An unpaired electron-based hole-transporting molecule: Triarylamine- combined nitroxide radicals

Article abstract of DOI:10.1039/b703790b

A durable nitroxide radical combined with a triarylamine moiety exhibited a hole-drift mobility of 6 × 10^-3 cm² V^-1 s^-1, to which the aminophenyl nitroxide structure contributed. The Royal Society of Chemistry.

Full text of DOI:10.1039/b703790b

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An unpaired electron-based hole-transporting molecule: Triarylamine-
combined nitroxide radicals{
Takashi Kurata,a Kenichiroh Koshika,a Fumiaki Kato,a Junji Kidob and Hiroyuki Nishide*a
Received (in Cambridge, UK) 13th March 2007, Accepted 10th April 2007
First published as an Advance Article on the web 14th May 2007
DOI: 10.1039/b703790b
application of nitroxide radicals was realized in the presence of
an electrolyte solution under wet conditions. Hole hopping based
on the redox reactions of radical molecules in dry or solvent-free
amorphous states have not been examined to date. Furthermore,
the ionization potential of typical nitroxide radical molecules has
been estimated to be around 25.2 eV,11 which is slightly higher
than those of previously reported hole transporting molecules, and
is expected to be appropriate for hole injection from an anode,
compared to other typical hole-transporting molecules.
A durable nitroxide radical combined with a triarylamine
moiety exhibited a hole-drift mobility of 6 6 1023 cm2 V21 s21
to which the aminophenyl nitroxide structure contributed.
,
Hole-transporting organic molecules have played an important
role in organic electronic devices, such as organic light-emitting
diodes,1 field-effect transistors2 and photovoltaic cells.3 The
requisites of such hole-transporting molecules include a reversible
redox property with a low ionization potential and a high hole
drift mobility in an amorphous state.4 Aromatic compounds, such
as arylamine5 and thiophene6 derivatives, have been explored as
hole-transporting molecules, and hole drift mobilities as high as
1023–1022 cm2 V21 s21 have been reported for triphenylamine-
In this paper, we describe for the first time the hole-transporting
properties of nitroxide radical molecules in an amorphous solid
state by synthesizing new radical molecules, i.e., N,N-bis(4-
methoxyphenyl)-4-(N-tert-butyl-N-oxyamino)phenylamine
(1),
combined thiophene derivatives,7
a value which has been
N,N-bis(4-methoxyphenyl)-4-(1-oxyl-3-oxide-4,4,5,5-tetramethyl-
imidazolin-2-yl)phenylamine (2) and N-{4-(N9-tert-butyl-
N9-oxyamino)phenyl}-N-(4-methoxyphenyl)-4-(1-oxyl-3-oxide-
4,4,5,5-tetramethylimidazolin-2-yl)phenylamine (3) (Fig. 1).
Nitroxide radical molecules 1 and 3 were expected to possess
considered the highest limiting value of hole drift mobility in the
amorphous state.4 These compounds possess a closed shell electron
configuration and form the corresponding cationic open shell
radicals (or ‘‘holes’’) via their one-electron oxidation, or by hole-
injection from an anode by an applied voltage. Hole-transporting
in these molecules is categorized as hopping conduction, which
could be described from the viewpoint of a chemical reaction
formula as an intermolecular repetitive redox reaction between the
neutral closed shell starting molecule and its cationic open shell
radical. However, to the best of our knowledge, hole hopping
using an unpaired electron on the singly-occupied molecular
orbital (SOMO) of an organic radical molecule with an open shell
electron configuration8,9 has not been reported. That is, we
expected hole hopping based on a repetitive redox or electron
exchange between the neutral radical molecule of the SOMO
configuration and the formed cationic molecule of the unoccupied
closed shell configuration. Such radical- or unpaired electron-
based hole hopping could be attractive because of the possible
realization of a new effective hole-transporting molecule, with a
lower oxidation potential compared to those of previously known
hole-transporting closed shell aromatic compounds.
the following characteristics: (i)
A high hole mobility,
additionally induced by an intermolecular overlapping among
the delocalized p-electrons over the entire molecule, (ii) a
possible redox property at a low oxidation potential due to the
aminophenyl nitroxide structure and (iii) a high thermal and
chemical durability during device fabrication. The nitronyl
nitroxide 212 was prepared as the control compound.
The radical molecules were prepared via several steps from
4-bromophenyl-N-tert-butyl-N-siloxyamine,13 N,N-bis(4-methoxy-
phenyl)aniline and 4-bromobenzaldehyde.{ These radicals were
durable under ambient conditions (half life time tK . 6 months).14
Very recently, we reported durable nitroxide radical molecules
as a new class of cathode-active materials for a secondary battery,
where the very rapid redox reaction of the unpaired electron on the
nitroxide radical was successfully utilized for the charging–
discharging process of a high rate battery.10 However, this
aDepartment of Applied Chemistry, Waseda University, Tokyo,
169-8555, Japan. E-mail: nishide@waseda.jp; Fax: +81-3209-5522;
Tel: +81-5286-3214
bDepartment of Polymer Science and Engineering, Yamagata
University, Yonezawa, 992-8510, Japan
{ Electronic supplementary information (ESI) available: Detailed synthetic
method, characterization, electrolytic ESR spectra and magnetic measure-
ments, and J–V–L plots of the organic devices. DOI: 10.1039/b703790b
Fig. 1 (a) Chemical structures of the radical molecules and (b) schematic
hole-transporting mechanism of 1.
2986 | Chem. Commun., 2007, 2986–2988
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The cyclic voltammograms of 1 and 3, with their typical
electrolytic ESR signals, are shown in Fig. 2. The monoradical 1
showed a reversible unimodal redox couple at 20.25 V (vs.
Fc/Fc+), and a three-lined ESR signal was observed at g = 2.006,
with the hyperfine coupling constant (aN) of 1.30 mT under
20.4 V, which disappeared with the application of an anodic
potential over 0.1 V. This redox couple of 1 was assigned to the
oxidation of the unpaired electron on the SOMO and the
reduction of the closed shell cation 1+ formed. On the other hand,
the diradical 3 displayed a multiplet signal at g = 2.0057 at a
potential of 20.50 V (Fig. 2(b)). The simulation, with a strong
exchange limit |J| . |a| (J = intramolecular exchange interaction)
between the unpaired electrons, gave hyperfine coupling constants
of aNO = 0.65 mT for the nitrogen of the nitroxide moiety and aNN
= 0.38 mT for the two equivalent nitrogens of the nitronyl
nitroxide moiety. The initial multiplet ESR signal changed to a
simple quintet signal with an intensity ratio = 1 : 2 : 3 : 2 : 1 and
aNN = 0.76 mT (twice that of the aNN for 3) in the potential region
20.10–0.20 V. These results indicated formation of the cationic
radical 3+ and the localization of the unpaired electron on the
nitronyl nitroxide moiety.15 While applying a potential of 0.20–
1.00 V, the ESR signal intensity decreased, ascribed to the
formation of the corresponding closed shell dication 32+. The
energy gap between the two SOMOs of the nitroxide moiety and
the nitronyl nitroxide moiety in molecule 3 was estimated to be
0.55 eV, based on the formal redox potentials (EK = 20.14 and
0.41 V). Cyclic voltammetry of monoradical 2 revealed two
oxidation potentials at 0.23 and 0.49 V. The former was assigned
to the redox of the triphenylamine, based on the electrolytic UV-
vis absorption spectroscopy,12 and the latter oxidation was due to
the nitronyl nitroxide; the first one-electron oxidation of 2 was
ascribed to the oxidation of the unpaired electron on the nitronyl
nitroxide moiety.
The ionization potentials (Ip) of 1, 2 and 3 obtained from
photoelectron spectroscopic measurements were 24.9, 25.4 and
25.1 eV, respectively. The Ip values of 1 and 3 were comparable to
that of copper phthalocyanine (Ip = 25.1 eV16), which could have
originated from the aminophenyl nitroxide structure of 1 and 3,
suggesting a low hole injection barrier at the indium tin oxide
(ITO) anode (24.9 eV)/radical layer interface.
The hole-drift mobility of radicals 1, 2 and 3 in polycarbonate
was measured by the time-of-flight method, as shown in Fig. 3.
Fig. 3 also shows the control hole-drift mobility data for the
equivalent mixture of tris(4-methoxyphenyl)amine and N-tert-
butyl-N-(4-tert-butylphenyl)oxyamine, which are the components
of the triarylamine-combined nitroxide radical molecule 1.
The hole-drift mobilities of 1 and 3 were 5.8 6 1023 and 7.8 6
1023 cm2 V21 s21 (at 0.47 MV cm21), respectively, which are
104 times higher than that of the control mixture (1027 cm2 V21 s21
)
and comparable to the previously reported mobility (1.1 6
1023 cm2 V21 s21) of a typical hole-transporting molecule,
e.g.,
N,N9-diphenyl-N,N9-bis(3-methylphenyl)-(1,19-diphenyl)-
4,49-diamine.17 This result indicates that the combined-molecular
structure of triarylamine and nitroxide radical or the aminophenyl
nitroxide structure in 1 and 3 has an excellent hole-transporting
property. The extended p-conjugation, involving an unpaired
electron over the aminophenyl nitroxide structure, could con-
tribute to intermolecular overlapping to yield the high hole-drift
mobility. On the other hand, the nitronyl nitroxide-combined
triarylamine 2 has a localized unpaired electron, resulting in a one-
order lower mobility than those of the nitroxide-combined
triarylamines 1 and 3. Current density (J) vs. electric field (E)
plots for 1 and 3 in the hole only devices ITO/radical:polycarbo-
nate/Au showed a gradual increase with the applied voltage, which
supports both the hole-injection and hole-transporting capabilities
Fig. 2 Normalized ESR intensity with cyclic voltammograms of (a) 1
Fig. 3 Hole-drift mobility vs. electric field plots of the monoradical 1
($), 2 (&), diradical 3 (#), and the blended film containing N-tert-butyl-
N-(4-tert-butylphenyl) nitroxide and tris(4-methoxyphenylamine) (e).
Device configuration: ITO/radical:polycarbonate (4–6 mm)/copper phtha-
locyanine (100 nm)/Al (20 nm).
and (b)
3
in dichloromethane solution (supporting electrolyte
(C4H9)4NBF4, Pt wire, Ag wire working electrode, quasi-reference
electrode referenced to Fc/Fc+). Inset: ESR signals during the electro-
chemical oxidation.
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Fig. 4 Current density–luminance–voltage characteristics of the OLED
device with configuration ITO/1:polycarbonate (20 nm)/NPD (40 nm)/
Alq3 (60 nm)/Ca (20 nm)/Al (100 nm).
of the nitroxide radical molecules. We have fabricated an OLED
device using 1 as the hole-injection layer, ITO/1:polycarbonate/
N,N9-di(1-naphthyl)-N,N9-diphenyl-(1,19-biphenyl)-4,49-diamine
(NPD)/tris(8-hydroxyquinoline) aluminum (Alq3)/Ca/Al. The
device including 1 exhibited a yellowish-green emission, with a
maximum intensity of 1800 cd m22 at 22 V (Fig. 4), which
exceeded that of a control device, ITO/CuPc:polycarbonate/
NPD/Alq3/Ca/Al (250 cd m22 at 22 V). This result indicates
that the triarylamine-combined nitroxide radical molecule is a
promising candidate for an efficient hole-transporting mate-
rial.
This work was partially supported by a Grant-in-Aid for
Science Research in Priority Area ‘‘Super Hierarchical Structures’’
from MEXT, Japan and the Research Project ‘‘Radical Polymers’’
at the Advanced Research Institute for Science & Engineering of
Waseda University. T. Kurata acknowledges the Research
Fellowships of the Japan Society for the Promotion of Science
for Young Scientists.
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indicated a triplet ground state for 3, with a exchange coupling
parameter of J/k = 26.1 K (x = magnetic susceptibility, T = temperature,
and k = Boltzmann constant): see ESI.{
Notes and references
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2988 | Chem. Commun., 2007, 2986–2988
This journal is ß The Royal Society of Chemistry 2007