Molecules and Materials with Unpaired Spins: In Search of Designer Magnetic Materials. I.
Since the time of Hückel, Hund,
Schlenk, Brauns, and Müller in the 1930's, scientists have been interested
in designing and understanding molecules which have sufficient atoms, but
insufficient bonds to fulfill the normal rules of valence (non-Kekulé molecules). In the 1950's,experimental techniques of cryogenic matrix isolation
and electron spin resonance (ESR) spectroscopy became sufficiently advanced
to allow major advanced in the investigation of organic diradicals during
the 1960's and early 1970's. While diradical generation and product trapping
studies dominated in this period, progress was being made on actual isolation
and observation of transient diradicals. Workers such as Dowd, Berson, and
Kothe (amongst others) blazed the way for people to generate and observe diradicals
in matrices by both ESR and UV-visible spectroscopy.
During this time various scientists became interested in
generating molecules with more than two unpaired electrons. Mataga and Itoh
in Japan developed a theoretical model based upon parity pairing of electrons,
whereby two-dimensional arrays of unpaired electrons could potentially be
incorporated into appropriate carbon-based sheets. Itoh (Osaka) and Wasserman
(ATT Bell) generated the first quintet ground state dicarbenes in matrix in
the 1960s. This triggered three decades of research by Itoh, Iwamura, and
their academic descendants into the generation of polycarbenes as models for
very high spin, organic ferromagnets. At present, organic molecules with ten
and more parallel, unpaired spins have been generated, using either polycarbene
generation techniques or Rajca's polyradical generation techniques. International
activity in the area is presently intense and varied, including the synthesis
of organic-inorganic complexes, charge-transfer complexes, and polymeric polyradicals.
Every two years since 1987 there has been an international symposium on the
chemistry and physics of designed magnetic materials.
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