A time-resolved study of the photoinduced tautomerization of 2-aminopyridine and derivatives within specific 1:1 complexes with carboxylic acids
Double-proton transfer in electronically excited complexes of 2-aminopyridine and carboxylic acid molecules has been investigated by time-resolved fluorescence spectroscopy. We show that the 1:1 complexes are formed in dilute solutions of the components in alkanes. In these solutions the proton transfer proceeds with a rate constant larger than 1011 s–1. The activation energy for the process could not be determined owing to the formation of ion pairs consisting of a 2-aminopyridinium cation and a carboxylic anion at low temperatures.
The formation of ion pairs is shown not to occur when the carboxylic acid is replaced by barbital which forms stable complexes with two 2-aminopyridine molecules in the solid state. The time-resolved fluorescence of these solid-state complexes reveals that the proton-transfer reaction rate is faster than the resolution of the detection system, even at low temperatures (77 K).
Complexes of 4-amino-2-methyl-5H- benzpyrano [3,4-c] pyridin-5-one (BPP) with carboxylic acids dissolved in alkanes exhibit ultrafast proton transfer both at room temperature and at 77 K. The proton-transfer rate decreases substantially in neat acetic acid as the solvent. An activation energy of 3.4 kcal mol–1 has been determined for the process in these solutions. The relatively small value of the rate constant in neat acetic acid is attributed to the fact that the cyclic structure of the solute-solvent complex, encountered in the solutions in alkanes, is disrupted owing to interaction of the solute molecule with more than one acid molecule. Substantial solvent reorganization is then required to achieve a suitable geometry for proton transfer.