Intensity enhancement of intraligand singlet–triplet π–π∗ transition for the coordinated β-diketonate in Ni(II) complexes with a nitroxide radical

Room temperature and low temperature magnetic circular dichroism (MCD) in the intraligand spin-forbidden singlet–triplet π–π∗ transition for the coordinated β-diketonate ligands were observed for the β-diketonato Ni(II) complexes with a chelated imino or nitronyl nitroxide radical, but not for the β-diketonato Ni(II) complexes without the radical ligands. This is elucidated by the borrowing mechanism from the singlet–singlet π–π∗ transition through the hypothetical interligand β-diketonate-to-radical charge transfer (LLCT) in contrast to the case of Cr(III) complexes.