Assemblage of Organic Polyradicals with the Aid of Magnetic Metal Ions and Ordering of Their Spins in Macroscopic Scales

Abstract

Both the high-dimensional structure and the strong exchange coupling are two most important factors for ordering spins in molecular systems at finite temperature. The coupling should be ferromagnetic for homogeneous spins and can be antiferromagnetic for alternating spins of different magnitude. Thus scrutinization of a number of crystals of organic free radicals has led to the discovery of several purely organic ferromagnets, e.g., 1 that order at nearly absolute-zero temperature (T < 1.5 K)[1]. Here the required magnetic structures are fortuitously satisfied but the magnitude of the ferromagnetic exchange coupling is quite limited as expected for intermolecular interaction. Efforts to use the intramolecular ferromagnetic coupling which can be stronger in an order or two of magnitude have led to the synthesis of unprecedentedly high-spin polycarbenes such as 2 [2] or very stable polyradical 3 [3]. The intramolecular spin alignment in these systems is based on orthogonality of the singly occupied orbitals either of the one-centered diradical, carbenes, or the m-phenylene diradicals having right topological symmetry [4]. Since the one-dimensional spin alignment cannot in principle afford the spontaneous magnetization at finite temperature and is vulnerable to chemical defects in practice [5], the two-dimensional network structure 4 has then been employed as a long-range goal. While some constituent units 5 and 6 contained in 4 have been synthesized and proved to be the highest-spin hydrocarbons {S = 6 and 9 for 5 (n = 6 and 9)} ever reported [6], intermolecular interactions between the polycarbene molecules were found to be very weak or mostly antiferromagnetic. Too much branching seemed to cause coupling of the two carbene centers between the chains [7]. Thus rigid polymer network 4 itself remained to be synthesized.