Crystal Control in Organic Radical Solids

Abstract

Recently, there has been substantial interest in the study of molecular-based magnetism. The synthesis of molecular crystals, macromolecules possessing organic radical, and/or paramagnetic metal ion as spin centers has been in progress based on various individual strategies [1-3]. More than a dozen purely organic radicals with bulk ferromagnetism caused by intermolecular magnetic coupling have been discovered by laboratories throughout the world [4-6]. Whereas these ferromagnetic orderings have been discovered fortuitously, methodology for designing intermolecular exchange coupling pathways has not been established yet. As a guiding principle of intermolecular magnetic coupling, McConnell suggested conditions for ferromagnetic interaction in stacked radical molecules that possess both positive and negative spin densities [7]. According to his mechanism, ferromagnetic interaction would be achieved when the product of the spin densities becomes negative between interacting spin sites, each belonging to the neighboring radical molecules. Figure 1 illustrates an allyl radical, which has positive spin densities on the terminal carbon atom and negative spin density on the central carbon atom. The product of spin densities at interacting sites between the two allyl radicals is positive in case (a), whereas it becomes all negative in case (b). Consequently, case (a) is predicted to be at ground singlet state, while case (b) would lead to a triplet ground state.