Photophysics of 7-Azaindole, Its Doubly-H-Bonded Base-Pair, and Corresponding Proton-Transfer-Tautomer Dimeric Species, via Defining Experimental and Theoretical Results

Photophysical research on the experimental and theoretical bases for the biprotonic transfer in the doubly-H-bonded dimer of 7-azaindole (7AI) is reported. The spectroscopic properties of the 7AI monomer, the normal-tautomer dimer, and the proton-transfer tautomer dimer are delineated by absorption, fluorescence and excitation spectra. The monomeric 7AI molecule is shown to exist at 10 -6 M in hydrocarbon solution at 298 K, and the pure dimer at temperatures below 227 K in 10 -4 M solution in 2-methylbutane (2MB). Deuteration of the pyrrolic sites yields dimers which exhibit only normal-tautomer fluorescence, whereas the nondeuterated 7AI dimer yields unique proton-transfer tautomer fluorescence, indicating quantum-mechanical tunneling as the principal mechanism for the biprotonic transfer. Density functional theory (B3LYP/6-31G** framework) calculations on the ground-state potential energy curves conform to the experimental results.