjugated sequences and their effect in the
spectroscopic properties. The results showed improved intermolecular cross-linking with the formation of more
emissive species in the visible region of the spectrum. Spectroelectrochemistry studies of these materials showed
a highly reversible electrochromic switching behavior for the films.
Introduction
In this case they facilitate high yield and short reaction times for
convergent dendrimer synthesis via etherification.
Dendrimers with electroactive groups at the periphery are of
current interest for research in designing efficient organic materi-
als for devices and sensor applications.1 π-Conjugated polymers
such as polythiophene, polypyrrole, polycarbazole, etc., have
been studied for their electrical conducting behavior through
doping of their linear π systems or by charge carrier generation.2
It has been assumed that the efficiency of the charge-carrier
transport properties depends on the conjugation length of the
“one-dimensional” polymers.3 A wide range of flexible-backbone
and rigid-backbone dendrimers such as polyphenylacetylene,4
polyphenylenevinylene,5 polythiophene,6 and polyphenylene7
dendrimers have been reported recently. Further studies indicate
the importance of persistence length and oligomeric derivatives in
determining electro-optical property control and limits.8 How-
ever, there is no clear understanding of 2D or even 3D charge
transport in a networked conjugated polymer structure including
energy transfer and emission properties. Recently, our group has
focused on electropolymerization approaches using both linear
and dendrimeric precursors to form conjugated polymer net-
works (CPN).9 It is expected that dendrimers canprovide another
insight into the mechanism of electropolymerization owing to
their monodispersed, highly branched, and structurally regular
macromolecular structures. For example, the synthesis of carbazole-
terminated dendrimers to form cross-linked nano-objects has
been reported.10
In this work, we report on the convergent synthesis and char-
acterization of a series of terthiophene-jacketed poly(aryl ether)
dendrimers. To our knowledge, this is the first attempt to synthesize
ꢀ
Frechet-type poly(aryl ether) dendrons and dendrimers with
terthiophene at the periphery. In our design, both the R-positions of
the terthiophene are made available for electropolymerization to
produce polythiophene-jacketed dendrimers. We have chosen the
terthiophene at the periphery of the dendrimers because of the
increasing importance of thiophene derivatives in organic electron-
ics research. Thiophene derivatives are one of the most attractive
chromophores in photochemistry and have been extensively studied
for other electro-optical devices.13 Dendrimers with terthiophene at
the periphery are also attractive due to their high electroactivity to
form a CPN upon electropolymerization or chemical oxidation.
There have been a few reports describing the electropolymerization
of the peripheral groups on a dendrimer. For example, in one of
our studies we have demonstrated the cross-linking of the peripheral
carbazole groups of a carbazole-terminated PAMAM dendri-
mers.14 Similarly, Crooks et al.15 have reported the cross-linking
of thiophene-terminated PAMAM dendrimers. In our present design,
we have chosen the poly(benzyl ether) dendrimer scaffold instead of
a linear chain polymer not only to enhance the solubility of the
resulting polymer but also to investigate the optoelectronic film
properties as a function of dendrimeric generation. Also, the poly-
(aryl ether) backbone possesses a lower Tg and could undergo
conformational changes as a function of generation, which could
allow fine-tuning of their electro-optical properties.
In order to expand the scope of dendrimer synthesis, recently,
we have developed a high yielding and faster poly(benzyl ether)
dendrimer synthetic route using sonochemistry, which led to
highly comparable yields to the conventional route within hours
rather than days.9b Sonication11 and microwave12 methods have
long been employed for enhancing the rate of chemical reactions.
Experimental Part
Materials. All the chemicals were purchased from Aldrich
Chemical Co. and were used without further purification unless
otherwise specified. Solvents were properly distilled to the right
purity and used immediately.
*Corresponding author. E-mail: radvincula@uh.edu. Fax: 713-743-
1755.
pubs.acs.org/Macromolecules
Published on Web 12/02/2010
r 2010 American Chemical Society

Article
Characterization. NMR spectra were recorded using a Gen-
Macromolecules, Vol. 43, No. 24, 2010 10415
Scheme 1. Synthesis of G0 OH Terthiophene Dendron
eral Electric QE 300 spectrometer (1H 300 MHz). UV-vis
spectra were recorded using an Agilent 8453 spectrometer. Fluo-
rescence spectra were obtained on a Perkin-Elmer LS-45 lumi-
nescence spectrometer. Elemental analysis was done (samples
sent) by Atlantic Analytical Microlabs. The size exclusion chro-
matography (SEC) or gel permeation chromatography (GPC)
analysiswas performedusing a Viscotek 270 quad detector equip-
ped with a VE3210 UV-vis detector and a VE3580 RI detector.
The columns used for finding SEC number-average molecular
weight (Mnsec) type are G3000HHR and GHMHR-M Viscogel.
To assess more clearly the identity of the chemical structures,
further characterization was performed using MALDI-TOF
(matrix-assisted laser desorption ionization time-of-flight) mass
spectroscopy (Perseptive Biosystems). The sample was prepared
by a method called “thin layer application” using trans-3-
indoleacrylic acid (IAA) as a matrix. All generations of the
dendrimers showed the corresponding ion peak of Mþ þ Na.
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 work-
ing 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) (Figure 1). The synthesis of 3T ET was
carried by first synthesizing ethyl 2-(2,5-dibromothiophen-
3-yl)acetate as reported in the literature.9a 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-(tributy-
lstanny1)thiophene (15 g, 20 mmol) were added to a 30 mL
dry DMF solution of dichlorobis(tripheny1phosphine)palladium
(1.3 g, 1.5 mmol). After three freeze-thaw cycles, the mixture was
heated at 100 °C for 48 h. The mixture was cooled to room tem-
perature and poured into a beaker containing 150 mL of water and
subsequently extracted with CH2Cl2. The extracted CH2Cl2 mix-
ture 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 ob-
tained in 85% yield as pale yellow oil. The characterization of the
compound was found in accordance with the literature.9a
Synthesis of 2-(2,5-Di(thiophen-2-yl)thiophen-3-yl)ethanol (G0
OH) (Scheme 1). The compound 3T ET (2 g, 5.9 mmol) in 10 mL
of THF was dropwise added 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 turns 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 turns yellow upon neutralization. The
solvent was evaporated, and the resulting mixture was extracted
three times using CH2Cl2. The combined CH2Cl2 extracts was
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:hex-
ane mixture as eluent. 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) 7.31-7.04 (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.
5 min. Under sonication, a solution of diisopropyl azodicarbox-
ylate (DIAD) was added dropwise under nitrogen. 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.
General Method for Lithium Aluminum Hydride (LAH) Re-
duction of Aromatic Esters to Benzylic Alcohols (Method B). The
aromatic ester 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 was removed under re-
duced pressure in a rotary evaporator. The resulting layer was
broughttoa neutral pH withthe additionof2 N HClsolution and
extracted with CH2Cl2. The organic layer werecombined, 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.
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), G0 OH (3 g,
10.2 mmol), and PPh3 (2.67 g, 10.3 mmol) in THF under sonica-
tion 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 purification.
1H NMR (CDCl3): δ (ppm) 7.33-7.02 (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 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 in 90%
yield. 1H NMR (CDCl3): δ (ppm) 7.33-7.00 (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 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 3).
Following the general synthesis method A as described above, a pre-
cooled solution of methyl 3,5-dihydroxybenzoate (0.221 g, 1.31
mmol), G1OH (2g, 2.9mmol), andPPh3 (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 soni-
cated for 3 h to afford a yellow solid product in 45% yield after
General Method for Etherification Reactions (Method A). The
Mitsunobu etherification method was performed using sonica-
tion conditions according to a literature procedure reported by
our group recently.9b The mixture of alcohol, phenol (or aro-
matic carboxylic acid), and triphenylphosphine (PPh3) in mini-
mal THF was cooled to 5 °C with an ice bath and sonicated for

10416 Macromolecules, Vol. 43, No. 24, 2010
Ponnapati et al.
Scheme 3. Synthesis of G2 OH Terthiophene Dendron
Scheme 2. Synthesis of G1 OH Terthiophene Dendron
1
purification. H NMR (CDCl3): δ (ppm) 7.33-7.00 (m, 30H),
4.14 (t, 24H, J = 8 Hz), 3.18 (t, 24H, J = 8 Hz). Calcd: C, 62.63;
H, 3.94; O, 8.34; S, 25.08. Found: C, 62.54; H, 4.17; O, 8.35; S,
24.95. MALDI-TOF of G2 D with Mþ = 4557.7.
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.
Electrochemical Synthesis of Cross-Linked Dendrimers. The
precursor dendrimers were electropolymerized using the CV
technique. In a three-electrode cell, 0.1 M tetrabutylammonium
perchlorate (TBAP) was taken as a supporting electrolyte along
with 5 mM of each dendrimer dissolved in 6 mL of methylene
chloride in separate cells. The electropolymerization was per-
formed 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 gold-coated slides were used
as a working electrode and also as a substrate.
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 3). Fol-
lowing 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 of THF afforded a
yellow solid in 76% yield. 1H NMR (CDCl3): δ (ppm) 7.33-7.00
(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 Tris(2-(2,5-di(thiophen-2-yl)thiophen-3-yl)ethyl)-
benzene-1,3,5-tricarboxylate [G0 D] (Scheme 4). Following the
general synthesis method A as described above, a precooled
solution of benzene-1,3,5-tricarboxylic acid (63 mg, 0.3 mmol),
G0 OH (292 mg, 1 mmol), and PPh3 (262 mg, 1 mmol) in THF
under sonication was treated with a solution of DIAD (202 mg,
1 mmol) in 2 mL of THF under nitrogen. The solution was soni-
cated for 8 h to afford yellow solid product in 55% yield after
purification. 1H NMR (CDCl3): δ (ppm) 8.81 (s, 3H), 7.33-7.00
(m, 21H), 4.58 (t, 6H, J = 8 Hz), 3.21 (t, 6H, J = 8 Hz). Calcd: C,
59.27; H, 3.51; O, 9.29; S, 27.93. Found: C, 59.44; H, 3.60; O,
9.24; S, 27.72. MALDI-TOF of G0 D with Mþ = 1032.1.
Synthesis of Tris(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)benzene-1,3,5-tricarboxylate [G1 D] (Scheme 4).
Following the general synthesis method A as described above, a
precooled solution of benzene-1,3,5-tricarboxylic acid (18 mg,
0.085 mmol), G1 OH (200 mg, 0.3 mmol), and PPh3 (76 mg, 0.3
mmol) in THF under sonication was treated with a solution of
DIAD (60 mg, 0.3 mmol) in 2 mL of THF under nitrogen. The
solution was sonicated for 12 h to afford yellow solid product in
Results and Discussion
Synthesis and Characterization. The synthesis was started
with ethyl-3-thiophene-3-acetate to produce G0 OH by using
a literature procedure.9a The G1 ester was synthesized by a
modified Mitsunobu etherification reaction under sonica-
tion conditions. The solution was sonicated for 2 h to afford
a pale yellow solid product in 54% yield after purification.
This is in contrast to several days of refluxing as has been
commonly done with this reaction.9 The G1 ester was then
reduced using LiAlH4 to produce G1 OH as a pale yellow
solid in 90% yield. G2 OH was synthesized in the same way
as G1 OH. Scheme 4 shows the route for the synthesis of G0,
G1, and G2 dendrimers from their corresponding Gn OH
dendrons and the trifunctional acid core. These esterifica-
tions were carried out under the same conditions used for the
etherification reactions under sonication. All these dendrimers
are highly soluble in common organic solvents such
as CHCl3, CH2Cl2, THF, toluene, ethyl acetate, etc. There is
good correspondence between NMR data and elemental
analysis for all the synthesis steps, indicating the correct
chemical structure and the right purity needed for further
physical analysis. Further characterization was performed
using GPC and MALDI-TOF (matrix-assisted laser desorp-
tion ionization time-of-flight) mass spectroscopy technique.
The MALDI-TOF analysis verified the monodispersed masses
of all G0, G1, and G2 dendrimers, giving the corresponding ion
peak of Mþ as 1032.1, 2222.0, and 4557.7, respectively. This is
consistent with the exact calculated MW values for each den-
drimer. Figure 1 shows the GPC peak results as a function of
retention volume. The polydispersity index was found on the
average to be less than 1.02 for all three generations with cor-
responding MW values not too different from MALDI data
with PS calibration: 1201, 2345, and 4611 g/mol for G0, G1, and
G2, respectively. More importantly, the elution curves were
1
45% yield after purification. H NMR (CDCl3): δ (ppm) 8.84
(s, 3H), 7.30-6.95 (m, 42H), 6.54 (s, 6H), 6.38 (s, 3H), 5.24 (s, 6H)
4.14 (t, 12H, J = 8 Hz), 3.18 (t, 12H, J = 8 Hz). Calcd: C, 61.13; H,
3.60; O, 8.80; S, 26.46. MALDI-TOF of G1 D with Mþ = 2222.0.
Synthesis of Tris(3,5-bis(3,5-bis(2-(2,5-di(thiophen-2-yl)thiophen-
3-yl)ethoxy)benzyloxy)phenyl)benzene-1,3,5-tricarboxylate [G2 D]
(Scheme 4). Following the general synthesis method A as described
above, a precooled solution of benzene-1,3,5-tricarboxylic acid
(21mg, 0.1mmol), G2 OH (510 mg, 0.35 mmol), and PPh3 (88 mg,
0.35 mmol) in THF under sonication was treated with a solution
of DIAD (72 mg, 0.35 mmol) in 2 mL of THF under nitrogen. The
solution was sonicated for 12 h to afford yellow solid product in
40% yield after purification. 1H NMR (CDCl3): δ (ppm) 8.84 (s,
3H), 7.30-6.95 (m, 84H), 6.55 (s, 12H), 6.36 (s, 6H), 5.21 (s, 12H)

Article
Macromolecules, Vol. 43, No. 24, 2010 10417
Scheme 4. Synthesis of G0 to G2 Terthiophene Dendrimers
broad absorption with λmax 345 nm is attributed primarily
to the π-π* transitions of a terthiophene monomer.16 The
emission spectra of the optically matched samples of the G0,
G1, and G2 dendrimers are also shown in Figure 2b. Again,
this represents the emission properties of a terthiophene unit
(excitation at 345 nm). The fluorescence emission intensities
of the G0 and G2 dendrimers seem to be slightly quenched
more than that of the G1 dendrimer. This quenching behav-
ior can be attributed to conformational changes due to the
dendritic wedges and the restriction that may be imposed on
G0 and G2 design.17a This suggests that there may be an
optimal conformational freedom required to allow intermo-
lecular interaction between the terthiophene chromophores
that may lead to quenching of the emission.17b-d Further studies
are underway to understand the influence of the dendrimer
generation on the quenching of the emission and fluorescence
lifetime measurements, including simulation studies with semi-
empirical modeling.
Figure 1. GPC chromatograms of dendrimers showing monomodal
distribution.
found to be narrow and monomodal in distribution. A decrease
in the retention volume was observed in going from the lowest
generation to the highest, which is consistent with an increase in
the hydrodynamic volume as the generation increases.
UV Absorption and Emission Studies. Shown in Figure 2a
are the optically matched UV-vis absorption spectra of the
G0, G1, and G2 dendrimers in methylene chloride. The
Electropolymerization. Electropolymerization of the den-
drimers was carried out by cyclic voltammetry (CV) using
indium tin oxide (ITO) as a working electrode, platinum as a
counter electrode, and Ag/AgCl as a reference electrode in a
three-electrode cell. The dendrimers were electrodeposited
(cross-linked) as films using 1.0 mM concentration in methylene
chloride containing 0.1 M tetrabutylammonium perchlorate as

10418 Macromolecules, Vol. 43, No. 24, 2010
Ponnapati et al.
Figure 4. Energy level diagram showing HOMO and LUMO levels for
all three dendrimers.
This suggests that there is an optimal conformational free-
dom required to allow extended conjugation during cross-
linking for these dendrimers.9b From the CV traces, the
growth of the conducting polymer is reflected by gradually
increasing currents in subsequent potential cycles and the
simultaneous appearance of the new onset peaks around
0.35 V, which corresponds to the formation of the polythio-
phene backbone.18 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.
These films showed very good stability toward repeated re-
dox cycling. A linear relationship between peak current and
scan rate was observed for all these films, suggesting that the
redox-active polymer was attached to the electrode with a
typical Fickian diffusion behavior as defined by the Cottrell
equation.9,10 The CV result also suggests that increasing the
dendrimer generation increases the amount of cross-linked
dendrimer units confirmed by increased peak current. Such
an evolution of the polymer structure could be related to the
increasing probability of inter- and/or intradendrimer cou-
pling of the terthiophene units as the Gn D increases. But
unlike previous studies on carbazole-jacketed poly(benzyl
ether) dendrimers, the peak current linearity for the three
generations are about the same.9b
Figure 2. (a) UV-vis absorption spectra and (b) emission spectra
excited at 345 nm of G0 D, G1 D, and G2 D in CH2Cl2.
In order to determine the band gap of these materials, we
performed CV on the cross-linked films using 0.1 mM
tetrabutylammonium phosphate (TBAP) as a supporting
electrolyte in methylene chloride and a Ag/AgCl reference
electrode. All three films showed quasi-reversible oxidative
behavior. The ionization potential can be taken as a measure
for the energy of the highest occupied molecular orbital
(HOMO). The onset oxidation potentials of these materials
were determined by the anodic scan. The measured oxidation
potentials can be converted into ionization potentials by
relating the electrochemical energy scale to the vacuum
energy scale.19 The calculated HOMO energy levels for G0 D,
G1 D, and G2 D are -4.77, -4.75, and -4.78 eV, respec-
tively. The LUMO level was estimated from the onset of the
UV-vis spectra and was found to be -2.17, -2.25, and
-2.25 eV for G0 D, G1 D, and G2 D, respectively.
The AFM images of the cross-linked films are shown in
Figure 5. Although it is hard to distinguish any marked dif-
ferences between the cross-linked films, sharp conical mor-
phological features and uniform surface coverage seemed to
be a function of the extent of cross-linking. While G0 D and
G2 D form relatively smooth films, G1 D appears to be
rougher and forms globular domains, indicating perhaps a
greater degree of quasi-particle formation during cross-
linking. The root-mean-square roughness (rms) of the den-
drimers G0 D, G1 D, and G2 D were found as 5.1, 5.9, and
2.5 nm, respectively. As mentioned earlier, the G1-D is less
quenched in the solution emission spectra than the G0 D and
G2 D, which indicate less pairing of the terthiophene units.
Figure 3. Cyclic voltammograms of 10 cycles for (a) G0, (b) G1, (c) G2,
and (d) trace of the first cycles of G0, G1, and G2 for clarity.
Table 1. Dendrimeric Cross-Linked Precursors Showing Peak Anodic
and Cathodic Currents/Potentials and Their Corresponding Onsets of
Oxidation Potential
dendrimers onsets Epa (V) ipa (mA) Epc (V) ipc (mA) ΔE (V)
G0 D
G1 D
G2 D
0.52
0.56
0.60
0.62
0.63
0.64
0.10
0.12
0.15
0.32
0.28
0.26
-0.09
-0.10
-0.14
0.20
0.28
0.34
a supporting electrolyte. Figure 3 shows different CV traces
recorded between 0 and 0.8 V vs Ag/AgCl for all the genera-
tions of the dendrimers. Table 1 summarizes the peak anodic
and cathodic currents and potentials. A clear decrease in the
onset potentials going from G0 to G2 (Figure 3d) is observed.

Article
Macromolecules, Vol. 43, No. 24, 2010 10419
Figure 6. Optical studies of electropolymerized dendrimers in solid
state thin films: (a) UV-vis absorption spectra and (b) emission spectra
of the different films excited at 440 nm.
relatively sharper peak and appears to have the highest Stokes
shift around 210 nm, indicating a large difference in the ground
and excited states. This G2D behavior is rather peculiar since
the narrow half-width maximum is less than 100 nm and may
indicate a very narrow distribution of conjugated species. On
the other hand, G1 D shows only two weak peaks with λmax at
525 nm with a shoulder at 590 nm. The peak at 525 nm indi-
cates a limited intermolecular cross-linking leading to a rela-
tively smaller amount of fluorescing conjugated units at this
particular excitation.22 This behavior may again be related to
the inability of the terthiophene units in G1 D to “reach out” to
neighboring dendrimeric species but have a tendency to react
only intramolecularly or within the dendrimer surface alone.
This is in contrast to what has been observed with polybenzy-
lether dendrimers bearing carbazole peripheral units in solu-
tion and solid-state films, where electropolymerizability and
the PL properties are highly correlated.9b PL properties should
also be a function of effective conjugation length and effective
torsional strain between thiophene units such that this can
be correlated with the inter- and intramolecular cross-linking
properties of the dendrimers.24 Future studies will include
other modes of excitation and fluorescence lifetime studies in
order to further probe the distinction of the respective PL be-
havior of the different generations and the narrow peak ob-
served with G2 D. Again, it will be interesting to correlate with
molecular modeling the dendrimer conformation in distin-
guishing these spectral features.
Figure 5. AFM images of the cross-linked dendrimer films: (a) G0 D,
(b) G1 D, and (c) G2 D.
It is possible that the eventual film formation has a correla-
tion to the inability of G1D to form more intermolecular
cross-linking, i.e., reaction of the terthiophenes with another
dendrimer. This is also evident in the fluorescence behavior
of the films as discussed in the next section. The cross-linked
solid-state films obtained by electropolymerization have
been characterized spectroscopically by UV-vis and fluores-
cence or photoluminescence (PL) measurements likewise to
understand their optical properties. The extent of increasing
π-orbital overlap between neighboring repeat units on con-
jugated polymers can be directly assessed from their electronic
spectra.20 Theextentofelectronicπ-conjugation directly affects
the observed energy of the π-π* transition, which appears as
the absorption maxima in these materials.23
As seen from Figure 6a, all three generations show broad
absorption peaks from 300 to 600 nm with a λmax value around
440 nm, attributed to π-π* transitions in a π-conjugated poly-
mer. These values are comparable to the absorption maxima of
typical polyterthiophenes obtained by electropolymerization21
and, in general, that of polythiophenes.24 Although the average
conjugation length cannot be determined from this data alone,
the red-shifted absorption relative to the terthiophene mono-
mer (oligomer of three thiophene units) indicates a significant
increase in conjugation length. It is also consistent with the
measured band gap, Eg, in Figure 2. Figure 6b shows the PL
spectra of the cross-linked dendrimer films excited at 440 nm.
The results of the photoluminescence of these cross-linked films
are more interesting. The λmax values for G0 D, G1 D, and G2
D are 590, 525, and 660 nm, respectively, and are normalized.
For G0 D film, the photoluminescence peak appears as a single
broad peak with a Stokes shift of 150 nm while G2 D shows
To understand this electrochemical cross-linking process
better, we have electropolymerized G1 D with various con-
centrations of the thiophene monomer, a copolymerization
process that introduces more intermolecular linking and
extends the amount of π-conjugated species. The emission
behavior for the electrodeposition of the monomer (terthio-
phene) alone is shown in the Supporting Information. Elec-
tropolymerization of G1 dendrimer with thiophene mono-
mer was done using the same conditions used for the G1

10420 Macromolecules, Vol. 43, No. 24, 2010
Ponnapati et al.
Figure 7. Optical properties of electropolymerized films of G1 with
thiophene monomer: (a) UV-vis absorption spectra and (b) emission
spectra excited at 440 nm for various ratios.
Figure 8. Spectroelectrochemical changes for the G1 D at different
constant voltages (a) forward and (b) backward directions.
other flat display device or “sandwich type” thin film device
or sensor applications.
dendrimer itself. We have made three different films by elec-
trochemical synthesis from pure G1 dendrimer as well as 1:1
and 1:18 molar ratios of G1 dendrimer and thiophene mono-
mer. The CV traces showed similar behavior to the G1 den-
drimer alone for all three films. Interestingly, the current
density of the film during scanning abruptly decreases with
the thiophene monomers are added, which does not follow
the equation for the current-concentration proportionality
relationship. This may be due to the relatively slow electron
transfer of thiophene monomers and intermolecular cross-
linking with G1 dendrimers. Nevertheless, Figure 7a shows
the absorption spectra of all three different films made from
G1 D and thiophene monomer which are very similar to G1
dendrimer itself. Figure 6b shows the photoluminescence
spectra of the cross-linked dendrimer films excited at 440 nm.
Interestingly, as we increase the concentration of thiophene
monomer, we can see an increase in the intensity of the peaks
at 580 and 660 nm, indicating the formation of more linear
π-conjugated species. This is a good indication that the limit-
ed intermolecular cross-linking ability of the G-1 dendrimer
is extended by copolymerization with monomeric species.
Interestingly, the fine spectra also indicate specific conju-
gated segments or species were formed with copolymeriza-
tion, which are usually the domain of PL spectral tuning with
the use of various side chain derivatives or by introducing
head-to-tail stereoregularity in polythiophenes.24 Another
method is by the use of polymer blends or block copolymer
formation.24 These results indicate that the dendrimer struc-
ture plays an interesting role when electrodeposited to form
distinct π-conjugated polymer species in a film. In principle,
facile control on the inter- and intramolecular cross-linking
with the use of dendrimers can potentially lead to another
route toward tuning the electro-optical property of poly-
thiophene films.24 The concept in general may be applicable
to spectral tuning of other linear π-conjugated polymers.
Finally, all cross-linked films formed were optically clear and
uniform and therefore could be well suited for OLED and
Electrochromic devices are another possible application
for these materials. The electrochromic properties of the G1
dendrimer were evaluated using spectroelectrochemistry. For
these investigations, the G1 dendrimer film was electropoly-
merized on an ITO-coated glass slide. The cross-linked G1 D
film was analyzed under monomer-free conditions by apply-
ing a constant potential. The results from the G1 D film are
presented in Figure 8 as a series of UV-vis absorbance curves
correlated to electrode potentials. In the neutral form at 0 V,
the film exhibited strong absorption at a wavelength near
445 nm, which is characteristic of a polythiophene solid-state
film.16 Upon oxidation of the G1 film (increasing the applied
voltage from 0 to 0.6 V), the intensity of the absorption at
445 nm gradually decreased while a new broad band having
its maximum absorption wavelength at 805 nm gradually in-
creased in intensity. We attribute this spectral change toward
the near IR to the formation of a stable radical cation or
polaronic and bipolaronic species.22 The observed UV-vis
absorption changes in the film of G1 D at various potentials
are fully reversible, as shown in Figure 8b, and are associated
with strong color changes going from bluish-green to orangish-
red, which can even be seen by the naked eye. Therefore, the
electrochromic switching behavior appears to be a highly
reversible process for these thin film materials. Further studies
will be made to compare their switching rates with PEDOT
derivatives.
Conclusion
In conclusion, poly(benzyl ether)-based terthiophene peripheral-
terminated dendrons and dendrimers have been successfully
synthesized by Mitsunobu coupling under sonication conditions.
All the characterization results, NMR, GPC, MALDI-TOF, and
elemental analysis confirmed the structures proposed in the
schemes. Optical studies of the thin films made bythe electropoly-
merization of these materials show that the dendrimer structure

Article
Macromolecules, Vol. 43, No. 24, 2010 10421
and generation play an important role when depositing these
materials to form a cross-linked conjugated polymer network
film. In particular, the generation of the dendrimer enabled selec-
tive energy transfer behavior that contributed to variations in
spectral emission as a function of cross-linking and dendrimer
generation. Moreover, copolymerization with monomeric species
can enable spectral tuning of the emission properties by facilitat-
ing more intermolecular reactions. Conversely, linear pi-conjugated
polymer species can be spectroscopically tuned by the addition of
dendrimeric species. Spectroelectrochemistry studies of these
materials show that the electrochromic switching behavior is
a highly reversible process. These materials are currently being
systematicallyinvestigated for their optical, redoxbehaviorunder
potential cycling and electropatterning as thin films
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Acknowledgment. The authors acknowledge funding from
the Robert A. Welch Foundation, E-1551. Technical support
from Agilent Technologies (formerly Molecular Imaging) and
Malvern Instruments (formerly Viscotek) is acknowledged.
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material is available free of charge via the Internet at http://
pubs.acs.org.
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