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.