Noboru Koga

Approaches to superparamagnetic polycarbenes and polynitrenes

Abstract Poly(m-phenylenemethynyls) and other conjugated polymers to which photolabile diazo or azido groups are attached in every repeating unit have been synthesized. When photolyzed and studied in situ by means of a Faraday balance equipped with a light guide at cryogenic temperatures, the hexa- and nonacarbenes obtained by stepwise synthesis were found to be in S = 6 and 9 ground states, respectively. Attempts to sythesize real polymer samples capable of showing S = 102∼3 were made. These materials would serve as prototypes for organic opto-magnetic recording devices.

Studies of the electronic structure of polyradicals by means of their magnetic properties

Having identified the limitations of the commonly used ESR spectroscopic method, we have employed a versatile method of measuring magnetic susceptibility and magnetization for the characterization of the ground state spins of di-, oligo- and polyradicals. Under these subjects are specifically included such items as the determination of a small singlet-triplet energy gap of < 1 K in conformationally fixed tetramethyleneethane (l), the demonstration of spin frustration or competing interaction in the ground state of antiferromagnetically coupled spins in triradical 3 and the dinuclear Mn(I1) complex 5, and the analyses of super high-spin (S = 7-9)

An Approach to Organic Ferromagnets. Synthesis and Characterization of Polymers Due to 1-Phenyl-1, 3-Butadiynes Carrying a Substituent in Doublet or Triplet State on the Phenyl Ring

Molecular design and attempted synthesis of superhigh-spin poly(l-phenyl-l, 3-butadiynes) carrying stable radicals, carbenes or nitrenes as ring substituents are reported. l-Phenyl-l,3-butadiynes having 4-chloro-3-(N-tert-butylhydroxylamino) (3a), 4-diazomethyl (4), 4-α-diazobenzyl (5) and 4-azido (6) groups have been prepared. The observed solid-state polymerization of 3a was estimated to take place along the a/c diagonal direction on the basis of its crystal structure. The solid-state polymerization of the diamagnetic precursors was monitored by IR absorptions and DSC. The exothermicity of 35 — 60 kcal/mol was found to be a sign of the expected 1, 4-polymerization reactions. Black microcrystalline polymer samples obtained by heating mixed crystals of 3a and the corresponding nitroxyl (3) (70 — 50%: 30 — 50%) at 140 ° C showed a broad X-band ESR signal at ca. 3100 G that showed geometrical anisotropy. The magnetic susceptibility showed an anomalous spin transition at ca. 250 K. Dark violet polymer samples from the diazo compounds 4 and 5 were photolyzed at 4.2 K within the cryostat of a Faraday balance and the magnetic measurements were made in situ. Both the magnetization curves and inverse X vs. temperature plots gave S = 1 with small negative Weiss constants. Similar results were obtained for the polyazide 6. The stackings of 4, 5 and 6 in crystals are considered not to be in line with the expected topochemical polymerization giving ferromagnetic coupling among the carbene and nitrene centers. Potentialities of these approaches to super high-spin molecules are also discussed.

Magnetic interaction between the photochemically generated triplet centers through the π-conjugated skeleton of PPV

Abstract PPV 1 is considered to be a high-spin polymer by theories on its topological symmetry and expected to have interesting magnetic properties. Dinitrenes 2 n and 3 n would serve as prototypes for evaluating the sign and magnitude of the intramolecular coupling in 1. 2 n and 3 n were photo-chemically generated from 2 a and 3 a, respectively, and found to have ground triplet states by means of ESR spectroscopy.

Approaches from High-Spin Organic Molecules to Organic Ferromagnets

Pseudo-two dimensional branched polycarbenes 1, 3, 5-[H(m-C 6 H 4 C:)n/3] 3 C 6 H 3 (2)(n = 6, 9) were prepared. The measurement of their magnetization on a Faraday balance equipped with a light guide has revealed that 2(n = 6) has a tridecet ground state (S = 6.0). The m-phenylene skeleton as a dependable ferromagnetic coupling unit has also been applied with success to the synthesis of persistent systems, i.e., 5, by replacing each carbene center in linear polycarbenes 1 with a nitroxide. A necessary degree of steric protection of the radical centers was disclosed. Extension to real polymers 4 gave an only partly successful result.

Photochemical production of highly ordered spins in organic solids

Abstract Poly(m-phenylenemethynyl)s and the related π-conjugated polymers to which photolabile diazo groups are attached as pendants have been prepared. They were photo-lyzed and studied in situ by means of ESR spectroscopy and magnetometry on a Faraday magnetic balance equipped with a light guide at cryogenic temperatures. Unprecedentedly high-spin polycarbenes have been characterized.