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.