High-Spin Organic Molecules and Molecular Assemblies Approaches to Organic Ferromagnets

Abstract

The electron spins in organic molecules and molecular assemblies have a tendency to pair off. Special molecular designs are necessary to achieve intra- and intermolecular spin alignments. Theories dictating a number of degenerate orbitals and the corresponding number of electrons in alternant hydrocarbons are briefly surveyed. Based on these theories, high-spin polycarbenes PhC(m-C₆H₄C)_n-1, Ph and 1, 3, 5-[PhC(m-C;₆H₄C)_n/3-1]₃C₆H₃ have been designed. The corresponding polydiazo compounds were synthesized, doped in single crystals of benzophenone, and photolyzed at cryogenic temperatures. The ground nonet state of the, linear tetracarbene, for example, was confirmed by its ESR fine structures and paramagnetic susceptibilities. The magnetic moment of the tetracarbene is the highest ever reported for molecular or ionic entities. Isomeric bis(diazo)[2.2]paracyclophanes were synthesized and photolyzed similarly to find, in good agreement with the McConnell's theory on the ferromagnetic intermolecular interaction between organic free radicals (1963), that the pseudoortho and pseudopara dicarbenes have the ground quintet state while the pseudometa isomer is in the ground singlet state. A strategy for increasing the spin ordering over the high-spin aromatic molecules by orienting the stacking mode was thus obtained. Relevance of these results to macroscopic ferromagnets is exemplified. Newer molecular design of high-spin polymers consisting of the polyacetylene, polydiacetylene or polyphenylene main chai n and carrying the radical centers in the side chains is quite promising from the point of view to construct new organic magnetic materials of practical use.