Organic Materials Chemistry

Keiji Okada (Professor)
http://www.sci.osaka-cu.ac.jp/chem/phyorg/POCweb/pg9.html

Current Research and Principal Research Interests
Selected Publications

1. Current Research and Principal Research Interests
Our interest is centered on the fundamental understanding of electronic properties of organicand related compounds especially with open shelled electronic structures. Design, synthesis,and characterization of new p-conjugated materials toward the development of high-spinchemistry, molecular magnetism, organic and organic-based conductors, and electroluminescentdevices have currently been investigated.
1) High-Spin Molecules
The study of high-spin molecules is very informative on how the electronic spins interact in themolecule. 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 uniqueground 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 ofsulfur atom which sits on the para-position of the spin center (nitrogen position) in the clasicalVB expression. The delocalization of the lone pair electrons alters the electronic nature verymuch 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 andtherefore even the detection of radical species frequently fails. This is particularly true for thespecies with multi-radical sites. We have developed a new method called SRSSP (Sequential1Redox Solid State Photolysis) to detect such species. The method consists of the following twosteps: 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 amatrix at low temperature, 2) cleavage of the weak bonds giving the desired diradical byphotolysis in the soft matrix which sometimes easily available by just cooling close to the usedmatrix melting point.

2) Molecular Magnetic Materials
Molecular magnetic materials (MMM) have many advantages when compared to the classicalmetal 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 molecularwire type magnets with quick photo-response may not be dream. To realize MMM, fundamentalstudies 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 ofOrganic Compounds;
The degree of metal-metal magnetic interaction
between the metal-ligand-metal typeorganic-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 without2nitronyl nitroxide showed no magnetic interaction under the same conditions.
2-2) Spin-Ligands Synthesis;
Shortening the distance between the coordination site and theorganic spin converts the above spin-introduced large bidentate
ligand into spin-chelating reagents. Spin-chelating reagents canbe 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 underaerated conditions. As a result, no stable radical-substituted radical ion with strong magneticinteraction 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 mostimportant 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 becauseof their 8 p electronic systems. The doubly condensed compounds mustbe much more superior donors. Hitherto, benzothiazinopheno- thiazine
and its radicalcation has been reported. However, the isolation of the radicalcation salts has notbeen reported probably because of their instability. N,N-Dimethylbenzoxazinophenoxazine
should have a lower oxidation potential than the corresponding sulfur analogue because of theanalogy 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 theirradical ion salts were prepared and their solid state properties were characterized. Theirconductivities were in semiconductor region (10-3 Å`10-6 Scm-1).

33-2) Electroluminescent Materials
Although the above mentioned
benzothiazinophenothiazine did not show goodelectron-donating ability, the N,N-phenyl derivative (NPD)
showed a superior electroluminescent property. Twodevices, 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 lightemitting 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 amaterial 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 abetter 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 andreduction 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 isa FET (Field Effect Transistor) technological development. The FET technique is applicable tothe 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 withphysicists in this university for deeper understanding molecular nature.
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Another is an idea of a molecular computer. We understand this term as “a single moleculewith controllable multi-functionality”. Characterization of a molecular (not bulk but a singlemolecule) 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 asa 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 PyridinylRadical”, Tetrahedron Lett., 39, 6315-6316 (1998).
4. Riho Suzuki, Masaji Oda, Atsushi Kajiwara, Mikiharu Kamachi, Masatoshi Kozaki, YoshikiMorimoto, Keiji Okada, “Preparation and Characterization of Novel OrganoboraneDianions”, 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 Stateof 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'-(por 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).

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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 ofDecarboxylation: Photosensitized Decarboxylation of N-Acyloxyphthalimides via ElectronTransfer 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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