Spectroscopic Properties of the Lower-Lying Excited States of Fluorenone

Abstract

The spectroscopic properties of the lower‐lying electronically excited states of fluorenone have been investigated. A test for n—π^* transitions, based upon the effect of oxime formation on the absorption spectra, is discussed. Application of the oxime test to fluorenone, in conjunction with an examination of the emission properties, the solvent effects on the absorption spectrum, the vibrational structure of the fluorescence spectrum, the absorption spectra of related molecules, and the results of MO calculations clearly establish that the lowest excited singlet state of fluorenone is a (π, π^*) state, rather than an (n, π^*) state, as previously assigned. The principal emission observed from fluorenone in solution at room temperature and at 77°K, and from crystalline fluorenone, is demonstrated to be fluorescence. Both the fluorescence spectrum and the quantum yield for fluorescence were found to be solvent and temperature dependent. At room temperature in solution, the fluorescence quantum yield is about 1% and increases to about 12% upon cooling to 77°K. The fluorescence quantum yield from fluorenone crystals is nearly temperature independent, but decreases by a factor of 14 when the crystal is melted. The relation between the quenching of the fluorescence in fluid solutions and the concommitant shift of the emission spectra is discussed in terms of the interaction between excited state fluorenone molecules and polar solvent molecules. Very weak delayed fluorescence is observed from concentrated rigid‐glass solutions and zone‐refined crystals of fluorenone. In fluorenone crystals doped with 3.7×10⁻² mole/mole pyrene, the delayed fluorescence is considerably stronger than in the ``pure'' crystals, and strong pyrene phosphorescence is observed. Normal phosphorescence from fluorenone could not be observed from thoroughly purified material, but estimates of the singlet—triplet splitting, combined with the pyrene‐doping experiments and solution absorption spectra obtained using heavy‐atom solvents, indicate that the lowest triplet state of fluorenone is located near 560 to 570 mμ. Anomalous phosphorescence (τ=3.6 sec, λ_0–0=435 mμ, Φ≃0.01) has been observed, but only upon excitation of the fluorenone to a high‐lying excited singlet state (<330 mμ). It is proposed that this anomalous phosphorescence originates from some new chemical species which is produced photochemically in its excited triplet state from fluorenone.