Polarization of Electronic Transitions of Aromatic Hydrocarbons

The polarizations of the fluorescence, phosphorescence, and triplet–triplet absorption of a series of aromatic hydrocarbons were determined by the method of “photoselection.” The compounds studied; phenanthrene ${\mathrm{(h}}_{10}\mathrm{and}{d}_{10})$ , chrysene ${\mathrm{(h}}_{12}\mathrm{and}{d}_{12})$ , and picene exhibit relatively intense and well‐characterized luminescence as well as clearly defined singlet–singlet and triplet–triplet absorption spectra. All spectra including the singlet–singlet absorption spectra were measured in dilute solutions of organic glasses at 77°K. For all five compounds the polarization of the fluorescence was primarily in‐plane short axis, the phosphorescence principally out of plane, and the T–T absorption mainly in‐plane long axis. Variations in the polarization of vibronic bands within a given transition were noted, as were differences in detail between light and heavy compounds. The polarization results permit the assignment of the states involved in these transitions in addition to confirming the $\pi$ , $\mathrm{\pi *}$ nature of the upper triplet state.