POLARIZATION MEASUREMENTS OF THE TRIPLET-TRIPLET ABSORPTION OF SOME AROMATIC HYDROCARBONS BY THE METHOD OF PHOTOSELECTION

Abstract

“The method of photoselection is modified to measure the polarization of the triplet-triplet absorption of naphthalene-$d_{8}$, phenanthrene and chrysene. The three main bauds of a given molecule are found to be similarly polarized. This, together with other spectroscopic arguments excludes the previously $proposed^{1}$ possibility that the absorption belongs to more than one electronic transition. While the absorption in naphthalene-$d_{8}$ is found to be perpendicular to the $^{1}\lambda_{a}$ band (Platt’s $notation^{2}$) that in phenanthrene is found to be parallel. Since the $^{1}\lambda_{a}$ band results from a transition which is polarized along the short in naphthalene-$d_{8}$ but is polarized along the long axis in phenanthrene, the triplet-triplet absorption must then be polarized along the axis in both molecules. Because the lowest triplet level of each molecule belongs to the $^{3} L_{a}$ level, the excited triplet is assigned to the $^{3} K_{a}$ level which corresponds to $^{3}B^{-}_{3g}$ in naphthalene and $^{3} B^{-}_{2}$ in Phenanthrene (they and z axes are taken as the long and short molecular axes respectively). These conclusions indicate that theoretical $predictions^{1}$ are good far the $^{3}B^{-}_{3g}$ level, but are off by more than one electron than one electron volt for the $^{3}A^{-}_{g}$ level. “