Investigating carbazole jacketed precursor dendrimers: Sonochemical synthesis, characterization, and electrochemical crosslinking properties

Article abstract of DOI:10.1021/ja074007t

Precursor carbazole terminated dendrons and dendrimers up to generation four (G4-D) were synthesized using a convergent approach. Sonication as a means of facilitating organic reactions in dendrimer chemistry was explored resulting in very facile and very fast (up to 50x) reaction times compared to those using traditional reflux conditions. The limits of peripheral group functionality were explored as a function of generation. The electrochemical cross-linking of the dendrimers as thin films revealed unusual cyclic voltammetry (CV) behavior depending upon the generations, which were significantly different from their linear counterpart, Poly(N-vinylcarbazole) (PVK). G1-D showed a higher extent of intermolecular cross-linking while G4-D showed a higher extent of intramolecular cross-linking. The formed films were optically clear and possess superior energy band gap properties making them an alternative candidate over PVK for future hole-transport layer materials in electro-optical devices.

Full text of DOI:10.1021/ja074007t

Published on Web 09/26/2007
Investigating Carbazole Jacketed Precursor Dendrimers:
Sonochemical Synthesis, Characterization, and
Electrochemical Crosslinking Properties
Prasad Taranekar, Timothy Fulghum, Derek Patton, Ramakrishna Ponnapati,
Gabriel Clyde, and Rigoberto Advincula*
Contribution from the Department of Chemistry and Department of Chemical Engineering,
UniVersity of Houston, Houston, Texas 77024
Received June 2, 2007; E-mail: radvincula@uh.edu
Abstract: Precursor carbazole terminated dendrons and dendrimers up to generation four (G4-D) were
synthesized using a convergent approach. Sonication as a means of facilitating organic reactions in
dendrimer chemistry was explored resulting in very facile and very fast (up to 50×) reaction times compared
to those using traditional reflux conditions. The limits of peripheral group functionality were explored as a
function of generation. The electrochemical cross-linking of the dendrimers as thin films revealed unusual
cyclic voltammetry (CV) behavior depending upon the generations, which were significantly different from
their linear counterpart, Poly(N-vinylcarbazole) (PVK). G1-D showed a higher extent of intermolecular cross-
linking while G4-D showed a higher extent of intramolecular cross-linking. The formed films were optically
clear and possess superior energy band gap properties making them an alternative candidate over PVK
for future hole-transport layer materials in electro-optical devices.
5
Introduction
for current carrier transport. Most organic semiconductors are
understood as one-dimensional conductors because the electrons
Electroactive polymer based dendrimers are of current interest
for developing new generation organic material devices and
other photonic applications. Compared to well-known conju-
gated linear chain polymers, the unusual electronic and photo-
physical properties of conjugated dendrimers, for example,
(
or holes) mainly travel through conjugated chains or stacks of
6
π-conjugated molecules. As such, a combination of comple-
mentary conjugated functional units into the periphery, branch-
ing units, and interior core of a dendrimer should give unique
structure-property relationships. The three-dimensional scaffold
of dendrimers would allow additional control over the structure
and morphology of these materials. This could lead to interesting
new properties and, conversely, identify new insight into
dendrimer structure-property relationships such as electronic
processes, inter- and intramolecular cross-linking, band gap
1
2
a,b
exciton and charge localization phenomena,
generation-
dependent carrier mobility,2 and cross-linking phenomenon,
are also of broad fundamental interest. Individual dendrimers
and dendrimeric functional groups at polymer chain ends or in
well-defined segments can determine the ultimate properties of
c
2d,e
3
macromolecules, providing highly controlled materials systems.
7
tunability, and interfacial effects. With reference to the precur-
For this purpose, several approaches have been attempted and
reported, including incorporation of functional units into the
periphery, the branching units, and the interior core of a
dendrimer using various synthetic methodologies.4
8
sor polymer concept for synthesizing conjugated polymers and
the desire to observe new optoelectronic effects from functional
dendrimers, we have synthesized various generations of poly-
(benzyl ether) type dendrimers functionalized with carbazole
Polymers based on carbazole units are interesting because
of their role in electrochromic devices, hole transport layers,
microcavity photoconduction, electroxerography, and as pho-
tovoltaic components which can provide a very efficient matrix
at the periphery and benzene-1,3,5-tricarboxylic as a core
material. In the process, we have discovered and utilized a
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10.1021/ja074007t CCC: $37.00 © 2007 American Chemical Society
J. AM. CHEM. SOC. 2007, 129, 12537-12548
9
12537

A R T I C L E S
Taranekar et al.
Scheme 1. Synthesis of G1 and G2 Dendrons
sonication mediated synthetic route which led to high yielding
and faster synthesis results. To our knowledge this is the first
attempt to synthesize Fr e` chet type dendrons and dendrimers
using a sonication method. Sonication9 and microwave
methods have long been employed for enhancing the rate of
chemical reactions. For better efficiency, microwave-assisted
synthesis is often performed without solvents, making this
method not ideally compatible for syntheses involving molecules
with high molecular weight or low solubility, as in the case of
dendrimer synthesis. Sonication, on the other hand, may provide
the needed energy (acoustic and thermal) to overcome the
different reactivities and enable the development of general
protocols for facilitating the synthesis of dendrons and den-
drimers. In this work, we demonstrate the use of sonication
methods in a Mitsunobu reaction, which involves both etheri-
fication and esterification reactions in solution to obtain the
desired dendrons and dendrimers in sufficiently high yield.
Conventionally, a reaction time of 3-4 days is required to
achieve synthetically useful yields (70-75%). But under the
present reaction conditions and submitted to sonication condi-
tions, we obtained comparative yields in only 2-4 h.
monomers such as N-ethylcarbazole and N-propylcarbazole are
13
poor photoconductors. Therefore, another purpose of this work
is to investigate the unique optoelectronic properties and
electropolymerizability phenomena by cross-linking peripheral
carbazole moieties as a function of dendrimeric generations that
would be significantly different from its linear polymer coun-
terpart, i.e., PVK. Although previously some groups have
reported dendrimers containing peripheral electropolymerizable
10
1
4
groups with thiophenes, the ultimate structure-property
relationship in a generational dendrimeric system remains highly
unexplored. In the past, we have used a linear “precursor
polymer” route to form conjugated network films of these
polycarbazoles which can be deposited and patterned by
1
5
electrochemical methods. A series of generational precursors
were synthesized, which by design contained the peripheral
electroactive carbazole monomer units.
The polyether possesses a low T and therefore in principle
g
could undergo conformational changes as a function of genera-
tion, which then allows one to fine-tune the properties such as
optical, electronic, morphological, and ion permeability within
this series. Electropolymerization or chemical oxidation will
result in a network conjugated polymer having both inter- and
intramolecular cross-linking possibilities. Thus in this report,
we present a design and synthesis strategy using sonication
chemistry and a detailed insight into the structure-property
relationship as a function of generation for a series of carbazole
terminated polybenzylether dendrimers.
We specifically targeted these carbazole jacketed precursor
dendrimers for several reasons: (i) the carbazole group is one
of the most attractive chromophores in photochemistry and has
11
been extensively studied; (ii) the carbazole groups act as good
hole transport materials for LED devices; and last (iii), the
carbazole group is electrochemically active forming insoluble
polycarbazole polymers based on 3,6 or 2,7 reactivity as a film
on the desired substrate.12 It is well-known that Poly(N-
Results and Discussion
vinylcarbazole) (PVK) is a good photoconductor, whereas
Structural Design and Synthesis. The synthetic routes for
the generation one (G1) and two (G2) carbazole terminated
dendrons are shown in Scheme 1. The synthesis was started
using carbazole and 1,4-dibromobutane to produce 9-(4-bro-
mobutyl)-9H-carbazole (CBZ-Br) in a modified procedure used
(
9) (a) Lepore, S. D.; He, Y. J. Org. Chem. 2003, 68, 8261. (b) Gholap, A. R.;
Venkatesan, K.; Daniel, T.; Lahoti, R. J.; Srinivasan, K. V. Green Chem.
2
003, 6, 693. (c) Chen, M.-Y.; Lu, K.-C.; Lee, A. S.-Y.; Lin, C.-C.
Tetrahedron Lett. 2002, 43, 2777.
(
10) Tierney, J. P.; Lidstrom, P. MicrowaVe Assisted Organic Synthesis;
Blackwell Publishing: Oxford, U.K., 2005; Vol. 280, p 89.
11) (a) Bo, Z.; Zhang, W.; Zhang, X.; Zhang, C.; Shen J. Macromol. Chem.
Phys. 1998, 199 (7), 1323. (b) Zhu, Z.; Moore, J. S. Macromolecules 2000,
1
1a
previously by Shen et al.
(
3
3, 801. (c) Kimoto, A.; Cho, J. S.; Higuchi, M.; Yamamoto, K.
(13) Ostroverkhova, O.; Moerner, W. E. Chem. ReV. 2004, 104 (7), 3267.
(14) (a) Sebastian, R.; Caminade, A.; Majoral, J.; Levillain, E.; Huchet, L.;
Roncali, J. Chem. Commun. 2000, 507. (b) Alvarez, J.; Sun, L.; Crooks,
R. M. Chem. Mater. 2002, 14 (9), 3995.
(15) (a) Taranekar, P.; Baba, A.; Fulghum, T. M.; Advincula, R. Macromolecules
2005, 38 (9), 3679. (b) Taranekar, P.; Fulghum, T. M.; Baba, A.; Patton,
D.; Advincula, R. Langmuir 2007, 23, 908.
Macromolecules 2004, 37, 5531. (d) Fu, Y.; Li, Y.; Li, J.; Yan, S.; Bo, Z.
Macromolecules 2004, 37, 6395. (e) Romero, D. B.; Schaer, M.; Leclerc,
M.; Ades, D.; Siove, A.; Zuppiroli, L. Synth. Met. 1996, 80, 271.
(
12) (a) Liu, X.-M.; Xu, J.; Lu, X.; He, C. Org. Lett. 2005, 7 (14), 2829. (b) Li,
J.; Liu, D.; Li, Y.; Lee, C.-S.; Kwong, H.-L.; Lee, S. Chem. Mater. 2005,
1
7 (5), 1208.
12538 J. AM. CHEM. SOC.
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Investigating Carbazole Jacketed Precursor Dendrimers
A R T I C L E S
Scheme 2. Synthesis of G3 and G4 Dendrons
Similarly, we have also modified the synthesis of (G1)
COOR as shown in Scheme 2. It took 4 h to obtain a
synthetically useful yield of 68.6%, which on the other hand
took 5 days under stirring conditions to produce comparable
yields. The G4-COOR was then further reduced to G4-OH in
81% yield.
1
1d
dendron ester (G1-COOR) reported earlier by Bo et al. in a
synthetically useful yield of 70%. The purification of G1-COOR
does not require any column chromatography. It can be easily
purified by recrystallization using ethyl acetate.
The G1-COOR was then reduced using lithium aluminum
hydride (LAH) to produce the G1 dendron alcohol (G1-OH) in
Direct coupling of these dendrons afforded the dendrimer
structures (Schemes 3 and 4). Compound Gx-OH (x ) 1, 2, 3,
4) was coupled with benzene-1,3,5-tricarboxylic acid (BTC) as
a core using Mitsunobu esterification, again under sonication.
BTC was purposely used as a core so that we can potentially
extend the expansion of applications for these dendrimers.
Recently we have demonstrated that these dendrimers can be
intramolecularly cross-linked both chemically and electrochemi-
90% yield. The G1-OH dendron was then dissolved in minimal
THF along with PPh3 and methyl 3,5-dihydroxybenzoate, and
DIAD was slowly added under sonication conditions to produce
G2-COOR in 84% yield within 75 min (Caution: Water in the
sonicator turns very hot if used for prolonged periods of time).
The amount of THF is very important. It should only be used
to barely dissolve the starting alcohol. Excess THF, temperature
fluctuations in the sonication bath, and scaling above 6 g caused
a significant lowering in the yields. The G2-COOR was further
reduced to G2-OH.
The synthesis protocol of generation three (G3) dendrons is
shown in Scheme 2. The G3-COOR was prepared by sonication
using G2-OH and once again under sonication for 3 h to afford
a white solid product in 70.8% yield after purification. The G3-
COOR was also eventually converted to G3-OH using LAH in
1
6
cally.
However, even after several repeated trials under sonication
with varying concentration or even prolonged time, disubstituted
product was isolated. Finally, under refluxing for 6 days and
under similar chemical conditions all three dendrons were
successfully grafted to the core yielding G4-D in 51% yield as
shown in Scheme 4.
Characterization of Dendrimers. The dendrimers were
found to be highly soluble in common organic solvents such as
8
5% yield. The G3-OH was coupled with methyl 3,5-dihy-
(
16) Taranekar, P.; Park, J.-Y.; Fulghum, T.; Patton, D.; Advincula, R. AdV.
droxybenzoate using sonication conditions to produce G4-
Mater. 2006, 18 (18), 2461.
J. AM. CHEM. SOC.
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A R T I C L E S
Taranekar et al.
Scheme 3. Synthesis of G1/G2/G3-Carbazole Terminated Dendrimers
Scheme 4. Synthesis of G4-Carbazole Terminated Dendrimer
CHCl3, CH2Cl2, and THF and insoluble in acetone and ethyl
acetate. The synthesized dendrimers were characterized using
NMR and elemental analysis. With increasing dendrimer
plicated and it is difficult to discern the structures. 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 spec-
1
13
generation, the H and C NMR signals become more com-
12540 J. AM. CHEM. SOC.
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Investigating Carbazole Jacketed Precursor Dendrimers
A R T I C L E S
reinforces the fact that the G4-D dendrimer structurally rear-
ranges and creates a dense phase resulting in more intermo-
lecular interactions within the macromolecule. In this case a
much higher degree of branching results in a lowering of the
Tg, perhaps due to a higher benzylether content compared to
1
1a
the carbazole. This unique behavior can only be expected
from dendrimers and not linear polymers. A low Tg value can
be highly desirable with a higher generation as it could in
principle lead to interesting optoelectronic trends correlated with
viscoelastic behavior.
Thermogravimetric analysis (TGA) studies reveal that all the
dendrimers except G1-D showed almost similar TGA onsets
indicating a relatively high thermal stability compared to that
for G1-D (Supporting Information Figure 1b). The ester groups
are more vulnerable in the case of G1-D, and the initial mass
loss was found to be equal to that of three CO2 groups, which
indicated the degradation of ester groups linked to the core.
The thermal degradation onset of G1-D starts at 336 °C, while
the higher generation dendrimers were found to be stable up to
3
75 °C. This is rational because, with increasing generations,
the ester groups are more shielded and therefore the degradation
occurs at higher temperatures.
Electrochemical /Morphological Studies. The different CV
traces for all the generations of dendrimers are shown in
Figure 2. The peak anodic and cathodic currents and potentials
are summarized in Table 1. The CV for G1-D shows an
unusually sharp reversible redox peak behavior as compared to
the conventional CV behavior of polycarbazole (Figure 2a).17
The sharp reversible redox behavior could be attributed to either
a very thin film, which in principle will show a homogeneous
electron-transfer rate, or a dominance of electron transfer from
either inter- or intramolecular species formed during cross-
linking (higher conjugated units) or a combination of both. As
a consequence, this material showed very fast doping-dedoping
characteristics among the different generations.
The dendrimers G2-D and G3-D, compared to G1-D and G4-
D, both show higher peak Epa values. This clearly suggests that
the there is an optimal conformational freedom required to allow
extended conjugation during cross-linking for these dendrimers.
This could also possibly lead to 3,6 and 2,7 type cross-linking
in the dendrimers. The presence of a small reduction peak at
Figure 1. Characterization of dendrimers using (a) GPC chromatogram
and (b) DSC.
troscopy. 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 corre-
+
sponding ion peak of M + Na (Supporting Information
Figure 1a).
The gel permeation chromatography (GPC) results are shown
in Figure 1a. The retention volume trace and the polydispersity
were found to be less than 1.02. The elution curves were found
to be narrow and monomodal in distribution. Furthermore, a
decreasing trend in the differences of retention volume was
observed in going from lowest generation to the highest. The
difference increases in going from G1-D to G2-D > G2-D to
G3-D . G3-D to G4-D, which indicates that the hydrodynamic
volume drastically changes after the third generation. This is
reasonable as the higher generation dendrimer behaves much
more like a large macromolecule with the exponential growth
of branching.
1
8a
1
.05 is indicative of some 2,7 cross-linking, but the majority
1
8b
of cross-linking still occurs via the 3,6-position as indicated
by strong reduction peaks observed between 0.6 and 0.7 V.
Although we did not observe any sharp redox peaks for the
other generations, the CV behavior is still very different
compared to that for the linear polycarbazole based polymer
(
PVK) (Supporting Information Figure 5) as evident by the
1
8c
shape and peak positions. The fact that both G2-D and G3-D
show a higher ∆E (Epa - Epc) value shown in Table 1 indicates
a more heterogeneous and slow electron-transfer rate for these
two. This heterogeneous electron transfer is also likely a
consequence of a thicker film deposited during each CV cycle
and is also indicative of a higher cross-linking.
The thermal properties of the dendrimers were investigated
using differential scanning calorimetry (DSC) as shown in
Figure 1b. The DSC traces of the second heating cycle (10 °C/
min) showed glass transition temperature (Tg) values of 74.6,
(
17) (a) Ambrose, Nelson J. Electrochem. Soc. 1968, 115, 1159. (b) Kakuta,
T.; Shirota, Y.; Mikawa, H. J. Chem. Soc., Chem. Commun. 1985, 553. (c)
Shirota, Y.; Nogami, T.; Noma, N.; Kakuta, T.; Saito, H. Synth. Met. 1991,
8
0.3, 82.1, and 79.2 °C corresponding to G1-D, G2-D, G3-D,
and G4-D, respectively. Interestingly, the G4-D dendrimer falls
off the increasing trend of Tg, suggesting that there is a huge
structural change consistent with the observed hydrodynamic
specific volume in GPC. Thus, we believe that this change
41, 1169.
(18) (a) Iraqi, A.; Wataru, I. Chem. Mater. 2004, 16 (3), 442. (b) Iraqi, A.;
Pickup, D. F.; Yi, H. Chem. Mater. 2006, 18 (4), 1007. (c) Fulghum, T.;
Karim, S. M. A.; Baba, A.; Taranekar, P.; Nakai, T.; Masuda, T.; Advincula,
R. C. Macromolecules 2006, 39 (4). 1467.
J. AM. CHEM. SOC.
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Taranekar et al.
Figure 2. Cyclic voltammograms: (a) G1-D, (b) G2-D, (c) G3-D, (d) G4-D dendrimers.
chain structure. Consequently, effective conjugation would
approach a hypothetical limit determined by the conformational
restrictions and connectivity imposed by the underlying dendritic
scaffold. In the present case, the limit seemed to be observed
for the G4-D dendrimer and is predicted to show predominantly
intramolecular cross-linking unlike the case in both G2-D and
G3-D. A decrease in the intermolecular interactions is indicative
of a decreasing capacitive behavior in these materials and is
attributed to a drop in electronic conductivity as observed. Thus,
the high thickness (as obtained by profilometry, Supporting
Information Figure 2) and relatively low intermolecular interac-
tions for G4-D (Figure 1d and Table 1) suggested a poorly cross-
linked film.
The high thickness and poor cross-linking behavior also result
in a less dense film. Another way to confirm this property is to
obtain the ratio of charge (Q) to the thickness of the electrode-
posited film during electrochemical cross-linking by CV under
similar conditions as those seen in Figure 3. The electropoly-
merization efficiency is expressed by the ratio Qmax/T (where
Qmax is the maximum amount of charge reversibly exchanged
upon cycling and T is the total thickness of the film).
Figure 3. Amount of charge per unit area as a function of time during
cross-linking.
The CV result also suggests that raising the Gn-D (n )
,2,3,4) generation increases the amount of longer conjugated
1
segments in the cross-linked dendrimer. Such an evolution of
the polymer structure could be related to the increasing
probability of inter- and/or intradendrimer coupling of the
carbazole units as the Gn-D augments. Thus, on going from
G1-D to G4-D, the cross-linking shows interesting trends
subjected to steric hindrance and the competitive nature of inter-/
intradendrimer linkages. It is expected that a higher content of
intramolecular linkages with higher Gn-D should also limit
intermolecular steric interactions in forming a polycarbazole
While an increase in the doping level of the polymer cannot
be totally excluded, these results strongly suggest greater
electropolymerization efficiency for G2-D/G3-D as 0.65%/
0.61% and for G1-D/G4-D as 0.59%/0.35%, respectively. This
phenomenon could reflect the increasing proximity of the
intermolecularly polymerizable carbazole groups and was found
to be highest in the case of G2-D and lowest for G4-D cross-
linked dendrimer films. These results clearly suggest that both
12542 J. AM. CHEM. SOC.
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Investigating Carbazole Jacketed Precursor Dendrimers
A R T I C L E S
Figure 4. EC-QCM studies showing frequency and viscoelastic changes as a function of time for (a) G1-D, G2-D and (b) G3-D, G4-D.
Table 1. Dendrimeric Precursors Showing Peak Anodic and
Cathodic Currents/Potentials and Their Corresponding Onsets of
Oxidation Potential
(Supporting Information Figure 3). The linear increase of ipa as
a function of scan rate with reversible redox peak indicates a
more homogeneous electron transfer or ion transport rate as seen
from Supporting Information Figure 3a for the case of G1-D.
In G2-D, G3-D, and G4-D, we did not observe linearity in the
peak current as a function of scan rate, which indicates more
heterogeneous and diffusion-limited electron-transfer kinetics
for these films. For instance, if the electronic transfer at the
surface is fast and the current is limited by the diffusion of
species to the electrode surface, then the current peak will be
proportional to the square root of the scan rate. The diffusion-
limited kinetics could result from the highly branched topology
and dendrimeric conformational changes influencing the film
morphology ion porosity and electron-transfer kinetics. This
could also be attributed to an increased distance between the
solution/film interface and the electrode or in other words the
higher thickness of the film with increasing Gn-D. Two-
dimensional (2D) and three-dimensional (3D) atomic force
microscopy images were also investigated for the cross-linked
films (Supporting Information Figure 4). Although it is hard to
distinguish any marked differences between the cross-linked
films, sharp conical features and uniform surface coverage
seemed to be a function of the extent of cross-linking. The root-
mean-square roughness (rms) of the dendrimers G1-D, G2-D,
E
(V)
pa
i
p
E
(V)
pc
i
pc
∆
(V)
E
thickness
(nm)
dendrimers
onsets
(mA)
(mA)
G1-D
G2-D
G3-D
G4-D
0.99
0.94
0.95
1.03
0.84
0.93
0.99
0.94
0.35
1.14
1.42
0.77
0.76
0.67
0.72
0.74
-0.36
-0.64
-0.65
-0.53
0.08
0.26
0.27
0.20
98.5
534.7
636.8
651.1
dendrimers G2-D and G3-D are good candidates for forming
uniform and highly dense polycarbazole films on the conducting
substrate. This study has demonstrated that confirmation and
connectivity (branching topology) play a huge role when
depositing to form a cross-linked conjugated polymer film. The
fact that one can have facile control of the inter/intramolecular
cross-linking with the use of dendrimers could potentially lead
to more interesting electro-optical property variations. Finally,
all cross-linked films formed were optically clear and uniform
and therefore could be envisioned as potential candidates for
OLED and other display device applications.
In an attempt to understand the electron-transfer kinetics in
the generational films, we have also performed CV in “monomer
free” or electrolyte solution conditions at a scan rate of 25, 50,
1
0
00, and 200 mV/s in methylene chloride solution containing
.1 mM tetrabutylammonium hexafluorophosphate [TBAP]
J. AM. CHEM. SOC.
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Taranekar et al.
3
density of the quartz (2.65 g/cm ), and µq is the shear modulus
6
2
of the quartz (2.95 × 10 N/cm ). This model equation is only
valid if the adsorbed film thickness is small in comparison with
the thickness of the crystal and if the ∆F is much smaller than
the resonant frequency of the bare crystal. Furthermore, the mass
sensitivity has previously been shown to be approximately the
same for liquid and air/vacuum measurements yielding the
ability to accurately monitor changes in mass adsorbed from
2
0
solution. However, an adsorbed viscoelastic overlayer in the
case of viscous liquids and some polymer thin films result in a
viscous coupling of the solution or film to the crystal, effectively
adding a mass component to the oscillating crystal. Thus, the
resulting resonant frequency change is more complex and was
first described by Kanazawa et al. as follows (eq 2):20
ηL ‚ F
L
3
q
/2
∆F ) -f
(2)
x
π ‚ µ
q
‚ F
q
where fq is the resonant frequency of unloaded crystal in Hz;
3
3
Fq is the density of quartz (2.648 × 10 kg/m ); µq is the shear
1
0
modulus of quartz (2.947 × 10 Pa); FL is the liquid density
3
kg/m ; and ηL is the liquid viscosity in contact with electrode
2
in N s/m . Hence, when the QCM operates under inelastic
conditions, it becomes difficult to differentiate contributions
attributed to the bound mass from that of the solution viscosity
2
1a
to the total change in frequency (∆F). In the present case
using dendrimeric precursors, this factor becomes very impor-
tant, since the solvent could essentially penetrate in the voids
that are present in the dendrimers especially with the higher
generation (G3-D/G4-D).
In this case, impedance analysis of the QCM resonator has
proven to be effective in providing additional information on
2
1b
the solid/liquid interface. This analysis is based on the well-
known Butterworth-van Dyke (B-VD) equivalent circuit
which provides the structure for relating the electrical properties
of the quartz resonator to the mechanical properties of the
deposited film.
Figure 5. EC-QCM: Amount of charge and simultaneous mass deposition
as a function of time.
In this regard, the B-VD circuit element R (motional
resistance) is useful in probing the energy loss or dissipation
factor (D) due to the damping process which occurs in
G3-D and G4-D were found to be 1.25, 0.756, 1.04, and 1.2
nm, respectively.
In Situ EC-QCM Studies. Electrochromism is another
possible application of these materials and therefore relies highly
on doping-dedoping characteristics, and in order to fully
understand the cross-linking process and film densification, in
situ electrochemical-quartz crystal microbalance (EC-QCM)
analysis was performed. EC-QCM is an exceptionally versatile
technique for in situ monitoring of gravimetric changes occur-
ring at electrode surfaces.1 The change in mass (∆m) is related
to the change in the fundamental resonance frequency ∆F (Hz)
of the QCM crystal by the well-known Sauerbrey equation
2
2
viscoelastic systems. Thus, an increase in ∆R is correlated
with an increase in the viscoelasticity of the layer adjacent to
the crystal surface while a small change in ∆R is indicative of
a more rigid adsorbed layer. So, monitoring the change in the
resonance resistance (∆R (Ω)) as a function of frequency shift,
in some cases, provides a means for quantitatively determining
the shear viscosity and elastic modulus of films and in others a
means for qualitatively comparing various thin film systems.
Here, E-QCM was performed to simultaneously monitor the
change in mass and resonance resistance at the working
electrode during the electrochemical deposition of the dendrimer
precursors. CV was utilized to study the electrochemical
behavior of the carbazole-modified dendrimers as deposited from
solution. ∆F changes in resonance resistance (∆R) as a function
of time during cyclic voltammograms (CVs) of various genera-
tion dendrimers deposited from a 0.1 M TBAP/CH2Cl2 (scan
9a,b
1
9c
(eq 1):
2
-
2Fq
∆
F )
∆m
(1)
Ae Fq µq
x
where Fq is the fundamental resonant frequency of the QCM
5 MHz), Ae is the area of the electrode (1.327 cm ), Fq is the
2
-1
(
rate 20 mV s ) solution (Figure 4). Increasing ∆F changes
(
19) (a) Abru nˇ a, H. D. Electrochemical Interfaces: Modern Techniques for In
situ Interface Characterization; VCH Publishers: New York, 1991. (b)
Buttry, D. A.; Ward, M. D. Chem. ReV. 1992, 92, 1355. (c) Sauerbrey, G.
Z. Phys. 1959, 155, 206.
(20) Kanazawa, K. Faraday Discuss. 1997, 107, 77.
(21) (a) Ward, M. D.; Buttry, D. A. Science 1990, 249, 1000. (b) Etchenique,
R.; Weisz, A. J. Appl. Phys. 1999, 86, 1994.
(22) Muramatsu, H.; Tamiya, E.; Karube, I. Anal. Chem. 1988, 60, 2142.
12544 J. AM. CHEM. SOC.
9
VOL. 129, NO. 41, 2007

Investigating Carbazole Jacketed Precursor Dendrimers
A R T I C L E S
indicates the deposition of G1-D dendrimer as a film up to six
cycles (Figure 4a). The dotted line represents the frequency shift
during a second CV cycle starting over the potential range for
the oxidation (1.1-1.2 V, anodic scan) and reduction (1.0-
0
.7 V, cathodic scan) of the polycarbazole formation and is
indicative of the deposition of a thin film. Simultaneously, ∆R
values were recorded during each CV cycle, which indicates
the change in the viscoelastic behavior. As seen from Figure
4a, initially the ∆R is small, but from the third cycle the change
becomes apparent showing considerable doping-dedoping
changes due to a thicker film.
On the other hand G2-D compared to G1-D shows a relatively
huge ∆F change, which indicates more mass deposition that
could be due to the higher extent of cross-linking and favorable
conformational changes. However, the ∆R values remain
unchanged, which indicates a more densely packed film. This
may be attributed to a decrease in free volume within the G2-D
thin film, which would result in a more rigid mechanical
response. Figure 4b shows a similar complex stepwise response
in frequency and resonance resistance for G3-D and G4-D with
each potential cycle indicating processes related to the deposi-
tion, doping, and dedoping of the polymer film. In the anodic
scan, the doping occurs at 0.6 V and one expects to observe a
decrease in ∆F, but we observed rather an opposite behavior.
This could possibly be due to a relatively higher change in ∆F
because of solvent expulsion rather than doping in the depositing
film.
With increasing CV cycles, the ∆F and particularly the ∆R
response resulting from the deposition, doping, and dedoping
processes becomes more prominent in the case of G3-D and
G4-D as compared to lower generations. A higher ∆R value as
compared to the lower generation indicates a more viscoelastic
film. In the case of G4-D, due to the conformational and
structural restraints a poor cross-linking occurs, which is
reflected in the highest ∆R change as compared to any other
case. This indicates that a poorly cross-linked film in the case
of G4-D also has a very low density.
Figure 6. Optical studies of dendrimers: (a) UV-vis spectra and (b)
photoluminescence.
Absorption and Photoluminescence Studies. The cross-
linked films were further characterized by studying their optical
properties. The UV-vis spectra of the cross-linked films are
shown in Figure 6a. The extent of increasing π orbital overlap
between neighboring repeat units on conjugated polymers can
be directly assessed from their electronic spectra. The extent of
electronic conjugation directly affects the observed energy of
the π-π* transition, which appears as the absorption maxima
in these materials. This could be due to the possibility of either
inter/intramolecular cross-linking dominating the π-π* transi-
tion. The values of absorption maxima for the different
generations are closely linked to their degree of polymerization.
As seen from Figure 6a, both G1-D and G4-D reveal a λmax
value between 308 and 310 nm.
These values are similar to the absorption maxima of 9-alkyl-
9H-carbazole dimers and indicate a restriction of electronic
conjugation along polymer chains in this series to about two
carbazole rings, regardless of the huge conformation and
branching topology difference between both dendrimers. This
data seem to be consistent with the CV trends, which shows a
limited cross-linking in both dendrimers. On the other hand both
G2-D and G3-D show a more red-shifted absorption value in
the range 322-325 nm, indicating an extended conjugation. This
could be due to the higher extent of intermolecular cross-linking
within these dendrimers consistent with the CV data. The band
gap of these materials, which are estimated from the onset of
the absorption peak, lies in the range 3.3 to 3.2 eV, which
The mass of the polymer depositing on the working electrode
was found to be linear as a function of the CV cycle, and a
corresponding increase in the amount of charge is indicative of
the extent of cross-linking in an electrochemically active thin
film as shown in Figure 5. The change in mass remains constant
for the anodic scan up to a potential of 0.6 V. At this point, the
onset of mass transport was observed with a sharp increase from
0.65 V, which is correlated to the injection of anions into the
film upon doping. This is supported by the simultaneous increase
in charge over this potential range. A second regime of mass
change was observed from ∼1.1 to 1.2 V in the anodic scan
and from 1.2 to 1.1 V in the cathodic scan which is mostly due
to the oxidation of the peripheral-carbazole monomers or
growing carbazole chain radical to form a more conjugated
polymer network. On the other hand, the overall increase in
mass is a combination of both doping and polymer deposition.
The ejection of anions from the film (dedoping) was observed
by a corresponding decrease in mass at potentials from ∼0.8 to
0
.5 V in the cathodic scan. Once again the polymerization
efficiency calculated from the mass to charge ratio (∆m/Q) was
found to be highest in the case of the G2-D dendrimer. These
values are consistent with the previously calculated efficiency
values during the CV discussion.
J. AM. CHEM. SOC.
9
VOL. 129, NO. 41, 2007 12545

A R T I C L E S
Taranekar et al.
possibly indicates the presence of both 3,6- and 2,7-linkages in
the conjugated polycarbazole network.18
Conclusion
In summary, benzyl ether based carbazole terminated den-
drons and dendrimers up to generation four have been success-
fully synthesized by Mitsunobu coupling under sonication
conditions. All the characterization results including NMR,
elemental analysis, GPC, and MALDI-TOF confirmed the
structures as proposed in the schemes. The electrochemical
cross-linking of the dendrimers as thin films revealed unusual
CV behavior depending upon the generations. G1-D showed a
higher extent of intermolecular cross-linking while G4-D showed
a higher extent of intramolecular cross-linking. On the other
hand, G2-D and G3-D tend to form a better balance between
intra- and intermolecular cross-linking resulting in dense and
more electrochemically active films. The cross-linked dendrim-
ers were found to show a diffusion controlled kinetic behavior
for all the generations except G1-D. The fluorescence of G1-D
compared to other dendrimers was found to be red-shifted. The
EC-QCM results confirmed that both G2-D and G3-D show
efficient deposition and cross-linking behavior. The G4-D
dendrimer shows the highest viscoelastic change during film
formation. A tunable HOMO level and optically clear films were
formed from these dendrimers making them very suitable and
promising candidates for future LED device applications and
materials for nanoscience. These materials are currently being
systematically investigated for their optical, electronic, and
conformation dependent band gap variation for improved hole
transport and patterning in ultrathin films.
The photoluminescence studies of un-cross-linked dendrimers
performed in methylene chloride solution gave a λmax at
65 nm (Figure 6b inset). The photoluminescence intensity due
3
to the un-cross-linked peripheral carbazole increases with
increasing generations, except in the case of G4-D. This could
be due to inter/intramolecular self-quenching of the carbazole
groups (aggregation), since this dendrimer has more carbazole
groups at the periphery. The results of the photoluminescence
of the cross-linked films are rather more interesting as seen from
Figure 6b. For the G1-D-clk film the photoluminescence peak
appears at a longer wavelength (474 nm) as compared to those
for all the other dendrimers. The peak shows relatively sharp
vibronic bands and appears to have the highest Stokes shift,
indicating a large difference in the ground and excited state.
The unique nature and highest red shift in the case of G1-D
indicate a total dominance of one type of cross-linking (perhaps
more intermolecular). The photoluminescence spectra of G2-
D-clk shows a shoulder peak at 412 nm, and its major peak
appears at 460 nm. The shoulder peak was also observed in
G3-D-clk and G4-D-clk films. The major intensity peak for G3-
D-clk and G4-D-clk films was found at 453 and 447 nm,
respectively.
Band Gap Studies. In order to determine the band gap of
these materials, we performed CV on the cross-linked films
using 0.1 mM TBAP as a supporting electrolyte in methylene
chloride and a saturated Ag/AgCl reference electrode (having
a potential of 0.2 V versus NHE) was used. All polymers
showed reversible oxidative behavior (Supporting Information
Figure 6). Supporting Information Figure 6 insets show the
oxidation onset (Eonset) during the anodic scan, which allows
us to estimate the HOMO level within these materials. The
measured oxidation potentials can be converted into ionization
potentials by relating the electrochemical energy scale (with
respect to NHE) to the vacuum energy scale. At room temper-
ature, the Fermi level of NHE is -4.5 eV with respect to the
Experimental Section
Synthesis of 9-(4-Bromobutyl)-9H-carbazole [CBZ-Br] (Scheme
1). The synthesis of 9-(4-bromobutyl)-9H-carbazole was accomplished
by a modified method as reported by Shen et al.1 A mixture of
1a
1
1
0.32 g (61.72 mmol) of carbazole in toluene (100 mL) containing
,4-dibromobutane (118.2 g, 547.4 mmol) and tetrabutylammonium
bromide (TBAB, 2 g) was stirred at 45 °C for 3 h and then left
overnight. After the aqueous layer was removed and washed three times
with water and brine, the organic layer was dried over Na SO . The
2 4
organic solvent was evaporated, and unreacted 1,4-dibromobutane was
removed by vacuum distillation. The residue was recrystallized from
ethanol to give 16.7 g (89.5%) of the product. The characterization
was found to be consistent with the literature.
2
3
vacuum level, which means that the ionization potential (IP)
can be calculated using IP [eV] ) 4.7 + Eonset. The ionization
potential can be taken as a measure of the energy of the highest
occupied molecular orbital (HOMO). All the dendrimers
displayed reversible oxidation peaks. The oxidizability of the
dendrimers did not show any regular trend as a function of
generation number. The HOMO level of G1-D, G2-D, G3-D,
and G4-D were found to be -5.4, -5.2, -5.3, and -5.4 eV.
For the present case the highest HOMO (-5.2 eV) value was
found for G2-D and, accordingly, is expected to have the lowest
barrier to hole injection. The LUMO level was estimated from
the onset of the UV-vis and was found to be -2.09, -1.97,
Synthesis of Methyl 3,5-Bis(4-(9H-carbazol-9-yl)butoxy)benzoate
[
G1-COOR] (Scheme 1). In a literature modified process as reported
11d
by Bo et al., a mixture of 15.22 g (50.37 mmol) of CBZ-Br, 21.0 g
of K CO , 3.94 g (23.4 mmol) of methyl 3,5-dihydroxybenzoate,
00 mL of acetone, and 45 mg of 18-crown-6 was stirred and refluxed
2
3
5
under a nitrogen atmosphere for 72 h. Acetone was evaporated under
vacuum, and the residue was partitioned between water (400 mL) and
CH
layer was extracted three times with CH
combined organic layer was dried over Na
2 2
Cl (500 mL). The organic layer was separated, and the aqueous
2
Cl
2
(250 mL). Finally, the
. After removal of the
SO
2 4
solvent, the residue was recrystallized from ethyl acetate to give a white
solid product in 70% yield. The characterization data were found to be
consistent with the literature.
-
2.03, and -2.12 corresponding to G1-D, G2-D, G3-D, and
G4-D, respectively. The LUMO level of G2-D is close to the
LUMO level of PVK (-1.8 eV), but the HOMO level of G2-D
General Method for Lithium Aluminum Hydride (LAH) Reduc-
tion of Aromatic Esters to Benzylic Alcohols (Method A). 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 aws stirred until the reaction was
complete as indicated by TLC. The reaction was quenched by adding
water, and the THF was removed under reduced pressure in a rotary
evaporator. The resulting layer was brought to a neutral pH with the
(
(
-5.2 eV) is far superior to its linear polymer analogue PVK
2
4
-5.9 eV).
(
23) Bockris, J. O. M.; Khan, S. U. M. Surface Electrochemistry. A Molecular
Level Approach; Kluwer Academic/Plenum Publishers: New York, 1993.
24) van Dijken, A.; Bastiaansen, J. J. A. M.; Kiggen, N. M. M.; Langeveld, B.
M. W.; Rothe, C.; Monkman, A.; Bach, I.; Stossel, P.; Brunner, K. J. Am.
Chem. Soc. 2004, 126 (24), 7718.
(
2 2
addition of 2 N HCl solution and extracted with CH Cl . The organic
12546 J. AM. CHEM. SOC.
9
VOL. 129, NO. 41, 2007

Investigating Carbazole Jacketed Precursor Dendrimers
A R T I C L E S
layers were combined, washed with water, dried over sodium sulfate,
filtered, and concentrated under reduced pressure. The product was
3
zoate, 2.3 g (1.8 mmol) of G2-OH, and 0.5 g (1.9 mmol) of PPh in
THF under sonication was treated with a solution of DIAD (0.38 g,
1.9 mmol) in 1 mL of THF under nitrogen. The solution was sonicated
purified by silica gel flash column chromatography using a 20/4/1 CH
Cl /hexane/ethyl acetate eluent.
General Method for Etherification Reactions (Method B). The
2
-
2
for 3 h to afford a white solid product in 70.8% yield after purification.
1
H NMR (CDCl ): δ (ppm) 8.09 (d, 16H, J ) 7.5 Hz), 7.47-7.35 (m,
3
Mitsunobu etherification and esterification method was performed using
sonication. The mixture of alcohol, phenol (or aromatic carboxylic acid),
and triphenylphosphine (PPh ) in minimal THF was cooled to 5 °C
3
with an ice bath and sonicated for 5 min. Under sonication, a solution
of diisopropyl azodicarboxylate (DIAD) was added dropwise under
nitrogen. The water temperature was allowed to warm to room
temperature, and precautions were taken to maintain the temperature.
16H), 7.30 (d, 2H, J ) 2.1 Hz), 7.25-7.20 (m, 16H), 6.78 (b, 1H),
6.69 (b, 4H), 6.59 (b, 2H), 6.51 (b, 8H), 6.31 (b, 4H), 4.97 (s, 4H),
4.92 (s, 12H), 4.30 (t, 16H, J ) 7.5 Hz), 3.88-3.81 (m, 19H), 2.07-
1.95 (m, 16H), 1.78-1.73 (m, 16H). 13C NMR (CDCl
): δ (ppm)
3
166.60, 160.17, 160.04, 140.25, 138.99, 138.80, 132.00, 125.60, 122.77,
120.31, 118.79, 108.59, 108.27, 107.07, 106.36, 105.77, 101.60, 100.77,
70.04, 69.97, 67.43, 52.23, 42.55, 26.85, 25.74. Anal. Calcd: C, 80.03;
H, 6.19; N, 4.19. Found: C, 79.97; H, 6.20; N, 4.28.
(
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 4/1 CH Cl /hexane
Synthesis of (3,5-Bis(3′,5′-bis(3′′,5′′-bis(4-(9H-carbazol-9-yl)bu-
toxy)benzyloxy)benzyloxy)phenyl)methanol [G3-OH] (Scheme 2).
Following the general synthesis for method A as described above, the
reaction of 0.051 g (1.33mmol) of LAH in 100 mL of THF with a
2
2
as an eluent by silica gel column chromatography. Note: In the case of
the G4-dendrimer (G4-D), sonication did not yield the desired product
and therefore a reflux condition was used to obtain the final product.
solution of 2.1 g (0.78mmol) of G3-COOR in 20 mLof THF afforded
1
The product was purified using neutral alumina and a 6/1 ratio of CH
Cl /hexane.
Synthesis of (3,5-Bis(4-(9H-carbazol-9-yl)butoxy)phenyl)methanol
G1-OH] (Scheme 1). Following the general synthesis method A as
2
-
a white solid product in 85% yield. H NMR (CDCl
3
): δ (ppm) 8.15
2
(d, 16H, J ) 7.5 Hz), 7.53-7.40 (m, 32H), 7.31-7.26 (m, 16H), 6.74
(b, 4H), 6.64 (b, 2H), 6.60 (b, 2H), 6.56 (b, 8H), 6.36 (b, 4H), 4.97 (s,
12H), 4.57 (s, 2H), 4.33 (t, 16H, J ) 7.0 Hz), 3.86 (t, 16H, J ) 4.2
[
Hz), 2.06-2.01 (m, 16H), 1.81-1.77 (m, 16H). 13C NMR (CDCl
): δ
described above, the reaction of 1.0 g (23.3 mmol) of LAH in 300 mL
3
of THF with a solution of 9.5 g (15.5 mmol) of G1-COOR in 400 mL
(ppm) 160.13, 160.01, 159.90, 143.58, 140.24, 139.25, 139.02, 125.58,
122.73, 120.27, 118.77, 108.58, 106.27, 105.75, 105.50, 101.48, 101.14,
100.75, 69.92, 69.80, 67.39, 64.97, 60.33, 42.49, 26.81, 25.69. Anal.
Calcd: C, 80.43; H, 6.25; N, 4.24. Found: C, 80.15; H, 6.28; N, 4.44.
Synthesis of Methyl 3,5-Bis(3′,5′-bis(3′′,5′′-bis(3′′′,5′′′-bis(4-(9H-
carbazol-9-yl)butoxy)benzyloxy)benzyloxy)benzyloxy)benzoate [G4-
COOR] (Scheme 2). Following the general synthesis for method B as
described above, a precooled solution of 0.13 g (0.75 mmol) of methyl
3,5-dihydroxybenzoate, 4 g (1.51 mmol) of G3-OH, and 0.4 g (1.52
1
of THF afforded a white solid in 90% yield. H NMR (CDCl
3
): δ
(ppm) 8.10 (d, 4H, J ) 7.5 Hz), 7.53-7.42 (m, 8H), 7.25-7.22 (m,
4
4
1
1
6
4
H), 6.45 (d, 2H, J ) 2.1 Hz), 6.28 (t, 1H, J ) 2.1 Hz), 4.59 (s, 2H),
.39 (t, 4H, J ) 6.9 Hz), 3.91 (t, 4H, J ) 6.2 Hz), 2.02-2.12 (m, 4H),
1
3
.87-1.78 (m, 4H). C NMR (CDCl ): δ (ppm) 160.26, 144.33,
3
40.38, 128.4, 125.7, 122.89, 120.44, 118.89, 108.69, 105.19, 100.57,
7.58, 65.34, 42.75, 27.02, 25.91. Anal. Calcd: C, 80.38; H, 6.57; N,
.81. Found: C, 80.12; H, 6.44; N, 4.95.
Synthesis of Methyl 3,5-Bis(3′,5′-bis(4-(9H-carbazol-9-yl)butoxy)-
3
mmol) of PPh in THF under sonication was treated with a solution of
benzyloxy)benzoate [G2-COOR] (Scheme 1). Following the general
DIAD (0.31 g, 1.52 mmol) in 1 mL of THF under nitrogen. The solution
synthesis for method B as described above, a precooled solution of
was sonicated for 4 h to afford a white solid product in 68.6% yield
1
0.86 g (5.15 mmol) of methyl 3,5-dihydroxybenzoate, 6 g (10.3 mmol)
after purification. H NMR (CDCl
3
): δ (ppm) 8.10 (d, 32H, J ) 7.8
of G1-OH, and 2.7 g (10.3 mmol) of PPh in THF under sonication
3
Hz), 7.48-7.35 (m, 64H), 7.30 (b, 2H), 7.25-7.20 (m, 32H), 6.79 (b,
was treated with a solution of DIAD (2.08 g, 10.3 mmol) in 3 mL of
1H), 6.71 (b, 8H), 6.69 (b, 4H), 6.61 (b, 6H), 6.51 (b, 16H), 6.32 (b,
THF under nitrogen. The solution was sonicated for 1.5 h to afford a
8H), 4.92 (b, 28H), 4.27 (t, 32H, J ) 7.5 Hz), 3.88-3.78 (m, 35H),
1
13
white solid product in 84% yield after purification. H NMR (CDCl
3
):
2.06-1.94 (m, 32H), 1.78-1.72 (m, 32H). C NMR (CDCl
3
): δ (ppm)
δ (ppm) 8.09 (d, 8H, J ) 7.5 Hz), 7.48-7.39 (m, 16H), 7.27-7.21
m, 8H), 7.19(d, 2H, J ) 2.1 Hz), 6.76 (t, 1H, J ) 2.1 Hz), 6.50 (d,
H, J ) 2.1 Hz), 6.31 (t, 2H, J ) 2.1 Hz), 4.94 (s, 4H), 4.37 (t, 8H,
J ) 7.5 Hz), 3.91-3.87 (m, 11H), 2.08-2.03 (m, 8H), 1.85-1.78 (m,
166.59, 160.16, 160.05, 159.65, 140.26, 139.15, 139.00, 138.85, 132.05,
125.60, 122.75, 120.29, 118.77, 108.79, 106.37, 105.79, 101.54, 100.78,
69.83, 67.41, 52.13, 42.50, 26.81, 25.71. Anal. Calcd: C, 80.24; H,
6.18; N, 4.14. Found: C, 80.26; H, 6.10; N, 4.22.
(
4
1
3
8
1
1
2
H). C NMR (CDCl
3
): δ (ppm) 166.70, 160.22, 159.87, 140.32,
Synthesis of (3,5-Bis(3′,5′-bis(3′′,5′′-bis(3′′′,5′′′-bis(4-(9H-carbazol-
9-yl)butoxy)benzyloxy)benzyloxy)benzyloxy)phenyl)methanol [G4-
OH] (Scheme 2). Following the general synthesis for method A as
described above, the reaction of 38 mg (1 mmol) of LAH in 100 mL
of THF with a solution of 3.2 g (0.6 mmol) of G4-COOR in 100 mL
38.72, 132.02, 125.63, 122.84, 122.47, 120.38, 118.82, 110.03, 108.63,
08.33, 107.34, 105.82, 100.87, 70.11,69.98, 67.98, 67.53, 52.27, 42.67,
6.93, 25.84, 25.62. Anal. Calcd: C, 79.60; H, 6.21; N, 4.32. Found:
C, 79.53; H, 6.12; N, 4.60.
1
Synthesis of (3,5-Bis(3′,5′-bis(4-(9H-carbazol-9-yl)butoxy)benzy-
loxy)phenyl)methanol [G2-OH] (Scheme 1). Following the general
synthesis for method A as described above, the reaction of 0.2 g
of THF afforded a white solid product in 81% yield. H NMR
(CDCl ): δ (ppm) 8.13 (d, 32H, J ) 7.5 Hz), 7.53-7.48 (m, 32H),
3
7.44-7.41 (m, 32H), 7.30-7.26 (m, 32H), 6.73-6.71 (m, 12H), 6.63
(5.5 mmol) of LAH in 100 mL of THF with a solution of 4.2 g (3.2
(b, 9H), 6.55 (b, 16H), 6.36 (b, 8H), 4.96 (b, 28H), 4.56 (b, 2H), 4.35
mmol) of G2-COOR in 25 mL THF afforded a white solid product in
(t, 32H, J ) 6.7 Hz), 3.86 (t, 32H, J ) 5.8 Hz), 2.07-2.00 (m, 32H),
1
13
9
7
6
0% yield. H NMR (CDCl
.42 (m, 16H), 7.31-7.25 (m, 8H), 6.59 (d, 2H, J ) 2.1 Hz), 6.50-
.52 (m, 5H), 6.32 (t, 2H, J ) 2.1 Hz), 4.95 (s, 4H), 4.59 (s, 2H), 4.36
3
): δ (ppm) 8.15 (d, 8H, J ) 7.5 Hz), 7.53-
1.82-1.77 (m, 32H). C NMR (CDCl
3
): δ (ppm) 160.19, 160.061,
160.02, 140.30, 139.21, 139.04, 125.62, 122.80, 120.32, 118.81, 108.61,
106.39, 105.84, 101.55, 100.81, 69.96, 67.35, 65.24, 65.03, 42.57, 26.96,
25.74. Anal. Calcd: C, 80.43; H, 6.21; N, 4.16. Found: C, 80.46; H,
6.29; N, 4.19.
(
t, 8H, J ) 6.9 Hz), 3.88 (t, 8H, J ) 5.7 Hz), 2.11-2.00 (m, 8H),
1
3
1
1
1
.87-1.76 (m, 8H). C NMR (CDCl ): δ (ppm) 160.25, 160.09,
3
43.39, 140.37, 139.2, 125.69, 122.88, 120.4, 118.86, 108.66, 105.84,
05.72, 101.33, 100.81, 69.98, 67.57, 65.3, 42.72, 26.96, 25.88. Anal.
Synthesis of Tris(3,5-bis(4-(9H-carbazol-9-yl)butoxy)benzyl)ben-
zene-1,3,5-tricarboxylate [G1-D] (Scheme 3). Following the general
synthesis for method B as described above, a precooled solution of
164 mg (0.78 mmol) of benzene-1,3,5-tricarboxylic acid, 1.5 g
(2.57 mmol) of G1-OH, and 0.79 g (3 mmol) of PPh in THF under
3
sonication was treated with a solution of DIAD (0.65 g, 3.2 mmol) in
2 mL of THF under nitrogen. The solution was sonicated for 6 h to
Calcd: C, 80.41; H, 6.35; N, 4.41. Found: C, 80.48; H, 6.30; N, 4.60.
Synthesis of Methyl 3,5-Bis(3′,5′-bis(3′′,5′′-bis(4-(9H-carbazol-9-
yl)butoxy)benzyloxy)benzyloxy)benzoate [G3-COOR] (Scheme 2).
Following the general synthesis for method B as described above, a
precooled solution of 0.15 g (0.9 mmol) of methyl 3,5-dihydroxyben-
J. AM. CHEM. SOC.
9
VOL. 129, NO. 41, 2007 12547

A R T I C L E S
Taranekar et al.
1
(t, 48H, J ) 5.4 Hz), 2.01-1.80 (m, 48H), 1.73-1.56 (m, 48H). 13
NMR (CDCl ): δ (ppm) 164.42, 160.13, 160.02, 140.24, 139.05,
138.97, 137.74, 134.61, 130.96, 125.60, 122.75, 120.30, 118.78, 108.58,
106.96, 106.32, 105.78, 101.71, 101.47, 100.69, 77.20, 69.89, 67.37,
67.20, 42.50, 26.92, 25.71. Anal. Calcd: C, 80.21; H, 6.13; N, 4.16.
Found: C, 80.00; H, 6.28; N, 4.20.
afford a white solid product in 71% yield after purification. H NMR
C
(
CDCl
m, 24H), 7.23-7.18 (m, 12H), 6.48 (d, 6H, J ) 1.5 Hz), 6.31 (b,
H), 5.22 (b, 6H), 4.33 (t, 12H, J ) 6.9 Hz), 3.85 (t, 12H, J ) 6.1
3
): δ (ppm) 8.86 (s, 3H), 8.07 (d, 12H, J ) 7.5 Hz), 7.45-7.36
3
(
3
1
3
Hz), 2.05-2.00 (m, 12H), 1.82-1.75 (m, 4H). C NMR (CDCl
ppm) 164.65, 160.26, 140.37, 137.68, 134.84, 131.23, 125.64, 122.90,
3
): δ
(
1
2
20.38, 119.48, 118.86, 108.63, 106.75, 101.28, 67.58, 67.19, 42.68,
6.92, 25.83. Anal. Calcd: C, 79.47; H, 6.03; N, 4.41. Found: C, 79.12;
Synthesis of Tris(3,5-bis(3′,5′-bis(3′′,5′′-bis(3′′′,5′′′-bis(4-(9H-car-
bazol-9-yl)butoxy)benzyloxy)benzyloxy)benzyloxy)benzyl)benzene-
1,3,5-tricarboxylate [G4-D] (Scheme 4). A precooled solution of 16
mg (0.75 µmol) of benzene-1,3,5-tricarboxylic acid, 1.35 g (0.25 mmol)
H, 5.84; N, 4.90.
Synthesis of Tris(3,5-bis(3′,5′-bis(4-(9H-carbazol-9-yl)butoxy)-
benzyloxy)benzyl)benzene-1,3,5-tricarboxylate [G2-D] (Scheme 3).
Following the general synthesis for method B as described above, a
precooled solution of 38.6 g (0.18 mmol) of benzene-1,3,5-tricarboxylic
of G4-OH, and 0.13 g (0.5 mmol) of PPh in 15 mL of THF was treated
3
with a dropwise addition of DIAD solution (0.12 g, 0.6 mmol) in
5 mL of THF under nitrogen. The solution was stirred for 1 h after
warming the reaction mixture to room temperature. The mixture was
then refluxed for 48 h. After cooling the reaction mixture, 0.5 mol %
acid, 0.77 g (0.6 mmol) of G2-OH, and 0.2 g (0.8 mmol) of PPh
THF under sonication was treated with a solution of DIAD (0.18 g,
.9 mmol) in 2 mL of THF under nitrogen. The solution was sonicated
for 6 h to afford a white solid product in 70.6% yield after purification.
3
in
0
equiv of both PPh and DIAD was added again and refluxed for another
3
72 h. The final product was obtained as a white solid in 50.9% yield
1
1
H NMR (CDCl
3
): δ (ppm) 8.81 (s, 3H), 8.05 (d, 24H, J ) 7.2 Hz),
after purification. H NMR (CDCl ): δ (ppm) 8.68 (s, 3H), 8.02 (d,
3
7
6
4
2
1
1
1
4
.43-7.32 (m, 48H), 7.21-7.16 (m, 24H), 6.64 (d, 6H, J ) 1.2 Hz),
.54 (b, 3H), 6.45 (d, 12H, J ) 1.2 Hz), 6.25 (b, 6H), 5.24 (b, 6H),
.85 (b, 12H), 4.26 (t, 24H, J ) 7.0 Hz), 3.77 (t, 24H, J ) 6.0 Hz),
96H, J ) 7.2 Hz), 7.37-7.12 (m, 288H), 6.62 (b, 42H), 6.51 (b, 21H),
6.42 (b, 48H), 6.20 (b, 24H), 5.12 (b, 6H), 4.77 (b, 84H), 4.15 (b,
1
3
96H), 3.63 (b, 96H), 1.96-1.76 (m, 96H), 1.68-1.52 (m, 96H).
C
13
.00-1.93 (m, 24H), 1.75-1.68 (m, 24H). C NMR (CDCl
3
): δ (ppm)
NMR (CDCl ): δ (ppm) 164.24, 160.09, 159.96, 140.19, 139.10,
3
64.57, 160.20, 160.08, 140.31, 140.17, 138.92, 137.74, 134.76, 131.01,
26.03, 125.64, 122.81, 121.96, 120.69, 120.36, 118.83, 108.62, 107.25,
07.14, 105.79, 101.93, 100.77, 100.66, 77.24, 70.01, 67.45, 67.13,
2.60, 26.88, 25.79. Anal. Calcd: C, 79.97; H, 6.10; N, 4.24. Found:
139.01, 137.84, 134.23, 130.46, 125.56, 122.68, 120.58, 120.26, 118.75,
108.57, 107.88, 106.82, 106.35, 105.72, 101.42, 100.62, 77.23, 69.83,
67.95, 67.27, 42.20, 26.75, 25.91, 25.62. Anal. Calcd: C, 80.33; H,
6.15; N, 4.12. Found: C, 80.00; H, 6.15; N, 4.21.
C, 80.01; H, 6.00; N, 4.61.
Synthesis of Tris(3,5-bis(3′,5′-bis(3′′,5′′-bis(4-(9H-carbazol-9-yl)-
butoxy)benzyloxy)benzyloxy)benzyl)benzene-1,3,5-tricarboxylate [G3-
D] (Scheme 3). Following the general synthesis for method B as
described above, a precooled solution of 40.9 mg (0.19 mmol) of
benzene-1,3,5-tricarboxylic acid, 1.7 g (0.64 mmol) of G3-OH, and
Acknowledgment. We are grateful for the partial funding
from the Robert A. Welch Foundation (E-1551) and NSF DMR-
0504435, NSF-DMR-0602896, and instrument support from
DMR-DMR-0315565. Technical support from Maxtek Inc. is
also acknowledged.
0
3
.24 g (0.9 mmol) of PPh in THF under sonication was treated with
a solution of DIAD (0.3 g, 1.5 mmol) in 2 mL of THF under nitrogen.
The solution was sonicated for 8 h to afford a white solid product in
Supporting Information Available: Experimental details,
characterization, and H NMR spectra for all the dendrons and
dendrimers reported. This material is available free of charge
via the web at http://pubs.acs.org.
1
1
5
8
7
4% yield after purification. H NMR (CDCl
3
): δ (ppm) 8.77 (s, 3H),
.02 (d, 48H, J ) 7.5 Hz), 7.40-7.35 (m, 48H), 7.30-7.26 (m, 48H),
.18-7.14 (m, 48H), 6.61 (b, 18H), 6.51 (b, 9H), 6.42 (b, 24H), 6.22
(b, 12H), 5.15 (b, 6H), 4.80 (b, 36H), 4.19 (t, 48H, J ) 6.6 Hz), 3.69
JA074007T
12548 J. AM. CHEM. SOC.
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VOL. 129, NO. 41, 2007