Acidity Dependences of the Photoluminescences of Benzaldehyde and Acetophenone

Aldehydes and ketones derived from benzene are generally thought to phosphoresce but not to fluoresce (1). Although fluorescence has been observed from aldehydes and ketones derived from medium and large polycyclic aromatic hydrocarbons in hydroxylic solvents (2), benzaldehyde and aceto-phenone do not fluoresce even in water. However, in concentrated acid media we have observed intense fluorescence from the conjugate acids of benzaldehyde and acetophenone, an observation which has potential analytical significance. Moreover, in concentrated sulfuric acid media, we have observed phosphorescence from the cations derived from benzaldehyde and acetophenone enabling calculation of the dissociation constants of these cations in the lowest triplet state. These observations prompted the examination of the acidity dependences of the electronic spectra of benzaldehyde and acetophenone.