Spin Alignment in High-Spin Carbenes of Heteroatomic π-Conjugation; Invalidity of Simple Extension of “π-Topology Rule” for Homoatomic Systems

Abstract Spin alignment of heteroatomic π-conjugated systems, 2,6-, 2,4-, and 3,5-pyridinebis(phenylmethylene) (3,5-PY), in which heterocycle plays a ferromagnetic or antiferromagnetic linker between carbenic units, has been studied by random orientation ESR spectroscopy. The electronic structures of the isomers are important for the understanding of spin alignment vs. topology of the heteroatomic π-electron networks. In 2,6-PY and 2,4-PY, the perturbing nitrogen atoms are in active positions, giving rise to a larger influence on their spin structure than in 3,5-PY. A simple application of molecular Hund's rule is invalidated.