THE NATURE OF N-]PI-STAR TRANSITIONS OF DI-2-PYRIDYLETHANEDIONE

The electronic absorption spectra of di-2-pyridylethanedione (2, 2′-pyridil) have been measured in EPA at room temperature and 101K, and compared with those of benzil. The polarized absorption spectra of 2, 2′-pyridil and 2-acetylpyridine have been measured in the stretched PVA film at 101 K, the latter molecule having a half jr-electron framework of the former. It is found that, for instance, the weak band system of 2, 2′-pyridil in the wavelength region 320$430 nm consists of two electronic transitions, whose 0-0 bands are at 398 nm and 352. Onm. Both electronic bands are assigned as the transitions from the n-orbitals of the oxygen atoms to the -orbitals. The comparison between the observed and calculated splitting values of the two n∗ transition energies indicates that the 2, 2,-pyridil molecule is twisted around the central C-C bond by about 80° in solution. From the configuration analysis, it is found that the 398 nm band is due to the transition from the n-orbitals of the oxygen atom to the ∗-orbitals in the 2-acetylpyridrne skeleton to which the oxygen atom belongs, and the 352. 0 nm band to the transition from the n-orbitals of oxygen atoms to the ∗-orbital in which the n- and ∗-orbitals belong to the different 2-acetylpyridine skeletons. © 1990, The Chemical Society of Japan. All rights reserved.