The Excited-State Triple Proton Transfer Reaction of 2,6-Diazaindoles and 2,6-Diazatryptophan in Aqueous Solution

3-Me-2,6-diazaindole (<b>(2,6-aza)­Ind</b>) was strategically designed and synthesized to probe water molecule catalyzed excited-state proton transfer in aqueous solution. Upon electronic excitation (λ_max ∼ 300 nm), <b>(2,6-aza)­Ind</b> undergoes N(1)–H to N(6) long-distance proton transfer in neutral H₂O, resulting in normal (340 nm) and proton-transfer tautomer (480 nm) emissions with an overall quantum yield of 0.25. The rate of the water-catalyzed proton transfer shows a prominent H/D kinetic isotope effect, which is determined to be 8.3 × 10⁸ s^–1 and 4.7 × 10⁸ s^–1 in H₂O and D₂O, respectively. Proton inventory experiments indicate the involvement of two water molecules and three protons, which undergo a relay type of excited-state triple proton transfer (ESTPT) in a concerted, asynchronous manner. The results demonstrate for the first time the fundamental of triple proton transfer in pure water for azaindoles as well as pave a new avenue for 2,6-diazatryptophan, an analogue of tryptophan exhibiting a similar ESTPT property with <b>(2,6-aza)­Ind</b>, to probe biowaters in proteins