Excited-state tautomerization of 7-azaindole in water

The authors have studied the solvent dependence of the steady-state absorption, emission, and time-resolved emission of 7-azaindole (7AI) and two nonreactive analogues, N-methyl-7-azaindole (NMAI) and 7-methyl-7H-pyrrolo[2,3-b]pyridine (7MPP), in an effort to understand the apparently anomalous behavior of 7AI in water. The author`s find that 7AI undergoes the same tautomerization reaction via solvent-catalyzed double proton transfer in water as it does in alcohol solvents. Kinetic modeling shows that the unusual features of the 7AI emission in water arise mainly from quantitative changes in two key rate parameters. In water the rate constant for tautomerization (H{sub 2}O 1.2 x 10{sup 9} s{sup {minus}1} at 24 {degrees}C) is much slower and simultaneously the nonradiative decay rate of the product is much faster (5 x 10{sup 9} s{sup {minus}1} in both H{sub 2}O and D{sub 2}O) than in most alcohols, making observation of the reaction difficult. The reason the reaction rate in water is unusually slow appears to result from differences in the hydrogen bonding structure and dynamics of water compared to monoalcohols. 43 refs., 12 figs., 4 tabs.