Design, synthesis, and isolation of a Kekulé hydrocarbon with a triplet ground state is described. Its triplet ground state was unambiguously confirmed by ESR experiments, and the structure and fundamental physical properties were also revealed. The key feature of the molecular design is the decrease in the bonding interaction in the singlet state by aromatic stabilization of benzene rings and the increase of the exchange interaction of unpaired electrons which are favorable for the triplet state. These results contribute to the development of hydrocarbon-based organic magnetic materials.

Design, synthesis, and isolation of a Kekulé hydrocarbon with a triplet ground state is described. Its triplet ground state was unambiguously confirmed by ESR experiments, and the structure and fundamental physical properties were also revealed. The key feature of the molecular design is the decrease in the bonding interaction in the singlet state by aromatic stabilization of benzene rings and the increase of the exchange interaction of unpaired electrons which are favorable for the triplet state. These results contribute to the development of hydrocarbon-based organic magnetic materials.

Hydrocarbons with open-shell singlet and triplet ground states have long been studied. In contrast to studies of Kekulé hydrocarbons with an open-shell singlet ground state, studies of non-Kekulé and Kekulé hydrocarbons with a triplet ground state are quite limited, and no hydrocarbon with a triplet ground state has been isolated as single crystals. In this review, our work on the synthesis, isolation, and properties of m-quinodimethane-based non-Kekulé polycyclic hydrocarbon, a kinetically stabilized triangulene derivative, and Kekulé polycyclic hydrocarbon, a kinetically stabilized bisdibenzo[3,4:5,6]cyclohepta[1,2-a:2,1-d]benzene derivative, are described. Triplet ground states of these hydrocarbons were experimentally confirmed by ESR and magnetic measurements, and these are the first example of polycyclic hydrocarbons with a triplet ground state whose structures were characterized by X-ray crystal structural analysis. These studies will enable the development of various polycyclic hydrocarbon multi-radicals with high spin multiplicity.