We have theoretically studied the stable spin state of selected heteroatom- substituted π-conjugated systems. We employed several treatments of Hückel approaches, ab initio calculations, and a Neel state (Heisenberg-Ising) model consideration. All of them elucidate the fact that the different heteroatom arrangements in the π system cause the different spin-stable states of the singlet and the triplet. The possibility of the singlet-triplet instable state is further predicted. These treatments also reveal that the π-electron deficiency due to the replaced group III atom functions as a spin hole and that the excess π electron due to the replaced group V atom functions as a spin cap. We theoretically demonstrate these features via quinodimethane isomers in which two carbon atoms are replaced by heteroatoms, boron and nitrogen.
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