Knowledge Bank

7-Azaindole forms doubly-hydrogen bonded dimers in saturated hydrocarbon solutions. These solutions exhibit two fluorescence bands in the near ultraviolet. In addition to the normal violet fluorescence of 7-azaindole $F1(\lambda max\sim 3300$ \AA), a green fluorescence $F2$ appears prominently ($\lambda max\sim 4750$ {\AA}). This green fluorescence is interpreted as arising from a tautometric form of the dimer in which two protons have been transferred during the lifetime of the excited state of the dimer. The relative intensities of $F2$ and $F4$ have been examined as a function of temperature, exciting wavelength and concentration. Effect of deuteration of the pyrrolic hydrogen on luminescence is investigated. The luminescence properties of N-methyl-7-azaindole and other related derivatives were compared with that of 7-azaindole. These studies confirm our interpretation of excited-state two-proton tautomerism in 7-azaindole hydrogen-bonded dimer.