1.
Current Research and Principal Research Interests
Our interest is centered
on the fundamental understanding of electronic properties of organic and
related compounds especially with open shelled electronic structures.
Design, synthesis, and characterization of new p-conjugated materials toward
the development of high-spin chemistry, molecular magnetism, organic and
organic-based conductors, and electroluminescent devices have currently
been investigated.
1) High-Spin Molecules
The study of high-spin molecules
is very informative on how the electronic spins interact in the molecule.
In addition, high-spin molecules are useful components as spin building
blocks to prepare the magnetic materials.
1-1) Heteroatomic Spin
Systems
These novel heteroatomic
di(radical cation)s have unique ground states.
The dication 1 has singlet
ground state, whereas 2 has triplet or pseudo-triplet ground state. The
unique behavior can be understood as electron-delocalization effect on
the lone pair electrons of sulfur atom which sits on the para-position
of the spin center (nitrogen position) in the clasical VB expression. The
delocalization of the lone pair electrons alters the electronic nature
very much and therefore the non-alternant hydrocarbon type prediction is
sometimes difficult.
1-2) SRSSP method
Moderately stable radicals
sometimes dimerizes in solution especially at low temperature and therefore
even the detection of radical species frequently fails. This is particularly
true for the species with multi-radical sites. We have developed a new
method called SRSSP (Sequential Redox Solid State Photolysis) to detect
such species. The method consists of the following two steps: 1)
reduction and oxidation of the monomeric dication or dianion to give diamagnetic
dimer or polymer type weakly bonded compounds in a solvent which can be
transformed into a matrix at low temperature, 2) cleavage of the weak bonds
giving the desired diradical by photolysis in the soft matrix which sometimes
easily available by just cooling close to the used matrix melting point.
2) Molecular Magnetic
Materials
Molecular magnetic materials
(MMM) have many advantages when compared to the classical metal magnets;
not only color or soft nature but also sensible nature toward outside stimulation
(light, field, pressure etc). Liquid crystalline magnets, very thin
film magnets, or molecular wire type magnets with quick photo-response
may not be dream. To realize MMM, fundamental studies based on currently
developing method and technologies; transition metal catalyzed organic
synthesis, crystal engineering, supramolecular architecture etc are essential.
2-1) Magnetic Interaction
and Molecular Size of Organic Compounds;
The degree of metal-metal
magnetic interaction between the metal-ligand-metal type organic-inorganic
hybrid complexes is strongly dependent on the metal-metal distance. Therefore
the size of over 10 diamagnetic element usually doesn't show any metal-metal
magnetic interaction. Introduction of spins in the diamagnetic organic
ligand may improve the poor magnetic interaction. Thus, one-dimentional
chain complexes 3 were designed and prepared. We could observe the
ferromagnetic interaction between Mn(II) and nitronyl nitroxide (below
10 K, J=+0.3 K). The ligand without nitronyl nitroxide showed no
magnetic interaction under the same conditions.
2-2) Spin-Ligands Synthesis;
Shortening the distance
between the coordination site and the organic spin converts the above spin-introduced
large bidentate ligand into spin-chelating reagents. Spin-chelating
reagents can be defined as chelating reagents with organic spins. This
project is now undergoing.
2-3) Stable Radical Introduced
Radical Cations
Radical substituted radical
ions are expected to be superior magnetic components. Some species
of this class have been synthesized. However, some species had very
weak magnetic interactions between the radical ion site and the organic
spin. Or some were very unstable under aerated conditions. As a result,
no stable radical-substituted radical ion with strong magnetic interaction
has been reported. We have recently synthesized a radical-substituted
radical cation with strong ferromagnetic interaction of J ª +300 K.
The species was air-stable at ambient or higher temperature.
3) Design and Synthesis
of Electronic Materials.
Electronic materials are
another class of target materials. Similar to the previous magnetic
materials, the design and synthesis of electronically unique p-conjugated
molecules are the most important step. Organic conductors involving
radical ion salts, oligomers and polymers, electroluminescent materials
are currently investigated.
3-1) Charge Transfer
Salts and CT-Complexes
Phenothiazine and phenoxazines
are well known superior donors because of their 8 p electronic systems.
The doubly condensed compounds must be much more superior donors.
Hitherto, benzothiazinopheno- thiazine and its radicalcation has been reported.
However, the isolation of the radicalcation salts has not been reported
probably because of their instability. N,N-Dimethylbenzoxazinophenoxazine
should have a lower oxidation potential than the corresponding sulfur analogue
because of the analogy of phenothiazine and phenoxazine analogues. We have
synthesized the oxigen analogues by multi-step reactions. The oxygen derivatives
were shown to have superior donating properties than the sulfur analogues
(0.2 V vs SCE for the sulfur N-Me derivative, +0.50 V vs SCE for the oxygen
N-Me derivative). The charge transfer complexes and their radical
ion salts were prepared and their solid state properties were characterized.
Their conductivities were in semiconductor region (10-3 Å`10-6 Scm-1).
3-2) Electroluminescent
Materials
Although the above mentioned
benzothiazinophenothiazine did not show good electron-donating ability,
the N,N-phenyl derivative (NPD) showed a superior electroluminescent property.
Two devices, ITO/DBP/NPD/Alq/LiF/Al and ITO/MTDATA /NPD/Alq/LiF/Al, were
prepared and compared. NPD and Alq are typical hole transporting
layer and light emitting layer, respectively. DBP was used as a hole
injection layer. This type multi-layered structure was recently introduced
to improve the light emitting efficiency. MTDATA is such a material
to enhance the efficiency and has been used frequently. The light
emitting efficiencies at 100 cd were 3.3 lm/W for DBP and 2.7 lm/W for
MTDATA. Thus DBP was proved to be a better hole injection material
than a typical hole injection material MTDATA.
3-3) Single Component
Conducting Materials
Usually conductivity of
organic compounds increases by hole injection with doping technique.
However, one can imagine single component conductive molecules without
doping. Such molecules should have small E1-sum. (the difference
of the first oxidation and reduction potentials) as well as small HOMO-LUMO
gaps in the molecular level. Molecular design and synthesis of such
amphoteric molecules are within our field and we are currently doing this
approach. The molecules with small E1 sum. will be also interesting
from the view point of conducting magnetic materials.
3-4) Forthcoming Projects
Recently, two new developments
have been achieved in the field of conducting materials. One is a FET (Field
Effect Transistor) technological development. The FET technique is
applicable to the wide range of molecules. Changing the applied voltage
of FET can freely alter the doping level of molecules. As a result,
some usual aromatic hydrocarbons have been converted superconductor states.
We are currently going to apply the FET approach by cooperation with physicists
in this university for deeper understanding molecular nature.
Another is an idea of a
molecular computer. We understand this term as “a single molecule
with controllable multi-functionality”. Characterization of a molecular
(not bulk but a single molecule) computer certainly needs nanotechnology.
The molecular computer must satisfy some fundamental requirements to be
characterized by nanotechnology in addition to its own electronic nature.
There are many unknown factors. However, this field can be considered
as a place where material science meets nanotechnology. The development
of this field is strongly dependent on the cooperation between physicists
and chemists and we as chemists like to do so.
2.
Selected Publications
1. Hiroki Mori, Osamai Nagao, Masatoshi Kozaki, Daisuke Shiomi, Kazunobu Sato, Takeji Takui, Keiji Okada, “Magnetic Behavior of Copper(II) Complexes of a Nitronyl Nitroxide-Substituted Pyrimidine”, Polyhedron, 20, 1663-1668 (2001).
2. Toshihiro Okamoto, Masatoshi Kozaki, Yoshiro Yamashita, Keiji
Okada, “Benzoxazinophenoxazines: Neutral and Charged Species”, Tetrahedron Lett., 42, 7591-7594 (2001).
3. Hiroki Mori, Masatoshi Kozaki, Kazunobu Sato, Takeji Takui, Keiji Okada, “A New Photochemical Approach to Benzylic Polyradicals through C-N Bond Cleavage of a Pyridinyl Radical”, Tetrahedron Lett., 39, 6315-6316 (1998).
4. Riho Suzuki, Masaji Oda, Atsushi Kajiwara, Mikiharu Kamachi, Masatoshi Kozaki, Yoshiki Morimoto, Keiji Okada, “Preparation and Characterization of Novel Organoborane Dianions”, Tetrahedron Lett., 39, 6483-6486 (1998).
5. Koji Nakatuji, Masaji Oda, Masatoshi Kozaki, Yoshiki Morimoto, Keiji
Okada, “4,4'-(Trimethylene)bis(2,6-t-butylphenoxy) Diradical:
An Application of the Sequential Redox-Solid State Photolysis (SRSSP) Method”, Chem Lett., 1998, 845-846.
6. Keiji Okada, Takaaki Imakura, Masaji Oda, Atsushi Kajiwara, Mikiharu Kamachi, Masakazu Yamaguchi, “Remarkable Heteroatom Dependence of the Spin Multiplicity in the Ground State of 9,9'-(m-Phenylene)dixanthyl and -dithioxanthyl Diradicals”, J. Am. Chem. Soc., 119, 5740-5741 (1997).
7. Keiji Okada, Kouzou Matsumoto, Masaji Oda, Kimio Akiyama, Yusaku Ikegami, “1,1'-(p- or m-Phenylene)bis(2,4,6-triphenylpyridinyl) Diradicals: Ground State in a Heteroatom-containing System in Relation to the Topology Rule”, Tetrahedron Lett., 38, 6007-6010 (1997).
8. Keiji Okada, Takaaki Imakura, Masaji Oda, Hisao Murai, Martin
Baumgarten, “10,10"-(m-and p-Phenylene)diphenothiazine Dication: Violation of Topology Rule in Heterocyclic High-Spin Systems”, J. Am. Chem. Soc., 118, 3047-3048 (1996).
9. Keiji Okada, Kazushige Okamoto, Masaji Oda, “A New and Practical Method of Decarboxylation: Photosensitized Decarboxylation of N-Acyloxyphthalimides via Electron Transfer Mechanism”, J. Am. Chem. Soc., 110, 8736-8738 (1988).
10. Keiji Okada, Hidekazu Sakai, Masaji Oda, “Stereocontrol by Energy Transfer: Photoisomerization of α-Arylalkyl Spiro-[cyclopropane-1,9'-fluorene]-2-carboxylate”, J. Am. Chem. Soc., 109, 5534-5535 (1987).
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