Electronic and Molecular Structures ofp-Phenylenebis(phenylmethylene) in its Thermally Excited Triplet State as Studied by Single-Crystal1H-ENDOR

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Abstract
The thermally excited triplet state of p-phenylenebis(phenylmethylene) (p-PBPM) which is a topological isomer of the first organic high-spin molecule, m-phenylenebis(phenylmethylene) (m-PBPM, S=2), was examined using singlecrystal 1H-ENDOR (Electron-Nuclear DOuble Resonance) spectroscopy. From the 1H-ENDOR spectrum of the triplet state we found that the absolute sign of the fine-structure parameter D was negative and that p-PBPM in the triplet state has eight negative π-carbon sites and six positive ones. The π-spin density distribution determined experimentally indicates that the carbon atoms of the central p-phenylene ring have negative π-spin density. The spin-spin interaction between the induced π-spins causes the fine-structure parameter D to be negative. It is concluded that the π-π spin interaction predominates in the triplet state of p-PBPM.