Spin-delocalization in charged states of para-phenylene-linked dendritic oligoarylamines

Article abstract of DOI:10.1021/cm102163w

Two kinds of basic dendritic (or starburst) all-para-phenylene-linked oligotriarylamines, which are generated from para-phenylenediamine and triphenylamine as core units, respectively, were prepared, and the electrochemical, spectroelectrochemical, ESR spectroscopic measurements were carried out with respect to their application as hole transport materials in optoelectronic devices such as organic lightemitting devices. The prepared dendritic oligoarylamines exhibits the multiredox-active properirs, and are stably oxidizable up to tetra-or hexacations. According to the degree of oxidation, it was suggested that the charge distribution of the charged states gradually change so as to reduce the electrostatic repulsion between increased charges on the basis of the spectroelectrochemical studies. Moreover, the stability of the generated radical cations and the spin distribution in their radical cations were confirmed by the ESR measurements.

Full text of DOI:10.1021/cm102163w

Chem. Mater. 2011, 23, 841–850 841
DOI:10.1021/cm102163w
Spin-Delocalization in Charged States of para-Phenylene-Linked
Dendritic Oligoarylamines†
Akihiro Ito,* Daisuke Sakamaki, Yusuke Ichikawa, and Kazuyoshi Tanaka
Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Nishikyo-ku,
Kyoto 615-8510, Japan
Received August 1, 2010. Revised Manuscript Received November 22, 2010
Two kinds of basic dendritic (or starburst) all-para-phenylene-linked oligotriarylamines, which are
generated from para-phenylenediamine and triphenylamine as core units, respectively, were prepared, and
the electrochemical, spectroelectrochemical, ESR spectroscopic measurements were carried out with
respect to their application as hole transport materials in optoelectronic devices such as organic light-
emitting devices. The prepared dendritic oligoarylamines exhibits the multiredox-active properirs,
and are stably oxidizable up to tetra- or hexacations. According to the degree of oxidation, it was
suggested that the charge distribution of the charged states gradually change so as to reduce the
electrostatic repulsion between increased charges on the basis of the spectroelectrochemical studies.
Moreover, the stability of the generated radical cations and the spin distribution in their radical
cations were confirmed by the ESR measurements.
Introduction
that the para-phenylene-linked bistriarylamines have the
delocalized and/or almost delocalized IV states.3-6
In this study, we focused on two kinds of dendritic (or
starburst) all-para-phenylene-linked oligotriarylamines 1
and 2, which are generated from para-phenylenediamine
and triphenylamine as core units, respectively (Chart 1).
Although the dendritic compound possessing the same7
and/or similar8-12 molecular backbone as 2 has been
Since the first report of the multilayered organic light-
emitting diodes (OLEDs) made from a bistriarylamine
and Alq3 by Tang and VanSlyke,1 triarylamine deriva-
tives have become one of the main components in OLED
devices.2 Because of their electron-rich property, the
positively charged (or hole) states of aromatic amines
are generally so stable that they act as good hole-trans-
port materials in various applications such as xerography,
solar cells, photorefractive systems, and so forth, in
addition to the use of hole-transport layer for OLED
devices.2 Hence, the efforts in the synthesis of many kinds
of oligotriarylamines and their polymers are being con-
tinued to clarify their characterization in device perfor-
mance. In particular, “star-shaped” oligoarylamines are
considered to be useful for the wide variety of electro-
optical applications because of their ability to form grassy
amorphous phases.2 In addition, the intervalence (IV)
compounds bearing two redox-active amino groups have
been exhausively examined so far, and it has been clarified
€
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2006, 110, 4866. (d) Hirao, Y.; Ishizaki, H.; Ito, A.; Kato, T.;
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† Accepted as part of the “Special Issue on π-Functional Materials”.
*Corresponding author. E-mail: aito@scl.kyoto-u.ac.jp.
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r
2010 American Chemical Society
Published on Web 12/21/2010
pubs.acs.org/cm

842 Chem. Mater., Vol. 23, No. 3, 2011
Ito et al.
Chart 1. Dendritic Oligoarylamines 1 and 2
extensively studied, we revisited to elucidate the spin
electronic structure of the charged species in more detail.
To enhance both the electron-donating property and the
stability of the generated charged species, the dendritic
oligotriarylamines 1 and 2 contain the methoxy groups at
all the para-positions of the peripheral phenyl rings.
Herein, we report the synthesis and electrochemical prop-
erties of 1 and 2, and in particular, the electronic struc-
tures including intramolecular spin distribution in the
charged states for 1 and 2, based on the absorption and
ESR spectroscopic studies.
124.7, 123.3, 115.1, 55.1. MS (FABMS): calculated [M-Hþ] for
C86H77N6O8, m/z = 1321.5590; found m/z = 1321.5803. Anal.
Calcd for C86H76N6O8: C, 78.16; H, 5.80; N, 6.36; O, 9.69.
Found: C, 78.35; H, 6.11; N, 6.06; O, 9.94.
Dendritic Oligoarylamine 2. Triamine 3 (1.25 g, 2.0 mmol), tris-
(4-bromophenyl)amine (0.24 g, 0.50 mmol), Pd(OAc)2 (0.0067 g,
0.030 mmol), DPPF (0.0033 g, 0.060 mmol) and NaOt-Bu (0.17 g,
1.8 mmol) was charged into a clean dry flask under an argon
atmosphere, and then anhydrous toluene (20 mL) was added. The
resulting solution was refluxed under an argon atmosphere for 4
days. The reaction mixture was washed with aqueous solution of
NaHCO3, and the organic layer was then dried over Na2SO4. After
evaporation of the solvent, column chromatography on silica gel
with toluene afforded 2 (0.32 g, 30.3%) as a greenish powder. 1H
NMR (400 MHz, THF-d8) δ (ppm) 6.96 (m, 24H), 6.89 (m, 24H),
6.79 (m, 36H), 3.72 (s, 36H). 13C NMR (100 MHz, THF-d8) δ
(ppm) 155.8, 143.9, 141.4, 141.3, 125.7, 125.6, 124.8, 124.6, 124.2,
123.4, 122.2, 114.4, 54.7. Anal. Calcd for C138H120N10O12: C,
78.54; H, 5.73; N, 6.64; O, 9.10. Found: C, 77.83; H, 5.73; N,
6.45; O, 8.96.
Experimental Details
Synthesis and Characterization. Commercial-grade reagents
were used without further purification. Solvents were purified,
dried, and degassed following standard procedures. The synth-
esis of triamine (3) was performed according to a previously
reported procedure for the unsubstituted triamine.9e Column
chromatography was performed using silica gel (Kanto Chemi-
cal Co., Inc., Silica gel 60N, spherical neutral). 1H and 13C
spectra were recorded on a JEOL JNM-AL400 FT NMR
spectrometer and chemical shifts are given in parts per million
(ppm) relative to internal tetramethylsilane (δ0.00 ppm). Elemental
analyses were performed by Center for Organic Elemental
Microanalysis, Kyoto University.
4-Methoxydiphenylamine 6. A mixture of bromobenzene
(0.250 g, 2.03 mmol), p-anisidine (0.314 g, 2.00 mmol), Pd(dba)2
(0.057 g, 0.10 mmol), P(t-Bu)3 (0.040 g, 0.20 mmol), and NaOt-
Bu (0.311 g, 3.24 mmol) in toluene (22 mL) was refluxed under
an argon atmosphere for 23 h. The resulting mixture was washed
with brine, and then the organic layer was dried over MgSO4.
After evaporation of the solvent, the crude product was chro-
matographed on silica gel with toluene as eluent to afford 6
Dendritic Oligoarylamine 1. Triamine 3 (0.62 g, 0.99 mmol),
1,4-dibromobenzene (0.078 g, 0.33 mmol), Pd(OAc)2 (0.0046 g,
0.020 mmol), DPPF (0.0022 g, 0.040 mmol), and NaOt-Bu (0.11 g,
1.2 mmol) was charged into a clean dry flask under an argon
atmosphere, and then anhydrous toluene (20 mL) was added.
The resulting solution was refluxed under an argon atmosphere
for 4 days. The reaction mixture was washed with aqueous
solution of NaHCO3, and then the organic layer was dried over
Na2SO4. After evaporation of the solvent, column chromatography
on silica gel with toluene afforded 1 (0.33 g, 76.2%) as a yellow
powder. 1H NMR (400 MHz, THF-d8) δ (ppm) 6.97 (m, 18H),
6.88 (m, 12H), 6.79 (m, 24H), 3.72 (s, 24H). 13C NMR (100 MHz,
C6D6) δ (ppm) 156.0, 144.3, 143.2, 142.3, 142.0, 126.2, 125.3,
1
(0.180 g, 45%) as faint yellow powder. H NMR (400 MHz,
acetone-d6) δ (ppm) 7.16 (m, 2H), 7.09 (d, 2H, J=9.0 Hz), 7.04
(br s, 1H), 6.95 (m, 2H), 6.87 (d, 2H, J = 9.0 Hz), 6.73 (t, 1H, J =
7.3 Hz), 3.76 (s, 3H). 13C NMR (100 MHz, C6D6) δ (ppm) 145.72,
136.10, 129.51, 128.56, 122.62, 119.71, 115.99, 114.93, 55.13.
4-Methoxy-40,400-bis[N-(4-methoxyphenyl)-N-phenylamino]-
triphenylamine 5. A mixture of 6 (0.166 g, 0.83 mmol), 4-meth-
oxy-N,N-bis(4-bromophenyl)aniline (0.132 g, 0.30 mmol),
Pd(OAc)2 (0.009 g, 0.04 mmol), 1,10-bis(diphenylphosphanyl)-
ferrocene (0.047 g 0.08 mmol), and NaOt-Bu (0.119 g, 1.24 mmol)
in toluene (8 mL) was refluxed under an argon atmosphere

Article
Chem. Mater., Vol. 23, No. 3, 2011 843
Scheme 1. Syntheses of 1, 2, and 5
for 19 h. The resulting mixture was washed with brine, and then
the organic layer was dried over MgSO4. After evaporation of
the solvent, the crude product was chromatographed on silica
gel with toluene as eluent to afford 5 (0.096 g, 47%) as white
powder. 1H NMR (400 MHz, acetone-d6) δ (ppm) 7.20 (dd, 4H,
J=8.8, 7.3 Hz), 7.08 (d, 2H, J=9.0 Hz), 7.05 (d, 4H, J=9.0 Hz),
6.92 (m, 20H), 3.79 (s, 6H), 3.78 (s, 3H). 13C NMR (100 MHz,
C6D6) δ (ppm) 156.59, 156.49, 152.03, 149.06, 143.72, 142.87,
141.33, 129.39, 128.57, 127.40, 127.12, 125.24, 124.21, 122.32,
121.52, 115.20, 55.05, 55.05.
Theoretical Calculations. Quantum chemical calculations
were carried out using a hybrid Hartree-Fock/density func-
tional theory (HF/DFT) method (B3LYP).13 Full geometrical
optimization of 1, 2, 5, and their radical cations were conducted
under D2, D3 and C1symmetrical constraints for 1, 2, and 5
respectively. Excitation energies for the radical cations of 1, 2,
and 5 were examined on the basis of the time-dependent density
functional method (TD-DFT).14 All the computations employed
(14) (a) Bauernschmitt, R.; Ahlrichs, R. Chem. Phys. Lett. 1996, 256,
454. (b) Casida, M. E.; Jamorski, C.; Casida, K. C.; Salahub, D. R.
J. Chem. Phys. 1998, 108, 4439. (c) Stratmann, R. E.; Scuseria,
G. E.; Frisch, M. J. J. Chem. Phys. 1998, 109, 8218.
(13) Ragavachari, K. Theor. Chem. Acc. 2000, 103, 361 and references
cited therein.

844 Chem. Mater., Vol. 23, No. 3, 2011
Ito et al.
the 3-21G basis set.15 All these quantum chemical approaches
are implemented in Gaussian 03 package of ab initio MO
calculation.16
Electrochemical and Spectroelectrochemical Measurements.
Cyclic voltammogarms were recorded using an ALS/chi electro-
chemical analyzer model 612A with a three-electrode cell using
a Pt disk (2 mm2) as the working electrode, a Pt wire as the
counter electrode, and an Ag/0.01 M AgNO3 (MeCN) as the
reference electrode calibrated against a ferrocene/ferrocenium
(Fc0/þ) redox couple in a solution of 0.1 M tetra-n-butylammo-
nium tetrafluoroborate as a supporting electrolyte (298 K, csan
rate 100 mV s-1). The absorption spectra were measured with a
Perkin-Elmer Lambda 19 spectrometer. Spectroelectrochem-
ical measurements were carried out with a custom-made opti-
cally transparent thin-layer electrochemical (OTTLE) cell (light
pass length = 1 mm) equipped with a platinum mesh, a platinum
coil, and a silver wire as the working, the counter, and the
pseudoreference electrodes, respectively. The potential was
applied with an ALS/chi electrochemical analyzer model 612A.
ESR Measurements. ESR spectra were measured using a JEOL
JES-TE200 X-band spectrometer in which temperatures were
controlled by a JEOL ES-DVT3 variable-temperature unit.
Results and Discussion
Synthesis and Electrochemical Studies. The syntheses of
the dendritic oligoarylamines 1 and 2 were carried out in
76 and 30% yield, respectively, by using a palladium-
catalyzed Buchwald-Hartwig amination reaction between
triamine 3 and the corresponding benzene halides
(Scheme 1).17 The redox properties of 1 and 2 were
evaluated by cyclic voltammetry (CV) in CH2Cl2 solution
at 298 K with 0.1 M tetra-n-butylammonium tetrafluoro-
borate as supporting electrolyte. Both 1 and 2 exhibited
five redox couples as shown in Figure S1 in the Supporting
Information. However, the first three reversible oxidation
processes are chemically stable after repeated potential
cycling in CH2Cl2 (Figure 1). The oxidation potentials [Eox
vs Fc0/þ (ne)] of 1 were determined to be -0.28 (1e), -0.03
(1e), and þ0.28 (2e) by digital simulations (see Figure S2a
in the Supporting Information), and therefore, 1 is stably
oxidizable up to tetracation. The first and third oxidation
potentials [Eox vs Fc0/þ (ne)] of 2 were determined to
be -0.30 (1e) and þ0.18 (3e), and the second oxidation
process was demonstrated to be two separate one-electron-
oxidation processes [-0.12 (1e) and -0.06 (1e)] by digital
simulations (see Figure S2b in the Supporting Information).
Hence, 2 is stably oxidizable up to hexacation. The lower
Figure 1. Cyclic voltamograms of (a) 1 and (b) 2, in CH2Cl2/0.1 M
n-Bu4NBF4 at 298 K (scan rate 100 mV s-1).
Table 1. Redox Potentials (V vs Fc0/þ) and Potential Differences of 1-5
and TAPD in CH2Cl2 at 298 K
E1
E2
E3
E4
E5
E6
ΔE1-2
1a
2a
3a
4e
5a
-0.28 -0.03 þ0.28 b
0.79
0.96
0.25
0.18
0.30
0.33
0.36
0.48
-0.30 -0.12
-0.06
0.18 c 0.76 0.99
0.90
-0.19 þ0.11 þ0.71 d
-0.16 þ0.17
TAPDf -0.13 þ0.35
þ0.56
-0.11 þ0.25 þ0.83 d
a Reversible oxidation potentials were digitally simulated. b Quasi-
two-electron oxidation. c Quasi-three-electron oxidation. d Oxidation
peak potential (irreversible). e See ref 11f. The reported oxidation
potentials vs SCE were corrected by the redox potential of Fc0/þ vs
SCE (þ0.48 V in CH2Cl2).19 f See ref 18.
Chart 2. Related Compounds 4, 5, and TAPD

Article
Chem. Mater., Vol. 23, No. 3, 2011 845
first oxidation potentials of 1 and 2 indicate that the
present dendritic oligoarylamines are promising as hole
transport materials because of strong electron-donating
properties. The results are summarized in Table 1, together
with those of the related compounds [3, 4,11f 5, N,N,N0,
N0-tetraanisyl-1,4-benzenediamine (TAPD)18 (Chart 2)]
under the same conditions. It should be noted that the
potential differences between first and second oxidation
potentials (ΔE1-2) of 1 and 2 are smaller than that of
TAPD, in which the radical cation is stabilized by charge
delocalization via π-conjugation. This strongly suggests
that planarity (or π-conjugation) of 1 and 2 decreases to
some extent as compared to that of TAPD, because of
hyperbranched structures in 1 and 2.
or 26þ by using an optically transparent thin-layer electro-
chemical cell. First of all, we started to check the spectral
change from 5•þ to 52þ, because 5 can be considered to be a
good reference compound for the dendron groups in 1and 2.
Moreover, the previous studies have already established
that the radical cation of the unsubstituted compound of 5 is
in a charge resonance (CR) intervalence (IV) state.9e It should
be noted that the CR states in IV compounds contain-
ing three redox-active centers like 5 differ from those in
IV compounds with two redox-active centers. CR IV
compounds with multiple redox-centers have inevitably
unsymmetrical charge distribution, originating from the
Spectroelectrochemical Studies. To investigate the charge
distribution in the charged states of 1 and 2, we measured
the optical absorption spectral changes of 1 and 2 in CH2Cl2
during the course of the oxidation going from neutral to 14þ
Figure 2. (a) UV/vis/NIR spectra of the stepwise electrochemical oxida-
tion of 5•þ (in red) to 52þ (in blue) in CH2Cl2/0.1 M n-Bu4NBF4 at room
temperature. It was confirmed by global analysis with HypSpec20 that no
other species than 5•þ and 52þ contain in this spectral change. (b)
Schematic Mulliken charge distributions (in yellow) of 5•þ and 52þ at
the B3LYP/3-21G calculations.
Figure 3. UV/vis/NIR spectra of the stepwise electrochemical oxidation
of (a) 1 to 1•þ (in red) and 1•þ to 12þ (in blue) and (b) 12þ (in red) to 14þ (in
blue) in CH2Cl2/0.1 M n-Bu4NBF4 at room temperature. It was con-
firmed by the global analysis with HypSpec20 that no other species than 1,
1•þ, 12þ, and 14þ contain in these spectral changes.
Table 2. TD-DFT (UB3LYP/3-21G) Calculations of the Low-Energy Excitation Energies for 1•þ, 2•þ, 5•þ, and 52þ
TD-DFT calcn
species
hν (obs) (eV)
hν (eV) (f)
assignment
1•þ
0.75
1.18
0.52 (0.52)
0.90 (0.36)
0.92 (0.16)
0.50 (0.41)
0.86 (0.28)
0.94 (0.49)
1.38 (0.18)
1.16 (0.99)
1.52 (0.44)
2.03 (0.18)
348β (HΟΜΟ) f 349β (LUMO)
347β ((HO-1)MO) f 349β (LUMO)
346β ((HO-2)MO) f 349β (LUMO)
555β, 556β (HOMOs) f 557β (LUMO)
554β ((HO-1)MO) f 557β (LUMO)
176β (HOMO) f 177β (LUMO)
175β ((HO-1)MO) f 177β (LUMO)
176 (HOMOs) f 177 (LUMO)
2•þ
5•þ
52þ
0.67
0.99
0.99
1.62
1.13
∼1.65(sh)
175 ((HO-1)MO) f 177 (LUMO)
173 ((HO-3)MO) f 177 (LUMO)
2.34

846 Chem. Mater., Vol. 23, No. 3, 2011
Ito et al.
unsymmetrical molecular structures, whereas CR IV com-
pounds with two redox-active centers have usually symmet-
rical charge distribution. Note that the electronic
structure for the CR IV compounds can be explained by
the quantum chemical calculations such as MO and/or DFT
calculations. As shown in Figure 2, the lowest energy bands
at 0.99 eV (1250 nm) in 5•þ was changed into more intense
band at 1.13 eV (1100 nm) in 52þ with an isosbestic point at
1.01 eV (1225 nm). Judging from the DFT calculations of
5•þ and 52þ, the charge distribution can be predicted to be
completely changed; the generated charge in 5•þ is distributed
mainly on the inner triarylamine moiety, whereas the charge
in 52þ is distributed mainly on the outer triarylamine
moieties to avoid the electrostatic repulsion between two
charges (Figure 2). Herein note that the delocalized electro-
nic state in CR IV compounds with multiple redox-active
centers do not necessarily have the symmetrical charge
distribution among the redox-active centers. As is the same
as the radical cation of the unsubstituted compound of 5,9e
the lowest energy band of both 5•þ can be assigned to the
transition from β(HOMO) to β(LUMO) on the basis of the
time-dependent density functional theory (TD-DFT) calcu-
lations. In addition, the TD-DFT calculations showed that
the lowest energy band of 52þ is also due to the transition
from HOMO to LUMO. The calculated results are sum-
marized in Table 2 together with those for the next lowest
band of 5•þ and 52þ. Note that the difference in relative
intensity of the lowest energy band between 5•þ and 52þ is
well-reproduced by the calculated oscillator strengths. The
fact that the TD-DFT calculations can explain the observed
absorption spectral change strongly suggests that both 5•þ
and 52þ are in the CR IV states: the charge distribution for
the CR state of 52þ is determined by the electrostatic
repulsion between the charged centers.
In view of change in charge distribution between 5•þ
and 52þ, let us then turn to the spectral change observed
during the stepwise electrochemical oxidations of 1 and 2.
During the electrochemical oxidation of 1, a lowest
energy band appeared at 1650 nm (0.75 eV) with a
shoulder at 1050 nm (1.18 eV). This lowest energy band
for 1•þ continued to grow up until the oxidation to 12þ has
been completed with an isosbestic point at 390 nm
(3.18 eV) (Figure 3a). This observation suggests that 12þ
dislikes the quinoid structure, which is seen in the
dicationic state of TAPD and its derivatives, probably
due to the bulky dendron groups. By increasing the
electrode potential to the value of the third oxidation
process (12þ to 14þ), both of the blue shift (1300 nm
Figure 4. UV/vis/NIR spectra of the stepwise electrochemical oxidation
of (a) 2 to 2•þ (in red), (b) 2•þ (in red) to 23þ (in blue), and (c) 23þ (in blue)
to 26þ (in red) in CH2Cl2/0.1 M n-Bu4NBF4 at room temperature. It was
confirmed by global analysis with HypSpec20 that no other species than 2,
2•þ, 22þ, 23þ, and 26þ contain in these spectral changes.
(15) Hehre, W. J.; Radom, L.; Schleyer, P. v. R.; Pople, J. A. Ab Initio
Molecular Orbital Theory; Wiley: New York, 1986.
(16) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.;
Kudin, K. N.; Burant, J. C.; et al. Gaussian 03; Gaussian, Inc.:
Pittsburgh, PA, 2003.
(17) (a) Wolfe, J. P.; Wagaw, S.; Buchwald, S. L. J. Am. Chem. Soc.
1996, 118, 7215. (b) Driver, M. S.; Hartwig, J. F. J. Am. Chem. Soc.
1996, 118, 7217.
(18) Ito, A.; Inoue, S.; Hirao, Y.; Furukawa, K.; Kato, T.; Tanaka, K.
Chem. Commun. 2008, 3242.
(0.95 eV)) and increase in the intensity of the lowest energy
band were observed together with several isosbestic points
(Figure 3b). This spectral change in the lowest energy band
from 1•þ to 14þ showed the same behavior as that from
5•þ to 52þ. Hence, it is suggested that the charge distribu-
tion changes on going from 1•þ to 14þ, asshowninFigure5a.
Similar spectral change was also observed for the
electrochemical oxidation of 2, in which the conjugated
π-system is extended by the core triphenylamine and the
number of dendron groups. During the electrochemical
(19) Kelly, S. L.; Kadish, K. M. Inorg. Chem. 1984, 23, 679.
(20) (a) Gans, P.; Sabatini, A.; Vacca, A. Talanta 1996, 43, 1739. (b)
Gans, P.; Sabatini, A.; Vacca, A. Ann. Chim. 1999, 89, 45.

Article
Chem. Mater., Vol. 23, No. 3, 2011 847
Figure 5. Hypothetical charge distributions (in yellow) of (a) 1•þ to 14þ and (b) 2•þ to 23þ
.
oxidation of 2, a lowest energy band appeared at 1850 nm
(0.67 eV) with a shoulder at 1250 nm (0.99 eV) (Figure 4a).
By increasing the electrode potential to the value of the
second oxidation process (2•þ to 23þ), both the blue shift
(1300 nm (0.95 eV)) and increase in the intensity of the
lowest-energy band were observed together with some
isosbestic points (Figure 4b). This spectral change in the
lowest energy band from 2•þ to 23þ showed the same
behavior as that from 5•þ to 52þ. Hence, it is suggested
that the charge distribution changes on going from 2•þ to
23þ, as shown in Figure 5b. Moreover, at the electrode
potential of the third oxidation process (23þ to 26þ), further
blue shift (1300 nm (0.95 eV)) and increase in the intensity
of the lowest-energy band were observed together with
several isosbestic points (Figure 4b). This process corresponds
to further charge up on each of the charged dendron groups.
It should be noted that the typical localized triarylamine
radical cation bands (∼750 nm)4a,21 are not observed in
all the absorption spectra for the charged states of 1, 2,
and 5. This strongly suggests that the observed charged
states are not in localized IV states but in CR IV states.
Furthermore, as will be clarified in the next section, the
observed lowest energy bands can be considered as the
charge resonance (CR) bands originating from the delo-
calized electronic states. This means that the usual quantum
chemical calculations allow us to assign the lowest energy
absorption bands of the charged states of 1 and 2 to specific
electronic transitions. Hence, we carried out the TD-DFT
calculations14 on 1•þ and 2•þ. As is shown in Table 2, it
was found that the TD-DFT-calculated results qualita-
tively elucidated the characteristics of the observed lowest
energy bands including relative intensity. The lowest energy
band of 1•þ can be regarded as an overlap of three
transitions: the peak at 0.75 eV can be assigned to the
transition from β (HOMO) to β (LUMO), whereas the
(21) (a) Neugebauer, F. A.; Bamberger, S.; Groh, W. R. Chem. Ber.
1975, 108, 2406. (b) Schmidt, W.; Steckhan, E. Chem. Ber. 1980,
113, 577. (c) Dapperheld, S.; Steckhan, E.; Grosse Brinkhaus,
K.-H.; Ecsh, T. Chem. Ber. 1991, 124, 2557.

848 Chem. Mater., Vol. 23, No. 3, 2011
Ito et al.
Figure 6. Relative energy levels of the frontier MOs for (a) 1•þ and (b) 2•þ based on the UB3LYP/3-21G calculations. The arrows with solid and dashed
lines represent the major contributions to the lowest and next-lowest energy transitions, respectively.
shoulder at 1.18 eV can be assigned to a mixture of two
transitions; (i) from β ((HO-1)MO) to β (LUMO), and
(ii) from β ((HO-2)MO) to β (LUMO). Similarly, The
lowest-energy band of 2•þ can be regarded as an overlap
of two transitions: the peak at 0.67 eV can be assigned to a
transition from doubly degenerate β (HOMOs) to β
(LUMO), while the shoulder at 0.99 eV can be assigned
to a transition from β ((HO-1)MO) to β (LUMO). As is
apparent from the MO energy diagrams in Figure 6, these
transitions are explainable to be charge resonance be-
tween the peripheral dendron groups and the core amine
moieties.
ESR Studies. To further evaluate the electronic struc-
ture of the charged states of 1 and 2, the ESR spectra of
the chemically oxidized samples in CH2Cl2 were re-
corded. In addition, we also checked the ESR spectral
change from 5•þ to 52þ. The radical cation and dication of
5 were generated by treating with 1 and 2 equiv. of tris(4-
bromophenyl)aminium hexachloroantimonate in CH2Cl2
at 195 K. As shown in Figure 7a, the ESR spectrum of 5•þ
showed the similar spectral shape to that of the radical
cation for the unsubstituted compound of 5.9e On the
basis of the spectrum simulation, the hyperfine coupl-
ing constants were determined: |aN| = 0.660 mT (1N),

Article
Chem. Mater., Vol. 23, No. 3, 2011 849
Figure 7. ESR spectrum of 5•þ: (a) in CH2Cl2 at 290 K; (b) simulated.
Figure 8. ESR spectrum of 1•þ: (a) in CH2Cl2 at 290 K; (b) simulated.
Figure 10. Temperature-dependence of the ESR spectrum for (a) 1•þ and
(b) 2•þ in CH2Cl2.
the negligible hydrogen nuclei were incorporated in the line
width of the spectral simulation (0.20 mT) (Figure 7b). On
the other hand, the dication 52þ was ESR-silent, and
therefore, we could not obtain any information on the
charge distribution (Figure S4).
The ESR spectra of 1•þ and 2•þ generated by chemical
oxidation of 1 by 1 equiv. of tris(4-bromophenyl)aminium
hexachloroantimonate in CH2Cl2 at 195 K showed a
multiplet hyperfine structures (Figures 8 and 9). Note that
1•þ and 2•þ were found to be stable under anaerobic
conditions at room temperature for a few months, as
indicated by there being no loss in the ESR signal
intensity. The splitting pattern of 1•þ can be explained
by the presence of two and four equivalent nitrogen nuclei
and 4 equivalent hydrogen nuclei originating from the
central para-phenylene moiety, although the broadness of
the signals suggests additional small hyperfine coupling
constants due to the other hydrogen nuclei (Figure 8a).
The optimum simulation of the observed spectrum gave
Figure 9. ESR spectrum of 2•þ: (a) in CH2Cl2 at 290 K; (b) simulated.
|aN| =0.275 mT (2N), |aH|=0.100 mT (2H), |aH|=0.050 mT
(4H), |aH|=0.030 mT (8H), and the contributions from

850 Chem. Mater., Vol. 23, No. 3, 2011
Ito et al.
nitrogen nuclei and 12 equivalent hydrogen nuclei origi-
nating from the central triphenylamine moiety, although
the broadness of the signals suggests additional small
hyperfine coupling constants due to the other hydrogen
nuclei (Figure 9a). The best fit simulated spectrum was
obtained by the following hyperfine coupling constants:
|aN| = 0.455 mT (1N), |aN| = 0.218 mT (3N), |aN| = 0.047
mT (6N) and |aH| = 0.033 mT (12H), and the contribu-
tions from the negligible hydrogen nuclei were incorpo-
rated in the line width of the spectral simulation (0.10 mT)
(Figure 9b). Like 12þ, the generated 22þ also gave an ESR
spectrum with no hyperfine nor fine structures, and
therefore, we could not extract any information on the
charge distribution from the spectrum (see Figures S5b
and S6b in the Supporting Information).
It is interesting to note that the ESR spectral shape for
both 1•þ and 2•þ did not change in the temperature range
from -60 to 20 °C (Figure 10), and hence, the observed
temperature independency clearly demonstrated that the
both 1•þ and 2•þ are in CR IV states. As a consequence, it
was found that the generated spins in 1•þ, 2•þ, and 5•þ are
distributed over the dendritic molecular backbones, jud-
ging from the hyperfine coupling constants. These spin
distributions are in good accordance with the DFT-
calculated spin densities on 1•þ, 2•þ, and 5•þ, as shown in
Figure 11. Again, note that the delocalized electronic state in
CR IV compounds with multiple redox-active centers like
1•þ, 2•þ, and 5•þ do not necessarily have the symmetrical
charge distribution among the redox-active centers.
Conclusion
We have reported the synthesis and electrochemical,
spectroelectrochemical, ESR spectroscopic results of two
kinds of basic dendritic (or starburst) all-para-phenylene-
linked oligotriarylamines 1 and 2, which are generated
from para-phenylenediamin and triphenylamine as core
units, respectively. We have demonstrated that 1 and 2 are
oxidizable up to tetra- and hexacations, respectively, and,
according to the degree of oxidation, the charge distribu-
tion of the charged states of 1 and 2 gradually change so as
to reduce the electrostatic repulsion between increased
charges. Moreover, spin distribution in the CR state for
the radical cations of 1 and 2 was confirmed by the ESR
measurements. These findings are of great interest in
connection with the intermolecular hole hopping transfer
in the solid state of the oligoarylamines utilized in hole-
transport layer in OLEDs.22
Figure 11. DFT-computed spin density distributions (black: positive
spin, white: negative spin; spin isosurface value = 0.0003 electron/au;3
UB3LYP/3-21G) of (a) 1•þ, (b) 2•þ, and (c) 5•þ
.
the following hyperfine coupling constants: |aN|=0.455 mT
(2N), |aN| = 0.100 mT (4N), and |aH| = 0.060 mT (4H),
and the contributions from the negligible hydrogen nuclei
were incorporated in the line width of the spectral simula-
tion (0.10 mT) (Figure 8b). On the other hand, the
generated 12þ gave an ESR spectrum with no hyperfine
nor fine structures, and therefore, we could not extract
any information on the charge distribution from the
spectrum (see Figures S5a and S6a in the Supporting
Information).
Acknowledgment. The present work was supported by a
Grant-in-Aid for Scientific Research (B) (20350065) from
the Japan Society for the Promotion of Science (JSPS).
Supporting Information Available: Additional figures (PDF).
This material is available free of charge via the Internet at http://
pubs.acs.org.
€
(22) (a) Borsenberger, P. M.; Pantmeier, L.; Bassler, H. J. Chem. Phys.
1991, 94, 5447.(b) Borsenberger, P. M.; Weiss, D. S. Organic
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On the one hand, the hyperfine structure of 2•þ was
clarified by the presence of one, three and six equivalent