A FLUORESCENCE STUDY OF AMINOPYRIDINES

The fluorescence properties of 2-, 3-, and 4-aminopyridines have been studied in a variety of solvents. The 2- and 3-isomers are very efficient fluorescers with quantum yields approaching unity in 0.1 N sulfuric acid. 4-Aminopyridine is characterized by a very weak fluorescence which is attributed to an n,π* lowest excited singlet and a second excited state which has large charge-transfer character. In contrast, the 2- and 3- isomers have more π,π* character in the lowest excited state. Determination of excited pK values for the 2- and 3-aminopyridines indicates that the excited state is a weaker acid than the ground state. Mirror-image plots of the fluorescence and absorption spectra for the 2- and 3- isomers were used to determine the O-O bands for a variety of solvents. Fluorescence quantum yields were measured by comparison to tryptophan.