On the evidence obtained by exciting 7-azaindole at 320 nm in 10-2 M solutions

Careful analysis of the UV absorption spectra for 2 x 10(-2) M solutions of 7-azaindole (7AI) in 3-methylpentane and n-hexane revealed that the onset of the first absorption band lies at ca. 335 nm, i.e., it is bathochromically shifted by 15 nm with respect to the onset of the band for the doubly-H-bonded dimer (D), so an additional aggregate species must exist in solution. Interestingly, this new species, like the doubly-H-bonded dimer, also emits in the region around 480 nm. The corresponding fluorescence excitation spectra confirm that the onset of the excitation lies between 335 and 340 nm. The presence of two different molecular structures in the red edge of the first absorption band, which undergo a proton transfer reaction on photoexcitation, seriously questions the model of Zewail et al. that because of two kinetic rates measured and ascribed to proton-transfer processes, the biprotonic transfer on photoexcitation in a 2 x 10(-2) M solution of 7AI excited at 320 nm is a nonconcerted mechanism.