Study of Some Excited Singlet and Triplet Electronic States of Fluorene

The absolute intensity of absorption parallel to the $a$ and $b$ axis of a fluorene crystal has been measured from 3000 to 2300 Å and allowed the following interpretation to be placed on the solution spectrum: The 3000‐Å system of medium intensity and the stronger 2600‐Å system are long‐axis polarized, and a rather weak band at about 2730 Å is short‐axis polarized. The onset of a stronger short‐axis polarized transition above 2400 Å was observed. This analysis shows that fluorene has quite different electronic properties from phenanthrene and carbazole. The low‐temperature absorption and fluorescence was entirely consistent with these electronic assignments. Vibrational analyses of the absorption and fluorescence spectra of fluorene in a polycrystalline n‐heptane matrix at about 15°K are presented; intervals that formed overtones were tentatively assigned as totally symmetric fundamentals. Crystalline fluorene did not measurably phosphoresce when pure, but intense, blue phosphorescence of fluorene could be induced at about 6°K by the deliberate addition of dibenzthiophene as an impurity. A vibrational analysis of this phosphorescence revealed intervals identical with those in the fluorescence and phosphorescence of fluorene in n‐heptane.