Fluorescence characteristics of benzodiazepines in strong acid

The fluorescence characteristics of 10 substituted 1,4-benzodiazepines in strong acid solution were investigated. The compounds that fluoresce in the Hammett acidity region possess, or can form by enolization, an azomethine linkage in the 1,2- or 4,5-position. All benzodiazepines that fluoresce in strong acid show increases in fluorescence intensity with corresponding blue shifts as acidity increases. Two pKa's in the Hammett acidity region were observed for both the fluorometric and absorptiometric titrations of the benzodiazepines possessing a carbonyl in the 2-position. No evidence of excited state prototropism was observed.