Polarization of the Triplet—Triplet Absorption Spectrum of Some Polyacenes by the Method of Photoselection

The method of photoselection is modified to measure the polarization of the triplet—triplet absorption of naphthalene-d8, phenanthrene, and chrysene. The three main bands of a given molecule are found to be similarly polarized. This fact, together with other spectroscopic arguments, excludes the previously proposed assignment that the absorption belongs to more than one electronic transition. While the absorption in naphthalene-d8 is found to be perpendicular to the 1La band (Platt's notation), that in phenanthrene is found to be parallel. Since the 1La band results from a transition which is polarized along the short axis in naphthalene-d8 but is polarized along the long axis in phenanthrene, the triplet—triplet absorption must then be polarized along the long axis in both molecules. Because the lowest triplet level of each molecule belongs to the 3La level, the excited triplet is assigned to the 3Ka level which corresponds to 3B3g— in naphthalene and 3B2— in phenanthrene (the y and z axes are taken as the long and short molecular axes respectively). Therefore, the 3Ag— level in naphthalene must have an energy which is either higher than 6.6 eV or lower than 4.2 eV. In either case, it shows that theoretical predictions of Pariser are good for the 3B3g— level but off by more than 1 eV for the 3Ag— level.