FLUORESCENT SPECIES OF 7-AZAINDOLE AND 7-AZATRYPTOPHAN IN WATER

A study of the fluorescence lifetimes and quantum yields of 7-azaindole and its methylated derivatives N1-methyl-7-azaindole (1M7AI) and 7-methyl-7H-pyrrolo[2,3-b]pyridine (7M7AI) in water is performed in order to explain the observation that the fluorescence spectrum of 7-azaindole apparently consists of one band (lambda(max) = 386 nm) whereas in alcohols the spectrum is bimodal (e.g., for methanol, lambda(max) = 374, 505 nm). Careful measurements of the fluorescence decay as a function of emission wavelength indicate a small amplitude of an approximately 70-ps decaying component at the bluer wavelengths and a rising component of the same duration at the redder wavelengths. The small amplitude component, which comprises no more than 20% of the fluorescence decay, is attributed to excited-state tautomerization that is mediated by the solvent. Particular attention is paid to the pH dependence of the fluorescence lifetimes and yields. We propose that upon tautomerization the basic 1-nitrogen (N1) of 7-azaindole is rapidly protonated giving rise to a species whose emission maximum is at approximately 440 nm. The fluorescence emission maximum and lifetime of 7-azaindole is dominated by the 80% of the solute molecules that are blocked by unfavorable solvation from executing excited-state tautomerization. It is proposed that greater than or similar to 10 ns is required for the surrounding water molecules to attain a configuration about 7-azaindole that is propitious for tautomerization.