Spin Alignment in Model Molecules for Organic Ferrimagnets as Studied by ENDOR/TRIPLE Spectroscopy

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Organic molecular based magnetism has been the focus of current topics in many fields of both pure and applied sciences. It is of crucial importance to substantiate the possible occurrence of various types of purely organic magnetism, particularly purely organic ferrimangetism which is defined as antiferromagnetically coupled organic hetero-spin assemblages with coherence length of long range.This program deals with the topologically spin-controlled molecular design and spin structure of the first organic hetero-spin system, 4-(diphenylmethylene)-4'-(m-phenylenebis (phenylmethylene)-ether 1, in which non-identical spins (S_i=1 and S_j=2) couple antiferromagnetically via an ether bridge in the ground state. The hetero spin system is a model for purely organic ferrimagnets via through-bond approaches, in which the underlining magnetic coupling is superexchange interaction via the bridge. The concept of the molecular design under study is associated with a counterpart of crystalline ferrimagnets via through-space approaches commonly exploited in magnetic materials. The single-crystal ENDOR spectroscopy has been applied to 1, featuring an unusually large negative spin polarization in the spin density distribution for the ground state (S=1). The finding discloses a physical insight into organic ferrimagnetism. The unusual negative spin polarization strictly arises from a salient feature of organic hetero-spin systems in the electronic ground state, directly showing that one molecular spin moiety with S_i (S_j=1) is inverted against the another with an inequivalent spin S_j (S_j=2) in antiferromagnetically coupled hetero-spin systems. The spin-inversion is responsible for such an unusually large negative spin polarization that chemistry so far has never expected. It is referred to a new aspect of negative spin polarization in chemistry.