A study of the excited electronic states of 9-fluorenone

Abstract

Some spectroscopic properties of the low-energy electronic states of 9-fluorenone have been examined. The spectra in paraffin matrices at 4.2°K show detailed vibrational spectra. Two fluorescence spectra are observed; a diffuse emission arises from 9-fluorenone crystals in the paraffin matrix, and a sharp emission is characteristic of the molecule. The sharp fluorescence is analyzed in terms of known a 1 vibrational fundamentals. The sharp absorption is a near mirror-image to the fluorescence, so Herzberg-Teller vibrations are not prominent. The polarization in the crystal spectrum allows this low-energy transition near 23 000 cm −1 to be assigned 1B 2 ← 1A 1 . Because there is no vibronic perturbation in fluorescence, and certainly no out-of-plane modes, a π ∗ ← n transition seen at about 26 000 cm −1 is tentatively assigned 1B 1 ← 1A 1 . Another sharp absorption system is seen at 31 000 cm −1 in the paraffin matrices at 4.2°K (linewidth 6 cm −1) but no fluorescence was detected. The polarized crystal spectrum indicated the assignment of this system and another very strong system at 40 000 cm −1 to be 1B 2 ← 1A 1 , while other systems at about 34 000 cm −1 and 44 000 cm −1 are 1A 1 ← 1A 1 . The phosphorescence spectrum of pyrene- d 10 held in a single crystal of 9-fluorenone at 4.2°K has been recorded. No delayed fluorescence from the host crystal is observed at 4.2°K but is intense at 77°K. The energy difference between host and guest triplet levels is estimated to be about 900 cm −1 allowing the lowest triplet state of 9-fluorenone to be placed at 17 800 cm −1.